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A study of interlayer assisted friction welding of incompatible materials Neelam, Jayanth R. 1984

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A STUDY OF INTERLAYER ASSISTED FRICTION WELDING OF INCOMPATIBLE MATERIALS by JAYANTH R. NEELAM B.E., UNIVERSITY OF MADRAS, INDIA, 1980 M.TECH., INDIAN INSTITUTE OF TECHNOLOGY, DELHI, INDIA, 1982 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n FACULTY OF GRADUATE STUDIES DEPARTMENT OF MECHANICAL ENGINEERING We a c c e p t t h i s t h e s i s as c o n f o r m i n g to- t h e r e q u i r e d s t a n d a r d UNIVERSITY OF BRITISH COLUMBIA NOVEMBER, 1984 © JAYANTH R. NEELAM, 1984 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an advanced degree a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I agree t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y purposes may be g r a n t e d by t h e head o f my department o r by h i s o r h e r r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f The U n i v e r s i t y o f B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 DE-6 (3/81) ABSTRACT F r i c t i o n w e l d i n g has become an a c c e p t e d method of j o i n i n g many d i s s i m i l a r m a t e r i a l s and i s now b e i n g e x t e n s i v e l y u s e d i n i n d u s t r y . Even t h o u g h i t i s t h e s i n g l e most v e r s a t i l e method o f j o i n i n g s i m i l a r and d i s s i m i l a r m a t e r i a l s , t h e r e a r e s t i l l many more m a t e r i a l c o m b i n a t i o n s t h a t c a n n o t be w e l d e d by t h i s p r o c e s s or w h i c h r e s u l t i n an u n s a t i s f a c t o r y b r i t t l e w e l d . T h e r e a r e s e v e r a l a p p l i c a t i o n s f o r t h e s e m a t e r i a l s i n i n d u s t r y . The o b j e c t i v e o f t h i s work was t o overcome t h e s e 'no-weld' o r ' b r i t t l e - w e l d ' 1 i m i t a t i o n s . The p r o p o s e d p r o c e s s i n v o l v e s i n t r o d u c t i o n o f a t h i r d e l e m e n t i n t o t h e c o n v e n t i o n a l two-element f r i c t i o n w e l d i n g p r o c e s s . The t h i r d e l e m e n t u s e d i s a m a t e r i a l w h i c h forms a good f r i c t i o n w e l d w i t h b o t h o f t h e o t h e r two e l e m e n t s when w e l d e d s e p a r a t e l y . I f t h e two p a r e n t m a t e r i a l s a r e not c o m p a t i b l e when t h e y come i n t o d i r e c t c o n t a c t due t o t h e f o r m a t i o n o f i n t e r m e t a l l i c s and l a c k of c o n d i t i o n s i n d u c i v e t o good b o n d i n g , t h e t h i r d e l e m e n t , w h i c h forms good w e l d s w i t h b o t h o f t h e p a r e n t p a r t s , a c t s as a b u f f e r between t h e o t h e r two. W h i l e k e e p i n g them a p a r t , i t forms a j o i n t between t h e two i n c o m p a t i b l e p a r e n t m a t e r i a l s . Among s e v e r a l p r o b a b l e c a n d i d a t e s f o r i n t e r m e d i a t e m a t e r i a l s , o n l y a few meet t h e r e q u i r e m e n t t h a t t h e y c r e a t e p r o p e r c o n d i t i o n s f o r b o n d i n g . A f t e r a f r i c t i o n w e l d i n g machine had been d e s i g n e d and b u i l t , i t was s u c c e s s f u l l y t e s t e d t o w e l d b o t h s i m i l a r and i i d i s s i m i l a r m a t e r i a l s by b o t h t h e c o n v e n t i o n a l and t h e p r o p o s e d t h r e e - e l e m e n t p r o c e s s e s . Welds between c o m p a t i b l e m a t e r i a l s o f h i g h e f f i c i e n c y were o b t a i n e d by t h e c o n v e n t i o n a l p r o c e s s . F o r m a t e r i a l c o m b i n a t i o n s n o r m a l l y i n c o m p a t i b l e , j o i n t s were made p o s s i b l e w i t h t h e a i d o f t h e i n t e r m e d i a t e e l e m e n t s . V a r i o u s d e g r e e s of j o i n t - s t r e n g t h d e p e n d i n g upon t h e m a t e r i a l c o m b i n a t i o n s u s e d were o b s e r v e d . The s t r e n g t h of t h e j o i n t s o b t a i n e d between b r o n z e and s t e e l , f o r example, was i n c r e a s e d by a s much as 40% by u s i n g a c o p p e r i n t e r l a y e r . The weld s were s u b j e c t e d t o t e n s i l e t e s t s . M i c r o s t r u c t u r a l e x a m i n a t i o n s were c o n d u c t e d on t h e s e p a r a t e d s u r f a c e s t o d e t e r m i n e t h e mode o f f a i l u r e . E l e c t r o n m i c r o - p r o b e a n a l y s e s were a l s o c o n d u c t e d t o d e t e r m i n e i f t h e r e were any d i f f u s i o n of e l e m e n t s a n d / o r f o r m a t i o n o f i n t e r m e t a l l i c s . Q u a n t i t a t i v e a n a l y s e s were a l s o made on t h e d i f f u s e d e l e m e n t s . Some h y p o t h e s e s were p u t f o r w a r d as t o t h e c o n d i t i o n s t h a t d i c t a t e t h e j o i n t s t r e n g t h o b t a i n e d by t h e t h r e e - e l e m e n t p r o c e s s and a l s o t h e r e q u i r e m e n t s of m a t e r i a l s w h i c h may p e r f o r m a d e q u a t e l y as i n t e r m e d i a t e m a t e r i a l s . Table of Contents ABSTRACT . i i LIST OF TABLES v i . LIST OF FIGURES v i i v ACKNOWLEDGEMENT i x 1. INTRODUCTION 1 1.1 CLASSIFICATION 3 1.2 PHASES DURING FRICTION WELDING 4 1.2.1 FRICTION PHASE 4 1.2.1.1 STAGE I 5 1.2.1.2 STAGE II 6 1.2.1.3 STAGE I I I , 6 1.2.1.4 STAGE IV 8 1.2.2 FORGE PHASE 8 1 . 3 FLASH 9 1.4 INERTIA FRICTION WELDING 10 2. LITERATURE SURVEY 14 2. 1 ADVANTAGES 14 2.2 APPLICATIONS 17 2.3 EARLIER RESEARCH 21 3. PARAMETER ANALYSIS 27 3.1 RELATIVE VELOCITY 27 3.2 PRESSURE 30 3.3 BURN-OFF RATE / DURATION OF HEATING 34 3.4 FORGE PRESSURE 36 4. PRELIMINARY TESTS 39 4.1 WELDING TRIALS 39 i v 4.2 TESTS ON WELDED SPECIMENS 43 5. THREE ELEMENT FRICTION WELDING 47 5.1 MATERIALS 47 5.2 REQUIREMENTS OF AN INTERMEDIATE MATERIAL 49 5.3 BONDING MECHANISMS 49 5.4 THE PROPOSED PROCESS 51 5.5 WELDING TRIALS 52 5.5.1 PRELIMINARY TRI-ELEMENT PROCESS 52 5.5.2 MECHANICS OF THE PROCESS 53 5.5.3 COPPER / BRASS COMBINATION 56 5.5.4 STEEL / BRONZE COMBINATION 59 5.5.5 TITANIUM / NICKEL COMBINATION 62 5.6 MICRO-PROBE ANALYSIS .64 6. CONCLUSIONS AND SCOPE FOR FUTURE WORK 69 BIBLIOGRAPHY 103 APPENDIX I 110 APPENDIX II . 118 APPENDIX I I I 119 v L I S T O F T A B L E S T A B L E P A G E I . R e l a t i v e w e l d a b i l i t y o f m a t e r i a l s 7 3 I I . M a t e r i a l c o m b i n a t i o n s w i t h i n t e r m e d i a t e m a t e r i a l s f r o m r e l a t i v e w e l d a b i l i t y c h a r t 7 4 I I I . E n v i s a g e d i n t e r m e d i a t e m a t e r i a l s 7 5 I V . P r o c e s s p a r a m e t e r s e t t i n g s f o r d i f f e r e n t m a t e r i a l c o m b i n a t i o n s 7 6 V . P a r a m e t e r s e t t i n g s f o r n a v a l b r a s s / s t e e l 7 7 V I . P a r a m e t e r s e t t i n g s f o r t i t a n i u m / a l u m i n u m / n i c k e l 7 8 V I I . Q u a n t i t a t i v e a n a l y s i s o f n i c k e l / a l u m i n u m i n t e r f a c e . . . 7 9 v i LIST OF FIGURES FIGURE PAGE 1. A l t e r n a t i v e arrangements f o r f r i c t i o n welding processes 80 2. C l a s s i f i c a t i o n of f r i c t i o n welding 81 3. Stages i n f r i c t i o n welding 82 4. Formation of the f l a s h 82 5. V a r i a t i o n of the s i z e and l o c a t i o n of the p l a s t i c i z e d r e g i on 83 6. R e l a t i o n between c o e f f i c i e n t of f r i c t i o n and speed....83 7. V a r i a t i o n of torque with pressure 84 8. V a r i a t i o n of r a t e of burn-off with pressure 84 9. Shape of heat a f f e c t e d zone with respect to speed and pressure 85 10. V a r i a t i o n of the c o e f f i c i e n t of f r i c t i o n with load....86 11. S t e e l specimens f a i l e d i n t e n s i o n with f l a s h 87 12. S t e e l specimens f a i l e d i n t e n s i o n without the f l a s h . . . 8 7 13. Schematic of the proposed three element process 88 14. F a i l e d aluminum specimens, welded by the three element method 88 15. F r a c t u r e s u r f a c e of s t e e l / n i c k e l / s t e e l specimen (X800 m a g n i f i c a t i o n ) 88a 16. F r a c t u r e s u r f a c e of copper/bronze/brass specimen (X1000 m a g n i f i c a t i o n ) .89 17. R e l a t i o n s h i p between s t r e n g t h and forge p r e s s u r e with b r o n z e / c o p p e r / s t e e l combination 90 18. F r a c t u r e s u r f a c e of s t e e l / c o p p e r / b r o n z e specimen (X40 magni f i c a t ion) 91 19. F r a c t u r e s u r f a c e of s t e e l / c o p p e r / b r o n z e specimen (X200 m a g n i f i c a t i o n ) 91 20. High spot on the f r a c t u r e s u r f a c e of s t e e l / c o p p e r / b r o n z e specimen (X400 m a g n i f i c a t i o n ) 92 v i i 21. X-Ray a n a l y s i s of a high spot on the steel/copper/bronze f r a c t u r e s u r f a c e 93 22. F r a c t u r e s u r f a c e of steel/copper/bronze specimen (X400 m a g n i f i c a t i o n ) 94 23. V a r i a t i o n of s t r e n g t h with f r i c t i o n time i n titanium/aluminum/nickel j o i n t s 95 24. V a r i a t i o n of s t r e n g t h with forge pressure i n titanium/aluminum/nickel j o i n t s 96 25. C o n c e n t r a t i o n p r o f i l e of titanium/aluminum i n t e r f a c e (Type III j o i n t ) .....97 2 6 . Scan f o r t i t a n i u m on titanium/aluminum i n t e r f a c e 98 27. Scan f o r n i c k e l on nickel/aluminum i n t e r f a c e (Type II j o i n t s ) 98 28. C o n c e n t r a t i o n p r o f i l e of nickel/aluminum i n t e r f a c e (Type III j o i n t ) 99 29. Scan f o r n i c k e l on nickel/aluminum i n t e r f a c e (Type I I I j o i n t s ) 1 00 30. Scan f o r n i c k e l on nickel/aluminum i n t e r f a c e (Type I I I j o i n t s ) 1 00 31. L o c a t i o n s of n i c k e l r e g i o ns s e l e c t e d f o r q u a n t i t a t i v e a n a l y s i s 101 32. Binary phase diagram of n i c k e l and aluminum ..102 v i i i ACKNOWLEDGEMENT The author would l i k e to express h i s s i n c e r e thanks to Dr. F. Sassani f o r the i n v a l u a b l e guidance and a s s i s t a n c e d u r i n g t h i s p r o j e c t and throughtout the program. S i n c e r e thanks are a l s o due to Dr. E. B. Hawbolt, department of me t a l l u r g y , f o r h i s help and expert comments d u r i n g the m e t a l l u r g i c a l a n a l y s e s . The most ab l e h e l p of Mr. Leonard Drakes i n f a b r i c a t i o n of the welding machine and of Mr. John Richards who put the e l e c t r o n i c c o n t r o l s tegether i s g r a t e f u l l y acknowledged. Thanks a l s o to Mrs. Mary Mager (dept. of me t a l l u r g y ) f o r the t e c h n i c a l a s s i s t a n c e while using the micro-probe a n a l y s e r and to Mr. John Hoar f o r p r o v i d i n g a l l the machine shop f a c i l i t i e s . Support f o r t h i s r e s e a r c h was pro v i d e d by the N a t u r a l Sciences and E n g i n e e r i n g Research C o u n c i l of Canada. i x 1. INTRODUCTION Welding can be broadly c l a s s i f i e d i n t o (1) Fusion welding and (2) Solid-phase welding. Fusion welding i s a very common process which i n v o l v e s m e l t i n g of parent metals, to which very f r e q u e n t l y a f i l l e r m a t e r i a l i n molten c o n d i t i o n i s added. Some of the so l i d - p h a s e welding processes are forge welding, e x p l o s i v e welding, r e s i s t a n c e welding and u l t r a s o n i c welding. Some of these processes, l i k e f o r g i n g , r e l y h e a v i l y on the flow of metals and hence they can only be a p p l i e d to d u c t i l e metals. J o i n t s can be made at room temperatures from very d u c t i l e m a t e r i a l s using h i g h l o c a l p r e s s u r e s r e s u l t i n g i n a p p r e c i a b l e deformations. There i s a minimum or ' t h r e s h o l d ' amount of deformation r e q u i r e d to ensure bonding. T h e o r e t i c a l l y , a l l d u c t i l e m a t e r i a l s can be j o i n e d i n t h i s f a s h i o n at room temperatures but the f o r c e s i n v o l v e d to o b t a i n a good weld would be p r o h i b i t i v e . The amount of p l a s t i c flow r e q u i r e d f o r a stron g j o i n t i s reduced markedly as the homologous temperatures are i n c r e a s e d . The i n c r e a s e i n temperature improves d u c t i l i t y , reduces the flow s t r e s s and the oxide f i l m s , and other contaminant s u r f a c e - l a y e r s p r e v e n t i n g bonding, are removed e a s i l y . S e v e r a l h e a t i n g methods l i k e flame h e a t i n g , r e s i s t a n c e , i n d u c t i o n e t c . are being used commercially and more r e c e n t l y FRICTION has been employed. When two work-pieces, of which at l e a s t one i s g e n e r a l l y symmetrical i n shape about i t ' s a x i s , are f o r c e d together and a r e l a t i v e motion i s imparted between them, there i s a 1 2 s u b s t a n t i a l g e n e r a t i o n of heat due to f r i c t i o n . The thermo-mechanical energy developed from the i n t e r a c t i o n of the two s u r f a c e s can be u t i l i z e d to develop c o n d i t i o n s c o n g e n i a l f o r welding. And t h i s process i s c a l l e d FRICTION WELDING. F r i c t i o n i s a very e f f i c i e n t source of heat and a l s o h e l ps i n s u r f a c e p r e p a r a t i o n by removing oxides and s u r f a c e f i l m s such as o i l and grease thus b r i n g i n g two v i r g i n s u r f a c e s together i n t o very c l o s e c o n t a c t and promoting bonding between them. The idea of welding by s e i z u r e caused by the rubbing of two s u r f a c e s together was f i r s t patented i n the United Kingdom d u r i n g e a r l y 40's but was not put i n t o use ^ 1 ^ . More than a decade l a t e r i n 1956, i t was proposed by A.I.Chudikov i n U.S.S.R to use f r i c t i o n as a source of r 2 ] heat to weld two metals . Subsequently a re s e a r c h team under the l e a d e r s h i p of V . I . V i l l , who i s c o n s i d e r e d as the pioneer of f r i c t i o n welding, made a comprehensive study of the p r o c e s s . The process was f i r s t p u b l i c i z e d i n the United S t a t e s i n 1959 by A.B.Tesmen, but the f i r s t a c t u a l work was c a r r i e d out i n l a t e 60's. I n d u s t r i a l a p p l i c a t i o n d i d not proceed i n Russia u n t i l the l a t e 50's, but by 1961 Russian l i t e r a t u r e r e p o r t e d t h a t over t h i r t y f i v e f r i c t i o n welding machines were i n use i n i n d u s t r y . 3 1 . 1 CLASSIFICATION F r i c t i o n welding can be broadly c l a s s i f i e d by the method by which the energy i s imparted to the work-pieces. One i s c a l l e d the 'continuous d r i v e ' method which i s more common and i s o f t e n c a l l e d the c o n v e n t i o n a l f r i c t i o n welding method. In t h i s method, the power i s p r o v i d e d c o n t i n u o u s l y by a motor and the r o t a t i o n i s maintained f o r a predetermined p e r i o d of time. The second method uses a flywhee l i n which the energy r e q u i r e d f o r the process i s f i r s t s t o r e d i n a flywh e e l and then r e l e a s e d as r e q u i r e d . Although the two processes are b a s i c a l l y the same, the thermal and mechanical p r o p e r t i e s of specimens produced by the two methods are d i f f e r e n t due to the c o n s i d e r a b l e d i f f e r e n c e s i n the r a t e of energy input and i n welding c y c l e times. The steps i n v o l v e d i n the continuous d r i v e f r i c t i o n welding are as f o l l o w s : A) One of the work-pieces i s r o t a t e d . B) The second p i e c e i s advanced a g a i n s t i t under some a x i a l t h r u s t . C) Contact pressure i s maintained f o r a predetermined d u r a t i o n of time or a x i a l displacement. The r e l a t i v e motion and the a x i a l p r e s s u r e cause the temperature t o r i s e at the i n t e r f a c e b r i n g i n g the m a t e r i a l to a p l a s t i c s t a t e . The extruded metal s t a r t s to form a c o l l a r , a l s o c a l l e d f l a s h . D) The r o t a r y motion i s stopped and the a x i a l f o r c e 4 i s maintained or i n c r e a s e d f o r a short d u r a t i o n to c o n s o l i d a t e the weld. S e v e r a l other a l t e r n a t i v e arrangements are p o s s i b l e f o r g e n e r a t i n g the r e l a t i v e motion and the a x i a l t h r u s t need not be l i m i t e d to the s t a t i o n a r y p i e c e . The c o n v e n t i o n a l method and some of the l e s s o f t e n used arrangements are shown i n f igure 1. ( i ) i s the most common arrangement, one p i e c e i s r o t a t e d while the other i s advanced a g a i n s t i t . In ( i i ) , one p i e c e i s spun and a l s o advanced. In ( i i i ) , two p i e c e s are advanced a g a i n s t a p i e c e r o t a t i n g between them. T h i s method i s e s p e c i a l l y u s e f u l when j o i n i n g two very long p i e c e s or two c o i l s of wire. In ( i v ) , two p i e c e s are spun and advanced a g a i n s t a common s t a t i o n a r y p i e c e . In g e n e r a l , f r i c t i o n welding can be c l a s s i f i e d as shown in f i g u r e 2. 1.2 PHASES DURING FRICTION WELDING The welding c y c l e can be d i v i d e d i n t o two main phases: the f r i c t i o n phase and the f o r g i n g phase. During the f r i c t i o n i n g phase, heat i s generated at the i n t e r f a c e and the metal i s made p l a s t i c , while the f o r g i n g stage c o n s o l i d a t e s the weld. 1.2.1 FRICTION PHASE The f r i c t i o n phase can be f u r t h e r d i v i d e d i n t o [3] four stages and these are summarized in f i g u r e 3. 5 1 . 2 . 1 . 1 STAGE I The two p i e c e s w i t h one of them r o t a t i n g a r e b r o u g h t t o g e t h e r . S l i d i n g t a k e s p l a c e between t h e u n l u b r i c a t e d s u r f a c e s . Due t o l a r g e a x i a l f o r c e s and i n h e r e n t r o u g h n e s s o f t h e two s u r f a c e s t h e r e i s i n t e r l o c k i n g o f a s p e r i t i e s . S t r o n g a d h e s i o n j o i n t s a r e formed a t t h e p o i n t s o f r e a l c o n t a c t and t h e r e i s s e i z u r e . S h e a r i n g t a k e s p l a c e a t a l l j u n c t i o n s where t h e m a t e r i a l i s weaker. T r a n s f e r o f m e t a l t a k e s p l a c e f r o m one s u r f a c e t o a n o t h e r . When one m a t e r i a l i s much h a r d e r t h a n t h e o t h e r , t h e h a r d e r m a t e r i a l p l o u g h s t h r o u g h t h e s o f t e r m a t e r i a l . The s i z e o f t h e m e t a l f r a g m e n t s and t h e r a t e o f t r a n s f e r d e p e n d on t h e t e m p e r a t u r e o f t h e i n t e r f a c e , t h e r u b b i n g s p e e d and t h e a x i a l f o r c e . A t h i g h e r t e m p e r a t u r e s , t h e r e i s an i n c r e a s e d a d h e s i o n o v e r l a r g e a r e a s due t o s o f t e n i n g o f m e t a l and a t s u s t a i n e d r o t a t i o n l a r g e r s e c t i o n s a r e s h e a r e d o f f . I n c r e a s i n g t h e s p e e d d e c r e a s e s t h e s i z e a n d f r e q u e n c y o f t r a n s f e r r e d p i e c e s and i n c r e a s i n g t h e a x i a l p r e s s u r e i n c r e a s e s t h e a r e a o f t h e t r a n s f e r r e d p i e c e s . The v a l u e of f r i c t i o n a l f o r c e ( w h i c h i n t u r n d i c t a t e s t h e t o r q u e ) depends on t h e o v e r a l l a r e a o f a c t u a l c o n t a c t and t h e s i z e and f r e q u e n c y o f t r a n s f e r o f m e t a l f r a g m e n t s . Hence r e d u c i n g t h e r u b b i n g s p e e d , i n c r e a s i n g t h e a x i a l 6 f o r c e r e s u l t i n an i n c r e a s e i n the r e s i s t i n g torque d u r i n g stage I. The areas of adhesion i n c r e a s e with the decrease i n speed and i n c r e a s e i n p r e s s u r e . The end of phase I i s c o n s i d e r e d to have a r r i v e d when the p i e c e s that are t r a n s f e r r e d and the m a t e r i a l at the i n t e r f a c e become p l a s t i c even as the temperature s t i l l r i s e s . 1 . 2 . 1 . 2 S T A G E I I Stage II i s a t r a n s i t i o n stage d u r i n g which the l a y e r of t r a n s f e r r e d fragments at the rubbing i n t e r f a c e become a l a y e r of p l a s t i c i z e d m a t e r i a l o f f e r i n g l e s s r e s i s t a n c e . The torque drops r a p i d l y d u r i n g t h i s stage. The adhesion p o i n t s c o a l e s c e to form a p l a s t i c i z e d l a y e r . Because of i n c r e a s e d p l a s t i c i t y of metal at the i n t e r f a c e , i t behaves almost l i k e a f l u i d of h i g h v i s c o s i t y and the r e s i s t i n g torque f a l l s . The m a t e r i a l s t a r t s to flow, forming a c o l l a r . 1 . 2 . 1 . 3 S T A G E I I I The temperature, r a t e of a x i a l s h o r t e n i n g , and the e u i l i b r i u m torque remain constant d u r i n g t h i s p e r i o d . A p l a s t i c i z e d l a y e r i s formed from the l a y e r of o r i g i n a l l y t r a n s f e r r e d m a t e r i a l s . The p l a s t i c i z e d l a y e r i s squeezed out i n c r e a s i n g the s i z e of the c o l l a r . At low speeds, the t h i c k n e s s of the c o l l a r i s even and l a r g e . At higher speeds, the t h i c k n e s s 7 of p l a s t i c i z e d l a y e r i s q u i t e narrow. At low speeds, the p l a s t i c i z e d l a y e r i s squeezed out but another p l a s t i c l a y e r i s formed immediately. The torque remains at a h i g h steady s t a t e as t h i s process of e x t r u s i o n , formation of new l a y e r c o n t i n u e s . A c o l l a r shown i n f i g u r e 4 c l e a r l y shows l i n e s on the c o l l a r which are r e p r e s e n t a t i v e of a a l t e r n a t i n g events of p l a s t i c l a y e r formation and e x t r u s i o n . As a s u b s t a n t i a l amount of heat i s l o s t due to the m a t e r i a l being squeezed out, there i s a steep temperature g r a d i e n t on e i t h e r f a c e s . The p l a s t i c i z e d l a y e r i s wide evenly at low temperatures and speeds. At higher speeds, the p l a s t i c zone moves inwards, towards the a x i s of r o t a t i o n , being narrow at the p e r i p h e r y and having a bulb shape near the r 3 ] c e n t r e . See f i g u r e 5. At low speeds, the deep p l a s t i c i z e d r e g ion extends o u t s i d e to the p e r i p h e r y of the specimen and the p l a s t i c i z e d m a t e r i a l i s not c o n f i n e d i n s i d e and i s squeezed out forming an unsymmetrical c o l l a r as seen i n f i g 5a. T h i s c o u l d be avoided with higher speeds and lower p r e s s u r e s where the deep p l a s t i c i z e d l a y e r i s c o n f i n e d w i t h i n the p e r i p h e r y as seen i n 5b and 5c. The m a t e r i a l j u s t behind the p l a s t i c i z e d r e g i o n i s heated to a hot-working temperature and t h i s metal comes out to form a uniform c o l l a r as i t c u r l s back under a x i a l p r e s s u r e . 8 i 1 . 2 . 1 . 4 STAGE IV D u r i n g t h i s s t a g e o f d e c e l e r a t i o n and s t o p p i n g , t h e d e e p p l a s t i c i z e d r e g i o n moves o u t w a r d s and t h e r e g i o n ' s t h i c k n e s s i s i n c r e a s e d . The f r i c t i o n a l b e h a v i o u r i s s i m i l a r t o t h a t o f low r u b b i n g s p e e d . The t o r q u e r i s e s c o n s i d e r a b l y a s t h e s p e e d f a l l s a s seen i n f i g u r e 3. The t o r q u e peak c o m p l e t e s t h e p l a t e a u b e f o r e i t f a l l s t o z e r o v e r y r a p i d l y . The r a t e o f a x i a l s h o r t e n i n g i n c r e a s e s due t o t h e i n c r e a s e d amounts o f m a t e r i a l b e i n g c h u r n e d o u t i n t o t h e c o l l a r . 1.2.2 FORGE PHASE D u r i n g t h e f o r g e phase i m m e d i a t e l y f o l l o w i n g f r i c t i o n i n g p h a s e , t h e a x i a l f o r c e i s i n c r e a s e d s u b s t a n t i a l l y t o f o r g e - w e l d t h e two s u r f a c e s . F o r g e w e l d i n g h e l p s i n h o t - w o r k i n g t h e j o i n t , i n c r e a s i n g t h e s t r e n g t h o f t h e w e l d z o n e . D i f f u s i o n i s one of t h e p r i m a r y mechanisms o f b o n d i n g i n f r i c t i o n w e l d i n g . Many s t u d i e s o f s o l i d - s t a t e d i f f u s i o n bonded j u n c t i o n s have shown t h a t p r e s s u r e s h i g h e r t h a n t h a t r e q u i r e d f o r c o m p l e t e a r e a c o n t a c t have a p r o n o u n c e d i n f l u e n c e on t h e bond q u a l i t y - h i g h e r p r e s s u r e s p r o d u c i n g bonds o f b e t t e r q u a l i t y . The h o t s t r e n g t h o f t h e m a t e r i a l s e t s t h e u p p e r l i m i t of t h e f o r g e p r e s s u r e . Too h i g h f o r g e p r e s s u r e s r e s u l t i n l a r g e c o l l a r s w i t h m e t a l b e i n g u n n e c e s s a r i l y 9 extruded out. The width of Heat A f f e c t e d Zone (HAZ) can e f f e c t i v e l y be reduced thus by pushing most of the HAZ i n t o the c o l l a r . But t h i s a l s o r e s u l t s i n e x c e s s i v e movement of metal t r a n s v e r s e l y , which i s u n d e s i r a b l e . The i m p u r i t i e s are r e o r i e n t e d i n t h i s t r a n s v e r s e d i r e c t i o n which r e s u l t s i n negative r 41 e f f e c t on f r a c t u r e and f a t i g u e p r o p e r t i e s . The pr e s s u r e s used f o r normal forge welding of m a t e r i a l s can be used as a guide to s e l e c t a forge p r e s s u r e , however the optimum s e t t i n g may be obtained e x p e r i m e n t a l l y . Too low pr e s s u r e s do not expel i m p u r i t i e s and there i s no proper c o n s o l i d a t i o n . Some m a t e r i a l s such as 1020 s t e e l do not respond to f o r g i n g and hence forge phase i s not r e q u i r e d while welding these m a t e r i a l s . 1 .3 FLASH The s o f t p l a s t i c m a t e r i a l i s extruded i n the process, r e s u l t i n g i n the formation of ' f l a s h ' c u r l or upset. The f l a s h i s coherent with the j o i n t but i t may not be r e q u i r e d . I t i s removed immediately a f t e r the weld i s completed while i t i s s t i l l hot, p l a s t i c and e a s i l y removable. T h i s i s done very f a s t and a u t o m a t i c a l l y i n most pr o d u c t i o n machines. The f l a s h c o u l d be removed by s h e a r i n g o f f or turned or ground down where a b e t t e r s u r f a c e f i n i s h i s r e q u i r e d . Even by shea r i n g method as i s done i n the case mentioned e a r l i e r , a f a i r l y good f i n i s h i s obt a i n e d . The process of shearing i s 10 as f o l l o w s . The a x i a l s l i p p a g e of the component i s prevented by using two backstops. A f t e r the normal f r i c t i o n , forge c y c l e s , one chuck r e l e a s e s the work-piece and one of the backstops i s removed. As the a x i a l p r essure i s s t i l l maintained, the welded p i e c e i s thus f o r c e d through a clamp with annular c u t t e r s and the f l a s h i s sheared o f f . In cases where a c c e s s i b i l i t y to the f l a s h poses a problem making i t i m p r a c t i c a l to remove i t , and the f l a s h c o u l d be an i n t e r f e r e n c e to normal o p e r a t i o n s , then a ' f l a s h - t r a p ' c o u l d be p r o v i d e d . F l a s h - t r a p i s a recess p r o v i d e d i n the m a t e r i a l adjacent to the j o i n t i n t e r f a c e so that the metal extruded c o u l d be accommodated i n t h i s a r e a . Thus e x t r a m a t e r i a l i n the area of concern c o u l d be avoided. The r e c e s s ' s volume depends on the kind of displacement / upset r e q u i r e d to achieve a good weld. T h i s method i s being e x t e n s i v e l y used in welding the ends of h y d r a u l i c c y l i n d e r bodies, d i e s e l [51 engine pre-combustion chambers, e t c 1.4 INERTIA FRICTION WELDING I n e r t i a f r i c t i o n welding or f l y w h e e l f r i c t i o n welding u t i l i z e s a d i f f e r e n t mode of energy source and the energy requirement f o r welding i s d e r i v e d from a r o t a t i n g f l y w h e e l . A f t e r the i n t r o d u c t i o n of continuous d r i v e method i n North America i n 1960, i t was m o d i f i e d by the C a t e r p i l l a r T r a c t o r Company i n 1962 to develop the so c a l l e d ' I n e r t i a F r i c t i o n Welding'. T h i s method i s more widely used i n the Un i t e d S t a t e s while the 'c o n v e n t i o n a l ' or 'continuous d r i v e ' method 11 i s more common elsewhere i n the world Due to inherent d i f f e r e n c e s i n the type of energy input r a t e s , c y c l e times, the weldment's mechanical p r o p e r t i e s vary even though both give 100% e f f i c i e n t j o i n t s . The rate of energy input i s r a t h e r high i n i n e r t i a f r i c t i o n welding. In i n e r t i a f r i c t i o n welding, the torques encountered are higher r e s u l t i n g i n higher r a t e s of b u r n - o f f . To produce the same amount of b u r n - o f f , the p r o c e s s time i n i n e r t i a welding i s s h o r t e r as compared to continuous d r i v e type. The method i s e s s e n t i a l l y the same as i n continuous d r i v e method with a r o t a t i n g p i e c e advanced a g a i n s t a s t a t i o n a r y p i e c e under constant p r e s s u r e . The s p i n d l e on which the chuck with the r o t a t i n g work-piece i s mounted a l s o c a r r i e s an interchangeable f l y w h e e l . Before the process i s s t a r t e d , the f l y w h e e l i s spun to a predetermined speed depending upon the amount of energy r e q u i r e d t o form a weld and then the two p i e c e s are f o r c e d t o g e t h e r . The f l y w h e e l energy i s consumed at the f r i c t i o n i n t e r f a c e and the speed of the s p i n d l e reduces r a p i d l y . In many cases when the speed of the f l y w h e e l f a l l s below a predetermined 'cut-out' speed, the f l y - w h e e l i s cut o f f from the s p i n d l e and the r e v o l u t i o n of the work-piece stops suddenly. A three stage model c h a r a c t e r i z e s the torque, upset (displacement) and speed h i s t o r y . I t i s r e c o g n i z e d to be more or l e s s s i m i l a r to the four stage continuous d r i v e ' s torque h i s t o r y but the displacement and speed c h a r a c t e r i s t i c s vary d r a s t i c a l l y . See f i g u r e 3. 1 2 Stage I t y p i f i e s dry f r i c t i o n where the a s p e r i t i e s are i n t e r l o c k i n g and are sheared o f f i n the process of r o t a t i o n . The torque has a peak value d u r i n g t h i s time and once the a s p e r i t i e s are l e v e l l e d o f f more and more areas are brought i n t o c o n t a c t . The torque drops from the peak as the e f f e c t s of i n i t i a l s t r a i n hardening are overcome by thermal s o f t e n i n g of the m a t e r i a l . Thermo-plastic flow i s i n i t i a t e d as soon as the s u r f a c e contaminations and a s p e r i t i e s are d i s r u p t e d . Due to a very r a p i d r a t e of energy i n p u t , p l a s t i c flow i s l i m i t e d to a very t h i n l a y e r of m a t e r i a l at the i n t e r f a c e without s i g n i f i c a n t i n f l u e n c e on most of the m a t e r i a l which amounts to a d i a b a t i c shear phenomenon. With a torque reaching a balance due to thermal s o f t e n i n g and s t r a i n hardening e f f e c t s , there i s a t r a n s i t i o n from stage I to stage I I . The process i s s e l f - a d j u s t i n g and the torque remains constant only f o r a short p e r i o d . There i s a steady d e c l i n e i n speed and a near constant r a t e of upset. During the l a t e r stages there i s a complex i n t e r p l a y between d e c r e a s i n g speed and power, and the i n c r e a s i n g s t r a i n r a t e and temperature. The torque does not remain constant f o r long as i n the case of continuous d r i v e because of c o n t i n u o u s l y d e c r e a s i n g speed. The second peak torque s e t s i n e a r l y due to the absence of the constant torque p e r i o d . The a d i b a t i c shear c o n d i t i o n does not e x i s t anymore with i n c r e a s i n g width of p l a s t i c zone and there i s a d e c r e a s i n g temperature g r a d i e n t along the l e n g t h of the work-piece. At t h i s time the t r a n s i t i o n from stage II to stage III o c c u r s . 13 T o r s i o n a l f o r g i n g p l a y s a major r o l e during stage I I I ; the temperature and speed continue to drop s t e a d i l y . As the temperature f a l l s , the m a t e r i a l becomes l e s s p l a s t i c and becomes s t i f f e r at the i n t e r f a c e which r e s u l t s i n another peak i n torque which i s c a l l e d ' t e r m i n a l torque' when the r o t a t i n g component st o p s . I t i s a l s o r e f e r r e d to by s e v e r a l other names such as 'plateau torque', ' a r r e s t torque' or [ 7 ] 'second torque peak' . Because of higher torques encountered d u r i n g the t e r m i n a l torque stage, the r a t e of burn-off i s higher i n f l y - w h e e l f r i c t i o n welding than i n the continuous d r i v e type. A c o l l a r i s formed with the s o f t m a t e r i a l f l u s h e d out i n a s p i r a l f a s h i o n from the c e n t r e of the i n t e r f a c e . The weld becomes r i g i d and the fl y w h e e l comes to a s t a n d s t i l l t e r m i n a t i n g the stage III and the pro c e s s . 2 . LITERATURE SURVEY 2 . 1 ADVANTAGES F r i c t i o n welding has found a wide range of a p p l i c a t i o n s i n recent years i n v a r i o u s i n d u s t r i e s . C u r r e n t l y an estimated 2 0 0 0 machines are i n use, producing more than 4 0 0 m i l l i o n F 8 1 j o i n t s a year 1 J . The reasons f o r adopting f r i c t i o n welding in p r e f e r e n c e to f l a s h butt welding, s h i e l d e d metal arc welding e t c . are many, important of them are as f o l l o w s : 1 . High e f f i c i e n c y i n the u t i l i z a t i o n of energy. 2 . Fast and r e l i a b l e way of making a j o i n t and very high r a t e s of p r o d u c t i o n c o u l d be achieved without l o s s of q u a l i t y . 3. E x c e l l e n t r e p r o d u c t i o n of j o i n t s i n terms of weld s t r e n g t h , low r a t e of r e j e c t i o n of p i e c e s due to d e f e c t s i n welds. A r e j e c t r a t e l e s s than 0 . 1 % f o r f g ] over 1 8 0 , 0 0 0 j o i n t s has been r e p o r t e d . 4. I t i s a process s u i t e d f o r mass p r o d u c t i o n , l e n d i n g i t s e l f w e l l to automation. 5. Very v e r s a t i l e i n j o i n i n g a wide range of m a t e r i a l s , both of s i m i l a r and d i s s i m i l a r combinations. Even n o n - m e t a l l i c substances l i k e p l a s t i c s , tungsten c a r b i d e can be j o i n e d . 6 . P o s s i b i l i t y of i n c o r p o r a t i n g an i n - p r o c e s s q u a l i t y c o n t r o l monitor. The present day machines sequenced i n p r o d u c t i o n l i n e s are operated by m i c r o p r o c e s s o r s . With d i g i t a l parameter s e t t i n g s and a v a i l a b i l i t y of 14 15 memory,-parameters can be changed and the machine can be set f o r d i f f e r e n t components i n very short p e r i o d s by car d s , tapes e t c . , thus making even small batch jobs economically f e a s i b l e . Very l i t t l e s u r f a c e p r e p a r a t i o n i s needed; saw-cut, sheared and even flame-cut s u r f a c e s can be j o i n e d . Grease and p a i n t do not pose a problem as they are e x p e l l e d from the i n t e r f a c e . M i l l s c a l e and some c o l d T 51 f o r g i n g l u b r i c a n t s are exceptions . But care must be taken to provide c l e a n , square-ended s u r f a c e s when j o i n i n g metals with s i g n i f i c a n t d i f f e r e n c e s i n the mechanical p r o p e r t i e s such as aluminum and s t e e l . Complete e l i m i n a t i o n of the use of f l u x e s or s h i e l d i n g gases and f i l l e r m a t e r i a l s . The process i s not very s e n s i t i v e to v a r i a t i o n s i n the process v a r i a b l e s . The process i n v o l v e s uniform heating a l l around the weld i n t e r f a c e hence e l i m i n a t i n g c e r t a i n welding s t r e s s e s and p a r t i c u l a r l y d i s t o r t i o n , l i k e those a r i s i n g from d i f f e r e n t i a l expansion, from a moving (p o i n t or l i n e ) heat source. As a s o l i d s t a t e p r o c e s s , i t i s c o n c e p t u a l l y s i m i l a r to a f o r g i n g process i n which base metal i s formed, c o n t r a s t i n g f a v o u r b l y with other processes which leave a cast s t r u c t u r e i n the weld zone. The process p h y s i c a l l y excludes a l l atmospheric a i r 16 from reaching and contaminating the weld; hence very c l e a n j o i n t s with minimum oxide formation r e s u l t . Even r e a c t i v e metals, l i k e t i t a n i u m , zirconium, t a n t a l l u m and niobium do not r e q u i r e any s p e c i a l a t t e n t i o n i n p r e p a r i n g f o r or d u r i n g the pr o c e s s . While t i t a n i u m needs a high degree of c l e a n l i n e s s f o r welding by other processes, i n f r i c t i o n welding i t i s t r e a t e d as any other m a t e r i a l . 12. A f a i r l y uniform weld over the e n t i r e i n t e r f a c e i s achieved by the normal f r i c t i o n welding process while f o r m a t e r i a l of s i m i l a r t h i c k n e s s , as i n l a r g e p i p e s , m u l t i p l e passes may be r e q u i r e d by a s h i e l d e d metal arc and other processes used. I t scores over the r i v a l f l a s h butt process ( f o r smaller j o i n t s ) with over 70-80% savings i n e l e c t r i c i t y alone and with b e t t e r q u a l i t y , l e s s e r r e j e c t i o n r a t e , h i g h e r p r o d u c t i v i t y . 13. Appearence of the weld i s not a f f e c t e d by s p a t t e r and undercuts and hence does not r e q u i r e a c l e a n i n g o p e r a t i o n . F l a s h may have to be removed f o r appearence or f o r design requirements but i s g e n e r a l l y sheared o f f or turned down and t h i s i s done very q u i c k l y . Many s i m i l a r and d i s s i m i l a r m a t e r i a l s can be j o i n e d e q u a l l y w e l l by t h i s p r o c e s s . While 100% e f f i c i e n t welds are not normally obtained, due to inherent g r a i n growth and the formation of b r i t t l e i n t e r m e t a l l i c s i n the welding area f o r 17 the welds made by the normal f u s i o n welding p r o c e s s e s , s e v e r a l i n s t a n c e s of a c h i e v i n g 1 0 0 % e f f i c i e n t welds by f r i c t i o n welding have been r e p o r t e d 1 1 ^  ' 1 1 1. For d i s s i m i l a r metal j o i n t s , the s t r e n g t h i s u s u a l l y as strong as the weaker m a t e r i a l , but at times, due t o the work-hardening e f f e c t , the j o i n t c o u l d be s t r o n g e r . However, the optimum p r o p e r t i e s can be achieved only by proper parameter s e t t i n g s . The process by i t s e l f i s q u i t e t o l e r a n t of reasonable v a r i a t i o n s i n parameter s e t t i n g s as compared with many other p r o c e s s e s . S t i l l , i t i s q u i t e necessary to determine optimum parameter s e t t i n g s f o r each s i m i l a r and d i s s i m i l a r m a t e r i a l combination due to inherent d i f f e r e n c e s i n m a t e r i a l p r o p e r t i e s and t h e i r responses when i n a p l a s t i c s t a t e e t c . 2.2 APPLICATIONS As the areas of a p p l i c a t i o n of f r i c t i o n welding are numerous, only those f i e l d s where the v i r t u e s of f r i c t i o n welding stand out over other p r o c e s s e s or where no other process can achieve the same r e s u l t s are given as examples. In the manufacture of t u r b i n e d i s c s f o r aero-engines, due to high temperatures and s t r e s s requirements, i t i s necessary to use c r e e p - r e s i s t i n g a l l o y s f o r d i s c s while the s h a f t s c o u l d be of a standard chromium - molybdenum s t e e l . These components were o r i g i n a l l y manufactured with e i t h e r as one p i e c e f o r g i n g s with design compromises or as a heavy b o l t e d c o n s t r u c t i o n , both o p t i o n s being u n d e s i r a b l e . Since 18 other welding processes, l i k e e l e c t r o n beam welding, reduce the s t r e n g t h of parent a l l o y m a t e r i a l , Rolls-Royce c o n s i d e r e d f r i c t i o n welding f o r j o i n i n g chromium -molybdenum s t e e l s h a f t with three other m a t e r i a l s used f o r the d i s c : Nimonic 90, J e t h e t e M152, Inconel 901 ^ 1 1 ^. The elements were then s u b j e c t e d to a 3-point bend t e s t and a l s o t e s t e d f o r f a t i g u e and t e n s i l e s t r e n g t h . The r e s u l t s of a l l three t e s t s were e n t i r e l y s a t i s f a c t o r y with f a i l u r e i n parent m a t e r i a l s . F r i c t i o n welding i s used i n the n u c l e a r e n g i n e e r i n g i n d u s t r y f o r the p r o d u c t i o n of t r a n s i t i o n j o i n t s , magnetic v a l v e s , beta chambers, vacuum v e s s e l s , heat t r a n s f e r equipment e t c . t 1 2 * ^ . T r a n s i t i o n j o i n t s comprise three metals, because of m e t a l l u r g i c a l reasons,'the e n d - s e c t i o n s cannot be welded together d i r e c t l y ; hence a t h i r d metal i s used as a sandwich. Some of the metal combinations used i n the n u c l e a r i n d u s t r y are given below with t h e i r a p p l i c a t i o n s : S t a i n l e s s S t e e l / 48% Ni i r o n E l e c t r o - m a g n e t i c v a l v e s S t a i n l e s s S t e e l / Cu (Oxygen f r e e ) Heat t r a n s f e r equipment S t a i n l e s s s t e e l / pure A l / ZW1 Mg a l l o y Cryogenic p i p e l i n e s S t a i n l e s s S t e e l / pure A l / Zr a l l o y Gas purge l i n e s , i n - p i l e vacuum v e s s e l s 1 9 The e l e c t r o - m a g n e t i c v a l v e s made by f u s i o n w e l d i n g d i d n o t meet t h e s p e c i f i c a t i o n s o f L l o y d ' s r e g i s t r y , t h e i n s u r a n c e company b a s e d i n London. The r e q u i r e d i n t e g r i t y was o b t a i n e d by f r i c t i o n w e l d i n g . As s t a i n l e s s s t e e l c a n n o t be w e l d e d t o z i r c o n i u m d i r e c t l y , i n making t h e i n - p i l e vacuum v e s s e l s , a s a n d w i c h o f p u r e aluminum was u s e d , a s i n t h e c a s e o f a b e t a chamber, where p u r e aluminum i s t h e s a n d w i c h between s t a i n l e s s s t e e l a nd H 6 aluminum a l l o y . When t h e l e n g t h o f t h e s a n d w i c h i n c r e a s e d , t h e t h e r m a l c y c l i n g p e r f o r m a n c e i m p r o v e d b u t a t t h e c o s t o f t e n s i l e s t r e n g t h . F r i c t i o n w e l d s have r e d u c e d c o n s i d e r a b l y t h e p r o b l e m s o f d e s i g n , o p e r a t i o n and r e l i a b i l i t y , e s p e c i a l l y w h i l e u s i n g d i s s i m i l a r m a t e r i a l j o i n t s , e l i m i n a t i n g complex f l a n g e s , g a s k e t s and b o l t e d c o n s t r u c t i o n s . A n o t h e r a r e a o f major i n t e r e s t i s i n t h e m a n u f a c t u r e o f e l e c t r i c a l c o n d u c t o r s . Aluminum has r e p l a c e d c o p p e r as c o n d u c t o r b e c a u s e of c o s t , w e i g h t and a v a i l a b i l i t y . But aluminum c a n n o t s u b s t i t u t e f o r c o p p e r c o m p l e t e l y b e c a u s e o f i t ' s s u s c e p t i b i l i t y t o c o r r o s i o n , t h e f o r m a t i o n o f i n s u l a t i n g o x i d e s and c r e e p u n d e r p o i n t s of s t r e s s . A l s o , t h e aluminum e q u i v a l e n t of a c o p p e r c o n s t r u c t i o n i s l a r g e r i n s i z e a nd c a n n o t be accommodated i n equipment d e s i g n e d f o r [ 1 2 3 ] c o p p e r c o n d u c t o r s or i n any e x i s t i n g e q u ipment . The e a r l i e r method, i n v o l v i n g t h e a r g o n p r o c e s s , p r o d u c e d a w e l d l a c k i n g i n i n h e r e n t r e p e a t a b i l i t y and q u a l i t y . F r i c t i o n w e l d s have overcome t h e s e s h o r t c o m i n g s o f e a r l i e r p r o c e s s e s , p r o d u c i n g sound j o i n t s , p r o v i d e d t h e c o p p e r p i e c e s a r e f a c e d 20 and degreased. In the automotive i n d u s t r y , the range of a p p l i c a t i o n s i s e x t e n s i v e . Renault of France uses over one m i l l i o n v a l v e s produced by f r i c t i o n welding each year with a b s o l u t e l y n e g l i g i b l e r e j e c t i o n r a t e s f 1 2 , 1 1 ^ . These v a l v e s , produced at the r a t e of over 700 u n i t s an hour by a double headed machine, are made of two d i f f e r e n t s t e e l s . The head i s made out of heat- and w e a r - r e s i s t a n t s t e e l , while the stem i s made out of l e s s expensive w e a r - r e s i s t a n t s t e e l . Axle cases fo r t r u c k s of a wide range of dimensions with c r o s s s e c t i o n a l areas from 2100 to 8400 sq.mm. are being produced by the same double headed machine at the r a t e of 120 u n i t s an hour [ 1 2 ' 8 ] . F r i c t i o n welding has become an accepted way of manufacture, e s p e c i a l l y where high p r o d u c t i o n r a t e s and j o i n i n g of d i s s i m i l a r m a t e r i a l s are i n v o l v e d . Anode hangers i n the aluminum i n d u s t r y , h y d r a u l i c j a c k s , h y d r a u l i c c y l i n d e r s , t w i s t d r i l l s (high speed s t e e l s with medium carbon s t e e l s ) , f l a n g e s f o r pipe work, a p p l i c a t i o n s i n the o i l and gas i n d u s t r y (pump s h a f t s , c o n t r o l v a l v e s - wear r e s i s t a n t m a r t e n s i t i c s t a i n l e s s s t e e l to c o r r o s i o n r e s i s t a n t a u s t e n i t i c s t a i n l e s s s t e e l ) , d r i l l p i p e s i n m i n e r a l and hydrocarbon e x p l o r a t i o n , heat exchanger p i p e s , e c c e n t r i c s h a f t s i n the packaging i n d u s t r y and innumerable p a r t s i n v o l v i n g s i m i l a r and d i s s i m i l a r m a t e r i a l s i n the automomile i n d u s t r y are some examples where f r i c t i o n welding i s being used e x t e n s i v e l y . 21 D i s s i m i l a r m a t e r i a l welds can be c h a r a c t e r i z e d as hard-to-hard or h a r d - t o - s o f t m a t e r i a l combinations, with or without the formation of i n t e r m e t a l l i c compounds as shown below . . . Hard / Hard ^ — : — ( ^ ) »-No i n t e r m e t a l l i c compound formed Hard / S o f t ( 4 ) ^ - I n t e r m e t a l l i c compound formed Examples of each one i s as f o l l o w s : 1. S t a i n l e s s s t e e l / Low a l l o y s t e e l 2. S t e e l / Titanium 3. S t e e l / Copper 4 . S t e e l / Aluminum The most d i f f i c u l t s i t u a t i o n to d e a l with i s hard / s o f t m a t e r i a l combination between which an i n t e r m e t a l l i c compound i s formed. The wide d i s s i m i l a r i t y i n s t r e n g t h of the m a t e r i a l s together with the formation of b r i t t l e i n t e r m e t a l l i c l a y e r s g e n e r a l l y r e s u l t i n a bad weld. 2.3 EARLIER RESEARCH T 1 3 1 Jessop et a l c a r r i e d out experiments i n the welding of s t a i n l e s s s t e e l with three other m a t e r i a l s , namely: aluminum, aluminum a l l o y and copper. T h e i r experiments were aimed at o b t a i n i n g optimum parameter s e t t i n g s and to determine the t o l e r a n c e l i m i t s of machine v a r i a b l e s . The 22 parameter s e t t i n g s were v a r i e d about the optimum s e t t i n g s to determine the l i m i t s w i t h i n which the bond s t r e n g t h i s s t i l l a c c e p t a b l e . T h e i r r e s u l t s show that the speed can be v a r i e d to a l a r g e extent f o r copper / s t e e l combination i n the range of 975 to 2190 rpm with l i t t l e e f f e c t on weld q u a l i t y . The v a r i a t i o n i n speed b r i n g s about a change i n the d u r a t i o n of the whole process but as there i s no formation of any i n t e r m e t a l l i c compound, extremes i n time and temperatures are t o l e r a t e d . I t i s a l s o r e p o r t e d that the process i s not so t o l e r a n t with respect to v a r i a t i o n s i n speed f o r s t e e l / aluminum combination. Any speed out of the range of 975±10% r e s u l t e d i n a bad weld. T h i s r e s u l t may be a s c r i b e d to the formation of b r i t t l e i n t e r m e t a l l i c m a t e r i a l s i n c e the process takes a longer time to complete at higher speeds. Growth of i n t e r m e t a l l i c s i s f a s t e r at higher temperatures and a l s o the growth i s d i r e c t l y p r o p o r t i o n a l to the time spent at that temperature. In l i n e with t h i s argument i t was shown that the i n t e r l a y e r was t h i c k e r f o r those aluminum / s t e e l j o i n t s produced at higher speeds. Some c o r r e l a t i o n between i n t e r m e t a l l i c t h i c k n e s s and weld s t r e n g t h was noted. When the t h i c k n e s s of the i n t e r m e t a l l i c was between the range of 0.2 and 1 mm, high s t r e n g t h welds r e s u l t e d . Jessop et a l a l s o c a r r i e d out r e s e a r c h as how the s u r f a c e c o n d i t i o n a f f e c t s the q u a l i t y of the weld. The q u a l i t y was observed to d e t e r i o r a t e q u i t e s i g n i f i c a n t l y i n case of aluminum / s t e e l weld. The r e s e a r c h e r s introduced contaminants at the i n t e r f a c e i n the form of grease and 23 machining l u b r i c a n t s . The q u a l i t y of the weld went down i n the case of aluminum but remained e s s e n t i a l l y the same f o r copper. The reason behind t h i s i s t h a t the temperature reached while welding aluminum i s around 600°c and lower, while i t i s almost 1 0 0 0 ° C f o r copper and most of the contaminants are burnt out. The main l i m i t a t i o n i n the s t r e n g t h of a j o i n t i n b i m e t a l l i c s i s the formation of i n t e r m e t a l l i c phases. While they a i d i n bonding d u r i n g the i n i t i a l stages, the i n t e r m e t a l l i c s are sought to be minimized as they are weak and b r i t t l e when they grow t h i c k e r ; c r a c k s i n i t i a t e and propogate e a s i l y and f a s t , l e a d i n g to an u n s a t i s f a c t o r y weld, weak i n t e n s i o n , bend or impact p r o p e r t i e s . In g e n e r a l , when using s o l i d - p h a s e p r o c e s s e s such as u l t r a s o n i c welding, e x p l o s i v e bonding, f r i c t i o n welding, i t i s sought to minimize ( i d e a l l y to prevent) the formation of T1 41 i n t e r m e t a l l i c phases. Wallach emphasizes on the knowledge of the r a t e s of n u c l e a t i o n and the growth of such u n d e s i r a b l e i n t e r m e t a l l i c s as e s s e n t i a l i f the welding techniques are to be opt i m i z e d to a c h i e v e a good weld. H i s paper o u t l i n e s a programme, the o b j e c t i v e of which was to prov i d e i n f o r m a t i o n on the r a t e s of n u c l e a t i o n and growth of i n t e r m e t a l l i c s and t h e i r e f f e c t on the j o i n t p r o p e r t i e s . The r e s u l t s of t h i s r e s e a r c h are u s e f u l i n a s s e s s i n g f a b r i c a t i o n techniques and s e l e c t i n g design c r i t e r i a f o r the f a b r i c a t i o n of s p e c i f i c b i m e t a l l i c composites of aluminum and copper welds. Even though the p r i n c i p l e s o u t l i n e d i n t h i s paper are 24 f o r an aluminum / copper combination, they have more general a p p l i c a t i o n than f o r t h i s system a l o n e . The aluminum / copper e u i l i b r i u m diagram shows that there are f i v e s o l i d phases which are s t a b l e at temperatures below 548°c. Crack propagation i s easy and r a p i d here at homologous temperatures of 0.65 Tffl . In some welding p r o c e s s e s , the time f o r h e a t i n g at the mating s u r f a c e s i s q u i t e s h o r t with minimal i n t e r d i f f u s i o n , hence no b r i t t l e media are formed. Cold r o l l bonding was used to f a b r i c a t e A l - C u - A l composites. In t h i s case, atomic c o n t a c t was made but i n t e r d i f f u s i o n i s minimized. The e l e c t r o n probe m i c r o - a n a l y s i s of Wallach of c o n c e n t r a t i o n - p e n e n t r a t i o n shows the c o n c e n t r a t i o n of each element at d i f f e r e n t d i s t a n c e s at the i n t e r f a c e . C e r t a i n p o i n t s were made from that a n a l y s i s such as 1. At low temperatures the i n t e r m e t a l l i c s d i d not grow as continuous l a y e r s ; there i s growth at d i s c r e t e r e g i o n s and only a f t e r c o ntinued growth f o r c e r t a i n p e r i o d s , a continuous l a y e r i s formed. 2. Due to the l a r g e c o n c e n t r a t i o n g r a d i e n t present at the i n t e r f a c e , the weld i s not a true r e p r e s e n t a t i v e of the e u i l i b r i u m diagram. Hence the e u i l i b r i u m diagram cannot be used to p r e d i c t the presence or compositions of i n d i v i d u a l phases. Since the growth of l a y e r s i s temperature-dependent and a l s o time-dependent, r a t e c o n s t a n t s are d e r i v e d f o r i n d i v i d u a l temperatures from g r a d i e n t s of l i n e s (from 25 c o n c e n t r a t i o n - p e n e n t r a t i o n curves) as Wn = Kt (2.1) where, W = width of i n t e r m e t a l l i c l a y e r t = d i f f u s i o n time K = r a t e constant f o r that p a r t i c u l a r temperature n = exponent, f o r volume d i f f u s i o n i s equal to 2 Hence the rate c o n s t a n t s allow us to p r e d i c t both i n d i v i d u a l and i n t e r m e t a l l i c phase widths. I n t e r d i f f u s i o n c o e f f i c i e n t s a l s o d e s c r i b e the r a t e s of m a t e r i a l t r a n s p o r t under i d e a l c o n d i t i o n s , such as i n pure metal or when one mechanism i s dominant, be r e l a t e d to a c t u a l p h y s i c a l atomic mechanisms of d i f f u s i o n . The temperature dependencies of both the r a t e constant and the i n t e r d i f f u s i o n c o e f f i c i e n t c o u l d be d e s c r i b e d by the T 1 4 ] A r r h e n i u s equation . E m p i r i c a l l y , D = K 0exp(-Q/RT) (2.2) where, D = d i f f u s i o n c o e f f i c i e n t (or r a t e constant) T = the p a r t i c u l a r temperature Q = a c t i v a t i o n energy K 0 = a constant R = the gas constant The two common d i f f u s i o n modes i n metals are volume d i f f u s i o n and s h o r t - c i r c u i t d i f f u s i o n , o c c u r i n g along c r y s t a l d i s c o n t i n u t i e s such as g r a i n boundaries, interphase boundaries and d i s l o c a t i o n s . While both modes operate 26 simultaneously, volume d i f f u s i o n predominates at higher temperatures and short c i r c u i t d i f f u s i o n at lower temperatures (below 1/2 T ). Hence to achieve and maintain adequate mechanical s t r e n g t h , the width of i n t e r m e t a l l i c phases at the i n t e r f a c e has to be c o n t r o l l e d . When data i s a v a i l a b l e on r a t e s of growth of i n t e r m e t a l l i c phases (using times and temperatures), i t can be used to ensure minimal i n t e r m e t a l l i c growth and produce j o i n t s with adequate p r o p e r t i e s . 3. PARAMETER ANALYSIS The f o l l o w i n g four parameters play an important r o l e i n the q u a l i t y of the weld i n the f r i c t i o n welding p r o c e s s : 1. R e l a t i v e v e l o c i t y 2. F r i c t i o n a l pressure 3. Burn o f f l e n g t h / D u r a t i o n of h e a t i n g 4. Forge pressure 3.1 RELATIVE VELOCITY For every m a t e r i a l there i s a minimum r e l a t i v e v e l o c i t y below which poor welds occur. The minimum speed f o r welding i s dependent upon the p h y s i c a l p r o p e r t i e s of the m a t e r i a l s to be welded i . e . , thermal c o n d u c t i v i t y , d e n s i t y , s p e c i f i c heat, m e l t i n g p o i n t e t c . Comparatively the y i e l d s t r e n g t h and the shape of the p a r t have l e s s e r i n f l u e n c e . The p e r i p h e r a l speeds u s u a l l y range from 15 m/min to 550 m/min. For example, a 2 mm diameter m i l d s t e e l stud r e q u i r e s a speed of about 76,000 rpm, (a p e r i p h e r a l speed of T1 51 477.5 m/min) J but f o r a bar of 76 mm diameter, the speed i s reduced to 320 rpm ( p e r i p h e r a l speed of 76.4 m/min). At lower v e l o c i t i e s , higher torques are produced, which c o u l d l e a d to work-holding problems and d i s t o r t i o n . Lower speeds c o u l d be advantageous while welding c e r t a i n d i s s i m i l a r metal combinations such as aluminum a l l o y (0.5% S i , 0.5% Mg) to m i l d s t e e l . Surface v e l o c i t i e s between 20 and 40 m/min minimize the formation of b r i t t l e i n t e r m e t a l l i c compounds at the rubbing s u r f a c e s r e s u l t i n g i n s t r o n g e r , more d u c t i l e 27 28 welds [ 8 ] . Radius r — Annular r i n g of 6r t h i c k n e s s C o n s i d e r i n g an elemental annular r i n g i n the weld s e c t i o n of r a d i u s r and r a d i a l t h i c k n e s s 6r, then the a x i a l f o r c e on t h i s area i s 27rpr.6r, where p i s the pressure d u r i n g f r i c t i o n phase. Hence the t a n g e n t i a l r e s i s t i n g f o r c e due to f r i c t i o n i s 2?rprM.6r where u i s the c o e f f i c i e n t of f r i c t i o n . T h i s has moment of 27rpMr 2.6r about the a x i s . Thus the amount of heat generated per u n i t time at t h i s elemental annular s u r f a c e when the angular v e l o c i t y i s co, i s given by 5H=27rcjpMr 2 6r If we assume u to be const a n t , f o r the moment, then the t o t a l heat generated at the weld s u r f a c e per,, u n i t time i s H=/6H= 2irtopvS* r 2 6 r H=2/3.7ropMR3 (3.1) The above equation shows that the r a t e of heat gener a t i o n v a r i e s p a r a b o l i c a l l y a c r o s s the s e c t i o n . An a l t e r n a t i v e r e l a t i o n has been suggested i n pr e f e r e n c e to one using p e r i p h e r a l v e l o c i t y and angular speed. Since the r e l a t i v e s l i d i n g v e l o c i t y i s p r o p o r t i o n a l to the d i s t a n c e from the c e n t r e of the bar to the o u t s i d e r a d i u s , "average p o i n t v e l o c i t y " v, d e s c r i b e d by the f o l l o w i n g equation, i s p r e f e r r e d [16] v = TTN[ (OD 3-ID 3)/(OD 2-ID 2) ] / l 080 (3.2) where v = average p o i n t v e l o c i t y i n fps N = angular speed i n rpm OD = Outside diameter i n inches ID = Ins i d e diameter i n inches T h i s equation e s s e n t i a l l y averages the v e l o c i t y at 2/3 of the r a d i u s of the bar. The average r a d i u s , i . e . , 2/3 r a d i u s i s used i n the c a l c u l a t i o n s r e l a t i n g to the sur f a c e s l i d i n g v e l o c i t y , torque and c o e f f i c i e n t of f r i c t i o n , while u n i t p r e s s u r e s r e f e r t o f u l l c r o s s - s e c t i o n a l a r e a . In the [13] s t u d i e s done by Jessop et a l on welding of s t a i n l e s s s t e e l with copper and aluminum, i t was observed that the gr e a t e s t t h i c k n e s s of the i n t e r m e t a l l i c formed always occured i n the region of mid-radius and not at the p e r i p h e r y d e s p i t e the f a c t that the r e l a t i v e v e l o c i t y i s maximum at the p e r i p h e r y . Thus, the heat g e n e r a t i o n may not be c o r r e s p o n d i n g l y maximum at the p e r i p h e r y . r 41 It has been observed by E l l i s t h a t t h e r e i s a s u b s t a n t i a l change i n other welding parameters with changes in speeds. E u i l i b r i u m torque i s i n v e r s e l y p r o p o r t i o n a l to speed. The r a t e of bur n - o f f which i s q u i t e important i n determining the q u a l i t y of the weld, i n c r e a s e s with the decrease i n speed. With r e s p e c t to p r o p e r t i e s , i t has been observed that there i s a s l i g h t decrease i n the u l t i m a t e t e n s i l e s t r e n g t h of s t e e l s with higher speeds while there i s no apparent i n f l u e n c e on the bending p r o p e r t i e s . 30 The changes i n p r o p e r t i e s may be a t t r i b u t a b l e to the v a r i a t i o n of f r i c t i o n with s u r f a c e speed, hence the r a t e of energy i n p u t . An i n c r e a s e i n speed a c t u a l l y b r i n g s down the e f f e c t i v e f r i c t i o n , hence the torque as shown by the graph r 31 i n f i g u r e 6 . T h i s i s due to the f a l l of f r i c t i o n v a l u e s at higher s l i d i n g speeds. Hence, g r e a t e r r a t e s of heat inputs are achieved by lower speeds r a t h e r than higher speeds. The d u r a t i o n of hea t i n g i s i n f l u e n c e d by speed. Longer times are r e q u i r e d with i n c r e a s e d speed because of lower f r i c t i o n c o e f f i c i e n t s , the heat input r a t e i s lower. When the d u r a t i o n of hea t i n g i s long due to high speeds, i t was . [ A ] shown by Voinov and V a v i l o v , r e f e r r e d by E l l i s t h a t the m i c r o s t r u c t u r e at the i n t e r f a c e as w e l l as the Heat A f f e c t e d Zone (HAZ) i s a l t e r e d . The longer time spent by the specimen at h i g h temperatures induces g r a i n growth and the width of HAZ i n c r e a s e s due to h e a t i n g . A l s o long h e a t i n g time causes u n d e s i r a b l e overheating f o r some m a t e r i a l s l i k e copper. Shorter c y c l e times are more d e s i r a b l e as we have minimum g r a i n growth, a narrow HAZ and a l s o h i g h e r r a t e s of p r o d u c t i o n . 3.2 PRESSURE The p r e s s u r e a p p l i e d between the two p a r t s i s important as i t c o n t r o l s the temperature g r a d i e n t i n the weld zone, governs the d r i v e power r e q u i r e d (torque developed) and a l s o the metal displacement c h a r a c t e r i s t i c s . The p r e s s u r e s used 31 depend on the metals being j o i n e d , t h e i r p h y s i c a l c o n f i g u r a t i o n s e.g., bar to bar, tube to bar, tube or bar to p l a t e e t c . The pressure generates f r i c t i o n and maintains the two s u r f a c e s i n in t i m a t e c o n t a c t . T h i s i n t i m a t e c o n t a c t a l s o h e r m e t i c a l l y excludes the weld from the surrounding atmosphere, thus minimizing the problems of o x i d a t i o n when the metal i s hot and a l s o e l i m i n a t i n g the need f o r s h i e l d i n g gases to o b t a i n an i n e r t atmosphere thus making the process simple. A d d i t i o n a l f o r c e at the end of the f r i c t i o n phase a c t s as a forge f o r c e which r e s u l t s i n stronger welds. No s i n g l e pressure i s good f o r a l l metals and every c o n f i g u r a t i o n of both s i m i l a r and d i s s i m i l a r metals and geometry c a l l s f o r an i n v e s t i g a t i o n i n t o the optimum s e t t i n g s . But a general trend can be e s t a b l i s h e d which s e t s g u i d e l i n e s i n the i n v e s t i g a t i o n to determine the optimum s e t t i n g s . Higher values of pres s u r e are g e n e r a l l y r e q u i r e d f o r metals of high hot s t r e n g t h . T y p i c a l l y , f o r m i l d s t e e l the h e a t i n g ( f r i c t i o n ) p r e s s u r e i s 20-60 N/mm2 (3,000-8,900 p s i ) and the forge phase's p r e s s u r e i s u s u a l l y of the order of 77-150 N/mm2 (11,500-23,000 p s i ) . As i t i s seen, the pressure s e t t i n g s have a very wide range of val u e s and good welds can be achieved over the f u l l range. Lower valu e s are used f o r m a t e r i a l s of lower hot s t r e n g t h . In m i c r o - f r i c t i o n welding, r e l a t i v e l y higher p r e s s u r e s are used 150 - 340 N/mm2 (22,350 - 50,660 p s i ) ^]5K The f r i c t i o n p r e ssure i s c o n s i d e r e d to be the more important parameter as i t i n f l u e n c e s the welding setup as 32 w e l l as the q u a l i t y of the weld by a great d e a l . I t determines the d r i v i n g torque and the d u r a t i o n of h e a t i n g . I t a l s o i n f l u e n c e s the maximum temperatures at the i n t e r f a c e , the m i c r o s t r u c t u r e and hence the s t r e n g t h . The p l o t of the e u i l i b r i u m torque versus pressure g i v e s a s t r a i g h t l i n e r e l a t i o n s h i p . See f i g u r e 7. These diagrams are a l s o i n d i c a t i v e of the v a r i a t i o n of e u i l i b r i u m torque with p r e s s u r e and diameter. The a p p l i e d p r e s s u r e i n f l u e n c e s the r a t e of metal consumption. The s t r a i g h t l i n e r e l a t i o n s h i p i s shown i n f i g u r e 8. The p r e s s u r e a l s o a f f e c t s the width of h e a t - a f f e c t e d zone s u b s t a n t i a l l y . At l a r g e r p r e s s u r e s we have hig h e r r a t e s of heat input r e s u l t i n g i n a narrow HAZ. T h i s i s p r e f e r a b l e f o r s t e e l s where the HAZ i s to be minimized whenever p o s s i b l e . The shape of the HAZ a l s o v a r i e s with p r e s s u r e as shown i n f i g u r e 9. Comparing a and b with c and d i n f i g u r e 9, i t i s seen that the HAZ at lower pressure i s even a c r o s s the c r o s s - s e c t i o n and i s wider too as compared to the HAZ of the p i e c e s c and d welded at higher p r e s s u r e s due to the lower heat input r a t e s at lower p r e s s u r e s . The c o n i c a l shape i s due to uneven r a t e of heat g e n e r a t i o n a c r o s s the f a c e . Due to the v a r i a t i o n i n r e l a t i v e motion as we move away from the c e n t r e , the heat input r a t e f i r s t i n c r e a s e s and then decreases a c c o r d i n g to the d i s t a n c e , see equation 3.1 and f i g u r e 6. A l s o , due to lower f r i c t i o n a l v a l u e s at high e r speeds, the r e s u l t i n g lower r a t e s of heat input f o r a and b gi v e s a r e l a t i v e l y wider HAZ. The c o e f f i c i e n t of f r i c t i o n 33 a l s o goes down with the i n c r e a s e i n loads as shown i n f i g u r e r 21 10 . F i g u r e 10 i n d i c a t e s that higher p r e s s u r e s have lower c o e f f i c i e n t s of f r i c t i o n which should c o n t r a d i c t the appearence of h e a t - a f f e c t e d zone of the specimens d i s c u s s e d p r e v i o u s l y . T h i s i s not so because higher p r e s s u r e s c o n t r i b u t e to the mechanical working of the s u r f a c e and there i s a high s u s t a i n e d r a t e of deformation which a l s o c o n t r i b u t e s to the genera t i o n of heat. Mechanical p r o p e r t i e s of the weld vary with pressure s e t t i n g . P r o g r e s s i v e i n c r e a s e of the u l t i m a t e t e n s i l e s t r e n g t h with the i n c r e a s i n g pressures has been r e p o r t e d . The metal i s hardened and becomes l e s s d u c t i l e with i n c r e a s e i n a x i a l p r e s s u r e . T h i s i s s u b s t a n t i a t e d by the f a c t that when the welded specimens were f a i l e d i n t e n s i o n , the degree of r e d u c t i o n i n c r o s s - s e c t i o n a l area at the i n t e r f a c e i s le s s e n e d with i n c r e a s e d p r e s s u r e . A l s o i n some cases the parent metal s t a r t s to neck before there i s a sudden f a i l u r e at the i n t e r f a c e . Examination of the f a i l e d s u r f a c e by other r e s e a r c h e r s has shown that there has been only a s l i g h t deformation at the re g i o n of the weld. T h i s i s e s p e c i a l l y t rue f o r hig h p r e s s u r e welds. For the low p r e s s u r e , low speed cases, t h e r e i s s u b s t a n t i a l g r a i n growth at the i n t e r f a c e and the specimens when sub j e c t e d t o t e n s i l e t e s t s f a i l i n a d u c t i l e mode of f r a c t u r e at the i n t e r f a c e . But the u l t i m a t e t e n s i l e s t r e n g t h i s r a t h e r low. 34 3.3 BURN-OFF RATE / DURATION OF HEATING Burn-off i s d e f i n e d as the amount the specimens are shortened p r i o r t o the stopping of the r o t a t i o n and i n i t i a t i o n of forge c y c l e . The burn-off r a t e / d u r a t i o n of he a t i n g vary with the diameter of the work-piece, r e l a t i v e v e l o c i t y , a x i a l p r e s s u r e and a l s o the hot s t r e n g t h of the m a t e r i a l . The b u r n - o f f s t a r t s at about the same time as the i n i t i a l peak torque. The burn-off r a t e depends on the torque, hence remains constant f o r a while (during stage I I I of continuous d r i v e type of f r i c t i o n w e l d i ng), see f i g u r e 3. The r a t e of burn-off i s higher at a l a t e r stage, e s p e c i a l l y d u r i n g t e r m i n a l peak torque. The burn-off f o r i n e r t i a welding s t a r t s about the same time as i n the continuous d r i v e f r i c t i o n welding process, d u r i n g the i n i t i a l peak torque, but because of higher i n i t i a l , t e r m i n a l torques, r 7 l s h o r t e r welding times, the rate of burn-off i s higher 1 . The burn-off r a t e decreases with the i n c r e a s e i n diameter f o r the same a x i a l p r e s s u r e . A n a l y s i s of the T1 7 1 r e s u l t s of experiments conducted by E l l i s c o u l d be i n t e r p r e t e d as i n d i c a t i n g that the r a t e of v o l u m e t r i c displacement i s d i r e c t l y r e l a t e d t o the diameter of the work-piece as f o l l o w s Bv = D/21 (3.3) where, Bv = Volumetric displacement r a t e d u r i n g b u r n - o f f i n c u . i n / s e c . D = Diameter of specimen i n inches For a sound weld t h e r e i s always a minimum d u r a t i o n of 35 h e a t i n g i n order to expel the contaminants at the i n t e r f a c e , to b r i n g the two rubbing s u r f a c e s to a p l a s t i c s t a t e , good enough f o r s o l i d - p h a s e pressure bonding. In cases where the d u r a t i o n of h e a t i n g does not a f f e c t the q u a l i t y of the weld very much, the amount of burn-off c o u l d be v a r i e d f o r a good dimensional c o n t r o l of the f i n i s h e d product. While the dimension of the two p a r t s to be welded c o u l d vary to a c e r t a i n degree before welding, the amount of burn-off during welding i s a d j u s t e d f o r each combination, s t o p p i n g the welding c y c l e as soon as the r e q u i r e d dimension i s achieved. An approximate value of burn-off i s given f o r commercial s t e e l s , 20% of the diameter of the specimens f o r s o l i d bars and 50% of the w a l l t h i c k n e s s i s recommended f o r t u b u l a r r 41 s e c t i o n s . And to achieve t h i s kind of burn-off f o r a given diameter of the specimen, i t can be determined that T, the time r e q u i r e d f o r the a p p r o p r i a t e displacement of a bar, i s given by T=3.25D 2. . . . . . (3.4) D i s the diameter of the bar i n in c h e s . I f the d u r a t i o n of h e a t i n g r a t h e r than burn-off time i s used to c o n t r o l the process time, the process time f o r an optimum weld i s determined e x p e r i m e n t a l l y . E x c e s s i v e times can r e s u l t i n wide HAZs, induce unwarranted g r a i n growth r e s u l t i n g i n lowering of t e n s i l e s t r e n g t h . Shorter welding times are p r e f e r a b l e , achieved by slower speeds and higher p r e s s u r e s , whose valu e s are both determined by the diameter. T h i s d u r a t i o n v a r i e s from a couple of seconds f o r a small 36 rod to over a minute f o r l a r g e p i p e s . [ 1 7 ] H a z l e t t and Gupta have concluded that no i n c r e a s e i n s t r e n g t h was observed beyond a burn-off of 0.035 inches f o r a s o l i d bar stock of r a d i u s of 3/8 i n c h heat t r e a t a b l e aluminum a l l o y . T h e i r r e s u l t s show that the t e n s i l e s t r e n g t h i n c r e a s e d with burn-off to a l i m i t and then l e v e l l e d o f f , remaining constant r e g a r d l e s s of the b u r n - o f f . Apart from burn-off alone, the r a t e of burn-off a l s o has an i n f l u e n c e on the p r o p e r t i e s of the weld. The r a t e c o u l d be used to T 1 8 1 monitor the q u a l i t y of the j o i n t . E l l i s i n h i s experiments with m i l d s t e e l bars observed that the j o i n t s t r e n g t h reached that of the parent metal when the burn-off r a t e exceeded 0.08 in / s e c ( 2 mm/sec). Hence, by c o n t r o l l i n g the burn-off r a t e by v a r y i n g other parameters l i k e p r essure and speed w i t h i n reasonable l i m i t s , the i n t e g r i t y of the weld c o u l d be monitored by an i n - p r o c e s s c l o s e d loop system f o r an automatic machine. Apparently the burn-off r a t e method i s q u i t e t o l e r a n t and g i v e s good welds over a range of welding c o n d i t i o n s and work-piece dimensions f o r a 5 to 1 range of p r e s s u r e , 2 to 1 range of speed and 2 to 1 range of c r o s s - s e c t i o n a l a r e a . 3.4 FORGE PRESSURE As soon as the two s u r f a c e s are ready f o r j o i n i n g , the r o t a t i o n i s stopped and the a x i a l p r e s s u r e i s e i t h e r maintained or i n c r e a s e d . The i n c r e a s e d a x i a l p r essure i s c a l l e d Forge Pressure. 37 The forge p r e s s u r e , though u s u a l l y a p p l i e d , i s not always e s s e n t i a l to achieve a good weld. For some m a t e r i a l s , 100% e f f i c i e n t welds can be obtained even without f o r g i n g , but i n general the forge pressure i s used to hot-work the weld region and c o n s o l i d a t e the weld. I t i s re p o r t e d that some s t e e l s (1020CR) do not respond to forge p r e s s u r e while an i n c r e a s e i n UTS was observed f o r some other type of s t e e l (4130) Hot s t r e n g t h of the m a t e r i a l d i c t a t e s the forge pressure s e t t i n g . Too l a r g e a pressure can a c t a d v e r s e l y with e x c e s s i v e metal deformation, unnecessary e x t r u s i o n of metal, severe r e o r i e n t a t i o n of g r a i n s r e s u l t i n g i n the l o s s of f a t i g u e s t r e n g t h and f r a c t u r e toughness p r o p e r t i e s . The forge pressure used should r e s u l t i n optimum m i c r o s t r u c t u r e , f r e e from e x c e s s i v e working due to high pressure or coarse or a c i c u l a r s t r u c t u r e due to low forge p r e s s u r e s . Apart from these four parameters which i n f l u e n c e the q u a l i t y of the weld, r e s e a r c h e r s have p o i n t e d out that s i n c e the r a t e of d e c e l e r a t i o n has a profound e f f e c t on the t e r m i n a l torque and r a t e of b u r n - o f f , r e g a r d l e s s of the speed of the s p i n d l e , i t e x e r t s good i n f l u e n c e on the T 7 1 s t r e n g t h of the weld produced. D u f f i n and Bahrani 1 J have a l s o concluded from t h e i r experiments that too high a rate of d e c e l e r a t i o n (2500 rad/sec, use of a brake) can r e s u l t i n unbonded areas at the p e r i p h e r y of the weld unless the process i s conducted at low speeds and high a x i a l p r e s s u r e s . They observed that as the d u r a t i o n of the t e r m i n a l torque i s prolonged, the notch between the two c o l l a r s i s l e s s sharp, 38 gets f i l l e d up with metal and w i l l be welded w e l l beyond the p e r i p h e r y . In cases of slow d e c e l e r a t i o n , the i n t e r f a c e i s prepared f o r the f i n a l forge phase f o r proper c o n s o l i d a t i o n . But too low a d e c e l e r a t i o n i n not recommended as toe c r a c k s can be i n i t i a t e d near the p e r i p h e r y between the burn-off c o l l a r and the specimen i n the HAZ. The plane of rubbing can move from the former i n t e r f a c e to the j u n c t i o n of the p l a s t i c c o l l a r and the specimen's s o l i d body. The t e r m i n a l torque i s l e a s t i n f l u e n c e d by the i n i t i a l speed of r o t a t i o n and d e c e l e r a t i o n and pressure have more e f f e c t , i n that order. 4 . PRELIMINARY TESTS 4.1 WELDING TRIALS A f t e r the f a b r i c a t i o n of the f r i c t i o n welding machine, s e v e r a l t r i a l runs were made to determine the a b i l i t y of the equipment. Some m a t e r i a l combinations which are co n s i d e r e d as those that are r e a d i l y weldable by the process of f r i c t i o n welding were t r i e d to evaluate the e f f e c t i v e n e s s of the machine. The motor used f o r the d r i v e d e l i v e r e d 10 hp which was q u i t e adequate f o r the specimens of the s i z e used (1/2 inch d i a m e t e r ) . The speed at the workpieces was 1120 rpm which was a l s o adequate. Since the motor was of AC type, the speed was e s s e n t i a l l y c o n s t a n t . No p u l l e y s were used to vary the speed; hence with a constant speed, the process v a r i a b l e s were reduced to 1. F r i c t i o n p r essure 2. D u r a t i o n of h e a t i n g 3. Forge pressure The h y d r a u l i c power u n i t used to f o r c e the work-pieces together a x i a l l y developed a maximum pressure of 2000 p s i i n the h y d r a u l i c c y l i n d e r of a c r o s s - s e c t i o n a l area of 3.14 square inches. Thus i t d e l i v e r e d a pressure of 32,000 p s i on a workpiece of h a l f i n c h diameter. The b u i l d - u p time f o r both f r i c t i o n and forge p r e s s u r e s was n e g l i g i b l e . Both of these p r e s s u r e s were h e l d constant throughout the two phases. Since very low p r e s s u r e s tend to extend the d u r a t i o n 39 40 of h e a t i n g , medium pressures of the range of 3000-4000 p s i were used on the components f o r the f r i c t i o n phase. At t h i s f r i c t i o n pressure range, the process time was about 2 seconds f o r s t e e l specimens of a diameter of h a l f an i n c h . The burn-off was j u s t over an e i g h t h of an i n c h which conformed to the recommended burn-off f o r a good weld f o r s t e e l s (see s e c t i o n 3.3). A s p e c i f i c burn-off d i s t a n c e i s u s u a l l y taken as a process v a r i a b l e , e s p e c i a l l y i n some p r o d u c t i o n s i t u a t i o n s when the in - p r o c e s s q u a l i t y c o n t r o l monitor takes a minimum burn-off l e n g t h f o r a weld to be a c c e p t a b l e . A l s o , i n cases where the f i n a l dimension i s c r i t i c a l , i t i s the f i n a l dimension which determines the d u r a t i o n of h e a t i n g . Since the f i n a l dimension i s not c r i t i c a l f o r r e s e a r c h specimens, as i n our case, the d u r a t i o n of h e a t i n g f o r the experiments was set such that the minimum burn-off l e n g t h was achieved f o r a l l those specimens where the d u r a t i o n of h e a t i n g i s not the process v a r i a b l e under i n v e s t i g a t i o n . F i n a l l y , the forge pressures were s e l e c t e d such that there was adequate working at the i n t e r f a c e of the two p i e c e s , improving the s t r e n g t h but keeping i n view that too h i g h a forge pressure c o u l d cause adverse e f f e c t s as d e s c r i b e d i n s e c t i o n 3.4. The f i r s t few t e s t s conducted had v a r i e d process d u r a t i o n as the e l e c t r o n i c u n i t c o n t r o l l i n g the process times had not yet been i n s t a l l e d . Hence the d u r a t i o n of h e a t i n g was c o n t r o l l e d manually. The c r i t e r i a adopted f o r determining i f there had been s u f f i c i e n t 41 f r i c t i o n i n g was the amount of e x t r u s i o n of the c o l l a r . During these experiments no forge pressure was being a p p l i e d a f t e r the r o t a t i o n had been stopped. The same f r i c t i o n i n g p ressure was maintained f o r an a d d i t i o n a l time of about 10 seconds and then the ram was r e t r a c t e d . See appendix I f o r a l l the d e t a i l s of the o p e r a t i o n of the machine. T e n s i l e t e s t s on the welded specimens were conducted a f t e r t u r n i n g down the c o l l a r of each specimen. Turning can r e s u l t i n e i t h e r removing the d e f e c t s i n the p e r i p h e r y , which can be the p o i n t s of i n i t i a t i o n of f r a c t u r e when subjected to l o a d i n g , or removing the a d d i t i o n a l m a t e r i a l forming the c o l l a r extending o u t s i d e the o r i g i n a l diameter of the t e s t p i e c e s . The true s t r e n g t h of the j o i n t cannot be assessed i n e i t h e r case. Turning down can be j u s t i f i e d i n the cases where the j o i n t needs to have the f l a s h removed fo r the sake of appearence or to s a t i s f y design requirements. R e t e n t i o n of the f l a s h c o u l d be advantageous as seen i n f i g u r e 11, where f a i l u r e took p l a c e i n the parent m a t e r i a l r a t h e r than i n the j o i n t when the specimens were sub j e c t e d t o the t e n s i l e t e s t s . The two m a t e r i a l s used f o r t e s t i n g the equipment were 1025 s t e e l and 6061 aluminum rods. See appendix II f o r the m a t e r i a l c ompositions. At the c o n s t r a i n t of constant speed, process v a r i a b l e s were changed over a wide range t o determine those v a r i a b l e s which had c o n s i d e r a b l e e f f e c t on the q u a l i t y of the weld. As e s t a b l i s h e d e a r l i e r , i t was observed that a minimum amount 42 of b u r n - o f f was always necessary to form a c l e a n j o i n t . J o i n t s were s t i l l o btained even i n the cases when the minimum time r e q u i r e d f o r s p e c i f i e d burn-off was not met, but when t e s t e d the j o i n t s f a i l e d at the i n t e r f a c e with much lower s t r e n g t h than what was observed f o r j o i n t s with a f a i r amount of burn-off f o r the same parameter s e t t i n g s . T h i s was due t o the presence of contaminants and inadequate p l a s t i c i t y f o r the working of the metal. P a r t i a l welding s t i l l took p l a c e over some are a s . S i m i l a r l y , the d u r a t i o n of f o r g i n g had some e f f e c t on the q u a l i t y of the weld. A very short d u r a t i o n i . e , l e s s than 1 second d i d not produce any notable i n c r e a s e i n s t r e n g t h compared with the j o i n t s made without any f o r g i n g at a l l . With the longer f o r g i n g times, the improvement i n s t r e n g t h was c o n s i d e r a b l e and more c o n s i s t e n t . The r a t e of improvement i n s t r e n g t h d i m i n i s h e d with the i n c r e a s e i n forge times; no s i g n i f i c a n t improvement was observed over 3 seconds of forge time f o r s t e e l s . The e f f e c t of f r i c t i o n p r e s s u r e was b a s i c a l l y on the d u r a t i o n of hea t i n g as mentioned i n s e c t i o n 3.2 and e f f o r t s were made to minimize t h i s p e r i o d to a v o i d unnecessary g r a i n growth and overhe a t i n g of the p a r t s . Forge pressure had a d i r e c t r e l a t i o n s h i p with the s t r e n g t h of the weld depending upon the m a t e r i a l combination. For example, while the forge pressure d i d not have any i n f l u e n c e on an aluminum - aluminum combination, i t showed marked e f f e c t on the UTS of the aluminum - s t e e l 43 combination. The s t r e n g t h i n c r e a s e d d i r e c t l y with the forge p r e s s u r e . 4.2 TESTS ON WELDED SPECIMENS The t e n s i l e t e s t s on the s t e e l specimens y i e l d e d i n t e r e s t i n g r e s u l t s . The f a i l u r e took p l a c e i n a l o c a t i o n o u t s i d e the weld plane and even o u t s i d e the h e a t - a f f e c t e d zone. T h i s was c o n s i s t e n t with most s t e e l specimens. Please see f i g u r e 12. The weld plane i s at the c e n t r e of the machined r e c e s s i n a l l . the specimens. T h i s 100% e f f i c i e n t weld was obtained over a wide range of f r i c t i o n and forge p r e s s u r e s and f r i c t i o n i n g times i n d i c a t i n g t h a t m i l d s t e e l i s q u i t e t o l e r a n t of f r i c t i o n welding. The same 100% e f f i c i e n t weld c o u l d not be ob t a i n e d i n the case of 6061 aluminum. Although short h e a t i n g times and high forge p r e s s u r e s r e s u l t e d i n r e l a t i v e l y high s t r e n g t h j o i n t s , the j o i n t s were not as s t r o n g as the parent m a t e r i a l . The forge pressure was i n c r e a s e d from 0 t o 16,000 p s i but no improvement i n the t e n s i l e s t r e n g t h was observed i n d i c a t i n g that the 6061 aluminum a l l o y does not respond to forge p r e s s u r e s . See appendix I I I . In the t e n s i l e t e s t s the f a i l u r e took p l a c e a c r o s s the weld plane a f t e r necking i n a d u c t i l e f a s h i o n . The reason f o r t h i s can be a t t r i b u t e d to the m i c r o s t r u c t u r a l changes d u r i n g h e a t i n g and c o o l i n g at the weld plane. The i n i t i a l l y wrought s t r u c t u r e has elongated g r a i n s with good t e n s i l e s t r e n g t h . A f t e r the welding c y c l e , the weld zone has much co a r s e r g r a i n s 44 r e s u l t i n g i n lowering of the s t r e n g t h of the j o i n t . The maximum t e n s i l e s t r e n g t h achieved f o r aluminum was of the order of 30,000 p s i (UTS) while the parent m a t e r i a l had a s t r e n g t h of about 42,000 p s i g i v i n g a j o i n t e f f i c i e n c y i n excess of 70%. M i c r o s t r u c t u r a l changes take p l a c e f o r s t e e l too, but these a i d the t e n s i l e s t r e n g t h i n some cases. Due to high r a t e s of c o o l i n g i n the weld area, a n o n - e q u i l i b r i u m s t r u c t u r e of mar t e n s i t e i s formed i n the HAZ. M a r t e n s i t e , a very hard s t r u c t u r e does not y i e l d and r e s i s t s w e l l i n t e n s i o n . Hence, when su b j e c t e d to a t e n s i o n t e s t the f a i l u r e takes p l a c e i n the parent m a t e r i a l . A l s o there are cases where the u l t i m a t e t e n s i l e s t r e n g t h of the parent m a t e r i a l i s gr e a t e r than the f r a c t u r e s t r e n g t h of the j o i n t . See f i g u r e 12. The bottom-most specimen shows that the parent m a t e r i a l had s t a r t e d to neck but a sudden f a i l u r e has taken p l a c e i n the j o i n t . Examination of the f a i l u r e showed that there had been a mixed mode of f r a c t u r e , d u c t i l e and b r i t t l e i n d i c a t i n g the presence of b r i t t l e n o n - e q u i l i b r i u m phases of s t e e l . T e s t s were a l s o conducted on f r i c t i o n welded j o i n t s of d i s s i m i l a r m a t e r i a l s . Aluminum was welded to s t e e l which i s not e a s i l y p o s s i b l e by normal welding methods. Due to l a r g e d i f f e r e n c e i n m e l t i n g p o i n t s , aluminum s t a r t s f l o w i n g very e a r l y and s t e e l i s never as much brought to a red hot temperature though a narrow HAZ i s formed. The weld's q u a l i t y depends mostly on the forge p r e s s u r e used, heavy f o r g i n g r e s u l t i n g i n b e t t e r welds. T e n s i l e t e s t s i n d i c a t e d 45 that the s t r e n g t h of these j o i n t s was b e t t e r than the best aluminum - aluminum j o i n t , by about 13%. The reason i s that the s t e e l - aluminum j o i n t f e a t u r e s a s h o r t e r welding time s i n c e the heat l o s t due to conduction i s l e s s . The f a i l u r e took p l a c e i n a d u t i l e f a s h i o n i n the aluminum adjacent to the weld plane; there was no s e p a r a t i o n at the i n t e r f a c e . The f o r g i n g pressure has a good i n f l u e n c e on the s t r e n g t h of the weld between aluminum and s t e e l . The s t r e n g t h of the j o i n t improved by over 20% when the forge pressure was i n c r e a s e d from 4,000 p s i to 12,500 p s i . C o s i d e r a b l e d i f f e r e n c e i n s t r e n g t h was a l s o observed when welding s t e e l t o aluminum, p a r t i c u l a r l y when the s t e e l specimen's s u r f a c e was not prepared f o r welding by f a c i n g o f f the saw-cut s u r f a c e . When the s t e e l specimen's s u r f a c e was not made to be f l a t and/or p e r p e n d i c u l a r to the a x i s of r o t a t i o n , the s t r e n g t h went down by over 70%. The e x p l a n a t i o n f o r t h i s i s that there i s no deformation on the s t e e l s u r f a c e and a l l the flow takes p l a c e on the aluminum s i d e . As the s u r f a c e i s n e i t h e r smooth nor p e r p e n d i c u l a r , the scavenging e f f e c t does not e x i s t , the s u r f a c e i s not prepared thermo-chemically f o r bonding and hence there i s la c k of bonding at the i n t e r f a c e . A l l the m a t e r i a l s have d i f f e r e n t requirements of the power needed f o r t h e i r f r i c t i o n welding. The power r e q u i r e d i s such t h a t , the thermal g r a d i e n t at the i n t e r f a c e i s so high that i t p l a s t i c i z e s only a t h i n l a y e r of m a t e r i a l without l o s i n g much heat to the surroundings, a process 46 amounting to a d i a b a t i c shear. Thus only a t h i n l a y e r i s p l a s t i c and i s welded. In cases where the torque or the speed are not adequate, there i s a p p r e c i a b l e deformation even i n areas away from the weld plane and t h i s i s to be avoided. As a rough measure of the power r e q u i r e d , the f o l l o w i n g equation has been put forward ^ l 2 . l ] . Q=T m p.v/Kpc/l0 4 where, K = thermal c o n d u c t i v i t y , p = d e n s i t y , c = s p e c i f i c heat, T m p = m e l t i n g p o i n t . T h i s i s only an approximation, but g i v e s an idea of the i n t e n s i t y of the power requirement. 10 s i s a s c a l e f a c t o r . A f t e r conducting the experiments mentioned e a r l i e r , i t was concluded that the machine c o u l d be used f o r f u r t h e r experimentations and the power developed was adequate f o r such purposes. 5. THREE ELEMENT FRICTION WELDING The welding of a wide range of both s i m i l a r and d i s s i m i l a r m a t e r i a l s c o n s i d e r e d hard i f not impossible to weld by most other processes prove to be q u i t e amenable by f r i c t i o n welding. T h i s i s more so when due to m e t a l l u r g i c a l reasons, f u s i o n welding i s i m p o s s i b l e , e s p e c i a l l y between f e r r o u s and non-ferrous m a t e r i a l s . With no m e l t i n g at the i n t e r f a c e i n the process of f r i c t i o n welding, problems of m i c r o - c r a c k i n g , d i f f e r e n t m e l t i n g p o i n t s , m i c r o - s e g r e g a t i o n , s l a g and other i n c l u s i o n s , d i f f e r e n t heat c a p a c i t i e s i n two d i s s i m i l a r m a t e r i a l s (to some extent) and a c a s t s t r u c t u r e at the i n t e r f a c e c o u l d a l l be avoided. 5 . 1 MATERIALS fl Table I g i v e s a comprehensive p i c t u r e of r e l a t i v e w e l d a b i l i t y of a wide range of m a t e r i a l s being used i n i n d u s t r y t 1 2 , 1 ^ # -phe t a b l e g i v e s d e t a i l s of only those m a t e r i a l s i n v e s t i g a t e d upon e n q u i r i e s from i n d u s t r i e s by the Welding I n s t i t u t e , England. R e f e r r i n g to t a b l e I, s e v e r a l m a t e r i a l combinations which do not weld are encountered. E v i d e n t l y there i s a requirement f o r these m a t e r i a l s to be welded and f r i c t i o n welding i s one of the processes that has been attempted or i s a p r e f e r r e d process f o r doing so. Keeping t h i s i n view, attempts were made to j o i n these m a t e r i a l s , p r e v i o u s l y c o n s i d e r e d unweldable, by i n t r o d u c i n g a t h i r d element i n t o the p r o c e s s . The t h i r d element i n t r o d u c e d a c t s as a b u f f e r 47 48 between the two primary m a t e r i a l s which are incompatible when brought i n t o d i r e c t c o n t a c t , by welding w e l l with both of them. Some such p o s s i b i l i t i e s f o r an intermediate m a t e r i a l evident from the c h a r t are summarized by t a b l e I I . The most l i k e l y i ntermediate m a t e r i a l s as seen from t h i s t a b l e seem to be l i m i t e d to aluminum and copper. T h i s i s due to the extremely l i m i t e d i n f o r m a t i o n a v a i l a b l e on the f r i c t i o n welding of d i s s i m i l a r m a t e r i a l s . In t a b l e I I I , another l i s t of incompatible m a t e r i a l s and those m a t e r i a l s that can be used as intermediate m a t e r i a l s which have not yet been t r i e d with one or both of the other two elements, are g i v e n . There are many combinations that can be t r i e d out, but many of them can be r u l e d out due to extreme d i f f e r e n c e s i n p r o p e r t i e s , formation of u n d e s i r a b l e i n t e r m e t a l l i c s e t c . For example, l e a d or aluminum can be r u l e d put as incompatible with tungsten because of vast d i f f e r e n c e s i n m e l t i n g p o i n t s and hardness v a l u e s . In the case of welding tungsten to i t s e l f , which forms a b r i t t l e weld, an intermediate m a t e r i a l l i k e i r i d i u m would be more a p p r o p r i a t e due to i r i d i u m ' s high m e l t i n g p o i n t and a l l o y i n g c h a r a c t e r i s t i c s with tungsten. For s i m i l a r reasons, bronze i s a good candidate to serve as an i n t e r f a c e between brass and copper. Since i t has a m e l t i n g p o i n t i n the same range of both copper and b r a s s , and good a l l o y i n g with both of them i s expected to r e s u l t i n a reasonable weld. 49 5.2 REQUIREMENTS OF AN INTERMEDIATE MATERIAL Some p r o p e r t i e s are d e s i r e d from the intermediate m a t e r i a l i f i t i s to perform as a good i n t e r f a c e , such as: 1. A b i l i t y to c r e a t e c o n d i t i o n s amenable to bonding. 2. A b i l i t y to form a good bond with both of the parent m a t e r i a l s or form a good a l l o y . 3. P a s s i v i t y to the formation of i n t e r m e t a l l i c s , even under the c o n d i t i o n s of prolonged e l e v a t e d temperatures. 4. To flow at the same temperature ranges as that of the parent m a t e r i a l s . 5. Have c e r t a i n p h y s i c a l p r o p e r t i e s s i m i l a r to that of the parent m a t e r i a l s . 6. R e s i s t a n c e to the formation of oxides and other u n d e s i r a b l e compounds. 5.3 BONDING MECHANISMS I d e a l l y a good j o i n t may be obtained by removing a l l the contaminants from the s u r f a c e such as f i l m s of o x i d e s , carbonates, s u l p h i d e s which are u s u a l l y found on almost a l l s u r f a c e s with v a r y i n g t h i c k n e s s e s (10 to 1000 Angstrom u n i t s ) and sometimes a l a y e r of adsorbed gases. The formation of the j o i n t under the a c t i o n of p r e s s u r e and heat can be c l a s s i f i e d i n t o three stages of topo-chemical \ 1 9] r e a c t i o n s , namely, 1. Development of p h y s i c a l c o n t a c t . 2. A c t i v a t i o n of s u r f a c e s i n c o n t a c t . 50 3. I n t e r a c t i o n w i t h i n the m a t e r i a l s being j o i n e d . The f i r s t stage i s brought about by shearing o f f of the a s p e r i t i e s to b r i n g l a r g e r areas i n t o c o n t a c t and, more important, to i n i t i a t e the p l a s t i c flow. The second stage c o u l d be delayed by a hard s u r f a c e r e s i s t i n g flow, but t h i s i s overcome when some areas are a c t i v a t e d on the harder s u r f a c e . During the t h i r d stage, strong chemical bonds are e s t a b l i s h e d s t a r t i n g at the a c t i v a t i o n c e n t r e s and these a ct as n u c l e i of i n t e r a c t i o n . The process i s terminated when the e n t i r e s u r f a c e i s u n i f i e d under t h e i r a c t i o n . There i s some i n t e r a c t i o n t a k i n g p l a c e i n the adjacent area too with complete or p a r t i a l r e l a x a t i o n of m i c r o - s t r e s s e s . A l s o some volume processes, such as r e c r y s t a l l i s a t i o n and heterogeneous d i f f u s i o n , may take p l a c e which may have e i t h e r p o s i t i v e or negative i n f l u e n c e on the bond s t r e n g t h . Since the two m a t e r i a l s are c o n s i d e r e d incompatible when brought i n t o c o n t a c t d i r e c t l y , the t h i r d element while s e p a r a t i n g the two should a l s o bond e f f e c t i v e l y with both of them. I t ' s flow at much d i f f e r e n t temperatures than that of the other two m a t e r i a l s would not c r e a t e c o n d i t i o n s r e q u i r e d f o r the formation of the bonds. A good intermediate m a t e r i a l i s expected to flow at m a r g i n a l l y higher temperatures / p r e s s u r e s than the parent m a t e r i a l s ; thus i t ( i ) s u s t a i n s at the j u n c t i o n without being e x p e l l e d completely from the i n t e r f a c e and ( i i ) i t s e l f flows to some exten t , thus exposing i t ' s own c l e a n s u r f a c e . The j o i n t thus formed i s expected to perform adequately i n a l l r e s p e c t s j u s t as do 51 the parent m a t e r i a l s . In many i n s t a n c e s where j o i n t s are made between d i s s i m i l a r m a t e r i a l s ( j u s t to e f f e c t savings i n m a t e r i a l c o s t s ) , and where these p a r t s are not sub j e c t e d t o severe working c o n d i t i o n s , the range f o r the s e l e c t i o n of inte r m e d i a t e m a t e r i a l i s l e s s c o n s t r a i n e d . In the proposed three-element f r i c t i o n welding process, the s t r e n g t h of the j o i n t i s p r i m a r i l y a f f e c t e d by 1. Mode of j o i n i n g . a. m e t a l l u r g i c a l bonding. b. mechanical i n t e r l o c k i n g . 2. Strength of the in t e r m e d i a t e m a t e r i a l . 3. Thickness of the i n t e r l a y e r formed. 4. Extent of the formation of b r i t t l e i n t e r m e t a l l i c s . Large intermediate p i e c e s or t r a n s i t i o n p i e c e s are being used c u r r e n t l y i n the i n d u s t r y to a c t as sandwiches i n s i m i l a r s i t u a t i o n s . One such example i s i n the nuclear i n d u s t r y . P r e s e n t l y , where a s t a i n l e s s s t e e l tube c o u l d not be d i r e c t l y welded to an aluminum a l l o y tube, a sandwich of pure aluminum i s used to s u c c e s s f u l l y j o i n the two. 5.4 THE PROPOSED PROCESS The proposed process i s q u i t e d i f f e r e n t from the one mentioned above as the t r a n s i t i o n p i e c e i s not present and t h i s i s r e p l a c e d by a much t h i n n e r l a y e r . The process i s i l l u s t r a t e d i n f i g u r e 13. The process i s complete with one run of the process t a k i n g but m a r g i n a l l y longer time than the r e g u l a r two element p r o c e s s . The other process takes a 52 longer process time as i t has to be done in two stages and r e q u i r e s almost twice the energy of the proposed method and a l s o more machining. Some of the m a t e r i a l combinations which l e a d to a 'no-weld' s i t u a t i o n as seen from t a b l e I are Brass / Copper Titanium / N i c k e l Bronze / S t e e l A l l of the above s i x m a t e r i a l s weld soundly with aluminum and hence aluminum can be thought of as a good i n t e r m e d i a t e . For the f i r s t combination, apart from aluminum, naval brass and phosphor bronze were a l s o t r i e d as i n t e r l a y e r s even though these were not l o g i c a l d e r i v a t i v e s from t a b l e I. The r e s u l t s are d i s c u s s e d i n the f o l l o w i n g sect i o n s . There are s e v e r a l other m a t e r i a l s which do not weld and t h i s f i e l d of r e s e a r c h i s open f o r e x p l o r a t i o n . In the work presented here, r e s e a r c h was l i m i t e d to the above three combinations of m a t e r i a l s to determine the e f f e c t i v e n e s s of i n t r o d u c i n g of a t h i r d element i n t o the f r i c t i o n welding process and the kind of weld-strength achieved. 5.5 WELDING TRIALS 5.5.1 PRELIMINARY TRI-ELEMENT PROCESS To begin with, experiments were conducted i n t o determining the e f f e c t s of i n t r o d u c t i o n of a t h i r d 53 element i n t o the process. The experiments were f i r s t conducted on s i m i l a r m a t e r i a l combinations. Two aluminum specimens were welded with a t h i n aluminum d i s c i n between them. A small recess was made i n the s t a t i o n a r y p i e c e to hold the d i s c i n p l a c e . S u p e r f i c i a l examination of welded aluminum specimens a f t e r the c o l l a r was removed showed that there was e x c e l l e n t bonding with no s i g n s of any non-adhesive areas. I t was a l s o not ev i d e n t i n any way that i t was a three-element weld. When the p i e c e s were su b j e c t e d to t e n s i l e t e s t s , i t was seen that the s t r e n g t h of the three-element j o i n t was equal to that obtained from the two-element p r o c e s s . Hence f r i c t i o n welding with three elements i s q u i t e e f f e c t i v e when a l l of the three elements have s i m i l a r p r o p e r t i e s . Some of the t y p i c a l r e s u l t s are shown i n t a b l e IV. F i g u r e 14 shows that an aluminum specimen, welded by the three-element method, f a i l e d i n the parent m a t e r i a l under t e n s i o n . 5.5.2 MECHANICS OF THE PROCESS The mechanics of the three-element f r i c t i o n welding process are d i f f e r e n t from the c o n v e n t i o n a l process as the r e l a t i v e speed v a r i e s between a l l of the three elements and the r a t e of heat g e n e r a t i o n i s not q u i t e the same as i n the two-element pr o c e s s . As the process s t a r t s , the d i s c i s f r e e to r o t a t e 54 and there i s a r e l a t i v e motion between a l l three elements. The r e l a t i v e v e l o c i t y between a l l the elements i s c o n t i n u o u s l y v a r y i n g but due to the rubbing and the a s p e r i t i e s s hearing o f f , the temperature r i s e s at the i n t e r f a c e . The r a t e of heat input though c o n s i d e r a b l e i s lower than the c o n v e n t i o n a l methods due to much lower r e l a t i v e speeds d u r i n g the i n i t i a l s t a g e s. The d i s c gets welded p a r t i a l l y to one of the other two elements, the one with which i t had g r e a t e r r e l a t i v e v e l o c i t y and thus there had been g r e a t e r heat g e n e r a t i o n at that i n t e r f a c e . The j o i n i n g of the i n t e r m e d i a t e m a t e r i a l to the p i e c e which has higher r e l a t i v e speed with respect to i t c o u l d be p e c u l i a r to t h i s machine as the s l i d i n g v e l o c i t i e s are comparatively on the lower s i d e . For the machines where g r e a t e r s l i d i n g speeds may be used, g r e a t e r heat g e n e r a t i o n takes p l a c e at the lower r e l a t i v e v e l o c i t y end as too h i g h s l i d i n g speeds c o n t r i b u t e to lower f r i c t i o n and hence a lower r a t e of heat g e n e r a t i o n . See s e c t i o n 3.1. A f t e r the i n t e r m e d i a t e l a y e r gets p a r t i a l l y welded to one of the l a r g e r p i e c e s , the p r o c e s s now behaves s i m i l a r to that of a two-piece f r i c t i o n welding p r o c e s s . The r e l a t i v e l y g r e a t e r r e l a t i v e speed allows h i g h e r r a t e s of heat input so that the heat i s c o n f i n e d to a narrow zone c r e a t i n g c o n d i t i o n s r e q u i r e d f o r f r i c t i o n welding, (see 55 s e c t i o n 4.2) and the m a t e r i a l s s t a r t to become more p l a s t i c . In the cases where the parent m a t e r i a l s s t a r t to flow at the same c o n d i t i o n s as the intermediate m a t e r i a l , a c o l l a r a k i n to the co n v e n t i o n a l process i s formed. In some cases there i s no flow of parent m a t e r i a l ( s ) at a l l as the i n t e r l a y e r flows much e a r l i e r and may act as a l u b r i c a n t without c o n t r i b u t i n g any f u t h e r to the gene r a t i o n of heat. In these cases where the parent m a t e r i a l s do not flow, the c l e a n i n g of the weld s u r f a c e s i s not p r o p e r l y accomplished and consequently the q u a l i t y of the weld s u f f e r s . At a p o i n t when the m a t e r i a l s ' c o n d i t i o n i s c o n g e n i a l f o r welding, the r o t a t i o n i s stopped and the pi e c e s are forged together. T h i s f o r g i n g c o n s o l i d a t e s the weld at the plane which had welded p a r t i a l l y at the be g i n i n g of the p r o c e s s . The forge phase i s more c r i t i c a l i n three element welding than i n the c o n v e n t i o n a l p r o c e s s . T h i s i s e s p e c i a l l y t r u e when using m a t e r i a l s with d i f f e r e n t hot s t r e n g t h s . When using a d i s c with higher hot s t r e n g t h or m e l t i n g p o i n t , high forge p r e s s u r e s become mandatory. The f o r g i n g i s then of b e t t e r h e l p e s p e c i a l l y i n cases where the d i f f e r e n c e i n m e l t i n g p o i n t s i s s u b s t a n t i a l . The next t r i a l i n v o l v e d u s i n g a d i s s i m i l a r m a t e r i a l as an i n t e r l a y e r . A n i c k e l d i s c was used as 56 an i n t e r l a y e r between two s t e e l rods. In t h i s p rocess, most of the i n t e r l a y e r was normally e x p e l l e d and only a t h i n l a y e r remained at the i n t e r f a c e . Examination of p o l i s h e d , unetched specimen showed no evidence of i n t e r f a c i a l o x i d e s . E t c h i n g with 2% n i t a l s o l u t i o n showed a very narrow HAZ. A very t h i n l a y e r of n i c k e l with a few uncoated regions were seen along with some n i c k e l r i c h a r e a s . Some other specimens welded under s i m i l a r c o n d i t i o n s proved to have extremely good t e n s i l e s t r e n g t h with a mean UTS of 87,000 p s i and a standard d e v i a t i o n of 2,547 p s i as compared to 89,500 p s i and 493 p s i r e s p e c t i v e l y f o r a pure s t e e l specimen. The f a i l u r e was a d u c t i l e f r a c t u r e and took p l a c e at the i n t e r f a c e a f t e r necking. F i g u r e 15 shows the f r a c t u r e s u r f a c e at a X10 m a g n i f i c a t i o n and the cup and cone f a i l u r e of s u r f a c e at a X400 m a g n i f i c a t i o n . Having s u c c e s s f u l l y welded m a t e r i a l s with s i m i l a r and d i s s i m i l a r washers, welding of incompatible m a t e r i a l s was embarked upon. 5.5.3 COPPER / BRASS COMBINATION F i r s t , i n attempting to weld copper to brass, three d i f f e r e n t m a t e r i a l s were used as intermediate elements, namely (1) aluminum (2) naval brass ( c o n t a i n i n g 1.5% t i n , hence t e c h n i c a l l y a bronze) and (3) phosphor bronze. See appendix II f o r the 57 composition of the m a t e r i a l s . The main problem encountered with t h i s combination, e s p e c i a l l y while using naval brass and phosphor bronze i n t e r l a y e r s was s t a n d a r d i z i n g the f r i c t i o n i n g p e r i o d . The v a r i a t i o n s i n speed caused very uneven r a t e s of heat i n g and the a x i a l s h o r t e n i n g was q u i t e u n p r e d i c t a b l e , v a r y i n g over by 0.125 inches i n some cases. In b o r d e r l i n e cases of low f r i c t i o n i n g time, non-formation of a j o i n t was a common occurence. The process time v a r i e d f o r the same a x i a l s h o r t e n i n g . Another problem encountered with t h i s combination was that when l a r g e amounts of burn-off were not allowed, the c h i p p i n g o f f of the brass which takes place during the i n i t i a l stages remained and r e s u l t e d i n a ragged p e r i p h e r y . The maximum u l t i m a t e t e n s i l e s t r e n g t h obtained f o r copper, b r a s s combination with aluminum i n t e r f a c e was about 21,500 p s i . In the t e n s i l e t e s t , the f a i l u r e took p l a c e by i n t e r f a c e s e p a r a t i o n , at both j u n c t i o n s i n d i s c r i m i n a t e l y . At times when the specimens were being loaded on the t e n s i l e t e s t i n g equipment, c l i c k i n g was heard as the specimens were s t r e s s e d , an i n d i c a t i o n of s e p a r a t i o n of mechanical i n t e r l o c k i n g s . Since the s t r e n g t h obtained was not reasonable, aluminum i n t e r f a c e was not pursued f o r t h i s combination. The maximum s t r e n g t h obtained with phosphor bronze as i n t e r l a y e r was 22,500 p s i and 58 t h i s s t r e n g t h was c o n s i d e r e d to be inadequate, hence t h i s m a t e r i a l combination was not pursued e i t h e r . Naval brass a c t i n g as i n t e r l a y e r improved the s t r e n g t h of the j o i n t between copper and b r a s s . The maximum UTS obtained f o r t h i s j o i n t was about 30,000 p s i . S u p e r f i c i a l examination of the t e n s i l e t e s t specimens showed that f a i l u r e took p l a c e at the i n t e r f a c e s by s e p a r a t i o n . One of the specimens that had f a i l e d at the bronze / copper i n t e r f a c e was examined more c l o s e l y under Scanning E l e c t r o n Microscope (SEM). A low m a g n i f i c a t i o n showed small amounts of copper bonding near the outer edges. Around t h i s area, copper seemed to have f a i l e d by d u c t i l e f r a c t u r e but i n t e r f a c e s e p a r a t i o n seemed to have taken p l a c e l a r g e l y . At a X20 m a g n i f i c a t i o n , the above o b s e r v a t i o n was confirmed. F i g u r e 16a shows the f r a c t u r e s u r f a c e of copper segments at a X800 m a g n i f i c a t i o n . The f i n e dimples give evidence of d u c t i l e f a i l u r e . The separated bronze i n t e r f a c e i s shown i n f i g u r e 16b at a X1000 m a g n i f i c a t i o n , the bronze face showing some l o c a l i z e d d u c t i l i t y , i n d i c a t i n g the e x i s t e n c e of some areas where good bonding had taken p l a c e . Table IV g i v e s the s e t t i n g s of the process parameters f o r welding of t h i s m a t e r i a l combination. These are only nominal values and the s e t t i n g s have to be f u r t h u r i n v e s t i g a t e d upon i n order to be o p t i m i z e d . 59 The main drawback working with t h i s combination was the i n c o n s i s t e n c y of the process as mentioned e a r l i e r . Each welding t r i a l responded d i f f e r e n t l y to the same m a t e r i a l s , p r e p a r a t i o n and setup. As the process proceeds e s s e n t i a l l y at the same r a t e a f t e r the formation of the c o l l a r , t h i s problem may be overcome by having a x i a l s h o r t e n i n g as the c o n t r o l l i n g parameter of the welding process r a t h e r than j u s t some p a r t i c u l a r f r i c t i o n i n g time as was the case i n the experiments conducted. 5.5.4 STEEL / BRONZE COMBINATION Attempts were made to weld naval brass to 1025 s t e e l , f i r s t without any i n t e r l a y e r . Welds averaging i n u l t i m a t e t e n s i l e s t r e n g t h of approximatly 33,100 p s i were ob t a i n e d . I n t r o d u c t i o n of an i n t e r l a y e r to improve the s t r e n g t h of the j o i n t between these two incompatible m a t e r i a l s was then proposed. An i n t e r l a y e r of copper was i n t r o d u c e d and welding was c a r r i e d out at the same parameter s e t t i n g s . The r e s u l t i n g j o i n t was stronger i n t e n s i o n and the d i f f e r e n c e between j o i n t s with and without an i n t e r l a y e r welded at the same parameter s e t t i n g s v a r i e d a p p r e c i a b l y . A l s o the j o i n t s with an i n t e r l a y e r responded much more s i g n i f i c a n t l y than those without an i n t e r l a y e r to i n c r e a s e s i n forge p r e s s u r e s by forming b e t t e r j o i n t s . No other process 60 parameter i n f l u e n c e d the s t r e n g t h of the j o i n t as much as the forge p r e s s u r e . Some of the r e s u l t s of the process f o r both of these combinations are given i n t a b l e V. The improvement i n s t r e n g t h i s d e p i c t e d i n f i g u r e 17. As i t i s seen, there was only a marginal improvement i n the t e n s i l e s t r e n g t h at lower forge p r e s s u r e s , and the g r e a t e s t improvement occured when the pressure was at 12,800 p s i and the UTS of 47,770 p s i was 13,770 p s i above the s t r e n g t h of weld made without an i n t e r l a y e r , an improvement of about 40%. V i s u a l examination of the f a i l e d s u r f a c e a f t e r the t e n s i l e t e s t showed that copper had welded to both parent m a t e r i a l s e q u a l l y w e l l and s e p a r a t i o n had p r i m a r i l y taken p l a c e w i t h i n the copper i n t e r f a c e , thus l e a v i n g a t h i n l a y e r of copper adhering over most of the s u r f a c e s of both the parent m a t e r i a l s . T h i s was the case with a l l the specimens without e x c e p t i o n , even those welded at low forge p r e s s u r e s . Low m a g n i f i c a t i o n ( s t e r e o ) examination r e v e a l e d an i n t e r f a c e s e p a r a t i o n of copper and s t e e l i n some small areas, e s p e c i a l l y at the c e n t r e where the j o i n t formation i s at i t ' s worst. At a X40 m a g n i f i c a t i o n , seen i n f i g u r e 18, flow l i n e s from e a r l i e r machining on s t e e l are c l e a r l y v i s i b l e . A l a r g e r m a g n i f i c a t i o n (X200) shows c l e a r l y both the 61 i n t e r f a c e s e p a r a t i o n wherever s t e e l i s exposed and mottled copper areas f a i l i n g i n a d u c t i l e f r a c t u r e mode. The cup and cone s u r f a c e of copper and flow l i n e s of s t e e l are shown i n f i g u r e 19. The same s u r f a c e of the s t e e l specimen had some high .spots of copper which seemed to be the areas that had f a i l e d by i n t e r f a c e s e p a r a t i o n on the bronze end j u s t as some areas on the s t e e l end when copper had separated. But a X200 m a g n i f i c a t i o n of t h i s area r e v e a l e d that there had been a d u c t i l e f r a c t u r e at t h i s i n t e r f a c e . An x-ray a n a l y s i s of elements on t h i s spot exposed the presence of z i n c which a l s o proves beyond doubt that f r a c t u r e was not e n t i r e l y w i t h i n copper or e x a c t l y at the j u n c t i o n . F i g u r e 20 shows a photograph which shows three d i s t i n c t areas showing a l l the three s u r f a c e s . . . . (1) flow l i n e s on s t e e l , (2) small dimples e n t i r e l y w i t h i n copper, and (3) l a r g e r dimples on copper / naval brass i n t e r f a c e . X-ray a n a l y s i s of elements on t h i s spot i s shown i n f i g u r e 21. F i g u r e 22 shows the co u n t e r p a r t s u r f a c e at a X400 m a g n i f i c a t i o n . The d u c t i l e f r a c t u r e has taken p l a c e evenly a l l over the s u r f a c e , i n both copper and bronze. T h i s method of i n t r o d u c i n g a t h i r d element hence can be adopted to improve s t r e n g t h of j o i n t s made between incompatible m a t e r i a l s by f r i c t i o n welding. 62 5.5.5 TITANIUM / NICKEL COMBINATION Attempts were f i r s t made to weld t i t a n i u m to n i c k e l by f r i c t i o n welding without any i n t e r f a c e and very b r i t t l e j o i n t s were o b t a i n e d . Titanium flows at a much lower temperature than n i c k e l , only to be extruded to an u n d e s i r a b l e extent without formation of any reasonable j o i n t . Welding was then attempted between n i c k e l and t i t a n i u m by i n t r o d u c i n g s e v e r a l m a t e r i a l s as i n t e r l a y e r s . With copper as an intermediate m a t e r i a l , the maximum UTS obtained was around 20,000 p s i . A naval brass i n t e r l a y e r y i e l d e d even lower s t r e n g t h , about 15,500 p s i . B e t t e r r e s u l t s were obt a i n e d from phosphor bronze as i n t e r l a y e r , with an UTS of 27,200 p s i i n one case. But none of these m a t e r i a l s gave c o n s i s t e n t r e s u l t s , hence were not pursued. The most c o n s i s t e n t r e s u l t s were obtained when usi n g aluminum as an i n t e r l a y e r . A l s o , two o b s e r v a t i o n s were made with respect to the process parameter s e t t i n g s . The s t r e n g t h of the j o i n t improved with the i n c r e a s e d forge pressure and a l s o with the d u r a t i o n of h e a t i n g . The e f f e c t of these two on the s t r e n g t h of the j o i n t i s shown i n f i g u r e s 23 and 24. T y p i c a l process parameter s e t t i n g s are given i n t a b l e VI. The s t r e n g t h of the j o i n t has two peaks, one o c c u r i n g at a very low f r i c t i o n i n g p e r i o d 63 (approximately 2 seconds) a f t e r which a very d r a s t i c drop i s observed. The s t r e n g t h then s t a r t s to in c r e a s e with the i n c r e a s e i n time and the maximum s t r e n g t h i s obtained at 15 seconds p e r i o d which i s a l s o the longest f r i c t i o n i n g time the specimens are sub j e c t e d t o . Hence the combination of hig h forge p r e s s u r e s with long f r i c t i o n i n g times r e s u l t e d i n the s t r o n g e s t welds. In t e n s i l e t e s t s , the weld f a i l e d at both t i t a n i u m and n i c k e l j u n c t i o n s by s e p a r a t i o n . There were no si g n s of f a i l u r e by a d u c t i l e mode. Unaided examinations of the s u r f a c e s showed t r a c e s of aluminum adhering to e i t h e r n i c k e l or t i t a n i u m whichever was not c a r r y i n g the major chunk of aluminum with i t . Examination under SEM at a X200 m a g n i f i c a t i o n a l s o showed that the f r a c t u r e had been b r i t t l e i n nature a c r o s s the i n t e r f a c e . I t i s expected t h a t the v a r i a t i o n of s t r e n g t h of the j o i n t with r e s p e c t t o time was due to the d i f f e r e n c e s i n bonding mechanisms. The i n i t i a l peak occured due to the bonding mechanism being p u r e l y a mechanical i n t e r l o c k i n g . As the d u r a t i o n of he a t i n g i n c r e a s e d , the a s p e r i t i e s are sheared o f f and then m e t a l l u r g i c a l bonding took p l a c e by d i s r u p t i n g the su r f a c e f i l m s and b r i n g i n g the s u r f a c e s t o a very c l o s e c o n t a c t . S u r f a c e f i l m s on aluminum are broken e a s i l y as i t flows e a s i l y . Titanium, l i k e aluminum, 64 forms a very s t a b l e oxide coat (which a c t u a l l y p r o t e c t s i t from f u r t h u r o x i d a t i o n and other h o s t i l e environment) and the d i s r u p t i o n of t h i s c o a t i n g i s more and more e f f e c t i v e with the passage of time thus exposing f r e s h e r areas. Hence, i n c r e a s e d e x p o s i t i o n of f r e s h e r s u r f a c e s at l a t e r stages r e s u l t i n stronger welds. 5.6 MICRO-PROBE ANALYSIS Phase diagrams show that b i n a r y phases can q u i t e e a s i l y be formed at the temperatures encountered d u r i n g the welding between t i t a n i u m and aluminum and a l s o between n i c k e l and aluminum. Hence the i n v e s t i g a t i o n was then focussed on determining the formation of i n t e r m e t a l l i c s (or a l l o y s ) at the weld area. I t was necessary to determine i f there was any 1. d i f f u s i o n of aluminum i n t o n i c k e l and/or t i t a n i u m , 2. d i f f u s i o n of n i c k e l i n t o aluminum, and 3. d i f f u s i o n of t i t a n i u m i n t o aluminum. T h i s a n a l y s i s was made on a l l the three types of welds obtained with t h i s combination, "(see f i g u r e 23) namely: 1. good welds r e s u l t i n g at low f r i c t i o n i n g time (2 seconds), 2. bad welds r e s u l t i n g from medium f r i c t i o n i n g time (7 seconds), and 3. good welds r e s u l t i n g from long f r i c t i o n i n g time (15 seconds). 65 The aforementioned j o i n t s w i l l be r e f e r r e d as type I, type II and type I I I j o i n t s , r e s p e c t i v e l y . An e l e c t r o n micro-probe a n a l y s e r was used to determine i f there was any d i f f u s i o n of aluminum i n t o n i c k e l or t i t a n i u m . A beam of spot s i z e of 1 micron was used f o r a l l the a n a l y s e s . Scans were made on specimens a c r o s s the i n t e r f a c e . No p o s i t i v e evidence of aluminum t r a v e l l i n g i n t o e i t h e r n i c k e l or t i t a n i u m was observed.. The i n t e r f a c e was q u i t e d i s t i n c t and the t r a n s f o r m a t i o n from aluminum to the other two m a t e r i a l s was sudden i n a l l of the three types of j o i n t s . The second a n a l y s i s on the movement of t i t a n i u m i n t o aluminum gave some p o s i t i v e r e s u l t s , but i n type I I I j o i n t s o n ly. No such movement was observed i n type I or type II j o i n t s . F i g u r e 25 shows a p l o t which i s a t r a c e of the c o n c e n t r a t i o n p r o f i l e s of aluminum and t i t a n i u m f o r a type III j o i n t which g i v e s a p o s i t i v e i n d i c a t i o n of a not too sudden t r a n s i t i o n from aluminum to t i t a n i u m and the e x i s t e n c e of a t i t a n i u m phase i n an aluminum medium. F i g u r e 26 shows a photograph of the scan made a c r o s s the i n t e r f a c e at one l o c a t i o n where a t i t a n i u m - r i c h phase was suspected. In the photograph, the dark r e g i o n on the top r i g h t corner i s pure t i t a n i u m and the the r e s t i n l i g h t e r shade i s aluminum. The exact l i n e of scan i s i n d i c a t e d by the h o r i z o n t a l l i n e . The scan made was f o r t i t a n i u m . As the scan t r a v e r s e d from the t i t a n i u m s i d e to the aluminum s i d e , the c o n c e n t r a t i o n p r o f i l e dipped from 100% (out of view) to bottom, i . e . , zero percent c o n c e n t r a t i o n . But as the scan 66 passed the suspected dark spot, the c o n c e n t r a t i o n p r o f i l e gave a peak i n d i c a t i n g i t to be a t i t a n i u m r i c h a r e a . Note that there was a delay between the scan and the p r o f i l e (due to the probe i t s e l f ) . Thus, i t was determined that there was a movement of t i t a n i u m i n t o aluminum at long f r i c t i o n i n g times. The n i c k e l scan r e v e a l e d s i m i l a r r e s u l t s but of l a r g e r magnitude. For type I j o i n t s , the boundary was very d i s t i n c t and there was no t r a n s f e r of m a t e r i a l as expected. Type II j o i n t s had mixing to a small extent of n i c k e l i n aluminum. F i g u r e 27 shows the t r a c e of n i c k e l scan. The darkened area i s n i c k e l and the t r a c e i s made at 45 degrees to the i n t e r f a c e . As suspected, the dark spots d i d prove to be n i c k e l r i c h a reas. S e v e r a l such spots were found along the j u n c t i o n at d i s t a n c e s v a r y i n g from 10 to 50 microns from the j u n c t i o n . The spot i n f i g u r e 27 i s at a d i s t a n c e of about 38 microns ( p i c t u r e o r i g i n a l l y m agnified X1200 t i m e s ) . Type I I I specimens proved to have maximum t r a n s f e r of n i c k e l i n t o aluminum and t h i s i s shown i n f i g u r e 28 g i v i n g the c o n c e n t r a t i o n p r o f i l e . F i g u r e s 29 and 30 give the extent of t r a n s f e r , l o c a t i o n , shape and s i z e of n i c k e l r i c h a r e a s . F i g u r e 29 i s i n an area c l o s e r to the c e n t r e than the areas shown i n 30. T h i s n i c k e l ( r i c h ) area i s more i n the p l a t e - l i k e form along the j u n c t i o n at a d i s t a n c e of 4 microns approximately, and 2 microns t h i c k . F i g u r e 30 shows areas more towards the p e r i p h e r y , at s l i g h t l y g r e a t e r than t w o - t h i r d s of the r a d i u s . N i c k e l peaks i n f i g u r e 30 are 67 extremely c l e a r i n showing high c o n c e n t r a t i o n areas deep i n s i d e aluminum. Now that the movement of n i c k e l i n t o aluminum at long f r i c t i o n i n g times has been c l e a r l y e s t a b l i s h e d , i t was r e q u i r e d to analyse these n i c k e l r i c h r e g i o n s q u a n t i t a t i v e l y to determine the percentage of n i c k e l so that the exact s t o i c h i o m e t r i c composition of the i n t e r m e t a l l i c can be determined from the phase diagram. The e l e c t r o n micro-probe was used f o r t h i s purpose. The q u a n t i t a t i v e a n a l y s i s was c a r r i e d out on type I I I specimens i n which the d i f f u s i o n of n i c k e l was the most, having a d i f f u s i o n i n t o r e g i o n s as deep as f i f t y microns i n some p l a c e s . The standard n i c k e l count was taken i n a region deep i n s i d e the n i c k e l end of the specimen. S i m i l a r l y the aluminum count was taken w e l l away from the i n t e r f a c e . Even though aluminum had a sma l l amount of a l l o y i n g elements ( 1% magnesium, 0.6% s i l i c o n ), f o r the sake of s i m p l i c i t y , t h i s count was taken to be of 100% pure aluminum. N i c k e l counts were taken at d i f f e r e n t spots as shown in the f i g u r e 31. A l l p r e c a u t i o n s were taken to ensure s t a b l e o p e r a t i o n of the micro-probe d u r i n g t h i s time by having a steady x-ray beam i n t e n s i t y to ensure a c c u r a t e measurements. Becuase of s i g n i f i c a n t d i f f e r e n c e i n the atomic numbers and d e n s i t i e s of the two m a t e r i a l s being analysed, a c c u r a t e counts were not p o s s i b l e i n areas r i g h t adjacent to the i n t e r f a c e . The atomic number c o r r e c t i o n , a d s o r b t i o n c o r r e c t i o n and the c o r r e c t i o n f o r f l u o r e s c e n t enhancement 68 were made using a computer program c a l l e d MAGIC (Micro-probe A n a l y s e r General I n t e n s i t y C o r r e c t i o n s ) . The exact c o n c e n t r a t i o n of each element at each of the spots was determined. Since only two elements were being analysed, c o n c e n t r a t i o n of aluminum was determined by d i f f e r e n c e . The r e s u l t s are shown i n t a b l e V I I . The c o n c e n t r a t i o n of n i c k e l at most of the spots was between 57% and 79% by weight. Looking at the bi n a r y phase diagram between aluminum and n i c k e l ^ ^ ( s e e f i g u r e 32) i t i s seen that there i s mostly one i n t e r m e t a l l i c compound, a 0 phase (NiAl) forming at t h i s composition range. T h i s phase i s s t a b l e without any phase t r a n s f o r m a t i o n s r i g h t from i t ' s m e l t i n g p o i n t to the room temperature. And s i n c e only one phase was observed to be predominant throughout and a l s o the t h i c k n e s s of t h i s i n t e r m e t a l l i c l a y e r was not s u b s t a n t i a l , i t was not co n s i d e r e d to be a dominating f a c t o r i n determining the s t r e n g t h of the j o i n t . A l s o s i n c e i t was not p o s i t i v e l y i d e n t i f i e d that t h i s i n t e r m e t a l l i c was d e t r i m e n t a l to the q u a l i t y of the weld, minimizing the formation of t h i s phase by v a r y i n g process parameters was not c o n s i d e r e d . However a t h i n l a y e r , l i g h t e r than both the elements was seen i n the absorbed e l e c t r o n p i c t u r e r i g h t at the i n t e r f a c e . I t was not c l e a r i f i t was an i n t e r m e t a l l i c or a f l u o r e s c e n c e e f f e c t a r i s i n g from the d i f f e r e n c e i n the d e n s i t i e s of the two elements. The counts taken at t h i s p l a c e were not c o n s i d e r e d r e l i a b l e , hence no f u r t h e r a n a l y s i s was made i n t h i s zone. 6. C O N C L U S I O N S A N D S C O P E F O R F U T U R E W O R K A f r i c t i o n welding machine was designed and f a b r i c a t e d . The equipment was s u c c e s s f u l l y t e s t e d and many s i m i l a r and d i s s i m i l a r m a t e r i a l s were welded. 100% e f f i c i e n t welds were common i n case of low carbon s t e e l s . Very good welds were obtained i n case of aluminum, which under t e n s i l e t e s t f a i l e d a c r o s s the i n t e r f a c e (and not along the i n t e r f a c e ) . The f a i l u r e took p l a c e i n a d u t i l e manner a f t e r necking with over 20% e l o n g a t i o n . The three element f r i c t i o n welding process was t r i e d on d i f f e r e n t m a t e r i a l combinations such as having an intermediate m a t e r i a l s i m i l a r to both of the parent m a t e r i a l s , s i m i l a r to one of the parent m a t e r i a l s and d i s s i m i l a r t o both the parent m a t e r i a l s . Strengths s i m i l a r to t h a t o f . p a r e n t m a t e r i a l s and i n some cases stronger than the parent m a t e r i a l s were observed. Good welds were not obtained i n some cases and i t was expected so. Good welds obtained proved that the three-element process was a f e a s i b l e method of f r i c t i o n welding d i s s i m i l a r m a t e r i a l s . The process was t r i e d on th r e e p a i r s of d i s s i m i l a r incompatible m a t e r i a l combinations. The j o i n t s o b t ained between t i t a n i u m and n i c k e l , a combination c o n s i d e r e d unweldable p r e v i o u s l y , when welded with an aluminum i n t e r l a y e r were stronger by 33% than the welds obtained between the int e r m e d i a t e l a y e r to i t s e l f . In the case of an attempt t o improve the s t r e n g t h of b r i t t l e welds between s t e e l and bronze using a copper i n t e r l a y e r , an i n c r e a s e of 69 70 40% i n s t r e n g t h was obt a i n e d . Hence the three element process was s u c c e s s f u l l y used to o b t a i n j o i n t s between f r i c t i o n unweldable m a t e r i a l s . Some s t a t i s t i c a l a n a l y s i s was a l s o c a r r i e d out whenever s u f f i c i e n t number of samples were a v a i l a b l e . Some hypotheses were put forward to act as g u i d e l i n e s i n s e l e c t i n g an intermediate m a t e r i a l that c o u l d achieve a bond with good s t r e n g t h along with some p r e d i c t i o n s r e g a r d i n g the s t r e n g t h of the j o i n t i t s e l f . E l e c t r o n micro-probe a n a l y s i s was c a r r i e d out on three types of j o i n t s a r i s i n g from three f r i c t i o n i n g times between nickel-aluminum-titanium combination to determine i f there was any p h y s i c a l movement or d i f f u s i o n of any of the three elements i n t o one another. While there was no movement of aluminum anytime, there were t r a c e s of movement of n i c k e l i n t o aluminum during medium f r i c t i o n i n g times and movement of t i t a n i u m i n t o aluminum i n j o i n t s obtained from long f r i c t i o n i n g times. There was an e x t e n s i v e formation of i n t e r m e t a l l i c s of aluminum and n i c k e l i n aluminum when the f r i c t i o n i n g was long, which when analysed showed a s i n g l e s t a b l e phase. I t d i d not appear t o a c t a d v e r s e l y on the q u a l i t y of the j o i n t . The q u a n t i t a t i v e a n a l y s i s of the i n t e r m e t a l l i c s at the n i c k e l r i c h regions showed that the composition of the i n t e r m e t a l l i c was mostly between 57% and 79%, a s t a b l e 0 phase. 71 As f o r the f u t u r e work, more i n v e s t i g a t i o n s are recommended i n t o determining the e f f e c t of the t h i c k n e s s of the i n t e r l a y e r before and a f t e r the process on the j o i n t p r o p e r t i e s . Ways and means of c o n t r o l l i n g the t h i c k n e s s of the i n t e r l a y e r can a l s o be i n v e s t i g a t e d . The e f f e c t of s u r f a c e roughness of the in t e r m e d i a t e m a t e r i a l on the process time ( f o r the same b u r n - o f f ) has to be i n v e s t i g a t e d , along with the f i n a l t h i c k n e s s of the intermediate m a t e r i a l with r e s p e c t t o the burn-off and/or the f r i c t i o n i n g time and a l s o the forge p r e s s u r e . T r a n s i e n t temperature d i s t r i b u t i o n i s a prime area f o r r e s e a r c h as i t g i v e s a p i c t u r e of the r a t e of heat g e n e r a t i o n at the s u r f a c e s and t h i s r a t e i n f l u e n c e s the q u a l i t y of the weld. C o n t r o l l i n g the weld time by a x i a l displacement i s v i s u a l i z e d as a b e t t e r process parameter e s p e c i a l l y when usi n g m a t e r i a l combinations that do not respond to the process i n a c o n s i s t e n t manner. A more complete understanding of three-element f r i c t i o n welding i s needed. T h i s c o u l d be achieved with more i n f o r m a t i o n obtained by moni t o r i n g torque, displacement c h a r a c t e r i s t i c s and the behaviour of the d i s c ( t h i r d element) i t s e l f , which c o u l d be done by determining the exact sequence with the a i d of a hi g h speed camera. A mathematical model can be evolved, using which one can p r e d i c t the q u a l i t y of three-element welds. A l s o the models should be ab l e to p r e d i c t the p o s s i b l e r e s u l t when j o i n i n g d i f f e r e n t m a t e r i a l s u s i n g t h i s p r o c e s s . In chapter 72 f i v e , there were s e v e r a l combinatons (with i n t e r l a y e r s ) which were i n c o n s i s t e n t or r e s u l t e d i n bad welds, and thus were abandoned. The reason was the l i m i t a t i o n of time and resources a l l o c a t e d to t h i s p r o j e c t . Such problem areas are worthwhile s u b j e c t s f o r research and a more complete understanding of the f r i c t i o n welding process i t s e l f . 73 Materials TABLE I. Relative w e l d a b i l i t y of d i f f e r e n t materials as reported by the Welding I n s t i t u t e , England (12.1). MATERIAL COMBINATION TYPE OF WELD FORMED POSSIBLE INTERMEDIATE MATERIALS Brass/Copper No weld Aluminum Bronze/Plain carbon s t e e l No weld Aluminum Magnesium alloys/Aluminum a l l o y s No weld Aluminum Nickel/Titanium No weld Aluminum Tungsten carbide/Plain carbon s t e e l B r i t t l e weld Aluminum Silver/Titanium No weld Copper Zirconium a l l o y s / P l a i n carbon s t e e l B r i t t l e weld Aluminum Sta i n l e s s steel/Titanium B r i t t l e weld Aluminum, copper Niobium/Stainless s t e e l B r i t t l e weld Aluminum Magnesium alloys/Magnesium a l l o y s No weld Aluminum P l a i n carbon steel/Titanium No weld Copper, aluminum TABLE I I . INCOMPATIBLE MATERIAL COMBINATIONS AND POSSIBLE INTERMEDIATE MATERIALS, AS SEEN FROM TABLE I. 75 MATERIAL COMBINATION TYPE OF WELD FORMED POSSIBLE INTERMEDIATE MATERIALS Brass/Copper No weld Bronze Cast i r o n / P l a i n carbon s t e e l No weld Brass,Copper,Nickel Invar/Brass No weld Bronze Nickel/Titanium No weld P l a i n carbon s t e e l Tung s t en/Tungs ten B r i t t l e weld Molybdenum,Iridium Zirconium alloys/Niobium No weld Iridium Tungsten carbide/Plain carbon s t e e l B r i t t l e weld Nickel TABLE I I I . DIFFERENT MATERIAL COMBINATIONS, POSSIBLE INTERMEDIATE MATERIALS NOT TRIED WITH ONE OR EITHER OF THE PARENT MATERIALS BEFORE. 76 MATERIAL COMBINATION FRICTION TIME (Seconds) FORGE FRICTION TIME PRESSURE (Seconds) (psi) FORGE PRESSURE (psi) U T S (Kpsi) S t e e l - S t e e l Aluminum-Steel Aluminum-Aluminum-Aluminum Steel-Nickel-Steel Copper-Phosphor bronze-Brass Copper-Naval brass-Brass 2 2.5 3.8 3.8 4 3.5 4.5 4 11 11.5 10.5 5 3 10 3 10 10 10 10 10 10 10 3200 4800 3200 3200 4000 3200 2560 3200 3600 3600 3200 6400 8000 3200 16000 9600 8000 6400 8000 8000 8000 7200 84.5 91.3 19.1 34.6 25.3 25.5 84.7 91.3 22.6 20.9 30.2 TABLE IV. SOME REPRESENTATIVE PROCESS PARAMETER SETTINGS FOR DIFFERENT MATERIAL COMBINATIONS AND PROCESSES. 77 MATERIAL FRICTION FORGE FRICTION FORGE COMBINATION TIME TIME PRESSURES PRESSURES U T S (Seconds) (Seconds) (psi) (psi) (Kpsi) NAVAL BRASS - STEEL 2.5 2.5 2.5 2.5 2.5 2.5 2.5 3600 3600 3600 3600 3600 3600 3600 6400 8000 8800 9600 11200 12800 14400 32.19 32.95 32.19 32.29 34.02 35.45 35.48 NAVAL BRASS - COPPER - STEEL 3 7 3.5 7 3 7 3 14 3 14 3 7 3440 3200 3600 3600 3600 3600 8000 8800 10400 11200 12800 14400 34.14 35.6 37.51 41.61 47.7 41.87 TABLE V. PROCESS PARAMETER SETTINGS FOR NAVAL BRASS - STEEL COMBINATION AND THE STRENGTH OBTAINED, WITH AND WITHOUT A COPPER INTERLAYER. FRICTION FORGE FRICTION FORGE U X o TIME TIME PRESSURE PRESSURE (Seconds) (Seconds) (psi) (psi) (Kpsi) 1.5 15 4000 16000 13 .4 2 ii 4 1 . 2 3 I I 1 0 . 3 5 5 1 2 . 3 7 15 11 . 8 10 I I „ 14 .9 13 I I 31 . 2 15 I I » 4 1 . 9 TABLE VI. VARIATION OF THE UTS WITH FRICTIONING TIME FOR A JOINT BETWEEN TITANIUM AND NICKEL WITH AN ALUMINUM INTERLAYER. SPOT NUMBER WEIGHT ATOMIC UNKNOWN STANDARD UNKNOWN STANDARD ELEMENT PERCENT PERCENT K ^ 1 1 0 INTENSITY INTENSITY BACKGROUND BACKGROUND I T E R A T I 0 N S 1 NI A l (K06) * 73.33 26.67 55.82 44.18 0.7203 56829 78896 109 189 2 2 Ni A l (KO) * 57.88 42.12 38.71 61.29 0.5269 44411 78896 109 189 2 3 Ni A l (Kft) * 78.48 21.52 62.63 37.37 0.7735 61026 78896 109 189 2 4 Ni A l (Kfc) 69.77 30.23 51.47 48.53 0.6838 53937 78896 109 189 2 5 Ni A l * 57.20 42.80 38.05 61.95 0.5560 43870 78896 109 189 2 6 Ni A l (KC6) * 39.97 60.03 23.43 76.57 0.3842 39314 78896 109 189 2 * Determined by difference TABLE VII. QUANTITATIVE ANALYSIS OF THE INTERFACE AT VARIOUS LOCATIONS FOR A TYPE I I I JOINT. 80 ( i i i ) (iv) FIGURE 1. Different mechanical arrangements for generating r e l a t i v e motion and a x i a l thrust i n the conventional f r i c t i o n welding process (12.1). ( i ) Conventional arrangement of the work-pieces ( i i ) The piece that i s spun i s also advanced ( i i i ) Centre piece i s spun, the end-pieces are advanced (iv) The end-pieces are spun and also advanced against a stationary centre piece 81 Relative motion classification Friction welding I Energy classification I Continuous drive The process variant where power or energy is provided by an infinite duration source and maintained for a preset period 1 Hybrid system Combining some features of both methods I Stored energy The process variant where the energy for welding is supplied by the kinetic energy stored in a rotating system or fluid storage system I Rotational A method in which one component is rotated relative to and in contact with the mating face of another component Symbol Linear oscillation A method in which one component is moved in a linear oscillating motion relative to and in contact with the mating face of another component Angular oscillation A method in which one component is moved in an angular oscillating motion about a common component axis relative to and in contact with the mating face of another component Orbital A method in which one component is moved in a small circular motion relative to and in contact with the mating face of another component either with neither component about its own central axis, or with components rotating in the same direction about their own axes and at the same speed but with their axes displaced ! FIGURE 2. A general c l a s s i f i c a t i o n of the f r i c t i o n welding process (12.1). time time (a) (b) FIGURE 3. V a r i a t i o n of torque, a x i a l displacement (burn-off), speed and a x i a l thrust during various stages of (a) continuous drive and (b) i n e r t i a f r i c t i o n welding (7). FIGURE 4. Fine l i n e s on the c o l l a r of a f r i c t i o n welded j o i n t showing the a l t e r n a t i n g events of the formation of a p l a s t i c i z e d layer and i t ' s extrusion. 83 FIGURE 5. V a r i a t i o n of the s i z e and l o c a t i o n of the p l a s t i c i z e d region with speed f o r a 3/4 inch diameter specimen ( 3 ) . . 1. P l a s t i c i z e d material (a) 1000 rpm (b) 2000 rpm (c) 4000 rpm T 1 1 1 1 1 r Mean rubbing speed m/sec FIGURE 6. Relationship between the c o e f f i c i e n t of f r i c t i o n and speed at various a x i a l l o a d s ( i n KN): A -3.8; •-6 .0; D-8.2; o-13 .5; V-19.0 (3). 84 I 156-H3-130-117-104-91-78-65-52-39-26-13-0-120\ 110\ 100 SO S QO I 701 I 60| I SOY 40 30| 20 ro| o Speed 97Sre*/min~ V * t Speed 1825 rev/min J 2000 6000 10000 MOO Pressure, tb/ki* 18000 FIGURE 7. V a r i a t i o n of equilibrium torque with pressure while welding two specimens of 0.75 inch diameter (18). 031 0-25 & 0-20 ~i 1 — i — i 1 1 — r Specimen size 0-75 in. did. 182Srev/min J 1 1 I I I i ' ' 2000 6000 10000 H000 18000 Pressure, lb/in2 FIGURE 8. Relationship between the rate of burn-off and pressure at two d i f f e r e n t speeds (18). 9. Effect of speed and a x i a l pressure on the shape and size of the heat-affected zone, a and c welded at 975 rpm, b and d at 1825 rpm. The 3/4 inch diameter specimen welded at a constant burn-off of 0.2 inches (18). 86 SLIDING SPEEDS FIGURE 10 . V a r i a t i o n of the c o e f f i c i e n t of f r i c t i o n with load ( 2 ) . FIGURE 11. Steel specimens f a i l e d i n tension i n base metal. FIGURE 12. Steel specimens f a i l e d i n tension a f t e r removing the f l a s h . F a i l u r e occured i n the base metal f or the top two specimens and i n the weld plane, a f t e r necking i n the base metal i n the bottom specimen. 88 Incompatible materials A groove made Third element to hold the (Buffer, intermediate material) t h i r d element i n place FIGURE 13. Schematic of the proposed f r i c t i o n welding method of introducing a t h i r d element (in t e r l a y e r ) into the process. FIGURE 14. A three element welding specimen (Aluminum - Aluminum -Aluminum) f a i l e d i n tension i n the base metal. The weld plane i s i n the middle of the recess machined. 88a FIGURE 15. The f a i l e d surface of s t e e l - n i c k e l - s t e e l specimen at a X10 magnification (above). The f a i l u r e took place i n the weld plane a f t e r necking. A X400 magnification below shows dimples of cup and cone fracture mode. FIGURE 16b. A X1000 magnification of the bronze surface of bronze/copper i n t e r f a c e showing i n t e r f a c e separation and some l o c a l i z e d d u c t i l i t y . 52 W I T H O U T I N T E R L A Y E R H 1 1 1 1 I 1 1 \ I I . 5000 7000 9000 11000 13000- 15000 FORGING PRESSURE (PSI) FIGURE 17. Vari a t i o n of strength with forge pressures of bronze/steel combination,, with and without copper i n t e r l a y e r , welded at 3600psi f r i c t i o n i n g pressure. , 3 seconds f r i c t i o n i n g time. 91 FIGURE 19. The above s t e e l specimen at a X200 magnification shows flow l i n e s on the s t e e l surface and f i n e dimples on the copper l a y e r . Locations of viewing of figures 18, 19 and 20 on s t e e l surface bronze/copper/steel specimen f a i l e d under tension. 92 FIGURE 20. A X400 magnification of a high spot on the s t e e l specimen of the brass/copper/steel combination, showing three regions; coarse dimples on the high spot on the l e f t , f i n e r dimples on the copper surface i n the middle, flow l i n e s of the s t e e l surface. Cu Cu Fe Cu Zn * •.r FIGURE 21. X-Ray analysis of a high spot on the f a i l e d s t e e l surface of s t e e l - copper - bronze specimen which appeared to be of interface separation from bronze surface. 94 FIGURE 2 2 . The f a i l e d bronze surface of bronze/copper/steel specimen showing an uniform d u c t i l e mode of f r a c t u r e on both, copper surface at the bottom, top l e f t corner of the p i c t u r e , and the rest of bronze surface. FIGURE 23. V a r i a t i o n of t e n s i l e strength with f r i c t i o n i n g f o r a titanium - aluminum - n i c k e l j o i n t , welded at 4000 p s i f r i c t i o n pressure and 16000 p s i forge pressure. © - Maximum and minimum strengths obtained at each parameter s e t t i n g . 42 w u u 6800 9600 12400 15200 18000 FORGE PRESSURE (PSI) FIGURE 24. Vari a t i o n of UTS with forge pressure of titanium/nickel j o i n t with aluminum i n t e r l a y e r , welded at a f r i c t i o n i n g pressure of 4000 p s i and a f r i c t i o n i n g time of 15 seconds. °^  97 FIGURE 25. Concentration p r o f i l e of titanium/aluminum i n t e r f a c e f or a type I I I j o i n t . SEE PAGE 98A. 98 FIGURE 26. Scan for titanium over a suspected titanium r i c h spot. The h o r i z o n t a l l i n e i s the scanning path. The l i g h t region i s aluminum. The concentration p r o f i l e on t h i s type I II j o i n t r i s e s to give a peak as i t traverses over the dark spot. FIGURE 27. A n i c k e l scan for a type II j o i n t over a dark area suspected to be n i c k e l r i c h . The dark area on the top r i g h t corner i s n i c k e l . ( X1200 magnification ) 98a Interlayer Reference l i n e s of scan for fi g u r e s 26 to 30. 1. Scan made at 45 degrees to the axis of the specimens' and moving into the aluminum i n t e r l a y e r , figures 26, 27 and 29. 2. Scan made p a r a l l e l to the specimen's axis, f i g u r e 30; moving into n i c k e l . 99 FIGURE 28. Concentration p r o f i l e f o r n i c k e l and aluminum f o r a type I I I j o i n t . 100 FIGURE 30. Nickel scan of a type III specimen at a region close to the periphery. Numerous dark regions i n d i c a t e extensive movement of n i c k e l into aluminum at long f r i c t i o n i n g times. 101 FIGURE 32. 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May 1963, pp 233-s to 240-s. 31. F.F.LING "Mechanics of s l i d i n g s u r f a c e s " , Metals e n g i n e e r i n g q u a r t e r l y , May 1967, pp 1 to 3. 32. K.ARMISHAW "Why f r i c t i o n welding gained acceptance", P r o d u c t i o n engineer, Sept. 1983, pp 17 and 18. 33. K.H.HOLKO " F r i c t i o n welding a l l o y 800", Welding r e s e a r c h supplement, May 1977, pp 140-s to 142-s. 34. K.H.HOLKO " F r i c t i o n welding h a s t e l l o y X to carbon s t e e l " , Welding j o u r n a l , March 1978, pp 19 to 27. 35. C.G.NESSLER, D.A.RUTZ, P.A.VOZZELLA 108 " F r i c t i o n welding of t i t a n i u m a l l o y s " , Welding research supplement, September 1971, pp 379-s to 385-s. 36. T.RICH, R.ROBERTS "The forge phase of f r i c t i o n welding", Welding res e a r c h supplement, March 1971, pp 138-s to 145s. 37. K.NAKAJIMA, S.KAWANAMI "On the movement of o r i g i n a l c o n t a c t face i n  f r i c t i o n welding", Proceedings of the second I n t ' l symposium of the Japanese welding s o c i e t y on advanced welding technology, paper 4-(4). 38. E.D.NICHOLAS " F r i c t i o n welding of copper to aluminum", Metal c o n s t r u c t i o n , March 1975, pp 135 to 141. 39. P.B.FOISTER "Heat Under Power (HUP) f r i c t i o n welding", Proceedings of the t h i r d I n t ' l conference on the advances i n welding, Harrogate, England, paper 36. 40. N.I.DOTSENKO, B.P.PEREGUDIN "The f r i c t i o n welding of c a s t i r o n to s t e e l with  brass i n t e r l a y e r " , 41. E.R.WALLACH " S o l i d - s t a t e j o i n i n g : Techniques & a p p l i c a t i o n " Conference of I n s t i t u t e of m e t a l l u r g i s t s on j o i n i n g of metals, Coventry, A p r i l 1981. 42. M.I.FLETCHER " F r i c t i o n welding". M i l l s and Boon Ltd.,1972. 109 Welding p r o d u c t i o n , v o l 21(9), Sept. 1974, pp 44 and 47. APPENDIX I I t i s a common p e r c e p t i o n t h a t a f r i c t i o n w e l d i n g machine i s s i m i l a r t o a l a t h e , w i t h a f a s t r o t a t i n g chuck and most of the p a r t s mounted on a f l a t bed; but the s i m i l a r i t y ends t h e r e . The f i r s t machines were i n f a c t d eveloped from l a t h e s , but a c o n s i d e r a b l e number of changes had t o be made l i k e a d d i t i o n of an o t h e r e q u a l l y r i g i d w o r k - h o l d i n g d e v i c e , b r a k i n g mechanism and p r o v i s i o n s t o f o r c e the w o r k - p i e c e s t o g e t h e r and disengagement of the main d r i v e t o f a c i l i t a t e f o r g i n g e t c . The f r i c t i o n w e l d i n g machine c o n s t r u c t e d i n the department was b a s i c a l l y d e s i g n e d t o c a r r y out w e l d i n g between p i e c e s of a p p r o x i m a t e l y h a l f an i n c h d i a m e t e r f o r e x p e r i m e n t a l p u r p o s e s . U n l i k e c ommercial machines, where f a c i l i t i e s f o r a u t o m a t i o n of l o a d i n g / u n l o a d i n g , removal of f l a s h e t c . a r e a l s o major f e a t u r e s i n d e s i g n , a l o n g w i t h r i g i d i t y , c a p a c i t y and c o s t , the machine b u i l t f o r t h i s work does not c a t e r f o r any au t o m a t i o n of t h i s k i n d . However the r i g i d i t y of the equipment was of prime importance. The m e c h a n i c a l p a r t of the machine can be b r o a d l y d i v i d e d i n t o two p a r t s ; (1) the power t r a n s m i s s o n s e c t i o n and (2) the w e l d i n g s e c t i o n . The t r a n s m i s s i o n s e c t i o n has a p u l l e y mounted a t the end of a s h a f t and i s d r i v e n by a 10 hp AC motor a t 1150 rpm. S i n c e the motor works on AC, the speed i s c o n s t a n t . At t h i s time no arrangements have been made w i t h the p u l l e y s t o v a r y the speed. A f r e q u e n c y i n v e r t e r can be used t o v a r y speed c o n t i n u o u s l y u n l i k e the use of d i s c r e t e 110 111 steps when using stepped p u l l e y s , but the c o s t of a frequency i n v e r t e r f o r the c a p a c i t y of the motor used i s p r o h i b i t i v e . Costs a l s o f o r b i d the use of a v a r i a b l e speed DC motor. However, t h i s should be made a v a i l a b l e f o r f u t u r e r e s e a r c h work. On the other end of the s h a f t , which i s mounted i n two p i l l o w b l o c k s , i s a p o s i t i v e d r i v e c l u t c h ( d i s c u s s e d more i n d e t a i l i n l a t e r s e c t i o n s ) . The power t r a n s m i t t e d by the c l u t c h to a second s h a f t i s d e l i v e r e d to the r o t a t i n g work-piece h e l d i n a chuck at the end t h i s s h a f t . The motion of t h i s s h a f t can be a r r e s t e d by using the c l u t c h and a h y d r a u l i c d i s c brake. The welding area e s s e n t i a l l y c o n s i s t s of two chucks; one has a r o t a r y motion, while the other, which i s h e l d by a t h i c k p l a t e , can be moved in a h o r i z o n t a l p l a n e . The p l a t e s l i d e s over two case-hardened s t e e l bars by means of two l i n e a r b e a r i n g s . These rods are secured to the box c a r r y i n g the b earings f o r the r o t a r y chuck on one end and an u p r i g h t r i b b e d member on the o t h e r . T h i s r i b b e d u p r i g h t member a l s o has the h y d r a u l i c c y l i n d e r used to f o r c e specimens together mounted on i t . The arrangement of having the s t e e l rods secured to these two members has an a d d i t i o n a l advantage of l o a d - s h a r i n g . The p r e s s u r e s r e q u i r e d f o r f r i c t i o n welding range from a few pounds to a few tons, depending upon the s i z e of the p i e c e s being welded. The i n s t a l l e d h y d r a u l i c u n i t i s capable of g e n e r a t i n g a p ressure of 2,000 p s i with a c y l i n d e r of a bore of 2 i n c h e s , which on a h a l f inch diameter specimen i s 1 12 transformed i n t o 32,000 p s i which i s adequate f o r j o i n i n g a l l the m a t e r i a l s . The b r a k i n g mechanism v a r i e s depending upon the type of process used, continuous d r i v e or i n e r t i a method. The brake i n i n e r t i a welding i s e s s e n t i a l l y used to enhance the r a t e of p r o d u c t i o n but serves no other purpose. The brake i n the c o n v e n t i o n a l method i s more c r i t i c a l and has to be used i n order to b r i n g the r o t a t i o n to a sudden stop to f a c i l i t a t e f o r g i n g . A d i s c brake i s used i n t h i s machine which i s acted upon by a system of c a l i p e r s and a master c y l i n d e r , both of these being d e r i v a t i v e s from an used automobile. The master c y l i n d e r i s a c t i v a t e d by a ram, (of a pneumatic c y l i n d e r , to be d i s c u s s e d with c l u t c h ) thus p r e s s u r i z i n g the o i l i n the c a l i p e r s to apply the brake. The c l u t c h used i s of p o s i t i v e d r i v e type. One h a l f of i t i s mounted f i r m l y on the d r i v e n s h a f t , c a r r y i n g the d i s c brake. T h i s p a r t has two p i n s which engage with h o l e s i n the other h a l f of the c l u t c h . These p i n s are a l s o designed to act as shear p i n s so that minimum damage i s caused i n case of any a c c i d e n t a l engagement to p r o t e c t more important p a r t s . The other h a l f of the c l u t c h , the s l i d i n g h a l f , i s c o n s i d e r e d more important of the two because of i t ' s c o n s t r u c t i o n and the amount of machining r e q u i r e d to r e p l a c e i t as compared to the f i x e d type. T h i s p a r t s l i d e s on the s h a f t engaging and disengaging the d r i v e power. The torque i s t r a n s m i t t e d by four keys which together are more shear r e s i s t a n t than the key of the other h a l f of c l u t c h . The 1 13 c l u t c h while i t s l i d e s over the keys and a l s o r o t a t e s with them i s h e l d i n place by a c o l l a r and a frame. This,frame while a l l o w i n g i t ' s r o t a r y movement f r e e l y , c o n t r o l s the l a t e r a l movement i n an h o r i z o n t a l a x i s along the shaft thus c o n t r o l l i n g the engagement. T h i s frame i n turn i s c o n t r o l l e d by a ram from a pneumatic c y l i n d e r . The c l u t c h i s engaged when the pneumatic c y l i n d e r has i t ' s p i s t o n r e t r a c t e d . At the end of f r i c t i o n phase, when i t i s time to stop the torque being t r a n s m i t t e d , a timer e n e r g i z e s a s o l e n o i d which all o w s compressed a i r i n t o the pneumatic c y l i n d e r , pushing out the ram with the frame (with the c l u t c h ) thus disengaging the c l u t c h . As soon as the c l u t c h i s disengaged, the master c y l i n d e r i s p r e s s u r i z e d to apply the brake. P r e c a u t i o n i s taken not to apply brake before the c l u t c h i s f u l l y disengaged. The process parameters are a l l set by using the c o n t r o l box. The c o n t r o l box has two d i a l s to set the times f o r f r i c t i o n i n i g phase and the forge phase, each can be set from zero to 15 seconds. Two buttons allow the s e t t i n g of the pr e s s u r e s f o r both f r i c t i o n i n g and f o r g i n g phases. With these f a c i l i t i e s a l l the process parameters are s e t . Two more buttons c o n t r o l the movement of c l u t c h , while one i s used to engage the c l u t c h , the other i s used to disengage i t i n case of an emergency. The l a s t button s e t s the process r o l l i n g by advancing the ram of the h y d r a u l i c u n i t a f t e r which a l l the sequences are a u t o m a t i c a l l y performed. 1 14 The C o n t r o l s : The process i s s t a r t e d by the operator a f t e r s e t t i n g the times f o r f r i c t i o n i n g and f o r g i n g and a l s o the pressures.The process s t a r t s with the advancing of the h y d r a u l i c ram which f o r c e s r o t a t i n g and s t a t i o n a r y work-pieces together. I t i s ensured that the timer device m o n i t o r i n g the f r i c t i o n phase i s a c t i v a t e d only a f t e r the two p i e c e s have come w e l l i n c o n t a c t . At the end of the phase I, the f r i c t i o n phase, the f i r s t timer a c t i v a t e s a s o l e n o i d which allows compressed a i r i n t o the pneumatic c y l i n d e r disengaging the c l u t c h through a ram. The ram a l s o a p p l i e s brake through the master c y l i n d e r almost s i m u l t a n e o u s l y . The frame moving the c l u t c h at, the end of i t ' s t r a v e l a c t i v a t e s a l i m i t s w i tch (#1) which e n e r g i s e s a s o l e n o i d f o r high (forge) p r e s s u r e i n the h y d r a u l i c c i r c u i t and a l s o switches on the second t i m e r . At the end of a set time t h i s timer changes the d i r e c t i o n of flow i n the h y d r a u l i c c i r c u i t and the ram r e t r a c t s and r e s e t s the c o n t r o l s f o r the next c y c l e through a l i m i t switch (#2). The time e l a p s e d between the end of the f r i c t i o n c y c l e and the b e g i n i n g of the forge phase i s l e s s than 1 second, i n c l u d i n g the time f o r forge p r e s s u r e b u i l d - u p . L i m i t switch #3 f u n c t i o n s only when the c l u t c h i s not p r o p e r l y engaged. During s i t u a t i o n s when the c l u t c h i s not p r o p e r l y engaged, the 'advance' button does not f u n c t i o n u n t i l the c l u t c h i s p r o p e r l y engaged and the l i m i t s w i t c h #3 i s s e t . FRICTION WELDING MACHINE SCHEMATIC OF THE FRICTION WELDING MACHINE 1. Rotating chuck 2. Stationary chuck 3. Work-pieces 4. Hydraulic cylinder 5. Clutch frame 6. Belt-drive from e l e c t r i c motor 7. (Bearing) Pillow blocks 8. Master cylinder 9. Disc brake calipers 10. Pneumatic cylinder 11. Case hardened steel bar 12. Hydraulic pressure controls 13. Control box (time settings, clutch, process controls) 14. Linear bearings 15. Limit switch 1 16. Limit switch 2 17. Limit switch 3 Hydraulic cylinder 1 17 Low pressure valve i To work-piece J High pressure valve 0 Hydraulic power unit CIRCUIT FOR THE HYDRAULIC LINES OF THE FRICTION WELDING MACHINE APPENDIX II The composition of v a r i o u s m a t e r i a l s used d u r i n g the course of the experiments are as f o l l o w i n g : S t e e l (1025) 0.25% carbon, 0.45% manganese, 0.04% phosphorous (max), 0.04% sulphur (max) Aluminum (6061) 0.6% s i l i c o n , 1% magnesium, 0.25% copper, 0.25% chromium, aluminum (balance) Phosphor bronze 90% copper, 10% t i n Naval brass 60% copper, 1.5% t i n , 38.5% z i n c Copper 99.9% copper (commercial p u r i t y ) N i c k e l 99.9% n i c k e l (commercial p u r i t y ) T i t a n i u m 99.9% t i t a n i u m (commercial p u r i t y ) Brass 70% copper, 30% z i n c (yellow brass) 118 APPENDIX III Due to a very l i m i t e d number of specimens welded at the same parameter s e t t i n g s , i t was not p o s s i b l e to formulate a mathematical model. S t a t i s t i c a l models were a l s o not t r i e d to be f i t t e d , however, some s t a t i s t i c a l a nalyses were made wherever s u f f i c i e n t data was a v a i l a b l e . One such a n a l y s i s i s given as an example below. I t i s to be t e s t e d s t a t i s t i c a l l y the hypothesis that higher forge pressures do not r e s u l t i n higher s t r e n g t h f o r j o i n t s between 6061 aluminum. J o i n t s made with forge p r e s s u r e s below 6400 p s i were c o n s i d e r e d as low forge pressure j o i n t s and the ones above 8400 p s i as high pressure j o i n t s . A 'run t e s t ' i s made. The s t r e n g t h s of the j o i n t s from high forge p r e s s u r e s are en c l o s e d i n the p a r e n t h e s i s . A f t e r they are arranged i n the order of ascending s t r e n g t h , they are as f o l l o w s : 22623 23125 (24378) (24854) (25123) (25396) 25789 (26030) 26417 26453 (27131) 27292 (28038) (28470) 30487 From the above, Number of specimens, n=l5, N,, number of specimens welded with low forge p r e s s u r e = 7 N 2, number of specimens welded with high forge p r e s s u r e = 8 * U = Number of runS = 9 (A sequence of o b s e r v a t i o n s from one sample or even a s i n g l e o b s e r v a t i o n from the ordered data i s c a l l e d as a run) [211 From the t a b l e s , the lower 0.025 c r i t i c a l value of the 119 120 * t e s t s t a t i s t i c U i s u T=4. Since U i s not l e s s than u , , c , 1 c , 1 the h y p o t h e s i s that higher forge p r e s s u r e s do not r e s u l t i n h i g h e r s t r e n g t h f o r j o i n t s of 6061 aluminum, i s NOT r e j e c t e d [21] 

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