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Three dimensional heat flow in the direct chill casting of non-ferrous metals

University of British Columbia

A three dimensional mathematical model has been developed to study heat flow and solidification in the Direct Chill casting of non-ferrous metals with rectangular as well as irregular cross-sections. The model which is based on an alternating direction, implicit finite-difference numerical method is capable of simulating heat flow both in the steady state and transient part of the casting operation. The validity of the model has been verified by comparing predicted pool profiles and pool depths with industrial measurements. The model has been used to study the importance of heat flows in the various directions, and the limitations of using two-dimensional heat flow models are brought out. This study has shown that a two-dimensional model which neglects heat flow normal to the narrow face can be used to simulate the solidification of slabs with aspect ratios greater than 2.5, cast under conditions of conventional D.C. cooling. Further it was demonstrated that with reduced secondary cooling, a two-dimensional model that neglects axial heat conduction is preferable. Model calculations show that in cooling large sections the unsteady state can occupy around 25% of the total casting cycle. The formation of cracks in jumbo ingots of Prime Western Grade zinc has been investigated with the aid of the mathematical model. It has been shown that the cracking is caused by reheating of the surface below the spray cooling zone, if this zone is short and characterized by a high water flux. The surface reheating generates tensile strains at the solidification front where cracking is aided by the presence of lead rich liquid in the inter-dendritic regions. A new spray assembly has been designed which attempts to cool the casting more uniformly from, the top of the mould to the bottom of the liquid pool. This new spray system has been tested in-plant for casting Prime Western Grade zinc and results have proven its effectiveness in preventing crack formation.

Text

2010-03-23

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http://hdl.handle.net/2429/22390

Materials Engineering

University of British Columbia

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