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A study on hot rolling of CP Titanium

University of British Columbia

A major source of concern in the direct breakdown rolling of titanium is the occurrence of surface cracks when the as-cast vacuum-arc or electron-beam melted ingots are hot-rolled for breakdown. Traditionally, this has been done by open-die forging, but recent developments in direct hot-rolling have revealed this problem. A possible reason why cracks are formed is that titanium forms a surface layer of alpha phase containing oxygen dissolved in solid solution through the diffusion of oxygen during the air soaking of the ingots prior to hot rolling. This layer cracks visibly during the rolling process. Available literature indicates the presence of a brittle surface layer, but its characterization has not been done. This work characterizes the solid solution case and measures the strain to failure of the material. Also, microstructural examination of the compression specimens showed cracks between Widmanstatten patterns which are formed when air-annealed samples are cooled. These results are helpful in understanding the reason for the formation of the cracks during direct rolling and could be used to remedy some of the problems. Constitutive equations for the hot working of CP.Titanium in the alpha and the beta phase have been developed by conducting axisymmetric compression tests on cylindrical specimens using a Gleeble 1500®* in the temperature range 750°C - 950°C for strain rates appropriate for direct hot-rolling. The above results are compared with the theoretical predictions of Ashby based on which Ashby had constructed deformation maps for CP. Titanium. The softening characteristics and the strain rate sensitivities of the two phases are noted to be different. Further, a simple model of the breakdown rolling using DEFORM®**, a finite element software package, has been studied to identify the criticality of some processing parameters. *This is a registered product name for a thermomechanical simulator marketed by Dynamic Systems Inc., Troy, New York. ** This is a registered product name for a software designed by Scientific Forming Technologies Corporation, Columbus, Ohio

Thesis/Dissertation

application/pdf

2009-02-06

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http://hdl.handle.net/2429/4277

Materials Engineering

University of British Columbia

UBCV

DSpace