E. Y. Chen, D. R. Bice, Transition45 Technologies, Inc., Orange, CA; Q. Li, University of Nevada, Reno, Reno, NV; D. C. Dunand, Northwestern University, Evanston, IL
One approach to expand the application of titanium alloys is to reduce their processing cost by near-net-shape castings. In near-net shape cast parts, an additional forming step is however sometimes still needed to achieve the final shape with the required tolerances and wall thickness. Superplastic forming is an attractive option, due to the low stresses needed for deformation and to the relatively low tooling costs. However, in the as-cast state, titanium and most titanium alloys have a grain structure too coarse to allow deformation by grain-boundary sliding through microstructural superplasticity. Thus, for most titanium alloys - and in particular for the commercially-dominant Ti-6Al-4V alloy - complicated and costly thermo-mechanical treatments are needed to produce the fine grains with equiaxed shape necessary for microstructural superplasticity by grain-boundary sliding, resulting in high tensile strains (>100%) and low strain-rate sensitivity. An alternative approach to achieve high tensile strains uses transformation superplasticity which has no grain-size requirement as it relies on the biasing by an applied external stress of internal stresses produced by cyclical phase transformation. This presentation explores the application of transformation superplastic forming to superplastic form coarse-grain cast titanium.
Summary: In the as-cast state, titanium and most titanium alloys have a grain structure too coarse to allow deformation by grain-boundary sliding through microstructural superplasticity. Thus, for most titanium alloys - and in particular for the commercially-dominant Ti-6Al-4V alloy - complicated and costly thermo-mechanical treatments are needed to produce the fine grains with equiaxed shape necessary for microstructural superplasticity by grain-boundary sliding, resulting in high tensile strains and low strain-rate sensitivity. An alternative approach to achieve high tensile strains uses transformation superplasticity which has no grain-size requirement as it relies on the biasing by an applied external stress of internal stresses produced by cyclical phase transformation. This presentation explores the application of transformation superplastic forming to superplastic form coarse-grain cast titanium.
