Hot forging design of as-cast NiTi shape memory alloy

A hot forging process of an as-cast NiTi shape memory alloy was designed by processing maps and a finite element method analysis. The NiTi alloy samples used in this study were made using a vacuum plasma arc melting process. In order to analyze the microstructural change and flow behavior of the as-cast NiTi alloy during the ingot breakdown process, high temperature compression tests were carried out at different temperatures and strain rates up to a true strain level of 0.9. Deformation processing maps were generated by using the dynamic materials model approach developed by Prasad to search for the optimum hot forging process conditions. The flow instability criterion proposed by Ziegler was utilized to evaluate the unstable deformation region in the processing map. The grain refinement was achieved by recrystallization during high temperature deformation of the NiTi alloy. Finally, the optimum hot forging design to obtain a sound NiTi billet without forging defects was identified with the actual ingot breakdown process.