Titanium Alloys: Thermomechanical Process Design to Achieve Superplasticity in Bulk Material

Titanium alloys subjected to suitable thermomechanical processing (TMP) schedules can exhibit superplasticity. Most studies on superplasticity of titanium alloys are directed to sheet materials while studies on bulk materials are rather limited. Bulk Superplastic materials require lower load for forging aeroengine components. It further facilitates forming using non-conventional processes such as superplastic roll forming (SPRF). Multi axial forging (MAF), is employed here to achieve bulk superplasticity by imparting large strain without any concomitant change in external dimension. A comparison between uniaxial and MAF with respect to strain, strain path, initial microstructure and heat treatment was carried out to ascertain the microstructure refinement in Ti-6Al-4V alloy. A fine-grained structure was obtained after 3 cycles of MAF followed by static recrystallization at 850°C. Grain boundary sliding was observed in identified processing domain along with strain rate sensitivity (SRS) of 0.46 and maximum elongation of 815%. Validation of established ther¬momechanical sequence on a scaled-up work piece exhibited 640% elongation in domain (T = 820°C, ε ̇= 3 x 10-4/s) which indicated that the established TMP scheme can be used on a reliable and repeatable basis to achieve superplasticity in bulk material.