Tri-modal Microstructure in Different Loading Zones Under TA15 Ti-alloy Isothermal Local Conventional Forging and Given Subsequent Heat Treatment

For TA15 Ti-alloy large-scale component, isothermal local conventional forging (ILCF) combined with subsequent heat treatment provides a feasible approach to obtain a tri-modal microstructure with attractive comprehensive performance. However the tri-modal microstructure characteristics and evolution laws in the first and second loading zones are lack of understanding. In this paper, the effects of ILCF conditions on the tri-modal microstructure in the first and second loading zones were investigated under given subsequent heat treatment and the reasonable ILCF conditions were proposed. As forging temperature increases, in both loading zones the content and aspect of equiaxed αp decrease obviously while the content and thickness of lamellar αs increase. The content and thickness of lamellar αs in the second loading zone are higher. Forged at a moderate strain rate of 0.1s-1, the content of lamellar αs is higher, and the differences in the content and morphology of constituent phases in both loading zones are relatively small. Compared with air cooling after forging, water quenching result in more and disordered lamellar αs, and less differences in obtained tri-modal microstructure in the first and second loading zones. Under three loading passes, the uniformity of tri-modal microstructure in the first and second loading zones is better.