Effect of heat treatment on microstructure evolution and mechanical properties of selective laser melted Ti–6Al–4V and TiB/Ti–6Al–4V composite: A comparative study

Abstract

Post heat treatment has been considered as an essential process to relieve residual stress, tailor the microstructure and obtain desired mechanical properties of the parts manufactured by selective laser melting (SLM) technology. The present work comparatively investigated the effect of heat treatment on microstructure evolution and mechanical properties of the SLM-fabricated Ti–6Al–4V alloy (Ti64) and 2 vol% TiB/Ti–6Al–4V titanium matrix composite (TMC). Microstructure analysis indicated that the as-built and heat treated TMC samples showed profoundly different microstructures compared to that of Ti64 samples. With heat treatment temperature increasing, the dominant acicular α′ martensite of Ti64 was decomposed into fine (α+β) lamella and microstructure coarsening occurred. A finer (α+β) dual-phase matrix microstructure, compared to that of Ti64, was achieved for TMC samples when heat treatment temperature exceeded 850 °C. The densely dispersed clusters of parallel nanoscale TiB precipitates of the TMC grew into individually separate TiB whiskers with larger nanoscale and sub-microscale size through a stacking up process of (100) planes based on an Ostwald ripening process. Mechanical testing results showed that the hardness and yield strength values of TMC samples before and after heat treatment were all higher than that of the Ti64 samples. A superior comprehensive mechanical performance with relatively high yield strength (1345 MPa) and significantly improved fracture strain from 17.6% to 31.3% was achieved for the TMC sample after heat treatment at 850 °C.