Effect of Solution Treatment and Cooling Rate on the Microstructure and Hardness of Ti-6Al-4V Alloy Manufactured by Selective Laser Melting Before and After Hot Isostatic Pressing Treatment

This paper extends our previous work to investigate the effect of heat treatment on the microstructure of Ti-6Al-4V fabricated by selective laser melting. A post-heat treatment at 930 °C for 15 min followed by three cooling rates before and after hot isostatic pressing (HIP) treatment was applied. The findings illustrated that the microstructure of the quenched samples before the HIP treatment was characterized by a mixture of α + α′ phase with a microhardness value of 336 ± 6 HV0.3. Air cooling produced a structure dominated by the α phase, with ~ 7.5% of the β phase and a microhardness value of about 330 ± 4 HV0.3. Furnace cooling led to a mixture of α phase and ~ 17% of the β phase and hardness of 327 ± 6 HV0.3. After HIP followed by post-heat treatment, acicular α′ martensite with microhardness value 377 ± 2 HV0.3 dominated the quenched specimen microstructure. Following air cooling, the microstructure consisted of a mixture of α-lamella and β with some needles of the α with a microhardness value of 336 ± 3 HV0.3. In the case of the furnace cooling, a complete transformation of β to a mixture of α + β phase was observed. The β volume fraction formed in the microstructure was estimated at ~ 8.5%, having microhardness 322 ± 4 HV0.3. Reasons for such behaviors are discussed.