Effects of heat treatment below beta transus temperature on the microstructure and mechanical properties of Ti–6Al–4V alloy fabricated by selective laser melting

Abstract

Ti–6Al–4V alloy fabricated via selective laser melting (SLM) inherently develops high residual stresses due to rapid cooling and steep thermal gradients. To address this, stress relief (SR) heat treatment was performed below the β-transus temperature (0.3–0.5 Tm) to enable atomic diffusion without triggering full phase transformation. This study investigates the effects of SR treatment on microstructural evolution, dislocation density, residual stress, and mechanical properties. Residual stress was quantitatively evaluated using X-ray diffraction, while dislocation density was indirectly assessed through Kernel Average Misorientation (KAM) from EBSD analysis. With increasing SR temperature, lattice distortion was gradually relieved, and a notable decrease in dislocation density was observed beginning at 550 °C. A significant reduction in residual stress approximately 50 % compared with the As-Built condition was achieved even at 350 °C and was nearly eliminated at 650 °C. Microstructural recovery of α′ martensite into equilibrium α and β phases commenced at 550 °C, leading to a dual-phase α + β structure above 650 °C. Correspondingly, tensile strength declined while ductility increased. Overall, sub-transus SR treatment effectively relieved internal stresses, refined the microstructure, and improved the strength–ductility balance, making it a practical approach for enhancing the performance of SLM Ti–6Al–4V components.