Additively manufactured Ti55511 alloy: Microstructure and residual stress effect on mechanical properties

Abstract

Stress relief (SR) is an effective heat treatment process that potentially changes microstructure and enhances the mechanical properties of additively manufactured parts. In this study, the influence of SR on the microstructure and mechanical properties of Ti-5Al-5Mo-5V-1Cr-1Fe alloy (Ti-55511), a near-β alloy fabricated by laser powder bed fusion (LPBF), was investigated. LPBF-built Ti-55511 samples were characterized and tested under tensile and low cycle fatigue loading in the as-built (AB) and SR conditions. The obtained results show that the LPBF sample was composed of columnar grains in a single bcc β-phase in the AB condition. In contrast, the SR condition resulted in a bi-modal microstructure comprising of α + β phases. This led to significant grain refinement of the β phase and reduced the residual stress (RS) up to 91 % compared to the AB condition. At the same time, the SR treatment increased the yield strength, ultimate tensile strength, and fatigue life at the expense of ductility. The improvement in mechanical properties is attributed to the grain refinement and phase transformation resulting in complex dislocation interactions during deformation.