Microstructure and mechanical properties of as-deposited and heat treated Ti-5Al-5Mo-5V-3Cr-1Zr (Ti-55531) alloy fabricated by laser melting deposition

In this work, the first successful attempt of additive manufacturing (AM) Ti-55531 near-beta alloy fabricated by laser melting deposition (LMD) is reported. Optimized processing parameters of LMD were used to manufacture high relative density (99.8%) Ti-55531 alloy. It was found that the microstructure of as-deposited sample is mainly composed of beta phase with heterogeneous distribution of a phase. The heterogeneity of microstructure of as-deposited sample was caused by the ubiquitous temperature gradient during LMD process. Thus, the as-deposited samples have a significant anisotropic tensile property which may not meet the aeronautical criterion. To overcome the issue of inhomogeneous microstructure and minimize the anisotropy, the microstructure was tailored by two kinds of heat treatment, i.e., beta solution treatment subsequent aging (beta-STA) and triplex annealing (alpha/beta-STA). A transformed beta microstructure was obtained in beta-SAT condition, which resulted in the high ultimate tensile strength (UTS) (> 1350 MPa) and low ductility (<6.5%). Besides, the bimodal structure was formed in alpha/beta-STA condition and the associated sample shows an excellent balance of strength (UTS> 1200 MPa) and ductility (>8%). Furthermore, the formation mechanism of microstructure and the subsequent impact on mechanical properties was systematically discussed. This work demonstrates the potential of fabricating Ti-55531 alloy parts by LMD and paves the way for enhancing LMD processed near beta Ti-alloys through heat treatment. (C) 2019 Elsevier B.V. All rights reserved.