Electron beam freeform fabrication of Ti6Al4V alloy and the role of post-heat treatment in the microstructure, texture, and mechanical properties

Abstract

Electron beam freeform fabrication (EBF3) is a promising manufacturing technique to fabricate large-scale and fully-dense metallic components, showing broad potential applications in the aerospace industry. A Ti6Al4V alloy block was produced using the EBF3 process at an ultra-fast wire feeding rate (3.5 m/min), and the role of post-heat treatments in the microstructure, texture, and mechanical properties was investigated. The results revealed that the as-deposited alloy was characterized by the large columnar prior β grains and the inhomogeneous periodic microstructures. Duplex annealing was considered to be an optimal post-heat treatment candidate to tune the as-deposited microstructure, although the columnar prior β grains maintain. Annealing at 950 ℃ for 1 h followed by air cooling created a unique bimodal microstructure consisting of coarse crab-like primary α and fine lamellar secondary α laths, and then secondary annealing at 630 ℃ for 2 h followed by air cooling facilitated the increased volume fraction of secondary α phase, thereby weakening the α texture. Such microstructure and texture evolution are favorable to a superior and isotropic overall tensile property. The ultimate tensile strength (UTS), yield strength (YS) and tensile fracture elongation (EL) were 889 MPa (transverse direction, TD)/875 MPa (verticle direction, VD), 821 MPa (TD)/801 MPa (TD) and 7.6% (TD)/8.1% (TD), respectively. Overall, the results offer a reasonable reference to the optimization in the microstructure and mechanical performance of EBF3-ed Ti6Al4V alloy.