Effects of Laser Welding and Post-Weld Heat Treatment on Microstructure and Mechanical Properties of Aged Ti55531 Alloy

As one of the relatively new titanium (Ti) alloys in the engineering field, β-Ti alloy–Ti55531 has attracted a great deal of attention due to its excellent mechanical properties, while a few research papers on weldability and the post-weld heat treatment (PWHT) process of Ti55531 have been reported. Based on an orthogonal experiment design, the parameters of laser beam welding (LBW) of Ti55531 alloy with a thickness of 2 mm were optimized. Moreover, the influences of welding parameters and PWHT on the microstructures and performance of the laser-welded joint of Ti55531 were analyzed. The results showed that, for microstructures in different zones of as-welded joints of Ti55531: three forms of α phases (i.e., equiaxial αp phase, lamellar αS phase, and αGB phase at grain boundaries) were observed in base metal (BM); in the heat affected zone (HAZ), part of lamellar αS phase had dissolved while equiaxial αp phase had grown; the fusion zone (FZ) mainly consisted of β phase, which presented as coarse columnar crystals. After the PWHT process, the microstructures of the welded joint were changed: in the BM zone, α phase at grain boundary disappeared and lamellar α phase decreased; in the HAZ, the edge of αp phase obviously dissolved; in the FZ, plenty of compact needle-like α phases were observed. The tensile strength of the as-welded joint was about 940 MPa and then increased to 1161 MPa after PWHT, which were 78.4% and 96.8% of that of the original BM respectively. The fracture position transformed from the interface between the FZ and HAZ to the BM during tensile tests after PWHT.