Friction stir welding of Ti-6Al-4V alloy: Friction tool, microstructure, and mechanical properties

Abstract

Abstract In this work, 2 mm-thick Ti-6Al-4V plates were successfully friction stir welded using a newly designed friction tool, and the microstructure evolution and mechanical behaviors of weld were investigated. The process parameters of friction stir welding (FSW) were explored and defect-free welds were produced. The cross-sections of the weld showed typical bowl profiles, which can be divided into four regions: base metal (BM), shoulder-affected zone (SAZ), stir zone (SZ), and heat-affected zone (HAZ). The peak temperature in the SZ exceeded the β-phase transition temperature and the materials in the SZ underwent phase transformation during FSW. The final microstructure of the SZ was a lamellar structure with (α + β) phase and the HAZ has a bimodal microstructure consisting of equiaxed prior α and lamellar (α + β) grains. The maximum tensile strength and yield strength of the FSWed samples reached up to 92 % of the base metal and the tensile failures occurred at the SZ. The micro-hardness varied significantly along the thickness of the plate. While the micro-hardness value of SZ was lower than that of BM due to the grain coarsening and dynamic recrystallization during the welding. The tensile fracture morphologies exhibited a typical quasi-cleavage fracture.