Mechanical Properties and Microstructure of a High-Power Laser-Welded Ti6Al4V Titanium Alloy

In this study, the high-power solid-state laser was used to weld the Ti6Al4V titanium alloy plates with a thickness of 8 mm. Microstructure and mechanical properties of the high-power laser-welded Ti6Al4V titanium alloy were investigated by the optical microscope, electron probe microanalysis, transmission electron microscope and nanoindentation technique. According to our findings, the average tensile strength of the joint is higher than 957 MPa, and the fusion zone possesses the highest microhardness in the laser-welded Ti6Al4V titanium alloy. The content of aluminum is reduced in the fusion zone. Equiaxed alpha + beta phases in the fusion zone are transformed into columnar beta and acicular alpha ' martensite grains by the martensitic transformation. Dynamic recrystallization of acicular alpha ' martensite occurs at the edge of columnar beta grains. Additionally, due to the extremely fast cooling rate and the shock of the laser, the crystal nucleus undergoes a phase transformation to form a staggered martensite structure. Meanwhile, ultrafine grains and nanograins formed at the martensite crossing are also preserved. The results of nanoindentation reveal that the fine acicular alpha ' martensite phase improves the creep resistance of the fusion zone.