Optimization of process parameters for ultrasonic-assisted laser welding of Ti6Al4V alloy and the impact of ultrasonic power on porosity, microstructure, and mechanical properties

In this study, the optimization of welding parameters, pore defects, microstructure and mechanical properties of Ti6Al4V alloy welded joints subjected to ultrasonic-assisted laser welding were studied. The optimal parameter combination and the most significant factor influencing the tensile strength of the joint were determined by three-factor and three-level orthogonal experiments with range and variance analysis methods. A control experiment was set up under the optimal parameters to reveal the reason of ultrasonic influence on joint strength from the pore defects and microstructure. The control test showed that the cavitation effect and stirring effect of ultrasonic could significantly improve the weld defects and microstructure. Compared with no ultrasonic, the porosity of the welded joint decreased from 3.06% to 0.08%, and the average grain size of prior-β and α' martensite were refined by 15.1% and 6.7% respectively under an ultrasonic power of 1000W. The ultrasonic vibration also contributed to the grain boundary transformation from low-angle to high-angle and significantly reduced the residual stress inside the joint. With the increase of ultrasonic power, the tensile strength increased by 4.37% and 8.38% respectively and the fracture mode changed from brittle-ductile mixed fracture to ductile fracture.