Melt flow and grain refining in ultrasonic vibration assisted laser welding process of AZ31B magnesium alloy

During the laser welding process of magnesium (Mg) alloy, it is easy to produce welding defects, which deteriorates the mechanical properties of welded joint. Since ultrasonic vibration can decrease weld defects considerably, the ultrasonic-assisted welding technology has become a hotspot in the current research field of welding and joining. In this study, the ultrasonic vibration assisted laser welding technology used for the joining of AZ31B Mg was carried out. Setting amplitude transformer and welding specimen in contact makes ultrasonic travel to the weld pool as well as promote melt flow and bubble escape by the cavitation, acoustic streaming and thermal effects. Owing to these effects, the weld porosity decreases significantly to less than 1%. Besides, in the crystallization process of the weld pool, ultrasonic vibration increases the number of nucleation particles and refine grain, which improve the mechanical properties of welded joint. As a result, the average grain area in weld center area decreases from 359.9 μm² to 213.7 μm². Also, compared with laser welding, the tension strength σ_b, and elongation A_gt increase from 235.1 MPa to 274.1 MPa and 6.6% to 7.5%, respectively.