Enhancing fusion welding in die-cast Mg alloys: A combined ultrasonic vibration and hot rolling approach for porosity reduction, microstructure refinement, and mechanical property improvement.

Severe weld porosity has led the industry to believe that die-cast magnesium alloys cannot be fusion welded. To address this challenge, a new process of ultrasonic vibration coupled with hot rolling was employed to eliminate porosity, and the mechanism associated with weld porosity in die-cast magnesium alloys was analyzed in detail. Unlike conventional welding processes, ultrasonic vibration coupled with hot rolling is effective and efficient. In this study, a roller with ultrasonic vibrations is synchronized with the molten pool to apply pressure to the weld, which is in a semi-solid state on the upper surface. The results showed that hot rolling and ultrasonic vibration coupled with hot rolling could eliminate 78.12 % and 85.5 % of the porosity, respectively. Compared to hot rolling, the ultrasonic vibration coupled with the hot rolling process resulted in smaller and fewer porosities and finer β-Mg 17 Al 12. This is attributed to the cavitation effect of ultrasonic vibration that promotes bubble overflow and breaks the β-Mg 17 Al 12 dendrites in the melt pool, as well as the thermal effect that improves the plasticity of the weld. Furthermore, after closing the porosity under pressure, recrystallized grains will be produced around it. Tensile test results indicated that hot rolling was able to increase the ultimate tensile strength by 84.21 % and elongation by 44.03, whereas ultrasonic vibration coupled with hot rolling was able to increase the ultimate tensile strength by 244.46 % and elongation by 71.70 %. Combining the micron-scale industrial computed tomography results with theoretical calculations, it is found that the porosity of die-cast magnesium alloys is mainly caused by nitrogen, and they originate from gas entrapment during the die-casting process; further, the formation mechanism of die-cast magnesium alloys' fusion-welding porosity is revealed in detail. A physical model of the formation process of welded porosity morphology was established, and the analysis suggested that the pattern of the inner wall of the porosity was related to the Kelvin–Helmholtz instability. This research is expected to improve the weld quality of alloys manufactured by the die-casting process. [Display omitted] • Proposed ultrasonic-vibration-coupled with hot-rolling reduces pore content by 85.51 %. • Ultrasonic-vibration-coupled with hot-rolling increases ultimate tensile strength by 244.46 % and elongation by 71.70 %. • The controversy of "the type of gas that causes porosity" has been resolved. • A complete mechanism for die-cast magnesium alloy weld porosity was proposed. • We explored common class of pore morphologies in metals to suggest physical models. [ABSTRACT FROM AUTHOR]