Physical mechanism of material flow in variable polarity plasma arc keyhole welding revealed by in situ x-ray imaging

Lightweight manufacturing reduces energy consumption and protects the environment; therefore, it offers an important direction of development in engineering. Variable polarity plasma arc (VPPA) welding is a light gauge aluminum and magnesium alloy joining technology capable of the highly efficient welding of medium-thickness metals via the keyhole effect, and without the complex pre-processing required by traditional methods. However, the stability of the keyhole weld pool is easily lost, resulting in a much narrower parameter window than that of conventional methods. Here, through keyhole morphology analysis and x-ray in situ imaging experiments, we reveal the material flow behavior, the mechanism behind the stability of the keyhole weld pool, and the reason for the very narrow process parameter window in VPPA welding processes. We discovered that the polarity pressure difference of the plasma arc, which is induced by the keyhole boundary shape and plasma arc intensity, drives the flow pattern to the top side of the keyhole, which is beneficial for keyhole filling and the formation of a stable weld bead. The influence of the difference in plasma arc pressure and the keyhole boundary on the flow field revealed in this study may guide the optimization process of light metal joining to achieve the highly efficient and defect-free manufacturing of large and complex structures.