The flow and forming mechanism in the bypass hybrid variable polarity plasma arc additive manufacturing

The bypass hybrid variable polarity plasma arc (VPPA) has the specific ability that could control the current through the wire and base metal independently. It offers a unique advantage for additive manufacturing for adjusting the heat input of wire and base metal freely. The deposition process of molten metal and the forming mechanism of the deposition layer are significantly different with the traditional wire and arc additive manufacturing (WAAM). Therefore, this paper investigates the effects of main current and bypass current on the morphology characteristics of the deposition layer, explores the surface and internal material flow of the molten metal, and constructs a multi-layer morphological model of the deposition layer under this process. The deposition process was observed by a high-speed camera, and the molten metal flow was tracked by slag and copper. It is found that the bypass hybrid VPPA can adjust the morphology of the deposition layer by changing the combination of the main and bypass current. The plasma arc establishes the deposition channel of the molten metal, and the surface flow in the deposition channel tends to stagnate further away from the arc and the mass transfer is in the form of internal filling. The depositing metal is not affected by the hybrid arc in solidification, and the surface tension plays a dominant role in forming. The flow pattern leads to an arch shape cross section of the deposition layer. The work lays a foundation for finely controlling and predicting the morphology of the deposition layer in WAAM.