Research of a Novel Non-Axisymmetric Side-Compressed Variable Polarity Plasma Arc and Its Pressure Distribution Characteristics

In the keyhole variable polarity plasma arc weldng (VPPAW) process at horizontal position, the metal driven by gravity gathered on one side of the molten pool, and the weld formation is difficult, especially for thick workpiece welding. A specially designed experiment to analyze the influence of gravity on weld formation and a novel nozzle structure with side holes was proposed to generate a novel non-axisymmetric side-compressed plasma arc and redistribute arc pressure. The arc shape and pressure distribution were studied, and the ratio of difference for arc pressure in different directions Rp was introduced to evaluate the effects of non-axisymmetric side compression for the plasma arc. The results indicate that the non-axisymmetric distributed side holes reshape the plasma arc both in the EN and EP phases. The pressure of the non-axisymmetric side-compressed plasma arc decreases relatively strongly in one direction (direction b) and relatively weakly in the other direction (direction a). Rp is significant at 1 mm to 5 mm from the arc center, with a relatively large Rp within this range. The compression effect is enhanced with an increase in welding current or plasma gas flow rate, and Rp increases from 24% to 49% as the plasma gas flow rate increases from 2 L/min to 4.5 L/min. Specially designed validation experiments confirm that the new plasma arc significantly affects the weld formation in keyhole VPPAW process. An aluminum alloy workpiece with 8 mm thickness and no groove preparation was welded by the novel plasma arc in a horizontal welding position, and the weld is well formed.