Effect of wire feed position on fluid flow and weld formation in variable-polarity plasma arc horizontal welding

Fluid flow around the keyhole in the molten pool is asymmetric due to the influence of gravity, which directly influences the formation of asymmetric welds and cutting in the variable polarity plasma arc horizontal welding of Al alloys. It is an efficient method used to improve molten metal flow and weld formation by changing the wire feed position (using asymmetric mass transfer to against the gravity effect). Numerical simulation results and direct observation (using a high-speed camera and tracer particles) showed that the stagnation point at the leading side of the keyhole was about at 1.5 mm above the centerline of the weld pool; gravity was the main force driving molten pool flow below the stagnation point. Indirect observation (using a tracer element) was provided to confirm the stagnation point. The metal distribution at the leading side greatly improved with changing wire feed position; the amount of metal at the upper wall of the keyhole was almost equal to that on the opposite side when the wire feed position was near the stagnation point. However, metal still tended to flow downwards to the lower side, at the rear side as the influence of gravity continued until the metal solidified. The wire feed position should be above the stagnation point. Filler material compensated for the lack of flow of base metal, and weld joints with good performance were obtained using asymmetric mass transfer (the solidification stripe was about 90°).