Abstract
The variable polarity plasma arc (VPPA) welding process is well suited for the welding of aluminum alloy, but the stable welding process and good-quality weld bead are difficult to be obtained in the flat position. The purpose of this work is to study the characteristic of variable polarity plasma arc pressure with different polarity time and find the key fact to achieve welding process stability and good quality welding joint in the flat position. The arc shape variation, arc pressure evolution and the molten pool behavior were observed and measured to discover the relationship to the weld joint formation. It was found that the small plasma arc pressure in the center with a similar affected region and enough upward shear force of arc plasma are the necessary conditions to get a stable keyhole process, which could drive the liquid metal of the back side weld pool converges at the bottom of the keyhole and flows upward and form a high-quality weld bead. Comparing the welding stability and quality, adjusting polarity time is the good method for completing the necessary conditions. With the variable polarity time decreasing, the stable keyholes process and acceptable weld quality were obtained in the flat position.
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