Research On Variable Polarity Plasma Arc Welding Technology Of Aluminum Alloy

With the development of transportation equipment manufacturing industry, aluminum alloy has been widely used in the lightweight production of traffic tools because of its excellent physical and mechanical properties. Welding technology of aluminum alloy is becoming more and more prominent. However, conventional arc welding methods have certain limitation in the welding of aluminum alloy. The variable polarity plasma arc welding (VPPAW) technology makes full use of the advantages of plasma arc and variable polarity. With the characteristics of high welding efficiency, stability and quality, it has become a new kind of welding technology with great industrial application prospect. At present, study on the VPPAW technology mainly focus on the thin plate. In this paper, the welding process of 6061 aluminum alloy with thickness of 15 mm with variable polarity plasma arc welding is studied. Main research contents and conclusions are as follows:(1) Welding torch control device and VPPAW platform are set up, with the purpose of meeting the adjustment of main welding parameters in VPPAW. Ensure the welding process is practicable.(2) Study the influence of welding parameters on the weld surface appearance through single factor experiment. Under the welding condition of arc current 160-260 A, welding speed 200-600 mm/min, plasma gas flow rate 1.5-2.5 L/min, frequency of variable polarity 100-200 Hz, current offset 30-50 A, cleaning time scale 20-25%, the ratio of weld depth and width is relatively small without keyhole formation. In terms of the influence on the surface appearance, heat input has no obvious effect, however, parameters of variable polarity have a significant influence on it. Concerned on the plasma gas flow rate range which meet the requirements for the stability of weld formation is quite narrow.(3) Under the optimized welding condition, analysis microstructure of weld seam. Compared with the base metal, grain size of weld zone was refined obviously. Microstructure of the weld center are mainly composed of equiaxial grains and near the fusion line there are columnar dendrite basically. The heat-affected zone is extremely small.(4) Investigate porosity defect of the weld seam. The formation mechanism of pores and the effect of welding parameters on porosity were analyzed. With an appropriate increase of current offset, we can achieve high quality weld seam with the porosity of 0.69%. In engineering practice, we can inhibit the welding porosity by reducing the source of hydrogen as far as possible, then making a reasonable welding technology, etc. Welding parameters range that keep a relatively low level of weld porosity are as follows:arc current 200-240 A, welding speed 300-400 mm/min, frequency of variable polarity 150-200 Hz, current offset 40-50 A.