Microstructure and mechanical properties of welds at keyhole closures in variable-polarity plasma arc welding of Al alloy

Al alloy is widely used in aerospace, military industries, and other fields due to its low density and high specific strength. Variable polarity plasma arc keyhole welding is a preferred method for processing large aluminum alloy structural components. However, keyhole closure is a challenge in plasma arc welding of light metal alloys. To address this issue, a gas-electric separation timed pulse plasma arc ring welding method named keyhole self-closing method is proposed. The microstructure and properties of the keyhole closure in the ring welds by the proposed method and the manual TIG welding filling method was compared. The results demonstrate that by adjusting the welding arc starting and ending strategies, the keyhole closure was achieved, with the formation of well-shaped weld joints exhibiting a uniform equiaxed crystal microstructure. The tensile strength was about 329.9 MPa, with an elongation of 16.3%, representing a 22.91% increase in tensile strength and a 55.24% increase in elongation compared to the keyhole filled by the TIG welding. The average hardness value was 78.9 HV0.2, with the heat-affected zone showing a decreasing trend in hardness, the fusion line exhibiting an increasing trend, and the weld zone hardness fluctuating in the range of 75 to 81 HV0.2