Microstructural and mechanical property gradients along the thickness direction of plasma arc welded thick aluminum alloy plate joints

To address the issue of insufficient strength in weld joints of thick aluminum alloys, a variable polarity plasma arc-tungsten inert gas (VPPA-TIG) composite welding process was proposed. The welding quality of VPPA-TIG composite welding was validated by conducting welding experiments on 2219 aluminum alloy plates with a thickness of 17 mm, followed by a detailed analysis of the weld microstructure and mechanical properties along the thickness direction. The mechanism of strength distribution characteristics was revealed by microstructure analysis. The results show that the VPPA-TIG composite welding process produces high-quality welds with a superior degree of metallurgical bonding. The hardness distribution in the weld fusion zone is uniform, with an average value of 67.6 ± 2.7 HV0.2. The tensile strength reaches 62.83% of the base material, while the elongation after fracture is only 24.8%. The increased thickness of the weldment expands the heat dissipation area of the molten pool, resulting in insufficient grain growth within the weld. This leads to the formation of “meandering” micropores in the interface region. These micropores will continue to grow when subjected to tensile loads, which is the main reason for affecting the tensile strength of the weld. This study further expands the application range of the VPPA-TIG composite welding, clarifies the mechanism of strength failure in welded joints of thick plates, and provides a theoretical and technological reference for welding large-thickness plates.