Microstructure and mechanical properties in VPPA keyhole welding of thick aluminum alloy

In this study, the 16 mm thick aluminum alloy was successfully welded using variable polarity plasma arc (VPPA) keyhole welding. The microstructure and microhardness distributions of the weld were systematically measured along its thickness and width. To evaluate tensile strengths along the weld thickness, tensile specimens were divided into upper, middle, and lower sections. The surface temperature distribution of the weld pool, along with the temperature profile through its thickness, was measured using thermometers and thermocouples. Numerical simulations were employed to calculate the pressure oscillations along the keyhole boundary and to analyze the temperature and flow field distributions of the VPPA. The findings initially revealed a homogeneous and refined microstructure distribution within the weld seam. The weld zone exhibited higher microhardness compared to the base metal (BM). The weld demonstrated a tensile strength of 187.86 MPa and an elongation of 24.76%, corresponding to 95.57% and 78.65% of the BM, respectively. The tensile strengths and elongations of the upper, middle, and lower sections displayed a similar distribution to that of the entire weld. The mechanism underlying the microstructure distribution of the weld was significantly influenced by the temperature distribution of the weld pool, dynamic pressure oscillations in the mushy zone, and fluid flow of the weld pool. Specifically, the mechanism whereby pressure oscillations fracture existing nuclei and generate multiple new nuclei has been proposed to explain the refined microstructure distribution. These findings provide crucial insights for optimizing VPPA keyhole welding technology, ensuring stable and high-quality welding of thick aluminum alloys.