Optimization of microstructure and properties of thick Al alloy welded joints for one-step formation by keyhole welding

To solve the problem of insufficient strength of thick aluminum alloy welded joints, this study introduces a pioneering technique known as variable-polarity plasma arc keyhole-tungsten inert gas cover hybrid welding (VPPA keyhole-TIG covering welding), which is designed for welding large-thickness aluminum alloys without any weld groove. Using 13-mm-thick 2319 aluminum alloy plates as experimental materials, plasma arc butt welding experiments were conducted by varying the plasma gas flow rate and application of cover welding. The research findings underscored that the use of cover welding, coupled with an increase in the plasma gas flow rate, markedly improved the overall performance of the weld. An average tensile strength of 299.0 +/- 4.0 MPa and an elongation rate of 4.5 +/- 0.3 % were achieved for the welded joint. A microscopic analysis revealed that the heightened plasma gas flow rate did not precipitate the 0' phase but instead resulted in undesired grain size enlargement, leading to reduced joint hardness. The secondary thermal cycle effect of covering welding played a pivotal role in facilitating the nucleation of needle-like 0' strengthening phases within the matrix, ranging in size from 50 to 150 nm. This phenomenon has emerged as a principal factor contributing to the substantial enhancement of the mechanical properties of welded joints.