Effect of a novel intermediate heat treatment process on abnormal grain growth in 2219 aluminum alloy friction stir welds

Abstract

The abnormal grain growth occurs in 2219 aluminum alloy friction stir welds during solution treatment, which results in the deteriorating mechanical properties of the weld. In this paper, a novel intermediate heat treatment process after friction-stir welding, including rapid heating and high-temperature treatment at 560 °C and annealing treatments, was proposed to suppress abnormal grain growth in the weld during subsequent solution treatment. The effect of the intermediate heat treatment process on the microstructure and mechanical properties of the weld was investigated and analyzed. The results show that the grains in the nugget zone grow continuously in a short time at a temperature higher than the eutectic line. There is no abnormal grain growth in most regions of the weld treated with the intermediate heat treatment after solution treatment. Compared with tailor-welded blanks directly with solution treatment, the elongation of the tailor-welded blanks treated with the intermediate heat treatment and solution treatment increases from 13.5% to 20% because of the improvement of weld strength. The increase in mechanical properties is more significant after artificial aging treatment. Because of the disappearance of the pinning effect of particles at high-temperature treatment, abnormal grain growth induced by particles cannot occur. The grains grow normally, improving their ability to resist abnormal grain growth during solution treatment. Fine grains result in a more uniform distribution of deformation in the solution-treated weld. Furthermore, it is more difficult for the particles at the grain boundary to cause crack propagation along the grain boundary after artificial aging treatment. This work helps guide how to suppress abnormal grain growth in the weld and further deepen the understanding of the phenomenon.