Mechanisms of abnormal grain growth in friction-stir-welded aluminum alloy 6061-T6

The relationship between the temperature conditions during friction-stir welding (FSW), stir-zone microstructure, and thermal stability of welded aluminum-alloy 6061 joints was established. To facilitate interpretation microstructural data, distributions generated FSW were quantified using a finite-element-modeling (FEM) approach. In all cases, microstructure found to be unstable against abnormal grain growth (AGG) post-weld solution annealing. Moreover, it that AGG always developed very rapidly, being nearly complete heating material from ambient condition temperature. conditions, annealing behavior followed Humphrey's cellular-growth model. particular, for low-heat-input combination fine-grain structure low content secondary particles gave rise competition normal AGG. As result, final size relatively small. By contrast, high-heat-input relatively-coarse grains high fraction in weld nugget, which provided stability. However, surface layer associated with tool shoulder triggered catastrophic coarsening eventually consumed entire zone resulted millimeter-scale grains. Remarkably, led 40o ?111? rotation crystallographic texture stir zone. It thus concluded governed by at least two mechanisms, viz., pinning effect exerted second-phase enhanced mobility < 111 > boundaries.