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

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