Effect of Dispersed Intermetallic Particles on Microstructural Evolution in the Friction Stir Weld of a Fine-Grained Magnesium Alloy

A fine-grained Mg alloy with copious dispersed Al₆Mn particles was friction stir (FS) welded, and microstructures at the stir zones (SZs) were investigated intensively and compared to those in a coarse-grained Mg alloy, which had identical chemical compositions with the fine-grained one but much less Al₆Mn particles. It is found that the grain size at the SZs is a little coarser than that of the base material in the fine-grained alloy, while it becomes much finer in the coarse-grained one. Under the same welding conditions, however, the grain size at the SZs in the fine-grained alloy is much smaller than that in the coarse-grained one. These experimental results are explained in terms of a grain growth mechanism proposed in the present study. Transmission electron microscopy (TEM) examination shows that the dispersed Al₆Mn particles can suppress grain growth via Zener pinning, and consequently result in smaller grains at the SZs in the fine-grained alloy.