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 Al(6)Mn 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 Al(6)Mn 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 Al(6)Mn particles can suppress grain growth via Zener pinning, and consequently result in smaller grains at the SZs in the fine-grained alloy.