Incipient melting during friction stir processing of AZ91 magnesium castings

The microstructural evolution in as-cast AZ91 ahead of the tool has been studied during friction stir processing (FSP) using the breaking pin method. In this method, the pin is deliberately broken during FSP followed by rapid quenching of the material to freeze the microstructure. Examination of the frozen microstructure has revealed that the β-Mg17Al12 phase undergoes eutectic melting during FSP ahead of the pin and before the surrounding matrix is heavily deformed. The molten regions then collapse under the severe shear strain imposed on the matrix, and this leads to a large increase in their surface area that facilitates rapid solidification by growth of α-Mg, consuming the liquid. This mechanism leads to the small volume fraction of β observed in the final FSP material. It is promoted by the low eutectic melting temperature in the Mg–Al alloy system compared to that of pure magnesium, and is thought more plausible than a solid state dissolution mechanism as it does not rely on an unrealistic acceleration of the solid state diffusion rate.