Microstructural evolution of a fine-grained 7075Al alloy processed by friction stir process during partial remelting

The success of thixoforming mostly depends on the preparation of semisolid slurry, consisting of non-dendritic and spherical solid grains surrounded by liquid. Initial microstructure of alloys before remelting, especially the initial grain size and the grain boundaries, can influence microstructural coarsening of semisolid alloys. The investigation focused on the microstructural evolution during reheating of a fine-grained 7075Al alloy processed by friction stir process (FSP) into the semisolid state. The results showed that a semisolid slurry containing fine and spherical solid grains was prepared and the distribution of solid grains and liquid phase is relatively uniform. Owing to the penetration of liquid phase into high angle grain boundaries (HAGBs) and a large fraction of HAGBs in the FSPed alloy, a network structure of intragranular liquid film in the semisolid state can be obtained in a short soaking time. The coarsening rate constant of the alloy increased with increasing liquid fraction. The grain coarsening mechanism in the semisolid state was discussed, and the effect of grain boundary characteristics in the initial microstructure on coarsening kinetics of semisolid alloys was also described.