Coarsening rate of microstructure in semi-solid aluminium alloys

In semi-solid metal processing, the temperature is between the solidus and the liquidus. To behave thixotropically, the microstructure must be non-dendritic and consist of spheroids of solid in a liquid matrix. Recrystallisation and Partial melting (RAP) and the cooling slope (CS) are two potential routes to suitable non-dendritic starting material. Here the rate of microstructural coarsening of such materials in the semi-solid state is compared with rates found in the literature. The rate of coarsening depends on the liquid fraction but RAP route 2014 alloy with 37% liquid coarsens slightly more slowly than the CS route 2014 alloy with a lower liquid fraction of 17%, contrary to expectations. For the CS route, an increase in liquid fraction resulted in faster coarsening. A modified 2014 alloy with the Fe, Mn and Zn stripped out of the composition gave a relatively high coarsening rate. The coarsening rate was also relatively high for CS 201 alloy in comparison with either RAP 2014 or CS 2014. Low coarsening rates are thought to be associated with the presence of particles which are inhibiting the migration of liquid film grain boundaries. This could be the result of pinning or of the liquid film impeding diffusion at the boundary.