Substantial grain refinement of Al-Mn-Si alloys mediated by collaborative effect of Al-5Ti-1B refiner and sub-rapid solidification

The mechanical properties of as-cast metallic materials depend strongly on the size and shape of grains, which are critical microstructural parameters dictated by the interplay of nucleation and growth of crystalline solids during solidification. In our experiments, the microstructure transition from coarse columnar crystals into fine equiaxed crystals for dilute Al-Mn-Si alloys was achieved by using sub-rapid solidification with the addition of Al-5Ti-1B grain refiner. The average grain size of Al alloy was reduced from a millimeter size to 73 µm. Through temperature gradient calculation, we found that the acquisition of fine equiaxed crystals could be attributed to the existence of a high number density of TiB2, acting as effective nucleation sites with an increase in total undercooling. Furthermore, the curvature supercooling, constitutional undercooling, thermal undercooling, and kinetic undercooling during sub-rapid solidification were quantitatively determined for given solidification rates. Our results showed that constitutional undercooling, rather than thermal undercooling, was primarily responsible for the formation of fine equiaxed grains, with the assistance of Al-5Ti-1B grain refiner. This work provides a new insight into the grain refining mechanism under sub-rapid solidification.