Solid fraction evolution characteristics of semi-solid A356 alloy and in-situ A356-TiB₂ composites investigated by differential thermal analysis

The key factor in semi-solid metal processing is the solid fraction at the forming temperature because it affects the microstructure and mechanical properties of the thixoformed components. Though an enormous amount of data exists on the solid fraction-temperature relationship in A356 alloy, information regarding the solid fraction evolution characteristics of A356-TiB₂ composites is scarce. The present article establishes the temperature-solid fraction correlation in A356 alloy and A356-xTiB₂ (x=2.5wt% and 5wt%) composites using differential thermal analysis (DTA). The DTA results indicate that the solidification characteristics of the composites exhibited a variation of 2℃ and 3℃ in liquidus temperatures and a variation of 3℃ and 5℃ in solidus temperatures with respect to the base alloy. Moreover, the eutectic growth temperature and the solid fraction (f_s) vs. temperature characteristics of the composites were found to be higher than those of the base alloy. The investigation revealed that in-situ formed TiB₂ particles in the molten metal introduced more nucleation sites and reduced undercooling.