Enhancement of mechanical properties in a cast hypereutectic Al–Si alloy via rapid cooling in heated mold continuous casting

To enhance mechanical properties in cast hypereutectic Al–Si alloys, Al–14.5Si samples were produced by the heated mold continuous casting (HMC). The HMC process was conducted under unidirectional solidification at cooling rates (CRs) ranging from 4.6 × 10 -1 to 1.3 × 10 3 K/s, which were obtained by controlling the casting speed (CS). The microstructure basically consisted of α -Al dendrite, Al–Si eutectic and primary Si, while their area fractions ( f α- Al , f e , and f p-Si ) varied depending on the CR. At the low CR of 4.6 × 10 -1 K/s, their area fractions were as follows: f α- Al = 0.2%, f e = 97.3%, and f p-Si = 2.5%, which is the typical microstructural morphology of a hypereutectic Al-Si alloy. In contrast, at the high CR of 1.3 × 10 3 K/s, those area fractions were altered to f α- Al = 32.3% and f e = 67.7%, while primary Si was strongly suppressed ( f p-Si ≈ 0%) despite the hypereutectic Al–14.5Si alloy. Moreover, at the high CR, the aluminum matrix appears to exhibit a nearly uniform crystal orientation along <101>. These microstructural changes may be related not only to high CR but also the difference in growth kinetics between primary Si and α -Al dendrites. Accordingly, an apparent eutectic composition shifted from 12.6 to 19.7wt% Si. The atypical microstructural formation led to a significant improvement in the tensile properties: the ultimate tensile strength of 400 MPa and fracture strain of 17%. These tensile properties are more than twice those of the conventional cast hypereutectic Al–14.5Si alloy.