Semisolid casting with ultrasonically melt-treated billets of Al-7mass%Si alloys

The demand for high performance cast aluminum alloy components is often disturbed by increasing impurity elements, such as iron accumulated from recycled scraps. It is strongly required that coarse plate-like iron compound of β-Al5FeSi turns into harmless form without the need for applying refining additives or expensive virgin ingots. The microstructural modification of Al-7mass%Si alloy billets with different iron contents was examined by applying ultrasonic vibration during the solidification. Ultrasonically melt-treated billets were thixocast right after induction heating up to the semisolid temperature of 583 oC, the microstructure and tensile properties were evaluated in the thixocast components. Globular primary α-Al is required to fill up a thin cavity in thixocasting, so that the microstructural modification by ultrasonic melt-treatment was firstly confirmed in the billets. With ultrasonic melt-treatment in the temperature range of 630 oC to 605 oC, the primary α-Al transforms itself from dendrite into fine globular in morphology. The coarse plate-like β-Al5FeSi compound becomes markedly finer compared with those in non-treated billets. Semisolid soaking up to 583 oC, does not appreciably affect the size of β-Al5FeSi compounds; however, it affects the solid primary α-Al morphology to be more globular, which is convenient for thixocasting. After thixocasting with preheated billets, eutectic silicon plates are extremely refined due to the rapid solidification arising from low casting temperature. The tensile strength of thixocast samples with different iron contents does not change much even at 2mass% of iron, when thixocast with ultrasonically melt-treated billets. However, thixocast Al-7mass%Si-2mass%Fe alloy with non-treated billets exhibits an inferior strength of 80 MPa, compared with 180 MPa with ultrasonically melt-treated billets. The elongation is also improved by about a factor of two in thixocastings with ultrasonically melt-treated billets for all iron contents of Al-7mass%Si alloys, for example, the elongation of 11% in thixocast of Al-7mass%Si-0.5mass%Fe alloy with ultrasonically melt-treated billets, 5% in that with non-treated billets.