2016 Volume 88 Issue 10 Pages 610-614
The Si content of α-Al and aggregation substance of P at the product surface of AC4CH (Al-7%Si-0.45%Mg) alloy castings which were cast with different semi-solid die casting conditions (Sleeve filling ratio : SFR and Shot time lag : STL) were investigated to determine the generation mechanism of very-fine globular α-Al (diameter : under 15μm). In the case of SFR of 10% with STL of 10 seconds, a number of very-fine globular α-Al (diameter : under 10μm) containing over 2at.% Si were observed at the product surface than with SFR of 30% and 50% and STL of 10 seconds. AlP which could be the nucleus of eutectic Si was eliminated when P was aggregated as AlP in the melt. Therefore, it was difficult to form eutectic Si even if the melt temperature reached the eutectic temperature. Consequently, eutectic undercooling occurred easily. Moreover, the degree of undercooling was greater because of the die-cast product. Furthermore, eutectic structures did not form easily due to the high content of Si in crystallized α-Al, suggesting that considerable ultrafine α-Al had occurred.
