The microstructure of AC4CH (Al-7%Si-0.45%Mg) alloy castings with different semi-solid die casting conditions (Sleeve filling ratio: SFR and Shot time lag: STL) was investigated to determine the generation mechanism of very-fine globular α-Al phases (diameter: under 15μm). In the case of SFR of 10% with STL of 10 seconds, a larger number of very-fine globular α-Al phases were observed between the globular α-Al (under 50μm) phases at the product surface compared to SFR of 30% and 50% with STL of 10 seconds. The size of the very-fine globular α-Al phases was about 5μm on the product surface and under 15μm at the product center. The very-fine globular α-Al phases at the product surface were coarser in the case of SFR of 10% with STL of 0 seconds than those of STL of 10 seconds, and their size was under 15μm. These results suggest that there exist conditions for promoting the generation of very-fine globular α-Al phases in the melt at the stage of reaching the eutectic temperature in case of the low sleeve filling ratio.

  Recently, to reduce the weight of automobiles, the thickness of aluminum alloy castings is increasingly being reduced. For the thickness reduction of the casting, Hypereutectic Al-Si alloy is considered promising for reducing the thickness of casting because it exhibits excellent fluidity. On the other hand, thinned castings are generally not machined and should be flat surfaced. The flatness of hypereutectic Al-Si alloy has rarely been reported to date. Accordingly, in this study, Al-17% Si alloy was pelted without restraint on a chill plate by changing the P content to determine the influence of P on the flatness of thin plate. The results are as follows. In alloys containing no or 25ppm P, spots without eutectic cells were formed on the surface. As small solidified shells between these spots transformed into waveform, the surface of thin plate was not flat. In contrast, eutectic cells were continuously formed on the surface in the alloy containing 112ppm P. Consequently, as large solidified shells were formed, no transformation occurred and the thin plate surface was flat.