On the formation mechanism of the ring-like microstructure of high-pressure die-cast A390 alloy

The microstructure of the A390 alloy during high pressure die casting (HPDC), including pores, primary silicon particles (PSPs) and Cu-rich phases was characterized using synchrotron X-ray tomography. On the cross section of the sample exhibited ring-like or band microstructure, which comprised multiple PSPs-free and PSPs-rich zones. The PSPs distributed evenly in both size and morphology regardless of the ring-like microstructure. The size of aluminum dendrites changed according to the location of the ring-like microstructure, and was highly dependent on the local latent heat released from the PSPs as well as the distribution of the 'net' Cu-rich phases. Along the radial direction of the sample exhibited an abrupt increase of pores, indicating a tendency for forming a defect band in but such tendency was inhibited because of the huge amount of latent heat released by the PSPs during solidification, which was quite different from that of either magnesium alloys or hypoeutectic aluminum alloys.