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Using the good criteria to predict hot tearing is very important during DC casting of aluminium alloys. Among all the hot tearing criteria, a fracture-mechanics based SKK criterion proposed by Suyitno et al. has made considerable improvements in the hot tearing prediction. However, its obtained hot tearing susceptibility (HTS) evolution during solidification is also not completely consistent with real industrial production circumstances, especially when approaching the solidus temperature. In this paper, some further modifications are made based on the SKK criterion to emphasise the important effect of solid bridging/grain coalescence on hot tear propagation. It is proved that the HTS evolution in freezing range predicted by the modified hot tearing criterion is in good agreement with casting practice.

Studies were carried out systematically on a series of Al-8.5 wt pct Zn-xMg-yCu alloys (x is about 1.5, 2.0, and 2.5 wt pct, and y is about 1.5, 2.0, 2.5, and 2.9 wt pct). The effects of alloying elements Mg and Cu on the microstructures of as-cast and homogenized alloys were investigated using the computational/experimental approach. It shows that Mg(Zn,Al,Cu) 2 (r) phase can exist in all the as-cast alloys without any observable Mg 32 (Al,Zn) 49 /Al 2 Mg 3 Zn 3 (T) or Al 2 CuMg (S) phase, whereas Al 2 Cu (h) phase is prone to exist in the alloys with low Mg and high Cu contents. Thermodynamic calculation shows that the real solidification paths of the designed alloys fall in between the Scheil and the equilibrium conditions, and close to the former. After the long-time homogenization [733 K (460°C)/168 hours] and the two-step homogenization [733 K (460°C)/24 hours + 748 K (475°C)/24 hours], the phase components of the designed alloys are generally consistent with the calculated phase diagrams. At 733 K (460°C), the phase components in the thermodynamic equilibrium state are greatly influenced by Mg content, and the alloys with low Mg content are more likely to be in single-Al phase field even if the alloys contain high Cu content. At 748 K (475°C), the dissolution of the second phases is more effective, and the phase components in the thermodynamic equilibrium state are dominated primarily by (Mg + Cu) content, except the alloys with (Mg + Cu) Z 4.35 wt pct, all designed alloys are in single-Al phase field.

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