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New 7xxx aluminum alloys with high alloying contents are being designed, which could induce serious hot tearing defects during direct-chill (DC) casting. Among all factors affecting hot tearing of 7xxx alloys, undoubtedly alloying elements play a significant role. In this study, the effect of main alloying elements (Zn, Mg, and Cu) on hot tearing of grain-refined Al-xZn-yMg-zCu alloys was investigated by a dedicated hot tearing rating apparatus simulating the DC-casting process. It was found that the minimum and maximum hot tearing susceptibilities occur for 4 to 6 and 9 wt pct Zn, respectively, indicating the complicated effect of Zn content. The hot tearing resistance of grain-refined Al-9Zn-yMg-zCu alloys is enhanced with increasing Mg content but is deteriorated with increasing Cu content. This can be attributed to the interaction of the thermal stresses, melt feeding, and final eutectics. The observed tendencies of the main alloying elements on hot tearing were also confirmed for four commercial 7xxx alloys. In addition, both the load value at non-equilibrium solidus and the SKK criterion proposed by Suyitno et al. using measured load developments were found to be good indicators in predicting hot tearing susceptibility. This study can provide a beneficial guide in designing 7xxx alloys considering the potential occurrence of hot cracks beforehand.

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In this work, a mixture of as-atomized and as-cryomilled powders instead of the pure as-cryomilled powder was used as feedstock to achieve high density nanocrystalline coatings by cold spraying. Cryomilled powder with nanocrystalline grains was produced by mechanical milling under liquid nitrogen and the grain size range was from 5 to 30 nm. A mixture of 30 wt.% as-atomized powder and 70 wt.% as-cryomilled powder was sprayed onto the aluminum substrates. High density coatings were attained by use of this kind of mixture, which was described as an effective method to decrease porosity in the coldsprayed nanocrystalline coating. The functions of the as-atomized powder in the coating were discussed. The mechanical behavior of the powders and the coating were studied using nanoindentation technique.

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.