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.

Cold cracking is a severe challenge during the direct-chill casting of high-strength 7××× series aluminum alloys. A finite element method (FEM) simulation combined with a cold cracking criterion has been demonstrated to possess obvious technical advantages in cold cracking prediction. However, the current absence of mechanical properties and effective criteria for 7××× series aluminum alloys inhibits the progress of the technique. In this study, the corresponding mechanical properties of four typical 7××× series aluminum alloys are investigated. The cold cracking tendencies of the alloys are evaluated by a new cold cracking index (CCI) developed by the authors. It is shown that AA7055 has the highest cold cracking propensity among the four alloys, followed by AA7050, AA7085, and AA7022, respectively. The cold cracking tendency is basically consistent with the amount of nonequilibrium eutectics of the alloys under the same casting process. It is also shown that the application of water wiper can effectively decrease the occurrence of cold cracking.

The effect of Zn addition on the hot tearing susceptibilities of non-refined Al-xZn-2Mg-2Cu (x = 2-12 wt pct) alloys was investigated via direct crack observations and load response measurements. The obtained experimental results were compared with the predictions made using a modified Rappaz–Drezet–Gremaud (RDG) hot tearing model. Both the minimum crack width and load at the non-equilibrium solidus (NES) temperature (which served as a good indicator of hot tearing response) were observed at a Zn concentration of approximately 4 wt pct, and the formation of cracks was highly correlated with the predictions made via the modified RDG hot tearing model (although the obtained relationship critically depended on the magnitude of fraction solid at which solid coalescence was expected to occur). Furthermore, it was confirmed from the load development pattern that the addition of Zn into the matrix of Al-xZn-2Mg-2Cu alloys promoted the formation of coalesced networks, which decreased their corresponding coalescence fraction solids.

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.