Effect of Main Elements (Zn, Mg, and Cu) on Hot Tearing Susceptibility During Direct-Chill Casting of 7xxx Aluminum Alloys

Abstract

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.

Grain Refinement of Alloys in Fusion-Based Additive Manufacturing Processes

“…The free growth model [12] developed by Greer et al is commonly used to determine the potency of particles. [13][14][15] Zhang et al further considered the importance of crystallographic matching by applying their edge-to-edge matching model to explain the effectiveness of grain refiners and have successfully applied this to Al and Mg alloys. [6,[16][17][18] More recently, Fan et al [19] explained the growth of grains by elucidating the mechanisms of heterogeneous nucleation by epitaxial growth from substrates.…”

Effects of Ni on the microstructure, hot tear and mechanical properties of Al–Zn–Mg–Cu alloys under as-cast condition

“…It is easy to apply in conventional foundry practice without significant changes in processing equipment and procedures. Previous studies confirmed that the modification of main alloying elements and their contents had complex and/or even contradictory effects on the hot tear tendency of AleZneMgeCu alloys [11,23,24]. Krajewski [25] and S. Shin et al [26] pointed that extra high Zn level (>20 wt%) could effectively suppress the formation of hot tear during solidification, however, the high density induced by high Zn is not favoured for light-weight requirement.…”

Recent advances in hot tearing during casting of aluminium alloys

“…Li et al [77] modified this T-shaped device to measure the size of the hot tears and load development of DC-cast billets. As shown in Fig.…”

“…2.5. Importantly, this device provides information on the hot tearing susceptibility and load developments, and it has been applied to different AA7xxx alloys [26,[76][77][78]. Recently, Pan et al [79] replaced the T-shaped graphite mould with a bronze mould to obtain higher cooling rates ranging from 11 to 24 K/s to further simulate the solidification during fusion welding.…”