Investigation on the Influence Mechanism of Mg Contents on the Hot Cracking Behavior of Al–Cu–Mg Alloys

The present work focuses on the mechanisms of liquid film feeding and intermetallic compounds interface property in the Mg content of Al–Cu–Mg alloys hot cracking behavior. Experimental results reveal that two types of hot cracking microstructure are observed when the mushy zone of Al–Cu–Mg alloys is constituted of liquid film and spherical solid grains. One type is attributed to the premature formation of a closed region which prevents the feeding of the liquid film and insufficient feeding of the liquid film, while the other is formed due to unstable intermetallic compounds. Combined with multiscale simulation calculations, it is demonstrated that in alloys which hot cracking is induced by the formation of a premature closed zone and insufficient liquid film feeding, the Al–4.0Cu–1.5Mg alloy exhibits the highest residual liquid film feeding volume (V_residual^*) value of 5.59 × 10⁴ µm³, indicating the lowest hot cracking susceptibility. Therefore, in the absence of dendrite bridging effects on hot cracking formation, the mechanism by which grain refinement affects hot cracking is to decrease the solid phase shrinkage volume (V_s^*) while increasing the liquid film feeding volume (V_feeding^*), thereby reducing the tendency for hot cracking initiation. Furthermore, molecular dynamics (MD) simulation is used to calculate the interface stability of different intermetallic compounds. The results demonstrate that the interfacial separation works for Al₂CuMg//Al and Al₂CuMg//Al₂Cu are 28.805 and 2.849 J/m², respectively. These values are higher than those for AlCuMg//Al and AlCuMg//Al₂Cu (26.939 and 2.473 J/m²), indicating that interfaces with AlCuMg as the matrix are more susceptible to be separated. When the Mg element content exceeds 1.5 wt pct, the formation of the unstable AlCuMg phase increases the sources of hot cracks. The research findings of this work provide design insights and theoretical guidance for development alloys with low hot cracking susceptibility.