Chemical segregation induces intergranular bridging to reduce the hot tearing susceptibility of as-cast Al-5.5Zn-2.4Mg-1.3Cu alloys

• Provides information on casting of Al-Zn-Mg-Cu-Ni alloys • Explained the mechanism of the relative intergranular bridging effect of Al 9 Fe 0 . 7 Ni 1 . 3 . • Provided a comparison of several methods for evaluating hot tearing. • The impact mechanism of Ni addition on the thickness of the intergranular liquid film is provided. • Provides direct evidence of the relationship between Ni element and solidification shrinkage stress and displacement. Al-Zn-Mg-Cu alloys are promising for lightweight automotive parts due to their high specific strength, but their high hot tearing susceptibility (HTS) has historically restricted the use of more economical and efficient casting techniques. This study systematically examines the effect of Ni addition on the hot tearing behavior of an as-cast Al-5.5Zn-2.4 Mg-1.3Cu alloy using a combination of solidification thermodynamic simulations, scanning electron microscopy (SEM), constrained rod casting (CRC) tests, and a self-designed setup for the simultaneous measurement of thermal history, contraction force, and displacement. The results indicate that Ni addition effectively lowers the coherency point temperature, narrows the vulnerable solidification temperature range, and reduces both the contraction force and displacement during solidification. These quantitative measurements align well with the qualitative CRC mold experiments. Critically, the presence of Fe facilitates the formation of AlNiFe intermetallic compounds. These compounds induce a beneficial intergranular bridging effect, strengthening grain-boundary cohesion, enhancing load transfer within the semi-solid structure, and consequently reducing the thickness and susceptibility of intergranular liquid films. This research clarifies the critical role of composition, specifically Ni and its interaction with Fe, in controlling HTS resistance. The findings establish a fundamental basis for designing cast Al-Zn-Mg-Cu alloys with minimized HTS for demanding lightweight applications.