Revealing the Role of Minor Zn and Zr Element Addition in Hot Tearing Susceptibility of Mg–Gd–Y Alloy

The hot tearing susceptibility (HTS) of Mg–5Gd–6Y alloy with 1 wt% Zn and 0.5 wt% Zr additions is investigated based on the constrained rod casting experiments and thermodynamic calculation. It shows that a minor Zn addition aggravates the HTS which cannot even be compensated by the grain refining effect of a further Zr addition. Moreover, the alloys with minor Zn and Zr addition show earlier hot tearing initiation (before the volumetric contraction force reaching minimum value). According to the calculation of prevailing criteria, minor Zn and Zr additions affecting the HTS has little relationship with the changing of the Scheil solidification curves. The mass formation of the X-Mg₁₂YZn phases in alloys with a minor Zn addition is the main cause of earlier hot tearing initiation and higher HTS. The phases with a higher elastic modulus result in poor liquid feeding among the α-Mg matrix and early dendrite bridging, causing more cavities, earlier strain accumulation at hot spots, and consequently earlier hot tearing initiation and higher HTS.