Effect of low-frequency electromagnetic casting on the castability, microstructure, and tensile properties of direct-chill cast Al-Zn-Mg-Cu alloy

Microstructure and grain boundary segregation of the direct-chill (DC) cast Al-Zn-Mg-Cu (7A60) alloy, with and without low-frequency electromagnetic field, were investigated. The surface quality, hot-tearing tendency, and tensile properties of the ingots manufactured by DC and low-frequency electromagnetic field casting (LFEC) were compared. The results show that LFEC significantly improves the surface quality and reduces the hot-tearing tendency of DC ingots. It is possible to generate a fine, uniform, and equiaxed microstructure with LFEC. Under optimum conditions, the average grain size varies from 30 mum near the surface to 45 mum at the center of the LFEC ingots. Decreasing electromagnetic frequency or increasing intensity significantly refines microstructure, suppresses grain boundary segregation, and increases as-east fracture strength and elongation. In the range of frequency and ampere turns employed in the experiments, the optimum frequency is found to be 15 to 25 Hz and the number of ampere turns to be 18,000 to 20,000 A(t).