Experimental Evaluation of Unidirectional Pulsating Traveling Magnetic Field Effectiveness for High-Strength 7xxx-Series Aluminum Alloy Slab Semi-Continuous Casting

This study investigates the application of a unidirectional pulsating traveling magnetic field (UPTMF) for the semi-continuous casting of high-strength aluminum alloy 7075 slabs (350×150 mm). The experimental setup involved a comparative analysis of direct chill (DC) casting and electromagnetic stirring (EMS) with a 5 Hz amplitude modulated U-PTMF and 22.5 Hz carrier frequency, focusing on microstructural evolution, chemical composition homogeneity, and defect formation. Microstructural analysis demonstrated that slabs cast under conventional DC conditions exhibited columnar dendritic structures with a grain size heterogeneity ranging from 90 to 125 μm in the center and up to 160 μm near the surface. In contrast, the EMS-treated slabs showed a more uniform and refined microstructure with equiaxed grains of 80~90 μm in the center and 90~125 μm at the surface. The electromagnetic stirring facilitated grain refinement by inducing forced convection, the fragmentation of dendritic arms, and mechanical electromagnetic vibration effects. Chemical composition measurements of the DC slabs indicated severe macro segregation, with relative deviations (RD%) of Zn (5.27%~44%), Mg (7.2%~33.6%), and Cu (18%~51%). Conversely, the EMS-treated slabs displayed significantly reduced segregation, with deviations limited to 0.18%~2.7% for Zn, 0.4%~2.8% for Mg, and 0.67%~8.7% for Cu. These improvements resulted from enhanced melt mixing and homogenization induced by the traveling magnetic field. Additionally, the DC-cast slabs developed internal cracks due to post-casting thermal stress accumulation, while the EMS-treated slabs exhibited no visible defects, suggesting that the U-PTMF method effectively mitigates stress formation. The findings confirm that pulsed electromagnetic processing enhances microstructural uniformity and chemical homogeneity while preventing casting defects, offering a viable approach for improving aluminum alloy slab quality for aerospace and automotive applications.