Effects of air on metallic sheet deformation by electromagnetic forming

Air is one of the most important processing parameters in electromagnetic sheet metal forming processes, especially when forming a large-scale sheet via single discharge or controlling dimensional accuracy with the die fitness of the deformed workpiece. Previous research on electromagnetic sheet forming mostly concentrated on the effects of magnetic pressure distribution, discharge energy, and material properties on workpiece deformation behavior. In this paper, the effect of air on electromagnetic sheet forming is investigated and the research contents are mainly divided into the following three parts: (1) investigating the influence of thickness, material strength, and die caliber on pre-deformation of the sheet when the die cavity is an approximate vacuum via simulation and experimentation; (2) studying the effect of workpiece pre-deformation on electromagnetic sheet forming with regard to the magnitude and distribution of the electromagnetic force, deformation velocity, workpiece profile, etc.; and (3) investigating the influence of the resistance generated by the residual die cavity air on workpiece deformation with experiments considering the die fitness of the deformed workpiece. The results show that atmospheric pressure results in workpiece pre-deformation when the die cavity is near vacuum and the pre-deformation is closely related to the workpiece thickness, material strength, and die caliber. The Lorentz force distribution moves toward the region near the chamfer of the die, and the peak value of the Lorentz force and the deformation velocity of the workpiece are reduced by 62% and 26.9%, respectively, when considering workpiece pre-deformation. The resistance caused by residual air will obviously affect workpiece deformation. The influence will weaken as air pressure decreases, and the maximum deviation of the workpiece formed by a single discharge is less than 1.5% when the air pressure is reduced to 30 Pa.