The effects of die counter-impact on aluminum alloy sheet during electromagnetic forming

Electromagnetic forming is a high-speed forming process, and it can generate high contact force between the sheet metal and die. Therefore, the effects of die counter-impact on aluminum alloy sheet during electromagnetic forming were numerically and experimentally investigated. Firstly, the numerical simulations of electromagnetic free forming and electromagnetic die forming were performed with the Gurson-Tvergaard-Needleman damage model. When the aluminum alloy sheet impacted the die, an unignorable normal stress occurred along the thickness direction. The void volume fraction was reduced due to the negative stress triaxiality, and the residual stress became smaller. Secondly, the effect of die counter-impact on the pre-deformed aluminum alloy sheet was numerically inspected for quasi-static and dynamic hybrid electromagnetic forming. Compared with the electromagnetic die forming of aluminum alloy sheet without pre-deformation, the void volume fraction and residual stress reductions were more significant for the pre-deformed aluminum alloy sheet. Finally, the aluminum alloy sheets after different electromagnetic forming experiments were compared by optical microscope and Vickers hardness. The results show that the damage suppression and hardness enhancement are attributed to the effect of die counter-impact, especially for the hybrid quasi-static and dynamic forming processes.