Abstract
A new method to determine electromagnetic forming limits curves (EM-FLCs) for sheet metals is proposed. The different strain paths (between uniaxial and biaxial tension) are achieved by specific tool coil and specimen designs. It is ensured that the apex of the specimen deforms on a constant strain path, and excess bending at the apex is avoided. This is done so that the determined EM-FLCs are comparable to their quasi-static counterparts. The method determines the EM-FLCs for the aluminum alloys AA-1050a-H24 and EN AW-5083-H111 and the magnesium alloy Mg AZ31-O. Overall, it is observed that the necking limits in electromagnetic forming (EMF) are higher compared to quasi-static forming. The fracture surfaces of electromagnetically deformed specimens are examined to reveal the existence of out-of-plane shear stresses. A numerical analysis corroborates this observation and their variation with strain rate. The presence of such stresses is proposed as a possible reason for the increased necking limits in EMF. As reasons for higher forming limits, previous research has identified inertial stabilization, strain rate hardening, die impact, and change in deformation mechanism. The current study reaffirms the positive effect of inertial stabilization and makes key observations in the increase of twinning in EMF of Mg AZ31-O.
Highlights
The prime intention to determine the forming limits curves (FLCs) is to have a robust failure criterion for sheet metal forming for a particular strain rate
Several attempts for the determination of FLCs for high-speed processes are found in the literature, which observed the change in material properties of various alloys at higher strain rates
The tool coil and specimen design are essential if the problems with FLC determination in high-speed processes observed in the literature are to be avoided
Summary
The tool coil and specimen design are essential if the problems with FLC determination in high-speed processes observed in the literature are to be avoided. The numerical modeling was used to predict the strain concentration, applied magnetic pressure, etc. To develop the coils and specimen forms for the experiments. A measured current curve is provided as the input. The model is created in LS-Dyna, where the electromagnetic solver is loosely coupled with the thermal–mechanical solver. The specimen and the coil provided temperature-dependent thermal and electrical properties such as the thermal conductivity, heat capacity, and electrical conductivity, but the mechanical properties of the specimen are neither strain-rate nor temperature-dependent, as only quasi-static flow curve data were available from tensile tests. The coils are modeled as rigid bodies
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