Two-step electromagnetic forming: A new forming approach to local features of large-size sheet metal parts

•Two-step electromagnetic forming combining forming and calibration is proposed.•A geometric design method for oblique hole-flanging is established.•Electromagnetic calibration improves forming accuracy by reducing the bending moment.

A new two-step electromagnetic forming (two-step EMF) which combines electromagnetic forming (EM forming) with electromagnetic calibration (EM calibration), is proposed for local features of large-size sheet metal parts, such as an oblique hole-flanging. During the process, the workpiece is firstly electromagnetic formed by a flat spiral coil and then electromagnetic calibrated by a helix coil with a similar shape to the final profile of the workpiece. To develop the process, an efficient geometric design method is established, the effects of key process parameters on forming quality are investigated, and the deformation behavior of the workpiece is revealed. The feasibility of two-step EMF is validated by the accurate oblique hole-flanging with a maximum die-fitting gap less than 0.2 mm. Moreover, experimental results show that there are critical discharge voltages for both EM forming and EM calibration which lead to the minimum die-fitting gap. Furthermore, EM calibration can reduce the die-fitting gap to 0.24~1 times of that for EM forming with more uniform distribution. In addition, simulation results show that during EM calibration, the stress distribution of the workpiece is improved, the bending moment is reduced which is responsible for the shape and size errors, then the rebound is restricted. Consequently, the fittability and forming accuracy are significantly enhanced.

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