Two-step method to improve geometry accuracy of elongated hole flanging by electromagnetic forming

Abstract

Electromagnetic forming (EMF) was used to conduct elongated hole flanging in this work. The investigation showed that the low geometry accuracy around the elongated hole flanging attributed from an uncoordinated deformation occurred, including unequal flanging heights and different rebound amounts between the straight edges and the circular edges. The uncoordinated flanging height resulted from different strain distribution. The strain distribution of the circular edges was similar to circular hole flanging, while the strain distribution of the straight edge regions was similar to sheet stretch bending. The uncoordinated rebound amount resulted from different deformation mechanisms, stress distribution, and structural stiffness within the forming regions. To improve the geometry accuracy of the elongated hole flanging, a two-step method combined geometric compensation electromagnetic forming (GC-EMF) and electromagnetic calibration (EM calibration) was proposed. GC-EMF was used to achieve equal flanging height by optimizing the prefabricated hole, while EM calibration after GC-EMF was used to improve the forming accuracy by reducing the amount of rebound. The difference in the flanging heights between the straight edges and the circular edges was only 0.1 mm after the GC-EMF process, when the compensation dimension on both sides of the straight edges of a prefabricated hole was 3.4 mm. The gap between the die and the straight edges was 0.2 mm, the gap between the die and the circular edges was 0.1 mm, after EM calibration. Therefore, the improved geometry accuracy of the elongated hole flanging was achieved by combining the GC-EMF with EM calibration. The results obtained and the method proposed in this work present significant guidance for the process of elongated hole flanging or other irregular hole flanging by EMF.