Electropermanent magnet blank holder technique in sheet metal deep drawing

A novel blank holder technique based on electropermanent magnet (EPM) technology is proposed, in which independent loading of blank holding force (BHF) can be realized in sheet metal forming process. The magnetic force and the resulting BHF are applied by permanent magnets rather than electromagnets, and the current is only required at the moment of loading or unloading the BHF. The BHF is convenient in control, low in energy consumption, and without overheating problem using this technique. In this paper, an experimental setup consisting of magnetic cushion, die, punch, and other components is designed based on the characteristics of EPM and requirements of deep drawing process. The magnetic cushion including blank holder and magnetic pole units is used to generate magnetic force and realize loading and unloading of BHF. Then, magnetic field analyses of a pair of magnetic pole units and the designed cushion are carried out by FEM. Also, the coupling problem involving magnetic field and stress field is further analyzed and solved by means of theoretical method and FEM, and the BHF and the blank holding pressure on the sheet using the new technique can be obtained. The deformation of the designed setup is small, and the stiffness can satisfy the requirements. The deep drawing processes of cylindrical cup of 08Al sheet with 0.98 mm thickness are simulated by using conventional blank holder and the new technique, respectively, and there are small differences of the strain distributions of the drawn parts with the two techniques. The experiments of cylindrical cup deep drawing applied four different BHFs, respectively, are carried out using EPM bank holder technique. The results indicate that the BHF applied by the designed cushion is large enough for the deep drawing process of the selected sheet.