Development of combined deep drawing and electromagnetic sharp edge forming of DP980 steel sheet

Abstract

DP980 steel sheet is one of advanced high strength steel sheets (AHSS) introduced in order to achieve both enhanced crashworthiness and lightweight design of auto-body. Due to its comparatively poor formability, the application of DP980 steel sheet to auto-body is limited with conventional deep drawing processes. One of remedies is a combined deep drawing and electromagnetic sheet metal forming process, which is one of the innovative forming methods to improve the formability. The process chosen in this paper consists of square cup drawing of DP980 1.2t steel sheet with punch radius of 30 mm firstly and electromagnetic sharp edge forming to reduce the radius to 10 mm. This paper proposes an approach for numerical optimization of combined deep drawing and electromagnetic sharp edge forming of DP980 1.2t steel sheet. The numerical optimization was conducted by following optimization procedures of electromagnetic forming force and efficient coil design. The mechanical properties and fracture forming limits of DP980 1.2t steel sheet were obtained experimentally for the numerical simulation. Conventional square cup drawing analysis was conducted firstly and the target function and the constraint function of a workpiece were established to minimize the deviation from an ideal shape without violating forming limits of the material. Then, the optimization of electromagnetic forming force applied to the workpiece was carried out using response surface method. In addition, coil design was carried out for enhancing the efficiency of the process since DP980 steel sheet has low electrical conductivity and very high flow stress. The feasibility of the combined forming process was investigated by experiment. Experimental and numerical results and conclusions are demonstrated for the applicability of this combined forming process.