| Targeting to meet the demand of lightening for automobile, using light weight alloys such as magnesium alloys and advanced high strength steels(AHSS) to form the auto parts has been obtained more and more attention in recent years. However, due to the close-packed hexagonal(HCP) crystal structure, magnesium alloys possess poor plasticity at room temperature. Due to the high flow stress, AHSS needs high forming force. In addition, the relative lower elastic modulus of magnesium alloys and higher yield strength of AHSS lead to serious spingback after bending. These problems seriously hinder the wide application of the light weight alloys in automobile industry. Developing new stamping processes are urgently needed to forming these light weight alloys.Electroplastic forming technology introduces electric current to the materials’ deformation zone so as to decrease the deformation resistance and increase the material plasticity. This technology has already been successfully applied in wire drawing, sheet metal rolling processes, etc., and has achieved favourable results. Meanwhile, electric current shows positive effect in grain refinement, damage repair as well as surface quality improvement. Therefore, it is quite promising to apply the electroplastic forming technology into the stamping process of the light weight alloys.Although the technology has been studied over the past few years, investigation of the relative theoretical models and the application of the electroplastic forming technology is still not enough. In the theoretical model aspect, some researchers proposed the flow stress models from the point of “physical mechanisms”. However, these models just aim to describe the physical significance of this technology and are not convenient to be used in engineering application. On the other hand, the exploration of the electrically-assisted stamping processes has just been initiated and the realted theories and technologies need to be further studied. Aiming at the above mentioned problems, this paper carries out the following tasks:(1) The influence of the electric pulses on the material flow behavior was studied through uniaxial tension of AZ31 B magnesium alloy and DP980 AHSS and found that the electroplastic effect(EPE) could decrease the flow stress and improve the plasticity for AZ31 B magnesium alloy while negative EPE was found for DP980 AHSS. Based on microstructure analysis, the observed phenomenon was explained and the influence of the electric pulses on microstructure was clarified. By modifying the Johnson-Cook flow stress model, a flow stress model considering the influence of the electric pulses was established for AZ31 B magnesium alloy. The proposed model was then validated by experimental data.(2) The influence of the electric pulses on the material stress relaxation behavior was studied through stress relaxation of AZ31 B magnesium alloy and QP980 AHSS. The influence of temperature on the stress relaxation of QP980 AHSS was analyzed and the suitable stress relaxation temperature was indicated. The mechanism of EPE on relaxation was explained by microstructure analysis. Based on the fundamental theory of creep mechanics, the stress relaxation models considering the influence of the electric pulses were deduced and established for AZ31 B magnesium alloy and QP980 AHSS. The proposed models were experimentally validated.(3) Based on the plain strain condition and the assumption that the materials obey the Mises yield criterion, through extending the stress relaxation model deserved in uniaxial stress state to multi-axial stress state, the springback prediction models considering the influence of the electric pulses were established for AZ31 B magnesium alloy and QP980 AHSS. The predictive ability of the models was then experimentally examined. The mechanism of the electric pulses assisted springback suppression was also discussed through microstructure analysis.(4) Based on the virtues of reducing the flow stress and improving the forming deformability, some electric pulses assisted plastic forming processes were developed, including electric pulses assisted cylindrical deep drawing process, roller hemming process and hole expansion process. During designing the experimental device, the forming movement, the current flow route as well as the insulation unit were well guaranteed. The material deformation capacity was tremendously improved with the help of the electric pulses. The electric pulses assisted forming technology supplies a new candidate for forming of materials with poor plasticity or hard to be deformed materials and is very promising in application. |
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