Deformation Behavior And Mechanism Of Electromagnetically Assisted Sheet Metal Stamping Of 5052 Aluminum Alloy

In recent years, the need to improve fuel economy and protect environment has led to extensive application of lightweight structural materials as represented by aluminum alloys in the field of advanced manufacturing. Unfortunately, the viability of the press forming of complex-shape aluminum parts is hindered by the fact that aluminum has much lower formability compared with steel at room temperature. Electromagnetically assisted sheet metal stamping (EMAS) integrates the high-speed forming advantages of electromagnetic forming (EMF) into the conventional quasi-static stamping process, which has provide a new way for room-temperature processing of complex-shape parts from lightweight, hard-forming materials as represented by aluminum alloys. As this technique is still in laboratory of initial realization, the deformation behavior and mechanism have not been well systematically studied. In this dissertation, the deformation behavior and mechanism of EMAS of 5052 aluminum alloy sheets are systematically investigated by theoretical analysis, coupled-field numerical simulation, experiments and microanalysis.Based on the basic characteristics of EMAS, the theoretical basis of FEA of stamping-EMF process is presented, including the elastic-plastic theory of sheet metal stamping, both the electromagnetic-field theory and the elastic-plastic theory of structural filed of EMF, and the coupled-field theory of electromagnetic and structural fields. A"multi-step"loose coupling numerical scheme for EMAS is proposed based on the ANSYS Multiphysics/LS-DYNA platform. The dynamic links of the successive deformation process is realized through establishing user-defined subroutines.Based on the hybrid quasi-static-dynamic deformation characteristics of sheets in EMAS, a series of quasi-static-dynamic forming limit experiments are established under the typical strain states (uniaxial tension, plane strain and equi-biaxial tension) of conventional FLDs, and the dynamic behaviors of material under complex-loading conditions are systematically investigated in traditional FLD space. Results show that the formability of 5052 aluminum alloy sheet undergoing a quasi-static-dynamic loading process is dramatically increased beyond that exhibited in quasi-static deformation process, and a little higher than that obtained in the fully dynamic EMF process. Analyzing from the loading conditions, we may reasonably attribute the hyperplasticity effect of quasi-static-dynamic process to high-velocity deformation. And with the increasing of quasi-static pres-training, the hyperplasticity effect of dynamic process increases.The plastic instability mechanism and deformation behavior of dynamic deformation process in EMAS are investigated by theoretical analysis and microanalysis. Results show that inertia force plays an important role in dynamic forming, which has the suppression effect on structural instability and thus improves the formability of sheet and spreads instability. Micromechanism shows that the nature of dynamic deformation is much similar with that of quasi-static deformation and no special deformation structures arise in dynamic process for 5052 aluminum alloy sheets. The deformation mechanism of both processes is dislocation slip mechanism. For quasi-static deformation, the dislocations show a uniform single-slip pattern, fracture combined with dislocation tangling and climbing. While for dynamic deformation, dislocation system tends to more slips, large areas showing clear cross-slip structures. The dislocation bands are more narrow and dense than those shown in the quasi-static process, and a much more uniform dislocation configuration is also exhibited after pulsed magnetic loadings. The characteristics of multi-slips and uniform effect of dislocations under pulsed magnetic loading conditions will result in much higher plasticity and strength of materials. In order to analyze the deformation behaviors of typical EMAS technology process, a limit forming problem (fracture at bottom corner) in conventional cylindrical deep drawing process is analyzed and the electromagnetically assisted cylindrical deep drawing scheme is established accordingly. On this basis, the former proposed FEA scheme of EMAS is firstly applied to investigate the hybrid deformation behaviors and deformation laws of sheets in EMAS, and then the efficacy and characteristics of EMAS on improving the sheet formability are established and validated experimentally. Results show that the proposed FEA scheme can successfully simulate the successive deformation process of EMAS, and the deformation law is validated by the experimental results. The forming limit experiments of EMAS of cylindrical parts shows that the idea of EMAS can successfully exploit the advantages of improving room-temperature formability of EMF, and the EMF phenomenon can successfully be integrated into conventional stamping process. In the deformation process of sheets metal, the effects of inertia and tool-sheet interaction are very remarkable, which can suppress damage evolution and stabilize deformation.Based on the idea of EMAS, the technology experiments of electromagnetically assisted bending of U-shape parts are established, and the involved springback-control effect of pulsed magnetic force is analyzed. Results show that the way of applying the pulsed magnetic force to the bending area can effectively reduce the springback. The springback control of magnetic force in EMAS is mainly presented in two aspects: the role of magnetic force changing the strain distributions of bending area and the role of tool-sheet interaction effect of high-speed loading. Both processes can successfully reduce the spingback. During the bending process, the way of many times of small discharges can not only control the springback, but also improve the deformation quality of bending area.