Plastic Deformation Behaviors Of Pre-strained Aluminum Under High Strain Rate Loadings

In recent years, the need to fuel economy and protect environment has led to theincreasing trend of extensive applications of lightweight structures and materials insuch advanced manufacturing fields of automotive, aeronautics and aerospace, forwhich, lightweight structural materials as represented by aluminum alloys have beencaused widespread concerns. Unfortunately, the viability of the press forming ofcomplex-shape aluminum parts is hindered by the fact that aluminum has muchlower formability at room temperature, while electromagnetic forming (EMF) andother high-speed forming technologies can significantly improve its plasticity atroom temperature. Electromagnetically assisted sheet metal stamping (EMAS) whichintegrates the high-speed forming advantages of EMF into the conventionalquasi-static stamping process has provide a new way for room-temperature precisionprocessing of complex-shape parts of aluminum alloys. Based on deformationcharacteristics of sheets during EMAS, in this work, the plastic deformationbehaviors of pre-strained5052aluminum sheet under high strain rate loadings areinvestigated by theoretical analysis, experiments and microanalysis, which will laythe foundations for the in-depth understanding of the dynamic deformationmechanisms of aluminum alloys and the promotion of engineering applications ofEMAS.According to the sequential deformation characteristics of quasi-staticpre-straining and high-speed dynamic re-deformation of sheets during EMAS, after ansystemic overview of the dynamic constitutive models of materials, the dynamicstress-strain data with different pre-straining and strain rates were obtained bySplit-Hopkinson Tension Bar (SHTB), the related dynamic materials responsecharacteristics were analyzed, and the dynamic constitutive models of5052aluminumalloy with pre-straining were established over further theoretical model analysis anddata processing. Results show that, under high strain rate loadings, the strain ratesensitivity of5052aluminum alloy sheets increases, with the increase of strain rate,the strain rate hardening effect of materials is strengthened. Under the same highstrain rate, the strain hardening effect increases with the increasing of pre-straining.For5052aluminum alloy, experimental data indicates that the Fields-Backofenconstitutive model can well describe its high strain rate materials responses with pre-straining.The dynamic plastic deformation mechanisms of pre-strained5052aluminumsheets were investigated by theoretical analysis, mechanical analysis, andmicroanalysis methods of SEM, TEM etc. Results show that, compared with thequasi-static deformation, the strength and plasticity of materials are improvedobviously after dynamic deformation, and pre-straining and high strain rate play acatalytic role in plasticity and strength. The larger pre-straining and the higher strainrate, the better plasticity and the higher strength of materials under high-speedloadings. Microanalysis indicates that dynamic deformation of the as-receivedaluminum also induces ductile fractures and dislocation slip mechanism of materials.While, different from the single-slip pattern of dislocations of quasi-staticdeformation, the dislocation system of dynamic deformation tends to more slips,large areas showing clear cross-slip structures, and much more dense and uniformdislocation configurations are also exhibited in dynamically deformed materials.Under high strain rate loadings, the existence of pre-strained structures is more likelyto induce cross-slip deformation and is conducive to coordinate the homogenizationof dislocation configurations.