| ECAP has been attracted considerable attention for it produces ultrafine-grained materials of grain sizes in a range0.1-1um with unique physical and mechanical properties. ECAP has a potential for applications in industry. Dual Equal Channel Lateral Extrusion (DECLE) is a new severe plastic deformation technology which based on ECAP. DECLE will enrich the ECAP process, at the same time, and it can provide a new method for the preparation of symmetrical ultrafine metal material. Up to now, there were very few report on producing ultrafine-grained metallic materials with DECLE.7000high strength aluminum alloy has excellent performance. So, it is widely used in aerospace, automobile, bridge and building areas. So far, there was lack of the reports which about enhancing the strength and toughness of Al-Zn-Mg high strength aluminum alloy using ECAP process.In this paper, according to varying grid of physical grid simulation during DECLE deformation, the macro-mechanical parameters were calculated. It is very important academic value for DECLE analysis. The DECLE process were systematically simulated by DEFORM software. It provides guidance for experimental die design and analyzing experimental datas. The homogenizing7003aluminum alloy were subjected to DECLE deformation at different routes and different passes, and then the deformed specimens were annealed. The deformed samples and annealed samples were investigated by x-ray diffraction (XRD) and transmission electron microscope(TEM). The evolution of microstructure and the second phase were observed, the grain refinement mechanism and recovery and recrystallization mechanism were discussed. The deformed samples and annealed samples were carried out on mechanical testing. It were compared to the traditional heat treatment method to improve the strength of7003aluminum alloy. The strengthening mechanism during DECLE process, the fracture were investigated by scanning electron microscope (SEM), the fracture mechanism was discussed. The main research contents are as follows:(1) Two different geometric models are builded in order to solving the per pass the shear strain during DECLE. Establishing two different slip line field models obtain the extrusion pressure of the DECLE deformation.(2) Physical grid simulation and finite element numerical simulation are conduced for DECLE. The deformation characteristic of DECLE is comprehensively explained, and discusses the main process parameters influence on the load, equivalent strain and equivalent stress under DECLE deformation. The process parameters include the friction coefficient, inner corner radius, back pressure, the number of ECCLP passes and pressing routes. The results show that deformation of the DECLE is not uniform. Increasing the friction coefficient, adding a back pressure and increasing the number of ECCLP passes conducive specimen deformation uniformity. Increasing the inner corner radius is not beneficial for grain refinemen. It easily lead to waste and crack during defromation. The uniformity of microstructure distribution and the refining effect of B route is better than A route. The simulation results can avoid cracks during DECLE.(3) The homogenizing7003aluminum alloy were subjected to DECLE with A path (not rotating or rotate180°) and B path (clockwise or anti-clockwise rotation of90°) for4passes at room temperature, separately. Ultrafine grains were obtained.The grain size of A route is200nm, B path’s is150nm, the refining effect of B route is better than A route. The microstructure of different regions after one pass shows DECLE deformation is not uniform. During DECLE defromation, the shear bands of A route easily bend and broken, The shear bands of B route easily cross breaking. As the passes increase, the second phase were successively splited refined and distributed in grains and grain boundaries. Second phase and grain refinement mechanism is dislocation segmentation mechanism.(4) To eliminate the microstructure instability after deformation, The DECLEed sample with4passes B route annealed at different temperatures for0.5h. The recovery and recrystallization nucleation and growth mechanisms of DECLEed sample were Observed by TEM. At recovery stage, high density dislocation wall polygons into sub grains, the subgrain grow up with mechanism of grain boundary bulge and mechanism of gain boundary merge; At recrystallization stage, nuclei formed at high strain zone and the second phase surrounded throughstrain induced grain boundary migration and subgrain coalescence coarsening.then nuclei grow up. The second phase MgZn2increased precipitation and grew with annealing temperature increases, and then disappeared after240℃.(5) After four passes DECLE deformation, the hardness increased from60.01HV to94.98HV, the yield strength is214.8MPa which increased by2.03times, the tensile strength is257.3MPa which increased by1.55times, the elongation is15%with A route; the hardness increasedto98.3HV, the yield strength is186.3MPa which increased by1.63times, the tensile strength is279.1MPa which increased by1.94times, the elongation is15.6%with B route. The samples with B route after four passes DECLE were treated at80℃for0.5h, the tensile strength is314MPa which increased by12.5%, the yield strength of294.9MP which increased by58.3%, the elongation is14.4%. At the same pass, the mechanical properties of DECLEed were higher than that of ECAPed. To improve the strength and toughness of7003aluminum alloy,"homogenization annealing+severe plastic deformation (DECLE)+low temperature annealing" process has the advantage than the traditional heat treatment "solution and aging" process.As the passes increas, the the morphology of tensile fracture has change from cup cone fracture to pure shear deformation. At the same passes, the number dimples of B route is more than that of A route, dimple small and shallow. Tensile fracture mechanism of deformation of the specimen is shear fracture mechanism, the fracture morhpology is related to the arrangemnent of the shear bands. |
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