Investigation On Deformation Mechanism Of Aluminum Alloy During Porthole ECAP Spread Extrusion

Developing load-saving, low cost and high efficient spread extrusion ofaluminum alloy sheet-metal is not only urgently needed to form high-performancelight-weight wide plate in high speed railway and aerospace industries, but also thefrontier challenge of precision forming of high-performance light-weight component.To this end, this dissertation proposed an innovative princple to achieve theload-saving, low cost and high efficient spread extrusion of aluminum alloysheet-metal, which integrates the porthole extrusion, equal channel angular pressing(ECAP) and labor-saving flat container. In this dissertation, the deformation behaviorand microstrcuture evolution of6005A aluminum alloy were studied first, based onwhich a systematic and thorough investigation on the extrusion theory, die structuredesign, macro-deformation characteristics, microstructure evolution and weldingquality in flat container porthole ECAP spread extrusion of6005A aluminum alloysheet-metal was carried out using finite element (FE) simulation combined withtheoretical analysis and experimental study. A brief introduction to the project and itsmain achievements obtained are as follows.Hot compression tests were carried out on a Gleeble-1500device attemperatures ranging from300℃to500℃, strain from0.69to1.39and strainrates from0.01-1to10s-1to investigate the deformation behavior and microstructureevolution of6005A aluminum alloy. The results show that the flow stress reachssteady state near the strain of0.2when temperature is higher than350℃, while theflow stress decreased continuously with strain until the strain reaches0.7when thetemperature is lower than350℃. The stress values at350℃~500℃are greatlydifferent to those at300℃~350℃. Based Arrhenius equation, constitutive model anddeformation activation energy of the hot compression of6005A aluminum alloy wereextracted in the above two temperature ranges. The dynamic recrystallization occurs(DRX) during hot compression test of6005A aluminum alloy under certainconditions. DRX is not obvious at lower temperature (350℃), while obvious attemperature range of350℃~500℃. The critical equation, grain size and volumefraction models of DRX at350℃~500℃were established.By the combination of the characteristics of conventional porthole-welding extrusion, grain refinement in ECAP, better metal flow and load-saving of flatcontainer extrusion, a new flat container porthole ECAP (P-ECAP) spread extrusionprincple was proposed to achieve the load-saving, low cost and high efficient spreadextrusion of aluminum alloy sheet-metal. The feasibility of P-ECAP spread extrusionwas proved by the analysis of material flow, and the extrusion die for P-ECAPspread extrusion was designed. The FE model for P-ECAP spread extrusion of6005A aluminum alloy was established by developing the geometric model, materialmodel, meshing and optimizing, boundary conditions and other key technologies.The realibility of FE model was verfied by comparisions of extrusion load,temperature distribution and grain size between physical experiment and FE model.It laid the foundation for prediction of macro-deformation and microstructure duringP-ECAP spread extrusion of6005A aluminum alloy.The effect laws of extrusion ratio (λ), welding chamber size depth (d), angle ofchannel (α), billet temperature (T) and ram speed (v) on the macro-deformationduring flat container P-ECAP spread extrusion of6005A aluminum alloy sheet-metalwas investigated systematically. Based on the effect laws, the die structure and itsparameter were optimized. The results shown that the optimized die structure are:λ=14.0, α=145°and d=150mm. The extrusion load varies from60to80MN indifferent extrusion conditions, which is lower than that in the conventional extrusionmethod. The influence of T and v on DRX volume fraction, average grain size andDRX grain size were investigated detailed. The recrystalization volume fraction ofwide sheet-metal reaches about100%. The average grain size of wide sheet-metal is15.7μm~19.9μm, which is greatly smaller than that of conventional extrusionmethod. Metallographic and TEM analysis suggested that the refinement mechanismis DRX. Above, the load-saving, low cost, and high efficent extrusion of fine grainsize wide sheet-metal was realized by the proposed P-ECAP method.Major influence factors for welding quality in flat container P-ECAP spreadextrusion and welding process were analyzed by theoretical analysis. Hightemperature welding test device and experiment were designed. The influences oftemperature, time and stress on welding quality (tensile strength and microstructureof weld-joint) were investigated systemically. The results show that the weldability islinearly proportional with temperature, time and stress, based on which, a set ofwelding strength equation was established. The temperature, mean stress and welding time were investigated by FE simulation. The processing window was predicted bywelding strength equation and verified by experiment. Through optimization, theproduced wide sheet-metal without weld seam presents the longitudinal strength of315MPa, and the produced wide sheet-metal with weld beam presents the beamstrength of265.9MPa. The quality of wide sheet-metal was improved significantly.