| 7xxx series aluminum alloy,which exhibits low density and high strength,has already been widely applied in areas of aircraft components,automotive body and electrical packagings.At present,wrought aluminum alloys are manufactured by forging,extrusion or rolling.However,problems such as complex process and high resource and energy consumption can not meet the demands of high efficiency and environmental protection for modern manufacturing industry.Thixoforming,as one of the most inmportant semi-solid processing(SSP)technologies,combines the advantages of thixotropic features of semi-solid materials and plastic forming abillity of wrought alloys,provides a new route for the research and application of high performance wrought aluminum components.However,for conventional thixoforming process,problems such as hot crack tendency,recrystallization difficulty and poor spheroidization process seriously limit the applications of thixoforming on the wrought aluminum alloys.Severe plastic deformation(SPD)exhibits excellent grain refining effect and improves the materials strength,ductility and microstructural uniformility simultaneously.In this paper,to solve the problems in conventional thixoforming process,a new thixoforming route was put forward by combining the SPD and thixoforming technologies.The effects of grain refining on the microstructural evolution and mechanical properties of 7075 wrought alloys were analyzed.In this paper,the equal channel angular pressing(ECAP)was carried out to pre-deform the billet in the strain induced melt activation(SIMA)route.The mechanisms of microstructural evolution and the influences of processing parameters on the grain morphology in the processes of ECAP-SIMA and conventional recrystallization and partial remelting(RAP)route were analyzed.The results show that after homogenization process and four-pass ECAP deformation,the fraction of high-angle boundaires increases to 39.3%while the average grain size decreases to 2.31 μm.The average particle size increases with the increasing semi-solid isothermal temperatures and soaking time,the average particle size reaches 67.19-71.09 μm and the shape factor ranges 0.84-0.86 after soaking 610-620℃ for 15 min.ECAP-SIMA is an effective route for producing the fine and uniform equiaxed semi-solid microstructure comparing with the conventional RAP route under the same SSIT parameters.The coarsening mechanisms of solid grains in the SSIT process(570-630℃)for 7075 aluminum alloy were characterized.The effects of liquid fraction,deformation degree,processing route,alloy composition and the diffusion rate for alloying elements on the coarsening rate constant K were analyzed.The results show that four different stages for the variation K can be identified and the corresponding coarsening mechanisms are coalescence-agglomeration,inhibited Ostwald ripening by solid-solid,conventional Ostwald ripening and convective diffusion caused by sample shape change.When the liquid fraction fL varies between 0.28 and 0.51,the coarsening rate constant K is proportional to the value(fL)-2/3;when the fL varies between 0.07 and 0.28,a modified coarsening rate model is established.Larger distortion energy and the finer solid particle caused by increasing the number of ECAP passes of 7075 aluminum alloy lead to a higher coarsening rate constant.Wrought aluminum alloys exhibits lower coarsening rate than that of cast alloys because greater number of insoluble particle inhibits the migration of the grain boundaries.The coarsening rate for the alloys prepared by SIMA and RAP shows a lower value than that processed by cast or stirring.The tensile properties and the corresponding fracture mechanisms of 7075 aluminum alloy at room temperature and in the semi-solid state were researched.The results show that the soaking temperatures and holding time have significant influences on the tensile properties and fracture mechanisms at room temperature.The fracture type transforms from a mixture form containing cleaved facets and dimples representing both brittle fracture and ductile fracture to intergranular form determining complete ductile fracture with the increasing volume fraction of liquid in the semi-solid 7075 aluminum alloy.For the lower liquid fraction,the micro-shrinkage and micro-porosity occur along the frozen liquid boundaries and insoluable second precipitates.For the higher liquid fraction,the bonding strength of the interface of solid grains and eutectic mixing depends the strength of the semi-solid specimens.The tensile and fracture behaviors for 7075 alloy in the semi-solid state can be divided into three stages:solid dominating stage,combined influences of solid and liquid stage and liquid dominating stage.Three kinds of segregation were investigated for semi-solid 7075 aluminum alloy:the composition segregation within an individual solid particle,the intergranular segregation during the semi-solid soaking and the liquid macro-segregation during the thixoforming.The cup-shaped components were formed by the backward extrusion and the influence of extrusion temperature and extrusion ratio on the liquid macro-segregation was investigated.The results show that the liquid macro-segregation occurs due to the difference of the flow ability for the liquid and solid.Higher extrusion temperature and larger extrusion ratio result in the larger macro-segregation degree.D’arcy’s law and Kozeny-Carman equation can be used to predict the the segregation degree during the thixoforming.The segregation degree can be reduced by decreasing the grain size,shortening the semi-solid holding time and increasing the extrusion velocity.In this paper,the preparation of semi-solid wrought aluminum alloys,the coarsening kinetic during SSIT,tensile behaviors and fracture mechanisms at room temperature and in the semi-solid state,and the liquid macro-segregation defects as well as the controlling measures were investigated and analyzed.The results and conclusions in this paper offer some theoretical and experimental foundation for the development of thixoforming for the wrought aluminum alloys. |
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