| Semi-solid processing(SSP)is known as a new near net shape technology that involves the processing of semisolid alloys consisting of non-dendritic solid particles between solidus and liquidus temperatures,and it combines the advantages of easy formability of liquid metal and high quality of solid metal forming,and it also opens up a new road for development of high strength and toughness aluminum alloys.Now,Recrystallization and partial remelting(RAP)is used to produce semisolid billet at a high solid fraction.However,the formation mechanism and microstructural evolution of semisolid alloys via RAP are lack of a systematic understanding,and the microstructural control of semisolid alloys and its deformation behavior has always been the challenge of forming technology.In this study,7075 Al alloy was used as the research object.First,the semisolid billet was prepared by RAP method.The effects of different processing parameters(such as extrusion ratio,isothermal temperature and soaking time)on semisolid microstructure were summarized.Furthermore,the compression behavior of semisolid alloys at a high solid fraction was carried out,and the microstructural evolution during deformation processing of semisolid alloys at a high solid fraction was studied and the effect of topological structure on deformation behavior was investigated.It provided a theoretical basis for SSP.The microstructure during partial remelting of extruded 7075 Al alloys experienced four stages including the partially recrystallization,liquid phase formation,intergranular liquid phase penetration and grain coarsening.With an increase of remelting temperature,the grain size decreases first and then increases,and the sphericity is getting higher and higher.Moreover,with an increasing of soaking time,grains continuously coarsened,and the grain coarsening rate increases.But the sphericity could decreases when soaking time increased to 25 min.In addition,semisolid microstructures had anisotropic characteristics.Comparing to the microstructures along the longitudinal section(parallel to the extrusion direction),solid grains along the transverse section(perpendicular to the extrusion direction)had smaller size and higher spherical shape.With an increase of the pre-deformation extrusion ratio,solid grain size of semisolid alloys was smaller,and the grain coarsening rate would increase.However,when the extrusion ratio is increased to a certain extent,there were large amounts of solid grains with a large diameter along the extrusion direction.After solution treatment of 7075 Al alloy with an extrusion ratio of 25,solid grains of the semisolid alloys had a better sphericity.With the increase of deformation temperature and soaking time,the peak stress of the stress-strain curve was obviously decreased,apparent viscosity is also reduced,and there was a shear thinning phenomenon.A reduction in strain rate can also cause a reduction in peak stress.In the same conditions,as the extrusion ratio increases,its peak stress also increases,and the phenomenon was contrary to the decrease of average grain size of semisolid alloy due to an increase in extrusion ratio.As extrusion ratio increased,the longitudinal to transverse spherical ratio(M=SFLongitudinal / SFTransverse)decreased,and the grain was more irregular in three-dimensional shape,so the deformation stress increases.Therefore,not only the grain size but also the grain's three-dimensional sphericity affects the thixotropic behavior of semi-solid billet.Topological structure of semisolid alloys greatly influenced the deformation behavior.Peak stress of 90 o sample(perpendicular to the extrusion direction)was significantly lower than that of 0o sample(parallel to the extrusion direction).0o and 90 o samples have the same three-dimensional structure,but the orientation difference resulted in the difference of the way of stacking.Under the same stress it will lead to changes in the microstructural response,thus affecting the semisolid deformation behavior. |
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