| During the conventional casting process,the CuSn10P1 alloy is easy to appear some casting defects such as intergranular segregation,even inverse segregation,which result in poor plasticity and low strength of castings.During semi-solid forming process,the spherical primary phase will give some intricate migration channels between the grains to hinder the migration of low melting point liquid phase.At the same time,it will contribute to prevent the intercrystalline segregation and inverse segregation in the castings,that the solid phase fraction in the semi-solid slurry and the tin element content in solid phase increase.But the tin-rich liquid phase in the semi-solid slurry is of a low melting point and a low viscosity,which is beneficial to lubricate soild phase grains.Therefore,between the CuSn10P1 alloy and the other alloys,such as aluminium alloy and magnesium alloy,there are some marked differences to control the flowing of liquid incorporating solid particles and macrosegregation of tin element in the mold filling process.In this paper,the technologies are sought that is to obtain CuSn10P1 alloy semi-solid slurry of high solid phase fraction,and to steadily control the tin element segregation.And the flowing and mold filling behaviors of the semi-solid slurry are researched,during thixotropy process,the interaction mechanism is revealed between solid phase and liquid phase.These studies can provide theoretical basis for semi-solid forming process plan formulation of CuSn10P1 alloy.In this paper,the CuSn10P1 alloy semi-solid slurries of hige solid phase fraction are obtain by cold rolling-partial remelting process.The thermal physics simulation tests of semi-solid slurry mold filling are carried out,in order to research the thixotropy deformation behaviors of slurriy under the uniaxial compression witiout restriction conditions,and to research flowing features of slurriy under the mould restriction conditions.Be guided through the simulation results,for the production of axle sleeves in CuSn10P1 alloy,the mould with a baffle ring and spring is designed and manufactured,and the semi-solid squeeze casting tests are carried out.With the help of optical microscope,scanning electron microscope,EBSD and transmission electron microscope,and so on,it is investigate that the effect of microstructure characteristics on microstructure evolution during cold rolling process,such as tin element distribution and dislocation density variation in as-cast samples,etc.It is analyzed that the effect of microstructure characteristics on semi-soid microstructure formation duting remelting process,such as tin element migration and recrystallization of primary phase in cold rolling samples,etc.And the mechanism of semi-soid microstructure formation is ulteriorly illuminated during cold rolling-partial remelting process.Under the thermal physics simulation conditions with different strains and stresses,the interaction characteristics is investigated between solid phase and liquid phase during flowing process of CuSn10P1 alloy semi-solid slurry.It is analyzed that the relations between stress-strain characteristics and solid-liquid separation,such as peak stress and corresponding strain value.The marked differences about mold filling behaviors of semi-solid slurries are illuminated between the CuSn10P1 alloy and aluminium alloy.And the effects of microstructure uniformity axle sleeves on plasticity and strength are investigated.The experimental results of four passes cold rolling show that the deformation starts from surface of a secondary dendrites,and the tin element content in these area is high.With accumulated deformation increases,the deformation extend into interior of secondary dendrites from surface,and the primary dendrites paralleling to each other gradually turn or slide.Because of the deformation,a lot of dislocations are stored into secondary dendrites,and the defects are formed such as dislocation tangle,slip band and twin crystal.The primary dendrites have a is relatively low dislocation density,and their deformation degree is relatively small.The experimental results of remlting in solid-liquid temperature interval show that the a primary dendrites merger interdendritic eutectoid a phase,for the sample of accumulated deformation about 20%in an early stage of remlting(about 1 min).With the holding time increasing to about 3 min,the static recrystallization happens in deformation a phase,so that the tiny polygonal recrystallized grains and annealing twins form.As holding time increasing to about 5 min,the annealing twins grow further.Meanwhile,the tiny a grains grow and merge,the intergranular mixture melt into liquid phase,and the liquid phase gather to borders among grains.During holding for about 5～8 min,the a short dendrites gradually necking,a part of rough interface of grains melt and become smoothness,and liquid phase gradually increase and connect with each other to form a network,and wrap the α grains simultaneously.When remelting to about 8～10 min,the a short dendrites fusing and transform into spheroidal grains,and the liquid phase fraction increase.When During holding for about 15 min,the semi-soid slurry of solid phase fraction 70～80%basically is gained,and the solid phase grains are rounding.During the remlting process,the tin element content of liquid phase generally have a downtrend,the tin element diffuses into interior of a phase.The tin element content of a phase increases,and the intergranular segregation decreases.Under the uniaxial compression witiout restriction conditions,the results of thermal physics simulation of semi-solid mold filling show that the solid phase grains close to each other in the center of deformation zone,the liquid phase extrude and flow to periphery of deformation zone,result in severe solid-liquid separation and solid phase grains plastic deformation.The liquid phase segregate in periphery of deformation zone.With strain rate increasing,the grain frameworks are depolymerized by rapidly flowing liquid phase,the degree of flowing of liquid incorporating solid grains increases in periphery of deformation zone.Under the mould restriction conditions,the results of thermal physics simulation show that the liquid phase segregate in the front of semi-solid slurry flowing direction,and the degree of flowing of liquid incorporating solid grains increases with filling rate increasing.In the back of slurry flowing direction,the solid phase segregate.And the separation between liquid and solid phase still is inevitable with filling rate increasing.When temperature increasing,the liquid phase fraction in semi-solid slurry increases,the strength of grain frameworks decreases,the stress to depolymerize grain frameworks decreases,result in the compressing peak stress decreases linearly.As strain rate increasing,the proportion of grain frameworks participating in deformation increases,and the solid grains cannot adequately and timely coordinate with each other through turn and deformation,so that it need a larger stress to depolymerize the grains frameworks,and the compressing peak stress also increases.The true stress-strain curves of CuSn10P1 alloy semi-solid compressing appear more peak at strain rate 10 s-1,it corresponds depolymerizing of grain frameworks repeatedly during semi-solid slurry flowing process.Therefore,the degree of flowing of liquid incorporating solid grains increases in the front of semi-solid slurry flowing direction.The compressing peak stress and corresponding strain value of CuSn10P1 alloy are much higher than that of aluminium alloy.It proves that the strength of grain frameworks in CuSn10P1 alloy semi-solid slurry is much higher than that of aluminium alloy,and it is more difficult and need consume more time to depolymerize the grain frameworks in CuSn10P1 alloy semi-solid slurry.Moreover,the viscosity of CuSn10P1 alloy liquid phase is much less than that of aluminium alloy liquid phase.So that before the grain frameworks depolymerizing,in CuSn10P1 alloy semi-solid slurry,there are more liquid phase have flowing longer distance.And after the grain frameworks depolymerizing,the liquid phase is easier to flow round solid phase grains.Therefore,compare with aluminium alloy semi-solid slurry,the CuSn10P1 alloy semi-solid slurry is more difficult to realize the flowing of liquid incorporating solid grains,only through increasing the filling rate.When the axle sleeves in CuSn10P1 alloy are produced by semi-solid squeeze casting process,it can obviously improves solid-liquid separation that adding a baffle ring and a spring in mould.With the forming pressure decreasing or the extrusion rate increasing,the microstructure uniformity of axle sleeves improves,the degree of flowing of liquid phase incorporating solid particles increases,and the strength and plasticity of axle sleeves increase.Under the forming pressure 250MPa,the extrusion rate 15mm/s,the microstructure uniformity of axle sleeves is the best,and the strength and ductility of axle sleeves by semi-solid squeeze casting process are respectively 371.1 MPa and 8.4%,and respectively increase 57.3%and 78.7%than that of axle sleeves by die casting process. |
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