| The CuSn10P1 alloy is easy to appear some casting defects such as intergranular segregation and inverse segregation.The low melting point liquid phase migrates to the surface through the solidified shell layer d ue to the solidification shrinkage force.The intergranular segregation and reverse segregation of tin elements greatly reduce the strength and plasticity of castings,also limit their application under low speed and heavy load conditions,such as axle sle eve in high-speed rail braking systems.Semisolid process can effectively control the solid-liquid interface morphology,interface propulsion speed,temperature field,and composition field etc.During melt solidification.The nearly equiaxed primary phase can be obtained.At the same time,the purpose of suppressing element segregation can be achieved.Based on the advantages of semisolid process technology and combined with the characteristics of peritectic reaction of CuSn10P1 alloy.The thermodynamic an d kinetic conditions of peritectic reaction are created,and the concentration of tin in primary phase is improved.Combined with the characteristics of semisolid slurry and reasonable design of rheological squeeze casting process.Preparation the axle sle eve with significantly improved strength and plasticity.In this paper,for the flow process in the melt constrained channel of CuSn10P1alloy,the ProCAST software and laminar turbulent flow Reynolds number were used to simulate the temperature field and calculate the laminar flow distance.Combined with the simulation and calculation results,a slit channel that constrains the melt flow is designed and manufactured.The channel is used to treat the melt and reveal the mechanism of microstructure evolution during its flow.The graphite crucible is used to collect the treated melt by the melt constrained channel,and realize the short-time similar isothermal treatment of the melt,revealing the migration discipline of tin in the similar isothermal process.An indirect rheological squeeze casting die was designed and manufactured to study the effect of forming process and solution treatment on the microstructure and properties of the axle sleeve.The melt flow temperature field was simulated at the initial melt temperature is1080°C and the flow thickness is 5 mm for the open sloping plate with a tilt angle is45°in the length is 300 mm.The results show that during the flow of the melt through the sloping plate,the lower surface temperature of the melt at the outlet is the lowest due to the heat conduction of the sloping plate,and the temperature of the melt thick layer only 0.7 mm below the liquidus.The cross-sectional area of the melt in the semisolid temperature range is approximately 15%,and an internal temperature difference is 85°C.The channel designed a s a slit flow channel,and the upper and lower surfaces simultaneously generate heat conduction to the melt.At the outlet,almost the entire cross-section melt temperature enters below the liquidus,and the internal temperature difference of the melt drop s to 60°C,which is in the semisolid temperature range.The cross-sectional area in the semi-solid temperature range was increased to 73.5%.In the slit flow channel,the laminar flow distance calculated by the Reynolds number is about 252.4 mm.According to laminar flow distance calculation and temperature field simulation results,a stainless steel melt-induced nucleation channel with a length is 400 mm,a thickness is 5 mm and a width is 100mm was designed and manufactured,and its upper and lower surf ace watercourses were designed for forced cooling.The optimal process parameters for preparing CuSn10P1 alloy semi-solid slurry through melt constrainted flow-induced nucleation channel are as follows:initial pouring temperature is 1080°C,cooling slope channel length is 300 mm and cooling slope channel angle is 45°.At this time,the primary?-Cu phase was transformed from the coarse dendrites by conventional metal mold casting into nearly spherical or equiaxed,with an average equivalent diameter of 46.6±5?m,and the shape factor is0.73,the number of grains in per unit area about 34900/cm2.The Sn element mass fraction in the center and edge of the primary?-Cu phase was increased to 1.87%and 14.09%,respectively,from 1.53%and 13.73%in conventional metal mold casting,and the Sn element mass fraction in intercrystalline structure was decreased from 27.94%to 21.14%.At the same time,the time when the same particl e of the melt flows through the channel was about 0.2s,the temperature dropped by 90°C,and the cooling rate reached about 450°C/s.At this time,the critical nucleation radius of the heterogeneous nucleation of the primary?-Cu phase was 14.33 nm,and the critical nucleation work was1.35?10-16J,which was 15%lower than that of the homogeneous nucleation.Melt constrainted flow-induced nucleation channel refinement mechanism of primary?-Cu phase:the melt flows through the constrainted flow-induced nucleation channel under strong chilling and gain super-cooling degree about 23.5°C,primary?-Cu phase is based on the channel wall,rapid nucleation and repeatedly to the free flow of melt inside.In addition,the primary?-Cu phase grews in a nearly spherical manner due to the uniform internal temperature field and the enrichment of the Sn element at the front of the solid-liquid interface.Rapid solidification causes Sn element to be dissolved in the primary phase without diffusion,and fine equiaxed or nearly spherical crystals with improved intergranular segregation are obtained.The slow cooling experiment of the alloy melt showed that the CuSn 10P1 alloy has a peritectic reaction platform near 990°C.The similar isothermal experiment of semisolid slurry at 990°C found that the local micro-thermal convection provides the kinetic conditions for the rotation and freeness of the primary?-Cu phase.The interface energy minimization principle to promote the merging and maturity of the primary?-Cu phase.At the same time,the concentration gradient drives the Sn element to diffuse from the liquid to the primary?-Cu phase,and improve overall content of the Sn element in the primary?-Cu phase and promotes the peritectic reaction.The mean equivalent diameter and shape factor and transition layer thickness of the primary?-Cu phase increase with the increase of the isothermal time.The Sn element mass fraction of the core and intergranular structure was positively and negatively exponential relationship,respectively,with similar isothermal time.In view of the uniform temperature of the lower part of the semisolid slurry after similar isothermal treatment,the bottom injection rheological squeeze casting method was adopted.Combined with the designed cross runner and inner runner to achieve further uniform cooling of melt and extrusion between slurry and reduce the hole in the axle sleeve.After similar isothermal treatment,the solid-liquid synergistic fluidity of the slurry,the axle sleeve compactness and elongation increase with the increase of the forming pressure at the same filling rate,while the equivalent diameter of the primary?-Cu phase decreases first and then increases.And the tensile strength increases first and then decreases.Under the same forming pressure,the solid-liquid synergistic fluidity increases with the increase of filling rate,the compactness and the tensile strength and elongation of the axle sleeve increased first and then decreased.The forming pressure is 100 MPa and the filling rate is 21 mm/s of semisolid squeeze casting,the comprehensive mechanical properties of the axle sleeve is the best.The tensile strength and elongation are 417.0 MPa and 12.6%,respectively,which are 21.9%and 90.6%higher than that of liquid squeeze casting.The difference of tin content between the surface and the core is about 0.35%,and the degree of reverse segregation is 61%lower than that of the liquid squeeze casting axle sleeve.After solution treatment?680°C+75 min?,the intragranular segregation was eliminated,and the tensile strength and elongation of the axle sleeve reached 439.5MPa and 38.5%,respectively,which are 5.4%and 210%higher than that of semisolid squeeze casting. |
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