Research On Semi-solid Microstructure And Properties Of Hypo-peritectic CuSn20 Alloy

Cu-Sn alloys with high Sn content are widely used in transportation,aerospace,navigation and other high-speed heavy haul fields because of its high strength,low friction coefficient and good wear and corrosion resistance.However,due to the low content of peritectic phase and the existence of intergranular hard brittle?phase in conventional casting,it has poor deformation ability and low plasticity,which greatly limits its application.Therefore,in order to improve the mechanical properties of this series of alloys,the semi-solid slurry of Cu Sn20 alloy was prepared by melt confined flow induced nucleation device.The effects of pulping process parameters(melt treatment temperature?cooling channel angle and circulating water flow velocity)on the microstructure of semi-solid slurry were studied,and the optimal pulping process parameters were obtained,and then the effects of extrusion parameters(forming pressure and filling rate)on the microstructure and mechanical properties of the castings were studied.Finally,the effects of short-time solution treatment on the microstructure and properties of the castings were discussed.The research results are as follows:During the preparation of semi-solid slurry,with the increase of melt treatment temperature,the microstructure of semi-solid slurry changes from wormlike to rosette or nearly spherical,and the average grain size of primary?phase decreases,but when the temperature is too high,there are a large numbeu of dendrites in the microstructure of semi-solid slurry,the average grain size of primary?phase increases,and the shape factor increases with the increase of treatment temperature;with the increase of cooling channel angle,the microstructure of semi-solid slurry changes from vermicular to nearly spherical or equiaxed,the average grain size of primary?phase first decreases and then increases,and the shape factor first increases and then decreases;with the increase of circulating water flow velocity,the average grain size of primary?phase gradually decreases and the shape factor gradually increases.The results show that the optimum preparation parameters of semi-solid slurry are as follows:melt treatment temperature 960?,cooling channel angle 45°,circulating water velocity 540L/h,shape factor of primary?phase 0.746,average grain size 36?m,peritectic phase content 34.47%of the total volume.The mass fraction of Sn in the primary?phase center increases from 8.07%to 12.5%,and the mass fraction of Sn in the intergranular structure decreases from 26.94%to 24.41%,indicating that the solid solution strengthening effect was significant.The influence order of process parameters on the microstructure of semi-solid slurry is melt treatment temperature>cooling channel angle>circulating water flow velocity.The transient and intense chilling effect of the inner wall of the channel on the semi-solid slurry can inhibit the diffusion of Sn elements,which leads to the nondiffusive martensitic transformation of the peritectic?-Cu₅Sn phase to metastable peritectic?'-Cu_5.6Sn phase.The martensitic transformation makes the lattice structure change,from the body-centered cubic structure to the simple tetragonal structure.The obvious convex or tilting occurs in the deformation area of lattice,which leads to the formation of a large number of needle and lamellar like tissues,and the lamellar like structure is relatively large and few;the needle like tissues are staggered,free growing and have more numbers.The TEM further confirmed that the lamellar like and needle like structures were?'phase.In semi-solid rheo squeeze casting,when the filling rate is constant,with the increase of forming pressure,the number of defects gradually decreases,and the densityof the casting gradually increases,but the average grain size of primary?phase and elongation decrease first and then increase,and the shape factor?Brinell hardness and tensile strength increase first and then decrease;when the forming pressure is constant,with the increase of filling rate,the average grain size of primary?phase decreases first and then increases,and the tensity,tensile strength and elongation of castings increase first and then decreases.When the forming pressure is 130MPa and the filling rate is 21mm/s,there is no obvious shrinkage porosity and other defects in the microstructure of the casting.The distribution of primary?phase is uniform,the average grain size is 27?m,the shape factor is 0.765,the grain size is small,the shape is complete,and the comprehensive properties are the best.The tensile strength?elongation and Brinell hardness are 421 MPa?2.8%and 150.1 HBW,respectively,which are 28.9%?833%and 35.5%,higher than liquid squeeze casting and the fracture mode changes from brittle fracture to mixed fracture.The order of the effect of extrusion parameters on the microstructure and properties of castings is as follows:forming pressure>filling rate.The casting is treated at 750?for a short time,when the solution time is 10min,the primary?phase grows up,and the average grain size is 54?m,but the grains become sharp and merge with each other,which leads to a large difference in grain size;when the solution time is 20 min,the primary?phase grows up successively,and the average grain size reaches 79?m,but the distribution becomes uniform,and there is no obvious agglomeration and coarsening;when the solution time is 30 min,the primary phase grains agglomerate,and individual grains coarsen obviously,which leads to the coarsening of the grains,the size difference between the grains is larger,and the uniformity of the microstructure is decreased.When the solution time is 20min,the microstructure is the most uniform,the shrinkage porosity and other defects disappear completely,the tensile strength and elongation are 657 MPa and 11.4%respectively,which are 56.1%and 307.1%higher than those before solution treatment,respectively.The fracture mode changes from mixed fracture to ductile fracture.