Effect Of Heat Treatment On Microstructure And Properties Of Hypereutectic Al-Si Alloy Prepared By CDS

Hypereutectic Al-Si alloy belongs to high strength casting Aluminum Alloy,due to its low density,good thermal conductivity,suitable strength,hardness and good wear resistance,which is used to manufacture friction resisting components in automobile,aerospace manufacturing and processing industr y.However,in the solidification process of alloy molding,the solidification temperature range of alloy is larger,when the temperature drops to near solidus temperature,alloy eutectic reaction,precipitation of coarse plate-like primary phase and long acicular eutectic phase in the alloy is nonuniform so that they can produce easily stress concentration and fragmentation of matrix,which may have a major impact on mechanical properties,reduce the machinability of materials and limit its large-scale production application.Grain refinement can improve the segregation of the aluminum alloy ingot,reduce the hot cracking tendency of the castings and improve the plastic deformation ability of the material.The hypereutectic Al-Si alloy was prepared by controlled diffusion solidification technology,the primary?-Al in the alloy possesses fine non dendrite morphology.Therefore,the alloy has good shrinkage effect,which can reduce the generation of defects such as hot cracking and shrinkage porosity.The all oy is then heat treated.In this paper,the hypereutectic Al-Si alloy was prepared by controlled diffusion solidification process.The morphology of primary Si phase and eutectic phase in the solidification microstructure of CDS?Controlled Diffusion Solidification?hypereutectic Al-Si alloy and the distribution of alloy elements in each phase were studied by heat treatment.The effects of different heat treatment methods on their microscopic structure were studied by XRD,SEM and EDS,respectively.The mac ro hardness is measured,then the standard samples with different heat treatment parameters are tested and the microscopic appearance of fracture is observed.Finally,the wear resistance of the sample is measured by friction and wear machine.The effects of different heat treatment methods on the microscopic structure and properties of hypereutectic Al-Si alloy were investigated by tensile test,hardness measurement and wear resistance test.The main conclusions are as follows:?1?Solid solution treatment can significantly improve the microstructure of CDS cast Al-18Si alloy,the effect of solution treatment on eutectic silicon is the larger than its effect on primary silicon in the solution treatment process,the primary Si size decreases from 34.23?m to 20.78?m;eutectic Si phase from acicular grain gradually for nearly globose,size decreases from 25.57?m to 5.44?m,and the distribution tends to be uniform.?2?Al-18Si alloy still maintained that hardness ranged from 34 to 35.5HRB under different solution treatment conditions)which is not sensitive to the change of the hardness of the Si morphology size;After 570?/6h solution treatment,alloy properties has improved significantly,the tensile strength reached 154.27MPa,compared to the as cast up about 21.1%;solution treatment enhanced the wear resistance of the alloy,and the alloy wear rate and friction coefficient than as cast was decreased.?3?CDS casting Al-18Si-5Cu-0.5Mg alloy after solid solution treatment,the solidification process of generating residual phase can be fully dissolved,the segregation phenomenon is also weakened obviously after solution treatment,with the increase of aging time,precipitation phase increased gradually until it reached a certain level but the Al₂Cu,Mg₂Si and Al₂Cu Mg phase present the phenomenon of growing gradually.When the solution system is 500/4h,and the aging system is 165?/12h,there are a large number of precipitates of Al ₂Cu,Mg₂Si and Al₂Cu Mg in the alloy.?4?After heat treatment,the hardness of Al-18Si-5Cu-0.5Mg alloy has been greatly improved.When 500?/4h solution treatment and 165?/12h aging treatment,Rockwell hardness reached the maximum value of 70.3HRB,which increased by 10%than the sample without heat treatment.With the increase of the holding time and temperature,hardness of the alloy decreased.After T6 heat treatment of the primary Si and the eutectic phase in the aging stage,the size of the second phase strengthening phase precipitation was reduced,the performance was improved significantly with the solid solution 500?/4h degrees and heat aging at 165?/12h process parameters,the tensile strength was 199.5 MPa,cast alloy increased by about 87.4%;the wear resistance of the alloy was improved significantly after T6 heat treatment.