| Cast Al-Si alloy has been widely used in piston material because of its good mechanical properties,wear resistance,low density,high specific strength,low thermal expansion coefficient and good casting performance.When the engine works,the piston operates at the temperature as high as 300?,and it also bears repeated alternating loads.Therefore,the research on high temperature strength and fatigue properties of piston Al-Si alloy has attracted much attention.In this paper,the multi-element alloyed eutectic Al-Si piston alloy was taken as an object of study,and the evolution and variation of its microstructures and mechanical properties with the prolongation of holding time were mainly studied after 25-400 hours holding at 300?.The fatigue properties of the alloy were also studied.The main results and conclusions are as follows:(1)Firstly,the modification treatment and technology of eutectic silicon in multi-component eutectic Al-Si alloy by Sr were studied.The best modification effect on eutectic silicon structure in eutectic Al-Si alloy is obtained by addition of 0.06wt.%Sr.The eutectic silicon is changed from coarse needle to coral and fibrous,and the shape factor is increased from 0.28 to 0.65.After modification,the average size of eutectic Si decreases dramatically from 38?m to 4?m.(2)The effects of heat treatment on the structure and properties of eutectic Al-Si alloys containing 1.5-3.5wt.%Cu,1.0wt.%Mg and 1.0-2.0wt.%Ni were studied.The results show that after heat treatment,the fibrous eutectic Si becomes independent granular.The local aggregation and average size of Si phase are reduced.The distribution of the Si phase become more uniformly in the matrix.After heat treatment,the metal compounds at the grain boundary of alpha-Al are decreased.And more amount of alloy element is dissolved into the matrix of alpha-Al.The hardness and tensile strength of the alloy are increased remarkably by solid solution strengthening effect.The optimum heat treatment parameters are 510?×6h solid solution and 185?×6h aging.The tensile strength of Al-12Si-3.5Cu-1.0Mg-2.0Ni(wt.%)is 332.34 MPa,Al-11Si-3.5Cu-1.0Mg-2.0Ni(wt.%)is 382.85 MPa,Al-12Si-1.5Cu-1.0Mg-1.0Ni(wt.%)is 299.84MPa.(3)The effect of high temperature and long time holding on the structure of eutectic Al-Si alloy containing 1.5-3.5wt.%Cu,1.0wt.%Mg and 1.0-2.0wt.%Ni was studied.It is found that the size of eutectic Si increases first and then decreases in the process of holding for a long time at high temperature,and the shape factor decreases first and then increases.The analysis shows that the size of eutectic Si increases first,because eutectic Si dissolved in high temperature insulation is precipitated and deposited on the existing Si particles,resulting in the increase of Si size and become massive.After a longer time of heat preservation,the bulk Si phase is dissolved and diffused for a long time.It dissolves in the matrix and nucleates and precipitates again during rapid cooling,so the size decreases.During high temperature and long time holding,intermetallic compounds in alloy continuously precipitate and aggregate,and Cu and Ni form needle-like Al-Cu-Ni or Al-Ni-Cu phases.(4)The effects of high temperature and long time heat preservation on the hardness,tensile strength and yield strength of the alloy were studied.The tensile strength of Al-12Si-3.5Cu-1.0Mg-2.0Ni(wt.%)alloy decreased by 44.1%,Al-11Si-3.5Cu-1.0Mg-2.0Ni(wt.%)alloy by 50.8%and Al-12Si-1.5Cu-1.0Mg-1.0Ni(wt.%)alloy by 44.6%.The decrease of tensile strength is mainly due to the precipitation and aggregation of alloy compounds and the coarsening of eutectic Si,especially the precipitation of alloy compounds.(5)The change of elongation of the alloy after holding for a long time at high temperature was studied.In the initial stage,eutectic Si coarsens with the prolongation of holding time,which leads to the decrease of elongation of Al-12Si-3.5Cu-1.0Mg-2.0Ni(wt.%)and Al-11Si-3.5Cu-1.0Mg-2.0Ni(wt.%)alloys.Then,with the prolongation of holding time,eutectic Si re-rounds,and alloy compounds precipitate,solid solution effect decreases,alloy plasticity increases,in addition,the crystallization of alpha-Al.Grain growth is also one of the reasons for the increase of plasticity.However,the elongation of Al-12Si-1.5Cu-1.0Mg-1.0Ni(wt.)alloy increases at first and then decreases,because the softening effect of alloy compound precipitation is greater than that of eutectic Si coarsening.(6)The fatigue properties of Al-12Si-3.5Cu-1.0Mg-2.0Ni(wt.%)alloy were studied.The fatigue strength limit of the alloy at 10~8 cycles is 128MPa.Three parts of fatigue fracture can be observed:fatigue source zone,fatigue propagation zone and instantaneous fracture zone.The fatigue source is located near the surface of the hole defect.In theory,if casting defects are reduced,the fatigue strength limit of the alloy can be improved. |
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