Investigation On Tensile And High-cycle Fatigue Properties Of Piston Aluminum Alloy

Cast Al-Si alloys are widely used to fabricate engine pistons with complex structures due to their excellent fluidity,low thermal expansion coefficient,high thermal conductivity,high strength-to-weight ratio,high wear resistance,high corrosion resistance,and so on.As is well known,pistons suffer high temperature and cyclic load at service which require them possessing outstanding high-temperature strength.As modern engines are going to be more powerful,operating temperature of piston will become higher which combustion chamber edge may be up to 425?.Enhancing high temperature mechanical properties is a key factor to ensure their long time service.According to this situation effect of temperature on microstructure and tensile properties,influences of second phase and strain rate on tensile properties,high-cycle fatigue crack initiation and propagation mechanism evolution as temperatures of Al-Si-Cu-Mg-Ni alloys which are used to produce pistons were investigated.Quantitative relationship between tensile strength and temperature,defect size and fatigue strength were build.The purpose of this thesis is to investigate the damage mechanisms of high temperature tension and high-cycle fatigue,figure out the dominant factors affecting static and dynamic properties of piston aluminum alloy and provide a reference for the optimization of mechanical properties.This thesis mainly includes the following results:1.Effects of temperature on tensile damage and tensile properties were investigated.The fracture mechanisms of piston alloy are quasi-cleavage below 350?and coexistence of quasi-cleavage and dimple above 350?.Since there are still a lot of precipitates in Al matrix between room temperature and 250? resulting in high strength and low ductility,fracture often occurs at work-hardening stage.The ductility of Al matrix is improved above 300? and the fracture occurs at the work-softening stage.The bifilm can cause decrease in strength below 300? while it has little influence on strength above 300?.It leads to cracking of grain boundary and phase/matrix interface during tension.The decrease of tensile strength may be attributed to the reduction of force area caused by bifilm.The variation of tensile strength with temperature can be divided into three stages,i.e.,drop lowly,drop repidly and then drop lowly again.The two slow change stages result from the strain softening,and the fast one is associated with combination of strain softening and precipitate dissolving.An empirical equation is proposed to describe the relationship between tensile strength and temperature.2.Effects of thermal exposure on tensile properties at room and high temperatures were investigated.The HRB hardness decreases rapidly firstly and then maintains stable with increasing exposure time.The decrease of hardness at 200-350?is attributed to the coarsening and dissolving of precipitates,while its evolution at 425? results from the combination effect of precipitate dissolving,solution strengthening and natural aging.Quantitative relationship between exposure parameters and strength or hardness is established based on the aging precipitating model.The tensile damage and fracture mechanisms at room temperature are confirmed by in-situ tension experiments.Second phases cracking is the damage mode and the alloy will fail when the damage accumulates to a certain degree.The damage and fracture mechanisms at room temperature are not be affected by exposure condition.Based on the fracture pattern a model predicting the tensile strength at room temperature by the fraction of second phases and matrix microhardness is proposed.Precipitates coarsening and dissolving also account for the evolution of high temperature tensile properties after exposure.It has more impact on yield strength than tensile strength.3.Effects of second phases and strain rate on tensile properties were explored.If the matrix strengths are identical,the tensile strength at room temperature should be determined by size of second phases.It decreases with increasing size.Tensile strength at 300? is predominately affected by total volume fraction of all second phase,it increases with increasing fraction.Tensile strength at 350? increases with volume fraction of eutectic Si and intermetallics net.Elongation to fracture is solely associated with size of second phases.The tensile strength increases with increasing strain rate in the range of 0.0001-0.1 s-1.However,when it increases to 1 s-1,the tensile strength at 250 and 300? decreases rapidly,and at 350? it keeps the same as that at 0.1 s-1.An equation predicting high temperature strengths is developed based on the constitutive equation about the flow stress at different strain rates.It is proved to be suitable for the strength prediction at 250-425?.4.Evolution of high-cycle properties and damage mechanism with temperature was carried out.Casting defects are the most preferential sites for fatigue crack initiation at room and high temperature.They are located at the surface of the specimens at room temperature,while at high temperature they can be distributed inside of the specimens besides surface.Fatigue crack growth mechanism depends on temperature.It propagates through Al matrix and second phases with equal chance at room temperature.Al matrix is easy to provide crack propagation path at 350? and the interfaces at grain boundary and phase/matrix are the preferential growth path at 425?.Assuming initiating defects as cracks,a model predicting fatigue strength by defect size is proposed based on the small crack growth.It provides well estimation of fatigue strength below 350? while not be suitable for that at higher temperature.