Studies On High Temperature Fatigue Behavior And Mechanism Of Near-eutectic Al-Si-Cu-Mn Heat-resistant Alloy

Aluminum alloy,as the material to manufacture the cylinder block of engine,invariably undergoes complex cyclic thermo-mechanical loading.In this study,the high-cycle fatigue behaviors of Al-12Si-4Cu-1.2Mn alloy at room temperature and high temperature were investigated mainly by fatigue tests,microstructure observation and nanoindentation technology detection.In addition,the effect of 1.0wt%Ni alloying on high temperature fatigue behavior was also discussed.The fatigue strength?10⁶ cycles,the same below?of Al-12Si-4Cu-1.2Mn alloy at room temperature is up to125.0MPa,better than that of most heat-resistant aluminum alloys of casting Al-Si system,exhibiting excellent fatigue resistance at room temperature.At 350^?,the fatigue strength is 47.5MPa,comparable to Al-Si-Cu-Mg system alloy.Ni-alloying results in an increase in fatigue strength at 350^??52.5MPa?by about 10%increment,which is better than that of Al-Si-Cu-Mg system alloy?The fatigue crack initiation sites of Al-12Si-4Cu-1.2Mn alloy at room temperature and at350^? are different:casting defects?shrinkage cavities in eutectic regions?at room temperature and primary Al₁₅Mn₃Si₂ phase at 350^?.The great difference of elastic modulus between the primary Al₁₅Mn₃Si₂ phase and the surrounding?-Al phase is further increased with the temperature,as a result,the primary Al₁₅Mn₃Si₂ phase becomes the crack initiation site at high temperature fatigue.The high cycle fatigue process produces the plastic hardening effect and residual compressive stress in the phases,which have a certain strengthening effect on the phases.The?-Al phase in Al-12Si-4Cu-1.2Mn alloy is obviously softened at high temperature.Its residual stress value in the fractured sample of high temperature fatigue comes back to zero.However,primary Al₁₅Mn₃Si₂ phase,as the main high temperature heat-resistant phase introduced to the Al-12Si-4Cu-1.2Mn alloy,can maintain stable morphology at 350^?.The unique dendritic structure of this phase can effectively induce the path deflection during crack propagation and thus reduce the crack growth rate.As a result,the primary Al₁₅Mn₃Si₂ phase in Al-Si-Cu-Mn alloy becomes the overriding part to undertake stress during high temperature fatigue.Ni alloying leads to a great increment on the reduced modulus of?-Al₁₅Mn₃Si₂ phase,and further increases the modulus difference between?-Al phase and?-Al₁₅Mn₃Si₂ phase,especially at high temperature.For the alloy with 1.0wt%Ni addition,fatigue crack still originates at the primary Al₁₅Mn₃Si₂ phase near sample surface.Addition of Ni forms a series of new Ni-rich phases that strengthen the eutectic structure,and suppresses the recrystallization of?-Al phase at high temperature,and also reduces the amount of the residual network-shape CuAl₂ phase that has little contribution to the fatigue load distribution.Ni alloying changes the stress distribution among phases at high temperature,from that?-Al₁₅Mn₃Si₂ suffers the main loading in the Al-Si-Cu-Mn alloy into that all phases suffer the loading together in the Al-Si-Cu-Mn-Ni alloy.As a result,1.0wt%Ni addition increases the fatigue strength at 350^? by about 10%.