Study On High Temperature Fatigue Properties And Strengthening Of Heat Resistant Cast Al-Si-Cu-Ni-Mg Alloys

Owing to their low density,high specific strength and stiffness,perfect casting performance and low thermal expansion coefficient,Al-Si-Cu-Ni-Mg cast aluminum alloys have been widely used in the field of engine pistons.Further increasing the output power of engine is an important way to solve the environmental and energy crisis,while the engine output power is strongly related to the performance of piston alloys at elevated temperature.Therefore,developing a new kind of heat-resistant Al-Si piston alloy is in strong demand,which will satisfy the service condition around300-400℃.Commercial cast Al-12Si-4Cu-2Ni-0.8Mg alloy（wt.%,M142）was chosen as the base material in this study.As-cast microstructure,evolution of mechanical properties and microstructure of M142 alloy after thermal exposure at different temperatures and time,as well as the fatigue properties and damage behavior at high temperature were systematically investigated.Furthermore,the strengthening methods were also proposed.Theα-Al phase was refined and strengthened by forming thermally stable nano-size Al₃Sc phase by introducing Zr,Ti,and Sc composite micro-alloying.The effect of micro-alloying on tensile and fatigue properties of M142 alloy at high temperature was studied.By investigating the effect of second phases on the tensile properties at elevated temperature,thermally stableε-Al₃Ni andδ-Al₃Cu Ni phases were selected and introduced through alloying.The effect of Cu-rich and Ni-rich phases on the high temperature tensile and fatigue properties of M142 series alloy were studied.Subsequently,the effect of microstructure refinement on tensile and fatigue properties of M142 alloy at elevated temperature was studied by employing high pressure die casting（HPDC）.Finally,three strengthening effects on tensile and fatigue properties for M142 series alloy at high temperature are compared.This study enriches the database of high temperature fatigue properties of piston alloys,and provides guidance for further development.Firstly,as-cast microstructure,evolution of mechanical properties and microstructure for M142 alloy after thermal exposure at different temperatures and time,as well as the fatigue properties and damage behavior at high temperature were systematically studied.Results show that the cast M142 alloy mainly consists ofα-Al,bulk primary Si,needle-like eutectic Si,Q-Al₅Cu₂Mg₈Si₆,θ-Al₂Cu,coarse Al Si（Mn,Fe）Ni Cu,δ-Al₃Cu Ni andγ-Al₇Cu₄Ni phase.When the alloy was exposed at 300-350℃,the Vickers hardness and tensile strength at room temperature decreased rapidly after 0.5 h,and then decreased slowly after the holding time was extended to 200 h.Increasing the temperature to 350-425℃,the trend for Vickers hardness and tensile strength variation is similar;however the decreasing is not that significant.In other words,the mechanical properties of studied alloy exhibit most degradation at 350℃.Therefore,the investigation at 350℃ is representative considering the evolution of microstructure and mechanical properties for M142 alloy.After thermal exposure at 350℃ for 200 h,the Nano sizeθand Q phases inα-Al are obvious coarsened,and the tensile strength at 350℃ decreases to 65.7±1.8 MPa from111.8±2.4 MPa for cast state.With the stress ratio of R=-1 and frequency of f=120Hz,the fatigue strength of cast M142 alloy is 41.5±1.5 MPa at 350℃（N=2.5×10⁷）.The debonding of coarse primary Si phase and Al Si（Mn,Fe）Ni Cu phase accelerates the fatigue crack initiation and propagation during high temperature fatigue cycles.Simultaneously,obvious coarsening of nanoscaleθand Q phases promotes the dislocation pilling up near brittle phases,leading to crack initiation and high temperature fatigue strength degradation.Secondly,to compensate the effect of coarseθand Q phases on high temperature properties ofα-Al,thermally stable nanoscale Al₃Sc phase was introduced to refine and strengthen theα-Al phase by means of Zr,Ti,and Sc composite micro-alloying.The effect of micro-alloying on microstructure and mechanical properties of Al-0.14Si-0.4Cu-0.05Mg,Al-12Si-0.4Cu-0.05Mg,Al-12Si-2Cu-2Ni-0.8Mg and M142 alloys was studied.Results show that nanoscale Al₃Sc phase precipitates fromα-Al and the grain size forα-Al decreases from 627±108μm for the base alloy to130±31μm after micro-alloying with Zr,Ti and Sc for Al-0.14Si-0.4Cu-0.05Mg alloy.The tensile strength at 350℃ increases from 20±1.8 MPa for base alloy to 35±2.1MPa for Zr,Ti,and Sc modified alloy,and the fatigue life dramatically increases from374 cycles to 247038 cycles under 20 MPa at 350℃.With the average radius of R=2.9 nm and volume fraction of f=0.35%,Nano size Al₃Sc phase obviously increases the tensile strength at ambient temperature from 152.7±3.8 MPa for the cast alloy to 201.1±2.8 MPa after holding at 350℃ for 200 h.Therefore,the precipitation of thermally stable and coherent Al₃Sc nanophase significantly improves the tensile strength and fatigue life at 350℃.After micro-alloying with Zr,Ti,and Sc,nanoscale Al₃Sc phase is still detected in Al-12Si-0.4Cu-0.05Mg alloy.Theα-Al grain size decreases from 1157±260μm for Al-12Si alloy to 265±55μm for Zr,Ti,and Sc modified alloy,and the average length for needle-like eutectic Si phase also decreases from 20.9μm to 10.7μm.The tensile strength at 350℃ increases from47.6±1.3 MPa for Al-12Si alloy to 58.8±1.6 MPa for Zr,Ti,and Sc modified alloy,and the fatigue life also increases from 550 cycles to 4250 cycles under 40 MPa at350℃.After micro-alloying with Zr,Ti,and Sc for Al-12Si-2Cu-2Ni-0.8Mg alloy,theα-Al grain size reduces from 973±240μm to 464±102μm,and the tensile strength at 350℃ increases from 79.6±1.6 MPa to 90.3±2.1 MPa.Finally,the effect of composite micro-alloying on M142 alloy was studied.The tensile strength at 350℃ increases from 97.3±2.1 MPa for M142 alloy to 108.2±1.5 MPa for Zr,Ti,and Sc modified M142 alloy,and the fatigue strength also increases from 41.5 MPa to 45.2MPa.In conclusion,composite micro-alloying with Zr,Ti and Sc was confirmed to be efficient for improving the tensile and fatigue properties of M142 alloy at high temperature.Thirdly,the effect of thermally stable micron sizedε-Al₃Ni andδ-Al₃Cu Ni phases on the tensile and fatigue properties of M142 series alloy at high temperature was studied by increasing Ni content.Results indicate that the tensile strength at350℃ monotonically increases from 88.1±1.1 MPa to 115.2±2.1 MPa when the volume fraction ofε-Al₃Ni phase increases from 1.1%to 8.2%.The relationship between the tensile strengthσ_UTS and the volume fraction ofε-Al₃Ni phase is established as follows:σ_UTS=4.02 f_v+82.91.The fatigue properties show a tendency of increasing before decreasing.With the volume fraction of 4.6%forε-Al₃Ni phase,the studied alloy shows better fatigue properties.Owing to obvious coarsening ofε-Al₃Ni phase,the fatigue properties were degraded when the content ofε-Al₃Ni phase is too high.Further introducing theε-Al₃Ni phase to M142 alloy,leading to the volume fraction ofε-Al₃Ni andδ-Al₃Cu Ni phase enhances simultaneously.The tensile strength at 350℃ increases from 97.3±2.1 MPa for M142 alloy to 105.6±2.1MPa for the modified alloy when increasing the volume fraction of（ε-Al₃Ni+δ-Al₃Cu Ni）phase increases from 6.77%to 10.38%,and the fatigue strength increases from 41.5 MPa to 42.9 MPa.The relationship between the tensile strengthσ_UTS and the volume fraction of（ε-Al₃Ni+δ-Al₃Cu Ni）phase was established:σ_UTS=5.85 f_v+49.36.Therefore,the tensile and fatigue properties for M142 series alloy at elevated temperature were improved by increasing the content of（ε-Al₃Ni+δ-Al₃Cu Ni）phase.Finally,to overcome the influences of coarse primary Si phase and intermetallic compounds on fatigue properties at elevated temperature,high pressure die casting（HPDC）was introduced to refine the microstructure for M142 alloy.The effect of microstructure refinement on the tensile and fatigue properties for M142 alloy was investigated.Moreover,the strengthening effect from microstructure refinement on tensile and fatigue properties are also compared with Zr,Ti,and Sc micro-alloying,and Cu-rich and Ni-rich intermetallic compounds strengthening.Results show that theα-Al grain size dramatically reduces from 781±170μm by permanent mold casting（PMC）to 32±6μm by HPDC,the length for eutectic Si phase decreases from5.4±1.7μm to 0.5±0.2μm,and the average of maximum length for intermetallics decreases from 5.2μm to 3.5μm.This indicates that the microstructure for M142alloy was significantly refined through HPDC.The tensile strength at 350℃ increases from 101.3±2.6 MPa by PMC to 135.4±1.7 MPa by HPDC,and the fatigue strength increases from 41.9 MPa to 50.8 MPa.Comparing the strength contribution from microstructure refinement,micro-alloying with Zr,Ti and Sc,and Cu-rich and Ni-rich intermetallic compounds strengthening,the enhancement in tensile strength at350℃ is 34.1 MPa,10.9 MPa and 8.3 MPa,respectively.Simultaneously,the fatigue strength increment is 8.9 MPa,3.7 MPa and 1.4 MPa,respectively.Therefore,the microstructure refinement greatly enhances the tensile and fatigue properties for M142 alloy at high temperature.