Study On The Creep Behavior And Strengthening Mechanism Of Cast Heat-Resistant Al-Si-Cu-Ni-Mg Alloys

Casting Al-Si alloys have been widely used in the automotive industry due to their excellent corrosion resistance,good casting property,friction and wear resistance,high specific strength,low coefficient of thermal expansion,and low density.Recently,with the development of the automotive industry,higher requirements have been proposed for the materials that used in the fabrication of piston,especially under higher temperatures and higher stresses.Current Al-Si-Cu-Ni-Mg alloys almost reach their upper limit of strength at elevated temperatures.Therefore,the development for new heat-resistant aluminum alloys is sought to serve at higher temperatures and stresses.The phase composition of the Al-12Si-3.8Cu-2Ni-0.8Mg alloy was studied to select thermal stable phases for subsequent alloy composition design.Mechanical properties including micro-hardness,tensile properties,and creep behavior were characterized through various instruments,such as computer data collection system,optical microscopy,inductively coupled plasma analyzer,image analysis software,differential scanning calorimetry,X-ray diffractometer,scanning electron microscopy,and transmission electron microscopy,etc.The phase composition of the Al-12Si-3.8Cu-2Ni-0.8Mg alloy consisteds of primary Si,eutectic Si,?-Al₃Cu Ni,?-Al₇Cu₄Ni,Q-Al₅Cu₂Mg₈Si₆,Ti₂Al₂₀Ce,and Al Si Fe Ni Cu phases at room temperature.The stress exponent of the as-cast Al-12Si-3.8Cu-2Ni-0.8Mg alloys at 250°C was 1.65 and the creep activation energy was 83.1 k J/mol.Therefore,the creep mechanism of as-cast alloy at low temperature and low stress(T?250°C;??40 MPa)was grain boundary creep.The sress exponent of the alloys at 350°C was 5.78 and creep activation energy was 240.9k J/mol.Therefore,the creep mechanism of the as-cast alloy at high temperature and low stress(T?250°C;??40 MPa)was dislocation creep.The creep stress exponent of the alloys did not change when exposure at 250°C,indicating that the creep mechanism was changeless during thermal exposure at 250°C.The Al-12Si-3.5Cu-2Ni-0.8Mg alloys with different microstructures were produced by permanent mold casting(PMC)and high pressure die casting(HPDC),respectively.Compared with the PMC alloy,the microstructures of the HPDC alloy was significantly refined.The the ultimate tensile strength(UTS)of the HPDC alloy at 350°C was 135 MPa,which was higher than that of the PMC.In low-temperature and low-stress regions,the minimum creep rate of HPDC alloy was higher than that of PMC alloy and the creep mechanism of both PMC and HPDC was grain boundary creep.In high-temperature and high-stress regions,the minimum creep rate of HPDC alloy was lower than that of PMC alloy and the creep mechanism of the PMC and HPDC alloys was dislocation climbing and dislocation slip,respectively.After thermal exposure at 350°C for 200 h,the eutectic silicon in both PMC and HPDC alloys exhibited different degrees of coarsening.The difference between the UTS of the HPDC alloy and that of PMC alloy at 350°C was minor,which was 68 MPa and 67 MPa,respectively.The minimum creep rate of the PMC alloy was lower than that of the HPDC alloy,indicating the creep resistance of alloys with refined microstructures was deteriorated after heat exposure.The effect of intermetallic compounds on Al-Si-Cu-Ni-Mg alloy(1)The Q-Al₅Cu₂Mg₈Si₆ phase was mainly dispersed in the?-Al.With increasing the mass fraction of Q-Al₅Cu₂Mg₈Si₆ from 3.2%to 6.4%,the UTS of the alloys increased from213 MPa to 275 MPa at room temperature.The UTS of the alloys increased from156 MPa to 200 MPa at 250°C.The UTS at 350°C increased only 9 MPa with increasing the Q-Al₅Cu₂Mg₈Si₆ phase,indicating that the Q-Al₅Cu₂Mg₈Si₆ phase strengthen alloys should be used below 350°C.With the mass fraction of the Q-Al₅Cu₂Mg₈Si₆ phase increasing from 3.2%to 6.4%,the creep stress exponent(n)at250°C fluctuated around 4.50,and the creep activation energy(Q)varied from 218k J/mol to 226 k J/mol.From the creep stress exponent and creep activation energy of the alloy,it can be concluded that the creep mechanism of the alloy was dislocation climbing.(2)The results showed that with increasing the volume fraction of?-Al₃Ni phase from 1%to 8%,UTS of the alloys increased from 179 MPa to 208 MPa,and the elongation decreased from 0.74%to 0.52%at the room temperature.During tensile at 250°C,the effect of?-Al₃Ni on the tensile properties of the alloy was consistent with that tensile at roomtemperture.With the increase of the volume fraction of?-Al₃Ni phase,the minimum creep rate of the alloy decreased obviously,indicating that?-Al₃Ni phase was effecient in improving the creep resistance of the alloys.The creep results showed that the creep mechanism of the alloy changed gradually from dislocation creep to grain boundary creep with increasing the volume fraction of?-Al₃Ni phase.(3)There were two kinds of?-Al₃Cu Ni phase in Al-Si-Cu-Ni-Mg alloy.One was lath-shape and the other was fishbone-shape,which was composed by many independent short rods.After thermal exposure at 350°C for 200 h,part of the short rod-shape changed to spherical-shape.The results of EDS and XRD showed that the short rod-shaped?-Al₃Cu Ni was transformed to globular?-Al₇Cu₄Ni during the heat exposure at 350°C.With the volume fraction of?-Al₃Cu Ni phase increased from 2.4%to 7.4%,the UTS of the alloy at 350°C increased from 49.2 MPa to 62.7 MPa,and the corresponding elongation decreased from 5.5%to 4.7%,indicating that?-Al₃Cu Ni phase was effective in improving the tensile properties of the alloys at elevated temperatures.Under the same creep conditions,the minimum creep rate of the material decreased by an order of magnitude with the increase of the volume fraction of?-Al₃Cu Ni phase,indicating that the?-Al₃Cu Ni phase was very effective in improving the creep resistance of the Al-Si-Cu-Ni-Mg alloy.With the volume fraction of?-Al₃Cu Ni phase increased from 2.4%to 7.4%,the stress threshold of the alloys increased from 11.8 MPa to 14.7MPa.The stress exponent of the alloys was in the range of 4?6,indicating that the main creep mechanism of the alloys was dislocation climbing.