Thermal Exposure Microstructures And Fatigue Properties Of Novel Aluminum Based Alloys

In order to research thermal stability and fatigue property of novel aluminum based alloy, the microstructures and properties of Al-13Si-xCu-1.0Mg-2.0Ni alloys with the different Cu contents,which were solution and aged first,then thermal exposured at 350℃and 400℃for different hours (0h～1000h),were studied using combination methods of thermal exposure, microstructural analysis and mechanical properties and fatigue S-N curve test.The results show that:With the elongation of thermal exposure time,the strength decreased obviously of the Al-13Si-xCu-1.0Mg-2.0Ni alloys,the strength declined faster in 0h～10h and tended to be stable after 10h.the 5%Cu alloy strength of room-temperature and high-temperature from 358 MPa and 85 MPa dropped to 210 MPa and 50 MPa when 10h.However,the elongation of the alloy increased quickly,and it rised faster in 0h～100h and tended to be stable after 100h, the 5%Cu alloy elongation of room-temperature and high-temperature from 0.5% and 2.5% increased to 2.8% and 8.9% when 100h.The residual strength and fatigue property of the alloy after thermal exposure at 400℃was more than it at 350℃about 15MPa and SMPa.With the change of the Cu content,the residual strength of alloys after thermal exposure was Al-13Si-4.0Cu-1.0Mg-2.0Ni>Al-13Si-5.0Cu-1.0Mg-2.0Ni>Al-13Si-3.0Cu-1.0Mg-2.0Ni。With the elongation of thermal exposure time, the primary silicon of Al-13Si-5.0Cu-1.0Mg-2.0Ni alloy structure grew up and the eutectic silicon didn't change, which aroused the little influence on the residual strength of the alloy.The Q phase gradually grew up. which resulted in the siguificant influence on the residual strength of the alloys. The strength decreased of the alloy was due to the transformation ofθ"â†'θâ†'θphase lead to the change of phase structure.The fatigue S-N curve was fitted by using the method of the composition and the up and down method, which showed that the fatigue limit S-1 of Al-13Si-3.0Cu-1.0Mg-2.0Ni, Al-13Si-4.0Cu-1.0Mg-2.0Ni and Al-13Si-5.0Cu-1.0Mg-2.0Ni alloy were 102.8 MPa.95.9MPa. and 92.8MPa.respectively.The fatigue fractures can be divided into three zones:fatigue crack source zone, crack propagation zone and fracture zone. The conchoidal pattern appeared in the fatigue crack area, and the fatigue striation appeared in the fatigue crack propagation area.The crack source of the sample surface is mainly due to the stress concentration induced by dislocations pileup of the second phase.the casting defects.surface finishment and axiality of the sample on crack formation can not be ignored.Therefore,the second phase of large size decreased by increasing the solution temperature and extending the solid solution time,which resulted in the reduction of crack source.However,the crack source was reduced by decreaseing the casting defects,improving the surface finishment and axiality of the sample,which improved the fatigue life of the alloy.