Strenghering Mechanism And Low Temperature Performances Of Composite Refined-modified Al-Si-Mg Alloy

Al-Si-Mg alloys(especially for A356.2 aluminum alloys)have been widely used in key components such as automobile hubs and engine cylinder heads due to their excellent overall performances.It is known that composite addition of Sc and Sr elements can achieve composite refinement-modification of A356.2 aluminum alloy,which improves its thermal stability and mechanical properties.However,aging strengthening mechanism of A356.2 aluminum alloy treated by Sc/Sr composite refinert-modifier is still unclear.Also,microstructure and mechanical properties evlutionof the alloy at low temperature and high-low temperature cycles are needed to be investigated.Based on thes,in this work we studied aging strengthening mechanism of A356.2 aluminum alloy treated by Sc/Sr composite refinert-modifier and also its microstructure,tensile properties and micro-hardness evlution at low temperature and high-low temperature cyclethrough OM,SEM,TEM,universal electronic testing machine and micro-hardness tester,the results showed that:(1)During the aging treatment,with the increase of aging temperature,secondary dendrite spacing(SDAS)of the alloy increased gradually,eutectic silicon thickened and micro-hardness decreased gradually,150°C was selected for the optimal aging temperature.At 150°C,with the increase of aging time,SDAS increased gradually,eutectic silicon experienced decomposition,rounding and roughening process.The micro-hardness increased first and then decreased.With the aging time of 5 h,micro-hardness reached the peak of 110 HV and Mg5Si6,Al Sc2 and Al2 Sc phases coexisted with the matrix,and sharp-angled Fe-containing phase passivated.(2)Under low temperature conditions(-60°C?20°C),SDAS of the alloys decreased with the decreasing of temperature,and the micro-hardness gradually increases.At the temperature of-60°C,the micro-hardness of the alloy increased about 20.75% compared to the alloy at room temperature(20°C).As the temperature decreasing,the tensile strength and yield strength of the alloys increased continuously with the elongation decreased gradually.At the temperature of-60°C,elongation of the alloy decreases 29.68% compared to the alloy at room temperature(20?).The tensile strength and yield strength of the alloy at-60? reached to 335.61 MPa and 237 MPa,respectively.(3)In the process of high-low cycle process,as the number of cycles increases,SDAS of the alloy increased,eutectic silicon coarsened and the micro-hardness decreased gradually whether it is air cooling or furnace cooling.With the same number of cycles,furnace cooling is beneficial to the diffusion of atoms due to its lower cooling rate,which makes the SDAS,eutectic silicon and micro-hardness of the alloy change more greatly than air cooling.Under air cooling conditions,the precipitation phases mainly included SrMgSi and Sr Si2 phases,while for the furnace cooling it was mainly SrMgSi2 phase.