Effect Of Sc And Sr Addition On Microstructure And Mechanical Properties Of A356.2 Alloy

A356.2 aluminum alloy has been extensively used in automotive and aerospace due to its excellent performance.However,the refinement and modification effect of Al-5Ti-1B and Al-10Sr master alloy in industrial application is not satisfactory,especially in improving the thermal stability of the alloy.In this work,A356.2 alloy with different contents of Sc addition and Sc/Sr ratios were prepared by melting and casting techniques.OM,SEM,TEM and Universal Material Testing Machine were used to investigate the effect of Sc and composite addition of Sc,Sr on microstructure and mechanical properties of A356.2 alloy.Meanwhile,microstructure and thermal stability evolution behavior of A356.2 aluminum alloy with the best Sc content and Sc/Sr ratio under cyclic thermal exposure conditions were researched.The researches show that the?-Al grain and secondary dendrite arm spacing of A356.2alloy were significantly refined by Sc elements,and the eutectic silicon was refined to rodlike or spherical,however,the effect of Sc compared with Sr on modification was not ideal,resulting the mechanical properties of A356.2 alloy treated by Sc was lower than that treated by conventional master alloys.The synergies of Sc and Sr enhances refinement and modification effects of A356.2 alloy.Because of the refined grains and the modified eutectic silicon,the mechanical properties of A356.2 alloy have been significantly improved,tensile strength,yield strength,elongation and Q are 208.39 MPa,108.7 MPa,10.23%and 358.97 MPa respectively,which are improved by about 4%,8.2%,14.3%and4.6%respectively compared with the traditionally treated alloy.Both secondary dendrite arm spacing and average area of eutectic silicon increased with exposure temperature and time increasing,resulting the thermal stabilities of ultimate tensile strength,yield strength and micro-hardness were decreased firstly.The thermal stabilities becomed stable owing to the precipitation of Al₃Sc phase under the later period of thermal exposure condition,which could be proposed for structural applications at elevated temperatures up to about 200?.Fuethermore,it is known that the formation of vacancies is difficult and more time is needed due to weak thermal movement of atoms in its equilibrium position under lower temperature,and more than 6×4 h were needed for the mechanical properties getting stable under 200?exposure temperature.However,vacancies can be quickly formed and precipitation of numerous nano Al₃Sc phases under higher cyclic thermal exposure temperature,and the thermal exposure time of mechanical properties tends to be stable were significantly reduced.