Microstructure Refinement And Mechanical Properties Regulation Of A356.2 Aluminum Alloy Through Heat Treatment

The automobile industry develops to energy saving and environmental protection,and reducing weight is one of the most effective ways.A356.2 aluminum alloy has become an ideal material for automotive lightweight owing to its excellent comprehensive performance.Traditional refiners and modifiers can significantly optimize the microstructure and improve its mechanical properties of A356.2 aluminum alloy,but the effects of refinement and modification are still unstable.In this paper,A356.2 aluminum alloy was used as matrix material,while Sr and La elements are used for composite refinement and modification.The OM,SEM,XRD,TEM,universal electronic testing machine,brinell hardness tester and microhardness tester were used to systematically study the influence of the Sr/La ratio on the microstructure and mechanical properties of as-cast alloys.On this basis,the effects of solution and aging heat treatment on the microstructure and mechanical properties of the as-cast alloys were studied.The results are as follows:1)In the as-cast condition,the content of Sr is constant?0.03 wt.%?.With the increase of La content in Al-Sr-La master alloy,the refinement and modification effects of alloy as well as brinell hardness increase firstly and then decrease.When the Sr/La ratio is 3:3.5?Al-6Sr-7La master alloy?,the microstructure and brinell hardness of the alloy are optimum.Because of the refiner Al₄La and the modifier Sr and La atoms are the best in content and distribution at this time.Continuing to increase the content of La,it led to the formation and growth of Al-Si-Sr-La intermetallic compounds,resulting in the reduction of composite refinement-modification effects;2)In the solution treatment process,the secondary dendrite spacing gradually increases and the eutectic silicon undergoes a process of decomposition,rounding,and coarsening,and the microhardness increases first and then decreases with the temperature and time increase.When the solution temperature is 540°C and the holding time is 3 h,the microstructure and microhardness of the alloy are the best.The reason is that the concentration of Mg and Si diffusion into matrix was the highest.The increase of temperature or time will lead to the growth of secondary dendrite arm space sharply,which contributing to decrease of the concentration of solid solution atoms in matrix,and then microhardness of the alloy decreases;3)During the aging treatment,the microhardness of the alloy first increases and then decreases with increase of temperature and time.When the aging temperature is 175°C and the holding time is 8 h,the micro-hardness reached the peak.It is because the precipitated metastable phase Mg₅Si₆ is coherent with the matrix,and forming a coherent distortion region in?-Al.With the distortion degree of coherent distortion area increases,the micro-hardness of the alloy reached the peak.The Al-6Sr-7La master alloy developed in this paper is an aluminum alloy composite refiner-modifier with practical application prospect,which can greatly improve microstructure and mechanical properties of A356.2 aluminum alloy.