Effect Of Rare Earth Sm On Microstructure And Mechanical Properties Of Al-Si-Mg Aluminum Alloy

Al-7Si-0.3Mg alloys have been widely used in many areas such as aerospace and automobile manufacturing because of their excellent combination of castability,high specific strength,good corrosion resistance and good rub resistance.In the condition of unmodified Al-7Si-0.3Mg alloy,the coarseα-Al dendrites and lamellar eutectic silicon can induce stress concentration and generate a crack at sharp point,which seriously damages the mechanical properties of alloys.The modification treatment can effectively improve the microstructure and increase mechanical properties.In this thesis,four kinds of Al-7Si-0.3Mg alloys with different Sm additions were designed.Except for one group free of Sm,other three groups were added Sm element with 0.33 wt.%,0.66 wt.%and 1.29 wt.%,respectively.The effect of different contents of rare earth Sm on the microstructure of Al-7Si-0.3Mg alloy and mechanism of Sm affecting eutectic silicon has been investigated by using multiscale microstructure characterization techniques（i.e.,OM,SEM,TEM）,XRD and DSC analyses.The effect of different contents of rare earth Sm on the mechanical properties of Al-7Si-0.3Mg alloy has been obtained by tensile experiment.The main conclusions are as follows:The addition of Sm has no significant effect on the grain size of the Al-7Si-0.3Mg alloy,however,secondary dendrite arm spacing is significantly reduced.The eutectic silicon size in cast alloys decreases with the increase of Sm content and the edge is gradually passivated.After heat treatment,the size of eutectic silicon decreases first,then slightly increases,and the degree of spheroidization increases with the increase of Sm additions.Sm is mainly consumed through the formation of Al₂Si₂Sm phase,which exists independently in the adjacent position of Si.When the content of Sm element is low,the primary phase is dominated by the smaller size of the flake,As the content of Sm increases,the primary phase accompanying growth and morphology shift to lath-like or needle-like.A small number of Sm maybe combines with P to form an SmP phase.Plate-like eutectic Si can be refined obviously by Sm addition.The formation of SmP phase could inhibite the formation of AlP phase,which deteriorates the function of AlP phase to eutectic Si in Al alloys.The undercooling of the Al and Si eutectic reaction can be increased with increasing Sm content.The toughness of the alloy is improved significantly with Sm additions.When the content of Sm is 0.3 wt.%,the alloy has the best comprehensive mechanical properties in T6 condition,and the tensile strength and the elongations are 310.77 MPa and 11.17%respectively.