Influence Of Trace Yttrium On The Structure And Properties Of A356 Prepared By Metal Mold Casting And T6 Treatment

Among cast aluminum alloys,A356 alloy has been widely used in various fields of industry because of its many advantages,such as excellent casting performance,corrosion resistance,high strength and toughness.Many experts have done a lot of scientific research,a large number of academic work to reveale the influence of the addition of rare earth elements on the microstructure and mechanical properties of Al alloy,but most of them focused on the deforming aluminum alloy,and few on casting aluminum alloy.In this paper,the effects of micro content of yttrium on microstructure and properties of A3 56 prepared by metal mold casting and T6 treatment were investigated.The main research results showed that:(1)The strength of as-cast A356 alloy conformed to Hall Petch equation,and the fitting relationship between yield strength and grain size was YS=-354.1+2875.2d-1/2,R2 was 0.83.When 0.05%Y was added,the optimal values of YS,UTS and El%were obtained after T6 treatment.Comparing with as-cast A356,YS,UTS and EI%were increased by 218%,77%and 155%,respectively.When 0.075%Y was added,the eutectic reaction temperature(Liquid? ?Al+?Si)was 2.3? higher than that without Y.With the addition of yttrium,the degree of undercooling increaseed,and the SDAS(?2)became thin and the microstructure was refined.After T6 heat treatment,the strength of A3 56 was significantly improved.Mg2Si precipitated in the ?-Al matrix,while Al3Y nanoparticles with an average particle size of 104.7±28.9 nm precipitated on the surface of Si phase.(2)After T6 treatment,the comprehensive data of yield strength,ultimate tensile strength and elongation of Y050 were better than those of other groups,and had a high microhardness value,and its comprehensive performance was the best.After T6 treatment,long sheets of primary Si were distributed in the ?-Al matrix,and large bulk,granular,or point-sheet Si/Mg2Si were distributed among the ?-Al grains,and the distribution of irregular polygons was discrete.Fracture morphology analysis showed that the point or granular Si/Mg2Si strengthening phase improved the toughness and plasticity of the alloy.Moreover,XRD analysis showed that Si phase and Mg2Si phase existed when 2? is about 28°,47°,56° and 68°,respectively.(3)Due to the addition of microalloyed yttrium,there are other ternary or multivariate phases in the alloy except ?Al and ?si.The yttrium content of X%(X=0.025%,0.05%,0.075%,0.1%)decreased the temperature of the eutectic reaction?Al+?si?Liquid(1.1?,2.0?,1.4?,2.2? lower than X=0 alloy)and the melting heat of the reaction.An appropriate amount of yttrium can promote the formation of ?Al heterogeneous nucleated substrate,which formed multiple phases with ?Si and Mg2Si,thus increasing the undercooling during solidification.Compared with these five alloys,alloy with 0.5%yttrium added showed the greatest effect.