Influence Of Sr-Y On Secondary Dendrite Arm Spaceing And Ageing Process Of A356 Alloy

In this paper, the influence of adding Sr-Y Intermediate alloy dendrite armspace and ageing process in A356 alloy is studied. The microstructure is observed by using the optical microscope and scanning electron microscopy(SEM). Using X-ray diffraction analysis (XRD). DSC curve is used to analyse the precipitation dynamics of A356. Experiments show that:DSC cures showed that, compared with Sr modification,the temperature of eutectic reaction decreased 2.7℃by composite modification of Sr-Y. The influence of adding Sr-Y in A356 alloy on the morphology of dendrite Phase was investigated and the amount of secondary dendrite arm spaceing. It was found that Sr-Y addition can promote the growth of columnar dendrite sceondary dendrite spaceing.It is rational to conclude that the present of Sr-Y in A356 alloy melts results in a decrease of the growth undercooling of dendrite tips and the nucleation of equaixed grains in the liquid in front of columnar dendrite tips is restrained. The reduetion of the solid-liquid interfacial energy also leads to the decreases of the seeondary dendrite arm spaceing.DSC curves showed that: the Sr-Y can accelerate ageing process (GP,β″,β′,β). And precipitated phase Mg2Si is more small scattered and more number that is observed by using the SEM (electrolytic polishing).So achieving the ageing peak time is sooner, ageing peak is higher(80HBW).The conductance can reflect the change of eutectic silicon shape during the solution period. During the time of solution,the eutectic silicon fused,turned to granulation and then grew up. At the same time,the conductance of A356 increased at the early time and deceased later. The best time of solution for the A356 was 6h,and the conductance reached 22.05×106S/m which was maximum.