Study On The Improvement Of Properties In Near-eutectic Al-Si-Mg Alloy

The role of strontium(Sr) in the Al-10%Si-0.5%Mg alloy has been studied.It was found that in the condition of metal mold,with increasing of Sr content in the alloy,the amount of dendriteαis increased remarkably at first,then tardily later,and the second dendrite arm spacing decrease at the same time;In the unmodified Al-10%Si-0.5%Mg alloy,the silicon phase grow separately in the ternary eutectic(Al+Si+Mg2Si),and the Mg2Si phase mainly take on meshwork structure in ternary eutectic(Al+Si+Mg2Si).However,the Mg2Si phase is rod-like structure in the well modified condition.Magnesium has some negative effect on Sr modification.In the study of the effect of Sr and Sr+B on the eutectic cell of Al-15.5%Si alloy,It was found that Sr and B have the ability of refining the eutectic grain.Sr and B also have mutual poison behavior,which is not only have related to the amount of Sr and B,but also affected by cooling rate.The Sr in near-eutectic Al-Si alloy have important effect on the ductility.The influence of eutectic cell refinement on the mechanical properties of Al-Si alloy have not been reported at present.However,in the condition of metal mold,addition of Sr lead to the large amount of dendriteα,which means that the mechanical properties of eutectic Al-Si alloy can be improved by dendriteα.as in the hypoeutectic Al-Si alloy.There are two ways of strengthening the dendriteα,One is dendriteαrefinement,and the other is alloying strengthening followed by heat treatment.The role of the Cu is similar to the Mg in Al-Si alloy.Both of the two elements make the Al-Si alloy strengthened by heat treatment.Mn is commonly used to neutralize the effect of iron and to changeβ-Fe phase to a less harmful form.The effect of Cu and Mn on the microstructure and mechanical properties of Al-12%Si-0.5%Mg have also been studied.In Al-12%Si-0.5%Mg-xCu-0.3%Mn alloy modified by Sr and refined by B,with increasing of Cu,the morphology of equiaxed dendrite changed from classⅠto classⅡ,and the amount of microporosity seem to increase.In addition,increasing of Cu in the alloy,the ultimate tensile strength increase,but the fracture elongation decreases,and hardness increases nearly linearly.The effect of Mn on the microstructure of Al-12%Si-0.5%Mg-2.5%Cu-xMn-Sr-B alloy is not obvious.Addtion of 0.5 percent of Mn in the alloy,the large amount of needle-like phase appear in microstructure.With increasing of Mn in the alloy,the ultimate tensile strength increase unconspicuously,but the increase of fracture elongation is obvious.The ultimate tensile strength and fracture elongation derease,when the content of Mn exceed 0.3 percent.