Effects Of Co-adding Sc And Trace Element On The Microstructure And Properties Of A356/7 Alloy

As a widely used cast alloy,Al-Si alloys are applied in aerospace,shipbuilding and automobile industry because of its good mechanical properties,out-standing corrosion resistance properties and excellent casting performance.But its low toughness makes the common cast alloys confront with a great challenge for the applications of high technology.Thus,developing new cast alloys with high integrated mechanical properties are needed by optimizing alloy composition and processing.Scandium?Sc?,a rare earth element,has received extensive attention due to the effects of grain refinement and precipitation strengthening as the micro alloying element.But Sc is rare and precious on the earth and the production cost is relatively high.In order to reduce manufacturing cost,co-addition of Sc with other elements is a challenging issue for developing high strength-ductility alloy.In this study,A356/7alloys,as matrix alloy,are added with different atom ratio and content of Sc,Zr and Ti.The effects of co-adding Sc,Zr and Ti on the microstructure and mechanical property have been investigated by analyzing the microstructure such as the grain size,size and distribution of eutectic silicon and secondary precipitation,as well as the processing state like casting,solution and aging.On this basis,the strengthening mechanism for Al-Si alloy is discussed.According to the result,the Al₃Sc changes to Al₃?Sc,Zr,Ti?,which has smaller mismatch with Al than Al₃Sc.As a result,the co-addition of Sc and Ti shows better refinement effect than single-addition of Sc.In addition,the morphologies of the eutectic silicon and of Fe phase exhibit obvious change?caused by Fe impurity from the raw material?.The flaky and coarse-fibrous Si transforms to the fine-fibrous Si,and the needle-like?-Fe phase is modified to Chinese-script?-Fe phase.Furthermore,Al₃?Sc,Zr,Ti?are formed during aging treatment and distribute densely and evenly,leading to precipitation strengthening.Compared with the original A356 alloy,the alloy with 0.19%Sc and 0.22%Zr has optimum mechanical property.The elongations of the modified A356 have increased from 2.2%to 15.4%at solution state and from0.3%to 8.4%at solution-aging?T6?state,resulting from the change of the fracture mechanism.In the presence of Sc,Zr and Ti,the fracture mechanism has transformed to transgranular fracture from the intergranular fracture,leading to apparent improvement in toughness.