Effect Of Cu And Zr Alloying On Microstructure And Properties Of Al-7Si-0.35Mg Alloy

Due to the advantages of low density and good corrosion resistance,hypoeutectic Al-Si cast alloy is widely used in automotive,aerospace and other industrial fields.However,with the continuous improvement of social demand,the performance requirements for aluminum alloy materials are also improved continuously.Improving the strength of hypoeutectic cast aluminum alloy and ensuring good toughness has become a research hotspot in the field of material lightweight.In this paper,based on the sub-eutectic Al-7Si-0.35Mg cast aluminum alloy,by adding Cu,Zr element modification,using metallographic microscope(OM),scanning electron microscope(SEM),transmission electron microscope(TEM)and material universal testing machine and other instruments.The effects of the addition of Cu and Zr on the microstructure and properties of the hypoeutectic Al-7Si-0.35Mg casting alloy were studied systematically.The main conclusions are as follows:(1)In the as-cast condition,the addition of Cu element has no significant effect on the secondary dendritic arm spacing of eutectic Si,while the addition of Zr element can reduce the secondary dendritic arm spacing to 32.6?m,significantly refining the size of eutectic Si and improving the morphology of eutectic Si.The addition of Cu and Zr elements precipitates Al-Si-Cu-rich phase,Al₂Cu and Al₃Zr primary phase in the solidification process,which can be used as heterogeneous nucleation particles to refine the grain,and can interrupt the eutectic Si to refine the Si phase.(2)By DSC test and aging hardening curve analysis,the optimal heat treatment process of the alloy is:535?×240 min+175?×240 min,and the alloy will appear obvious overaging phenomenon after aging treatment at 200?for 600 min.After the T6heat treatment,the spheroidization effect of eutectic Si in the as-cast alloy is obvious,and the long and needle-like eutectic Si disappears basically.Most of the eutectic Si in the alloy structure has been refined into short rod-like and worm-like.The aspect ratio of eutectic Si in the alloy without alloying element is 2.42.However,the aspect ratio of eutectic Si in the alloy with Cu and Zr elements decreases to 1.61-1.87,and the distribution is more dispersed.The addition of Zr element can effectively bind the impurity Fe element in the alloy,forming the Al-Si-Zr-Fe phase rich in Zr,and reducing the harm of the impurity Fe element.After heat treatment,secondary precipitates of Al₃(Ti,Zr)can be used as heterogeneous nucleation particles to refine the grain size.(3)The addition of Cu element can significantly improve the tensile strength and hardness of the alloy,but at the cost of the plastic toughness of the alloy,while the addition of Zr element can improve both the strength and the plastic toughness of the alloy.The alloy with 0.24 wt.%Zr element has the highest elongation,which is 8.8%in the as-cast condition and 11.8%in the T6 heat treatment condition.The alloy with 1.2wt.%Cu element and 0.24 wt.%Zr element has the best comprehensive mechanical properties.The hardness,tensile strength and elongation of the alloy are 82.29 HBS,239.4 MPa and 7.9%,respectively,under the as-cast condition.After T6 heat treatment,the hardness,tensile strength and elongation of the alloy reach 117.64 HBS,342.4 MPa and 10.1%,respectively.