Effect Of Rolling Deformation On The Microstructure And Mechanical Properties Of Mg-8Al-1Sm-xZn Alloy

Magnesium alloy has the advantages of light weight,high specific strength and high specific stiffness,and can be lightweight in transportation,aerospace and other fields.Mg-Al series magnesium alloy is one of the mainstream magnesium alloy series at present and has broad application prospect.Mg-Al alloy alloyed by Sm shows better comprehensive properties,and Mg-Al-Sm alloy has attracted much attention in recent years.But the effect of Zn on the alloy is not clear.To solve the above problems,this study focuses on the effect of Zn content on the microstructure and mechanical properties of Mg-8Al-1Sm alloy in as-cast and heat-treated states,and on the homogenization treatment and rolling deformation of Mg-8Al-1Sm-1Zn alloy in heat-treated states with the best comprehensive properties.The deformation law and strengthening mechanism of Mg-8Al-1Sm-1Zn alloy under conventional rolling and liner rolling with high pressure were described.The evolution of microstructure and mechanical properties of as-cast Mg-8al-1sm-xZn（x=0,0.5,1,1.5wt.%）alloy was investigated by adjusting the content of Zn,and the conclusion that Zn is helpful for refining the grain size of Mg-8Al-1Sm alloy was confirmed.When the content of Zn is 1wt.%,The grain size of the alloy was refined to 85μm and about 29%.The as-cast Mg-8Al-1Sm alloy mainly consists ofα-Mg,β-Mg₁₇Al₁₂ and Al₃Sm phases.After Zn alloying,part of Al in Mg₁₇Al₁₂ phase in Mg-8Al-1Sm alloy is replaced by Zn to form more complex Mg₁₇（Al,Zn）₁₂ phase.The strength of Mg-8Al-1Sm alloy is enhanced by fine crystal strengthening and solid solution strengthening of Zn,but the hot cracking tendency of Mg-8al-1sm-xZn alloy is aggravated by excessive Zn.Therefore,the strength of Mg-8al-1sm-xZn alloy increases first and then decreases with the increase of Zn content.The ultimate tensile strength（UTS）,yield strength（YS）and elongation（EL）are 185 MPa,145 MPa and 7.7%,respectively,when the amount of Zn is 1 wt%.The dendrite segregation of the as-cast Mg-8Al-1Sm-1Zn alloy is obviously weakened after being held at 410℃for 16h.The UTS,YS and EL of the alloy are increased to 242 MPa,167 MPa and 10.3%,respectively,which are increased by15.2%,30.8%and 33.7%compared with as-cast.The homogenized alloy was rolled at 400℃by multiple passes conventional rolling and single pass lining rolling.It is found that after conventional rolling with 60%cumulative deformation,the average grain size of the alloy is relatively large（20.8μm）,and UTS,YS and EL of the alloy are 248 MPa,221 MPa and 11.5%,respectively,due to multiple heating between rolling passes.After 60%deformation of liner rolling,the microstructure of the alloy is mixed crystal and fine recrystallized grains are produced.The average grain size of the liner rolling alloy is 7.2μm.UTS,YS and EL of the alloy are 285 MPa,255 MPa and 12.7%,respectively,which are 15.3%,14.9%and 10.4%higher than those of conventional rolled alloy.Multi-pass conventional rolled plate and single-pass liner rolled plate with the same deformation（60%）were aged respectively to investigate the hardening response and precipitated phase morphology at 200℃.It is found that the optimal age-hardening response of the two kinds of rolled plates occurs at 24 h,and the age-hardening response is 99 HV and 106 HV,respectively.The main reason for the above phenomenon is that the dynamic recrystallization degree of alloy in liner rolling samples is higher,which makes the average grain size smaller and promotes the precipitation of discontinuous precipitated phase.The precipitated phase of conventional rolled alloy presents different morphological characteristics in different regions,showing chain shape near the grain boundary or twins,and diamond shape flake inside the grain.The precipitated phase of liner rolling alloy has two obvious forms,one is point-like or chain-like precipitated phase,and the point-like phase usually appears with cellular structure,showing discontinuous precipitated behavior（DP）.The other is the fine rhomboidal phase,which shows continuous precipitation behavior（CP）.