The Research Of Microstructure And Mechanical Properties In Magnesium Alloys With Rare-earth Addition For Automotive Wheels

As the lightest metal structure material,magnesium alloy has the advantages of high specific strength,high specific stiffness,good machinability,green environmental protection and recyclable.It has a good application prospect in the automobile industry.In order to promote the application of magnesium alloy for automobile wheel,based on the alloy of NZ30K which was developed in Shanghai Jiaotong University,this project is willing to develop new magnesium alloys which meet the requirement of automobile wheel(130 MPa-210 MPa-7%)by reducing the content of Nd.This article systematically researched the microstructure and mechanical properties of Mg-1Nd-1Ce-xZn-0.4Zr and Mg-6Gd-xZn-0.4Zr(x=0,0.2,0.5,1.0 wt.%)alloys.The influence rule of Zn content to the microstructure and mechanical properties in these two alloys is revealed and the relationship between microstructure and mechanical properties is discussed.The main conclusions in the present study are summarized below:Zn element can significantly change the age-hardening behavior and the compositions of the precipitated phases in Mg-1Nd-1Ce-0.4Zr alloy.In 1Zn alloy,the age-hardening response decreases significantly(AHV=7).In Zn-free alloy,the precipitated phases are mainly comprised of disk-like phase on prismatic plane and point-like phase at aged state.In 0.2Zn alloy,the fine plate-like basal phase appears apart from disk-like phase and point-like phase.In 0.5Zn alloy,the basal plane phase becomes the main precipitated phase.The content of disk-like phase and point-like phase significantly decreases.In 1Zn alloy,the prismatic plane phases disappear and only small amount of basal plane phase and point-like phase appear.Compared with Zn-free alloy(92 MPa),small amount of Zn content(0.2,0.5%)can enhance the yield strength in aged stated(+30,+44 MPa).The enhancement of yield strength is related with basal plane phase formation.When the Zn addition is 1%,the enhancement of the yield strength decreases which is only+19 MPa.Zn element can significantly change the age-hardening behavior and the compositions of the precipitated phases in Mg-6Gd-0.4Zr alloy.At 200℃,the age-hardening response decreases as Zn content increases while the 0.5Zn or 1Zn alloys exhibit nearly no age-hardening response.In Zn-free alloy,the precipitated phases are comprised of prismatic P’ phase andβ" phase which is attached to P’ phase,and the density of them is high.In 0.2Zn alloy,the density of prismatic precipitated phases decreases significantly,the morphology of connection between β’ phase and β1 phase forms.The fine y’ phase on basal planes forms at the same time.In 0.5Zn alloy,the size of β’ phase decreases compared with 0.2Zn alloy and the basal y’ phase coarses significantly.In 1Zn alloys,the plate-like γ" phase on basal plane becomes the main precipitated phases.Compared with Zn-free alloy(145 MPa),the addition of Zn element significantly enhances the ductility at room temperature,decreases the yield strength of alloys(-9,-31,-29 MPa)in aged state.The decrease of the yield strength is caused of the formation of basal precipitated phases.The mechanical properties of Mg-1Nd-1Ce-0.5Zn-0.4Zr alloy at room temperature in peak-aged state are:136 MPa for yield strength,237 MPa for ultimate tensile strength and 9%for elongation.As for Mg-6Gd-0.2Zn-0.4Zr in aged state,the mechanical properties are:134 MPa for yield strength,222 MPa for ultimate tensile strength and 7.4%for elongation.Compared with NZ30K developed by Shanghai Jiaotong University,the mechanical properties at room temperature decrease slightly.As the cost of alloys decrease significantly because of the lower Nd content,both of the alloys is expected to be applicable in magnesium wheel.