Microstructure And Mechanical Properties Of The Laser-welded Mg-3Nd-0.2Zn-0.4Zr(NZ30K) Magnesium Alloy

Magnesium alloy is widely used in the field of the aviation and aerospace,mechanical manufacture,electronic products and so on.However,its development is limited by low tensile strength and bad corrosion property.To increase the property of magnesium and widen its applicatiom field,rare-earth magnesium alloy and its welding technology are widely concerned.There exist many defects such as large deformation and bad heat affected zone property by using traditional welding methods.Laser beam welding is one of effective approaches to solve above problems.4mm NZ30 K magnesium alloy by using laser welding and the effect of laser welding parameters on weld geometry and microstructure of the joint were mainly studied in this paper.Besides,distribution regularities of alloy elements and mechanical properties after laser welding were also studied.What's more,through the comparison of rare-earth magnesium alloy NZ30 K and conventional magnesium alloy AZ31,the difference between microstructure,mechanical properties and corrosion property were emphatically compared,as well as growth pattern of grains after laser welding.This study has certain significance in industrialized application,with which high-quality welding technology of rare-earth magnesium alloy can be solved,as a result of which technical assistance of rare-earth magnesium alloy on industrial production and application was provided.Mechanical properties of NZ30 K under normal temperature and high temperature(200°C)are studied.Metallographic microscope,SEM?EDS?XRD were utilized to analyze the microstructure and distribution of alloy elements in laser welding of rare-earth magnesium alloy.Besides,tensile testing machine and microhardness tester were used to analyze the mechanical properties under normal temperature and high temperature(200°C)of laser welding joint.The results showed that weld microstructure consists of large amount of equiaxed grain,and element Nd is mainly assembling on the boundary of the grain.The boundary of grain is mainly Mg12 Nd,for which softening phenomenon will not occur in weld seam.When laser power was 3kW,welding speed 3m/min,defocusing amount-5mm,tensile strength of weld joint can reach to 228 MPa,which accounts for 90% of base metal,and tensile strength under high temperature can reach to 70% of that under normal temperature.Mechanical properties were pretty good after laser welding.Welding joint of AZ31 is studied,and mechanical properties and corrosion property of AZ1 are also studied.The results showed that welding structure of AZ31 is mainly dendritic crystal,whose tensile strength under normal temperature can reach to 224 MPa and under high temperature can be only 105 MPa,which showed that softening phenomenon occurred in weld joint of AZ31,as a result,the property under high temperature decreased obviously.Immersion corrosion and electrochemical corrosion were adopted to analyze the corrosion property of these two magnesium alloys.The result indicated that,the corrosion property of rare-earth magnesium alloy NZ30 K is better than that of AZ31 because of the existence of element Nd,which increases the corrosion property of ?-Mg phase.At the same time,there are large amount of fine equiaxed grain,and Mg12 Nd distributing continuously on the boundary of grain,which impeded the progress of the corrosion,as a result of which decreased the whole corrosion rate of weld structure.