Study On The Weldability Of Laser Welds In Fine-grained ZK60-Y Magnesium Alloys

As the lightest metal structure materials,magnesium alloys have been widely used in aeraspace,automobile and electronics field etc,due to their high specific strength,good toughness,low density and excellent machining property.The wrought magnesium alloys have a wider application in manufacturing the structural parts for their better mechanical properties than that of the cast magnesium alloys.The ZK60 magnesium alloy is considered as one of the most excellent commercial magnesuim alloys with a better integrated performance.However,the ZK60 magnesium alloy is very difficult to be welded because of its high content of Zn and high sensitivity to hot cracking,which restricts its application seriously.Though there are many kinds of welding methods for magnesium alloys,the characteristics of the ZK60 alloy limit its welding methods.Laser beam welding(LWB)is one of the preferred methods to ZK60 alloy owing to its low heat input and high welding speed.The mechanical properties of the ZK60 alloy can be efficiently improved by the addition of rare earth(RE).In this study,the laser beam weldability of the fine-grained ZK60-Y plates prepared by the high strain-rate rolling process has been investigated.Four kinds of the laser beam welded joints of fine-grained ZK60 magnesium alloys with different contents of Y(0,0.5,1.0 and 1.5wt.%)were investigated,and the effects of Y addition on the microstructure and mechanical properties of the joints were studied.The results reveal that the formation of porosity can be avoided in the fusion zone(FZ)efficiently,when the content of Y is more than 1.0wt.%.The addition of Y element changes the distribution of the second phases and promotes the dendrites growth,which results in refining the microstructure in the FZ and improving the microstructure stability in the heat-affected zone(HAZ).The width of the partial melted zone is reduced and the grain growth is inhibited with the increase of Y addition.When the Y content is over 1.0wt.%,the average grain size of the larger grains in the HAZ and the microstructure near the PMZ are no more than 4.5μm and 3μm respectively.When the content of Y reaches 1.5wt.%,the width of the PMZ is only 85μm up to 46%of that of the ZK60 joints and the average grain size in the HAZ is just 2.2μm without significant grain growth.On the other hand,the results show that Y addition has little effect on the ultimate tensile strength of the laser beam welded ZK60 joints.The different second phases may influence the ductility of the as-welded ZK60 joints.The plasticity is increased with the volume fraction increase of the quasicrystalline I-Mg3YZn6 phase,while reduced by the formation of the brittle W-Mg3Y2Zn3 phase.When the Y content is 1.0wt.%,the elongation of the welded joint reaches 11.1%The effects of the low heat input(13.1,14.4,16.8 and 21.0J/mm)on the laser beam weldability of the fine grained ZK60-1.0Y alloy have been discussed.The increase of the heat input results in wider space of the FZ,lower depth-to-width ratio of the weld seam and a little coarsening of the fine equiaxed dendrites.The width of the columnar zone is increased from 0 to 18μm.The width and the average grain sizes of the partially melted zone(PMZ)and the HAZ are both increased with the increase of the heat input.When the heat input is above 16.8J/mm,the tensile property of the joint at ambient temperature is improved significantly,and the elongation reaches more than 10%.The FZ of welded joint for ZK60-1.0Y is characteristic of equiaxed dendrites and columnar zone which are formed by the heterogeneous nucleation and the epitaxial growth respectively.The grain coarsening is invisible in the HAZ due to its excellent thermal stability.There are three liquiation mechanisms as followed:a)The liquid film along grain boundaries(GBs)is caused by the constitutional liquation of the dispersed MgZn particles,and the ghost GB network is formed in the grain of the PMZ.b)The a-Mg matrix closed to the FZ line melts at the eutectic temperature(TE),resulting in the formation of the dispersed eutectic precipitate after solidification.c)The large residual eutectic phases melt under the weld thermal cycle.And the complete or incomplete resolidification microstructure are formed with the variation of the degree of heat effect.