Study On The Weldability And Liquation Mechanism Of Laser Welding Of Fine-grained Magnesium Alloys Sheets With High Hot Cracking Tendency

Magnesium alloys have been widely applied to the light weight structural and functional parts in the automotive,computer,communication and consumer electronic products due to their low density,high specific strength and excellent damping capacity.Magnesium alloy sheets are mainly used as the shell,panel,stamping and welding parts and so on.It's difficult to manufacture the high strength magnesium alloy sheets with high alloying content due to the poor plastic deformation capacity.It's reported that the plastic deformation capacity and mechanical properties of magnesium alloy can be effectively improved by grain refinement.The AZ(Mg-Al-Zn)and the ZK(Mg-Zn-Zr)series are the most important kinds of magnesium alloys widely used in many fields.AZ91 and ZK60 magnesium alloys,featured with high alloying elements are two kinds of widely used magnesium alloys.Their applications are greatly limited because they are difficult to prepare the sheets and have high welding hot cracking tendency.In most cases,welding is unavoidable for the ZK series magnesium alloys due to their high welding hot cracking tendency.It's reported that the mechanical properties and weldability of ZK series magnesium alloys can be improved by the Y addition.So far,fine-grained magnesium alloy sheets with good mechanical properties can be fabricated by the large strain rolling(LSR)technology,while the corresponding welding technology needs futher study.As compared with the other welding technologies,laser welding is considered to be a promising method for welding of magnesium alloy thin sheets due to its higher power density,the narrower heat affected zones(HAZs),the deeper fusion zones(FZs),the higher welding speed and so on.In this paper,the fine-grained AZ91 and ZK60-0.5Y alloy sheets with 1mm in thickness are welded by autogenous laser beam welding(LBW)and laser beam welding with filler strip,respectively.The weldability and the liquation behavior of both alloys are investigated.The main conclusions are drawn as the following:1.The laser welding parameters are opmtimized by welding experiments for two alloys.As for LBW with filler strip,the optimized laser welding parameter of AZ91 thin sheets is that the laser power of 1100W and the welding speed of 6.5 m/min.As for the ZK60-0.5Y alloy,they are 1300 W and 6.5 m/min,respectively.2.Both welded joints are characterized by fine equiaxed dendrites in the fusion zones and free from significant grain coarsening in the heat affected zones.3.The liquation mechanisms in PMZ are clarified for two alloys.As for the AZ91 welded joint,two liquation mechanisms are involved in the partially melted zone,which are related to the size and the shape of the ?-Mg17A112 precipitates.For the bulky ?-Mg17Al12 precipitates,liquation results from the melting of the residual?-Mg172Al12 precipitates.For the sub-micron size P-Mg17Al12 precipitates,constitutional liquation occurs.As for the ZK60-0.5Y welded joint,there are four kinds of liquation mechanisms involved in the partially melted zone.They are the melting of the a-Mg matrix,segregation-induced liquation,constitutional liquation and the melting of the residual I-Mg3YZn6 phase.As far as the liquation of I phase is concerned,the resolidified microstructure is composed of Mg7Zn3 and I-Mg3YZn6.Liquation can be effectively prevented or eliminated by modifying the size and the amount of the second phase and reducing the heat input.4.Both welded joints of AZ91 and ZK60-0.5Y obtained by LBW with filler strip have good mechanical properties.As for the AZ91 welded joints,the ultimate tensile strength is 301MPa,about is 82.7%of the base metal and 16.9%higher than that of the autogenous laser beam welding.As for the ZK60-0.5Y alloy welded joint,the ultimate tensile strength is 269.7MPa,about is 93.5%of the base metal and 4%higher than that of the autogenous laser beam welding.