Microstructure And High Temperature Mechanical Properties Of The Laser-welded Rare-Earth Magnesium Alloy

At present,magnesium alloys are widely used in aerospace,mechanical manufacturing and electronic products,but their poor high-temperature performance seriously restricts its development.In order to improve the performance and promote the application of magnesium alloys,the rare earth magnesium alloys and their connection technologies have received widespread attention in recent years.Traditional fusion welding methods can cause some shortcomings,such as large welding deformation and large heat affected zone performance changes.Laser welding as an advanced connection method has become one of the preferred methods for welding magnesium alloys.In this paper,laser self-fusion welding is used to weld the T6 state Mg-10Gd-3Y-0.5Zr rare earth magnesium alloy sheet.In addition,the effects of laser process parameters?laser power,welding speed,and defocusing amount?on weld formation,joint structure,and high-temperature mechanical properties of joints are also studied.The SEM,EDS,TEM,and HRTEM test methods were used to analyze the metallographic structure,the types of precipitated phases,the interface and the phase relationship between the precipitated phase.In the orthogonal test,taking the high-temperature tensile strength of200?as the reference index,the welding process parameters were optimized by the range method and the variance method.Finally,the influence degree of each process parameter and the optimal welding process were determined.Moreover,the high temperature mechanical properties,fracture morphology and fracture mechanism of the base metal and welded joint under the optimal process parameters at 200?were compared and analyzed.The results show that:By properly adjusting the laser welding process parameters Mg-10Gd-3Y-0.5Zr rare earth magnesium alloy can achieve a good connection with no excessive surface burning,no melt pool collapse,cracks,pores and other defects.The weld center and heat-affected zone grain size and the width of heat-affected zone are all increase with the increase of heat input.The 200?high-temperature tensile strength of welded joints decreases with the increase of heat input.The horizontal and vertical microhardness of the weld joint also gradually decreased with the increase of heat input.The microstructure of the weld seam is strengthened by grain boundary strengthening,followed by the second phase strengthening.The EDS results show that the second phase precipitated along the grain boundary is Mg₂₄?Gd,Y?₅.After using HRTEM to observe the interface between it and?-Mg,it was determined that the interface between Mg₂₄?Gd,Y?₅and?-Mg was a semi-coherent interface.However,the original?'-strength phase in the heat-affected zone dissolves during the welding process,resulting in softening of the heat-affected zone.Orthogonal experiment results show that the laser power has the most significant effect on the high-temperature mechanical properties of the welded joint.Under the optimal process parameters?laser power 3.3k W,welding speed 4.2m/min,defocus amount-2mm?,the welded joints are well formed,the average high temperature tensile strength at 200?is 285.4MPa,which is 82.7%of the base material,and the elongation is 8.6%which is 71.1%of the base material.Tough dimples can be observed at high temperature tensile fractures,so the fracture mode of the joint and the base material are all ductile fractures.