Wrought Magnesium Alloy Welding By Laser Filling Wires And Laser-arc Hybrid Techniques

Being a kind of light alloy, magnesium alloy is widely used in manufacturingindustry for its high specific strength, large modulus and shock resistance, as well as goodmechanical properties, However, there is still some difficult problems about how toimprove the weldability of magnesium alloys. In order to overcome the welding defectsand enhance the welding performance of magnesium alloys, studies on the weldingprocess of the AZ31 wrought magnesium alloy was performed in this paper by means ofthe techniques of laser welding with filler wire and laser-MIG hybrid welding. The mainresults showed as below:About laser welding with filler wire of the AZ31B-T5 wrought magnesium alloy sheetwith 2mm-thickness, the effects of laser welding parameters on bead shapes,microstructure characteristics, and mechanical properties of welding joints were studied.The result demonstrateted that the transverse reverse-taper shape of the welding seam wastransformed to cylinder shape gradually as the heat input increasing; and both the frontwidth and the back width of the welding seam increased as the laser power increasing oras the welding speed decreaseing. While the distance between the end of the wire and thelaser spot was maintained between +0.5mm to+1mm, and the ratio of the filler wire feedrate to welding speed was kept between 0.3 and 0.4, a good welding bead quality could beobtained with no apparent defects. In the welding zone, the microstructures were mostlythe tiny dendrite equiaxed grains whereas the columnar grains existed near the fusion lines,and the Mg-Al-Zn phases precipitated among the crystal grains. The value of the ultimatetensile strength of the welding joints could reach 83% ~94% of that of the base metal. Theporosity was the main factor which reduced mechanical properties of the welding joints.The process of laser-are hybrid welding for the AZ31-F wrought magnesium alloysheet with thickness of 10mm was studied systematically. It could be found that the arc oflaser-arc hybrid welding for the AZ31 magneisum alloy was stabilized by means of thesynergic effects of both the laser and the arc, and the hybrid welding could obtain a stableprocess and a good weld shape; whereas.a very unstable state existed in the MIG weldingof magnesium alloys. The grains were dendrite with the Mg-Al-Zn precipitation phases in the weld zone. The grain sizes and the PMZ width in the arc-acting zone were larger thanthose in the laser-acting zone. The value of the ultimate tensile strength of the weldingjoints could reach 262MPa at best, which was 97.8% of that of the base metal. It was foundthat the grain sizes and the porosities in the welding zone were the primary factors whichdetermined the tensile strength vaule of the welding joints.Comparing with the pure laser welding, laser filling wire and laser-arc hybridwelding possessed their own advantages for effectively improving the surface defects inthe welding seam, reducing the porosities in the welding line to a certain extent andleading to higher tensile strength welding joints by taking into account of the bead shapes,the microstructure and the mechanical properties of the AZ31B-T5 wrought magnesiumalloy joints. Due to the arc existed, the heat input of laser-arc hybrid welding was higherthan that of laser filling welding in the same laser power and the same welding speed, andthe coarse grains were obtained in the welding seam which led to reduce.the tensilestrength value of the welding joint. The value of the ultimate tensile strength of the laserfilling welding joints was 120% of that of the base metal owing to the better weld shape,the highest value of the ultimate tensile strength of laser-arc hybrid welding joints was98% of that of the base metal.