Effects Of The Welding Parameters And Hot Treatment On The Microstructures And Mechanical Properties Of Tungsten Inert Gas Welded AZ31B Magnesium Alloy Joints

Magnesium and its alloys have a higher specific strength and stiffness, an excellentdamping capability, electromagnetic interference shielding property, machiningperformance and castability compare with other alloys and engineering plastics.Magnesium becomes the most important lightweight structural material in the21stcentury. However, the wider application of magnesium alloys needs reliable weldingprocesses. The tungsten inert gas arc welding （TIG） is widely used in magnesium alloysbecause of the good weld appearance and high quality. In the paper, hot-extrudedmagnesium alloy AZ31was selected and the effects of welding current andmix-activated fluxes （TO₂-CaF₂） on the macro-morphologies, microstructures andmechanical properties of the activating flux TIG welded （A-TIG） AZ31B magnesiumalloy joints were systematically studied by adjusting the welding parameters andchanging the composition of the mix-activated fluxes. The mechanisms of the increasein the weld penetration of the A-TIG welded magnesium alloy joints with the additionof activating fluxes and the formation of defects were discussed in details. Finally, dueto the bad mechanical properties of the A-TIG welded AZ31B magnesium alloys joints,the relationships between microstructures and mechanical properties of the TIG weldedmagnesium alloy joints were systematically studied by adjusting the parameters of hottreatment. Based on the experimental facts and combined with theoretical analysis, mainconclusions may be summarized as follows:The effects of welding current on the macro-morphologies, microstructures andmechanical properties of the A-TIG welded AZ31B magnesium alloy joints wereinvestigated by microstructural observations, microhardness and tensile tests. Theresults showed that the welded joint with TiO₂coating exhibited a deeper weldpenetration compared with the welded joint without TiO₂coating. An increase of thewelding current resulted in an increase in the depth/width （D/W） ratio and weldpenetration of the welded seams with TiO₂coating, however, too high welding currentdecreased the D/W ratio and deteriorated the surface appearance of the welded seams.Moreover, the size of α-Mg grains and the amount of the granular β-Mg₁₇（Al, Zn）₁₂particles in the welded seams also increased. In addition, the microhardness of theA-TIG welded seams was lowered than that of the BM due to the coarsening of α-Mggrains in the welded seams and the average microhardness of the heat-affected zone and fusion zone of the welded joints decreased gradually with the increase of the weldingcurrents. With an increase of the welding current, the ultimate tensile strength （UTS）value of the welded joints with TiO₂coating increased gradually and then decreasedwhen too high welding currents were adopted. The fact that the element of Ti wasdetected in the middle and bottom of the weld seam indicated that the TiO₂coating hadan effect on the convection in the weld pool.The effects of mix-activated fluxes （TO₂-CaF₂） on penetrations, microstructuresand mechanical properties of the TIG welded AZ31B magnesium alloy joints werestudied by microstructural observations, microhardness and tensile tests. The resultsshowed that an increase in the amount of the TiO₂coating improved the weldpenetration and the depth/width （D/W） ratio of the TIG welded AZ31B magnesiumalloy seams. However, the CaF₂have limited ability to increase penetration and thedepth/width ratio of the weld bead and with a poor formation of the weld surface. Themix-activated flux of30%CaF₂+70%TiO₂led to the great improvement in the D/Wratio capability, up to118%compared with the normal welded seam without flux. Theα-Mg grains of the fusion zone （FZ） became fine gradually with the addition of CaF₂coating but coarsened sharply when the amount of TiO₂coating was over70%. Inaddition, the porosities and total length of solidification cracks in the FZ were reducedby the CaF₂and TiO₂coating. With an increase in the amount of the TiO₂coating, themicrohardness of FZ of the AZ31B magnesium alloy welded joints decreased slightlyinitially and then decreased sharply when more TiO₂coating was adopted. The UTSvalue of the welded joints increased with the addition of TiO₂coating but decreasedsharply when more TiO₂coating was adopted.The effect of heat treatment on the microstructures and mechanical properties ofthe TIG welded AZ31B magnesium alloy joints with Cr₂O₃flux coating wereinvestigated by microstructural observations, microhardness and tensile tests. Theresults showed that the seams welded coating with Cr₂O₃flux exhibit smooth surface.The activating flux of Cr₂O₃improved the weld penetration and the depth/width （D/W）ratio of the TIG welded AZ31B magnesium alloy joints obviously, up to84%comparedwith the normal welded seam without flux. The solution treatment led to the β-Mg₁₇（Al,Zn）₁₂particles dissolve into the α-Mg grains. With an increase in the aging temperature,the average microhardness of the fusion zone increased gradually （higher than that ofthe untreated seam when the aging temperature was over463K） due to the increasedvolume fraction of theβ-Mg₁₇（Al, Zn）₁₂particles. Also, the UTS value of the welded joints increased with the increase in aging temperature but decreased sharply when theaging temperature was too high. The elongation of the aging treated welded jointsincreased slightly due to the increase in the volume fraction of theβ-Mg₁₇（Al, Zn）₁₂particles in the FZ.