Effects Of Particle Reinforcement On The Microstructures And Mechanical Properties Of Activating Flux Tungsten Inert Gas Welded AZ31B Magnesium Alloy Joints

In the paper, AZ31B magnesium alloy was selected and the effects of the additionof SiC particles and activating fluxes on the macro-morphologies, microstructures andmechanical properties of activating flux tungsten inert gas （A-TIG） welded joints weresystematically studied by microstructural observations and mechanical tests. Thestrengthening mechanism of SiC particles and the relationships between microstructuralevolutions and mechanical properties of the welded joints were revealed. Finally, twodifferent sizes of SiC particles were studied for SiC particle strengthening A-TIGwelding of magnesium alloy and the intrinsic mechanisms of particle sizes improvedmechanical properties of SiC particle strengthening A-TIG welded joints werediscussed in details. Based on the experimental facts and combined with theoreticalanalysis, main conclusions may be summarized as follows:Cr₂O₃activating flux mixed with SiC particles were selected as the flux coatings.The effects of SiC particles on the macro-morphologies, microstructures andmechanical properties of A-TIG welded joints were studied by microstructuralobservations, microhardness and tensile tests. The results showed that an increase in theamount of SiC particles contained in the Cr₂O₃activating flux led to the weldpenetrations decreased gradually, however, all the welded penetrations of welded seamswith Cr₂O₃activating flux were obviously higher than that without flux coating. Theaddition of SiC particles in Cr₂O₃flux coating improved the the surface appearance ofthe welded joints, but when the mass percentage of SiC particles in the flux was reachedto45%, the surface appearance of the welded seam was deteriorated. Grain refinementwas obtained in the welded seams when SiC particles were involved and distributeddispersed in the weld bead because of the change of the liquid flow in the weld pool.The mechanical properties （ultimate tensile strength and microhardness） of the weldedjoints increased gradually due to the dispersion strengthening effect of SiC particles andgrain refinement of the welded seams. However, too many SiC particles led to thedecrease of the ultimate tensile strength value because of the dispersion ability of SiCparticles was suppressed.TiO₂and CaF₂powder were selected as the activating fluxes and SiC particles wereadded into the fluxes with the mass percentage of40%to achieve flux mixture.Theeffects of activating fluxes （TiO₂and CaF₂） on the microstructures and mechanical properties of SiC particle strengthening A-TIG welded joints were studied.The resultsshowed that the surface appearances of welded joints were improved in the case of theaddition of SiC particles in TiO₂activating flux. However, when the addition of SiCparticles in CaF₂activating flux was adopted, the surface appearances of the weldedjoints were deteriorated. The α-Mg grains in fusion zone （FZ） of the welded jointsrefined with the addition of SiC particles in TiO₂flux. Because the CaF₂flux cannotinfluence the Marangoni convection to transport SiC particles into the welded seamsand the addition of SiC particles in the CaF₂flux did not refine the α-Mg grains sizecompared with the welded sample with pure CaF₂flux. The microhardness of the FZand ultimate tensile strength of the welded joints increased with the addition of SiCparticles in TiO₂flux and nano-sized particles have more strong efficiency on theimprovement of mechanical properties of the welded joints. However, the addition ofSiC particles in CaF₂activating flux decreased the ultimate tensile strength value ofA-TIG welded joints.TiO₂was selected as the activating flux and SiC particles with average sizes of10mm and40nm were added into the flux to achieve flux mixture.The effects of SiCparticle size on the microstructures and mechanical properties of SiC particlestrengthening A-TIG welded joints with TiO₂activating flux were studied. The resultsshowed that an increase in the amount of nano-sized SiC particles （40nm） in the TiO₂flux coating improved the depth/width （D/W） ratio of the welded joints obviouslycompared with the weded joints with the micro-sized SiC particles （10μm） in the flux.The weld forming was more sensitive to the change of the addition of nano-sized SiCparticles. The α-Mg grains of FZ of welded joints refined gradually with the addition ofnano-sized SiC particles （<40%） in flux coatings，and then coarsened when more SiCparticles added and brought agglomeration effect. Meanwhile, the effect of nano-sizedSiC particles on grain refinement was found to be more impressive. In addition, themechanical properties of the welded joints increased with the addition of nano-sizedSiC particles （<40%） in flux coatings. The ultimate tensile strength value of the weldedjoints reached the maximum value （98%of that of base material） when the amount ofthe nano-sized SiC particles in the flux coating reach to40%.