Effects Of Activating Flux On The Microstructures And Mechanical Properties Of Tungsten Inert Gas And Laser Beam Welded AZ31 Magnesium Alloy Joints

In the paper, hot-extruded magnesium alloy AZ31 was selected and the effects of TiO₂ coating and CaF₂ coating on the macro-morphologies, microstructures and mechanical properties of tungsten inert gas (TIG) welded AZ31 magnesium alloy joints were systematically studied by adjusting the welding parameters and the amount of the flux coating. The microstructural evolutions and the relationships between microstructures and mechanical properties of welded joints were revealed; the inner relationships between the amount of active flux coating and welding defects were clarified. Finally, due to the low energy utilization of laser welding of magnesium alloy, a new laser welding technology with active flux was developed. Three different types of active fluxes were studied for laser welding of AZ31 magnesium alloy, the intrinsic mechanisms of the increased weld penetration and improved mechanical properties of welded joints were discussed in details. Based on the experimental facts and combined with theoretical analysis, main conclusions may be summarized as follows:The effects of TiO₂ coating on the macro-morphologies, microstructures and mechanical properties of TIG welded AZ31 magnesium alloy joints were investigated by microstructural observations, microhardness tests and tensile tests. The results showed that an increase in the amount of the TiO₂ coating improved the weld appearance, the weld penetration and the depth/width (D/W) ratio of the TIG welded AZ31 magnesium alloy seams (however, too much TiO₂ coating decreased the D/W ratio and deteriorated the surface appearance of the samples). Moreover, with an increase in the amount of the TiO₂ coating, theα-Mg grains and theβ-Mg17Al12 intermetallic compound (IMC) coarsened while the volume fraction of theβ-Mg17Al12 phase decreased. The microhardness of the fusion zone (FZ) and the HAZ was lower than that of the BM due to the coarsening ofα-Mg grains in the FZ and the HAZ. With an increase in the amount of the TiO₂ coating, the microhardness of the FZ of the AZ31 magnesium alloy welded joints decreased slightly at first because it was dominated by the dispersion strengthening effect ofβ-Mg17Al12 particles. Then, it decreased sharply due to the effect of the coarsening ofα-Mg grains. The ultimate tensile strength (UTS) value and elongation of the welded joints increased with an increase in the amount of the TiO₂ coating because the decrease in the porosities and cracks. However, too much TiO₂ coating caused a significant decrease in the UTS value and elongation of the welded joints because of the coarsening ofα-Mg grains and the formation of cracks in the FZ.The effects of CaF₂ coating on the macro-morphologies of the TIG welded AZ31 magnesium alloy seams were studied. Microstructure and mechanical properties of the butt joints welded with different amounts of CaF₂ coatings were investigated by optical microscopy observation and tensile tests. The welding defects formed in welded seams and the tensile fracture surfaces were analyzed by a scanning electron microscopy. The results showed that an increase in the amount of the CaF₂ coating deteriorated the appearance of the welded seams but it improved the weld penetrations depth and the D/W ratio of the TIG welded joints. With an increase in the amount of the CaF₂ coating, theα-Mg grains and theβ-Mg17(Al,Zn)12 IMC coarsened while the volume fraction of theβ-Mg17(Al,Zn)12 particles decreased. With the increase in the CaF₂ coating, the porosities and the average diameters of hydrogen pores declined because the CaF₂ coating slowed down the solidification of welding pool and fluorin ion reacted with hydrogen ion. The total length of the solidification cracks decreased gradually. However, it increased slightly when the largest amount of the CaF₂ coating (6.35 mg cm-2) was added. The UTS value and elongation of the welded joints increased gradually at first due to the decrease of the porosities and the total length of the solidification cracks. However, too much amount of the CaF₂ coating caused serious coarsening of theα-Mg grains and led to the formation of oxides and cracks, which resulted in the decrease of the mechanical properties of the welded joints significantly.The effects of activating fluxes on the macro-morphology, microstructure and mechanical property of low-power pulse laser beam welding of AZ31 magnesium alloy joints were investigated by microstructural observations, microhardness tests and tensile tests. The results showed that three types of fluxes all can improve the weld appearances and increase the weld penetration depths and the D/W ratios. The SiO₂ flux can make the penetration increased as much as 200%. However, the addition of fluorides deteriorated the weld appearances and had little effect on the weld penetration. Moreover, there is a little change of the grains in the welded joints prepared with fluxes and without fluxes, the morphologies of grains in the band zone were characterized by dendritic crystals. In addition, the mechanical properties (including the UTS value and microhardness) of the welded joints decreased compared to the welded joint without flux.