| Magnesium alloys are widely used in aerospace, automotive industry, electronic communication and other fields because of the advantages of light weight, high specific strength, good electromagnetic shielding performance and so on. At present, more and more magnesium alloy welded parts have been applied in the above fields, so it is very important to study the high efficiency and good performance of magnesium alloy welding technology. In the actual production process, tungsten inert gas welding(TIG) was preferred for magnesium alloy welding. However, the welded joints of magnesium alloy have coarse grain, porosity and low mechanical properties, which limit their applications in engineering. In order to solve the above problems, the addition of strengthening particles and rare earth powder can be used as research means. In this paper, the effects of welding speed on the distribution of SiC particles, microstructure and mechanical properties of AZ31 magnesium alloy NSA-TIG welded joints were studied by using the optical microscope(OM), scanning electron microscope(SEM), X-ray diffraction(XRD), hardness testing machine and tensile testing machine. Moreover, the temperature field and flow field distribution of weld joints were simulated. In additional, the composite strengthening particles(SiC+Ce) were proposed as TIG welding coating. The effect of the content of rare earth Ce and the amount of composite strengthening particles on the macro morphologies, welding defects, microstructures and mechanical properties of AZ31 magnesium alloy TIG welded joints were studied.The SiC particles were mainly distributed in the central area and the upper part of the weld pool of AZ31 mgnesium alloy NSA-TIG welding joints. With the increase of welding speed, the content of SiC in the welding pool decreases gradually. When the speed was more than 180 mm/min, part SiC particles agglomerated in the weld pool. The hardness value of the central part and the upper part of the weld pool were higher than that of the other regions. With the increase of welding speed, the ultimate tensile strength of the joint firstly increased and then decreased, and the ultimate tensile strength reached 230 MPa at the welding speed of 180 mm/min. The change law of ultimate tensile strength was mainly caused by the fine grain strengthening, thermal mismatch strengthening and Orowan strengthening of SiC particles.Numerical simulation of AZ31 magnesium alloy NSA-TIG welding was carried out, and the behavior of weld pool at different welding speed was studied. Results shown that with the increase of welding speed, weld pool surface temperature gradient increased gradually, and the liquid metal flow properties of molten pool decreased gradually. Simulation results shown that the liquid metal flow performance of central region of molten pool was the best.The different proportions of SiC particles and Ce powders were used as composite strengthening particles for AZ31 magnesium alloy particles strengthening TIG welded. The effects of the content of rare earth Ce in composite strengthening particles on the microstructure, weld defects and mechanical properties of AZ31 magnesium alloy particles strengthening TIG welded joints were studied. The results showed that a small amount of Ce could refine ?-Mg grain, reduce the porosity and improve the mechanical properties of the joints. After joining the Ce and SiC, the microstructure of the welded joint was mainly composed of the ?-Mg, ?-Mg17Al12, SiC and Al3 Ce phases. With the increase of Ce content, the ?-Mg grains coarsen and the Al content in the ?-Mg decreased, which resulted in the decrease of the mechanical properties of the joint.After obtaining the optimal proportion of composite strengthening particles, the effects of the coating amount on the microstructure,welding defects and mechanical properties of AZ31 magnesium alloy TIG particles strengthening welded joints were studied. Experimental results showed that with the increase of the amount of coating, the surface quality of the joint is gradually getting worse. When the amount of coating was 2.4 mg/cm2, some black oxide and collapse were formed on the weld surface. With the increase of the amount of the composite strengthening particles, the distributions of SiC particles in the molten pool were more uniform. With a small amount of composite strengthening particles, the ?-Mg grains of the welded joint are obviously refined, and the mechanical properties of the welded joint are greatly improved. The optimum mechanical property of welded joint was obtained with the 1.2 mg/cm2 coating. With the further increase of the coating amount, the ?-Mg grains in the fusion zone coarsen, which reduced the mechanical properties of the joints. |
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