The Research On Gas Tungsten Arc Welded Magnesium Alloy AZ31 Plates

In this thesis, the influence of welding factors (welding current, surface coating and oxide film, shielding gas, welding wire, post weld heat treatment) on geometry and quality of welded joints of gas tungsten arc (GTAW) welded magnesium alloy AZ31 plates with 1.2 thickness was discussed. The microstructure was investigated with optical microscopy. Mechanical properties were determined by standard tensile tests on small-scale specimens. Fractography characteristics were observed by scanning electron microscopy (SEM). Meanwhile, the factors leading to the forming of porosity and cracking in welded joints were analyzed and the methods to avoid porosity and cracking were described.The result indicated that welding parameters have great impact on the shape and microstructure of the joints. When oxide film on the surface of magnesium alloy is not erased, the weld is wide and shallow. The width and penetration of weld increases as the welding current increasing. The weld broadens firstly, then narrows as the shielding gas increasing, and the same is the penetration. When the welding parameters are feasible, it can get well weld formation, no porosity and cracking exists in the welded joints.The microstructure is fine equiaxed grains in welded zone, which is shown to consist of a-Mg and Mg₁₇Al₁₂, while the microstructure of heat affected zone is coarser. Tensile specimens fracture at heat affected zone. The content of Mg in welded zone is a little reduced, while the content of Al increased.Welding current has great impact on mechanical properties of the joints. Mechanical properties of the joints decrease at low or high welding current. Without welding wire, the joint strength is about 79 percent of the base material at 45A welding current. The joints strength is increased with welding wire. The joints strength increases with the content of Al in welding wire. The joints strength can be improved with post weld heat treatment.Magnesium has higher hydrogen solubility in liquid than in solid. The difference in the solubility between the solid and liquid phases is major cause of the porosity of magnesium alloys during welding. The low melting element of magnesium alloy is easily to vaporize and the presence of porosity in the base metal coalesced and expanded, which is another cause. Because of low melting...