Study On Microstructure And Properties Of Weld Joint Of AZ31B Magnesium Alloys By FSW/TIG

With the rapid development of aerospace, automotive and traffic transportation industries, high demand is required for lightweight materials. Magnesium alloys, acting as desirable lightweight materials due to their advantages such as low density, high specific strength and specific stiffness, excellent machinability, good recyclability, are regarded as green environmental engineering material for the 21st century. With the extensive application of magnesium alloys in engineering structures, welding process is certain to be required. Friction stir welding (FSW) is a new solid state joining technique, which is suitable for magnesium and aluminum lightweight metal welding owing to fume free, spatter free, porosity free, small deformation after welding. However, some disadvantages (expensive equipments, low flexibility, patent protection) restricts its wide application, whereas tungsten inert gas (TIG) becomes the most common welding method for its economization and practicality.Both FSW and TIG welding process experiments of hot extrusion AZ31B magnesium plate are studied in the paper. Microstructure and properties of weld joint in two process are analyzed by optic microscopy (OM), scanning electron microscopy (SEM), Energy Dispersive Analysis (EDS), X-ray diffractometry (XRD), tensile test machine and micro-hardness instrument.The optimum welding process parameter of FSW on magnesium alloy with 6.1 thickness is confirmed by process experiment and Orthogonal experiment:Welding speed is 200mm/min, spin rotating speed is 800r/min, shoulder insertion is 0.25mm. The results of tensile strengths indicate that the tensile strength is up to 230.5MPa (92.9% of base metal) and elongation is 7.1%. Fracture with mixed toughness and brittleness occurs in the HAZ. Microstrucrures show that microstructure of weld nugget zone has fine equiaxed grains, which consist ofα-Mg andβ-Al12Mg17 phase. Whereas dynamic recrystallization occur and grains are coarsened and some grains are deformed in the microstructure of thermo-mechanically affected zone (TMAZ); microstructure of HAZ is coarse. The micro hardness of the joint shows a W-shaped distribution and it gradually reduces from bottom to top of weld nugget zone.AC-TIG welding experiments on AZ31 magnesium alloy plate with 7.4mm thickness under three welding currents (160A,190A,220A) were carried out. The results indicate that the weld microstructures are almost fine equiaxed a-Mg matrix. Moreover, fineβ-Al12Mg17 phase are found and precipitated on grain boundaries. Microstructure of heat-affected zone coarsen; the effect of welding current on joint appearance and mechanical properties is obvious, the grains of weld and heat-affected zone become coarser and tensile strengths increase first and reduce afterwards with the increase of welding current. Weld joint with good formation and mechanical properties can be obtained under 190A welding current. The tensile strength of joint is up to 218.3MPa (88.1% of base metal) and elongation is 6.2%. the fracture with mixed toughness and brittleness all occurs in the heat-affected zone. The hardness test indicates that the micro hardness of the joint shows a W-shaped distribution and it gradually increases from weld bottom to top and fluctuates heavily. Generally speaking, mechanical properties of FSW is better than that of TIG.