Study On Friction Stir Spot Welding Procedure And Microstructure Property Of AZ31 Magnesium Alloy

Magnesium alloy is the preferred material for manufacture of lightweight structures thanks of a series of extraordinary features. However, the fusion welding method results easily defects like cracks, slag inclusion, splashing and bigger deformation. FSSW (friction stir spot welding) is a solid-phase joining process newly developed based on friction stir welding. Compared with the traditional welding methods, FSSW has the feature of low absorption of welding heat, smaller deformation of work-piece, stable welding quality, cost reduction and the welding process could be implemented by simple programming, provides research prospect for magnesium alloy welding.In this paper, the pin tool was optimal designed for the hot extruded AZ31 magnesium alloy sheet, and then the friction stir spot welding test was carried out. The microstructure and properties of the welding joints with optimal parameters were tested by optical microscope, scanning electron microscopy, tensile testing machine and micro-hardness tester, and the welding defects were also analyzed.By means of the friction stir welding process and orthogonal test, the optimal FSSW welding parameters have been determined for the AZ31 magnesium alloy with 3mm thick. The optimal welding parameters are as follows, the rotation speed is 2450r/min, welding time is 8s and the downward compression at the shaft shoulder is 0.25mm.While for the AZ31 magnesium with compound radius, the optimal FSSW welding parameters are as follows, the radius of gyration is 0.5mm, the press amount of the shoulder is 0.2mm, the rotation speed is 2450r/min and welding time is 8s. The microstructure studies show that tiny and even equiaxial grains are formed in WNZ (weld nugget zone) and coarse grains with uneven sizes are formed in TMAZ(thermo-mechanically affected zone) and HAZ (heat affected zone). The micro-hardness tests show that the micro-hardness of WNZ is slightly lower than the base metal's hardness, and the micro-hardness of the TMAZ is higher than the hardness of HAZ, the minimum hardness appears in the heat affected zone. The hardness of the joints in upper plate distributes in the shape of double-V, while it distributes in W-shaped in the nether plate joints. The shear resistance force of magnesium alloy welded joint prepared by FSSW reveals that the shear strength with composite radius is higher than the normal joints, about 34.5%. The formation mechanism and features of the welding defects have been analyzed comprehensively in the FSSW joints. The defects in friction stir welding of magnesium alloys are main flying edge, burr, oxidation wear rings, involute and weak connections, while the involute are main in three forms, namely linear shaped, S-shaped, V-shaped. Defects significantly affect the mechanical properties of welding joints, and when shape and size of pin tool are determined, the defects have a great relationship with the welding parameters.