| Magnesium and its alloys have the prospect of wide application in the automotive, aircraft and electronic consumer industries because of their low density in combination with a high strength, an excellent castability, a perfect electromagnetic interference shielding property, a high thermal conductivity and a high damping capability. However, the production of complicated workpieces of magnesium alloys is usually difficult and expensive because of their poor ductility and cold processability at room temperature. This is because magnesium has a hexagonal close-packed (HCP) crystal structure, which has insufficient slip systems at room temperature. Therefore, welding technology of magnesium alloys play an important role in the board application of magnesium alloy in structural parts.In the paper, hot-extruded magnesium alloy AZ31 and AZ61 were selected and the microstructural evolutions and the relationships between microstructures and mechanical properties of joints of laser welding (LW), tungsten inert gas arc weldin (TIG) and friction stir spot welding (FSSW) of magnesium alloys were systematically studies by adjusting the heat inputs and the welding speeds. Moreover, the increase of yield strength of joints were predicted by strengthening mechanism models. Lastly, due to the poor ductility of magnesium alloy at the room temperature, a new FSSW was applied to improve the weldability of magnesium alloy AZ31. The characterization of welded joint was analyzed and the strengthening mechanism of joint was discussed in details. Main conclusions may be summarized as follows:The effects of the heat input on the microstructures and mechanical properties of TIG butt-welded AZ61 magnesium alloy plates were investigated by microstructural observations, tensile tests and microhardness tests. The results show that an increase of heat input resulted in an increase of the width of heat affected zone (HAZ) and the grain coarsening of theα-Mg in both the HAZ and the fusion zone (FZ). Moreover, continuousβ-Mg17Al12 phase decreased while granularβ-Mg17Al12 phase increased in both the HAZ and the FZ with an increase of the heat input. In general, the ultimate tensile strength (UTS) of welded joints increased with an increase of the heat input because a too low heat input led to the presence of partial penetration and pores. However, a too high heat input decreased the UTS of the welded joints slightly due to the evaporation of the zinc from the magnesium alloy AZ61. The tensile fracture of the welded joints usually occurred in the HAZ and the fracture surfaces of the welded joints were characterized by brittle and ductile components.The microhardness of the HAZ was lower than that of the base material (BM) and FZ due to the grain coarsening ofα-Mg in the HAZ. With an increase of the heat input, the microhardness of both the HAZ and the FZ decreased sharply at first and then decreased slightly due to the formation of the granularβ-Mg17Al12 phase when a relatively high heat input was used.The effects of the heat input on the microstructures and the weldability of welded seams in a low-power ND:YAG pulse laser welded AZ61 magnesium alloy plate were investigated by microstructural observations. The results show that the penetration depths of the welded seams increased with an increase of the heat input. Moreover, a more stable and thin plasma formed on the surface of the molten pool helped to increase the penetration depth. In agreement with the classical theory of solidification, the morphologies of grains in a band zone, which was located in the FZ and close to the fusion boundary, evolved with an increase of the heat input as: cellular crystals→cellular dendritic crystals→dendritic crystals→equiaxed crystals. An increase of the heat input reduced the tendency of the formation of solidification cracks but increased the amount of liquation cracks. A welded seam with a minimum length of hot cracks was obtained when the value of the heat input was 68 J.mm-1. The porosity and average diameter of hydrogen pores in the welded seams increased at first with an increase of the heat input and reduced sharply when a relatively large heat input was used. The degree of the formation of craters increased linearly with an increase of the heat input due to the loss of elements and the formation of high metallic vapor pressures, which were caused by elemental evaporation.The microstructures, mechanical properties and strengthening mechanism of high power laser welded AZ61 magnesium alloy plates were investigated by microstructural observations, microhardness tests and tensile tests. The results show that compared with the BM contains the fineα-Mg grains, the microstructure in FZ consisted of fineα-Mg equiaxed dendrite crystals and some distributed dispersedlyβ-Mg17Al12 phases, which were characterized by strip and granule. Increasing the welding speed, the strip-likeβ-Mg17Al12 decreased while the granularβ-Mg17Al12 phase increased in the FZ. With an increase of the welding speed, the sizes of bothα-Mg grains andβ-Mg17Al12 phase particles in FZ decreased while the volume fractions ofβ-Mg17Al12 phase increased. The UTS, yield strength (YS) and elongation (EL.) of the welded joints increased when the welding speed increased from 1800 mm.minute-1 to 2800 mm.minute-1. The increase in YS can be attributed to the grain boundary strengthening, solid solution strengthening and precipitation strengthening. It is suggested that precipitation strengthening is the largest contributor to the strength of joints of the laser welded magnesium alloy AZ61. In addition, compared with the microhardness in HAZ and BM, the microhardness in FZ significantly increased due to the grain refinement and the increase ofβ-Mg17Al12 phase particles. When the welding speed increased from 1800mm.minute-1 to 2800mm.minute-1, the microhardness of the FZ up to the BM increased from 103% to116%. Compared with the friction stir welding of magnesium alloy, the fusion welding of magnesium alloy shows a stronger grain size dependence of hardness.Due to the poor ductility of magnesium alloy at the room temperature, a new FSSW was used to improve the weldability of magnesium alloy AZ31. Comparative evaluation of magnesium alloy AZ31 joints welded by FSSW with and without heating were conducted by microstructural observations and tensile tests. The results show that compared with the joint of FSSW magnesium alloy without heating, the width of joining zone of the joint with heating increased significantly while the width of partial metallurgical joint in the joining zone decreased. In addition, the pores near the interface between top and bottom sheet reduced remarkably due to the use of continuous heating source, which enhanced the plastic deformation capacity and flowability of magnesium alloy during the FSSW process. The tensile property of the joint of FSSW magnesium alloy AZ31 with heating significantly increased due to the increase in width of joining zone and the decrease in pore.
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