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Effects of welding current on macromorphologies, microstructures and mechanical properties of nano-SiC particles strengthening activating flux tungsten inert gas welded AZ31 magnesium alloy joints were investigated by scanning electron microscope and energy dispersive X-ray spectrometer observations and microhardness and tensile tests. The results showed that SiC particles mainly dwelled in the centre and bottom zone of welding pool. Moreover, with the welding current increasing, the mechanical properties of the welding joints were improved by the increased SiC particles in welding pool, the depth/width ratios of weld pool and the refined a-Mg grain size. However, the a-Mg grains coarsened and the mechanical properties of joints decreased due to excessive heat input in welding process when welding current over 115 A.

In this work, a new water bath friction stir spot welding (WB-FSSW) process was developed. The effects of dwell time on the microstructures and mechanical properties of the WB-FSSW-welded AZ31 magnesium alloy joints were investigated by microstructural observation, tensile tests, and microhardness tests. It was found that compared with those of ordinary FSSW welds, the WB-FSSW welds have poorer tensile shear force, almost equal tensile shear strength per unit area τ and higher microhardness mainly due to the narrower bonded zone width of the hooks and microstructures in the welded joints, respectively. Moreover, with the increase in the dwell time, the microhardness and the tensile shear strengthen per unit area τ of the WB-FSSW-welded joints increased firstly and then decreased because of the fluctuant evolution of grain size of α-Mg and the volume fraction of the β-Mg 17 Al l2 . The optimal dwell time for the WB-FSSW welding was from 10 to 15 s whereby the finest grains and the highest mechanical properties of the welded joints can be achieved.

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