In this work, microstructure and mechanical properties of Mg weld with addition of carbon nanotubes (CNTs) and TiC particles were investigated. The results showed that the weld microstructure was mainly presented as equiaxed grains with almost high angle grain boundaries. The introduction of reinforcements promoted the formation of precipitates and refined the grains effectively, the average grain size was refined by 51% and 23% with addition of CNTs and TiC particles, respectively. The dislocation density and the fraction of CSL boundaries were increased with addition of CNTs, while those were decreased with addition of TiC particles. Besides, the infrequent {1013} contraction twins formed within the weld due to the stress concentration caused by dislocation accumulation, which contributed to the Σ29 CSL boundary. The ultimate tensile strength and elongation rate were increased by 13.5% and 40% with addition of CNTs, while the ultimate tensile strength and micro-hardness were increased by 14.8% and 20.9% with addition of TiC particles.

Oscillating laser-arc hybrid welding of AZ31B magnesium alloy was carried out, the effects of beam oscillation parameters on pore inhibition, microstructure, grain boundary characteristics and tensile properties were investigated. The results showed that the pore formation can be inhibited with oscillating frequency higher than 75 Hz and radius smaller than 0.5 mm. The columnar grains neighboring the fusion line can be broken by the beam oscillation behavior, while the grain growth was promoted with the increase of frequency or radius. It should be noted that the coincidence site lattice (CSL) boundaries were mainly Σ13b and Σ29 boundaries, which were contributed by {1012} tensile twins and {1122} compression twins, respectively. The total fraction of CSL boundaries reached maximum at radius of 0.25 mm and frequency of 75 Hz, which was also confirmed as the optimized parameters. In this case, the elongation rate increased up to 13.2%, 12.8% higher than that of the weld without beam oscillation. Finally, the pore formation and inhibition mechanisms were illustrated according to the state of melt flow and keyhole formation, the abnormal growth was discussed basing on secondary recrystallization, and the relationship among the pore formation, grain size, boundary characteristics and weld toughness were finally established.

Laser-arc hybrid welding of AZ31B magnesium alloy was carried out, the effects of welding parameters on weld formation, microstructure homogeneity and mechanical properties were investigated. The results showed that laser-arc hybrid welding was beneficial to improve the weld formation of magnesium alloy by inhibiting the defect of undercut and pores. The weld microstructure was mainly columnar grains neighboring the fusion line and equiaxed grains at the weld center. It was interesting that the grain size at the upper arc zone was smaller than that at the lower laser zone, with the difference mainly affected by laser power rather than welding current and welding speed. The welding parameters were optimized as laser power of 3.5 kW, welding current of 100 A and welding speed of 1.5 m/min. In this case, the weld was free of undercut and pores, and the tensile strength and elongation rate reached 252 MPa and 11.2%, respectively. Finally, the microstructure homogeneity was illustrated according to the heat distribution, and the evolution law of tensile properties was discussed basing on the weld formation and microstructure characteristics.