Laser-CMT (Cold Metal Transfer) and plasma-CMT hybrid welding are two promising alternative joining technologies for traditional Metal-Inert-Gas (MIG) welding of the aluminum alloy joints in the high speed trains manufacturing industry. In this work, a comparative study on the weld formation, microstructure, micro-hardness, and mechanical properties of the butt joints in the two welding methods was conducted. The results indicate that the overall quality of the laser-CMT and plasma-CMT welds were good, especially of the laser-CMT hybrid weld, and the laser-CMT hybrid welding process needed a lower heat input. The width of the partially melted zone of the laser-CMT hybrid weld was narrower than that in the plasma-CMT hybrid weld. Micro-hardness test results show that two distinct softening regions were identified in the heat affected zone, and the micro-hardness values of each zone in the laser-CMT hybrid weld were lower than that in the plasma-CMT hybrid weld. The tensile strength of the laser-CMT hybrid welded joints was higher than that of the plasma-CMT hybrid welded joints, which could reach up to 79.4% and 73.7% of the base materials, respectively. All the fractures occurred in the softening region and exhibited a ductile shear fracture with a shear angle of approximately 45°. The fractographs manifested that the laser-CMT and plasma-CMT hybrid welded joints presented ductile fracture and ductile-brittle fracture features, respectively.

In this study, 45 carbon steel was boronized and borosulphurized at 950°C for 2, 3， 4, 5, 6 and 8 h, respectively. The samples were characterized by scanning electron microscopy, optical microscope, microhardness tester and ring-on-block wear tester. It is found that the surface of borosulphurized samples was dense, compact and relatively smooth; Although the boride layers produced by boro- sulphurizing at 950°C showed a lower microhardness value compared with that produced by boronizing, the wear resistance of the borosulphurized carbon steel is higher than that of boronized sample due to formation of FeS phase in the boride layer.