Microstructures, Wear and Corrosion Behaviors of Laser Cladding In Situ Synthetic Al3Ti/AlNi/AlNi3/MgNi2 Composite Coatings on Magnesium Alloy using Al as Middle Layer

In situ synthesized Al3Ti/AlNi/AlNi3/MgNi2-reinforced composite coatings were fabricated on AZ91D magnesium alloy by laser cladding using a mixture of Al, Ti and Ni-C powders. Al layer was adopted as a middle layer for avoiding much evaporation of Mg substrate at high heat input. Phase composition, microstructures and surface properties of coatings and the substrate were studied. The interfaces between the substrate, middle layer, and cover layer presented good metallurgical bonding, and no crack was found in the interior. Tiny gray Al3Ti and large white axiolitic AlNi + Al3Ti particles were synthesized in 10 wt.% and 20 wt.% TiNiC coatings, while AlNi + Al3Ti was replaced by MgNi2 + AlNi3 + Al3Ti with polygonal and irregular shape in 30 wt.% TiNiC coating, and in this coating tinier Al3Ti tended to aggregate. Matrix phases in three coatings were α-Al + Al3Mg2 and Al12Mg17 + Al3Mg2, respectively. The formation of intermetallic compounds increased the hardness of the coating. The maximum hardness value is 368.27HV0.1 in the coating with 30 wt.% TiNiC, which was about 5.93 times that of the AZ91D substrate (62HV0.1). The friction coefficient, wear rate and wear track also showed that wear resistance of coatings was better than substrate and increased with the increase in TiNiC content. Corrosion resistance of the coatings is all improved significantly compared with the substrate.