Design and characterization of gradient coating on AZ91D alloy: Reinforcing effect of in-situ synthetic TiB ceramic phase, mechanical properties, corrosion-wear characteristics

The application of magnesium alloys is mainly hindered by their poor wear and corrosion resistance. In this work, the gradient coatings regulated by rare earth (La 2 O 3 and LaB 6 ) were prepared on the surface of AZ91D magnesium alloy using laser cladding. Numerical simulations were performed to identify the optimal rare earth doping concentration and laser parameters. Furthermore, the influence mechanism of rare earth on the microstructure, microhardness, corrosion resistance and wear properties of the coating was systematically explored. Following the addition of La 2 O 3 /LaB 6 , the microstructural inhomogeneity of the Ti 3 Ni 2.5 Al 2 Cu 1.5 Zr top layer was significantly reduced, accompanied by a phase transformation from BCC to FCC structures. Particularly, the introduction of LaB 6 triggered the in-situ generation of TiB strengthening phase. FIB was adopted to prepare TEM specimens for the detailed phase identification. TEM observations clearly revealed distinct and defect-free interfacial boundaries between the two solid-solution phases and TiB phase. Compared with the Ti 3 Ni 2.5 Al 2 Cu 1.5 Zr top layer, the I corr of the coating after rare earth addition decreased by 26.1% (for La 2 O 3 ) and 47.59% (for LaB 6 ), respectively. Notably, the initiation and propagation of pitting could be effectively suppressed by the in-situ formed TiB phases. The top layer incorporating LaB 6 exhibited superior tensile strength and microhardness, as well as a remarkably reduced wear rate and cumulative wear volume.