Deposition behavior and microstructure of WC-12Co coating deposited by HVOF on 7075 aluminum alloy substrates

WC-12Co particles were deposited on a polished AA7075 (7075 aluminum alloy) substrate, and a WC-12Co coating was prepared on grit-blasted AA7075 substrate by HVOF (High Velocity Oxy-Fuel) spraying. The deposition behavior of a single particle and the stacking behavior of multiple particles were studied, as well as the bonding state between the coating and substrate, and that among particles within the coating. The phases, microstructure, and composition of the coating were analyzed by XRD, SEM, EDS and TEM. Results show that the particles impact the AA7075 substrate at a high speed in a liquid-solid bi-phase state, and they are embedded in the substrate and metallurgically bonded with it; the main elements W and Al exhibit gradient changes across the area. Extrusion zones, extrudates, teared zones, sideslip zone, and splashes appear on the substrate when the particles are being deposited and stacked. In the stack zone, the particles exhibit better melting and splash appear in them, and then the stocked particles have melted together. The deposited particles on the substrate are approximately 30 μm in size, which are in an optimal state (temperature and velocity), which possess strong deformation ability, allowing them to embed into rough surface with the depth of about 108 μm. The obtained coating is dense without large pores, cracks, delamination and other defects, which is tightly bonded to the substrate. The phases of the coating include WC, a small amount of W₂C (in fine WC) and Co₃W₃C (at the edge of large WC), while no element Co is detected; however, an amorphous phase appears, which consists of a mixture γ + W_xC. Finally, the reasons for the formation of these phases were analyzed.