Microstructure Characterization of Nickel Matrix Composite Reinforced with Tungsten Carbide Particles and Produced by Laser Cladding

Detailed experimental characterization of a laser-clad metal matrix composite (MMC) consisting of hard tungsten carbide particles embedded in a comparatively soft nickel-based matrix is provided. Special focus of the investigations is placed on the relationship between the microstructure of the as-deposited reinforcing particles and their hardness. Therefore, thermally induced dissolution of carbides caused by laser metal deposition (LMD) processing is studied. The as-received powder blend mainly consists of fused spherical particles of ditungsten carbides (W ₂C, W ₂C _1−x) and monotungsten carbides (WC, WC _1−x). Dissolution of large particles at the matrix/particle interfaces and fragmentation or even complete dissolution of small particles due to the high process temperature of LMD is observed. Primary W ₂C/W ₂C _1−x phases are partially dissoluted, and layers of secondary WC/Ni _xW _yC phases are formed at the matrix/particle interfaces. As these surface layers are less hard than the as-received particles, the local hardness gradually decreases from the inner region of the particles across the surface layer toward the matrix, which is supposed to improve bonding of the particles inside the matrix. The hardness depends on the grain size and on the crystal structure of the carbides. Even when the crystal structures are identical, particles consisting of fine grains have considerably higher hardness than particles consisting of coarse grains.