| Firstly, this paper reviews the research status about heat treatment in the field of laser cladding. The Ti C-WC reinforced composite coatings were produced on the surface of Ti6Al4 V alloy substrate by laser cladding Ni/WC, Ni Cr BSi/WC mixed powder respectively. For the cracking problem of laser cladding technology, the low temperature post-heat treatment with different holding time were applied for pure Ni/WC coatings, and the high temperature post-heat treatment with different temperatures and cooling medium were applied for Ni Cr BSi/WC coatings. Effects of post heat treatment on microstructure,microhardness and fracture toughness of the coatings were investigated by scan electron microscopy(SEM), X-ray diffraction(XRD), energy dispersive spectroscopy(EDS),optical microscopy(OM), especially focused on the microstructure evolution and fracture toughness. On this basis, wear resistance of the coatings were evaluated under the dry sliding reciprocating friction condition at room temperature.The results indicated that the pure Ni/WC coatings possess excellent bonding with the substrate and uniformly dense microstructure. The coatings mainly consist of a certain amount of coarse white equiaxed WC particles surrounded by the white-bright W2 C, a great deal of fine dark spherical Ti C particles and the matrix composed of the α(Ti), Ti2 Ni and Ti Ni phases. Effects of the post heat treatment on phase constituents and microstructure of the coatings were not significant. However, the post heat treatment decreased the residual stress and microhardness, and increased fracture toughness of the coatings. The average microhardness values of the coatings were reduced from 782 HV0.1to 710 HV0.1 and 687 HV0.1 with the heat treatment for 1 h and 2 h, respectively. And the fracture toughness of the coatings were improved from 2.77 MPa·m1/2 to 3.80 MPa·m1/2and 4.43 MPa·m1/2 with the heat treatment for 1 h and 2 h, respectively. The dominant wear mechanism for the coatings was abrasive wear. The appropriate heat treatment(heated at500 °C for 1 h) can significantly reduce the value and fluctuation of the friction coefficient,and had small effects on wear mass and volume loss.Laser cladding Ni Cr BSi/WC coatings on Ti6Al4 V alloy substrate were fabricated.The microstructure of the coatings mainly comprised coarse, gray, and equiaxed WC particles accompanied by white(W, Ti)C particles and black Ti C cellular/dendrites. The matrix comprised Ni Ti and Ni3 Ti phases. The microstructural evolution of the coatings at various temperatures was analyzed. Some Ni- and Ti-rich unknown precipitates were generated and uniformly distributed in the Ni3 Ti phase to form a thin granular layer. The layer spread from Ni Ti to Ni3 Ti with increasing temperature. A Cr23C6 secondary phase precipitated from the Ni Ti supersaturated solid solution at 900 °C. Meanwhile, the average microhardness and the fracture toughness were significantly increased to about 1395 HV0.1and 5.31 MPa·m1/2 from 1142 HV0.1 and 3.05 MPa·m1/2 of as cladding coating. The post-heat treatment not only reduced wear volume and friction coefficient, but also decreased cracking susceptibility during sliding friction. Comparatively speaking, the heat-treated coating at 900 °C presented the most excellent wear resistance.The effects of post-heat treatment(about 900 °C for 1 h) with different cooling medium on laser cladding Ni Cr BSi/WC coatings were further investigated. The results indicated the relative content and size of Cr23C6 precipitate phase was reduced significantly in the water-cooling coating compared to the air-cooling coating. The Cr23C6 phase was precipitated from the Ni Ti supersaturated solid solution at 900 °C and increased or grown up in the subsequent cooling process. Due to the quenching effect caused by the rapid cooling rate, the brittleness and hardness were improved markedly. As the result, the fracture toughness was decreased and the micro-cracks were easily produced during friction and wear process, leading to delamination. |
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