Situ Synthesis Of Composite Particulate Reinforced Nickel-based Laser Cladding Layer

In situ synthesized particulate-reinforced metal matrix composites (MMCs) coatings has broad application prospects by combining the high-intensity and high toughness of metal with the ceramics properties such as high melting point, high chemical stability, high hardness and high wear resistance, and by making high performance working surface with the low-cost traditional substrates. The eminent advantage of the in situ synthesis technology is that the reinforcements nucleate and grow in the matrix. Therefore the particulates are much more compatible with the matrix and the surface of particulates are much more cleaner. In addition, the in situ synthesized reinforcements are finer in size and their distribution in the matrix is more uniform.The in situ particulate-reinforced composites include single particulate-reinforced and multiple particulate-reinforced ones. Compared to the single particulate-reinforced one, the multiple particulate-reinforced composite coating has the better mechanical properties. The reason is given as follows. There are two or more kinds of ceramics reinforcements in the multiple particulate-reinforced composite coating. They have different melting point. During the solidification, a kind of ceramics phase must be formed firstly and others following. The previous precipitated ceramics phase serve as heterogeneous nucleations and the precipitation of subsequent ones is probably to start from their surface by epitaxial growth, resulting to multiple particulates. In the case, the clustering or combining each other for the previous precipitated ceramics particulates is likely to be repressed. So, the multiple particulate-reinforced composite coating has the finer microstructure and better performance.Based on these thesis, laser cladding in situ synthesized TiC-ZrC multiple particulate-reinforced Ni-based coatings and VC-VB-B₄C multiple particulate-reinforced Ni-based coatings were fabricated. The microstructure and wear properties have been intensively studied. The main conclusions are as the follows:1. In Situ Synthesized TiC-ZrC Multiple Particulate-reinforced Ni-based Composite Coating by Laser Cladding(1) The in situ synthesized TiC-ZrC multiple particulate-reinforced Ni-based composite coating was fabricated on 45# steel using laser cladding. The clad layer which has good macromorphology and fine microstructure can be obtained with a content of 15 wt.% (TiO₂+ZrO₂+C)-doping at 1.8 kW laser power, 50 mm out-of-focus and 2 mm/s transverse scanning velocity.(2) The phase constituents of the upper and center consists of a large number of the TiC-ZrC multiple particulates and Cr₃C₂ acicular phases, which are distributed uniformly in theγ(Ni) dendrite matrix. The microstructure of the bottom layer consists mainly of Cr₃C₂ acicular phases dispersed in the orientedγ(Ni) dendrites.(3) The compatibility is excellent between the matrix and fine TiC-ZrC multiple particulates which play an important part in strengthening and refining the microstructure during laser cladding process. The hardness of the TiC-ZrC multiple particulate-reinforced composite coating was enhanced to Hv_0.31300. Compared with a pure NiCrBSiC coating, the wear rate in a block on ring test against hardened steel was reduced by one fourth.2. Research on In Situ Synthesis of VC-VB-B₄C Multiple Particulate-reinforced Ni-Based Composite Coatings by laser cladding(1) Continuous and in situ synthesized VC-VB-B₄C multiple particulate-reinforced Ni-based composite coatings can be produced on A3 steel by laser cladding. The optimal process conditions in this work are 20 wt.% content of (V₂O₅+B₂O₃+C)-doping, 1.6 kW laser power, and 50 mm defocusing length and 2 mm/s scanning velocity.(2) The phase constituents of the upper and center mainly consists of a large number of fine VC-VB-B₄C multiple white particulates and Cr₃C₂ acicular phases, which are distributed uniformly in theγ(Ni) dendrite matrix. The microstructure of the bottom layer consists Cr₃C₂ acicular phases and a small amount of VC-VB-B₄C multiple particulates dispersed in the orientedγ(Ni) dendrites.(3) The hardness of the VC-VB-B₄C multiple particulate-reinforced composite coating was enhanced to HV_0.31350. And the wear rate in a block on ring test against hardened steel was reduced by one third than a pure NiCrBSiC coating. This can be attributed to the presence of in situ synthesized VC-VB-B₄C multiple particulates and their well distribution in the coating.