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Thermodynamic Calculations and Microstructural Verification of a Laser Clad CrxSy/Ni Composite Coating on a Ti6Al4V Substrate
Y.W. Lei, M.L. Liu, R.L. Sun, Y. Tang and W. Niu

CrxSy/Ni composite coatings were prepared on Ti6Al4V substrates by laser cladding. NiCrBSiC+Ni/MoS2 powder mixture was used as clad material and preplaced on the substrate. The phase constituent and microstructure of the coating were investigated. A coating with no cracks, few pores and metallurgically bonded with the substrate was fabricated under the optimized experimental parameters. During laser cladding process, MoS2 decomposed and S reacted with Cr to form a nonstoichiometric CrxSy. C reacted with Ti coming from the molten substrate to form TiC phase. The coating is mainly composed of bright dendritic TiC, needle-like M23C6, dark near-spherical CrxSy and grey γ-Ni matrix. The TiC, M23C6, CrxSy phases are uniformly dispersed in the γ-Ni matrix. To understand the phase transformation in the coating during laser cladding process, the thermodynamics of the molten pool was calculated using Thermo-Calc software. It shows that the solidification process and reaction scheme are Liquid→Liquid+CrxSy→Liquid+CrxSy+TiC→Liquid+CrxSy+TiC+γ-Ni→CrxSy+ TiC+γ-Ni→CrxSy+TiC+γ-Ni+Cr23C6. The calculation results are helpful to understand the microstructure evolution in the coating and agree well with the experimental data.

Keywords: CO2 laser, CrxSy/Ni composite coating, titanium alloy, Ti-alloy, Ti6Al4V, laser cladding, microstructure, Thermo-Calc

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