Thermodynamic Calculations and Microstructural Verification of a Laser Clad CrxSy/Ni Composite Coating on a Ti6Al4V Substrate

摘要

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 gamma-Ni matrix. The TiC, M23C6, CrxSy phases are uniformly dispersed in the gamma-Ni matrix. To understand the phase transformation in the coating during laser cladding process, the thermodynamics of the molten pool was calculated using Thermo-Cale software. It shows that the solidification process and reaction scheme are Liquid - Liquid+CrxSy - Liquid + CrxSy + TiC - Liquid + CrxSy + TiC + gamma-Ni - CrxSy + TiC + gamma - Ni - CrxSy+TiC+gamma-Ni+Cr23C6. The calculation results are helpful to understand the microstructure evolution in the coating and agree well with the experimental data.