Study On Parameter Optimization And Corrosion Resistance Of Laser/plasma Remelting Plasma-sprayed Ni-coated WC Coatings

Plasma spraying is one of the most commonly used thermal spray techniques, by which cermet coatings with good function could be obtained on metal surface. However, some defects, such as lamellar structure and porosity were existed in plasma sprayed coating unavoidably. The post-processing of remelting was applied to eliminate the defects and the microstructure and corrosion resistance of cermet coatings could be improved effectively.In this paper, laser remelting and plasma remelting techniques were employed to remelt the plasma sprayed coatings. The orthogonal experiment was designed to get the optimum parameters of remelting process. The optimum parameters of laser remelting were Power 600W, Scanning velocity 1.2m/s and Moving distance 1.8mm. That of plasma remelting were Current 60A, Velocity 25cm/min. The sequence of factors that affected the laser remelting coatings were Scanning velocity , Power and Moving distance, while the sequence for the plasma remelting coatings were Current and Velocity.Microstructures and phase compositions of the coatings both before and after the remelting postprocessing, before and after the parameter optimization as well were studied by the help of scan electron microscope (SEM), energy disperse spectroscopy (EDS), X-ray diffraction (XRD), optical microscope (OM), and Vickers hardness tester. The phase composition of plasma sprayed coatings contained Ni, WC, FeW₃C, (Fe, Ni)₂₃C₆ and NiC_x. Laser remelting induced some new phases of W such as WC_X and W₃C, while plasma remelting caused the oxidization of WC and WO₃ phase appeared.The plasma sprayed coatings contained a lot of defects, such as unmelted particles, inclusions and composition segregations. Laser remelting improved the microstructure of the coatings, while some pores and microcracks remained. Plasma remelting minished the porosity to 1.5% and coatings with compact microstructure and uniformity can be attained. The post-processing of remelting enhanced the microhardness of the coatings, and plasma remelting using the optimum parameters brought the highest microhardness and a more smooth variation as well.The corrosion resistance of coatings were examined by salt spray corrosion test and electrochemical corrosion test. The results indicated that plasma sprayed coatings had a comparatively poor corrosion resistance, while remelting coatings under the optimum parameters showed a better corrosion resistance, among which plasma remelting the best. The corrosions were caused by the pores, microcracks, inclusions and lamellar structures in the coatings. According to the experiment results, plasma remelting proved to be quite effective on eliminating the defects and improving the corrosion resistance of coatings.