| In aviation, petroleum, chemical engineering, metallurgy and other industries, most of the working conditions of the mechanical parts are very poor, which leads to higher requirements for the materials that are used. In the actual working condition, wear, corrosion, oxidation and other behaviors often start from the contact surface of movement pair. Therefore, using advanced surface engineering technology to make excellent coating of wear resistance, corrosion resistance and oxidation resistance on the surface of the mechanical parts is one of the most economical, flexible and effective method to fundamentally solve these inherent properties of the substrate defects. During the exploration of a new type of alloy coating, laser cladding is carried out by using mixed element method, which makes the composition design of alloy coating more flexible.In this paper, the Ni, Si, Mo and Cr element powders were used as the raw material of alloy composition, and the alloy coatings were made by pre-and powder laser cladding respectively. OM, SEM, EDS and XRD were employed in the analyses of alloy coating microstructure and phase composition. The micro hardness tester was used to measure the hardness of coating and evaluate the toughness of the coating. The alloy coatings took wear-resisting, corrosion resistance and high temperature oxidation resistance tests under different experimental conditions, and the wear, corrosion and oxidation mechanism was analyzed. Research results show that:(1) Based on the theory of gas-solid two-phase flow, the movement model of powder particles in powder feeding tube was established. The influences of carrier gas flow rate^ particle size and feeding voltage on powder transport characteristics were discussed.(2) The change of the powder-feed laser cladding coating’s length and thickness alongside different laser power and scanning speed was analyzed. The process of laser cladding was carried out at the selected process parameters. Consider the probability distribution of the powder in the molten pool, the actual ratio of N60 coating is:mNi=56.3%、mMo-36.6%、mSi=7.1%. The modified component improved the content of Si element and reduced the deviation of theory composition.(3) In this study, PVA solution with a concentration of 4% is used in pre-laser cladding. The laser cladding coatings of different composition ratio were composed by y-Ni and Mo2Ni3Si and the nickel-based solid solution makes the coating both hard and tough. As the proportion of Ni in the coating reduced from 65%to 50%, the proportion of Mo2Ni3Si phase was increased from 31.7% to 70.3% and the average micro-hardness of the coating also increased. As for N50-Cr coating, Cr was used as the main solid solution element, which made the coating micro-hardness reach 660.6 HV.(4) The wear resistance of laser cladding y-Ni/Mo2Ni3Si alloy coating was greatly improved compared to that of the substrate, and the wear weight loss decreased as the volume fraction of Mo2Ni3Si increased. The alloy coating had higher hardness and was better in strength and toughness. The surface scratches were relatively shallow, and the main scratch was micro-cutting. As the test loaded and the relative sliding speed increased, the wear rate of the alloy coating increased insignificantly which showed that the wear resistance of the alloy coating was better.(5) The results of corrosion immersion and electrochemical corrosion revealed that the corrosion resistance of laser cladding y-Ni/Mo2Ni3Si alloy coating showed excellent corrosion resistance in 3.5 wt.% NaCl、0.5 mol/L H2SO4 and 1 mol/L HaOH solution compared with 0Cr18Ni9Ti stainless steel.(6) During 120 h isothermal oxidation at the temperature of 973 K, the measured antioxidant capacities of coating were:N60<N55<N50. The oxide film was mainly composed of NiO, MoO3 and Fe2Si04. N50 coating and N50-Cr coating barely oxidized at the temperature of 773 K. When 1173 K took isothermal oxidation, in the initial oxidation, the reaction rate was quite fast and the coating gained weight significantly. Then it gained weight slower and lost weight in the late stage. By comparison, the oxide film of N50-Cr sample was more compact, mainly composed of NiCr2O4 with spinel structure. |
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