Objective  Mechanical components in marine and mining fields have long served in harsh environments of mechanical wear and electrochemical corrosion. The interaction of friction and corrosion will accelerate the damage of the component surface and reduce its service life. At present, in order to improve the wear resistance of components, the method of preparing ceramic particle reinforced metal-based composite coating on the surface of the substrate is widely used. Due to the excellent chemical stability, wettability and adhesion, WC particles have become one of the most commonly used ceramic particles in reinforced coating produced by laser cladding. However, under the action of high-energy laser beam, the dissolution of WC particles will change the phase composition and microstructure of the reinforced coating, thus affecting its corrosion resistance. In order to solve the problem that the WC particles reinforced coating is difficult to have both high wear and high corrosion resistance produced by laser cladding, ultrasound is introduced into laser melt injection process. The effects of ultrasonic vibration on the microstructure, microhardness, wear and corrosion resistance of the coating were analyzed. The study provides reference for the preparation of WC particles reinforced coating with high wear and high corrosion resistance.

  Methods  The experimental setup for ultrasonic-assisted laser cladding (Fig.1) is mainly composed of fiber-coupled semiconductor laser, cooling system, motion control system, powder feeder and ultrasonic vibration device. The substrate used in the experiments is 316L stainless steel plate. The powder used in the experiments is a mixed powder of 316 powder and WC particles with the mass ratio of 1 : 4, while the particles size are 70-100 μm and 50-100 μm (Fig.1). Based on the developed experimental setup, the laser cladding experiments with and without ultrasound are carried out. After the experiments, the cross section (perpendicular to the laser scanning direction) and the longitudinal section (parallel to the laser scanning direction) of the laser cladding layer are sampled, polished and etched. The microstructure of the sample was characterized by optical microscope, scanning electron microscope and the chemical composition was determined by EDS analysis. Meanwhile, the hardness, wear and corrosion resistance of the cladding layer were tested.

  Results and Discussions   After ultrasonic assisted laser cladding, the average grain size around WC decreased from 101.0 μm to 59.6 μm, and the surrounding structure and elements are more uniform (Fig.2-4). Due to the effect of ultrasound, the precipitation of fishbone carbide around WC is inhibited. At the same time, the alloy reaction layer on the surface of the WC is dissolved, resulting in the average microhardness of the sample increasing from 310 HV_0.1 to 425 HV_0.1, and the hardness distribution around the tungsten carbide particles is more uniform (Fig.6). The wear resistance of the composite coating was further improved by increasing the hardness (Fig.7-8). The mass loss and wear rate of the sample without ultrasonic assisted laser cladding were 8.8 mg and 0.043 8 mg/m, respectively, and the maximum depth of the wear mark was about 53 μm. The mass loss and wear rate of the sample with ultrasonic are 6.5 mg and 0.032 3 mg/m, respectively, and the maximum depth of the wear marks is about 26 μm. The addition of ultrasound reduced the wear rate by 26.2%. In addition, the introduction of ultrasound did not change the overall corrosion open circuit potential and pitting potential of the cladding layer, but it reduced the corrosion current density (Fig.9), improved the penetration resistance of the corrosive medium on the surface of the coating (Fig.10), and improved the corrosion resistance. Ultrasonic vibration assisted laser cladding can dissolve the alloy reaction layer on the surface of tungsten carbide and increase the hardness of the coating through the uniform distribution of acoustic flow, thus improving the wear resistance of the cladding layer. At the same time, due to the cavitation of ultrasound, the epitaxial growth of columnar dendrites is broken, the grains are refined, and a denser grain boundary is formed. In a corrosive environment, a stable and continuous passivation film can be formed faster because of the increase of the grain boundary, thus improving the corrosion resistance of the WC particles reinforced coating.

  Conclusions  In this paper, ultrasonic assisted laser cladding technology was used to prepare WC particles reinforced coating. The microstructure, hardness, wear and corrosion resistance of the coating under the influence of ultrasound were compared and analyzed. In the non-ultrasonic cladding layer, a large number of columnar crystals existed around WC, accompanied by some element segregation bands, due to the acoustic cavitation effect of ultrasound. The average grain size around WC in the ultrasonic cladding layer is refined from 101.0 μm to 59.6 μm, and there is no obvious segregation phenomenon; The average microhardness of WC particles strengthened coating without ultrasonic is 310 HV_0.1, and the hardness around WC decreases from 480 HV_0.1 to 320 HV_0.1. The average microhardness of WC particles strengthened coating with ultrasonic is 425 HV_0.1, and the hardness around WC decreases from 426 HV_0.1 to 413 HV_0.1. The weight loss and wear rate of samples without ultrasound were 8.8 mg and 0.0438 mg/m, respectively. The mass loss and wear rate of samples with ultrasound were 6.5 mg and 0.0323 mg/m, respectively. The maximum depth of samples without ultrasonic scratches was about 53 μm, and the maximum depth of samples with ultrasonic was only about 26 μm. The introduction of ultrasound reduced the wear rate by 26.2%. The corrosion current densities of the electrochemical samples with and without ultrasonic are 2.13 μA /cm² and 5.20 μA /cm², respectively. Ultrasonic assisted laser cladding of WC particles reinforced coating has better wear and corrosion resistance.