Core components of high-end equipment are prone to surface damage owing to harsh service environments. Reinforced coatings with great surface properties are able to be prepared via laser melt injection to prolong service life of core components. However, particles can easily locally aggregate during the process of laser melt injection, resulting in stress concentration and crack initiation in the coating layer. Presently, the approaches to control particle distribution mainly include process optimization, material optimization, adding reinforcement or rare earth elements, and applying external energy field. Because the acoustic cavitation and acoustic flow produced by the ultrasonic energy field in the molten pool have significant effects on microstructure regulation, defect suppression, and performance improvement, ultrasonic vibration has been applied to the fields of laser cladding and laser welding. Meanwhile, several studies on the microstructures and properties of the coating layer deposited by ultrasonic-assisted laser melt injection have been carried out. However, there are few reports on the effect of ultrasound on the distribution of laser melt injected reinforced particles. In this study, ultrasound is introduced into laser melt injection process to realize the distribution regulation of enhanced particles. Meanwhile, the variability coefficient of Voronoi cell area is adopted to analyze the distribution uniformity of WC particles, which provides a novel approach to evaluate the particle distribution in the laser melt injection layer.
The experimental setup for ultrasonic-assisted laser melt injection (Fig. 1) is mainly composed of fiber-coupled semiconductor laser, cooling system, motion control system, powder feeder, and ultrasonic generator. The substrate used in the experiments is 316L stainless steel plate with size of Φ 100 mm×4.8 mm. The particles used in the laser melt injection are spherical WC particles with phase compositions of WC and W2C with an average particle size of 75 μm (Fig. 2). Based on the developed experimental setup (Fig. 1), the laser melt injection experiments with and without ultrasound considering various powder feeding rates are carried out. The process parameters are reported in Table 1. After conducting the laser melt injection experiments, the cross-section (perpendicular to the laser scanning direction) and longitudinal section (parallel to the laser scanning direction) of the laser melt injection layer are sampled, polished, and etched. The number density and distribution of WC particles in the melt injection layer are observed and analyzed using optical microscope.
Aggregation position of WC particles in the coating layer is analyzed using the quadrat method (Figs. 4 and 5). With an increasing powder feeding rate, WC particles gather at the edge of the coating layer without ultrasound, while the number densities of WC particles in different cells of laser melt injection layer are relatively uniform with ultrasound. Meanwhile, a Voronoi diagram of the laser melt injection cross-section is constructed (Figs. 7 and 8). It is found that the area distribution of Voronoi cell is more concentrated with ultrasound (Fig. 9) and variability coefficient of Voronoi cell area is significantly reduced (Fig. 10). These results indicate that ultrasound significantly improves the local aggregation of particles and uniformity of particle distribution. The effects of ultrasonic vibration on the distribution of laser melt injected reinforced particles are revealed. The acoustic cavitation and acoustic flow produced by ultrasound significantly promote the flow of the molten pool and increase the drag force of particles. WC particles continuously move from the edge of the molten pool to the center of the molten pool with a large drag force. Furthermore, the uniform distribution of particles prevents the stress concentration and inhibits the crack initiation in the laser melt injection layer. Under the condition of non-ultrasound, the macroscopic cracks appear in the laser melt injection layer with a powder feeding rate of 6 g/min, while the macroscopic cracks appear in the laser melt injection layer when the powder feeding rate reaches 8 g/min with ultrasound; accordingly, the number of cracks is significantly reduced (Figs. 11 and 12).
In this study, laser melt injected WC particle strengthening layer in a 316L substrate is prepared. The results demonstrate that WC particles gather at the edge of both sides of the laser melt injection layer. Accordingly, numerous macroscopic cracks appear on the surface of the laser melt injection layer without ultrasound with a powder feeding rate of 8 g/min. The application of ultrasonic vibration suppresses the local aggregation of particles and promotes the uniform distribution of WC particles. The Voronoi cell area dispersion coefficient of molten pool decreases by 18.7%-43.52% with ultrasound with powder feeding rates of 2-8 g/min. Meanwhile, improving the WC particle distribution uniformity prevents the initiation of cracks in the coating layer; accordingly, the number of cracks in the laser melt injection layer are significantly reduced with ultrasound.
