## 3 The Experimental Results and Analysis
### The Effect of Ultrasonic Vibration on CBN Abrasive Grain Distribution  
The morphology of a single CBN abrasive grain with good protruding height and sharp cutting angle is shown in Figure 3a. Figure 3b shows the height distribution section profile of the CBN abrasive grain. The surface of the height distribution curve is smooth, which indicates that there is a good bond between the Cu-Sn-Ti metal bond and the CBN abrasive grain. Moreover, the transition between the two bonding surfaces is smooth, and no gap is observed, which indicates that the metal bond reacts with the CBN abrasive grains to form a chemical-metallurgical bond when melting and diffusion occur [17]. The holding power of the bond on the abrasive grain is enhanced, thus the grinding performance and wear resistance of the CBN grinding wheel can be improved. The distribution of the CBN abrasive grains under different ultrasonic amplitudes during the laser cladding process can be seen in Figure 4. As shown in Figure 4a, when no ultrasonic vibration assistance is applied, the surface contour of the printing layer is not flat, and the protruding heights of the CBN abrasive grains are different. Laser cladding technology has the characteristics of rapid cooling and heating. The printing layer has residual heat after laser scanning and has not fully solidified. The metal powder and CBN abrasive particles fall on the surface of the printing layer, resulting in the phenomenon of sticky powder. The CBN abrasive grains on the surface of the printing layer are divided into two parts, one of which is the CBN abrasive grains floating up to the surface in the melt pool, and the other part is the CBN abrasive particles bonded to the surface. This causes the CBN abrasive grains on the surface of the printing layer to be unevenly distributed in terms of their protruding height. Figure 3: The morphology of a single CBN abrasive: (**a**) three-dimensional; (**b**) profile of cross-section. Figure 2: The macro-morphology of the printing layer: (**a**) the metal bond; (**b**) the mixture of metal bond and CBN abrasive grains. With the addition of ultrasonic vibration and the gradual increase in the amplitude, the surface of the printing layer tends to be smooth. Figure 4b-f shows the surface morphology of the printing layer and the distribution of the CBN abrasive grains with different ultrasonic amplitudes applied. The ultrasonic vibration induces the strong vibration of the molten metal in the melt pool, while the thermal effect of the ultrasonic vibration induces the temperature increase in the melt pool, which delays the solidification time of the molten metal. After falling on the surface of the printing layer, part of the powders melts into the melt pool, and part of it bounces away from the printing layer surface due to the strong vibration, so the printing layer surface gradually becomes smooth. For CBN abrasive grains, the vibration causes the CBN abrasive grains in the melt pool to float. The CBN abrasive grains falling on the surface of the printing layer will also enter the melt pool due to the strong vibration of the melt pool. The CBN abrasive grains in the melt pool are subjected to gravity, buoyancy, fluid drag, and ultrasonic vibration force. Among them, the ultrasonic vibration force plays a dominant role, causing the protrusion height of the CBN abrasive grains to become uniform gradually, as shown in Figure 4d. When the ultrasonic amplitude continues to increase, the protruding CBN abrasive grains falling on the surface of the melt pool or inside the melt pool are detached from the printing layer due to the strong vibration. This resulted in a low number of abrasive grains on the surface of the printing layer, and the remaining abrasive grains were almost flat with the surface of the printing layer, as shown in Figure 4e,f. Too high a protruding height of the abrasive grain will lead to the abrasive grain breaking or even falling off during the grinding process.