## 3 Results and Discussion
### Microstructure and Mechanical Performance of Repaired Gear  
#### 3.5.1 Phase and Microstructure Analysis  
The phase in the material is an important indicator that determines its microstructure and mechanical properties [(40; 41)]. As seen in Figure 14, X-ray diffraction (XRD) results show that the repaired gear contains \(\gamma\)-Ni, Ni3B, Cr7C3, Cr23C6, CrB, and Cr5B3; the same compound was also detected in the nickel-based cladding layer [(42)]. The matrix of the  
Figure 13: The machining process of gear teeth. repaired gear is composed of the \(\gamma\)-Ni phase. All peaks contain Cr-B-C, indicating that various Cr-B-C compounds exist in different lattices. Figure 15a-e indicates representative microstructures from the root to the top of the repaired gear. As shown in Figure 15a, a white bright band presents at the interface, indicating metallurgical bonding was formed between the substrate and cladding layers [18; 43]. After the white bright band, many layers of columnar dendrites are formed. The microstructure at the bottom of the repaired zone consists of plane crystal, columnar crystal, and dendrite. When the metal powder is combined with the substrate, it will nucleate and grow on the substrate at a low temperature. At the same time, materials fall off and free from it, so the fine grains and metallurgical bonding are forming, as shown in Figure 15f. After the formation of a fine grain area, there is a large number of high-temperature liquid metal powders deposited. Because of the direction of crystallization during solidification led by the direction of the temperature gradient, the columnar crystal region perpendicular to the bonding surface is found [44], as shown in Figure 15a. During the formation of columnar crystals, new equiaxed crystals may also grow due to local undercooling or free crystals elsewhere. With the solidified front moving inward and upward, the ability of peripheral heat dissipation gradually weakens, and the internal temperature gradient tends to be gentle. Therefore, equiaxed crystals are formed internally to replace large dendrites because of heterogeneous nucleation caused by increased supercooling, as shown in Figure 15a, the same as [18; 45]. As the increase in the cladding layers occurs, the transition zone that connects the dense dendrite structure with the precipitate region is shown in Figure 15b, and the enlarged zone is shown in Figure 15g. It can be observed that the microstructure in zone G is finer than the upper and lower parts without precipitates. Considering that there are large amounts of material precipitates in the cladding layer at the top of the transition zone, the exothermic chemical reaction provides a lot of heat for grain refinement. Moreover, the heat released was limited, so grains in the transition zone are refined. After the cladding zone of precipitate-free dendritic structure adjacent substrate, three major morphologies of phases were recognizable in the BSE images: bulk-shaped (see Figure 15c,d), butterfly-or wing-shaped (see Figure 15j,j), and dendritic-shaped (see Figure 15h), the same precipitate having been reported in [30; 42; 43; 46]. From Figure 15a-e, these precipitates have gone through processes from thin to coarse and from variable to less. Figure 15(1-7) is the EDS diagram of the corresponding precipitate, and the DES diagram can specifically characterize the atomic weight percentage and atomic centrifuge content [47]. Point 6 indicates that the cladding layer is composed of crystal structures of columnar Ni and Fe dendrites. Points 2, 3, 4, 5, and 7 indicate that the precipitates are various compounds formed by Cr-B-C.  
Figure 14: XRD results of the repaired zone. Moreover, the volume of the precipitate and content of the Cr element is increased. The precipitates of points 2, 5, and 7 are all black, and their B atomic percentage fluctuates at 45.30%. The precipitates of points 3 and 4 are all gray, and their B atomic percentage fluctuates at 24.38%.