## 3 Results and Discussion
### Effect of Remannufacturing Strategies on the Quality
#### 3.3.2 Effect of Scanning Strategies on the Multi-Tracks Multi-Layers  
Figure 11 depicts the morphologies of deposition of nine layers under scanning strategies II-1 and II-2. A common phenomenon in Figure 11(a1,b1) is bulging and collapsing at both ends of the scanning route. It can be seen from the cladding path map. After beginning a path or completing a path, the coaxial powder feeding nozzle would be lifted a certain height in the Z-axis direction stopping briefly at the path endpoint. At this time, the powder will be continuously ejected from the nozzle (the laser is off). It is equivalent to the simultaneously proceeding powder-feeding LC and preset powder-feeding LC at both endpoints. Each layer will be slightly higher at the endpoints. When multilayer LC is carried out, the height difference of each layer will be superposed to form a bulge. On the other hand, when the nozzle stays at the endpoint of a just finished path, the continuous carrier gas and protective gas blowing to the molten pool will generate an impact effect on the liquid cladding layer, and the coating material will also bounce or fly out during the process, resulting in less powder entering the molten pool. Then, the liquid surface of the cladding layer will collapse under the action of gravity and surface tension. When multilayer LC was carried out, the collapse of each layer would be superposed to form an inclined surface at the end face. The obvious phenomenon is that the bulge and collapse are serious in the same directional scanning; the paper [36] also explains this phenomenon. The metal powder, carrier gas, and protective gas at both ends of the route have a larger amount of accumulation, so there is more powder to support bulging and more gas to  
Figure 10: Morphologies of deposition layers under different lapping strategies: (**a**) Strategy I-1, (**b**) Strategy I-2, (**c**) Strategy I-3.  
support collapsing in the same directional scanning. Another reason is that the end point of one path adjacent to the starting point of the next route is on the same end face of the cladding layer in the reverse scanning. The collapse and bulge depressions alternately appear so the defects of collapse and bulge are complemented. In summary, the end face defects of the reverse scanning surface are not obvious.  
Compared with the scanning in the same direction as shown in Figure 11(a2,a3,b2,b3), the other phenomenon is that the middle part cladding layer of the reverse scanning is relatively flat. The powder-sticking phenomenon of the cladding layer in the same direction is also serious. From the scanning path, in the same direction when a path is completed, the nozzle will turn back to the next path starting point. The nozzle produces an oblique path on the cladding layer, as shown in Figure 11(a2). A lot of redundant powder falls on the surface first, resulting in the powder sticking phenomenon due to the residual temperature of the cladding layer. The redundant powder secondly results in the cladding surface being uneven. Then, residual powder in the middle part would be clad in the next LC. However, the reverse powder feeding path will find the nearest starting point for the next scanning path with no redundant powder spray on the surface of the cladding. To sum up, scanning in the reverse direction has a flatter surface and less sticky powder.