# A Study on the Application of Laser Cleaning on the Weld of the Gas Fuel Supply Pipe for DF Engine
## 3 Results and Discussions
### Characteristics of Laser Cleaning Applied to the Butt Weldment
#### 3.1.2 Effects of Defocused Distance  
When laser cleaning is applied to a gas fuel supply pipe, a defocus of the laser beam occurs because of the curvature of the pipe. Provided laser cleaning of the prototype is manually performed, it is significantly important to identify a range of defocused distance, which can derive a similar quality of the laser cleaned prototype in the defocused distance to that in the focal position. The laser cleaning experiment according to the defocus distance was performed while changing the defocus distance from \(-\)10 mm to \(+\)10 mm at 2 mm intervals. At this time, energy density was fixed as 5 J/cm\({}^{2}\). Figure 6 illustrates the surface of the laser cleaned specimen according to the defocused distance. As illustrated, the influence of the defocused distance is symmetrical to its increase in both positive and  
Figure 4: SEM images, profile and roughness of the surface before and after laser cleaning according to energy density and 1 and 2 positions of EDS spot analysis; (**a**) Before laser cleaning, (**b**) 3 J/cm\({}^{2}\), (**c**) 5 J/cm\({}^{2}\), (**d**) 7 J/cm\({}^{2}\).  
Figure 5: EDS spot analysis results at positions 1 and 2 shown in Figure 4; (**a**) Pos. 1, (**b**) Pos. 2.  
negative directions. In the focal position, contaminants, which remained on the surface of the specimen, were completely cleaned. On the other hand, as the focal position increases to + or \(-\), the removal force of the contaminants decreases, and it can be seen that there is no significant difference compared to the specimen before laser cleaning.  
A precise surface analysis on the specimen under the condition of the defocused distance ranging from 0 to +7 mm was conducted to identify the threshold of defocused distance for laser cleaning. Figure 7 illustrates the morphology of the weld surface before and after laser cleaning according to the defocused distance. In the focal position, the surface of the weldment was cleaned uniformly and clearly because of the overlapping of laser beams. Such morphology of the surface was similarly observed under the condition of the defocused distance ranging from +1 to +2 mm. However, when the defocused distance was +3 to +5 mm, the size of laser beams irradiated on the material decreased as well as the laser beam overlap. Moreover, as laser beam patterns were distorted, the influence of laser heat sources on the material decreased gradually. However, the surface of the base metal was all scanned with the laser beam. When the defocused distance was +6 mm or higher, no trace of the laser beam irradiation can be observed on the surface of the base metal. In other words, since the laser energy did not reach the laser cleaning threshold energy, we could not obtain the desired cleaning result. Therefore, the range of conditions for cleaning work was derived from a defocus distance of \(-\)5 mm to +5 mm.  
Figure 6: Photos of the laser cleaned surface according to defocus distance.  
The depth of focus (D.O.F.) can be calculated based on Equation (2) [19].  
\[\text{D.O.F.}=\frac{8\cdot f^{2}\cdot\lambda}{\pi\cdot D^{2}}\cdot M^{2} \tag{2}\]  
Here,\(f\) is the focal length of a lens, \(\lambda\) is the wave length, \(D\) is the input beam diameter and \(M^{2}\) is a constant that evaluates the quality of the laser beam. The depth of focus is theoretically calculated as 6.9 mm. Such a value is lower than 10 mm, which is equal to the depth of focus obtained in experiments. Thus, it was determined that laser cleaning for eliminating oxide layer formed on the weld surface was successfully conducted, although the depth of focus was different from its theoretical value to a certain extent.