# 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  
#### 3.1.1 Effects of Energy Density  
In this experiment, energy density was adjusted in a range from 3 to 9 J/cm\({}^{2}\) to analyze its effect on laser cleaning of the butt weldment. Figure 3 illustrates the surface of the laser cleaned butt weldment according to energy density. The welded specimen before laser cleaning included defects, such as an oxide layer, heat tinting, and slag. After laser cleaning, such defects were completely removed. However, as the energy density increased, the amount of heat input increased as well. Accordingly, the surface of the laser cleaned specimen was tainted by thermal effect under the condition of energy density of 9 J/cm\({}^{2}\).  
Figure 4 illustrates the enlarged SEM image, profile, and roughness of the weld surface before and after laser cleaning in Figure 3a-d. Before laser cleaning, a nonuniform oxide layer was observed on the surface of the weldment. After laser cleaning, traces of the irradiated circular laser beam were observed on the specimen. As energy density increased, the thermal effects of laser energy also increased. Accordingly, both the spot size  
Figure 3: Photos of the laser cleaned surface according to energy density; (**a**) Before laser cleaning, (**b**) 3 J/cm\({}^{2}\), (**c**) 5 J/cm\({}^{2}\), (**d**) 7 J/cm\({}^{2}\), (**e**) 9 J/cm\({}^{2}\).  
Figure 2: Laser cleaning experimental method according to the shape of the weld; (**a**) Butt weld, (**b**) Fillet weld.  
irradiated on the material, and the influence of laser energy on the behaviors of molten metal increased. Therefore, under the condition of the energy density of 3 J/cm\({}^{2}\), the size of laser beams irradiated on the specimen became smaller than 97 \(\upmu\)m, leading to an absence of overlapped laser beams. However, when the energy density was increased to 5 J/cm\({}^{2}\) or more, there was a 50% overlap between the laser beams, enabling uniform cleaning. The surface roughness of the specimen was the highest at approximately 9.7 \(\upmu\)m before laser cleaning. The nonuniform oxidized layer was removed after laser irradiation, and the roughness of the surface decreased. Under the condition of the energy density of 3 J/cm\({}^{2}\), the surface profile with the shape of a crater was observed. When energy density was 5 J/cm\({}^{2}\) or more, the surface roughness slightly decreased because of the overlapped laser beams.  
Figure 5 illustrates the EDS spot analysis results at position 1 and 2 illustrated in Figure 4. Before laser cleaning, components of oxides, such as SiO\({}_{2}\) and iron oxides, were detected in the specimen. After laser cleaning, these oxide components were removed. Subsequently, components of stainless steel, which was the base metal, were detected. These results verify that oxides on the surface of the specimen were completely removed without thermal damage caused by laser heat sources under the condition of energy density ranging from 5 to 7 J/cm\({}^{2}\). In addition, overlapping of laser beams formed a roughness of approximately under 5 \(\upmu\)m on the surface of the specimen.