# Influence of Surface State in Micro-Welding of Copper by Nd:YAG Laser
## 3 Results and Discussion
### Influence of Surface Shape on Absorption Rate
Figure 4: Molten area appearance under transitional processing condition between heat conduction and keyhole welding modes. In comparison to the flat surface, the light absorption rate under concave holes increases in all the welding modes for both irradiation positions. However, for irradiation on the concave walls under keyhole welding mode, the absorption rate was not affected by increased depth of concave and thus showed a constant value. Therefore, to clearly show the influence on surface shape, irradiation should be done at the bottom center of the concave holes. Figure 7 shows the observed surfaces and cross-sections under keyhole welding mode for flat surface and when various concave depths were prepared, with irradiation at the bottom center of the concave holes. Also shown are the appearances of the surfaces and cross-sections before laser irradiation. Figure 5: Variations of absorption rate with concave depths for irradiation at bottom center of concave. Figure 6: Variations of absorption rate with concave depths for irradiation on concave walls. Using high peak power of 1.67 kW, stable keyhole welding condition could be achieved irrespective of changes in light absorption rate under conditions of flat surface and different concave depths, but there was the disadvantage of porosity. Lowering the power to 1.13 kW led to a condition of transitional processing between heat conduction and keyhole welding modes. As shown in Figure 4, this transitional condition is unstable, but it was stabilized leading to deep penetration with no porosity generation. Figure 8 shows variations of light absorption and molten volume for different concave depth under transitional processing condition. The horizontal lines indicate changes in the absorption rate and molten volume for a flat surface. When irradiating the laser beam on a flat surface, heat conduction and keyhole welding modes are combined, and the standard deviations of light absorption rate and molten volume are large. For depths of concave at 10 \(\upmu\)m and 20 \(\upmu\)m, only heat conduction welding mode exists and the molten volume is small compared to that under flat surface. On the contrary, the light absorption increases in the concave depths between 30 \(\upmu\)m to 50 \(\upmu\)m, and its standard deviation becomes small. The molten volume also increases corresponding to the change of absorption rate, and stable keyhole welding mode can be obtained. Molten volume decreases in the concave shape condition with depth of more than 100 \(\upmu\)m. This is because as the laser is focused on the specimen surface, the position of the convergence point is deviated by the depth of the concave hole, and the power density is lowered. Therefore, under transitional processing condition, concave holes of a certain appropriate depth help to stabilize the absorbed energy, hence stabilizing the process. Figure 7: Surface and cross-section appearances for flat surface and various concave holes before and after keyhole welding, with irradiation at bottom center of concave. Figure 9 shows the variations of absorption rates and molten volumes with spot diameters of 30 \(\upmu\)m, 40 \(\upmu\)m, and 50 \(\upmu\)m, under conditions of flat surface and with concave diameter of 40 \(\upmu\)m. The power density was maintained at a constant by using different peak powers for each case. This part of the study aimed at showing how positional accuracy of the laser irradiation on the work-piece surface influences the absorption rate and molten volume. Under flat surface condition, the standard deviations were large and the process was unstable. On the other hand, with 40 \(\upmu\)m concave diameter, the absorption rate was stabilized, leading to a stable molten volume. Thus, it is evident that, by maintaining the power density and providing a suitable concave hole, the micro-welding process can be stabilized.