# Influence of Surface State in Micro-Welding of Copper by Nd:YAG Laser
## 3 Results and Discussion
### Influence of Surface Roughness on Absorption Rate  
High surface roughness means that reflection of light to the surrounding is reduced and keyhole multi-reflections effect is increased, as illustrated in Figure 1. This results in an increase of the absorption rate. Consequently, high absorption rate results in higher molten volume, hence leading to an effective welding process. Thus, proper control of surface roughness can contribute to improvement of 1064 nm laser absorption rate and its stability. Figure 10 shows typical appearances of copper surfaces and cross-sections for different roughness values, before and after laser irradiation. Figure 11 shows the variations of absorption rate and molten volume with surface roughness \(R_{z}\) under transitional processing condition between heat conduction and keyhole welding modes for 1064 nm wavelength Nd:YAG laser. When surface roughness \(R_{z}\) is small, unstable welding prevails where heat conduction and keyhole welding modes are combined. The values of both light absorption rate and molten volume are small. However, as surface roughness \(R_{z}\) increases, the occurrence rate of keyhole welding mode prevails against heat conduction welding mode, and light absorption rate and molten volume increase. When surface roughness \(R_{z}\) is about 27 \(\upmu\)m, the light absorption rate and molten volume are the largest, and stable welding can be obtained. Considerably higher values of surface roughness above 30 \(\upmu\)m result in unstable keyhole welding phenomenon, with reduced molten volume. It is expected that with roughness \(R_{z}\) above 30 \(\upmu\)m, the depth of surface undulations is considerably increased and this might lead to deviation of laser convergence point, hence reduction in absorption rate leading to an inefficient process. Therefore, stable welding can be achieved when surface roughness \(R_{z}\) is around 30 \(\upmu\)m, a depth similar to concave, as indicated in Figure 8.  
Figure 10: Observed surfaces and cross-sections for different surface roughness values. Figure 12 shows the observed surfaces and cross-sections, when different power densities were used, with specimen surfaces set to flat condition and roughness \(R_{z}\) of 27 \(\upmu\)m.  
In deep penetration condition with power density up to 2.3 \(\times\) 10\({}^{8}\) W/cm\({}^{2}\), porosity and scattering occur, leading to a bad weld. On the contrary, transitional processing condition between heat conduction welding and keyhole welding modes, with power density of around 1.4 \(\times\) 10\({}^{8}\) W/cm\({}^{2}\), generates a stable keyhole and surface texture can be controlled. Under this condition, porosity and scattering are reduced compared to deep penetration condition. This is an ideal condition to obtain sufficient penetration and good surface quality. Therefore, with low power density, it is possible to reduce weld defects by controlling the surface topology, and thus obtain a stable welding process. The tilt of the specimen surface and slope variations on the specimen surface affect the effective cone angle of light from a lens. This cone angle is directly related to the effective numerical aperture (N.A. ), which determines the size of the focus spot [23; 24; 25]. The focus spot diameter is directly proportional to the wavelength of the laser and inversely proportional to the N.A. of the focusing lens [19; 20]. Consequently, the relationship between surface profile variations and N.A. is of great influence to the process. Thus, the relationship of surface roughness and N.A., and the resulting influence on laser absorption rate should be investigated. Figure 11: Variations of absorption rate and molten volume with roughness under transitional processing condition between heat conduction and keyhole welding modes. Figure 12: Observed surfaces and cross-sections for different power densities in the case of flat surface and \(R_{z}=27\)\(\upmu\)m.  
It is considered that surface topology has a great influence on absorption rate for 1064 nm laser.