# Influence of Surface State in Micro-Welding of Copper by Nd:YAG Laser
## 3 Results and Discussion  
### Influence of Surface Shape on Absorption Rate  
High power density is required for deep penetration. Under no-melting condition, the power density is not enough to cause melting of the material. In this study, this condition was experienced when the power density was below 1.0 \(\times\) 10\({}^{8}\) W/cm\({}^{2}\). Higher power density sufficient to melt the material but insufficient to cause boiling leads to heat conduction welding mode. Heat is transferred into the material by heat conduction. Convection phenomenon also plays a role once a weld pool is formed. This conduction mode welding does not penetrate into the material. The process takes place in both vertical and horizontal directions along the surface of the material, hence the semi-circular shape of the weld bead cross-section. This condition was achieved in the power density region between 1.0 \(\times\) 10\({}^{8}\) W/cm\({}^{2}\) and 1.7 \(\times\) 10\({}^{8}\) W/cm\({}^{2}\). The heat conduction welding mode is stable since there is no vaporization, and there is no further absorption below the surface of the material. With considerably high power density, boiling of the material is achieved, hence the creation of a keyhole in the melt pool. This leads to keyhole welding mode. The keyhole is filled with plasma and can extend over the full \begin{table}
\begin{tabular}{c c c} \hline
**Compression Force (N)** & **Depth of Concave (\(\upmu\)m)** & **Diameter of Concave (\(\upmu\)m)** \\ \hline
100 & 300 & 320 \\
50 & 185 & 220 \\
10 & 50 & 110 \\
8.0 & 40 & 90 \\
6.0 & 30 & 80 \\
4.0 & 20 & 70 \\
2.5 & 10 & 50 \\
2.0 & 8.0 & 40 \\ \hline \end{tabular}
\end{table} 
Table 2: Variation of concave dimensions with applied load. thickness of the material. The plasma flows out of the keyhole and forms plasma plume above the material. To protect the weld from oxidation, a shielding gas is employed. In this study, nitrogen was used as the shielding gas. The keyhole can be unstable owing to its intermittent oscillations and closing. This instability causes porosity due to the gas being held within the keyhole, and this may lead to embrittlement of joint [19; 20]. Keyhole welding mode was achieved with power densities above \(1.7\times 10^{8}\) W/cm\({}^{2}\). In this study, transitional processing condition refers to the region between the two modes of heat conduction and keyhole welding. In this region, the processing consists of a mixture of the two modes. This transition region was in the power density range between \(1.35\times 10^{8}\) W/cm\({}^{2}\) and \(1.85\times 10^{8}\) W/cm\({}^{2}\). Under this transitional processing condition, it is possible to stabilize the process, and thus generate large penetration depth with no porosity, and to achieve good surface quality. By using peak power around 1.13 kW, transitional processing condition between heat conduction and keyhole welding modes was achieved using a rectangular waveform with a pulse duration of 1.2 ms. Figure 4 shows photographs for the irradiated flat surface and cross-section under transitional processing condition. [PERSON] et al. have reported the possibility of obtaining good surface quality through process stabilization at the transition region between heat conduction and keyhole welding [21]. [PERSON] et al. have noted that low process stability has disadvantages of porosity and spatter generation [22]. Improvement of welding stability makes 1064 nm wavelength laser useful in copper welding. Therefore, in this study, a stable keyhole was generated under transitional processing condition by utilizing the effect of increasing light absorption rate with surface undulation. Figure 5 shows the light absorption rate of 1064 nm wavelength laser on a flat surface and under concave holes of different depths. Irradiation was done at the bottom center of the concave holes. Figure 6 shows processing results under same conditions as Figure 5, but with irradiation on the walls of the concave holes.