# Investigation of Intermetallics Formation and Joint Performance of Laser Welded Ni to Al
## 2 Materials and Methods  
In this study, 0.3 mm thick pure Ni sheets (50 mm \(\times\) 7 mm) and 1.5 mm thick pure Al sheets (50 mm \(\times\) 40 mm) were used with a lap-joint configuration, as shown in Figure 1. The dimensions of the samples were determined according to the real system of the adaptor busbar. Before laser welding, the materials were cleaned using IPA to remove any grease that remained on the surface. A set of mechanical clamps were applied to minimize the gap between two welding materials. During welding, argon gas was supplied to protect melting pools from avoiding external atmospheric contamination. The chemical compositions and physical properties of both materials are listed in Tables 1 and 2, respectively. The Ni sheet was placed on top of the Al sheet in order to minimize the melting of the Al material, as Al has a much lower melting point than Ni, as shown in Table 2.  
Figure 1: Test specimen size. The abovementioned materials were welded using a continuous wave (CW) and a pulsed wave (PW) laser source, respectively. The CW laser source from IPG (model: YLS \(-\)6000) has a nominal wavelength of 1070 nm and a focused circular beam size of 0.2 mm in diameter, while the PW laser source from the LASAG (model: FLS 542 CL-307 HP) has a nominal wavelength of 1064 nm and a focused circular beam size of 0.5 mm in diameter. The process parameters were studied and optimized based on our previous study [20] and preliminary investigations. When using the PW laser, the experiments were performed at different levels of laser power whilst welding speed, pulse duration time and pulse frequency were kept fixed. For the experiments performed using the CW laser, the laser power and the welding speed were fixed, whereas the focal point position was varied. The process parameters are presented in Table 3.  
Multiple samples were welded for process optimization and to guarantee reproducibility. After laser welding, the surface appearance, macrostructure and microstructure of the welds were observed using an optical microscope (OM, Olympus, Japan). Hardness distribution was evaluated by using a MMT-X3 digital micro-Vickers hardness tester (MAT-SUZAWA, Japan) with a 25 g loading force and a testing duration of 10 s. Chemical composition across welds was analyzed using a scanning electron microscope (SEM, Carl Zeiss, German) equipped with energy-dispersive X-ray spectroscopy (EDS) (X-Max, Oxford Instruments) to determine the intermetallics formed. Both a shear test and 90-degree peel test were performed to evaluate weld strength. As shown in Figure 2, the universal testing machine Instron 3367 and a fixture were used to conduct the shear test and 90-degree peel test. For each set of process parameters, 8 pieces of samples were welded: 4 pieces for the shear test and 4 pieces for the 90-degree peel test. A crosshead speed of 0.5 mm/min was set. The testing was conducted continuously until the Ni sheet was completely detached from the Al sheet and the maximum force was recorded. The average force was adopted to evaluate the properties of the welded samples. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Material**} & \multicolumn{8}{c}{**Chemical Composition (wt. \%)**} \\ \cline{2-9}  & **Si** & **Fe** & **Cu** & **Mn** & **Mg** & **Ni** & **Zn** & **Ti** & **Al** \\ \hline
1060 Al & 0.25 & 0.35 & 0.05 & 0.03 & 0.03 & & 0.05 & 0.03 & Bal. \\ Ni200 & & & & & 0.02 & Bal. & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of two welding materials. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Material**} & **Melting Point** & **Thermal Conductivity** & **Density** & \\  & (\({}^{\circ}\)**C**) & **(W/m k)** & **(g/cm\({}^{3}\))** & **Hardness (Hv)** \\ \hline
1060 Al & 660 & 247.0 & 2.70 & 20 \\ Ni200 & 1453 & 82.9 & 8.90 & 140 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Physical properties of two welding materials.