# Diode Laser Welding/Brazing of Aluminum Alloy to Steel Using a Nickel Coating
## 2 Experimental Procedure  
In this study, 1.5-mm thick 5754 aluminum alloy and DP980 steel sheets were used as the base materials. Their chemical compositions and mechanical properties are given in Tables 1 and 2. A 1.6-mm diameter Zn-based filler metal (Zn-22 Al) filler metal with solidus and liquidus temperatures of 441 \({}^{\circ}\)C and 482 \({}^{\circ}\)C was used. An anticorrosive brazing flux Superior No. 20 was used. Base materials were machined as rectangular strips of 50 \(\times\) 60 mm\({}^{2}\). The aluminum alloy sheets went through a very strict two-step cleaning process, which was shown in our previous study [19]. The steel sheets were cleaned in acetone, and then ground to 1000 gr \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**Materials** & **Mg** & **Cr** & **Mn** & **Si** & **Cu** & **Zn** & **Ti** & **Fe** & **Mo** & **C** & **Al** & **B** \\ \hline DP 980 steel & - & 0.15 & 2.1 & 0.05 & - & - & - & Bal. & 0.35 & 0.135 & 0.45 & 0.007 \\ Al 5754 & 2.6-3.6 & 0.3 & 0.5 & 0.4 & 0.1 & 0.2 & 0.15 & 0.4 & - & - & Bal. & - \\ Zn-22 Al & - & - & - & - & -0.003 & Bal. & - & - & 0.02 & - & - & 22 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of the materials in wt.%. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Materials** & **Yield Strength (MPa)** & **Ultimate Strength (MPa)** & **Elongation (\%)** \\ \hline DP980 steel & 666 \(\pm\) 36 & 1005 \(\pm\) 7 & 12.5 \(\pm\) 0.7 \\ Al 5754 & 85 \(\pm\) 11 & 239 \(\pm\) 3 & 16.2 \(\pm\) 1.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mechanical properties of the base materials. paper and again ultrasonically cleaned in acetone. The prepared surfaces were then immediately electroplated with electrolytic pure Ni. In the Ni electroplating process, the clean steel sample was the cathode, and the pure Ni sheet was the anode. The composition of the electroplating solution and the electroplating conditions are listed in Table 3. Figure 1a shows a schematic of the Ni electrodeposition process. To get a uniform 45-mm thick Ni layer on the steel, different cathode current densities and plating times were tested. The electrodeposition of Ni using a cathode current density of 300 mA/cm\({}^{2}\) for 20 min was found to provide a 45 \(\pm\) 3-mm-thick pure Ni coating layer on the steel with a defect-free interface. Figure 1b shows the SEM images of Ni-coated steel. Energy-dispersive X-ray spectrometer (EDS) analysis of the electrodeposited layer on the steel showed a pure Ni coating. An integrated Panasonic six-axis robot and a 4-kW Nuvonyx diode laser system were used for laser welding/brazing. A 1 \(\times\) 12 mm\({}^{2}\) rectangular laser beam intensity profile was obtained at the focal point. The filler metal was placed on the surface of the steel sheet. Shielding gas (99.99% Ar) was applied with a flow rate of 15 L/min to limit oxidation. The experimental setup of the laser welding/brazing is shown in Figure 2a. Before the experiment, preliminary trials were conducted to obtain a visually acceptable joint. The process parameters are listed in Table 4.  
Figure 1: (**a**) Schematic of the Ni electrodeposition process on steel; (**b**) cross-section of the Ni electrodeposited layer on the steel substrate. Figure 2: (**a**) Schematic description of the laser welding/brazing layout used for joining the aluminum alloy to Ni-coated steel sheets in the lap joint configuration and (**b**) schematic illustration of the tensile-shear test samples. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Plating Solution Composition (g/L)** & \multicolumn{2}{c}{**Electrodeposition Parameters**} \\ \hline NiSO\({}_{4}\cdot\)6H\({}_{2}\)O & 180 & Cathode current density & 300 mA/mm\({}^{2}\) \\ NH\({}_{4}\)Cl & 25 & Temperature & 43–60 \({}^{\circ}\)C \\ H\({}_{3}\)BO\({}_{3}\) & 30 & pH & 5.6–5.9 \\  & & Time & 10–25 min \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Composition of Ni electroplating solution and electroplating parameters.