# Diode Laser Welding/Brazing of Aluminum Alloy to Steel Using a Nickel Coating
## 4 Discussion  
The above results of the microstructure and mechanical properties analysis indicated that the presence of Ni coating leads to the change of interfacial reaction products from Fe\({}_{2}\)Al\({}_{5}\) and FeZn\({}_{10}\) to Ni\({}_{5}\)Zn\({}_{21}\) and Al\({}_{3}\)Ni, which resulted in the improvement of joint mechanical properties. Thus, Ni coating has a significant effect on the joint strength enhancement, which will be discussed further below.  
With the tailored energy distribution, the complete melting of Ni coating was avoided. Thus, the element mixing between Al and Fe could be thoroughly prohibited as the present of this physical barrier. Consequently, the formation of brittle Fe\({}_{2}\)Al\({}_{5}\) was deterred. Instead, less brittle Ni\({}_{5}\)Zn\({}_{21}\) and Al\({}_{3}\)Ni were formed at the interface. Less brittle reaction products had higher resistance to crack initiation and propagation during tensile loading. Thus, the joint strength was improved.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Area** & **Al** & **Zn** & **Ni** & **Possible Phases** \\ \hline A1 & 2.9 \(\pm\) 0.4 & 74.0 \(\pm\) 2.7 & 23.1 \(\pm\) 3.0 & Ni\({}_{5}\)Zn\({}_{21}\) \\ A2 & & & 100.0 & Ni \\ A3 & 4.3 \(\pm\) 0.5 & 72.2 \(\pm\) 1.8 & 21.9 \(\pm\) 1.6 & Ni\({}_{5}\)Zn\({}_{21}\) \\ A4 & 64.5 \(\pm\) 2.5 & 8.5 \(\pm\) 3.3 & 27.0 \(\pm\) 1.3 & Al\({}_{3}\)Ni \\ A5 & 66.1 \(\pm\) 2.3 & 10.0 \(\pm\) 3.1 & 23.9 \(\pm\) 2.2 & Al\({}_{3}\)Ni \\ \hline \end{tabular}
\end{table}
Table 6: EDS analysis of the marked areas in Figure 7 in at.%.  
Figure 8: Fracture surface morphology and XRD patterns of the laser Al/bare steel joint: (**a**) FZ side; (**b**) steel side; and (**c**) XRD pattern at the FZ side.  
Based on the analyses above, the joining mechanism of Al to Ni-coated steel was clarified with the aid of the schematic shown in Figure 9.  
Firstly, with the application of laser irradiation, melting of the filler metal occurred upon heating to the melting point of the filler metal (\(\sim\)482 \({}^{\circ}\)C). Then, the liquid filler metal wetted the Ni substrate and intimately contacted each other (Figure 9a). When the temperature approached the melting point  
Figure 9: Schematic of joining mechanism: (**a**) melting of filler metal and its spreading on Ni coating; (**b**) partial melting of the Al base metal, excessive dissolution of Ni, and element mutual diffusion; (**c**) formation of Ni\({}_{5}\)Zn\({}_{21}\) at the direct irradiation zone; (**d**) formation of layered and dispersed Al\({}_{3}\)Ni at different zones; and (**e**) fractured joint.  
of the Al base metal, parts of the Al base metal started to melt, and then mixed with molten filler metal, forming the FZ. As the Gaussian distribution of the laser beam, it would result in a non-uniform thermal distribution at this region. It in turn led to various metallurgical reaction products at the interface. At the direct irradiation zone, the temperature would be higher, which resulted in the excessive dissolution of Ni and intense elemental diffusion (Figure 9b). Consequently, the L + Ni\(\rightarrow\)Ni\({}_{5}\)Zn\({}_{21}\) transformation happened at 881 \({}^{\circ}\)C [24] (Figure 9c). While, at the FZ root, the lower temperature along with the limited dissolution of Ni and elemental diffusion was obtained, which only allowed the formation of Al\({}_{3}\)Ni through the transformation of L + Ni\(\rightarrow\)Al\({}_{3}\)Ni at 640 \({}^{\circ}\)C [25] (Figure 9d). A similar phase transformation occurred adjacent to Ni\({}_{5}\)Zn\({}_{21}\) at the direct irradiation zone by a metallurgical reaction between dissolved Ni and FZ metal.