# Diode Laser Welding/Brazing of Aluminum Alloy to Steel Using a Nickel Coating
## 3 Results  
### Microstructures  
The top surface appearance and typical cross-section of the laser Al/Ni-coated steel joint are presented in Figure 3. With a 3-mm defocusing amount above the filler metal, a uniform FZ with good wetting (wetting angle 53.5\({}^{\circ}\)) on Ni coating substrate was observed. As marked in Figure 3b, the FZ/Ni coating interface was mainly divided into two zones: direct irradiation zone and the FZ root. The microstructure was observed to vary from the direct irradiation zone to the FZ root. The local heating characteristics of the temperature gradient along the interface were considered as the main reason for the evident variation of microstructure along the FZ/Ni interface [20; 21].  
Figure 4 displays the backscattered electron image (BEI) of the interfacial area in the Al/Ni-coated steel joint. The Ni coating was clearly observed at the interface, indicating that only part of the Ni  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Welding Parameters** & **Value** \\ \hline Laser power (kW) & 2.6 \\ Defocusing amount (mm) & 3 \\ Welding speed (m/min) & 0.5 \\ Flow rate of shielding gas Ar (L/min) & 15 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Experimental parameters used in the laser welding/brazing.  
Figure 3: Laser Al/Ni-coated steel joint: **(a)** top surface appearance and **(b)** cross-section.  
coating was dissolved into the FZ during laser irradiation. A nonhomogeneous reaction layer was formed at the interface. In the direct irradiation zone, a wavy light layer with a thickness of 3.5 \(\upmu\)m was observed (Figure 4a,c). To analyze this reaction layer, EDS point analysis was performed at point 1. According to Al-Ni-Zn ternary phase diagram [22; 23], the possible phase of the reaction layer was Ni\({}_{5}\)Zn\({}_{21}\), since it consisted of 6.1 \(\pm\) 0.7 at.% Al, 74.4 \(\pm\) 0.3 at.% Zn, and 19.4 \(\pm\) 0.5 at.% Ni (Table 5). This phase was then confirmed by XRD, which will be shown later. Between Ni\({}_{5}\)Zn\({}_{21}\) and FZ, a large number of dark particles was obvious. The EDS results shows that they had the average chemical composition of 68.9 \(\pm\) 0.9 at.% Al, 3.5 \(\pm\) 0.3 at.% Zn, and 27.6 \(\pm\) 0.4 at.% Ni, which corresponded to the possible phase of Al\({}_{3}\)Ni. In the FZ root, a 1.5-\(\upmu\)m thick uniform dark layer was formed at the FZ/Ni interface (Figure 4b,d). Base on EDS analysis, it had the average composition of 69.5 \(\pm\) 5.1 at.% Al, 7.2 \(\pm\) 0.9 at.% Zn, and 23.2 \(\pm\) 4.5 at.% Ni, which was identified as Al\({}_{3}\)Ni.  
Figure 4: Backscattered electron image (BEI) images and energy-dispersive X-ray spectrometer (EDS) line scanning of the laser Al/Ni-coated steel joint at different zones: (**a,c**) direct irradiation zone; (**b,d**) fusion zone (FZ) root.  
Figure 5: BEI images of laser Al/bare steel joint at different zones: (**a**) direct irradiation zone; (**b**) FZ root.  
Figure 5 shows the BEI of the interfacial region in the Al/bare steel joint. In the direct irradiation zone, two phases were seen in an 8-\(\upmu\)m thick reaction layer. Based on EDS analysis, the average chemical composition of the dark matrix was 71.6 \(\pm\) 0.5 at.% Al, 23.8 \(\pm\) 0.1 at.% Fe, and 4.6 \(\pm\) 0.5 at.% Zn, while the light granular phase had an average composition of 10.8 \(\pm\) 3.6 at.% Al, 9.7 \(\pm\) 1.8 at.% Fe, and 79.5 \(\pm\) 3.1 at.% Zn. According to the Al-Fe-Zn phase diagram and related studies [23], the possible phases were Fe\({}_{2}\)Al\({}_{5}\) and FeZn\({}_{10}\) (Table 5). In the FZ root, the same phase components, i.e., Fe\({}_{2}\)Al\({}_{5}\) and FeZn\({}_{10}\), were formed at the reaction layer; however, the layer thickness decreased to around 4 \(\upmu\)m. It suggested that the temperature variation along the interface of the laser Al/steel joint would not lead to the change in phase component, but rather only decrease the thickness of the interfacial layer. Thus, a homogeneous reaction layer was formed in the Al/bare steel joint.