# Investigation of Intermetallics Formation and Joint Performance of Laser Welded Ni to Al
## 3 Results
### IMCs Formation  
In this section, the phases formed in the laser welded samples were studied. During the laser welding process, Ni and Al were melted in a short time and then mixed in the melting pool. After rapid cooling, IMCs formed in the weld. According to Al-Ni phase diagram as shown in Figure 4[21], a variety of IMCs are possibly produced during the laser welding of Ni and Al, including Al\({}_{3}\)Ni, Al\({}_{3}\)Ni\({}_{2}\), AlNi, Al\({}_{3}\)Ni\({}_{5}\) and AlNi\({}_{3}\), etc., depending on the temperature and element constitution. IMCs formation could be qualitatively identified using the energy-dispersive X-ray spectroscopy (EDS) in the case of dissimilar metal welding. Hence, the EDS of the scanning electron microscope (SEM) was used to determine the composition of the compounds formed in a different weld, as shown in Figures 5-8.  
Figure 4: Al-Ni phase diagram. Reprinted/adapted with permission from Ref. [21]. 2019, Springer.  
Figure 3: Macro view of the laser welded Ni to Al: (**a**) PW (150 W), (**b**) CW (1800 W, focal point position 0), (**c**) PW (120 W) and (**d**) CW (1800 W, focal point position +5 mm).  
The SEM images with higher magnification in Figures 5-8 are enlarged from the marked areas of the inserted images. The cross represents the areas used to perform the EDS analysis. It can be seen from the EDS measurement value in Figure 5 that Ni remains the major element near the fusion zone with a content of about 89 wt.%, while the Al element is always lower than 10 wt.%. Based on the EDS measurement results and the Al-Ni phase diagram in Figure 4, AlNi\({}_{3}\) would be formed near the fusion zone when the PW laser was used to weld Ni and Al with 150 W laser power.  
When Ni and Al were welded using a focused CW laser beam with 1800 W laser power, various IMCs could be formed near the fusion zone, as shown in Figure 6. Areas A and C with similar microstructure were recognized as Al\({}_{3}\)Ni IMC. Area B was identified as AlNi IMC, while area D was considered the rich-Al phase. It can also be seen from Figure 6 that the area C near the bottom layer of the weld mainly generated Al\({}_{3}\)Ni due to the lower temperature, while the area B near the upper layer of the weld generated AlNi due to the higher temperature. In addition, the melted Al flowed and mixed with Ni due to the dynamics of the melting pool and then formed the soft Al\({}_{3}\)Ni during the solidification process, which can be identified by the measured hardness, as shown in Figure 9.  
As shown in Figure 7, when Ni and Al was welded with 120 W PW laser power, a significant reduction in material mixing and reduction of defects was observed. The weld penetration and the melting of the Al sheet were limited and the Ni content in the melting pool was significantly lower than that produced using 150 W PW laser power.  
Figure 5: SEM image with high magnification, EDS results in areas A-D of the Al-Ni joint welded using PW laser with 150 W laser power.  
Figure 6: SEM image with high magnification, EDS results in areas A–D of the Al-Ni joint welded using a focused CW laser with 1800 W laser power.  
Further improvement in the weld quality can be observed in Figure 8 using 1800 W CW laser power at focal point position of +5 mm. The melting of Al is further reduced to form a weld- braze joint with an intermetallic layer thickness of less than 30 \(\upmu\)m. Based on the EDS results and in accordance with the Al-Ni phase diagram analysis, it could be seen that the Al\({}_{3}\)Ni generated in the fusion zone should be dominant. This is beneficial to the improvement of weld strength.