# Research on a Low Melting Point Al-Si-Cu (Ni) Filler Metal for 6063 Aluminum Alloy Brazing
## 3 Results and Discussion
### Bazeability of 6063 Aluminum Alloy with Al-Si-Cu (Ni) Filler Metal  
Figure 9 shows the microstructure of the aluminum alloy brazed joints after heat treatment strengthening. The microstructures of the brazed joints were compact and showed good metallurgical bonding. Table 6 shows the results of the EDS analysis of the six points marked in Figure 9a,c. It can be concluded that the microstructure of the Al-6.5 Si-20 Cu brazed joint was mainly composed of the \(\alpha\)-Al phase, a coarse \(\theta\)(Al\({}_{2}\)Cu) phase, and a Si phase. There was a small amount of Al\({}_{2}\)Cu intermetallic compound at the brazed joint interface, and some Si particles were embedded in the matrix [14,24]. Figure 9b-d show the microstructure diagrams of Al-6.5 Si-20 Cu-Ni brazed joints. The addition of Ni produces the \(\delta\)(Al\({}_{3}\)Ni\({}_{2}\)) phase, which makes the Al\({}_{2}\)Cu at the brazing seams transition from a bulk to a lamellar-type structure, and the microstructure distribution of the joints is more monotonous. Figure 10 shows the line scan distribution diagram of the elements in Figure 9a. In the brazing process, Cu and Si were enriched at the interface between the brazing seam and the base metal, and the post-welding heat treatment promoted the interdiffusion between atoms, so the contents of Al and Mg at the interface of the brazing seam were significantly reduced [4,11]. Figure 11 shows a schematic diagram of the surface scanning consequences of the microstructure of Al-6.5 Si-20 Cu-1.5 Ni-brazed 6063 aluminum alloy joints. Cu and Ni were uniformly distributed in the central area of the brazing seams, the segregation of Si was reduced, which was conducive to reducing the Si phase, and the \(\theta\)(Al\({}_{2}\)Cu) phase influenced the brittleness of the brazed joints. The results of the EDS analysis (Table 6) show that the Si and Cu contents at B and F were higher than those of the base material at the joint interface, which indicates that during the brazing process of Al-Si-Cu (Ni) filler metals and 6063 aluminum alloy, the elements between the liquid solder and the base metal diffused, forming a diffusion layer. The post-weld heat treatment enhanced the diffusion of elements and effectively promoted the metallurgical bonding reaction between the filler metal and the base metal [20]. Figure 8: Line-EDS analysis of the spreading interface in Figure 7d: (**a**) Cu, Ni elements; (**b**) Al, Si, Mg elements. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Testing Points** & **Al** & **Si** & **Cu** \\ \hline A & 90.27 & 1.70 & 8.03 \\ B & 40.36 & 1.15 & 58.49 \\ C & 51.06 & 6.77 & 42.17 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Chemical composition (wt.%) of the points in Figure 7c. Figure 12a shows that the shear strength of the Al-6.5 Si-20 Cu brazed joint was approximately 117.2 MPa, and the shear strength of the Al-6.5 Si-20 Cu-(1.0, 1.5, 2.0) Ni brazed joints gradually increased with the increase in Ni content to 123.6 MPa, 136.3 MPa, and 150.4 MPa, compared with the Al-6.5 Si-20 Cu brazed joints whose shear strength increased by 5.46%, 16.30%, and 28.32%, respectively. Compared with the joint shear strength (62.6 MPa) obtained by brazing 6063 aluminum alloy with the Al-8.5 Si-25 Cu-1.5 Ni-0.2 Re filler metal studied by [PERSON] et al. [5], the shear strength (136.3 MPa) of the joint brazed with the Al-6.5 Si-20 Cu-1.5 Ni filler metal developed in this experiment increased by 117.73%. Figure 12b shows the shear strength of the Al-6.5 Si-20 Cu-1.5  
shows the microhardness curve of the brazed joints. There were Al\({}_{2}\)Cu intermetallic compounds and primary Si in the brazed seam center area of the weld [2,7,25], and the hardness of this zone was 281-310 HV. During the brazing process, the elements between the filler metal and the base metal diffused to form a diffusion zone whose hardness was slightly higher than that of the base metal. The hardness of the interface area of the brazed seam was 107-216 HV, and the hardness of the base metal was 102-105 HV.