# Research on a Low Melting Point Al-Si-Cu (Ni) Filler Metal for 6063 Aluminum Alloy Brazing
## 2 Materials and Methods  
The base material used in this experiment was 6063 aluminum alloy plates with a thickness of 1 mm. Its chemical composition is listed in Table 1. The melting temperature of the base metal was approximately 617.8-657.7 \({}^{\circ}\)C, and the tensile strength at room temperature was 257.94 MPa. A series of Al-Si-Cu and Al-Si-Cu-Ni filler metals were prepared using the master alloys Al-50 Cu (\(\geq\)99.970 wt.%), Al-30 Si (\(\geq\)99.970 wt.%), and Al-10 Ni (\(\geq\)99.000 wt.%) and high-purity aluminum particles (\(\geq\)99.900 wt.%) as raw materials, which were melted in a well-type resistance furnace covered with molten salt with a mass ratio of NaCl:KCl = 1:1. Hexachloroethane was used the refining agent. The nominal compositions of the investigated filler metal are listed in Table 2. To ensure the compositional uniformity of the brazing alloys, each group was remelted twice, and the obtained castings were heat-retained at 450 \({}^{\circ}\)C for 12 h for the diffusion annealing treatment. The melting temperature of the filler metal was determined by differential thermal analysis (DTA, TG/DTA7300, Japan). The filler metal was heated from room temperature to 700 \({}^{\circ}\)C under an argon atmosphere at a heating rate of 10 \({}^{\circ}\)C/min. The fluidity and wettability of filler metals were assessed by wetting experiments. After the filler metals with a size of 3 mm \(\times\) 3 mm \(\times\) 3 mm were weighed, they were placed at the center of the \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Alloy** & **Si** & **Mg** & **Fe** & **Cu** & **Mn** & **Cr** & **Zn** & **Ti** & **Al** \\ \hline
6063 & 0.462 & 0.603 & 0.173 & 0.016 & 0.018 & 0.006 & 0.005 & 0.053 & Bal. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition (wt.%) of 6063 aluminum alloy. base material 6063 aluminum alloy (40 mm \(\times\) 40 mm \(\times\) 1 mm) and completely covered with QJ201 flux (with a melting range of 460-520 \({}^{\circ}\)C) (Figure 1). The process parameter of the spreading test was 570 \(\pm\) 3 \({}^{\circ}\)C for 5 min. The spread sample was washed with warm water (50-60 \({}^{\circ}\)C), photographed with a camera, and imported into Image-Pro Plus to measure the spreading area of the filler metals. The results were averaged by the spreading area per unit mass. The brazing experiment was completed in a box-type resistance furnace. A schematic illustration of the brazed joint is shown in Figure 2. The specification of the base material was 50 mm \(\times\) 20 mm \(\times\) 1 mm, the specification of the filler metal was 15 mm \(\times\) 1 mm \(\times\) 0.2 mm, the lap length of the joint was 1.5 mm \(\pm\) 0.3 mm, the gap was 0.1 \(\pm\) 0.05 mm, and the flux used for brazing was QJ201. Before brazing, the surface of the base metal and the filler metal was polished with sandpaper. After the acetone was degreased, the surface oxide film was removed with a 10% NaOH solution and then neutralized with a 10% HNO\({}_{3}\) solution. Finally, the samples were cleaned with ethanol, dried, and assembled. The brazing technological parameters were as follows: raise the furnace temperature to 570 \({}^{\circ}\)C, put the prepared sample into the furnace after the furnace temperature becomes stable, keep the temperature at 570 \({}^{\circ}\)C for 10 min, stop heating, open the furnace door to take out the sample for air cooling when the furnace temperature drops to approximately 500 \({}^{\circ}\)C, and finally clean the sample repeatedly with hot water (50-60 \({}^{\circ}\)C) to remove the flux residue. Two step-aging treatments were adopted to improve the mechanical properties of the brazed parts. The heat treatment system was as follows: after solid solution treatment (530 \({}^{\circ}\)C \(\times\) 3 h) and room temperature water quenching of the brazed parts, the brazed parts were immediately subjected to primary aging (160 \({}^{\circ}\)C \(\times\) 1 h) and water cooling.