# Research on a Low Melting Point Al-Si-Cu (Ni) Filler Metal for 6063 Aluminum Alloy Brazing
## 3 Results and Discussion  
### Melting Characteristics of the Al-Si-Cu (Ni) Filler Metal  
Figure 3 shows the melting characteristics of filler metals with different Cu and Ni contents. The melting temperatures of the filler metals in this study, determined according to the DTA curve in Figure 3, are summarized in Table 2. Combined with Table 2 and the Figure 3 analysis, when the Cu content was reduced from 10 wt.% to 20 wt.%, the liquidus temperatures of Al-Si-Cu alloys decreased from 587.8 \({}^{\circ}\)C to 533.4 \({}^{\circ}\)C, the solidus temperature had no significant effects, and the melting temperature ranges of Al-6.5 Si-(10, 15, 20) Cu filler metals gradually became narrower (66.5 \({}^{\circ}\)C, 18.1 \({}^{\circ}\)C, and 12.0 \({}^{\circ}\)C, respectively). With increasing Cu content, the composition of the Al-Si-Cu filler metal gradually approached the eutectic composition point of the Al-Si-Cu ternary alloy, noncongruent melting gradually changed into congruent melting, thus reducing the melting temperature range [15]. It can be seen from Figure 3 that the DTA curve of the Al-6.5 Si-10 Cu filler metal was similar to that in previous research [4; 7]; it was not a single endothermic peak but a certain number of endothermic peaks, and the corresponding temperatures were a: 528.5 \({}^{\circ}\)C, b: 552.4 \({}^{\circ}\)C, and c: 587.8 \({}^{\circ}\)C. According to the Al-Si-Cu, Al-Cu, and Al-Si phase diagram [16; 17; 18], it was inferred that endothermic peak a was related to the reverse reaction of the L\(\rightarrow\)\(\alpha\)(Al) + Si + \(\theta\)(Al\({}_{2}\)Cu) reaction, and endothermic peaks b and c were relatively smooth, which was presumed to be related to the Al-Cu eutectic and Al-Si eutectic reactions.  
When 1.0 wt.%, 1.5 wt.%, and 2.0 wt.% Ni were added to the ternary filler metal Al-6.5 Si-20 Cu, the solidus temperatures of the filler metals did not change obviously, but the liquidus temperature was somewhat reduced. The solidus and liquidus temperatures of the Al-Si-Cu-Ni alloys were 519.8-528.4 \({}^{\circ}\)C, 520.8-528.2 \({}^{\circ}\)C, and 519.2-526.8 \({}^{\circ}\)C, respectively. Because the addition of Ni introduces a novel low melting temperature alloy reaction, the reverse reaction of L\(\rightarrow\)\(\alpha\)(Al) + Si was inhibited, and the liquidus temperature of the brazing alloy was reduced [18]. The composition of the Al-6.5 Si-20 Cu-2.0 Ni filler metal was close to the eutectic point of the Al-Si-Cu-Ni quaternary alloy, so the DTA curve of the filler metal was relatively gentle between 526.8 and 530.1 \({}^{\circ}\)C [4; 15; 19], and the melting temperature range of Al-Si-Cu-Ni filler metals was narrower than that of Al-Si-Cu. Compared with previous scholars' research on Al-10 Si-15 Cu-4 Ni(\(\Delta\)T = 24.9 \({}^{\circ}\)C) [4],  
Figure 2: Schematic illustration of the spreading testing.  
Al-8.5 Si-25 Cu-1.5 Ni-0.2 Re(\(\Delta\)T = 18.0 \({}^{\circ}\)C) [5], Al-10.8 Si-10 Ge(\(\Delta\)T = 6.10 \({}^{\circ}\)C) [10] and Al-6.5 Si-42 Zn-Sr (\(\Delta\)T = 28.1 \({}^{\circ}\)C) [11], the melting temperature ranges of the Al-6.5 Si-20 Cu, and Al-6.5 Si-20 Cu-(1.0, 1.5, 2.0) Ni filler metals were narrower, which will facilitate the filling of the joint gaps with the liquid filler metals during the brazing process [4,20].