# Research on a Low Melting Point Al-Si-Cu (Ni) Filler Metal for 6063 Aluminum Alloy Brazing
## 1 Introduction  
Brazing holds the advantages of low deformation of the weldment, high dimensional accuracy, and high production efficiency. Brazing is one of the commonly used connection methods for heat exchangers [1, 2, 3]. As a light alloy material with excellent weldability and heat conductivity, 6063 aluminum alloy can be used as the main material of the heat exchanger [4, 5, 6]. Al-Si system filler metal is typically used in the brazing of aluminum and its alloys. The content of Si is 7-13 wt.%, and the liquidus temperature is approximately 570 \({}^{\circ}\)C [7, 8]. To obtain excellent brazed joints, the brazing temperature must be higher than 595 \({}^{\circ}\)C, which is close to the solid phase temperature of 6063 aluminum alloy (approximately 615 \({}^{\circ}\)C), which may cause base metal grain growth or over-burning [2, 4, 7]. In addition, an excessive brazing temperature not only enhances the manufacturing cost but also causes dissolution of the base metal, and the quality of the brazed joint obtained is poor [5, 9]. Therefore, it is becoming increasingly important to develop a new type of low melting point, aluminum-based filler metal for low-cost and high-quality aluminum alloy brazed joints. To solve the problem of an excessively high brazing temperature, in recent years, many researchers have developed a series of new low melting point, aluminum-based filler metals by alloying methods with the theoretical support of eutectic components of Al-Si,Al-Cu, Al-Ge, Al-Zn, etc., and the corresponding properties have been studied [2, 7, 10, 11, 12]. The addition of Ge to the Al-Si filler metal can narrow the melting point of the Al-Si-Ge filler metal, but the melting range of the filler metal will widen with increasing Ge content, and the fluidity of the filler metal will decrease. In addition, Ge is costly; as a result, Al-Si-Ge filler metal is not suitable for large-scale applications [10]. [PERSON] et al. [8, 11] studied the Al-6.5 Si-42 Zn filler metal, the melting range of which was found to be 27.7 \({}^{\circ}\)C. When it was used to braze 6061 aluminum alloys, the joint tensile-strength was improved to 129 MPa compared with Al-Si filler metal. However, the content of Zn in Al-Si-Zn filler metal is too high, which easily causes the dissolution of the base metal and reduces the strength of the brazed joint. Moreover, the vapor pressure of Zn is too high, which may have adverse effects on the vacuum-brazing process [13]. Al-Si-Cu filler metal has the benefits of a low melting point and good wettability, which is a hot research direction of aluminum alloy filler metals [4, 5, 7]. [PERSON] et al. [7] studied the brazing of 6061 aluminum alloy with Al-10.8 Si-10 Cu and Al-9.6 Si-20 Cu filler metals at 560 \({}^{\circ}\)C, and found that there are more \(\theta\)(Al\({}_{2}\)Cu) brittle phases in the brazed joints, leading to a lower shear strength of the brazed joints (47 MPa and 67 MPa, respectively). The results indicate that adding an appropriate amount of Ni to Al-Si-Cu filler metal can replace part of the Cu and reduce the adverse effect of \(\theta\)(Al\({}_{2}\)Cu) on joint properties [4, 5, 14]. [PERSON] et al. [4] developed Al-10 Si-15 Cu-4 Ni filler metal with a melting range of 24.9 \({}^{\circ}\)C and successfully brazed 6061 aluminum alloy at 570 \({}^{\circ}\)C. Meanwhile, the joint shear strength was as high as 144.4 MPa, which was higher than that of Al-Si-Cu brazing filler metal [2, 7]. However, Al-Si-Cu filler metal still has the problems of a wide melting interval and a low shear strength of the brazed joint, which urgently needs to be solved. Based on the pseudoeutectic point composition of Al-Si-Cu ternaries [15], a new type of Al-Si-Cu (Ni) multicomponent aluminum-based filler metal with a low melting point and a narrow melting interval was developed. The optimal Al-Si-Cu (Ni) filler metal was obtained by a comprehensive analysis after studying the melting characteristics, the spreading wettability, the microstructure, and the brazeability of different Al-Si-Cu (Ni) filler metals.