**1. Introduction**

Currently, there is a range of alloys available on the specialized market, with different chemical compositions, used as filler materials for brazing. Of these alloys, a Cu-Ag class is used to obtain similar or dissimilar joints, operating in moderate corrosive environments at low or high temperatures. The Cu-Ag class brazing alloys yield specific characteristics such as high fluidity and ability to spread fast in very narrow interstices, high ductility and mechanical resistance, chemical stability in different environments and temperatures, good adhesion and excellent wetting capacity, suitable for a wide variety of metal or metal-ceramic materials [1, 2]. To determine the optimal operating temperature (melting point and eutectic temperature), the mutual

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

solubility of chemical elements and the types of main phases formed (such as solid solutions, eutectic phases, compounds, etc.), see the phase diagrams of the alloying system (binary or ternary diagrams) in **Figure 1**.

In **Figure 1(a)**, the eutectic point is at about 780°C with the specific concentrations of 72 wt% Cu and 28 wt% Ag, whereas in **Figure 1(b)**, other values for temperature and chemical composition are shown. The differences between the values specified by different charts are determined by the different purity values of the elements and by the accuracy of determining the transformation temperatures. If other chemical elements are added in the metallic matrix of the filler material, in order to obtain some special characteristics (lower temperature of the eutectic point, narrower or wider solidification domain), it becomes difficult to determine the exact value of the eutectic point position. The temperature range for Cu-Ag class brazing alloys is between 500 and 1000°C [2]. These alloys are used for brazing parts made of carbon steel, galvanized steel or stainless steel, some copper alloys (Cu, Cu-Sn and Cu-Zn) or others. Since December 2011, the introduction of Cd in these brazing alloys has been banned, in accordance with the EU regulation 494/2011.

Besides Cu (22–40 wt%) and Ag (25–56 wt%), brazing alloys may contain elements such as Sn (2–2.5 wt%) and Zn (17–35 wt%). The melting domain is 620–790°C, depending on the chemical composition [2].

The advanced brazing filler materials contain some chemical elements added in the coating or in the brazing rods, which give them a great capacity for the wetting of metallic surfaces coated with oxide layers. At the lowest temperature value of the melting domain (650–700°C), the ceramic mixture from the coating starts to melt, resulting in deoxidation of the surface for the parent material. At higher temperatures, between 700 and 800°C, the second ceramic coating, containing a mixture of silver particles, starts to melt, completing the final chemical composition of the filler material. In this way, the filler metal can spread rapidly and it can completely fill the gap between the joined components [3–6]. The most used filler material brands are 244 Ag, 134 Ag and 145 Ag, which are designed to perform connections for drinkable water pipes

**Figure 1.** Phase diagram for the Cu-Ag alloying system. Cu: 71.9%wt, Ag: 28.1%wt [1, 2]; a) Melting Temperature: 778.1<sup>ᵒ</sup> C b) Melting Temperature: 780<sup>ᵒ</sup> C

with brazing temperatures of approximately 650–830°C, bundles or tubular exhaust systems. The operating conditions involve corrosive effects and oxidation at high temperatures (300°C), tensions and contractions in the joint. For these applications, the increase in the tin content can improve the wettability and mechanical properties of the joints. However, when the tin content exceeds 5% Sn, the shear strength of the joints decreases [7]. Lower temperatures desired for the diffusion brazing of stainless steels are not determined only by the melting point of the filler materials. In this case, the end product of the reaction between the filler material and parts is a solid solution, and residual interfacial tensions do not occur if the inter-metallic phases are not present. For that reason, the solubility limit of the minor metal constituents in the primary phase is crucially important [8].

Generally speaking, the stainless steel surface is wetted with difficulty by some brazing alloys, because the superficial oxide (chromium oxide) protects it against corrosion. Therefore, the chemical composition of the brazing flux is designed to ensure a rapid dissolution of oxides and to ease the wetting of the molten filler material on the base material. Due to the fact that additional surfaces of the brazing zone are affected by the dissolution effects favoured by the brazing fluxes, it is important to rapidly clean the surrounding surfaces after brazing [9]. The brazing flux preferred for this type of filler material is FB 10 [2, 3]. To ensure a good mechanical resistance of the joint, the brazing alloy must be sufficiently fluid, in order to penetrate the gaps between the components, on large distances. Especially in the case of dissimilar joints, the melting temperature of the brazing alloy that contains silver and copper must be well correlated with the type of base material. In the case of austenitic stainless steels, the holding in the 600–800°C temperature range is critical to avoid the inter-crystalline corrosion. In the case of very thin components, the effects of high temperatures are more critical [6].

This chapter presents some results obtained by using experimental brazing filler materials (ecological brazing rods, cadmium free, obtained under reproducible manufacturing conditions and at reasonable costs, metallic glass filler material and metallic foils) for different types of materials and applications.

The new class of coated rods for brazing can provide high deposition efficiency, chemical compatibility in relation to a number of metal and alloys currently used in industry and high corrosion resistance in different media. Such diverse characteristics were obtained by achieving a special coating that contains a mixture of materials having chemical activation role and catalyst effects, as well as contributing to increasing the adherence to unmolten interfaces. There are highlighted some aspects on the diffusion effects of chemical elements in the soldering interface of stainless steel 304 used for high temperature applications, dissimilar bonding of ceramics (tungsten carbide with steel), alumina bonding using metallic foil and some issues related to the effects of chemical elements from the brazing material. Some other results refer to the possibilities of joining ceramic materials using the various types of filler metal alloys.
