BrazeSkin Pre-brazing techniques for nickel-based and CuproBraze brazing alloys Dr. H. Schmoor, BrazeTec GmbH - Degussa Löttechnik - Hanau 1. Introduction The BrazeSkin technology for applying nickel-based brazing alloys by spraying and screen printing techniques is an advantageous alternative to existing melt-spin films, brazing alloy tapes and brazing alloy pastes. The first applications are currently being used for largevolume manufacture or are being prepared for such use. The expansion of the BrazeSkin product range to flux-free brazing of copper-brass radiators (CuproBraze) led to the development of new binder systems for brazing alloy pastes, to enable these to be used for mixed metal radiator construction via simple application techniques. 2. BrazeSkin application of nickel-based brazing alloys Conventional manufacture of tapes and wires is not possible due to the melting point lowering metalloids B, Si and P required for the nickel-based brazing alloys and the associated brittle phase development. For this reason, nickel-based brazing alloys are applied in the form of melt-spin films, brazing alloy tapes or brazing alloy pastes. As a result of the manufacture of special suspensions of alloy powder and modified solventbased binder systems, BrazeSkin technology now allows uniform large-area application of brazing alloy by conventional spraying methods and contoured-application of brazing alloy on complex shapes by screen printing techniques /1/. 2.1 BrazeSkin spraying technique Conventional paint spray-guns (Figure 1) are used in the BrazeSkin spray technique. By careful selection of the parameters such as the pressure, amount dosed, etc., uniform layers of alloy covering large areas can be applied. The thickness of the layers is in the range between 50 µm and 200 µm. The automation of the process has been tested by a spraying plant manufacturer and can be realised. Figure 1: BrazeSkin spraying technique Page 1
2.2 BrazeSkin screen printing technique If the components to be brazed have complex shapes which must be pre-brazed in a contoured way with line-definition, the BrazeSkin products can be applied by screen printing, whereby point-shaped and line-shaped layers of alloy can also be realised (Figure 2). The alloy suspensions required for this have a higher metal content and a slightly modified binder system compared to sprayable variants. In particular, the ability to easily automate the screen printing process and the optimal use of materials have already resulted in this technology being used for large-volume manufacture. Figure 2: BrazeSkin screen printing technique 2.3 Applications One of the first large-volume applications of BrazeSkin technology involved the production of contoured built-up layers for cylinder head gaskets /2/. For the multi-layer steel gaskets used in the car manufacturing industry, assembly is made easier by replacing a layer by making a contoured coating on one of the intermediate layers around the cylinder bore. For this, a BrazeSkin alloy suspension is applied to the steel gasket by the screen printing technique. The alloy suspension consists of both the nickel-based brazing alloy NI 102 and the higher melting point brazing alloy Ni80Cr20. During the brazing process, which is carried out in a hydrogen conveyor belt furnace, the nickel-based alloy melts and reacts with the Ni80Cr20 alloy. This guarantees the required contouring and edge-stability of the coating (Figure 3). The width of the coating can be between 1.0 mm and 4 mm and the height can vary between 40 µm and 120 µm. Page 2
A Figure 3: Cylinder head gasket with brazed layer (A) Other applications of BrazeSkin technology with nickel-based brazing alloys are chiefly in the area of thin sheet stainless steel, principally in the heat exchanger market. Due to its corrosion resistance, nickel-based alloy brazing NI 105 is used for heat exchangers. Heat exchangers for fuel cell systems and plate heat exchangers are currently at the trial phase /3/. Figure 4: BrazeSkin spraying technique: plate heat exchanger brazed with NI 105 Page 3
3. BrazeSkin for CuproBraze - Applications Copper-brass radiators have been used for years in cars and commercial vehicles. Soldering is used as the joining method because the base material strength achieved in a brazing process using standard copper and brass materials and standard alloys does not meet the requirements for radiators. Soldering is carried out with PbSn-alloys, with application of a flux. This necessitates a series of process steps, including washing away corrosive flux residues. Compared to brazing aluminium, this manufacturing process will no longer be competitive in the longer term from an economic and environmental point of view /4/. The objective of one of the projects carried out by the International Copper Association was hence to increase the base material strength of copper and brass alloys and to decrease the required brazing temperature by developing brazing materials in the temperature range around 600 C. 3.2 Materials 3.2.1 Copper and brass alloys Conventional copper and brass alloys currently used for soldering cannot be used as radiator materials for the manufacture of brazed mixed metal radiators due to the high brazing temperature. The exposure to high temperatures during brazing adversely affects the mechanical properties of the copper fins and brass pipes. As part of the above-mentioned research project, copper and brass alloys were developed having a higher temperature resistance than standard materials. This was achieved by introducing small amounts of dopants. Table 1 summarises the mechanical properties of these copper and brass alloys before and after a brazing process at 630 C. Property Brass Copper before after before after Yield point MPa 340 270 340 260 Tensile strength MPa 420 400 400 330 Hardness HV 0.1 130 115 120 105 Elongation break Table 1: at 3.2.2 Brazing alloys A L0=50mm 20 30 1 10 Mechanical properties of radiator materials before and after a brazing process at 630 C /2/ Brazing alloys used for brazing copper-brass radiators must not reduce the corrosion resistance of the complete radiator system. They must also possess working temperatures in the range between 600 and 650 C so that they do not affect the strength characteristics of the base materials. On the basis of these requirements, a brazing alloy suitable for brazing copper and brass was developed from the copper-phosphorus system. Table 2 shows the composition and melting range of this brazing alloy. Composition: Melting range: 580 600 C 75% Cu, 15% Sn, 5% P, 5% Ni Table 2: Composition and melting range of the brazing alloy OKC 600 Page 4
Using this self-flowing brazing alloy (self-flowing due to the phosphorus content), a flux-free brazing process in an inert gas atmosphere (nitrogen) can be carried out. In contrast to soldering, the use of corrosive fluxes is avoided. With no need for flux and hence also no cleaning processes, this also means that there are cost advantages compared to soldering. In preliminary experiments, the application of the brazing alloy was carried out by using rapidly solidified films (melt-spin films, 40-60 µm thick). Due to their low ductility, these did however cause handling problems in the production and were hence not pursued further for reasons of cost. The further search for an application method which was favourable in cost led to the development of brazing alloy pastes. 3.2.3 Brazing alloy pastes The following requirements were put on the brazing alloy pastes, brazed coating and brazing process to be developed: - Application of the brazing alloy pastes in thin layers, - Quick-drying and adherent brazing alloy pastes, - Residue-free burn-off of the binder system in an inert gas, - No reaction of the binder systems with the brazing alloy powder, - Satisfactory storage stability for the brazing alloy pastes and - A brazing process which is environmentally friendly and favourable in cost. Due to the requirement for an environmentally friendly brazing process, conventional solventcontaining binder systems for the brazing alloy pastes were totally excluded from the outset of the development work. During the course of the latter, a brazing alloy paste with an aqueous binder system and a brazing alloy paste with a thermoplastic binder system both emerged as being very suitable. These two systems are described in more detail below. 3.2.3.1 BrazeSkin SCu 200 brazing alloy paste with aqueous binder system Water-based binder systems such as the polyglycol-cellulose system are known and are stateof-the-art. Results from preliminary experiments on the relevant systems did not show these oxygen-containing polymer systems to be suitable as binders for the special copperphosphorus brazing alloy. This was firstly because they did not fully burn off in the inert gas atmosphere and secondly because they caused corrosion. In addition the alloy-flow was hindered, as shown in particular by the non-filled joint seams and the incompletely developed rounded corners between the copper fins and the brass tube. The conversion of a non water soluble polymer into a stable aqueous dispersion did however bring considerable progress. In this binder system, the brazing alloy particles are kept homogeneously in suspension for a long time, resulting in easy application of the brazing alloy paste. Thin brazing alloy layers can be applied to the surface of the brass pipes by conventional spraying methods. These dry quickly at high temperatures (ca. 100-120 C) to form an abrasion-resistant film. Figure 5 shows a scanning electron micrograph of a dried brazing alloy layer. The polymer film produced on drying is clearly visible, cross-linking the individual powder particles and guaranteeing adhesion to the brass pipes. Page 5
Due to the lack of organic solvents, the use of this brazing alloy paste means that emissions of volatile organic compounds (VOCs) during the application and drying can be largely avoided. The use of specifically selected surfactants and polymers allows total residue-free burn off in the brazing process. Polymer film Figure 5: Scanning electron micrograph of a dried brazing alloy layer using the waterbased brazing alloy paste BrazeSkin SCu 200 The cross-sectional view of a radiator segment (Figure 6) shows that by using water-based brazing alloy paste all fins are brazed to the pipes over their entire width. Figure 6: Radiator segment, manufactured by using water-based brazing alloy paste BrazeSkin SCu 200 3.2.3.2 Thermoplastic brazing alloy paste BrazeSkin RCu 200 A totally different concept was used to develop a thermoplastic brazing alloy paste. The basic idea here was to take a brazing alloy paste which was solid at room temperature and liquefy it by heating it to ca. 70-100 C. This was then applied to the components and it was then solidified again by cooling. This was achieved by developing a paste which contained finely distributed brazing alloy powder in a thermoplastic polymer matrix (Figure 7). Before application of the brazing alloy, the paste is liquefied by heating. It is then applied to the fins to be brazed using the roller-coating process (Figure 8). Page 6
Compared to conventional pastes, the use of a thermoplastic paste has the following advantages: - Virtually unlimited storage times for the paste in the solid state because the alloy powder is completely bedded in the polymer matrix (protection against oxidation, no settling of the alloy powder); - The viscosity of the liquid paste can be modified to the relevant application system by varying the set temperature; - The solvent-free binder system allows rapid solidification of the applied alloy layer by simple cooling; - No solvent emissions; - Residue-free burn off of the binder system allows unhindered flow of the alloy. Figure 7: Scanning electron micrograph of a dried alloy layer of the thermoplastic brazing alloy paste BrazeSkin RCu 200 Hot rollers 95 to 100 o C Wiper Wiper Cold air jet Cold air jet Figure 8: Principle of the roller-coating process Hot brazing alloy paste Page 7
3.4. Brazing process To braze copper-brass radiators the following equipment is employed: inert gas chamber furnaces for batch operations (discontinuous working method) or especially for largevolume series continuous furnaces with regulated atmospheres. Radiator manufacturers are hence able to use the same furnaces that are used for brazing aluminium radiators. Brazed mixed metal radiators can thus be manufactured with relatively little reconstruction of existing production plants and with the same plant efficiency as for manufacturing aluminium radiators. The following points must be heeded when carrying out the CuproBraze process: - The residual oxygen concentration in the furnace atmosphere should be less than 50 ppm because flux is not used, - The exposure to temperatures above the melting point of the alloy (above 600 C) should not exceed 4 minutes, so that too strong alloying of the thin copper fins is avoided, - The maximum brazing temperature should be between 630 and 650 C. The newly developed brazing alloy pastes have been successfully used in different pilot plants to braze copper-brass radiators (Figure 9). In the meantime about 60 projects all over the world are being carried out to manufacture brazed mixed metal radiators. Figure 9: Copper-brass radiator brazed using the CuproBraze process Page 8
4. Summary BrazeSkin technology has established itself as an advantageous alternative to existing methods for applying nickel-based brazing alloys. In particular, screen printing application methods mean that uses for nickel-based brazing alloys are being found in other industrial applications. Compared to soldering technology, the CuproBraze technology for brazing copper-brass radiators for cars allows mixed metal radiators to be made having higher strength, improved corrosion resistance and lower weight. The brazed mixed metal radiators contain no lead and can therefore be totally recycled. As a result of the small radiator size which can be produced using the CuproBraze technology, and having the same performance characteristics, this is a serious competitor for aluminium radiators in the car industry. Due to space restrictions, this is of special interest for smaller cars. 5. Literature /1/ J. Koch and H. Schmoor: BrazeSkin Application of nickel-based brazing alloys (in German) DVS Berichte, Band 192, DVS-Verlag, Düsseldorf, 1998 /2/ K. Cierocki, M. Heilig, J. Koch and M. Koschlig: Production of contoured built-up layers for cylinder head gaskets (in German) DVS Berichte, Band 192, DVS-Verlag, Düsseldorf, 1998 /3/ H. Schmoor: Brazing with Nickel-Based Brazing Alloys applied by the BrazeSkin Technology International Brazing and Soldering Conference, Albuquerque, 2000 /4/ J. Koch, S. Wittpahl and L. Staab: The new generation of copper radiators materials and processes for their manufacture (in German) DVS-Berichte 192, DVS-Verlag, Düsseldorf, 1998 /5/ Outokumpu Technical Manual Page 9