Aluminium brazing

Aluminium-silicon alloys are used for the brazing of aluminium and aluminium alloys. The composition of a commonly used aluminium brazing alloy is shown in Table 12.4. 

Potassium fluorometalates and in particular fluorozincate and fluorosilicate have applications in the field of aluminium brazing as so-called reactive fluxes. The nomination reactive stands for the redox reaction that takes place between the flux (KZnp3 or K2SiF6) and the aluminium surface to be brazed. These reactions give rise to formation of elemental Zn or Si and potassium fluoroaluminate, which acts as the actual flux. The elements formed in this way combine and alloy with aluminium, diffusing through the... 

Table 7.1 Steps during an aluminium-brazing process...

There are several factors determining the success of aluminium brazing ... 

U. Seseke, New developments in non-corrosive fluxes for innovative brazing. First International Congress Aluminium Brazing, Diisseldorf, May 2000, conference proceedings. 

The addition of beryllium and silicon to nickel-palladium alloys gives very good high-temperature brazes, especially for alloys containing aluminium and titanium. 

Heat-resisting steels have limited uses in contact with molten metals. They are not recommended for use with molten zinc, cadmium, aluminium, antimony or copper, because of excessive attack and embrittlement effects. In brazing and silver soldering, contact between the molten non-ferrous alloy and the steel occurs for only a very limited period of time. 

Alpema. The standards of the brazed aluminium plate-fin heat exchanger. Houston Association Alpema, 1994. 

The fluxes used when brazing are alkali halide and borate mixtures and compounds, and they have two main functions first, to dissolve the oxide film on the component surface or at least to degrade its adhesion by penetration of naturally occurring flaws and electrolytic action at the oxide-substrate interface, and secondly to prevent formation on the liquid surface of oxide skins which would restrict braze flow. Fluxes can be contained in a bath held at the brazing temperature in which the, usually aluminium, component is placed or else applied as a paste to surfaces of the component or braze. 

Each manufacturer has specialized design procedures for their equipment. The manufacturers have formed ALPEMA, the Brazed Aluminium Plate-Ein Manufacturers Association, which has issued The ALPEMA Standards [63] for the design, construction, operation, and maintenance of this equipment. 

Brazed Aluminium Plate-Fin Heat Exchanger Manufacturers Association (ALPEMA), The ALPEMA Standards, 2nd ed. (2000) available on-line at http //www.alpema.org/stand.htm, accessed 14 June 2007. 

Plate-fm heat exchangers are manufactured by assembling plates separated by corrugated sheets, which form the fins. The plates are made from aluminium sealed by brazing. The operation of these devices requires clean fluids. The main applications can be found in cryogenic and natural gas liquefaction for pressures and temperatures up to 60 bars and 150 °C. 

The oxy-acetylene welding process can be applied in steels up to and exceeding 25 mm thick, but is mainly used on gauges up to 16 SWG (1.6mm), where the heat input needs to be flexible, and for welding die castings and brazing aluminium where the heat must be maintained within a critical range. 

The bonded assembly of aluminium or stainless steel pipes, tubes or sleeves, and the fitting of threaded or flanged collars to pipe-ends, is simplified by eliminating the problems of soldering, brazing and welding. 

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