Brazing is a metal joining process, similar to soldering, where a joiner or filler metal is melted down and used to fix together two (or more) closely fitting parts. The process uses capillary action to draw the molten filler into the narrow gap between the work parts. The molten metal then cools, joining the work parts together.
The process has several advantages over other traditional joining techniques including its cost effectiveness, tight tolerances and clean joint production. The base metal of the joint does not melt during the process, leaving a clean finish and little to no need of post-production finishing. Moreover, there is minimal thermal distortion.
Brazing also allows complex assemblies to be fitted together with relative ease. The simplicity of the process, coupled with its insensitive to variation parameters, makes brazing suitable for mass production and automation.
On the other hand, brazed joints are renowned for lacking strength in comparison to welded joints. This is largely due to the fact that a soft metal must be used as the filler. Moreover, brazed work-parts cannot be used in environments that experience high temperate as they would quickly melt and fail.
Still, brazing remains a popular joining process in many manufacturing industries; examples of brazed products can be found in automobiles, air conditioning units and pipe work. In fact, there are a number of brazing techniques around.
Brazing Techniques
Manual torch brazing is by far the most common technique. A brazing torch, which looks similar to a soldiering iron, is used to apply a gas flame to the joint between the work-pieces. The flame melts the filler material, which is also applied manually, without melting the work-pieces and capillary action draws it into the joint. The method is labour intensive and hence suited to small production volumes, minor repairs and DIY.
Many companies invest in automatic or semi-automatic brazing torches, such as those manufactured by Burner and Flame Technology Ltd based in Chesire, which removes the need for a skilled operator. These torches enable a company to reduce labour costs and increases production rate whilst maintaining a uniform braze.
Companies wishing to fully industrialise their brazing process are likely to invest in a furnace brazing machine. These machines are geared up for mass production and can be operated by semi-skilled workers rather than trained technicians. The machines maintain a controlled atmosphere (either inert or a partial vacuum) to protect the work-pieces from oxidation and a constant temperature, to ensure uniformity of the joint. Operating in a vacuum also produces clean, flux-free joints with improved mechanical and thermal properties, produced by controlled, uniform cooling.
The downside of brazing furnaces are the initial high capital investment required and the need to more considered product design, to ensure the pieces are suitable to be manufactured on a production line.
Pre-Processing
Before any brazing takes place it is necessary to carefully clean the work pieces being joined. Any dirt, grease, oil or rust will prevent the brazing filler bonding to the work piece. Once the work pieces are cleaned, it is important to flux and braze them as quickly as possible to minimise the chance of re-contamination.
Once the cleaning is complete is necessary to apply a chemical compound to the joint surfaces in order to protect it from oxidization, known as fluxing the work pieces. Apply heat to a metal surface accelerates oxidization, which then prevents the brazing material from bonding the work pieces together. The coating of flux shields the surface area of the work pieces, forming a barrier between them and the oxygen in the air.
The flux comes in the form of a viscous liquid or paste and is simply brushed onto the work pieces. During the brazing process it will absorb and dissolve any remaining oxides on the work piece. Many companies automate the fluxing process, using either dipping techniques or applicator guns to get a smooth, consistent covering.
Post-Processing
Once the brazing is complete the product must be cleaned to remove any leftover flux and oxidization that has occurred. The flux is simply removed by flushing the joint with hot water or brushing with a wire brush for stubborn flakes. Finally, if there is any evidence oxidization, the joint should be placed into a mild acid path to dissolve any remaining flux and oxide scale.
