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5.4 Brazing and soldering

Brazing is defined as the joining of metals using a filler rod which melts at temperatures above 450°C but below the melting temperature of the metals being joined. Typical features of the brazing process are:

  • the brazing alloy can be significantly different from the base material because the base material does not melt;

  • the strength of the brazing alloy is substantially lower than the base metal;

  • bonding requires capillary action, where the brazing liquid is drawn into the joint.

And because of these differences, the brazing process has several distinct advantages over welding:

  • virtually all metals can be joined by some type of brazing metal;

  • the process is ideally suited for dissimilar metals;

  • the lower temperature than that needed for welding (welding is discussed shortly) means the process is quicker and more economical;

  • the low working temperature reduces problems with distortion that can occur during welding, so thinner and more complex assemblies can be joined successfully;

  • brazing is highly adaptable to automation and performs well in mass production.

Soldering is defined as a brazing-type operation where the filler metal has a melting temperature below 450°C. The bond strength is relatively low compared to brazing. The 'traditional' solder alloy is based on a tin/lead mixture, but lead-free alloys are becoming more commonplace.