The brazing process has five steps: cleaning work pieces, fluxing them, securing them in place, brazing them and cleaning them again. Depending on a metal's composition (e.g. aluminum vs. steel), each step has different requirements that affect the quality of the finished piece, of the work pieces. For those who understand brazing, but not how to braze aluminum, listed below are important considerations regarding the latter.

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Cleaning Considerations

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The base metal should be free of residue, dirt and oxidization before being brazed. When high quality joints are required, aluminum's oxide finish should be removed as well-a process that amounts to thinning the finish, not removing it. For small pieces, a stainless steel brush can be used, with care taken not to imbed the oxide in the base metal. Large pieces and pieces that have thick coatings are usually dipped in acid solution.

Heating Concerns

There are three things to consider about the heating process: what heating method is ideal, what temperature is necessary based on the composition of the base metal, and clearance between work pieces.

Four heating methods are commonly used for brazing aluminum: handheld torch, used for repairs and small production runs; flux-dip heating, used to join pieces that have different wall thicknesses or other variations; furnace heating, preferred for heating alloys and large production runs; and vacuum and controlled atmosphere heating, creating the highest quality joints and lowest corrosion.

The necessary temperature, and in turn the necessary filler metal, depend on the base metal's composition. When choosing a heat source or filler, know the type of metal you have concerning its alloy, gauge and temper. This will determine the temperature and whether high or low temperature filler will be used.

When calculating clearance between work pieces (.0015 inches is recommended), calculate the thermal expansion effect for your metal. Unlike most metals used in fixtures, aluminum features a high coefficient of expansion, making it easy to not achieve the right clearance between work pieces.

Fluxing Concerns

Most fluxes for aluminum contain alkali and alkaline chlorides and fluorides, although cryolite and chlorides of heavy metals are also used. Usually, a flux is chosen based on one or more of the following needs: a flux whose melting point is below the flow point of the filler metal, a flux that does not attack the base metal, a flux that is cost effective, a flux that can be removed with ease, and/or a flux that limits gaseous emissions.

Conclusion

Brazing stainless steel, aluminum and other metals requires careful consideration of a base metal's properties.When the process for one metal is used for another one, weak joints, corroded surfaces, or dangerous emissions could result. In addition to compromising the assemblies in which brazed parts are used (e.g. vehicle assemblies and support structures); improper brazing can also be dangerous to the worker.

The guidelines above are not exhaustive. Rather, they communicate concerns that everyone should know before they braze aluminum. For more information, consulting a brazing company is an excellent option.

Brazing Aluminum: What You Need to Know

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