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1)
What is brazing?
The American Welding Society defines brazing
as the coalescence of materials by heating and introducing a filler
metal which melts at a temperature greater than 840°F and below
the melting temperature of the material being joined. The filler
metal is distributed between closely fitted base metals by capillary
attraction.
2) What is Capillary Attraction?
The answer to this question is complex and requires understanding
both metallurgical and chemical processes as well as the ability
of liquids to flow and wet solid surfaces or base metals. Simply
put, it is a pulling force created by two close fitting surfaces
when heated distributes a liquid filler metal into the area between
the close fitting or faying surfaces.
3) What affects ‘Capillary Action or Attraction’?
Again, this is a complex question. The principle influences are
joint gaps or spacing of faying surfaces and surface energy of
both the liquid filler or brazing alloy and the surface of the
base metals the alloy contacts. The term ‘wetting’ simplifies
this discussion.
4)
What is Wetting?
The AWS defines wetting as the phenomenon where a liquid filler
alloy or flux spreads and adheres in a thin continuous layer on
a solid base metal.
5)
What are the major influences that affect surface energy or wetting?
Typically foreign contaminants and ocides are present on the base
metal surfaces. In addition, wetting or flow is assisted by heating
the base metals. Generally, heating is a rapid process minimizing
time and temperature.
6)
What are these contaminants and how do they inhibit wetting?
These include oils, greases, cutting fluids, polishing compounds
and drawing compounds. The brazing alloy will not displace these
foreign materials when liquid; this will create voids, pitting
in the joint or limit the joint penetration of the liquid filler.
7)
How do you prevent base metal contamination?
These forgein materials are best removed by either chemical (degreasing,
acid dipping, alkaline cleaning, etc.) or mechanical processes
(grinding, sandpaper, wirebrushing, etc.). This is one of the
fundamental steps of brazing or precleaning.
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