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8)
What are oxides and how do they inhibit flow?
Oxides are formed when metals come in contact
with oxygen in air. Oxygen atoms attach to the metals forming
a surface condition that the molten alloy cannot metallurgically
attach to. This actually increases when heat is increased.
9) How do you prevent oxidation?
By using a reducing or fluxing material on the base metals.
10)
What is the primary function of fluxes?
Fluxes dissolve, remove and prevent oxides from forming during
the heating of the base metals. In additon, fluxes reduce surface
tension on the base metals thus improving wetting of the base
metals. The molten brazing alloy will follow this flux path.
11)
Will flux eliminate other contaminants?
While flux might improve the surface condition it will not clean
or remove oil or greases; again this emphasizes precleaning of
the base metals with solvents, degreasers, grinding, etc.
12)
What types of fluxes are there?
There are chemical fluxes, controlled reducing atmospheres and
vacuum. In addition, phosphorous is a reducing element that is
added to silver-copper alloy when joining copper to itself. Generally,
the type of flux will be determined by the base metals being joined,
the preferred filler metal and method of applying heat.
13)
Must fluxes be removed after brazing?
Chemical fluxes containing fluoride leave residues after brazing.
These residues are corrosive and will attack the joint formed
over time.
14)
How are fluxes removed?
The flux residues form a glass-like layer as they absorb oxides
during heating. When using a sufficient amount of flux, the fluxes
may be water soluable. After heat is removed and the molten alloy
has solidified, the part, while hot, can be sprayed or placed
in water. This will thermally shock the glassy surface possibly
assisted by water agitation or lightly brushing. Some residues
may be difficult to remove thus requiring a pickling bath.
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