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The
Road to Successful Brazing
The
"Road" to brazing success can have many potholes. The key to avoiding
these problems is to follow the same basic steps on each and every
brazed assembly. The following information is provided as a general
guideline; please contact a Bellman-Melcor representative with
any questions.
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1.
Proper Clearances / Proper Joint Design
During
the brazing process, two closely fitted surfaces or parent metals
are heated and a filler metal is introduced. As the filler metal
becomes liquid, a pulling force draws the molten filler between
the surfaces of the parent metals. This is known as capillary
action. The coalescence of materials when cooled is a strong,
void-free braze joint. This sounds easy but the first step to
insure success begins in the design engineer's office. The design
engineer has a working knowledge of what the braze joint will
face in the field. With this input a joint is designed with as
little stress and the greatest strength possible. The joint integrity
will be maximized by maintaining good fits or clearances between
the parent metals. The tensile strength of a braze joint is directly
related to the clearance as indicated below:
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The
graph above page will change slightly depending on the material
chosen and the method of brazing. These choices are based upon
the type of base metals to be brazed. For instance, if copper
brazing a low carbon steel to itself in a controlled atmosphere,
the clearances call for a press fit to maximize joint strength.
The
following Table lists standard clearances recommended for the
American Welding Society filler metal classifications.
All metals expand/contract upon heating/cooling.
When joining dissimilar metals, the expansion rate of each parent
metal must be calculated and introduced into the joint design.
If this is not included, a joint may be too tight or too wide
during the heating process leading to lower strength conditions.
Expansion rates of various metals are available. Please consult
a Bellman representative for further information.
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Step 2. Pre-Braze Cleaning of Parent Metals
Any form of contaminant on the parent metals
or in the joint itself will reduce capillary action and affect
the joint strength. The cleaning process can be as simple as using
an emery pad or as intricate as multiple emulsion sprays or alkaline
soaks. There are primarily two cleaning methods: A) Chemical:
1) spraying petroleum or chlorinated solvents; 2) vapor degreasing
with chlorinated or trichlor solvents; 3) acid pickling cleaning;
4) using a phosphate type acid. Care must be taken when working
with acids and using chlorinated solvents as both might be operator
and/or environmentally unfriendly. B) Mechanical: 1) grinding;
2) machining; 3) sandblasting; 4) wire brushing. Beware of burnishing
or types of sandblast media which can embed themselves on the
facing surfaces. This can limit capillary action or reduce the
bonding of filler metal reducing the strength of the joint.
Step
3. Assembly and Fixturing
After cleaning, maintaining alignment
of the base metals during the heating cycle will assist capillary
action. The easiest method is using gravity. In most cases the
parts are self-supporting. More intricate methods might include
fixtures such as clamps or vises. For larger production runs,
fixtures can use spring loaded pins which provide easy loading/unloading
of the assemblies. When designing a fixture, keep the mass to
a minimum and try to use materials which will prevent a heatsink.
This will keep the heat on the assembly and not in the fixture.
Step
4. Proper Fluxing of Parent Metals
The fluxing process is largely misunderstood.
The flux is not a surface cleaner. Fluxes and brazing atmospheres
will: A) stop the formation of oxides on parent metals and in
the joint area during the heating process. Oxygen in the air and
from the gas flame create a chemical reaction on the base metal
surface. A layer of oxidation forms which limits capillary action
and decreases joint strength. B) reduce surface tension improving
capillary action. There are certain filler metals, which have
components in them, which act as a fluxing agent. The BCuP alloys
containing phosphorous, prevent oxidation from forming on copper
to copper applications.
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Step 5. Brazing the Assembly
This is the point where heat is introduced.
Torch Brazing using a fossil fuel such as oxy-acetylene is a reliable
method. This is most common in single assemblies or smaller production
levels. In larger operations, multiple station turntables with
multi-tip torches can increase production levels. Automation can
preplace preforms, introduce heat and post-clean the assembly
reducing labor costs. In addition, Induction, Resistance, Vacuum
and Atmosphere Furnace Brazing can be cost effective alternatives.
Bellman-Melcor can assist in making your choice easier based upon
your production needs. The heat must be applied uniformly. Mass
differences and conductivity of the base metals will affect the
amount of heat and how much time is required. The heat is directed
to a broad area surrounding the joint. Because filler metals follow
the greater heat source, the key is getting the interior facing
surfaces to proper temperature. DO NOT direct heat solely on the
joint surface. This can lead to premature flow of the alloy but
not necessarily into the length of the joint. The joint might
look adequate but it will have little strength. When using preforms,
the alloy is preplaced as close as possible to the joint.
Step 6. Post Braze Cleanup
After completing the brazed assembly,
the flux residues must come off. Fluxes are corrosive. If not
removed they can eventually weaken a braze joint. The quickest
and most economical method is a water quench. Once the filler
has solidified, place the warm assembly in a hot water bath. This
will normally "crack" the residue off. For more tenacious residues,
agitate the water bath or use a jet spray to knock the flux off.
Or simply wire brush the assembly while submerged in the bath.
If the flux has been saturated during the heating cycle, the assembly
will have a blackish discoloration. In most cases, an acid bath
will be needed to assist the flux removal. Care must be taken
in choosing a mild acid to avoid etching the joint.
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