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The information on
these pages will be advantageous to the average brazing operator.
Select Brazing Alloy
When brazing Copper to Copper, low cost PhosCopper 6D is recommended or PhosCopper 15 may be used. These alloys
contain phosphorus and are self-fluxing on copper. When brazing brass or
bronze fittings, White SilverFlux is required. When brazing iron, steel
or other ferrous metals, select a high silver brazing alloy and use White
SilverFlux flux. Do not use phosphorus bearing alloys as the welded joint
may be brittle.
For more information on selection of brazing alloys please
refer to our proper alloys selection
chart.
Select Proper Flux
Flux is a chemical compound applied to the joint surfaces before brazing.
Its use is essential in the brazing process, because heating a metal
surface accelerates the formation of oxides, the result of chemical
combination between the hot metal and oxygen in the air. These oxides must
be prevented from forming or they'll inhibit the brazing filler metal
from wetting and bonding to the surfaces. A coating of flux on the joint
area, however, will shield the surfaces from the air, preventing oxide
formation. And the flux will also dissolve and absorb any oxides that
form during heating or that were not completely removed in the cleaning
process. When using White SilverFlux, apply it only with a brush. To
prevent excess flux residue inside refrigeration lines, apply a thin
layer of flux to only the male tubing and, if possible, rotate the
fitting once or twice on the tube to ensure uniform coverage.
For more information on the selection of brazing fluxes
please refer to our flux selection
page.
Proper Clearances / Proper Joint Design
Joint clearance is a principal factor in determining the mechanical
strength of brazed joints. It is also a factor in eliminating harmful
voids in the joint area and in establishing the capillary force required
to fill the joint.
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.
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.
Aufhauser LayerMet (a silver-copper-silver “sandwich”)
should be used to braze Large Carbides to steels or stainless steels. ( By “large” we mean bigger
than 0.5 sq. inches or 12.7mm2.) When brazing with LayerMet, the silver
on each side of the sandwich bonds with the Carbide and the Steel. The the
copper in the middle (which melts at a higher temperature), acts to relieve
the stresses caused by unequal rates of thermal expansion of the joined
metals.
Cleaning of Metals to be welded
The joint surface areas should be clean and free from oil, grease, or
oxide contamination. Surfaces may be properly cleaned for brazing by
brushing with a stainless steel wire brush, or by a stiff rubbing with
emery cloth. If oil or grease is present, clean with a commercial
solvent. Remember to remove small foreign particles, such as emery dust,
by wiping with a clean, dry cloth. The joint surfaces MUST be clean.
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. If you have a
number of assemblies to braze and their configuration is too complex for
self-support or clamping, it may be a good idea to use a brazing support
fixture. In planning such a fixture, design it for the least possible
mass, and the least contact with the parts of the assembly.
Try to use materials in your fixture that are poor heat conductors, such
as stainless steel, Inconel or ceramics. Since these are poor conductors,
they draw the least heat away from the joint. Choose materials with
compatible expansion rates so you won't get alterations in assembly
alignment during the heating cycle.
Fluxing of Parent Metals
Since most of flux from Aufhauser is conventionally made in a paste
consistency, it's usually most convenient to brush it on. But as
production quantities increase, it may be more efficient to apply the
flux by dipping. When using White SilverFlux,
however, apply it only with a brush. To prevent excess flux residue
inside refrigeration lines, apply a thin layer of flux to only the male
tubing and, if possible, rotate the fitting once or twice on the tube to
ensure uniform coverage.
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. It involves heating the
assembly to brazing temperature, and flowing the filler metal through the
joint. In larger operations, multiple station turntables with multi-tip
torches can increase production levels. Automation can replace with
preforms, introduce heat and post-clean the assembly reducing labor
costs, for more information of Aufhauser performs please visit our Preforms page,or contact an knowledgeable
Aufhauser customer representative for more details. 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 If you're brazing a small assembly, you
may heat the entire assembly to the flow point of the brazing filler
metal. If you're brazing a large assembly, you heat a broad area around
the joint. DO NOT direct heat solely on the joint surface as it 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.
Post Braze Cleanup
After completing the brazed assembly, it must be
cleaned, 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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