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Brazing fluxes
remove oxides and contaminants from base materials to ensure
good-quality brazed joints.
Flux selection
depends on the base-material and filler-metal type, heat source, and
application method. Brazing joins similar and dissimilar materials by
heating them in the presence of filler metal having a liquidus above
840º F and below the solidus of the base material. During
brazing, filler metal flows between fitted surfaces of the joint by
capillary action. Brazing permits the joining of dissimilar
materials. Also, heat from brazing is less damaging than that from
welding, causing little or no metal vaporization, grain growth,
intergranular precipitation, stress corrosion, or distortion.
Further, brazed joints have higher strength than do soft-soldered
joints. Flux plays an important role in nearly all air-brazing
processes. Use of the wrong flux can compromise joint quality. This
article offers information on selecting the right brazing flux.
What
Fluxes Do
When heated,
fluxes dissolve surface oxides and protect the cleaned surfaces from
re-oxidation, transfer heat from the heat source to the joint; and
remove oxidation products, allowing filler metal to contact and wet
the base materials.
Brazing
fluxes—pastes or powders—fuse at temperatures below those
needed to melt filler metals. Because fluxes must be in close contact
with the joint surfaces, they are liquid or gaseous at brazing
temperatures. They remove only surface oxides and tarnish; other
contaminants—oil, grease, lubricants, lacquer, and
paint—must be removed either mechanically or chemically before brazing.
Fluxes classify by
form (powder, liquid, or paste), base materials and filler metals
they can be used with, heat source, application method, and active
temperature range. The five categories: aluminum, aluminumbronze,
silver, magnesium, and high temperature flux. With few exceptions,
fluxes from one category will not work with base materials and filler
metals of another category.
Aluminum- and
magnesium-brazing fluxes contain alkaline chlorides or fluorides.
Lithium salts give these fluxes low melting points,
1,000º-1,140º F. and high chemical activity, enabling the
fluxes to dissolve stubborn aluminum oxide.
Silver-brazing
fluxes contain boric acid and potassium borates, combined with
complex potassium fluoborate and fluoride compounds. Fluorides, up to
40 percent in flux content, give these fluxes their
characteristically low melting points, 1,050º F. and high
activity for dissolving metal oxides. Silver-brazing fluxes that
contain elemental boron offer improved protection on carbides and on
materials that form refractory oxides such as chromium, nickel, and cobalt.
High-temperature
fluxes, based on boric acid and alkaline borates, sometimes contain
small additions of elemental boron or silicon dioxide to increase
activity and protection, good up to 2,200 F. Fluoride content of
these fluxes is usually low, at most 2 to 3 percent. These fluxes
braze ferrous and high-temperature alloys and carbides.
Factors
Influencing Flux Selection
Base-material type
determines flux selection more than any other factor. To braze
aluminum alloys, coat parts with aluminum brazing fluxes. Too,
aluminum-bronze and magnesium fluxes braze only with their respective
base metals. To braze ferrous and nickel alloys, two flux types can
be used: silver-brazing or high-temperature fluxes.Which of the two
is better depends on base and filler material type, brazing
conditions, and cost. Fabricators call on silver-brazing fluxes, more
expensive than high temperature fluxes, to minimize heat input and
distortion to the work. These also braze copper alloys. To braze
carbides—tungsten carbide infiltrated with cobalt to impart high
strength and toughness—coat with boron-modified fluxes and fill
the joint with silver-brazing alloys containing nickel.
High-temperature fluxes and fillers also braze carbides, when the
carbide-steel combination can tolerate the high brazing temperatures,
near 2,000º F.
Specifying
Flux-Temperature Range
To be effective,
flux must be molten and active before the filler metal melts, and it
must remain active until the filler metal flows through the joint and
solidifies on cooling. Therefore, filler-metal solidus determines
minimum working temperature of the flux and filler-metal liquidus
dictates maximum brazing temperature that the flux must withstand.
Generally, select a flux that is active at least 100º F below
the solidus of the brazing filler metal and that remains active at
least 200º Fahrenheit above the filler-metal liquidus.
If overheating is
likely to occur during brazing, as when torch brazing, select a flux
active at 250º-350º F above the filler-metal liquidus. This
gives the flux a wide temperature range to remove surface oxides
before the filler metal melts and will keep it effective at brazing temperatures.
Brazing time
affects flux performance. Molten flux forms a semi-protective blanket
that prevents oxidation only for a finite period—oxygen will
eventually diffuse through the flux to the base materials. Flux must
continue to remove newly formed oxide until the end of the heating
cycle. Because flux can dissolve only a limited amount of oxide, the
longer the heating cycle the greater the likelihood that the flux
will become saturated with oxide, a condition called flux exhaustion.
Rated temperature
range of a flux, which depends on brazing temperature, flux type and
volume, and base material type, assumes a brazing cycle of 15-20
seconds. With a longer heating cycle, flux exhaustion may occur even
when brazing below the maximum operating temperature, because over
time the flux becomes saturated with metal oxide. To avoid flux
exhaustion over prolonged heating cycles, switch to a flux with a
higher working temperature range. When the heating cycle is short, a
fabricator can braze with a flux above its maximum rated working
temperature. Using a low-temperature flux above the maximum working
temperature eases flux removal, since these fluxes are more soluble
in water than are high-temperature fluxes.
Applying
Flux
Ideally, apply
flux to both joint surfaces; for some applications, coating only one
surface suffices—the flux will transfer to the mating surface on assembly.
Application method
depends on joint design, production volume, and joint-heating
technique. Operators brush to apply paste flux to the joint and to
surrounding surfaces, or they may dip parts into a container of flux.
Flux for dipping is of a thinner consistency than that used for
brushing. In some cases, parts are dipped in boiling flux solutions
in which the solids are completely dissolved. Automatic application
of flux can be carried out by spraying, pumping, blotting, or dipping.
Fluxes may be in
paste, liquid, or slurry form. Most slurries are waterbased- some
organic-based fluxes— petroleum- or polyethylene-glycolbased for
example—suit precision dispensing due to lower evaporation rates
and better viscosity control. Hot rodding, used to braze-weld,
plunges a hot brazing rod into powder flux. Heat from the rod causes
a small amount of flux to adhere to the rod surface. This method is
best suited to brazing of shallow joints, up to 1/4 inch in steel, as
it results in poor capillary penetration in deep joint areas.
Torches apply one
of the high-temperature fluxes FB3K. This flux is a flammable Liquid
containing trimethyl borate. A dispenser installed in the fuel-gas
line feeds flux vapor into the flame.
How
Much to Apply
Apply enough flux
to coat the joint faces and adjacent surfaces with a thin layer.
Excess flux will not compromise joint quality and may even assist
flux removal since residues will be less loaded with metal oxide and
more soluble in water. Also, applying flux to surfaces adjacent to
the joint helps to prevent oxidation of the workpiece and may act as
a flux reservoir, draining flux into the joint. Using too little
flux, however, can lead to premature flux exhaustion and inadequate
coverage, producing unsound or unsightly brazed joints. Better to err
on the side of too much rather than too little flux.
The choice of heat
source has little effect on flux selection. Exceptions include
salt-bath heating, which requires dip-brazing fluxes; specialized
high-temperature torches using a flammable-liquid flux, and furnace
brazing, which often calls for a powder flux to minimize the amount
of vapor. Boron-modified fluxes are often preferred for induction heating.
Post-Braze
Cleanup
Remove flux
residues after brazing—they can hydrolyze when exposed to
moisture, causing corrosion. Avoid overheating of the
joint—excess heat impairs flux removal. Spent flux residues,
saturated with metal oxides, are the most difficult to remove. To
avoid flux exhaustion, apply excess flux to ease removal of residues
from the base material.
When practical,
after brazed assemblies have cooled to black heat, quench them in
water to crack off most of the flux by thermal shock. Remaining flux
residues can be removed by one of the following treatments:
Filler
Metal-Flux Combinations
...can be either a
brazing paste or flux-coated rod. Pastes, mixtures of filler-metal
powder and flux, and sometimes an organic binder to ease dispensing,
work well for automated processes; aluminum, silver, and
high-temperature brazing pastes are most popular. Flux-coated rods
perform brazing and braze welding. The most common flux-coated
filler-metal rods are silver-brazing and low-fuming bronze, used
primarily to braze-weld.
Safety
Precautions
Try to prevent
brazing fluxes from contacting skin. Occasional contact is not
dangerous, but all flux should be thoroughly washed off before eating
or smoking. Cuts or breaks in the skin must be properly covered with
a dressing. Flux, especially if it contains fluorides or chlorides,
can delay the healing of wound.
Fluxes produce
fumes when heated, especially above the temperatures given as their
maximum. Braze in work stations with large air space into which fumes
can escape. Ventilate with fans or exhaust hoods to carry fumes away
from workers or equipment operators with air-supplied respirators.
Consult manufacturers' Material Safety Data Sheets for specific
safety and health procedures connected with flux use.
Dr. Baskin is
president of Superior Flux & Manufacturing Company, 6615 Parkland
Blvd., Cleveland, OH 44139, 440/349-3000, Web
site.
H&N
Electronics supplies non-hazardous fluxes for silversmiths.
Superior's brazing paste flux #6 embodies "an entirely new
chemical principle," and differs widely from ordinary
silver-brazing fluxes. It is neutral, not acid, absolutely non
corrosive, and will not burn skin.
Superior's soft
soldering flux #30 is super-safe and supplied in a liquid or gel.
They are organic and water soluble. Superior's #6 and #30 eliminate
all occupational hazards inherent in acid fluxes. For more
information, contact H&N Electronics, 10937 Rome Beauty Dr.,
California City, CA 93505, 760/373-8033, Web
site.
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