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WHAT
IS DIP BRAZING |
HOW TO DESIGN
WHAT
IS DIP BRAZING?
To
braze is to join two or more pieces of metal by means of flowing
a filler metal between the joint interfaces at a temperature below
the melting point of the base metal but above 900°F. The filler
metal, upon cooling to the solid state, forms a strong metallic
bond throughout the joined area.
In
aluminum dip brazing the filler metal is basically 88% aluminum
and 12% silicon.
The
parts to be brazed after being chemically cleaned, are assembled
with the filler metal preplaced as near the joints as possible.
The assembly is then preheated in an air furnace to 1,025°F
to insure uniform temperature of dissimilar masses in the assembly.
The part is then immersed in a molten salt bath. These salts are
actually aluminum brazing flux. The bath is maintained at 1,095°F±5°F
in a salt bath furnace. As the assembly is immersed or dipped, the
molten flux comes in contact with all internal and external surfaces
simultaneously. This liquid heat is extremely fast and uniform.
Since
the bath is a flux, complete bonding on oxide-free surfaces assures
unusually high quality joints. The time of immersion is determined
by the mass to be heated but is seldom over two minutes in duration.
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HOW
TO DESIGN FOR DIP BRAZING
| SEEK
EXPERT ADVICE |
If
you have had no experience or have any doubts, please consult DIP
BRAZE, INC. before proceeding. We offer free engineering
service and encourage its use. |
| SELECT
CORRECT ALLOY |
Use
only alloys recommended for dip brazing as other alloys
tend to break down at brazing temperatures. Some brazable
alloys are as follows:
| WELD
ROD |
ALLOY
DESIGNATIONS |
| 1100 |
1100 |
6053 |
6062 |
PERMANENT
MOLD C712 |
| 5154 |
3003 |
6061 |
6063 |
SAND
CAST A712 |
|
| AVOID
FIXTURES IF POSSIBLE |
Design
parts to be self jigging wherever possible. Spot welding,
heli-arc tack welding and pinning often serve as good substitutes
of self jigging designs. Fixtures can be used, but their
design is of critical importance. Consult DIP BRAZE, INC.
on fixture design. |
| PROVIDE
PROPER PLACEMENT FOR FILLER METAL |
Prepositioning
of the filler metal immediately adjacent to the joint must
be provided for in the joint design. The use of wire rings,
stamped washers, specially formed rod, strips or paste can
be utilized to suit the type of joint selected. When no practical
means of replacing the filler metal is available, such as
in heat exchanger or other multi-joint applications, brazing
sheet can be used to a great advantage and is often used
for its economic advantage alone. This material is available
in sheet with the filler metal clad on one or both sides. |
| USE
LAP JOINTS FOR BEST RESULTS |
Design
brazed assemblies for lap joints for best results. Butt joints
have been made to develop strength equal to the base metal
but conditions must be especially suitable. To achieve maximum
efficiency in lap joints the overlap should be about twice
the thickness of the thinnest part. Consult DIP BRAZE,
INC. for suggested joint designs. |
| ALLOW
SUITABLE JOINT CLEARANCE |
Joint
clearance is governed by the fact that capillary action causes
the filler metal to be drawn into the joint area. Consequently,
the proper fit must be controlled to allow good flow and
uniform quality joints. For dip brazing, .001 to .010 clearance
is considered a general tolerance range. This is somewhat
governed by the length and type joint chosen. It is advisable
to run tests if there is reason for doubt. As a rule of thumb,
the greater the depth of joint over 1/4" the looser
the fit should be to avoid flux inclusion. |
| CONSIDER
FILLING AND DRAINING OF INTERIORS |
Keep
in mind that a way must be provided for the flux to flow
into and drain out of the brazed assembly. Two or more openings
in good drain positions will usually suffice for this purpose. |
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