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.
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.
