Mech-Tronics
1635 N 25th Avenue
Melrose Park, IL 60160

Aluminum Dip Brazing

The dip brazing process allows simultaneous joining of multiple joints with varying material thicknesses. The resultant joints are leak-tight and EMI shielded. As the parts are uniformly heated with minimum fixturing, only minor distortion results. This minor distortion is easily managed with proper tolerancing and design of components.

The parts are cleaned of excess oxides. Parts are then assembled with the proper filler metal applied. The form of filler alloy varies to suit the type of joint. At Mech-Tronics, we utilize our in-house Photo Chemical Etching process to make our brazing shims. The precision achieved with this process yields excellent control over excess filler metal flow. Parts are placed onto stainless steel fixtures or baskets. The fixtured assembly is heated to 985°F in a pre-heat furnace and then immersed in a bath of molten salt that contains a flux. The molten flux (1100°F) serves a multi-purpose role: providing heat transfer, supporting the assembly, and of course, fluxing the joints. Immersion time required will vary, but generally is less than two minutes. During the course of this cycle, filler metal will melt and flow to all joints through a capillary action. The assembly is removed from the bath, cooled, and cleaned; ready for further processing.

• Reduced Tooling Cost. The dip brazing process uses little if any specialized tooling. Fixturing is generally accomplished with common fixtures that allow for quick and inexpensive configuration changes.
• Material Savings. Unlike castings or machined parts, dip brazing can be done at near net size.
• Improved Structural Integrity. Dip brazing offers a continuous leak-tight and EMI shielded joint even with a variety of material thicknesses.
• Better Conductivity. As the dip brazed joint is aluminum, conductivity is clearly better than that achieved with an adhesive bonded or mechanically attached assembly.
• Less Distortion. The braze process heats all components uniformly thus yielding distortion less than that of a welded part.
• Lower Cost. The dip brazed process yields the lowest cost given the superior characteristics achieved.
• Design Freedom. The design engineer will enjoy a greater range of options using the dip brazed process.

• Reduced Tooling Costs
• Reduced Design Change Cost and Implementation Time

• Design components to be self-fixturing and self-locating. Aluminum's rate of expansion is approximately 1/3 greater than that of the fixturing material. As such, fixturing is generally kept simple and to a minimum. This sketch shows some of the many construction features that can be used in the dip brazing process. Additional construction detail is available by selecting this picture.
  
  
• Some distortion will be present on brazed parts. Aluminum reaches annealed condition at brazing temperatures. Parts are quenched immediately after brazing. Some straightening can be performed to improve the distortion. In some cases, closely toleranced features must be removed from the component part and machining stock may need to be added so that these features can be machined after the brazing operation.
• Parts must be vented. Brazing salts are very dense. The density of the salt makes the aluminum very buoyant. The salt density, combined with the soft state of the aluminum, dictate a specific fixturing orientation to allow for easy immersion and removal from the bath. In some cases we may ask that vent holes be present to help this condition. The vent holes can be welded closed after the brazing operation. No closed cavity can be present due to the pressure created by expanding gases generated by the brazing flux.
• Filler metal comes in different forms.

• .003 thick shim
• paste (powder mixed with water)
• wire of varying diameters
• clad brazing sheet

• Joint clearance must be controlled. Dip brazing depends on the capillary action created by the proper joint clearance. The following are general guidelines only. Please consult us if you are designing your own component parts for dip brazing.

1. Clean component parts prior to assembling.
2. Perform subassembly work (i.e., installation of stainless steel hardware).
3. Assemble all components, inserting brazing shims at lap joints of .187 or greater. Butt joints of less than .187 thick will have paste applied later in the process. Assembly may take a variety of forms:

• Slot and Tabs
• Self-locking Joints
• Tackwelding
• Spotwelds
• Aluminum Screws
• Aluminum Rivets
• Staking

4. Verify dimensions and straighten as required.
5. Apply filler metal paste to all joints.
6. Load on brazing fixture.
7. Preheat at 985°F (Cycle time is dependent on mass and part configuration).
8. Remove from preheat furnace and immediately immerse in salt bath. The bath operates at 1100°F. (Immersion must be performed at a constant rate to minimize distortion and filler metal run-off. Time within the bath is dependent on mass and part configuration.)
9. Remove from bath taking care to allow for flux drainage.
10. Move to quenching area.
11. After part has cooled, remove part from fixture and clean to remove flux residue.
12. Inspect braze joints.
13. Grind construction aids (screw heads, rivet rollover, tabs, tack welds).
14. Straighten.
15. Age harden at 350°F for 8-10 hours.
16. Inspect.

Mech-Tronics
1635 N. 25th Avenue
Melrose Park, IL 60160
Voice (708) 344-9823 Fax (708) 344-0067

A leader in
Precision Metal Fabrication | Photo Chemical Etching
Aluminum Dip Brazing | Nuclear Instrumentation

Email us at info@mech-tronics.com
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