Aluminum Dip Brazing

Process Overview

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 — easily managed with proper tolerancing and design.

Parts are cleaned of excess oxides, then assembled with the proper filler metal applied. At Mech-Tronics, we use our in-house photo chemical etching process to make custom brazing shims, yielding excellent control over excess filler-metal flow. Parts are placed on stainless steel fixtures, heated to 985°F in a pre-heat furnace, then immersed in a bath of molten salt (1100°F) containing flux. The molten bath transfers heat evenly, supports the assembly, and fluxes the joints. Immersion is generally less than two minutes, during which filler metal melts and flows to all joints by capillary action. The assembly is then removed, cooled, and cleaned — ready for further processing.

Typical Aluminum Dip Brazing Process

1. Clean & assemble. Clean the component parts, complete any subassembly work, then assemble with brazing shims at lap joints (.187 or greater) using slots and tabs, tack/spot welds, aluminum screws, rivets, or staking.
2. Apply filler & fixture. Verify dimensions, apply filler-metal paste to all joints, and load onto the brazing fixture.
3. Preheat & braze. Preheat at 985°F, then immerse in the 1100°F salt bath at a constant rate so filler metal flows to every joint with minimal distortion.
4. Remove & quench. Withdraw the assembly allowing for flux drainage, then move to the quenching area to cool.
5. Clean & finish. Remove from the fixture, clean off flux residue, grind construction aids, and straighten as needed.
6. Age harden & inspect. Age harden at 350°F for 8–10 hours, then perform final inspection of the braze joints.

Design Considerations

• Design components to be self-fixturing and self-locating. Aluminum's rate of expansion is about 1/3 greater than the fixturing material, so fixturing is kept simple and minimal.
• Some distortion will be present. Aluminum reaches annealed condition at brazing temperatures. Parts are quenched immediately; some straightening can improve distortion. Closely toleranced features may need to be machined after brazing.
• Parts must be vented. Dense brazing salts make aluminum buoyant, dictating specific fixturing orientation. Vent holes may be required and can be welded closed afterward. No closed cavity can be present due to pressure from expanding gases.
• Filler metal comes in different forms: .003 thick shim, paste (powder mixed with water), and wire of varying diameters / clad brazing sheet.
• Joint clearance must be controlled. Dip brazing depends on capillary action created by proper joint clearance (general guidelines below).

Suggested Joint Clearance

When clad brazing sheet is used, the clad side must be in direct contact with the mating part. Please consult us if you are designing your own components for dip brazing.

Materials

Brazeable Aluminum Alloys

Composition of Clad Aluminum Brazing Sheet