Brazing Tips

1. Proper Joint Clearances

During the brazing process, 2 closely fitted surfaces or base metals are heated and a filler metal is introduced.  As the filler metal becomes molten it is drawn into the joint by capillary action.  The joint strength is a function of the joint design.  The joint must have the proper amount of clearance between the base metals.  See the table below for recommended clearance at the brazing temperature.  Remember that the metals will expand when heated.

Recommended Joint Clearances at brazing Temperatures

2.  Pre-cleaning of Base Metals

The metals to be joined must be cleaned prior to brazing.  Cleaning can be accomplished by chemical or mechanical means.  Chemical cleaning can be done by vapor degreasing, chlorinated solvents, acid pickling cleaning products.  Mechanical cleaning can be done by grinding, machining, sandblasting or wire brushing.

3.   Proper Fluxing of the Base Metals

Fluxing and atmosphere brazing are used to stop/prevent the formation of oxides on the base metals during the heating process.  Oxidation limits capillary action and decreases joint strength.  Copper Filler metals containing Phosphorus act as a fluxing agent to prevent oxidation.  In many case an additional flux is not required.

4.  Brazing

This is where the heat is introduced to the assembly.  There are several basic heating methods; torch, resistance, induction, vacuum and atmosphere furnaces.  Torch heating can be done with any fuel such as oxy-acetylene.   The filler metal will tend to flow toward the heat.  Do not directly heat the filler metal, this can lead to a very low strength joint.  Heat must be applied uniformly to both parts to be joined. 

Induction and resistance heating can be cost effective for medium volume production.

Furnace heating is usually used for higher production volumes and difficult to join materials.   

5.  Post Cleaning

After brazing, the flux residue must be removed because they are corrosive.  The most common was is to water quench the assembly into a hot water bath.   This will normally break up the flux residue.  The residue can also be removed by mechanical means.