What is Brazing:
Brazing is the joining of metals through the use of heat and a filler metal – one whose melting temperature is above 840°F(450°C) but below the melting point of the metals being joined. It is distributed between two or more close-fitting parts by capillary action. At its liquid temperature, the molten filler metal interacts with a thin layer of the base metal, cooling to form an exceptionally strong, sealed joint due to grain structure interaction. The brazed joint becomes a sandwich of different layers, each metallurgically linked to each other.
Basic steps in brazing
- Good fit and proper clearances
- Cleaning the metals
- Fluxing the parts (Torch Brazing)
- Assembly for brazing
- Brazing the assembly
Brazing, as we’ve seen, uses the principle of capillary action to distribute the molten filler metal between the surfaces of the base metals. Therefore, during the brazing operation, you should take care to maintain a clearance between the base metals to allow capillary action to work most effectively. This means, in almost all cases – a close clearance. The following chart is a general guide.
If the gap is wider than necessary, the strength of the joint will be reduced almost to that of the filler metal itself. Also, capillary action is reduced, so the filler metal may fail to fill the joint completely – again lowering joint strength.
Remember, brazed joints are made at brazing temperatures, not at room temperature. So you must take into account the “coefficient of thermal expansion” of the metals being joined. This is particularly true when dissimilar metals are being joined.
|BAISi group||0.000-0.002 Vacuum Furnace Brazing|
|BAg group||0.002-0.005 Flux Brazing|
|0.000-0.002 Vacuum Brazing|
|BAu group||0.002-0.005 Flux Brazing|
|0.000-0.002 Vacuum Brazing|
|BCu group||0.000-0.002 Vacuum Brazing|
|BNi group||0.000-0.002 Vacuum Brazing|
One of the important steps is to ensure metals are clean from any impurities. Since Capillary action is needed when fitting parts, metal surfaces face difficulty during this fitting process if they are contaminated. For example, if they are coated with oil, grease, rust, scale or just plain dirt, those contaminants will form a barrier between the base metal surfaces and the brazing materials.
Cleaning the metal parts is seldom a complicated job, but it has to be done in the right sequence. Oil and grease should be removed first, because an acid pickle solution aimed to remove rust and scale won’t work on a greasy surface. (If you try to remove rust or scale by abrasive cleaning, before getting rid of the oil, you’ll wind up scrubbing the oil, as well as fine abrasive powder, more deeply into the surface).
Flux is a chemical compound applied to the joint surfaces before brazing. Its use is essential in the brazing process. The reason? Heating a metal surface accelerates the formation of oxides, the result of chemical combination between the hot metal and oxygen in the air. These oxides must be prevented from forming or they’ll inhibit the brazing filler metal from wetting and bonding to the surfaces.
A coating of flux on the joint area, however, will shield the surfaces from the air, preventing oxide formation. And the flux will also dissolve and absorb any oxides that form during heating or that were not completely removed in the cleaning process.
The parts of the assembly are cleaned (and fluxed if torch brazed). Now you have to hold them in position for brazing. And you want to be sure they remain in correct alignment during the heating and cooling cycles, so that capillary action can do its job.
If the shape and weight of the parts permit, the simplest way to hold them together is by gravity. Or you can give gravity a helping hand by adding additional weight.
If you have a number of assemblies to braze and their configuration is too complex for self-support or clamping, use a brazing support fixture. In planning such a fixture, design it for the least possible mass, and the least contact with the parts of the assembly. Use pin-point and knife-edge design to reduce contact to the minimum.
Try to use materials in your fixture that are poor heat conductors, such as stainless steel, Inconel or ceramics. Since these are poor conductors, they draw the least heat away from the joint. Choose materials with compatible expansion rates so you won’t get alterations in assembly alignment during the heating cycle. However, if you’re planning to braze hundreds of identical assemblies, then you should think in terms of designing the parts themselves for self-support during the brazing process.
At the initial planning stage, design mechanical devices that will accomplish this purpose, and that can be incorporated in the fabricating operation. Typical devices include crimping, tacking, interlocking seams, swaging, peening, riveting, pinning, dimpling or knurling. Sharp corners should be minimized in these mechanically held assemblies, as such corners can impede capillary action. Corners should be slightly rounded to aid the flow of filler metal.
This is the actual accomplishment of the brazing joint. It involves heating the assembly to brazing temperature, and flowing the filler metal through the joint.