In the brazing process, the filler metal is drawn into the joint by a pulling force known as capillary action during the heat cycle. So it is particularly important to maintain right amount of space between the parts to allow this to happen. Usually, the strongest joints are made by allowing just enough space for the filler metal to flow into the joint area, typically in the range of .001” to .005” (0.25 mm to 0.127 mm). Wider spacing will generally result in a weaker joint.
It is also important to remember that metals expand and contract at different rates when heated and cooled. Particularly when joining dissimilar metals, expansion/contraction rates must be allowed for when positioning the parts.
Silver, copper and aluminum alloys are commonly-used filler metals; silver is frequently chosen because it has a relatively low melting point. Copper braze has a higher melting point but is generally more economical. Depending on the application, the alloy may be in the form of a stick, paste or preform. A pre-formed brazing alloy is normally the best choice when even distribution and repeatability are paramount considerations.
The braze material will not flow properly if grease, dirt or rust blocks its path. So the first procedure is to remove any oil or grease with a degreasing solvent or other method. Then remove rust and scaling with a chemical bath, stainless steel wire brush or emory cloth. The joint area MUST be clean.
When brazing is done in the open air, the joints are normally pre-coated with flux, a chemical compound which protects the part surfaces from air. A flux coating helps prevent oxidation when the metal heats up, protects the braze alloy and improves its flow. As heat is applied to the joint, the flux will dissolve and absorb the oxides that form. A variety of fluxes are available for use at different temperatures, with different metals and for a variety of environmental conditions. The point to remember is that the flux should melt and become completely liquid before the alloy melts. Most often flux is sold in paste form so it can be brushed on to the parts just before the actual heating cycle.
To eliminate flux, eliminate the presence of oxygen and braze your parts in a protective atmosphere such as nitrogen, hydrogen or dissociated ammonia. This type of brazing is usually completed in a controlled atmosphere glove box or a vacuum furnace. Without oxygen in the surrounding atmosphere, there is no potential for oxidation and the finished joint retains a clean, high quality appearance. The utilization of a protective atmosphere also eliminates any need for the post-braze acid cleaning bath.
Because of these advantages, protective atmosphere brazing has great appeal for manufacturers concerned with high throughput in a lean, continuous flow manufacturing environment.
Before applying heat to the parts, make sure they are properly-positioned and braced to remain in proper alignment. Particularly with lap joints, the laws of gravity help in this regard. Clamps, vises, additional weights and supports are sometimes needed. When choosing support materials, select those that are poor conductors of heat; stainless steel, inconel or ceramics will draw minimal heat away from the joint and preserve the efficiency of the heating process. Also look for support materials with compatible expansion rates so that the alignment is not disturbed.
Most brazing processes run at temperatures between 800°F and 2,000°F. For a strongest braze joint, the metals that are being joined together need to be at close to the same temperature. Slow heat cycles generally produce better results than fast heat cycles.
In many brazing procedures, the filler metal is applied to the joint after the proper temperature is reached. Alternatively, brazing preforms can be positioned around the joint before the heat cycle begins. The melting filler metal will tend to flow toward areas of higher temperature, so it is good practice to apply heat to the side of the assembly opposite to where the filler metal is positioned. The heat then helps draw the molten metal down into the joint area.
Parts which are brazed in an open-air atmosphere require a two-step cleaning operation. Flux residues are chemically corrosive and may weaken the joint if not completely removed. After the filler has solidified, a hot water quench immediately after the heat cycle is recommended. To remove residual oxidation, the parts can be dipped in hot sulfuric or hydrochloric acid. Care should be taken to avoid etching the joint with too strong an acid solution.
Parts brazed in a protective atmosphere require no cleaning.