The essential principle of brazing is the flow of the braze alloy due to capillary action. Capillary flow is the ability of a liquid to flow in a narrow gap against traditional forces like gravity. The force of the capillary action is the same as the wicking of paint in between the bristles of a brush, or in induction brazing the wicking of the braze alloy between the two surfaces of the joint.
As the braze alloy melts during the heating cycle, the molten alloy is drawn through the joint due to this phenomenon. It is typically recommended to have a clearance joint of 0.002 to 0.004 thousandths of an inch for good capillary action for silver braze alloys. A number of different braze alloys are available depending on the metals to be joined and the temperature required.
Brazing materials are alloys of two or more metals. The dominant metals in the braze alloys typically determine the melting temperature of the alloy. However, when these metals are mixed in particular ratios the combination typically melts at a temperature that is much lower than the alloying metals. The mixture is called a Eutectic.
Holding of the parts during the induction heating process is an art and a science all by itself. It’s important for the holding devices to be capable of withstanding multiple heat cycles without deteriorating. The holding device should also not become a heat sink, robbing the heat from the actual parts being heated. In some applications the fixture is what is heated because the parts are so small that it is impractical to heat them directly.
Flux in a brazing process is an add-on paste or liquid that is applied to the joint area for better flow of the braze alloy. Fluxes are inert salts and do not interfere with the quality of the brazing process. The purpose of the flux is to provide a shield to the joint area so that the metal surfaces being heated do not oxidize.
Appearance and the size of the braze fillets has been a topic of considerable debate for some time. While a good visible braze fillet is reassuring that the braze alloy has flowed, the fillet itself does little to improve the strength of the braze joint. The strength of the braze joint is obtained from the lap joint area between the parts being joined, however the appearance of a good braze fillet may act as a simple quality control check for the brazing process.
In a number of brazing applications it is difficult to rely just on the capillary action to limit the flow of braze alloy to the joint area. It is often exasperated by the fact that the braze alloy flows towards the heat source, pulling it away from the joint. A few brazing applications can benefit from the use of a braze stop-off compound.
Voids in braze joints are undesirable. They are also unpredictable and result in quality problems. They reduce the strength of the brazed parts. Often leaks in the braze joints are a result of the voids in the braze joints. The following are common practices for the elimination of voids in braze joints.
A number of applications, especially in the aerospace and automotive markets, require temperatures in the vicinity of 2000 °F (1093 °C). Generic fluxes do not work at these elevated temperatures.
In order to protect the oxidation of the metal at these elevated temperatures, vacuum or other inert gas environments are needed. Typical inert gasses include cracked ammonia, nitrogen or argon. Other gasses may also be used for specialized applications.