Brazing is a metal joining process where a metal filler is heated and distributed evenly between two or more close fitting parts. The filler metal, usually bronze or copper, is heated to slightly above it’s melting point. It is then allowed to flow over the base metal and cooled to join the pieces together. This is known as “wetting”. Usually a flux is placed on the metal. Flux is required to prevent oxides from forming while the metal is heated. The flux also serves the purpose of cleaning of any contamination left on the brazing surfaces. The brazing process is a similar process to soldering, except the temperatures used to melt the filler metal is above 800 degrees F. Torch brazing is the type of brazing that is used with Castaloy, Steelaloy, and Alumaloy. This is the most common form of brazing and works best when working with a small repair.
The two most important factors in obtaining a solid repair are surface preparation and temperature control. There are two main methods for cleaning parts prior to brazing. One is to use an abrasive method to scuff up the surface area and the second is chemical cleaning. When using mechanical cleaning it is important to maintain the proper surface roughness as wetting on a rough surface occurs much more readily than on a smooth surface. The next area of concern is the effect of temperature and time on the quality of brazed joints. As the temperature of the braze alloy is increased, the alloying and wetting action of the filler metal increases as well. In general, the brazing temperature selected must be above the melting point of the filler metal. In general, the lowest possible braze temperature is used in order to minimize brazing time and the associated costs.