Brazing is the ideal process to create durable, dependable joints that are as strong or stronger than either of the metals forming the components being joined. It is important to realize that there are several steps in brazing metals and as with any other process, each step is essential increasing those solid, dependable and extremely strong joints.
The unique aspect of brazing metals is that it doesn’t include melting the sides of the metals to create the joint. Instead, a carefully selected filler metal with a melting point lower than the melting point of the metal or metals forming the joint is heated and flows into the joint through capillary action.
The use of a specially selected flux that works to assist the flow of the melted filler metal assures a smooth flow into the joint. Since the metal actually enters and forms the joint and also bonds with the components, the result is a uniform, strong connection that will cool to create a seamless joint.
Space and Capillary Action
For capillary action to occur there has to be a slight space between the two sides of the joint. This space, which is known as clearance, is essential to allow the melted filler metal to be drawn into the joint. The clearance is very slight, but it has to be carefully calculated based on the type of metal and the desired tensile strength of the joint after brazing.
The area where the flux and the filler metal are applied, as well as the area where the joint will form, has to be completely clean and free from any type of surface contamination. This can include oil and grease, paints, chemicals, dirt and even flakes of rust or areas of irregular, rough surfaces.
When brazing metals anything that is different between one area of the metal and another will cause the flux and the melted filler metal to flow differently in that area, resulting in a weaker overall braze.
The Choice of Flux
Think of flux as a coating over the area when brazing metals that prevent oxygen from contacting the surface. This, in turn, prevents oxides from forming at the joint during the heating of the filler metal. If flux is not used, these oxides will form almost instantly with the application of the heat, resulting in poor flow into the capillaries by the filler metal.
In brazing metals, there are many essential steps to consider, each of equal importance to the end product. The process actually starts with the design of the joint, the selection of the optimal clearance, and the choice of flux and filler metal based on the specific application.