So, What Then Is “Brazing”?
Brazing is a process that permanently joins two or more metals/materials together to form a single assembly by heating them in the presence of a filler metal that begins to melt above 450°C (840°F). The liquid filler metal is drawn into the gap between the closely fitted faying surfaces of the joint by capillary action.
Thus, in brazing, the base materials (metals and/or ceramics) are not melted. Only the brazing filler metal (BFM) becomes liquid, and flows by capillary action between the surfaces of the non-melted base materials, as shown in Fig. 1. These surfaces of interest here are called the “faying surfaces”, that is, the surfaces inside the joint that are to be joined together.
Surfaces outside of the joint are generally irrelevant. The same holds true for the fillets formed at the outside edge of the joint by the BFM—they don’t really contribute to the strength of the joint. Thus, only what happens INSIDE the joint, i.e., between the faying surfaces, matters. And that, in turn, depends on the ability of capillary action to do its work. Therefore, it is very important to understand what variables will enhance or hinder its work.
Capillary action is the force that causes liquids to rise in a straw sitting in a glass, or what causes spilled coffee to be absorbed by a paper napkin. Capillary force is very strong—stronger than gravity inside a properly designed joint — thus enabling a liquid brazing filler metal (BFM) to be drawn between the faying surfaces of the joint in any direction, irrespective of the orientation of the assembly (horizontal or vertical)For capillary force to work, however, the base metal and the BFM must be compatible, that is, they must be able to alloy with (diffuse into) each other. The BFM will then be able to “wet” (spread out along the surface of) the base metal. The extent of alloying doesn’t have to be much, but it must occur.
Nickel and chromium, however, are able to alloy nicely with iron, and thus, the nickel-based BFMs are able to “wet” steel well, resulting in low wetting angles, as shown in Fig. 2 (top.) The smaller the wetting angle, the better. Pure silver, on the other hand, doesn’t alloy well with pure iron. So when pure silver is melted on the surface of a steel (iron alloy), it tends to ball up, as illustrated in Fig. 2 (bottom), showing non-wetting of surfaces. It does not “wet” the steel, and has a large (poor) “wetting” angle (greater than 90°).