Handbook-Brazing & Soldering

The key to successful brazing is joint design. Because the process depends on capillary attraction, the spacing

between the parts to be joined must be small and accurately controlled. While it is possible to make a square-butt joint (similar to that we talked about for fusion welding steel sheet) the two edges must be very carefully prepared, and spaced precisely, if the joint is to be successful. Such butt joints are occasionally employed, but are exceptions. Almost all brazed joints can be classified as lap joints. Several examples are illustrated in Fig. 21-2. The joint between a piece of copper tube and a socket-type fitting is a kind of lap joint. In such a joint, the separation between the two parts should usually be between 0.050 mm (.002 in.) and 0.150 mm (0.006 in.) While clearances greater than 0.150 mm can usually be filled with brazing alloy, the joint will lose some of its strength when subject to tensile forces. In brazing operations, as in braze welding, a flux is required. The purpose of the flux is to remove from the surfaces of the parts any oxides which may remain after thorough mechanical cleaning, or which form as the parts are being heated to brazing temperature. The fluxes used with silver- or copper- based brazing alloys are all based on boric acid and other boron or flourine compounds. They are somewhat similar to the fluxes used in braze welding, but should not be considered interchangeable with such fluxes. The active ingredients are dry powders which are then mixed with a vehicle to form a paste or thick liquid which can be readily applied to the joint surfaces. When the joint is heated, the vehicle evaporates, and the flux then liquefies to do its work of dissolving oxides.