Handbook-Braze Welding

3 The Nature of the Braze Welded Bond Braze welding works because a molten metal with low surface tension will flow easily and evenly over the surface of solid metal which has been well cleaned and heated to the right temperature. You can compare the action to the even flow of water over a truly clean glass surface. If the glass is not perfectly clean, the water will form a series of puddles or drops. Similarly, if the metal to be welded isn’t really clean, or cannot be made clean enough by the action of the welding flux, and if the metal is not at the right temperature, the brazing filler metal will not flow, or tin, properly. Whenever proper tinning has been achieved, the bond between the base metal and the brazing alloy, after the weld has cooled to room temperature, should have a strength at least equal to the strength of the brazing metal deposit itself, and equal to the strength of the base metal in many cases. Although we say there is no true ”fusion” of the two metals, there is actually a very narrow zone, observable only at high magnification under a microscope, in which mixing of the atoms of the base metal and brazing alloy has taken place. Braze Welding Technique Successful braze welding requires careful attention to three factors: 1. Preparation and mechanical cleaning of the joint. 2. Proper tinning of every bit of the joint surface, which results from the combination of chemical cleaning of the surface by the flux and maintenance of the proper metal temperature. 3. Complete fusion between layers of weld metal when the weld is made in two or more passes. Joint Preparation. In general, the joint design for braze welds is the same as for fusion welds in base metals of like thickness. Square edges may be used if the base metal is sheet not thicker than about 3-4 mm (1/8 to 3/16 in.) Bevelled edges should be used with all thicker material. Generally, avoid using an included angle of less than 90 degrees.