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Before selecting a brazing material, three basic characteristics of the brazing material must be understood and evaluated: the physical properties, the melting process and the forms that can be used.
The physical properties of a brazing material are based on its metallurgical composition. (Brazes are generally alloys, made of two or more "pure" metals.) These components determine whether the braze is compatible with the metal to be joined - it wets the base material, flows through the joint area, and does not produce harmful metallurgical components. In addition, special properties may be required due to production needs. For example, in the case of vacuum brazing, the brazing material must be free of any volatile elements, such as cadmium or zinc. Some electronic components, for example, require very high purity brazing materials. Corrosion-resistant joints require a brazing material that is both corrosion-resistant and that works in harmony with the base material being joined.
The melting process of the brazing material is also based on the metallurgical composition. Since most braze materials are alloys, they generally do not go from solid to liquid at the same temperature as pure metal melting. There is, however, one very important exception. There are alloys, called eutectics, which also melt like pure metals. A silver-copper alloy, 72% silver and 28% copper. This brazing material melts completely at a single temperature of 780°C. In metallurgical terms, this means that the melting point (solid phase point) and the fluid phase point (liquid phase point) are the same.
Note that in the 72% silver, 28% copper fraction, the liquid and solid phase line temperatures are the same, while the alloy to the left or right of this eutectic fraction does not go directly from the solid to the liquid state, but through a "paste" range - where the solid and liquid phases coexist. This range is the difference between the "solid phase point" temperature and the "liquid phase point" temperature. The "solid phase point" temperature is the highest temperature at which the alloy is fully solidified, i.e., the point from which it melts when heated. The "liquid phase point" temperature is the temperature at which the alloy is completely liquid (i.e., when the alloy is cooled, it starts to solidify from this point).
In all brazing applications, the "liquid point temperature" of the brazing brazing material is a very important temperature. Since you never want - or need - to melt the base material in brazing, you should choose a brazing material that has a lower liquid point temperature than the solid point temperature of both base materials. There are also brazing welds where the liquid phase point factor must be taken into special consideration. For example, when brazing a component in steps, i.e., re-brazing near a previously brazed joint. The second brazing operation must not disturb the first joint. The way to prevent this is to use another brazing material again. The liquid phase point temperature of the brazing material for the second joint is lower than the liquid phase point temperature of the brazing material used for the first joint. This method ensures that the first joint is not melted when the second brazing is performed.
Common forms of brazing media include wire, sheet, wire, ring, powder, paste, foil, preform, core, etc. The appropriate brazing media form can be determined according to the structural characteristics of the workpiece to be brazed, the specific brazing method to be used, and the brazing productivity requirements.
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