Weld My World - Welding News

All About Brazing


Brazing is a process that joins metal. Filler metal is heated and distributed between at least two close-fitting parts by a process called capillary action. A flux is usually used to protect the atmosphere around the filler metal as it is brought just above its melting point. The filler then flows over (called wetting) the base metal where it is cooled and joins the workpieces. The process is similar to soldering but the temperature used is above 800 ºF (450 ºC).

For joints to be high quality, the parts have to be fitted closely and the base metals must be free of any oxides and be extremely clean. For the most strength and best capillary action, a clearance of 0.03 to 0.08 mm (0.0012 to 0.0031 in.) is required. Any contamination will cause poor wetting, and the main cleaning methods are abrasive (mechanical) cleaning or chemical cleaning.

Time and temperature have effects on quality. When the temperature of the braze alloy increases the wetting and alloying action in the filler metal also increases. The brazing temperature needs to stay above the melting point of the filler metal and there are ways to select the best temperature. Choose the lowest braze temperature possible, minimize heat effects, keep the base and filler metal intereactions to a bare minimum, and maximize the life of the jigs or fixtures that are used. This will give you the best temperature.

The flux prevents the formation of oxides during heating. A flux is required when brazing is not done in an environment that reduces the atmosphere or produces an inert environment, such as a furnace. The flux will also clean off any contamination that may be left on the surface. It can be applied in many forms like a paste, powder, or liquid. It also can be applied using brazing rods that have a flux core or a flux coating. It flows into the join and is displaced by the filler. Excess should be removed as it can lead to corrosion and other problems.

Filler materials can be any of a wide variety of alloys such as copper, nickel, silver, or aluminum-silicon. Most braze alloys have 3 or more metals in them in order to make the alloy that has the properties required. It is available as rods, pastes, creams, wires, powders, ribbons, and preforms. It can either be placed before heating at the right location or it can be applied while heating. If doing manual brazing, rods and wires are most often used because they are the easiest.

Atmosphere is very important in brazing. Oxidation occurs where there is oxygen so sometimes an environment without oxygen is required. Some of the commonly used atmospheres are air, ammonia, vacuum, and hydrogen. Air is simple, economical, and leaves materials prone to a buildup of scales and oxidation. It can be removed with mechanical cleaning or an acid bath. Flux can be used to head off oxidation but it can actually weaken the joint. Ammonia is used for copper, nickel, silver, copper-zinc and copper-phosphorus fillers. It is used for brazing brass, low-nickel alloys, copper, and some steels. A vacuum is used for the highest-quality joints like in aerospace brazing but is very expensive. Hydrogen is used for some kinds of steel, nickel alloys, copper, and brass and the same filler metals as ammonia. It is also used for carbides, cobalt and tungsten alloys, and chromium alloys.

Brazing is a popular method that many welders use in their jobs or in their own hobbies. Brazing is often taught in classes and there are seminars to help further welders' education in this method.

Ed C.


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