Brazing is defined as a process of joining similar or dissimilar metals in which molten filler metal is drawn by capillary action into the space between closely adjacent surfaces of the parts to be joined.
In general, the melting point of the filler metal lies above 500C but below the melting point of the parent metals being joined. The brazing alloy is often called spelter and can be plain brass (50% copper and 50% zinc). Increasing of the zinc content decreases the melting point of the spelter. Borax is used as a flux for brazing.
Brazing alloys containing phosphorus are referred as ‘self-fluxing brazing alloys’. These alloys contain silver, phosphorus and copper or copper and phosphorus. These alloys are very much in use for manufacturing refrigerator and electrical assemblies etc.
Applications of Brazing
There are various applications of brazing. Some of these are given below:
(a) Brazing is used for fastening of pipe fittings, tanks, carbide tips on tools, radiators, heat exchangers, electrical parts, axles, etc.
(b) It can join cast metals to wrought metals, dissimilar metals and also porous metal components.
(c) It is used to join band saws, parts of bicycle such as frame and rims.
Brazing Alloy: Composition and Melting Temperature
The brazing alloys are mainly of copper and zinc composition. The melting point of the alloy varies according to the percentage of copper and zinc. As the zinc percentage increases, melting point decreases.
Composition and Melting Temperature of Brazing Alloy
|Percentage (%)||Melting Temperature(0 C)|
Brazing Torch. A special torch is required for brazing. The usual type is supplied with main gas and air from a blower. It is fitted with adjustable taps for controlling the gas, air flow and mixture of gases.
Brazing Hearth. Brazing is best carried out in a hearth designed for the purpose. The hearth is made from steel lined with firebricks and has an electrically driven air blower mounted on it.
Brazing Lamp. Brazing lamp is also known as blowlamp. It is used to heat up the parts to be soldered and for heating the common soldering iron. These are most often applicable to soft soldering, but are sometimes used for brazing with hard solders having a relatively low melting point (e.g. silver solders). These are available in many sizes (e.g. 1 pint, 2 pints, 5 pints etc.) as per their container capacity and are operated on kerosene. The use of inappropriate fuel deteriorates the operation of a blowlamp. The normal working pressure is 20/30 lb. per sq. in. The container should be filled upto maximum ¾th of its capacity with clean kerosene to accommodate the air pressure.