Welding is a process of joining similar metals by application of heat, with or without application of pressure, with or without adding the filler material, with or without using flux. The welding processes generally used are classified into two main groups:
(a) Pressure Welding
(b) Fusion Welding or Welding without Pressure
In pressure welding, the surfaces to be joined are heated to a plastic state. Then forced together by external pressure to make the joint. This process is used in Forge Welding, Resistance Welding and Pressure Thermit Welding.
FUSION WELDING OR WELDING WITHOUT PRESSURE
In fusion welding, the metals to be welded are heated to a molten state and allowed to solidify. This includes gas welding and arc welding.
Fusion welding is accomplished by melting the edges or surfaces to be joined and allowing the molten metal to flow together. Filler rod is used to make a joint wherever required. A solid continuous joint is formed after cooling. This process is particularly suitable for joining metal sheets and plates having thickness of 2mm to 50mm. The composition of the filler rod is usually the same or nearly the same as that of the part being welded. To obtain a satisfactory bond, a flux is always employed during welding except for mild steel.
TYPES OF WELDED JOINTS
Following are some of the common joints employed in welding:
(a) Edge Joint. This joint is used to weld two parallel plates by means of a weld. This joint is often used in sheet metal work. The two edges can be easily and quickly melted down, eliminating the need of any filler metal.
(b) Butt Joint. Butt joint is used to join the ends or edges of two plates or surfaces lying in the same plane.
(c) T–Joint. This is used for welding two plates or sections whose surfaces are approximately at right angles to each other.
(d) Lap Joint. This joint is obtained by overlapping the two plates and welding each plate on the surface of other.
(e) Corner Joint. This is used to join the edges of two plates whose surfaces are at 90° to each other.
Brazing is similar to soldering, but it gives a much stronger joint than soldering. The main difference is the use of a harder filler material, which is known as spelter/brazing rod. It melts at a temperature above red heat (450˚C) but below the melting temperature of the parts to be joined.
In this process the parts to be joined are carefully cleaned. Then the flux (Bronzotectic) applied. The parts are heated to red heat and brazing rod (filler rod) is added. A brazed joint is obtained after cooling.
Bronzotectic flux is used in brazing. It helps in free flow of molten filler material and prevents the surface oxidation.