The welding processes may be broadly classified into the following two groups.
1) Welding processes that use heat alone e.g., fusion welding.
2) Welding processes that use a combination of heat & pressure e.g., forge welding.
1) Fusion Welding:
In case of fusion welding, the parts to be jointed are held in position while molten metal is supplied to the joint. The molten metal may come from the parts themselves (i.e., parent metal) or filler metal which normally has the composition of the parent metal. The joint surface becomes plastic or even molten because of the heat from the molten filler metal or other source. Thus, when the molten solidifies or fuses, the joint formed.
The fusion welding further classified as:
a) Thermit welding
b) Gas welding
c) Electric Arc welding
a) Thermit welding:
In thermit welding, a mixture of iron oxide and aluminium called thermit is ignited and the iron oxide is reduced to molten metal. The molten metal is poured into a mould made around the joint and fuses with the parts to be welded. The thermit welding is often used in joining iron & steel parts that are too large to be manufactured in one piece, such as rails, truck frames, locomotive frames etc.
b) Gas welding:
A gas welding is made applying the flame of an oxy-acetylene or hydrogen gas from a welding torch upon the structures of the prepared joint. The intense heat at the white cone of the flame heats up the local surfaces to fusion point while the operator manipulates a welding rod to supply the metal for the weld. A flux is being used to remove the slag. Since the heating rate is in gas welding is slow, therefore it can be used on thinner materials.
c) Electric Arc Welding:
In electric arc welding, the work is prepared in the same manner as for gas welding. In this case the filler metal is supplied by metal welding electrode. The formation of a joint between metals being are welded may or may not require the use of pressure or filler metal. The arc is struck between the workpiece and an electrode that is manually or mechanically moved along the joint or that remains stationary while workpiece is moved underneath it. The slag is brushed off after the joint has cooled.
There are two kinds of arc welding depending upon the type of electrodes.
1. Un-shielded arc welding.
2. Shielded arc welding.
2) Forge welding:
In forge welding, the parts to be jointed are first heated to a proper temperature in a furnace or forge and then hammered. This method of welding is rarely used now-a-days. An electric resistance welding is an example of forge welding.
In this case, the parts to be joined are pressed together and an electric current is passed from one part to the other until the metal is heated to the fusion temperature of the joint. The principal of applying heat and pressure, either sequentially or simultaneously, is widely used in the process known as spot, seam, upset and flash welding.