Welding is broadly classified into two types:

I. Plastic Welding:

In this case the two pieces of metals to be joined are heated to plastic state and welding is completed by forcing them. In this case no filler material is added.

II. Fusion Welding:


In this case the two pieces of metal are heated and brought to molten state and filler material is added and allowed to solidify.

Plastic and fusion welding processes are further classified as follows:

I. Plastic Welding:

(a) Forge Welding:

It is the oldest method of joining two pieces of metals. In this case heat is created by blacksmith’s fire and the two pieces of metals to be joined are heated and brought to plastic state. Then they are superimposed and hammered together to form the joint.


(b) Resistance Welding:

In this case a heavy electric current is passed through the pieces of metals to be joined. Then because of electric resistance, the metals are heated to plastic state and by applying force, welding can be completed.

In this case, no additional filler material is required and the metal pieces are pressed in the two copper electrodes.

(i) Spot Welding:


It is used for welding ferrous and non-ferrous sheets upto a thickness of 8 mm. The sheets to be welded are held between the fixed and movable electrodes as shown. Then the electrodes are pressed by pressing the foot lever. When the electrodes are pressed, current flows and the two parts at the pressed points are brought to plastic state. Then, this plastic metal mixes, solidifies to form the joint.

(ii) Projection Welding:

It is the slight modified form of spot welding process. In this case current and pressure are localised at the welding points by making projections for the upper sheet of metal as shown in Fig. 40.9. The two metal sheets are held in position between fixed arm and upper movable arm. Then the current is passed and welds at all the points of projections are obtained due to flattering of projections.

For producing continuous welds between two over lapping sheets this seam welding is used. In this case the two sheets to be seam welded are held as shown between the electrodes. When the electrode wheels rotate continuous weld will be produced.


(iii) Seam Welding:

(II) Fusion Welding:

(a) Gas Welding:


In gas welding heat energy required to heat the surfaces is obtained by the combustion of mixture of two gases. Generally oxygen and acetylene are used for gas welding purpose. These gases are mixed in proper proportions in the welding torch. This welding torch is provided with two regulators to regulate the gases.

The flame produced due to the combustion is used to heat the metal surfaces to plastic state and welding is completed by adding the filler material as shown.

(i) Oxyacetylene Welding:

As the name implies in this case the two gases used are oxygen and acetylene. It is to be noted that oxygen and acetylene are commercially available in cylinders. If required acetylene can be produced by the chemical reaction between calcium carbide and water as under-


And the chemical reaction for the combustion of acetylene is,

When the combustion takes place, we get the flame at the tip of the torch as shown.

Mainly there are 3 types of flames:

(i) Neutral Flame:

It is obtained by mixing equal quantities of acetylene and oxygen. It is used for welding all the metals like ferrous metals, Cu, and Aluminium alloys.

When we change the proportions of Oxygen and acetylene. Carburising and oxidising flames can be obtained.

(ii) Carburising Flame:

It is obtained by more quantity of acetylene. It is very much suitable for welding of steel as the rate of welding is faster by this flame.

(iii) Oxidising Flame:

It can be obtained by more quantity of oxygen. It is mainly used for welding of brass. It is also very much suitable for cutting operations.


(1) Gas welding is more suitable for thin sheets.

(2) The equipment is portable, so suitable for outdoor repair works.

(3) By changing the nozzle in the torch, the torch can be used for gas cutting.


(1) It is a slow process compared to arc welding.

(2) Gases used in the gas welding are more costly.

Note that we will be studying welding torch, and other welding equipments while studying gas cutting.

Air acetylene welding. As the name implies in this case air and acetylene are used as the gases to produce the flame. In this case temperatures produced are low compared to other gas welding processes. This method is generally used for Lead welding.  

(b) Arc Welding:

(i) Metal Arc Welding:

Figure 40.8 shows an arc welding circuit. It mainly consists welding rectifier, electrode (or welding rod) holder, welding rod, and work piece.

When the welding rod touches the work piece, an arc is produced and tremendous amount of heat is liberated. The temperature of the arc is about 3600° C. This heat energy is utilised for melting work piece and welding rod. So, a small pool of molten metal is formed. This molten metal is agitated by the action of arc and the metal is perfectly mixed and after cooling it produces a sound joint.


(i) Arc initiation voltage is 60-100 V.

(ii) Arc maintenance voltage is 25-45 V.

(iii) Power source may be a.c. or d.c.

(iv) The gap between welding rod and work piece is to be 3 mm.

(v) Molten metal in the pool when exposed to air forms oxides and nitrides in the steel, when it reacts with oxygen and nitrogen in air. This weakens the welded joint also it reduces resistance to corrosion. So in order to avoid this, the welding rods are provided with flux coating. This flux after welding forms a slag covering on the welded joint and prevents from oxidation. This slag has to be chipped off afterwards.


This arc welding is most commonly used in the fabrication of tanks, vessels, trusses, frames, boilers, automobile chassis and body buildings etc.

(ii) Submerged Arc Welding (SAW):

It is the improved arc welding process, which is used for the production of butt welds of thick steel plates.

In this case the arc produced is submerged (covered) in the flux, hence the name submerged arc welding. Through the flux hopper, flux is supplied and wire feeder feeds the bare wire continuously.

In this case arc produced is submerged in the flux. Because of heat of arc the wire and parent metals melt and form a pool of molten metal. Thus molten pool produces a welded joint after cooling.

(iii) Gas Tungsten Arc Welding (GTAW):

It is also known as Tungsten Inert Gas welding (TIG).

It is a fast process, produces clean welds and it can weld metals considered to be impossible to weld. TIG-welding utilizes a non-consumable electrode in a special holder, a separate filler material and an inert gas i.e., argon gas cylinder, power supply source.

When the tungsten electrode strikes the work piece, an arc will be produced. Around the arc inert gas shielding is formed as the gas is coming out from the torch. Because of heat of arc, work piece and filler material melt and form a molten pool. This molten pool after cooling forms a sound welded joint in the shielding of inert gas.

Advantages of TIG Welding:

1. Welded joints are stronger more ductile and corrosion resistant than the other weld made by other methods.

2. The welding of non-ferrous metals is simplified as no flax is required.

3. Dissimilar metals can also be welded easily.

4. Due to presence of inert gas, there is less smoke.

5. The arc is transparent due to shielding inert gas, so the welder gas can clearly observe the weld as it is being made.

Limitations of TIG Welding:

1. The process is relatively slow in operation.

2. The back side of weld joint has to be protected from the atmosphere.

3. The cost of inert gas is quite high.

4. A1 joints require proper cleaning before welding as the inert gas does not provide any cleaning or fluxing action.

(iv) Gas Metal Arc Welding (GMAW):

It is also known as Metal Inert Gas (MIG) welding. In MIG welding we get clean and good welds also fast filler metal deposition rates. It uses high welding current which is used to break the globules of molten metal into fine spray.

Figure 40.11, shows schematic diagram of MIG welding. MIG uses a consumable electrode. It is supplied through an electrode holder into the arc. Then at the same speed electrode is melted and deposited into the weld. A small motor with adjustable speed will be used to remove the wire from the reel and fed into the arc.

Generally CO2 or Argon are used as shielding gases. Mainly the process was developed for welding Aluminium and Titanium. But nowadays it has wide application since it can be used for welding in all the positions; and less skilled operators are required to operate this set up.

(c) Thermit Welding:

This welding process is used in the repair of heavy parts such as spokes of driving wheels, broken connecting rods, other motor parts etc.

So, oxygen from iron oxide unites with aluminium and forms aluminum oxide or slag and a superheated thermit steel will be formed. The temperature of thermit-will be around 2700°C which is almost double when compared to the melting point of steel.

In thermit welding process, the parts which are to be welded are placed in the mould as shown Then the superheated thermit steel is poured from the refractory lined crucible as shown. Due to the heat of thermit steel, the surfaces of metals to be welded are brought to plastic state and if required some mechanical pressure is applied to complete the weld.