The following points highlight the five main types of resistance welding. The types are: 1. Spot Welding 2. Projection Welding 3. Seam Welding 4. Butt Welding 5. Flash Butt Welding.
Type # 1. Spot Welding:
It is used for joining two or more sheets of metal. The sheets to be welded are held overlapping between the two electrodes, one moveable and the other fixed. Heavy current is passed through the electrodes. Pressure is applied from the top electrode by moving it downward.
The fusion of material takes place at the spot. The sheet is then moved to have another spot weld at the desired place. The time period for each weld may be 1/100th of a second or more. The current may be of the order of 5000 amperes or more. The voltage between electrodes about 2 volts. The open circuit voltage is less than 12 volts.
Fig. 4.4 shows how spot welding is done. Rods, wires and small pieces can also be welded by this method. For the sheets the time required is about 1/50sec. for each 0.25 mm of total thickness of the two sheets to be welded.
Spot Welding Machine:
A spot welding machine is shown in Fig. 4.5 AC supply is given to the primary of the welding transformer through the timer which controls the time for which the current flows. The primary has got tappings and a particular tapping is selected by the selector switch S depending upon the current required for welding.
For maximum welding current the selector switch should be kept in position No. 1. The secondary is connected to the two electrodes made of copper or bronze and they are cooled by water. When pressure is not applied on the foot pedal, the electrodes are kept apart by the spring.
The sheets to be welded are placed over-lapping each other between the two electrodes and pressure is applied by pressing the foot pedal. The spot weld is made between the sheets. Thickness and composition of the plates determine the duration and magnitude of the current. It varies from 1000 to 10,000 amperes and may flow for a fraction of a second to some seconds. Spot welding is mostly employed in the manufacture of automobiles, refrigerators and other metal stamping assemblies.
Advantages of Spot Welding:
(i) It has low initial cost.
(ii) Higher productivity.
(iii) Less skilled worker can operate this welding.
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(iv) No edge preparation is required.
(v) Operation may be made automatic or semi-automatic.
Disadvantages of Spot Welding:
1. Job of more thickness cannot be welded.
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2. High initial cost of equipment.
3. Skilled labour is required for maintenance purpose.
Applications of Spot Welding:
(i) It can be frequently used in automobile and aircraft industries.
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(ii) It is used for making containers.
(iii) Fabrication of steel household furnitures.
Type # 2. Projection Welding:
It is infact a modified form of the spot welding. One of the pieces to be welded this way has projections produced by pressure. The electrodes are flat. The electrodes are placed on work piece and current passed between them. Heat is produced at the contacts and work piece gets welded at these points.
It is easy to weld certain parts which cannot be welded by spot welding. When two plates to be welded are of different cross-section then in order to obtain desirable strength it is necessary to have the projections on the thicker plate.
Advantages of Projection Welding:
(i) More than one spot can be weld at a given time.
(ii) Proper heat balance can be obtained easily.
(iii) The welds may be placed closer than spot welding.
(iv) Electrode life is much longer than the life of electrode in spot welding.
(v) Appearance and uniformity of the weld is better than spot welding.
(vi) With the projection welding, it is easy to weld certain parts which cannot be welded by spot welding.
Disadvantages of Projection Welding:
1. Metals, which cannot support projection, cannot be welded satisfactory.
2. To make projection is an extra operation.
3. The initial cost of the equipment required for projection welding is high as press type machines are required for this type of welding.
Applications of Projection Welding:
(i) It is usually employed on punched where the projection automatically exists.
(ii) Small fasteners, nuts etc. can be welded to larger components.
(iii) Projection welding is used for welding of refrigerator condensers, crossed-wire welding grills etc.
(iv) Welding of stainless steel parts.
Type # 3. Seam Welding:
In Fig. 4.7 shows how seam welding is carried out. In this case wheel or roller type electrodes are used and the pressure between them remains constant. The seam welding is done in order to produce a continuous joint which may be required for the construction of tanks, transformer’s refrigerators, gasoline tanks, air craft, containers and cylindrical pieces. Seam weld is actually a series of overlapping spot welds. Usually numbers of spots obtained per meter of the joint are between 200 to 400.
The two sheets on which seam welding is required are placed overlapping each other. The current is passed between the two moving electrodes as shown in Fig. 4.7. The electrodes are to be shape of continuously rotating wheel which also applies pressure on the job. It is used when a continuous joint required between two overlapping metal sheets.
Advantages of Seam Welding:
(i) Less overlap than spot or projection welds.
(ii) Several parallel seams may be produced.
(iii) Gas tight or liquid tight joints can be produced.
Disadvantages of Seam Welding:
(i) High cost of equipment as compared to spot welding set.
(ii) It is difficult to weld thickness greater than 3 mm.
(iii) Welding can be done only along a straight line.
1. It is used for welding of stainless steel, aluminium and its alloys nickel and its alloys etc.
Type # 4. Butt Welding:
In Fig.4.8 shows how butt welding is done. The welding transformer has a comparatively larger number of turns on the primary and lesser number of turns on the secondary because heavy current is required on the secondary side. The two parts to be welded together are placed touching each other end to end so as to form a butt joint.
A pressure is also applied in the axial direction of weld. The jobs are securely clamped and heavy current is passed through them. The heat produced is sufficient to raise the temperature of material to plastic state and fusion takes place at the points of contact when pressure is applied on both sides.
This method is used for welding pipes, wires and rods etc. The voltage required for welding is 2 to 10 volts and current varies from 50 A to several hundred amperes, depending upon the material and the area to be welded at a time.
Type # 5. Flash Butt Welding:
This is similar to butt welding except that the parts to be welded are joined together under light pressure and a heavy current passed through the joint. Due to poor contact at the joint arcing takes place. When sufficient heat has been produced the two parts are suddenly pressed together and the current is simultaneously stopped. A thin film is produced around the joint which is subsequently removed to give a weld of this type. It is used for welding chains, rail ends, shaft axles etc.
Note: Resistance welding can be applied to iron and steel, brass, copper, aluminium and certain other metals with eminently satisfactory results.
Advantages of Flash Butt Welding:
(i) It is a faster process.
(ii) Preparation of weld surface is not required.
(iii) Power requirement is less.
(iv) Many dissimilar metals with different melting temperatures can be flash welded.
(v) The process is cheap.
(vi) It offers strength factor upto 100%.
Disadvantages of Flash Butt Welding:
(i) More chances of fire hazards.
(ii) Metal is lost during flashing and upsetting.
(iii) Concentricity and straightness of the work pieces during welding process is often difficult to maintain.
Applications of Flash Butt Welding:
(i) It is used for welding metal sheets, bars, rods fittings etc.
(ii) Flash welding finds applications in automotive and air craft products.
(iii) It is also employed in household appliances, refrigerators and farm implements.