- Spot Welding
- Projection Welding
- Seam Welding
- Butt Welding
Types of Resistance 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 movable and the other fixed. Pressure is applied from the top electrode by moving it downward.
The fusion of material takes place at the spot. Then the sheet is moved to have another spot weld at the desired place. The time period for each weld may be 100th of a second or more. The current may be of the order of 5000 amperes or more.
The voltage between electrodes is about 2 volts. The open circuit voltage of the welding machine is less than 12 volts. For the sheets, the time required is about 1/50 sec. for each 0.25 mm of the total thickness of the two sheets to be welded. Wires, rods and small pieces of a sheet can also be welded by this method.
Spot Welding Machine
The spot welding machine consists a welding transformer and required accessories to perform the welding. When the machine is connected to the AC supply, transformer of welding machine gets the supply through a timer which controls the time of flow of current.
The primary winding of the transformer has got tappings and a particular tapping is selected by a selector switch depending upon current required for welding.
The secondary winding of the transformer is connected to the two electrodes made of copper or bronze and they are cooled by water. The movable electrode of the machine can be moved by a foot pedal. When pressure is not applied to the foot pedal, the electrodes are kept apart by a spring. The sheets to be welded are placed overlapping each other between the two electrodes and pressure is applied by pressing the foot pedal. The spot weld is made between the sheets.
The composition and thickness of the plates determine the magnitude and duration of the current. It varies from 1000 to 10,000 amperes and may flow for a fraction of second to some seconds. Spot welding is mostly employed in the manufacture of automobiles and other metal stamping assemblies.
Advantages of Spot Welding
- It has low initial cost.
- High productivity.
- A less skilled worker can also operate this welding.
- The operation can be made fully automatic or semi automatic.
Disadvantages of Spot Welding
- Jobs of high thickness cannot be welded.
- Skilled workers are required for maintenance purpose.
Applications of Spot Welding
- It is used frequently in automobile and aircraft industries.
- It is used for making containers.
- It is used in the fabrication of steel furniture.
It is a modified form of spot welding. One of the pieces to be welded by this method has projection produced by the pressure. The electrodes used in projection welding are flat. The electrodes are placed on the work pieces 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 jobs which cannot be welded by spot welding. When two plates to be welded are of different cross-section then in order to obtain high strength, it is necessary to have the projections on the thicker plate.
Advantages of Projection Welding
- More than one spot can be welded at one time.
- Proper heat balance can be obtained easily.
- Welds can be placed closer than spot welding.
- Electrode life is much longer than the life of electrode used in spot welding.
- Appearance and uniformity of this weld are better than spot welding.
- With the projection welding, it is easy to weld certain jobs which cannot be welded by spot welding.
Disadvantages of Projection Welding
- Metals, which cannot support projections, cannot be welded satisfactorily.
- To make projection is an extra operation.
- The initial cost of equipment required for projection welding is high.
Applications of Projection Welding
- It is usually employed on punched jobs, where the projection automatically exists.
- Welding of stainless steel parts.
The figure shows how seam welding is carried out. In this case, roller type electrodes are used and the pressure between them remains constant. The seam welding is done in order to obtain a continuous joint.
The two sheets on which seam welding is required are placed overlapping each other. The current is passed between the two moving electrodes and sheets get welded. It is used for stainless steel, aluminum and its alloys, nickel and its alloys etc.
Advantages of Seam Welding
- Less overlap required of two parts than spot or projection welding.
- Several parallel seams can be produced.
- Gas tight or liquid tight welds can be produced.
Disadvantages of Seam Welding
- Seam welding sets are more costly than spot welding sets.
- It is difficult to weld thickness greater than 3mm.
- Welding can be done only along a straight line.
The figure shows how butt welding is done. The two parts to be welded together is placed touching each other end to end so as to form a butt joint. A pressure is also applied in the axial direction of the job. The jobs are securely clamped and heavy current is passed through them. The heat is produced to raise the temperature of material and fusion takes place at the points of contact.
This method is used for welding pipes, wires and rods etc. The voltage required for welding is 2 to 12 volts and current varies from 50 A to several hundred amperes, depending upon the kind of material and the area to be welded at a time.
Flash Butt Welding
It is similar to butt welding except that the parts to be welded are joined together under light pressure and heavy current is passed through the joint. Due to poor contact arcing takes place at the joint. When sufficient heat has been produced the two parts are suddenly pressed together and current is stopped simultaneously.
A thin film is produced around the joint which is subsequently removed to give a weld of this type. It is used for welding rail ends, shaft axles, chain etc.
Advantages of Flash Butt Welding
- It is a fast and cheap process.
- Preparation of weld surface is not required.
- Many dissimilar metals with different melting temperatures can be welded.
- It offers 100% strength factor.
Disadvantages of Flash Butt Welding
- More chances of fire hazards.
- Metal is lost during welding.
- Concentricity and straightness of work pieces are often difficult to maintain during the welding process.
Thanks for reading about types of resistance welding.
Electric Welding – 2 | Objective Type Question Answers
#1 Spot welding is used for
thin metal sheets
#2 Galvanizing is a process of applying a layer of
#3 A seamless pipe has
#4 Plain and butt welds may be used on materials up to thickness of nearly
#5 In argon arc welding argon is used as a
shield to protect the work from oxidation
#6 During carbon arc welding
electrode is negative with respect to the work if D.C. is used
#7 In welding, weld spatter defect is generally the result of
too high current during welding
#8 Steel pipes are manufactured by
#9 A rectifier for welding has voltage/current characteristic as
#10 TIG welding is
tungsten inert gas welding
#11 Flux used in TIG welding is
#12 For the welding of aluminum alloys which of the following methods would you recommend ?
Tungsten arc welding
#13 The danger of shock is maximum
while inserting electrode into the holder
#14 The welding electric circuit is
#15 Steel rails are welded by
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