Resistance welding is a type of electric welding in which heat produced to weld the two metal pieces will be due to the resistance offered to the flow of current by the two metal pieces at the point of joint. In this article let us see the working principle, types, advantages, disadvantages, and applications of resistance welding.
Principle of Resistance Welding :
In resistance welding sufficiently strong electric current is sent through the two metal pieces to be welded using electrodes, the resistance offered by the two metal pieces at the contact area develops heat and melts the metal to a plastic state or liquid state. Then after a high mechanical pressure is applied by electrodes to press the two metal pieces together which completes the weld.
The heat developed is given by I2Rt, where 'I' is the current flow, 'R' is the resistance (in ohms) and 't' is the time for which the current flows. The resistance in the equation is made of,- Resistance to the current path in the workpiece.
- The resistance between the contact surfaces of parts being welded.
- The resistance between the electrodes and workpieces.
For resistance welding a very high value of current (above 100A) is used to generate the heat and voltage in the range of 4-12V is used depending upon the composition, area, and thickness of metal pieces. Also, the resistance between the electrodes and metal pieces should be minimum so that a higher temperature is obtained at the point to be welded rather than at the surface of the metal piece in contact with the electrode.
Due to the availability of desired voltage and current combinations using transformers, ac power supply is well suited for resistance welding. The magnitude of the secondary current can be easily controlled by varying primary voltage using an autotransformer or tap-changing transformer. Usually, automatic arrangements are made to control the power supply since the time for which current flows is very important.
Types of Resistance Welding :
Depending upon the manner in which the weld is obtained and the type of electrodes used, the resistance welding is of four types,- Spot Welding,
- Projection Welding,
- Seam Welding, and
- Butt Welding.
Spot Welding :
Spot welding is a form of resistance welding in which the metal parts or pieces are joined in spots by heating relatively small sections between suitable electrodes under pressure. This type of welding technique provides high mechanical strength, but cannot provide water or air-tight sealing. The below diagram represents the spot welding process.
The welding process consists of two electrodes between which metal pieces to be welded are placed. The two electrodes are connected to the supply source through a transformer. When electrodes are excited, heat will be generated at the tip of the electrodes and between the two workpieces to be joined. The heat developed will melt the metal pieces at the place of the joint and are fused together under the pressure of electrodes.
The electrodes used are made up of copper or copper alloy. Generally, currents in the range of 1000 to 10000A with a voltage value of around 2V are used for spot welding. The value of current depends upon the thickness and composition of the metal pieces. For obtaining good welds having strength, the metal pieces are cleaned thoroughly.
The high current for the welding process is obtained from a step-down transformer, that step-downs high voltage low current supply to low voltage high current supply. The welding process can be controlled by varying welding current, duration of welding current, and pressure of electrodes. Spot welding is well-suited for joining two or more overlapping pieces where the number of welds required is relatively small. The applications of spot welding are joining automobile sections, cabinets, etc.
Projection Welding :
Projection welding is similar to spot welding but with some modifications. In this welding, electrodes of flat metal plates are used. These metal plates are known as platens in which one is movable and the other is fixed. The metal pieces to be welded are held together between platens as shown below.
The metal pieces placed between platens contains projections or bumps of required shape and size on one of the piece. When current starts flowing through the metal pieces, the projection area gets heated up and changes to a plastic state. Then after a high mechanical pressure is applied by platens, the heated and softened projection collapses under the pressure of the electrodes, thus completing the weld.
Projection welding has the following advantages over spot welding,- The projection made the welding process simple.
- With the projection welding, it is easy to weld certain parts where spot welding is not possible.
- The projections made before the weld makes the position of welds locate automatically.
- As the electrodes used are flat type, it is possible to join several welding points in projection welding.
This type of welding technique is used for mass production. Some of the applications of projection welding are welding refrigerators, condensers, crossed-wire welding, grills, etc.
Seam Welding :
Seam welding can be defined as a series of spot welds made progressively along a joint between the two metal pieces. In this type of welding electrodes in the form of a wheel or roller type are used instead of tipped electrodes as shown below.
As seen in the above figure, in seam welding the metal pieces are kept between two circular electrodes. Before welding, the two pieces are cleaned and overlapped suitably. As the wheel-type electrodes rotate keeping the metal pieces under pressure, current passes through them and heats up. When these metal pieces reach welding temperature, they change into a plastic state, and due to pressure from electrodes, continuous spot welds are obtained.
In seam welding, a timer is used for controlling the weld current, since the flow of continuous current can build up high heat that causes burning and wrapping of the metal piece. The timing controller controls the flow of current by switching ON and OFF of supply at rapidly repeated intervals for a sufficient period. The production of a series of current pulses and the speed of electrode rotation decides the weld spots.
The whole process is usually made automatic by an electronic controller. This type of welding provides pressure-tight or leak-proof joints. Seam welding is employed for welding pipes, conduits, tanks, transformers, refrigerators, aircraft, and various types of containers.
Butt Welding :
In butt welding, no electrodes are used to join the metal pieces, instead, the workpieces to be joined are connected to the supply. There are two types of butt welding,- Upset butt welding.
- Flash butt welding.
Upset Butt Welding :
In this welding two metal pieces are joined end to end. The two pieces to be welded are gripped firmly one in each clamp and are connected across the secondary of the transformer as shown below.
When current is made to flow through the metal pieces, due to the resistance of metal pieces at their contacts, the faces are heated to fusion temperature. Then after a force is applied from both sides of the metal pieces and remains under pressure to complete the weld.
The voltage required is 2 to 8V and currents vary from 50A to several hundred amperes depending upon the material and the area to be welded. This type of welding is used principally on non-ferrous metals for welding bars, rods, wires, tubing, etc.
Flash Butt Welding :
The flash butt welding makes use of the resistance of metal pieces and the arc formed between them to make the joint. The faces of the two metal pieces where the joint is to be made are placed very nearer to each other as shown below.
When a high current is passed through the metal pieces, a small arc is formed between them which burns away some portion of the material and the temperature goes on increasing until the final welding temperature is reached. Now the supply is switched OFF and the two metal pieces are brought in contact very rapidly by applying high pressure.
Once the contact is made, molten metal is expelled thereby making a solid weld. The metal expelled forms a flash around the joint which is removed later on by cutting or grinding. The advantages of flash butt welding are,