In this article we will discuss about:- 1. Definition of Resistance Welding 2. Machines for Resistance Welding 3. Power Supply 4. Electronic Control.
Definition of Resistance Welding:
By definition, resistance welding is that process in which a sufficiently strong electric current is sent through the two metal pieces in contact to be welded which melts the metals by the resistance they offer to the flow of the electric current.
Resistance welding includes butt welding, spot welding, projection welding, seam welding and percussion welding. All are alike in the principle of resistance heating but differ in the details of application.
In resistance welding a heavy current (above 100 A) at a low voltage is passed directly through the work-piece and heat developed by the resistance to the flow of current, given by the expression 12R/ (where I is the current in amperes, R is the resistance in ohms and t is the time or duration of flow of current in seconds) is utilised. The heat developed at the contact area between the pieces to be welded reduces the metal to a plastic state; the pieces are then pressed together to complete the weld. In this process, preferably two copper electrodes are incorporated in a circuit of low resistance and the metals to be welded are pressed between the electrodes.
The electrical voltage required ranges from 4 to 12 volts depending on the composition, area, thickness etc. of the metal pieces to be welded. The amount of power supplied to the weld usually ranges from about 60 watts to 180 watts for each sq. mm of area. Alternating current is found to be most suitable for resistance welding as it can provide any desired combination of current and voltage by means of a suitable transformer.
In order to avoid the surface distortion, the portion of the metal adjacent to the weld or joint should not be allowed to be overheated.
Resistance to the flow of current is made of:
(i) Resistance of the current path in the work
(ii) Resistance between the contact surfaces of the parts being welded and
(iii) Resistance between the electrodes and the surface of the parts being welded.
In order to develop higher temperature between the interfaces of the work to be welded rather than at the surface of the work in contact with the electrodes it is necessary to keep the resistance between the electrodes and the surface of the body being welded to minimum.
In order to obtain a good weld it is necessary to maintain the contact resistance uniform which depends upon the surface condition.
For welding thin materials the resistance of the current path in the work is kept minimum. For welding thick materials of low conductivity the resistances of the current path have a comparatively greater value and the control of contact resistance is not necessary. For welding thick materials of high conductivity either reduced pressure or high resistance electrodes having melting point higher than that of metal to be welded, can be used. For welding two dissimilar metals having different conductivity, low conductivity electrodes on high conductivity metal side and vice versa are used in order to prevent overheating on the low conductivity metal and to develop sufficient heat to melt high conductivity metal side.
The pressure which is to be applied on the weld is also an important factor. At high pressure, low temperature plastic welds can be obtained and where as if the pressure is lowered the resistance to the welding current is to be increased. There is a limit up to which the resistance can be increased and after that there will be surface burning. The pressure necessary to effect the weld varies from 2.5-5.5 kgf/mm2.
The magnitude of current is controlled by varying either the primary voltage of the welding transformer (by using auto- transformer between supply and the welding transformer) or changing the primary turns of the welding transformer. Alternative method of controlling the current to weld is to vary the magnitude and wave of the primary as well as secondary current by using Thyratron or Ignitron tubes in the primary circuit.
In resistance welding, the time for which current flows is very important. Usually automatic arrangements are devised which switch off the supply after a predetermined time from applying of pressure (starting of weld). The pressure may be applied manually, by air pressure, by springs or by hydraulic means. After switching off the supply, the pressure is maintained on the electrodes until the weld cools. In machines which are operated continuously, the electrodes are cooled by water circulating through hollow electrodes.
Electrical circuit diagram for resistance welding is shown in Fig. 6.1. The machine employed for resistance welding contains a transformer provided with necessary taps, a clamping device for holding the metal pieces, and a mechanical means for forcing the pieces, to be welded, together to complete the weld.
Resistance welding has the advantage of producing a large volume of work at high speeds that are reproducible with high quality. Resistance welds are made very quickly; however, each process has its own time cycle. Resistance welding operations are automatic. Good-quality welds do not depend on welding operator skill but more on proper set-up and adjustment of the equipment and adherence to weld schedules.
Resistance welding is employed mainly for mass production. It is easily adapted to those components which can be moved to the machine and are light. The operation is extremely rapid and simple. This is the only process where heat can be controlled and which permits a pressure action at the weld. Metals of medium and high resistance, such as steel, stainless steel, monel metal and silicon bronze are easy to weld. Special control gear is required, however, in the case of high- carbon steels and special equipment providing very high current impulses (stored energy welding) is used in case of materials of low electrical resistance.
The automotive industry is the major user followed by the appliance industry. It is used by many industries manufacturing a variety of products made of thinner-gauge metals and for manufacturing pipes, tubing, and smaller structural sections.
When specifying the material intended to be resistance welded, consideration must be given to the state in which this to be supplied to the welding shop. Whilst slight rust, mill scale etc., on the material may not affect the efficiency of arc welds to a considerable degree, lack of cleanliness will be fatal to resistance welded joints. Pickling or shot-blasting immediately prior to the resistance welding operation is essential for making this latter method a success.
Material up to 5 mm thickness which is to be used on resistance-welded jobs is usually purchased in a pickled and slightly oiled condition, and should be carefully stored in order to keep it clean. It can then be used without removing the oil film provided that the oil is clean. Material above 5 mm thickness should be shot-blasted before being taken to the resistance welding machines.
No long delay should occur between the shot blasting and the welding, in order to avoid new corrosion which might eliminate the advantage gained by the former. Sandblasting is not recommended, as particles of the siliceous material may be embedded in the steel surface and influence its electrical resistance.
High frequency resistance welding is done with 400 to 450 kHz current usually supplied by an oscillator. The high frequency current readily breaks through oxide film barriers and produces a thin heat-affected zone because it travels on the surface of the material.
However, resistance welding has got some limitations and drawbacks also, as enumerated below:
(i) The initial cost of equipment required is high.
(ii) Skilled persons are required for the maintenance of equipment and its controls.
(iii) In some materials, special surface preparation is required.
(iv) Certain resistance welding processes are limited to lap joints. A lap joint has an inherent device between the two metal pieces, which cause stress concentration in applications where fatigue is present. The device may also cause trouble when corrosion is present.
Machines for Resistance Welding:
The machine for resistance welding incorporates a transformer, suitable electrodes for supplying current to the weld and arrangement for controlling the mechanical pressure, and finally, means for controlling the duration of weld current flow. The mechanical pressure may be exerted through levers and clutch by an electric motor or by compressed air. The magnitude of pressure required depends upon the type of work and may vary from a few kg for thin sheets or wires up to a tonne or more for heavy work.
In the older type of welding machines, the electrodes were brought on to the work and the electrical circuit closed by the operation of a pedal. Thus application of pressure and the time duration of current flow used to be controlled by the operator and for this operator needs to be experienced and skilled. The modern practice is to pass heavy currents for shorter time durations (ranging from 10 ms to 100 ms). The equipment used for this purpose may be constant time, current-actuated, or energy-actuated types.
Constant time equipment is employed in high speed production where the work has a consistently clean surface. Constant time equipment may be provided with mechanical control or electrical control. In mechanical control providing up to 300 welds per minute, the device employed is a cam- operated switch, connected in the primary circuit of a welding transformer, driven from the welding machine.
For a large number of welds per minute the mechanical arrangement becomes unsuitable because it is not capable of providing consistently accurate timing, due to wear of the cam and operating mechanism, arcing and burning of the contacts and irregularities caused by closing of switch at different instants in the cycle.
An alternative arrangement is to control the timing through grid controlled ignitrons or thyratrons. It is easier to build tubes for a high voltage and small current than with a low voltage and high current. An arrangement using valves in the secondary circuit of a series transformer is shown in Fig. 6.18. When the tubes conduct, the series transformer secondary is almost short- circuited, and whole of the supply voltage is available across the welding transformer primary.
But when the tubes are not conducting, the series transformer primary winding offers high impedance in the circuit of the welding transformer and the current is reduced to a negligible value. Auxiliary valves are employed for controlling the timing of the negative potential applied to the grids of the main tubes.
Constant-time method of control does not yield consistently good results when there may be variations in the conditions under which successive welds are made, due to variations in supply voltage or mechanical pressure, wearing of electrodes, surface irregularities etc. The energy-actuated control, wherein definite amount of energy is supplied to the weld, is used.
Constant-time method of control did not prove to be successful, particularly with modern high-speed welding. The energy- actuated control, which permits the current to flow until a predetermined amount of energy has been supplied to the weld, is theoretically an ideal method. However, the control equipment is quite complicated.
Power Supply for Resistance Welding:
AC supply is used for resistance welding because of the ease and convenience with which the required high current at a low voltage can be obtained by means of a transformer. The kVA required for resistance welding, when actually making a weld, ranges from a few kVA to as much as 1 MVA. The power factor will be about 0.25 or 0.3 lagging. The power factor is low mainly due to the high ratio of reactance to resistance of the loop formed by the jaws of the welding machine. Such heavy intermittent single-phase loads may cause serious voltage drop difficulties in the supply network.
Such problems can be overcome to some extent by connecting capacitors of suitable capacity in parallel with the welding transformer so as to improve the power factor. But with this arrangement the power factor will become leading when welding current is not being drawn. This problem can be avoided by connecting the capacitors in series with the welding transformer to neutralize the reactance drop in the supply circuit.
Electronic Control in Resistance Welding:
As there are several factors, i.e., current, pressure, heat, time, to be considered, manual control does not yield good results in case of resistance welding. For the precise control of these factors electronic control welding circuits are used.
Some of the electronic control circuits are given here:
i. Ignitron Contactor:
We will now discuss the theory of ignitron contractor use as a contractor for controlling heavy currents.
A simple line contactor using two ignitrons is shown in Fig. 6.19. If the switch S is closed at the instant the line 1 is positive, current will rush through the primary of welding transformer, rectifier a, switch S, rectifier b, the ignitron I2 and back to the line 2. The current will strike an arc in ignitron I2 and the tube starts conducting. Now voltage drops across I2 to a low value, causing a voltage drop in the ignitron circuit. Therefore, the ignitron will conduct just long enough to strike an arc. Similarly, during next half cycle, the line 2 will be positive and the current will flow from line 2 through rectifier c, switch S, rectifier d to ignitron I1.
During this half cycle, as the anode of ignitron I2 becomes – ve, it stops conducting. Metallic rectifiers are used in this circuit. They conduct the current in proper direction, thus, preventing the application of negative voltages to the electrodes and saving the ignitrons from inverse current damages. A solenoid is used to apply suitable pressure through the upper movable electrode. Manual control of the contactor switch is possible only if the welds are of long duration. But for the accurate time control of the welds of short duration, thyratrons are used to ignite the ignitrons, as shown in Fig. 6.20. The grid circuits of the thyratrons are controlled by a suitable timing control circuit.
Since very heavy current flows through the ignitrons, say 1,000 amperes, and arc drop be taken constant at 10 volts, therefore, losses to the tune of 10 kW will take place. So the ignitrons are always water cooled. In case of very heavy load, temperature of water becomes too high, normally closed thermostat contacts open and the ignitrons stop conducting.
ii. Heat Control Unit:
It is an electronic circuit which helps the delay in the firing of the ignitrons by a definite, predetermined angle in each cycle and operates in conjunction with the line contactor. A typical circuit employed for the heat control is shown in Fig. 6.21. This is essentially a phase shift control circuit and delays the firing of the ignitrons, thus reducing the magnitude of welding current as per requirements.
iii. AC Timer Circuit:
When the condenser C is discharged through a resistor R the voltage across the condenser falls exponentially as given by the expression-
From the above expression it is clear that greater the capacity of condenser and value of resistor, greater will be the time required for voltage to fall by given amount. In all timer circuits provision is, therefore, made for charging a condenser to a particular value of voltage and then discharge by short- circuiting switch till condenser is discharged to a particular value when relay will operate and particular contact will open or close.
Typical ac timer circuit is shown in Fig. 6.22. Such a timer circuit is used to control the number of cycles for which power may be supplied into the weld. The action of such a timer circuit is explained as- when switch S is open and supply terminal 1 is positive w.r.t. terminal 2, the cathode and anode of thyratron are at the same potential, and grid is – ve w.r.t. cathode, and, therefore no current flow between cathode and grid. When terminal 1 is negative w.r.t. terminal 2, the potential at a is positive, grid becomes positive w.r.t. cathode and anode and electronic current flows through R2, R1 from grid to cathode, through R3 and to terminal 1.
In a few cycles the capacitor C2 will get charged to the maximum voltage between a and 1. This is due to the large value of time constant. The capacitor does not discharge much during the negative half cycle of the grid voltage. Resistance R, limits the grid-cathode circuit current to a safe value and also determines the number of cycles in which C2 will be fully charged. So long switch S remains open, capacitor C2 remains charged by the grid rectification action.
As soon as switch S is closed, the grid becomes very much negative w.r.t. cathode and there is no capacitor charging current through grid rectification. Capacitor will, therefore, start discharging through R2 and -ve bias of the grid will gradually decrease depending upon the time constant R2C2 of the discharge circuit. Conduction in thyratron tube will start during positive half cycle of anode voltage when the grid voltage instantaneously rises to critical grid voltage. Current through relay coil is rectified half waves. As such to avoid relay terminals chatter, a capacitor C1 is connected across relay coil.
iv. Energy Storage Welding Processes:
To meet the demand of heavy current of very high conductivity metals such as aluminium and magnesium energy storage welding circuits are used. There are basically two such circuits namely electrostatically stored energy circuits and electromagnetically stored energy circuits.
1. Capacitor Discharge Welding Circuit:
As shown in Fig. 6.23, condenser C (capacitor bank of capacity of 2,000 to 3,000 µF) is charged to about 3,000 volts from grid controlled rectifier. When the condenser is connected to the primary of welding transformer by ignitron contactor, it will discharge and thus high transient current will be produced in the secondary to weld the material.
The noteworthy points in connection with this circuit are:
(i) As the voltage of condenser approaches the voltage of the source of supply, charging rate becomes lower, therefore to charge condenser to about 3,000 V at high charging rate voltage of about 5,000 V to 6,000 V will be required. A voltage regulating circuit cuts off the rectifier from the bank when the voltage of the bank becomes 3,000 V.
(ii) If there is residual magnetism near saturation, it will result in low rate of change of flux linkages in the secondary and, therefore, in production of low heat. Hence in the welding transformer core flux should not be present.
2. Magnetic Energy Storage Welding Circuit:
In this type of welding, energy stored in magnetic circuit is used in the welding operation. The dc voltage of the rectifier is suitably controlled so that the current in the primary of the transformer rises gradually without inducing large current in the secondary. This is necessary to avoid preheating of metals at the weld joint. Preheating in aluminium, magnesium etc. is undesirable as it causes deformation.
When sufficient energy has been stored up in the transformer core, the contactor opens, dc flow ceases and there is a rapid collapse of magnetic field. The decay of flux induces heavy currents in the secondary of the transformer for welding.
The kVA demand on the line in magnetic energy storage welding is higher as compared to that in capacitor discharge welding but a high voltage rectifier and costly capacitor bank are not required.