In describing this group of processes, the word electric is usually omitted and simply the term resistance welding is used. In these processes, the coalescence is achieved by passing electric current through the metallic parts by inserting them into a circuit. The electric resistance produces heat at the junction of the parts. Finally, pressure is applied.
Resistance Welding Processes Features
Resistance welding processes have been placed between fusion and solid phase processes, because it is difficult to decide whether they are fusion processes or solid state processes as evident from the following examples:
- Resistance butt welding is a solid state process in which melting of the joint is totally avoided by controlling the parameters.
- Spot, seam and projection welding are also solid state processes, except for a small molten nugget which is formed in the weld. The size of the nugget is kept as small as possible.
- In flash, percussion and high frequency welding, the surfaces to be joined do get fused, but the fusion zone is controlled to an extremely narrow zone, almost to 0.005 mm. If the fusion is not restricted, molten metal will fly off under pressure and the weld will not take place.
Resistance Butt Welding
This process is also known as upset welding or simply butt welding. Here, the temperature of the joint is raised by the resistance to the passage of an electric current across the interface of the joint.
The parts to be joined (usually wires and rods) are held in clamps, one stationary and the other movable, which act as conductors for the low-voltage electric supply and also apply force.
This force or pressure is applied only after the abutting surfaces have reached a temperature slightly below the melting point, which results in the upsetting of the metal.
Uniform and accurately mating surfaces are desirable to exclude air and give uniform heating.
The process is commonly used during rod rolling and wire drawing operations to join the ends.
Flash Butt Welding
This process is an extension of resistance butt welding. The parts to be joined are gripped in the clamps, and their interfaces are gradually brought into contact to complete the secondary circuit.
When the welding voltage of up to about 10 is applied at the clamps, current flows through the initial points of contact causing them to melt. These molten bridges are then ruptured and small short-lived arcs are formed.
The platen on which the movable clamp is mounted is moving forward while this takes place, and fresh contacts are then made elsewhere so that the cycle of events can be repeated.
This intermittent process, during which much of the metal contained in the molten bridges is expelled violently in a spectacular manner, is called flashing.
Flashing is allowed to continue until the surfaces to be joined are uniformly heated or molten.
By this time the moving platen will have advanced, at an increasing rate, to close the gap as metal is expelled, the total distance up to the point of upset being known as the flashing allowance.
At this point the rate of movement of the platen is rapidly increased and a high force applied to forge the parts together and expel the molten metal on the surfaces.
Typical applications of this process are: rails, steel strips, window frames and automobile rear axle casings. Special purpose machines are designed for each of these applications.
Spot Welding
in this process, a spot of weld is made between overlapping sheets by means of two cylindrical copper-alloy electrodes, one on top and the other at the bottom, which carry a high current.
The electrodes also clamp the work and apply pressure when the metal at the joint gets sufficiently heated by electrical resistance.
A tiny button of fused metal results at the sheet interface which is called the nugget.
The electrodes are retracted after the weld is complete. Spot welding is always performed with a machine in which all the parameters can be controlled.
The process is used on a large scale in automotive production, and in sheet-metal fabrication.
Stitch Welding
This is an extension of spot welding, in which a series of overlapping spot welds are made in the same manner as stitching cloth.
Stitch welding may be performed with a normal spot welding machine or with a specially designed one, which automatically makes spot welds in a continuous series.
Multiple Spot Welding
This is a modification of spot welding, in which two or more welds can be obtained from each transformer secondary.
Series Spot Welding
In series welding, a portion of the secondary current bypasses (shunts) any weld nugget being formed.
This shunt current passes through one of the panels being welded. Generally, two welds are made per transformer secondary.
Roller Spot Welding
In this process, a series of intermittent spot welds are made using wheels or rollers as electrodes.
The rollers are power driven and are stopped while individual welds are made Current is passed intermittently when the electrodes are stationary.
Seam Welding
Seam welding is similar to spot welding, except that the copper-alloy electrodes are in the form of circular rollers.
The overlapping sheets are held under constant pressure between the roller electrodes, which rotate at constant speed and carry current.
A series of spot welds, whose nuggets overlap each other, are formed which give the appearance of a continuous weld seam. A common application of seam welding is in the manufacture of steel drums.
Butt Seam Welding
This is similar to seam welding, except that the sheets to be joined are in the same plane instead of being overlapped.
The joint edges are in intimate contact and the roller electrodes travel directly over the seam.
Foil Butt-Seam Welding
This is a modification of seam welding, in which thin narrow strips of metal are introduced between one or both of the circular electrodes and the workpiece.
The joint edges are held in the same plane instead of being overlapped. The strips help to localize the melting and to avoid reduction of section thickness at the joint.
The process is being used successfully on the shells of rail coaches.
Projection Welding
This is a modified method of making single or multiple spot welds. Projection welds are made by providing an embossment or projection on one or both of the contacting base metal surfaces to localize the pressure and current flow at a particular point.
This reduces the amount of current and pressure required to produce a spot weld between two pieces of metal. Other advantages would include:
- Maintaining proper heat balance between two workpieces of markedly different mass,
- Better control of size and shape of fused area,
- Making of multiple welds simultaneously between a single set of electrodes or contact platens.
- Forming of welds from one side ofthe assembly by series welding two separated projections, provided adequate pressure can be applied.
Percussion Welding
Percussion (or percussive) welding is unique and of considerable interest, because butt welds can be made with incredible speed, in almost any combination of dissimilar materials, and without the expulsion of a fin or flash around the joint.
The pieces to be joined are held a short distance apart in clamping dies which carry current and apply pressure.
The ends to be welded are prepared for accurate mating. An extremely heavy electric current is delivered (electrical energy from discharging condensers or capacitors) to the pieces as a very short impulse, perhaps for only 0.001 second, and flows across the gap between the pieces as an arc.
The heat of this high-energy arc produces superficial melting over the entire end surfaces of the bars.
An instant after the arc has struck, the pieces are brought together with an impact blow (hence the term percussion) to complete the weld.
High Frequency Resistance Welding
This process is mainly used in the upset butt welding of tubes as a continuous process.
The tube formed out of a continuous strip passes under the electrodes, which transfer the high frequency welding current (alternating currents ranging from 200 to 4,50,000 cycles) into the material being welded and through a set of rolls, which provides the welding force.
The amount of upset is regulated by the relative position of the welding electrodes and the rolls applying the upset force.
The required welding heat is governed by the current passing through the work and speed at which the tube goes through the roll.
