What is Resistance Welding
In ancient times, metal welding was done in the form of forge resistance welding (metals heated up to melting point are pressed together) and brazing (weld using alloy of low melting point).
FUNDAMENTALS OF RESISTANCE WELDING
Resistance welding processes are pressure welding processes in which heavy current is passed for short time through the area of interface of metals to be joined with the application of pressure. In other way, it may be defined as “Resistance welding is a thermo-electric process in which heat is generated at the interface of the parts to be joined by passing an electrical current through the parts for precisely controlled time and under a pressure”.
The various types of resistance welding are as follows :
(a) Spot welding
(b) Seam welding
(c) Projection welding
(d) Resistance Butt welding
(e) Flash Butt welding
(f Percussion welding.
Key advantages of the resistance welding process include :
>Very, short process time.
>No consumables such as brazing materials, solder, or welding rods.
>Operator safety because of low voltage.
>Clean and environmentally friendly.
>A reliable clectro-mechanical joint is formed.
Features of Resistance
- No flux such as solder is necessary. So, welded parts can’ be easily recycled.
- Easy operation as only pressing buttons facilitates process automation and it does not require trained skills unlike arc welding and gas welding.
- As this welding is performed efficicntly in a short period of time, it is suited for a high-volume production of low-cost products.
- Since welding is done in short time duration, it gives less heat-affected workpicces by resulting a beautiful appearance with less indentation.
- Electric welding facility is required in some cases due to ust of large current: Optimum parameters must be figured out before parameters depend on actual welding since those condition setting material and thickness of parts to be welded. Welding must be prepared.
- Visual inspection is difficult because the welded portion cannot be cheeked from outside.
Principle of Resistance Welding
The name “resistance” welding derives from the fact that the resistance of the workpieces and electrodes are combination or generated by the contrast to generate the heat at their interface. Heat is generated in localized area which is enough to heat the metal to sufficient temperature so that the parts can be joined with the applocation of pressure. In resistance welding, the parts to be joined are heated to plastic state by their resistance to the flow of electric current and mechanical pressure is applied to complete the weld.
The mechanical pressure is applied to complete the weld. The heat developed by. the current is proportional to the electric resistance of the weld.
AC with a suitable transformer is used for the power supply. Usually, 4V to 12V is used dependent on the composition, area and thickness of the metal to be welded. The power supply ranges from 6 to 18kW, per cm’ area used.
Resistance welding processes differ from other welding processes in the aspect that no fluxes are used and filler metal rarely used. All resistance welding operations are automatic. Therefore, all process variables are preset. and: maintained constant. Resistance: equipment utilizes programmers for controlling current, time cycles, pressure and movement.
- The amount of current that passes through the work
- The pressure that the electrodes transfer to the work
- The time the current flows through the work.
The important characteristics of the resistance welding process is the transfer of heat to two parts being welded for obtaining a proper fusion even if the plates are dissimitar from the stand point of material or thickness.
If the proper heat balance is existed only, the proper fusion can be obtained by providing an electrode with a smaller contact area at the thinner sheet and a thicker electrode at the thicker sheet together with very high current densities for short times. If two dissimilar metals with different electrical conductivities or thermal conductivities are to be joined,
- Large contact area electrode should be used for the one which has higher electrical conductivity, and
- Small contact area electrode should be used for the one which has higher thermal conductivity.
Welding current:
The welding current is the important parameter in resistance welding which determines the heat generation by a power of square as shown in the above equation. The size of the weld nugget increases rapidly with increasing welding current. At the same time, too high current will result expulsions and electrode deteriorations.
Welding machine characteristics:
The electrical and mechanical characteristics of the welding machine have a significant influence on resistance welding processes. The electrical characteristics include the dynamic reaction time of welding current and the magnetic / inductive losses due to the size of the welding window and the amount of magnetic materials in the throat. The up-slope time of a welding machine can be very critical in micro resistance welding as the total welding time is often extremely short. The mechanical characteristics include the speed and acceleration of the electrode follow-up as well as the stiffness of the loading frame/arms. If the follow-up of the electrode is too slow, expulsion may easily occur in projection welding.
Weldability in Resistance Welding
- Resistivity
- Thermal conductivity
- Melting temperature
Metals with a high resistance to current flow and with a low thermal conductivity and a relatively low melting temperatures would be easily weldable.
Functions of Electrode in Resistance Welding
- Electrodes keep the parts aligned and in place.
- They are used to apply the required pressure to develop the correct surface resistance at the interface for containing the molten metal to avoid weld expulsion and to forge the nugget near the end of the cycle.
- They convey the electric welding current to the electrodes.
- They also dissipate excess heat to avoid the surface melting.
Advantages, Limitations and Applications
- Less skill is required to operate the resistance welding machine.
- This type of welding is well suited for mass production as it gives a high production rate.
- There is no need of using consumables such as brazing materials, solder or welding rods in this process except for the electrical power and a relatively smaller electrode wear.
- Heating the workpiece is confined to a very small part which results less distortion.
- It is possible to weld dissimilar metals as well as metal plates of different thickness.
- It has a short process time.
- It offers more safety to operator because of low voltage.
- It produces clean and environmentally friendly weld.
- A reliable electro-mechanical joint is formed.
- The heat is localized, action is rapid and no filler metal is required.
- The operation can be easily mechanized and automated due to the need of less skill.
- Both reliability and reproducibility can be obtained with high degree.
- The welding process IS more economical.
SPOT WELDING
It is one type of electrical resistance welding processes. Spot welding is used for making lap joints. By using this method, the metal sheets ranging from 0.025 mm to 1.25 mm thickness can be easily welded. The metal pieces are assembled and placed between two copper electrodes and then current is passed. The parts are heated at their area of contact by electrical resistance as shown in image.
The electrodes must possess high electrical and thermal conductivity and they retain the strength at high temperature.
