It is simplest form of resistance welding. In this process, a pair of water-cooled copper electrodes is used. Two overlapping metal plates are held between these electrodes’ jaws as shown in Figure 20.42.
Figure 20.42 Resistance Spot Welding
The pressure is applied to a very small area, which is known as spot. The resistance at the inner face is very high so applying low voltage and high current melts the inner surface and after solidification make a spot joint. The current used may be 3,000–40,000 A; this depends on melting point of the material to be welded. The voltage applied may be 20–90V. The diameter of spot welds (d).
Spacing of spot weld should be three times of diameter of spot weld. The complete weld cycle is divided into four parts.
- Squeeze time
- Weld time
- Hold time
- Off time
Squeeze Time: During this time, workpieces are under pressure and the electrodes are in contact with them. The squeeze time is used to bring two pieces together in contact just before current flow. The pressure gradually increases from zero to a certain value during this time interval.
Weld Time: During this period the current is switched on and the temperature at the interface starts rising and attains welding temperature to melt the metal at the interface. The pressure should then be increased considerably just as the proper welding heat is attained. Pressure is the most important variable in resistance welding. Pressure is inversely proportional to resistance. Pressure is high if resistance is low otherwise pressure is low if resistance is high.
Hold Time: During this time weld starts cooling and pressure is further increased.
Off Time: During this period pressure is released and workpiece is removed for other spot.
Advantages
- Similar and dissimilar metals can be welded very easily.
- The time involved in spot welding is very less.
- Sheets of different thickness can be joined easily.
- It can be used for large production run with the help of multiple spot welding machines.
Limitations
- Silver and copper are especially difficult to weld because of their high thermal conductivity.
- Spot welding is limited to overlap welding only.
Leave a Reply