**2.1 Theory**

Resistance spot welding is a fusion welding process that works on the principle of Joule's law of heating, which states that: *Q = I2 Rt*, where '*Q*' is the amount of heat generated during RSW, '*I*' denotes the welding current used, '*R*' is the resistance setup at the interface of the metal sheets, and '*t*' is the welding time employed. RSW technique uses two truncated cone/dome-shaped copper alloy electrodes to concentrate the welding current into a fixed small spot and to simultaneously clamp the sheets together without any misalignment. Thin sheets of metals used as workpieces are held together under pressure exerted by the electrodes. The thickness of the metal sheets generally varies between 0.5 and 3 mm. The enforcement of a large amount of current through the spot will melt the metal and form a weld. RSW allows a large amount of energy to be distributed to a specific location in a short period (approximately 10–100 milliseconds). This allows the welding to take place without overheating the rest of the metal sheet.

The resistance between the electrodes and between the electrodes and metal sheets, as well as the amplitude and duration of the welding current, control the amount of heat energy transferred to the produced spot. The amount of energy is chosen to match the sheets' material properties like thermal conductivities, coefficient of thermal expansion, electrical conductivity, etc. Applying too little energy will not melt the localized region and sufficient strength will not be developed. Whereas, applying too much energy will melt too much metal, eject molten material and make a void rather than a spot [1]. **Figure 1** depicts a schematic of a resistance spot welding process.

#### **2.2 Technique and equipment used**

There are generally three stages in the resistance welding process which are stated as follows: (a) the electrodes are being brought to the surface of the metal sheets being welded and a slight amount of pressure is applied and (b) the welding current from the electrodes is then applied for a very short time after which the current is removed but the electrodes maintain the pressure to allow the weld metal to cool and solidify. The applied weld times normally range from 0.01 to 0.8 s depending on the thickness of the metal, the electrode force, and the electrode tip diameter [2, 3].

The resistance spot welding setup mainly consists of tool holders and copper alloy electrodes. The tool holders act as a mechanism to hold the electrodes firmly in place

**Figure 1.** *Schematic of resistance spot welding.*

### *Resistance Spot Welding: Principles and Its Applications DOI: http://dx.doi.org/10.5772/intechopen.103174*

and also to support the cooling water hoses that are used for cooling the electrodes. Tool holding techniques generally include paddle-type light-duty, universal and regular offset. The electrodes are made up of low resistant highly conductive metals like copper and are manufactured in numerous designs such as truncated cone, dome, flat shapes depending on the application needed.

The metal sheets to be welded together are known as workpieces and should be a good conductor of electricity. The width of the metal sheets is limited by the throat length of the welding equipment and ranges typically from 5 to 50 inches (13–130 cm). The thickness can vary between 0.20 and 3 mm.

In the case of RSW, there are two critical components of the tooling system whose characteristics have a significant impact on the entire process: the gun and its kind, as well as the size and form of the electrode. The C-type gun is commonly employed in applications where the gun layout must be as stiff as possible due to large applied forces. This design provides great stiffness and tooling flexibility, as well as collinear electrode motion. The X-type arrangement, like the C type, provides minimal stiffness, even though the reachable workspace is significantly greater than the C-type. As a result, this architecture is highly frequent where tin and flat objects are processed. However, low flexibility is provided in terms of tooling, because the paths of the moving electrodes are not collinear, hence a dome-shaped electrode tip should be used.

Electrodes that are used in spot welding also vary in terms of their uses. For high heat applications, radial type electrodes are used, truncated tip electrodes are used for high pressure, eccentric electrodes for welding corners, offset eccentric tips for reaching into corners and narrow places, and lastly offset truncated into the workpiece itself.

#### **2.3 Features of resistance spot welding**

Resistance spot welding tends to work harden the material during the application of electrode force causing it to warp. This phenomenon leads to the reduction of the materials fatigue strength and may stretch the material as well as anneal it. The various defects of spot welding include internal cracking, liquation cracking at the interior of the weld nugget, and a bad appearance. The chemical properties affected include the metal's internal resistance and its corrosive properties.

The welding times used are very short, which can cause electrode wear- they cannot move fast enough to keep the material clamped. During the first pulse, the electrode contact may not be able to make a good weld. The first pulse will soften the metal. During the interval between the two pulses, the electrodes will come closer and make better contact. Also, a higher welding current creates a huge magnetic field, and when the electric current and magnetic field intersect, a large magnetic force field is produced, which causes the melted metal to move very quickly, up to 0.5 m/s. As a result, the fast motion of the melted metal could substantially alter the heat energy distribution in spot welding. A high-speed camera can be used to observe the rapid motion of spot welding [4–6].

### **2.4 Power supply**

The basic spot welding setup consists of a power supply, an energy storage unit (e.g., a capacitor bank), a switch, a welding transformer, and the welding electrodes. The capacitor bank acts as a supplier of high instantaneous power levels. The accumulated energy is dumped into the welding transformer when the switch is pressed. This transformer then

reduces the voltage while increasing the current. The transformer's main feature is that it reduces the amount of electricity that the switch can tolerate. The transformer's secondary circuit includes the welding electrode. A control box is also present, which controls the switch and may also monitor the welding electrode voltage or current.

A large number of resistances are being set up in different regions, thus making the resistance offered quite intricate. Secondary winding, cables, and welding electrodes all have their resistances. The contact resistance between the welding electrodes and the workpiece is also a factor. There's also the resistance of the workpieces and the resistance of the workpieces' contact. Because contact resistances are typically high at first, the majority of the energy is wasted there. The heat created by the clamping force softens and smoothens the material at the electrode-material interface, resulting in better contact and lower contact resistance. As a result, more electrical energy will be transferred into the workpiece, and the junction resistance between the two workpieces will increase. The electrodes and the workpiece conduct the heat away as electrical energy is provided to the weld and the temperature rises. The most important need is to provide enough energy to melt a piece of the material within the spot without melting the entire spot. The perimeter of the spot will channel considerable heat away and keep the perimeter at a lower temperature due to thermal conductivity. Because less heat is transferred away from the inside of the spot, it is the first to melt. When a significant welding current is used, the entire spot melts, the material pours out, and a hole instead of a weld is formed.

The working voltage needed for welding is dependent on the resistance of the material to be welded, the sheet thickness, and the desired size of the nugget. When welding a 2 mm lapped joint, the voltage between the electrodes is only about 1.5 V at the start of the weld but can fall as low as 1 V at the end of the weld. This voltage reduction is due to the reduction in resistance owing to the workpiece melting. The open-circuit voltage from the transformer is higher than this which ranges from 5 to 22 V typically.
