Spot Resistance Welding

Spot welding is a process where overlapping surfaces are joined at one or more points through heat generated at those spots. This heat is produced by the flow of electrical current established between electrodes and passing through the parts. Meanwhile, the electrodes apply specific pressure to bring the surfaces closer together. (Resistance welding)

Figure 1-1: General Diagram of Spot Resistance Welding

Figure 1-1: General Diagram of Spot Resistance Welding

As mentioned, metals heat up due to the passage of electrical current, and according to Joule’s law, the generated heat is determined by Equation (1-1):

Equation 1-1: Q = KRI²t
Where:

$I$ is the current,
$R$ is the resistance,
$t$ is the time, and
$Q$ is the heat.

The factors of current intensity and time are controllable via the resistance welding machine. However, electrical resistance depends on factors such as material type and thickness, workpiece properties, pressure between electrodes, the size, shape, and material of the electrodes, and the smoothness and cleanliness of the surface.

In Figure 1-1, different resistances along the current path are depicted. In Joule’s formula (Equation 1-1), $R$ is typically the sum of all resistances in the system:
Equation 1-2: R = Σrj T

$r_1$ and $r_5$ represent the contact resistances between electrodes and the workpiece, which are unwanted resistances causing heat loss and potential adhesion of electrodes to the surface. Factors such as electrode material, pressure application, and surface cleanliness reduce these resistances. Additionally, cooling the electrodes effectively can reduce $r_1$ and $r_5$ since increasing a material’s temperature raises its electrical resistance.
$r_2$ and $r_4$ represent the resistance of the two parts (sheets), which are generally negligible due to the low electrical resistance of metals compared to other resistances.
Equation 1-3:
$r_2, r_4 = S(S/L)$
Where $S$ depends on the sheet material, and $L$ is the sheet thickness.

Finally, $r_3$ is the interfacial resistance, which is the highest among these resistances because:

It is not directly cooled by the cooling system, leading to a temperature rise and increased resistance.
It is the resistance at the junction of two parts, making it higher than the contact resistance between electrodes and parts ( $r_1$ and $r_5$ ).

It is important to note that there are continuous resistances in the current flow path from the power source to the electrodes. These resistances cause heat and current loss, which must be considered when calculating the required current for welding. Strategies to reduce this heat generation or prevent overheating of the current transmission components will be discussed in detail in subsequent chapters.

Resistance welding machines consist of two main units:

Electrical (Thermal) Unit: To raise the temperature at the welding site.
Mechanical (Pressure) Unit: To apply the required pressure to join the two parts at the welding site.

The applied force can be manual, hydraulic, pneumatic, or hydro-pneumatic. Pressure is maintained until the molten area solidifies, ensuring that edges align, preventing air from entering the molten pool, and avoiding spillage. The effects of pressure and its impact on weld quality are discussed in the next chapter.

It is crucial to consider the resistances present in the current transmission path from the power source to the electrodes, as these can cause heat generation and current losses. These factors must be accounted for when determining the current required for welding. However, by implementing specific measures, it is possible to reduce heat generation or prevent overheating of the current-transmission components. These topics will be explored comprehensively in later chapters.

Resistance Welding Machines
Resistance welding machines consist of two main units:

Electrical (Thermal) Unit: This unit is responsible for increasing the temperature at the welding spot.
Mechanical (Pressure) Unit: This unit applies the necessary force to join the two pieces at the welding point.

The applied force can be generated manually, hydraulically, pneumatically, or through hydro-pneumatic systems. This force is maintained until the molten area solidifies, ensuring that the edges fit closely together, preventing air from entering the molten pool, and avoiding the spilling of molten material. The details of force application and its impact on weld quality will be discussed in the next chapter.

Electrodes in Resistance Welding
In resistance welding processes, electrodes come in various shapes and are designed to:

Conduct the electrical current to the welding joint.
Hold the sheets together with the required force.
Focus heat quickly at the welding joint.

Characteristics of Suitable Electrodes:

They must have adequate strength and hardness to withstand pressure without deforming.
They should have a high annealing temperature.
They need a suitable electrical conductivity coefficient.
They must possess high thermal conductivity.

Electrode Materials and Design
Electrodes are manufactured from special materials and alloys and are available in various shapes. The spot welding process achieves the highest thermal concentration (approximately 95%) among welding methods while minimizing energy loss. This process also minimizes the distortion of the workpiece caused by heat and the heat-affected zone (HAZ). Additionally, it has no position limitations, allowing for welding in various orientations with appropriately designed equipment.

Advantages of Resistance Spot Welding:

No need for consumables, shielding gas, or filler metals.
Environmentally friendly due to its clean operation.

Limitations of Resistance Spot Welding:

Welding is challenging for metals with high thermal and electrical conductivity, requiring special equipment and strategies.
Metals sensitive to rapid heating and cooling may require customized equipment and are limited to maximum thicknesses (typically 3-4 mm).
When welding dissimilar metals with different electrical resistances, the molten zone tends to grow toward the material with higher resistance, as it generates more heat. To counter this, smaller-diameter electrodes or electrodes with higher resistance can be used on the side with greater resistance. Care must also be taken to evaluate the chemical composition, mechanical properties, and microstructure of the alloy formed in the molten pool, ensuring it is free from brittle phases.

Applications of Spot Resistance Welding:
This method is widely used for joining sheet metal in automobile bodies, home appliances, construction products, and, to a lesser extent, in the aerospace industry. Interestingly, modern cars contain approximately 10,000 resistance spot welds, which are increasingly performed by advanced robotic and computer-controlled systems.

By automating the control of current, time, and electrode force, manufacturers can achieve high-quality welds, increase production rates, and reduce costs by employing less-skilled labor.

Process Variations in Spot Welding:
Depending on the specific requirements and working conditions, variations in the spot welding process may be implemented. While these changes do not alter the core principles of welding, familiarity with them is essential. Some examples include:

Multi-Electrode Resistance Welding:
To minimize the number of transformers, reduce equipment size, and lower costs, systems are developed to create two or more welds simultaneously using a single electrode setup. This approach varies based on energy supply methods and electrode configurations.

Energy Supply Methods in Multi-Electrode Welding:
Multi-electrode welding uses two main designs:
- Parallel Configuration: A single transformer is used, and its secondary circuit is connected to create multiple welds simultaneously, as shown in Figure 1-2.
- Series Configuration: Multiple transformers are used to generate several weld points simultaneously, as depicted in Figure 1-2.
The advantage of the series method is its ability to produce higher voltage at the weld location, or to use smaller transformers for a given voltage. However, to achieve consistent weld quality, the transformers, electrode resistances, and surface conditions must be precisely uniform.

Figure 1-2: Methods of energy supply in multi-electrode resistance welding.

شکل ۱-۲ – انواع روشهای تامین انرژی در فرآیند جوشکاری مقاومتی با الکترودهای چندتایی

Button or Disc Welding:
For welding thicker or heavier sheets, additional small metal discs are inserted between the sheets. These discs concentrate the electric current, reducing the contact area and localizing the heat. As the discs melt, the sheets are joined with lower electrical energy and pressure requirements.
Bridge Welding:
As shown in Figure 1-3, additional sheets are used to reinforce the joint between two parts, enhancing the connection strength.
Mash Welding:
This technique is widely used for manufacturing wire mesh, lamp guards, and attaching wires to sheets. Wires are positioned on jaws or flat electrodes with pre-designed grooves. As pressure is applied and current passes through the overlapping wires, localized melting occurs at the contact points, resulting in a strong bond upon solidification.

Figure 1-3: Strengthening a weld connection.

شکل ۱-۳ استحکام اتصال جوش

Stitch Resistance Welding:
In this process, one of the electrodes is designed to move up and down in a controlled, reciprocating motion, while the workpiece moves horizontally beneath it, much like fabric under a sewing machine. This creates a series of evenly spaced spot welds between the sheets, resembling stitched seams.

If the distance between the weld points is reduced to the extent that they overlap, a continuous seam-like connection can be achieved. However, this requires a higher-than-normal current because some of the current bypasses through the neighboring welds.

شکل ۱-۴: جوش له کردنی

Roller Spot Resistance Welding:
In this method, a series of individual spot welds is created along a line using one or two rotating electrodes. During this process, the welding force is maintained consistently between the spots.

Key Features of Roller Spot Welding:
- The principle is similar to standard spot welding.
- The radius of the electrodes determines the contact length.
- The spacing between welds depends on the speed of electrode movement and the current timing settings.
Compared to manual methods, this approach significantly reduces welding time. Due to the movement of the electrodes, the resulting welds form elongated weld nuggets.

Note: Some references classify this method as a form of seam welding due to its continuous nature.

Advantages of Resistance Welding:

High thermal concentration and low energy waste compared to other welding methods.
Minimal distortion of the workpiece due to reduced heat-affected zones (HAZ).
No restrictions on welding positions; welding can be performed in any orientation with the appropriate equipment.
The process does not require consumables, shielding gases, or filler materials, making it environmentally friendly.

Limitations of Resistance Welding:

Welding highly conductive metals is challenging, requiring special equipment and adjustments.
Metals sensitive to rapid heating and cooling may require custom setups, and there are limitations on maximum sheet thickness (3–4 mm).
Welding dissimilar metals with varying resistances poses challenges, as the molten zone tends to grow toward the higher-resistance material, leading to uneven heat distribution. Solutions include using smaller-diameter electrodes or higher-resistance electrodes on the side with greater resistance.

Applications of Resistance Welding:
Resistance welding is widely used in automotive, domestic appliance manufacturing, construction products, and, to a lesser extent, in the aerospace industry. Approximately 10,000 resistance spot welds are present in a typical car, performed using advanced robotic or computerized systems.

By automating the control of welding current, timing, and electrode force, manufacturers can achieve high product quality, enhance production rates, and reduce costs by employing less-skilled workers.

Multi-Electrode Resistance Welding

This method is used to minimize the number of transformers, reduce equipment size, and lower costs. Multiple welds are created simultaneously in this process.

Energy Supply Methods in Multi-Electrode Welding:

Parallel Configuration: A single transformer powers the system, and its secondary circuit enables simultaneous welding at multiple points (see Figure 1-2).
Series Configuration: Multiple transformers are used, providing higher voltage at the weld points or enabling the use of smaller transformers for a given voltage.

Advantages and Challenges:
The series configuration can achieve consistent weld quality, but uniform conditions must be maintained across transformers, electrode resistances, and surface quality to ensure similar weld properties.

Other Methods of Resistance Spot Welding

Button or Disc Welding:
Used for thicker sheets where small metal discs are placed between them. These discs localize the current flow, reducing the required energy and pressure.
Bridge Welding:
Additional material is added to increase joint strength, as shown in Figure 1-3.
Mash Welding:
Commonly used for wire mesh production and wire-to-sheet connections.

1- Contact resistance

2- Annealing Temperature

3- Distortion

4- Heat Attested zone

5- Position

6- Parallel Welding

7- Direct Weld

8- Over and Under Welding

9- Button or Disc Welding

10- Bridge Welding

11- Mash Welding

12- Stitch Welding