Resistance Welding and Resistance Spot Welding

In this tutorial, we will get a clear insight about Resistance Welding (RW) and Resistance Spot Welding (RSW). We will start with the definition, advantages and the power source used in resistance welding. Moreover, we will describe about the process, materials used as electrodes and applications of resistance spot welding.

Contents:

  1. What is Resistance Welding?
  2. Advantages of Resistance Welding
  3. Power Source Used in Resistance Welding
  4. What is Resistance Spot Welding?
  5. Electrode Material for Resistance Spot Welding
  6. Steps Involved in Resistance Spot Welding Cycle
  7. Factors Affecting Resistance Spot Welding
  8. Applications of Resistance Spot Welding

What is Resistance Welding?

Resistance welding is a thermo-electric welding / joining process of two metallic materials that produces coalescence of the surfaces in contact using a combination of heat and pressure. An extremely high value of current is passed through the parts for a precisely controlled time as well as under a controlled pressure to carry out the welding process.

Advantages of Resistance Welding

Resistance welding has a lot of advantages over arc welding. The following are some of the advantages.

  • Wide range of materials can be employed.
  • Noticeably short process time is required. Moreover, distortion produced is minimum hence the parent properties of the metal does not get affected during this process.
  • No consumables or electrodes are required in this process.
  • This method is simple compared to arc welding and does not involve high expert labor.
  • Large scale and high rate of production is possible.
  • It is a clean and environment friendly process. Moreover, the process does not involve any shielding gas.

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Power Source Used in Resistance Welding

Power source plays an important role in the resistance welding process. Alternating current (AC) is typically used in the process. This is because any desired combination of current and voltage can be obtained by means of a suitable transformer. The following facts must be noted regarding the power source used in resistance welding.

  • The current used in resistance welding operations is extremely high (5000–20,000 A, typically). Voltage used in the process is relatively low (usually below 10 V).
  • The heat energy supplied to the welding operation depends on current flow(I), resistance of the circuit(R), and duration(t) for which the current is applied. The heat produced can be given by the formula:
    H=I2Rt
  • The reason why such a high current is used in resistance welding is because the squared term in heating equation amplifies the effect of current. Hence a large amount of heat is produced. Further the resistance of the metal piece is quite low (around 0.0001 ohm)
  • The resistance in the welding circuit is the sum of electrode resistance, workpiece resistance, contact resistance between electrode and workpiece and contact resistance of the contact surface.
  • Metals with extremely low resistivity such as copper, are commonly used to minimize electrode resistance. In addition, the electrodes are usually cooled with water to dissipate the heat generated there.
  • The resistance of the workpiece is a function of the resistivity of the base metal and the thickness of the workpiece.
  • The contact resistance between the electrode and the workpiece depends on the contact area (i.e., the size and shape of the electrode) and the condition of the surface (such as the cleanliness of the working surface)
  • Finally, the resistance of the contact surface depends on the surface finish, cleanliness, contact area and pressure. One must take care that there should be no paint, oil, dirt, or other contaminants that separate the contact surfaces.

What is Resistance Spot Welding?

Resistance Spot Welding (RSW) is a resistance process in which the fusion of the mating surfaces of the lap joint is done in one location by opposing electrodes. The size and shape of weld spots are determined by the tip of the electrode.

The following diagram shows the assembly of resistance spot welding process.

resistance spot welding process
  • The assembly comprises of welding workpieces (usually sheet metal parts), two opposing electrodes, a device that applies pressure to squeeze the parts between the electrodes, and an AC power source from which the current can be controlled.
  • The most common electrode shape is round. Hexagonal, square, and other shapes are also used.
  • The resulting weld nugget is, usually 5 to 10 mm (0.2-0.4 inch) in diameter, and the heat-affected zone extends from beyond the weld nugget to the base metal.
  • If the welding is carried out properly its strength will be comparable to the surrounding metal.

Electrode Material for Resistance Spot Welding

Electrode materials used in resistance spot welding are mainly of two types.

  • Copper-based alloys: These are the commonly used electrodes which are comprised of alloys that have copper as their principal component. They have excellent properties like high thermal conductivity leading to uniform heating. High strength, ductility and excellent corrosion resistance are other features of these alloys.
  • Refractory metal compositions: These electrodes are comprised of copper along with composites. They are famous for superior wear resistance and have high hardness at room temperature.

Steps Involved in Resistance Spot Welding Cycle

The resistance spot welding cycle involves 5 major steps which is repeated in a sequential manner to obtain the acquired welding.

The following diagram illustrates the 5 steps of resistance spot welding cycle.

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5 steps of resistance spot welding cycle

The 5 stages are described below:

  • Stage 1: Off time:
    • The electrodes are far away from the workpiece.
    • This region is also called no pressure zone.
  • Stage 2: Squeeze time:
    • The pressure is increased gradually during this stage.
    • The electrodes block the workpiece and provide a constant force (F) to ensure good fixation.
    • No current is passed through the electrodes in this stage of the welding cycle.
  • Stage 3: Weld time:
    • While still under pressure, AC current (I) passes through the system.
    • This leads to the formation of molten metal as shown in the diagram above.
  • Stage 4: Hold time:
    • During this stage, the molten metal is allowed to solidify under mechanical pressure. leading to the formation of weld nugget.
    • The current is switched off and the pressure is gradually reduced.
  • Stage 5: Off time:
    • The electrodes will no longer block the part and the welding process is completed.
    • This cycle is repeated if required.

Factors Affecting Resistance Spot Welding

The significant factor that affects resistance spot welding is weldability. It basically refers to its ability to be welded.

  • Weldability primarily depends on three factors of the metal being welded.
    • Electrical resistivity
    • Thermal conductivity
    • Melting point of the metal
  • Weldability(W)= \(\frac{Electrical\, resistivity × 100}{Thermal\, conductivity × melting \,point}\)
    Here, electrical resistivity is taken in micro-ohm/cm and relative thermal conductivity is taken with respect to copper.
  • If W denotes the weldability, then the following facts can be noted:
    • “W” more than 2 represents excellent weldability.
    • “W” more than 0.75 and less than 2 denotes good weldability.
    • “W” less than 0.75 is moderate weldability while less than 0.25 is poor weldability.
    • “W” for mild steel is above 10 and for aluminum it lies between 1 and 2.

Applications of Resistance Spot Welding

Resistance welding has numerous applications at industry level. A few of them are enumerated below.

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  • It is widely used in automobile and aircraft industry.
  • It is used in mass production of weld sheet metal, wire, and tubes.
  • They are commonly used by industrial robots to weld different parts of vehicles.
  • A car body may have as much as 10,000 individual spot welds. This shows the economic impact of resistance spot welding worldwide.
  • It is also used to manufacture cutting tools, nuts, and bolts.

Key Points to Remember

Here is the list of key points we need to remember about “Resistance Welding”.

  • Resistance welding (RW) is a group of fusion-welding processes which employs a combination of heat and pressure to accomplish coalescence.
  • Resistance welding has many advantages compared to arc welding technique.
  • The resistance welding is characterized by high current and low voltage.
  • This is important since a large amount of heat must be produced to melt the metal sheets as per Joules law of heating.
  • Resistance spot welding is comparatively a newer welding process that involves the formation of a nugget to weld materials.
  • Copper alloy or refractory metal can be used as electrodes in the process based on different needs.
  • The resistance welding cycle consists of 5 major steps namely Off time, Squeeze Time, Weld time and Hold time which are performed sequentially to obtain the required welding.
  • Weldability (W) plays a significant role in the success and efficiency of resistance spot welding.
  • Automobile industries and industrial robots rely largely on resistance spot welding for manufacturing various parts.

If you find any mistake above, kindly email to [email protected]

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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