What is Resistance welding?

Resistance welding (RW) is the joining of two metals using pressure and electrical current, for a set length of time, through the area of the metal to be joined. The key advantage of resistance welding is no other materials such as shielding gasses, fluxes or filler rod are required in the union of the metals.

There are four main process variables that are used to perform resistance welding

  • Current (amperage) that flows through the materials to be welded

  • A set length of time the current flows through the work pieces

  • Pressure that the electrodes deliver to the work pieces to be joined

  • The cross-section area of the electrode tip surface in contact with the work pieces


Let’s look at the different types of resistance welding, there are six main resistance welding processes:

  1. Resistance Spot Welding (RSW)

  2. Resistance Projection Welding (RPW)

  3. Resistance Seam Welding (RSEW)

  4. Flash Welding (FW)

  5. Upset Welding (UW)

  6. Percussion Welding (PW)


Resistance Spot Welding (RSW)

Resistance spot welding is performed using heat generated by the resistance to the flow of welding current through the faying surfaces, plus the force of the work pieces being pushed together, over a defined set time. The equipment can be simple or complex, a typical set up is shown in Figure 1.

Image of resistance spot welder


Figure 1, Image of typical resistance spot welder

The typical resistance spot welder uses a set of electrodes on the end of arms that are used to press the materials together to be welded and the current is then charged through the workpiece at a set time and the workpieces are fused together from the inside out, as shown in the macro section illustrated in Figure 2.

 an image of  a cross section of a spot weld

Figure 2, Cross section of a typical resistance spot weld


Resistance Projection Welding (RPW)

Resistance projection welding is another variation of resistance welding, here the current flow is introduced at one or more points of contact at selected projections on either one or both parts being welded. These projections are used to focus the heat at the point of contact. This process typically uses lower currents, lower forces, and shorter cycle times than does the similar resistance spot welding application. Figure 3 shows typical projection joint configurations.

 Diagram, of projection configurations

Figure 3, Types of projection configurations


Resistance Seam Welding (RSEW)

Resistance seam welding is used to convey a continuous or non-continuous weld in a butt seam, lap seam or mash seam joint configuration. The electrodes are in the form of an upper and lower wheel that squeeze the workpieces together while turning and feeding along the workpieces. The process uses force and a continuous flow of current while the workpieces are moving. Figure 4 shows the typical lap seam welding.

 Diagram of a Lap Seam Weld

Figure 4, Lap Seam Weld


Flash Welding (FW)

Flash welding uses the heat generated by the resistance to the flow of the welding current along with the forces when the workpieces are pushed together causing a flashing action. This action is produced using very high current intensity at very small contact points between the workpieces. At a fixed point after the flashing process has begun a force is applied to the workpiece, and they are moved together at a precise rate and, thus,  bonded together. Figure 5 shows the flash welding process.

 Diagram of the flash welding process

Figure 5, Flash welding process


Upset Welding (UW)

Upset welding appears like Flash Welding, except that in the upset welding process the workpieces are already in firm contact with one another, so no flashing occurs. Pressure is applied before the current is started and is maintained until the process is complete. When the workpieces are heated to the welding temperature the material softens and deformation occurs when the upsetting force is applied. Figure 6 shows the upset welding process.

 Diagram of the Upset Welding process

Figure 6, Upset welding process


Percussion Welding (PEW)

Percussion welding uses an arc produced by a rapid electrical discharge across a declining air gap and pressure is applied “percussively” during or instantly after the discharge of electrical energy. This process is like flash and upset welding, similar cross-section areas are used.

Resistance welding is widely used in automotive industries, projection welding in the production of nuts and bolts, seam welding is used to produce leak proof joints required in small tanks, boilers etc. and flash welding can be used for the welding pipes and tubes.

All told, the six forms of resistance welding can be found in the following industries:

  • Electrical

  • Automotive

  • Construction

  • Office furniture

  • Architectural and structural metal manufacturing

  • Mining and agriculture

  • Aerospace

  • Shipbuilding

There are others, but the above are the major users for this welding process.

Bill Eccles, CSA W178.2 Level 2, IWS

Vice President,  PPC & Associates

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How-It Works content is submitted by Industry experts to the CWB Association and does not necessarily reflect the views of the CWB Group. When testing for CWB Certification or CWB Education, please refer to CWB Education textbooks or CSA standards as the official source of information.