What is Resistance Welding?
Resistance welding is the joining of metals by applying pressure and passing current for a length of time through the metal area which is to be joined. The key advantage of resistance welding is that no other materials are needed to create the bond, which makes this process extremely cost-effective.
There are several different forms of resistance welding (e.g., spot and seam, projection, flash, and upset welding) which differ primarily by the types and shapes of weld electrodes that are used to apply the pressure and conduct the current.
The electrodes, typically manufactured from copper-based alloys due to superior conductive properties, are cooled by water flowing through cavities inside the electrode and the other conductive tooling of the resistance welding machine.
Resistance welding machines are designed and built for a wide range of automotive, aerospace, and industrial applications. Through automation, the action of these machines is highly controlled and repeatable allowing manufacturers to staff production readily.
How Does Resistance Welding Work?
Resistance welding is generally used to join two plain metal workpieces together. An electric current is delivered to the metal sheets (or any workpieces being joined) through weld electrodes which apply force to the sheets.
This force is then converted to heat. The heat is generated so that it melts the metal at the point where it joins the point of ‘resistance’ between the faying surfaces. The electrode then extracts heat from the molten weld area which forms a weld nugget at the point where it solidifies.
A force is applied before, during, and after a current is applied, which confines the contact area.
Types of Resistance Welding
1. Spot Welding and Seam Welding
Resistance Spot Welding, like all Resistance Welding Processes, creates welds using heat generated by resistance to the flow of welding current between the faying surfaces, as well as force to push the workpieces together, applied over a defined period of time.
Resistance Spot Welding uses the face geometries of the welding electrodes themselves to focus the welding current at the desired weld location, as well as to apply force to the workpieces. Once sufficient resistance is generated, the materials are set down and combined, and a weld nugget is formed.
Resistance Seam Welding is a subset of Resistance Spot Welding using wheel-shaped electrodes to deliver force and welding current to the parts. The difference is that the workpiece rolls between the wheel-shaped electrodes while weld current is applied.
Depending on the particular weld current and weld time settings, the welds created may be overlapping, forming a complete welded seam, or may simply be individual spot welds at defined intervals.
2. Projection Welding
Like other Resistance Welding Processes, Projection Welding uses heat generated by resistance to the flow of welding current, as well as force to push the workpieces together, applied over a defined period of time.
Projection Welding localizes the welds at predetermined points by using projections, embossment, or intersections, all of which focus heat generation at the point of contact. Once the weld current generates sufficient resistance at the point of contact, the projections collapse, forming the weld nugget.
Solid Projections are often used when welding fasteners to parts. Embossments are often used when joining sheet or plate material. An example of Projection Welding using material Intersections is cross-wire welding.
In this case, the intersection of the wires themselves localizes heat generation, and therefore resistance. The wires set down into one another, forming a weld nugget in the process.
3. Flash Welding
Like other Resistance Welding Processes, Flash Welding uses heat generated by resistance to the flow of welding current, as well as force to push the workpieces together, applied over a defined period of time. Flash Welding is a Resistance Welding Process that generates resistance using flashing action.
This action is created using very high current density at very small contact points between the workpieces. At a predetermined point, after the flashing process has begun, force is applied to the workpiece, and they are moved together at a controlled rate. Rapid upset created by this force expels oxides and impurities from the weld.
4. Upset Welding
Like other Resistance Welding Processes, Upset Welding uses heat generated by resistance to the flow of welding current, as well as force to push the workpieces together, applied over a defined period of time.
While similar to Flash Welding, in Upset Welding 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.
Applications Of Resistance Welding
- Resistance welding is used in mass production for welding sheet metal, wire and tubes.
- It is used in welding bars, boxes, cans, rods, pipes and frames metals of medium and high resistance materials such as steel, stainless steel and silicon bronze which are easy to weld.
- It is used in welding aircraft and automobile parts
- It is used for making cutting tools.
- It is used for making fuel tanks of cars, tractors etc.
- It is used for making wire fabrics, grids, grills, mesh weld, containers etc.
Advantages of Resistance Welding
- Similar and dissimilar metals are capable of being welded
- Highly automated
- Efficient with a high production rate and high welding rate
- Cost-effective
- Environment-friendly, produces little waste or pollution
- No need for filler metal or extraneous materials such as rods, fluxes, inert gasses, oxygen, or acetylene are required
Disadvantages of Resistance Welding
- Complex and often high-cost machinery a resistance welding machine generally requires a high level of technically trained personnel to use
- The thickness of the workpiece is often limited
- It is less efficient for high-conductive materials
- High electric power required