About Resistance Welding

What is resistance welding?

Resistance welding is a type of welding which takes place between two metals under a certain load and heat without any demand of additional materials. The heat required during resistance welding process is generated from the electrical resistance of the materials. The advantage of Resistance welding is its high operation speed, high volume, low cost and the elimination of filler material in every welds.

There are five types of welding processes in resistance welding:
Spot Welding, Projection Welding, Seam Welding, Flash Butt / Butt Welding and Resistance heating or upsetting.

Among five of the resistance welding process variants, Flash Butt or Butt Welding requires no overlapping of the joining metals. Heating is similar to a fusion process where current flow through the part without force applied.

There are two main types of power conversion on resistance welders. AC to AC conversion and AC to DC conversion.

AC to AC power conversion

  • Single Phase AC machines

AC to DC power conversion

  • Capacitor Discharge DC
  • Three Phase Rectified DC
  • Three Phase DC Inverter (MFDC)

Capacitor Discharge DC is not applicable in Seam Welding and Flash Butt / Butt Welding processes due to its intrinsic nature of instantaneity on current flow time.

Schematic Diagram Spot Welding Projection Welding Seam Welding Flash Butt /
Butt Welding
Heating /
Single Phase AC




Capacitor Discharge DC

Three Phase Rectified DC




Three Phase Inverter DC





Welding Processes

Spot Welding is mainly used in welding of sheet metal . The equipment for Spot Welding can be in the form of stationary or bench type welding machines, portable spot or robotic welding guns, and multi head spot welding machine designed for different applications. The recommended maximum weldable per sheet thickness for resistance spot welding is 5mm. Weldable material includes mild steel, stainless steel, galvanized steel, aluminum alloy, copper alloy, titanium alloy, high tensile steel, carbon steel etc. Generally speaking, spot welding welds one point at a time while Projection Welding welds multiple points at a time at the embossed points on the sheet metal.

Projection welding is characterized by joining the predetermined weld point or so called projections or embossments on the weld part with the parent material. Projection welding current and weld force are localized on the weld points unlike spot welding where the weld is determined by the size or shape of electrode. Projections for projection welding can be produced by embossing on sheet metal, machining or cold forging on a solid piece and press punch on the edge of a piece. Projection welding has wider applications than spot welding; from welding nuts and bolts to sheet components to as big as welding bearing bars and cross bars to form a grating panel.

Unlike Spot and Projection welding which is static welding, Seam welding is welding through motion. One or two wheels rotate to make a series of overlapping spot welds on sheet components to achieve air or liquid proof on the welded part. There are Roll Spot and Seam in seam welding. Roll spot are an interchange between a spot or a series of spot welds and a period of cool time but without opening the electrode during the welding process. Roll spot is well depicted in the brake shoe seam welding process where the embossments on the rim are roll spot welded. Seam in seam welding separates into continuous seam and pulse seam. Continuous seam is overlap of spot welds without cool time while pulse seam is overlap of spot welds with cool time but so short that the spots are still connected together. Both continuous seam and pulse seam sometimes can run on same machine. The selection between continuous seam and pulse seam can be decided based on the sheet component material and the weld appearance. The seam welding machine also has Internal and External cooling on the electrode wheels. Selection between Internal and External cooling on seam welding vary among sheet component material, weld strength to be achieved and application of the weld part.

Flash Butt Weld / Butt Weld is a resistance welding that does not require overlap of weld material. It is commonly used to join same or different cross section in tube, bar, rod, sheet or plates. Butt weld is also called Upset weld. Heat for welding is the resistance to electric current created through contacts between the moveable part and the fixed part. As the weld part touches, the material starts to flash and sparks (metal loses) from the weld area. The flashing process heat up the cross section of the weld part, then comes the butt weld (Upset) process. Upset process is to accelerate the movable part towards the fixed part with higher current. Rapid speed and continuous pressure will weld two pieces together.

Resistance Heating machine is to generate heat on metal part by utilizing resistance welding electrical system. Upsetting is to apply force to the resistance generated heat zone to forge the metal part to shape. Refer to Heating & Fusing Machine section to find out more.

Resistance Welding Power Conversions

Single Phase AC is the most common type of power conversion. Electric current levels return to zero every half cycle creating a “no current flow” period and inducing an intermittent heating reaction. AC welding machines with constant current Controllers supply primary voltage compensation and primary/secondary constant current feedback control which automatically adjusts power fluctuations.

Capacitor Discharging DC Power uses tens or hundreds of capacitors to store energy in its own electric field and returns energy to the circuit whenever required. The stored energy is rapidly discharged to the transformer producing a flow of electrical current for welding. Capacitor Discharge solution is the best suited welding application for hot rolled sheet or thick steel sheet with multiple embossments.

Three Phase Rectifier-DC Power is a Three Phase Power before Three Phase DC Inverter becomes common. The Three Phase Rectifier DC power converts three-phase balanced AC input into direct current for welding. This method saves 40% of energy in comparison to a Single Phase AC solution.

Three Phase DC Inverter (MFDC) utilizes three-phase balanced input, current wave is fully rectified to DC and switched to 1000Hz to produce AC supply to primary transformers. The current is then rectified into DC for welding operation.

Three Phase DC Inverter power has following advantages:

  • 1.No "blank period" in the current wave form. The Three Phase Rectifier DC power converts three-phase balanced AC input into direct current for welding. welding quality and controls welding splash.
  • 2.Lower Power consumption in welding. Frequency converted from 50/60Hz to 1000 Hz. Higher frequency gives high current density. Higher frequency easily generates the required welding heat in the shortest time interval, thus welding is completed in a faster approach with lower power consumption. High frequency switching technology reduces 60-70% of power consumption compared to Single Phase AC solutions.
  • 3.High-frequency transformers make wide-range welding possible. AC transformers may induce insufficient current dispensation leading to poor welding quality.