Types of Welding - Resistance Welding

Resistance welding is a fusion process which takes place between two metals under a certain load and heat without any demand of additional materials. The heat required during welding process is generated from the electrical resistance of the materials.

There are four types of welding methods in resistance welding: Spot Welding, Projection Welding, Seam Welding, & Flash Butt or Butt Welding.

Flash Butt or Butt Welding requires no overlapping of the joining metals among them. There are four types of Output Current solutions. Capacitor Discharge DC is not applicable in Seam Welding and Flash Butt / Butt Welding methods due to its intrinsic nature of instantaneity on current flow time.
 
Resistance Welding can be divided into four welding methods and four output current solutions.
 
Spot Welding / Projection Welding / Seam Welding / Flash Butt Welding Spot Welding is mainly used in sheet metal welding. Spot Welding can be in form of stationary type spot welder, bench type spot welder, portable spot welding gun, robotic spot welding gun, multi head spot welding machine designed for special applications. . The maximum weld-able thickness is 4mm under resistance spot welding. Weld-able material includes mild steel; stainless steel; galvanized steel, aluminum alloy, copper alloy, titanium alloy, high tensile steel, carbon steel etc. A stationary Spot welding can only perform a spot weld one at a time, to perform multiple spots, embossing on the sheet metal would be a convenient way.
   
Spot Welding / Projection Welding / Seam Welding / Flash Butt Welding Projection Welding is typically for welding thick to thin sheet. That is, for heat balance, the thick sheet are embossed to have projections for welding. Through projections, the current flow path is limited, hence current density is higher, and welding is possible with smaller current. Further, in spot welding, electrode cap requires dressing after 3000 spot welds, but in case of Projection welding, the electrode is flat and current required in projection welding is lower, the electrode life is high. Projection sizes changes as the sheet thickness changes, however, as the sheet becomes thicker, it may become difficult to make embossments. In this case, it is also possible to make embossments on the thinner sheet. Projections on sheet metals enable to weld multiple projections at a same time. It is widely used when the part is to be welded on an automated welding machine. Projections on sheet metal is one type of the projection welding application, but not all. There are projections on nuts, bolts, sheet edge, cross wire, wire to sheet, T joint or even overlap of material etc. Applications such as Grating panels, Cap Nuts, Brake Shoes, Compressor Shell body, Oil Filters, Gas Spring end fittings etc. Projection welding is a very useful resistance welding method for joining sheet metal.
   
Spot Welding / Projection Welding / Seam Welding / Flash Butt Welding Seam Welding is continues overlap of spot welds nuggets to form a welded line. Through rotation of two seam weld wheel discs to allow current flowing through and form overlapped weld nuggets. The seam weld housing is important in performing a good weld, the design of the seam weld housing affects how current can passes through the brush and the shaft to the weld wheel discs. Dahching have different types of seam weld housing and among those, it is separated into passive type housing and active type housing. Active type housing is driven by motor to give motion to the seam weld wheel while passive type housing does not have motion applied to it. A common seam welder use active type housing and passive type housing together, to drive one seam weld wheel disc while the other seam weld wheel disc is being driven. There are 4 active type housing; knurl driven housing, worm gear driven housing, differential gear driven and chain driven housing. Dahching will recommend the suitable housing base on the parts provided. During seam welding process, the seam weld wheel disc can be cooled by either external cooling or internal cooling. In external cooling process, Water are sprinkled onto the seam weld wheel disc and the weld part for cooling during welding while internal cooling process avoids water sprinkled onto the welded part. Internal cooling process ensures a clean working environment and avoid rust on the weld part.
   
Spot Welding / Projection Welding / Seam Welding / Flash Butt Welding Flash Butt Weld / Butt Weld is the only weld process under resistance welding that does not require overlap of weld material. It is commonly used to join same cross section in tube, bar, rod, sheet or plates. Butt weld is also so-called Upset weld. To heat up the cross section of the weld part then apply pressure.

Flash Butt Weld is widely used for welding larger cross section such as bar diameter over 12mm or thick sheet metal with wide width. During Flash Butt Weld process, voltage is applied to the weld part while the movable part advances towards 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.
 

Electrical Output Methods used in Welding

Single Phase AC is the most common type of output solution, 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.
 
DC Current Capacitor Discharging. The capacitor stores 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 Output. Widely applied before the emergence of Three Phase DC Inverters, this method uses a three-phase balanced AC input operating at 50 or 60Hz and converts it to DC. This method saves 40% of energy in comparison to a Single Phase AC solution.
 
Three Phase Inverter. 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 Inverter DC solution has the following advantages:

1.No “blank period” in the current wave form. Heat produced is continuous and efficient. The fast feedback control (roughly 0.1 milliseconds) ensures stable welding quality and controls welding splash.

2.High frequency means high current density, which easily generates the required welding heat in the shortest time interval, thus welding is completed in a faster approach with minimum power consumption.

3.High-frequency switching technology reduces 60-70% of power consumption compared to Single Phase AC solutions.

4.High-frequency transformers make wide-range welding possible. Traditional AC transformers may induce insufficient current dispensation leading to poor welding quality.