Capacitor discharge (CD) stud welding

Capacitor discharge (CD) stud welding allows to weld metal studs or other welding elements to thin sheet metal with a minimum thickness of 0,5mm. The welding process is executed by discharging a large amount of electrical energy, stored in a powerful bank of electrolytical capacitors, through a special ignition tip on the head of the stud. When the welding process is triggered, the energy stored in the capacitors is instantaneously discharged through the ignition tip, which burns away and ignites an arc between the stud and the base material; this electric arc completely melts the end of the stud and the workpiece’s surface, which are thus homogeneously joined. The entire weld cycle is completed in 1-3 milliseconds, the weld is uniform and has complete fusion across the stud flange, with a penetration of a few tenths of a millimeter. Due to very low thermal load, the welding zone is minimal.

These features enable studs to be welded on very thin and clean sheets without blackening, distortion or reverse-side marks, guaranteeing excellent aesthetic results and good strength of the joint.

Welding range: from M3 to M8 (up to M12 with special equipment and adaptors).

The process

There are two types of process: contact welding and gap welding.

In contact welding, when the process is triggered, the stud is pressed by a spring in the welding gun/head against the workpiece. Contact welding is recommended for high-grade steel and zinc-coated steel (<15µ).

In contrast to contact welding, in gap welding, when the process is triggered, the stud is first lifted to a preset distance and then pushed against the workpiece surface by a spring, previously attracted by a magnet inside the welding gun/head. Welding time is generally shorter than in the contact welding, therefore gap welding process is recommended for stainless steel and for materials with low melting point such as aluminium and brass.

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Drawn arc (DA) stud welding

Drawn arc (DA) stud welding is a welding process used to weld studs up to 25 mm in diameter (M24) to thick sheet metal (at least 2 mm), with a stud penetration into the base material from 1 to 3 mm: this process allows welds with a sheet thickness/stud diameter ratio up to 1:4. The welding cycle lasts up to 1000 ms with currents up to 3000 A, depending on stud diameter. Ceramic ferrules positioned around the end of the stud are used in order to protect and contain the weld pool; ferrules are removed breaking them with a hammer after weld pool solidification. However, for particular applications (e.g. for aluminium and stainless steel) and for stud diameters up to 8-10 mm, shielding gas can be used to replace the ceramic ferrule.

Drawn arc welding guarantees best-quality welded connections on very thick base materials and it is widely used in civil and industrial constructions, shipbuilding and heavy metal carpentry.

Welding range: from M3 to M24.

Short cycle (SC) stud welding

Short cycle (SC) drawn arc stud welding is a variant of drawn arc welding that allows studs up to M12 to be welded, with the same quality and strength of the drawn arc welding process but without deformation, even on thin sheets (thickness from 0,6 mm upwards): stud penetration is about 0,4 mm, therefore it is such as not to excessively deform the sheet. This stud welding process uses high currents and very short welding times (up to 100 ms), even though they are longer than capacitive discharge welding times.

Short cycle stud welding guarantees higher strength welded joints compared to capacitive discharge welding process and it is also suitable for welding studs on oxidized, dirty or galvanized sheet metal. Thanks to the high strength and reduced penetration features mentioned above, the short cycle drawn arc technology is widely used in the automotive industry, in particular for stud welding on car body shells. Up to 8 mm stud diameter, it is possible to weld studs without weld pool protection. For larger diameters, or in order to achieve higher welding quality (reduced pore formation) and better aesthetic results, use of inert shielding gas (generally a mixture of 82% Argon and 18% CO2) is recommended. It is supplied to the welding gun/head via a suitable shielding gas attachment. Short cycle stud welding on aluminium materials is possible only to a limited extent and requires the use of special gas mixtures.

Welding range: from M3 to M12.

The process

The stud is first placed in contact with the workpiece, when the welding process is triggered the stud is lifted to a preset height by a magnet and a low current pilot arc is ignited; this pilot arc is followed by a main current arc that completely melts the end of the stud and the workpiece surface; at the end of the welding cycle, the welding gun spring pushes the stud into the molten weld pool, the material solidifies and the stud is permanently welded.

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Rotating magnetic field stud welding (SRM®)

The rotating magnetic field stud welding process (SRM®) is a technology patented by the German company Heinz Soyer Bolzenschweißtechnik GmbH that allows to weld large-diameter steel and stainless steel studs (up to M16) even on thin sheets, without using ceramic ferrules for the protection of the weld pool. The SRM® technology uses a rotating magnetic field that allows to achieve the following benefits:

  • welds with a sheet thickness/stud diameter ratio up to 1:10 (instead of 1:4 as previously with drawn arc welding process);
  • small-sized and regular weld beads;
  • no blowholes, no weld spatter;
  • 70% less energy consumption, 60% reduction in welding time;
  • excellent aesthetic appearance due to stud penetration depth reduced by 60%.

This innovative technology makes it possible to weld also common stainless steel nuts from M8 to M12 onto punched and non-punched sheet metal, simplifying and considerably speeding up the welding of these elements.

Welding range: from M3 to M16.

The process

The stud is placed in contact with the base material and the current circuit is closed; when the welding process is triggered, the stud is raised from the workpiece and the electric arc is ignited; the arc is guided spirally through the magnetic field and melts the end of the stud and base metal over their entire surface; then the stud is immersed with low penetration in the liquid weld pool; after weld pool solidification, the stud is inseparably connected to the workpiece. The entire process takes place in a controlled inert gas atmosphere.

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