Application of TRUMPF laser welding technology to inflatable cabinets

A cabinet-type gas insulated switchgear enclosure holds pressurized SF6 or a dry-air mix. The shell is the pressure boundary, so every joint has to hold a leak rate spec, not just a mechanical load. Bolted flanges with gaskets work, but each gasket is a potential leak path and a service item that ages. A continuous welded seam removes that path.

The walls are thin. Most inflatable cabinet shells run 1.5 mm to 3 mm in 304 or 316L stainless, sometimes with a 4 mm to 6 mm reinforcement ring at the flange. Heat input has to stay low or the panel distorts and the flange face will not seat. This is the constraint that shapes every process choice downstream.

The enclosure also carries internal busbars, insulators, and a bursting disc. Weld spatter inside the tank is a dielectric risk, and grinding dust is worse. A process that welds without spatter and needs little post-weld cleanup has a real advantage here.

TRUMPF solid-state laser sources operate at 1030 nm to 1080 nm, which couples well into stainless steel. For a 2 mm 304 wall, a typical bead-on-plate weld runs 1.5 kW to 3 kW at 3 m/min to 6 m/min, with a spot of 0.3 mm to 0.6 mm. Those are starting points, not fixed recipes. Welding speed is set by penetration depth and by how much distortion the fixture can hold.

Keyhole mode is used where full penetration is needed, such as the corner joints of the tank body. Conduction mode is used on visible outer seams and on thin return flanges, where a smooth cap bead matters more than depth. Filler wire is usually not needed on 1.5 mm to 2 mm sheet, but 1.4301 and 1.4404 grades can crack in the crater without a small amount of 316L filler or a ramp-down profile.

The joint design is the part most often wrong on first drawings. A square butt with zero gap works up to about 2 mm. Above that, a small V or a stepped lap gives the beam somewhere to go. A lap joint on 1.5 mm sheet is forgiving on gap, which is why many cabinet corner details are laps rather than butts.

Thick flanges and cast bosses are the first case. A 10 mm 316L flange welded to a 2 mm wall needs either a hybrid process or a machined step so the beam sees a consistent thickness. The laser does not care that the wall is thin; it will punch through if the parameter set is tuned for the flange.

Deep, closed geometries are the second case. A beam needs line of sight. If a stiffener sits inside a 200 mm deep box behind a lip, no fiber delivery head reaches it. Those joints go to TIG, or the design is changed to move the stiffener outside.

The third case is cost. A laser cell earns its keep on repeat production with a stable fixture. For a one-off prototype shell, TIG plus a machined flange is usually cheaper and faster, even with the extra cleanup.

Welding does not produce a finished cabinet. The flange faces, the busbar penetrations, the insulator mounting holes, and the seam where the cover seats all need metal removed to a tolerance. A welded shell that has been distorted by heat is a poor starting point for a tight bore, so the sequence matters.

The usual order is: form the sheet, weld the shell, stress-relieve if the grade allows, then machine the flange face and the penetration bores. Flange flatness on a C-GIS tank is commonly held to 0.05 mm to 0.1 mm across the sealing face. Bore position for busbar penetrations is often ±0.05 mm, and the bore itself may be held to ±0.005 mm on diameter when it takes an O-ring or a ceramic insulator.

GreatLight machines these welded shells on 3-axis and 4-axis mills and on mill-turn centers, with a Ø400 mm rotary table for the flange work. Wall thickness on a welded tank is not uniform, so we touch off on the actual surface rather than trusting the drawing. A 4,000 mm maximum processing size covers most cabinet lengths. If the shell arrives already welded, we check flatness before the first cut.

304 and 316L are the default grades. 316L resists pitting better in coastal or humid switchgear rooms, and it welds with less sensitization because of the low carbon. 304 is cheaper and fine for indoor, dry conditions.

Aluminium shells appear on some dry-air designs. ADC12 die castings are used for end plates and small covers rather than the main tank, because cast aluminium porosity creates leak paths that a laser seam cannot close. If a casting is welded, the porosity has to be mapped first.

After welding and machining, the inside of the tank is usually cleaned and passivated. Outside surfaces get bead blasting, brushing, or powder coating. Laser marking for nameplates and terminal IDs is done after finishing, with a minimum character height of 1.5 mm so it stays legible after coating.