Fabricating a vacuum chamber is not the same as fabricating a pressure vessel. A pressure vessel is judged on strength; a vacuum chamber is judged on the absence of a path for gas, and the size of the path that matters is measured in microns.
What makes a weld vacuum-tight
Full penetration with no undercut, no crater at the stop, and no micro-porosity. Porosity is the usual culprit: a gas pore that does not reach the surface is harmless, but a chain of pores that links the two faces of the weld is a leak path. It is caused by contamination — oil, oxide, moisture or a wrong shielding gas — which is why cleanliness before welding matters more than technique during it.
Welding environment
Welds on vacuum components are made in a controlled environment with the joint cleaned immediately beforehand. On stainless, the heat tint that forms on the outside of a weld is an oxide layer, and on the inside it is a contamination source that has to be removed mechanically or chemically before the chamber can be considered clean.
Distortion and why it is a vacuum problem
Welding a large chamber pulls it out of shape. That is a dimensional problem first and a vacuum problem second: a flange that is no longer flat will not seal, which is why sealing faces are machined after welding rather than before. Distortion also closes up gaps in ways that make the weld harder to complete, so the fit-up matters as much as the welding.
Verification
Every welded chamber is helium leak tested by mass spectrometer before it leaves, with the method and detection limit recorded per chamber. Visual inspection of a weld tells you almost nothing about whether it will hold UHV — a joint can look perfect and leak at 10⁻⁶ mbar·l/s.
Materials and thickness
304 and 316L stainless from thin instrument cans to heavy-wall chamber shells, and aluminium for large lightweight chambers. Thinner material needs more care with heat input because distortion scales badly as wall thickness falls, which is where a controlled welding procedure rather than operator judgement earns its keep.
What we control in-house
- 304 / 316L stainless steel and 6061 / 5083 aluminium fabrication
- Clean-environment TIG welding with full penetration and no micro-porosity
- Helium mass-spectrometer leak testing to 1 × 10⁻⁹ mbar·l/s
- In-house electropolishing and passivation — not subcontracted
- Chamber bake-out to 150–400 °C for UHV outgassing control
- Precision-machined ISO-KF, ISO-K and CF (ConFlat) flanges
- Zeiss CMM verification to ±0.02 mm across large work envelopes
- 12 m gantry CNC machining — 12 m × 4 m × 2 m envelope
- SEMI-referenced documentation and inspection protocols
Frequently asked questions
How do you weld a vacuum chamber so it does not leak?
Full-penetration welds, clean joints, correct shielding gas, and no crater or stop defects. The single biggest cause of leaking welds is contamination before welding rather than poor technique during it, so joint preparation and cleaning on vacuum work get more attention than they would on general fabrication.
What is the difference between vacuum chamber welding and pressure vessel welding?
A pressure vessel is designed against strength and is tested at pressure; a vacuum chamber is designed against leak rate and is tested with a mass spectrometer. A weld that comfortably passes a hydrostatic test can still leak at a rate that prevents UHV.
Can you weld thin stainless without distortion?
Yes, with controlled heat input and a sequenced weld pattern. Thin-section work is where welding procedure control matters most, because distortion grows quickly as the wall gets thinner and a warped flange will not seal.
How do you check a vacuum weld?
By helium mass-spectrometer leak testing, not visually. The test is run on the finished chamber with the method and detection limit recorded, and the result is supplied as a per-chamber record.
Do you machine the chamber after welding?
Yes. Sealing faces and mounting surfaces are machined after welding, once the distortion has occurred, so they end up flat and parallel. Machining before welding would put the distortion directly onto the sealing face.
Related capabilities
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