Sealing Options for Aluminum Vacuum Chambers

The goal of any sealing option for a vacuum chamber, whether aluminum or another material, is to provide a leak free (or acceptable leak rate) transition from the interior vacuum of the chamber to the exterior environment. For all types of vacuum chambers these options fall into two broad categories: permanent joints and demountable joints.

Permanent sealing options are achieved through welding, brazing or soldering. Demountable sealing options for high vacuum or ultra-high vacuum chambers employ design variations that use either elastomer or metal seals. Other methods of sealing such as epoxies or adhesives will not be discussed here.

Permanent Seals: Welding
Like proper welding of stainless steel for vacuum service, welding of aluminum to achieve vacuum leak tight joints requires stringent adherence to proper cleanliness, proper selection of joint design, and use of the correct weld technique protocols. However, most of the familiar techniques and experiences utilized in stainless steel welding are not transferable to aluminum welding.

Techniques Common to SSTL and Aluminum
Welds should be made on the vacuum side of the pressure boundary whenever possible. When not possible they should be full penetration welds.
The GTAW process should be used for all root (initial pass) or single pass welds. Single pass welds are preferred, as multiple pass welds can create trapped volumes.
If, due to strength requirements, a double weld is necessary, the inside weld should be the vacuum leak tight weld, the outside weld should be a “skip” weld (i.e. discontinuous) to avoid creating trapped volumes.

Internal structural welds should also be discontinuous to avoid the possibility of trapped volumes.

Techniques Common to Aluminum Only
Vacuum tight welds of aluminum without the use of filler metal are almost impossible to achieve and should be avoided in design.
Because of the properties of aluminum, the minimum sized aluminum weld you should expect to make is a 3/16″ fillet weld.

In designing aluminum weld joints, it is critical to match the mass of the mating parts. Aluminum conducts heat quickly; uneven cooling of mating parts causes stresses that induce cracks in welds.
Initial cleanliness of aluminum weld joints is critical. Aluminum forms a surface oxide that must be removed; the joint and adjacent area must be completely cleaned of contaminants.
Avoid machining of aluminum welds. Structurally sound and leak tight welds can develop leak paths from the removal of cover material.

Welding of the common 6000 series aluminum alters the temper and lowers the tensile strength of the weld metal and heat affected zone. This should be taken into consideration when designing chambers with these alloys. The 5000 series alloys, while also affected, have much higher tensile strengths even at zero temper. Thus, using the 5000 series alloys can result in chambers with thinner wall thickness for the pressure boundaries.

6000 series aluminum weldments might require heat treating to meet structural requirements. Heat-treating should always take place before final machining. Unacceptable distortion from the heat treatment to retemper the aluminum is a real risk.

Proper amperage, voltage, weld speed, selection of purge technique, pre-heat requirements, a.c. or d.c. polarity are all critical and dependent factors much less forgiving in welding of aluminum. Proper weld procedure specifications considering these and other factors, coupled with the practical experience of welders qualified to such Standards as the ASME Section IX, ensure a vacuum leak tight weld.

Demountable Seals: Elastomers
The most common demountable seal used on vacuum chambers is a flanged or door joint sealed with an elastomer gasket. Sometimes a flat gasket design, but most often (and a better choice) the gasket is an o-ring captured in a groove. There are a number of good sources of information from o-ring companies on the selection and details of o-rings and o-ring grooves for vacuum. These details apply equally well to stainless steel or aluminum chambers. We highly recommend the Parker O-ring Handbook. This resource will give you detailed information on proper o-ring material selection and the precise design details of the groove itself. This handbook is well organized and thorough; any attempt to summarize it here would be superfluous.

We have two cautions when utilizing one of these sources for design.
Select the maximum recommended squeeze (the compression of the o-ring seal). This will ensure good positive vacuum seal.
When utilizing a double dovetail o-ring groove, confirm that the selected o-ring will compress and fit into the design detail opening. We have noticed that some, but not all, of the sizes in the handbooks are excessively tight.

​Commercially available aluminum sealed flanges and half nipple nozzles are available and meet the design criteria of the ISO2861/1 Specification (more commonly referred to as KF or NW flanges). They consist of a centering ring containing an o-ring that is captured between two flanges. The flanges might be sealed by a clamp ring, a set of clamps or by bolts.

When assembling or disassembling an aluminum elastomer seal, one should be aware of the ease with which aluminum seal surfaces can be damaged.

Demountable Seals: Metal
Metal seals are normally used in vacuum applications when higher vacuum levels (better than 10-7 std cc/sec leak rates and base pressures below 10-8 torr) are desired. Common metal seals include the knife-edge (Conflat™), wire seal and metal jacketed spring seals, such as Helicoflex™ seals. Custom “diamond shaped” aluminum gaskets and indium wire seals are less common.

Metal jacketed spring seals consist of a helical spring enclosed within a ductile outer metal jacket which yields under compression to create a seal. Properly designed seals of this type can be used directly with aluminum flanges, the characteristics of which are similar to an o-ring groove. However, the manufacturer’s design handbook and technical sales staff must be consulted when designing seals of this type to ensure proper choice of the seal materials and groove dimensions.

The 6000 series and 5000 series aluminum alloys commonly used to fabricate vacuum chambers are not hard enough to withstand multiple uses before the seal surfaces of knife-edge or wire seal designs are no longer usable without repair. Also, in chambers that are baked out there is the possibility of bonding between the aluminum metal gaskets and the flanges.

Where metal seals are necessary, two common workarounds are used.
Where stainless steel is permissible, an explosion-bonded aluminum/stainless steel transition joint is welded in the flange nozzle or the flange itself is machined directly from this material. The explosion-bonded material is readily available, and this technique has been used successfully in vacuum applications in accelerators for decades. With the transition from aluminum to stainless steel, standard metal seals can be utilized.

Aluminum Knife-edged Flanges with a hard coating of titanium-nitride, chromium-nitride or electroless nickel over the knife-edge seal surfaces are used in applications where stainless steel (commonly because of radiation activation issues) cannot be utilized. These flanges utilize the same design details for the knife-edge as the common stainless-steel flanges. Dead soft aluminum gaskets are used. While the hard coating protects the knife-edge from wear, if the joint is accessed often, we would recommend the use of the transition joint method unless the use of stainless steel is prohibited.

Uncoated aluminum knife-edge flanges with aluminum gaskets, paired with another aluminum or stainless-steel knife-edge flange are sometimes used. We see no reason why this solution wouldn’t work in dismountable joints that were seldom used and carefully assembled and disassembled.

Achieving leak tight permanent welded joints in aluminum is routine when performed by competent, experienced welders following stringent procedures.
The use of o-ring grooves with elastomer seals is the preferred method for designing demountable seals in aluminum vacuum chambers.
Metal seals can be utilized successfully in aluminum vacuum chambers. The most common method, however, is to transition to stainless steel using explosion bonded aluminum/stainless steel transitions.

​Suggested Reading
1. O’Hanlon, A Users Guide to Vacuum Technology
2. Roth, Vacuum Systems
3. Roth, Vacuum Sealing Techniques
4. ASME B&PV Code Section IX
5. Parker O-Ring Design Handbook

The experienced engineers and manufacturing staff at Meyer Tool understand the proper welding and sealing techniques for leak free aluminum pressure and vacuum chambers. This knowledge ensures product quality and is just one aspect of how we apply our Reduce Project Risk Process to your aluminum chamber requirements. Meyer Tool has more than 50 years of experience manufacturing cutting-edge components and assemblies. Our experience spans a wide range of materials, processes, and project sizes. Whether you’re building a single prototype or need production manufacturing support for your unique components, Meyer Tool’s engineering and manufacturing team approaches each challenge using our Reduce Project Risk Process to support your needs.