Atlas UHV Explosive Bonding of Dissimilar Metals

Bonding for Ultra High Vacuum

Over the past several years Atlas Technologies has concentrated in developing methods and processes specifically intended for bonding dissimilar metals for use in Ultra High Vacuum (UHV). The details of this process are proprietary, but the following report provides a working understanding of the process. Atlas has filed patents on several applications of this technology, for example, Atlas Flange‰ and Atlas VCR‰ connector.

Metallurgical Bond:

The Atlas Flange‰ is metallurgically bonded using an explosion bonding process. The flanges blanks are cut from plates after the plates are bonded. Final machining of the blanks assures the UHV quality of the materials.

The preparation for bonding requires that the plates lay flat to each other, the flyer plate on top of the base plate separated by a small distance. An explosive material (i.e. ammonium nitrate) is placed on top of the flyer plate and detonated from a point at edge of the plate.

The explosion is a controlled progressive ignition starting from one point on the surface of the flyer plate progressing like the ripples on a pond from the drop of a rock.

The energy from the explosion accelerates the flyer plate against the base plate progressing at impact velocities of 1800-2200 m/sec.

A High energy surface plasma is formed moving ahead of the collision point striping electrons from the two bonding surfaces. The electron hungry metals are then thrust against each other at extreme pressures forming an electron sharing bond.

The process is a cold process inducing cold work into the flyer plate, and slight increase magnetic properties when bonding to stainless steels. This effect can be mitigated if requested by the customer.

Bonding Parameters:

Atlas has the capability of bonding many similar and dissimilar metals, but concentrates on those metals that are commonly needed for UHV applications. The formation of multi-laminates by explosive welding involves a working knowledge of the process variables and the ability to control them. The controllable bonding parameters are 1) explosive detonation velocity, 2) explosive load and the 3) interface spacing. An understanding of the metallurgy of dissimilar metals at the bond interface is an additional factor to be considered. The three bonding parameters are quantifiable and careful preparation of the metals to be bonded make for a successful bonding event.

The two constituent metals can be imagined to act as a viscous fluid in the reaction zone (bondline interface). By controlling the three bonding parameters the interface turbulence can be controlled. An Ultra High Vacuum interface requires that a smooth flowing wave pattern be developed. Excessive turbulence results in leak pathways and virtual leak possibilities.

Multi-layer Composites:

Metals such as copper and stainless are readily bondable. However other metals such as aluminum and stainless are incompatible. Many materials are not directly bondable without the formation of brittle intermetallic compounds. Atlas has develop patented multi-layer composites technologies that provide metallurgical compatibility to aluminum and stainless as well as other metals.

Multi-layer composites also provide diffusion barriers eliminating the possibility of the formation of the brittle intermetallic compounds during weld up or through repeated heat cycles such as bakeout and high heat processes. Titanium and copper are the typical materials used for to achieve diffusion protection for aluminum/stainless flanges. All materials used are UHV compatible and are metallurgically bonded, no adhesives are used.

On close examination you will notice a wave pattern at the bond line. The pattern is developed in the bonding process as the flyer plate is accelerated against the base plate, like ripples in a pond. The wave pattern is more noticeable depending upon whether you are observing a crossing section or parallel section. The wave pattern increase the mechanical strength of the bond, in shear by offering mechanical interference, and tension by offering more bonded surface area. Base metals break before the bond fails.

The explosion bonding process is a solid state process. The time duration of the explosion welding event is small and the heated reaction zone between the metals microscopic. The remaining thickness of the metal remains near ambient temperature and acts as a heat sink to the reaction zone. Therefore, the bondline is an abrupt transition between the metals with little, if any degradation of the metals.

This process allows Atlas to produce stainless/aluminum and stainless/copper flanges while retaining a T-6 temper in the aluminum and, in the case of copper, maintaining a half hard RF78 condition. Other processes would anneal these materials leaving them too soft for many UHV applications

Bondline Wonder:

The explosion bonding process is violent leaving the bonded plate with obvious extrusions, deformations and warpages. Atlas takes great care to flatten the plate after bonding. As each flange is produced careful attention is given to positioning the bondline. This typically can be done ± 0.04” and as small as ± 0.012”.

But, with careful examination, comparing one flange to another, you will observe that even though the flanges are dimensionally alike, no two flanges are the same when it comes to bondline location. Bondline wonder does not affect the mechanical integrity nor the function of the flange for UHV application.

UHV Metal Bonding Capability:

The explosive bonding process has been used as an industrial process for over 40 years. However, the industrial applications for large heat exchanges and for ship building are not suited for UHV where bonds must operate at leak rates <1x10-9 to 1x10-12 Torr.

Atlas Technologies has developed UHV bonding processes for a group of metals commonly used in UHV applications. (See Bonding Matrix). Although the Atlas Flange‰ and Atlas VCR‰ are most commonly used, Atlas also had developed successful bonding architectures for many metal combinations. Common Semiconductor and Particle Physics applications are:

Atlas Flange™:

• Aluminum Vacuum Chambers
• Copper Beam Tubes

Atlas VCR‰:

• Heated Aluminum gas lines
• MOCV applications

Water cooled Flanges:

• Cryo-pump protection
• Beryllium Windows
• Cooled View Ports

Wafer Heater Plates:

• Copper/Stainless Discs

Beam Stops & Absorbers:

• Molybdenum/copper
• Tungsten/copper
• GlidCop/copper
• GlidCop/Stainless
• Aluminum/copper

Superconducting Flanges & Fittings:

• Niobium/Titanium

Cryogenic Fittings:

• Stainless/Titanium
• Invar/Stainless

Bonding Architectures:

Plate-to-plate bonding has been discussed above. Tube to tube coaxial bonding technology has also been developed.

References:

For further questions on the subject of explosive bonding there are now many excellent reviews , , describing the process.

Cowman, G.R., O.O. Bagmen and A.H. Holtzman. Metal Transitions, 1971, vol 2, pp 3145-55

Shribman, V., A.S. Bahrani and B. Crossland, Production Engineering, Feb. 1969, pp 69-83

Blazinski, T.Z., Explosive Welding, Forming and Compaction, Applied Sciences Ltd., New York, London (1983), pp189-343