New "window" of opportunity for e-beam welding

Senior Editor

Those considering electron-beam welding (EBW) have three basic techniques from which to choose: nonvacuum, partial vacuum, and high vacuum. These methods use a stream of high-energy electrons to locally heat metals to welding temperatures exceeding 10,000°C. And all need a high vacuum at the beam source. The difference between the three lies downstream of the beam source.

Nonvacuum EBW uses large-capacity vacuum pumps and blowers with differentially pumped sections to separate the vacuum from atmospheric pressure. Weld quality suffers because gas molecules scatter and diffuse the electron beam as it travels from the source to the work surface.

Partial-vacuum electron beam welding is typically done at a chamber pressure of about 75 to 300 microns. The gun portion where the beam is generated is held at about 10 ^-14 Torr. The chief advantage of partial-vacuum EBW is a short pump downtime required prior to welding. Welds made with the method are not as narrow or clean as from high-vacuum EBW.

High-vacuum EBW avoids the scattering problem and produces higher-quality welds because welding takes place in a hard vacuum (about 10 ^-4 Torr). As such, part size is limited to that of the vacuum chamber. The pumps that evacuate the chambers are expensive, slow, and consume a lot of power. And all equipment associated with the welding process inside the weld chamber must be capable of operating in a vacuum, which further raises costs.

But a Plasma Arc Window being codeveloped by Acceleron LLC, East Granby, Conn., and Brookhaven National Laboratory gets around these limitations and permits high-quality electronbeam welding at atmospheric pressure. The " window" is a stabilized gas-plasma arc that preserves the hard vacuum yet is effectively transparent to the beam. Only the beam source runs in a hard vacuum, which is a comparatively small volume. Eliminating the large weld-chamber vacuum pumps cuts power consumption by up to 80%.

Though an electron source must run in a vaccum the resulting beam is able to travel a few centimeters in air without significant loss of energy or focus. The problem has been how to pass an electron beam from high vacuum to atmosphere — without the dispersion of electrons — and keep it tightly focused. That is where the Plasma Arc Window comes in.

Plasma in this context is a hot, ionized (electrically charged) gas. Creating plasma in the 5-mm-wide circular window is fairly straightforward. Inert gas feeds into a ceramic-lined cavity containing a cathode and anode to which a potential of a few hundred volts is applied. This voltage potential strips electrons from the gas molecules and accelerates the resulting ions from anode to cathode and electrons from cathode to anode, heating them in the process and filling the window with plasma.

Keeping the plasma stable is tricky because the ionization process that creates it becomes more energetic and difficult to confine with increasing temperature. Conversely, cooling the plasma makes it less energetic and electrically conductive. The plasma window takes advantage of these properties and surrounds the cavity walls with a system of water-cooled, thin copper tubes. The tubes pull heat from the plasma to mainuum,tain a low-temperature outer ring while the core remains hot. It is through this hot plasma core that the electron beam passes from a vacuum to ambient air and to the work surface.

How does a Plasma Arc Window keep air at atmospheric pressure on one side separated from vacuum on the other? A rigorous explanation of the plasma physics is beyond the scope of this article, but there are three basic reasons why it works.

Plasmas exhibit many of the same properties as the gases they are made from. For example, both pressure and viscosity increase with temperature. The plasma-arc window operates at about 15,000°K, or about 50 X hotter than air at room temperature. Therefore, plasma pressure in this case can balance atmospheric pressure at about 1/50th density. Low density means fewer electron collisions so the beam passes through the window essentially unimpeded. A high viscosity lowers plasma flow rate and leakage while the electric and magnetic fields confining the plasma window exert a "pumping" effect on the ionized gas.

Together these factors make plasma arc windows about 22,000 X more effective at maintaining a vacuum than current differential pumping methods. In fact, experiments conducted by plasmaarc-window inventor and physicist Ady Hershovitch at Brookhaven show they are capable of withstanding pressure differentials up to nine atmospheres without leaking.

A plasma window has another benefit for EBW: It acts as a strong lens that focuses charged particles such as electrons. A magnetic field generated by the plasma arc current — as the electron beam passes through the plasma — exerts a strong inward radial force that focuses the beam into a small spot size, much smaller than possible with nonvacuum EBW.

There are obvious advantages to the method, not the least of which is reduced power consumption. Acceleron President Rory Montano estimates that eliminating the vacuum pumps needed for conventional high-vacuum ebeam welding could save the company about $14,000 monthly in electricity costs alone. Also eliminated would be special, $1,000/gallon diffusion-pump oil. The Acceleron system instead uses a turbomolecular pump for hard vacuum and a pair of scroll pumps to maintain vacuum upstream of the plasma window. Both pump types are oilless.

Customers would benefit from greater production efficiencies. For example, a standard, large vacuum chamber is 108 X 56 X 56 in. and can take 12 min to pump down. Eliminating this "dead time" would more than double production and lower costs to customers by 60%, says Montano.

Though several technical hurdles remain, the Acceleron plasma-arc-window welder has successfully drilled holes in, cut, and recently welded through 0.125-in.-thick 316 stainlesssteel plates. An unwanted by-product of this and all EBW processes is the emission of X-rays. Here, a lead shield can contain the X-rays.

MAKE CONTACT
Acceleron Inc.,
(860) 651-9333,
www.acceleron-enbeam.com