FOR the first time, an electron beam has been used to weld thick metal plates in air, rather than inside a vacuum chamber. The novel welding gun that made the feat possible could free aircraft designers, for instance, from having to make airframe parts that fit in the restricted space of a vacuum chamber 鈥� allowing them to make stronger parts more cheaply.
Electron beam welding can be used to join metal plates up to 20 centimetres thick 鈥� far thicker than is possible with a blowtorch or electric arc welder 鈥� because the electrons penetrate deep into the metal.
But because electron beams can only be generated in a vacuum, commercial electron-beam welding could until now only be done inside a vacuum chamber. As even the largest vacuum chambers are only a few metres wide, this limits the size of the components that can be welded.
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The new welding gun, which was demonstrated last month by the welding company Acceleron in East Granby, Connecticut, promises to combine the advantages of electron-beam welding with the ease of use of a blowtorch. This could help aircraft makers, says Acceleron, who could use it to make airframe, wing support or engine support structures from bigger components. These would be stronger because they have fewer joins 鈥� and yet be easier and cheaper to make because they require fewer manufacturing stages.
The open-air electron welding gun was dreamed up by Ady Hershcovitch, a physicist at the Brookhaven National Laboratory in Upton, New York. The key problem he solved was how to devise a window that would get an electron beam out of a vacuum tube and into the air (快猫短视频, 12 April 2003, p 42). A solid window would absorb too much of the beam鈥檚 energy, but he reasoned that a charged gas or plasma held in place by a magnetic field would keep air out of the vacuum tube, yet still let the electron beam through. Acceleron has licensed Brookhaven鈥檚 patent.
The 1-centimetre-thick window in Acceleron鈥檚 welding gun is made by ionising a stream of argon gas by applying a voltage to produce plasma at 10,000 掳C that can be held in place by a magnetic field. Air molecules can鈥檛 get through the fast-moving ions in the plasma, but a beam of electrons generated by a source inside the tube can.
It has taken a while to perfect the new technology. 鈥淲e have been working on this for over two years,鈥� says Acceleron鈥檚 president Rory Montano. 鈥淚t has been frustrating because we have been so close for so long. We were able to drill holes with the beam. We were able to cut metal, but we couldn鈥檛 weld.鈥� Now they can. The breakthrough came when they realised that refraction in the plasma was making it act like a lens 鈥� focusing the beam too finely for welding. By taking this effect into account, they have compensated for it.
Although the first in-air welds are only 32 millimetres deep, the firm is confident that boosting beam power will eventually allow in-air welds up to 20 centimetres deep.
