Basic features of electron beam welding

Profily typických svarů elektronovým svazkem

The kinetic energy of fast moving electrons can be utilised as a source of heat for welding. The free electrons in vacuum can be accelerated by electric field and formed by magnetic lens into a narrow ray carrying high energy, which is transformed into heat at the spot of impact. Thanks to high power concentration (e.g. 104 mm−2) the velocity of temperature increase in the spot of impact is extremely high, making any material melt very rapidly. Consequently, the electrons penetrate rapidly into the material, so producing a deep but narrow weld. Using high enough power, the depth of electron penetration can be as high as e.g. 10 cm in stainless steel, at the depth to width ratio up to 30:1.

Atmosphere density in the space where electron beam is generated and formed must be as low as possible. Also in working space of electron beam welder high vacuum is preferable. Electron beam generator (electron gun), vacuum working chamber, vacuum pumping system, high voltage power supply and control electronics are inevitable parts of each electron beam welder. That's why electron beam welders are very expensive. The acquisition costs may be compensated by low operation costs by some applications, like in mass production. Electron beam in vacuum may be inevitable in some cases, e.g. if highly reactive metals like Titanium or Zirconium, are to be welded.

The construction and dimensions of the working chamber are to be adjusted individually to the dimensions of welding parts. Their volume may vary between litres and hundreds of cubic meters. The EB welders in our laboratory are small sized – the bigger one has capacity about 150 litres, the smaller ones about 7 litres.

Free electrons in the electron gun are obtained by emission from a tungsten wire or strip heated to high temperature by electric current. The electric field between the negative emitter (cathode) and positive electrode (anode) not only accelerates the emitted electrons,but also forms their orbits into a narrow space around the axes.The accelerating voltage is usually about 60 kV, but can be as high as 200 kV. To achieve highest possible power concentration at the spot of impact, the beam is furthermore focused by one or two „magnetic lenses“ (magnetic field rotationally symmetrical around the beam axes). For more information see e.g. the book 1 or 2.

In spite of high purchase costs EB welding found, thanks to its exclusive capabilities, wide use in many branches of industry. To its strong points belong:

Fig.1: Shapes and dimensions of electron beam melted zones.Fig.1: Shapes and dimensions of electron beam melted zones.

Obr. 2: Metallography of test welds.Obr. 2: Metallography of test welds.

  • high ratio depth/width (depending on welding parameters up to 25/1) knife-like weld cross-section, see Fig.2;
  • deep welds in one passing;
  • no melting point limitations;
  • minimal temperature affected zone;
  • minimal deformations;
  • good productivity;
  • purity of the weld, thanks to remelting in vacuum.

Construction of joints for EB welding

The parts which are to be welded by electron beam should be in good contact, without any gap. The contact surfaces are to be fine machined, and perfectly clean (without remnants of cooling liquid or abrasives, et.) Most often the EB welding is realized by melting only both joining parts, without adding any other material. Two very thin or, thin to thick parts cannot usually be welded without special jigs securing mutual contact and preventing thermal deformations.

Our technological equipment

Fig.3: Upgrade ES2D of ES2 welder built in 1970Fig.3: Upgrade ES2D of ES2 welder built in 1970

Fig.4: Desk-top welder SES-2Fig.4: Desk-top welder SES-2

At present our department operates three electron beam welders, all of them designed and built in our institute.

One of them, the oldest, was built in 1969–70 ES2 and later upgraded. Its working chamber is cylindrical, horizontally oriented, with inner diameter 600 mm, 500 mm deep. It is equipped with 50 kV/1,5 kW electron gun.

The other welder SES-1 has also cylindrical working chamber with dimensions: 235 mm in diameter and 165 mm long. The vacuum working space can be extended vertically or horizontally in both directions. Position of the gun can be horizontal or vertical. With the gun vertical and vacuum chamber extended horizontally, zirconium tubes 4 meters long has been welded in this small welder.

The third welder MEBW-60/2-E is of similar construction and dimensions like SES-1, but is equiped with 60 kV 2 kW gun and ingenious electronics. This makes possible to control all functions of the welder with the help of a PC. Among others, it can be operated in the mode of scanning electron microscope, which can be exploited for inspection of realized welds, or for pre-welding preparations.

Very useful is the function „Draw'a'weld“, which can be applied for welding along quite irregular paths or in cases when the work-piece can not be moved (see Fig.6).

The computer control of the welder in EM scanning mode enables also to „engrave“ on metal surface any picture (drawn and digitalised or taken by digital camera, see Fig.7).

All our welders are equipped with positioning mechanism for rotational or linear movement. Both can be computer controlled.

Fig.5: MEBW-60/2-welderFig.5: MEBW-60/2-welder

Fig.6: Example of "Function Draw'a'weld" applicationFig.6: Example of „Function Draw'a'weld“ application

Fig. 7: Photo engraved on stainless steel sheetFig. 7: Photo engraved on stainless steel sheet

Other services

We have all necessary workshop means for manufacture of welding jigs. We can also do the full metallurgical evaluation of welds, i.e. sawing, grinding, polishing and etching, taking photos in light stereo-microscope (Nikon SMZ1000) or in scanning electron microscope, for optimization of welding parameters.

Short review of EB welding history in ISI Brno

We have made first practical experience with EB welding in 1965, when we have built a multi-purpose vacuum furnace with different possibilities of heating, including electron beam 3. By the construction of the electron-gun we have applied our previous experience in the field of electron microscopes. The good results and experience with electron beam welding led us to the design of electron beam welder ES-2, mentioned earlier. Brief history of EB welders in ISI Brno you can also find in dedicated article.

In the past 45 years of electron beam welding in our institute, we have gained a lot of experience in this field. It helped to solve not only our technological problems in ultra high vacuum engineering and cryogenics, but also in many other institutions and laboratories.

Photogallery of EB welded parts

In our gallery you can see examples of EB-welded components from our own welding practice.