Electron beam welder MEBW-60/2

In the year 2005 the representatives of a German firm expressed their interest to include one of our electron beam welders into their production program. It was agreed that building of the prototype will be started at the beginning of the next year, but soon was realized that the existing documentation needs to be thoroughly revised and digitalized. It was also clear that the current electronics is outdated and must be substantially renewed, if it should meet our new demands and the EU standards.

Working chamber of MEBW-60/2

Only slight changes have been made in electron gun construction, the vacuum working chamber and positioning manipulator. The working chamber is provided with four identical openings in the cylinder wall,with axis perpendicular to each other, making possible to set up the electron gun on the working chamber horizontally or vertically. With the gun in vertical position, the chamber can be extended on both sides horizontally, enabling to weld tubes as long as, e.g. 5 meters.

The inner dimensions of the chamber are 235 mm in diameter and about 125 mm in length. The axis of the chamber is horizontal. The front opening of the chamber can be closed by a small door fitted with glass window for observation of the welding procedure. To the opposite opening a turbo-molecular pump is connected.

Working chamber and electron gun of the MEBW-60/2

The electron gun of the new welder, MEBW-60/2, is similar to the gun designed by J. Dupak for EB welder SES-1 (see1), with only minor improvements, e.g. the high voltage electrode is automatically connected to earth potential while the gun is being vented. The security of the operator is so increased.

The source of electrons in this gun is a V-bent tungsten wire of 0.15 mm diameter. With the highest accelerating voltage of 60 kV and the beam current about 30 mA, the maximum beam power is about 2 kW. The beam is focused by one magnetic lens. The focused beam may be deflected in X-Y directions proportionally to the current in two pairs of coils, controlled by a computer. In this way the welder can, among others, be operated in the „scanning microscope mode“. In this mode,with a low beam power, the ready made weld can be inspected at higher magnification and, if needed, immediately repaired. The small diameter of the beam (about 0.1 mm) determines the useful magnification of the scanned area to about 30X. (see2. With more power in the beam some surface thermal treatments, like localized hardening or annealing, can be realized.

Draw'a'weld function

The manipulator of the MEBW-60/2 welder is designed first of all for revolving, as this kind of work-piece movement in our practice prevails. Our manipulator enables also linear movement, which is usually used only for positioning of the work-piece with respect to the beam. The movements are realized by two electric motors. Each of them can be independently controlled by the computer.

The pumping system consists of a rotary pump and two turbo-molecular pumps, one for working chamber, the other for the electron gun. The welder is sufficiently evacuated in less than two minutes. The venting takes less than 30 seconds. The MEBW-60/2 is operated without any water cooling. The operator is sufficiently protected against the X-rays produced during welding, by the welder construction.

The MEBW-60/2 welder is designed as a desk-top equipment. The vertical dimensions of the working table are similar to the usual office furniture. The construction is divided into three modules. This makes easier its transport and enables three different sets: either linear, or in L shape, or „minimalistic“, using only two of the modules.

Control electronics

The electronic equipment of the MEBW-60/2 comprises six units: the high voltage power supply, the supply for electron optical elements, a unit for scanning mode operation, vacuum control unit, supply for manipulator motors, and a panel for control of the complete equipment. The electronic units, except the control panel, are build into two consoles under the desk of the working table.

Common feature of all electronic units is microprocessor control. This brought possibility of full digitalization, automation of some functions, auto-diagnostic, possibility of remote control, and last but not least, more comfort for the operator.

Control program for MEBW-60/2

All control elements for manual control are centralized on a small, light movable panel comprising a LCD display. If needed, the control of the welding process can be turned over to the computer with adequate program facilities.

Manufacturer

The MEBW-60/2 is manufactured in our licence by German company Focus GmbH.

What is electron beam welding?

By fusion electron beam welding, the two parts which are to be joined, are locally melted in the vicinity of their contact by means of a focused electron beam. Thanks to high power concentration at the spot of impact, any metal is melted very quickly, so enabling the beam to penetrate rapidly deep into the material, producing a deep but narrow weld. The depth to width ratio can be up to about 30:1.

Almost all metals, as well as many of their combinations , can be welded by this method.The process takes place in high vacuum, which prevents oxidation, so enabling to weld even highly reactive metals like titanium, zirconium and many others.

The electrons for electron beam welding are gained by thermal emission, accelerated by high voltage (60 kV) and formed by means of electron optics (electric and magnetic fields) into a narrow beam with high power concentration.

Note

This paper is a reduced version of a paper published in the Czech magazine Jemná mechanika a optika3.

1. 1. Dupák, J. - Vlček, I.  Elektronové dělo pro svařování v přístrojové technice. Jemná mechanika a optika, , roč. 44, 1999, p. 239–241, ISSN 0447-6441.
2. 2. Zobač, M.  Zobrazování svarů v elektronové svářečce. Brno: Fakulta elektrotechniky a informatiky VUT v Brně, Ústav radioelektroniky, 1998.
3. 3. Vlček, I. - Zobač, M. - Dupák, L. - Dupák, J. - Kapounek, P.  Prototyp stolní elektronové svářečky MEBW–60/2. Jemná mechanika a optika, , 53, 2008, p. 27–29, ISSN 0447-6441.