Electron beam in technologies

Introduction

In the past, as well as today, people looked for ways how to make use of that what the nature offered. The knowledge how to gain, process and use metals, up today belongs to the most important skills. Names of the first known metals, bronze and iron, had given names to the two important historic epochs. The processing of metals can not be done without heating. To the classical methods, fire, gas flame, electric current and electric arc, the new science added two other methods, the laser and electron beams. The technological applications of the latest we shall pursued in the following paper.

The history of electrons

The existence of elementary particles carrying electric charge has been discovered by english physicist J. J. Thomson in 1897, who was awarded by Nobel prize for this invention in 1906. This discovery explained the nature of so-called cathode rays observed by experiments with electrical discharge in rarefied gases. Later it was found that the electric current can flow even in high vacuum if one of the electrodes is heated to high temperature, but in only one direction, when the heated electrode, the cathode, is negative. It was further observed, that the current can be strongly influenced by voltage on a third electrode in the form of a grid, positioned between the cathode and anode. In this way the first vacuum valves, the current rectifying diode and voltage amplifying triode has been invented. Their enormous contribution to the technical progress in 20. century is today, in the age of semiconductors, almost forgotten. The diode was patented in Britain in 1904 under number 24 805.

Electrons in metallurgy

The thermal effects of „cathode rays“ have soon been exploited for metallurgical purposes, i.e. melting metals in vacuum. The method has been patented by by Marcello Pirani, first in 1905 in Germany, and two years later in USA (see Fig. 1).

For metallurgical purposes the electrons emitted from the heated cathode need not be formed into a narrow beam but only accelerated to the metal which is to be melted. This method is applied first of all for preparation of metals with high melting point or highly reactive, like titanium and its alloys, tantalum, niobium, molybdenum, tungsten, zirconium, hafnium, vanadium, uranium, silicon, platinum or iridium. Electron melting is also used for preparing of steel or other metals if highest purity is required. The power of electron beam needed in metallurgy for remelting of large quantities (tons) of metal is measured in megawatts.

Fig. 1: Equipment for electron beam melting in vacuum, detail from American patent of M. von Pirani.

Focused electron beams

In the following paper we shall be concerned with such technologies, where high concentration of power is needed. If this should have been achieved with electrons experience gained with electron microscopes had to be used. That's why the first electron beam welders have been constructed by first designers of electron microscopes, the Germane scientists (e.g. von Aredene, von Ruska, Steigerwald).

The device which can produce a narrow electron beam is in English literature called Electron gun. Two possible constructions of electron guns are schematically shown in Fig. 2, one being designed for black-and-white TV cathode ray tubes, the other for technological applications. In each electron gun we can find similar construction elements: In Fig. 2 is: K – cathode, A – anode, C – beam adjustment coils, O – beam focusing coil, V – deflection coils, I – high voltage insulator, S – electron beam.

Fig. 2: Electron gun for: technological applications (left) black-and-white CRT (right).

Because the attainable power concentration (in kW/mm²) in the cross-section a focused beam, which is needed in technological applications, is inversely proportional to the dimensions of the emitter, the cathode is a thin tungsten wire in CRT, or a tungsten strap in a technological gun. The emitted electrons are accelerated by a strong electric field applied between the negative cathode surrounded by a control electrode (called Wehnelt cylinder), and the anode, kept at zero electrical potential. The electric field not only accelerates the electrons, but also forms their trajectories into a narrow space around the axis. The beam passes through an opening in the anode. The number of electrons in the beam can be controlled by negative voltage on Wehnelt cylinder U_W from zero to any desired value. The power of the beam is the product of accelerating voltage U_A and the beam current.

To prevent electrical discharges and burning of the hot cathode, „high“ vacuum (pressure lower than 0.001 Pa) must be kept in the working space of the gun. The other effect of high vacuum is, that the electrons are not scattered by collisions with the gas molecules. Also the working chamber of the equipment must be evacuated, though not high vacuum is inevitable here. To save the costs of the pumping equipment, the working chamber is sometimes evacuated only to a „low“ vacuum, that is pressure of about 0.1 Pa.

Another positive effect of vacuum is that also highly reactive metals can be welded. Just the need of such materials in atomic research and industries led to widespread use of electron beam welders after 1960.

After leaving the anode, the electrons move with constant velocity in a slightly divergent cone. To achieve the highest possible Power density kW/mm² in the cross-section of the beam, the electrons are focused by a magnetic lens (magnetic field rotationally symmetrical around the axis). In this way power density as high as 1 MW/mm² can achieved.

The beam passes through other two magnetic fields produced by two pairs of coils. The first one, positioned before the focusing lens adjusts the beam precisely around the axis. The other, positioned behind the focusing lens, deflects the focused beam from the axis by an angel linearly proportional to the intensity of the current in the deflecting coils. This current can be controlled by a computer, which enables to move the beam very precisely and very rapidly over a wide field. This possibility finds its use in some practical applications.

EB machining

At extremely high power densities, of the order of 0.1 to 1 MW/mm², any material is in a very short time evaporated. This was first observed in electron microscope and used for making very small molybdenum diaphragms used in these devices, which were also used for their production (in 1938 by Manfred von Ardene). In 1942 German physicist von Boris patented the first electron beam drilling device. In 1949 another German physicist, Karl-Heinz Steigerwald, started scientific research of thermal effects of electron beams, resulting in 1952 in equipment for electron beam „machining“. Its capabilities were respectable, for example it could „bore“ 3000 holes into a metal sheet in one second. Yhe electron beam can also be applied for boring non-metal materials, like corundum or diamond.

The attainable power concentration in the beam is proportional to the accelerating voltage, therefore the drilling machines use 150 kV. Holes of about 25 microns at depth to diameter ratio 25/1 can be produced. The holes need not be perpendicular to the surface. For more information see http://www.steigerwald-eb.de/ and http://www.ebdrilling.com/.

EB welding

Though the thermal effects of electron beams were known already before the second world war, the era of wider use of EB welding started only in the fifties of the 20th century. In 1957 J. A. Stohr in France reported the first wider use of this method and its application for welding reactive metals. Soon the new method spread into many other countries like Germany, Great Britain, USA, Russia and others. In Czechoslovakia the first experiments with EB heating and welding have been made in 1963 in ISI Brno, in a multi-purpose vacuum furnace.

When the power density is about 10 kW/mm², the electron beam penetrates fast into the metal, only melting the material without considerable evaporation. Thanks to the high speed of penetration (tens of millimetres per second), the melted zone is very narrow. The depth to width ratio can be as high as 30:1. The thermal affected zone and deformations are minimal. Welds in aluminium up to 300 millimetres deep, or 100 mm in stainless steel have been realized. On the other side, very thin metal parts, in the range of tens of a micrometer can also be electron beam welded (see Fig. 3).

Fig. 3: Photo of a membrane below, made by electron beam welding of 0.05 mm thick stainless steel membranes.

To other characteristics of electron beam welding belongs high reproducibility.

Electron beam can also be applied for welding of metals with highest melting points, like tungsten, molybdenum, tantalum and many others. The mechanical strength of the weld is not much different from the basic material, sometimes even better thanks to getting rid of impurities by remelting in high vacuum. Electron beam can also enable to join two quite different materials, like e.g. copper to stainless steel, aluminium to titanium, nickel, silver, and many other combinations.

At least the vacuum working chamber and work-piece positioning mechanisms must be „tailored“ to meet the wide variety of demands. The volume of working chamber may vary betwen litres and hundreds of cubic metres, e.g. 630 m³. They also differ in the value of acceleration voltage; today there are two categories, with 60 kV or 150 kV, the latest rather expensive. The higher voltage is chosen when the highest power concentration is necessary.

Thermal surface treatment

The possibility to scan the electron beam with exactly defined parameters over an exactly defined area brings other possibilities of its application in technologies. It makes possible to harden or anneal locally a thin surface layer, without changing the properties of the basic material. The speed of cooling of the affected layer into the base may be as high as 10⁴ K/s.

The remelting of the surface layer can be used e.g. for compacting the surface of the cast iron or aluminium and its alloys, or porous deposited metal layers. In this way, the surface layer can be „doped“ by another material, to improve its mechanical properties, e.g. abrasion resistance.

Texturing and engraving

Computer controlled positioning of the beam can be used for creation of textures by remelting locally the surface with intensity dependent on beam parameters. In this way the welded components can be „stamped“ by any mark or inscription. Even a photo can be „engraved“ on the metal surface (see Fig. 4).

Fig. 4: Photo transferred on a metal sheet by electron beam. The remelted spots are seen as darker. Real size of the picture is about 60×40 mm.

Additive prototyping

By successive melting, layer after layer, of powder material by electron beam controlled according to a virtual 3D model in a computer, prototype models can be realized in EB welder. The beam is scanned over the a thin layer of powder causing it to fuse with the previous one, already solidified. The thickness of a layer can be 0.05 up to 0.2 mm, with lateral accuracy up to 0.4 mm. In this way, about 60 cm³/hour can be remelted.

This method is applied for creating models made of titanium, or alloys of cobalt, chromium and molybdenum, used in surgery. For this purpose, the implants may be adjusted to the results of tomography.