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List of some abbreviations<br />

* divergence angle (full angle) rd oder °<br />

β beam angle of deflection rd oder °<br />

a fillet weld thickness mm<br />

A W ** working distance mm<br />

A F ** focus distance mm<br />

b*** weld seam width, nominal mm<br />

b P oscillation width mm<br />

b S gap width mm<br />

d*** maximum dimension of an imperfection (pore, cavity) mm<br />

d FL beam spot diameter mm<br />

d Fo beam focus diameter mm<br />

F force N<br />

f*** maximum pore or cavity cross-sectional area mm 2<br />

f oscillation frequency Hz<br />

h weld seam height mm<br />

h*** size of imperfection (height, depth, edge misalignment) mm<br />

I B ** beam current mA<br />

I BD continuous beam current mA<br />

I BP impulse beam current mA<br />

I d deflection current mA<br />

I D transmitted current mA<br />

I H heating current A<br />

I L ** lens current mA<br />

j e emission current density A/cm 2<br />

j eT temperature dependent current density A/cm 2<br />

l*** maximum imperfection dimension mm<br />

l centre lip length mm<br />

L beam power density W/cm 2<br />

m** load on the work table or rotating device kg<br />

P beam power kW<br />

p A pressure in working chamber mbar<br />

p E pressure in beam gun mbar<br />

Q** leak rate mbar/dm 3 · s<br />

s*** depth of fusion zone mm<br />

s 1 *** depth of the melted zone for a T-joint mm<br />

SR slew rate mA/ms, °/ms<br />

t time s<br />

t weld thickness, sufficient for strength mm<br />

t*** workpiece thickness mm<br />

TIG tungsten inert gas (welding) –<br />

t V pulse ratio –<br />

U A ** accelerating voltage kV<br />

U St bias voltage (Wehnelt voltage) kV<br />

v** welding speed mm/s, cm/min<br />

V volume m 3 , dm 3<br />

z centre lip thickness mm<br />

According to: * DIN 32533, ** ISO 14744, *** ISO 13919

1 Introduction<br />

1.1 History<br />

Modern society takes for granted the technical achievements that make life easier, and may<br />

overlook that most useful things were laboriously invented, developed and tested by many different<br />

ingenious predecessors. Although an electron beam welding equipment is not an item of our<br />

everyday life, the products manufactured with it contribute to our high standard of living. It has<br />

been a long road from the pioneering work of British physicists Hittorf and Crookes from 1871 to<br />

1872, generating cathode rays in gases and melting metals, to the modern computer-controlled<br />

manufacturing equipment in the aviation and aerospace industry [1-1].<br />

Cathode rays were first described as “fast-moving electrons” by their discoverers Wilhelm Röntgen<br />

(1895), Thompson (1897) and Milikan (1905). They were viewed as a special type of radiation and<br />

an interesting physical phenomenon but they were not considered as relevant for material<br />

processing. On the contrary, in all these experiments, the heat generated by the collision of electrons<br />

on the anode or the target were a great inconvenience and water cooling was used to prevent<br />

melting [1-2]. Marcello von Pirani was the first to take advantage of this effect, building an X-ray<br />

tube in an electron beam vacuum furnace to melt tantalum powder and other metals, patenting the<br />

process in 1905 and 1907, Figures 1-1 and 1-2.<br />

Figure 1-1. Excerpt from the Marcello<br />

von Pirani patent “Production of homogenous<br />

bodies from tantalum or other<br />

metals” dated 26.3.1907<br />

Figure 1-2. Marcello von Pirani, German physicist,<br />

1880 – 1968<br />

In the following decades, many scientists investigated the properties of electron beams. Among others,<br />

Langmuir, Child, Dushman and Wehnelt explored the parameters of beam generation, while Bush,<br />

Rogowski, Flegler, Davisson and Calbrik, just to name a few, worked out the basics of electron optics.<br />

The first technically meaningful electron beam applications were in oscilloscopes and electron<br />

microscopes. Although von Ardenne and Rühle (1938) began to drill metals by evaporation and<br />

melting, a larger industrial application was not possible due to the lack of sufficiently powerful<br />

vacuum pumps.<br />


In 1949 the German physicist K. H. Steigerwald ushered in a new epoch of material processing<br />

with electron beams, Figure 1-3. He was engaged in the development of high performance electron<br />

microscopes and had the idea that the electron beam could be used in vacuum as a thermal tool for<br />

drilling watchmakers’ jewels and extrusion dies, and also for brazing, melting and welding under<br />

vacuum, Figure 1-4 [1-3; 1-4].<br />

Figure 1-3. Karl-Heinz Steigerwald,<br />

German physicist, 1920 – 2001<br />

2<br />

Figure 1-4. 1952 Steigerwald developed the<br />

first electron beam drilling machine<br />

The first experimental results proved very promising, leading to a license agreement with an<br />

American investor [1-5]. At this time it was thought that the only advantage that the heat input from<br />

electron beam welding would have over an electric arc or gas flame (for thermal conduction<br />

welding), would be that gas-sensitive materials such as niobium, tantalum or titanium would be<br />

protected by the vacuum from reactions with the atmosphere. In 1958 there was a breakthrough for<br />

industrial electron beam welding when Steigerwald successfully made a butt weld in 5 mm thick<br />

Zircaloy [1-6]. Using gradually increasing beam current, very deep but still unexpectedly narrow<br />

weld seams were generated. Although these deep welds awakened a new worldwide interest, the<br />

technical importance of the procedure was first recognised in the United States. In Germany,<br />

Steigerwald successfully built the first two electron beam welding machines, one of which was<br />

delivered to Pittsburgh, USA, for welding submarine components, while the other was operated for<br />

many years in Germany.<br />

After the discovery of the deep welding effect, a vigorous development of new equipment [1-7]<br />

began, in particular in France and the United Kingdom. Since the electron beam had been only able<br />

to weld thin workpieces, the impetus was to increase the power density and beam current to be able<br />

to increase the seam thickness. The first practical applications for electron beam welding were in the<br />

aerospace and nuclear industries.<br />

Milestones of technical developments in process and equipment include:<br />

– a high-voltage cable connection without insulation oil in the beam gun<br />

– precision exchange of the cathode heating ribbon using a clamping cartridge<br />

– beam gun vacuum separated from the working chamber vacuum

– welding equipment with larger working chamber and vacuum pumps<br />

– transfer and cycle welding equipment for mass production<br />

These improvements enabled many new electron beam welding applications. Today even a<br />

specialist may find it difficult to get a complete overview of the extent of electron beam processing<br />

in industrialised countries.<br />

1.2 Special characteristics of electron beam welding processes<br />

A number of special features and benefits of electron beam processing in comparison to other<br />

methods of welding by melting is shown in Table 1-1.<br />

Table 1-1. Technical characteristics<br />

Characteristic<br />

Description<br />

Power density<br />

Very high power density, more than 10 5 W/mm 2 in beam focus<br />

Beam power<br />

0.5 to 300 kW in research and development, 1 to 30 kW in industrial use<br />

Thermal input<br />

Kinetic energy of accelerated electrons converted into heat with material<br />

vaporisation (deep welding effect). Not heat conduction welding<br />

Welding environment Normally in vacuum; shielding gases used for welding in atmosphere<br />

Controlling<br />

Control of mechanics and welding parameters with computer. Automatic<br />

beam and focusing correction and seam tracking with electron optical<br />

camera systems<br />

Weld depth<br />

0.5 to 100 mm in a single pass, from 3 to 30 kW respectively<br />

Edge preparation<br />

Butt welds for all depths without bevelled edges<br />

Seam shape<br />

Narrow melting and heat affected zones. Seam width to depth ratio: from<br />

1:10 to 1:50<br />

Workpiece and beam motion Welding of longitudinal, circular and 3-D curved seams with computerprogrammed<br />

work piece and beam motion<br />

Energy distribution<br />

Control of melting and solidification processes of the weld by inertia-less<br />

movement of the electron beam in many oscillation shapes, directions and<br />

frequencies<br />

Multi-beam technology Many material processes are possible using extremely fast electron beam<br />

motion in the kHz range. Almost simultaneous, but locally separated beams<br />

appear for tacking, welding and heat treatment<br />

Filler material<br />

Since no filler material requirement, workpiece preparation is reduced.<br />

Exceptions: deposit welding and metallurgical conditions<br />

Weldable metals<br />

Large range of suitable welding metals: steels, alloyed steels, non-ferrous<br />

metals (aluminium, copper, titanium, etc.)<br />

Distortion<br />

Lower longitudinal, transverse and angular distortion in comparison to<br />

other fusion welding processes. Since the welds have so little distortion it<br />

is possible to maintain small tolerances. Very little or no reworking of the<br />

workpiece needed<br />


Table 1-1. Continued<br />

Characteristic<br />

Welding seam access<br />

Workpiece construction<br />

Working chamber<br />

Quality assurance<br />

Description<br />

Possible to weld even difficult open seams, for example with narrow gaps<br />

(1 to 2 mm wide) with variable working distance (in practice about 50 to<br />

1000 mm) possible<br />

In order to reduce manufacturing costs, complex components can be<br />

divided into structurally simple parts and welded to final or near-final<br />

dimensions<br />

It is possible to adapt the working chamber shape, size and the quantity of<br />

workpieces to reduce the evacuation time to a few seconds or zero (with<br />

continuous systems)<br />

The quality is controlled due to the high reducibility and stability of the<br />

welding parameters and automatic monitoring of welding data<br />

Electron beam welding equipment construction designs can be varied according to requirements to<br />

suit diverse production tasks, Table 1-2.<br />

Table 1-2. Classification of electron beam welding equipment 1) concepts<br />

Identifying feature Classification Remarks, application examples<br />

Accelerating voltage<br />

Up to 60 kV<br />

100 kV to 150 kV<br />

Up to 175 kV<br />

Low-voltage 2) equipment<br />

High-voltage 2) equipment<br />

Atmosphere equipment<br />

Working pressure<br />

Workpiece shape and quantity<br />

Beam gun<br />

1)<br />

2)<br />

About 10 –4 mbar<br />

About 10 –2 mbar<br />

Universal equipment<br />

Cycle and transfer equipment<br />

Multi-chamber equipment<br />

Continuous equipment<br />

Fixed on the working chamber<br />

In the working chamber<br />

applications associated with motion<br />

devices; beam gun performs all or<br />

part of the weld feed motion<br />

High vacuum 2) equipment<br />

Partial vacuum equipment<br />

Gear parts<br />

Small parts<br />

Bimetallic blades (for sawblades<br />

production)<br />

For particularly large components<br />

Although they are often called “machines”, we will refer to them in this book as “equipment” to remind<br />

us of their complicated combination of machines<br />

Commonly used, but not the terms used in standards<br />

Electron beam welding has been utilised successful in industry for almost 60 years. Today it<br />

competes strongly with other welding processes due to its flexibility, reliability and profitability.<br />

1.3 Other beam welding processes<br />

In early laboratory tests, other beam welding processes, such as incoherent light beam or ion beam<br />

welding were investigated but they remain without practical application [1-8].<br />


In contrast, the laser beam is already known in data transmission, communication technology,<br />

medicine, instrumentation, etc. It has become important for cutting and welding and finds extensive<br />

application in surface layer modifications. The laser light, being monochromatic (single wavelength)<br />

and coherent (in phase), can be produced using various media (gas, liquid and solid<br />

phases). In contrast to electrons, which transform most of their kinetic energy in the form of heat<br />

in the first 0.06 mm of materials, the laser beam is absorbed in a thin surface layer of only 0.01<br />

µm. Moreover, in addition to other effects, the laser light is reflected and absorbed in the ionised<br />

metal vapour (plasma plume) above the weld, reducing the energy transfer and overall efficiency<br />

compared to electron beam welding [1-9].<br />

Like the electron beam, the laser beam focus can reach power densities of 10 5 W/mm 2 , and is also<br />

used for deep welding to produce seams which are much deeper than wide. Since the laser beam<br />

also has an extremely low beam divergence due to its particular light characteristics, it can travel<br />

over large distances without significant beam diameter enlargement. The advantage for the laser<br />

beam is that it can normally be used for welding or cutting in atmosphere. This fact is always emphasised<br />

when comparisons are made with electron beam welding, which usually takes place in a<br />

vacuum. Although laser beam welding does not require a vacuum working chamber, along with<br />

other thermal joining processes, the melt must be protected from contact with oxygen and nitrogen<br />

using argon and/or helium inert gas mixtures.<br />

Recent studies have demonstrated that laser beams can also be used in vacuum. At operating pressures<br />

from 10 to 50 mbar, narrower and deeper welds can be achieved than under atmosphere,<br />

similar to the values achieved with electron beam welding [1-10]. While the weld pool dynamics<br />

and microstructure solidification characteristics can be readily modified by oscillating the inertialess<br />

electron beam at extremely high frequency, this is not possible with the laser beam and so<br />

must be carried out by other measures.<br />

[1-11] reports an investigation of pore frequency and distribution of laser and electron beam in<br />

welds under vacuum of pure titanium and nickel. The laser beam seams were welded both at<br />

working pressure of 10 –1 mbar and also under argon shielding gas. A comparable seam width was<br />

achieved with an electron beam acceleration voltage of about 80 kV. Since the welds were all<br />

partial penetration, the seams without under beads. It was found that both types of welding under<br />

vacuum have very similar melting efficiencies, transferring similar amounts of energy into the base<br />

material. The laser welds under vacuum showed much-reduced pore density than under the<br />

shielding gas. The same results can be achieved with electron beam welding by implementing one<br />

of the many possible combinations of beam oscillation and defocusing.<br />


2 Generation of the electron beam<br />

2.1 Free electrons<br />

It is not obvious on first reflection about the fundamental physics of processing materials with a<br />

beam of electrons that this lightest of atomic particles can provide enough kinetic energy for a heat<br />

source. Like all atomic orders of magnitude, the numerical value of the electron rest mass of<br />

9.1 · 10 –28 grams is beyond our comprehension (about 1836 times lighter than the proton with<br />

opposite charge). For kinetic energy conversion the disadvantage of the low mass of the electron is<br />

compensated for by its electrical charge. The numerical value of the elementary negative charge of<br />

an electron at 1.6 · 10 –19 coulombs is the source of the beam current. The advantage of the electrical<br />

charge is that by using magnetic fields one can readily accelerate the electrons to the highly<br />

energetic velocities required for welding. As Figure 2-1 shows, in a vacuum of better than<br />

10 –4 mbar, a typical welding accelerating voltage of U A = 150 kV = 1.5 · 10 5 V brings the electrons<br />

up to a velocity of 2 · 10 8 m/s, about two-thirds the velocity of light. At this high velocity the<br />

Newtonian kinetic energy of 1 / 2 m · v 2 is supplemented by a relativistic mass increase of about 35 %<br />

[2-1].<br />

6<br />

Figure 2-1. Relationship<br />

between accelerating voltage<br />

and electron velocity<br />

Electrons bound to atoms normally only occupy fixed energy orbits, called shells. Metals have high<br />

electrical conductivity because the outer electrons are only weakly bonded to the atomic nuclei and<br />

are able to move around quite freely in the atomic crystal lattice (conduction electrons). These<br />

electrons are not normally available for welding because they cannot leave the metal surface. The<br />

potential threshold binding them to the metal lattice can be overcome by applying additional energy<br />

in the form of a heat input leading to a temperature rise. When the energy of the free electrons<br />

increases sufficiently, they overcome the potential threshold and collect near the metal surface,<br />

initially as an electron cloud, Figure 2-2. Heating is applied to the cathode together with other

measures to form the electron source. Leaving the metal surface for emitting is normally impossible<br />

for the electrons due to the mutual electrical attraction forces. The metal binds the electrons to itself<br />

and so cannot deliver the negatively charged electrons, as it would then become positively charged.<br />

2.2 Cathode<br />

Figure 2-2. Hot metal causing free electrons emission<br />

The cathode has a number of other tasks to fulfil besides providing electrons. It should have<br />

minimum heating power with a long service life and provide a high beam current by emitting a<br />

maximum number of electrons. The cathode is normally made from a high temperature material<br />

since, according to the Richardson thermionic emission law [O. W. Richardson, English physicist,<br />

1879 – 1959], the electron flow increases rapidly with the temperature of the emitter. Tungsten,<br />

often alloyed with rhenium, is the preferred material as it has a low vapour pressure in vacuum and<br />

a long cathode service life. Directly heated tungsten cathodes in electron beam welding equipment<br />

can achieve emission current density of j e = 5 A/cm 2 .<br />

Figure 2-3. Example of the shape and dimensions (mm) of a ribbon cathode for<br />

beam powers up to P = 15 kW<br />

7<br />

Y X slurC B C M Y X C 20 B C M Y X C 40 C 80 B C M Y X CM B C M Y X CMY B C M Y X B C M Y X B C M Y X

−−−−−−−−− 2 −−−−−−−−−−−−−−− 3 −−−−−−−−−−−−−−− 4 −−−−−−−−−−−−−−− 5 −−−−−−−−−−−−−−− 6 −−−−−−−−−−−−−−− 7 −−−−−−−−−−−−−−− 8<br />

The heat input needed to bring the cathode to the desired temperature depends on the dimensions<br />

and form of the cathode and the heating method. Ribbon cathodes, Figure 2-3, are connected as an<br />

ohmic resistor in a circuit and heated directly by passing a current, I H directly through them. They<br />

are the most common cathode type in electron beam welding equipment, providing stable beam<br />

characteristics with a high heating efficiency by using a clamp cartridge to ensure a geometrically<br />

stable emission surface position. They are practically the only consumable in an electron beam gun<br />

and can be quickly and easily exchanged (section 10.3). After switching off the heating current,<br />

directly heated ribbon cathodes cool down quickly due to their low mass, which is necessary to<br />

prevent oxidation to allow rapid venting of the beam gun.<br />

8<br />

Figure 2-4. Indirectly heated cathode<br />

Indirectly heated cathodes are heated by an auxiliary cathode providing electron bombardment.<br />

Because they are not heated by a large electric current, they can be made from solid with a more<br />

adaptable design. Indirectly heated cathodes, Figure 2-4, can operate with lower acceleration<br />

voltages, and are mainly included in electron beam welding equipment with high beam currents.<br />

They can be easily refurbished and have a longer lifetime than ribbon cathodes. However, a<br />

disadvantage is that the rapid wear of the auxiliary cathode necessitates more frequent exchange<br />

than the indirectly heated cathode itself.<br />

2.3 Anode<br />

The freely moving electrons thermally emitted from the cathode are insufficient for electron beam<br />

welding. The electrons must be accelerated to a high velocity to reach the required kinetic energy by<br />

the cathode, which is at a high negative acceleration voltage. An anode at earth potential, situated<br />

away from the cathode, attracts the electron cloud, Figure 2-5. The electric field between the<br />

cathode and anode accelerates the electrons and gives them the required kinetic energy. A highvoltage<br />

generator powers the cathode continuously with new electrons and therefore with current.<br />

This electrons flow has a safe return current with a closed circuit to the high voltage generator while<br />

the workpiece and the machine are connected to earth potential.<br />

In the arrangement shown in Figure 2-5, the accelerated electrons would impact with high kinetic<br />

energy onto the anode instead of the workpiece to be welded. Therefore, a hole is included in the<br />

anode, through which the electrons can pass uninterrupted at high velocity to the workpiece.

2.4 Bias cup<br />

Figure 2-5. Accelerated electrons<br />

in a diode system<br />

The simplest construction of beam gun has only a cathode and anode. Although this diode system,<br />

called the Pierce system [G. W. Pierce, German physicist, 1872 – 1956] was used for some<br />

equipment in the early years of electron beam welding, it has one significant drawback: the electron<br />

stream can only be controlled by changing either the acceleration voltage or the cathode temperature,<br />

which is completely unsuitable for industrial welding. A third electrode with a higher and<br />

separately controllable negative voltage than the cathode surrounds the cathode and significantly<br />

improves the electron beam adjustment and current. The electrons can overcome the potential<br />

difference between the cathode and anode following the law of physics of mutual rejection<br />

of charges of the same polarity. This third electrode is called bias cup or Wehnelt cylinder<br />

[A. R. Wehnelt, German physicist, 1871 – 1944] Together with the anode and cathode it is<br />

nowadays the practically universally employed triode system, Figure 2-6.<br />

Figure 2-6. Cathode, anode and bias<br />

cup<br />

9<br />

−−−−−−−−−−−−−− 9 −−−−−−−−−−−−−−− 10 −−−−−−−−−−−−−−− 11 −−−−−−−−−−−−−−− 12 −−−−−−−−−−−−−− B = B −−−−−−−−−−−−−− 14 −−−−−−−−−−−−−− C = C −−−−−−−−−−−−−− 16

C M Y X slurM B C M Y X M 20 B C M Y X M 40 M 80 B C M Y X CMY B C M Y X CMY B C M Y X Y 20 B C M Y X<br />

Figure 2-7. Graph showing how the cathode emission area modifies the beam current I B with the bias voltage<br />

U St<br />

a) beam current I B = 0<br />

b) low beam current I B<br />

c) high beam current I B<br />

d) electron-optic beam distortion due to the bias voltage U St being too low<br />

e) dependence of the beam current I B on the bias voltage U St<br />


A sufficiently high bias voltage can completely block the beam current I B such that the cathode<br />

emits no electrons, Figure 2-7a. As the bias voltage is reduced, the emission area increases in size<br />

and the beam current I B increases, Figure 2-7b and 2-7c. For welding, the adjustment of the beam<br />

current I B is thus independent of the acceleration voltage U A and the cathode temperature and is<br />

only controlled by the bias voltage U St with an inverse relationship: high bias voltage = low beam<br />

current, low bias voltage = high beam current, Figure 2-7e. The emission surface on the cathode<br />

must be placed at a sufficient distance from the ribbon edges and bends, otherwise the cathode<br />

flanks emit also electrons. Figure 2-7d. The consequences are an excessive beam divergence,<br />

significant distortion of the beam and a lack of rotational symmetry.<br />

The shape of the beam in a typical triode system in an electron beam welding equipment is<br />

indicated in Figure 2-8. Note that a constriction of the electron beam, called real crossover, occurs<br />

underneath the cathode emission surface as a result of both, the geometry of the bias cup and the<br />

bias voltage. The electron beam passes through the anode bore and reaches field-free space where<br />

its diameter increases constantly due to the electrons, with equal negative charge, being mutually<br />

repulsed. A backwards projection of the beam tangents creates a further point of intersection with<br />

the beam axis, called virtual crossover for electron microscopy calculations. Figure 2-9 shows<br />

enlarged the paths of electrons under the influence of the field distribution. You can see how the real<br />

crossover is in reality without a unique point of intersection but is rather a region with lines crossing<br />

the axis at different points. From this calculations can be made to show that the electrons are<br />

accelerated to slightly different velocities.<br />

Figure 2-8. Electron beam geometry in a triode<br />

system<br />

Figure 2-9. Electron beam with real and virtual<br />

crossovers<br />

The focal length of a beam system has the same definition in electron optics as in light optics. In<br />

particular at the beginning of the electron beam process development there was a differentiation<br />

between short focal length systems (Rogowski short focal) [W. Rogowski, German physicist,<br />

1881 – 1947] and long focal length systems (Steigerwald long focal). Most modern electron beam<br />

welding equipment are installed with modified Rogowski systems.<br />

B C M Y X X 20 B C M Y X X 40 X 80 B C M Y X MY B C M Y X CMY B C M Y X B C M Y X B C M Y<br />


−−−− M = M −−−−−−−−−−−−− 18 −−−−−−−−−−−−−− Y = Y −−−−−−−−−−−−−− 20 −−−−−−−−−−−−−− X = X −−−−−−−−−−−−−− 22 −−−−−−−−−−−−−−− 23 −−−−−−−−−−−−−−− 24<br />

2.5 Space charge effects<br />

The physics principles on which the electrons move between the cathode, anode and the bias cup<br />

are very complex and need not be explained in detail to understand the later technical points. Only<br />

one relationship is worth highlighting, that is for the adjustment of the cathode heating current I H .<br />

During operation the cathode has a finite service life because it experiences a small but significant<br />

material removal by sputtering and evaporation. Although this cathode temperature (about 2800°C)<br />

is still well below the melting temperature (approximately 3200°C), the evaporation rate is<br />

enhanced by the high vacuum. Sputtering occurs due to the impact of positively charged ions via the<br />

ion return current (ion back-streaming) produced by the collision of the electrons on their way to the<br />

weld with the remaining gas molecules, producing metal vapour, Figure 2-10.<br />

This slow material removal decreases the profile of the ribbon cathode and its emitting area,<br />

reducing the heating current I H (at a constant supply voltage), thus reducing the cathode temperature.<br />

According to Richardson’s law, the emission current density j eT decreases rapidly as the cathode<br />

temperature T falls, so a quite low temperature change will significantly affect the value of the beam<br />

current I B , Figure 2-11. Such quality variations would be unacceptable for electron beam welding<br />

with high requirements for seam quality reproducibility.<br />

Figure 2-10. Ribbon cathode emission area damaged<br />

by ion impacts<br />

12<br />

Figure 2-11. Strong dependency of emission current<br />

density j eT on cathode temperature T<br />

We may assume that the acceleration voltage U A , is specified with a maximum value dependent on<br />

geometrical parameters such as the distance between cathode and anode. The equation of the<br />

Langmuir law of physics [J. Langmuir, American chemist and physicist, 1881 – 1959] states that<br />

the emission current J eR from the cathode is space-charge limited with only two variables, U A and<br />

the distance from anode to cathode . It is therefore necessary to select enough cathode heating<br />

current I H to achieve a high enough temperature to make available a sufficient number of electrons<br />

at maximum acceleration voltage; that is j eT and thus the cathode heating temperature can be<br />

limited. The cathode will be surrounded by a cloud of "surplus" electrons which have their own<br />

charge and restrict any further electron emission so that the cathode is affected by an electric<br />

space-charge field. The graph in Figure 2-12 shows that when above a certain cathode temperature,<br />

the beam emission current is unaffected by fluctuations in cathode temperature.

Figure 2-12. Definition of space-charge area<br />

In practice, automatic control and regulation devices facilitate setting and monitoring of the<br />

heating current I H (section 7.6). At the same time, these controls verify that the life of the cathode<br />

is not affected by excessive temperature. As already mentioned the cathode life is more influenced<br />

by the vacuum inside the beam gun chamber, the beam current and the material to be welded. A<br />

cathode life of 8 to 10 hours is normal if we measure only the cathode heating time during actual<br />

welding. However, if the entire lifecycle is taken into account, including all pauses in welding<br />

when the cathode temperature is reduced, 60 to 100 hours is the average.<br />

2.6 Focussing lens<br />

The geometric shapes of the cathode, bias cup and anode influence the more or less curved<br />

trajectories of electrons not only inside the beam gun but also further away towards the welding<br />

zone. As already mentioned, the electric field in the triode system shapes the beam to the first<br />

crossover focus, while the electrons are mutually negatively repulsed, passing through the anode,<br />

Figure 2-8. The electrons are accelerated after passing the anode to their final velocity, but the<br />

beam lacks the necessary power density for welding and so has still to be focused.<br />

The trajectories of the electrons are focused onto the workpiece surface by the magnetic field of a<br />

ring coil which consists of many copper windings encased on three sides by high permeability iron,<br />

Figure 2-13. A direct current passes through the ring windings, which generate a magnetic field<br />

that acts in the middle of the coil and focuses the electron beam like an optical convex lens. In<br />

order to achieve the small diameter focus of 0.1 to 1.0 mm (depending on beam performance and<br />

focus distance) required for deep electron beam welding, the magnetic lens induces curved, largeradius<br />

spiral paths which do not affect the electron velocity. The focus is not a mathematically<br />

exact point but has the appearance of a circular surface, referred to as the beam spot, which is the<br />

precondition for the required power density of L > 10 5 W/mm 2 for welding metals with a thickness<br />

of several centimetres.<br />

The beam focus can be adjusted onto the surface of the workpiece to be welded with the lens<br />

current I L . The melt process may also be modified by focussing the beam slightly above or below<br />

the surface (Chapter 6). Figure 2-14 shows the dependence of the focus distance A F on the lens<br />

current I L for two typical acceleration voltages U A . The power of the lens must be increased with<br />

increasing acceleration voltage to maintain the focus distance.<br />

13<br />

−−−−−−−−−−−−−−− 25 −−−−−−−−−−−−−−− 26 −−−−−−−−−−−−−−− 27 −−−−−−−−−−−−−−− 28 −−−−−−−−−−−−−−− 29 −−−−−−−−−−−−−−− 30 −−−−−−−−−−−−−−− 31 −−−−−−−−−−−−−−−

B C M Y X slurB B C M Y X B 20 B C M Y X B 40 B 80 B C M Y X CMY B C M Y X slurX B C M Y X C 20 B C M Y X C 40 C 80 B<br />

It should be noted that the exact path of the electrons from the cathode to the workpiece is in<br />

reality rather more complex than just described. Just as with light optics, beam aberrations<br />

influence the shape and position of the beam focus. In addition to stray electrical and magnetic<br />

interference fields in the beam gun and focus lens, the aberrations are caused by the physics of<br />

electron beam generation and transportation (thermal expansion, space-charge effect, aperture<br />

angle error, astigmatism, scattering effects, etc.). With the exception of astigmatism, electron beam<br />

is not corrected because the other aberration corrections are very expensive and these errors hardly<br />

affect the welding process. For astigmatism correction, see section 3.1.2.<br />

Figure 2-13. Electron beam focused by an electromagnetic<br />

lens.<br />

14<br />

Figure 2-14. Example of the effect of changing the<br />

accelerating voltage U A on the relationship between<br />

the focus distance A F and the lens current I L

3 Shaping and deflecting the electron beam<br />

3.1 Static shaping and deflecting<br />

Downstream of the anode there are several coil systems which interact via electromagnetic fields<br />

with the negative charge carriers to shape and deflect the electron beam, Figure 3-1.<br />

3.1.1 Centring<br />

Figure 3-1. Beam gun with coil systems for<br />

centring, astigmatism correction, focussing,<br />

and deflection<br />

For the best welding results the incoming electron beam should be normal to the surface of the<br />

workpiece, which only occurs if the axis of the electron beam is aligned with the electron-optical<br />

axis of the focus lens. To calibrate this electron beam parameter, it is first “through focused”, which<br />

involves modifying the lens current so that the beam goes from upper to under focus or vice versa<br />

while observing the beam spot on a target. The condition for a vertical beam axis is when the beam<br />

spot maintains its location when changing the focus. The beam is eccentric if the beam spot moves<br />

to the side and changes its shape, Figure 3-2. The electron beam can be brought back to a vertical<br />

position using a centring coil system consisting of four coils with unequal polarity arranged<br />

opposite each other, Figure 3-3. These coils steer the beam with respect to the vertical axis until the<br />

beam spot no longer shifts when changing the focus. During process operation the centring currents<br />

are usually adjusted only if a beam check requires it.<br />

15<br />

−−−−−−−−−−−−−−− 25 −−−−−−−−−−−−−−− 26 −−−−−−−−−−−−−−− 27 −−−−−−−−−−−−−−− 28 −−−−−−−−−−−−−−− 29 −−−−−−−−−−−−−−− 30 −−−−−−−−−−−−−−− 31 −−−−−−−−−−−−−−−

B C M Y X slurB B C M Y X B 20 B C M Y X B 40 B 80 B C M Y X CMY B C M Y X slurX B C M Y X C 20 B C M Y X C 40 C 80 B<br />

Figure 3-2. Spot diameter variation in upper focus<br />

(d FÜ ) and under focus (d FU )<br />

a) eccentric and oblique beam axis<br />

b) centric and vertical beam axis<br />

3.1.2 Astigmatism correction<br />

16<br />

Figure 3-3. Coil system for beam centring<br />

At its focus the electron beam hits the workpiece surface like a cone standing on its tip. The beam<br />

in-plane angle is known as the divergence, convergence or aperture angle, Figure 3-4. According to<br />

[3-1], the focused beam angle is measured between its outer limits (asymptotes). If this aperture<br />

angle is not the same in all axial beam levels there is an astigmatic aberration which will adversely<br />

affect the possibility of an imperfection-free weld seam. The electron paths do not meet together in<br />

a well-defined spot but form elliptically shaped and separately oriented ovals at different levels.<br />

Astigmatism ovals appear when monitoring the focus by changing from upper to under focus or<br />

vice versa (through focussing), Figure 3-5. To avoid this an astigmatism correction coil (stigmator)<br />

system is installed with pairs of coils arranged with the same poles facing each other in the beam<br />

gun chamber. It acts by shaping, that is pushing and pulling the electron paths, Figure 3-6, to restore<br />

a circular beam with a uniform aperture angle, Figure 3-5. The value of each coil current must be<br />

calibrated so as not to exceed a maximum value and over time may have to be increased to<br />

overcome impurity effects in the beam gun.

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