Leseprobe_180017

List of some abbreviations
* divergence angle (full angle) rd oder °
β beam angle of deflection rd oder °
a fillet weld thickness mm
A W ** working distance mm
A F ** focus distance mm
b*** weld seam width, nominal mm
b P oscillation width mm
b S gap width mm
d*** maximum dimension of an imperfection (pore, cavity) mm
d FL beam spot diameter mm
d Fo beam focus diameter mm
F force N
f*** maximum pore or cavity cross-sectional area mm 2
f oscillation frequency Hz
h weld seam height mm
h*** size of imperfection (height, depth, edge misalignment) mm
I B ** beam current mA
I BD continuous beam current mA
I BP impulse beam current mA
I d deflection current mA
I D transmitted current mA
I H heating current A
I L ** lens current mA
j e emission current density A/cm 2
j eT temperature dependent current density A/cm 2
l*** maximum imperfection dimension mm
l centre lip length mm
L beam power density W/cm 2
m** load on the work table or rotating device kg
P beam power kW
p A pressure in working chamber mbar
p E pressure in beam gun mbar
Q** leak rate mbar/dm 3 · s
s*** depth of fusion zone mm
s 1 *** depth of the melted zone for a T-joint mm
SR slew rate mA/ms, °/ms
t time s
t weld thickness, sufficient for strength mm
t*** workpiece thickness mm
TIG tungsten inert gas (welding) –
t V pulse ratio –
U A ** accelerating voltage kV
U St bias voltage (Wehnelt voltage) kV
v** welding speed mm/s, cm/min
V volume m 3 , dm 3
z centre lip thickness mm
According to: * DIN 32533, ** ISO 14744, *** ISO 13919

1 Introduction
1.1 History
Modern society takes for granted the technical achievements that make life easier, and may
overlook that most useful things were laboriously invented, developed and tested by many different
ingenious predecessors. Although an electron beam welding equipment is not an item of our
everyday life, the products manufactured with it contribute to our high standard of living. It has
been a long road from the pioneering work of British physicists Hittorf and Crookes from 1871 to
1872, generating cathode rays in gases and melting metals, to the modern computer-controlled
manufacturing equipment in the aviation and aerospace industry [1-1].
Cathode rays were first described as “fast-moving electrons” by their discoverers Wilhelm Röntgen
(1895), Thompson (1897) and Milikan (1905). They were viewed as a special type of radiation and
an interesting physical phenomenon but they were not considered as relevant for material
processing. On the contrary, in all these experiments, the heat generated by the collision of electrons
on the anode or the target were a great inconvenience and water cooling was used to prevent
melting [1-2]. Marcello von Pirani was the first to take advantage of this effect, building an X-ray
tube in an electron beam vacuum furnace to melt tantalum powder and other metals, patenting the
process in 1905 and 1907, Figures 1-1 and 1-2.
Figure 1-1. Excerpt from the Marcello
von Pirani patent “Production of homogenous
bodies from tantalum or other
metals” dated 26.3.1907
Figure 1-2. Marcello von Pirani, German physicist,
1880 – 1968
In the following decades, many scientists investigated the properties of electron beams. Among others,
Langmuir, Child, Dushman and Wehnelt explored the parameters of beam generation, while Bush,
Rogowski, Flegler, Davisson and Calbrik, just to name a few, worked out the basics of electron optics.
The first technically meaningful electron beam applications were in oscilloscopes and electron
microscopes. Although von Ardenne and Rühle (1938) began to drill metals by evaporation and
melting, a larger industrial application was not possible due to the lack of sufficiently powerful
vacuum pumps.
1

In 1949 the German physicist K. H. Steigerwald ushered in a new epoch of material processing
with electron beams, Figure 1-3. He was engaged in the development of high performance electron
microscopes and had the idea that the electron beam could be used in vacuum as a thermal tool for
drilling watchmakers’ jewels and extrusion dies, and also for brazing, melting and welding under
vacuum, Figure 1-4 [1-3; 1-4].
Figure 1-3. Karl-Heinz Steigerwald,
German physicist, 1920 – 2001
2
Figure 1-4. 1952 Steigerwald developed the
first electron beam drilling machine
The first experimental results proved very promising, leading to a license agreement with an
American investor [1-5]. At this time it was thought that the only advantage that the heat input from
electron beam welding would have over an electric arc or gas flame (for thermal conduction
welding), would be that gas-sensitive materials such as niobium, tantalum or titanium would be
protected by the vacuum from reactions with the atmosphere. In 1958 there was a breakthrough for
industrial electron beam welding when Steigerwald successfully made a butt weld in 5 mm thick
Zircaloy [1-6]. Using gradually increasing beam current, very deep but still unexpectedly narrow
weld seams were generated. Although these deep welds awakened a new worldwide interest, the
technical importance of the procedure was first recognised in the United States. In Germany,
Steigerwald successfully built the first two electron beam welding machines, one of which was
delivered to Pittsburgh, USA, for welding submarine components, while the other was operated for
many years in Germany.
After the discovery of the deep welding effect, a vigorous development of new equipment [1-7]
began, in particular in France and the United Kingdom. Since the electron beam had been only able
to weld thin workpieces, the impetus was to increase the power density and beam current to be able
to increase the seam thickness. The first practical applications for electron beam welding were in the
aerospace and nuclear industries.
Milestones of technical developments in process and equipment include:
– a high-voltage cable connection without insulation oil in the beam gun
– precision exchange of the cathode heating ribbon using a clamping cartridge
– beam gun vacuum separated from the working chamber vacuum

– welding equipment with larger working chamber and vacuum pumps
– transfer and cycle welding equipment for mass production
These improvements enabled many new electron beam welding applications. Today even a
specialist may find it difficult to get a complete overview of the extent of electron beam processing
in industrialised countries.
1.2 Special characteristics of electron beam welding processes
A number of special features and benefits of electron beam processing in comparison to other
methods of welding by melting is shown in Table 1-1.
Table 1-1. Technical characteristics
Characteristic
Description
Power density
Very high power density, more than 10 5 W/mm 2 in beam focus
Beam power
0.5 to 300 kW in research and development, 1 to 30 kW in industrial use
Thermal input
Kinetic energy of accelerated electrons converted into heat with material
vaporisation (deep welding effect). Not heat conduction welding
Welding environment Normally in vacuum; shielding gases used for welding in atmosphere
Controlling
Control of mechanics and welding parameters with computer. Automatic
beam and focusing correction and seam tracking with electron optical
camera systems
Weld depth
0.5 to 100 mm in a single pass, from 3 to 30 kW respectively
Edge preparation
Butt welds for all depths without bevelled edges
Seam shape
Narrow melting and heat affected zones. Seam width to depth ratio: from
1:10 to 1:50
Workpiece and beam motion Welding of longitudinal, circular and 3-D curved seams with computerprogrammed
work piece and beam motion
Energy distribution
Control of melting and solidification processes of the weld by inertia-less
movement of the electron beam in many oscillation shapes, directions and
frequencies
Multi-beam technology Many material processes are possible using extremely fast electron beam
motion in the kHz range. Almost simultaneous, but locally separated beams
appear for tacking, welding and heat treatment
Filler material
Since no filler material requirement, workpiece preparation is reduced.
Exceptions: deposit welding and metallurgical conditions
Weldable metals
Large range of suitable welding metals: steels, alloyed steels, non-ferrous
metals (aluminium, copper, titanium, etc.)
Distortion
Lower longitudinal, transverse and angular distortion in comparison to
other fusion welding processes. Since the welds have so little distortion it
is possible to maintain small tolerances. Very little or no reworking of the
workpiece needed
3

Table 1-1. Continued
Characteristic
Welding seam access
Workpiece construction
Working chamber
Quality assurance
Description
Possible to weld even difficult open seams, for example with narrow gaps
(1 to 2 mm wide) with variable working distance (in practice about 50 to
1000 mm) possible
In order to reduce manufacturing costs, complex components can be
divided into structurally simple parts and welded to final or near-final
dimensions
It is possible to adapt the working chamber shape, size and the quantity of
workpieces to reduce the evacuation time to a few seconds or zero (with
continuous systems)
The quality is controlled due to the high reducibility and stability of the
welding parameters and automatic monitoring of welding data
Electron beam welding equipment construction designs can be varied according to requirements to
suit diverse production tasks, Table 1-2.
Table 1-2. Classification of electron beam welding equipment 1) concepts
Identifying feature Classification Remarks, application examples
Accelerating voltage
Up to 60 kV
100 kV to 150 kV
Up to 175 kV
Low-voltage 2) equipment
High-voltage 2) equipment
Atmosphere equipment
Working pressure
Workpiece shape and quantity
Beam gun
1)
2)
About 10 –4 mbar
About 10 –2 mbar
Universal equipment
Cycle and transfer equipment
Multi-chamber equipment
Continuous equipment
Fixed on the working chamber
In the working chamber
applications associated with motion
devices; beam gun performs all or
part of the weld feed motion
High vacuum 2) equipment
Partial vacuum equipment
Gear parts
Small parts
Bimetallic blades (for sawblades
production)
For particularly large components
Although they are often called “machines”, we will refer to them in this book as “equipment” to remind
us of their complicated combination of machines
Commonly used, but not the terms used in standards
Electron beam welding has been utilised successful in industry for almost 60 years. Today it
competes strongly with other welding processes due to its flexibility, reliability and profitability.
1.3 Other beam welding processes
In early laboratory tests, other beam welding processes, such as incoherent light beam or ion beam
welding were investigated but they remain without practical application [1-8].
4

In contrast, the laser beam is already known in data transmission, communication technology,
medicine, instrumentation, etc. It has become important for cutting and welding and finds extensive
application in surface layer modifications. The laser light, being monochromatic (single wavelength)
and coherent (in phase), can be produced using various media (gas, liquid and solid
phases). In contrast to electrons, which transform most of their kinetic energy in the form of heat
in the first 0.06 mm of materials, the laser beam is absorbed in a thin surface layer of only 0.01
µm. Moreover, in addition to other effects, the laser light is reflected and absorbed in the ionised
metal vapour (plasma plume) above the weld, reducing the energy transfer and overall efficiency
compared to electron beam welding [1-9].
Like the electron beam, the laser beam focus can reach power densities of 10 5 W/mm 2 , and is also
used for deep welding to produce seams which are much deeper than wide. Since the laser beam
also has an extremely low beam divergence due to its particular light characteristics, it can travel
over large distances without significant beam diameter enlargement. The advantage for the laser
beam is that it can normally be used for welding or cutting in atmosphere. This fact is always emphasised
when comparisons are made with electron beam welding, which usually takes place in a
vacuum. Although laser beam welding does not require a vacuum working chamber, along with
other thermal joining processes, the melt must be protected from contact with oxygen and nitrogen
using argon and/or helium inert gas mixtures.
Recent studies have demonstrated that laser beams can also be used in vacuum. At operating pressures
from 10 to 50 mbar, narrower and deeper welds can be achieved than under atmosphere,
similar to the values achieved with electron beam welding [1-10]. While the weld pool dynamics
and microstructure solidification characteristics can be readily modified by oscillating the inertialess
electron beam at extremely high frequency, this is not possible with the laser beam and so
must be carried out by other measures.
[1-11] reports an investigation of pore frequency and distribution of laser and electron beam in
welds under vacuum of pure titanium and nickel. The laser beam seams were welded both at
working pressure of 10 –1 mbar and also under argon shielding gas. A comparable seam width was
achieved with an electron beam acceleration voltage of about 80 kV. Since the welds were all
partial penetration, the seams without under beads. It was found that both types of welding under
vacuum have very similar melting efficiencies, transferring similar amounts of energy into the base
material. The laser welds under vacuum showed much-reduced pore density than under the
shielding gas. The same results can be achieved with electron beam welding by implementing one
of the many possible combinations of beam oscillation and defocusing.
5

2 Generation of the electron beam
2.1 Free electrons
It is not obvious on first reflection about the fundamental physics of processing materials with a
beam of electrons that this lightest of atomic particles can provide enough kinetic energy for a heat
source. Like all atomic orders of magnitude, the numerical value of the electron rest mass of
9.1 · 10 –28 grams is beyond our comprehension (about 1836 times lighter than the proton with
opposite charge). For kinetic energy conversion the disadvantage of the low mass of the electron is
compensated for by its electrical charge. The numerical value of the elementary negative charge of
an electron at 1.6 · 10 –19 coulombs is the source of the beam current. The advantage of the electrical
charge is that by using magnetic fields one can readily accelerate the electrons to the highly
energetic velocities required for welding. As Figure 2-1 shows, in a vacuum of better than
10 –4 mbar, a typical welding accelerating voltage of U A = 150 kV = 1.5 · 10 5 V brings the electrons
up to a velocity of 2 · 10 8 m/s, about two-thirds the velocity of light. At this high velocity the
Newtonian kinetic energy of 1 / 2 m · v 2 is supplemented by a relativistic mass increase of about 35 %
[2-1].
6
Figure 2-1. Relationship
between accelerating voltage
and electron velocity
Electrons bound to atoms normally only occupy fixed energy orbits, called shells. Metals have high
electrical conductivity because the outer electrons are only weakly bonded to the atomic nuclei and
are able to move around quite freely in the atomic crystal lattice (conduction electrons). These
electrons are not normally available for welding because they cannot leave the metal surface. The
potential threshold binding them to the metal lattice can be overcome by applying additional energy
in the form of a heat input leading to a temperature rise. When the energy of the free electrons
increases sufficiently, they overcome the potential threshold and collect near the metal surface,
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
for the electrons due to the mutual electrical attraction forces. The metal binds the electrons to itself
and so cannot deliver the negatively charged electrons, as it would then become positively charged.
2.2 Cathode
Figure 2-2. Hot metal causing free electrons emission
The cathode has a number of other tasks to fulfil besides providing electrons. It should have
minimum heating power with a long service life and provide a high beam current by emitting a
maximum number of electrons. The cathode is normally made from a high temperature material
since, according to the Richardson thermionic emission law [O. W. Richardson, English physicist,
1879 – 1959], the electron flow increases rapidly with the temperature of the emitter. Tungsten,
often alloyed with rhenium, is the preferred material as it has a low vapour pressure in vacuum and
a long cathode service life. Directly heated tungsten cathodes in electron beam welding equipment
can achieve emission current density of j e = 5 A/cm 2 .
Figure 2-3. Example of the shape and dimensions (mm) of a ribbon cathode for
beam powers up to P = 15 kW
7
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
The heat input needed to bring the cathode to the desired temperature depends on the dimensions
and form of the cathode and the heating method. Ribbon cathodes, Figure 2-3, are connected as an
ohmic resistor in a circuit and heated directly by passing a current, I H directly through them. They
are the most common cathode type in electron beam welding equipment, providing stable beam
characteristics with a high heating efficiency by using a clamp cartridge to ensure a geometrically
stable emission surface position. They are practically the only consumable in an electron beam gun
and can be quickly and easily exchanged (section 10.3). After switching off the heating current,
directly heated ribbon cathodes cool down quickly due to their low mass, which is necessary to
prevent oxidation to allow rapid venting of the beam gun.
8
Figure 2-4. Indirectly heated cathode
Indirectly heated cathodes are heated by an auxiliary cathode providing electron bombardment.
Because they are not heated by a large electric current, they can be made from solid with a more
adaptable design. Indirectly heated cathodes, Figure 2-4, can operate with lower acceleration
voltages, and are mainly included in electron beam welding equipment with high beam currents.
They can be easily refurbished and have a longer lifetime than ribbon cathodes. However, a
disadvantage is that the rapid wear of the auxiliary cathode necessitates more frequent exchange
than the indirectly heated cathode itself.
2.3 Anode
The freely moving electrons thermally emitted from the cathode are insufficient for electron beam
welding. The electrons must be accelerated to a high velocity to reach the required kinetic energy by
the cathode, which is at a high negative acceleration voltage. An anode at earth potential, situated
away from the cathode, attracts the electron cloud, Figure 2-5. The electric field between the
cathode and anode accelerates the electrons and gives them the required kinetic energy. A highvoltage
generator powers the cathode continuously with new electrons and therefore with current.
This electrons flow has a safe return current with a closed circuit to the high voltage generator while
the workpiece and the machine are connected to earth potential.
In the arrangement shown in Figure 2-5, the accelerated electrons would impact with high kinetic
energy onto the anode instead of the workpiece to be welded. Therefore, a hole is included in the
anode, through which the electrons can pass uninterrupted at high velocity to the workpiece.

2.4 Bias cup
Figure 2-5. Accelerated electrons
in a diode system
The simplest construction of beam gun has only a cathode and anode. Although this diode system,
called the Pierce system [G. W. Pierce, German physicist, 1872 – 1956] was used for some
equipment in the early years of electron beam welding, it has one significant drawback: the electron
stream can only be controlled by changing either the acceleration voltage or the cathode temperature,
which is completely unsuitable for industrial welding. A third electrode with a higher and
separately controllable negative voltage than the cathode surrounds the cathode and significantly
improves the electron beam adjustment and current. The electrons can overcome the potential
difference between the cathode and anode following the law of physics of mutual rejection
of charges of the same polarity. This third electrode is called bias cup or Wehnelt cylinder
[A. R. Wehnelt, German physicist, 1871 – 1944] Together with the anode and cathode it is
nowadays the practically universally employed triode system, Figure 2-6.
Figure 2-6. Cathode, anode and bias
cup
9
−−−−−−−−−−−−−− 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
Figure 2-7. Graph showing how the cathode emission area modifies the beam current I B with the bias voltage
U St
a) beam current I B = 0
b) low beam current I B
c) high beam current I B
d) electron-optic beam distortion due to the bias voltage U St being too low
e) dependence of the beam current I B on the bias voltage U St
10

A sufficiently high bias voltage can completely block the beam current I B such that the cathode
emits no electrons, Figure 2-7a. As the bias voltage is reduced, the emission area increases in size
and the beam current I B increases, Figure 2-7b and 2-7c. For welding, the adjustment of the beam
current I B is thus independent of the acceleration voltage U A and the cathode temperature and is
only controlled by the bias voltage U St with an inverse relationship: high bias voltage = low beam
current, low bias voltage = high beam current, Figure 2-7e. The emission surface on the cathode
must be placed at a sufficient distance from the ribbon edges and bends, otherwise the cathode
flanks emit also electrons. Figure 2-7d. The consequences are an excessive beam divergence,
significant distortion of the beam and a lack of rotational symmetry.
The shape of the beam in a typical triode system in an electron beam welding equipment is
indicated in Figure 2-8. Note that a constriction of the electron beam, called real crossover, occurs
underneath the cathode emission surface as a result of both, the geometry of the bias cup and the
bias voltage. The electron beam passes through the anode bore and reaches field-free space where
its diameter increases constantly due to the electrons, with equal negative charge, being mutually
repulsed. A backwards projection of the beam tangents creates a further point of intersection with
the beam axis, called virtual crossover for electron microscopy calculations. Figure 2-9 shows
enlarged the paths of electrons under the influence of the field distribution. You can see how the real
crossover is in reality without a unique point of intersection but is rather a region with lines crossing
the axis at different points. From this calculations can be made to show that the electrons are
accelerated to slightly different velocities.
Figure 2-8. Electron beam geometry in a triode
system
Figure 2-9. Electron beam with real and virtual
crossovers
The focal length of a beam system has the same definition in electron optics as in light optics. In
particular at the beginning of the electron beam process development there was a differentiation
between short focal length systems (Rogowski short focal) [W. Rogowski, German physicist,
1881 – 1947] and long focal length systems (Steigerwald long focal). Most modern electron beam
welding equipment are installed with modified Rogowski systems.
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
11

−−−− M = M −−−−−−−−−−−−− 18 −−−−−−−−−−−−−− Y = Y −−−−−−−−−−−−−− 20 −−−−−−−−−−−−−− X = X −−−−−−−−−−−−−− 22 −−−−−−−−−−−−−−− 23 −−−−−−−−−−−−−−− 24
2.5 Space charge effects
The physics principles on which the electrons move between the cathode, anode and the bias cup
are very complex and need not be explained in detail to understand the later technical points. Only
one relationship is worth highlighting, that is for the adjustment of the cathode heating current I H .
During operation the cathode has a finite service life because it experiences a small but significant
material removal by sputtering and evaporation. Although this cathode temperature (about 2800°C)
is still well below the melting temperature (approximately 3200°C), the evaporation rate is
enhanced by the high vacuum. Sputtering occurs due to the impact of positively charged ions via the
ion return current (ion back-streaming) produced by the collision of the electrons on their way to the
weld with the remaining gas molecules, producing metal vapour, Figure 2-10.
This slow material removal decreases the profile of the ribbon cathode and its emitting area,
reducing the heating current I H (at a constant supply voltage), thus reducing the cathode temperature.
According to Richardson’s law, the emission current density j eT decreases rapidly as the cathode
temperature T falls, so a quite low temperature change will significantly affect the value of the beam
current I B , Figure 2-11. Such quality variations would be unacceptable for electron beam welding
with high requirements for seam quality reproducibility.
Figure 2-10. Ribbon cathode emission area damaged
by ion impacts
12
Figure 2-11. Strong dependency of emission current
density j eT on cathode temperature T
We may assume that the acceleration voltage U A , is specified with a maximum value dependent on
geometrical parameters such as the distance between cathode and anode. The equation of the
Langmuir law of physics [J. Langmuir, American chemist and physicist, 1881 – 1959] states that
the emission current J eR from the cathode is space-charge limited with only two variables, U A and
the distance from anode to cathode . It is therefore necessary to select enough cathode heating
current I H to achieve a high enough temperature to make available a sufficient number of electrons
at maximum acceleration voltage; that is j eT and thus the cathode heating temperature can be
limited. The cathode will be surrounded by a cloud of "surplus" electrons which have their own
charge and restrict any further electron emission so that the cathode is affected by an electric
space-charge field. The graph in Figure 2-12 shows that when above a certain cathode temperature,
the beam emission current is unaffected by fluctuations in cathode temperature.

Figure 2-12. Definition of space-charge area
In practice, automatic control and regulation devices facilitate setting and monitoring of the
heating current I H (section 7.6). At the same time, these controls verify that the life of the cathode
is not affected by excessive temperature. As already mentioned the cathode life is more influenced
by the vacuum inside the beam gun chamber, the beam current and the material to be welded. A
cathode life of 8 to 10 hours is normal if we measure only the cathode heating time during actual
welding. However, if the entire lifecycle is taken into account, including all pauses in welding
when the cathode temperature is reduced, 60 to 100 hours is the average.
2.6 Focussing lens
The geometric shapes of the cathode, bias cup and anode influence the more or less curved
trajectories of electrons not only inside the beam gun but also further away towards the welding
zone. As already mentioned, the electric field in the triode system shapes the beam to the first
crossover focus, while the electrons are mutually negatively repulsed, passing through the anode,
Figure 2-8. The electrons are accelerated after passing the anode to their final velocity, but the
beam lacks the necessary power density for welding and so has still to be focused.
The trajectories of the electrons are focused onto the workpiece surface by the magnetic field of a
ring coil which consists of many copper windings encased on three sides by high permeability iron,
Figure 2-13. A direct current passes through the ring windings, which generate a magnetic field
that acts in the middle of the coil and focuses the electron beam like an optical convex lens. In
order to achieve the small diameter focus of 0.1 to 1.0 mm (depending on beam performance and
focus distance) required for deep electron beam welding, the magnetic lens induces curved, largeradius
spiral paths which do not affect the electron velocity. The focus is not a mathematically
exact point but has the appearance of a circular surface, referred to as the beam spot, which is the
precondition for the required power density of L > 10 5 W/mm 2 for welding metals with a thickness
of several centimetres.
The beam focus can be adjusted onto the surface of the workpiece to be welded with the lens
current I L . The melt process may also be modified by focussing the beam slightly above or below
the surface (Chapter 6). Figure 2-14 shows the dependence of the focus distance A F on the lens
current I L for two typical acceleration voltages U A . The power of the lens must be increased with
increasing acceleration voltage to maintain the focus distance.
13
−−−−−−−−−−−−−−− 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
It should be noted that the exact path of the electrons from the cathode to the workpiece is in
reality rather more complex than just described. Just as with light optics, beam aberrations
influence the shape and position of the beam focus. In addition to stray electrical and magnetic
interference fields in the beam gun and focus lens, the aberrations are caused by the physics of
electron beam generation and transportation (thermal expansion, space-charge effect, aperture
angle error, astigmatism, scattering effects, etc.). With the exception of astigmatism, electron beam
is not corrected because the other aberration corrections are very expensive and these errors hardly
affect the welding process. For astigmatism correction, see section 3.1.2.
Figure 2-13. Electron beam focused by an electromagnetic
lens.
14
Figure 2-14. Example of the effect of changing the
accelerating voltage U A on the relationship between
the focus distance A F and the lens current I L

3 Shaping and deflecting the electron beam
3.1 Static shaping and deflecting
Downstream of the anode there are several coil systems which interact via electromagnetic fields
with the negative charge carriers to shape and deflect the electron beam, Figure 3-1.
3.1.1 Centring
Figure 3-1. Beam gun with coil systems for
centring, astigmatism correction, focussing,
and deflection
For the best welding results the incoming electron beam should be normal to the surface of the
workpiece, which only occurs if the axis of the electron beam is aligned with the electron-optical
axis of the focus lens. To calibrate this electron beam parameter, it is first “through focused”, which
involves modifying the lens current so that the beam goes from upper to under focus or vice versa
while observing the beam spot on a target. The condition for a vertical beam axis is when the beam
spot maintains its location when changing the focus. The beam is eccentric if the beam spot moves
to the side and changes its shape, Figure 3-2. The electron beam can be brought back to a vertical
position using a centring coil system consisting of four coils with unequal polarity arranged
opposite each other, Figure 3-3. These coils steer the beam with respect to the vertical axis until the
beam spot no longer shifts when changing the focus. During process operation the centring currents
are usually adjusted only if a beam check requires it.
15
−−−−−−−−−−−−−−− 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
Figure 3-2. Spot diameter variation in upper focus
(d FÜ ) and under focus (d FU )
a) eccentric and oblique beam axis
b) centric and vertical beam axis
3.1.2 Astigmatism correction
16
Figure 3-3. Coil system for beam centring
At its focus the electron beam hits the workpiece surface like a cone standing on its tip. The beam
in-plane angle is known as the divergence, convergence or aperture angle, Figure 3-4. According to
[3-1], the focused beam angle is measured between its outer limits (asymptotes). If this aperture
angle is not the same in all axial beam levels there is an astigmatic aberration which will adversely
affect the possibility of an imperfection-free weld seam. The electron paths do not meet together in
a well-defined spot but form elliptically shaped and separately oriented ovals at different levels.
Astigmatism ovals appear when monitoring the focus by changing from upper to under focus or
vice versa (through focussing), Figure 3-5. To avoid this an astigmatism correction coil (stigmator)
system is installed with pairs of coils arranged with the same poles facing each other in the beam
gun chamber. It acts by shaping, that is pushing and pulling the electron paths, Figure 3-6, to restore
a circular beam with a uniform aperture angle, Figure 3-5. The value of each coil current must be
calibrated so as not to exceed a maximum value and over time may have to be increased to
overcome impurity effects in the beam gun.