Leseprobe_180017

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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.

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