Table Of ContentElectron beam welding
Dipl.·lng. H. Schultz
ABINGTON PUBLISHING
WoodheadPublishingLtd inassociationwithTheWeldingInstitute
Cambridge England
Published byAbingtonPublishing,
WoodheadPublishingLimited, Abington Hall,Abington,
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FirstpublishedinGerman, 1989,DVS,ISBN3-87155-111-2
FirstpublishedinEnglish 1993,WoodheadPublishingLtd
Reprinted 2000,2002, 2003,2004
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SteigerwaldStrahltechnik
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Preface
The literature on electron beam welding isvery extensive and increasing at the
rate of about a hundred newpublications a year. In spite of this, there isas yet
no book which summarises all the processes and machines employed, and
describes the many uses to which these are put.
This book attempts to fill thisgap without aspiring to be a complete scientific
description of the mechanisms and processes involved. I have set myself the
task of explaining the trials, developments and manufacturing processes in
volved in electron beam welding in a practically orientated way aimed specifi
callyat the average engineer. In doing so, the current state of the technology is
described, not only with reference to the processes used but also including a
great deal of basic welding information which should help the engineer in his
own deliberations. Our highly developed and rapidly changing world demands
that today's engineer has a broad general knowledge to enable him as a
specialist to solve both present and future problems successfully.
I would like to express my thanks both to my numerous colleagues and to
the companies which have assisted me, and especially to Messer Griesheim
GmbH Steigerwald Strahltechnik/FRG, for their generous support and for
their advice in the drafting of the manuscript. I also wish to thank them for
sponsoring the translation of the book into English and thereby making it
accessible to an international readership.
My special thanks go posthumously to Dr mont Walther Hiller whose
valuable groundwork was an encouragement to me to write this book.
I would also like to thank the German Welding Society (DVS) and the
Dokumentation SchweiBtechnikfor their valuable assistance and willingness to
help in obtaining specialist literature.
Krailling, April1993 Helmut Schultz
vi Preface
List of abbreviations
Shortestdistance betweenporesor voids mm
Thicknessoffilletweld mm
Deviationof the electron beamfrom the jointor
ofthe focus from the surfaceofthe position
ofwelding in the X-, Y-and Z-directionsas
ameasureofthe accuracyofguidance mm
Weld convexity mm
Rootconvexity mm
Depthof undercut mm
Workingdistance mm
Focal distance mm
Widthof weld mm
Amplitudeofbeam oscillation mm
Widthof beamoscillation mm
Widthofweld gap mm
Widthof fusion zone at any position mm
Diameter mm
Beamspot diameter mm
Focal diameterofbeam mm
Diameterofcircumferentialweld mm
Beamdiameterat any position mm
Edge misalignment mm
Frequencyofoscillation Hz
Force N
Greatestcross sectional areaofporesor voids mm2
Areaof the beamspot at the pointofimpingement mm2
Areaoffocal spot ofbeam mm2
Gravitationalacceleration m S-2
Magneticfluxdensity V's'm-2
Heightofweld seam mm
Pulse pausetime ms
Pulse frequency Hz
Pulse duration ms
Pulse ratio
Beamimpingementcurrent rnA
Transmittedbeamcurrent rnA
Cathodeheatingcurrent A
Beamdeflectioncurrent rnA
Lens current A
Beamoscillatingcurrent rnA
Beamcurrent rnA
Continuousbeamcurrent rnA
Pulsedbeamcurrent rnA
Emissioncurrentdensity A·cm-2
Emissioncurrentdensity limited byspace charge A·cm-2
Listofabbreviations vii
jeT Temperaturedependentemission currentdensity A·cm?
k Distancebetweenthe cathodeand the control
electrode mm
I Length ofweldseam mm
2
L Power density W'cm-
PA Workingpressure, the pressure inthe working
chamber mbar
PE Pressureinthe beam generator mbar
P Beam power kW
Q Vacuum leakage rate mbar :dm3•S-I
R Brightness A·cm-2. sr-I
R. Centrelineroughness 11m
RK Brightnessat cathode A .ern"? .sr"
2.
R Radiance W'cm- sr"
L
Rp<J,2 0.2% proofstress N· mm?
Rw Brightnessat workpiece A .em"? .sr-1
s Depthoffusion zone mm
t Thicknessofworkpiece mm
t Strengthdeterminingthickness ofthe weld mm
t Time s
T Temperature °C;K
u Focalshift inthe axisofthe beam mm
U Acceleratingvoltage kV
B
U Controlvoltage kV
St
v Beam offset mm
v, Welding speed mm.S-I; m.min-I
V Volumeofworking chamber m3
X,Y,Z Systemofco-ordinates
z Crossovershifting mm
z Thicknessofthe centeringlip mm
U Apertureangle rd
U Tangentialangle to weldseam
UK Apertureangle towards cathode rd
Uw Apertureangle towardsworkpiece rd
p Angle ofdeflection °
t Density g:cm?
s
Gradientofthe integralcurve °
a Standarddeviation
CJ) Solidangle sr
viii Listof abbreviations
1 Introduction
1.1 History
Nowadays, the technical achievements whichmake lifeso convenient are taken
for granted, and the invention, development and testing carried out byour pre
decessors are often overlooked. Although an electron beam weldingmachine is
of course not an item of every day use, it can be used to manufacture products
which in one way or another add to the high standard of living experienced
today. Over a hundred years have passed since the first experiments by the
physicists Hittorf and Crookes to produce cathode rays in gases (1869)and to
use them to melt metals (1879) which have led to today's CNC manufacturing
machines for electron beam welding of jet engine components, to name just
one example of many.
Initially cathode rays were little more than an interesting physical phenom
enon which, in 1895,led to the discovery of one particular type of radiation by
Rontgen and which was described byThompson (1897)and Millikan (1905)as
'rapidly moved electrons'. As far as processing of material wasconcerned, such
beams were of no significance. Indeed quite the opposite wasthe case since, in
all the experiments carried out at that time, the heat produced by the collision
of the electrons with the anode was regarded as a great disadvantage and
continued attempts were made usingwater cooling to prevent the anode target
from being melted [1].ItwasMarcello von Pirani!'who firstthoughtof making
use of this effect byadapting acathode ray to construct a type of electron beam
furnace (Fig. 1)for melting tantalum powder and other metals, and whichwas
patented in 1905and 1907.
Fig.1 Extract from apatentbyMarcellovon Pirani regardingthe
'Productionof homogeneousbodiesfromtantalum orothermetals'
takenout onthe 26March 1907(1).
1)Pirani, M. V., 1880-1968,German physicist
Introduction
1
In the following decades many scientists became involved with electron
beams. Amongst others, Langmuir, Child, Richardson, Dushman and Wehnelt
investigated the laws governing the production of such beams, whilst Busch,
Rogowski, Flegler, Davisson, Calbrick and others worked on the basis of
electron optics. The first important uses of electron beams were in the con
struction of oscilloscopes and microscopes. Von Ardenne (1938)used them for
drilling metals, and together with Ruhle (1939) for melting and vaporising
metals. Sufficientlypowerful vacuum pumps were still unavailable, however, to
permit use of such beams in larger scale industrial applications.
A new epoch in the working of materials using electron beams began in
1948. It occurred to the physicist Steigerwald", who at this time was involved
indevelopmentofmore powerful beam sources for useinelectron microscopes,
that electron beams could be employed as thermic tools, in particular for
drilling precious stone bearings forwatches and clocks, for drillingwiredrawing
dies and for soldering, melting and welding metals under vacuum [2J.
The results ofthe firsttrials proved verypromising and led to the signingof a
licensing agreement with an interested American party. At that time electron
beam welding was thought of in the same terms as arc or gas welding, namely
as a process usinga suitable source of heat, whichhad the particular advantage
that gas sensitive metals were protected from reaction with the atmosphere.
The breakthrough in industrial electron beam welding came in 1958 with
the requirement to butt weld together 5mm thick plates of Zircaloy [3J. By
gradually increasing the beam current it finallybecame possible to completely
penetrate the full thickness of the workpiece to produce weld seams which, as
was required, were of considerably greater depth than they were wide. This
deep penetration welding process immediately drew worldwide interest, but it
was in the USA that its real technical significancewas most rapidly perceived.
Fig.2 Thefirstelectron beamwelding machine
for deepwelding(1958),Us= 55kV,Is= 20mA, at
present inthe DeutschesMuseum, Munich.
2)Steigerwald, K. H., 1920,German physicist
2 Electronbeam welding
Following this success Steigerwald sold two such electron beam machines. One
was installed in Pittsburg/USA for welding submarine components, whilst the
other was used for many years in industry in Germany, and today can be seen
in the Deutsche Museum in Munich (Fig. 2).
After the discovery of the deep welding effect an upsurge in development of
new machines occurred, in particular in France and Great Britain [4J. Initially
the electron beam had been considered only in relation to the surface of the
material, but now attempts were made to increase the power density and beam
current in order to be able to weld even thicker components. The nuclear and
aerospace industries were the first to control the use of electron beam welding.
Amongst the milestones in the subsequent development of the process and
equipment were coupling of the high voltages in the beam chamberwithout the
use of insulating oil, changing the cathode using a clamping system, isolating
the vacuum in the beam gun from that in the working chamber, the con
struction of welding machines with much higher beam powers and larger
working chambers, and the use of cycle type welding machines for mass
production techniques. These all led to a multitude of new applications for
electron beam welding. Today, even specialists have difficulty in appreciating
the wide field of use of electron beam welding in materials processing.
1.2 Special characteristicsof electron beam welding
As an introduction, a number of the special characteristics of electron beam
welding are summarised and discussed in greater detail in the following sec
tions. Incomparison with other methods of joining, electron beam welding is
characterised by:
- an extremely high power density of about 107W.em-2 at the focus of
the beam, Table 1;
- energy transfer occurs not by conduction of heat across the surface of
the workpiece, but much more efficiently within the workpiece itself;
- as no edge preparation is necessary, regardless of the thickness of
workpiece, filler metal is not required;
- the high welding speed results in narrow welds and heat affected zones
with little distortion of the workpiece;
- inertia free oscillation of the electron beam makes it possible in many
cases to join materials otherwise considered unsuitable for welding;
- the variable working distance allows workpieces of widely differing
shapes to be welded;
- as welding iscarried out under vacuum, no consumables (gases, fluxes)
are required to protect the weld pool from oxidation;
- short evacuating times can be achieved by adapting the working
chamber to suit the number and size of the workpieces; ,
- computer monitoring and control of the electrical and mechanical
welding parameters is possible;
- the welding parameters, and thus the quality of the welds produced,
are highly reproducible and consistent;
Introduction 3
- at accelerating voltages above 60kV lead shielding is directly bonded
to the welding machine in order to prevent X-ray emission.
Table1 The maximum power densities ofthevarious heat
sourcesused forwelding
Maximum power density
Heat source W·cm?
Gas ftame
Electricarc
Plasma
Laser beam (continuous)
Electronbeam
Unlike other manufacturing processes, a relatively limited range of electron
beam welding machines is capable of meeting the various power requirements
of the most diverse applications:
- beam powers of far less than 1kW up to 300kW are available for
welding material thicknesses of less than O.5mm up to 300mm [5];
- machines are available for weldingvariable one-offcomponents aswell
as for use in massproduction operations as in the automotive industry;
- simple longitudinal weld seams can be welded as well as complicated
three dimensional components requiring use of programmed welding
parameters and workpiece manipulation;
- structural steels, alloy steels, non-ferrous metals and even gassensitive
special metals can be successfullywelded.
Unfortunately there are no statistics available showing exactly how many
electron beam welding machines are inuseworld-wide. In1979itwasestimated
that 3000machines were in use [6].Itcan be said with certainty, however, that
the economic viability of every electron beam welding machine sold today is
proven, and that it isthis singlefact whichisthe best possible argument for the
use of this process.
1.3 Other beamwelding processes
Laser beams, best known for their applications in measurement, eye surgery
and for data transfer, etc, have now also found a place in cutting and welding
technology. A laser produces a monochromatic (one wavelength) and coherent
(in phase) beam of light with the aid of particular laser active media (gases,
liquids, solids). The medium is excited to a higher energy state such that an
incident quantum of light will cause emission. Mirrors are then used both to
arrange and amplify the stimulated emission into a collimated beam. One of
the properties of a laser beam resulting from this method of producing light
is the extremely low divergence of the beam. The laser beam can thus be
transmitted across large distances and can be focused to very small diameters
4 Electronbeam welding
with high power densities. Thus, usingthe deep weldingeffect, laser beams are
also capable of producing weld seams which are considerable deeper than they
are wide. However, how in detail the energy of the laser beam is transferred
into the material is not yet fully understood. An electron transfers its kinetic
energy to the material over a distance of about 0.01mm, largely in the form
of heat, and the same process appears to occur in the case of laser beams, but
within a distance of as little as 0.01urn. In addition, processes such as the
absorption occurring inthe ionised metal vapour (plasma) alsoplayasignificant
role in transferring energy.
For welding applications, CO is the main laser active medium. Incontrast
2
to solid media, it is particularly suited to producing high continuous beam
powers. Laser welding is normally carried out in air. Therefore there is no
need for a working chamber which has to be evacuated. But it isnecessary, as
with other welding processes, to protect the weld pool from ingress of the
surrounding atmosphere. Inlaser welding, both argon and helium separatelyor
a mixture of both are used as the shielding gas. Even without an evacuated
working chamber, the capital cost of high power laser welding machines is
considerable, so that most laser welding machines at present in use in the
automotive and electro-technical industry have a beam power of less than
5kW.
Prior to the introduction of laser beam welding, light beam soldering and
welding was also of some importance. The light was produced by a gas dis
charge lamp, collimated and focused on to the workpiece with the aid of
elliptical mirrors. The relatively poor ability to focus such light, even with
beam powers of 5kW, permitted only thin components of ca 1mm thickness to
be soldered or welded [7].
Other trials, for example to weld with ion beams, have never progressed
beyond the laboratory stage and today are no longer of any real practical
significance [8].
Introduction 5
