Download Electron Beam Welding (EBW)

Electron Beam Welding (EBW)
Professor Pedro Vilaça *
Materials Joining and NDT
* Contacts
Address: P.O. Box 14200, FI-00076 Aalto, Finland
Visiting address: Puumiehenkuja 3, Espoo
[email protected] ; Skype: fsweldone
January 2015
Electron Beam welding (EBW)
Process Overview
• EBW is a fusion joining process that produces coalescence of materials with heat
obtained by impinging a beam composed primarily of high energy electrons onto
the joint to be welded: transformation of Mechanical energy into Heat energy
Principles of Operation
• The heart of the EBW process is the electron beam gun/column assembly
• Electrons are generated by heating a negatively charged emitting material to its
thermionic emission temperature range, causing electrons to "boil off" this emitter
or cathode and be attracted to the positively charged anode.
• Bias cup surrounding the emitter provides the electrostatic field geometry that then
simultaneously accelerates and shapes these electrons into the beam. The beam
then exits the gun through an opening in the anode.
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Generation
Acceleration
Focusing
Guidance
Working zone
Simplified Representation of a Triode
Electron Beam Gun Column
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• Upon exiting the gun, this beam of electrons accelerates to speeds in the
range of 30 to 70 % of the speed of light, when gun operating voltages are
in the range of V = 25kV to 200kV.
• The mechanical power of a beam of electrons can be calculated from the
following analysis, based on the kinetic energy of the beam per unit of time:
Pkinetic 
E kinetic 1
2
 me  n   v e  
t
2
Typical Power are in the range of:
 1 to 5 kW
Where :
me  9.109  10
 31
kg
v e  v luz  3  10 m / s
8
typicaly : v e  0.3;0.7 v luz
n – number of electrons per unit of time
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In practice, the rate of Heat Input to the weld joint is controlled by the following
four basic variables:
1. Number of electrons per second being impinged on the workpiece (beam current)
2. Magnitude of velocity of these electrons (beam accelerating voltage)
3. Degree to which this beam is concentrated at workpiece (focal beam spot size)
4. Travel speed with which workpiece or electron beam is being moved (welding
speed)
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Selection Of Welding Variables
The rate of energy input to the workpiece during EBS is:
Energy input (heat input), J/mm
Where:
Heat input  
V = beam accelerating voltage, V
I = beam current, A
P = beam power, W or J/s
v = travel speed, mm/s
 = fusion efficiency
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P
V I
  beam
v
v
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Change in individual welding variables will affect the penetration and bead
geometry in the following manner:
• Accelerating voltage: as the accelerating voltage is increased, the depth of
penetration achievable will also increase. (V = 25 to 200 kV)
• Beam current: for any given accelerating voltage: the penetration achievable will
increase with beam current. (I = 0.05 to 1 A)
• Travel speed: for any given beam power level, the weld bead will become
narrow and the penetration will decrease as the travel speed is increased
• Beam Spot size: sharp focus of the beam will produce a narrow, parallel-sided
weld geometry because the effective beam power density will be maximum
(diameter = 0.25 to 0.75 mm)
Note: Defocusing the beam, either by over focusing or under focusing, will
increase the effective beam diameter and reduce beam power density
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• Equipment: Classifications of EBW Equipment
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• Equipment: Level of Vacuum in Gun and Working Cameras
Variants:
-3
-6
 High-Vacuum: 10 to 10 Torr
-3
 Fine-Vacuum: 25 to 10 Torr
 Non-vacuum (1 atm  760 Torr)
High-vacuum chamber equipment for EBW
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• Equipment: Level of Vacuum in Gun and Working Cameras.
High-vacuum, medium-vacuum, and non-vacuum (atmospheric pressure) EBW
equipment employs: i) electron beam gun / column assembly; ii) one or more vacuum
pumping systems; iii) power supply.
Gun
Working
camera
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• Equipment: Importance of Level of Vacuum in Gun and Working Cameras.
 Increased divergence of electron beam
 Reduce the speed of electrons by friction and collision with atmospheric particles
 To avoid oxidation of the cathode the compressions in beam source are always
needed and are always equal or higher than the ones in the working chamber
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• Equipment: Influence of Level of Vacuum in EBW Penetration.
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• Equipment: Influence of Level of Vacuum in EBW quality.
Fine-Vacuum
Non-vacuum
High-Vacuum
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Effect of Travel Speed on
Penetration of Non-vacuum
Electron Beam Welds in Steel
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• Equipment: Different machine concepts
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• Equipment: Different machine concepts
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• Equipment: Geometric shape and type of cathodes.
Filament
Tape
Helicoidally
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Spiral
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• Equipment: Material of cathodes.
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• Equipment: Sample of an electronic scanning for joint tracking.
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• Once the beam exits from the
gun, it will gradually broaden
with distance travelled
• In order to counteract this
inherent divergence effect, an
electromagnetic lens system is
used to converge the beam,
which focuses it into a small
spot on the workpiece
• The beam divergence and
convergence angles are
relatively small, which gives
the concentrated beam a long
usable focal range, or depth
of focus
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• The resulting beam power levels and power densities attainable from these units
can reach values as high as 100 kW and 1.55 x 104 W/mm². Such power densities
are significantly higher than those possible with arc welding processes
• At power densities on the order (1.55 x 10² W/mm²), and greater, the electron
beam is capable of instantly penetrating into a solid workpiece or a butt joint and
forming a vapour capillary (keyhole) which is surrounded by molten metal
• As the beam advances along the joint molten metal from the forward portion of the
keyhole flows around its periphery and solidifies at the rear to form weld metal.
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The weld penetration formed is much deeper than it is wide, and the heat affected
zone produced is very narrow. For example, the width of a butt weld in 13 mm thick
steel plate may be as small as 0.8 mm.
This stands in remarkable contrast to the weld zone produced in arc and gas
welded joints, where penetration is achieved primarily through conduction melting.
1. EBW
2. GTAW
3. OGW
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Advantages
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Advantages
The following advantages of electron beam welding:
- The EBW directly converts electrical energy into beam output mechanical energy.
Thus the process is extremely efficient
- Electron beam weldments exhibit a high depth-to-width ratio. This feature allows
for single-pass welding of thick joints
- The heat input per unit length for a given depth of penetration can be much lower
than with arc welding. The resulting narrow weld zone results in low distortion, and
fewer deleterious thermal effects
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- A high-purity environment (vacuum for welding) minimizes contamination of the
metal by oxygen and nitrogen
- The ability to project the beam over distance of several centimetres in vacuum
often allows welds to be made in otherwise inaccessible locations
- Rapid travel speeds are possible because of the high melting rates associated
with this concentrated heat source. This reduces welding time and increases
productivity and energy efficiency
- Reasonably square butt joints in both thick and relatively thin plates can be
welded in one pass without filler metal addition
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- Hermetic closures can be welded with the high or medium-vacuum modes of
operation while retaining a vacuum inside the component
- The beam of electrons can be magnetically deflected to produce various shape
welds and magnetically oscillated to improve weld quality or increase penetration
- The forced beam of electrons has a relatively long depth of focus, which will
accommodate a broad range of work distances
- Full penetration, single-pass welds with nearly parallel sides, and exhibiting nearly
symmetrical shrinkage, can be produced
- Dissimilar metals and metals with high thermal conductivity such as copper can be
welded
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Limitations
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Limitations
• Capital costs are substantially higher than those of arc welding equipment
depending on the volume of parts to be produced, however, the final per piece
part costs attainable with EBW can be highly competitive
• Preparation for welds with high depth-to-width ratio requires precision machining
of the joint edges, exacting joint alignment, and good fit-up
• The rapid solidification rates achieved can cause cracking in highly constrained,
low ferrite stainless steel
• For high and medium vacuum welding, work chamber size must be large enough
to accommodate the assembly operation. The time needed to evacuate the
chamber will influence production costs
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• Partial penetration welds with high depth-to-width ratios are susceptible to root
voids and porosity
• Because the electron beam is deflected by magnetic fields, non-magnetic or
properly degaussed metals should be used for tooling and fixturing close to the
beam path
• With the non-vacuum mode of electron beam welding, the restriction on work
distance from the bottom of the electron beam gun column to the work will limit the
product design in areas directly adjacent to the weld joint
• With all modes of EBW, radiation shielding must be maintained to ensure that there
is no exposure of personnel to the X-radiation generated by EB welding
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• Adequate ventilation is required with non vacuum EBW, to ensure proper removal
of ozone and other harmful gases formed during this mode of EB welding
•
• Full penetration, single-pass welds with nearly parallel sides, and exhibiting nearly
symmetrical shrinkage, can be produced
• Dissimilar metals and metals with high thermal conductivity such as copper can be
welded
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Characteristics of Welds
• Produces weld metal geometries that differ significantly from made by
conventional arc welding process. The geometry of typical electron beam weld
exhibits a weld depth-to-width ratio that is very large in comparison to that of
an arc weld
• This feature results from the high-power density of the electron beam. The high
depth-to-width ratios of electron beam welds account for two important
advantages of the process:
• Relatively thick joints can be welded in a single pass
• For a given thickness, the travel speed is much greater than can be attained
with arc welding
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Welding Procedures
Joint Designs
• Butt, corner, lap, edges, and T-joints can be made by EBW using square-butt joints or
seam welds
• Fillet welds are difficult to make and are not generally attempted
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Joint Preparation and Fit-Up
• When no filler wire is added, the fit-up of parts must be more precise then for
arc welding processes. The beam must impinge on and melt both members
simultaneously, except for seam welds where the beam penetrates through the
top sheet
• Underfill or incomplete fusion will result from poor fit-up, and lap joints which are
not clamped sufficiently will burn through
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• Typical Joint Designs for EBW
Seam Appearance at
Atmospheric EB-Welding
Seam Appearance for
EB-Welding in Vacuum
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Metals Welded
In general, all metals and its alloys that can be fusion welded by other welding
processes can also be joined by EBW
This includes similar and dissimilar metal combinations that are metallurgically
compatible
However, if EBW is applied to metals that are subject to hot cracking or porosity, the
welds will often contain such discontinuities
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Steels
Rimmed and Killed Steels
Note: Rimmed steels differ from killed steels in that the amount of deoxidising agent (manganese, ferrosilicon and aluminium,
resulting in Al2O3) added is less. Killed steels are totally deoxidised, whereas rimmed steels are only partially deoxidised.
• Chemical reaction that occurs between carbon and oxygen to form carbon
monoxide gas (CO) will occur in the molten weld pool
• As a result, violent weld pool action, spatter, and porosity in the solidified weld
metal are expected with this type of steel
• Electron beam welds in rimmed steel can be improved if deoxidizers, such as
manganese, silicon, or aluminium, are incorporated through filler metal
additions
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Hardenable Steel
• Thick sections of hardenable steels may crack when electron beam welded without
preheat
Stainless Steels
• Austenitic Stainless Steels- EBW helps to inhibit carbide precipitation in stainless
steels because of the short time that the weld zone is in the sensitizing temperature
range. However, the high cooling rate may cause cracking in highly constrained,
low ferrite grades of material
• Martensitic Stainless Steels- Hardness and susceptibility to cracking increase
with increasing carbon content and cooling rate. Prevented by preheating the base
materials before welding
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Precipitation-Hardening Stainless Steels- The semi-austenitic types, such as 177PH and PH14-8 Mo, can be welded as readily as the 18-8 types of austenitic stainless
steels
In the more martensitic types, such as 17-4 PH and 15-5 PH, the low carbon content
precludes formation of hard martensite in the weld metal and heat-affected zone
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• Aluminum Alloys- In general, aluminum alloys that can be readily welded by gas
tungsten arc and gas metal arc welding can be electron beam welded
• Titanium and zirconium- These materials and their alloys must be welded in an
inert environment. High vacuum electron beam welding is best for both metals, but
medium vacuum and non-vacuum welding with inert gas shielding may be
acceptable for some titanium applications.
• Refractory metals- Excellent process for joining the refractory metals, because the
high-power density allows the joint to be welded with minimum heat input. (Ta, Mo)
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Weldability Dissimilar Metal Combinations
1.
2.
3.
4.
5.
Very desirable (solid solubility in all combinations)
Probably acceptable (complex structures may exist)
Use with caution (Insufficient data for proper evaluation)
Use with extreme caution (data not available)
Undesirable combinations (intermediate compounds formed)
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3
Molybdenum
3
5
2
5
5
3
2
3
Niobium
4
5
4
5
5
2
5
4
1
Nickel
2
5
1
5
1
1
2
5
5
Aluminium
Gold
Beryllium
Cobalt
Copper
Iron
Magnesium
Molybdenum
5
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Tungsten
5
Titanium
2
5
Tin
2
5
Tantalum
5
5
Rhenium
2
2
Platinum
5
5
Nickel
3
Magnesium
Niobium
Iron
Silver
Weldability Dissimilar Metal Combinations (continued)
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1
1
1
5
2
5
1
Rhenium
3
4
4
5
1
3
5
4
5
5
3
2
Tin
2
2
5
3
5
2
5
5
3
5
5
5
3
Tantalum
5
5
4
5
5
3
5
4
1
1
5
5
5
5
Titanium
2
5
5
5
5
5
5
3
1
1
5
5
5
5
Aluminium
Gold
Beryllium
Cobalt
Copper
Iron
Magnesium
Molybdenum
Platinum
Rhenium
Nickel
1
Titanium
5
Tungsten
1
Tantalum
5
Tin
1
Niobium
Platinum
Silver
Weldability Dissimilar Metal Combinations (continued)
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5
5
3
5
3
1
1
5
1
5
3
1
2
5
5
5
5
5
3
5
1
5
5
5
5
2
1
5
Aluminium
Gold
Beryllium
Cobalt
Iron
Magnesium
Platinum
Rhenium
Tin
Tantalum
Titanium
Tungsten
Nickel
4
5
Niobium
5
5
Molybdenum
3
Zirconium
Copper
Tungsten
Silver
Weldability Dissimilar Metal Combinations (continued)
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Applications
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Applications
• EBW is primarily used for two distinctly different types of applications:
 high precision
 high production
 low weldability advanced engineering materials
 high penetration/width with low heat input
• Types of applications are mainly in the nuclear, aircraft, aerospace, and
electronic industries:
 Typical products include nuclear fuel elements, special alloy jet engine
components, pressure vessels for rocket propulsion components,
pressure vessels for rocket propulsion system, and hermetically sealed
vacuum devices
• Others examples are gears, frames, steering columns, and transmission and
drive-train parts for automobiles; thin-wall tubing; band saw and hacksaw blades;
and other bimetal strip applications
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Application Sample 1
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Application Sample 2
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Application Sample 3
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Application Sample 4
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Other Electron Beam Processing
Cladding and
Surfacing
Welding
Cutting
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Other Electron Beam Processing: Production of metal ingot by fusion
Electron beam gun
Visualization
Electron beam
Vacuum
System
Base Material
(BM)
BM pool
Fusion Chamber
BM liquid drops
Refrigeration system
with water
Ingot extraction
Ingot
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Other Electron Beam Processing: Drilling and thermal machining
Liquid BM
Expelled out BM
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Other Electron Beam Processing: Cladding by evaporation projection
Substrate heating
resistance
Subtract
Thin cladding
Electron
beam gun
Vacuum
System
Surface beamed
by electrons
BM pool
water-cooled melting pot
Working chamber
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Material A
melting pot
Material B
melting pot
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Health & Safety
• The process requires users to observe safety precautions not normally necessary
with other types of fusion welding equipment
• The four primary potential dangers associated with electron beam equipment are:
1. Electric shock
2. X-ray radiation
3. Fumes and gases
4. Explosion danger of vacuum systems
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