Electron beam machining
(EBM) – MM461
Dr. Dermot Brabazon
Sch. Of Mech. and Manu. Eng.
Dublin City University
EBM -Introduction
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Electrons generated in a vacuum
chamber
Similar to cathode ray tube
EBM – main elements of m/c
EBM – main elements
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10-4 torr
Electron gun
Cathode - tungsten filament at 2500 –
3000 degC
Emission current – between 25 and
100mA (a measure of electron beam
density)
Electron beam
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Emmision current increases with
increases in temperature and
accelerating voltage (kV). It is also
dependent on cathode material.
Electrons are focused by the field
formed by the grid cup and by a
magnetic or electrostatic lens system.
Material removal
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Electrons therfore hit workpiece in a well
defined manner, over a circular area typically
0.025 mm in diameter.
Kinetic energy of the electrons is rapidly
translated into heat, causing a
correspondingly rapid increase in the
temperature of the workpiece, to well above
its boiling point. Material removal by
evaporation then occurs. Power densities of
1.55 MW/mm2.
Welding / Automation
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The elements of the EBM centre can
also be use for welding and can be
automated with the addiation of moving
tables or robotic manipulators.
Emission current
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Je = AT2 exp(-b/kT)
1
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Je = 2.33E-6  (Va3/2 / dac2 )
2
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Ie = KVa3/2
3
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Je = Ie / Ae
4
Material removal
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In the region where the beam of electrons
meet the workpiece, their energy is converted
into heat
Workpiece surface is melted by a
combination of electron pressure and surface
tension
Melted liquid is rapidly ejected and vaporized
to effect material removal
Temperature of the workpiece specimen
outside the region being machined is reduced
by pulsing the electron beam (10kHz or less)
Theory
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Ep = VoIetp
1
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fp = 1/( tp+ ti)
2
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w = IeV/Ae
3
Advantages
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Large depth-to-width ratio of material
penetrated by the beam with applications of
very fine hole drilling becoming feasible
There is a minimum number of pulses ne
associated with an optimum accelerating
voltage. In practice the number of pulses to
produce a given hole depth is usually found
to decrease with increase in accelerating
voltage.
Hole formed in an alloy steel
after a single pulse of EBM
Kaczmarek, 1976
Rates of material removal
(power 1kW)
Material
Volumetric removal rate (mm3 s-1)
Tungsten
1.5
Aluminium
3.9
(Adapted from Bellows, 1976)
Limit of accelerating voltage
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Increasing the hole depth requires a much
greater rise in the number of pulses at low
voltage, due mainly to a relative rise in heat
losses resulting from conduction and melting
of the adjacent metal layers.
For a given number of pulses little
improvement in material removal rate is
obtained from increasing the accelerating
voltage above 120 kV.
Surface roughness
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Depends on material being machined
Pitting is common – depends on thermal
properties of material and pulse energy
Ra increases from 5-10m to 8-15m
have been reported in nickel, tungsten
and gold for an increase in pulse charge
from 10E-9 to 200E-9 As.
Heat affected zone
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Microstructure of the surface layer
around the hole can be severly altered
by the EBM process (e.g. the formation
of an amorphous white layer in steels)
This layer increases with pulse duration
and hole diameter.
Can be as much as 0.25 mm
Applications of EBM
1.
Drilling
2.
Perforating of sheet
3.
Pattern generation (associated with
integrated circuit fabrication)
Drilling
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Electron beam machines are fitted with
systems for numerically controlling the beam
power, focus and pulse duration, and
mechanical motion
Cylindrical and other configurations, such as
conical and barrel shaped holes, of various
diameters can now be drilled with consistent
accuracy at rates of several thousand holes
per second.
Perforation
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Usually lined with an auxiliary material
The electron beam first penetrates
through the sheet forming a vapour
channel within the fused material, and
then enters the auxiliary lining.
An eruption of vapour occurs, causing
ejection of molten material.
Perforation
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104 to 105 holes per second have to be
produced (s pules needed)
Ceramic and syntethic filter material
applications (e.g 620 holes/mm2)
Pattern generation
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The beam is positioned accurately by means of
deflection coils at the location where a pattern is to
be written, by exposing a film of electron resist
coated on either a chrome mask blank or a wafer, for
the production of the lithographic definition required.
Resist film is immersed in the developer, usually a
solvent. Due to the difference in solubility between
the original and exposed resist polymers, differential
material removal occurs.
A fine pattern of polymer is thus obtained. This
pattern is then used as an active mask to avoid
unwanted etching of the integrated circuit mask or
wafer .
Pattern generation
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Line widths of a few hundred Ǻ
Writing speed 20 MHz
Across a 125mm mask an accuracy of
0.125m can be achieved in about 60
minutes.