Laser Beam Machining (Non-Traditional Machining)

1. Laser Beam Machining
(Light Amplification by Stimulated
Emission of Radiation)
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2. Introduction
• Laser-beam machining is a thermal material-
removal process that utilizes a high-energy,
• Monochromatic (coherent) light beam to melt and
vaporize particles on the surface of metallic and
nonmetallic workpieces.
• The electrical energy supplied to the laser is
converted into light energy and then thermal energy
• Lasers can be used to cut, drill, weld and mark.
• LBM is particularly suitable for making accurately
placed holes.
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3. EQUIPMENT
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4. a) Ruby crystal
• The cylindrical shaped ruby crystal forms the
important part of the laser beam equipment.
• Ruby is aluminum oxide with chromium
dispersed throughout it.
• Both the ends of the ruby crystal are made
absolutely parallel to each other.
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5. Cont…
• One of the end faces of the crystal is highly
silvered, so that it reflects nearly 96% of the
incident light.
• In order to tap the laser output, the other end face
of the crystal is partially silvered and contains a
small hole through which the laser beam emerges.
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6. Cont…
b)Xenon flash tube
• The ruby crystal is surrounded by a helical flash
tube containing inert gas xenon.
• The flash tube is connected to a pulsed high
voltage source by which the xenon transforms the
electrical energy into white light flashes (light
energy).
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7. Cont…
c)Cooling system
• A cooling system, which utilizes water, air or
liquid nitrogen, is provided to protect the ruby
crystal from the enormous amount of heat
generated.
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8. Cont…
d)Focusing lens
• The light beam or laser beam, which escapes
through the tiny hole of the ruby crystal possess
low power densities.
• The beam is useless for material processing
applications until its power density is increased.
• This is achieved by means of a focusing lens. The
lens focuses the laser beam to converge to a
narrow spot thereby increasing its power density.
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9. Cont…
LBM Operation
• In operation, when the xenon flash tube is
connected to a pulsed high voltage source, the
inert gas xenon transforms the electrical energy
into white light flashes (light energy).
• Since the ruby crystal is exposed to the intense
light flashes, the chromium atoms of the crystal
are excited and jumped to a high-energy level.
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11. Cont…
• These chromium atoms immediately drop to an
intermediate energy level with the evolution of
heat and eventually drop back to their original
state with the evolution of a discrete quantity of
radiation in the form of red fluorescent light.
• As the red light emitted by one excited atom hits
another excited atom, the second atom gives off
red light, which is in phase with the colliding red
light wave.
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12. Cont…
• The effect is enhanced as the silvered ends of the
ruby crystal cause the red light to reflect back and
forth along the length of the crystal.
• The chain reaction collisions between the red
light wave and the chromium atoms become so
numerous that, finally the total energy bursts and
escapes through the tiny hole as a laser beam.
• The beam is focused with a simple lens to obtain
high power densities in small areas of the work
surface.
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13. Cont…
• The intense heat of the laser beam is used to melt
and, or evaporate the work piece material being
cut.
• A stream of gas, like oxygen, nitrogen or argon is
often used to blow the molten metal through the
cut, cool the work piece and minimize the heat
affected zone.
• The type of gas used depends on the work piece
material being cut.
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14. Cont…
• Oxygen is used for mild steel work pieces;
nitrogen or oxygen for stainless steel; nitrogen for
aluminum, and inert gas like argon for titanium
metals.
• The table carrying the work piece can be moved
in three dimensions with respect to the laser beam
to obtain the desired profile of cut on the work
piece.
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15. Cont…
Process Parameters of LBM
• Power density and laser beam-work piece
interaction time are the most important variables
determining whether the beam will weld, cut,
mark or heat treat.
• For rapid heating of a surface without melting, a
highly focused beam producing power densities
of only 1.5x102- 1.5xl04 w/ cm2 is used.
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16. Cont…
• If melting is desired, as in the case of welding or
cladding applications, power densities ranging
from 1.5 x 104 - 1.5 x 105 W cm2 is used.
• Cutting and drilling action will occur for power
densities ranging from 1.5 x 106 - 1.5 x 108 W
cm2.
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17. Cont…
ADVANTAGES OF LBM:
• Any material, including non-metals, and
irrespective of their hardness and brittleness can
be machined by laser.
• Apart from cutting, drilling and welding
materials, lasers can also be used for marking,
scribing, heat- treating of surfaces and selectively
clad materials.
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18. Cont…
• The process can be easily automated
• Can remove material in very small amounts
• Laser beam machining is a force-less machining
process.
• This allows very thin and fragile parts to be easily
cut.
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19. Cont…
DISADVANTAGES OF LBM
• Costlier
• Low thermal efficiency
• Low metal removal rates
• Process is limited to thin parts
• High reflectiv materials are difficult to machine
• Difficult to drill exact round holes
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20. Cont…
APPLICATIONS OF LBM
• Laser beam machining is used to perform
precision micro- machining on all materials such
as steel, ceramic, glass, diamond, graphite etc.
• It is used for cutting, drilling, welding of
materials, marking, scribing, heat treating of
surfaces and selectively clad materials.
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21. Electron Beam Machining
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22. Principle of EBM
• The ability of electron beam to cause drastic
thermal effects has been known since a long time.
• An electron beam is controlled stream of
electrons that are generated and accelerated to
move at between 30-70 % the speed of light,
providing the energy to heat and melt the metal. A
heated tungsten filament is used to emit the
electrons.
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23. Cont…
In electron beam machining, the stream of electrons
generated is directed against a precisely limited
area of the workpiece, on impact the KE of the
electrons is converted into thermal energy that
melts and vapourizes the material to be removed,
there by forming holes or cuts.
The process is performed in a vacuum chamber, as
the electrons would lose energy by collision with
the air molecules in atmosphere.
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24. EBM Equipment
1. Electron beam gun:
The electron gun, which is mainly responsible for
emission of electrons, consists of three parts, a
tungsten filament, grid cup and anode. The
tungsten filament is connected to the terminal of
the power supply and is acts as a source for
electrons.
The grid cup is negatively biased with respect to the
filament.
The anode part is kept at ground potential and
through it, the high velocity electrons passes.
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25. Cont…
2. Magnetic Deflection Coil.
A magnetic deflection coil (lens) fitted below the
electron gun is used to make the electron beam
circular in cross section and deflect it anywhere
on the work surface.
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26. Cont…
3. Vacuum chamber:
The vacuum chamber enclose all the parts, and is
required for three reasons
• The emitter (Filament) would rapidly oxidze
when incandescent, at anything like atmospheric
pressure
• The electrons would lose energy by collision
with air molecules in the atmosphere.
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27. EBM Operation
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28. Cont…
• The tungsten filament is electrically heated in vacuum
to about 2500°C, due to which a cloud of electrons
are emitted by the filament.
• The electrons are guided by the grid cup to travel
downwards, towards the anode.
• The flow of electrons is controlled by the negative
bias applied to the grid cup.
• A potential difference of 50 – 150 kV is maintained
between the filament and the anode.
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29. Cont…
• The electrons passing through the anode are
accelerated to achieve as high velocity as around
2/3 rd of light.
• This high velocity stream of electrons are
collected into a concentrated beam by means of
diaphragm and focusing lens and further directed
towards the workpiece with the help of magnetic
forces resulting form the deflector coils.
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30. Cont…
• The high velocity beam of electrons impinges on
the workpiece where its kinetic energy is
converted into heat energy.
• The concentrated heat raises the temperature of
the workpiece material to its melting point and
vapourizes a small amount of it, resulting in
removal of metal from the workpiece.
• The table on which the workpiece is mounted can
be traversed to feed the workpiece as needed.
•
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31. Cont…
• Alternately, by focusing and turning – off the
beam in a specific direction, the cutting process
can be continued till the desired profile is
achieved.
• A suitable viewing device is usually incorporated
so as to enable the operator to observe and control
the progress of the machining operation.
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32. Mechanism of Metal removal
• In EBM the electrons impinge on the work
surface with velocities exceeding one half the
speed of light; their kinetic energy is transformed
into thermal energy.
• The workpiece surface is melted by combination
of electron pressure and surface tension.
• The melted liquid metal on the worksurface is
rapidly ejected and vapourized to effect material
removal
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33. Process Parameters
• Metal removal rate in electron beam machining is
evaluated in terms of the number of pulses
required to evaporate a particular amount of work
material.
Following are the critical parameters that affect the
metal removal rate
1. Beam Current
2. Pulse duration
3. Lens current
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34. Cont…
• Beam Current:
Beam current is directly related to the number of
electrons emitted by the cathode.
Beam current can range from 200 µa to 1A.
Increasing the beam current directly increases the
energy per pulse delivered to the workpiece.
Electron beam machining can generate pulse
energies in excess of 120 J/Pulse, a value which is
200 to 400% greater than that available in laser
drilling system.
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35. Cont…
This extreme high energy pulse available with EBM
explains the capability of the process to rapidly
drill very deep and large diameter holes.
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36. Cont…
2. Pulse Duration :
Pulse duration affects both the depth and diameter
of the hole.
Increase in pulse duration enhances the energy per
pulse.
High energy pulse can machine larger and deeper
holes on thick plates.
Shorter pulse duration will allow less interaction
time for thermal effects to materialize
Electron beam systems can generate pulses as
shorter as short as 50µ sec or as long as 10 m sec.
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37. Cont…
3. Lens current :
Lens current is used as the parameter to determine
the working distance between the focal point and
the electron beam gun.
It also determines the spot size of the focused part
of the beam on the workpiece material.
The diameter of the focused beam spot dictates the
diameter of the hole produced.
A higher power density results in increased energy
per pulse however the spot size is smaller.
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38. Cont…
The material removal would be faster in such a case,
although the size of the hole would be smaller
It must be noted that power density must be higher
to generate a high beam power, else heating and
vaporization of the work surface will be poor
leading to inefficient machining.
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39. Advantages
• Any material, hard or soft, can be successfully
machined
• No tool wear problems
• Heat can be concentrated on a particular spot, hence
effective machining
• Does not apply any cutting forces on the workpiece
material.
• Simple work holding device is sufficient, helps in
machining of thin and fragile work part
• Can machine holes of any shape by combining beam
deflection using electromagnetic coils and the CNC
table with high accuracy.
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40. Disadvantage
• High investment cost
• Skilled operator is required for machining
• Vacuum requirement tend to limit the workpiece
size and production rate
• High maintenance cost
• Suitable for small and fine cuts only
• Low metal removal rate
• High power consummation
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41. Application
• Micro finishing operations of thin sections, dies for
wire drawing parts of electron microscopes, fiber
spinners, injector nozzles for diesel engines, gas
orifices for pressure differential devices used in
nuclear reactors, rotors and aircraft engines etc.
• Depending on the beam intensity the process can
also be used for heat treatment applications.
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42. Electron Beam Machining
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43. Electron Beam Machining – Process
• Electron beam is generated in an electron
beam gun.
• Electron beam gun provides high velocity
electrons over a very small spot size.
• Electron Beam Machining is required to be
carried out in vacuum.
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44. Cont…
• Otherwise the electrons would interact with
the air molecules, thus they would loose their
energy and cutting ability.
• Thus the work piece to be machined is located
under the electron beam and is kept under
vacuum.
• The high-energy focused electron beam is
made to impinge on the work piece with a spot
size of 10 – 100 μm.
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45. • The kinetic energy of the high velocity
electrons is converted to heat energy as the
electrons strike the work material.
• Due to high power density instant melting and
vaporisation starts and “melt – vaporisation”
front gradually progresses.
• Finally the molten material, if any at the top of
the front, is expelled from the cutting zone by
the high vapour pressure at the lower part.
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46. • Unlike in Electron Beam Welding, the gun in
EBM is used in pulsed mode.
• Holes can be drilled in thin sheets using a
single pulse.
• For thicker plates, multiple pulses would be
required.
• Electron beam can also be maneuvered using
the electromagnetic deflection coils for drilling
holes of any shape.
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47. Localized heating by
focused electron beam
Gradual formation of
hole
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48. Penetration till the
auxiliary support
Removal due to high
vapour pressure
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49. TSN, JSSATEB

50. TSN, JSSATEB

51. Electron Beam Gun
• An electron beam gun is the heart of any electron
beam machining facility.
• The basic functions of any electron beam gun are
to generate free electrons at the cathode,
accelerate them to a sufficiently high velocity and
to focus them over a small spot size.
• Further, the beam needs to be maneuvered if
required by the gun.
• The cathode is generally made of tungsten or
tantalum.
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52. • Such cathode filaments are heated, often
inductively, to a temperature of around 2500 0C.
• Such heating leads to thermo-ionic emission of
electrons, which is further enhanced by
maintaining very low vacuum within the chamber
of the electron beam gun.
• Moreover, this cathode cartridge is highly
negatively biased so that the thermo-ionic
electrons are strongly repelled away form the
cathode.
• This cathode is often in the form of a cartridge
so that it can be changed very quickly to reduce
down time in case of failure
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53. Electron Beam Gun
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54. • Just after the cathode, there is an annular bias grid.
• A high negative bias is applied to this grid so that the
electrons generated by this cathode do not diverge and
approach the next element, the annular anode, in the
form of a beam.
• The annular anode now attracts the electron beam and
gradually gets accelerated.
• As they leave the anode section, the electrons may
achieve a velocity as high as half the velocity of light.
• The nature of biasing just after the cathode controls the
flow of electrons and the biased grid is used as a switch
to operate the electron beam gun in pulsed mode.
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55. • After the anode, the electron beam passes through a
series of magnetic lenses and apertures.
• The magnetic lenses shape the beam and try to reduce
the divergence.
• Apertures on the other hand allow only the convergent
electrons to pass and capture the divergent low energy
electrons from the fringes.
• This way, the aperture and the magnetic lenses
improve the quality of the electron beam.
• Then the electron beam passes through the final
section of the electromagnetic lens and deflection coil.
• The electromagnetic lens focuses the electron beam to
a desired spot.
• The deflection coil can maneuver the electron beam,
though by small amount, to improve shape of the
machined holes.
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56. • Generally in between the electron beam gun and the work
piece, which is also under vacuum, there would be a series
of slotted rotating discs.
• Such discs allow the electron beam to pass and machine
materials but helpfully prevent metal fumes and vapour
generated during machining to reach the gun.
• Thus it is essential to synchronize the motion of the
rotating disc and pulsing of the electron beam gun.
• Electron beam guns are also provided with illumination
facility and a telescope for alignment of the beam with the
work piece.
• Work piece is mounted on a CNC table so that holes of
any shape can be machined using the CNC control and
beam deflection in-built in the gun.
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57. Diffusion Pump
• One of the major requirements of EBM operation of
electron beam gun is maintenance of desired vacuum.
• Level of vacuum within the gun is in the order of 10-4 to
10-6 Torr. {1 Torr = 1mm of Hg} Maintenance of suitable
vacuum is essential so that electrons do not loose their
energy and a significant life of the cathode cartridge is
obtained.
• Such vacuum is achieved and maintained using a
combination of rotary pump and diffusion pump.
• Diffusion pump is attached to the diffusion pump port of
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58. • Diffusion pump is essentially an oil heater.
• As the oil is heated the oil vapour rushes upward where
gradually converging structure is present.
• The nozzles change the direction of motion of the oil
vapour and the oil vapour starts moving downward at a
high velocity as jet.
• Such high velocity jets of oil vapour entrain any air
molecules present within the gun.
• This oil is evacuated by a rotary pump via the backing
line.
• The oil vapour condenses due to presence of cooling
water jacket around the diffusion pump.
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59. Working of a Diffusion Pump
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60. Electron Beam Process – Parameters
The process parameters, which directly affect the
machining characteristics in Electron Beam Machining,
are:
• The accelerating voltage
• The beam current
• Pulse duration
• Energy per pulse
• Power per pulse
• Lens current
• Spot size
• Power density
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61. • EBM the gun is operated in pulse mode.
• This is achieved by appropriately biasing the biased grid
located just after the cathode.
• Switching pulses are given to the bias grid so as to achieve
pulse duration of as low as 50 μs to as long as 15 ms.
• Beam current is directly related to the number of electrons
emitted by the cathode or available in the beam.
• Beam current once again can be as low as 200 μ amp to 1
amp.
• Increasing the beam current directly increases the energy per
pulse.
• Similarly increase in pulse duration also enhances energy per
pulse.
• High-energy pulses (in excess of 100 J/pulse) can machine
larger holes on thicker plates.
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62. • The energy density and power density is governed by
energy per pulse duration and spot size.
• Spot size, on the other hand is controlled by the degree
of focusing achieved by the electromagnetic lenses.
• A higher energy density, i.e., for a lower spot size, the
material removal would be faster though the size of the
hole would be smaller.
• The plane of focusing would be on the surface of the
work piece or just below the surface of the work piece.
• This controls the kerf shape or the shape of the hole
• Final deflection coil can maneuver the electron beam
providing holes of non-circular cross-section as
required the surface of the work piece
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63. Typical kerf shape of electron beam
drilled hole
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64. Electron Beam Process Capability
• EBM can provide holes of diameter in the range of 100
μm to 2 mm with a depth up to 15 mm, i.e., with a l/d
ratio of around 10.
• The hole can be tapered along the depth or barrel
shaped.
• By focusing the beam below the surface a reverse taper
can also be obtained.
• Typically there would be an edge rounding at the entry
point along with presence of recast layer.
• Generally burr formation does not occur in EBM.
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65. • A wide range of materials such as steel, stainless steel,
Ti and Ni super-alloys, aluminium as well as plastics,
ceramics, leathers can be machined successfully using
electron beam.
• As the mechanism of material removal is thermal in
nature as for example in electro-discharge machining,
there would be thermal damages associated with
EBM.
• However, the heat-affected zone is rather narrow due
to shorter pulse duration in EBM.
• Typically the heat-affected zone is around 20 to 30
μm.
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66. • Some of the materials like Al and Ti alloys are more
readily machined compared to steel.
• Number of holes drilled per second depends on the hole
diameter, power density and depth of the hole as well as
material type as mentioned earlier.
• Depicts the variation in drilling speed against volume of
material removed for steel and Aluminium alloy.
• EBM does not apply any cutting force on the
workpieces. Thus very simple work holding is required.
• This enables machining of fragile and brittle materials
by EBM.
• Holes can also be drilled at a very shallow angle of as
less as 20 to 300.
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67. Variation in drilling speed with volume of material removal for
steels and aluminium
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68. Advantages
• EBM provides very high drilling rates when small
holes with large aspect ratio are to be drilled.
• Moreover it can machine almost any material
irrespective of their mechanical properties.
• As it applies no mechanical cutting force, work
holding and fixturing cost is very less.
• Further for the same reason fragile and brittle materials
can also be processed.
• The heat affected zone in EBM is rather less due to
shorter pulses.
• EBM can provide holes of any shape by combining
beam deflection using electromagnetic coils and the
CNC table with high accuracy
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69. Limitations
• The primary limitations are the high capital cost of the
equipment and necessary regular maintenance
applicable for any equipment using vacuum system.
• Moreover in EBM there is significant amount of non-
productive pump down period for attaining desired
vacuum.
• However this can be reduced to some extent using
vacuum load locks.
• Though heat affected zone is rather less in EBM but
recast layer formation cannot be avoided.
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