A typical SEM micrograph hole drilled by laser beam machining process employed in

making a hole is shown in Figure

Figure: SEM micrograph hole drilled in 250 micro meter thick Silicon Nitride with
3rd
harmonic Nd: YAG laser
5.2. Laser beam cutting (milling)
• A laser spot reflected onto the surface of a workpiece travels along a prescribed
trajectory and cuts into the material.
• Continuous-wave mode (CO2) gas lasers are very suitable for laser cutting providing
High-average power, yielding high material-removal rates, and smooth cutting
surfaces
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Advantage of laser cutting
• No limit to cutting path as the laser point can move any path.
• The process is stress less allowing very fragile materials to be laser cut without any
support.
• Very hard and abrasive material can be cut.
• Sticky materials are also can be cut by this process.
• It is a cost effective and flexible process.
• High accuracy parts can be machined.
• No cutting lubricants required
• No tool wear
• Narrow heat effected zone
Limitations of laser cutting
• Uneconomic on high volumes compared to stamping
• Limitations on thickness due to taper
• High capital cost
• High maintenance cost
• Assist or cover gas required
5.3. ELECTRON BEAM MACHINING (EBM)
As has already been mentioned in 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 μamp 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. 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.
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1. Electrons generated in a vacuum chamber
2. Similar to cathode ray tube
3. Electron gun
4. Cathode - tungsten filament at 2500 – 3000 degC
5. Emission current – between 25 and 100mA (a measure of electron beam density)
MRR:
In the region where the beam of electrons meets 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 (10 kHz or less)
31 -
Material Volumetric removal rate (mm s )
Tungsten 1.5
Aluminium 3.9
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Advantages of Ebm:
1. Large depth-to-width ratio of material penetrated by the beam with
applications of very fine hole drilling becoming feasible
2. There are 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.
5.4. PLASMA ARC MACHINING (PAM)
The plasma welding process was introduced to the welding industry in 1964 as a
method of
bringing better control to the arc welding process in lower current ranges. Today,
plasma retains the
original advantages it brought to industry by providing an advanced level of control
and accuracy to
produce high quality welds in miniature or precision applications and to provide long
electrode life
for high production requirements.
The plasma process is equally suited to manual and automatic applications. It has
been
used in a variety of operations ranging from high volume welding of strip metal, to
precision
welding of surgical instruments, to automatic repair of jet engine blades, to the
manual
welding of kitchen equipment for the food and dairy industry.
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5.5. PLASMA ARC WELDING (PAW):
Plasma arc welding (PAW) is a process of joining of metals, produced by heating with
a
constricted arc between an electrode and the work piece (transfer arc) or the electrode
and the
constricting nozzle (non transfer arc). Shielding is obtained from the hot ionized gas
issuing
from the orifice, which may be supplemented by an auxiliary source of shielding gas.
Transferred arc process produces plasma jet of high energy density and may be used
for high
speed welding and cutting of Ceramics, steels, Aluminum alloys, Copper alloys,
Titanium
alloys, Nickel alloys.
Non-transferred arc process produces plasma of relatively low energy density. It is
used for welding of various metals and for plasma spraying (coating).
Equipment:
(1) Power source. A constant current drooping characteristic power source supplying
the dc
Welding current is required. It should have an open circuit voltage of 80 volts and
have a duty
cycle of 60 percent.
(2) Welding torch. The welding torch for plasma arc welding is similar in appearance
to a gas
tungsten arc torch but it is more complex.
(a) All plasma torches are water cooled, even the lowest-current range torch. This is
because the arc is contained inside a chamber in the torch where it generates
considerable
heat.During the non transferred period, the arc will be struck between the nozzle or
tip with the
orifice and the tungsten electrode.
(b) The torch utilizes the 2 percent thoriated tungsten electrode similar to that used
for gas tungsten welding.
(3) Control console. A control console is required for plasma arc welding. The plasma
arc
torches are designed to connect to the control console rather than the power source.
The
console includes a power source for the pilot arc, delay timing systems for
transferring from
the pilot arc to the transferred arc, and water and gas valves and separate flow meters
for the
plasma gas and the shielding gas. The console is usually connected to the power
source. The
high-frequency generator is used to initiate the pilot arc.
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Principles of Operation:
The plasma arc welding process is normally compared to the gas tungsten arc process.
But in the TIG-process, the arc is burning free and unhandled, whereas in the plasma-
arc
system, the arc is necked by an additional water-cooled plasma-nozzle. A plasma gas
– almost
always 100 % argon –flows between the tungsten electrode and the plasma nozzle.
The welding process involves heating a gas called plasma to an extremely high
temperature and then ionizing it such that it becomes electrically conductive. The
plasma is
used to transfer an electric arc called pilot arc to a work piece which burns between
the
tungsten electrode and the plasma nozzle. By forcing the plasma gas and arc through a
constricted orifice the metal, which is to be welded is melted by the extreme heat of
the arc.
The weld pool is protected by the shielding gas, flowing between the outer shielding
gas
nozzle and the plasma nozzle. As shielding gas pure argon-rich gas-mixtures with
hydrogen or
helium are used.
The high temperature of the plasma or constricted arc and the high velocity plasma jet
provide an increased heat transfer rate over gas tungsten arc welding when using the
same
current. This results in faster welding speeds and deeper weld penetration. This
method of
operation is used for welding extremely thin material and for welding multi pass
groove and
welds and fillet welds.
Uses & Applications:
Plasma arc welding machine is used for several purposes and in various fields. The
common
application areas of the machine are:
1. Single runs autogenous and multi-run circumferential pipe welding.
2. In tube mill applications.
3. Welding cryogenic, aerospace and high temperature corrosion resistant alloys.
4. Nuclear submarine pipe system (non-nuclear sections, sub assemblies).
5. Welding steel rocket motor cases.
6. Welding of stainless steel tubes (thickness 2.6 to 6.3 mm).
7. Welding of carbon steel, stainless steel, nickel, copper, brass, monel, inconel,
aluminium,
titanium, etc.
8. Welding titanium plates up to 8 mm thickness.
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9. Welding nickel and high nickel alloys.
10. Melting, high melting point metals.
11. Plasma torch can be applied to spraying, welding and cutting of difficult to cut
metals and
alloys.
5.6. Plasma Arc Machining (PAM):
Plasma-arc machining (PAM) employs a high-velocity jet of high-temperature gas to
melt and displace material in its path called PAM, this is a method of cutting metal
with a
plasma-arc, or tungsten inert-gas-arc, torch. The torch produces a high velocity jet of
hightemperature
ionized gas called plasma that cuts by melting and removing material from the
work piece. Temperatures in the plasma zone range from 20,000° to 50,000° F
(11,000° to
28,000° C). It is used as an alternative to oxyfuel-gas cutting, employing an electric
arc at very
high temperatures to melt and vaporize the metal.
Equipment:
A plasma arc cutting torch has four components:
1. The electrode carries the negative charge from the power supply.
2. The swirl ring spins the plasma gas to create a swirling flow pattern.
3. The nozzle constricts the gas flow and increases the arc energy density.
4. The shield channels the flow of shielding gas and protects the nozzle from metal
spatter.
Principle of operation:
PAM is a thermal cutting process that uses a constricted jet of high-temperature
plasma gas to
melt and separate metal. The plasma arc is formed between a negatively charged
electrode
inside the torch and a positively charged work piece. Heat from the transferred arc
rapidly
melts the metal, and the high-velocity gas jet expels the molten material from the cut.
Applications:
The materials cut by PAM are generally those that are difficult to cut by any other
means,
such as stainless steels and aluminum alloys. It has an accuracy of about 0.008".
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UNIT I
PART-A (QUESTIONS WIRH ANSWERS)
1. What is meant by Conventional Machining Processes?
In conventional machining processes, metal is removed by using some sort of the
tools which is harder than the work piece and is subjected to wear. In this process,
tool and work piece being in direct contact with each other.
2. What is meant by Unconventional Machining Processes?
The Unconventional machining processes do not employ a conventional or
traditional tool for metal removal; instead, they directly utilize some form of energy
for metal machining. In this process, there is no direct physical contact between the
tool and the work piece.
3. What is a thermal energy method of unconventional machining machining?
In these methods, heat energy is concentrated on a small area of the work piece to
melt and vaporize the tiny bits of work material. The required shape is obtained by
the continued repetition of this process.
Examples: LBM, PAM, EBM, IBM
4. What is an electro chemical energy method of unconventional machining
machining?
In these methods, material is removed by ion displacement of the work piece
material in contact with a chemical solution.
Examples: ECM, ECG, ECH, ECD
5. What is a chemical energy method of unconventional machining?
The chemical energy methods involve controlled etching of the work piece
material in contact with a chemical solution.
Example: CHM
6. What is a mechanical energy method of unconventional machining?
In mechanical energy methods, the material is removed by mechanical erosion
of the work piece material.
Examples: USM, AJM, WJM
7. List the unconventional machining process which uses mechanical energy?
1. Ultra Sonic Machining (USM)
2. Abrasive Jet Machining (AJM)
3. Water Jet Machining (WJM)
8. List the unconventional machining process, which uses thermal or heat energy?
1. Laser Beam Machining (LBM)
2. Plasma Arc Machining (PAM)
3. Electron Beam Machining (EBM)
4. Ion Beam Machining (IBM)
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9. List the Unconventional machining process, which uses Electro chemical energy?
1. Electro Chemical Machining (ECM)
2. Electro Chemical Grinding (ECG)
3. Electro Chemical Honing (ECH)
4. Electro Chemical Deburring (ECD)
10. What are the characteristics of unconventional machining process?
1. The Unconventional machining processes do not empty a conventional or
traditional tool for metal removal; instead, they directly utilize some form of
energy for metal machinery.
2. The tool material need not be harder than the work piece material.
3. A harder and difficult to machine materials such as carbides, stainless steel,
nitralloy hastalloy and many other high strength temperature resistant alloys can
be machined by unconventional machining processes.
11. Name the unconventional machining processes which are used to remove
maximum
material?
1. Electro Chemical Machining (ECM)
2. Plasma Arc Machining (PAM)
12. Name the unconventional machining process which is used to remove minimum
material?
Electron Beam Machining
13. Name the unconventional machining process which consumes maximum power?
Laser Beam Machining
14. Name the unconventional machining process which consumes minimum power?
Plasma Arc Machining
15. Name the unconventional machining processes for machining following material?
1. Non metal like ceramic , Plastics and glass
2. Refractories
3. Titanium
4. Super alloys
5. Steel
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