MODERN MANUFACTURING PROCESSES (PEME 5306)
Module-III
PLASMA ARC MACHINING (PAM):
Introduction
A plasma is defined as a superheated, electrically ionized gas. Plasma Arc
Cutting(PAC) uses a plasma stream operating at temperatures in the range
from 10,000 to 14,000 ºC to cut metal by melting. The cutting action takes
place by directing the high velocity plasma stream at the work, thus melting
it and blowing the molten metal through the kerf. Plasma is encountered in
electrical discharges, such as fluorescent tubes and electric arcs, lightning,
high temperature combustion flames and the sun. Most application of PAC
involve cutting of flat metal sheets and plates. Operations include hole
piercing and cutting along a defined path. It was initially employed to cut
metals that are difficult to machine by conventional methods. However, in
recent years, PAC has also been used to cut plain carbon steel, stainless
steel and aluminium.
Principle:
When heated to elevated temperatures, gases turn into a distinctly different
type of matter, which is plasma. When gases are heated by an applied
electric field, an igniter supplies the initial electrons, which accelerate in the
field before colliding and ionizing the atoms. The free electrons, in turn, get
accelerated and cause further ionization and heating of the gases. The
avalanche continues till a steady state is obtained in which the rate of
production of the free charges is balanced by recombination and loss of the
free charges to the walls and electrodes. The actual heating of the gas
takes place due to the energy liberated when free ions and electrons
recombine into atoms or when atoms recombine into molecules
Types of Plasma Arc Cutting system:
There are different types of plasma arc cutting operations are here. So
there are 2 main configurations are there.
non-transferred mode,
transfer mode
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DR P.K.Parida, CET, BBSR
MODERN MANUFACTURING PROCESSES (PEME 5306)
So this plasma gas is flowing surrounding this cathode. So this surrounding
this cathode this plasma gas is coming and it is passing, this high velocity
plasma gas is passing through this nozzle here. So it is passing through the
nozzle. When it is passing through this nozzle, it is ionized, this plasma gas
high pressurized, high velocity plasma gas is ionized and using this non-
transferred mode any kind of material whether it is electrically conducting or
electrically non-conducting, any kind of material can be cut or machined.
So this non-transferred arc, arc between electrode and nozzle so this is arc
is generated between this electrode and this nozzle.
So here electrode this cathode and nozzle is connected to the anode and
electrothermal efficiency of this kind of non-transferred arc is 65 to 70%. So
it has a very low efficiency, low efficiency than this transferred mode. So
ionizes high velocity gas streaming towards the workpiece when it is
passing through this cathode or electrode and the nozzle so high velocity
plasma jet is actually here, it is ionizes. So workpiece conductivity is not a
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DR P.K.Parida, CET, BBSR
MODERN MANUFACTURING PROCESSES (PEME 5306)
constant. So any kind of material, any kind of workpiece material, whether it
is electrically conducting or non-conducting, any kind of material can be cut
by this plasma, non-transferred mode of plasma arc cutting operation.
But in this transferred mode you can see here this is the electrode here and
this positive terminal is connected to the workpiece. So here the main
constant is that this workpiece should be electrically conducting material.
So this kind of transferred mode can be used only for electrically
conducting mode of electrically conducting workpiece material. So here
plasma gas is coming surrounding this electrode and while it is passing
through this so it is ionized here in this zone it is ionized while it is passing
through this nozzle it is ionized.
So electrical efficiency, electrothermal efficiency of this kind of transfer
mode, it is higher than this non-transfer mode, here it is 85 to 90%
electrothermal efficiency for transferred mode or plasma arc nozzle. So
arcing is generated between the electrode and the workpiece and arc heats
of this coaxial-flowing gas so this is coaxial flowing plasma gas okay so
maintains it in a plasma state. So here this workpiece is electrically
conducting. So these are the 2 modes arc mode, one is the non-transferred
and second one is the transferred arc mode.
Equipment
Elements of Plasma Arc system are
power supply
Gas supply
Cooling water system
Control console
Plasma tourch
So first one is the power supply. Second one is the gas supply, plasma gas
supply system, then cooling water system. So you have to cool down this
nozzle as well as the plasma jet you have to cool down and then control
console. So this plasma jet can be controlled by using a CNC machining
system or this working table may be controlled by using a CNC machining
system, CNC system so that any complicated contour can be cut from the
workpiece material and also the fifth one is the fifth one of the plasma arc
system, plasma arc cutting system is the plasma torch.
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DR P.K.Parida, CET, BBSR
MODERN MANUFACTURING PROCESSES (PEME 5306)
Mechanism of material removal:
The metal removal in PAM is basically due to the high temperature
produced. The heating of the work piece is, as a result of anode heating,
due to direct electron bombardment plus convection heating from the high
temperature plasma that accompanies the arc. The heat produced is
sufficient to raise the work piece temperature above its melting point and
the high velocity gas stream effectively blows the molten metal away.
Process Parameters:
Parameters that govern the performance of PAM can be divided into three
categories:
1. Those associated with the design and operation of the torch –
electrical power delivered , the gases used to form the plasma, the
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DR P.K.Parida, CET, BBSR
MODERN MANUFACTURING PROCESSES (PEME 5306)
flow rate of the gases through the torch, the orifice diameter through
the nozzle duct.
2. Those associated with the physical configuration of the set up – torch
standoff, angle to the work, depth of cut, feed into the work and
speed of the work toward the torch.
3. Environment in which the work is performed – cooling that is done on
the bar, any protective type of atmosphere used to reduce oxidation
for the exposed high temperature machined surface and any means
that might be utilized to spread out or deflect the arc and plasma
impingement area.
Gas Cutting PAM
1.Oxidation of the work piece melted 1. Plasma is generated by subjecting
generates the heat to melt the a volume of gas to electron
material for (e.g) in cutting steel, fuel bombardment of an electric arc. The
gas is used to heat it to 7600-8700C anode heating due to direct electron
at while steel reacts rapidly with bombardment plus convective heating
oxygen to form iron oxide. The heat from the high temp plasma raises the
generated by the burning iron is material to the molten point and the
sufficient ot melt the iron oxide. high velocity gas stream effectively
2.Oxy-fuel gas cutting is mostly blows the material away.
limited to only ferrous metal 2.Because of the high temp involved,
especially plain carbon steels. the process can be used on almost all
3.Cutting speed are lower for (e.g) in material including those white are
cutting mild steel 19mm thick can be resistant to oxy- fuel gas cutting
cut at 500 mm /min. 3. Cutting speeds are higher and
4.Operating costs are higher leave a narrower kerf. They can cut
5.Limited to the max. temperature of mild steel 19mm thick at the rate of
the chemical reaction (burning) 1775mm /min.
6.Cost of equipment is lower. 4. Operating costs are lower. Ratio of
Surfaces are less smoother than savings in favor of PAM is about 3:1
those cut by PAM 5. Seems to be unlimited. The greater
the power used, the greater the
volume of kerf material that can be
removed.
6. High initial cost of the equipment.
7.Surfaces cut by plasma torch are
smoother but the edges are rounded.
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DR P.K.Parida, CET, BBSR
MODERN MANUFACTURING PROCESSES (PEME 5306)
Advantages
The main advantage of PAM is speed. For example, mild steel of
6mm thick can be cut at 3m/min
The plasma arc can be used to cut any metal or even to non
conducting materials like concrete etc., since it is primarily a melting
process
Due to high speed of cutting the deformation of sheet metals is
reduced while the width of the cut is minimum
Owing to the high productivity of the plasma arc cutting coupled with
the tendency to use cheap and easily available plasma-forming media
(air, water, ammonia etc.,), PAC is finding ever increasing
application.
Smooth cuts free from contaminants are obtained in the process
Profile cutting of metals especially of stainless steel and aluminium
can be very easily done by PAM
Operating costs are less when compared to oxy-fuel torch
Can be automated
Limitations
The main disadvantage of PAC is the high initial cost of the
equipment. However, it can be made economical, if the quantity
involved is large and the thickness is up to 50mm.
Well-attached drops on the underside of the cut can be a problem
and there will be heat affected zone (HAZ). The depth of HAZ
depends on the material and its thickness
Smoke and noise
Sharp corners are difficult to produce because of the wide diameter of
the plasma stream
Burr is often produced
Taper on the work-piece may occur
Applications
Chiefly used to cut stainless steel and aluminium alloys. It is preferred
to oxy-fuel cutting because it produces comparatively smoother cuts
and is free from contamination
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DR P.K.Parida, CET, BBSR
MODERN MANUFACTURING PROCESSES (PEME 5306)
Other metals which are resistant to oxy-fuel cutting and hence cut by
PAC are magnesium, titanium, copper, nickel and alloys of copper
and nickel
PAC can be used for stack cutting, plate beveling, shape cutting and
piercing.
It can also be used for underwater cutting.
The plasma jets are used for welding materials like titanium, stainless
steel etc.,
Plasma arc is used for depositing filler metal on surface to obtain
desired properties like corrosion resistance, wear resistance,
toughness or anti-friction properties – Plasma arc surfacing
The plasma arc can also be used for spraying a prepared surface of
the base material with droplets of molten metal to obtain a surface of
required thickness
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DR P.K.Parida, CET, BBSR
