Detailed document of Laser Beam Machining

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CHAPTER 5
LASER BEAM MACHINING (LBM)
5.1 INTRODUCTION
The word LASER stands for “Light Amplification using Stimulated Emission of
Radiation.”Lasers provide intense and uni-directional beams of light. This light is
coherent in nature, whereas ordinary light is particularly incoherent because in the later
case, the source is generally a hot matter. The laser in short pulses has power output of
nearly 10 kW/cm2
of the beam cross-section. (The sun’s total radiation is 7 kW/cm2
of its
surface.) By focusing a laser beam on a spot 1/100 of a square mm in size, the beam can
be concentrated in a short flash to a power density of 100,000 kW/cm2
. This can provide
enough heat to melt and vaporize any known high strength engineering materials and
permit the fusion and welding of refractory materials. In a laboratory test when a laser
beam was focused on a piece of carbon, a spot was heated to approx 8000K in 0.0005
seconds. Thus, it is seen that laser promises to be a valuable tool for drilling, cutting, or
milling virtually any metal, ceramic, or other organic material. Any material that can be
melted decomposition cut with the laser beam. Laser beam machining (LBM) uses an
intensely focused, coherent stream of light (a laser) to vaporize or chemically ablate
materials. A schematic of the LBM process is shown in Figure 28-27. Lasers are also
used for joining (welding, brazing, soldering), heat treating materials. Power density and
interaction time are the basic parameters in laser processing as shown in Figure 28-28.
Drilling requires higher power densities and shorter interaction times compared to most
other applications.
The material removal mechanism in LBM is dependent upon the wavelength edia is
struck by a photon of light, it ergized atoms, a he most common industrial laser is the CO
2 laser. The CO 2 Lasers produce highly collimated, coherent (in phase) of the laser
used. Laser light is produced within a laser cavity which is a highly reflective cavity
containing a laser rod and a high intensity light source, or laser lamp. The light source is

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used to "pump up" the laser rod which includes atoms of a lasing media which is capable
of absorbing the particular wavelength of light produced by the light source. When an
atom of lasing m becomes energized. When a second photon strikes the energized atom,
the atom gives off two photons of identical wavelength moving in the same direction and
with the same phase. This process is called stimulated emission. As the two photons now
stimulate further emission from other en cascading of stimulated emission ensues. To
increase the number of stimulated emissions, the laser rod has mirrors on both ends that
are precisely parallel to one another. Only photons moving perpendicular to these two
mirrors stay within the laser rod causing additional stimulated emission. One of the
mirrors is partially transmissive and permits some percent of the laser energy to escape
the cavity. The energy leaving the laser rod is the laser beam. T laser is a gas laser which
uses a tube of helium and carbon dioxide as the laser rod. Output is in the far infrared
range (10.6 µm) and the power can be up to 10 kW. Nd:YAG lasers are called solid state
lasers. The laser rod in these lasers is a solid crystal of yttrium, aluminum, and garnet
which has been doped with neodymium atoms (the lasing media). The output wavelength
is in the near infrared range (1064 nm) and power up to 500 W is common. L light which
when focused to a small diameter produce high power densities which are good for
machining. It is generally accepted that in order to evaporate materials, infrared power
densities in excess of 105 W /mm are needed. For CO2 lasers, these levels are directly
achievable. However, in Nd: YAG lasers, these high power conditions would
significantly decrease the life of the laser lamp

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5.2 APPRATUS
The most important part of the laser apparatus is the laser crystal. Many materials
with laser action have been developed, for example calcium fluoride crystals doped with
neodymium (Ca + F2 + Nd) and glass doped with various earths. The most commonly
used laser is a man-made ruby consisting of aluminium oxide into which 0.05 per cent
chromium has been introduced (Al2O3 + Cr2). The crystals rods are usually round and the
end surfaces are made reflective. A laser rod for a 3 Joule unit is about 6 mm in diameter
and 70 mm in length.
Fig. 5.1 Schematic Diagram of Laser Beam Machining
The laser rod is excited by the xenon filled flash lamp which surrounds it. Both are
enclosed in a highly reflective cylinder which directs the light from the flash lamp into
the rod. The chromium atoms in the ruby are thus excited to high energy levels. The
excited ions emit energy (photons) when they return to the normal state. In this way, very
high energy is obtained in short pulses. The ruby rod becomes less efficient at higher
temperature; it is thus continuously cooled with water, air, or liquid nitrogen.

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The mechanism by which a laser removes material from the surface being worked
involves a combination of the melting and evaporation process. However, with some
materials, the mechanism is purely one of evaporation. Laser machining is essentially a
thermal process; the heat requirements and the heat utilization involved are discussed
below.
The radiant energy delivered to a surface by a focused laser is consumed in 4 ways:
a) A part is reflected and lost.
b) Most of the energy which is not reflected is used for melting metal.
c) A relatively small part of the energy is used to evaporate the liquid metal.
d) A very small part of the energy is conducted into the un melted base material.
5.3 Interaction of Laser Beam with Work
The relative magnitudes of the heat consumption at these 4 avenues strongly
depend upon thermal and optical properties of the material being worked and the
intensity and the pulse duration of the laser beam. So, it is obvious that the work surface
should not reflect back too much of the incident beam energy. Figure below shows a laser
beam falling on a solid surface. The absorbed light propagates into the medium and its
energy is gradually transferred to the lattice atoms in the form of heat.
(Fig. 5.2) Laser beam falling on work surface
and variation of intensity below surface
The absorption is described by Lambert’s Law as
I (z) = I (0) e- µz

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Where I (z) denote the light intensity at depth z and µ is the absorption coefficient. Most
of the energy is absorbed in a very thin layer at the surface (typical thickness (0.01 µm)).
So, it is quite reasonable to assume that the absorbed light energy is converted into the
heat at the surface itself, and the laser beam may be considered to be equivalent to a heat
flux.
5.4 Thermal Analysis
Machining by laser is a high speed ablation process. As uniform high intensity
energy is delivered to a work surface, there is an initial transient period, after which the
steady state ablation is established, and this continues as long as the energy continues to
be supplied at uniform intensity. This steady state ablation is characterized by a constant
rate of material removal and by the establishment of a steady temperature distribution in
the solid immediately in advance of the ablating surface.
In case where the material being worked is both melting and evaporating the steady
temperature distribution is given by
T-T1 = exp {-Vabx }
Tm-T1 β
Where
T --------- temperature at distance x below the ablating surface,
T1 -------- initial temperature (uniform) of the material being worked,
------- temperature at a large distance from the ablating surface,
Tm ------- melting point of the material being worked,
Vab ------ steady ablation velocity
β = s/ρCp ---- thermal diffusivity of material being worked,
s, ρ, Cp ------- thermal conductivity, density, and specific heat, respectively of the
material being worked.
Since x is measured from a moving boundary, that is, the ablating surface, the
temperature distribution is in terms of moving coordinate system.It can be seen that the
exponential temperature distribution represented by the above equation satisfies the
boundary condition that T=T1 when x is very large, and T=Tm when x=0. This depth up
to which heat penetrates beyond the ablating surface is of considerable practical
importance, and it is required to be as shallow as possible.The term ‘characteristic depth’

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xc is defined as the depth during steady ablation which has experienced a temperature
rise of 1/2.718 times (Tm-T1) from the temperature T1. This xc decreases with increasing
ablation velocity and decreasing thermal diffusivity.
xc = β/ Vab
During the initial transient period, when the ablation is just beginning, part of the heat
delivered to the work surface is used to establish the temperature distribution within the
solid. Once steady conditions are obtained, the heat contained in the solid does not
increase nay further and the value of this steady heat content is given by
∞
(H/A)0 = ∫ρ.Cp (T-T1) dx
0
= s(Tm-T1)
Vab
Where (H/A)0 is the heat per unit area contained in the solid beneath the ablating surface.
After the heat contained in the solid has reached its steady value, all the heat at the
material surface (less the amount reflected) is used for melting and vaporizing material.
The relationship governing it is
f’= Vab.ρ.H
Or
Vab = f’/ ρ.H
Where
f’ = net heat flow rate per unit area (total-reflected)
H = heat requirement for melting and vaporization or the heat/unit weight of the
material required to elevate the temperature of the solid from its initial value to the
melting point, plus the heat of fusion and the additional heat supplied to boil a small
fraction of the metal which has been melted. The two parameters f’ and H are, however,
most difficult to assess quantitatively. The first requires knowledge of not only the beam
intensity but the average effective reflectivity of the material during the pulse; the second
requires a good estimate of exactly how much metal is vaporized and how much remains
in the liquid state.
5.5 Cutting Speed and Accuracy of Cut

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The latest commercially available laser unit of 800W capacity can cut most metal
plates up to 3 mm thick at speeds of about 1 to1.25 m/min. A general cutting accuracy of
this unit is claimed to be within 0.8 mm. a feature of this cutting process is that a square
and straight edge is obtained.
5.6 Metallurgical Effects
The heat affected zone is relatively narrow, about 1 mm for 3 mm thick sheet
metal. It is reported that at this thin sheets, very little melting occurs and there is almost
instantaneous vaporization. Therefore, there is no significant effect on metallurgical
properties of work material.
5.7 Advantages
Any solid material which can be melted without decomposition can be cut with the laser
beam. The other major advantages of the laser beam as a cutting tool are:
1. There is no mechanical contact between the tool and the work.
2. Large mechanical forces are not exerted upon the work piece.
3. The laser operates in any transparent environment, including air, inert gas,
vacuum, and even certain liquids.
4. The laser head need not be in close proximity for performing cutting and drilling
operations of difficult accessibility.
5. The beam can be projected through a transparent window.
6. Unlike other thermal machining devices, the laser can be used with materials
sensitive to heat shocks such as ceramics. ( This advantage is also obtained with
electron beam)
All features of laser beam machining improve with increased intensity. The higher the
intensity, the laser is the heat resonant in the uncut material.
5.8 Limitations
One main limitation of the laser cutting is that currently it cannot be used to cut
metals that have high heat conductivity or high reflectivity. This means that it cannot be
used to cut aluminium, copper and their alloys satisfactorily. This limitation, although not
very important, can even be regarded as an advantage, in that the laser beam is a self-

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selective cutter for different materials. This will permit the metal that is to be cut placed
on a work table made from another metal which is not affected by the laser beam, and
thus give complete maneuverability in cutting out complex profiles without damage to
the work table.
During operation, the work piece to be cut is placed on the aluminium work table (which
is resistant to being cut by laser beam). The laser head is traversed over the work piece
and an operator visually inspects the cut while manually adjusting the control panel. The
actual profile s obtained from a linked mechanism which copies the master drawing or
actual profile place on a nearby bench.
The other limitations of this method are:
1. The machined area can be irregular due to off-axis modes that may be generated
during laser action.
2. The least diameter to which laser beam can be focused depends upon the laser
beam divergence; this in turn, is a function of the quality of the laser material and
the laser cavity length.
3. Output energy from the laser is difficult to control precisely.
4. The laser system is quite insufficient.
5. Pulse repetition rates are low.
6.
Summary of Laser Beam Machining Characteristics
Mechanics of material removal Melting, Vaporization
Medium Normal atmosphere
Tool High Power Laser Beam
Maximum material removal rate 5 mm3/min
Specific power consumption 1000 W/mm3/min
Critical Parameters Beam Power Intensity, beam diameter,
melting temperature
Material Application All Materials
Shape Application Drilling fine holes
Limitations Very large power consumption,
cannot cut materials with high
heat conductivity and reflectivity