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Transcript
Laser machining – MM461
Dr. Dermot Brabazon
Sch. Of Mech. and Manu. Eng.
Dublin City University
Laser machining - Introduction

Interaction of an intense, highly
directional,
coherent,
and
monochromatic beam of light with a
workpiece, from which material is
removed by vaporization.
Laser Fabrey-Perot
interferometer cavity



Two plane highly parallel half silvered
mirrors
Between which a monochromatic beam
of light undergoes multiple reflections
Cavity between the mirrors would be
filled with an amplifying medium, gas
molecules excited to high energy levels
Laser Fabrey-Perot
interferometer cavity
Spontaneous and stimulated
emission
When an atom in an excited state of energy
Ei falls to a lower level Ej, it emits a quantum
of radiation of frequency, vij, where
Ei - Ej = h vij
where h is Planck’s constant.
 The same atom can be stimulated to emit this
radiation if it receives radiation of the same
frequency.

Stimulated emission


The rate at which stimulated jumps in radiation occur
is proportional to the energy density uvij of the
radiation, and to the difference in the population (that
is number per unit volume) of atoms between the
upper and lower states. Both the stimulated and
stimulating radiation have the same directional and
polarization characteristics.
This process is the basis of the laser phenomenon.
Stimulated emission

Most sources of the radiation emit through
spontaneous transactions, and since these occur
in a random fashion ordinary sources of visible
radiation are incoherent. In comparison, in a
laser the radiation density builds up such that
induced transitions become completely dominant,
and the emitted radiation is very coherent.
Moreover, the spectral radiation of the laser at its
operating frequency is much greater than that of
ordinary light.
Stimulated emission

A condition, called population inversion, is needed
to obtain this effect with lasers. If the population
inversion exists, the intensity of a light beam can
be shown to increase as it traverses the lasing
medium. That is, the beam will be amplified, since
the gain due to the induced emission exceeds the
loss due to absorption.
Stimulated emission


The induced radiation is emitted in the same
direction as the primary beam. The two have a
definite phase relationship. That is, the induced
and primary radiations are coherent.
Population inversion may be achieved by
several methods including optical pumping,
direct electron excitation, and inelastic atomatom collisions.
Production of population
inversion. (After Fowles, 1975.)
By (a) Optical pumping, (b) Direct electron excitation,
and (c) Inelastic atom-atom collisions.
Types of machining laser

Gas


Carbon dioxide
Optically pumped solid-state
Gas laser


External electrical excitation can be
obtained from direct and alternating
current discharges
The latter is very simple: the power
source can be an ordinary high voltage
transformer to which are connected cold
metal electrodes in the tube.
Carbon dioxide laser
Carbon dioxide laser


Very high continuous power levels (hundreds of
watts) became possible with the C02:N2:He laser
Detailed studies have revealed that the nitrogen
molecules in the discharge are excited to a
vibrational level (v = 1) which is very close to the
001 level in the CO2. This excitation in N2 is
effectively transferred to the upper laser level of the
CO2, creating an excess of molecules there. This
preferential population is a necessary condition for
the occurrence of lasing action. The helium in the
discharge helps to maintain the population inversion,
as well as improving heat conduction to the walls.
Optically pumped solid state
lasers

The active atoms of the laser medium are
embedded in a solid, typically a rod of crystal
or glass, with parallel, flat ends which are
optically ground and polished. The rod may
have coated ends to form the optical cavity
needed; alternatively external mirrors can be
used.
Laser beam characteristics






Spatial profile
Beam divergence
Focusing
Temporal behaviour
Brightness
Power.
Spatial pattern

Lasers have a characteristic spatial pattern
called Transverse Electromagnetic Modes
(TEM). Briefly the transverse mode
determines the propagation and focusing of
the beam. The TEM are a consequence of
resonance within the laser cavity, and are a
measure of the configurations of the
electromagnetic field determined by the
boundary conditions in the cavity.
Conditions for TEM00 mode
Divergence

The lower limit of beam divergence t is
given by t = K /d
where K = 2/  for a Gaussian beam, d
is the aperture diameter through which
the beam emerges, and  is the
wavelength of the beam.
Focusing

The diameter of the unfocused laser beam can
be several mm wide. Focusing is needed to
provide sufficient power density, so that the
temperature of the materials to be treated is
raised above the melting or boiling point. The
diameter df of a Gaussian beam, focused by a
simple lens, is given by df = 2 f  /  d = f 
where f is the focal length of the lens, d is the
beam diameter, and  is the laser wavelength.
Power

When the radiant energy of a laser is focused
by a lens, the power density P at the focal
plane of the lens can often be represented by
the expression:
P = 4 E /  f2 2 t
where E is energy output from the lens, f is
the focal length,  is full angle beam
divergence, and t is the laser pulse length.
Effect of laser on materials


When the laser beam meets the workpiece,
several effects arise, including reflection,
absorption, and conduction of the light energy.
Material removal by melting and vaporisation.
Effect of laser
on materials
Reflectivity

The amount by which the beam is reflected
depends on the wavelength of the laser radiation,
and on the condition and properties of the material,
such as its surface finish, the amount to which it is
oxidized, and its temperature. In particular, the
high reflectivity of many materials at certain laser
wavelengths renders them unsuitable for
machining. Generally, the longer the wavelength of
the laser beam, the higher becomes the reflectivity
of metals.
Absorption

Laser energy which is not reflected at the surface is
absorbed into the material. The absorption of the
light in metals takes place by an internal photoelectric effect which raises the electrons to higher
energy states in the conduction band of the metal.
The mean free time between collisions for electrons
in a conductor is of the order of 10-14 to 10-13 s. Thus
in 1 nanosecond (ns), the electrons will have made
1014 to 1015 collisions among themselves. Since
this is a very short period compared to even the
shortest laser pulse, the energy absorbed by the
electrons from the laser beam is rapidly passed to
the lattice.
Conduction



The conduction of the heat from the laser into the
workpiece material is an extremely complex effect no adequate theory yet.
Since the workpiece is assumed to be composed of
an isotropic material, the heat flow through it can be
described by the diffusion equation: T/ t =  2 T
Here T is absolute temperature (K) and t is time (s).
, the diffusivity, is given by  = k /  c
k is the coefficient of thermal conductivity (Wm-1 K-1),
 is the density (kgm-3), and c the specific heat (J kg1K-1) of the solid material.
Melting

For heat fluxes of 10-4 Wcm-2, melting times
of about 0.1 seconds are common. Melting
times are proportional to the square of the
incident power.
Vaporisation

Very rapidly after melting by the laser, vaporization
of the workpiece surface commences. The rate of
vaporization may be related to the incident flux F of
the laser by the expression F = (dx/dt)C
where (dx/dt) is rate of recession of the workpiece
surface, and C is the energy needed to vaporize a
unit volume of the workpiece. Typically C is about
103 Jcm-3.
Plastic vs metal machining


Comparatively low energy is needed to vaporize
plastics, compared with metals. Radiation of the
wavelength of the CO2 laser (10.6 m) is readily
absorbed by most non-metals, which also usually
have low thermal conductivity. Thus, plastic
materials can be readily melted by low power
(several watts) CO2 lasers. They can be cut at high
speeds with slightly higher powers. For instance, a
400W CO2 laser can cut through 0.1 mm thick
plastic at a rate of more than 4 ms-1.
Since metals have a higher reflectivity and thermal
conductivity than plastics, greater power densities
are usually needed to cut them.
Gas-assistance of the laser

Allen, Spalding and Whittle (1975) – point out that
the higher reflectivities and thermal conductivities
of metals might suggest that high power density
lasers should be needed for cutting these
materials. Nevertheless, they draw attention to
experimental evidence for the existence of a critical
thermal threshold, above which a sharp drop
occurs in the surface reflectivity. Also they describe
the enhancement of machining efficiency by gasassistance of the laser action.
Gas-assistance of the laser
Gas-assistance of the laser

The efficiency of metal machining by laser is often
increased by oxygen-assisted gas cutting. This
technique is based on the exploitation of
exothermic chemical reactions, which are utilized
in the well-established oxy-acetylene torch cutting
of metals. With the latter effect, the initial melting
and oxidation of the metal are caused by the heat
from the torch. The cutting is achieved by the
release of heat from the oxidation process, and the
flow of the gas stream also contributes, by
removing the oxide from the cutting area.
Gas-assistance of the laser

Both the focused laser beam and the oxygen
jet emerge coaxially through a nozzle at the
foot of the pressure chamber. The workpiece
is cut as it traverses past this focal point. With
this technique, titanium of 0.5 mm thickness
has been cut with a CO2 laser of 135 W at 15
m min-1, the narrow heat-affected zone (or
kerf) width being only 0.375 mm.
Applications





Drilling
Cutting
Scribing
Controlled fracturing
Trimming of electronic components