1. Dublin City University
Laser machining – MM461
Dr. Dermot Brabazon
Sch. Of Mech. and Manu. Eng.
Dublin City University

2. 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’ is an acronym of light amplification by stimulated emission of radiation
predicted in 1958 by Schawlow and Townes

3. 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
For example, when a ruby laser is excited by a flash lamp, emitting 1 kJ of electrical energy in 1 ms, a laser beam of 3 J energy at 6934 Å having a cross-section of 5 mm, and a divergence of 10-3 rad is produced. The beam, on focusing, can provide power densities of 1 MWcm-2.

4. Laser Fabrey-Perot interferometer cavity

5. 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.

6. 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.
Further insight can be derived from work by Einstein on the interaction of matter and radiation. He proposed that a proper description of this interaction required the inclusion of conditions whereby an excited atom is induced by radiation in order to emit a photon which thereby decays to a lower energy state.

7. 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.

8. 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.

9. 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 atom-atom collisions.

10. Production of population inversion. (After Fowles, 1975.)
By (a) Optical pumping, (b) Direct electron excitation,
and (c) Inelastic atom-atom collisions.

11. Types of machining laser
Gas
Carbon dioxide
Optically pumped solid-state

12. 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.
Many high-powered lasers use high voltage pulses, especially when steady population inversion cannot be maintained.

13. Carbon dioxide laser

14. 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.
C02:N2:He laser was devised in the 1960s, with the output rising from a few milliwatts to hundreds of watts.
The laser transition occurs, between two (001 and 100) vibrational energy levels of the carbon dioxide molecules, the difference between which corresponds to an output wave­length of 10.6 m.
After the lasing action at a wavelength of 10.6 m, the CO2 molecule decays to the 01ºO level, and then radiates to the ground state.

15. 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.
Xenon-flash and high-pressure mercury-discharge lamps are the usual external light sources used to produce the optical pumping of the active atoms.
E.G. Ruby laser
The three-level-type lasing action in a ruby laser requires high pumping, normally provided by pulsing action. The typical output wavelength and power of the ruby laser are respectively m and 400J.
In the four level system only a small percentage of the ground state ions needs exciting to provide the population inversion between levels 3 and 2; cf. the three level system which needs more than 50%. Consequently four level systems have much lower pumping needs and laser thresholds. A well-known member of four level group is the Nd3+ ion, which is a key constituent of the YAG laser (crystalline Y3A15012 doped with Nd3+).

16. Laser beam characteristics
Spatial profile
Beam divergence
Focusing
Temporal behaviour
Brightness
Power.

17. 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.
The subscripts ‘mm’ are often used to refer to the number of nulls in the spatial pattern that occur in each of two orthogonal directions transverse to the direction of beam propagation. The TEM00 mode is often used for machining.
The intensity of the laser beam in this mode follows a Gaussian distribution as a function of radius r, from the centre of the beam:
I(r) = I0exp(—2r2 / ro2)
where I0 is the intensity of the beam at the centre, and ro is the radius at which the intensity is reduced from its central value by the factor e2. (Sometimes ro is called the ‘Gaussian’ radius.)

18. Conditions for TEM00 mode

19. 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.
For a laser operating in the fundamental mode, the beam divergence is typically rad. A high-powered laser operating in a multi-mode pulsed fashion will have an angle of divergence of to rad.

20. 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.
The smallest minimum spot sizes for higher-mode, gas lasers, which cannot be focused to the diffraction limit can be about 0.1 mm.

21. 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.
The minimum beam divergence min of a light beam is a function of its wavelength and beam diameter. The minimum beam divergence of a spatially coherent beam can be deduced, from the Rayleigh criterion, as
min = 1.22  / R
where  is the wavelength, and R is the radius of the beam or aperture. With higher energy lasers, the minimum beam divergence can be within a factor of three of that predicted by the Rayleigh criterion.

22. 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.

23. Effect of laser
on materials

24. 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.
For wavelengths greater than 5 m, most metals reflect about 90% of the incident radiation at low power densities.
The amount of reflectivity can be substantially reduced by modification of the surface condition of the workpiece. For example, the reflectivity of copper at a wavelength of nm has been reported to be reduced from 95% to less than 20% by oxidizing the surface.
Reduction in reflectivity leads to increase in absorption of the laser energy by the surface, with subsequent effects on the material

25. 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 photo-electric 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 to 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.
Typically the energy is absorbed in a depth of about 0.1 m (for visible and infra-red wavelengths). For most organic compounds, absorption is found to take place in less than 1 m (for CO2 for infra-red radiation). In summary, the laser energy may therefore be regarded as having a surface effect, with the energy penetrating further into the material by thermal conduction.

26. 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 kg-1K-1) of the solid material.
The higher the thermal diffusivity the lower the penetration time required for a given depth.

27. 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.
The temperature rise due to heat flux incident on the surface (x=0) of the workpiece, still assumed to be semi-infinite, can be calculated as T(x,t)x=o = [2Fo / k](t /  )1/2 where Fo is the constant heat flux (Js-1cm-2).

28. 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.
Note: the use of this simple equation assumes that vaporization does not interfere with the laser beam as it meets the workpiece.

29. 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.

30. 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 gas-assistance of the laser action.
Typical cutting speeds of greater than 600 m/min can be obtained using a 0.4 kW laser for cutting paper. A similar powered laser can achieve cutting rates of 1.2 m/min for cutting stainless steel, or 4.6 m/min for mild steel. The higher the laser cutting feed rate, the smaller is the thickness of the heat-affected zone (HAZ). A HAZ of 0.13, 0.20, and 0.25mm respectively for the cutting scenarios just described has been presented.

31. Gas-assistance of the laser

32. 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.

33. 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 mm.

34. Applications Drilling Cutting Scribing Controlled fracturing
Trimming of electronic components