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

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 atom-atom 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 001 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 Electro magnetic 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 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 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 / ck 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.

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 gas-assistance 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