Laser machining – MM461

1. 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.

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

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 uvijof 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.

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.

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.

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.

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.

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, dis the aperture diameter through which the beam emerges, and  is the wavelength of the beam.

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, dis the beam diameter, and  is the laser wavelength.

21. Power • When the radiant energy of a laser is focused by a lens, the power density Pat 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, fis the focal length,  is full angle beam divergence, and t is the laser pulse length.

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.

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 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.

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 Tis absolute temperature (K) and tis time (s). , the diffusivity, is given by  = k /  c kis 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.

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.

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 Fof the laser by the expression F = (dx/dt)C where (dx/dt)is rate of recession of the workpiece surface, and Cis the energy needed to vaporize a unit volume of the workpiece. Typically Cis about 103 Jcm-3.

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.

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 0.375 mm.

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