Technology suitable for small quantity production

Laser micromachining removes material in a layer-by-layer fashion. It is an ablation operation causing vaporisation of material as a result of interaction between a laser beam and the workpiece being machined. The removal of material during laser ablation is affected by the characteristics of the laser beam and the workpiece but is mainly determined by the way the two interact. The most important laser radiation features are the pulse length (duration) and repetition rate (frequency). This allows the accumulated energy to be released in very short time intervals, which is what generates the extremely high power. Additionally, the laser beam can be focused on a very small spot. Thus, extremely high intensities (1013-1018 W/cm2) are achievable. The most important substrate material features are the absorptivity in the wavelength of the transmitted radiation, and the thermal conductivity of the substrate. Thus, material transition energies such as the latent heat of melting and the latent heat of vaporisation are most significant. This is demonstrated by the fact that metals melt more easily than ceramics but are considerably more difficult to vaporise and consequently respond less well to laser ablation.

As previously mentioned, when laser radiation impacts on the substrate, electrons in the latter are excited by the laser photons. This absorbs the energy of the photons and generates considerable heat which is transferred to the material lattice in picoseconds resulting in very high temperatures that can create local melting or vaporisation. Energy loss through electron heat transport to the bulk of the substrate is undesirable as it can raise the temperature of the surrounding material and create heat-affected zones. However, for very high intensities, which can be achieved by laser processing, non-linear effects take place and become a factor for stronger energy absorption. In the case of extreme intensities, as in ultrashort pulse ablation, the bond electrons of the material can be directly dislocated. The laser ablation regimes depend on the laser pulse length. In the case of femtosecond pulses, resolutions of 1-2 µm are achievable without a heat affected zone.

Laser micromachining provides a new method of producing parts in a wide range of materials directly from CAD data. Laser micromachining [A_49] is a relatively new process that removes material in a layer-by-layer fashion. It is an ablation operation causing vaporisation. of material as a result of interaction between a laser beam and the workpiece being machined. The removal of material during laser ablation is affected by the characteristics of the laser beam and the workpiece but is mainly determined by the way the two interact. The most important laser radiation features are the pulse length (duration) and repetition rate (frequency). This allows the accumulated energy to be released in very short time intervals, which is what generates the extremely high power. Additionally, the laser beam can be focused on a very small spot. Thus, extremely high intensities (1013-1018 W/cm2) are achievable.

The most important substrate material features are the absorptivity in the wavelength of the transmitted radiation, and the thermal conductivity of the substrate. Thus, material transition energies such as the latent heat of melting and the latent heat of vaporisation are most significant. This is demonstrated by the fact that metals melt more easily than ceramics but are considerably more difficult to vaporise and consequently respond less well to laser ablation. As previously mentioned, when laser radiation impacts on the substrate, electrons in the latter are excited by the laser photons. This absorbs the energy of the photons and generates considerable heat which is transferred to the material lattice in picoseconds resulting in very high temperatures that can create local melting or vaporisation. Energy loss through electron heat transport to the bulk of the substrate is undesirable as it can raise the temperature of the surrounding material and create heat-affected zones. However, for very high intensities, which can be achieved by laser processing, non-linear effects take place and become a factor for stronger energy absorption. In the case of extreme intensities, as in ultrashort pulse ablation, the bond electrons of the material can be directly dislocated. The laser ablation regimes depend on the laser pulse length. In the case of femtosecond pulses, resolutions of 1-2 μm are achievable without a heat affected zone.

Laser ablation is a cost-effective process for manufacturing small batches of parts. It allows parts with complex shapes to be produced without the need for expensive tooling. Laser milling is most suitable for machining parts with one-sided geometry or for partial machining of components from one side only. Complete laser milling of parts is also possible but difficulties in accurately re-positioning for additional set-ups have to be addressed. The influence of the process parameters is complex and must be optimised to obtain the highest part quality. [A_48]

[A_48] A Comparison between Microfabrication Technologies for Metal Tooling, L. Uriarte, A. Ivanov, H. Oosterling, L. Staemmler, P. T. Tang and D. Allen. Multi-material Micro Manufacture, W. Menz & S. Dimov, 4M2005, pp. 351 - 358.

[A_49] Pham D.T., e.a. Laser milling as a rapid micro manufacturing process. Journal of Engineering Manufacture, vol. 218, January 2004.