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Laser milling of ceramic components
ARTICLE IN PRESS
International Journal of Machine Tools & Manufacture 47 (2007) 618–626 www.elsevier.com/locate/ijmactool
Laser milling of ceramic components D.T. Pham, S.S. Dimov, P.V. Petkov Manufacturing Engineering Centre, Cardiff University, Queen’s Building, Newport Road, The Parade, Cardiff, CF24 3AA, UK Received 10 September 2004; received in revised form 16 March 2006; accepted 3 May 2006 Available online 30 June 2006
Abstract Conventional methods of producing ceramic components are based on sintering technology which requires expensive tooling making it uneconomic for small batch fabrication. Laser milling provides a new method of producing parts in a wide range of materials, including ceramics, directly from CAD data. This paper considers the technical capabilities of laser milling when applied to the machining of microcomponents from alumina and silicon nitride ceramics. The main parameters affecting the material removal characteristics of laser milling are reviewed. A new technique for machining alumina components is proposed emphasising the importance of correct set-up design in achieving a high level of accuracy. Process parameters influencing part quality are analysed and guidelines for machine set-ups are formulated. The paper concludes with an assessment of the accuracy of the laser milling process. r 2006 Elsevier Ltd. All rights reserved. Keywords: Laser micro machining; Micro machining; Milling
1. Introduction Conventional technologies for the manufacture of ceramic components are based on sintering processes requiring expensive tooling and for this reason are best suited to high-volume production. Recently, new uses have been found for ceramics, for example in medical and surgical devices, where small batches in a diversity of complex shapes incorporating fine intricate features are required [1]. This paper examines the feasibility of laser milling as a method of producing ceramic components in low volumes from blanks that can be manufactured easily, for example, by sintering or reaction bonding. Laser milling is a relatively new machining 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. Provided appropriate process parameters are adopted and the material is not transparent to the laser, laser milling can be readily utilised to remove materials, such as ceramics, that are difficult to machine by conventional means [2,3]. Corresponding author. Tel.: +44 29 20874429; fax: +44 29 20874003.
E-mail address: [email protected] (D.T. Pham). 0890-6955/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2006.05.002
The products chosen for this study are: a prototype ceramic cavity tool and a microsurgical tool in which the ceramic part acts as an insulator located between a tissuecutting electrode and the metal housing. The geometry of the ceramic cavity tool is shown in Fig. 1. The cavity has overall dimensions: 3 mm 1.5 mm 0.35 mm.The properties of the materials used are listed in Table 1 ( for alumina [4]) and Table 2 (for silicon nitride [4]). The geometry of the ceramic microsurgical component, which has overall dimensions 6 mm 2.9 mm 1.18 mm, is shown in Fig. 2. Note that both sides of the part have cavities, as well as a hole and a slot, which means that repositioning of the workpiece is necessary during manufacture. Because the laser spot diameter is very small (approximately 45 mm), very high set-up accuracy is required and repositioning is not desirable. The properties of the material used (alumina 23) are listed in Table 3 [5]. The paper considers the technical capabilities of laser milling when applied to the machining of the parts. Parameters influencing part quality are analysed and guidelines for machine set-ups are formulated. The paper concludes with an assessment of process accuracy.
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Fig. 1. Ceramic cavity tool geometry.
Table 1 Material properties of alumina [4] Material Alumina Al2O3 (Goodfellow)
Units
Apparent porosity Density Volume resistivity at 25 1C Thermal conductivity at 20 1C Upper continuous use temperature
% g/cm3 O cm W/m K 1C
0% 3.9 41014 26–35 1700
Table 2 Material properties of silicon nitride [4] Material Silicon Nitride SI3N4 (Goodfellow)
Units
Apparent porosity Density Volume resistivity at 25 1C Thermal conductivity at 20 1C Upper continuous use temperature
% g/cm3 O cm W/m K 1C
15–23 2.4 4107 10–16 1200–1500
Fig. 2. Ceramic component. Table 3 Material properties of alumina 23 [5] Material DIN VDE 0335/IEC 672
2. Physical factors affecting the process Ceramics are materials with strong bonds. These bonds are either covalent or ionic in nature. The main difference between ceramics and metals is related to the fact that the Fermi energy of the electrons is between the valence and conduction bands. For ceramic materials that are considered to be semiconductors, the gap between the bands is narrow, so that a relatively small amount of energy is sufficient to promote electrons to the conduction band. For ceramics that are insulators, a larger amount of energy is required to excite electrons to the conduction band. The removal of material during laser milling is affected by the characteristics of the laser beam and the workpiece
Principal constituent Apparent density Main grain size Open porosity Compressive strength Bending strength Modulus of elasticity Melting point/maximum working temperature Specific heat Thermal conductivity at 100 1C/ 1000 1C
Units g/cm3 mm % N/mm2 N/mm2 GPa 1C
99.7% Al2O3 3.7–3.95 10 0 3500 300 380 2030/1950
J/kg K W/m K
900 30/5
but is mainly determined by the way the two interact. The most important factors influencing this interaction are discussed below.
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2.1. Laser radiation features Laser radiation can be controlled and modulated in an ordered sequence of pulses with a predetermined 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. This significantly increases the energy density (fluence) and power density (intensity) in the area of the spot. Thus, extremely high intensities (1013–1018 W/cm2) not achievable by any conventional machining technique can be unleashed in the laser-material interaction zone. This explains why laser milling can successfully process materials that are difficult to machine using conventional methods. The wavelength of the laser is also a factor that affects laser milling. Laser radiation is monochromatic, which means that stable processing conditions can be maintained because the substrate response and properties remain relatively uniform. The ablation mechanism that governs the material removal process is affected by an important laser radiation feature, its photon energy. Finally, interaction between laser radiation and the plasma formed in the process must be taken into account as it reduces overall machining efficiency [6]. 2.2. Substrate material features The response to laser radiation is influenced by a number of characteristics of the workpiece material. Laser ablation occurs only when the substrate material absorbs strongly in the wavelength of the transmitted radiation. Hence, to optimise machining results, it is essential to match the laser source with the material being worked. Generally, higher absorption efficiency leads to more effective laser milling. Laser absorptivity [7] can be increased in a number of ways, for example, by varying the surface finish or applying a surface coating. The thermal conductivity of the substrate is another key factor. Material removal efficiency is affected by the dissipation of absorbed energy within the bulk of the material, other energy losses, and the dimensions of the heat affected zone. Ceramic materials, which have the highest melt and vaporisation temperatures but the lowest thermal conductivity are most easily machined by laser radiation. During laser ablation the affected material passes through several transition phases depending on the process parameters. 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. 2.3. Laser radiation absorption mechanisms 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 [8]. 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 heataffected zones. The Beer–Lambert law which represents the classical understanding of light absorption states that, when a particular wavelength of light is transmitted through a material, its absorption is a function of the material path length and independent of the incident intensity. 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. Effects such as avalanche ionisation and multiphoton absorption can be observed. With an electron avalanche, electrons of an intermediate energy level are excited into the conduction band by the absorption of single photons. These excited electrons collide with bound electrons in increasing numbers leading to a cascade of electrons leaving the valence band. Multiphoton absorption occurs when an electron transfers from the valence band to the conduction band by absorbing several photons. In general, when a material reaches a critical electron density it starts absorbing photon energy at a sufficient level to undergo ablation [8]. A laser radiance fluence threshold exists which is material dependent. Material removal by laser ablation cannot begin until this threshold has been exceeded as shown in Fig. 3 [9]. 2.4. Laser ablation mechanisms Three of the most important factors controlling the laser ablation process are:
te—electron cooling time, ti—lattice heating time, tL—laser pulse duration.
Material removal
620
Optimal range
No material removal
Material removal
Fig. 3. Laser beam intensity.
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As a rule teooti and for most materials ti is in the picosecond range. Three different ablation regimes exist depending on the laser pulse length.
femtosecond pulses—tLoteotI, picosecond pulses—teotLoti, nanosecond and longer pulses—teotiotL.
The laser milling machine used in this work operates with the longer type of pulses for which process conditions are summarised in Fig. 4. In this case, the absorbed energy melts the material and heats it to the vaporisation temperature but there is a significant time delay whilst the thermal wave propagates into the material. Evaporation occurs from the liquid material. Secondary effects include a heat-affected zone, a recast layer, microcracks, surface damage caused by shockwaves and debris from ejected material. Additionally, the vaporised material forms a hot plasma from the leading edge of the pulse and this plasma is sustained during the rest of the pulse. Due to the plasma shielding effect (the plasma absorbs and defocuses the pulse energy) a higher fluence or irradiance for deeper penetration is required. 3. Set-up design The blanks (workpieces) for the cavity tool were rectangular blocks measuring 50 mm 50 mm 5 mm. Fig. 5 shows the workpiece and the three locating datums A, B and C. As the cavity is fully enclosed in the workpiece, there were no special requirements for set-up. The cavity was produced in one sequence, employing closed–loop control with a touch–probe and a light sensor as feedback means [10]. The blank for the microsurgical part was a rectangular block measuring 10 mm 4 mm 2 mm. The component was virtually centred in the blank and manufactured in three operations. In the first two operations, closed-loop control (by means of a touch-probe to control depth) was employed in order to achieve the required part accuracy in the Z direction (perpendicular to the milling plane). In the third operation, the pulse Ejected molten material
Lens Plasma plume Damaged ajacent structure
Recast layer Surface debris
Heat affected zone
Shock waves Microcracks Melt zone
Heat transfer
Fig. 4. Nanosecond and longer pulse laser ablation.
Fig. 5. Cavity blank set-up. Datums A, B and C are locating datums.
through hole and slot were machined. This did not require accuracy in the Z direction and for this reason open-loop (without touch-probe feedback) machining was performed. Fig. 6 shows the sequence of operations, intermediate component geometry and set-up datums during each operation. The following notation was used to label the datums: datum A—locating datum, datum B—temporary datum and datums C and D—check datums. For the first operation, datum A was the ground flat surface of the blank. For the second and third operations, the uppermost flat surface was used as datum A. In all operations, the straight side wall of the blank was utilised as the temporary datum B. The actual location of the workpiece (datums C and D) in the XY plane was determined by a co-ordinate measuring machine, fitted with a CCD camera, to obtain the required positioning accuracy. Normally, a metal plate would be used as a support base. However, it could not be employed here as the third operation involved producing a through hole. This would have brought the laser beam into contact with the metal plate, creating vapour, which would cause the part to shift and the operation to fail. To avoid this situation a glass support plate was used. Glass is transparent to the laser employed, a Nd:YAG laser in the 1064 nm waveband [10], and therefore does not absorb sufficient energy for vapour to be produced when the laser beam pierces through the material. As a further precaution, a transparent adhesive was used to bond the workpiece firmly to the glass plate and the plate itself was held sufficiently far above the metal base of the jig to ensure that the laser was not focused when it reached the base. 4. Influence of process parameters on final part quality The main requirements to be observed when selecting values for the process parameters are:
a stable process, the specified dimensional accuracy, an acceptable surface finish, reasonable cycle times.
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Fig. 6. Sequence of operations.
Several single-factor experiments were carried out to determine the influence of the process parameters on the process output. Although they do not provide a complete picture of the different phenomena involved because interactions between the factors are not considered, the experiments give an idea of the influence of the process parameters on a particular target process output, facilitating process tuning and reducing the amount of trial and error involved in process adjustment.
Fig. 7 presents the results obtained. It can be inferred that the most important process parameter with respect to the layer thickness and the removal rate is the current in the lamp used to generate the laser beam (Fig. 7(a)). This is because the lamp current directly influences the energy of the laser pulses and the pulse peak power. This was the reason for using the lamp current as an approximate process tuning parameter.
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The effect of pulse duration (Fig. 7(b)) is also linked to the pulse peak power. It was noticed that for longer pulses (tL44 ms) with lower peak powers the surface finish was poor and there was also a change in the surface colour. The latter meant that the chemical structure might have been altered. In addition, for pulses longer than 4 ms, the process was not stable resulting in a wide variation in layer thickness. The process window with regard to pulse duration was found to be in the range 2–4 ms. In general, shorter pulses are preferred for the machining of ceramics, because they give higher pulse peak powers, when the other parameters are fixed. Pulse frequency (Fig. 7(c)) has two main influences on laser milling results. It directly controls the degree of overlapping of the craters produced by the laser pulses. In machining ceramic materials, lower pulse frequencies are more efficient, because the pulse energy required for material removal is higher than that for other materials. The scanning speed (Fig. 7(d)) also influences the overlap between craters, the lower the speed, the grater the overlap. When the overlap increases, the depth per layer also increases, because more material is subject to multipulse radiation. From the single-factor experiments, it can be concluded that in order to achieve a stable process for machining ceramics, the following should be adopted:
small pulse duration for higher pulse peak power, low pulse frequency for adequate pulse energy, low scanning speed for better surface finish.
Generally the lower the scanning speed, the better the surface finish. However, below a speed threshold, the opposite effect can be observed—the roughness increases when the scanning speed reduces. It should be stated that pre-machining (to remove one or a few layers of material using the laser milling machine) is required to assure uniform absorption and process stability. 5. Analysis of the machining results
Fig. 7. Influence of the process parameters on the process output.
For the cavity tool, two kinds of ceramics with different laser light absorption properties were selected (Fig. 8 [4])— Alumina ceramic and Silicon Nitride ceramic. Alumina ceramic absorbs approximately 10% of the laser energy for the wavelength employed, while the absorption for Silicon Nitride is about 90%.In order to determine the process repeatability, three cavities were produced in each workpiece. Fig. 9 shows the errors obtained for the Alumina ceramic and Silicon Nitride ceramic parts. Clearly, the higher absorption properties of the latter generally helped machining accuracy. Furthermore, secondary effects (HAZ, recast layer) of laser milling were reduced due to the lower laser intensity adopted when processing materials with such good absorption properties.
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A batch of 23 microsurgical components was manufactured. The key dimensions of the components were measured. The results are given in Table 4. The dimensions in the slice plane XY are divided into two groups—internal and external(I/E). For the internal dimensions (I), hatching (material removal by laser as shown in Fig. 10) was performed from the inner side of the contour containing the corresponding dimension, while for the external dimensions hatching was from the outer side. For dimensions in the Z direction, depth control using the touch probe was employed. It was observed that for most internal dimensions, negative deviations from the nominal value were obtained, indicating that the actual dimensions were smaller than the nominal. For external dimensions
Fig. 8. Laser light absorption [2].
Fig. 9. Errors for the two ceramic materials (mm).
Table 4 Measurement results for the microsurgical tool Nominal
Average
Standard deviation
Median
Range
Deviation from nominal
Type
6s
IT Grade
X 1.35 3.75 4.75 5 6
1.375 3.715 4.687 4.853 6.039
0.028 0.034 0.021 0.029 0.025
1.373 3.714 4.679 4.844 6.035
0.124 0.161 0.088 0.14 0.108
0.025 0.035 0.063 0.147 0.039
I I I I E
0.167 0.205 0.125 0.176 0.153
IT14 IT14 IT12 IT13 IT13
Y 0.5 0.5 1.7 2.5 2.9
0.464 0.468 1.565 2.54 2.971
0.019 0.019 0.031 0.047 0.029
0.468 0.473 1.557 2.546 2.978
0.094 0.09 0.125 0.167 0.122
0.036 0.032 0.135 0.04 0.071
I I I E E
0.115 0.111 0.186 0.283 0.175
IT13 IT13 IT13 IT14 IT14
Z 0.25 0.3 0.52 1.18
0.231 0.284 0.503 1.178
0.020 0.021 0.02 0.018
0.24 0.29 0.51 1.18
0.07 0.1 0.09 0.06
0.019 0.016 0.017 0.002
0.119 0.129 0.122 0.107
IT13 IT13 IT13 IT12
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1
3
625
6. Conclusion
4
2
5
6
1. Border cuts 2. Hatching cuts 3. Laser spot with its diameter 4. Border cut track displacement (step-over) 5. Hatch track displacement (step-over) 6. Distance between the end of the hatch line and the innermost border cut Fig. 10. Material removal in the laser milling process. 1. Border cuts. 2. Hatching cuts. 3. Laser spot with its diameter. 4. Border cut track displacement (step-over). 5. Hatch track displacement (step-over). 6. Distance between the end of the hatch line and the innermost border cut.
(E), the situation was the opposite and the actual dimensions were larger than the nominal. This could be explained by the fact that the effective laser spot diameter was smaller than the value employed in generating the laser path. It was observed that, as the machined dimension increased, the absolute value of the deviation from the nominal also increased. This is likely to have been caused by changes in both the diameter and shape of the effective laser spot as the beam moves away from the optical axis of the system. However, the dimensional changes were not significant as shown by the measurements. In addition, the deviation from the nominal was also due to losses of accuracy during the generation of the laser path. Table 4 shows that most of the actual tolerances for the measured dimensions were between ISO Grades IT12 and IT14, which is within the prescribed tolerances for the component. For a small number of non-critical dimensions, the measured values were outside the specified limit, but this did not affect part functionality.
Laser milling is a cost-effective process for manufacturing small batches of ceramic 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 setting-up of the workpiece is most critical when small parts are to be manufactured. In cases where a through feature has to be machined, it is recommended that the support be made from a material that cannot be laser milled as vapour generated from the support may damage the part or influence its quality. It is suggested that, for a machine employing a laser with a wavelength l ¼ 1064 nm, a glass support plate be used to hold the workpiece. The influence of the process parameters on laser milling is complex and must be optimised to obtain the highest part quality. Approximate tuning of the process for machining of ceramics can be achieved by controlling the lamp current and fine adjustment made by manipulating the scanning speed and/or pulse frequency. It is strongly recommended that closed-loop control of the depth be used whenever possible in order to avoid instability in the layer thickness and the associated accumulated errors. Acknowledgements The authors would like to thank the Welsh Assembly Government, the Department of Trade and Industry and the Engineering and Physical Sciences Research Council for funding this research under the ‘‘SUPERMAN’’ and ‘‘Micro Tooling Centre’’ ERDF Programmes, the ‘‘MicroBridge’’ MNT Programme and the ‘‘Cardiff Innovative Manufacturing Research Centre’’ Innovative Manufacturing Programme. Also, this work was carried out within the framework of the EC FP6 Networks of Excellence ‘‘Innovative Production Machines and Systems (I*PROMS)’’ and ‘‘Multi-Material Micro Manufacture: Technologies and Applications (4M)’’. The authors gratefully acknowledge the support given to the Networks by the European Commission. References [1] K. Chen, Y.L. Yao, Process optimisation in pulsed laser micromachining with applications in medical device manufacturing, International Journal of Advanced Manufacturing Technology 16 (2000) 243–249. [2] D. Hellrung, L.Y. Yeh, F. Depiereux, A. Gillner and R. Poprawe, High-accuracy micromachining of ceramics by frequency-tripled Nd:YAG-lasers, in: Proceedings of the SPIE Conference on Laser Applications in Microelectronics and Optoelectronics, San Jose, California, 1999, pp. 348–355.
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[3] H.K. Tonshoff, D. Hesse, J. Mommsen, Micromachining using excimer lasers, Annals of the CIRP 42/1 (1993) 247–251. [4] Goodfellow, Material properties of Alumina and Silicone Nitride, Goodfellow Cambridge Ltd., Ermine Business Park, Nuntingdon, PE29 6WR, UK, 2003. [5] Friatec, Material properties of Alumina 23, Aktiengesellchaft Postfach 710261, Steinzeugstrasse, D-68222 Mannheim, Germany, 2000. [6] C. Do¨rbecker, H. Lubatschowski, S. Lohmann, C. Ruff, O. Kermani, W. Ertmer, Influence of the ablation plume on the removal process during the ArF-excimer laser photoablation, Proceedings SPIE 2632 (1996) 2–9.
[7] J. Takacs, Laser technology—technology for the future, in: Proceedings of the Conference on Mechanical Engineering, vol. 2, Budapest, Hungary, 1998, pp. 515–519. [8] M.D. Shirk, P.A. Molian, A review of ultrashort pulsed laser ablation of materials, Journal of Laser Applications 10 (1) (1998) 18–28. [9] J.H. Yoo, S.H. Jeong, R. Grief, R.E. Russo, Explosive change in crater properties during high power nanosecond laser ablation of silicone, Journal of Applied Physics 88 (2000) 3. [10] Lasertech GmbH. Operation manual, DMG Lasertec GmbH, Tirolerstrasse 85, D 87459 Pfronten, Germany, 1999.
International Journal of Machine Tools & Manufacture 47 (2007) 618–626 www.elsevier.com/locate/ijmactool
Laser milling of ceramic components D.T. Pham, S.S. Dimov, P.V. Petkov Manufacturing Engineering Centre, Cardiff University, Queen’s Building, Newport Road, The Parade, Cardiff, CF24 3AA, UK Received 10 September 2004; received in revised form 16 March 2006; accepted 3 May 2006 Available online 30 June 2006
Abstract Conventional methods of producing ceramic components are based on sintering technology which requires expensive tooling making it uneconomic for small batch fabrication. Laser milling provides a new method of producing parts in a wide range of materials, including ceramics, directly from CAD data. This paper considers the technical capabilities of laser milling when applied to the machining of microcomponents from alumina and silicon nitride ceramics. The main parameters affecting the material removal characteristics of laser milling are reviewed. A new technique for machining alumina components is proposed emphasising the importance of correct set-up design in achieving a high level of accuracy. Process parameters influencing part quality are analysed and guidelines for machine set-ups are formulated. The paper concludes with an assessment of the accuracy of the laser milling process. r 2006 Elsevier Ltd. All rights reserved. Keywords: Laser micro machining; Micro machining; Milling
1. Introduction Conventional technologies for the manufacture of ceramic components are based on sintering processes requiring expensive tooling and for this reason are best suited to high-volume production. Recently, new uses have been found for ceramics, for example in medical and surgical devices, where small batches in a diversity of complex shapes incorporating fine intricate features are required [1]. This paper examines the feasibility of laser milling as a method of producing ceramic components in low volumes from blanks that can be manufactured easily, for example, by sintering or reaction bonding. Laser milling is a relatively new machining 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. Provided appropriate process parameters are adopted and the material is not transparent to the laser, laser milling can be readily utilised to remove materials, such as ceramics, that are difficult to machine by conventional means [2,3]. Corresponding author. Tel.: +44 29 20874429; fax: +44 29 20874003.
E-mail address: [email protected] (D.T. Pham). 0890-6955/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2006.05.002
The products chosen for this study are: a prototype ceramic cavity tool and a microsurgical tool in which the ceramic part acts as an insulator located between a tissuecutting electrode and the metal housing. The geometry of the ceramic cavity tool is shown in Fig. 1. The cavity has overall dimensions: 3 mm 1.5 mm 0.35 mm.The properties of the materials used are listed in Table 1 ( for alumina [4]) and Table 2 (for silicon nitride [4]). The geometry of the ceramic microsurgical component, which has overall dimensions 6 mm 2.9 mm 1.18 mm, is shown in Fig. 2. Note that both sides of the part have cavities, as well as a hole and a slot, which means that repositioning of the workpiece is necessary during manufacture. Because the laser spot diameter is very small (approximately 45 mm), very high set-up accuracy is required and repositioning is not desirable. The properties of the material used (alumina 23) are listed in Table 3 [5]. The paper considers the technical capabilities of laser milling when applied to the machining of the parts. Parameters influencing part quality are analysed and guidelines for machine set-ups are formulated. The paper concludes with an assessment of process accuracy.
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Fig. 1. Ceramic cavity tool geometry.
Table 1 Material properties of alumina [4] Material Alumina Al2O3 (Goodfellow)
Units
Apparent porosity Density Volume resistivity at 25 1C Thermal conductivity at 20 1C Upper continuous use temperature
% g/cm3 O cm W/m K 1C
0% 3.9 41014 26–35 1700
Table 2 Material properties of silicon nitride [4] Material Silicon Nitride SI3N4 (Goodfellow)
Units
Apparent porosity Density Volume resistivity at 25 1C Thermal conductivity at 20 1C Upper continuous use temperature
% g/cm3 O cm W/m K 1C
15–23 2.4 4107 10–16 1200–1500
Fig. 2. Ceramic component. Table 3 Material properties of alumina 23 [5] Material DIN VDE 0335/IEC 672
2. Physical factors affecting the process Ceramics are materials with strong bonds. These bonds are either covalent or ionic in nature. The main difference between ceramics and metals is related to the fact that the Fermi energy of the electrons is between the valence and conduction bands. For ceramic materials that are considered to be semiconductors, the gap between the bands is narrow, so that a relatively small amount of energy is sufficient to promote electrons to the conduction band. For ceramics that are insulators, a larger amount of energy is required to excite electrons to the conduction band. The removal of material during laser milling is affected by the characteristics of the laser beam and the workpiece
Principal constituent Apparent density Main grain size Open porosity Compressive strength Bending strength Modulus of elasticity Melting point/maximum working temperature Specific heat Thermal conductivity at 100 1C/ 1000 1C
Units g/cm3 mm % N/mm2 N/mm2 GPa 1C
99.7% Al2O3 3.7–3.95 10 0 3500 300 380 2030/1950
J/kg K W/m K
900 30/5
but is mainly determined by the way the two interact. The most important factors influencing this interaction are discussed below.
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2.1. Laser radiation features Laser radiation can be controlled and modulated in an ordered sequence of pulses with a predetermined 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. This significantly increases the energy density (fluence) and power density (intensity) in the area of the spot. Thus, extremely high intensities (1013–1018 W/cm2) not achievable by any conventional machining technique can be unleashed in the laser-material interaction zone. This explains why laser milling can successfully process materials that are difficult to machine using conventional methods. The wavelength of the laser is also a factor that affects laser milling. Laser radiation is monochromatic, which means that stable processing conditions can be maintained because the substrate response and properties remain relatively uniform. The ablation mechanism that governs the material removal process is affected by an important laser radiation feature, its photon energy. Finally, interaction between laser radiation and the plasma formed in the process must be taken into account as it reduces overall machining efficiency [6]. 2.2. Substrate material features The response to laser radiation is influenced by a number of characteristics of the workpiece material. Laser ablation occurs only when the substrate material absorbs strongly in the wavelength of the transmitted radiation. Hence, to optimise machining results, it is essential to match the laser source with the material being worked. Generally, higher absorption efficiency leads to more effective laser milling. Laser absorptivity [7] can be increased in a number of ways, for example, by varying the surface finish or applying a surface coating. The thermal conductivity of the substrate is another key factor. Material removal efficiency is affected by the dissipation of absorbed energy within the bulk of the material, other energy losses, and the dimensions of the heat affected zone. Ceramic materials, which have the highest melt and vaporisation temperatures but the lowest thermal conductivity are most easily machined by laser radiation. During laser ablation the affected material passes through several transition phases depending on the process parameters. 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. 2.3. Laser radiation absorption mechanisms 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 [8]. 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 heataffected zones. The Beer–Lambert law which represents the classical understanding of light absorption states that, when a particular wavelength of light is transmitted through a material, its absorption is a function of the material path length and independent of the incident intensity. 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. Effects such as avalanche ionisation and multiphoton absorption can be observed. With an electron avalanche, electrons of an intermediate energy level are excited into the conduction band by the absorption of single photons. These excited electrons collide with bound electrons in increasing numbers leading to a cascade of electrons leaving the valence band. Multiphoton absorption occurs when an electron transfers from the valence band to the conduction band by absorbing several photons. In general, when a material reaches a critical electron density it starts absorbing photon energy at a sufficient level to undergo ablation [8]. A laser radiance fluence threshold exists which is material dependent. Material removal by laser ablation cannot begin until this threshold has been exceeded as shown in Fig. 3 [9]. 2.4. Laser ablation mechanisms Three of the most important factors controlling the laser ablation process are:
te—electron cooling time, ti—lattice heating time, tL—laser pulse duration.
Material removal
620
Optimal range
No material removal
Material removal
Fig. 3. Laser beam intensity.
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As a rule teooti and for most materials ti is in the picosecond range. Three different ablation regimes exist depending on the laser pulse length.
femtosecond pulses—tLoteotI, picosecond pulses—teotLoti, nanosecond and longer pulses—teotiotL.
The laser milling machine used in this work operates with the longer type of pulses for which process conditions are summarised in Fig. 4. In this case, the absorbed energy melts the material and heats it to the vaporisation temperature but there is a significant time delay whilst the thermal wave propagates into the material. Evaporation occurs from the liquid material. Secondary effects include a heat-affected zone, a recast layer, microcracks, surface damage caused by shockwaves and debris from ejected material. Additionally, the vaporised material forms a hot plasma from the leading edge of the pulse and this plasma is sustained during the rest of the pulse. Due to the plasma shielding effect (the plasma absorbs and defocuses the pulse energy) a higher fluence or irradiance for deeper penetration is required. 3. Set-up design The blanks (workpieces) for the cavity tool were rectangular blocks measuring 50 mm 50 mm 5 mm. Fig. 5 shows the workpiece and the three locating datums A, B and C. As the cavity is fully enclosed in the workpiece, there were no special requirements for set-up. The cavity was produced in one sequence, employing closed–loop control with a touch–probe and a light sensor as feedback means [10]. The blank for the microsurgical part was a rectangular block measuring 10 mm 4 mm 2 mm. The component was virtually centred in the blank and manufactured in three operations. In the first two operations, closed-loop control (by means of a touch-probe to control depth) was employed in order to achieve the required part accuracy in the Z direction (perpendicular to the milling plane). In the third operation, the pulse Ejected molten material
Lens Plasma plume Damaged ajacent structure
Recast layer Surface debris
Heat affected zone
Shock waves Microcracks Melt zone
Heat transfer
Fig. 4. Nanosecond and longer pulse laser ablation.
Fig. 5. Cavity blank set-up. Datums A, B and C are locating datums.
through hole and slot were machined. This did not require accuracy in the Z direction and for this reason open-loop (without touch-probe feedback) machining was performed. Fig. 6 shows the sequence of operations, intermediate component geometry and set-up datums during each operation. The following notation was used to label the datums: datum A—locating datum, datum B—temporary datum and datums C and D—check datums. For the first operation, datum A was the ground flat surface of the blank. For the second and third operations, the uppermost flat surface was used as datum A. In all operations, the straight side wall of the blank was utilised as the temporary datum B. The actual location of the workpiece (datums C and D) in the XY plane was determined by a co-ordinate measuring machine, fitted with a CCD camera, to obtain the required positioning accuracy. Normally, a metal plate would be used as a support base. However, it could not be employed here as the third operation involved producing a through hole. This would have brought the laser beam into contact with the metal plate, creating vapour, which would cause the part to shift and the operation to fail. To avoid this situation a glass support plate was used. Glass is transparent to the laser employed, a Nd:YAG laser in the 1064 nm waveband [10], and therefore does not absorb sufficient energy for vapour to be produced when the laser beam pierces through the material. As a further precaution, a transparent adhesive was used to bond the workpiece firmly to the glass plate and the plate itself was held sufficiently far above the metal base of the jig to ensure that the laser was not focused when it reached the base. 4. Influence of process parameters on final part quality The main requirements to be observed when selecting values for the process parameters are:
a stable process, the specified dimensional accuracy, an acceptable surface finish, reasonable cycle times.
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Fig. 6. Sequence of operations.
Several single-factor experiments were carried out to determine the influence of the process parameters on the process output. Although they do not provide a complete picture of the different phenomena involved because interactions between the factors are not considered, the experiments give an idea of the influence of the process parameters on a particular target process output, facilitating process tuning and reducing the amount of trial and error involved in process adjustment.
Fig. 7 presents the results obtained. It can be inferred that the most important process parameter with respect to the layer thickness and the removal rate is the current in the lamp used to generate the laser beam (Fig. 7(a)). This is because the lamp current directly influences the energy of the laser pulses and the pulse peak power. This was the reason for using the lamp current as an approximate process tuning parameter.
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The effect of pulse duration (Fig. 7(b)) is also linked to the pulse peak power. It was noticed that for longer pulses (tL44 ms) with lower peak powers the surface finish was poor and there was also a change in the surface colour. The latter meant that the chemical structure might have been altered. In addition, for pulses longer than 4 ms, the process was not stable resulting in a wide variation in layer thickness. The process window with regard to pulse duration was found to be in the range 2–4 ms. In general, shorter pulses are preferred for the machining of ceramics, because they give higher pulse peak powers, when the other parameters are fixed. Pulse frequency (Fig. 7(c)) has two main influences on laser milling results. It directly controls the degree of overlapping of the craters produced by the laser pulses. In machining ceramic materials, lower pulse frequencies are more efficient, because the pulse energy required for material removal is higher than that for other materials. The scanning speed (Fig. 7(d)) also influences the overlap between craters, the lower the speed, the grater the overlap. When the overlap increases, the depth per layer also increases, because more material is subject to multipulse radiation. From the single-factor experiments, it can be concluded that in order to achieve a stable process for machining ceramics, the following should be adopted:
small pulse duration for higher pulse peak power, low pulse frequency for adequate pulse energy, low scanning speed for better surface finish.
Generally the lower the scanning speed, the better the surface finish. However, below a speed threshold, the opposite effect can be observed—the roughness increases when the scanning speed reduces. It should be stated that pre-machining (to remove one or a few layers of material using the laser milling machine) is required to assure uniform absorption and process stability. 5. Analysis of the machining results
Fig. 7. Influence of the process parameters on the process output.
For the cavity tool, two kinds of ceramics with different laser light absorption properties were selected (Fig. 8 [4])— Alumina ceramic and Silicon Nitride ceramic. Alumina ceramic absorbs approximately 10% of the laser energy for the wavelength employed, while the absorption for Silicon Nitride is about 90%.In order to determine the process repeatability, three cavities were produced in each workpiece. Fig. 9 shows the errors obtained for the Alumina ceramic and Silicon Nitride ceramic parts. Clearly, the higher absorption properties of the latter generally helped machining accuracy. Furthermore, secondary effects (HAZ, recast layer) of laser milling were reduced due to the lower laser intensity adopted when processing materials with such good absorption properties.
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A batch of 23 microsurgical components was manufactured. The key dimensions of the components were measured. The results are given in Table 4. The dimensions in the slice plane XY are divided into two groups—internal and external(I/E). For the internal dimensions (I), hatching (material removal by laser as shown in Fig. 10) was performed from the inner side of the contour containing the corresponding dimension, while for the external dimensions hatching was from the outer side. For dimensions in the Z direction, depth control using the touch probe was employed. It was observed that for most internal dimensions, negative deviations from the nominal value were obtained, indicating that the actual dimensions were smaller than the nominal. For external dimensions
Fig. 8. Laser light absorption [2].
Fig. 9. Errors for the two ceramic materials (mm).
Table 4 Measurement results for the microsurgical tool Nominal
Average
Standard deviation
Median
Range
Deviation from nominal
Type
6s
IT Grade
X 1.35 3.75 4.75 5 6
1.375 3.715 4.687 4.853 6.039
0.028 0.034 0.021 0.029 0.025
1.373 3.714 4.679 4.844 6.035
0.124 0.161 0.088 0.14 0.108
0.025 0.035 0.063 0.147 0.039
I I I I E
0.167 0.205 0.125 0.176 0.153
IT14 IT14 IT12 IT13 IT13
Y 0.5 0.5 1.7 2.5 2.9
0.464 0.468 1.565 2.54 2.971
0.019 0.019 0.031 0.047 0.029
0.468 0.473 1.557 2.546 2.978
0.094 0.09 0.125 0.167 0.122
0.036 0.032 0.135 0.04 0.071
I I I E E
0.115 0.111 0.186 0.283 0.175
IT13 IT13 IT13 IT14 IT14
Z 0.25 0.3 0.52 1.18
0.231 0.284 0.503 1.178
0.020 0.021 0.02 0.018
0.24 0.29 0.51 1.18
0.07 0.1 0.09 0.06
0.019 0.016 0.017 0.002
0.119 0.129 0.122 0.107
IT13 IT13 IT13 IT12
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1
3
625
6. Conclusion
4
2
5
6
1. Border cuts 2. Hatching cuts 3. Laser spot with its diameter 4. Border cut track displacement (step-over) 5. Hatch track displacement (step-over) 6. Distance between the end of the hatch line and the innermost border cut Fig. 10. Material removal in the laser milling process. 1. Border cuts. 2. Hatching cuts. 3. Laser spot with its diameter. 4. Border cut track displacement (step-over). 5. Hatch track displacement (step-over). 6. Distance between the end of the hatch line and the innermost border cut.
(E), the situation was the opposite and the actual dimensions were larger than the nominal. This could be explained by the fact that the effective laser spot diameter was smaller than the value employed in generating the laser path. It was observed that, as the machined dimension increased, the absolute value of the deviation from the nominal also increased. This is likely to have been caused by changes in both the diameter and shape of the effective laser spot as the beam moves away from the optical axis of the system. However, the dimensional changes were not significant as shown by the measurements. In addition, the deviation from the nominal was also due to losses of accuracy during the generation of the laser path. Table 4 shows that most of the actual tolerances for the measured dimensions were between ISO Grades IT12 and IT14, which is within the prescribed tolerances for the component. For a small number of non-critical dimensions, the measured values were outside the specified limit, but this did not affect part functionality.
Laser milling is a cost-effective process for manufacturing small batches of ceramic 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 setting-up of the workpiece is most critical when small parts are to be manufactured. In cases where a through feature has to be machined, it is recommended that the support be made from a material that cannot be laser milled as vapour generated from the support may damage the part or influence its quality. It is suggested that, for a machine employing a laser with a wavelength l ¼ 1064 nm, a glass support plate be used to hold the workpiece. The influence of the process parameters on laser milling is complex and must be optimised to obtain the highest part quality. Approximate tuning of the process for machining of ceramics can be achieved by controlling the lamp current and fine adjustment made by manipulating the scanning speed and/or pulse frequency. It is strongly recommended that closed-loop control of the depth be used whenever possible in order to avoid instability in the layer thickness and the associated accumulated errors. Acknowledgements The authors would like to thank the Welsh Assembly Government, the Department of Trade and Industry and the Engineering and Physical Sciences Research Council for funding this research under the ‘‘SUPERMAN’’ and ‘‘Micro Tooling Centre’’ ERDF Programmes, the ‘‘MicroBridge’’ MNT Programme and the ‘‘Cardiff Innovative Manufacturing Research Centre’’ Innovative Manufacturing Programme. Also, this work was carried out within the framework of the EC FP6 Networks of Excellence ‘‘Innovative Production Machines and Systems (I*PROMS)’’ and ‘‘Multi-Material Micro Manufacture: Technologies and Applications (4M)’’. The authors gratefully acknowledge the support given to the Networks by the European Commission. References [1] K. Chen, Y.L. Yao, Process optimisation in pulsed laser micromachining with applications in medical device manufacturing, International Journal of Advanced Manufacturing Technology 16 (2000) 243–249. [2] D. Hellrung, L.Y. Yeh, F. Depiereux, A. Gillner and R. Poprawe, High-accuracy micromachining of ceramics by frequency-tripled Nd:YAG-lasers, in: Proceedings of the SPIE Conference on Laser Applications in Microelectronics and Optoelectronics, San Jose, California, 1999, pp. 348–355.
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[7] J. Takacs, Laser technology—technology for the future, in: Proceedings of the Conference on Mechanical Engineering, vol. 2, Budapest, Hungary, 1998, pp. 515–519. [8] M.D. Shirk, P.A. Molian, A review of ultrashort pulsed laser ablation of materials, Journal of Laser Applications 10 (1) (1998) 18–28. [9] J.H. Yoo, S.H. Jeong, R. Grief, R.E. Russo, Explosive change in crater properties during high power nanosecond laser ablation of silicone, Journal of Applied Physics 88 (2000) 3. [10] Lasertech GmbH. Operation manual, DMG Lasertec GmbH, Tirolerstrasse 85, D 87459 Pfronten, Germany, 1999.