Laser microsintering of tungsten in vacuum

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1 Laser microsintering of tungsten in vacuum Robby Ebert, Frank Ullmann, Lars Hartwig, Tino Suess, Sascha Kloetzer, Andre Streek, Joerg Schille, Peter Regenfuss, Horst Exner Hochschule Mittweida, Technikumplatz 17, Mittweida, Germany ABSTRACT Laser microsintering of tungsten powder is investigated as a function of laser output power, pulse interval and vacuum level. The intensities are calculated for the evaporation thresholds of tungsten powder particles of various sizes. In addition, the powder layer generation and the resulting layer thicknesses are calculated. The powder abrasion occurring during the process was taken into consideration. Polished sections and REM images were prepared in order to analyse the experimental outcomes. The dependence of sinter density on the parameters is discussed. 1. INTRODUCTION Laser microsintering is a relatively recent process for fabricating microstructured components from metals and ceramics [1-3]. By using short, high-intensity pulses, even metals with a high melting point such as e.g. tungsten can be machined. The achievable resolution of this approach is better than 30 µm. This method, therefore, is currently the only one that permits direct fabrication of free-formed microstructures with undercuts. Tungsten is difficult to work mechanically. µmim is the only method available at present for fabricating microparts. It is used predominantly for making microstructural components for the illuminant industry. Due to the material s unusual physical and chemical properties, new manufacturing methods could be exploited to develop further fields of application in the high-temperature range, in x-ray technology, in microsystem engineering and in medical engineering. Up to now, laser microsintering of tungsten has usually been carried out in a protective gas environment. Our investigations, carried out under different vacuums, are intended to provide further information about the mechanism involved and about possible approaches for influencing the sintered structures. 2. EXPERIMENTAL PRINCIPLES An existing laser microsintering facility was used for the investigations [4]. It consists of a Nd:YAG laser (Table 1), a modulator for controlling the pulse, a scanner for beam deflection and focusing, and a vacuum sintering chamber for performing the experiments under defined conditions. Tab. 1: Laser parameter. wavelength pulse duration pulse frequency beam radius in focus λ [nm] t [ns] f [khz] ω 0,86 [µm] µm The variable parameters included the incident laser power P, which was varied between W and consequently the pulse energy W p ranged between mj, the pulse power P P between W, the pulse interval a between µm and the vacuum pressure p between mbar. Helium was used as residual gas. In order to adjust the intensity, the incident beam spot was defocused vertically in all the experiments by 175 µm away from the powder s surface, which leads to a calculated beam radius ω 86 (z = -1,75*10-6 ) of 17.5 µm at the powder s surface V. 3 (p.1 of 12) / Color: No / Format: A4 / Date: :04:56 AM

2 Each layer was irradiated with a regular pulse pattern at equal pulse intervals over an area of 1000x1000 µm 2. Relatively uniform irradiation was achieved gradually by rotating the pulse pattern by 139 and performing 8 passes per layer. A pulse interval of 45 µm produced 3950 pulses per dispensed layer area (1 mm 2 ); a 60 µm interval produced 2222 pulses, and a 75 µm interval 1422 pulses respectively. The variation in power and pulse intervals led to variation in the mean laser radiation fluence E m incident on the surface being sintered, ranging between J/cm 2. The intensity I F at the focus varied between 1.46 and 4.38 x 10 8 W/cm 2, and the intensity I O at the powder s surface between 0.58 and 1.74x10 8 W/cm 2. The effective laser power is subject to some uncertainty in terms of an undefined dirt contamination of the chamber s coupling pane. The sintering process can cause a coating of the pane with material. This results in decreased laser power incident on the process site. At higher laser powers, in turn, self-cleaning of the pane may occur. 3. THE POWDER DISPENSING The experiments were carried out with tungsten powder supplied by Goodfellow. The measured bulk density was 2.8 g/cm 3. The grain shape is polyhedral. The powder has a strong agglomeration tendency. The manufacturer specifies a grain size < 1 µm. Our own measurements showed that there is a maximum in the number of particles at a grain size of 300 nm (Fig. 1), and a maximum in terms of volume percentage at 2 µm. Fig. 1: Number of particles against particle size for Goodfellow tungsten powder < 1 µm. The powder s layer generation was performed with a special cylindrical dispenser. In this way, it was possible to achieve a relatively dense, uniformly thin layer. Before the dispensing, the carrier platform was lowered by 1 µm at a time. To determinate the thickness of the resulting dispensed powder d a mathematically description of the cyclical process of powder dispensation and lowering of carrier platform is given below with following variables: d - thickness of dispensed powder layer d Z - lowering of carrier platform ρ - density ρ P - bulk density of powder ρ S - density of sintering body V. 3 (p.2 of 12) / Color: No / Format: A4 / Date: :04:56 AM

3 ρ P = D P ρ - relative powder density ρ S = D S ρ - relative density of sintering body D P = D S ρ P = D - density coefficient. ρ S In general assuming no powder losses during laser sintering process (Δm = 0) it is in force: d 1 = dz d 2 = dz + d 1 (1-D), can be transformed in d 2 = dz (1 + (1-D) 0 (1-D)) d 3 = dz + d 2 (1-D), can be transformed in d 3 = dz (1 + (1-D) 1 (2-D)), d 4 = dz + d 3 (1-D), can be transformed in d 4 = dz (1 + (1-D) 2 (3-D) + D (1-D)) d 5 = dz (1 + (1-D) 3 (4-D) + D ((1-D) + 2(1-D) 2 )) d 6 = dz (1 + (1-D) 4 (5-D) + D ((1-D) + 2(1-D) 2 +3(1-D) 3 )) For k > 2 a polynominal series can be arranged as follows: d k = dz (1 + (1-D) k-2 (k-1-d) + D ((1-D) + 2(1-D) 2 +3(1-D) 3 + +k-3(1-d) k-3 )) d k = dz (1 + (1-D) k-2 (k-1-d) + D n 3 ( n 3 )( 1 D ) ). k 3 For 0 < D < 1 and k gravitating infinite summing up of the dispensed powder thickness d is in force: n 3 d = dz ( D ( n 3 )( 1 D ) ). Because of the partial sum 3 x nx n = 2 0 ( x 1) dispensed powder thickness can be summarized as follows: 1 D d = dz (1 + D ) 2 (1 D 1) d = dz ( d = D dz. D +1 D ) D Considering the mathematical derivation above, powder layer thickness d is inversely proportional to powder density factor D and proportional to the lowering of carrier platform dz. With tungsten powder bulk density and the relative density of the sintered body approximated to 0.7, the density coefficient can be calculated to D = Respectively a constant dispensed powder thickness of d = 4.81 µm can be expected after infinite dispensing and sintering cycles. The calculated powder layer thickness is shown in Fig. 2; after 20 dispensing and sintering cycles a constant is reached. During the laser sintering process the powder layer will be compacted to a 1 µm thick sintering layer. In general, after some tens dispensing cycles the supplied powder height always manifests itself as a sintered layer thickness V. 3 (p.3 of 12) / Color: No / Format: A4 / Date: :04:56 AM

4 6 powder layer thickness (µm) number of dispensing and sintering cycles Fig. 2: Calculated tungsten powder layer thickness vs. the number of dispensing und sintering cycles (lowering per dispensing cycle dz = 1 µm, rel. powder bulk density D P = , rel. sinter density D S = 0.7, Δm = 0) In all 176 experiments carried out, layer dispensing took place 400 times after lowering the active space by 1 µm. Hence, assuming no mass losses and a relative laser sintering density of D S = 0.7, this would imply a built-up height of µm (total dispenser height 400 µm powder layer thickness 4.81 µm + sinter layer thickness 1 µm). 4. THE LASER MICROSINTERING PROCESS The use of high pulse powers and intensities in laser microsintering brings about a very complex process [5]. When the laser pulse irradiates the powder s surface, some of the powder is melted by absorbed beam energy, and in turn part of that material is vaporised. The result is that partly vaporised powder particles and separate molten material are accelerated into the powder bed through recoil during vaporisation [6, 7]. Thus far, energy absorption and dissipation have remained largely unexplored. Our own previous experiments have established that the intensity required for sintering depends very strongly (up to a factor of 20) not only on the powder s grain size but also on its size distribution and on the grains shape. It is conjectured that in dependence on the proportion of nanoscale powder particles present and on the particles shape (sharp edges and vertices), optical near-field effects such as e.g. the local field amplification at nanoparticles play a part [8]. The literature contains a temperature estimate for laser microsintering, applicable to metal particles with a diameter of 10 µm and a laser beam focus diameter of 100 µm [9]. At the intensity used, the temperature rise with a single pulse was too low for sintering. By using overlapping pulses, however, it proved possible to bring the material to melting. The intensities required to reach the boiling point, across different particle sizes with diameters ranging between 0.05 and 5 µm, were calculated with the help of a temperature field program [10], as a first approximation for the effect of powder grain size on a laser microsintering process utilising single pulses. The program takes into account both the melting and the vaporisation energy. Individual tungsten particles were calculated; which were assumed to be freely present in the space, without taking into consideration heat conduction into the powder bed or into the agglomerate such as occurs during a real sintering process. The calculated intensities can thus be regarded as an estimate for the lower limit. The particle shape was cylindrical, with the diameter and height set to be equal. The laser pulse duration was 100 ns and the laser beam radius 10 µm. The absorption rate was set to Particles with a diameter > 2 µm require an almost constant intensity of 5-6*10 7 W/cm 2 in order to reach the vaporisation temperature at the surface (Fig. 3). In particles with a diameter below 2 µm, the required intensity decreases almost linearly with particle size V. 3 (p.4 of 12) / Color: No / Format: A4 / Date: :04:56 AM

5 7 6 intensity (10 7 W/cm 2 ) particle diameter (µm) Fig. 3: Calculated intensity to reach the vaporisation threshold as a function of particle size in tungsten, with a laser wavelength of 1064 nm When the powder s particle size distribution includes nanoscale components, it would therefore be difficult to find parameters under which the powder could be processed uniformly, since the nanoparticles would vaporise completely even at relatively low intensities of < 1*10 7 W/cm 2. But since the temperature gradients that are present between the powder particles in the real powder bed are dissipated through heat conduction and radiation, processing of powders that exhibits a wider range of grain sizes is possible also. The calculated intensities reflect the experimentally used parameters, taking the boundary conditions into consideration. Simultaneously with the powder s heating, and as a function of the pressure and of the type of residual gas present, a process plasma is formed which exhibits a far greater impact duration (> 100 µs) than the laser pulse itself. The plasma expands and initially exerts a pressure of up to 100 MPa on the powder s surface and on the molten material produced [6], after which it leaves behind a temporary vacuum. As a result of the processes taking place, one obtains a horizontally and vertically cross-linked stable structure with a density of 40-95%. 5. RESULTS AND DISCUSSION The experiments were analysed in terms of the morphology of the fabricated sintered structures and the achieved sintered structure height, using polished sections; selected specimens also underwent REM investigations. An attempt was made to use optical methods to determine the exact sinter density. This led to a very high degree of variance between different methods, such that no absolute values can be stated. Nevertheless, it was possible to observe a number of trends that depend on the parameters chosen. At the lowest laser power of 0.8 W, it was only with a pressure of 500 mbar and a pulse interval of 45 µm that a complete layer buildup could be observed, which nonetheless led to a structure with low density and visible cracks (Fig. 4, left). At lower pressures or longer pulse intervals, the buildup of the sinter structure is broken off at small heights (Fig. 5). In other words, complete layer buildup with a laser power of 0.8 W required, in addition to an intensity of 0.58*10 8 W/cm 2 at the powder s surface and a pulse interval of 45 µm, also a corresponding residual gas pressure of > 100 mbar. At the next investigated power of 1.2 W, however, a complete sinter structure was generated at all pulse intervals and pressures other than 0.01 mbar. From a power of 1.5 W upwards, complete generation was generally possible at all pressures. Based on visual assessment, sinter density depends on laser power, pulse density and pressure V. 3 (p.5 of 12) / Color: No / Format: A4 / Date: :04:56 AM

6 Please verify that (1) all pages are present, (2) all figures are acceptable, (3) all fonts and special characters are correct, and (4) all text and figures fit within the P = 0.8 W P = 1.2 W P = 1.5 W P = 1.8 W P = 2.1 W P = 2.4 W h = 381 µm h = 391 µm h = 368 µm h = 336 µm h = 327 µm h = 314 µm Fig. 4: Macro images of polished sections, section width 200 µm, p = 500 mbar, a = 45 µm p = 0.01 mbar, P = 0.8W, a = 45µm p = 500 mbar, P = 0.8W, a = 75µm Fig. 5: Macro images of polished sections of incomplete sinter structures It can be seen clearly that at a pressure of 500 mbar, the sinter density increases with an increasing applied laser power (Fig. 4). Since frequency and pulse duration were always held constant, this means that the pulse energy, the pulse power and ultimately the intensity were increased in equal measure and thus affect the sintering density (Table 2). Tab. 2: process parameter laser power P [W] pulse energy Wp [µj] pulse power PP [W] intensity in focus IF [108 W/cm2] intensity at the powder s surface IO [108 W/cm2] An increase in sinter density at smaller pulse intervals was also observed at this pressure (Fig. 6, left). At a pressure of 0.01 mbar, however, this was no longer observed (Fig. 6, right) V. 3 (p.6 of 12) / Color: No / Format: A4 / Date: :04:56 AM

7 Please verify that (1) all pages are present, (2) all figures are acceptable, (3) all fonts and special characters are correct, and (4) all text and figures fit within the p = 500mbar p = 0.01 mbar a = 75µm a = 60µm a = 45µm a = 75µm a = 60µm a = 45µm h = 329 µm h = 343 µm h = 336 µm h = 218 µm h = 222 µm h = 215 µm Fig. 6: Macro images of polished sections, section width 200 µm, P = 1.8 W Furthermore, it was established that the carrier platform was abraded at the start of the build-up processes (Fig. 7). This can be explained inter alia by the fact that aluminium, the platform material used for technical reasons, has a considerably lower evaporation temperature than tungsten. As regards parameter variation, it was observed in particular (as expected) that higher power contributed to higher platform abrasion. p = 0.01 mbar, a = 45µm, P = 1.8 W p = 0.01 mbar, a = 75µm, P = 1.8 W p = 500mbar, a = 45µm, P = 2.4 W p = 500mbar, a = 75µm, P = 2.4 W Fig. 7: Macro images of polished sections, section width 300 µm The pulse interval had no clear effect in this regard. At low pressure, a shorter pulse interval led to higher platform abrasion (highest measured value was 67 µm). Sometimes the sintered body s lower boundary areas were strongly rounded off (Fig. 7, p = 500 mbar, a = 45 µm, P = 2.4 W). This cannot be explained logically by the procedure used, since the same areas were always irradiated successively. The REM images of polished sections in Fig. 8 show that at moderate power and low pulse intervals, the sinter density increases with pressure. It is estimated that when the pressure varies from 0.01 mbar to 500 mbar, it increases approximately between 10 and 20%. Additionally, the built-up height of the sinter cubes as a function of pressure was determined from images of polished sections (Fig. 9). In the pressure range of 1 to 500 mbar there is a linear increase in the built-up height, whereas before that the built-up height increased in the direction of even lower pressures, such that a local minimum in built-up height was observed around a pressure of 1 mbar. The pulse interval had no significant effect on the height V. 3 (p.7 of 12) / Color: No / Format: A4 / Date: :04:56 AM

8 p = 0.01 mbar p = 1 mbar p = 10 mbar p = 100 mbar p = 500 mbar Fig. 8: REM images of polished sections, section width 50 µm, P = 1.8 W, a = 45 µm 400 built-up high (µm) µm 60µm 75µm ,001 0,01 0, chamber pressure (mbar) Fig. 9: Measured height of the sintered body as a function of chamber pressure, P = 1.8 W, Parameter Pulse interval a As a further result, at a pressure of 500 mbar the built-up height decreased with increasing laser power (Fig. 4, measured with a microscope), but again it was independent of pulse interval (Fig. 6, left). Correspondingly the powder layer calculation (Capt. 3), the height of the sintered body was expected to 396µm. Even with a assumed sintering density of 100% the height of the sintering body should not sink below 394 µm. However, calculated sintering height is in opposite to measured values in Fig. 9 and reduction in height seems to be mainly caused by ablation of both, tungsten powder or sintering layer during laser sintering built-up process. In consequence equation to calculate the powder layer thickness is modified with Δm > 0. It was assumed that the dispensed powder and the layer already sintered are reduced by a fixed amount during each sintering cycle, since it can be assumed that after a start-up phase the abrasion occurs relatively uniformly V. 3 (p.8 of 12) / Color: No / Format: A4 / Date: :04:56 AM

9 60 powder layer thickness (µm) number of dispensing and sintering cycles Fig. 10: Calculated powder layer thickness against the number of dispensing and sintering cycles (active space lowering per dispensing cycle 1 µm, rel. tungsten powder bulk density D P = , rel. sintering density D S =0.7, max. powder abrasion 20 µm, max. sinter layer abrasion 5 µm) The estimated values were 20 µm for the abraded powder layer thickness per irradiation cycle and 5 µm for the abraded sinter layer thickness. These values were estimated from previous experience. Since the layer thicknesses present during the first dispensing cycles are not yet sufficient, 90% abrasion was estimated for each powder layer thickness < 20 µm and sinter layer thickness < 5 µm. Considering the calculation, the equilibrium between material application, abrasion and sintering is reached after ca. 30 dispensing cycles at a powder layer thickness of 49 µm (Fig. 10). The height of a sintered part would be accordingly 352 µm and correlates to measured height obtained after sintering process at 1.8W laser power and 500mbar chamber pressure (Fig. 9). Therefore the ratio of body height losses caused by powder or sintering layer ablation can only be approximated, currently experimental data are not available. built-up high (µm) abrasion (µm) Fig. 11: Calculated built-up height as a function of abrasion during the build-up process (400 dispensing cycles, active space lowering dz =1 µm each time, rel. tungsten powder bulk density D P = , rel. sintering density D S =0.7, max. powder layer abrasion = 20 µm, max. sinter layer abrasion = 5µm) Accordingly the calculation the sintered part s built-up height decreases linearly with the abrasion as shown in Fig. 11. The abrasion of the dispensed powder layer goes into the calculation directly, and the abrasion of the sinter layer with 1/D (here 4.81) V. 3 (p.9 of 12) / Color: No / Format: A4 / Date: :04:56 AM

10 Additional abrasion of the substrate at the start of the sintering cycle as it occurs in practice, e.g. at a thickness of 50 µm, means that the equilibrium between powder dispensing, ablation and sintering is reached as early as after 10 dispensing cycles, with the sintered part s height remaining unaffected. The measured built-up heights as a function of the parameters and the calculations of layer heights, indicate that pressure- and power-dependent abrasion mechanisms exist during the build-up process. Essentially, these are direct vaporisation of powder and sinter material through the effect of the laser pulses, and indirect abrasion by the generated process plasma. The process plasma consists predominantly of material vapour plasma, and to some extent of residual gas plasma. The observations established that changing the type of residual gas affects the process plasma. Without material vapour, no residual gas plasma can be ignited under normal pressure at the intensities used. The threshold for generating the process plasma drops with pressure, since the mean free path of the particles increases. At lower pressure, a smaller field strength is needed to accelerate the particles up to their ionisation energy, since longer acceleration paths can be travelled without collisions. The laser focus has a diameter of 22 µm in the region of its greatest intensity, ca. 175 µm above the powder s surface. A mean free path of 22 µm is present at a pressure of 2.2 mbar. This was confirmed by optical measurements of the built-up height, where the smallest value of 175 µm was recorded at 2 mbar chamber pressure due to highest ablation (Fig. 9). This is likely to be caused by the strong plasma formation in this pressure range. At pressures < 1 mbar, the field strength needed to generate a residual gas plasma increases once again. Thus, plasma intensity drops again in this pressure range. This is consistent with the experimental results, according to which the sinter height increases slightly once again below a pressure of 1 mbar (Fig. 9). Generally speaking, the effect of process plasma increases with increasing laser power, such that ever greater vaporisation of the powder and sinter materials takes place and the built-up height decreases continuously. According to the model of the process, the sintering product s compaction should increase at the same time. With decreasing pressure, however, a more complex behaviour can be observed. At a pulse interval of 45 µm, the proportion of vaporised material increase, i.e. the built-up height decrease, and at the same times the sinter density also (Fig. 6 and 8). This is attributed to the effect of the plasma, contrary to the behaviour observed thus far at a pressure of 500 mbar. At a pulse interval of 75 µm, in contrast, although the proportion of vaporised material increases again (with a maximum at 1 mbar), the sinter density increases also, as judged visually (Fig. 12). This would correspond to the current model of the process. p = 500mbar p = 100 mbar p = 10 mbar p = 1 mbar p = 0.01 mbar h = 329 µm h = 303 h = 232 h = 186 µm h = 218 µm Fig. 12: Macro images of polished sections, section width 200 µm, P = 1.8 W, a = 75 µm This behaviour could indicate that the plasma s expansion at 500 mbar is too low to result in sufficient compaction at a pulse interval of 75 µm. At lower pressures, an increasing expansion of the plasma leads to higher compaction V. 3 (p.10 of 12) / Color: No / Format: A4 / Date: :04:56 AM

11 At a pulse interval of 45 µm, in contrast, apparently the plasma s overlapping is so great already that reducing the pressure, with the associated expansion of the plasma, has no effect. p = 0.01 mbar p = 1 mbar p = 10 mbar p = 100 mbar p = 500 mbar Fig. 13: REM images of the sintered body s surface, section width 50 µm, P = 1.8 W, a = 45 µm It was also observed that different sinter structures are formed as a function of chamber pressure (Fig. 13). At low pressures it was possible to note fine-grained, smooth structures on the sintered body s surface. As a result of the thick powder layers that need to be penetrated due to the low built-up heights, the laser pulses no longer impact the surface with such a high energy. The appearance of the resulting structure is as though the material had been sprayed on. It is open-pored, and therefore can be infiltrated or re-sintered. At high pressures, in contrast, smelt-like, relatively dense and ragged structures are formed. The laser beam s impact points can be recognised. Due to the thick necks and the closed pores, it is unlikely that these structures could be re-sintered and infiltrated. 6. SUMMARY AND PROSPECTS Mathematical calculations show that after some ten dispensing cycles a constant thickness of dispensed powder layer d will be obtained correlating to the density coefficient D and the height of periodical space lowering of the carrier platform dz. Furthermore sintering thickness is influenced by laser ablation and vaporization of powder layer or sintering body. The degree of particle vaporisation during laser microsintering depends on particle size, applied laser power and residual gas pressure. Despite relatively large vaporised layer thicknesses, after a few dispensing cycles the result is a constant dispensed layer thickness. The exhibited effects in terms of sinter density are as follows: - at 500 mbar: o higher power leads to greater abrasion and higher sinter density o higher pulse density does not lead to greater abrasion but does lead to higher sinter density - in vacuum: o the abrasion is generally higher o higher laser power leads to slightly greater abrasion and correspondingly higher sinter density o higher pulse density does not lead to greater abrasion nor to higher sinter density The sinter structure can be influenced by the variation in chamber pressure. A finer adjustment of the laser parameters will be necessary in future, in order to minimise the proportion of vaporised material. The possibility exists of building up bodies with partially modified structural properties. The resulting open porosity at low chamber pressures permits subsequent infiltration of the tungsten body by other materials. The investigations will continue with shorter pulses (6-10 ns, 250 fs) and even lower chamber pressures for a better plasma elimination caused by the remaining gas. Furthermore it will be examined to what extent post-processing (furnace sintering, infiltration) leads to modified properties (density, sinter structure, strength) of the sintered tungsten body V. 3 (p.11 of 12) / Color: No / Format: A4 / Date: :04:56 AM

12 ACKNOWLEDGEMENTS The authors wish to thank BMBF for sponsoring the Innoprofile project Rapid microtooling using laser-based methods (Ministry ref. no. 03IP506), A. Fischer and F. Ospald for providing the temperature field calculation program and H. Fritsch for assistance to calculate the dispensing powder layer thickness. REFERENCES [1] P. Regenfuss, L. Hartwig, S. Klötzer, R. Ebert, H. Exner, SME Technical paper TP04PUB185 (2004) [2] P. Regenfuss, L. Hartwig, S. Klötzer, R. Ebert, Th. Brabant, T. Petsch, H. Exner, Rapid Prototyping Journal, Vol.11, No. 1, (2005) [3] H. Exner, M. Horn, A. Streek, P. Regenfuß, F. Ullmann, R. Ebert, Proceedings of 3rd International Conference on Advanced Research in Virtual and Rapid Prototyping, Leiria (Portugal), (2007) [4] R. Ebert, P. Regenfuss, L. Hartwig, S. Klötzer, H. Exner, Proceedings of SPI, 4th International Symposium on Laser Precision Microfabrication, München (Germany), Vol. 5063, (2003) [5] P. Regenfuss, A. Streek, L. Hartwig, S. Klötzer, Th. Brabant, M. Horn, F. Ullmann, R. Ebert, H. Exner, Proceedings of the 5th LANE 2007, Erlangen (Germany), (2007) [6] R. Glardon, N. Karapatis, V. Romano, Annals of the CIRP 50, (2001) [7] J.P. Kruth, L. Froyen, J. Vaerenbergh,P. Mercelis, M. Rombouts, B. Lauwers, Journal of Materials Processing Technology 149 (1-3), (2004) [8] J.R. Krenn, F.R. Aussenegg, Physik Journal, No. 3, (2002) [9] P. Fischer, N. Karapatis, V. Romano, R. Glardon, H.P.Weber, Appl. Phys. A 74, (2002) [10] G. Reisse, R. Ebert, Applied Surface Science, Vol. 106, (1996) V. 3 (p.12 of 12) / Color: No / Format: A4 / Date: :04:56 AM