INDUSTRIAL LASER MICRO SINTERING

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1 INDUSTRIAL LASER MICRO SINTERING P. Regenfuß 1, R. Ebert 1, S. Klötzer 1, L. Hartwig 1, H. Exner 1, Th. Brabant 2, T. Petsch 2 1 Laserinstitut Mittelsachsen e.v. / Hochschule Mittweida, Technikumplatz 17, D Mittweida, Germany, exner@htwm.de 2 3D - Micromac AG, Max-Planck-Str.22b, D Chemnitz, Germany, info@3d-micromac.com Abstract As mechanical engineering industry encounters a growing demand of µ m-sized or µmstructured components and tools for an increasing range of applications, miniaturization is presently ranking among the most important goals in product and tool development. Compared to still higher resolving techniques, selective laser sintering (SLS) still bears the advantages of relatively low production costs and short processing times for uniques as well as small series productions of micro parts. Prismatic or tapered microstructures can be applied as electrodes for electro erosion, as tools for direct shaping of plastic materials or as molds for injection molding. Furthermore freeform - meaning tool-independent - undercuts and hollows can be realized, allowing e.g. the fabrication of miniature tools and components with hydrodynamic functions. Therefore SLS remains an attractive tool for the mentioned size range. Until recently commercial SLS units with a laser focus diameter of µ m were unable to generate metal micro parts smaller than 100µm. The Laser Institut Mittelsachsen e.v (LIM) in Mittweida, Germany, has developed a procedure and a device, which makes feasible the selective laser sintering of solid and structured parts out of metals and in the future ceramics. The obtained structures show a resolution of less than 30 µm for overall resolution. Contingent on the parameters, the generated bodies are either firmly attached to the substrate or can be dissevered by a non-destructive method. The technique has been successfully applied to produce functional micro tools or micro-components for tools from powders of refractory as well as lower melting metals in steps of 1µm thick sintered layers. Keywords: Freeforms, Micro Structures, Nanopowders, Selective Laser Sintering, SLS, Tungsten 1 Introduction In early 2003 the Laserinstitut Mittelsachsen e.v. announced a newly developed modification of SLS meanwhile referred to as laser micro sintering - that shifted the resolution of selective laser sintering below the limits commercial SLS devices had been confined to [1-3]. The obtained structures show a resolution of less than 30 µm for overall resolution, of 20µm for ligaments and of 10µm for notches at aspect ratios of 12 and above and presently a minimal roughness R a of 1.5µm.

2 The achieved progress was due to innovations in two fields: - A novel technique and equipment for the handling and processing of sub-µm sized metal powders. - A new laser sintering technique of applying q-switched pulses in such a way that 3Dbodies could be generated with the above mentioned resolution in geometry. The innovation concerning the powder handling and processing unit consists firstly of the realization of the idea to perform SLS with powder kept under vacuum or a keenly controlled shielding or reaction atmosphere. The origin of this development dates back to a patent applied in 1999 [4]. Secondly, the powder coating equipment and regime is also crucial for successful laser micro sintering, especially when executed with sub-µm-sized powder. The improvement in the laser regime consist of the choice of an Nd:YAG laser operated in mono-mode, the employment of q-switched pulses and the strategy of setting the pulses to generate the cross sections of the micro body in the respective powder layers. Most metal SLS techniques employ continuous wave laser radiation. But there are also reports on SLS with laser pulses: In 1999 Y. Kathuria [5] used a pulsed (not q-switched) Nd:YAG laser to sinter a powder blend of copper and tin and a cermite powder containing nickel, molybdenum, tungsten and borine with an average grain size of 30-60µm. He generated prismatic structures with a minimal height of 2.5mm and a resolution of minimally 221µm. He described the type of the process as liquid phase sintering. With a frequency doubled q-switched Nd:YAG laser J. Chen et. al. [6] produced structures from lead powder with an announced resolution of 100µm and a height of 2mm. P. Fischer et al [7] reported and described the structure and consistency of titanium layers sintered with ns-pulses using an Nd:YAG laser with a focal diameter of 100µ m for sintering and an Nd:glass laser with a spot of 2mm for the finishing of the surface. The powder material consists of titanium grains <30µm. The results are evaluated by the authors to verify their model [8] which is based on the selective heating of the grain shell by repetitive nspulses leaving the core of the grains at a temperature below the temperature of fusion. There is not yet evidence in the literature of micro bodies generated with this method. Micro laser sintering of LIM is a unique combination of equipment and laser sintering parameters and strategies. The ideas and applications of the innovation are registered in Germany and worldwide as patents and utility models. The technique is marketed under the brand name microsintering together with the equipment by 3D-Micromac AG, Chemnitz, Germany. 2 Characteristics of the technique 2.1 Sinter chamber and powder coating device The once innovative set up [1] - consisting of a hermetically closed sinter chamber and a special rake which already fitted the needs for the handling and selective sintering of sub-µm grained metal powders - has been upgraded ( chamber type 2 ) for higher efficacy and industrial applicability. A schematic of the process assembly is displayed in Fig.1. The set-up consists of a sinter chamber, an attached turbo molecular vacuum pump, gate valves for various shielding and reaction gases as well as the power supply and the control unit for the coater and piston

The lid on top of the casket holds an integrated quartz glass window with transmission for the applied laser radiation. Two or more rakes [Fig.

3 Fig. 1: Schematic of equipment chamber type 2 drives. The vacuum tight stainless steel casket is bisected into an upper and a lower compartment. The bisection allows the drives and the controls in the lower compartment to be kept unaffected by the reaction atmosphere. The lid on top of the casket holds an integrated quartz glass window with transmission for the applied laser radiation. Two or more rakes [Fig. 2a] sweep the powder materials in a circular motion onto the sample piston (the sintering platform). This technique allows realizing vertical gradients of material or of grain sizes along the vertical axis of a 3D-micro body. The blades of the rakes are metal cylinders; they also serve as intermediate powder reservoirs [Fig. 2b]. The pistons are tight for powders and liquids, which allows to process also emulsions and ceramic slurries. The chamber can be evacuated down to pressures of 10-3 Pa and it can be charged with shielding gases or reaction gases at any pressure in the range between 10-3 Pa up to 4x10 5 Pa. Flushing with reaction gases is possible at pressures of 1Pa and makes the device applicable for the combination of laser micro sintering with laser chemical vapor deposition (Laser CVD). Fig. 2a: Multiple rakes and powder supplies allow varying compositions Fig.2b: Ringblade serves as rake and powder storage

2.2 Laser material processing Fig. 3: Laser micro sintering with seperately set pulses The source of the laser pulses is a Q-switched Nd:YAG laser (λ = 1064nm) with an average power of 0.

4 2.2 Laser material processing Fig. 3: Laser micro sintering with seperately set pulses The source of the laser pulses is a Q-switched Nd:YAG laser (λ = 1064nm) with an average power of W in TEM 00 mode [1] and kHz pulse frequencies, lately multimode pulses and other lasers with wavelengths in the near infrared are being applied for laser micro sintering. A ScanLab beam scanner with focal length of 54 mm and a scan field of 25x25mm 2 steers the pulses over the micro body cross that corresponds with the actual powder layer [Fig. 4]. The density and intensity of the pulses are adjusted even during the process - according to the requirements of the micro body s aspired properties, the geometry of the cross section and the material. This is achieved by application of the proprietary software IVS STL Converter (Version 1.0) that was developed especially for this purpose. It controls the actual sinter process. STL data can be processed with a high resolution on a micrometer scale. Especially curves are executed at fast rates with high precision. Outline and filling parameters can be adjusted arbitrarily. Another - home-developed - program allows flexible control of the raking routine. The programs are accessed by the software via an interface, which facilitates automatic performance of a complex SLS - process. This feature enables the operator to realize structural or density gradients in the micro body (see also section 3.2). Continuously repeated calibration of the scanner is integrated into the software, which accounts for the fidelity and precision of the technique even at high aspect ratios. 3 The performance of laser micro sintering 3.1 The postulated mechanism of laser micro sintering With single component powders the texture of the resulting solid area is not a closed coating of metal, but is more a network of craters or wedges that root about 5-10µm below the mean surface level with crests above between 1 and 3µm. Densities between 40% and 75% arise from those materials. Blends, especially those consisting of a refractory and a lower melting metal, yield densities of 90% and above [1]. The mechanism of pulsed sintering is believed to be a synergism of melting and evaporation or ablation respectively. The recoil, the molten and solid powder material receives from the evaporated or erupting material, creates vertical interconnections between the sintered layers and the material being processed. The effect is, that with each pulse, the new material to a large extend is attached to or into the already sintered body below. Especially in the first stage of a sinter layer

the areas of influence of the individual pulses do not overlap.

responsible for the deterioration of the resolution, takes place.

advantage of this regime. SEM views in Figs. 4 and 5 show examples of the pulsing effects. Fig. 4a: High intensity pulses Fig.

in a higher degree of vertical cross linking and a higher roughness. Low intensity pulses as occur Fig.

5 the areas of influence of the individual pulses do not overlap. It is only in the last stage of the sintering of a layer that horizontal cross linking, which usually is also responsible for the deterioration of the resolution, takes place. Consequently - next to the high resolution- the noticeable lack of thermally induced stresses is also an advantage of this regime. SEM views in Figs. 4 and 5 show examples of the pulsing effects. Fig. 4a: High intensity pulses Fig. 4b: Low intensity pulses High intensity pulses confer to the process an increased depth of impact, resulting in a higher degree of vertical cross linking and a higher roughness. Low intensity pulses as occur Fig.5a: Cuboid sintered with constantly high intensities Fig. 5b: Cuboid sintered with constantly low intensities e.g. with higher pulse frequencies yield better surface smoothness with the price of reduced

vertical cross linking. The above mentioned software (section 2.

2 Generation of detachable or firmly interlocked micro bodies Knowing the interrelationships between pulse parameters

generate breaking zones in order to remove a micro part in a non-destructive way from another sintered body or the

On the other hand, given the appropriate parameters and regime, the micro body can be firmly interlocked with the

3.3 Resolution of laser micro sintering Figs 7 show a multishape micro structure with a height of 400µm.

6 vertical cross linking. The above mentioned software (section 2.1) enables the operator to generate micro bodies with a sufficient stability and still good surface quality. 3.2 Generation of detachable or firmly interlocked micro bodies Knowing the interrelationships between pulse parameters and sinter regime on the one and the morphology of the resulting solid body on the other side, it is possible to generate breaking zones in order to remove a micro part in a non-destructive way from another sintered body or the substrate (platform) [Fig. 6a]. On the other hand, given the appropriate parameters and regime, the micro body can be firmly interlocked with the substrate [Fig. 6b]. Fig. 6a: View of a cross section preparation of a micro body interlocked with the substrate. Fig. 6b: View of a cross section preparation of a micro body, attached via a breaking zone. 3.3 Resolution of laser micro sintering Figs 7 show a multishape micro structure with a height of 400µm. The blow up in Fig. 7c gives evidence of a 40µm notch complying with an aspect ratio of 10 Fig. 7a: Tungsten multishape structure; 400µm high Fig. 7b: Pyramid with a base of 100µmx100µm Fig. 7c: Blow up view with a notch width 40µm

Figures 8a and b show ligaments made of tungsten with a width of 300µm and a gauge of 35µm.

distinctive or does not occur at all. Fig. 8a: Concentric circular ligaments made of tungsten; gauge: 35µm Fig.

9a and b also demonstrate the resolving power of the technique.

7 Figures 8a and b show ligaments made of tungsten with a width of 300µm and a gauge of 35µm. At close observation one can detect the typical texture of laser micro sintered bodies, generated from a single component high melting metal powder. With lower melting metals as well as blends of higher and lower melting metals the porous character of the sintered material is less distinctive or does not occur at all. Fig. 8a: Concentric circular ligaments made of tungsten; gauge: 35µm Fig. 8b: Coiled tungsten ligament; gauge 35µm, The blow-up views of the three nested spheres in Figs. 9a and b also demonstrate the resolving power of the technique. The structure is somewhat less porous than that of the above presented ligaments as it was generated with slightly different parameters, though of the same single component tungsten powder. Still, however, the typical single pulse stitches dominate the appearance of the surface. Fig. 9a: Blow-up of three nested spherical shells structure [Fig. 12b] Fig 9b. Blow-up of three nested spherical shells structure [Fig. 12b]

3.4 Undercuts The first undercuts that were generated in laser micro sintering were achieved with the help of support

Later on procedures that did not require support structures were developed [Figs. 11, 12].

In the case of the samples presented in Figs.

8 3.4 Undercuts The first undercuts that were generated in laser micro sintering were achieved with the help of support structures, which were disintegrated after the end of the process e.g. by ultrasonification [1] [Fig. 10]. Later on procedures that did not require support structures were developed [Figs. 11, 12]. With those procedurs, however, the angle of undercut is limited to a parameter dependent maximum value. In the case of the samples presented in Figs. 11 and 12 the achieved maximum undercut is 70 which complies with an angel of 30 relative to the horizontal substrate surface. In Fig. 12b, the generation of the outer shell of the three nested spheres was started with that maximal undercut angle of 70. Fig. 10: Undercuts generated via support structures Fig. 11a: Open work semisphere, builtwithout support structure Fig. 11b: Blow-up view of open work semisphere Fig. 12a: Bihelical tungsten coil Fig. 12b: Three nested spherical shells

4 Laser micro sintered products of industrial relevance The functional parts displayed below [Figs.

The tools are employed as grip bits for micro manipulators or micro positioning devices. Fig.

13c] is a blow up of a highly resolved detail).

The drawing was released to our disposal by MiLaSys Technologies. The tool has an open and hollow cylindrical base.

Fig 14a: Grip bit for the positioning of an optical lens Fig 14b: Lens positioner; view into aspiration channels Fig

9 4 Laser micro sintered products of industrial relevance The functional parts displayed below [Figs. 13a-14b and 15a] are generated from a blend of metal powders. The tools are employed as grip bits for micro manipulators or micro positioning devices. Fig. 13a shows the aspiration box of a fiber holder and Fig. 13b a pair of tweezers for mechanical fixation of fibers ([Fig. 13c] is a blow up of a highly resolved detail). Fig 13a: Aspiration box for a fiberholder of a micro manipulator Fig 13b: Tweezers for mechanical fiber fixation Fig 13c: Blow up of tweezer detail Figs. 14 show a bit for the lifting and positioning of small lenses and its functional environment [Fig. 14c]. The drawing was released to our disposal by MiLaSys Technologies. The tool has an open and hollow cylindrical base. Via the three visible channels vacuum can be applied. Fig 14a: Grip bit for the positioning of an optical lens Fig 14b: Lens positioner; view into aspiration channels Fig 14c: Lens positioner; functional environment A photograph view of a mechanical stop lock is presented in Fig. 15a and b together with a drawing presenting of a cut open view of the construction [Fig 15c], which describes the openings and hollows of the part. The generation process had to be interrupted in the first assay because of a power breakdown in the beginning [Fig 15b], yielding the operator the chance of a photograph of the part at an unfinished stage.

Fig 15a:Stop lock Fig 15b: Inside view of stop lock (unfinished part) Fig 15c: Stop lock; drawing of

Frequency [1/kHz] Total processing time [1/h] Total sintering time [1/h] Total coating time [1/h]

the example of the stop lock [Figs. 15] an overview of the process times is given in Table 1.

frequency, whereby certain requirements regarding the pulse properties have to be fulfilled.

2] and a production of 6 samples per cycle, which in the case of the stop lock can be realized on a

10 Fig 15a:Stop lock Fig 15b: Inside view of stop lock (unfinished part) Fig 15c: Stop lock; drawing of inside view Table 1: Dependence of the process time for the stop lock upon the laser frequency Laser Frequency [1/kHz] Total processing time [1/h] Total sintering time [1/h] Total coating time [1/h] Process time per part [1/h] 35 11,95 11,34 0,61 1, ,31 5,70 0,61 1, ,39 3,78 0,61 0,73 For the example of the stop lock [Figs. 15] an overview of the process times is given in Table 1. The process times for the generation of a single item are listed up as a function of the laser frequency, whereby certain requirements regarding the pulse properties have to be fulfilled. The data is valid for a chamber with circularly activated rakes [Fig. 2] and a production of 6 samples per cycle, which in the case of the stop lock can be realized on a platform of 16mm in diameter. Fig. 16a: The laser micro sintered mold remained unaffected after 500 casts with POM. Fig. 16b: POM-cast from laser micro sintered structure. Photo: PORTEC GmbH

11 Of course, as in all other versions of selective laser sintering, combination of different work pieces on a single platform is possible. Vertical stacking of several layers of work pieces, however, has not been performed yet. Besides the above presented applications, laser micro sintered tools have been tested amongst others as electrodes in EDM (electro-discharge machining) [9] and as micro structured inserts for PIM (pressure injection molding). In the latter application a tool, consisting of a blend, was still intact after several hundred casts. Figs. 16 show over 500 samples and the mold after casting. Indentations in the mold are due to an initial positioning failure. 5 Conclusion and Perspectives Since its introduction in May 2003 laser micro sintering has gained advertence and acknowledgement among toolmakers and users. The once innovative set up - consisting of a hermetically closed sinter chamber and a special rake which already fitted the needs for the handling and selective sintering of sub-µm grained metal powders - has been upgraded to higher efficiency and industrial applicability. Commercialization has been initiated by 3D-Micromac GmbH, Chemnitz [Fig. 17]. The specific procedure including the laser sintering regime has Figure 17: Prospective design of the commercial microsintering device which is built and will be marketed by 3D-Micromac AG, Chemnitz (Germany) become a reproducible routine by which functional micro-freeforms are obtained from a number of powder materials. The technique has proved suitable for the production of functional tools; material qualities can be achieved that meet the requirements for injection molds. Profitability estimations anticipate a reasonable cost range for the up scaled production. Ceramics laser micro sintering is still under development.

12 6 References [1] P. Regenfuss, L. Hartwig, S. Klötzer, R. Ebert, H. Exner: Microparts by a Novel Modification of Selective Laser Sintering. Rapid Prototyping and Manufacturing Conference, May 12 15, 2003, Chicago (IL), published on CD [2] H. Exner, P. Regenfuss, L. Hartwig, S. Klötzer, R. Ebert: Selective Laser Micro Sintering with a Novel Process. LPM 2003, 4th International Symposium on Laser Precision Microfabrication, June 21-24, 2003, Munich, Proceedings of SPI, Vol. 5063, S [3] R. Ebert, P. Regenfuss, L. Hartwig, S. Klötzer, H. Exner: Process Assembly for µm- Scale SLS, Reaction Sintering, and CVD. LPM 2003, 4th International Symposium on Laser Precision Microfabrication, June 21-24, 2003, Munich, Proceedings of SPI, Vol. 5063, S [4] R. Ebert; H. Exner: Vorrichtung und Verwendung von Vakuum und/oder einer zusätzlichen Wärmequelle zur direkten Herstellung von Körpern im Schichtaufbau aus pulverförmigen Stoffen. Patent, reference number DE , date of application [5] Y.P.Kathuria: Microstructuring by selective laser sintering of metallic powders. Surface and Coating Technology, (1999) [6] J. Chen, X. Wang, T. Zuo: The micro fabrication using selective laser sintering micron metal powder. Proc. of SPIE Vol 5116 (2003) [7] P. Fischer, V. Romano, H.P. Weber, S. Kolossov: Pulsed laser sintering of metallic powders. Thin Solid Films (2004) pp [8] P. Fischer, V. Romano, H.P. Weber, N.P. Karapatis, E. Boillat, R. Glardon: Sintering of commercially pure titanium powder with a ND:YAG laser source. Acta Materialia 51 (2003) [9] H. Exner, P. Regenfuss, L. Hartwig, S. Klötzer, R. Ebert: Microsintering of Miniature and Precise Components and Tools. Proc. of the Euro-uRapid, Frankfurt/Main, Dec. 1-2, 2003, B/3

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