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Microparts by a Novel Modification of Selective Laser Sintering Peter Regenfuss, Lars Hartwig, Sascha Klótzer, Robby Ebert, Horst Exner Authors are with Laserinstitut Mittelsachsen e. V. at the University of Applied Sciences Mittweida, Technikumplatz 17, 09648 Mittweida, Germany, email: pregenfu@htwm. de Keywords: Freeforms, Micro Structures, Nanopowders, Selective Laser Sinterlng, SLS, Tungsten. Abstract: Microparts with a structural resolution of IO have been generated by selectiv or7 sintering under condi ns pressures.

A novel se p an material is processed allows the work Piec hnique includes ced shield gas re is employed; the er. The procedure powders of high melting metals like tungsten as well as lower melting metals like aluminium and copper. Contlngent on the parameters, the generated bodies are either firmly attached to the substrate or can be dissevered by a non-destructive method. The set- up is equipped with a device suited to rake thin layers of sub- micrometer grain sized powders as well as slurries; it also fulfils the requirements for selective reaction sintering and selective laser sintering of ceramics 1.

Introduction: Selective laser sintering (SLS), a familiar technique in rapid prototyping and rapid tooling, was heretofore referentially applied for the generation of macroscopic freeforms. Commercial devices with a laser focus diamet diameter of 40-500pm still do not allow generation of microparts smaller than 100pm. Therefore since its first application, efforts have not ceased to increase the resolving power of SLS, aiming for dimensions in the range of 20pm. This is beyond the bounds of classical chip removing or milling processes.

Compared to still higher resolving techniques, SLS sintering still bears the advantages of relatively low production costs and short processing times for uniques or small sample numbers. Prismatic r 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 easily, allowing e. g. he fabrication of miniature tools and components with hydrodynamic functions. Therefore SLS remains an attractive tool for the mentioned size range. As SI_S is a layenuise material structuring process, the approach of finer details requires thinner layers and consequently powders with smaller grain sizes. The realization of these requirements is not always trivial, s finer grained solids are more reactive than coarse materials. Precautions have to be taken to avoid corrosion of the powder by oxygen or humidity.

Moreover, the finer the powder gets, the poorer becomes its “rakeabliõf’. The packing of the fine powder layers are very loose as gravitational forces succumb to the inter particle forces. Especially during simple recoating procedures e. g. by sweeping a blade across the modelling p PAGFarl(F7 Especially during simple recoating procedures e. g. by sweeping a blade across the modelllng platform, the powder forms agglomerates which are more than one order of magnitude larger than a single graln.

This behaviour can be partly overcome by a special raking strategy; the remaining lack of layer density has to be taken account ofby an adequate laser sintering regime. The Laser Institut Mittelsachsen e. V. in Mittweida, Germany, has developed a procedure and a sintering machine which makes feasible the generation of solid and structured parts out of metals and ceramics by direct selective laser sintering.

To overcome the difficulties from oxidation and humidity, the complete vacuum process was transferred into a vacuum tight 2 chamber 111. The obtained structures show a resolution of less than 30 pm and a minimal roughness of3. pm can be achieved. 2. Process Performance 2. 1 Process Assembly: The process assembly consists of the sintering chamber (SC), an attached turbo molecular vacuum pump, a ScanLab beam scanner with a scan field of 25x25mm, a Q-switched Nd:YAG — laser (? 1064nm) with an output of 0. -10W in TEMOO mode and 0. 5-50kHz pulse frequencies, the mounting and gate valves for various shielding and reaction gases as well as the power supply and the control unit for the coating and positioning bench (CPB). The coating and positioning bench (CPB) – the core of the SC, where the sintering takes place – is mounted inside a vacuum tight stainless steel asket, the lid of which has an integrated quartz glass window with transmission for the PAGF3rl(F7 With transmission for the applied laser radiation.

The casket has electrical feed throughs for the sintering platform and an internal process observation camera. Several valves allow for the exchange of the shielding and reaction gases; at a major and a minor connection respectively, the pump and a manometer are attached to the SC. The CPB has an aluminium frame, holding three piezzo ceramic drives with a resolution of 0. 1 pm, and the sintering platform. The platform is positioned horizontally and as two vertical cylindrical bores for the powder piston and the probe piston. Each of it has its separate drive.

With the third drive the powder rake, which is actually a stainless steel blade, is swept across the platform. The position ofthe blade is manually adjustable, it is supposed to run as low over the platform surface as possible. The pistons are tight for powders and liquids which allows to process also emulsions and ceramic slurries. The SC can be evacuated by the attached turbo molecular pump 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 and 10-3 Pa up to 4×105 Pa..

A second – chemically resistant – pump can be connected to the chamber and, with a system of flow controls and pressure reducers, reaction gases can be flushed through at pressures of Pa, which makes the SC applicable for laser chemical vapour deposition (Laser CVD). The presented results were achieved with the above described laser in the pul (Laser CVD). The presented results were achieved with the above described laser in the pulse regime. Micro freeforms generated with continuous wave radiation and different wavelengths Will be the subject of upcoming presentations.

Fig. 1a Fig. Ib Figures 1 : View ofthe SC during operation (a) and after remova’ of the Iid (b). 3 2. 2 Materials: For the generation of metallic free forms single component powders were used (Table 1), in addition, metal sintering was performed with mixtures of copper and tungsten. Table 1: Processed Metal Powders and their Grain Sizes Metal Grain Size Tungsten 300nm Aluminium 3pm Copper 10pm Silver 2gm AII metals are relatively inert materials at low and normal temperatures.

When processed with laser radiation under a normal atmosphere, however, most of them show considerable oxidation. Presently direct sintering of ceramics is probed with luminium nitride powder and a porcelain raw material as a nonoxide ceramic and an oxide ceramic with a glassy component respectively. Selective reaction sintering is being done with aluminium powder under nitrogen. The results presented in this article confine to selective sintering of metal powders, especially tungsten. 2. SLS process: The process atmosphere: To provide the proper atmosphere for the process, the SC is evacuated to 10-3 Pa. Depending on the condition of the powder the vacuum is applied for several hours to allow desorption of water. Subsequently, the chamber is charged with the shielding gas at the appropriate ressure between 104 and Ilv the gas charged with the shielding gas at the appropriate pressure between 104 and 105 Pa. Usually the gas does not need flushing or exchange in the course of a process even if this extends over more than one day.

The raking procedure: As mentioned above, the raking of a thin layer of fine grained powder causes problems, because the material does not sediment in a dense packing but – partly supported by the raking -forms agglomerates which in the case of a submicrometer tungsten powder often occur in the shape of polyhedrons with a preference for certain angles. The agglomerates, which are approximately an arder of magnitude larger than the grain size, do not pack densely either. The mass ofthe particles is too low for gravity to suffice for a dense sedmentation.

To overcome this drawback a special raking regime was developed to generate a thin layer by first applying a thicker one which is sheared off by successive raking from opposite directions. The nature of the interparticular forces is not quite clear, but obviously the amount of absorbed water plays a certain role, as exposition of the powder to a vacuum of 10-3 Pa for several hours improves the result of the raking procedure. The raking speed was 50mms -1 . Still, however, the density of the resulting layer is very poor, estimations are in the range of 1 so that further condensation has to be achieved during sintering.

Sintering: Laser pulses with powers from 0. 5kW-2kW were applied at frequencies in the range of 520kHz. The cross sections of the microparts are processed with the pulsed radiation in PAGFsrl(F7 range of 520kHz. The cross sections of the microparts are processed with the pulsed radiation in a way that the pulses are distributed evenly across the selected areal segments. The resulting solid area is not a closed coating of metal, but is more network of craters or wedges that root about 10vm below the mean surface level with crests above between 1 and 3pm.

This effect accounts for the higher quality of the generated vertical surfaces compared to the horizontal 4 surfaces. Specific regimes are applied for bottom and top horizontal surfaces respectively. Metallic micro free forms can be either fused to the metal substrate or attached to the substrate surface by narrow sinter necks, frail enough to be sheared off without destruction of the generated free form but stable enough to fix the part throughout the raking and sintering process.

Also, or stable positioning, the adjacent powder zone is processed with low power embedding the freeforms in a crust, that can be removed completely by subsequent ultrasonification. 3. Results: Prismatic and tapered microstructures were generated from tungsten metal powder. Figures 2 and 3 show an arrangement cuboids fixed to the substrate (2) and another one separably attached to the stainless steel base plate (3a,b). Figure 3b shows a cuboid that was Split off the substrate by a slight chop with a spatule. Figure 4a shows SEM views of a tungsten structure with a depth of 400pm. It contains notches with a width of