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Generation of ceramic green bodies in the additive manufacturing by Laser Induced Slip-casting (LIS)
(2016)
For the additive manufacture of large components usually powder-based methods are used. A powder is deposited layer wise by a recoater, then, the component structure is printed to the powder bed or sintered by a laser. In slurry based methods, the slurry is deposited by a doctor blade and dried before the printing of binder or the laser treatment. The new method of laser-induced slip casting is also a slurry-based method and the layers deposited sequential. However the slip is not dried and the structure is written directly by a laser into the suspension. The wall thickness of the ceramic material can be adjusted by the laser spot size and treatment time. The water is evaporated by the laser and a green body is formed locally. Because of its porosity, water is taken from the surrounding suspension and the wall thickness of the green body increases with treatment time. Due to the use of highly filled suspensions, the green body is stable in the ceramic slurry. Large green bodies can be built which have no visible layers in the microstructure.
Generation of ceramic green bodies in the additive manufacturing by Laser Induced Slip-casting (LIS)
(2016)
For the additive manufacture of large components usually powder-based methods are used. A powder is deposited layer wise by a recoater, then, the component structure is printed to the powder bed or sintered by a laser. In slurry based methods, the slurry is deposited by a doctor blade and dried before the printing of binder or the laser treatment. The new method of laser-induced slip casting is also a slurry-based method and the layers deposited sequential. However the slip is not dried and the structure is written directly by a laser into the suspension. The wall thickness of the ceramic material can be adjusted by the laser spot size and treatment time. The water is evaporated by the laser and a green body is formed locally. Because of its porosity, water is taken from the surrounding suspension and the wall thickness of the green body increases with treatment time. Due to the use of highly filled suspensions, the green body is stable in the ceramic slurry. Large green bodies can be built which have no visible layers in the microstructure.
The production of ceramic parts by an additive manufacturing process is still a challenging task. Especially in powder-based technologies, such as 3D printing or laser sintering, the feedstock material is commonly spread out as thin layers of a dry powder/granulate by a roller or a shaker system. As consequence the layers are characterized mostly by a low packing rate. On the other hand, from a ceramic processing viewpoint, appreciable densities can be achieved by the use of ceramic slurries. In this context, the so-called Layer-wise Slurry Deposition (LSD) process has been developed. An overview about the LSD process and latest advances in the development of this technology will be presented.
Selective laser sintering is a well-established technology for the additive manufacture of metallic and polymeric parts. For the additive manufacture of ceramic parts some examples for its successful application do exist, also. In this context, the selective laser sintering of SiC ceramic powders has been studied extensively in the past. Despite its low sintering activity, SiC is particularly suited for the selective laser sintering process, because of the possibility to form SiO2 at elevated temperatures in oxidizing ambient. The SiO2 formed can act as a binder within the laser sintering process.
In the present work densely packed powder beds generated by the Layerwise Slurry Deposition (LSD) technology are selectively sintered by a cw. fiber laser. The layerwise slurry deposition is an innovative process for the deposition of layers in additive manufacturing. A slurry with no or very small organic content is repetitively spread as thin layers on each other by means of a doctor blade. During the deposition process, the ceramic particles settle to form thin layers of about 100 µm which have a high packing density (typical 55-60%). This high powder packing density is the result of a slip casting process: When a layer is deposited on a previously dried porous layer, the water is drawn into the pores by capillary forces. The LSD process therefore shares aspects of tape casting and slip casting. An additional benefit of the LSD technology is the free choice of the size of the ceramic particles used. Compared to the processing of dry flowable powders, especially small particles can be very well processed.
The use of water based ceramic slurries as feedstock for the additive manufacture of ceramics has many advantages which are not fully exploit yet. In the layerwise slurry deposition (LSD) process a slurry with no or low organic content is repetitively spread as thin layers on each other by means of a doctor blade. During the deposition, the ceramic particles settle on the previously deposited and dried material to form thin layers with a high packing density (55-60%). The LSD therefore shares aspects both of tape casting and slip casting. The LSD differentiates from the classical powder-based AM layer deposition, which typically achieves with a flowable coarse grained powder a low packing density (35-50%) only, consequently hindering the ability of sintering ceramic parts to full density. The LSD is coupled with the principles of selective laser sintering (SLS) or binder jetting, to generate novel processes which take advantage of the possibility of achieving a highly dense powder-bed. Contrary to the LSD process, which requires drying of each individual layer, the direct interaction of ceramic slurries with intense laser radiation is a promising approach for the additive manufacture of ceramics, also. This presentation will provide a detailed discussion of the specific features of the slurry based processes, potentialities and issues connected to the layer deposition and describe the most recent developments in their application to technical ceramics.