5 Werkstofftechnik
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The Laser-Induced Slip Casting is an additive manufacturing technology specifically developed for ceramic materials using water-based ceramic slurries. The process takes place layer-by-layer in a similar fashion as top-down vat photopolymerization, selectively consolidating each layer by means of a laser energy source positioned on the top. Contrary to vat photopolymerization, in which the consolidation is achieved by selectively cross-linking a ceramic-filled resin, LIS uses water-based slurries with a low amount of organic additives (typically < 5 wt%) as feedstocks.
In LIS, a green body is formed by local evaporation of water which causes the suspension to collapse forming a cast, following a mechanism similar to slip casting. Only a small content of organic additives is needed to effectively disperse the ceramic particles and to increase the green strength.
The technology is very versatile and can be applied to all ceramic systems that can be dispersed in water. One of the main advantages is that even dark materials such as silicon carbide can be processed without issues related to light scattering and absorption.
The presentation will discuss strengths and limitations of LIS compared to other AM technologies and will highlight the latest results for alumina and for silicon carbide ceramics.
Here we demonstrate that by applying exclusively Martian resources a processing route involving suspensions of mineral particles called slurries or slips can be established for manufacturing ceramics on Mars. We developed water-based slurries without the use of additives that had a 51 wt. % solid load resembling commercial porcelain slurries in respect to the particle size distribution and rheological properties. These slurries were used to slip cast discs, rings and vases that were sintered at temperatures between 1000 and 1130 °C using different sintering schedules, the latter were set-up according the results of hot-stage microscopic characterization. The microstructure, porosity and the mechanical properties were characterized by SEM, X-ray Computer tomography and Weibull analysis. Our wet processing of minerals yields ceramics with complex shapes that show similar mechanical properties to porcelain and could serve as a technology for future Mars colonization. The best quality parts with completely vitrificated matrix supporting a few idiomorphic crystals are obtained at 1130 °C with 10 h dwell time with volume and linear shrinkage as much as ~62% and ~17% and a characteristic compressive strength of 51 MPa.
In diesem Artikel werden die verschiedenen additiven Fertigungsverfahren und ihre Anwendungsmöglichkeiten vorgestellt. Der Fokus liegt hierbei auf Silikatkeramik. Für jedes 3D Druckverfahren werden die Möglichkeiten mit Silikatkeramik, Hochleistungskeramik und Dentalkeramik aufgezeigt. Auf den Nutzen der verschiedenen 3D Druckverfahren für industrielle Anwendungen, den privaten Bereich, sowie die Verwendung für Kunst und Design wird eingegangen. Das Potenzial der Additiven Fertigung wird auch für den Bereich der Keramik in der dentalen Anwendung bewertet.
Additive manufacturing (AM) is developing rapidly due to itsflexibility in producing complex geometries and tailored material compositions. However, AM processes are characterized by intrinsic limitations concerning their resolution and surface finish, which are related to the layer-by-layer stacking process. Herein, a self-organization process is promoted as an approach to improve surface quality and achieve optimization of 3D minimal surface lightweight structures. The self-organization is activated after the powder bed 3D printing process via local melting, thereby allowing surface tension-driven viscous flow.The surface roughness Ra (arithmetic average of the roughness profile) could bedecreased by a factor of 1000 and transparent lenses and complex gyroid structures could be produced for demonstration. The concept of self-organization is further elaborated by incorporating external magnetic fields to intentionally manipulate magnetic particles, which are mixed with the polymer before printing and self-organization. This concept can be applied to develop programmable materials with specific microtextures responding to the external physical conditions.
Frass (fine powdery refuse or fragile perforated wood produced by the activity of boring insects) of larvae of the European house borer (EHB) and of drywood termites was tested as a natural and novel feedstock for 3D-printing of wood-based materials. Small particles produced by the drywood termite Incisitermes marginipennis and the EHB Hylotrupes bajulus during feeding in construction timber, were used. Frass is a powdery material of particularly consistent quality that is essentially biologically processed wood mixed with debris of wood and faeces. The filigree-like particles flow easily permitting the build-up of woodbased structures in a layer wise fashion using the Binder Jetting printing process. The Quality of powders produced by different insect species was compared along with the processing steps and properties of the printed parts. Drywood termite frass with a Hausner Ratio HR = 1.1 with ρBulk = 0.67 g/cm3 and ρTap = 0.74 g/cm3 was perfectly suited to deposition of uniformly packed layers in 3D printing. We suggest that a variety of naturally available feedstocks could be used in environmentally responsible approaches to scientific material sciences/additive manufacturing.
The present work addresses powder bed binder jetting additive manufacturing by selective magnesium phosphate cement activation. Despite the potential of this technology to aid the digitalization of the construction industry, the effect of processing parameters on the mechanical performance of printed materials has not yet been studied to generate a guideline for the further development of the technology. Statistical methodologies were used to screen the effect of four printing process parameters (printing speed, layer thickness, raster angle, and build direction on flexural and compressive strength). As the exploited technology works with constant fluid pressure, the physical interpretation of the effect of each factor can be considered taking into account the interactions between the binder materials in the powder bed. Analysis of variance (ANOVA) indicated that printing speed and layer thickness significantly affect mechanical performances. Furthermore, the layout of samples for the printing process is preferable to be parallel the printhead movement. An anisotropic behavior was observed, and the samples subjected to compressive forces parallel to the layer plane possessed lower strength values. This effect can be interpreted as a result of a weak area of low density in between layers, leading to a pronounced delamination under compression. Even though the strength of the printed material is not suitable for a structural concrete, it can be marginally improved by design of experiment and optimized for non-structural applications, such as for porous artificial stone. Design of experiment coupled with ANOVA methods can be used in the future to support the development of novel material mixtures, thus expanding the fields of application of this novel additive manufacturing technology.
In powder-based Additive Manufacturing (AM) processes, an object is produced by successively depositing thin layers of a powder material and by inscribing the cross section of the object in each layer. The main methods to inscribe a layer are by binder jetting (also known as powder 3D printing) or by selective laser sintering/melting (SLS/SLM).
Powder-based AM processes have found wide application for several metallic, polymeric and also ceramic materials, due to their advantages in combining flexibility, easy upscaling and (often) good material properties of their products.
The deposition of homogeneous layers is key to the reproducibility of these processes and has a direct influence on the quality of the final parts. Accordingly, powder properties such as particle size distribution, shape, roughness and process related properties such as powder flowability and packing density need to be carefully evaluated.
Due to these requirements, these processes have been so far precluded to find commercial use for certain applications. In the following, two outstanding cases will be presented.
A first example is that powder-based AM processes are widely used for many metallic and polymeric materials, but they find no commercial application for most technical ceramics.
This seemingly contradicting observation is explained by the fact that in powder based AM, a dry flowable powder needs to be used. The processing of technical ceramics in fact typically requires very fine and poorly flowable powder, which makes them not suitable for the standard processes. There have been several approaches to adapt the raw materials to the process (e.g. by granulation), but in order to maintain the superior properties of technical ceramics it seems necessary to follow the opposite approach and adapt the process to the raw materials instead.
This was the motivation for developing the Layerwise Slurry Deposition (LSD), an innovative process for the deposition of powder layers with a high packing density. In the LSD process, a ceramic slurry is deposited to form thin powder layers, rather than using a dry powder. This allows achieving high packing density (55-60%) in the layers after drying. It is also important, that standard ceramic raw materials can be used. When coupled with a printing head or with a laser source, the LSD enables novel AM technologies which are similar to 3D printing or selective laser sintering, but taking advantage of having a highly dense powder bed.
The LSD -3D printing, in particular, offers the potential of producing large (> 100 mm) and high quality ceramic parts, with microstructure and properties similar to traditional processing. Moreover, due to the compact powder bed, no support structures are required for fixation of the part in the printing process.
Figure 1 shows the schematics of the working principle of the LSD-3D print and illustrates some examples of the resolution and features achievable.
The second outstanding case here described is the application of powder-based AM in environments with reduced or zero gravity. The vision is to be able to produce repair parts, tools and other objects during a space mission, such as on the International Space Station (ISS), without the need of delivering such parts from Earth or carrying them during the mission. AM technologies are also envisioned to play an important role even for future missions to bring mankind to colonize other planets, be it on Mars or on the Moon. In this situation, reduced gravity is also experienced (the gravitational acceleration is 0.16 g on the Moon and 0.38 g on Mars).
These environments cause the use of AM powder technologies to be very problematic: the powder layers need to be stabilized in order to avoid dispersion of the particles in the chamber. This is impossible for standard AM powder deposition systems, which rely on gravitation to spread the powder.
Also in this case, an innovative approach has been implemented to face this technological challenge. The application of a gas flow through a powder has a very strong effect on its flowability, by generating a force on each particle, which is following the gas flow field. This principle can be applied in a simple setup such as the one shown in Figure 2.
In this setup, the gas flow causes an average pressure on the powder bed in direction of the arrows, generating a stabilizing effect which acts in the same direction of the gravitational force. This effect can be used in addition to normal gravity on Earth to achieve a better stabilization of 3D printed parts in the powder bed. In this case, even a significant increase of packing density of the powder was measured, compared to the same experimental setup without gas flow. This is due to the fact that the force on each single particle follows the gas flow field, which is guiding the particles to settle between the pores of the powder bed, thus achieving an efficient packing.
The same principle can be applied in absence of gravitation, where the gas flow acts to stabilize the powder layers. It has been shown that ceramic powder could be deposited in layers and laser sintered in µ-gravity conditions during a DLR (Deutsches Zentrum für Luft- und Raumfahrt) campaign of parabolic flights, as shown in Figure 2. A follow-up campaign is dedicated to the deposition of metallic (stainless steel) powder in inert atmosphere and to study the effects of laser melting in µ-gravity.
In conclusion, the description of these two example cases shows how the development of novel technological processes can address some of the limitations of standard powder-based AM, in order to enable the use of new materials, such as technical ceramics, or to tackle the challenges of AM in space.
The Global Exploration Strategy of the International Space Exploration Coordination Group (ISECG) describes a timeframe of 2020 and beyond with the ultimate aim to establish a human presence on Mars towards the 2040ies. The next steps lie on the Moon with a focus on the coming 10 years. Early lunar surface missions will establish a capability in support of lunar science and prepare and test mission operations for subsequent human exploration of Mars and long-duration human activities on the Moon.
Given the extreme costs involved in the shipping of material from Earth, a prerequisite for future human exploration is the manufacturing of elements directly on the Moon’s surface. Unlike the equipment, which at the beginning will have to be brought from Earth, raw materials and energy could be available following the concept of In-Situ Resource Utilization. The ESA OSIP PAVING THE ROAD (PAVER) study investigates the use of a laser to sinter regolith into paving elements for use as roadways and launch pads thus mitigating dust issues for transport and exploration vehicles.
The ESA-funded study examines the potential of using a laser (12 kW CO2 laser with spot beam up to 100 mm) for layer sintering of lunar and martian regolith powders to manufacture larger 3D elements and provide know-how for the automatic manufacture of paving elements in the lunar environment. The project contributes to the first step toward the establishment of a lunar base and will lead to the construction of equipment capable of paving areas and manufacturing 3D structures.
PAVER project sets the starting point for an examination of the larger context of lunar exploration. Mission scenarios will look at different phases of lunar exploration: Robotic Lunar Exploration, Survivability, Sustainability, and Operational Phase. A proposed Technology Roadmap investigates the mission scenario and analyses how, and to which extent, laser melting/sintering will play a role in the various phases of exploration. The paper contextualizes laser sintering within selected mission scenarios and discusses the different kinds of infrastructure that can be produced at each phase of the mission. The outcome of the study includes the detailing of the TRL steps in the project and an outline of a timeline for the different elements. Covered aspects include terrain modelling such as operation pads, roadways, or towers, non-pressurized building structures to protect machinery, and habitat envelopes, to protect and shield humans against dust, micrometeoroids, and radiation.
The microstructure of an apatite-wollastonite (code name AP40) glass-ceramic is analyzed in this study by combining 2D microscopy, phase analysis, X-ray absorption and synchrotron X-ray refraction computed tomography (XCT and SXRCT, respectively). It is shown that this combination provides a useful toolbox to characterize the global microstructure in a wide scale range, from sub-micrometer to millimeter. The material displays a complex microstructure comprising a glassy matrix with embedded fluorapatite and wollastonite small crystals. In this matrix, large (up to 200 μm) spike-shaped structures are distributed. Such microstructural features are oriented around a central sphere, thereby forming a structure resembling a sea urchin. A unique feature of SXRCT, in contrast to XCT, is that internal interfaces are visualized; this allows one to show the 3D distribution of these urchins with exceptionally good contrast. Furthermore, it is revealed that the spike-shaped structures are not single crystals, but rather composed of sub-micrometric crystals, which are identified as fluorapatite and diopside phases by SEM-EDX analysis.
Future lunar exploration will be based on in-situ resource utilization (ISRU) techniques. The most abundant raw material on the Moon is lunar regolith, which, however, is very scarce on Earth, making the study of simulants a necessity. The objective of this study is to characterize and investigate the sintering behavior of EAC-1A lunar regolith simulant. The characterization of the simulant included the determination of the phase assemblage, characteristic temperatures determination and water content analysis. The results are discussed in the context of sintering experiments of EAC-1A simulant, which showed that the material can be sintered to a relative density close to 90%, but only within a very narrow range of temperatures (20–30 °C). Sintering experiments were performed for sieved and unsieved, as well as for dried and non-dried specimens of EAC-1A. In addition, an analysis of the densification and mechanical properties of the sintered specimens was done. The sintering experiments at different temperatures showed that the finest fraction of sieved simulant can reach a higher maximum sintering temperature, and consequently a higher densification and biaxial strength. The non-dried powder exhibited higher densification and biaxial strength after sintering compared to the dried specimen. This difference was explained with a higher green density of the non-dried powder during pressing, rather than due to an actual influence on the sintering mechanism. Nevertheless, drying the powder prior to sintering is important to avoid the overestimation of the strength of specimens to be fabricated on the Moon.