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- Additive manufacturing (2)
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Eingeladener Vortrag
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The most successful additive manufacturing (AM) technologies are based on the layer-by-layer deposition of a flowable powder. Although considered as the third industrial revolution, one factor still limiting these processes to become completely autonomous is the often necessary build-up of support structures. Besides the prevention of lateral shifts of the part during the deposition of layers, the support assures quality and stability to the built process. The loose powder itself surrounding the built object, or so-called powder-bed, does not provide this sustenance in most existent technology available. Here we present a simple but effective and economical method for stabilizing the powder-bed, preventing distortions in the geometry with no need for support structures. This effect, achieved by applying an air flow through the powder-bed, is enabling an entirely autonomous generation of parts and is a major contribution to all powder-based additive manufacturing technologies. Moreover, it makes powder-based AM independent of gravitational forces, which will facilitate crafting items in space from a variety of powdery materials.
Porous silicon nitride ceramics were prepared via sintered reaction bonded silicon nitride at 1680 °C. The grain size of nitrided Si3N4 and diameter of post-sintered ß-Si3N4 are controlled by size of raw Si. Porosity of 42.14–46.54% and flexural strength from 141 MPa to 165 MPa were obtained. During post-sintering with nano Y2O3 as sintering additive, nano Y2O3 can promote the formation of small ß-Si3N4 nuclei, but the large amount of ß-Si3N4 (>20%) after nitridation also works as nuclei site for precipitation, in consequence the growth of fine ß-Si3N4 grains is restrained, the length is shortened, and the improvement on flexural strength is minimized. The effect of nano SiC on the refinement of the ß-Si3N4 grains is notable because of the pinning effect, while the effect of nano C on the refinement of the ß-Si3N4 grains is not remarkable due to the carbothermal reaction and increase in viscosity of the liquid phase.
Additive Manufacturing (AM) covers a wide range of processes, ranging from rapid prototyping technologies for polymers to directed energy deposition and powder bed fusion processes for metals and ceramics. In all cases, AM processes involve the layer-by-layer deposition of material from a digital file. Even though these processes may be known by a variety of commercial names, the general characteristics of the processes are similar, as are their impacts on different material systems.
The flexibility of the process provides unprecedented design freedom by allowing the direct fabrication of complex geometries with unique material combinations. However, the interactions between the high energy density sources (laser and electron beams) and the materials being deposited create complex processing conditions that have a significant impact on the material properties. With the wide ranging possibilities inherent in AM produced components, a fundamental understanding of the process/structure/property relationships across different material systems will allow for specific material properties to be obtained.
This Focus Issue of Journal of Materials Research is dedicated to the most recent advances in the characterization of processing/structure/property relationships in AM produced metallic, ceramic, and polymer systems. The breadth of different properties and behaviors across these different materials systems makes the characterization of AM materials extremely complex and a fertile subject for investigation. Many of these issues are addressed in this collection of papers. Such a wide range of unique research areas and the breadth of materials examined in this issue is evidence of the scope of materials issues in additive manufacturing and a glimpse into the future.
We are grateful to both the authors and reviewers of the many high-quality manuscripts submitted to this JMR Focus Issue on The Materials Science of Additive Manufacturing.
The present study is dealing with the basic physics for a novel way to generate a free-formed ceramic body, not like common layer by layer, but directly by Selective Volume Sintering (SVS) in a compact block of ceramic powder. To penetrate with laser light into the volume of a ceramic powder compact it is necessary to investigate the light scattering properties of ceramic powders. Compared with polymers and metals, ceramic materials are unique as they offer a wide optical window of transparency. The optical window typically ranges from below 0.3 up to 5 µm wave length. In the present study thin layers of quartz glass (SiO2) particles have been prepared. As a function of layer thickness and the particle size, transmission and reflection spectra in a wave length range between 0.5 and 2.5 µm have been recorded. Depending on the respective particle size and by choosing a proper relation between particle size and wave length of the incident laser radiation, it is found that light can penetrate a powder compact up to a depth of a few millimeters. With an adjustment of the light absorption properties of the compact the initiation of sintering in the volume of the compact is possible.