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An interlaboratory study, involving eigth international laboratories and coordinated by COMTES FHT (Czech Republic), was conducted to validate tensile measurements obtained using miniature specimens on additively manufactured (AM) components and artifacts. In addition to AM 316L stainless steel (316L SS), a wrought highstrength steel (34CrNiMo6V, equivalent to AISI 4340) was also used. Based on the results, a precision statement in accordance with ASTM E691 standard practice was developed, intended for inclusion in a proposed annex to the ASTM E8/E8M tension testing method. The primary outcomes of the study highlighted the agreement between yield and tensile strength measured from miniature and standard-sized tensile specimens. Furthermore, most tensile properties exhibited similar standard deviations, offering users insight into the efficacy of miniature specimen applications.
Wollastonite (CaSiO 3 ) – diopside (CaMgSi 2 O 6 ) glass-ceramic scaffolds have been successfully fabricated using two different additive manufacturing techniques: powder-based 3D printing (3DP) and digital light processing (DLP), coupled with the sinter-crystallization of glass powders with two different compositions. The adopted manufacturing process depended on the balance between viscous flow sintering and crystallization of the glass particles, in turn in fluenced by the powder size and the sensitivity of CaO – MgO – SiO 2 glasses to surface nucleation. 3DP used coarser glass powders and was more appropriate for low temperature firing (800 – 900 °C), leading to samples with limited crystallization. On the contrary, DLP used finer glass powders, leading to highly crystallized glass-ceramic samples. Despite the differences in manufacturing technology and crystallization, all samples featured very good strength-to-density ratios, which bene fit theiruse for bone tissue engineering applications. The bioactivity of 3D-printed glass-ceramics after immersion in simulated body fluid and the similarities, in terms of ionic releases and hydroxyapatite formation with already validated bioactive glass-ceramics, were preliminarily assessed.
Recently, additive manufacturing of ceramics has achieved the maturity to be transferred from scientific laboratories to industrial applications. At the same time, research is progressing to expand the boundaries of this field into the territory of novel materials and applications. This feature issue addresses current progress in all aspects of additive manufacturing of ceramics, from parts design to feedstock selection, from technological development to characterization of printed components.
The determination of residual stresses becomes more complicated with increasing complexity of the structures investigated. Additive manufacturing techniques generally allow the production of ‘lattice structures’ without any additional manufacturing step. These lattice structures consist of thin struts and are thus susceptible to internal stress-induced distortion and even cracks. In most cases, internal stresses remain locked in the structures as residual stress.
The determination of the residual stress in lattice structures through nondestructive neutron diffraction is described in this work. It is shown how two difficulties can be overcome: (a) the correct alignment of the lattice structures within the neutron beam and (b) the correct determination of the residual stress field in a representative part of the structure. The magnitude and the direction of residual stress are discussed. The residual stress in the strut was found to be uniaxial and to follow the orientation of the strut, while the residual stress in the knots was more hydrostatic. Additionally, it is shown that strain measurements in at least seven independent directions are necessary for the estimation of the principal stress directions. The measurement directions should be chosen according to the sample geometry and an informed choice on the possible strain field. If the most prominent direction is not measured, the error in the calculated stress magnitude increases considerably.
Laser melting manufacturing of large elements of lunar regolith simulant for paving on the Moon
(2023)
The next steps for the expansion of the human presence in the solar system will be taken on the Moon. However, due to the low lunar gravity, the suspended dust generated when lunar rovers move across the lunar soil is a significant risk for lunar missions as it can affect the systems of the exploration vehicles. One solution to mitigate this problem is the construction of roads and landing pads on the Moon. In addition, to increase the sustainability of future lunar missions, in-situ resource utilization (ISRU) techniques must be developed. In this paper, the use of concentrated light for paving on the Moon by melting the lunar regolith is investigated. As a substitute of the concentrated sunlight, a high-power CO2 laser is used in the experiments. With this set-up, a maximum laser spot diameter of 100 mm can be achieved, which translates in high thicknesses of the consolidated layers. Furthermore, the lunar regolith simulant EAC-1A is used as a substitute of the actual lunar soil. At the end of the study, large samples (approximately 250 × 250 mm) with interlocking capabilities were fabricated by melting the lunar simulant with the laser directly on the powder bed. Large areas of lunar soil can be covered with these samples and serve as roads and landing pads, decreasing the propagation of lunar dust. These manufactured samples were analysed regarding their ineralogical composition, internal structure and mechanical properties.
Additive manufacturing is well established for plastics and metals, and it gets more and more implemented in a variety of industrial processes. Beside these well-established material platforms, additive manufacturing processes are highly interesting for ceramics, especially regarding resource conservation and for the production of complex three-dimensional shapes and structures with specific feature sizes in the µm and mm range with high accuracy. The usage of ceramics in 3D printing is, however, just at the beginning of a technical implementation in a continuously and fast rising field of research and development. The flexible fabrication of highly complex and precise 3D structures by means of light-induced photopolymerization that are difficult to realize using traditional ceramic fabrication methods such as casting and machining is of high importance. Generally, slurry-based ceramic 3D printing technologies involve liquid or semi-liquid polymeric systems dispersed with ceramic particles as feedstock (inks or pastes), depending on the solid loading and viscosity of the system. This paper includes all types of photo-curable polymer-ceramic-mixtures (feedstock), while demonstrating our own work on 3D printed alumina toughened zirconia based ceramic slurries with light induced polymerization on the basis of two-photon absorption (TPA) for the first time. As a proven exemplary on cuboids with varying edge length and double pyramids in the µm-range we state that real 3D micro-stereolithographic fabrication of ceramic products will be generally possible in the near future by means of TPA. This technology enables the fabrication of 3D structures with high accuracy in comparison to ceramic technologies that apply single-photon excitation. In sum, our work is intended to contribute to the fundamental development of this technology for the representation of oxide-ceramic components (proof-of-principle) and helps to exploit the high potential of additive processes in the field of bio-ceramics in the medium to long-term future.
Additive manufacturing of alkali-activated materials currently attracts a lot of attention, because of the possibility to produce customized high-performance elements for a range of applications, potentially being more resource-efficient than conventionally produced parts. Here, we describe a new additive manufacturing process for alkali-activated materials that is based on selective laser-heating of lithium aluminate/microsilica slurries. The new process-material combination allows to manufacture elements with complex geometries at high building rates and high accuracy. The process is versatile and transferrable to structures of sizes differing by orders of magnitude. The mechanical strength of the obtained materials was in the range of values reported for conventional metakaolin-based geopolymers, and superior to what has been hitherto reported for alkali-activated materials produced by additive manufacturing. This mechanical performance was obtained despite the fact that the degree of reaction of the lithium aluminate and the microsilica was low, suggesting that significant reactions took place only at the surface of the microsilica particles.
The wet processing of regolith simulant for clay in situ resource utilization (ISRU) on Mars is presented. The two raw materials from the Mars global simulant family, one without clay (MGS-1) and one with clay - sodium montmorillonite smectite - (MGS-1C) were milled and mixed to produce a simulant with small particle size and reduced clay content (MGS-1C/8). All three simulants and the pure clay raw material were extensively characterized using XRF, synchrotron XRD, gas adsorption and gas pycnometry methods. In a straightforward processing approach, MGS-1C/8 was mixed with water and different dispersant approaches were investigated, all of which gave stable slurries. Particle size distribution, rheology, ion concentration, pH and electrical conductivity of these slurries were characterized. The slurry systems can easily be adapted to fit all typical ceramic shaping routes and here parts of varying complexity from slip casting, throwing on a potter's wheel and additive manufacturing, including material extrusion (robocasting) and binder jetting (powder bed 3D printing) were produced.
The unique properties of the sodium montmorillonite clay, which is readily accessible in conjunction with magnesium sulfate on the Martian surface, acted as a natural nanosized binder and produced high strength green bodies (unfired ceramic body) with compressive strength from 3.3 to 7.5 MPa. The most elaborate additive manufacturing technique layerwise slurry deposition (LSD) produced water-resistant green bodies with a compressive strength of 30.8 ± 2.5 MPa by employing a polymeric binder, which is similar or higher than the strength of standard concrete. The unfired green bodies show sufficient strength to be used for remote Habitat building on Mars using additive manufacturing without humans being present.
Mean-field modeling and phase-field simulation of grain growth under directional driving forces
(2024)
Directional grain growth is a common phenomenon in the synthetic and natural evolution of various polycrystals. It occurs in the presence of an external driving force, such as a temperature gradient, along which grains show a preferred, yet competitive, growth. Novel additive manufacturing processes, with intense, localized energy deposition, are prominent examples of when directional grain growth can occur, beneath the melting pool. In this work, we derive a phenomenological mean-field model and perform 3D phase-field simulations to investigate the directional grain growth and its underlying physical mechanisms. The effect of the intensity of driving force is simulated and systematically analyzed at the evolving growth front as well as various cross-sections perpendicular to the direction of the driving force. We found that although the directional growth significantly deviates from normal grain growth, it is still governed by a power law relation <R> α tⁿ with an exponent n ~ 0.6–0.7. The exponent n exhibits a nontrivial dependence on the magnitude of the directional driving force, such that the lowest growth exponent is observed for intermediate driving forces. We elaborate that this can originate from the fact that the forces at grain boundary junctions evolve out of balance under the influence of the directional driving force. With increasing the driving forces, the growth exponent asymptotically approaches a value of n ≈ 0.63, imposed by the largest possible grain aspect ratio for given grain boundary energies. The current combined mean-field and phase-field framework pave the way for future exploration in broader contexts such as the evolution of complex additively manufactured microstructures.
Manufacturing defects, high residual stress (RS), and microstructures affect the structural integrity of laser powder bed fusion (LPBF) Ti-6Al-4V. In this study, the individual effect of these factors on fatigue performance at elevated temperature (300 °C) was evaluated. Material in as-built condition and subjected to post-processing, including two heat treatments and hot isostatic pressing, was investigated. It was found that in the absence of tensile RS, the fatigue life at elevated temperature is primary controlled by the defects; and densification has a much stronger effect than the considered heat treatments on the improvement of the mechanical performance.