Additive manufacturing letters最新文献

Additive manufactured (AM) 94 % alumina was successfully 3D printed using the Lithography Ceramic Manufacturing (LCM) technique. Each 3D printed sample was exposed to a different stage of the thermal post-process to identify changes in chemical composition at each stage. The thermal phases studied were the as printed green state, preconditioning at 120 °C, debinding at 600 °C, debinding at 1100 °C, and sintering at 1650 °C. Fourier Transform Infrared Spectroscopy (FTIR), Raman Spectroscopy, Thermogravimetric Analysis (TGA), and X-Ray Fluorescence (XRF) were used to evaluate the changes in composition at each stage of the thermal post-process. Cross-sectional images of 3D printed alumina samples after thermal exposure were captured using scanning electron microscopy (SEM).

In-situ microstructural control is desirable in additively manufactured metal parts due to limited post-processing options for net-shaped components. Here, we introduce a novel selective rescanning approach to control the local solidification conditions and the microstructure in metal parts produced by laser powder-bed fusion (LPBF). We show that the melt pool dimensions, grain size, and sub-grain cell structure can be selectively varied in three dimensions to engineer the mechanical response of LPBF parts. The lattice-based rescanning strategy enables the formation of an interpenetrating microstructure comprised of fine and coarse grains. The localized heating and cooling-induced thermal stresses increase the hardness and tensile strength of rescanned specimens. The study shows the potential of selective rescanning strategy as a promising avenue for achieving precise control of microstructure and properties in as-printed LPBF parts without subsequent processing.

This study involves a comparative analysis of additively manufactured GRCop-42 specimens produced using two processes: laser-powder bed fusion (L-PBF) and laser powder direct energy deposition (LP-DED). The investigation characterizes a range of material attributes, including surface topography, internal defects, microstructural features, quasi-static mechanical properties, and fractographic characteristics. The findings demonstrate that, despite the specimens being fabricated with the same base material, the resulting material properties vary significantly between the two additive manufacturing processes. As such, material properties cannot be presumed to be uniform across different manufacturing methods. Consequently, material characterization must be conducted for individual manufacturing processes based on specific parameters.

In extrusion-based additive manufacturing, achieving high surface quality typically involves using small layer heights to reduce the size of grooves between layers. However, this approach can be both less effective and time-consuming in big-area additive manufacturing. Therefore, the current focus is on investigating methods for printing with fewer layers without compromising surface quality. In this study, single-strand walls were printed using a two-component thermoset material, where different nozzle designs and printing strategies are explored to achieve the flattest possible surface. The success of each approach was evaluated by measuring the percentage of material that required removal to achieve a perfect vertical flat wall. The results suggested that incorporating vertical wings to contain the material in the desired shape was beneficial. Furthermore, the study introduced the idea of adjustable layer heights to mitigate layer deformation. This deformation is most noticeable in the initial layers but largely affects all subsequent printed layers. Finally, making the wings have an angle with regard to the printing direction or trapezoidal wings, served as a pressure funnel that produced the greatest improvement in surface quality. These changes allowed for a reduction of the amount of material which would need to be removed to achieve a flat wall without grooves from 14.3% for a standard print from a round nozzle, to 2.5% for an optimized strand. The research shows a promising path to producing entirely flat vertical structures, even when printing with still-deformable, thermoset materials in the context of big-area additive manufacturing.

This study investigates the microstructural characteristics and the high-temperature mechanical behavior of Hastelloy X, fabricated via laser powder-bed fusion (LPBF) technology. Hastelloy X, a solid solution-strengthened nickel-based superalloy known for its high strength and oxidation resistance at elevated temperatures, has gained significant interest for the fabrication of complex aerospace components through LPBF technology. The study initially focuses on the impact of solution annealing heat treatment at 1227 °C on the alloy microstructure, based on scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigations. It then explores the fatigue and cyclic deformation response of the alloy at 750 °C across different strain ranges, comparing the as-built and solution-annealed conditions. To understand the observed differences in the cyclic mechanical response of as-built and solution-annealed LPBF HX, for a particular condition, a set of dedicated tests have been performed and interrupted at selected numbers of cycles in the different stages of the mechanical response. At each interruption point, specimens have been examined by TEM to provide an in-depth understanding of the effect of dislocation microstructural evolution on the high-temperature cyclic mechanical response of the alloy.

The oxidation behavior of copper-silver (Cu–Ag) alloy with the structure of triply periodic minimal surfaces (TPMS) processed by laser powder bed fusion (LPBF) was investigated at 300 °C and 600 °C. The lightweight TPMSs increase surface area, boosting measurement sensitivity in oxidation studies. The presence of silver enhances oxidation resistance of Cu–Ag alloy compared to that of pure copper by slowing down the oxidation process and thinning the oxide layer. This suggests that silver in the alloy potentially suppresses the outward diffusion of copper from the substrate to the oxide layer. This effect is evident in the oxidation rate curves, where the introduction of silver changes the oxidation kinetics from a linear rate in Cu to a parabolic rate in Cu–2 wt.% Ag at 300 °C. Moreover, at 600 °C, silver induces a slower parabolic rate in Cu–2 wt.% Ag compared to Cu.

Electron beam powder bed fusion offers the unique opportunity to observe the process by measuring scattered electrons on a metal detector. This technique is the state of the art in generating electron optical images of the build area after melting using single- or multi-detector setups. The images enable the detection of surface defects like porosity or material transport by reconstructing the surface topography. Internal defects such as layer-bonding defects cannot be identified. Many of these defects, particularly layer-bonding defects, often originate from an irregular distribution of the powder bed.

This work introduces an additional process step by recording an electron optical image after the distribution of the powder bed. Combining this with an electron optical image after melting the previous layer enables extraction of powder bed features such as the current powder bed height. The underlying method bases on the correlation of experimental measurements and numerical simulations of the intensity of the electron optical signal for different powder bed heights. With this approach, it is possible to identify irregular powder distributions, such as uncovered areas of previously molten material or locally varying powder bed heights. This information is crucial for online monitoring and real time process control. Exemplary, this opens the opportunity of healing the powder bed by an additional raking step.

Multi-Material Additive Manufacturing (MMAM) enables the grading of material properties and the integration of functions within printed parts. While most MMAM methods are limited to process single-component or pre-mixed multi-component materials, the in-process mixing and extrusion of multi-component materials enables innovative material properties and use cases. When processing liquid multi-component materials, the individual component streams need to be homogenized in-process, but the required volume in conventional passive mixing hinders rapid transitions in material composition. In this paper, a two component printhead is presented which combines an active mixing approach with a continuous composition adjustment for a third additive. The approach to control the mixing composition is to influence the hydrodynamic equilibrium of individual material streams before merging them near the point of extrusion. The printhead’s functionality is verified in terms of mixing homogeneity and transition speed between material compositions.

This study investigates the impact of atmospheres (Ar and N₂) on Fe-12Cr-6Al alloy fabricated using laser powder bed fusion (L-PBF) in terms of melt pool shape/size, microstructure, precipitate characteristics, and mechanical properties. The sample built in the N₂ atmosphere exhibited lower porosity, wider melt pools, and no Al₂O₃ agglomeration. Oxygen content decreased from 0.012 to 0.0045 (wt.%), and nitrogen content increased from 0.013 to 0.02 (wt.%). The Ar-printed sample had a yield strength (YS) of 232 ± 15 MPa, ultimate tensile strength (UTS) of 286 ± 10 MPa, and total elongation (TE) of 6.4 ± 1.3 %, while the N₂-printed sample showed significant improvements of the mechanical properties: YS of 315 ± 11 MPa, UTS of 401 ± 11 MPa, and TE of 7.8 ± 1.1 %. Therefore, N₂ might be considered to replace Ar as a cost-effective shielding gas for FeCrAl alloys, with improved properties.

Using theory and simulations, the challenge of gravity-induced distortions during sintering is addressed and a mitigation strategy is proposed. Based on the continuum theory of sintering, the finite element simulation demonstrates the advantages of a rotating furnace to counteract gravity forces during sintering. Its application for stainless steel hollow parts produced by additive manufacturing (binder jetting) is demonstrated, numerically, for reliable industrial production of complex shapes. Sintering a tube in a very slow rotating motion exhibits an improvement in the final deformation ratio compared to a conventional sintering process.

The same concept has been adapted for higher furnace revolution speeds and the centrifugal force is now surpassing the effects of gravity. An extended study of sintering under microgravity for space-borne applications is also widely depicted with the same model. Indeed, it shows the possibility of reproducing Earth's sintering conditions at places where gravity is insufficient to provide acceptable densification and shape conservation during sintering.