This study focused on the potential of topology optimization (TO) for metallic tertiary structures of spacecrafts produced by the additive manufacturing (AM) technique laser powder bed fusion. First, a screening of existing conventionally manufactured products was carried out to evaluate the benefits of a redesign concerning product performance and the associated economic impact. As a result of the study, the most suitable demonstrator was selected. This reference structure was redesigned by TO taking into consideration the AM process constraints. Another major aim of this work was to evaluate the possibilities and challenges of AM (accuracies, surface quality, process parameters, postmachining, and mechanical properties) in addition to the redesign process. A comprehensive approach was implemented including detailed analysis of the powder, mechanical properties, in-process parameters, and nondestructive inspection (NDI). All measured values were used for a back loop to the design process, thereby providing a final robust redesign. Finally, the fine-tuned demonstrator was built up in an iterative process. The parts were tested under representative conditions for the application to verify the performance. The demonstrator qualification test campaign contained thermal cycling, vibration testing, static load testing, and NDI. Thus, an improvement in technology readiness level up to “near flight qualified” was reached.
Recently, additive manufacturing (AM) by laser metal deposition (LMD) has become a key technology for fabricating highly complex parts without any support structures. Compared to the well-known powder bed fusion process, LMD enhances manufacturing possibilities to overcome AM-specific challenges such as process inherent porosity, minor build rates, and limited part size. Moreover, the advantages aforementioned combined with conventional machining enable novel manufacturing approaches in various fields of applications. Within this contribution, the additive manufacturing of filigree flexure pivots using 316L-Si by means of LMD with powder is presented. Frictionless flexure pivot bearings are used in space mechanisms that require high reliability, accuracy, and technical cleanliness. As a contribution to part qualification, the manufacturing process, powder material, and fabricated specimens were investigated in a comprehensive manner. Due to its major impact on the process, the chemical powder composition was characterized in detail by energy dispersive X-ray spectroscopy (EDX) and inductively coupled plasma optical emission spectrometry (ICP-OES). Moreover, a profound characterization of the powder morphology and flowability was carried out using scanning electron microscopy (SEM) and novel rheological investigation techniques. Furthermore, quantitative image analysis, mechanical testing, laser scanning microscopy, and 3D shape measurement of manufactured specimens were conducted. As a result, the gained knowledge was applied for the AM-specific redesign of the flexure pivot. Finally, a qualified flexure pivot has been manufactured in a hybrid manner to subsequently ensure its long-term durability in a lifetime test bench.
This study reports on the laser-assisted reduction of iron ore waste using Al powder as a reducing agent. Due to climate change and the global warming situation, it has become of paramount importance to search for and/or develop green and sustainable processes for iron and steel production. In this regard, a new method for iron ore utilization is proposed in this work, investigating the possibility of iron ore waste reduction via metallothermic reaction with Al powder. Laser processing of iron ore fines was performed, focusing on the Fe2O3–Al interaction behavior and extent of the iron ore reduction. The reaction between the materials proceeded in a rather intense uncontrolled manner, which led to the formation of Fe-rich domains and alumina as two separate phases. In addition, a combination of Al2O3 and Fe2O3 melts, as well as transitional areas such as intermetallics, was observed, suggesting the occurrence of incomplete reduction reaction in isolated regions. The reduced iron droplets were prone to acquire a sphere-like shape and concentrated mainly near the surface of the Al2O3 melt or at the interface with the iron oxide. Scanning electron microscopy, energy-dispersive x-ray spectroscopy, and wavelength-dispersive x-ray spectroscopy analyses were employed to analyze the chemical composition, microstructure, and morphological appearances of the reaction products. High-speed imaging was used to study the process phenomena and observe differences in the movement behavior of the particles. Furthermore, the measurements acquired from x-ray computed microtomography revealed that approximately 2.4% of iron was reduced during the laser processing of Fe2O3–Al powder bed, most likely due to an insufficient reaction time or inappropriate equivalence ratio of the two components.
Advanced Manufacturing (AM) has the potential to improve existing technologies and applications in terms of performance, light-weighting and costs. In the context of the SME4ALM initiative, launched by DLR and ESA, the company Kampf Telescope Optics GmbH (KTO) in cooperation with the Fraunhofer Institute for Material and Beam Technology (IWS) have assessed the feasibility of AM to build a high-performance optical mirror for space applications.For the assessment of the AM potentials, a mirror design concept for cryogenic instruments for observations in the IR and NIR range was baselined. In a second step, Nickel-Phosphorus (NiP) was selected as optical coating. The combination of coating and mirror material is a primary design driver for optical performance. Both materials must have a very similar CTE as well as be compliant to modern optical manufacturing (diamond turning, polishing). As a promising candidate for NiP coating the AlSi40 was selected for the mirror structure.The potential advantages of AM for optical mirrors in terms of mechanical performance, cost, and manufacturing time were exploited. The achievement of those objectives was / will be demonstrated by:1. verifying AM material properties and manufacturability of AM mirrors by material sample tests and subcomponent tests 2. designing AM mirror demonstrator by structural, thermal, and optical performance analysis 3. applying and elaborating AM specific design methods (topology optimization, sandwich structures with internal microstructures, monolithic design, etc.) 4. manufacturing, assembling, and testing AM mirror demonstrator to verify manufacturability and optical performance 5. comparing optical and mechanical performance of the AM mirror demonstrator to a conventional mirror by numerical analysis to exploit potential advantages of AM
Most optical instruments for space applications in the areas of earth observation, science and optical communication require high performance optical mirrors. Driving requirements are typically low Surface Form Error (SFE), high structural stiffness over a wide temperature range, a low micro roughness and low mass.A promising material combination for optical mirrors is the usage of the aluminium alloy AlSi40 as mirror substrate and electroless Nickel phosphorous (NiP) as coating for the optical surface. The main advantage of this combination is the CTE compatibility of AlSi40 and NiP over a large temperature range. In addition, NiP coated surfaces can be easily diamond turned and polished. This material combination thus enables relatively fast production cycles even for a mirror surface roughness in the few-nm range.Within the ESA GSTP SME4ALM program, the feasibility to produce optical mirrors from AlSi40 by additive manufacturing (AM) was demonstrated. Main objectives were the determination of AlSi40 AM parameters and material properties as well as the demonstration of advantages of AM (e.g. topology optimization, monolithic design, lattice structures). By consequent improvement of the Laser Powder Bed Fusion (LPBF) manufacturing process, similar material properties to bulk AlSi40 were obtained. Up to 40% improvement were achieved for relevant material properties such as stiffness and SFE.Based on the good results of the first project, a follow up program was launched by ESA (also in the context of ESA-GSTP) with the objective to design and print an optical mirror with TRL ≥ 5. This paper will summarize the development program and results of the follow-up project, in particular:• Design and testing of lattice structures• Definition of a cleaning process • Transfer of AlSi40 processing strategies to industrial-scale machines• Design and testing of a mirror demonstrator, which is representative for space applications
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