FG Hybride Fertigung
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The Wire Arc Direct Energy Deposition (WA-DED) process is highly
regarded as part of additive manufacturing. Compared to other additive manufacturing
processes, it is characterized above all by its high deposition rate and low
system costs. Despite many years of experience in the build-up welding process,
WA-DED still holds a number of challenges in terms of process stability. This
article analyses the interactions in the WA-DED process. To this end, the process
was visualized and described with the help of Structured Analysis and Design
Technique (SADT). Building on this, a process Failure Mode and Effects Analysis
(FMEA) was presented to identify and priorities risks. Finally, the results of
the Taguchi tests were analyzed and visualized. The results illustrate the strong
interactions between the influencing factors. These have a material-specific effect
on the production results. Each new material composition therefore requires a
systematic analysis in order to determine quantitative correlations. In future, these
can be supported by machine learning approaches.
Welding with an electric arc is mainly used as a joining technology, where a high energy input is required locally for the joining process. Due to the heat input, the welded bead is characterized by a heat affected zone, that consists of an inhomogeneous microstructure of different phases and grain sizes. This leads to non-optimal mechanical properties.
One way to improve the microstructure is to induce plastic deformation (e.g. by rolling), which can enable recrystallization mechanisms, that homogenize the microstructure and therefore can optimize the material properties. Yet, the recrystallization mechanisms require a minimum temperature to be activated. On one hand, the in-situ heat input during welding can be used for this in a process combination of welding and rolling but might not be enough, to reach or hold this recrystallization temperature during rolling. An excessive heat input on the other hand leads to too high temperatures, causing significant grain growth, that also negatively affect the mechanical properties. In any case, a certain temperature range over time has to be maintained.
The temperature can be controlled by burners for pre-heating and post-heating, and the heating power can be controlled separately. In this numerical study, burner heat models are used to simulate the temperature-controlled process combination of welding one single bead with a subsequent rolling step for the mild steel St37/S235JR. The influence of the pre- and post-heating could be numerically proven and suitable heating combinations were found, that allow a full recrystallization with a nearly homogenous grain size distribution.
Additive technologies are now widely used for the production of complex precise parts and have high potential for the production of forming dies. In this work, hot-forming dies optimized for additive manufacturing were developed and produced with wire arc direct energy deposition (WA-DED) and laser powder bed fusion (L-PBF) technologies. The concept of lightweight hot-forming dies with a 2D-lattice structure was developed, which reduced the weight of each die by 56%, from 14.2 kg to 6.1 kg, in production using L-PBF. Maraging/precipitation-hardened steel 17-4PH was used as an alternative to traditional hot-working steels with slightly lower mechanical properties and a much higher processability in the additive manufacturing process. The workability of the manufactured dies was confirmed by forging tests on an industrial screw press.
Many processes may be used for manufacturing functionally graded materials. Among them, additive manufacturing seems to be predestined due to near-net shape manufacturing of complex geometries combined with the possibility of applying different materials in one component. By adjusting the powder composition of the starting material layer by layer, a macroscopic and step-like gradient can be achieved. To further improve the step-like gradient,
an enhancement of the in-situ mixing degree, which is limited according to the state of the art, is necessary. In this paper, a novel technique for an enhancement of the in-situ material mixing degree in the melt pool by applying laser remelting (LR) is described. The effect of layer-wise LR on the formation of the interface was investigated using pure copper and low-alloy steel in a laser powder bed fusion process. Subsequent cross-sectional selective electron microscopic analyses were carried out. By applying LR, the mixing degree was enhanced, and the reaction zone thickness between the materials was increased. Moreover, an additional copper and iron-based phase was formed in the interface, resulting in a smoother gradient of the chemical composition than the case without LR. The Marangoni convection flow and thermal diffusion are the driving forces for the observed effect.
Press-hardening is an important metal sheet manufacturing process to improve the metal sheet properties during forming with an inline quenching process. This requires higher cooling rates often obtained by cooling channels within the tools, that enable the formation of martensite for a high strength. The manufacturing of those forming tools with internal cooling channels is quite complex, time and material consuming and therefore expensive. Optimal cooling channel geometry cannot be realized by conventional machining operations, that limits cooling efficiency too.
Wire Arc Additive Manufacturing (WAAM) is a layer-wise welding process, that allows the manufacturing of near net shapes and internal cooling channels. In contrast to conventional machining, manufacturing of a complex lightweight design forming tool can be realized by WAAM. This will further reduce the WAAM process time and material consumption. However, the lightweight design reduces on one hand the thermal mass and thus the capability of heat transfer, making cooling via the cooling channels more crucial. On the other hand, elastic tool
deformation has to be as low as possible.
In this study, a press-hardening forming tool made of S235JR is designed and manufactured by means of WAAM. FEM analysis are performed to optimize the design of the forming tool regarding lightweight aspects. Simple near net shapes of cooling channels are considered for a simplification of the WAAM process. The forming tool is mechanically tested to compare and evaluate the stiffness with the FEM analysis.
Classic forming technologies can make a difference to a climate-neutral economy. This also applies to the production of innovative hydrogen coolers (recuperators). To improve the efficiency of hydrogen coolers, heat transfer and heat flux can be increased by tube structures made by pressing spherical elements on their surface. Hydrogen is passed through the tubes of the recuperators and the cooling medium flows across it (Figure 1a). The tube serves as an atmosphere separator. Structured tubes can increase the power den-sity with the same dimensions. Figure 1b shows a process for introducing the structure into the pipe using radially and symmetrically arranged tools. Sheet metal forming usually involves pressing with structural tools, bending into a tube, and longitudinally welding. However, producing small tube diameters in the range of one inch presents a technical challenge. The challenge considered in this paper is to develop a new structuring process while maintaining the structural integrity and wall thickness of the tubes. This study aims to determine the feasibility of a multi-stage vault structuring process for recuperator tubes us-ing heat-resistant semi-finished products.
Die Dauerschwingfestigkeit ist eine wichtige sowie für die Auslegung von zyklisch belasteten Bauteilen in Umformmaschinen notwendige Materialkenngröße. Diese Kenngröße ist nicht nur material- sondern auch fertigungsprozessabhängig.
Aufgrund der rapiden Entwicklung von additiven Fertigungsanlagen und Werkstoffen mangelt es an hinreichenden Erfahrungswerten zu den Betriebseigenschaften von additiv gefertigten Bauteilen.
Daher ist es wichtig, die Eignung von additiv gefertigten Bauteilen für Umformmaschinen im Vorfeld technologisch und wissenschaftlich zu untersuchen. Die Ermittlung der Dauerschwingfestigkeit von additiv gefertigten Komponenten für Umformmaschinen liefert außerdem einen Mehrwert für die konstruktive Auslegung anderer AM-Bauteile mit einem vergleichbaren Belastungskollektiv und
ermöglicht eine Topologieoptimierung von Komponenten von Umformmaschinen.
Im Rahmen dieses Forschungsvorhabens wurde die Dauerschwingfestigkeit additiv gefertigter Bauteile in Abhängigkeit von der Gesamtheit der Herstellungsparameter eines WAAM-Prozesses sowie diverser Nachbehandlungsmethoden erfolgreich anhand des Fallbeispiels
Pleuel ermittelt.
Außerdem wurde die Anwendbarkeit von WAAM-Prozessen zur Herstellung von individuellen Anlagenbauteilen für den Bereich Umformmaschinen erfolgreich belegt. Mit den ermittelten Dauerschwingfestigkeitswerten konnte eine Grundlage zur Auslegung weiterer AM-Bauteile mit vergleichbaren Belastungskollektiv sowie Topologieoptimierung genannter Bauteile geschaffen werden.
Die Untersuchungen erfolgten an Laborproben geeigneter Geometrie in Bezug auf das Realbauteil und unter Gewährleistung der thermischen Übertragbarkeit auf Realbauteile. Abschließend ist mittels eines skalierten Demonstratorbauteils die Machbarkeit nachgewiesen worden.