Fraunhofer-Institut für Produktionsanlagen und Konstruktionstechnik IPK

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Numerical investigation into cleanability of support structures produced by powder bed fusion technology

2022 , Campana, G. , Uhlmann, E. , Mele, M. , Raffaelli, L. , Bergmann, A. , Kochan, J. , Polte, J.

Purpose: Support structures used in laser powder bed fusion are often difficult to clean from unsintered powder at the end of the process. This issue can be significantly reduced through a proper design of these auxiliary structures. This paper aims to investigate preliminary the airflow within differently oriented support structures and to provide design guidelines to enhance their cleanability, especially the depowdering of them. Design/methodology/approach: This study investigates the cleanability of support structures in powder bed fusion technology. Digital models of cleaning operations were designed through computer-aided engineering systems. Simulations of the airflow running into the powder entrapped within the thin walls of auxiliary supports were implemented by computational fluid dynamics. This approach was applied to a set of randomly generated geometrical configurations to determine the air turbulence intensity depending on their design. Findings: The resul ts, which are based on the assumption that a relationship exists between turbulence and powder removal effectiveness, demonstrated that the maximum cleanability is obtainable through specific relative rotations between consecutive support structures. Furthermore, it was possible to highlight the considerable influence of the auxiliary structures next to the fluid inlet. These relevant findings establish optimal design rules for the cleanability of parts manufactured by powder bed fusion processes. Originality/value: This study presents a preliminary investigation into the cleanability of support structures in laser powder bed fusion, which has not been addressed by previous literature. The results allow for a better understanding of the fluid dynamics during cleaning operations. New guidelines to enhance the cleanability of support structures are provided based on the results of simulations.

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Efficient abrasive water jet milling for near-net-shape fabrication of difficult-to-cut materials

2020 , Uhlmann, E. , Männel, C. , Braun, T.

The utilization of materials with high strength to density ratio enables efficiency improvements and is therefore demanded for many applications, particularly in the aerospace and other mobility sectors. However, the machining of these typically difficult-to-cut materials poses a challenge for conventional manufacturing technologies due to the high tool wear. Abrasive water jet (AWJ) machining is a promising alternative manufacturing technology for machining difficult-to-cut materials, since the tool wear is low and material independent. However, AWJ machining is limited regarding the producible geometries when conducting cuts through a material. This limitation can be resolved with AWJ milling operations which on the other hand are time-consuming. To approach this challenge, an enhanced AWJ milling operation is presented and investigated in this paper with the aim to expand the producible geometries. This operation consists of two kerfs, inserted from different sides of the workpiece, which intersect at their kerf ground. Consequently, a piece of material is separated without the cut material being entirely chipped. Thus, the operation possesses a high aggregated material removal rate. The investigations presented in this paper show and evaluate the effects that occur during the milling of kerfs with variable depths on titanium aluminide TNM-B1. Furthermore, a method to compensate these effects is introduced and thus the producible geometries for effective AWJ milling could be enhanced.

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Erhöhung der Genauigkeit beim Fräsen mit Industrierobotern

2018 , Uhlmann, E. , Reinkober, S. , Hollerbach, T.

Aufgrund des seriellen kinematischen Aufbaus und den serienmäßig antriebsseitig verbauten Winkelmesssystemen ist die Form- und Lagegenauigkeit von Industrierobotern bei Fräsaufgaben limitiert. Diese wird maßgeblich von den auftretenden Prozesskräften F in Kombination mit den geringen nicht-linearen Steifigkeiten c der Getriebe serieller Kinematiken beeinflusst. Ein vielversprechender Ansatz die Genauigkeit kostengünstig zu erhöhen, ist die Verwendung von abtriebsseitigen Winkelmesssystemen. Für statische Prozesse ist die Nutzbarkeit bereits validiert. In diesem Beitrag wird erstmalig die Anwendbarkeit solcher Systeme bei Bahnprozessen untersucht und anhand von Bearbeitungsversuchen in Stahl validiert.

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Gyroscopic and mode interaction effects on micro-end mill dynamics and chatter stability

2013 , Shi, Y. , Mahr, F. , Wagner, U. von , Uhlmann, E.

This paper investigates the gyroscopic and mode interaction effects on the micro-end mill dynamics and the stability behavior due to regenerative chatter. A high-speed spindle system for micro-milling is modeled using finite elements. The transfer functions and the mode shapes are studied to gain a deep insight into the dynamic characteristics. The experimentally identified chatter states and operational vibration modes are given to verify the analytical results. It is shown that, due to the small rotary inertia of the micro-end mill, the gyroscopic effect considered in the inertial frame is less significant despite high rotational speeds. The mode interaction strongly affects the dynamics and the chatter stability. Moreover, piezoelectric elements are applied to in-process excitation in order to identify the transfer behavior of the micro-end mill in the operating state.

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Analysis of influencing factors for the dressing process in double side face grinding

2021 , Uhlmann, E. , Lichtschlag, L.

In grinding, the design of the dressing process is an essential part of work preparation and restoration of the grinding wheel's profile and cutting ability. In contrast to most grinding processes, the choice of dressing parameters in double face grinding with planetary kinematics has so far only been experience-based. As a consequence, the dressing process causes a higher degree of tool wear than the machining of the workpieces. A focused design of the dressing process based on a scientific data could help to improve the ecological and the economic efficiency by reducing tool wear and the amount of dressing tools used. In this paper, methods for determining the wear condition and the result of the dressing process, including macro- and microscopic characteristic are presented. This includes a correlation analysis between parameters of wear characteristics and workpiece surface quality. Furthermore, technological investigations are carried out in order to systematically limit the main influencing factors on the dressing process. As a result, the parameters dresser grain size dgd, rotational speed ratio nld and the machined dresser height ∆hd are identified as significant for dressing. The knowledge about their principal influence on the dressing result could provide the basis for further research.

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Influence of the fatty acid profile on the lubricating film formation in micro-milling process on 7050-T7451 aluminum alloy

2020 , Chanes de Souza, M. , Wiesner, H.M. , Kuche, Y. , Polte, J. , Oliveira Gomes, J. de , Uhlmann, E.

This work studies the technical performance of vegetable-based metalworking fluids as a more environment-friendly alternative for micro-milling on 7050-T7451 aluminum alloy. Micro-milling investigations were performed using two vegetable-based oils, two emulsions prepared with the same vegetable-based oil and a commercial emulsion for comparison. It was noted that the vegetable-based oils with high oleic fatty acid profile provide better lubrication films than mid-oleic fatty acid profiles. However, the wear of the cutting tools is slightly more pronounced when the high oleic oil was used. When these oils were used for the preparation of vegetable-based emulsions, the high oleic emulsion presents better lubricating potential and provides high surface quality than the mid-oleic emulsion. However, the wear of the cutting tool was similar. The vegetable-based metalworking fluids showed similar performance than the commercial reference. Therefore, the oil with high oleic fatty acid profiles presents good technical performance and can be used in vegetable-based metalworking fluids formulations.

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3D-Vorkonturierung mittels Wasserabrasivstrahl

2018 , Uhlmann, E. , Männel, C.

Die notwendige Reduzierung von Kraftstoffverbrauch und Treibhausgasemissionen sind nur zwei der Gründe für die steigenden Anforderungen an die Werkstoffeigenschaften. Nicht immer können die konventionellen Fertigungsverfahren mit den Entwicklungen neuer Werkstoffe Schritt halten. Der Einsatz der Wasserstrahltechnologie, deren Verschleiß vom Werkstoff unabhängig ist, bietet für diese Materialien ein hohes Potenzial. Der Wasserabrasivstrahl nutzt diesen Vorteil heute für vielfältige materialabtrennende Aufgaben. In diesem Beitrag werden die Vorkonturierung von dreidimensionalen Strukturen mittels Wasserabrasivstrahl sowie die Randbedingungen für eine wirtschaftliche Umsetzung vorgestellt. Ein Vergleich zwischen konventionalen Bearbeitungsverfahren und innovativer Wasserstrahlbearbeitung zeigt die Vorteile des Wasserabrasivstrahls bei der Bearbeitung von schwer zerspanbarem Titanaluminid auf.

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Steuerung von Laser-induzierten periodischen Oberflächenstrukturen

2021 , Uhlmann, E. , Schweitzer, L. , Schneider, P. , Michel, A. , Hein, C.

Laser-induzierte periodische Oberflächenstrukturen (LIPSS) weisen ein hohes Potenzial für Anwendungen in den Bereichen der Oberflächenfunktionalisierung auf. Die Steuerung der Richtung dieser Nanostrukturen kann nur durch Änderung der Laserpolarisation erfolgen. Auf dem Markt gibt es kein System zur automatischen Änderung der LIPSS-Orientierung. Für den industriellen Einsatz ist dies vom Vorteil, um Inhomogenität im Strukturverlauf zu vermeiden. In diesem Beitrag wird eine Systemlösung vorgestellt, indem die Steuerung der Richtung von Nanotexturen ermöglicht wird.

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Electrical discharge dressing of honing stones

2020 , Uhlmann, E. , Yabroudi, S. , Zimmermann, S. , Gerlitzky, G. , Polte, M. , Klink, U. , Sihling, B.

Honing processes require a systematic preparation and a regeneration of the honing tool to guarantee a sufficient cutting behaviour and prevent unintended wear of the tool. These dressing processes are particularly important for the regeneration of the macro- and microscopic topography of the honing stones. The established method of manual shaping subsequent to a cylindrical grinding process demands a high skill level, a great amount of non-productive time and is not reproducible. In this paper the preparation of honing stones by use of electrical discharge machining (EDM) is presented. The aim is to ensure an automatic, systematic and reproducible process of resetting the bond of honing stones. Different tool electrode materials were analysed systematically for two different bond specifications of the honing stones. EDM process parameters were studied within a design of experiments (DoE) strategy and a subsequent effect analysis to assure the identification of most appropriate EDM process parameters. After this procedure various cycles of long-stroke honing processes were carried out. As a result, the new approach makes it possible to prepare honing stones with reproducible sharpness and macroscopic topography within an automated process. The dressing time td was reduced by 88 % compared to the established method.

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Trochoide Fräsbearbeitung mit Industrierobotern

2018 , Uhlmann, E. , Reinkober, S. , Hoffmann, M. , Käpernick, P.

Der hohe Bedarf an einer dynamischen und kundenindividuellen Produktion rückt die Flexibilität von Produktionssystemen zunehmend in den Fokus. Industrieroboter leisten heute einen entscheidenden Beitrag zur Befriedigung dieses Bedarfes. Obwohl sie in der Handhabung bereits Stand der Technik sind, finden sie in der mechanischen Fertigung, nach wie vor nur selten Anwendung. Das Trochoidfräsen bietet ein Potenzial zur Steigerung der Zerspanleistung Pc [1, 2]. Diese hochdynamische Frässtrategie erzeugt deutlich geringere Schnittkräfte Fc als konventionelles Fräsen und steigert gleichzeitig das Zeitspanvolumen Qw [2, 3, 4].

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