Publications and presentations

Das wire arc additive manufacturing (WAAM) unter Verwendung des Metall-Schutzgasschweißprozesses zeichnet sich vordergründig durch eine kostengünstige und flexible Fertigung von großvolumigen, metallischen Bauteilen aus. Dabei führt der hohe Wärme- und Materialeintrag des Lichtbogenverfahrens jedoch zu einer periodischen Wiedererwärmung und teilweisen Wiederaufschmelzung des bereits platzierten Schweißguts. Bei umwandlungsfähigen Werkstoffen resultiert die Anwendung des Verfahrens dementsprechend in einer komplexen Wechselwirkung zwischen Temperatur-Zeit-Regime, Gefüge und technisch-mechanischen Bauteileigenschaften. Die Beschreibung der zugrundeliegenden Wirkmechanismen liegt im derzeitigen Stand der Technik nur ansatzweise vor. Im Rahmen der Arbeit erfolgt die systematische Untersuchung der Wärmeführung bei WAAM unter Einsatz von niedriglegiertem Stahl. Dabei wird die Adressierung eines breiten Anwendungsbereichs durch den Substratwerkstoff S355J2+N sowie den Schweißzusatzwerkstoff G4Si1 erreicht. Die zielgerichtete Beschreibung der Wärmeführung bei WAAM findet zunächst durch die experimentelle Untersuchung des prozessseitigen Wärmeeintrags und die Analyse der Wärmeabführung während der Bauteilabkühlung statt. Die allgemeingültige Darstellung der Ergebnisse gelingt durch die Einführung der t8/5-Zeit zur Beschreibung der Bauteilabkühlung je aufgetragener Schweißraupe im WAAM-Prozess. Auf Grundlage der experimentellen Versuchsdurchführung findet die Umsetzung einer transienten, thermischen Simulation des WAAM-Prozesses statt. Der Fokus liegt dabei auf der örtlich und zeitlich aufgelösten Analyse von Wärmefeldern und Abkühlraten zur fundierten Beschreibung von Phasenumwandlungen während der zyklischen Wiedererwärmung des Stahls. Als auschlaggebender Mechanismus zur Gefügeausbildung kann die letztmalige Überschreitung der Ac3-Temperatur sowie die anschließende Abkühlrate identifiziert werden. Die Verifizierung der Erkenntnisse gelingt einerseits durch den Abgleich mit metallografischen Querschliffen und andererseits durch ausgewählte Analyseverfahren zur Charakterisierung der technisch-mechanischen Bauteileigenschaften wie Härtemessungen oder Zugversuche. Zusammenfassend können die experimentell und numerisch ermittelten Erkenntnisse für die Erarbeitung eines Prozessmodells zur erstmaligen, quantitativen Beschreibung der Wärmeübertragungsmechanismen bei WAAM herangezogen werden. Zudem werden Prozessstrategien zur örtlichen Einstellung von technisch-mechanischen Bauteileigenschaften aufgezeigt.

This study investigates the effects of sputtering and electron beam evaporation (e-beam) on the microstructure and reactive properties of Al/Ni reactive multilayers (RMLs). The intermixing zone, a critical factor influencing reaction kinetics, is characterized using high-resolution transmission electron microscopy and found to be consistently 3 nm for both fabrication methods. Differential scanning calorimetry reveals that e-beam samples, with thicker Al layers, exhibit slightly higher total molar enthalpy and maintain high reaction temperatures despite reduced reaction velocities in comparison to sputtered samples. X-ray diffraction confirms the formation of both Al3Ni2 and AlNi phases in the e-beam samples. These findings indicate that while thicker bilayer structures reduce reaction velocity, they keep thermal output and mitigate the impact of intermixing zones, leading to similar total molar enthalpy. This analysis underscores the significance of deposition technique and bilayer thickness in optimizing the performance of Al/Ni RML, offering the possibility to establish different phase formations in thicker RML. It advances the control over the reactive properties of RMLs in their applications, for example, reactive bonding.

The chemical energy released as heat during the exothermic reaction of reactive multilayer systems has shown potential applications in various technological areas, e.g., in joining applications. However, controlling the heat release rate and the propagation velocity of the reaction is required to enhance their performance in most of these applications. Herein, a method to control the propagation velocity and heat release rate of the system is presented. The sputtering of Al/Ni multilayers on substrates with periodic 2D surface structures promotes the formation of growth defects into the system. This modification in the morphology locally influences the reaction characteristics. Tailoring the number of 2D structures in the substrate enables the control of the velocity and maximum temperature of the propagation front. The morphology of the produced reactive multilayers is investigated before and after reaction using scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. In addition, the enthalpy of the system is obtained through calorimetric analysis. The self-sustained and self-propagating reaction of the systems is monitored by a high-speed camera and a high-speed pyrometer, thus revealing the propagation velocity and the temperatures with time resolution in the microsecond regime.

During the layer-by-layer build-up in the Direct Energy Deposition (DED) - Arc additive manufacturing (AM) process, the distance between the contact tube and the workpiece, effectively the welded layer, changes. Since the weld paths are predefined by the path planning software, a constant Contact Tube to Workpiece Distance (CTWD) and weld bead height is assumed. However, even small changes in geometry, such as crossovers of weld paths, result in higher weld beads than assumed. Similarly, an incorrectly assumed bead height as input to the path planning will result in a change in the CTWD. The sum of the deviations of the real weld geometries from the assumed ones in the path planning can greatly influence the CTWD. This implies that the dimensional accuracy may be significantly compromised. This research presents an approach for a general indirect measurement method using the welding current to obtain the CTWD during the actual welding process. A real-time process control method is implemented and validated using the mechanically controlled short arc and the pulsed arc process. Varying process parameters are used to validate the general applicability for a specific material. For the mechanically controlled short arc process, the model underestimates the measured CTWD by a mean error of 3.4 mm. The pulse process is overestimated by a mean error of 2.2 mm. The standard deviation for the pulse process with 1.3 mm is slightly smaller than for the short arc process with 1.7 mm.

Friction stir welding (FSW) is subjected to process-specific challenges including comparatively high process forces and tool wear resulting from thermomechanical stresses. As a result, the acting loads and the geometric-related tool wear can cause tool failure. The tool (shoulder) design, whether it is concave or flat, with or without geometrical elements, is mainly responsible for the related failure mechanism and tool life. Therefore, this study systematically analyzes the failure mechanisms as a function of the process temperature, during FSW of AA-6060 T66 using tools made of H13 tool steel, with different shoulder designs, namely a concave contour and a scroll contour. The mechanism responsible for tool failure was induced by repeated welding at rotational speeds of 4000 rpm and 2000 rpm, at process temperatures within the range of the secondary hardness maximum (552 ˚C and 555 ˚C) and below the temperature of the secondary hardness maximum (488 ˚C and 499 ˚C). The experimental investigation showed that reducing the rotational speed of the scrolled shoulder from 4000 rpm to 2000 rpm resulted in less wear and therefore an increase in tool life from 474 m to up to 1400 m. In this context, it has also been shown that the shoulder geometry affects the mechanism relevant to failure due to the free length of the probe.