Bestimmung der Verbindungseigenschaften beim punktförmigen Schweißen von Aluminiumlegierungen mit Stahl. - Ilmenau : Universitätsverlag Ilmenau, 2023. - 1 Online-Ressource (XV, 153 Seiten). - (Fertigungstechnik - aus den Grundlagen für die Anwendung ; Band 18)
Technische Universität Ilmenau, Dissertation 2023
Stahl wird in Leichtbaustrukturen durch Aluminiumlegierungen ergänzt. Die Fügetechnik nimmt hierbei eine Schlüsselrolle ein. Die Werkstoffe weisen unterschiedliche thermophysikalische Eigenschaften auf, was die schweißtechnische Verarbeitung erschwert. Die Bildung intermetallischer Phasen während des Schweißens führt bei unkontrolliertem Wärmeeintrag und unzureichender Anbindung zu sprödem Verbindungsversagen bei geringen Lasten. Das Verfahren des Ultraschall-Element-Widerstandspunktschweißens ermöglicht den Einsatz des Widerstandspunktschweißens als Hauptfügeprozess. Dazu wird auf das Aluminiumblech an der Fügestelle mittels Ultraschallschweißen ein Fügehilfselement aufgeschweißt, welches die Verbindung zum Stahl durch eine Schweißlinse erreicht. Während der Widerstandserwärmung wachsen intermetallische Phasen in der Ultraschallschweißung über das prozessual einstellbare Temperatur-Zeitregime während des Widerstandsschweißprozesses an. Es gelang mit Hilfe einer eigens entwickelten Zugprobe die Fügezonenfestigkeit nach der Durchführung der Schweißprozesse zu bestimmen. So konnte nachgewiesen werden, dass ein Verbindungsversagen im Zusammenhang mit der intermetallischen Phase nur dann eintritt, wenn die Fügezonenfestigkeit kleiner ist als die Festigkeit des Aluminiumwerkstoffs. Als Grenze wurde eine Gesamtphasendicke von 8 µm herausgearbeitet. Es gelang, ein Prognoseverfahren für das Abgrenzen von Ausknöpfbrüchen zu Scherbrüchen bei Scherzugbelastung auf Stahl/Aluminium-Verbindungen unter Berücksichtigung intermetallischer Phasen anzuwenden. Zur Für die Scher- und Kopfzugverbindungen wurde ein Berechnungskonzept entwickelt, was die intermetallischen Phasen als Bemessungsgröße berücksichtigt. Die Erkenntnisse konnten auf bauteilähnliche Werkstücke angewendet und auf weitere Verbindungen übertragen werden.
https://doi.org/10.22032/dbt.58335
Porenbildung beim Schweißen von Kupferwerkstoffen unter Verwendung von Laserstrahlung im grünen Wellenlängenbereich. - Ilmenau : Universitätsverlag Ilmenau, 2023. - 1 Online-Ressource (153 Seiten). - (Fertigungstechnik - aus den Grundlagen für die Anwendung ; Band 16)
Technische Universität Ilmenau, Dissertation 2023
Der Einsatz von herkömmlicher, infraroter Laserstrahlung für das Schweißen von Kupfer ist mit zahlreichen Herausforderungen, wie der Bildung von Prozessporen, Spritzern und Materialauswürfen verbunden. Ein Optimierungsansatz stellt diesbezüglich die Verwendung von grüner und blauer Laserstrahlung dar. Aufgrund der höheren Absorption von Laserstrahlung in diesem Wellenlängenbereich lassen sich geringe Aspektverhältnisse der Dampfkapillare beim Schweißen realisieren (Verhältnis Tiefe zu Breite: < 5), was zur Stabilisierung der Dampfkapillare und zur Reduzierung auftretender Prozessinstabilitäten und Nahtdefekte führt. Jedoch tritt in diesem Zusammenhang eine Art der Porenbildung auf, die bisher für das Laserstrahlschweißen von Kupferwerkstoffen nicht beobachtet wurde. Im Rahmen dieser Arbeit wurden die zugrunde liegenden Wirkmechanismen dieser sogenannten Gasporenbildung untersucht. Durch die Applizierung definierter Umgebungsgasatmosphären während des Schweißprozesses konnte experimentell nachgewiesen werden, dass die Gasporenbildung auf zwei unterschiedlichen Wirkmechanismen beruht, die der metallurgischen Porenbildung und der Reaktionsporenbildung. Es wurde gezeigt, dass die Gasporenbildung im Zusammenhang mit der Existenz von Stickstoff und Sauerstoff im Umgebungsgas steht. Während die Aufnahme von Stickstoff zur metallurgischen Porenbildung führt, ist die gleichzeitige Aufnahme von Stickstoff und Sauerstoff, beispielsweise beim Schweißen unter atmosphärischer Luft, mit der Bildung von Reaktionsporen verbunden. Mittels der Methode der Heißgasextraktion konnte die Existenz dieser Elemente im Schweißgut nachgewiesen, und damit deren Aufnahme während des Schweißens im schmelzflüssigen Kupfer gezeigt werden. Das Auftreten von metallurgischer Porenbildung und Reaktionsporenbildung wurde im Rahmen dieser Arbeit in einer modellhaften Vorstellung zusammengeführt, welche durch die Ergebnisse unterschiedlicher experimenteller Untersuchungen gestützt wird. Ferner konnte gezeigt werden, dass zudem ein werkstoffseitiger Einfluss auf die Gasporenbildung besteht, der auf den Restsauerstoffgehalt des Grundmaterials zurückzuführen ist. Aufbauend auf diesen Erkenntnissen wurden zahlreiche Maßnahmen zur Reduzierung oder Vermeidung der Porenbildung abgeleitet.
https://doi.org/10.22032/dbt.57726
Ni/Al multilayer reactions on nanostructured silicon substrates. - In: Journal of materials science, ISSN 1573-4803, Bd. 58 (2023), 31, S. 12811-12826
Fast energy release, which is a fundamental property of reactive multilayer systems, can be used in a wide field of applications. For most applications, a self-propagating reaction and adhesion between the multilayers and substrate are necessary. In this work, a distinct approach for achieving self-propagating reactions and adhesion between deposited Ni/Al reactive multilayers and silicon substrate is demonstrated. The silicon surface consists of random structures, referred to as silicon grass, which were created by deep reactive ion etching. Using the etching process, structure units of heights between 8 and 13 µm and density between 0.5 and 3.5 structures per µm^2 were formed. Ni and Al layers were alternatingly deposited in the stoichiometric ratio of 1:1 using sputtering, to achieve a total thickness of 5 µm. The analysis of the reaction and phase transformation was done with high-speed camera, high-speed pyrometer, and X-ray diffractometer. Cross-sectional analysis showed that the multilayers grew only on top of the silicon grass in the form of inversed cones, which enabled adhesion between the silicon grass and the reacted multilayers. A self-propagating reaction on silicon grass was achieved, due to the thermally isolating air pockets present around these multilayer cones. The velocity and temperature of the reaction varied according to the structure morphology. The reaction parameters decreased with increasing height and decreasing density of the structures. To analyze the exact influence of the morphology, further investigations are needed.
https://doi.org/10.1007/s10853-023-08794-9
Arc welding process monitoring using neural networks and audio signal analysis. - In: SMSI 2023 Conference - Sensor and Measurement Science International, (2023), S. 249-250
This paper investigates the potential of airborne sound analysis in the human hearing range for automatic defect classification in the arc welding process. We propose a novel sensor setup using microphones and perform several recording sessions under different process conditions. The proposed quality monitoring method using convolutional neural networks achieves 80.5% accuracy in detecting deviations in the arc welding process. This confirms the suitability of airborne analysis and leaves room for improvement in future work.
https://doi.org/10.5162/SMSI2023/D7.2
The study of screw extrusion-based additive manufacturing of eco-friendly aliphatic polyketone. - In: Journal of materials research and technology, ISSN 2214-0697, Bd. 25 (2023), S. 4125-4138
Aliphatic polyketone is a new-age eco-friendly, high-performance engineering thermoplastic. However, its potential for replacing other polymers depends on its ability to be processed. Considering that the first aliphatic polyketone suitable for processing was developed relatively recently (2015), the material gained new research potential. In this paper screw extrusion-based process was developed for additive manufacturing of aliphatic polyketone. A detailed characterisation of the process and printed samples was done. It was shown that the extruder-base process can produce stable additive-manufactured parts depending on printing speed (process parameters). The interpass temperature has a significant influence on printing properties and it depends on printing speed (travel speed of building platform and extruder rotational speed). With the increase in the printing speed, the interpass temperature increases as well. If it is low causes insufficient heat for diffusion to occur causing delamination and if it is too high causes geometrical deviation of workpieces which leads to defects causing a reduction in inter-road strength. The tensile strength of specimens with raster angle 0˚ was 62.7 ± 1.4 MPa, which is slightly higher than the tensile strength of base material guaranteed by the supplier (60 MPa) while the elongation up to the first crack was 32.8 ± 4.6%. Iinter-road strength in specimens with a raster angle of 90˚ was 37.2 ± 0.8 MPa which is 62% of the base material while interpass temperature was 189 ± 3.3 ˚C.
https://doi.org/10.1016/j.jmrt.2023.06.223
Influence of extrinsic induced tensile stress on the self-propagating high-temperature synthesis of nanosized Al/Ni multilayers. - In: Journal of materials science, ISSN 1573-4803, Bd. 58 (2023), 24, S. 10085-10095
Reactive multilayer systems consisting of alternating nanoscale Al and Ni layers are applicable in joining, various pyrotechnic applications and thermal batteries. Since diffusion based high-temperature synthesis occurs without the presence of air, efforts have focused on investigating the understanding of the fundamental reaction processes and characteristics. The aim of this study is to expose the reactive multilayers to extrinsic induced tensile stress so that the self-propagating synthesis can proceed under these conditions. Further, the properties during and after the reaction will be investigated. Multilayers deposited by sputtering on Kapton® substrates with different bilayer- and total thicknesses as well as commercial Nanofoils® with thicknesses of 40 µm and 60 µm were used as samples. The investigations focused on the propagation velocity measured with a high-speed camera, the temperature regime determined with a high-speed pyrometer, and the formed phases after the synthesis examined via X-ray diffraction. The gained results of this study reveal important insights for the application of the reactive Al/Ni multilayer system in terms of stability or reliability related to propagation front velocity, maximum temperature and formed phases under induced external tensile stresses.
https://doi.org/10.1007/s10853-023-08618-w
The GMAW process using a two-dimensional arc deflection with AC hot wires. - In: Welding journal, ISSN 0043-2296, Bd. 102 (2023), 4, S. 88-s-96-s
Heat input in gas metal arc welding (GMAW) directly correlates with the applied current. As a result, welding irregularities, such as incomplete fusion and excessive penetration, increase and mechanical properties decrease. One way for adjusting heat input is to use hot wire technology. In this article, a two-dimensional arc deflection in GMAW was presented by simultaneous application of two alternating current (AC) hot wires. It is shown how the positioning of the hot wires and the signal characteristics of the current intensity influenced the deflection pattern and weld quality. It was found that the magnetic fields of the two hot wires overlapped due to the narrow opening between. Therefore, an increased one-dimensional deflection resulted. To obtain a two-dimensional deflection, it was necessary to shield the magnetic fields from each other by means of a ferritic material. By pulsing or phase shifting the current signals, individual deflection patterns were possible. The effect of arc deflection was visualized with high-speed recordings and metallographic investigations. Different deflection patterns were generated to adjust heat input and counteract weld irregularities. The use of hot wire technology allowed an increase in deposition rate by simultaneous improvement of weld quality.
Effects of ultrashort pulsed direct laser writing on Ni/Al reactive multilayer foils. - In: Applied Sciences, ISSN 2076-3417, Bd. 13 (2023), 7, 4313, S. 1-13
Reactive multilayer foils (RMFs) for joining processes have attracted a great deal of attention over the last few years. They are capable of exothermic self-propagating reactions and can serve as localized heat sources for joining applications when ignited by suitable means. Using short and ultrashort pulsed lasers with carefully selected parameters, cutting and shaping of RMFs makes it possible to tailor heat release characteristics without triggering the reaction. The present study is an investigation of microstructural changes induced by femtosecond laser machining of a commercially available Ni/Al-based RMF. The effects of the specific laser parameters pulse duration and repetition rate on the heat-affected zone (HAZ) are investigated by scanning and transmission electron microscopy. Debris consisting of oxide deposits can be found at a distance of several tens of microns from the cut edge. A negligible HAZ extending to less than 100 nm was observed for all parameters tested and no signs of ignition of a self-propagating reaction were observed. These results underline the suitability of femtosecond lasers for metal machining with minimal heat input.
https://doi.org/10.3390/app13074313
Influence of the temperature-time regime on the mechanical properties during the DED-Arc process of near-net-shape Ti-6Al-4 V components. - In: Welding in the world, ISSN 1878-6669, Bd. 67 (2023), 7, S. 1643-1665
In a research project, the additive manufacturing process of components made of Ti-6Al-4 V using gas metal arc welding (GMAW), which is classified into the directed energy deposition-arc (DED-Arc) processes, was investigated. The project focused on the systematic development of economical additive build-up strategies and the analysis of the temperature-time regime during the build-up process, as well as the investigation of the resulting properties. A welding range diagram was created with recommendations for process settings for additive manufacturing with the controlled short circuit, as well as a presentation of possible defect patterns outside the range shown. For the fabrication of thick-walled structures, various build-up strategies were investigated by modifying the welding path and evaluated with regard to their suitability. Based on the results, additive structures were fabricated by varying the temperature-time regime in order to gain insights into selected geometrical, metallurgical, and mechanical properties. Different energy inputs per unit length, structure dimensions, and interpass temperatures (IPT) were used for this purpose. The research project provides comprehensive findings on the additive processing of the material Ti-6Al-4 V using metal inert gas welding, in particular with regard to the temperature-time regime and the resulting properties.
https://doi.org/10.1007/s40194-023-01513-7
Gap and force adjustment during laser beam welding by means of a closed-loop control utilizing fixture-integrated sensors and actuators. - In: Applied Sciences, ISSN 2076-3417, Bd. 13 (2023), 4, 2744, S. 1-17
The development of adaptive and intelligent clamping devices allows for the reduction of rejects and defects based on weld discontinuities in laser-beam welding. The utilization of fixture-integrated sensors and actuators is a new approach, realizing adaptive clamping devices that enable in-process data acquisition and a time-dependent adjustment of process conditions and workpiece position by means of a closed-loop control. The present work focused on sensor and actuator integration for an adaptive clamping device utilized for laser-beam welding in a butt-joint configuration, in which the position and acting forces of the sheets to be welded can be adjusted during the process (studied welding speeds: 1 m/min, 5 m/min). Therefore, a novel clamping system was designed allowing for the integration of inductive probes and force cells for obtaining time-dependent data of the joint gap and resulting forces during welding due to the displacement of the sheets. A novel automation engineering concept allowed the communication between different sensors, actuators and the laser-beam welding setup based on an EtherCAT bus. The subsequent development of a position control and a force control and their combination was operated with a real time PC as master in the bus system and proved the feasibility of the approach based on proportional controllers. Finally, the scalability regarding higher welding speeds was demonstrated.
https://doi.org/10.3390/app13042744