Evaluation of the effect of local gas flows on keyhole geometry by means of a half-section setup. - In: 12th CIRP Conference on Photonic Technologies, (2022), S. 385-390
Increasing beam power of solid-state lasers enables high welding speeds for laser welding processes. However, increasing welding speeds lead to imperfections, especially spatter formation while processing high-alloy steels. A recent and novel approach to reduce the spatter formation is the utilization of a local gas flow to manipulate the keyhole pressure balance beneficially. To get a better understanding of the effect of the gas flow on the keyhole and its geometry during deep penetration welding, a half-section setup was developed. The laser beam was positioned partially on a glass plate and the metal sheet to provide an insight into the processing zone by means of high-speed recordings. Thus, it was possible to measure the keyhole geometry and to quantify the effect of different welding speeds and gas flows on keyhole length for full penetration welds.
Characterization of keyhole dynamics in laser welding of copper by means of high-speed synchrotron X-ray imaging. - In: 12th CIRP Conference on Photonic Technologies, (2022), S. 501-506
Laser welding of copper is of great importance for industrial applications, e.g., for manufacturing of electrical components such as hairpins. Solid state lasers are widely used due to the high power and beam brilliance, but the implementation can be challenging in terms of process instabilities and resulting weld defects, i.e., spatter and pore formation. In order to understand the formation thus avoid these defects, the development of the understanding of the keyhole and its interaction with the surrounding melt pool is required. In this paper, high-speed synchrotron X-ray imaging (frame rate: 20,000 Hz) demonstrated its capability to study the keyhole geometry to quantify and describe the dynamic behavior inside the workpiece. Thus, novel insights into fundamental processes are provided and a new methodic approach was introduced to describe the time-dependent behavior of the keyhole and its interaction with the melt pool during laser beam deep welding of copper (Cu-ETP/CW004A).
P3HT:PCBM polymer solar cells from a didactic perspective. - In: Journal of photonics for energy, ISSN 1947-7988, Bd. 12 (2022), 3, S. 035501-1-035501-19
Here, we studied the influence of pre- and post-thermal annealing on the performance of polymer:fullerene bulk heterojunction solar cells using the conventional architecture, comprising a conjugated polymer, namely, poly(3-hexylthiophene-2,5-diyl) (P3HT) and a fullerene derivative [6,6]-phenyl-C60-butyric acid methyl ester (PC60BM) as a photoactive layer. The non-annealed active layer device exhibited a power conversion efficiency of <1 % , which was significantly lower than the pre- and post-annealed devices. To investigate the impact of pre- and post thermal annealing on the natural morphological state of the polymer, regiorandom (P3HT-I) and regioregular (P3HT-II) type P3HT were compared in photoactive layers. In general, P3HT-I is amorphous, whereas P3HT-II is semi-crystalline. Changes in solar cell performance were associated with changes in carrier extraction efficiencies influenced by the annealing conditions. The charge photogeneration processes were investigated using spectroscopic techniques, including electroluminescence, steady-state, and time-resolved photoluminescence spectroscopy. Finally, to explore the morphological changes upon annealing, atomic force microscopy and electroluminescence imaging measurements were performed on films and solar cells, respectively.
Influence of solidification rate on hot crack behavior in heat conduction laser beam welding of EN AW-6082. - In: 12th CIRP Conference on Photonic Technologies, Bd. 111 (2022), S. 490-495
Joining of thin aluminum sheets (<1mm) is widely used in the industry for electronic housings or EMC shielding. Aluminum alloys of the 6xxx series are particularly relevant for such applications. However, their use is very challenging in welding processing due to the hot cracking phenomenon. Hot cracks are known to form during the solidification phase and are a major defect regarding mechanical properties and tightness of the weld. For this reason, the formation of hot cracks must be prevented and solidification rate is known to be a key parameter. In this paper, the solidification rate is specifically manipulated by means of pulse shaping. Metallographic analysis and high-speed recordings of welds carried out with different pulse shapes that allowed the determination of a threshold solidification rate to avoid hot cracks. A further pulse shape optimization considered the determined threshold solidification rate and minimized the required pulse energy to realize sound welds. On the one hand, the study proved which simplifications can be assumed for the modeling and optimization of the pulse shapes and that solidification rate can be used as sole parameter to control crack formation of bead on plate welds. On the other hand, the experimental procedure enabled the identification of critical properties of laser beam sources regarding stability, threshold and fast adaptability of beam power.
Effect of local pressure distribution on spatter formation for welding high alloy steel at high welding speeds. - In: 12th CIRP Conference on Photonic Technologies, Bd. 111 (2022), S. 391-396
Laser beam welding with solid-state lasers leads to the formation of spatters at relevant processing speeds (≥ 8 m/min). The use of local gas flows proved to be adequate to avoid these phenomena. This publication examines the mechanical effect of a local supply of argon, nitrogen and helium to provide a deeper understanding of the local pressure distribution and its effect on the welding process. A dynamic pressure measurement was performed to quantify the two-dimensional pressure field in and around the keyhole aperture. By varying flow rate, nozzle distance and flow angle of the nozzle, the size of pressure field and the position of maximum pressure could be varied. In order to describe the separation of the dynamic pressure from the composition of the shielding gas on spatter formation, equal dynamic pressures were considered for different shielding gases. It was shown that the effect of local gas flows cannot be solely attributed to the influence of the dynamic pressure.
Effective design concepts for welded mixed connections in steel structures :
Effiziente Nachweiskonzepte für geschweißte Mischverbindungen im Stahlbau. - In: Stahlbau, ISSN 1437-1049, Bd. 91 (2022), 10, S. 660-670
Mischverbindungen aus normalfesten und hochfesten Stählen ermöglichen einen optimierten Materialeinsatz durch die Anpassung an die einwirkenden Kräfte. Das führt ökologisch und wirtschaftlich zu Vorteilen durch einen reduzierten Materialverbrauch und kleinere Schweißnahtgeometrien. Da normativ derzeit keine Regelungen für stumpfgeschweißte Mischverbindungen aus normalfesten und hochfesten Stählen bestehen, soll im laufenden Forschungsprojekt „Effiziente Nachweiskonzepte für Mischverbindungen im Stahlbau“ ein Bemessungsansatz dafür entwickelt werden. Darüber hinaus werden die bereits bestehenden Bemessungskonzepte für Stumpfnahtverbindungen und Kehlnahtverbindungen bis zur Stahlsorte S960 erweitert, sodass die geplante neue Fassung des Teils EN 1993-1-12 entsprechend ergänzt werden kann. Dafür werden umfangreiche experimentelle Untersuchungen zur Tragfähigkeit durchgeführt, bei denen die Einflussgrößen Grundwerkstoff, Schweißzusatz, Energieeintrag, Blechdicke und Nahtgeometrie variieren. Mit den erzielten Ergebnissen und weiterführenden Begleituntersuchungen können die Auswirkungen variierender Einflussgrößen auf die Tragfähigkeit der Schweißverbindungen identifiziert werden. Über erste Ergebnisse wird im Folgenden berichtet.
Investigation of the directional characteristics of the emitted airborne sound by friction stir welding for online process monitoring. - In: 2nd International Conference on Advanced Joining Processes (AJP 2021), (2022), S. 41-56
Currently, destructive or non-destructive testing methods are used to verify the weld seam quality subsequent to the manufacturing process. Therefore, pre-processes such as visible or mechanical testing require additional efforts, which can lead to expensive reworking or rejection of the components. The acoustic process characterization for Friction Stir Welding (FSW) applications permits a comparatively new approach of process monitoring to detect weld seam irregularities by the characterization of the emitted noise in the audible frequency range (airborne sound signal). In previous publications, the acoustic detection of weld seam irregularities was mostly based on structure-borne sound sensors. Although a correlation between weld defects and audio signals has been demonstrated, there are process-related deficits in the use of structure-borne sound sensors. These include a fixed installation position and limited applicability for large-scale components such as battery cases. In contrast airborne sound sensors (microphones) can be mounted directly in the area of the joining process and thus influences of component size, joining materials, and weld seam geometry can be reduced. However, the use of airborne sound sensors for FSW applications requires preparatory considerations on the sensor position towards the joining process (sidely, in front of or behind the processing tool). Therefore, in this study an approach will be presented to evaluate the directional characteristic of the airborne sound emitted by the FSW process. First, the positioning of the microphone for the various welding directions were investigated. This was done to determine a suitable microphone orientation during the process. Then, the general determination of audio signals from the FSW process will be considered and compared to the process force feedback. Further, it was demonstrated that acoustic analysis can be used for detection of weld seam irregularities such as flash formation on 5 mm AA 5754 H111 sheets. All experiments were performed with a robotized FSW setup that was modified by a self-developed acoustic measuring device.
Effect of friction stir welding tool hardness on wear behaviour in friction stir welding of AA-6060 T66. - In: Proceedings of the Institution of Mechanical Engineers, ISSN 2041-3076, Bd. 236 (2022), 6, S. 1333-1345
Friction stir welding is a solid-state joining process with a wide range of industrial applications in the e-mobility, automotive, aerospace and energy industries. However, friction stir welding is subjected to process-specific challenges, including comparatively high process forces and friction stir welding tool wear resulting from tribological interaction between the tool and workpiece. The geometric-related friction stir welding tool wear can cause varying material flow conditions, lateral path deviations and premature tool failure, with detrimental economic and technological consequences. This study systematically analyses the wear behaviour of friction stir welding tools as a function of tool hardness. To compare and differentiate the geometric-related tool wear as a function of tool hardness, experiments were carried out with a hardness of 240 HV, 410 HV and 580 HV. Whereas 240 HV is non-hardened, 410 HV is 50% of the secondary hardness maximum and 580 HV is the secondary hardness maximum of the tools made of H13 tool steel (hot-working steel, X40CrMoV5-1). During the experimental tests, the shoulder and probe exhibited varying wear and geometrical deviations. The investigations were carried out with a force-controlled robotized welding setup in which AA-6060 T66 sheets with a thickness of 8 mm were joined by weld seams up to a total length of 80 m.
Influence of tool and welding parameters on the risk of wormhole defect in aluminum magnesium alloy welded by bobbin tool FSW. - In: Metals, ISSN 2075-4701, Bd. 12 (2022), 6, 969, S. 1-14
Bobbin tool friction stir welding (BTFSW) utilizes a special tool that possesses two shoulders interconnected by a pin instead of one: the top shoulder and the pin in the conventional FSW tool. This greatly simplifies the kinematics in the otherwise complicated setup of FSW since the bottom shoulder forms the bottom surface of the weld, without the need for a backing plate. Moreover, the tool enters the base metal sideways and travels, forming the joint in a straight line while rotating, without the need for downward and upward motion at the beginning and end of the process. This paper presents a study on the BTFSW tool geometry and parameters on the risk of wormhole defect formation in the AA5005 aluminum–magnesium alloy and the wormhole effect on mechanical properties. It was shown that higher stress imposed by the tool geometry on the joint has a significant influence on heating, an effect similar to the increased rotational speed. Optimal kinematic and geometrical tool properties are required to avoid wormhole defects. Although weld tensile strengths were lower (between ˜111 and 115 MPa) compared with a base metal (137 MPa), the ductile fracture was obtained. Furthermore, all welds had a higher impact strength (between ˜20.7 and 27.8 J) compared with the base material (˜18.5 J); it was found that the wormhole defect only marginally influences the mechanical properties of welds.
Automatic detection and prediction of discontinuities in laser beam butt welding utilizing deep learning. - In: Journal of advanced joining processes, ISSN 2666-3309, Bd. 6 (2022), 100119, S. 1-11
Laser beam butt welding of thin sheets of high-alloy steel can be really challenging due to the formation of joint gaps, affecting weld seam quality. Industrial approaches rely on massive clamping systems to limit joint gap formation. However, those systems have to be adapted for each individually component geometry, making them very cost-intensive and leading to a limited flexibility. In contrast, jigless welding can be a high flexible alternative to substitute conventionally used clamping systems. Based on the collaboration of different actuators, motions systems or robots, the approach allows an almost free workpiece positioning. As a result, jigless welding gives the possibility for influencing the formation of the joint gap by realizing an active position control. However, the realization of an active position control requires an early and reliable error prediction to counteract the formation of joint gaps during laser beam welding. This paper proposes different approaches to predict the formation of joint gaps and gap induced weld discontinuities in terms of lack of fusion based on optical and tactile sensor data. Our approach achieves 97.4 % accuracy for video-based weld discontinuity detection and a mean absolute error of 0.02 mm to predict the formation of joint gaps based on tactile length measurements by using inductive probes.