Publications and presentations

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.

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.

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.

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.

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.