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Petronijevic Sarcev, Branislava; Balos, Sebastian; Markovic, Dubravka; Sarcev, Ivan; Vukcevic, Marija; Labus Zlatanovic, Danka; Miletic, Vesna;
Effect of the degree of conversion on mechanical properties and monomer elution from self-, dual- and light-cured core composites. - In: Materials, ISSN 1996-1944, Bd. 14 (2021), 19, 5642, insges. 14 S.

The objective of this work was to measure and correlate the degree of conversion (DC), mechanical properties and monomer elution from self-, dual- and light-cured core composites. Five samples of each of the following materials were prepared for each test: Clearfil (Core, Photo Core, Automix), Bisco (Core-Flo, Light-Core and Bis-Core). DC was determined using FTIR, compressive and flexural strength and modulus of elasticity using a universal testing machine and microhardness using Vickers hardness. Elution was measured using HPLC. One-way ANOVA with Tukeys post-test and Pearson's correlation were used to statistically analyze the data. DC of Clearfil-Dual (70.1%) and Clerafil-Photo (66.8%) were higher than Clearfil-Self (55.4%) and all Bisco materials (51.4-55.3%). Flexural strength of Clearfilwas higher than that of Bisco composites. The Microhardness of Clearfil-Dual (119.8VHN) and Clearfil-Photo (118.0VHN) were higher compared to other materials. The greatest elution was detected from self-cured materials. DC positively correlated to microhardness and compressive/flexural strength and negatively to BisGMA elution. Clearfil-Photo and Automix showed higher conversion, lower monomer elution and, generally, better mechanical properties. Self-cured composites should not be recommended for routine clinical use as their performance was inferior to dual- and light-cured composites. Microhardness may be used as an indicator of elution.
Bergmann, Jean Pierre; Köhler, Tobias; Pöthig, Pascal;
Ultrasonic welding. - In: Advanced joining processes

This chapter gives an overview of ultrasonic welding, especially ultrasonic metal welding. The principles of ultrasonics are briefly described following by the assembly of an ultrasonic metal device with functions of the individual components as well as a survey about typical joint configurations and materials. Further, current challenges and applications in ultrasonic welding are summarized. Namely hybrid joints, the broad field of aluminum to copper welding and simulation and modeling approaches are depicted. Concluding, a short summery is given as well as perspectives of how ultrasonic welding could develop in the future.

Alam, Shahidul; Nádaždy, Vojtech; Váry, Tomáš; Friebe, Christian; Meitzner, Rico; Ahner, Johannes; Anand, Aman; Karuthedath, Safakath; Castro, Catherine S. P. De; Göhler, Clemens; Dietz, Stefanie; Cann, Jonathan; Kästner, Christian; Konkin, Alexander; Beenken, Wichard J. D.; Anton, Arthur Markus; Ulbricht, Christoph; Sperlich, Andreas; Hager, Martin; Ritter, Uwe; Kremer, Friedrich; Brüggemann, Oliver; Schubert, Ulrich Sigmar; Ayuk Mbi Egbe, Daniel; Welch, Gregory C.; Dyakonov, Vladimir; Deibel, Carsten; Laquai, Frédéric; Hoppe, Harald;
Uphill and downhill charge generation from charge transfer to charge separated states in organic solar cells. - In: Journal of materials chemistry, ISSN 2050-7534, Bd. 9 (2021), 40, S. 14463-14489

It is common knowledge that molecular energy level offsets of a type II heterojunction formed at the donor-acceptor interface are considered to be the driving force for photoinduced charge transfer in organic solar cells. Usually, these offsets - present between molecular energy levels of the donor and acceptor - are obtained via cyclic voltammetry (CV) measurements of organic semiconductors cast in a film or dissolved in solution. Simply transferring such determined energy levels from solution or film of single materials to blend films may be obviously limited and not be possible in full generality. Herein, we report various cases of material combinations in which novel non-fullerene acceptors did not yield successful charge transfer, although energy levels obtained by CV on constituting single materials indicate a type II heterojunction. Whilst the integer charge transfer (ICT) model provides one explanation for a relative rise of molecular energy levels of acceptors, further details and other cases have not been studied so far in great detail. By applying energy-resolved electrochemical impedance spectroscopy (ER-EIS) on several donor-acceptor combinations, a Fano-like resonance feature associated with a distinctive molecular energy level of the acceptor as well as various relative molecular energy level shifts of different kinds could be observed. By analyzing ER-EIS and absorption spectra, not only the exciton binding energy within single materials could be determined, but also the commonly unknown binding energy of the CT state with regard to the joint density of states (jDOS) of the effective semiconductor. The latter is defined by transitions between the highest occupied molecular orbitals (HOMO) of the donor and the lowest unoccupied molecular orbitals (LUMO) of the acceptor. Using this technique among others, we identified cases in which charge generation may occur either via uphill or by downhill processes between the charge transfer exciton and the electronic gap of the effective semiconductor. Exceptionally high CT-exciton binding energies and thus low charge generation yields were obtained for a case in which the donor and acceptor yielded a too intimate blend morphology, indicating π-π stacking as a potential cause for unfavorable molecular energy level alignment.
Manzke, S.; Trautmann, Marcus; Spaniol, Erik; Füssel, Uwe; Gierth, Maximilian; Reimann, Jan; Bergmann, Jean Pierre;
Numerische Berechnung und Optimierung einer WAAM-Gaskühlung. - In: DVS Congress 2021, (2021), S. 308-314

Grätzel, Michael; Hasieber, Michael; Bergmann, Jean Pierre; Weigl, Markus;
Einfluss der Eindringtiefe auf Prozess- und Verbindungseigenschaften von Rührreibschweißverbindungen in der Elektromobilität. - In: DVS Congress 2021, (2021), S. 217-224

Wagner, R.; Siewert, Erwan; Schein, Jochen; Hussary, Nakhleh; Eichler, Stefan; Fehrenbach, L.; Pfreuntner, M.;
Einfluss von Schutzgas auf die Emissionen beim Lichtbogenschweißen. - In: DVS Congress 2021, (2021), S. 141-148

Grätzel, Michael; Sieber, Felix; Schick-Witte, Konstantin; Bergmann, Jean Pierre;
Advances in friction stir welding by separate control of shoulder and probe. - In: Welding in the world, ISSN 1878-6669, Bd. 65 (2021), 10, S. 1931-1941

Friction stir welding (FSW) has developed into a reliable and increasing used industrial joining technology. Various tool configurations can be used for FSW, each of which has advantages and challenges. State-of-the-art FSW employs various tool configurations, including the conventional, the stationary shoulder, and the dual-rotational configuration which is characterized by separate control of shoulder and probe. In this study, an innovative method to combine various tool configurations was developed by a novel FSW spindle stack construction. With an additional servomotor, existing FSW systems can be extended by separate control of shoulder and probe so that varying rotational speeds and rotational directions can be set. This allows enhanced possibilities (a) to adjust frictional heat generation and (b) to apply several tool configurations. The main advantages of this enhanced type of FSW are demonstrated in three ways: increased weld penetration depth, reduction of undesirable machine vibrations, and the combination of varying tool configurations such as stationary shoulder and conventional FSW. The investigations were carried out with 2-mm EN AA 5754 H22 sheets and performed on a robotized FSW setup.
Günther, Karsten; Bergmann, Jean Pierre;
Experimental approach to determine the impact of the droplet transfer mode on the degradation of fused tungsten carbides during GMAW. - In: International journal of refractory metals & hard materials, ISSN 0263-4368, Bd. 101 (2021), 105692

The application of fused tungsten carbides (FTCs) in nickel-based alloys is important for improving the wear resistance of tooling equipment in the mining industry. However, FTCs are thermally unstable and will dilute under excessive energy input during welding. The parameters affecting dilution in this context are diverse and not yet completely understood. To date, the existing scientific literature focuses on the impact of the melt bead characteristics to explain the degradation during gas metal arc welding (GMAW). The degradation-promoting influence of the droplet transfer mode has not yet been considered. A methodology was developed to experimentally quantify the dependence of the degradation kinetics of FTCs on the droplet transfer mode. The established experimental model demonstrated that the globular transfer mode leads to increased degradation of FTCs in comparison to that of the short-arc mode, which can be attributed to the higher process power and hence higher droplet temperature. In this context, the quantifiable impact of the droplet transfer mode was determined.
Reimann, Jan; Hammer, Stefan; Henckell, Philipp; Rohe, Maximilian; Ali, Yarop; Rauch, Alexander; Hildebrand, Jörg; Bergmann, Jean Pierre;
Directed energy deposition-arc (DED-Arc) and numerical welding simulation as a hybrid data source for future machine learning applications. - In: Applied Sciences, ISSN 2076-3417, Bd. 11 (2021), 15, 7075, insges. 16 S.

This research presents a hybrid approach to generate sample data for future machine learning applications for the prediction of mechanical properties in directed energy deposition-arc (DED-Arc) using the GMAW process. DED-Arc is an additive manufacturing process which offers a cost-effective way to generate 3D metal parts, due to its high deposition rate of up to 8 kg/h. The mechanical properties additively manufactured wall structures made of the filler material G4Si1 (ER70 S-6) are shown in dependency of the t8/5 cooling time. The numerical simulation is used to link the process parameters and geometrical features to a specific t8/5 cooling time. With an input of average welding power, welding speed and geometrical features such as wall thickness, layer height and heat source size a specific temperature field can be calculated for each iteration in the simulated welding process. This novel approach allows to generate large, artificial data sets as training data for machine learning methods by combining experimental results to generate a regression equation based on the experimentally measured t8/5 cooling time. Therefore, using the regression equations in combination with numerically calculated t8/5 cooling times an accurate prediction of the mechanical properties was possible in this research with an error of only 2.6%. Thus, a small set of experimentally generated data set allows to achieve regression equations which enable a precise prediction of mechanical properties. Moreover, the validated numerical welding simulation model was suitable to achieve an accurate calculation of the t8/5 cooling time, with an error of only 0.3%.
Reimann, Jan; Henckell, Philipp; Ali, Yarop; Hammer, Stefan; Rauch, Alexander; Hildebrand, Jörg; Bergmann, Jean Pierre;
Production of topology-optimised structural nodes using arc-based, additive manufacturing with GMAW welding process. - In: Journal of civil engineering and construction, ISSN 2051-7777, Bd. 10 (2021), 2, S. 101-107

The desire to generate a stress optimised structural node with maximum stability is often coupled with the goal of low manufacturing costs and an adapted and minimal use of material. The complex, three-dimensional free-form structures, which are created by means of topology-optimisation, are only partially suitable for conventional manufacturing. The wire arc additive manufacturing (WAAM), by means of arc welding processes, offer a cost-effective and flexible possibility for the individual production of complex, metallic components. Gas metal arc welding (GMAW) is particularly suitable to produce large-volume, load-bearing structures due to build-up rates of up to 5 kg/h. The generation of strength and stiffness adapted support structures by means of the numerical simulation method of topology-optimisation was investigated in this study to generate topology-optimised structural nodes. The resulting node is transferred into a robot path using CAD/CAM software and manufactured from the filler material G4Si1 using WAAM with the GMAW process. Based on the boundary conditions of the WAAM process, the path planning and thus the manufacturability of the topology-optimised supporting structure nodes is evaluated and verified using a sample structure made of the welding filler material G4Si1. Depending on the path planning, an improvement of the mechanical properties could be achieved, due to changes in t8/5 times.