Local shielding gas supply in remote laser beam welding. - In: Journal of manufacturing and materials processing, ISSN 2504-4494, Bd. 5 (2021), 4, 139, insges. 19 S.
The use of shielding gases in laser beam welding is of particular interest for materials interacting with ambient oxygen, e.g., copper, titanium or high-alloy steels. These materials are often processed by remote laser beam welding where short welds (e.g., up to 40 mm seam length) are commonly used. Such setups prevent gas nozzles from being carried along on the optics due to the scanner application and a small area needs to be served locally with inert gas. The article provides systematic investigations into the interaction of laser beam processes and parameters of inert gas supply based on a modular flat jet nozzle. Based on the characterization of the developed nozzle by means of high-speed Schlieren imaging and constant temperature anemometry, investigations with heat conduction welding and deep penetration welding were performed. Bead-on-plate welds were carried out on stainless steel AISI 304 for this purpose using a disc laser and a remote welding system. Argon was used as shielding gas. The interaction between Reynolds number, geometrical parameters and welding/flow direction was considered. The findings were proved by transferring the results to a complex weld seam geometry (C-shape).
https://doi.org/10.3390/jmmp5040139
Detecting process anomalies in the GMAW process by acoustic sensing with a convolutional neural network (CNN) for classification. - In: Journal of manufacturing and materials processing, ISSN 2504-4494, Bd. 5 (2021), 4, 135, insges. 14 S.
Today, the quality of welded seams is often examined off-line with either destructive or non-destructive testing. These test procedures are time-consuming and therefore costly. This is especially true if the welds are not welded accurately due to process anomalies. In manual welding, experienced welders are able to detect process anomalies by listening to the sound of the welding process. In this paper, an approach to transfer the “hearing” of an experienced welder into an automated testing process is presented. An acoustic measuring device for recording audible sound is installed for this purpose on a fully automated welding fixture. The processing of the sound information by means of machine learning methods enables in-line process control. Existing research results until now show that the arc is the main sound source. However, both the outflow of the shielding gas and the wire feed emit sound information. Other investigations describe welding irregularities by evaluating and assessing existing sound recordings. Descriptive analysis was performed to find a connection between certain sound patterns and welding irregularities. Recent contributions have used machine learning to identify the degree of welding penetration. The basic assumption of the presented investigations is that process anomalies are the cause of welding irregularities. The focus was on detecting deviating shielding gas flow rates based on audio recordings, processed by a convolutional neural network (CNN). After adjusting the hyperparameters of the CNN it was capable of distinguishing between different flow rates of shielding gas.
https://doi.org/10.3390/jmmp5040135
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 Tukey’s 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.
https://doi.org/10.3390/ma14195642
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.
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.
https://doi.org/10.1039/D1TC02351A
Numerische Berechnung und Optimierung einer WAAM-Gaskühlung. - In: DVS Congress 2021, (2021), S. 308-314
Einfluss der Eindringtiefe auf Prozess- und Verbindungseigenschaften von Rührreibschweißverbindungen in der Elektromobilität. - In: DVS Congress 2021, (2021), S. 217-224
Einfluss von Schutzgas auf die Emissionen beim Lichtbogenschweißen. - In: DVS Congress 2021, (2021), S. 141-148
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.
https://doi.org/10.1007/s40194-021-01136-w
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.
https://doi.org/10.1016/j.ijrmhm.2021.105692