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.
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.
Temperature-based quality analysis in ultrasonic welding of copper sheets with microstructural joint evaluation and machine learning methods. - In: Welding in the world, ISSN 1878-6669, Bd. 67 (2023), 6, S. 1437-1448
Ultrasonic metal welding (USMW) is a highly attractive joining technology due to high energy efficiency and solid-state joint formation. Various joining solutions for conductor materials can be realized with USMW. Still, a big challenge for complex industrial applications is an adequate process monitoring that allows to cope with inevitable and complex process fluctuations. In this work, the suitability of contactless temperature measurements for process monitoring of copper sheet welding is examined and compared with the suitability of vibration measurements by means of machine learning methods. Different sensor signals acquired during welding on a metrological test rig are used for predicting the tensile shear strength of the joint. Results show that quality predictions based on temperatures exceed the state-of-the-art monitoring based on the welding energy. Yet, solely temperature-based predictions are exceeded by quality predictions based on either welding machine signals or tool vibration measurements. To further explore temperature-based quality analysis, joint microstructure analyses are carried out. These reveal concurring joint formation mechanisms associated with the mechanical and thermal process domains. To finally cover both domains in quality prediction, a sensor-fusion-based regression model is set up relying on vibration and temperature measurements. This fusion model exceeds all previously considered regression models with a mean absolute percentage error of 7.4% on the test data set. These results stress the importance of both process domains and suggest the combination of temperature and vibration measurements as a good starting point for future industrial monitoring of USMW processes.
Effect of surface oxide layers in solid-state welding of aluminium alloys - review. - In: Science and technology of welding and joining, ISSN 1362-1718, Bd. 28 (2023), 5, S. 1-21
This review sheds novel insights on the residual oxide behaviour of solid-state weld joints of aluminium alloys. Understanding the influence of oxides on the aluminium surface before and during welding, its impact on the weld structure and possible solutions for reducing its impact were addressed. The solid-state techniques most relevant to the transportation sector namely, diffusion bonding, friction stir spot welding and ultrasonic welding were surveyed, analysed and reviewed. During this analysis, the implication of the presence of oxides on aluminium substrate affecting the metallurgical characteristics of the weld joints was reviewed. Visible defects such as voids, delamination, kissing bond and hook defects, and problems associated with these defects were analysed and few suggestions are made to partially overcome these issues.
Effects of cryogenic cooling on machining of acrylonitrile-butadiene rubber. - In: Journal of manufacturing processes, Bd. 90 (2023), S. 429-442
The basic idea of cryogenic machining of elastomers is to lower the process temperature under glass transition temperature, causing the transformation of the viscoelastic properties of elastomers into brittle with better machinability outcomes. However, because of the heat generated by plastic deformation and chip formation in the primary shear zone and friction between the tool and the workspace, even with cryogenic cooling, the resulting temperature in the cutting area is often higher than the glass transition temperature. As a result, it can cause a partially rubbery state of the workpiece. In this paper, the effect of cryogenic cooling on the milling of acrylonitrile-butadiene rubber was analysed. Three different cooling setups, namely, indirect, direct and flow cooling, were proposed and their influence on temperature distribution in the cutting zone was studied in conjunction with different tool geometries and parameters (depth of cut, rotational speed, and feed rate). Direct cooling provides the best-resulting temperature distribution with the lowest surface roughness (1.27 to 1.47 μm) as it acts as a lubricant between the tool and workpiece and cools the tool and workpiece simultaneously.On the other hand, increasing the depth of cut and rotational speed also increases surface roughness. The best results show samples with grooves obtained with a rotational speed of 5000 rpm, depth of cut 0.25 mm and feed rates between 75 and 300 mm/min with surface roughness between 0.86 and 1.29 μm. Those samples show clean grooves with sharp edges, minimal surface roughness and geometric deviation, with defined ductile chip formation.
The SCALEX facility - an apparatus for scaled fluid dynamical experiments. - In: Technisches Messen, ISSN 2196-7113, Bd. 90 (2023), 5, S. 296-309
The working conditions of the Scaled Convective Airflow Laboratory Experiment (SCALEX) at Technische Universität Ilmenau and sample experiments are reported. The SCALEX facility is a pressure vessel which allows for downscaling of laboratory experiments up to a factor of 20 by compression of gaseous working fluids, air or sulfur hexafluoride, to change the material properties of the fluid. The requirements and conditions for downscaling of fluid dynamical problems are discussed in detail. Long-term high and low pressure tests are conducted to screen the stability of the experimental environment inside the vessel against pressure and temperature fluctuations. Finally, a Rayleigh-Bénard convection experiment at an aspect ratio 10 is performed inside the SCALEX facility as a proof of concept. The reference experiment was conducted under 4.5 bar pressure for Ra = 1.9 × 10^5. However, the Rayleigh number could be varied in a wide range of Ra = 10^4 … 10^8. The flow investigation was pursued with stereoscopic particle image velocimetry in horizontal mid-plane through the convection cell. To improve the image quality the cameras were placed inside the pressure cell and tested up to 6 bar. Thus the feasibility of optical flow measurements at elevated pressures is shown.
Influence of different surface conditions on mechanical properties during ultrasonic welding of aluminum wire strands and copper terminals. - In: Welding in the world, ISSN 1878-6669, Bd. 67 (2023), 6, S. 1427-1436
Ultrasonic metal welding (USMW) has become considerable attention in terms of its suitable applications compared to conventional fusion welding techniques. The main advantage of USMW results from the comparatively low process times and joining temperatures below the melting point. Thus, USMW is particularly used for the joining of dissimilar material combinations, e.g., aluminum and copper (Al/Cu), in battery cell production or wiring harness applications. However, process fluctuations in USMW of Al/Cu joints can occur due to varying surface conditions of the joining materials. Therefore, this study investigated different surface conditions of copper terminals and their effects on mechanical properties. At first, three different surface conditions were generated, respectively: surface cleaning (sulfuric acid and ethanol), structuring process by laser, and structuring process by milling. These modifications are compared with the terminals in the initial state (contaminated). The characterization of the terminal surfaces was carried out with 3-D laser scanning microscopy as well as light microscopy. The mechanical conditions were examined with shear tensile tests. The tensile tests showed a significant influence of the surface condition on the resulting failure loads compared to the initial state. The highest failure loads could be achieved with the structured terminals (+ 48%), whereas contaminated terminals and terminals with notches exhibited comparatively poor failure loads (- 28%). This can be explained by varying interface formations between the terminal and the wire, which was detected by metallography and SEM analysis. Furthermore, it was figured out that the interface between aluminum and copper exhibits a firm and formed closure bond and hence increased failure loads for laser-structured terminals. Additional investigations by SEM revealed no detectable occurrence of intermetallic phases.
Prozessmodell für das Hinterspritzen von Dekorfolien in der In-Mould-Labeling Technik. - Ilmenau : Universitätsverlag Ilmenau, 2023. - 1 Online-Ressource (XIV, 207 Seiten). - (Fertigungstechnik - aus den Grundlagen für die Anwendung ; Band 15)
Technische Universität Ilmenau, Dissertation 2022
Das Folienhinterspritzen ist ein spezielles Spritzgießverfahren zur Dekoration und Funktionalisierung von Kunststoffoberflächen. Dabei wird eine transparente Kunststofffolie bedruckt, verformt und hinterspritzt. Die Vorteile des Verfahrens liegen in der Oberflächenqualität und den zahlreichen Dekorationsmöglichkeiten. Nachteilig sind die hohen Investitionskosten, die sich meist nur durch hohe Stückzahlen und einer Vollauslastung der Maschine rechtfertigen lassen. Ansatzpunkte, die Wirtschaftlichkeit zu erhöhen, liegen in der Reduzierung der Zykluszeiten und der Ausschussraten. Typische Fehlerbilder beim Folienhinterspritzen sind die Auswaschung der aufgedruckten Dekore durch den Hinterspritzvorgang, Gestaltabweichungen wie Verzug und die Ablösung der Folie vom Träger. Der Arbeit liegt die These zugrunde, dass die Formteilqualität auf die Wechselwirkung zwischen thermischen und mechanischen Randbedingungen zurückgeführt werden kann. Basierend auf einer systematischen Verfahrensanalyse und einer analytischen Betrachtung der thermischen und mechanischen Einflussfaktoren, wird ein grundlegendes Prozessverständnis erarbeitet. Dies erfolgt unter Berücksichtigung von Prozess-, Material- und Geometriegrößen. Auswaschungen resultieren aus thermischen und mechanischen Folienbelastungen. Dabei können zwei wesentliche Entstehungsmechanismen abgeleitet werden, die von der Wandschubspannung und den Temperaturen abhängen. Es wird gezeigt, dass die Wirkungsweise von Schmelzetemperatur und Einspritzgeschwindigkeit von der Höhe der Wandschubspannung abhängt. Die Verbundfestigkeit wird anhand von bedruckten und unbedruckten Folien untersucht, wobei sich unterschiedliche Zusammenhänge zeigen. Während bei unbedruckten Folien die Verbundfestigkeit mit der thermischen Energie in der Grenzschicht steigt, liegt der Einfluss der Prozessparameter bei bedruckten Folien im Bereich der doppelten Standardabweichung. Ferner wird in der vorliegenden Arbeit erstmals die Orientierung betrachtet. Dabei zeigt sich ein vom Fließweg abhängiger Orientierungsverlauf sowie eine Abnahme der Orientierung mit den Temperaturen. Analog hierzu nimmt auch der Formteilverzug mit einem steigenden Temperaturniveau ab, da hohe Temperaturen zu einer Reduzierung der Schwindungsdifferenzen zwischen Folie und Träger führen. Abschließend werden die Erkenntnisse in einem qualitativen Prozessmodell zusammengefasst, das die Haupteinflussfaktoren auf die Formteilqualität beinhaltet und eine Prozessoptimierung zulässt.
Hybrid thermoplastic-metal joining based on Al/Ni multilayer foils - analysis of the joining zone. - In: Materials and design, ISSN 1873-4197, Bd. 226 (2023), 111561, insges. 16 S.
Multi material pairings like metal-plastic hybrid compounds are becoming increasingly important across all industrial sectors. However, the substitution of metals by plastics leads to a multitude of challenges based on the combination of dissimilar materials. The variations in the chemical and physical properties of the used materials require innovative joining processes. The application of reactive multilayers represents an advanced joining method for flexible and low-distortion joining of dissimilar joining partners by means of a short-term and localized application of thermal energy. In the context of this publication, the joining process between semi-crystalline polyamide 6 and austenitic stainless steel X5CrNi18-10(EN 1.4301 / AlSI304) based on reactive Al/Ni multilayers is investigated. In addition to evaluation of resulting joint strength, the focus of the work is in particular the characterization of the resulting failure behavior at the fracture interface under tensile load and the deriving binding mechanisms in the joint. From the results obtained, it is estimated that a direct bond can be generated between plastic and metal despite the presence of a residual reacted foil in the joining area. The structures present in the metal surface have a particularly positive influence on crack initiation and the resulting increased bond strength.