Veröffentlichungen und Vorträge

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.

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.

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.

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.

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.