Andreas Nestler

Ultrasonic vibration superimposed turning represents a highly efficient method for surface microstructuring, which enables a combination with finish machining. However, there are almost no industrial applications of this process due to the special kinematics. Furthermore, the effects of the varying cutting conditions combined with the tool geometry on the resulting surfaces and process stability are not yet fully understood. In experimental investigations, specimens consisting of bronze (CuSn7Pb15-C) are machined by ultrasonic vibration superimposed turning. The influence of the geometry of the MCD-tipped indexable inserts on the surface microstructure is analyzed. Indexable inserts with different rake angles (0°, −10°, and −20°) and artificially generated flank wear lands (widths 50 µm and 100 µm) are used. Moreover, the influences of the cutting speed (120 m/min, 480 m/min) and the feed (0.05 mm, 0.1 mm) are analyzed. While machining, the strain of the sonotrode is detected by an integrated fiber Bragg grating. Subsequent to machining, geometrical surface properties are determined by SEM and 3D surface analysis using focus variation. Furthermore, kinematic simulations are realized, enabling the comparison with the generated surfaces. Generally, there is a high concordance between the simulated and the generated surfaces. However, in particular when the tool flank face gets in contact with the specimen, deviations are visible, especially the formation of burr. Summarized, the research improves the understanding of the mechanisms in ultrasonic vibration superimposed turning and the formation of the surface microstructures.

Aluminium matrix composites (AMCs) represent an important group of high-performance materials. Due to their specific strength and a high thermal conductivity, these composites have been considered for the large-scale production of brake discs. However, preconditioning the friction surfaces is necessary to avoid severe wear of both the brake discs and the brake linings. This can be achieved through controlled friction against commercially available brake-lining materials and the formation of transfer or reactive layers (tribosurfaces). Homogeneous tribosurfaces allow for nearly wear-free brake systems under moderate brake conditions. In this work, preconditioning was carried out with a pin-on-disc tester, aiming for the fast creation of homogeneously formed and stable tribosurfaces. The influence of surface microedges perpendicular to the direction of friction on the machined AMC surfaces on the build-up speed and homogeneity of the tribosurfaces was investigated. The microedges were generated using ultrasonic-vibration-superimposed face turning. Thereby, the vibration direction corresponded to the direction of the passive force. For research purposes, the distance of the microedges was changed by varying the cutting speed and feed. The experiments were carried out using AMC disc specimens with a reinforcement content of a 35% volume proportion of silicon carbide particles. Machining was realised with CVD-diamond-tipped indexable inserts. The evaluation of the generated surfaces before and after preconditioning was achieved using 3D laser scanning microscopy and scanning electron microscopy. It was demonstrated that ultrasonic-vibration-superimposed face turning effectively generated microedges on the AMC surfaces. The results show that larger distances between the microedges enhanced the formation of stable tribosurfaces. Thus, the tribosystem’s steady state was reached quickly. Therefore, the benefits of AMC-friction-surface microstructuring on the generation of tribosurfaces under laboratory conditions were proven. These findings contribute to the development of high-performance AMC brake systems.

Surface properties significantly influence the performance of machined parts. However, they cannot be measured directly during machining. For surface conditioning based on a soft sensor, it is necessary to monitor process characteristics like temperatures and forces, which influence the surface state. Soft-sensor development in machining requires a robust methodology, which is adaptable to different materials and machining processes. In addition, a measurement system that combines hardware sensors to measure temperatures and process forces has to be implemented into the machine tool. In the present study, a suitable methodology is proposed and tested using a tool-workpiece thermocouple and a dynamometer to determine the thermomechanical workpiece load during turning of the aluminum alloys EN AW-2017 and EN AW-7075. Experimental investigations are performed according to a D-optimal statistical design of experiments. For this, the machining parameters cutting speed, feed, depth of cut, as well as the flank wear land width are varied on four levels. Subsequent measurements of residual stresses and the surface roughness are used to correlate the surface state with input parameters and their resulting thermomechanical workpiece load by multiple regression based on analysis of variance (ANOVA). It is found that the methodology is applicable and allows for the prediction of surface states. The developed soft sensors enable an in-process control of machining parameters, which enables a robust prediction and targeted conditioning of the addressed surface properties during machining.

AlSi7Mg/SiC_p aluminium matrix composites (AMCs) with a high ceramic content (35 vol.%) that were produced by using the field-assisted sintering technique (FAST) were subjected to tribological preconditioning and evaluated as a potential lightweight material to substitute grey cast iron brake discs. However, since an uncontrolled running-in process of the AMC surface can lead to severe wear and thus to failure of the friction system, AMC surfaces cannot be used directly after finishing and have to be preconditioned. A defined generation of a tribologically conditioned surface (tribosurface) is necessary, as was the aim in this study. To simulate tribological conditions in automotive brake systems, the prepared AMC samples were tested in a pin-on-disc configuration against conventional brake lining material under dry sliding conditions. The influence of the surface topography generated by face turning using different indexable inserts and feeds or an additional plasma electrolytic treatment was investigated at varied test pressures and sliding distances. The results showed that the coefficient of friction remained nearly constant when the set pressure was reached, whereas the initial topography of the samples studied by SEM varied substantially. A novel approach based on analysing the material ratio determined by 3D surface measurement was developed in order to obtain quantitative findings for industrial application.

Das Einsatzverhalten von Bauteilen wird von deren Oberflächenzustand bestimmt. In diesem Zusammenhang haben bei der spanenden Endbearbeitung die Werkzeuggeometrie und der daraus resultierende Spanfluss beziehungsweise -bruch eine große Bedeutung. Im Rahmen der experimentellen Untersuchungen werden die Auswirkungen der Spanleitstufengeometrie auf Temperaturen und Kräfte beim Drehen der Aluminiumlegierung EN AW-2017 sowie die resultierenden Eigenspannungen und die Oberflächenrauheiten vertieft betrachtet. The performance of parts is significantly influenced by their surface condition. In this context, the tool geometry and the resulting chip flow or chip breakage are of great importance in finish machining. The experimental investigations focus on the impact of the chip breaker geometry considering temperatures and forces during turning of the aluminum alloy EN AW-2017 as well as the resulting residual stresses and surface roughness.

More and more forming and punching tools as well as wear parts are made from cemented carbide. The large variety and the geometry of these components are often reasons for finish machining by milling, but the tool wear is very high. For the investigations, end mills with different diamond materia...