Volume 87, Issue 12 - Surface Conditioning in Machining Processes (Part 2). Oberflächenkonditionierung in der Zerspanung (Teil 2) / Gastherausgeber: Volker Schulze

During turning of quenched and tempered AISI4140 surface layer states can be generated, which degrade the lifetime of manufactured parts. Such states may be brittle rehardened layers or tensile residual stresses. A soft sensor concept is presented in this work, in order to identify relevant surface modifications during machining. A crucial part of this concept is the measurement of magnetic characteristics by means of the 3MA-testing (Micromagnetic Multiparameter Microstructure and Stress Analysis). Those measurements correlate with the microstructure of the material, only take a few seconds and can be processed on the machine. This enables a continuous workpiece quality control during machining. However specific problems come with the distant measurement of thin surface layers, which are analyzed here. Furthermore the scope of this work is the in-process-measurement of the tool wear, which is an important input parameter of the thermomechanical surface load. The availability of the current tool wear is to be used for the adaption of the process parameters in order to avoid detrimental surface states. This enables new approaches for a workpiece focused process control, which is of high importance considering the goals of Industry 4.0.

The surface quality and the subsurface properties such as hardness, residual stresses and grain size of a drill hole are dependent on the cutting parameters of the single lip deep hole drilling process and therefore on the thermomechanical as-is state in the cutting zone and in the contact zone between the guide pads and the drill hole surface. In this contribution, the main objectives are the in-process measurement of the thermal as-is state in the subsurface of a drilling hole by means of thermocouples as well as the feed force and drilling torque evaluation. FE simulation results to verify the investigations and to predict the thermomechanical conditions in the cutting zone are presented as well. The work is part of an interdisciplinary research project in the framework of the priority program “Surface Conditioning in Machining Processes” (SPP 2086) of the German Research Foundation (DFG). This contribution provides an overview of the effects of cutting parameters, cooling lubrication and including wear on the thermal conditions in the subsurface and mechanical loads during this machining process. At first, a test set up for the in-process temperature measurement will be presented with the execution as well as the analysis of the resulting temperature, feed force and drilling torque during drilling a 42CrMo4 steel. Furthermore, the results of process simulations and the validation of this applied FE approach with measured quantities are presented.

The development of thin-film sensors for temperature and wear measurement in machining operations is presented in this work. A functional thin-film system, consisting of an Al 2 O 3 insulation layer, a chromium sensor layer structured by photolithography and an Al 2 O 3 wear-protection and insulation layer, is deposited by physical vapor deposition (PVD) processes onto the surface of cemented carbide cutting inserts. First specimen of the sensors are successfully fabricated and tested in laboratory experiments as well as in machining operations to demonstrate their functionality. These tool-integrated sensors can be used as an in-process monitoring device to determine the temperatures on the rake face at or close to the tool-chip contact area and to measure the progress of the flank-wear land width. The knowledge of these important process parameters opens up the possibility to develop new in-process control mechanisms in order to modify and improve the surface integrity of manufactured components. Thereby, their performance and lifetime can be enhanced.

The surface integrity strongly affects the performance properties of parts. Therefore, it is of great importance to be able to measure and adjust the surface-layer properties during the manufacturing process. In particular, cutting operations are characterized by high mechanical loads and temperature gradients in the area of chip formation. To enable a targeted control of the surface-layer properties, a fundamental comprehension of the interrelationships between the thermomechanical impact and the thereby induced material modification is required. Hence, the subject of this study is to measure the thermomechanical changes during turning of the aluminium alloy EN AW-2017 and find correlations thereof to the surface integrity. In order to achieve a large variation of the thermal and mechanical loads, the feed f (0.04 mm to 0.2 mm) and the depth of cut ap{a_{\mathrm{p}}} (0.4 mm to 2 mm) are varied over a wide range. The cutting speed vc{v_{\mathrm{c}}} is kept constant (300 m/min). For the in-process measurement of the temperatures and contact conditions at the interface of the tool and the specimen, a tool-workpiece thermocouple is used. Additionally, the components of the resultant force are measured by a dynamometer. The characterization of the surface layer is performed by the measurement of the residual stresses using X-ray diffraction and supplemented by the determination of the geometrical properties of the machined surface using a stylus measurement instrument. The results show an increase in temperature and the components of the resultant force with the enlargement of the cross-section of the undeformed chip. Due to the temperature gradient, tensile residual stresses are introduced in the tangential direction of the surface layer. Compressive residual stresses occur only in the axial direction and can be correlated with the in-process measurement data by introducing the C -value. Consequently, the calculation of the presence of compressive residual stresses allows for a targeted control of the surface-layer properties during machining.

Grinding processes are often the last step in the value-added chain of high-performance hardened steel components. However, thermo-mechanical loads which can take place during the process can have a detrimental effect on the surface integrity of ground parts, which are generally tested by post-process measurements. In the present study, two different approaches for an in-process inspection of the workpiece surface integrity were assessed using magnetic Barkhausen noise analysis during cylindrical grinding of hardened workpieces. The results showed that both measuring systems are able to detect changes in the surface state of workpieces in-process or directly after grinding in the grinding machine. After preparations to protect the sensors from influences during the grinding process, changes in the residual stress state and a decrease of hardness could be reliably detected. Due to constant contact conditions between sensor and workpiece a high reproducibility of the measurements was achieved.