Grundlagenuntersuchungen und simulationsbasierte Optimierung zur fünfachsigen Mikrofräsbearbeitung von NiTi-Formgedächtnislegierungen
(Fundamental Analysis and Simulation-based Optimization for the Five-Axis Micromilling of NiTi Shape Memory Alloys)
Kahleyß, F.
The potential of functional materials like NiTi shape memory alloys is of great interest in various fields of scientific research. This material offers many different application possibilities, especially in medical technologies, due to its versatile properties like biocompatibility, superelasticity, shape memory and damping. Applications like actuators, joining components and, especially, medical implants that rely on these properties, call for reliable manufacturing technologies to create and machine various freeform surfaces. It is a fact that the machining of NiTi is very demanding in terms of tool wear and workpiece quality. This is due to the high ductility of the material and the strong work hardening that results after the deformation processes. Therefore, the five-axis micromachining of NiTi parts has to be specifically designed in order to meet these challenges. This production process has not been studied so far.
This doctoral thesis presents a fundamental analysis of five-axis micromilling processes of NiTi shape memory alloys using ball nose cutters. Different aspects like the influence of the cutting parameters, the cutting materials and geometries and different NiTi alloys as workpiece material on the tool wear and the workpiece quality are investigated for tools with diameters as small as 1.0 mm and 0.4 mm. Though, the main emphasis in the first part of the thesis is laid on the investigation of the influence of different tool inclination angles. The second part focuses on the development of a simulation-based optimization for NC-programs that follows the findings of the fundamental experiments concerning the tool inclination angles. The developed new algorithm is not only capable of detecting unfavorable tool inclinations in the machining of freeform surfaces, rather also can readjust them. The verification of the algorithm is accomplished by machining a reference workpiece.
The results of the fundamental experiments show that a reliable machining process of NiTi with a very good workpiece quality is possible. However, the allowable window of the cutting parameters is narrow. The setup of the tool inclination angles affects the tool wear and the machining quality, too. It is recommended to apply a down-milling strategy with a high tool inclination angle in the feed direction to achieve a good chip formation and a low tool wear. Thus, a volume removal of up to Vz = 55 mm³ is possible. If the tool diameter is reduced, it has to be considered that the maximum chip thickness should not be lower than the cutting edge rounding of the tool tip. The strong work hardening effects would cause an increase of the specific cutting force otherwise. The transfer of the experimental results to a simulation model to readjust unfavorable tool inclinations was successful. The verification of the developed algorithm shows that the transformation of a three-axis NC-program into a five-axis NC-program, which avoids unfavorable inclination angles, improves the process results. The surface finish as well as the shape accuracy of the workpiece is considerably improved.
Published as
Dissertation, Technische Universität Dortmund, Vulkan Verlag, Essen, 2010, ISBN 978-3-8027-8753-9