Deformation mechanisms of hard to machine metal alloys at the microscale

NOWAG, Kai Taal: Engels


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The machinability of a material is an important property, with strong influence on the production costs of the machined parts. Besides the material properties, the deformation mechanisms are influenced by the process parameters like cutting speed, penetration depth and the tool geometry in macroscopic machining. In micromachining, additionally the crystallographic features of the sample have to be taken into account. If the scale of the process is in the range of the grain size of the workpiece or smaller, the crystal orientation as well as size effects may have a fundamental influence on the deformation mechanisms, as the plastic deformation generally takes place by dislocation slip in confined planes. Therefore, this manuscript deals with the question, if it is possible to utilize anisotropy effects of the workpiece material and concomitant inhomogeneous deformation mechanisms to improve the efficiency of micromachining processes, especially in single crystals and textured materials. Two technical alloys, the body centered cubic titanium alloy "Ti 15-3" and the face centered cubic nickel based superalloy "Alloy 625" were investigated. As machining processes lead to highly complex stress fields in the sample, rendering a detailed analysis difficult, the mechanical properties and deformation mechanisms of the aforementioned alloys were first investigated by nanoindentation and compression of FIB machined micropillars. Thus, the fundamental mechanisms concerning the deformation behavior were determined, and applied to explain the results of microscratching experiments, which were considered as representatives of a micromachining process. The mechanical behavior of the investigated alloys was determined with respect to the crystal orientation, by performing compression experiments in crystals with different orientations. Additionally, the indentation moduli and hardness of the materials were studied with respect to the applied load to identify possible size effects. In indentation, the deformation mechanisms were determined by analyzing the asymmetry of material pile-up around the imprint, as well as the orientation of the slip lines at the material surface, determined by EBSD. Moreover, the deformation mechanisms were also determined by pillar compression, allowing the calculation the critical resolved shear stress, leading to the activation of dislocation slip in the active slip system. It was therefore possible to investigate, if the peculiar deformation response of pure body centered cubic materials was also observed for highly alloyed systems like Ti 15-3. In particular, the peculiarities lie in the violation of Schmid law by the dislocation slip in a system with low resolved shear stress, as well as the twinning-antitwinning asymmetry, which describes the dependency of the critical resolved shear stress on the sense of slip. With the knowledge about the deformation mechanisms, the microscratching experiments were conducted in both materials. In scratching, the deformation can take place by two basic mechanisms, chip formation and plowing. The effect of the process parameters on the deformation mechanisms was studied by the variation of tool- and sample geometry, as well as the scratching speed. Moreover, the main focus was put on the analysis of the influence of the crystallographic features, by systematically changing the crystal orientation and scratching direction. The deformation mechanisms were found to be strongly dependent on the scratching direction, and the surface finish could be significantly improved by choosing the direction of the process in a way that chip formation was the favored mechanism. When the deformation took place exclusively by chip formation, the deformed material was completely removed from the sample, while it stayed in contact with the workpiece in the case of plowing, leading to a significant increase of the surface roughness and the formation of a nondesirable burr. Hence, the manipulation of the deformation mechanisms is possible, when the crystallography of the sample and the possible slip systems are known. This methodology can be used to improve the process efficiency, especially for single crystalline and textured materials.
Van Ditmar Boekenimport B.V.
211 pp.
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