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PhD Defense: Microstructural analysis of atomic mechanisms of metal plasticity under machining conditions: case study of AISI 1045 steel and 7475 aluminum

Wednesday, July 18, 2018 - 11:00
Mondragon University
Bentejui Medina, Electron Microscopy Group
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The goal of this PhD thesis is discovering the potential of nanostructure characterization for revealing atomic mechanisms of metal plasticity under machining conditions. This approach has been used to reconstruct the phenomena in the tool-chip contact by a composite study of chips utilizing advanced microscopy techniques in combination with a fundamental description of plasticity. There are collected multiple evidences for existence of two qualitatively different cutting regimes in orthogonal machining of annealed AISI-1045 steel with uncoated P15 carbide cutting tool in dry conditions at cutting speed between 5 and 200 m/min. These regimes are characterized by two different phenomena controlling the tool-chip contact, i.e. severe plastic hardening at low velocities, and dynamic recrystallization at high speed. This last phenomenon has induced a structure in the tool-chip contact area with unique properties. Morphological, chemical and mechanical studies have revealed a nanostructured material with shifted properties in comparison with the original material. 
The analysis of previously cut chips provides an average picture of the mechanisms that govern tool-chip contact. However, some aspects like grain orientation and microstructural features may have differentiated roles. Hence, the analysis of the cut over individual features could feed a deeper understanding of metal cutting. In order to address this, the present thesis studies the potential of direct observation of the machining process in high-magnification microscopes. For that, a device to perform lineal cutting of aluminum inside an electron microscope’s vacuum chamber has been designed and constructed. Based on this device, experiments of machining in-situ have been successfully performed, shown a coherent chip generation. First results have shown that the crystallographic orientation may increase the layer of material deformed under the cutting tool. Furthermore, experiments of cutting in the sub-micrometer regime has demonstrated that this layer is proportionally larger in smaller cuts, consequence of a deeper tool-chip interaction in comparison with macroscopic cutting. 
At deeper scales, a method for simulations down to the atomic level has been proposed to model machining. This initiative relies in the capability of atomistic simulations to reproduce the mechanisms of plasticity that govern the deformation of crystalline materials, like dislocations and grain boundary effects, which cannot be reproduced by other techniques based on continuum mechanics. In this study a set of molecular dynamics simulations in different conditions of tool-chip friction and feed size have been carried out. The results have shown the effects of localized recrystallization observed experimentally, largely dependent on the friction with the tool. Moreover, it has been observed that a large friction value reduces the mobility of the atoms up to certain distance to the tool. This produces a gradient of velocities in the proximity of the cutting edge, thus supporting the appearance of specific deformations mechanisms in the area of contact.
In summary, the present work describes the mechanics of machining based on different atomic mechanisms of plasticity, what has been afforded by experimental and simulation approaches. In addition, this thesis provides new methodologies for the research of the cutting process, with potential application to study other conditions and materials of interest.

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