Severe plastic deformation and nanostructured materials by large strain extrusion machining
Abstract
Large Strain Extrusion Machining (LSEM), a constrained chip formation process, is proposed and demonstrated as a method of Severe Plastic Deformation (SPD) for metals and alloys. It is shown that SPD parameters such as deformation strain, macro-texture and strain rate can be controlled by varying the tool rake angle and chip compression (thickness) ratio, while imposing large strains in the chip in a single step of deformation. This level of parameter control is not achievable in any of the conventional SPD processes. By controlling the geometry of the chip (extrudate) a priori using a constraining tool, production of bulk material in foil, sheet and bar forms, and with nanostructured and ultrafine grained (UFG) microstructures, is demonstrated using commercially pure copper and aluminum 6061-T6 as model systems. The effects of secondary deformation arising from friction at the tool-chip contact and of the incipient (transient) stage of chip formation on the foil/sheet microstructure are characterized. A characterization of the deformation field is carried out using the method of slip line fields (SLFs). Unlike upper-bound models, it is possible to estimate strain profiles, hydrostatic pressure and modes of deformation using an SLF model. By using an SLF model, estimates are made of SPD parameters such as strain, machining force and hydrostatic pressure, and their dependence on controllable input variables of tool rake angle and chip compression ratio. This analysis provides a theoretical framework for the use of LSEM as a method of SPD, and for its implementation as a process for making bulk nanostructured materials. The experimental results and the SLF model suggest a number of exciting possibilities for the LSEM. These include creation of bulk nanostructured and UFG alloys from a variety of materials, scaling of the process to make sheet, foil, wire and bar, study and control of material texture, and the possibility of a new method for consolidation of particulates and foils.
Degree
Ph.D.
Advisors
Compton, Purdue University.
Subject Area
Mechanical engineering
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