Mechanics of Ductile Fracture and Segmented Chip Formation in Cutting of Metals

Mojib Saei, Purdue University

Abstract

The machining of metals is of significant technological importance for discrete products manufacturing encompassing automotive, aerospace, energy systems and biomedical sectors. Any gains obtained by improving surface quality and increasing tool life can have important impacts in this vital manufacturing sector. Obtaining such improvements is closely tied to reducing forces and specific energy, and, hence, directly to the mechanics of the material removal process (chip formation). Typically, there are four principal chip types in metal cutting, each corresponding to a different (underlying) plastic flow mode with characteristic deformation attributes. In this dissertation, we examine mechanics of segmented chip formation, one of the principal chip types, with a view to understanding the underlying flow dynamics and instabilities, forces and deformation, development of chip morphology and attributes, and how segmentation can be controlled. This is done using high speed in situimaging of material flow and deformation, complemented by force and surface topography characterization, using varied material systems that show a strong propensity for segmentation. It is shown that ductile fracture plays a key role in the segmentation, with periodic fracture events being nucleated on the back (free) surface of the chip, and each fracture then propagating towards the tool cutting edge, to create the segmented chip/flow. Furthermore, the crack nucleation triggering the fracture occurs at a critical strain, that is independent of the deformation geometry. Two types of segmentation morphologies are uncovered, each with its distinct (characteristic) force signature. In the first type, the segmentation runs straight across the chip width, and the force profile shows an oscillation at the same frequency as the segmentation frequency. In the second type, the segmentation meanders across the chip width, and the force trace in this case is fairly steady with no oscillation. The strain field with segmented flow is non-uniform, with significant localization of the strain occurring in the crack propagation zone. Based on an understanding of the deformation conditions and stress state prevailing in the fracture zone, we demonstrate material-agnostic strategies to control the segmentation. This control includes 1) suppression of the segmentation by application of a second constraining die (constraint) located directly across from the cutting tool – Hybrid Cutting Extrusion (HCE); this results in the chip being formed as a continuous strip of uniform thickness with uniform laminar plastic flow and homogeneous deformation; and 2) inducing segmentation to occur in ductile metals (e.g., Cu) that would normally not segment in metal cutting, by utilizing a surface medium, applied to the initial workpiece surface, to locally embrittle the metal in the chip-formation zone – a mechanochemical effect. The HCE, by suppressing the segmentation, offers opportunities for use of machining as a forming process to produce sheet metals, even from low-workability alloys, in a single step of deformation, by chip formation. The deliberate triggering of segmentation via a mechanochemical effect offers opportunities to cut the so-called “gummy” metals with smaller forces and energy. Our understanding of deformation mechanics of segmentation has other benefits for machining process, including improvement of surface finish, cutting force control/reduction, and for study of ductile fracture phenomena.

Degree

Ph.D.

Advisors

Trumble, Purdue University.

Subject Area

Energy|Mechanics|Industrial engineering|Materials science

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