ABSTRACT

The machining of metals is of significant technological importance. Any gains obtained by reducing forces and energy, improved surface quality, and lower system vibrations can have important impacts in the discrete products manufacturing sector. Typically, there are four principal chip types in metal cutting, each corresponding to different flow modes and with characteristic attributes. 

In this dissertation, we examine the mechanics of segmented chip formation, one of the principal chip types, with a view to understanding the underlying flow/deformation dynamics, development of the chip morphology and its attributes, microstructural origins of the flow, and how this segmentation can be controlled. This is done using high speed in situ imaging of the deformation, complemented by force and surface topography characterization, using different material systems.

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 the fractures propagating towards the tool cutting edge. The fracture is nucleated at a critical strain, that is independent of the deformation geometry. Based on an understanding of the deformation conditions prevailing in the fracture zone, we demonstrate control of this segmentation. This includes 1) suppression of the segmentation by application of a second constraining die (constraint) located directly across from the cutting tool, resulting in the chip being formed as a continuous strip of uniform thickness with no fracturing; and 2) inducing segmentation to occur in metals that would normally not segment in metal cutting (e.g., Cu) by utilizing a mechanochemical effect.

The mechanochemical effect is based on application of a chemical medium to the initial workpiece surface remote from the tool-chip interface. We examine application of mechanochemical effect to promote segmentation in annealed copper. Combination of mechanochemical effect and high speed in situ imaging provided an opportunity to study microstructural origins of segmentation in thermally etched copper. 

Implications of the results for ductile fracture of metals and for enhancing the performance of the cutting process are discussed. It is presented that control of segmentation flow promotes surface topography and cutting force control.