Date of Award
Doctor of Philosophy (PhD)
Professor S.V. Subramanian
The influence of dynamic behaviour of materials (i.e. the response of materials to large strain, high strain rate, deformation under large hydrostatic pressure occurring during metal cutting) on machinability (i.e., chip morphology, tool wear and surface finish) has been investigated in ferrous alloys with a wide range of matrix and volume fraction of second phase particles. With the increase of cutting speed, there is change in the tribological phenomenon at the tool-chip interface from sliding to seizure. A physical model for seizure is proposed based on atomic contact at the tool-chip interface. The model predicts the critical cutting speed for onset of seizure from force measurements. Seizure is said to occur when the normal stress exceeds yield strength of asperities such that the true area of contact approaches the apparent area of contact. The tribological phenomenon of seizure is shown to cause thermoplastic shear localisation. In consequences, the temperature at the tool-chip interface rises, leading to dissolution wear of the tool into the chip by a diffusion mechanism which causes chemical wear of the tool. The technique of ICP-MS has been developed and used to separate the physical and chemical wear aspects of the tool by measurement of minute concentrations of tungsten present in the chips as WC as distinct from tungsten atomically dissolved in the chips. The results have confirmed that chemical crater wear dominates at high cutting speeds. The temperature distribution at the tool-chip interface has been predicted by finite element analysis and used to compute diffusion wear. A comparison of theoretical and experimental values of diffusion wear suggests that high diffusivity paths operate at the tool-chip interface to enhance the diffusivity by more than two orders of magnitude. The maximum depth of the measured crater depth profile has been found to coincide with the phase transformation temperature of the workpiece material rather than the maximum predicted temperature at the tool-chip interface. The amount of dissolution wear, as measured by the amount of tungsten transferred into the chips is attributed to dislocation pipe diffusion. It is further suggested that dislocations generated by deformation concomitant with phase transformation provide high diffusivity paths that contribute to enhanced diffusion wear. The implication is that dissolution crater wear of the tool is phase transformation coupled. Dissolution crater wear can be suppressed if the tribological phenomenon of seizure can be prevented. This can be achieved by in-situ lubrication at the tool-chip interface through inclusion engineering of the workpiece. Alternatively, the diffusion wear can be minimized by coating the tool with a compound which hasa the least solubility in the workpiece. The microstructural response to changes of metal cutting variables during the machining of a range of iron alloys with varying heat treatment condition and microstructural constituents of the matrix has been investigated to establish the inter-relationship among chip morphology, tool wear and surface finish. The microstructural changes in the chips have been analysed by optical microscopy, scanning electron microscopy, transmission electron microscopy and x-ray diffractiion techniques. The results from the chips formed during machining of martenistic Fe-28.9%Ni-0.1%C alloy confirmed the presence of austenite, exhibiting grains as fine as 40-100 nm in the white shear bands. This structure is attributed to a sequence of events: the reverse phase transformation of martensite to austenite, shear localisation, formation of the transformation shear bands, and probably dynamic recrystallisation. This it is demonstrated that thermal softening due to phase transformation causes shear localisation leading to chip segmentation in the primary shear zone. Phenomenological observations are presented to confirm that shear localisation is caused by (i) geometrical softening due to second phase particles (graphite inclusions in cast iron, inclusions in free cutting steels), (ii) thermal softening of the matrix due to phase transformation or recrystallisation and (iii) a combination of the above. Shear localisation causes step temperature rise in a narrow band, referred to as shear band. The interaction of the priary shear band with cutting edge of the tool is found to cause dissolution wear of the cutting edge of the tool. The loss of the cutting edge in turn is shown to impair surface finish. The effect of metal cutting variables, i.e., speed, feed, depth of cut, external lubricants on shear localisation, chip morphology and tool wear is investigated. It is shown that chip segmentation can be suppressed by decreasing the feed. This is analyzed in terms of damage concepts underlying chip fracture behaviour.
Gekonde, Haron Ogega, "Influence of Dynamic Behaviour of Materials" (1998). Open Access Dissertations and Theses. Paper 1165.