The first part of this study involve an evaluation of the performance of TiN and AlZ03 single layer coated cemented carbide tools when threading inclusion modified, 708M40T (En 19T) 817M40T (En 24T) and Jethete steels at high cutting conditions by monitoring tool wear, failure modes, post threading workpiece properties, micro and macro-surface alterations and subsurface microhardness variation of threaded surfaces. Test results show that flank wear was the dominant failure mode, increasing rapidly when machining at the top speed of 225 m min,l due to the high temperature generated which accelerates thermally related wear mechanisms. Tool life, surface finish, hardness variation and component forces during threading were influenced by the geometry of the cutting edge, shape of wear/length of wear along tool nose/cutting edge after threading. Formation of flake-like oxide debris on the worn inserts was found to increase with nickel content in the workpiece material. The Al20) coated carbide inserts with K05 - K20 substrate gave longer tool life, lower cutting forces, better surface finish! damages as well as minimum hardness variation after threading compared with the TiN coated VSX grade with P20-P30 substrates. This can be related to their superior hardness, density, transverse rupture strength as well as the unalloyed WC fine grained substrate (1/lm) in addition to the high hot hardness, excellent chemical stability and low thermal conductivity of the AlZ03 coating at elevated temperatures. A formula for tool rejection was also developed during this study based on the average flank wear (VBb) and growth in thread root (GTR) in order to establish a scientific basis for assessing wear of threading tools. The second part of this study involve single point turning of a nickel base, G263, alloy using rhomboid-shaped PVD coated (TiN/TiCN/TiN, TiAIN and TiZrN) carbide tools at high speed cutting conditions. The worn tool edges revealed adhesion of a compact fin-shaped structure of hardened burrs with saw-tooth edges. The compact structure also formed on the cut surface of the workpiece material. The use of coolant during machining tend to work harden the root of the burr thereby restricting tool entry at the cutting zone leading to the generation of excessive feed force which subjects the tool edge to premature fracture and consequently lower tool life. The serrated/saw-tooth like edges of the burr encourages abrasion wear on the tool flank face and the formation of shallow cavities/lateral cracks where fragments of hardened workpiece material are deposited causing deterioration of the machined surfaces. Tool life was generally influenced by the cutting conditions employed as well as the insert geometry. Increasing cutting conditions (speed, feed and depth of cut) led to chipping of the cutting edge and/or flaking of coating layers as well as notching and fracture of the cutting edge. These failure modes jointly contributed to lowering tool life during machining. The TiN/TiCN/TiN coated KC732 (Tool A) inserts with positive sharp edges gave overall performance at the optimum cutting conditions established under finishing operation. This is followed by the TiN/TiCN/TiN coated KC732 (Tool B), TiAlN coated KC313 (Tool C) and lastly the TiZrN coated KC313 (Tool D) inserts' with razor sharp edges. Under roughing operation, the ranking order of tool performance is the TiZrN coated KC313 (Tool D), TiN/TiCN/TiN coated KC732 (Tool A), TiAlN coated KC313 (Tool C) and lastly the TiN/TiCN/TiN coated KC732 {Tool B). The difference in tool geometry and coating materials contributed to the relative order of tool performance.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:297919 |
| Date | January 1999 |
| Creators | Okeke, Christopher Igwedinma |
| Publisher | London South Bank University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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