Two aerospace superalloys, Inconel 718 and IMI 318, were machined with different grades of PVD (KC730 and KC732) and CVD (KC950) coated tools in order to evaluate their performance under various cutting conditions and to further investigate the effect of the machining conditions on surface finish and surface integrity of the work materials. A self-propelled rotary tool was also developed and used for machining under the finishing conditions. Tool wear, component forces and surface roughness were recorded and analysed during the machining trials. Study of the surface integrity involved physical as well as metallographic examination and analysis of the machined surfaces. The results of the machining trials show that the multi-layer (TiN/TiCN/TiN) PVD coated KC732 tools gave the best overall performance when machining both Inconel 718 and IMI 318, especially at lower feed conditions. Flank wear, excessive chipping, flaking of tool materials close to the cutting edge or on the rake face were the dominant failure modes when machining with the PVD coated tools while flank wear and notching were dominant when cutting with the CVD coated tools. These failure modes are associated with attrition, abrasion, diffusion and plastic deformation wear mechanisms acting individually or in combination during machining. The statistical regression analysis of the tool life data shows that wear of the PVD and CVD coated tools used for machining Inconel 718 was mainly affected by cutting speeds employed while cutting speed and feed rate exhibited similar influence on tool performance when machining IMI 318 with PVD coated tools. Tool life equations for each of the three coated grades when machining both superalloys under the cutting conditions investigated were derived. Severe plastic deformation and hardening of the machined surfaces occurred after machining both materials due to a combined action of increased component forces, thus increased stresses, and high temperature. Softening of the top surface layer when machining IMI 318 can be attributed to overaging of the titanium as a result of highly localised surface heating during machining. Tearing of the machined surfaces occurred when machining IMI 318 with the PVD coated tools, particularly with KC732 tools as a result of irregular flank wear and excessive chipping of KC732 tools. The self-propelled rotary tool (SPRT) incorporating K68 straight grade carbide exhibited superior wear-resistance when machining IMI 318 due to the absence of thermally related wear mechanisms caused by reduced temperature and the use of the entire edge of a round insert during rotary cutting. The minimal subsurface alterations (such as plastic deformation and hardness) when machining Inconel 718 and IMI 318 with the SPRT can also be attributed to lower cutting temperature with rotary action.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:321824 |
| Date | January 1997 |
| Creators | Wang, Zhiming |
| Publisher | London South Bank University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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