Machining of aerospace superalloys with coated (PVD and CVD) carbides and self-propelled rotary tools.
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
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.