High-speed machining of nickel-base, Inconel 718, alloy with ceramic and coated carbide cutting tools using conventional and high-pressure coolant supplies
The first part of this study involve an evaluation of the performance of recently developed
nano-grain size ceramic tool materials when machining nickel base, Inconel 718, with
conventional coolant flow in terms of tool life, tool failure modes and wear mechanisms as
well as component forces generated under different roughing conditions. Comparison tests
were carried out with commercially available ceramic tool materials of micron-grain
composition. The test results show that the micron grain size commercially available tool
materials generally gave the longest tool life. The dominant failure mode is nose wear, while
some of the nano-ceramic tools were rejected mainly due to chipping at the cutting edge. It is
also evident that chemical compositions of the tool materials played significant role in their
failure. The alumina base ceramics performed better than the silicon nitride base ceramics.
Severe abrasion wear was observed on both rake and flank faces of the cutting tools while
cutting forces increased with increasing cutting speed when machining with the silicon nitride
base nano-ceramic tools. This is probably due to the lower superplastic flow temperature of
the nitride base nano-ceramics.
The second part of this study involve turning of Inconel 718 with commercially available
ceramic and PVD coated carbide tools with conventional and high-pressure coolant supplies
at cutting speeds up to 300 and 60 m min" respectively. Increasing the coolant pressure
results in shorter tool life when machining Inconel 718 with ceramic tools, suggesting that the
high-pressure coolant supply reduces temperature at the cutting zone below a critical level
where ceramic tools can perform satisfactorily. The inadequate fracture toughness of ceramic
tools makes them more susceptible to failure by mechanical action such as notching at the
depth of cut line and premature fracture. The notch wear rate increases with higher coolant
supply pressure due to significant erosion of the tool material by the high-pressure coolant jet.
Machining Inconel 718 with a triple PVD coated (TiCNI AI20iTiN) carbide tool at speeds up
to 60 mlmin using conventional and various high coolant pressures, up to 203 bar was found
to be encouraging. The test results show that acceptable surface finish and improved tool life
can be achieved when machining Inconel 718 with high coolant pressures. Compared to
conventional coolant supplies, tool life improved as much as 7 folds when machining at 203
bar coolant pressure at high speed conditions. Tool life generally increased with increasing
coolant supply pressure due to the ability of the high-pressure coolant to lift the chip and gain
access closer to the cutting interface.
Chip breakability during machining is dependent on the depth of cut, feed rate and cutting
speed employed as well as on the coolant pressure employed. Machining Inconel 718 with
lower coolant pressures did not produce chip segmentation. Tool wear increased gradually
with prolong machining with high coolant pressures. Nose wear was the dominating tool
failure mode when machining with coated carbide tools due probably to a reduction in the
chip-tool and tool-workpiece contact length/area.
SEM micrographs of the machined surfaces show that micro-pits are the main damage to the
machined surfaces. Microhardness analysis show evidence of hardening of the top machined
surfaces. In most cases the microhardness readings tend to approach the hardness of the base
material at 0.25 mm under rough and 0.2 mm under finish machining below the machined
surface. This is due to the austenitic structure of Inconel 718 which promote work hardening
when machining as a result of the high temperature generated at the cutting interfaces. The
hardening effect decreased under finishing conditions and with increasing coolant pressures
up to 203 bar as the coolant gain access closer to the cutting interfaces, thus minimising the
cutting interface temperature. Analysis of the microstructure shows that severe plastic
deformation occurred when machining with conventional coolant supply than with highpressure
coolant supplies. There was also mild plastic deformation under finish machining.
Surface damage or phase transformation was absent when machining Inconel 718 under highpressure
coolant supplies. Generally the surface integrity of the finish machined surface is in
accordance with CME 5043.