High-speed machining of nickel-base, Inconel 718, alloy with ceramic and coated carbide cutting tools using conventional and high-pressure coolant supplies

• Main Author: Bonney, John
• Published: London South Bank University 2004
• Subjects: 671.35
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.410701

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.