Experimental Studies on Machinability of Inconel 718 and 825 Superalloys under Application of Different Coolant Media and Cutting Inserts

Abstract

INCONEL refers to a particular family of nickel-chromium-iron based superalloys (trademarked by the Special Metals Corporation). Inconel alloys possess excellent oxidation-corrosion resistance appropriate for service under aggressive operating conditions subjected to intense pressure and heat. Inconel 718 and 825 both belong to the category of Ni-Cr-Fe based superalloys with small amount of Mo, Ti and Al. The addition of Ti and Al imparts high strength and hardness to these alloys. The major constitutional difference between these two superalloys is the presence of Nb (within Inconel 718) and Cu (within Inconel 825). The addition of Nb within Inconel 718 imparts age-hardenability; increases strength and fatigue resistance due to formation of the stable intermetallic compound (Ni3Nb) within base matrix-microstructure. Mo and Cu added within Inconel 825 provide outstanding resistance towards corrosive environments. Inconel 718 is extensively used for applications in jet engine and gas turbine operations whilst the major application of Inconel 825 is found in chemical processing industries. Superalloys are often experienced as ‘difficult-to-cut’ through conventional machining routes. Dry machining of superalloys with uncoated cemented carbide tool is not encouraged due to enormous cutting heat generation which causes significant plastic deformation of the tool point. Owing to high toughness of carbide, thermally softened tool point (altered tool geometry) requires higher cutting force, causes rapid tool wear and finally degraded machined surface quality. Research endeavors are being put to determine an efficient method towards improving machinability of Inconel. Application of coolants, harder tool materials (harder than carbides); depositing coating material(s) over tool substrate and usage of coated tools, precise tuning of cutting parameters and appropriate combination of the above may ensure satisfactory machining yield with acceptable balance amongst productivity, part quality and economy. In the present dissertation, the machinability of two nickel-based superalloys such as Inconel 718 and 825 are analyzed through a few case experimental studies. Owing to the fact that Inconel 825 possesses higher degree of chemical reactivity (towards tool substrate/ binder/ coating materials) than Inconel 718; machining experiments on Inconel 825 work alloy are planned under application of Nanofluid Minimum Quantity Lubrication (NFMQL) environment. On the other hand, the machinability of Inconel 718 is studied under dry condition as well as conventional MQL (sunflower oil-based) with application of single/ multi-layered coated carbide tools (in comparison to that of uncoated counterparts). The entire dissertation is divided into four modular clusters. In the first module, longitudinal finish turning operations are carried out on Inconel 825 under (distilled water + nano-Al2O3 powder)-based NFMQL using a PVD multi-layered (TiN-TiCN-TiN) coated cermet insert. In the second module, performances of two nanofluids are compared during turning of Inconel 825 workpiece using uncoated WC-Co tool. Nanofluids are prepared by dispersing nano-Al2O3 powder and Multi-Walled Carbon Nanotubes (MWCNTs) separately, within biodegradable sunflower oil (as base fluid). In the third module, conventional MQL (sunflower oil-based) machining of Inconel 718 is attempted under application of uncoated/ coated WC-Co tools. Performances of MT CVD multi-layered TiCN-Al2O3-TiOCN coated and MT CVD double-layered TiCN-Al2O3 coated carbide tools are compared to that of uncoated WC-Co counterpart. The fourth module focuses on use of a coated tool (tool with deposited advanced coating material) during dry machining of Inconel 718. Instead of an uncoated WC-Co tool, a HSN2 (2nd generation TiAlxN supernitride; TiAlN doped with Si) coated WC-Co tool is used. The WC-Co tool substrate is coated with PVD HSN2 layer through DC magnetron sputtering technique. In HSN2 coating, the blend of titanium and silicon nitride provides high hardness to the deposited coating layer, refractoriness and resistance towards thermal oxidation. The silicon content develops a strong resistive barrier to thermal diffusion which restricts oxidative and diffusive wear at elevated cutting temperatures. In the present dissertation, machinability of Inconel 718 and 825 are studied in perspectives of magnitude of cutting force components (offset mean value), approximate tool-tip temperature, tool flank wear (width) and quantitative estimates of chip’s micro-morphology (segmentation spacing and frequency, equivalent chip thickness, segment shear angle, segmentation ratio, segment included angle, microhardness, etc.). In addition to detailed tool wear morphology, effects of cutting velocity on influencing machinability of the said work alloys are investigated therein. In relation to machinability of Inconel 825, the severity of machining induced vibrations (absolute value of the maximum amplitude of acceleration) is experienced relatively less during (nano-Al2O3 powder + distilled water) nanofluid assisted machining than dry machining condition. It is also experienced that (MWCNTs + sunflower oil) nanofluid outperforms (nano-Al2O3 powder + sunflower oil) nanofluid due to possession of higher thermal conductivity (imparting better cooling action) and nano-structured cylindrical morphology (ensuring better lubrication effect) of dispersed Carbon Nanotubes (CNTs) causing improved thermo-physical and tribological properties of the resultant nanofluid. While exercising ease of machining of Inconel 718, it is experienced that under sunflower oil-based MQL environment, TiCN-Al2O3-TiOCN multi-layered coated carbide tools may be a better choice than TiCN-Al2O3 double-layered coated carbide tool for operation up tov = 80 m/min. When cutting velocity exceedsv = 80 m/min, TiCN-Al2O3 double-layered coated carbide tool outperforms due to dominance of thermal properties of coating layers thus subsiding frictional effects at tool-work and tool-chip interfacial regions. Dry machining of Inconel 718 with application of HSN2 coated carbide tool is suggested in comparison with usage of uncoated WC-Co counterpart. Application of HSN2 coated carbide tool causes lower cutting force, feed force and thrust force; lower progression width of tool flank wear, better morphology at chip’s underside surface and lower microhardness of chips when compared to that obtained using uncoated tool.