PCBN Performance in High Speed Finishing Turning of Inconel 718

Inconel 718 is a Ni superalloy widely used in high responsibility components requiring excellent mechanical properties at high temperature and elevated corrosion resistance. Inconel 718 is a difficult to cut material due to the elevated temperature generated during cutting, its low thermal conductivity, and the strong abrasive tool wear during cutting process. Finishing operations should ensure surface integrity of the component commonly requiring the use of hard metal tools with sharp tool edges and moderate cutting speeds. Polycrystalline cubic boron nitride (PCBN) tools recently developed an enhanced toughness suitable for these final operations. This paper focuses on the study of PCBN tools performance in finishing turning of Inconel 718. Several inserts representative of different manufacturers were tested and compared to a reference carbide tool. The evolution of tool wear, surface roughness, and cutting forces was analyzed and discussed. PCBN tools demonstrated their suitability for finishing operations, presenting reasonable removal rates and surface quality.This research was funded by Ministry of Economy, Industry and Competitiveness and FEDER program grant number DPI2014-56137-C2-2-R

Future research directions in the machining of Inconel 718

Inconel 718 is the most popular nickel-based superalloy, extensively used in aerospace, automotive and energy industries owing to its extraordinary thermomechanical properties. It is also notoriously a difficult-to-cut material, due to its short tool life and low productivity in machining operations. Despite significant progress in cutting tool technologies, the machining of Inconel 718 is still considered a grand challenge.This paper provides a comprehensive review of recent advances in machining Inconel 718. The progress in cutting tools’ materials, coatings, geometries and surface texturing for machining Inconel 718 is reviewed. The investigation is focused on the most adopted tool materials for machining of Inconel 718, namely Cubic Boron Nitrides (CBNs), ceramics and coated carbides. The thermal conductivity of cutting tool materials has been identified as a major parameter of interest. Process control, based on sensor data for monitoring the machining of Inconel 718 alloy and detecting surface anomalies and tool wear are reviewed and discussed. This has been identified as the major step towards realising real-time control for machining safety critical Inconel 718 components. Recent advances in various processes, e.g. turning, milling and drilling for machining Inconel 718 are investigated and discussed. Recent studies related to machining additively manufactured Inconel 718 are also discussed and compared with the wrought alloy. Finally, the state of current research is established, and future research directions proposed.<br/

Future research directions in the machining of Inconel 718

Inconel 718 is the most popular nickel-based superalloy, extensively used in aerospace, automotive and energy industries owing to its extraordinary thermomechanical properties. It is also notoriously a difficult-to-cut material, due to its short tool life and low productivity in machining operations. Despite significant progress in cutting tool technologies, the machining of Inconel 718 is still considered a grand challenge.This paper provides a comprehensive review of recent advances in machining Inconel 718. The progress in cutting tools’ materials, coatings, geometries and surface texturing for machining Inconel 718 is reviewed. The investigation is focused on the most adopted tool materials for machining of Inconel 718, namely Cubic Boron Nitrides (CBNs), ceramics and coated carbides. The thermal conductivity of cutting tool materials has been identified as a major parameter of interest. Process control, based on sensor data for monitoring the machining of Inconel 718 alloy and detecting surface anomalies and tool wear are reviewed and discussed. This has been identified as the major step towards realising real-time control for machining safety critical Inconel 718 components. Recent advances in various processes, e.g. turning, milling and drilling for machining Inconel 718 are investigated and discussed. Recent studies related to machining additively manufactured Inconel 718 are also discussed and compared with the wrought alloy. Finally, the state of current research is established, and future research directions proposed.<br/

State-of-the-art cooling and lubrication for machining Inconel 718

Inconel 718 is the most used nickel superalloys with applications in aerospace, oil&amp;gas, nuclear and chemical industries. It is mostly used for safety-critical components where the condition of the surface is a significant concern. The combination of mechanical, thermal and chemical properties of Inconel 718, has made it a difficult-to-machine material. Despite recent advances in machining Inconel 718, achieving desired surface integrity with prescribed properties is still not possible. Different machining environments have been investigated for improving the machinability of Inconel 718 and enhance the surface integrity of machined components. This paper provides a new investigation and classification into recent advances in the machining of Inconel 718 regarding surface integrity, mostly concentrated on turning applications. The major findings and conclusions provide a critique of the state-of-the-art in machining environments for Inconel 718 together with future directions for research. Surface integrity has been evaluated in terms of surface topology as well as mechanical and microstructural properties. The impact of various cooling and lubrication methods has been investigated. It has been found that surface integrity is affected by the thermomechanical conditions at the cutting zone which are influenced by the cutting parameters, cutting tool, tool wear and cooling/lubrication condition. The current technologies are incapable of delivering both productivity and sustainability whilst meeting surface integrity requirements for machining Inconel 718. High-pressure cooling has shown the potential to enhance tool wear at the expense of higher power consumption

State-of-the-art cooling and lubrication for machining Inconel 718

Inconel 718 is the most used nickel superalloys with applications in aerospace, oil&amp;gas, nuclear and chemical industries. It is mostly used for safety-critical components where the condition of the surface is a significant concern. The combination of mechanical, thermal and chemical properties of Inconel 718, has made it a difficult-to-machine material. Despite recent advances in machining Inconel 718, achieving desired surface integrity with prescribed properties is still not possible. Different machining environments have been investigated for improving the machinability of Inconel 718 and enhance the surface integrity of machined components. This paper provides a new investigation and classification into recent advances in the machining of Inconel 718 regarding surface integrity, mostly concentrated on turning applications. The major findings and conclusions provide a critique of the state-of-the-art in machining environments for Inconel 718 together with future directions for research. Surface integrity has been evaluated in terms of surface topology as well as mechanical and microstructural properties. The impact of various cooling and lubrication methods has been investigated. It has been found that surface integrity is affected by the thermomechanical conditions at the cutting zone which are influenced by the cutting parameters, cutting tool, tool wear and cooling/lubrication condition. The current technologies are incapable of delivering both productivity and sustainability whilst meeting surface integrity requirements for machining Inconel 718. High-pressure cooling has shown the potential to enhance tool wear at the expense of higher power consumption

Review of current best-practices in machinability evaluation and understanding for improving machining performance

Machinability is a generalized framework that attempts to quantify the response of a workpiece material to mechanical cutting, which has been developed as one of the key factors that drive the final selection of cutting parameters, tools, and coolant applications. Over the years, there are many attempts have been made to develop a standard evaluation method of machinability. However, due to the complexity of the influence factors, i.e., from work material and cutting tool to machine tool, that can affect the materials machinability, currently there is no uniquely defined quantification of machinability. As one of the outcomes from the CIRP's Collaborative Working Group on “Integrated Machining Performance for Assessment of Cutting Tools (IMPACT)”, this paper conducts an extensive study to learn interacting machinability parameters to evaluate the overall machining performance. Specifically, attention is focused on recent advances made towards the determination of the machinability through tool wear, cutting force and temperature, chip form and breakability, as well as the surface integrity. Furthermore, the advanced methods that have been developed over the years to enable the improvement of machinability have been reviewed

Optimization of Process Parameters for CNC Turning using Taguchi Methods for EN24 Alloy Steel with Coated/Uncoated Tool Inserts

Coated and uncoated tool inserts offers certain degrees of control on the desired rate of tool wear and surface roughness to an extent. This work pursues the quest for realizing the optimal values for the significant process parameters that bears an influence on the response parameters. Experiments were conducted on the samples of EN 24 alloy steel material with the help of PVD coated TiAlN insert and uncoated carbide insert. The experimental runs carried out with proper variation in the levels. Levels are selected with the help of manufacturing catalogue and by pilot experimentation and results are recorded for further analysis. For this study, 9 runs designed using L9 orthogonal array of Taguchi Design of Experiment. Surface roughness was measured using a Mitutoyo surface tester at test lab and material removal rate is calculated by mathematical equation. The data was compiled into Minitab 17 software for analysis. The relationship between the machining parameters and the response variables were analyzed using the Taguchi Method. Optimization of process parameters is carried out by Grey Relational Analysis method (GRA). GRA method is a powerful and most versatile tool which can manipulate the input data as per requirement and comes with results that can be used to have best multi-objective in respective concerns

Face milling of nickel-based superalloys with coated and uncoated carbide tools

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Machining of nickel based superalloys using coated PCBN tooling

Following a comprehensive literature review on the machinability of nickel based superalloys using conventional carbide, coated carbide and ceramics including uncoated/coated PCBN, the research details statistically designed experimental work to assess the tool life/wear performance and workpiece surface integrity of a range of uncoated and coated PCBN tools, when turning solution treated and aged Inconel 718. Typically, the use of carbide tooling is limited to < 60 m/min cutting speed, even with the use of high pressure cutting fluids (JetStream systems). Hardmetal coatings provide some productivity improvements although cutting speed restrictions still operate. Details of PCBN use at up to 600 m/min have been published but at the expense of tool life. Experimental testing of a number of uncoated PCBN grades involving both high and low CBN concentrations, indicated a preferred operating window of ~ 300 – 400 m/min. Advanced ceramic coatings provided no significant benefits. Primary tool wear mechanisms related to abrasion, workpiece adhesion/diffusion and fracture depending on the specific operating parameters employed. In depth workpiece integrity evaluation involving surface roughness, microstructure, microhardness and residual stress measurement suggested only limited damage when operating with PCBN tooling at preferred/optimised conditions

Hard turning of martensitic AISI 440B stainless steel

Hard turning has been in use for some time to achieve close dimensional tolerances to eliminate time consuming and costly grinding operations. The most widely used cutting tools for finish machining of hardened steels under dry cutting conditions are the ceramics and PcBN cutting tools. The purpose of this study was to investigate the machinability of hardened martensitic AISI 440 B stainless steel (HRC 42-44) using commercially available cutting tools: alumina based ceramic and PcBN, by hard turning under different machining conditions, by providing an in-depth understanding of wear mechanisms of these cutting tools. The study also developed a serrated chip formation mechanism of the workpiece and provided a deep understanding of the chemical interaction between workpiece and cBN cutting tools, through microstructural analysis of the adhered layer on the worn cutting tool. Experimental studies on the effects of cutting parameters on the tool wear mechanism, cutting forces; surface roughness, dimensional accuracy, and chip formation mechanism were investigated. The characterization of the workpiece, cutting tools, chips and wear scars on the cutting tools was performed using an X-ray diffractometer, and optical, scanning and transmission electron microscopes, as well as an energy dispersive spectroscope (EDS). The cutting speeds selected for testing the cutting tools were in the range of 100 m/min and 600 m/min, depending on the type of parameter investigated. Two depths of cut, 0.1and 0.2 mm, and three feed rates, 0.05, 0.1 and 0.15 rev/min, were selected for the experiments. Experimental results showed that the flank wear in the PcBN cutting tool is lower than that of the mixed alumina, with PcBN showing better wear resistance at all cutting conditions (about five times longer in some instances). Apart from the cutting speed, the feed rate was found as a parameter that directly influences the flank wear rate of the cutting tool. The wear mechanism for the ceramic cutting tool is predominantly abrasive wear, and for PcBN tools it was adhesive wear and abrasive wear. The abrasive wear was caused by hard carbide particles in the workpiece material resulting in grooves formed on the flank face. There was formation of a transferred layer followed by plastic deformation of the workpiece material on the rake face of the PcBN tool when cutting at low cutting speed and feed rate. At much higher cutting speeds, some form of chemical wear preceded by adhesion and abrasion was the main tool wear resulting from the chemical affinity between the PcBN tool and the workpiece. Better surface finish (Ra) was recorded for mixed ceramics but with deteriorating surface topography. The increase in the cutting speed results for improvement in the surface finish produced by both cutting tools was investigated. The final part, using the PcBN cutting tool, produced better dimensional accuracy resulting from its better wear resistance at the flank face. The results also show that good dimensional accuracy can be achieved with cBN tools using a CNC machine with high static and dimensional stiffness coupled with high precision hard turning. The influence of cutting conditions on the chip formation showed production of continuous chip at a cutting speed of 100 m/min and segmented chip at higher cutting speeds above 200 m/min by both cutting tools. The increasing cutting speed affects the formation of shear localised chips with rapid increase in shear strain rate and degree of segmentation at cutting speeds higher than 200 m/min. The microstructure of the chip produced shows the distinct carbide grain in the martensite of the work material with intense shear localisation in the primary deformation zone of the cutting tool and formation of white layer in the secondary deformation zone. The microstructure of the crater of the worn PcBN cutting tool at cutting speeds of 100 m/min and 600 m/min were studied in detail. A situ lift-out technique, in a Focused Ion Beam/SEM instrument, was used to produce thin foil specimens, which were taken out of the crater face of the PcBN tool and observed using SEM and TEM. The SEM and TEM study showed evidence of chemical interaction between the work material and the PcBN tool. Fe from the work material was found in the vicinity of TiC and AlB grains of the PcBN tool, with TiC having greater affinity for Fe. Oxidation of the elements was common in all Fe-rich areas. The microstructure of the worn PcBN cutting tool at the cutting speed of 600 m/min showed deeper penetration of Cr and Fe into the cBN tool, which was not easily detected by SEM at the cutting speed of 100 m/min. The hard turning operations using the PcBN cutting tool for substituting traditional machining operations was successfully performed in the industrial environment. The overall surface finish and dimensional accuracy generated during the application of CBN-100 for machining within the industrial environment on specified mass produced shape showed a component acceptable tolerance range with good surface finish similar to that of the grinding operation