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Top1. Research Background
Inconel 825 is a superalloy with major elemental composition of nickel, iron, and chromium. It has excellent corrosion resistance, and hence, it is an appropriate candidate for wide variety of industrial applications such as nuclear plants, acid-producing plants, chemical industries, and petroleum refineries. Inconel 825 has certain useful properties such as retention of hardness, and strength at elevated temperature which makes machining of this alloy difficult. Other properties which are responsible for its poor machinability are low thermal conductivity, strong reactivity with tool material (at higher temperature), and strain hardening ability (Ezugwu et al. 1999; Aggarwal et al., 2015). Therefore, proper selection of cutting tool is necessary for improved machining efficiency in terms of tool life, specific power consumption, product surface finish, and machining cost.
Different machining environments (wet) including flood cooling (Ezugwu and Bonney, 2005), Minimum Quantity Lubrication (MQL) (Marques et al., 2016; Travieso-Rodriguez et al., 2015), Nanofluid Minimum Quantity Lubrication (NFMQL) (Singh et al., 2018) were attempted by pioneers. But due to elevated cost of lubricants as well as nano-additives, and problem related to disposal of used cutting fluids (Bart et al., 2013); traditional dry machining is considered to be a better choice by the industries. Dry machining is environment friendly, and economical (Sreejith and Ngoi, 2000).
Desired properties of tool materials include: high toughness, and hot hardness; high thermal shock and wear resistant; chemical stability at elevated temperature. Due to low wear and oxidation resistance, application of traditional (uncoated) tungsten carbide tools is limited for machining of Inconel 825. In addition, due to lower thermal shock resistance, machining with carbide tool is restricted to lower cutting speeds (Olovsjo and Nyborg, 2012). These limitations lead to use of coated carbide tools, for dry machining of superalloys. The most vital function of tool coating is to reduce coefficient of friction which results in lower friction at tool-chip, and tool-work contact surfaces. Coating also provides improved hardness; wear as well as oxidation resistance to the tool (Chinnasamy et al. 2019). Common tool coating materials are metallic carbides, and nitrides such as TiN, TiC, TiCN, TiAlN, ZrN, CrAlN, TiAlSiN, cBN, etc. However, beyond 100 m/min cutting speed, coated carbide tool is not recommended for dry machining of superalloys (Devillez et al. 2007, Cantero et al. 2013). Hence, manufacturing sectors are inclined towards application of harder tool material, especially, for high speed machining of superalloys. Harder tool materials include cermet, ceramic, Polycrystalline Cubic Boron Nitride (PCBN), and diamond.