International Journal of Machine Tools & Manufacture 44 (2004) 879–887 www.elsevier.com/locate/ijmactool Residual stress and surface roughness when facing age hardened Inconel 718 with CBN and ceramic cutting tools R.M. Arunachalam a , M.A. Mannan a,à , A.C. Spowage b a Mechanical Engineering Department, National University of Singapore, Block EA-07-08, 10 Kent Ridge Crescent, Singapore 119260, Singapore b Precision Measurement Group, Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore Received 21 October 2003; received in revised form 16 February 2004; accepted 24 February 2004 Abstract The demand for increasing productivity when machining heat resistant super alloys has resulted in the use of advanced cutting tools such as ceramics and cubic boron nitride (CBN). However, the effects of these tools on the surface integrity, especially the residual stresses created, in the high speed facing operation of Inconel 718 has not been dealt with. In this paper, the residual stresses and the surface roughness when facing age hardened Inconel 718 using CBN and mixed ceramic cutting tools at their respective optimum performance based on productivity has been investigated. The residual stress and surface finish generated during facing with CBN cutting tools have been investigated as a function of speed, depth of cut, coolant, tool geometry and nat- ure of the tool coating. In addition, mixed ceramic cutting tools have been investigated for comparison. The results show that mixed ceramic cutting tools induce tensile residual stresses with a much higher magnitude than CBN cutting tools. The residual stresses and the surface roughness generated by CBN cutting tools are more sensitive to cutting speeds than depth of cut. The use of coolant results in either compressive residual stresses or lowers the magnitude of the tensile residual stresses, whereas dry cut- ting always resulted in tensile residual stresses. From this investigation, it is suggested that round CBN cutting tools should be used at slow cutting speeds (150 m/min) and small depths of cut (0.05 mm) and with the use of coolant to achieve compressive or minimal tensile residual stresses and good surface finish. # 2004 Published by Elsevier Ltd. Keywords: Inconel 718; Residual stresses; Surface roughness; Facing; CBN; Ceramic tools 1. Introduction Among the nickel based heat resistant super alloys (HRSA), Inconel 718 is the most extensively used alloy [1], principally because it maintains excellent mechan- ical properties and is corrosion resistant over a wide temperature range (À250 to 705 v C) [2]. Consequently, Inconel 718 finds a wide range of applications in the manufacture of components for aero engines such as discs, casing, engine mounts and so on. The major problem in manufacturing these components is associa- ted with the machining processes, due to the alloys’ otherwise desirable properties such as high strength retention even at elevated temperatures, work harden- ing nature and low thermal conductivity in addition to its tendency to adhere to the cutting tools. These aspects lead to high temperature at the cutting zone and so restrict the cutting speed of carbide cutting tools to a maximum of 30 m/min. The introduction of new coated carbide tools has increased this range to 100 m/min, but still the time required to machine this alloy is considerably higher when compared to many other commonly used materials such as steels, thus resulting in higher costs. High speed machining (HSM) using ceramic or cubic boron nitride (CBN) tools is one possible alternative for improving productivity. HSM provides added advantages of workpiece softening and chip control [3]. Ceramic and CBN tools have superior hot hardness and so could be used at speeds an order of magnitude higher than the coated carbide cutting tools. To date, there has been a considerable amount of research published on the performance of these tools in the à Corresponding author. Tel.: +65-6874-6889; fax: +65-6779-1459. E-mail address: mpemma@nus.edu.sg (M.A. Mannan). 0890-6955/$ - see front matter # 2004 Published by Elsevier Ltd. doi:10.1016/j.ijmachtools.2004.02.016