Study on Micro-Drilling Characteristics of a Ni Based Superalloy

Abstract

Accomplishment of good dimensional accuracy in micro drilling of Ni-based superalloys has always been a challenge. Hence, ample research, especially on the micro machinability of the superalloy, is required for the low cost, large scale utilisation of the material in different industrial fields. At the very outset, the FEM simulation of micro-drilling of Incoloy 825 using a 0.4 mm drill made up of cemented carbide (WC-Co) was carried out. The aim of this initial phase of work was to explore fundamental insights into various micro-drilling characteristics of the same workpiece material. The effect of feed was simulated on various cutting forces, dynamic fluctuations and specific cutting energy under dry environment. Thrust force and radial forces along with their dynamic components obtained during experimentation were in close agreement with the results of simulation with a deviation in the range of 7 to 8 %. Moreover, efforts were also made to experimentally study the various features of the micro holes including oversize error and circumferential deformation layer under the same condition. A comparative study of micro-drilling of Incoloy 825 with pilot and without prior pilot under minimum quantity lubrication (MQL) condition was carried out to understand the effect of pilot hole. The next phase of study sought to explore the mechanism of physical vapour deposition (PVD)-based TiAlN coating in micro-drilling of Ni-based superalloy. Results indicated that increase in edge radius due to coating hindered the performance of coated micro-drill while machining under very low feed (1 μm/rev). This was attributed to ‗size effect‘. However, elevation of feed, cutting speed and also machining duration (number of holes drilled) was found to be effective for TiAlN coated micro-drill. Furthermore, a comparative study of different cooling techniques was undertaken to investigate their influences on thrust force, tool wear, oversize error and circumferential deformation layer under constant cutting parameters (spindle speed of 20000 rpm and feed of 2.5 μm/rev). The experimental results demonstrated remarkable improvement in micro-machining characteristics under MQL which caused significant reduction in thrust force of around 8 % and average flank wear of 60 % compared to flood cooling. Therefore, MQL may successfully replace conventional flood cooling process during micro-drilling difficult-to-cut materials like Ni-based superalloys. The final phase of experiment aimed at investigating the effects of CrAlN and TiAlN coatings deposited using high power impulse magnetron sputtering (HiPIMS). Influence of feed which plays a major role on size effect was studied for the uncoated and coated micro-drills in terms of thrust force along with its bandwidth, oversize error and average flank wear. Particular emphasis was placed on the exploration of the mechanism of wear of the coated micro-drills in comparison with their uncoated counterpart. HiPIMS coated micro-drills were successful in mitigating thrust force, bandwidth and rate of progression of wear. However, TiAlN consistently outperformed the CrAlN coating in micro drilling of Incoloy 825 owing to the superior hardness, coating adhesion and chemical inertness of TiAlN towards Ni based superalloys.