Effect of Pilot Hole on Micro Drilling Characteristics of a Ni-Based Superalloy

Micro drilling on Ni-based superalloy is a challenging process due to the material properties, operating conditions, low thermal conductivity and high quality requirements. Incoloy 825 belongs to the family of Ni-based superalloy and is widely used in containers for acid production, pickling operations, nuclear fuel reprocessing, handling of radioactive wastes, chemical processing, oil and gas recovery. Mechanical micro drilling of these alloys has found application in the manufacture of cast turbine blades. The study focuses on the micro drilling of Incoloy 825 alloy under MQL (minimum quantity lubrication) cutting condition. In order to analyze micro drilling operation, an initial study has been undertaken aiming at investigating the influence of spindle speed (5000, 10000, 15000 and 20000 rpm) and feed (1, 2.5 and 4 μm/rev) on thrust force and torque. The appropriate process parameters for 12 different through holes are used for each twist drill and pilot hole micro-drills. Constant depth of 1.25 mm and drill diameter of 400 μm were considered throughout the experimentation. The effect of cutting parameters on entry and exit diameter of hole, taper angle of hole, circumferential deformation layer and white layer has been analysed. Comparative study has been carried out using twist drill and pilot hole micro-drills. It is concluded that pilot hole micro-drill is preferable for micro drilling and the optimal results in the form of superior dimensional accuracy, minimum circumferential deformation layer, minimum torque and reasonably less value of thrust force can be accomplished under the feed of 4 μm/rev and spindle speed of 20000 rpm.