Experimental investigations on single-lip deep hole drilling of superalloy Inconel 718 with small diameters

Abstract

Recent trends in downsizing, as well as miniaturization of components, increase the importance of deep hole drilling with small diameters in various industrial applications. For the manufacturing of deep holes, many processes are used. In addition to mechanical cutting, processes based on thermal material removal mechanisms like electrical discharge machining (EDM), laser drilling and electron-beam drilling are established. Whereas non-mechanical processes can be used for the machining of extremely hard and high-strength materials, substantial disadvantages are the restrictions in the realizable hole diameter and depth dimensions, the long production times, the requirement for special-purpose machinery, as well as the limited bore hole quality. In the aerospace industry, where nickel-based superalloys are widespread, high requirements lie on the surface integrity due to high stress loads during operation. Hence, the major drawback of these non-mechanical processes is the alteration of the subsurface zone, which could affect the part safety. In contrast, the mechanical machining of nickel-based superalloys is extremely difficult due to material properties such as high strength, high tendency to work hardening and low thermal conductivity which results in increased mechanical tool loads and heavy abrasive and adhesive tool wear. An additional challenge is constituted by the chip removal and the unfavorable ratio of cutting edge rounding and undeformed chip thickness caused by the limitation in the feed rates, if deep hole drilling with small diameters and high-length-to-diameter ratios of difficult-to-cut materials like nickel-based superalloys is required. Up to now, the use of standard tool geometries in small diameter single-lip deep hole drilling of Inconel 718 has not been productive and leads to limited process stability and drilling lengths as well as insufficient bore hole quality. Concerning this matter, this paper presents a process adaption using a tailored cutting edge design to accomplish the predominant challenges. Substantial analysis on gun drilling in Inconel 718 regarding mechanical tool loads, chip formation, the tool wear, as well as bore hole quality will be presented.