Assessment of thread-cutting strategies to enable damage-tolerant surfaces on an advanced Ni-based aerospace superalloy

The global aerospace manufacturing industry is continually developing higher strength superalloys to increase engine efficiency. With this comes the challenge of machining these difficult-to-cut materials at high productivity rates and increased surface accuracy requirements. Despite the technical challenges faced, it seems that the research area of thread cutting in aerospace alloys has generally been neglected. This paper addresses reports on a critical assessment of two thread-cutting methods (i.e. tapping and thread milling) applied to a 'next-generation' high-temperature Ni-based aerospace superalloy. Carrying out tool life tests revealed tapping (of small thread dimensions) to be particularly difficult to perform due to high and continuous friction incurred at the cutting edge-workpiece interface, which resulted in various surface anomalies (i.e. severe plastic deformation, laps). The optimized thread-milling strategies have, however, shown a significant improvement in tool life and surface integrity of the machined component. To support the understanding of the performances for the investigated thread-making methods, this paper discusses the interrelationship between the specific characteristics of thread tapping and milling sensory signals (cutting forces and torques) with further associations on the quality (metallurgical integrity and residual stresses) of threaded surfaces in notoriously difficult-to-cut Ni-based superalloys.