An evaluation of the evolution of workpiece surface integrity in hole making operations for a nickel-based superalloy

White layers and extensive material drag introduced during rough machining are regarded as detrimental to surface integrity. As such a sensible method for determining the amount of material to be removed in a roughing process would be to understand the relationship and interaction between roughing (i.e. drilling) and finishing (i.e. plunge milling) operations. Within this work non-standard cutting parameters were employed during the roughing process to generate a white layer and material drag up to a depth of 20 mu m. Various plunge milling cutting strategies followed, with radius removal ranging from 25 mu m to 250 mu m in order to identify the amount of material removal necessary to eliminate the anomalies previously generated from mistreated surface history. The results show that finishing with a depth of cut between 50 mu m and 125 mu m removes all anomalies from the roughing process, leaving behind a negligible amount of material drag (3-4 mu m). X-ray diffraction demonstrates significant tensile residual stresses (1000-2000 MPa) were generated in the axial and hoop direction by abusive hole drilling while subsequent plunge milling operation leaves compressive surface stresses in the region of -500 MPa in both the axial and hoop directions: in both cases the depth of the surface stresses extended to around 125 mu m from the drilled surface. It was also found that a depth of cut of 25 mu m was not sufficient to recover the abused surface: this was due to intense material drag accompanied by surface cracking (i.e. 2 mu m depth). The research shows that understanding the interaction between successive cutting operations can provide a suitable machining route to fulfil the industrial quality requirements in terms of the machined surface mechanical/metallurgical properties. (C) 2012 Elsevier B.V. All rights reserved.