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

AbstractWhite 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 20μm. Various plunge milling cutting strategies followed, with radius removal ranging from 25μm to 250μm 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 50μm and 125μm removes all anomalies from the roughing process, leaving behind a negligible amount of material drag (3–4μm). X-ray diffraction demonstrates significant tensile residual stresses (1000–2000MPa) 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 −500MPa in both the axial and hoop directions; in both cases the depth of the surface stresses extended to around 125μm from the drilled surface. It was also found that a depth of cut of 25μm was not sufficient to recover the abused surface; this was due to intense material drag accompanied by surface cracking (i.e. 2μm 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