The sensitivity of Ni-based superalloy to hole making operations: Influence of process parameters on subsurface damage and residual stress

Higher strength high temperature Nickel superalloys are required to increase the operational efficiency of gas turbine aeroengines through higher operating temperatures. This paper reports on research to assess the workpiece surface sensitivity of a new Ni-based superalloy, RR1000, to hole making by drilling (roughing) and plunge milling (finishing). Coarse grain (CG) RR1000 displays a unique combination of mechanical and thermal properties making it even more difficult-to-machine than traditional Ni-based alloys (e.g. Inconel, Waspaloy). Cutting speed, feed rate, cutting edge quality (i.e. flank wear level) and cooling conditions have been varied across a wide range for drilling and plunge milling to produce hole surfaces. For the first time, the sub-surface damage and axial and hoop residual stress distribution have been quantified. The experimental trials have been directed towards an assessment of the proclivity for surface malformations and detrimental residual stress profiles under controlled machining conditions. Material deformation from drilling was found to be most significant in the hoop direction where increasingly severe levels of material drag, cracking and white layering occurred as the material removal rate was increased. Surface hardening occurred around all of the drilled holes. The amplitude and the depth below the free surface of the tensile residual stress increased with the material removal rate most probably due to a rise of temperature at the tool-workpiece interface during drilling. Plunge milling was found to be an effective finishing operation, successfully removing undesirable material properties introduced by abusive drilling processes provided that adequate stock of material was removed from the superficially damaged workpiece. This enabled the generation of essentially damage-free surfaces, as are necessary for safety critical components used in gas turbine aeroengines.