Surface integrity and tool life when turning Inconel 718 using ultra-high pressure and flood coolant systems

The use of high-pressure cutting fluids has been shown to increase tool life or enable higher cutting speeds to be used when machining advanced aerospace materials such as Inconel 718. Cutting fluids act to provide cooling and to reduce friction between the tool and the chip. When supplied at high pressure, greater cooling and penetration of the fluid into the interface between the tool and the chip occur, when compared to conventional flood supply. Both these factors have the potential to alter the resulting surface integrity of the machined component. Following a brief review of the general machinability of Inconel 718, a series of experiments examining the effects of varying the cutting fluid supply pressure and direction of the jet are detailed when finish turning. Surprisingly, no increase in tool life was observed when using up to 450 bar pressure. When the high-pressure jet was directed at the flank face, reduced flank wear occurred but the level of notching was not affected, resulting in a similar life. Effective chip-breaking occurred at 150 bar; below this, continuous tubular-type chips were produced. Cutting fluid pressure and direction had relatively little influence on the level of surface integrity obtained. However, the use of ultra-high-pressure coolant provided a clear reduction in the level of tensile residual stresses developed in the near-surface layer. The lowest level of residual stress was induced with 450 bar coolant supplied to the flank face. Flood coolant induced a higher tensile stress in the near-surface layer and a larger compressive stress zone beneath this layer.