On Shear Instability in Machining a Nickel-Iron Base Superalloy

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Abstract
The deformation in chips produced in machining a nickel-iron base superalloy (Inconel 718), at various speeds up to 213.5 m/min [700 surface feet per minute (SFPM)] has been investigated. In addition to slip, considerable twinning in the chips is observed at all speeds. Up to a cutting speed of about 30.5 m/min (100 SFPM), the chips formed are essentially continuous and ribbon-like, although deformation in the chip is inhomogeneous. At cutting speeds above 61 m/min (200 SFPM), shear-localized chips form. The longitudinal midsections of the chips show gross inhomogeneous deformation with shear localization between any two segments, and relatively low deformation within any individual segment. With an increase in speed the extent of contact between segments decreases rapidly, until a speed is reached where the individual segments become completely detached. The speed at which this occurs for other difficult-to-machine materials, such as AISI 4340, was found in an earlier study, to depend upon the metallurgical condition of the material and its hardness. Based on this study, the mechanism of chip formation when machining Inconel 718 is very similar to that reported earlier for machining both titanium alloys and hardened AISI 4340 steels at higher speeds. While the hcp crystal structure of titanium alloys in addition to titanium’s poor thermal properties (kρc) is believed to be partly responsible for the intense shear localization in that material, results with Inconel 718 (fcc) and AISI 4340 steel (bcc) indicate that the effect of structure on shear localization is not yet well understood.

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