Identification and effect of chip shear band on chatter vibration in the turning of Nickel Alloy 718

Abstract

Shear bands occur during the machining of high-strength materials such as Nickel and Titanium alloys. Shear bands lead to segmented chips, which in turn causes fluctuation in the force that excites vibrations. In some cutting conditions, it is difficult to distinguish regenerative chatter from the shear bands because both originate from chip thickness oscillations. This paper presents the relationship between the chip segmentation and regenerative chatter in turning Alloy 718. The tests are conducted with tool holders having different flexibilities, hence natural frequencies. The chatter stability diagrams with process damping effect are predicted first, followed by experiments in stable and unstable zones. It is observed that, at low-speed machining of Alloy 718, the chip segmentation is caused by chatter. At high speeds, thermo-mechanical induced adiabatic shear dominates the chip segmentation but may become coupled with the relative vibrations between the tool and workpiece. While chatter occurs close to a dominant natural frequency of the structure, shear banding frequency is dependent on the cutting speed and chip thickness and may occur at a distinct frequency.