High frequency cutting force measurement during saw tooth chip formation of Al-7075-T6 for critical flow stress estimation

Abstract

Shear banding is an unstable mode of deformation caused by material beginning to soften or weaken with additional strain, resulting in narrow bands of highly deformed material called shear bands. Shear bands occur in metal cutting when deforming material in the primary shear zone is subject to weakening by ductile damage and/or thermal softening by adiabatic heating. Recent work [1] has shown that the force supported by shear bands induced in hat-shaped test specimens subject to high-speed compression can be measured during the evolution of shear bands. This has given rise to the question whether very high band-width force measurement can be used to quantitatively measure the strain softening of material during high-speed machining. The challenge impeding measurement of cyclic changes in cutting and thrust force during shear banding cycles in orthogonal cutting is the rate at which these shear bands typically occur – from a few kHz to 100 kHz. This is well above the lowest natural frequency of even stiff cutting force dynamometer setups. This paper describes the development and validation of a high bandwidth dynamometer. This is used to measure both the cutting and thrust forces during orthogonal cutting of AA 7075-T6 with unprecedented fidelity. The critical flow stress at which flow softening initiates is calculated from the measured forces and chip morphology.