Theoretical study on brittle-ductile transition behavior in elliptical ultrasonic assisted grinding of hard brittle materials

The elliptical ultrasonic assisted vibration can lead to a higher comprehensive performance with better machined quality and lower grinding force during brittle materials machining. The undeformed chip thickness in grinding can be defined as the distance between two consecutive grinding surfaces formed by the adjacent abrasive grain trajectories. The ductile-brittle transition point of chip formation in brittle materials machining is believed to be represented by the threshold value of undeformed chip thickness (critical undeformed chip thickness). In this paper, an energy based method is proposed to predict the critical undeformed chip thickness in elliptical ultrasonic assisted grinding (EUAG). The effects of vibration parameters on grinding force and specific grinding energy are analyzed in detail. Results show that the axial vibration amplitude leads to a slight reduction in grinding force, whereas the vertical vibration amplitude results in a significant reduction of grinding force. The increase of wheel speed and ultrasonic vibration frequency can reduce the grinding force as well. Nevertheless, the grinding force ratio remains relatively steady fluctuating from 1.37 to 1.56. Additionally, the critical undeformed chip thickness increases with the increasing grinding speed, axial vibration amplitude and ultrasonic vibration frequency, while firstly increases and then decreases with the increasing vertical vibration amplitude. Especially, a ductile-mode grinding in micron-level can be achieved when the axial vibration amplitude is above 3 μm and a totally brittle fracture occurs when the axial vibration does not exist. Thus, a reasonable selection of vibration parameters in EUAG process is required. The above theoretical evaluations coincide well with the variable experimental results.