Understanding the abnormal effects of ultrasonic vibration on tool wear and surface generation in Zr-based bulk metallic glass cutting

Bulk metallic glasses (BMGs), which represent an emerging popular class of alloys, are regarded as difficult-to-cut materials. Ultrasonic vibration-assisted cutting (UVAC) has been reported as an effective method to improve the machinability of such materials. In UVAC, it is widely accepted that, the larger the ultrasonic amplitude, the better the processing outcome, i.e., lower cutting force, lower surface roughness and longer tool life. However, this study demonstrates that the cutting performance of BMGs can be enhanced only if the ultrasonic amplitude is within a carefully-determined window. Due to the high hardness, high elastic limit, and inhomogeneous deformation of BMGs, an excessive ultrasonic amplitude would violate the cutting performance of UVAC. On the one hand, the increase of ultrasonic amplitude can increase the cutting-in speed and actual cutting distance of the tool, which, due to the extreme hardness and elastic limit of BMGs, might lead to severe tool wear in the form of abrupt fracture and rapid flank abrasion, respectively. On the other hand, the increase of ultrasonic amplitude can increase the instantaneous maximum cutting speed of the tool, which, due to the inhomogeneous deformation of BMGs, might lead to local melting of the BMG. When the instantaneous cutting speed reaches a critical value, local material melting takes place on the newly generated surface, which will induce the deterioration of surface quality due to the formation of dimples and periodic micro-pools during UVAC. Based on the theoretical and experimental results, the process window of UVAC of BMGs has been identified. A process optimization strategy is proposed by synergistically considering the surface quality, tool life, and machining efficiency. Additional confirmatory experiments reveal that UVAC can effectively reduce the cutting force and surface roughness of BMGs if the process parameters are well-matched within the identified window. • UVAC improves the machinability of BMGs, unless an excessive vibration amplitude. • An analytical model of UVAC tool failure was established and verified by experiments. • The unique properties of BMGs induce the abnormal failure of UVAC tool. • Feathered dimples and micro-pools were found, the mechanism is revealed. • the process window and optimization strategy of UVAC of BMGs were proposed.