Investigation on chip deformation behaviors of Zr-based bulk metallic glass during machining

Machining is an essential process for manufacturing high precision bulk metallic glass (BMG) components. It requires a fundamental understanding of the BMG chip deformation behaviors and their underlying mechanisms. In this study, chip deformation in orthogonal cutting of zirconium (Zr)-based BMG was investigated. A series of cuts was obtained to analyze morphological characteristics of chip and its related cyclic force. Further, the shear yielding stress, temperature, and strain rate during cutting were evaluated. By analyzing the separation of chip-workpiece and the fracture of chip, the formation mechanism for BMG chip was revealed. Based on the improved Atkins' model, the energy consumption in chip formation was calculated, and its relations to chip morphology were discussed. The results showed that BMG cutting produced serrated chip. Individual chip segment deformation involved the formation of the primary shear zone (PSZ) and the secondary shear bands (SSBs). The fracture of the PSZ was found to be responsible for chip serration, and the SSBs were involved in the enhancement of the material plasticity. During low-speed cutting, the PSZ experienced high temperature (> 995 K) and high strain rate (> 2400 s(-1)). An increase in cutting speed or depth of cut led to the increase in the temperature and strain rate; however, reduced the shear yielding stress. The serrated chip formation is attributed to the fact that two cracks originated from the chip root and the free surface of the chip, which propagated face to face along the PSZ, eventually leading to the fracture of the PSZ. The generated serration energy accounted for the maximum cutting power during chip formation, while the elastic deformation energy within the PSZ accounted for the least. Increasing depth of cut led to an increase in generated serration energy. Increasing cutting speed resulted in significant increase in friction work at tool-chip contact interface, exacerbating chip material deformation.