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UBC Theses and Dissertations

Mechanics of machining zirconium-based bulk metallic glass Maroju, Naresh Kumar


Bulk Metallic Glasses (BMGs) are amorphous metal alloys with applications in biomedicine, electronics, sports equipment, and aerospace. In the machining of BMGs, inhomogeneous deformation and shear localization occur, which are influenced by the temperature and free volume in the deformation zone. Understanding the mechanics of machining BMGs is essential to improve the machining efficiency and surface quality. In this thesis, a physics-based model of chip formation is developed for orthogonal cutting of Zirconium (Zr) - based BMG considering the process kinematics, material constitutive law, heat, and free volume evolution. The model predicts the segmented chip formation, oscillations of stress and temperature in the primary shear zone. The simulation results of chip segmentation are validated from experimental measurements of chip morphology. The model explains the material deformation mechanism in orthogonal cutting of Zr-BMG associated with the amorphous structure of the workpiece material. Vibration-assisted machining has been used to improve the machining efficiency for high-strength metal alloys. This thesis presents a new mechanistic model of elliptical vibration-assisted machining (EVAM), which determines the shear angle with respect to the ratio between the original cutting speed and the amplitude of the horizontal vibration speed. The effects of intermittent tool-workpiece contact, speed ratio, and friction reversal on the chip formation mechanism in EVAM are investigated. A 2-D vibration assistance stage is developed to perform the experimental study of the EVAM process. The chip morphology in EVAM experiments of Zr-BMG is measured to validate the predicted chip formation from the mechanistic model. In the milling process of Zr-BMG, light emission can occur, and result in oxidation and crystallization of the machined surface. The orthogonal cutting mechanics model is extended to oblique cutting, and the predicted temperature is used to determine the amorphous-crystalline transition of Zr-BMG. The predictions are validated by X-ray diffraction examinations on the machined surface. Furthermore, experimental studies are performed to investigate the effect of milling-induced stress on surface microstructure property. The proposed model and the experimental study are used to understand the chip formation mechanism, microstructure evolution, and surface quality in the milling of Zr-BMG.

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