Theoretical and experimental analysis of machinability and surface integrity of bulk metallic glass in ultra-precision diamond cutting

Abstract

Single point diamond turning (SPDT) is a cutting technique which can produce optical-quality components with submicron form accuracy and surface roughness within a few tens of nanometers with the use of mono-crystal diamond tools. The mechanism in ultra-precision cutting is different from conventional cutting as the depth of cut in ultra-precision cutting lies in the sub-micrometric to a few tens of micrometric region, which is smaller than the average grain diameter of the work materials. It is 100 times smaller than the depth of cut used in conventional cutting, so the material behavior plays an important role on the dimensional accuracy and stability of the machined surface. Most previous research on the effect of metallurgical properties on SPDT was mainly focused on crystalline materials, and little attention has been paid to the cutting mechanics and surface generation of diamond turned amorphous metals. Therefore, a systematic study of the machinability and surface integrity in ultra-precision diamond cutting of bulk metallic glasses (BMGs) was been conducted though experimental and theoretical analysis. The machinability of a material is well-known for being affected by the cutting conditions, cutting tools and material properties. The main contributions of the theoretical and experimental studies described in this report are shown below: (1) The cutting characteristics of BMGs in single point diamond cutting were studied by experimental investigation. The obtained results showed that the machined surface roughness is sensitive to the cutting speed and depth of cut. A finer surface can be obtained by employing a small depth of cut and slow cutting speed in the diamond cutting process. The built-up material formed on the tool edge was observed after diamond cutting BMG, and continuous serrated chips were formed. In contrast with conventional machining of BMG, as reported in the literature, the amorphous microstructure remained unchanged after the diamond turning process. (2) The shear band morphology has been investigated under various cutting speeds for exploring the effects of cutting speed on the surface generation in the micro-cutting process. This study investigates the formation of multiple shear bands in the micro-cutting of zirconium-based bulk metallic glass in where classic models have not been studied. A series of slip-steps were observed in various cutting directions which were significantly affected by cutting speed. Electron microscopy studies further confirmed the mechanism of the formation of nanocrystals with the formation of shear bands within the primary deformation zone (PDZ) in the micro-cutting process under various cutting speeds. (3) Experimental and theoretical investigations into chip formation in diamond cutting of BMG were successfully conducted. The generation of twinned-serrated chips (TSCs) in ultra-precision micro-cutting (UPMC) of BMG was studied. The intrinsic cause was due to BMG exhibiting a strongly adiabatic effect, which was confirmed by a proposed finite element model. A series of UPMC tests have been carried out to verify the results of simulation, revealing the effect of rake angle on the BMG cutting process. The simulation and experimental results of chip formation proved that the adiabatic effect is the key factor leading to serrated chips in the UPMC of BMG. In addition, a comparison of the simulation and experimental results under different tool rake angles showed that the serrated chips and cutting force are sensitive to the rake angle of the tool. The originality and significance of this study can be summarized as (i) providing a comprehensive understanding of the cutting characteristic in SPDTof BMG; (ii) the mechanism of multiple shear bands formation and propagation in SPDT of BMG is revealed by the experimental analysis; (iii) finite element analysis (FEA) modelling of the chip formation in SPDT of BMG is realized for the first time, revealing the mechanism of generating twinned-serrated chips in SPDT of BMG.