| Author: | Zhang, Canbin |
| Title: | Theoretical and experimental investigation of high frequency ultrasonic vibration-assisted machining of three dimensional microstructured surfaces |
| Advisors: | Cheung, C. F. Benny (ISE) |
| Degree: | Ph.D. |
| Year: | 2025 |
| Department: | Department of Industrial and Systems Engineering |
| Pages: | 1 volume (various pagings) : color illustrations |
| Language: | English |
| Abstract: | Ultra-precision machining of complex microstructured surfaces is receiving more and more interest in both academic research and industrial manufacturing. However, conventional ultra-precision cutting (CC) of difficult-to-machine materials with good surface quality and accuracy is not amenable due to serious surface defects and chemical tool wear, which restricts the widespread application of microstructured functional surfaces. To address the problems faced in CC of difficult-to-machine materials, high-frequency ultrasonic vibration-assisted cutting (HFUVAC) is used due to its advantages in terms of improved material machinability, higher geometry precision, better surface quality, and reduced tool wear according to the extensive research on the experimental phenomena. However, these studies lack a comprehensive investigation of the material removal mechanisms of HFUVAC, to explain the chip removal mechanism and the surface/subsurface formation mechanism of the cutting process. On the other hand, in slow-slide-servo diamond turning of large-scale complex 3D microstructures, the spiral tool path generation strategy could cause intolerable linear error and non-uniform surface quality. Sharp edge geometry is prone to induce tool vibration due to the quick acceleration of machine slides. In this regard, this study aimed to carry out a theoretical and experimental investigation of the material removal and surface generation mechanisms for the fabrication of 3D microstructured surfaces using HFUVAC of ferrous materials. The effects of the high-frequency vibration on chip formation, surface integrity, and microstructure evolution were analyzed by using various characterization methods at multiple scales. In addition, a novel tool path generation strategy was developed to reduce the linear error and stabilize the slide acceleration in order to fabricate 3D microstructures with high form accuracy and surface quality. This thesis consists of three parts. In the first part, HFUVAS with a smoothed tool path was developed to reduce the linear error and stabilize the slide acceleration in the fabrication of large-scale optical 3D microstructured surfaces on steel workpieces. One pass with a round tool nose was used to cut a spheric lenslet, while multiple passes to envelop the workpiece contour were used to cut aspheric or freeform lenslets. Modelling of 3D microstructure, tool path generation, surface generation, and tool parameter selection were comprehensively investigated. The results found that HFUVAS with a smoothed tool path can achieve an extremely small linear error and stable acceleration, thus eliminating tool vibration in the fabrication of 3D microstructures with sharp edges. HFUVAS with one pass is applicable to the highly efficient fabrication of spheric microlens arrays with micro feature size, in which a surface roughness with an arithmetical mean height Sa of less than 10 nm and a form error within 1 μm can be achieved. In contrast, HFUVAS with multiple passes could fabricate aspheric and freeform microlens arrays, achieving a surface roughness Sa of 5 nm or below and a form error of within 0.5 μm. The second part of the thesis focuses on carrying out a comprehensive study of the ultrasonic cutting kinematics, the chip formation, and surface generation process of HFUVAC. First, the ultrasonic cutting kinematics and the surface texture of HFUVAC were analyzed and verified by comparing the theoretical and experimental results. Based on the cutting kinematics, the chip formation and surface generation between CC and HFUVAC were analyzed. Compared with the continuous cutting mode of CC, the incremental cutting mode of HFUVAC achieved superior machinability due to the suppressed material adhesion and friction effect. The machinability improvements of HFUVAC were shown by decreasing cutting force, tool wear reduction, surface defect suppression, and chips undergoing discontinuous quasi-shear extrusion into a continuous multiple shear stable state. Furthermore, the incremental cutting mode of HFUVAC was investigated with various nominal cutting speeds to find the critical cutting speed, and thereby verify the technical advantages with a high vibration frequency. With a small cutting stroke smaller than the critical value in each vibration cycle, the incremental cutting characteristics can be achieved to obtain a defect-free surface. The results not only help to improve the surface quality but also allow the optimization of the ratio of cutting speed to vibration frequency so as to enhance the efficiency of HFUVAC. In the third part, a more in-depth study was carried out on the multi-physics-induced material deformation mechanism of HFUVAC. Firstly, combining cutting experiments with finite element analysis, the reliability of finite element modelling for HFUVAC of 316L stainless steel was validated by comparing the chip morphology. Hence, the multi-physical field distributions of instantaneous tool force, strain, and stress were established and analyzed. After this, the microstructural features of the chip and the machined surface were observed to clarify the multi-physics-induced subsurface microstructure deformation mechanism of CC and HFUVAC. The grain refinement and severe torsional deformation were observed at the chip bottom and machined surface in CC. The elongation deformation was associated with strong mechanical loads and friction effect resulting from the contact interface of the tool surface. The deformation mechanisms of CC occurred via dislocation slip due to the inhomogeneous characteristics of material flow and plastic deformation. In contrast, the chip bottom and machined surface in HFUVAC identified the formation of grain refinement with equiaxed nanocrystals via dislocation intersection through the additional high-frequency ultrasonic impact loads. The local deformation region exhibited ultrasonic impact marks, a large number of mechanical twinning structures, and stacking faults inside the elongated sub-grains due to the limited slipping. These structures could alleviate the severe deformation and stress concentration whilst changing the crystal orientation. The results lay a solid theoretical and experimental basis for better understanding the material deformation and chip/surface generation mechanisms of HFUVAC. This thesis presents an original study of the theoretical and experimental investigation of HFUVAC of 3D microstructured surfaces made of difficult-to-machine materials based on the comprehensive characterization of form accuracy, surface roughness, chip morphology, surface integrity, and subsurface microstructure at multiple scales. The originality and significance of this thesis can be identified as: (i) Developing novel strategies for generating tool paths with a small linear error and reliable dynamic response in UVAS of microstructures with sharp edges; (ii) Exploring machinability improvement of incremental cutting of HFUVAC compared with CC based on the cutting kinematics, tool force, tool wear, chip morphology, and surface integrity; (iii) Comparing the multi-physics fields distribution between HFUVAC and CC and clarifying the subsurface microstructure deformation behaviours; (iv) Revealing the material removal and surface generation mechanisms in HFUVAC of ferrous materials by analyzing the chip/surface formation and the subsurface microstructure evolution. The results of this study focus on the material removal and surface generation mechanisms of HFUVAC, which will contribute significantly to a better scientific understanding of this machining technique and an optimized selection of machining parameters for improving the machining efficiency. Moreover, the tool path generation strategies in UVAS help to fabricate 3D microstructured moulds with high machining accuracy and good surface finish to meet the optical application requirements. |
| Rights: | All rights reserved |
| Access: | open access |
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