Published papers

Fused silica is an important optical material with important applications, where the surface must be precisely machined without subsurface damage. In this study, multi-cyclic indentations under incremental loads were performed on fused silica using two kinds of indenters to clarify the mechanisms of crack generation and propagation induced by precision grinding. It was found that incremental loading cyclic nanoindentation induced various patterns of subsurface cracking and surface spalling. Four kinds of surface spalling were identified at different locations around an indent, the temporal formation mechanisms of which were clarified by microscopic observation and topographical measurement. Load–displacement curve analysis demonstrated that incremental propagation of lateral cracks during early indentation cycles caused large-scale brittle fractures during later cycles. Compared with a Berkovich indenter, a cube-corner indenter caused more significant brittle fractures and surface spalling. The findings in this study will deepen the understanding of subsurface damaging mechanism of fused silica and other brittle solids caused by cyclic tool-workpiece interactions in grinding and other mechanical machining processes.

Trochoidal milling has been developed to enhance tool life during slot machining. It is characterized by reduced cutting forces, cutting temperature, and tool wear as compared to conventional milling processes. It is effective in machining difficult-to-cut materials, high-speed machining, and groove corners. However, this process has not been deeply investigated enough to discover its advantages and optimize its parameters. A full factorial design of 144 experiments has been applied in this paper to investigate extensively the effects of axial depth of cut, feed rate, and trochoidal step on material removal rate, cutting forces, and specific energy in trochoidal milling. Trochoidal step and axial depth of cut almost have the same contributions on cutting forces by 32% and 31% respectively, followed by feed rate by 25%. Feed rate, trochoidal step, and axial depth of cut influence the material removal rate by 37%, 30%, and by 19% respectively. The contributions of feed rate, trochoidal step, and axial depth of cut on relative specific energy are 57%, 24%, and 8% respectively. The increase of axial depth of cut increases the maximum resultant force till a threshold value, then it stabilizes. This behavior occurred due to the increase of the maximum engagement angle to a certain limit, then it does not increase any more. Both feed rate and trochoidal step linear affect maximum resultant force and material removal rate, while the relationships are non-linear for specific energy. It is recommended to machine slots in full depth with the highest possible trochoidal step and feed rate, considering the increase of tool wear and surface roughness. It is preferred to use a cutting tool of a large helix angle and small diameter to reduce the threshold axial depth of cut. Overall, this study is significant in characterizing, designing, and optimizing of trochoidal milling through experimental work.

Single-crystal calcium fluoride (CaF2) is considered a superior optical material with good transmittance in a wide transmitted wavelength region. However, the production of high surface quality freeform surfaces on CaF2 is difficult owing to its soft, brittle, and anisotropic properties. This study attempted to fabricate spherical and hexagonal microlens arrays on CaF2 using diamond turning with a slow tool servo. In addition, the machining characteristics of CaF2 were systematically investigated. It was found that microcracks tended to be generated in the cut-in and cut-out regions of the lenses. When cutting along the [− 211] and [11 − 2] directions, the machined surfaces were very prone to cracking. When reducing the maximum undeformed chip thickness to less than approximately 60 nm, continuous ribbon-like chips were formed throughout the cutting process, and all lenses were machined in ductile mode without surface cracking. In addition, a spherical microlens array was successfully fabricated in ductile mode with a surface roughness of approximately 2 nm Sa. Consequently, a hexagonal microlens array with sharp edges and crack-free surfaces was achieved using the proposed depth-offsetting segment cutting method. The surface roughness was approximately 4.1 nm Sa with a form error of approximately 147 nm P-V (peak to valley). Tool wear was insignificant until a machining distance of approximately 50 m; however, with further increase in the machining distance, crater wear and microchipping were observed. This study demonstrated the feasibility of fabricating high-quality freeform microstructured surfaces on CaF2 and other soft-brittle materials via diamond turning.

Due to its exceptional performance, the Ti6Al4V alloy finds extensive use in aviation, biomedical, and petrochemical sectors. Nonetheless, its limited machinability and subpar wear resistance have hindered its widespread application. Therefore, this study underlooks a novel approach by employing nitrogen (N2) gas–assisted electrical discharge machining (NA-EDM) to machine Ti6Al4V. In this context, N2 gas serves a dual purpose. On the one hand, the flow of N2 has a beneficial impact on enhancing the surface properties of the workpiece. On the other hand, N2 reacts with titanium (Ti) to create titanium nitride (TiN), enhancing the component’s performance. Also, this study focuses on investigating how peak current, pulse-on time, electrode rotation speed, and gas pressure influence various aspects, including surface morphology, surface roughness, the microstructure of the recast layer, the microhardness of the recast layer, the thickness of the recast layer, and material removal rate (MRR). Experimental findings show that the NA-EDM process results in a smoother surface morphology, a uniform recast layer, and a reduced MRR. The microhardness in NA-EDM exceeds that of EDM by 42.1%, attributed to the formation of TiN and martensite. Ra is decreased by 21.2% compared to EDM. This study offers novel insights into machining Ti6Al4V alloy, opening up possibilities for broader-scale applications.

Featured Application: Functional surfaces of zirconia with antibacterial properties. Femtosecond pulsed laser irradiation was performed to investigate the feasibility and fundamental characteristics of embedding silver nanoparticles onto zirconia ceramic surfaces. By irradiating laser, nanopores were fabricated on the surface of the yttria-stabilized zirconia (YSZ) substrate, and silver nanoparticles were infiltrated and immobilized into the pores using a commercial nano-silver dispersion solution. Numerous nanopores embedded with silver nanoparticles were successfully obtained on the YSZ surface while keeping the grains’ shapes unchanged by controlling laser parameters. Optimizing laser fluence and scanning speed near the ablation threshold made it possible to remove only the excess dispersant that remained on the surface while keeping silver in the pores and without causing machining of the surface of the YSZ substrate. In addition, about 60% embedding in the nanopores was achieved. It was found that the shorter pulse width was suitable to avoid evaporating both dispersant and silver. Cross-sectional observation revealed that the silver nanoparticles were agglomerated to form clumps and were embedded without a gap at the bottom of the pores at a depth of about 600 nm. After laser irradiation, no significant laser-induced phase change was observed in the YSZ substrate, indicating that there was no in-process thermal damage to the bulk. These findings demonstrated the possibility of adding a metal nanoparticle to the zirconia surface by using only a laser process without damaging the properties of the base material during the process. New applications of zirconia, such as the generation of functional surfaces with antibacterial properties, are expected.

Reaction-bonded silicon carbide (RB-SiC) has gained significant attention owing to its exceptional physical and chemical properties. Improving the photovoltaic efficiency and mechanical properties of RB-SiC is conducive to further enhancing its applicability. Laser gas nitriding is an effective strategy to simultaneously reduce the surface reflectivity and enhance the surface hardness of materials. Accordingly, the evolution of microstructure, mechanical and optical properties of RB-SiC composite ablated by nanosecond pulsed laser in nitrogen atmosphere was comprehensively investigated. After laser gas nitriding, the surface hardness of RB-SiC composite was increased by 10.6–44.6 %. The chemical composition analysis indicated that Si-based nitride was formed on the laser-ablated surface, which was mainly responsible for the hardness enhancement. Additionally, the reflectivity of the laser-ablated surface was reduced by up to 95.2 % compared to the original RB-SiC composite. This study provides a straightforward and effective method to simultaneously improve the mechanical and optical properties of RB-SiC composites.

Multifunctional reaction-bonded silicon carbide (RB-SiC) surfaces with tailored wetting behaviors have garnered notable interest due to their potential applications across various fields. However, the inherent hard-brittle nature of RB-SiC ceramics poses substantial challenges for the facile and controllable preparation of homogeneous and heterogeneous wetting surfaces via conventional techniques. Herein, taking advantage of the fact that laser irradiation of RB-SiC can induce the formation of SiO2 phase and that SiO2 with high surface energy can inhibit the adsorption of organic matter, the surface wetting characteristics of RB-SiC ceramics are flexibly tuned through a hybrid processing technique combining laser irradiation and annealing treatment. The SiO2 content of the laser-irradiated surface can be regulated by varying the laser processing parameters. In the subsequent annealing process, the laser-irradiated surface with low SiO2 content achieves a wettability transition from superhydrophilicity to superhydrophobicity, whereas the laser-irradiated surface with high SiO2 content remains superhydrophilic. Inspired by annealing-induced significant difference in wettability, gradient wetting and extreme wettability contrast surfaces are prepared, enabling the directional transport of droplets. The homogeneous superhydrophobic surfaces produced at different annealing temperatures exhibit distinct anti-icing capabilities and adhesion properties. Importantly, the prepared superhydrophobic surfaces with varying adhesion forces can achieve no-loss droplet transport. This study provides a straightforward, high-efficient, and low-cost method for fabricating RB-SiC surfaces with diverse wetting behaviors, thereby promising to expand their utility spectrum and practical applicability.

Preparing amorphous pure metals has long been significantly challenging because of their stringent transformation requirements. Here, we report a surprising discovery: the preparation of a layer of amorphous aluminium by ultraprecision cutting a polycrystalline bulk material. The findings indicated that under specific cutting conditions, polycrystalline aluminium was transformed into a completely amorphous phase on the surface layer. Additionally, the material matrix exhibited shear bands, including an amorphous phase, a 9 R phase, and high-density SFs. The modification resulted in a significant increase in surface micro-hardness, with the average value increasing from 0.93 ± 0.13 GPa to 2.21 ± 0.92 GPa. The amorphization in the ultraprecision cutting process is attributed to the high shear strain and strain rate, as confirmed by molecular dynamics simulation and theoretical calculation. This study provides a novel approach for preparing an amorphous layer on a crystalline aluminium bulk material to enhance its surface properties.

A lens array is often used for optical components of sensing devices, requiring high surface quality and form accuracy. Fast tool servo (FTS)-based diamond turning is one of the technologies for manufacturing complicated shapes, such as freeform optics, structured surfaces, and microlens arrays, with high machining efficiency. In this study, lens array machining was performed on copper using an FTS on a diamond turning machine. For evaluating the lens array surface topography, the focus was on surface waviness formation. As a dominating factor of surface waviness, the system dynamics behavior was investigated by capturing and analyzing the position signal. It was found that a specific waviness pattern could be formed on the surface due to the servo response. By considering the dynamics of the FTS system from the captured signals, the FTS system behavior was identified, and optimal machining parameters for the lens array were proposed. A machining test under the optimized cutting conditions reduced the average Sdq used to quantify the waviness amount from 93 to 50 µrad and the standard deviation from 33 to 3 µrad, which greatly improved the consistency in accuracy for all lens arrays. This study will contribute to the appropriate utilization of FTS systems in the ultraprecision machining of various advanced optics, such as microlens arrays.

Zr-4 alloy, extensively employed as cladding in nuclear reactor fuel rods, stands as the foremost line of defense for reactor safety. Given its susceptibility to defects such as micro-pores during casting and its long-term exposure to extreme conditions during practical service processes, enhancing the surface and subsurface properties of Zr-4 alloy is essential. Herein, a nanosecond pulsed laser is used to irradiate the Zr-4 alloy in a nitrogen environment to investigate the evolution of its surface and subsurface properties under the combined effects of laser heat treatment and laser gas nitriding. The experimental results reveal a remarkable augmentation in surface hardness and scratch resistance of the Zr-4 alloy after laser irradiation. Notably, the hardness of the laser-irradiated surface obtained with a laser power of 23.0 W and a scanning speed of 150 mm/s reaches 19.10 GPa, which is more than 7 times higher than that of the untreated surface (2.37 GPa). These improvements are attributed to the synergistic effects of the introduction of hard ZrN phase, the reduction of subsurface porosity, and grain refinement. This study demonstrates a promising approach for significantly improving the mechanical properties of Zr-4 alloy, holding considerable practical significance for its applications in the nuclear energy industry and other relevant fields.

Studying single particle impact is crucial to understanding the erosion mechanism in abrasive waterjet machining (AWJM). In this paper, the finite element method (FEM) and smoothed particle hydrodynamics (SPH) were combined to investigate the mechanism of single particle impact under the waterjet influence. The particle (garnet) and workpiece (Al 7075-T6) were represented by FEM, while the waterjet was discretized using SPH. Linear elasticity and the Johnson-Cook (JC) model were selected to model the abrasive and workpiece materials, respectively. The simulation was conducted in ABAQUS where the effects of particle shape, position, and impact angle on material removal were investigated. The results of the developed model revealed that the erosion mechanism and mechanics significantly differed when considering the waterjet effect. This was primarily attributed to the dynamic phenomena resulting from the waterjet impact, including stagnation zone and drag forces. It was found that the impact angle corresponding to maximum erosion depended on the particle shape and radial position within the jet stream. The presented model has proved its capability since it offers a novel perspective on the single particle impact mechanism by incorporating the realistic dynamic phenomena accompanying the impact in AWJM.

Conventional polishing methods face significant challenges for achieving excellent performance on complex surfaces. The magnetorheological shear thickening polishing (MRSTP) method, with its dual-stimulus response of shear thickening and magnetization enhancement, offers an effective solution for polishing complex surfaces. However, existing models fail to elucidate the cross-scale material removal mechanisms in MRSTP owing to the coupling of magnetic and flow fields. In this study, a comprehensive model was proposed to address the the challenge of predicting the cross-scale material removal rate (MRR) in MRSTP for complex surfaces. Analytical and finite difference methods were employed to solve the pressure distribution during the MRSTP process using magnetohydrodynamics. By incorporating the pressure distribution, an MRR predictive model was developed for arbitrary points on the workpiece surface based on the indentation theory for active abrasive grains. The material removal mechanism was explored by considering elastic and plastic deformation under fluid pressure. The experimental validation showed a relative error of 14.9 % between the theoretical and experimental MRR. Experiments on aero-engine blade tenons demonstrated that the established MRR model is well suited for application to complex surfaces. This study ultimately reveals the material removal mechanism of MRSTP with coupled magnetic and flow field, providing a new foundation for predicting MRR on complex surfaces.

The demand for wearable device applications has continuously grown in recent years, especially with the significant rise of augmented and virtual reality technologies. Freeform optics plays a crucial role in these devices by enhancing optical performance, shortening the light path, and reducing the weight, all while allowing for smaller, lighter systems with higher efficiency. The independent fast tool servo (FTS)-based diamond-turning method stands out as a highly effective technique for fabricating freeform shapes with high accuracy and productivity. However, microsecond-order time delays occur within the system, significantly impacting form accuracy as machining speeds increase. This study explores the sources of form errors in freeform surface fabrication associated with the FTS diamond-turning process, with particular attention to the effects of clocking angle errors caused by the time delay. These errors were found to greatly affect form accuracy, especially at higher machining speeds. The FTS position data were analyzed, and time delays under various operational conditions due to servo control were confirmed. To precisely identify the extent of the time delay, a cylindrical surface was machined under high-speed conditions, and the clocking angle error was measured using a non-contact chromatic probe. Results showed that time delays originating from the machine platform had a significant effect on form accuracy. By accurately identifying and compensating for these time delays, the clocking angle error was eliminated. To validate the effectiveness of the time-delay compensation strategy, a cylindrical freeform surface was machined after the compensation, and the clocking angle error was minimized down to 0.00014° evaluated by on-machine measurement. The form accuracy of the freeform machining result after compensation was achieved at 0.85 μm PV. This study establishes a methodology for identifying and compensating for time delays in an independent FTS system, contributing to improved form accuracy in freeform optics fabrication.

Fused silica is an important material for applications requiring high temperature resistance, low thermal expansion coefficient, and excellent optical properties. The machining of micro-cavities on fused silica surfaces is of particular interest for micro-fluidic manipulation and miniaturization of high-quality optical waveguides, etc., but it still remains technically challenging for traditional manufacturing techniques. In the present study, machining of square cornered semienclosed micro-cavities on fused silica surfaces by femtosecond laser has been investigated experimentally. The effects of laser machining conditions including laser power, laser scanning speed, laser incidence angle, and laser-off delay time on the sidewall slope and bottom surface roughness of the micro-cavities were comprehensively investigated. The results indicated that laser power played an important role in determining the sidewall slope of the micro-cavity, while the laser scanning speed had a significant influence on the bottom surface roughness and subsurface damage. Furthermore, the sidewall slope of the micro-cavity was linearly increased as the laser incidence angle increases. By using a laser incidence angle of 10 deg and a laser-off delay time of 280 ms, a micro-cavity with sidewall slopes close to right angles (90 deg) was fabricated. This study demonstrates that femtosecond laser machining is an effective method for fabricating sharp cornered micro-cavities in fused silica, and the appropriate selection of laser machining conditions based on practical application scenarios is important.

Highlights A micro-amplitude vibration-assisted scratching method was proposed. One step and controllable fabrication of microscale V-groove and nanoscale ripples was achieved. The effects of processing parameters on the resultant structures were explored. Relationships between processing parameters and dimensions of the resultant structures were established. Complex patterns were fabricated on flat and curved surfaces, achieving various structural colors.

The application of titanium alloys still faces some challenges such as relatively low hardness and insufficient wear resistance. Nanosecond pulsed laser irradiation in a nitrogen atmosphere has proven to be effective for fabricating micro/nano structures on titanium alloys, enhancing surface hardness and wear resistance. However, micro/nano structures produced by line-by-line laser scanning often exhibit directional differences and surface defects such as re-deposited particles and pores. Herein, vertically crossed laser scanning in a nitrogen atmosphere was employed to address the directional difference, enabling the simultaneous fabrication of microcone arrays and nitriding layers on Ti6Al4V surface. To eliminate the surface defects on the microcone structure, laser surface finishing via defocused scanning was performed, followed by morphological analysis, bonding force evaluation, and tribological performance testing. The results showed that the area percentage of re-deposited particles on the laser-finished microcone array surface decreased from 24 % to 5 %. The bonding force of the nitriding layer was assessed using linear loading scratch tests, and the highest value of 69 N was obtained. To investigate the wear resistance of the laser-finished microcone array surface, friction and wear tests were performed. From 25 °C to 600 °C, nearly no wear grooves were observed on the tested surfaces, demonstrating excellent high-temperature wear resistance. The wear mechanism was mainly related to the cutting action of the microcone structures on the counterpart ball. This study advances the fabrication of micro/nano structures and simultaneous nitriding on titanium alloys, demonstrating the significant potential for practical applications as surface contact materials.

The present research work amis to develop a controlled vibratory system for improving the performance of slotting process based on the vibration-assisted milling technique. In addition, the establishment of a statistical model for predicting the process efficiency was a key aim target. In the current investigation, an ultrasonic vibratory device was designed to provide vibration oscillations in the cutting feed direction to the 7075 aluminum alloy workpiece side at 34.7 kHz frequency and 10 μm amplitude. A LabView code has been developed to control the input vibration parameters. A full factorial design of experiments with 4 factors and 2 levels was conducted. An in-depth statistical analysis is then implemented to study the effect of machining and vibration parameters on the process performance. Then, a statistical model for predicting the process response in terms of cutting force, surface roughness, and machining accuracy has been established. Some experiments showed a significant reduction in cutting force up to 50% in the feed direction. Additionally, an improvement in workpiece surface roughness was recorded. Regarding the machined surface accuracy, introducing ultrasonic vibrations significantly reduces the produced slot width error compared to Conventional Milling (CM). The developed statistical model shows a very well agreement with the experimental results. Therefore, the developed VAM technique is suitable for manufacturing parts that require high geometrical and dimensional accuracy.

Deep learning has impacted defect prediction in additive manufacturing (AM), which is important to ensure process stability and part quality. However, its success depends on extensive training, requiring large, homogeneous datasets - remaining a challenge for the AM industry, particularly for small- and medium-sized enterprises (SMEs). The unique and varied characteristics of AM parts, along with the limited resources of SMEs, hamper data collection, posing difficulties in the independent training of deep learning models. Addressing these concerns requires enabling knowledge sharing from the similarities in the physics of the AM process and defect formation mechanisms while carefully handling privacy concerns. Federated learning (FL) offers a solution to allow collaborative model training across multiple entities without sharing local data. This article introduces an FL framework to predict section-wise heat emission during laser powder bed fusion (LPBF), a vital process signature. It incorporates a customized long short-term memory (LSTM) model for each client, capturing the dynamic AM process's time-series properties without sharing sensitive information. Three advanced FL algorithms are integrated - federated averaging (FedAvg), FedProx, and FedAvgM - to aggregate model weights rather than raw datasets. Experiments demonstrate that the FL framework ensures convergence and maintains prediction performance comparable to individually trained models. This work demonstrates the potential of FL-enabled AM modeling and prediction where SMEs can improve their product quality without compromising data privacy.

High-entropy alloys (HEAs) are solid-solution alloys composed of multiple elements, exhibiting excellent mechanical properties. The unique plastic deformation mechanism induced by their specific solid solution structures has attracted considerable attention but remains incompletely understood, particularly at the micro-scale. In this study, the surface morphology, chemical composition, and microstructures of CoCrFeNiV HEA before and after laser remelting repair were investigated. Nanoindentation testing was employed to characterize the surface hardness and creep behavior of the repaired surface. The distribution of surface hardness before and after laser remelting, as well as the indentation creep behavior under different loads, were studied. The mechanism of indentation creep on the repaired surface was discussed and analyzed. The effect of microstructures of HEAs, including precipitated phases and sub-grain boundaries, on dislocation-dominated micro-scale plastic deformation was elucidated by the transmission electron microscope (TEM). This study contributes to an in-depth understanding of the creep behavior and micro-scale deformation mechanisms in HEAs.