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The mechanochemical effect on the microcutting of AA6061 alloy is studied through characterization on the chip. A pronounced reduction of machining forces and chip thickness was observed with mechanochemical effect during microcutting. Furthermore, electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) observations were performed on the chips and shear bands. The result reveals much coarser grains (24.6 μm in size) in the surfactant-affected chip than that in the surfactant-free chip (13.5 μm). Different grain orientations are induced by microcutting. {100} and {110} grain orientations are majority for surfactant-free chip, and {110} and {100} dominate most for all grain orientations for surfactant-affected chip. Additionally, since less localized shear strain and lower temperature are generated inside the shear band with mechanochemical effect, almost no recrystallization phenomena can be observed in this region. TEM analysis shows that fewer subgrains and dislocations could be observed inside the shear band of the surfactant-affected chip in comparison with the surfactant-free chip. Based on the high-resolution transmission electron microscopic (HRTEM) observations, dislocations were observed at the atomic scale. The results show that the main dislocation motion mode in shear bands of surfactant-free and surfactant-affected chips are dislocation climb and dislocation glide, respectively.

Delaying the onset of brittle mode cutting does not only involve the augmentation of slip deformation but could also be tackled from the perspective of suppressing the leading causes for crack formation. This work studies the micro-cutting of calcium fluoride single crystals with diamond turning and orthogonal plunge-cutting. An applied coating shows promise to enhance ductile mode cutting during diamond turning and also alters the crack morphology during cutting in the brittle regime. Conventionally, calcium fluoride exhibits anisotropic pyramidal cracks that were associated with the activation of the { 111 } cleavage plane while the adoption of the coating prior to micro-cutting alters the cracks to become laminar in nature associated with the ( 1 ‾ 1 1 ‾ ) cleavage plane. The resistance to deformation of the coating is proposed to create reaction stress along the shear plane of the workpiece to reduce the effective tensile stresses acting on cleavage planes. Crystal plasticity finite element method modelling demonstrates the relationship between the coating stress distribution and the intensified concentration of shear stresses in the workpiece, which confirms the notion of the reaction stress acting along the shear plane. Moreover, the simulations with the coating present the affected tensile stresses acting on a majority of the { 111 } cleavage planes with the exception of the ( 1 ‾ 1 1 ‾ ) cleavage plane, which supports the crack orientations in brittle mode cutting.

The mechanochemical effect on the microcutting of AA6061 alloy is studied through characterization on the microgroove surface. There is a reduction in cutting and thrust forces with the application of ink during microcutting. Moreover, the microhardness of the ink-affected microgroove is lower than that of the ink-free microgroove. Numerous substructured grains exist in the ink-affected microgroove zone whilst deformed grains dominate in the ink-free microgroove zone produced by microcutting. Furthermore, the mechanochemical effect can facilitate the nucleation of precipitates in the microgroove zone and induce the formation of subgrains with multiple orientations. According to the analysis and calculation, the main texture components of the ink-affected sample are Goss {110} and R {124} , and that of the ink-free sample are Brass {110} , Copper {112} and S {123} . Besides, a clear difference of slip systems is found between the ink-free and ink-affected microgrooves, and the results show that R texture is easier to form on the ink-affected microgroove.

The mechanical properties, physical properties and electrochemical behavior of metal components produced by selective laser melting can be influenced by the relative density and building direction. To this end, the optimization of the building process was conducted by identifying the influence of process parameters on the relative density and determining the ideal combination of parameters using the Box–Behnken design response surface methodology to achieve a relative density of 99.303 pct. With the ideal process parameters, material strength, thermal, and electrochemical performance were evaluated in a series of experiments. Anisotropic characteristics were displayed due to the differences in build-direction, microstructural features, and phase composition. The 0 deg possessed the highest tensile strength measured to be 1263.03 ± 8.71 MPa, while the 45 deg demonstrated the highest ductility with an elongation of 13.21 ± 0.34 pct. Thermal expansion was governed by the heat treatment process, such that anisotropic traits were eliminated after solution treatment. Strip melt tracks on the X–Y plane differed from the strip and arcuate melt tracks observed in the X–Z and Y–Z planes, leading to significant deficiencies in electrochemical reactance with an open circuit potential of − 645.8 mV in comparison to the latter measured at − 397.7 and − 396.7 mV, respectively.

The Rehbinder effect (R-effect) on microcutting has been successful to augment machinability of ductile metals; however, there is a lack in understanding of its influence on texture evolution in the machined surfaces. This study validates the R-effect on AA6061-T6 and characterises the influence of the R-effect on the texture transformation in the machined surface. Microhardness tests reveal that the R-effect produces softer surfaces (87–96 HV) as compared with the surfaces produced conventionally (96–108 HV), which is attributed to the differences in grain types. A large fraction of substructured grains (96.4%) exists in the surfactant-affected subsurface while a higher proportion of deformed grains (42.8%) forms during conventional microcutting. In addition, the main texture components of the conventionally produced surface are Cube, Goss and S textures, which differ from the dominant volumes of Copper and R textures under the R-effect. The differences are results of the stress states induced during microcutting, which is exemplified through numerical simulations that consider the reduction in fracture toughness of the material with the R-effect. The advanced understanding of the R-effect better positions the use of surfactants as a cleaner near-dry metalworking fluid.

Diamond is a highly favourable material for cutting tools, particularly in ultra-precision machining to produce highly accurate freeform surfaces with mirror-like finishing. Unfortunately, diamond cutting tools undergo catastrophic wear when machining transition metals such as iron, cobalt, nickel, etc. The underlying wear mechanism is understood to involve dissociation of carbon atoms from the diamond lattice, i.e. graphitization. Existing approaches of wear reduction aim at reducing chemical reactivity generally through process modifications. In this work, the effectiveness of ion implantation as a tool modification methodology is studied on the gallium ion irradiated diamond tools. Wear occurrences are compared between irradiated diamond and unmodified diamond using thermal analytical techniques and micromachining experiments. Calorimetric tests showed a more than 40% increase in activation energy required for graphitization with a Ga ion dose of 1 × 1013 ions/cm2 at 30 keV. Significant improvements in the wear resistance of an irradiated diamond tool are also observed in micromachining tests with the reduction in workpiece adhesion that indicates potentially lower heat generation at the tool-chip interface for the graphitization process. Ab initio calculations suggest an increased stability against exfoliation with the reduction in surface energy, which influences the surface-to-surface interaction between diamond and catalytic iron.

Positive improvements have been observed during machining brittle materials under high hydrostatic pressure, but the techniques to achieve such desirable effects often utilize complex and expensive equipment or tools. This work presents a cost-efficient method to achieve ductile-mode machining of brittle materials at higher uncut chip thicknesses, by the application of a solidified coating on workpiece surface before a machining process. Orthogonal microcutting experiments were conducted on calcium fluoride single crystals oriented with the (111) plane and an increase in critical uncut chip thickness was observed with the solidified coating. The primary cause has been resolved to be mechanical-related and results in a stabilized microcutting process. Transmission electron microscopy provided evidence of slip deformation occurring in the machined subsurface regions and a layer thickness of subsurface damages reduced by ~ 45% under the influence of the solidified coating. In addition, erratic fluctuations in direction of the resultant machining force were subdued with the applied coating, which is proved to be caused by the compressive stresses induced from the sandwiching of the CaF2 material between the tool and the solidified coating. The proposed technique successfully reduces the cost and pollution in the fabrication process of optical components from the use of coolant in an ultraprecision machining process to the time consumed by eliminating the subsurface damage with abrasive slurries in post-machining polishing.

A mechanochemical effect is investigated to reduce diamond tool wear by means of applying a surfactant to low-carbon magnetic iron during diamond turning. Orthogonal microcutting demonstrates the manifestation of the mechanochemical effect through the reduction of cutting forces by 30%, which supports the notion of lower cutting temperatures for reduced tribo-chemical wear. This is affirmed by the reduction in tool flank wear by up to 56% with the mechanochemical effect during diamond turning. While wear suppression increases by 9.4&ndash, 16.15% with feeds from 5&ndash, 20 &mu, m/rev, it is not proportional to the reduction in cutting forces (31&ndash, 39.8%), which suggests that the reduction in cutting energy does not directly correspond with the reduction in heat energy to sustain tribo-chemical tool wear. The strain localization during chip formation is proposed to serve as a heat source that hinders the wear mitigation efficiency. Finite element simulations demonstrate the heat generation during strain localization under the mechanochemical effect, which counteracts the reduced heat conversion from the plastic deformation and the transfer from tool&ndash, chip contact. Hence, this paper demonstrates the effectiveness of the mechanochemical method and its ability to reduce tool wear, but also establishes its limitations due to its inherent nature for heat generation.

Innovative techniques have been proposed to overcome the challenge of the strong tendency of brittle materials to crack during machining. One of them is the use of an epoxy coating to serve as a crack formation restraint. However, this only serves to achieve ductile-mode grinding along the uncut shoulders. Therefore, this study evaluates the effect of a layer of epoxy resin on the machined surface perpendicular to the micro-cutting direction of a brittle material, single-crystal calcium fluoride. An increase in the ductile–brittle transition was observed in micro-cutting experiments on the (111) plane of calcium fluoride with the use of a 4-μm thick epoxy resin coating, ranging from 167–347 nm to 213–476 nm for the 0° rake angled tool. A similar increase was also observed with the +5° rake angle, ranging from 91–233 nm to 187–310 nm. An energy-based ductile–brittle transition predictive model is introduced, which characterises the anisotropic behaviour of the single crystal and incorporates the additional stress induced during cutting with a coating as a fraction of the coating hardness. The analytical model accurately predicts a consistent range of improvement from 69–325 nm to 257–485 nm for the 0° rake angle and from 62–285 nm to 215–460 nm for the +5° rake angle. The ductile-mode cutting energy increases to preserve its dominance over brittle-mode cutting and delays the onset of brittle-mode activation. The validity of the model extends the understanding of a surface coating as a restraining technology to include the beneficial stress acting in the deformation zone during cutting.

The excellent mechanical properties of maraging steel 3J33C make it desirable for a wide spectrum of high-strength applications, which would also mean that it is difficult to machine. The application of surface-active media onto the pre-machined surface is explored to improve the machinability of this high-strength material. This mechanochemical effect, named after its discoverer as the Rehbinder effect, was implemented using a marker ink to reduce machining forces and deformed chip thicknesses, and significantly improve the machined surface quality. Machined subsurface microstructure characterization reveals the occurrence of grain refinement with evenly distributed orientations and the microstructure texture components when cutting with the mechanochemical influence. This work also demonstrates with an analytical model the effectiveness of the Rehbinder effect when coupled with the tool-edge radius effect. Lastly, numerical simulations are developed for chip evolution and cutting forces in machining a polycrystalline workpiece material with the mechanochemical influence by modifying the fracture energy.

Optical materials, such as calcium fluoride single crystals, are widely used across various industries for their light transmission capabilities. These materials possess excellent mechanical, physical, and chemical resistant properties but also tend to be very brittle, which poses a challenge to the microcutting of complex freeform shapes with optical surface quality. Ultraprecision single-point diamond turning is commonly used in ductile-regime machining of hard and brittle materials, and polishing is employed as a secondary process to achieve optical quality surfaces, which can be time consuming. To improve the machining efficiency of high-strength materials, hot machining techniques have been developed to improve workpiece plasticity and surface integrity after machining. Surface quality and subsurface damage evaluation of the machined material along with finite element analysis allow a deeper understanding of the effectiveness in heat-assisted machining. In this chapter, an introduction to ultraprecision single-point diamond turning and the fundamentals of ductile-regime machining of hard and brittle materials will be discussed, followed by its application in fabrication of calcium fluoride single crystal lenses. Subsequently, the anisotropic characteristics of calcium fluoride single crystal will be investigated through experimentally validated crack formation models and surface generation morphology to gain detailed appreciation of the challenges faced during production of brittle single-crystal materials. To conclude the chapter, the effects of elevated temperatures on the material properties and machinability will be evaluated using experimental and numerical solutions.

Machining processes have made technological leaps in achieving ultraprecision material removal and surface finishing at the submicrometric scale. At this level of precision, size effects have dominated machining investigations while surface effects have often been overlooked. The majority of micromachining research works are ignorant of the potential implications that these phenomena bring as the characteristic length approaches the ultraprecision machining level. In this review, physicochemical and physical surface effects are discussed by examining the theoretical developments and applications of each phenomenon in machining. These effects include the Rehbinder effect, solid coatings, and extrusion-cutting. Substantial mesoscopic analyses have been performed on metals with the Rehbinder effect and extrusion-cutting but the inherently different material deformation characteristics in microcutting invite further investigations. While solid coating effects have been reported at the microscale, its discovery questions the influence of other inevitably formed surface coatings (i.e. oxide layers). To these ends, key areas for future research in the microcutting of engineering metals and brittle materials are proposed in addition to the integration with unavoidable size effects. As machining technology and material characterization techniques progress into the nanometric scale, there is a need to rally efforts towards the embrace of surface phenomena in microcutting.

Additive manufacturing technology provides a feasible solution to directly manufacture optical components with complex functional structure. However, the poor surface quality and low relative density result in the limitation on its rapid application. In order to overcome the above shortcomings, process optimization and ultrasonic elliptical vibration-assisted machining (UEVAM) were used in the fabrication of optical surfaces on selective laser melted (SLM) AlSiMg0.75 alloy. The optimised energy density in the SLM process was identified ranging from 65 to 130 J/mm3 with the highest achievable relative density of 99.6 %. Post-processing heat treatment changed the cellular/dendritic microstructure of as-built samples to an α-Al matrix embedded with Si particles, which reduced the microcutting forces by 27.67 % and improved the machined surface roughness (Ra) by 8.7 % during conventional microcutting. In contrast, the UEVAM process is capable of further improving the surface quality from 11.03–5.1 nm Ra, without heat treatment. It is also evident that poor machined surface quality was attributed to the formation of oxide particles during SLM. Chip morphology analysis and finite element method simulations revealed the benefits of UEVAM in tackling the issue of precipitation and extended our understanding of the applications of UEVAM.