Muti-Energy Field-Assisted Grinding of Hard and Brittle Materials: Tools, Equipment and Mechanisms

Traditional grinding processes often fail to meet the manufacturing requirements of precision components in terms of processing accuracy and high quality in some high-tech fields such as aerospace, electronic information, and biomedicine. Moreover, with the increase demand for hard, brittle, and difficult-to-machine materials, the introduction of energy fields for auxiliary grinding is of great significance. However, as an emerging technology, it still faces difficulties to varying degrees in many aspects [3].

(1)

Precise regulation of energy field parameters. Energy field-assisted grinding involves various parameters, including laser power, pulse frequency, spot size within the laser field, as well as magnetic field intensity and direction etc. These parameters interact with each other and have a complex coupling relationship with the process parameters of grinding. To precisely control these parameters and achieve the expected processing effect, a large number of experiments and detailed data analysis are indispensable.

(2)

Monitoring and evaluation of energy field effects. The microscopic mechanism of material removal under the influence of the energy field is rather complex. It requires numerous physical, chemical, and other changes, and it is difficult to understand it relying only on a single measurement method fully. On the other hand, at present, there is still a lack of an effective and comprehensive evaluation index system to measure the overall impact of energy field-assisted grinding. The existing traditional indicators such as surface roughness and material removal rate, often fail to fully reflect the profound impacts brought by the energy field, such as improving processing accuracy and quality. This situation is not benefit to the further optimization of the energy field-assisted grinding process.

3.1. Physical Energy Field-Assisted Grinding The implication of physical energy field-assisted grinding in hard and brittle materials depends on various unique physical effects. Aiming to alter the interaction between abrasive particles and the workpiece, and improve the influence of the physical state of the material. Through these manners, the grinding effect and processing quality can be improved. Physical energy field-assisted grinding requires the transfer and conversion of different forms of energy within the machining system. For example, in an ultrasonic field, electrical energy is converted into high-frequency mechanical vibration energy via an ultrasonic transducer. Once physical energies are transferred to the grinding area, they will interact with the abrasive particles and the workpiece, thereby changing the traditional grinding mode that relies solely on mechanical grinding force. 3.1.1. Mechanism Ultrasonic-assisted grinding is a composite processing technology that ingeniously combines ultrasonic vibration with traditional grinding methods. During this process, high-frequency electrical signals are converted into high-frequency mechanical vibrations by ultrasonic transducers and then amplified by amplitude rods before being transmitted to the grinding tool. Under the influence of ultrasonic vibration, abrasive particles adhering to grinding tools will repeatedly impact and rub against the surface of hard and brittle materials at high speed. This dynamic interaction can effectively remove materials, therby enabling the processing of hard and brittle materials that are usually difficult to handle only through traditional grinding. Usually, high-frequency vibration is generated in a single direction, such as the axial or radial direction of grinding wheel. During the cutting process of the workpiece, the abrasive particles only have additional motion caused by the vibration in that specific direction. This is one-dimensional (1D) ultrasonic vibration-assisted grinding [4], and the ultrasonic vibration is simultaneously applied to the grinding tool or workpiece in two mutually perpendicular directions. If one direction can be along the feed direction of grinding and the other direction can be perpendicular to the surface of the workpiece, it namely two-dimensional (2D) ultrasonic vibration-assisted grinding [8]. At this time, the cutting trajectory of abrasive particles becomes complex and diverse. Compared with 1D ultrasonic vibration, this method can further improve surface quality, accuracy and material removal efficiency. Meanwhile, the three-dimensional (3D) ultrasonic vibration-assisted grinding includes the feed direction during the grinding process, the direction perpendicular to the workpiece, and another lateral direction perpendicular to the first two directions. In 3D space, the cutting trajectory of abrasive particles presents an extremely complex form, featuring high dynamics, intermittence and irregularity. Ding et al. [47] analyzed the kinematics of a single diamond grain by considering the influence of the compound grinding system and trajectory, and the 3D ultrasonic-assisted grinding highly reduced the surface roughness compared with 2D ultrasonic-assisted grinding. In addition, processing parameters have a significant impact on the performance of ultrasound-assisted grinding. Li et al. [48] analyzed the influence of vibration direction on grinding characteristics and surface morphology through the simulation of abrasive motion. The processing results under different amplitudes show that as the amplitudes of the workpiece and tool increase, the normal force in the grinding force decreases at a faster rate than the tangential force. Wen et al. [49] proposed that when the ultrasonic amplitude is 4 μm, the surface contact performance of the workpiece is the best. At this time, the surface roughness of the processed workpiece is reduced by ~24% compared to the conventional grinding surface. Under the same load, the contact stiffness of the ultrasonic-assisted grinding surface is increased by at least 68%, and the maximum local contact pressure is reduced by ~17%. Abdollahi et al. [50] found that the vibration amplitude has the most significant effect on improving the material removal rate and enhancing the thickness of the recast layer. The optimal parameters, that is, the vibration amplitude of 40 μm, the modulation frequency of 1200 Hz, and the gas pressure of 0.25 MPa, a material removal rate of 2.0 m³/min, are optimized with the response surface method. Taking the single-particle cutting behavior during ultrasonic-assisted grinding of hard and brittle materials as the research object, Zheng et al. [51] proposed to establish an ultrasonic-assisted scratch model and explained the variation law of related mechanical parameters, as shown in Figure 4a. To predict the cutting force during the ultrasonic-assisted grinding process of hard and brittle materials, Kang et al. [52] established a cutting force model considering the influence of diamond abrasive grains on the transition fillet of grinding tool (Figure 4b). To evaluate the influence of process parameters on the surface quality of ultrasonic-assisted end-face grinding of SiCp/Al composites, Zheng et al. [53] conducted a single-factor experiment in an ultrasonic vibration machining center and found that the 3D surface fractal dimension increased nonlinearly with the spindle speed and amplitude and decreased with the increase of feed rate and cutting depth. The grinding schematic diagram and abrasive grain trajectory are shown in Figure 4c. This method is suitable for processing workpieces with complex shapes, such as molds for components in the aerospace field. For instance, when processing aircraft engine blades with spatially distorted surfaces, 3D ultrasonic vibration-assisted grinding enables abrasive particles to fully utilize ultrasonic vibration in all directions, which can better ensure high precision and good surface quality of the blade surface, and meet the processing requirements of complex-shaped workpieces in various positions. Chen et al. [54] studied the changes in surface roughness during ultrasound-assisted dry grinding of 12Cr2Ni4A steel with large-diameter grinding wheels. According to the Kinematic characteristics, feed depth and speed have a significant influence on ultrasound-assisted grinding. Furthermore, due to the Poisson effect, the ultrasound-assisted dry grinding of large-diameter grinding wheels is a combination of axial and radial ultrasound-assisted grinding. Radial vibration improves surface roughness by deepening individual grinding trajectories, while axial vibration smooths the surface topography by increasing the interaction and overlap of adjacent cutting trajectories, as shown in Figure 4d. To study the influence of the vibration direction on the processing effect of SiCp/Al composites, Li et al. [48] proposed the ultrasound-assisted grinding-electrolysis-electrical discharge machining technology. The workpiece is removed through electrolytic dissolution, grinding, and discharge corrosion. The surface became smoother through tangential grinding and electrolytic-electrical discharge machining, resulting in a reduction of grinding force and surface roughness, as shown in Figure 4e. Therefore, controlling the ultrasonic frequency properly can play an auxiliary role in grinding and is expected to achieve high-quality and high-precision processing [55]. Selecting the appropriate ultrasound-assisted grinding method provides a theoretical basis for the high-precision processing of hard and brittle materials. Sun et al. [56] conducted a theoretical analysis to establish the kinematic function of a single abrasive grain (Figure 4f) and collected data such as grinding force and surface roughness. It has been found that axial ultrasonic vibration can increase the dynamic contact length and reduce the thickness of grinding chips. Xiang et al. [57] suggested that the ultrasonic-assisted grinding significantly changes the movement characteristics of abrasive grains during ordinary cylindrical processing. The trajectory of abrasive grain movement becomes longer, the grinding efficiency increases, and the surface quality of the workpiece is significantly improved. By establishing a kinematic function model, it is found that under the same material removal rate, Ultrasonic vibration-assisted grinding enhances material removal rate, reduces surface roughness, and minimizes damage by superimposing high-frequency and micro-amplitude ultrasonic vibrations [58]. Yang et al. [59] established the contact rate model during intermittent machining based on the kinematic analysis of ultrasonic vibration-assisted grinding. The motion trajectory of ultrasonic vibration-assisted grinding is longer than that of ordinary grinding, which is helpful for improving the surface quality of certain aviation materials [60]. Frank et al. [61] analyzed the kinematics of meshing between grains and workpieces through single-grain scratch tests. When ultrasonic vibration is superimposed on the grinding process, the material removal amount of each grain is increased, and the impact effect of ultrasound-assisted grinding would cause fragmentation around the generated scratches. Chen et al. [54] revealed that ultrasound-assisted dry grinding can reduce the negative impact of cutting fluid on the environment and improve the surface quality of the machined object. Mao et al. [62] improved the performance of the grinding wheel by altering the structure of the working surface, allowing the coolant to enter the grinding area for effective lubrication. When using CBN grinding wheels for ultrasound-assisted dry grinding of 12Cr2Ni4A steel, the surface roughness can be reduced by 30% compared with ordinary dry grinding. Moreover, the physical effects of ultrasound-assisted grinding are often combined with other processing methods, such as plasma electrolytic oxidation technology, etc., all of which have achieved favorable results [63]. Magnetic field is another important physical-assisted grinding method. During the processing, a magnetic field is applied to the working area. Material removal can be achieved by taking advantage of the specific distribution and movement patterns formed by abrasive particles under the influence of a magnetic field. Magnetic abrasive particles are subjected to magnetic force, causing them to aggregate, arrange and present chain or brush-like structures. Then, these abrasive particles come into contact with the workpiece, under the combined action of grinding force and magnetic force, to cut, scrape and squeeze the workpiece, thereby effectively removing the material. Among numerous experiments, magnetic field assistance is often used as a means to optimize processing properties rather than directly participating in cutting operations. Of course, the role of the physical energy field in the micro-grinding of hard and brittle materials cannot be ignored. Zhang et al. [64] used magnetic field-assisted technology to reduce energy consumption and thermal deformation in wire electrical discharge machining to improve processing performance and reduce energy consumption and thermal deformation. In the optical instrument manufacturing industry, the demand for optical components in astronomical telescopes is constantly increasing. Because in a space with an extremely large temperature difference, the diffraction lens is very sensitive to reducing chromatic aberration caused by thermal expansion. To solve this problem and achieve the fine processing of synthetic silica lenses, Suzuki et al. [65] proposed a magnetic-assisted polishing method. The optical lenses were grounded using sharp-edge diamond grinding wheels, and then the shape deviation and changes in surface roughness were evaluated through grinding and polishing tests. During the ultrasonic vibration and magnetic field-assisted diamond composite cutting of titanium alloys, Deng et al. [66] discovered that the magnetic field could suppress the surface ripples and unevenness caused by material recovery during ultrasonic assisted diamond cutting of titanium alloys, as shown in Figure 5a. Through theoretical modeling and experimental verification of the movement trajectory of magnetic abrasive grains, Peng et al. [67] proposed a method for abrasive wire-saw in a magnetic field was proposed. The magnetic field environment and the different movement trajectories of magnetic abrasive grains in the magnetic field are shown in Figure 5b. Ye et al. [68] found that the magnetic field-assisted laser drilling process can produce a deeper drilling depth, and generate confined plasma plumes and relatively less residue, as shown in Figure 5c. To clarify the differences in magnetic fields during the processing of different materials, Ming et al. [69] took the magnetic material SKD11 and the non-magnetic material Ti6Al4V as workpieces, and conducted a series of electrical discharge machining experiments with different magnetic intensities and servo voltages. When the magnetic intensity increases, the energy of the discharge channel becomes more concentrated, so as to improve the energy utilization efficiency and material removal rate. Because the debris of the magnetic material is more likely to be discharged from the discharge gap under the action of the magnetic field, as shown in Figure 5d. Singh et al. [70] optimized the process parameters to achieve offline monitoring in the magnetic field-assisted abrasive flow machining process. The experimental device system is shown in Figure 5e. Kanish et al. [71] investigated the key parameters of magnetic field-assisted abrasive finishing based on the Taguchi orthogonal array, including the electromagnetic voltage, machining clearance, electromagnet speed, abrasive size, and feed rate. The contact situation of the magnetic abrasive brush with the workpiece is shown in Figure 5f. Ding et al. [72] conducted comparative experiments using brazed grinding wheels with specific grain distribution to explore the material removal mechanism of ultrasound-assisted grinding of SiC ceramics (Figure 5g). As the thickness of the undeformed chips increased, the material removal mechanism changed from ductile removal to brittle fracture. Suzuki et al. [65] utilized the magnetic field-assisted polishing method to precisely process the silica diffraction lens, visually presenting the geometric interference problem existing when grinding convex lenses, as shown in Figure 5h. Zhao et al. proposed the magnetic-assisted ultra-precision grinding technology for processing nickel-based alloys, which can directly suppress the vibration of the grinding wheel by utilizing the magnetic field [73]. In Figure 5i, Tang et al. [74] investigated the auxiliary processing effect of the magnetic field through the meshing of periodic and concentrated magnetic fields. Du et al. [75] explored the evolution mechanism of the solidification structure and the changes in mechanical properties during the selective laser melting process of AlSi10Mg alloy under a static magnetic field. A static magnetic field could lead to a reduction in the dendrite spacing in the molten pool, refine the grains and improve the mechanical properties. Cui et al. [76] reported the frictional performance of graphene nanofluid lubricants at the interface between grinding wheels and workpieces, providing a basis for the collaborative optimization of processing parameters and lubrication conditions in magnetic field-assisted grinding. According to the above research, it can be evident that ultrasonic assistance can effectively reduce grinding force, optimize the material removal mechanism, and thus make the grinding process more stable and efficient. Meanwhile, the magnetic field assistance can regulate the movement trajectory of abrasive grains, further improving the stability and accuracy of the grinding operation. These physical effect energy field-assisted methods have successfully solved problems such as microcracking and precision control during the grinding process of hard and brittle materials. 3.1.2. Devices and Performance Most of the experiments on ultrasound-assisted grinding are conducted on CNC machines. Yang et al. [59] carried out grinding tests on CNC lathes and used ceramic-bonded CBN grinding heads. The device diagram is shown in Figure 6a. Wu et al. suggested that the grinding force and surface roughness decrease with the increase of spindle speed and ultrasonic amplitude, but increase with the increase of feed rate and cutting depth in ultrasonic vibration-assisted grinding [77]. Suzuki et al. [65] conducted different magnetic field-assisted grinding experiments, the differences in their devices mainly lie in the magnetic field generation devices, except for the common electromagnetic coils and permanent magnets, as shown in Figure 6b. The ultrasound-assisted grinding device adopted by Rattan et al. [78] mainly consists of a processing chamber, electrode wire, workpiece feed unit and DC power supply, and uses neodymium iron boron disc permanent magnets. A brass wire with a diameter of 100 μm is used as the tool electrode (cathode), and a graphite wire with a diameter of 20 mm is used as the circular counter electrode (anode). The experimental setup is depicted in Figure 6c. Heike et al. [61] adopted an ultrasonic generator that regarded as specially designed plate with multiple resonant frequency responses, which could vibrate workpieces of different sizes and weights during the grinding process (Figure 6d). Li et al. analyzed the potential formation mechanisms of surface integrity under different ultrasonic-assisted grinding proceses and found that the abrasive particles have spatial movement trajectories and a gradually increasing cutting depth [79]. In addition to CNC machine tools, there are also cases where ultrasonic vibration processing is carried out on precision grinding machines, as shown in Figure 6e. Sun et al. [56] equipped the precision grinding machine with an ultrasonic vibration air spindle system, which generates axial ultrasonic vibration by the spindle vibrator and applies it to the grinding wheel. Wang et al. [80] provided the magnetic field by an external Nd-Fe-B permanent magnet that was directly placed below the machining gap to assist in electrochemical machining (Figure 6f) As shown in Figure 6g, the design by wang et al. [81] can present different magnetic flux densities at different target positions around the roller, enabling the magnetic abrasive to generate a reorganization mechanism each time. Khali et al. [82] used a composition of four neodymium magnets, which were fixed on a custom steel frame to generate a magnetic field in the grinding experiment, as shown in Figure 6h. When building the energy field-assisted grinding device, it is necessary to ensure that the structural design is reasonable and the energy field parameters are precisely regulated to meet the process requirements and stability. To address the issues of low energy conversion efficiency and heat generation in ultrasonic transducers, the industry typically employs a cooling structure combining thick copper electrode plates with compressed air to reduce temperature rise through forced convection. At the same time, a pre-stressed sleeve design is adopted to minimize energy loss caused by mechanical vibration [79]. Yang et al. [83] introduced different ultrasonic generators and auxiliary processing devices. The relationship among their ultrasonic frequencies, amplitudes, and contact times is shown in Figure 7a. Yang et al. [77] performed the influence of amplitude on the grinding force of CNC lathes, as shown in Figure 7b. Xiang et al. [57] studied the conditions of ultrasound-assisted high-speed grinding on a five-axis machining center. The motion characteristics, grinding force, surface quality of the workpiece, and abrasive wear of single-CBN abrasive grains. The worktable moves up and down to achieve tool feed, and the workpiece rotates at high speed through the machine tool spindle. The wear conditions of a single CBN abrasive grain after ultrasonic-assisted grinding and conventional grinding are depicted in Figure 7c. Intelligent and sustainable manufacturing systems provide more convenient tools for the processing of hard and brittle materials [84]. Chen et al. [85] adopted an improved CNC machining center and installed a specially designed ultrasonic vibration device to conduct 3D ultrasonic vibration-assisted grinding of zirconia ceramics (Figure 7d). Heike et al. [79] conducted ultrasonic scratch experiments on plates with multiple resonant frequency responses. The influence of material accumulation and removal is shown in Figure 7e. Ming et al. [69] constructed a magnetic field on the electrical discharge machining tool to process some complex-shaped and high-performance materials. The movement trajectory of the electron beam is demonstrated in Figure 7f. The main difference of the device developed by Suzuki et al. [65] lies in the magnetic field generation device. The working principle and structure of the magnetic field-assisted polishing system on the four-axis (X, Y, Z, C) ultra-precision machine tool (Figure 7g). In summary, the device selected for magnetic field-assisted grinding experiments is based on the material, such as precision grinding machines, micro-nano abrasive grinding wheels, CNC machine tools, etc. Most magnetic field generating devices have both magnetic field control and regulation capabilities, and are finally equipped with measurement and monitoring devices. At present, with the assistance of physical energy fields, there are several problems with the experimental devices of ultrasonic waves and magnetic fields. The energy conversion efficiency of ultrasonic transducers is rather limited. During high-energy output, unstable phenomena such as heating may occur. In the magnetic field experimental device, it is quite difficult to precisely control and rapidly adjust the magnetic field. Some devices for processing techniques that require dynamic changes in the magnetic field. At the same time, the magnetic field has a certain degree of interference with the surrounding equipment, which can affect the normal function of electronic components. Therefore, effective shielding measures need to be taken. 3.2. Chemical Energy Field-Assisted Grinding Chemical energy field-assisted grinding shows significant advantages in the micro-grinding of hard, brittle, and difficult-to-machine materials. For instance, a magnetic field forms a flexible grinding tool through the magnetorheological effect, reducing the impact of abrasive grains on the workpiece. High-frequency vibration generates impact crushing and micro-forging effects, reducing the critical cutting force and minimizing the depth of subsurface damage and edge chipping. The laser field, with its high-energy beam, can precisely control the action area, efficiently and intensively process specific areas. At the same time, by altering the chemical properties of the material surface, it makes hard materials relatively fragile, thus enhancing grinding efficiency and reducing time costs. The electrochemical field, by gently and precisely controlling the chemical state of the material surface, enables it to reach the softened state before grinding, reducing grinding force and energy consumption. The combination of the two mechanisms makes chemical energy field-assisted grinding a highly promising method for micro-grinding of hard and brittle materials in industrial applications. 3.2.1. Mechanism Hard and brittle materials are generally composed of specific chemical bonds, such as ionic and covalent bonds contained in ceramic materials. For laser-assisted grinding, when the laser is irradiated onto the surface of hard and brittle materials, its high energy density enables the materials to absorb photon energy. Once the photon energy reaches or exceeds the bond energy of the chemical bonds in the material, it will trigger the breaking of these chemical bonds. From a chemical perspective, this change in the chemical structure of the material makes the previously stable material have higher chemical activity in specific local areas, thus laying a solid foundation for the subsequent removal of the material. For instance, under laser irradiation, the chemical bonds within ceramic materials are broken, weakening the connections between atoms. Therefore, through mechanical means such as grinding, it becomes much easier to remove the materials in which these chemical bonds are activated in this way Furthermore, for some composite hard and brittle materials, lasers can selectively act on specific chemical bonds of different components. This selective effect leads to obvious chemical structural changes among various components, which in turn helps to control the material removal process more precisely and effectively, and meet the requirements of microfabrication of hard and brittle materials. Meanwhile, in specific processing scenarios, chemical reagent gases can be deliberately introduced into the processing area. The high temperature generated by lasers can promote chemical reactions between these gases and hard and brittle materials, thereby generating new compounds. These newly formed compounds are either easier to remove during the subsequent grinding process or help improve the surface quality. If some compounds with lubricating properties are produced, this can effectively reduce the friction and material damage in the grinding process. When cutting silicon nitride, due to the large tool wear and cutting force, the surface integrity is poor, and the material removal rate is limited. Therefore, Bahman et al. [86] proposed a laser-assisted grinding process to overcome the technical limitations existing in the grinding of silicon nitride. Wang et al. [87] demonstrated that compared with conventional grinding processing, when laser-assisted processing of AL₂O₃P/Al composite materials, the cutting force and tool wear were reduced by 30–50% and 20–30%, respectively. Hu et al. [88] proposed that laser-assisted dry micro-grinding aims to improve the surface quality of difficult-to-machine materials. The grinding model and the steady-state temperature distribution are shown in Figure 8a. Liu et al. [89] studied the influence of laser-assisted processing on the processing mechanism of RB-SiC. The schematic diagram of the laser-assisted slotting experiment, the path of the laser beam in the diamond tool, and the molecular dynamics model of the RB-SiC nano-cutting simulation are depicted in Figure 8b. The subsurface damage mechanism is mainly manifested as the tool geometric parameters influencing the degree of damage by regulating the stress field, temperature field and phase transformation evolution. To explore the role and mechanism of laser-assisted processing in reducing damage during the finishing process of sapphire surfaces, Langan et al. [90] employed an universal micro-friction meter to process the sapphire crystals with different laser wavelengths and powers. Compared with the traditional single-point diamond turning, the surface damage of sapphire processed with laser assistance is greatly reduced, and the residual stress condition is improved. The higher laser power can effectively alleviate the problems of fracture, crack, and fragmentation, as shown in Figure 8c. The parameters of laser processing include power, wavelength, pulse frequency, and scanning speed etc. Among them, a higher laser power can cause the material to heat up and soften more quickly, but it is also prone to causing thermal damage. Fortunato et al. [91] established a finite element model to analyze the thermal stress generated by laser irradiation and predict crack formation. At the same time, attention should also be paid to the influence brought by grinding parameters. Ma et al. [92] found that when laser-assisted grinding alumina ceramics, the processing parameter that laser power has the greatest impact on surface roughness. The second key parameters are the grinding depth and the rotational speed of the grinding wheel. Therefore, the optimal surface roughness value can be obtained using appropriate processing parameters. Rashid et al. [93] studied the processing states under different laser powers and concluded that the cutting force used for titanium alloys in laser-assisted processing was significantly reduced. Singh et al. [94] investigated the changes in microstructure and microhardness of the heat-affected zone (HAZ) caused by different scanning speeds. A 3D transient finite element model of the moving laser heat source was developed to predict the temperature distribution in the workpiece. Commercial lasers can be classified into blue, green, and purple lasers based on their different laser wavelengths. Among them, green laser has been widely used in laser-assisted etching of silicon due to its ideal absorption effect [95]. Ultraviolet lasers with shorter wavelengths can theoretically be better absorbed by metallic materials [96], but currently there are few commercial applications in this regard. So far, laser-assisted processing technology for hard and brittle materials has only been attempted under laboratory conditions [97]. Due to its inevitable thermal instability and relatively immature technology, its application is still limited. For instance, Song [98] explored that the processing effect was not stable under different spindle speeds, feed rates, pulse duty cycles, and different grinding tools. As for the electrochemical field, the method of removing materials by controlling electrochemical reactions is fundamentally different from mechanical materials removal relying on mechanical grinding. This method effectively avoids the physical damage that mechanical force may cause to the workpieces. For hard and brittle materials such as quartz and sapphire which are highly sensitive to processing stress, this is a gentle yet effective material removal method. During the electrochemical process, a passivation film forms on the workpiece. This is a dense oxide or other compound film formed by the chemical reactions of atoms on the surface under the conditions of electrolyte and potential. This passivation film exhibits stable chemical properties, preventing excessive dissolution of the material and thereby protecting the workpiece. On the other hand, during the subsequent grinding process, when the abrasive particles come into contact with and remove this film, a new surface will be exposed. Subsequently, this newly exposed surface once again participates in the electrochemical dissolution and passivation process. This cyclic process keeps the material surface in a chemically controlled and favorable state during the grinding process, which is conducive to obtaining a smoother and higher-quality processing surface. After the electrochemical grinding technology was introduced in 1952, the feasibility of its application in stainless steel titanium alloys has been confirmed in past studies [99]. For example, Li et al. [100] used electrochemical grinding to process Ti-6Al-4V alloy. Tehrani et al. [101] investigated four processing states in electrochemical assisted grinding by considering the intensity of electrochemical anodic dissolution and the degree of electrochemical overcutting that occurs during the material removal process. Among them, the application of direct current (DC) pulse power supply can effectively regulate the electrochemical overswitching phenomenon. Kong et al. [102] proposed a new type of ultrasonic-assisted electrochemical grinding technology that combines electrochemical drilling, mechanical grinding, and ultrasonic vibration to create high-quality holes on superalloys. Ucak et al. [103] employed electrochemical assisted grinding of nickel-based superalloys and discussed the influence of abrasive grain types and electrical parameters on roughness and overcutting. Zhu et al. [104] proposed an electrochemical drilling and grinding method for precision machining of small holes. Keeping the balance between electrochemical effects and mechanical grinding is the key to improving the machining accuracy and surface quality of the complex parts. Xu et al. [105] used brazed diamond grinding wheels instead of electroplated diamond grinding wheels in electrochemical assisted grinding could extend the durability of tools, and the adoption of optimized voltage and electrolyte temperature could highly improve the material removal rate. Recently, Zhu et al. [106] adopted electrochemical grinding technology on stainless steel materials and the flowchart of the working method and the processing procedure are shown in Figure 9a. To explore the electrochemical jet machining technology, Wirtz et al. [107] improved the quality and accuracy of the workpiece through micromachining and surface modification. The relevant situation of mass transfer and the control effect during the electrodeposition process is shown in Figure 9b. Compared with electrical discharge machining, Ahmet et al. [108] found that electrochemical grinding could effectively improve surface roughness, which is closely related to the oxide film. The schematic diagram of the processing procedure and surface morphology processed under different pulse currents is shown in Figure 9c. Ming et al. [69] investigated the influence of magnetic field-assisted electrical discharge machining on the processing properties of magnetic and non-magnetic materials. The influence of the magnetic field on the movement of debris is shown in Figure 9d. The discharge state is monitored in real time through the programmable system-on-chip, and the servo feed speed and planetary motion parameters are dynamically adjusted by the indirect self-tuning regulator. Meanwhile, the rotational magnetic field is utilized to accelerate the discharge of molten material. As for other difficult-to-machine alloys, Niu et al. [109] processed Inconel 718 alloy through electrochemical assisted milling and grinding. The schematic diagram of the electrolyte flow and the flow velocity distribution of the cross-section during the machining process are shown in Figure 9e. To explore new deep grinding technologies, improve processing performance, and solve problems such as grinding heat and wheel wear during the deep grinding process, Ge et al. [110] carried out a series of electrochemical deep grinding experiments. During the electrochemical assisted grinding process, the anode material removal mechanism changes significantly. Mechanical grinding can not only scrape off the electrolytic products on the machined surface but also grind the insoluble components in the substrate, as shown in Figure 9f. The chemical energy field holds significant value in the micro-grinding of hard and brittle materials. Through technologies such as laser fields and electrochemical fields, the grinding accuracy, efficiency, and surface quality have been enhanced, providing an effective approach for the precise processing of hard and brittle materials. However, some challenges still need to be overcome. In the field of laser-assisted grinding, precisely controlling laser parameters is complex, and a large number of experiments are required to determine the optimal parameters. As for the electrochemical assisted grinding, issues such as electrolyte selection, electrode corrosion, and polarization could highly affect the processing stability. In the future, with the development of intelligent control systems, it is expected to achieve real-time optimization of chemical energy-field parameters. Meanwhile, the emergence of new materials will promote the development of more eco-friendly and stable electrolytes and corrosion-resistant electrodes, further enhancing processing stability and environmental friendliness. 3.2.2 Devices and Performance During the laser-assisted grinding, various types of lasers are usually used to emit light beams, and most of them are equipped with scanners to inspect the surface condition of the workpiece. According to the positional relationship between the optical path and the spindle, lasers can be roughly classified into side-axis lasers (where the laser beam is in a position parallel to the grinding wheel), coaxial lasers (where the laser beam is coaxially arranged with the grinding wheel), and rear-mounted laser-assisted micro-grinding devices (where the laser emission device is set behind the grinding wheel). Hu et al. [88] proposed a novel laser-assisted dry micro-grinding technology for fabricating micro-grooves of various workpieces such as cemented carbide, die steel, and quartz glass. The laser beam is focused on the chip rather than the workpiece. Through the coupling of the thermal energy output by the laser and the shear heat, the chip is removed to obtain an intact surface (Figure 10a). Yehia et al. [112] employed an electrochemical assisted grinding machine, which is composed of an electrolyte tank, an electrolytic pump, a cathode wheel, a DC power supply, etc., and is capable of mechanical operation (Figure 10b). The NaCl solution was used as the electrolyte to study the influence of the electrolyte concentration and feed rate. In Figure 10c, Ma et al. [92] combined the fiber laser system with the three-axis grinding machine to achieve laser-assisted grinding. Song et al. [98] adopted a CO₂ laser processing system, adjusted to a diameter of 3 mm through an optical system, and was used to irradiate the workpiece, as shown in Figure 10d. Xu et al. [115] used a 6-degree-of-freedom robotic arm equipped with a laser generator to ensure the accurate focusing position of the laser beam. In fact, it replaced the function of a machine tool with a robot. Xu et al. optimized the thermal environment of CNC grinding machines based on an improved neural network and designed an interface that could simplify the operation process, thereby streamlining the operation procedures [116]. Zhang et al. [113] adopted a five-axis CNC milling machine, which could achieve translation along the X, Y, and Z axes, rotation along the A and C axes, and rotation along the Z axis. The positive end of DC power supply is connected to a conductive rod, and the negative end is connected to an electrode. The power supply voltage, gap voltage and current are monitored through a signal acquisition system (Figure 10e). Wang et al. [114] adopted the self-developed electrochemical grinding system as the experimental device, which mainly includes a marble platform, three motion axes, an electric spindle, a machine tool control system, as well as an electrolyte circulation and filtration system. The workpiece is used as the anode, made of steel, and the grinding head serves as the cathode. In electrochemical assisted grinding, the processing quality and efficiency is improved by superimposing translational motions (including linear and circular translation), as shown in Figure 10f. The processing equipment developed by Ge et al. [110] mainly consists of the machine tool, power supply, motion control and data acquisition unit (Figure 10g). Furthermore, Liu et al. [89] fabricated microgrooves on polished RB-SiC using a five-axis ultra-precision machine tool and found that compared with traditional processing, the cutting force and fluctuation of laser-assisted processing of silicon carbide (RB-SiC) were lower. Among them, the grain orientation and distribution fluctuate with the cutting force, and as the cutting temperature increases, as shown in Figure 11a. Zhang et al. [113] established a five-axis CNC milling machine. A box for circulating working fluid was placed on the clamping table, with fixtures installed inside and conductive rods connected. The flow field under different flushing pressures is demonstrated in Figure 11b. It includes the flow velocity distribution cloud map, curve, and the movement trajectories of particles, etc. Speidel et al. [111] developed an electrolyte injection device in combination with a power supply and auxiliary equipment to achieve electrochemical injection grinding. The mass transfer control during the electrodeposition process and the dissolution process of titanium alloys in electrochemical jet machining were analyzed, as shown in Figure 11c. In electrochemical field-assisted grinding, the traditional DC power supply is usually adopted to drive the electrochemical-assisted grinding device [117,118]. Direct current is transmitted through the electrolyte to the processing area, composed of the workpiece and grinding tools. In addition, there is a pulsed electrochemical-assisted grinding device with an output of intermittent pulsed current. There are some dynamic electrochemical-assisted grinding devices, which adopt a power supply combining direct current and pulse power to ensure that the material can undergo stable electrochemical dissolution [119]. The positive and negative electrodes and electrolyte are generally selected based on the experimental purpose.