A review of research on material removal mechanisms for laser-assisted machining of difficult-to-machine materials

With the development of technology in aerospace, medical devices and other fields, high-performance difficult-to-machine materials have been widely used in these fields due to their good comprehensive mechanical properties. However, when using traditional machining methods, it is difficult to ensure the machining accuracy and surface quality, and at the same time, there are problems such as serious tool wear and low machining efficiency. Laser-assisted machining (LAM) technology is an advanced manufacturing process that softens the material in the machining area through the preheating effect of the laser, thus reducing the surface hardness of the material and improving the machinability of the material, which has the advantages of high efficiency and economy in machining difficult-to-machine materials. This paper introduces the common methods of establishing thermal models and simulation modeling of removal behavior in the LAM material removal process, summarizes the research progress on the removal behavior of LAM processing of various difficult-to-machine materials, and analyzes the shortcomings and challenges of the current research. Finally, the key issues of LAM material removal mechanism are proposed, and the development direction of LAM material removal technology is envisioned in order to provide a reference for the research and development in this field.


Introduction
At present, the high-end manufacturing field has higher and higher expectations and requirements for materials, "Made in China 2025" and the "14th Five-Year Plan" have also mentioned the need to actively promote the research and development of important materials and efficient processing.Difficult-to-process materials such as titanium alloys and high-temperature alloys occupy an important position in the manufacturing industry due to their excellent mechanical properties.The use of traditional precision machining technology to process difficult-to-machine materials, there are serious tool wear [1], low machining efficiency [2], high production costs, accompanied by cracks, ablation and other damage phenomena [3], which seriously affects the use of material performance.High-performance manufacturing of difficult-to-machine materials is particularly important for promoting the innovative development of aerospace, marine and offshore engineering equipment, construction machinery and other industries.Laser Assisted Machining (LAM) technology, as an advanced manufacturing process, provides a potential method to improve material removal rate and reduce machining damage, which is important for obtaining high-performance parts machined surfaces for research.
Laser-assisted machining technology was first proposed by Stephen M.Copley and Michael Bass et al. [4] in the 1980s, mainly used for metal cutting.In general, LAM is divided into preheating or in-process heating depending on whether the laser preheats the material near the front cutter face or irradiates the subsurface material passing through the transparent cutter.According to the process of laser equipment application, preheating LAM can be divided into preheating laser-assisted turning [5], preheating laser-assisted milling [6], preheating laserassisted drilling [7] and preheating laser-assisted grinding [8].Since preheating LAM inevitably introduces undesirable thermal effects, the absence of cutting fluid is also not conducive to controlling machining quality and tool life [9], thus generating the heating LAM in the process, and accurately controlling the heat-affected area through the integration of laser beam and transparent cutting tool, so as to obtain better machining quality.The commonly used in-process heating LAM includes in-process heating laser-assisted turning [10] and in-process heating laser-assisted drilling [11].The schematic diagram of inprocess heating laser-assisted turning process and experimental bench is shown in Fig. 1.
The laser-assisted processing process uses the laser beam to focus on the surface of the workpiece, so that the material is heated to a certain temperature in a short time, thus softening, making the material removal process more efficient.LAM process covers the characteristics of material melting or gasification, friction and wear, extrusion molding, etc.The interaction between thermal multienergy fields is complex and coupled with each other, resulting in changes in material removal mechanism, and the material removal process is difficult to characterize.Summarizing the research methods and results of LAM material removal behavior is of great significance for obtaining high performance parts machining surfaces.
In this paper, the research methods and conclusions of material removal mechanism of laser-assisted machining are overviewed, which provides the method and theoretical reference for the realization of laser-assisted machining for high performance components.Firstly, the thermal model of material removal process of laserassisted machining is sketched from three aspects: analytical method, finite element method and finite difference method.Secondly, three commonly used simulation modeling methods for material removal in laser-assisted machining are introduced, and the mechanical and thermal changes, material removal form changes, stressstrain distribution and sub-surface damage involved in material removal process simulation are reviewed.Then, the material removal behavior of laser-assisted machining based on experimental studies is summarized from three aspects: hard and brittle materials, composite materials, and titanium and nickel-based superalloys.Finally, the development trend of laser-assisted machining material removal technology is prospected.

Thermal model of material removal process
In the process of laser-assisted processing, Variation in temperature will cause changes in material characteristics, the temperature of the material directly affects the removal of the material, cutting temperature and the formation of chips, when the temperature is too high, it may damage the surface of the material and lead to subsurface damage or negative impact on the tool life.At present, domestic and foreign scholars mainly use analytical Method [12], Finite Element Method (FEM) [13] Fig. 1 Schematic of μ-LAM process and experimental test bed [10] and Finite Difference Method (FDM) [14] to calculate and predict the temperature field distribution in laserassisted machining.

Analytical method
Considering the initial and boundary conditions, the heat sources in the laser-assisted machining process mainly include laser heating, heat generated by the auxiliary processing mode, thermal radiation, natural convection and forced convection.The heat conduction equation of the LAM process can be solved by applying the appropriate Green function to infinitely small heat flow elements, and integrating Green's function and the convolution of heat flow with time and space [15].
Rozzi et al. [16,17] first established a transient thermal model of rotating composite isotropic workpiece under laser heating and material removal, and found that the temperature distribution inside the raw workpiece was most affected by laser power, laser tool distance and laser/ tool translation speed.Tian and Shin [18,19] improved the thermal model established by Rozzi by changing the fixed laser/moving workpiece to a fixed workpiece/moving laser, and obtained a three-dimensional transient thermal model of the workpiece with complex geometric features, and verified the accuracy of the model through the online measurement of surface temperature by infrared camera.Based on the above research, Kashani et al. [20] analyzed and calculated the analytical solution of the transient temperature field of a rotating cylinder under the action of a local laser heat source, and the error between the simulation results and the actual test results of the pyrometer was less than 10%.Fang et al. [21] added the non-translational kinetic energy to the atoms in the laser hot zone, proposed a process thermal laser-assisted cutting model that considered both the cumulative thermal effect and the thermal boundary conditions during laser processing, and calculated the laser heat flux of each atom.Ren et al. [22] established a three-dimensional transient heat transfer model of fused quartz LAM, and verified the accuracy of the model by measuring the temperature of the workpiece and the polycrystalline diamond tool.The geometric and thermal modeling of fused silica LAM is shown in Fig. 2. The governing equation is shown in formula (1): Among them, t is the time, T is the temperature, ρ is the density, c p (T ) is the specific heat, ω is the workpiece angular velocity, V z is the workpiece feed speed, k(T ) is the thermal conductivity, is the volume of heat generation, is the proportion of cutting heat absorbed by fused quartz and the tool, q machine is the cutting heat, q cont is the energy flowing into the tool due to the con- tact heat transfer, z, φ, r is the three-dimensional column coordinates. (1) Fig. 2 Geometry and thermal model of LAM of fused silica [22] Because there are many complex factors affecting the actual processing, analytical methods rarely provide accurate results.With the development of electronic computers, finite element method and finite difference method can effectively deal with various complex boundary conditions and nonlinear problems, so as to obtain accurate num-erical solutions.

Finite element method
The finite element method, which can add multiple initial temperatures and boundary conditions at the same time, is able to create more accurate thermal models, thus helping to efficiently solve heat transfer problems in machining processes.
In terms of laser-assisted turning, Song Panpan et al. [23] established a mathe-matical model of the temperature field of laser-assisted turning of silicon nitride ceramics combined with the heat conduction theory, and found that the isotherm on the surface of the workpiece presented an elliptical distribution, the thickness of the radial softening layer was small, and the temperature gradient was large.The experimental apparatus of laserassisted machining is shown in Fig 3 .Wang Zhida et al. [24] established a cutting simulation model of aluminumbased silicon carbide material with a volume fraction of 45%, and found that the stress value of the heated cutting model was smaller than that of the conventional cutting model, the laser-assisted cutting technology could effectively reduce the surface roughness of the workpiece, reduce surface topography defects, and reduce tool wear.Shen [25] developed a transient 3D thermal model of silicon nitride ceramics based on finite element analysis and verified the results experimentally.The model demonstrates that the heat generated during processing has little effect on the LAM and that laser power is a key parameter for successful LAM operation.Based on the simulation and analysis of the temperature field changes of laser-assisted heating Ti6Al4V, Wang Lihao et al. [26] adopted the finite element method to carry out a simulation study on the turning process of titanium alloy, and found that compared with traditional cutting, laserassisted cutting can obtain stable chips more quickly, with smaller main cutting force and longer tool life.In order to more accurately simulate the thermal field distribution of laser-assisted machining under thermal coupling, Germain G et al. [27] adopted ABAQUS/Standard subroutine to establish time-dependent moving heat flux on the top surface of the workpiece, established a thermodynamic coupling model to analyze the influence of laser on chip formation, and found that the temperature field could explain the reduction of cutting force.Xie et al. [28] developed a finite element model of thermodynamically coupled moving laser heating assisted turning process.Under the optimal laser preheating parameters, the critical cutting speed formed by the white layer on the processed surface of hardened steel is increased by 26.09% compared with con-ventional cutting.Roostaei et al. [29] used the finite element method to establish a three-dimensional transient heat transfer model during laser-assisted turning of SCFS ceramics, obtained the temperature field distribution of SCFS cylindrical specimens, and verified the validity of the numerical model through comparative experiments.Geng et al. [30] developed a three-dimensional transient heat transfer model based on finite element theory and performed machining experiments.It was found that the surface roughness and the thickness of the amorphous layer decreased with increasing rotational speed.Ductile mode removal was realized in μ-LAM without any decrease in surface integrity.
In terms of laser-assisted milling, Xu et al. [31] established a three-dimensional temperature field finite element model for laser multi-pass milling of Al2O3 ceramic material and simulated the movement of heat Fig. 3 Experimental apparatus of laser-assisted machining [23] source during multi-pass milling.The results show that with the increase of the laser scanning milling passes, the temperature of the spot center rises and the heat-affected zone gradually becomes larger, and the temperature gradient changes the most in the region where the laser scanning direction is changed.Liu et al. [32] adopted the sequential thermo-mechanical coupling finite element method and took a series of discrete time step temperature distributions as the initial temperature conditions, and found that the established analytical model of cutting force and cutting zone temperature had a good predictive effect to a certain extent.Shi Yuhao et al. [33] conducted finite element analysis on the milling process, milling force and milling area temperature of TC4 titanium alloy material under laser heating, and concluded that the optimal material removal temperature was 200-450℃.It was found that the feed force was most significantly reduced when the workpiece surface temperature increased from 200℃ to 450℃.Dong et al. [34] performed numerical modeling and experimental evaluation of laser-assisted micromachining of SiC/SiC ceramic matrix composites.Using a finite element-based three-dimensional transient thermal model, appropriate process parameters were predicted based on material removal temperature (Tmr) analysis, and the effect of LAMM on tool wear and tool life was evaluated.
In terms of laser-assisted turn-milling, Kim et al. [35] used laser heating assisted turn-milling to process SM45C.The finite element simulation and experimental results of temperature field showed that, compared with traditional turn-milling combined machining, the cutting force was reduced by 8%-14% and the surface roughness by 25%-40%.The experimental set-up for the laser-assisted turn-mill is shown in Fig. 4. Cha et al. [36] established a three-dimensional transient temperature field model for laser heating assisted turn-milling AISI 1045 steel, and the errors between the simulation results and the experimental results of cutting force and surface roughness and the experimental results were 2.42% and 9.82%, respectively.

Finite difference method
FDM is an approximate solution that replaces a continuous fixed solution region with a finite grid of discrete points to simplify model solving [37].FEM mainly consists of three basic difference formats, forward difference, backward difference, and center difference, the most common of which is center difference [38].Yan Cuo et al. [39] established a quasi-steady-state heat transfer model for laser heating assisted cutting of alumina ceramics by using FDM method, simulated the influence of different laser parameters on the temperature field distribution, and found that higher laser power, lower laser scanning speed and smaller laser spot radius were more conducive to softening the materials in the cutting area and improving the material removal rate.The temperature field distribution in the heated area of the workpiece is shown in Fig 5 .Rebro et al. [40] used FDM method to numerically evaluate the radial temperature gradient of LAM processed mullite material, and obtained the optimal laser power range of 170-190W.Jen et al. [7] established a quasi-steady state heat conduction model based on FDM and calculated the LAM model of thermal field of hot-sintered alumina ceramics.Wang et al. [41] used a 193 nm laser to irradiate a MgF2 window material and calculated the laser electric field distribution inside Fig. 4 Experimental set-up for the laser-assisted turn-mill [35] it using the three-dimensional time-domain finite-difference method.The simulations revealed that the back surface of the material was more easily damaged during laser irradiation, and the mechanical strength of the material was reduced.
From the above literature analysis, it can be seen that the current domestic and foreign scholars focus on the thermal model of LAM material removal process, from the optimization of thermal model to the establishment of the finite element model of thermal-coupled moving laser-assisted processing to the numerical evaluation of temperature gradient.The influence of temperature distribution on process parameters such as laser power, laser scanning speed, spot diameter, cutting speed and cutting depth, as well as the effect of temperature on material properties, force changes, chip changes and tool wear were studied.However, LAM process is a complex material removal process involving various mechanisms such as ablation, spallation, phase explosion and evaporation.Many fitting parameters and assumptions are required for thermal model construction, which will seriously affect the accuracy of numerical simulation.Under the premise of ensuring the simultaneous effect of laser technology and auxiliary machining methods, it is one of the key problems to solve at present to carry out thermal model numerical simulation under LAM thermodynamic coupling, considering the defects such as nucleation dynamics at the micro scale, and achieve efficient and high-precision numerical simulation Table 1.

Material removal process simulation
At present, in the simulation analysis of laser-assisted machining material removal behavior, modeling methods mainly include Finite Element Method (FEM), Smoothed Particle Hydrodynamics (SPH) and Molecular Dynamics method (MD).So far, most of the laser-assisted machining technology models are about thermal field assisted machining technology.Although progress has been made in two-dimensional and three-dimensional processing modeling, there are still many challenges in processing modeling.Commonly used simulation software include ANSYS, ABAQUS, LAMMPS and so on.

Material removal model based on FEM
At present, LAM material removal research based on finite element method modeling mainly includes laserassisted turning, laser-assisted milling, laser-assisted grinding and laser-assisted drilling, according to the classification of auxiliary processing methods.
In terms of laser-assisted turning, Song Hua Wei [42] established a cutting model with thermodynamically coupled non-uniform temperature field based on the finite element method, and found that the non-uniformity of laser heating leads to the change of the interfacial Fig. 5 Temperature field distribution in the heated area of the workpiece.[39] Table 1 The study of thermal modeling of material removal processes  It is found that laser scanning speed, laser power and undeformed chip thickness have the greatest influence on the metallurgical performance of the workpiece.Xu et al. [45] developed a fully coupled thermal model for the material removal process of Ti6Al4V laser-assisted turning, and found that the thermoplastic instability of the material and the thermal expansion of the material triggered by the laser heating are the main reasons for the formation of jagged chips.
In terms of laser-assisted milling, Niu Yin [46] established a mathematical model of the tool wear rate of laser-assisted micro-milling and found that, compared with conventional micro-milling, the milling force was reduced by 30%-40% and the tool wear was reduced by 50%.The comparison of chip morphology under feed per tooth when milling cutter angle is 150° is shown in Fig 7 .Feng Weiwei [47] studied the relationship between cutting force and tool wear in the process of laser-assisted micro-milling TC4, and found that compared with conventional micro-milling, the reduction of milling force Fx, Fy and Fz in laser-assisted micro-milling was 36%, 23.07% and 5.56%, respectively.The stress nephogram of different tool tip radius is shown in Fig 8.
In terms of laser-assisted grinding, Kim et al. [48] explored the LAM mechanism of silicon nitride ceramics and found that with the increase of laser power, the material temperature increased, and the amorphous material changed from shear chips to floating chips.The hardness of the material is reduced, the fluidity is enhanced, the material removal method is changed from brittle collapse to plastic removal, the processing difficulty is reduced, and the processing surface quality is improved.Ma et al. [49] established a grinding force prediction model in laser-assisted grinding process, which considered the comprehensive effects of mechanical properties of zirconia ceramic materials with temperature changes, grain-material microscopic interaction state, particle shape and random distribution, etc.The simulated distribution of workpiece surface grinding force under different laser powers was obtained, which was in good agreement with the measured force and the error could be controlled within 12%.The experimental set-up is shown in Fig 9 .He Yi et al. [50] performed laser-assisted scratching on a typical high-strength TC17 titanium alloy material using molecular dynamics simulations and experiments at different laser powers.The scratch force, material removal efficiency based on the scratched surface, and subsurface damage were analysed to determine the laser effects.A smaller scratch force can be achieved by laser assistance, and an appropriate laser power can enhance the material removal efficiency.The schematics of experimental setup is shown in Fig 10.
In terms of laser-assisted drilling, Zheng et al. [51] found that laser-assisted drilling mainly softens the surface material by laser, so as to avoid the situation of bit deflection, large axial force and severe bit wear caused by factors such as excessive hardness and strength of the material.The experimental setup for the laser-assisted drilling is shown in Fig 11 .From the above literature analysis, it can be seen that at present, domestic and foreign scholars mainly use finite element method to study the change of chip form, material removal form, surface hardness change and force change in the LAM process, lack of systematic modeling and analysis of LAM material removal behavior, and finite element method has high requirements on computer memory, large calculation amount and relatively time-consuming.How to build a simulation model for material removal in laser-assisted machining under the premise of ensuring the simultaneous effect of laser technology and auxiliary machining technology, and to achieve reliable simulation analysis, is one of the key problems to be solved at present.

Material removal model based on SPH
There are also many studies on LAM material removal based on the SPH method of modeling.Dan [52] explored the laser-assisted cutting performance of fused silica glass and used SPH to establish a cutting simulation model, and the simulation results showed that under high temperature conditions, the cutting force was reduced, the plastic fluidity of the workpiece material was enhanced, the chip morphology was gradually transformed to a continuous manner, and the stress concentration was alleviated.He et al. [53] used SPH to simulate and analyze the laser-assisted in-situ cutting process of monocrystalline silicon at different temperatures, and carried out experiments to prove that laser-assisted processing can effectively improve the brittle-plastic transition depth of monocrystalline silicon, thus realizing the processing of monocrystalline silicon in the plastic domain.The 3D SPH model is shown in Fig 12 .Balbaa et al. [54] investigated the effect of laser-assisted machining on the surface residual stresses in the cutting direction of CrNiFe alloy 718 using SPH modeling.The results show that LAM produces surface compressive residual stresses in the cutting direction, whereas conventional machining produces surface tensile residual stresses.The effect of LAM on residual stresses is due to the thermal softening effect of the laser beam in front of the tool tip, resulting in an overall greater tensile-plastic strain in the cutting direction.As a result, fewer tensile (and more compressive) residual stresses.
Wei et al. [55] established a micromechanical model based on multiphase modeling, and found that different relative positions between the tool and SiC particles would lead to different removal modes of SiC particles, including rolling or penetrating in Al matrix mode, breaking mode and pulling mode.Liu et al. [56] established a finite element model for high-speed grinding of particle-reinforced titanium matrix composites (PTMCs), and found that the grinding force during PTMCs grinding can be divided into matrix removal region and reinforcement particle removal region.It is suggested that the grinding removal behavior of PTMCs can be divided into four stages: matrix material shaping removal, initial crack formation of reinforced particles, crack propagation of reinforced particles and brittle removal of reinforced particles.The simulation results of material removal behavior of PTMCs under different grinding speeds is shown in Fig. 13.
From the above literature analysis, it can be seen that at present, when domestic and foreign scholars use SPH method to analyze the removal behavior of laserassisted machining materials, SPH particle refinement is limited by computer performance, and there is no hierarchical definition for materials processed by laser processing and auxiliary technology.Pulsed laser single point energy concentration, high local temperature, material properties will change with the temperature increase during processing.How to ensure the simultaneous function of laser and auxiliary machining Fig. 9 The experimental set-up.[49] technology, refine the parameters of contact materials, and realize the collaborative machining simulation of the two is one of the key problems to be solved at present.

Material removal model based on MD
Molecular dynamics simulation is a method based on classical mechanics, quantum mechanics and statistical mechanics, which uses computers to simulate the interaction of molecules and atoms and record their behaviors [57].MD model was first proposed by Alder [58] and is now widely used in various fields such as medicine, chemistry, biology, materials and machinery.
Chen et al. [59] studied the subsurface damage and phase transformation of single crystal Si caused by laser-assisted nano-cutting, and found that compared with the processing without laser assistance, the dislocation activity increased by about 8 × 10 14 times and the  Fig. 12 The 3D SPH model of grooving experiment of single-crystal silicon [53] improvement of the material removal rate, but also effectively reduced the stress concentration phenomenon and the degree of sub-surface slippage, and improved the surface morphology and quality.
Fang et al. [21] studied the removal mechanism of conventional cutting and process thermal laser-assisted cutting of binder-free polycrystalline tungsten carbide materials through simulation, and found that process thermal-laser-assistedcutting can help avoid subsurface crystal bending and reduce subsurface damage.The subsurface activities of In-LAC is shown in Fig 16 .Meng et al. [62] analyzed the removal behavior of singlecrystal SiC by the micro-laser-assisted method using a molecular dynamics approach, and analyzed in detail the microdeformation mechanism and processability under the micro-laser-assisted method.The results show that laser irradiation can directly affect the material removal rate and the depth of subsurface damage of 3C-SiC, and increasing the peak temperature in the processing region can improve the removal efficiency of single-crystal SiC.LAM has become a powerful method for cutting quartz glass, Liu et al. [63] investigated the effect of temperature on the structural evolution of silica glass through MD simulations.Cutting simulations were performed at different temperatures.The results show that the plastic deformation increases when the temperature is increased to 1500 K.The results of the simulations show that the temperature of the glass is not affected by the temperature of the glass.In addition, although the densification range is almost unaffected, the degree of densification near the machined surface is greatly reduced at high temperatures, which contributes to the theoretical study of precision machining of quartz glass by LAM.Yue et al. [64] simulated the EDM process by using a two-micron scale dual-temperature model, and found that the pressure generated inside the molten pool is one of the removal mechanisms of molten materials during EDM.Compared with single-crystal copper, the discharge on polycrystalline copper can produce more defect structures and larger denating-layer.
From the above literature analysis, it can be seen that MD model can accurately reproduce the material removal deformation process at the microscopic atomic level, but its calculation is very large and how to match the microscopic scale with the actual deformation, resulting in time consuming and certain unreliability of MD simulation.At present, there is a lack of analysis of molecular layer changes, lattice deformation and dislocation slip during LAM material removal at home and abroad, and there is no systematic analysis of the internal characteristics of materials processed by laser processing and auxiliary technology.Surface and subsurface defects in the process of laser-assisted machining have always been one of the difficulties to be studied.How to ensure Fig. 13 Simulation results of material removal behavior of PTMCs under different grinding speeds [56] the simultaneous effect of laser and assisted machining technology to refine the lattice deformation during processing is one of the key issues to be solved at present Table 2.

Experimental study of material removal behavior
Laser-assisted processing technology, that is, adding a laser module on the basis of the traditional processing system, utilizes the characteristics of non-contact laser technology to achieve efficient processing of difficult-tomachine materials, which does not cause serious damage to the material structure in the process [65], and through automated control, it can also achieve accurate removal of the damaged parts and process precise geometrical shapes [66].When Wahab et al. [67] conducted cutting tests on composite laminates using Nd:YAG laser, it was found that the effects of pulse energy and pulse frequency on the kerf width, heat affected zone and taper angle were significant, and that reducing the laser interaction time with the composite material could reduce the heat affected zone and kerf width.The interfacial bonding properties between the repair matrix and the repair material after removal of the damage site are also critical in determining the strength of the damage repair [68].
Rauh [69], C. Leone [70], RanTao [71], and Schmutzler [72] have found that the use of laser pretreatment can be effective in removing the surface resin layer of the material and exposing the internal carbon fibers.The schematic of laser treatment and strategy is shown in Fig. 17.It was also found that laser pretreatment improved the interfacial bonding strength between the repair matrix and the patch and the final repair results [73].Apply the above characteristics of laser technology to the actual processing, to realize the synergistic effect of laser and mechanical processing, which can ensure the processing accuracy and improve the production efficiency.
As an effective technology to improve the processing performance of materials, laser-assisted machining technology can well improve the mechanical properties of hard and brittle materials, composite materials, titanium and nickel-based superalloys and other difficult-to-process materials, and has been widely used in machinery, chemical industry, aerospace, nuclear industry and other fields [74][75][76][77].
In the processing of hard and brittle materials, Song et al. [78] investigated the effect of normal cutting and laser-assisted cutting on the surface quality of fused silica machining, and the experimental results showed that, relative to normal cutting, the laser-assisted cutting of the surface of the workpiece cracks, grooves and other defects are reduced, and the surface quality is higher, and the laser-assisted effectively improves the cutting performance of fused silica glass.Ren et al. [22] conducted LAM high-temperature and conventional low-temperature experiments to study the effects of machinability on sub-surface damage, chip morphology and tool wear.The results show that the machining performance of fused quartz in LAM is greatly improved, which is attributed to the changes of hardness, strength and viscoplasticity of fused quartz.The SEM images of chip morphology in LAM and CM is shown in Fig 18 .Jin et al. [79] conducted thermodynamic simulations and experimental studies using laser irradiated quartz glass.The results show that the laser-assisted micro-milling process, which can expand the ductile domain of the material, effectively improves the surface roughness of the fused silica workpiece, improves the surface machining quality, and reduces the tool wear at the same time.Liu Feng et al. [80] established a surface micro-texture profile evolution model based on ion beam assisted laser to achieve highquality controllable preparation of WC/Co cemented carbide micro-texture.Alessendro et al. [81] analyzed the formation mechanism of hot cracks caused by continuous laser ablation and the diffusion form of thermal stress in the ablation process, and found that by controlling the depth and expansion form of hot cracks, laserassistedgrinding can reduce the grinding force by about 30% on the premise of obtaining excellent surface quality.Chen et al. [59] studied the subsurface damage and phase transition of monocrystalline silicon caused by laser-assisted nano-cutting, and found that the critical tough-brittle transition depth of monocrystalline silicon increased from 150 to 395 nm when laser-assistedmachining was applied.The microgrooves machined using diamond cutting and the critical ductile-brittle transition depth of cut measured using WLI is shown in Fig 19 .Fang et al. [21] experimentally studied the removal mechanism of binder-free polycrystalline tungsten carbide material by conventional cutting and process thermal laser-assistedcutting, and found that the critical depth of non-crack on the surface of binder-free WC increased from 26.6 nm to 106.3 nm.The rotation taper cutting microgrooves and NOSC depths of the binderless WC is shown in Fig. 20.
Through the cutting test analysis of silicon nitride ceramics, Wu Xuefeng et al. [82] found that the machinability of the material could be improved by appropriately increasing the material temperature during laser-assisted processing, and continuous chips with local cracks could be obtained, and tool wear could be reduced, resulting in fewer surface cracks.
In terms of composite material processing, Ma et al. [83] found that the processing parameters that have the greatest impact on surface roughness are laser power, followed by grinding depth and grinding wheel speed, and finally feed speed.The optical images of machined surfaces is shown in Fig 21 .Compared with conventional grinding, the removal mode of alumina ceramics under laser-assistedgrinding changes from brittle fracture to plastic fracture.Li et al. [84] used laser structure grinding to process grooves on the surface of silicon nitride, and the normal grinding force could be reduced by up to 63%.Clemens et al. [85] used laser induced surface periodic structure of Al 2 O 3 -ZrO 2 -Nb composite ceramic materials to improve the wetting angle of the material, and found that the wetting angle of the composite material could be adjusted by controlling the energy density of the laser.Li Shiyu et al. [86]    3.38 × 10 6 W/cm 2 , two orders of magnitude higher than the previous LAM study.It can achieve a cutting speed of 565 m/min, increase productivity by more than 2.7-140 times, and in addition to reducing tool wear, surface quality is also significantly improved.Chen et al. [88] proposed a laser ablation pretreatment milling (LAPM) process for milling experiments on SiCf/SiC composites, and the results showed that compared with the traditional milling process, the cutting allowance could be obtained only once after a depth-controllable laser ablation treatment, which greatly improved the machining efficiency.The material removal rate is also greatly improved under the premise of ensuring the machining quality.
In terms of difficult-to-process materials such as titanium alloy and nickel-based superalloy, Rashid et al. [89] performed laser assisted turning of Ti-6554 alloy at different feeds and cutting speeds for laser-assisted turning of Ti-6554 alloy.It was found that compared with the conventional cutting results, the cutting force can be reduced by up to 15 percent.The improvement of machining performance has contributed to the improvement of machining efficiency, surface integrity and reduction of tool wear of Ti-6554 alloy.machining efficiency, surface integrity, and tool wear reduction of titanium alloys.Kalantari et al. [90] conducted experimental studies on conventional turning and laser-assisted turning of Ti-6Al-4 V, and found that the microhardness, surface roughness and grain size increased under LAM processing method decreased.Kong et al. [91] conducted an experimental and simulation study of laser-assisted machining of TC6 titanium alloy, considering the effect of LAM (laser-assisted machining technology) on reducing the magnitude of fluctuations in cutting forces, and the experiments showed that LAM resulted in flatter The profile line location is determined by the first surface crack position.The bottom of groove refers to the entrance position of taper cutting [21] machined surface profiles and reduced the number of surface defects.Du et al. [92] proposed synchronized nanosecond laser-assisted electrochemical machining for efficient fixed-domain removal of titanium alloy materials.Under the parameters of electrolysis voltage of 20 V, laser power of 3-5 W, and feed rate of 1.8 mm/min, high efficiency and low surface roughness machining of titanium alloy can be realized.He et al. [93] proposed a new method for picosecond laser-assisted belt grinding of TC17 titanium alloy.Compared with single-grit scribing, picosecond laser-assisted single-grit scribing significantly reduces the main cutting force, and appropriate laser power-assisted machining improves the material removal capability, and the force-heat coupling in the grinding area is reduced, which improves the surface finish quality.Razavi et al. [94] evaluated the machining specific energy, surface roughness, tool wear and chip morphology of Inconel718 during pulsed LAM.The results show that the surface roughness and tool wear are reduced by 22% and 23% respectively, and the specific machining  (c, d) laser-assisted grinding.[83] energy is reduced by 35% (temperature 540 °C) compared with the traditional machining method.Pan et al. [95] conducted experiments to explore the law of the machining parameters on the heat affected area of the workpiece surface in laser-assisted milling Inconel718.Venkatesan et al. [96] further found that the thermal influence of laser-assisted machining can soften the surface material, thereby reducing the cutting force, surface roughness and sub-surface damage, and improving the tool life.The same conclusion also appeared in the study of Habrat et al. [97], and the high temperature and rapid cooling of the surface during laser-assisted processing would lead to martensitic phase transformation on the surface of titanium alloy, thus changing the structure of the material surface.Xu et al. [98] combined the results of traditional milling and single laser scanning machining Inconel718, conducted a comprehensive study on the surface integrity of laser-assisted milling process, and found that the processed workpiece had less damage in terms of white layer and material resistance, but the fatigue performance was reduced.Liao et al. [99] further found that compared with Inconel718 roughed samples using the same milling parameters, the laser-assisted milling samples had much smaller lattice deformation layers, indicating that the machining properties and surface integrity of LAM were improved.Shang et al.The workpiece machined with different methods is shown in Fig 23 [100] proposed a new reverse heat laser-assisted milling method to process Inconel 718, which solved the problem of reverse heat conduction in laser thermal placement.Compared with the conventional dry milling process, the peak and average main cutting forces of laser-assistedmilling after path Fig. 22 Influence of different laser pulse width on the morphology of the kerf.[86] optimization are reduced by 55% and 47.8%, respectively, and the surface roughness is increased by at least 14%.The microstructure of path-optimised LAMill and dry milling subsurface is shown in Fig 24 .Shelton et al. [101] conducted the results of Laser-assisted micromachining side-cutting tests of Ti64 and Inconel718, which showed that the burrs on the surface of the material were substantially reduced and the surface quality was improved by using the laser-assisted.
From the above literature analysis, it can be seen that there is currently a lack of collaborative control analysis of laser-assisted processing material removal process at home and abroad.Due to the single-point energy concentration of the pulsed laser, the local temperature increases, and the material properties change.How to ensure the simultaneous role of laser and auxiliary processing technology, to realize the collaborative processing of the two, and to refine the concept of hierarchical design of tools and materials is one of the key issues to be solved at present.

Laser-assisted processing material removal
problem to be solved (1) Precise control of the temperature distribution of the workpiece is one of the key technologies of laserassisted machining.Analytical method and finite element method are effective research methods for analyzing laser-assisted machining, but the nucleation defect law of material removal process needs to be further explored.Laser-assisted processing involves coupling of multiple physical phenomena, and scholars at home and abroad mainly use analytical method, finite element method and finite difference method to calculate and predict the temperature field distribution in laser-assisted machining.The researchers explored how the temperature distribution is affected by process parameters such as laser power, laser scanning speed, spot diameter, cutting speed, and cutting depth, and studied the effects of temperature on changes in material properties, force changes, chip changes, and tool wear.However, the numerical simulation of large area or long time temperature field in the laser-assistedmachining process still has some shortcomings, which is one of the key problems to be solved at present.(2) The simulation modeling method can intuitively and clearly obtain the material removal results of laser-assisted machining, but the correlation law of contact material parameters needs to be further clarified.Domestic and foreign scholars mainly used FEM, SPH and MD simulation methods to study the changes of chip morphology, material removal forms, surface hardness and force changes during LAM process.However, the definition of molecular layer change, lattice deformation, dislocation slip and material stratification during LAM material removal has not been systematically ana- lyzed.The single point energy concentration of the pulsed laser makes the local temperature higher, and the material characteristics will change with the temperature increase during the processing.How to ensure the simultaneous function of laser and auxiliary machining technology, refine the parameters of contact materials, and realize the collaborative machining simulation of the two is one of the key problems to be solved at present.(3) Experimental study can obtain the actual results of laser-assistedmachining accurately and effectively, but the precision control of material removal needs to be further analyzed.For the detection of LAM material removal behavior, domestic and foreign scholars mainly explore the experimental factors including laser parameters and auxiliary processing parameters.Such as the speed of the cutting system, feed speed, cutting depth and pulse frequency of the laser system, pulse duty ratio, heating time, defocusing amount, the angle of the tool and the spot, the distance between the tool tip and the spot, etc.The detection of the experimental results mainly includes the composition analysis of energy spectrometer, the surface topography detection, the dynamic thermal detection, the material removal detection and so on.However, most of the existing experimental studies are sequential processing, and how to realize the collaborative experimental processing of the two is one of the key problems to be solved at present.

Outlook and conclusion
In combination with the above analysis, in order to solve the key problems of laser-assistedtechnology and adapt to the major development needs of the country, laserassisted processing material removal technology needs to develop in the direction of intelligent, high-performance processing, surface performance strengthening and green.

Intelligent laser-assistedprocessing
At present, laser-assistedmachining is mostly controlled by manual input parameters, but parameter optimization in the machining process has a non-negligible advantage in obtaining high-performance surfaces.And laser-assisted machining has great advantages in surface microstructure creation, surface precision control, extending tool life, etc.In order to reduce production costs, improve efficiency and ensure processing quality, it is necessary to integrate laser-assistedprocessing with information technology, and introduce big data, cloud computing, machine vision and other technologies.Intelligent processing reasoning and decision, process parameter prediction and optimization, error analysis and compensation have become the main development trend of intelligent processing.

Laser-assistedprocessing of high performance surfaces
Laser-assistedmachining can reduce surface damage, reduce cutting force and extend tool life through toughness removal.However, due to the diameter of the light source and other reasons, there are great limitations in the processing efficiency of laser-assisted machining, and usually the thermal stress of the laser will degrade the surface properties to a certain extent.In view of the outstanding advantages of ultrasonic vibration assisted machining in processing efficiency and surface strengthening, it is an important research direction in the future to integrate laser-assistedmachining and ultrasonic vibration assisted machining technology, design a "laser-ultrasonic" multi-energy field assisted machining device for efficient and high-performance surface processing, and reveal the law and mechanism of material removal under the action of multi-energy field.

Laser aided machining surface performance enhancement
At this stage, laser-assistedprocessing is mostly used for the removal of hard and brittle materials, only using the advantages of laser processing in temperature field control, laser processing technology has more excellent properties to be developed in the field of laser-assistedprocessing.Laser induced directional deposition and powder bed laser cladding are widely used in additive coating manufacturing, and the mechanical properties and binding strength of coatings are the key technologies that restrict their application.The abrasive belt grinding and laser induced directional deposition technology can simultaneously complete the production of increasing and decreasing materials, reshape the surface of the substrate to strengthen the coating bonding force, and repair the coating thickness to prevent the coating from falling off.Laser sand belt collaborative surface strengthening technology can improve the surface of the coating and strengthen the mechanical properties of the coating surface through rapid laser impact.The lasersand belt grinding composite processing technology can realize collaborative and mutual assistance processing according to demand, and further deepen the coating processing and strengthening technology, which is one of the important development directions of laser-assistedprocessing technology in the future.

Laser-assistedprocessing greenization
Laser-assistedmachining, as a heat-assisted machining method, has an incomparable advantage in improving the performance of difficult-to-process materials due to its portable disassembly and strong operability.Compared with other processing methods, laser-assistedprocessing can be selective and real-time processing of the processing area, avoid unnecessary energy loss and thermal damage during workpiece processing, can also reduce the cutting force, wear, improve the workpiece surface processing quality, greatly saving energy consumption and improve the material removal rate and the service life of the workpiece.It is a feasible method to achieve green manufacturing and can promote the realization of the national strategic goal of "carbon peak and carbon neutrality" .
friction coefficient of tool chips and the non-uniformity of residual strain in the cutting zone, resulting in the transformation of chips from spiral to various forms, including collapse, block, curl, bifurcation, etc.The effect of temperature inhomogeneity on chip morphology is shown in Fig 6.Ayed Y et al.[43] studied the chip formation process during laser-assisted machining.The model built can better optimize the laser and cutting parameters, and improve the understanding of the physical phenomena of chip formation and cutting force reduction during laser-assisted machining.Khatir et al.[44] developed a finite element model composed of mechanical model, thermal model and thermo-mechanical coupling model, and analyzed the heat affected zone, hardness change of machished surface and white layer formation during laser-assisted turning of AISI4340 hardened steel.

Fig. 10
Fig. 10 Schematics of experimental setup.a Laser-assisted scratching.Surface/subsurface alterations occur at the affected layer.b Laser scanning trajectory generated by the motion of workpiece.c Morphology of laser-assisted scratching with increasing laser power.Ablation occurred when the laser power was 4.8 W. d Laser-assisted belt grinding for verification.[50]

Fig. 11
Fig.11lmage of the experimental setup for the laser-assisted drilling process.[51]

Fig. 14 Fig. 15 Fig. 16
Fig. 14 MD simulation of thesc-Si nanometric cutting process.a and (b) are the snapshot of the cutting process with the temperature in the contact region of 300 K and 700 K, respectively; (c) and (d) are the distribution of the dislocations presented in (a) and (b), respectively.[59] conducted laser planarization orthogonal experiments on CVD polycrystalline diamond films to analyze the influence of laser parameters on cutting quality.The results show that the factors affecting kerf taper are pulse width, pulse frequency, feed speed and laser current in turn, and the factors affecting line roughness are feed speed, laser current, pulse frequency and pulse width in turn.The influence of different laser pulse width on the morphology of the kerf is shown in Fig 22.Wei et al.[87] adopted high-speed and high-power density laser-assistedturning of Al-SiC metal matrix composites, and the maximum laser power density reached

Fig. 19
Fig.19 Microgrooves machined using diamond cutting and the critical ductile-brittle transition depth of cut measured using WLI: (a) without laser assistance; (b) with laser assistance.[59]

Fig. 20
Fig. 20 Rotation taper cutting microgrooves and NOSC depths of the binderless WC.(a) Conventional taper cutting result and (b) In-LATC result.The profile line location is determined by the first surface crack position.The bottom of groove refers to the entrance position of taper cutting[21]

Fig. 24
Fig.24 Microstructure of path-optimised LAMill and dry milling subsurface: at edge area gotten by path-optimised LAMill (a) and dry milling (b), at centre area gotten by path-optimised LAMill (c) and dry milling (d).[100]

Table 2
The Study of material removal process simulation