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Article

Surface Integrity and Machining Mechanism of Al 7050 Induced by Multi-Physical Field Coupling in High-Speed Machining

by
Wei Lu
1,
Chenbing Ni
1,2,*,
Youqiang Wang
1,2,*,
Chengguo Zong
3,
Dejian Liu
1 and
Xingbao Huang
4
1
School of Mechanical and Automotive Engineering, Qingdao University of Technology, Qingdao 266520, China
2
Key Laboratory of Industrial Fluid Energy Conservation and Pollution Control, Ministry of Education, Qingdao 266520, China
3
College of Mechanical and Electronic Engineering, Shandong University of Science and Technology, Qingdao 266590, China
4
State Key Laboratory of Bridge Safety and Resilience, College of Civil Engineering, Hunan University, Changsha 410082, China
*
Authors to whom correspondence should be addressed.
Lubricants 2025, 13(2), 47; https://doi.org/10.3390/lubricants13020047
Submission received: 6 December 2024 / Revised: 17 January 2025 / Accepted: 21 January 2025 / Published: 22 January 2025
(This article belongs to the Special Issue Friction and Wear of Alloys)
Browse Figures
Versions Notes

Abstract

:
Improving the surface quality and controlling the microstructure evolution of difficult-to-cut materials are always challenges in high-speed machining (HSM). In this paper, surface topography, defects and roughness are assessed to characterize the surface features of 7050 aluminum alloy (Al 7050) under HSM conditions characterized by high temperature, strain and strain rate. Based on multi-physical field coupling, the mechanism of microstructure evolution of Al 7050 is investigated in HSM. The results indicate that the surface morphology and roughness of Al7050 during HSM are optimal at fz = 0.025 mm/z, and the formation of surface defects (adherent chips, cavities, microcracks, material compression and tearing) in HSM is mainly affected by thermo-mechanical coupling. Significant differences are observed in the microstructure of different machined subsurfaces by electron backscatter diffraction (EBSD) technology, and high cutting speeds and high feed rates contributed to recrystallization. The crystallographic texture types on machined subsurface are mainly {110}<112> Brass texture, {001}<100> Cube texture, {123}<634> S texture and {124}<112> R texture, and the crystallographic texture type and intensity are significantly affected by multi-physical field coupling. The elastic–plastic deformation and microstructural evolution of Al7050 alloy during the HSM process are mainly influenced by the coupling effects of multiple physical fields (stress–strain field and thermo-mechanical coupling field). This study reveals the internal mechanism of multi-physical field coupling in HSM and provides valuable enlightenment for the control of microstructure evolution of difficult-to-cut materials in HSM.

1. Introduction

High-speed machining technology is applied widely in the machining of difficult-to-cut (DTC) materials due to its outstanding advantages such as high productivity, machining accuracy, good surface quality and low production cost [1,2]. However, the HSM of DTC materials still faces the challenges of improving surface quality and controlling microstructure evolution. Therefore, exploring the surface quality and subsurface microstructure evolution of DTC materials in HSM is crucial [3,4,5].
Research on the cutting of DTC materials has long been favored by experts and scholars [6,7]. Huang et al. [8] investigated the effect of machining parameters on the cutting force of Al 7050-T7451, and they found that the feed rate and the rake angle of the tool played a major role. Shi et al. [9] pointed out that the starting/ending point, rotation sense and trajectory radius of the tool path significantly affect the cutting force of Al 7050 by altering the thickness of uncut chips. Zhang et al. [10,11] studied the machining features of Al 7050-T7451 by cutting test and finite element (FE) simulation, and they found that the tool rake was negatively correlated with the force and temperature, and the roughness and residual stress were significantly affected by the cutting speed and feed rate. Yao et al. [12] explored the surface morphology of Al 7075 by high-speed milling and found that increasing the cutting speed and reducing the feed can reduce the surface roughness, the feed per tooth has the most serious effect on the surface morphology and the milling depth has a smaller impact on the surface quality. Patel et al. [13] analyzed the surface quality of Al 7075 and optimized its machining performance by principal component analysis combined with the JAYA algorithm, and they found that the absolute error percentage was 7.97% through experimental verification. Imbrogno et al. [14] conducted high-speed cutting of AA7075-T6 under dry and cryogenic conditions, and they found that the surface roughness value was higher under cryogenic conditions, and the grain refinement caused by high-speed machining was the main contribution to hardness. Ates et al. [15] analyzed the machining deformation of 2050 and 7050 aluminum alloys and found that the spindle speed and feed rate had the most obvious effect on the workpiece deformation. Szablewski [16] analyzed the surface morphology of high-speed cutting Inconel 718 alloy with ceramic and cemented carbide tools, and found that ceramic tools can obtain a better machined surface and obtain the optimal processing parameters. Based on molecular dynamics simulation, Li et al. [17,18] ceramic tools were used to grind gallium nitride crystals, and they found that the grinding force, stress and subsurface damage depth were significantly affected by the lateral interaction distance, while the dislocation length had no obvious correlation with it. In addition, a theoretical model of normal scratch force and surface morphology was further established.
During material processing, the generation of plastic deformation invariably results in a change in microstructure, which can have a significant impact on the material’s properties [19]. Li et al. [20,21] investigated the plastic deformation, microstructure texture evolution and tribological behavior of Ti-6Al-4V alloy during HSM. The study showed that cutting speed does not play a positive role in the orientation density of the texture, and the evolution of crystal texture caused by machining would change surface integrity and mechanical performance. Ni et al. [22] experimentally studied the cutting performance of Al 7050 based on various crystal orientations, and they found that the initial crystal orientation significantly affects the cutting performance. Under high-speed machining, Liu et al. [23] explored the microstructure evolution of oxygen-free high-conductivity copper, and it was pointed out that the physical fields are inhomogeneous, resulting in a gradient distribution of microstructure. Wang et al. [24,25] studied the crystal texture evolution and stress–strain evolution of Ti-6Al-4V alloy on chips and surfaces based on cutting experiments, FE simulation, and the visco-plastic self-consistent (VPSC) model. The study showed that shear deformation determines the activation of the slip system and its relative activity, which helps to control the texture change and predict the dynamic behavior.
The actual processing of difficult-to-machine materials is subject to complex multi-field coupling, including mechanical forces, thermal forces, electromagnetic forces and chemical reactions [26,27,28,29]. Qi et al. [30] established the mapping relationship between the multi-physical field (equivalent plastic strain, temperature) and the microstructure evolution of Inconel 718 during the cutting process, and the study indicated that high cutting speed contributes to grain refinement, and that high temperature and equivalent plastic strain are the main factors for grain refinement. Zhou et al. [31] constructed the cutting force model of milling from the perspective of a multi-physical field, explored the influence of milling parameters on cutting force and revealed the interaction mechanism of tool geometry and tool angle on cutting force components. Li et al. [32] analyzed the synergistic effect of thermal field and electrochemical field coupling on the removal mechanism of Ti6Al4V material processing, and revealed the internal mechanism of the combined action of laser field and electric field in composite processing. Gao et al. [33] used an experimental combined with finite element simulation method to cut ATI 718 plus alloy at high speed, analyzed the multi-physical field distribution and EBSD test results under different parameters and indicated that the microstructural evolution mechanism was CDRX-induced grain flake refinement and DDRX-induced grain growth.
Literature review shows that the machining of DTC materials faces problems in improving the surface quality and controlling the microstructure evolution, and research on the surface integrity and machining mechanism under the coupling of multiple physical fields during the cutting process is not available. Therefore, considering the influence of the coupling of multi-physical fields on surface integrity and machining mechanism in the machining process, a comprehensive study combining simulation and experiments is conducted to reveal the machining mechanism under the coupling effect of multiple physical fields. This paper firstly characterizes and analyzes the surface quality and microstructure of Al 7050 under different cutting parameters utilizing high-speed dry cutting experiments and advanced characterization tools. Then, combined with FE simulation, the machining and microstructure evolution mechanisms under the coupling of multiple physical fields of cutting force, temperature field, stress field and strain field were revealed. It provides a reference basis for improving the surface quality and controlling the microstructure evolution of DTC materials under high-speed dry-cutting conditions.

2. Experiments and Modeling

2.1. Experimental Equipment and Method

Al 7050 was used as the experimental material for machining, and its chemical composition and mechanical properties are given in Table 1 and Table 2, respectively. To explore the effect of cutting parameters on surface quality and microstructure evolution, its specific experimental scheme is shown in Table 3. The high-speed machining experiment was conducted on the MV-820 CNC machining center (nmax = 8000 r/min) using the APKT1604PDER-MA H01 carbide insert (KORLOY Corp., Seoul, Republic of Korea) with a tip angle of 85°, a normal back angle of 11° and a corner radius of 0.2 mm, as illustrated in Figure 1. The diameter of the cutter head used in the high-speed cutting experiment is 80 mm, and only one cutting insert is clamped on the cutter head to mill a workpiece with dimensions of 15 mm (L) × 15 mm (W) × 10 mm (H) using end-face milling. During the cutting process, the cutting force was captured with a YD15-111 three-axis dynamometer with a sampling frequency of 500 kHz.
After the HSM experiment, the surface morphology and defects were tested using an optical microscope produced by Shanghai Optical Instrument Factory No. 1 and a MERLIN compact field emission scanning electron microscope. The microstructure characteristics of the machined surface layer were measured by the EBSD technique, and the specimens were treated by sandpaper friction electrolysis and mechanical polishing before testing. The specimens were taken for electrolytic polishing and the electrolytic solution was a 1:9 mixture of perchloric acid and anhydrous ethanol with an electrolytic voltage of 30 V. Immediately thereafter, the specimens were placed in anhydrous ethanol for ultrasonic cleaning of residual adhesions, and then the microstructure was measured using MERLIN compact field emission scanning electron microscope.

2.2. FE Simulation

Based on the actual insert specifications and cutting conditions, a 3D FE simulation model for high-speed cutting of Al 7050 was established by ABAQUS 2020 software, as shown in Figure 2. The tool is in rotational motion relative to the workpiece and the tool is defined as a rigid body. For the meshing of the model, the thermo-mechanical coupling plane strain four-node quadrilateral element (CPE4RT) was employed to divide the grid, and the high-density grid was used in the tool–work contact area, and the sparse grid was used in other areas. The simulation employed explicit thermomechanical coupling analysis and described the formation of chips and material failure based on strain-based material fracture and separation criteria from a physical standards perspective. The contact between the tool and workpiece was modeled using a modified Coulomb cohesive sliding friction model.
HSM is an extremely complex thermo-mechanical coupling process. In order to establish the above model, simplify the modeling process and improve efficiency, the following basic assumptions need to be made [34,35,36,37,38,39]:
(1)
The tool is a rigid body, only considering the heat conduction of the tool.
(2)
The changes in metallographic structure and other chemistry caused by temperature change during processing are not considered.
(3)
The material is isotropic, and the vibration of the tool and the workpiece is not considered.
The constitutive equation describes the dynamic mechanical properties of the material, and the constitutive equation adopted the Johnson–Cook (J–C) equation [39], whose specific form is given in Equation (1).
σ 0 = ( A + B ε n ) ( 1 + C ln ( ε ˙ ε ˙ 0 ) ) ( 1 − ( T − T 0 T m − T 0 ) m )
where ε , ε ˙ and ε ˙ 0 are equivalent plastic strain, equivalent plastic strain rate and reference plastic strain rate, respectively; T , T 0 and T m are the temperature of the cutting part of the workpiece, the initial temperature of the workpiece and the melting point of the material, respectively; A , B and C are the initial yield stress, the hardening coefficients and the strain rate coefficients, respectively; n and m are the strain-hardening index and the thermal softening index.
The parameters of the J–C constitutive for Al 7050 are presented in Table 4.

3. Results and Discussion

3.1. Surface Quality in HSM

3.1.1. Effect of Cutting Speed on Surface Morphology and Defects

In HSM, the effect of cutting speed on surface morphology and defects is presented in Figure 3. The machined surface exhibits more prominent apophysis and furrows at Vc = 500 m/min, resulting in poor surface flatness and quality. This is because the built-up edge in front of the tool tip is easy to form at low-speed cutting, and the built-up edge replaces the cutting edge to cut the workpiece [10,41]. It is further found that there are cavities and microcracks on the surface, and the microcracks extend outward from the cavities, and EDS analysis shows the composition is the same as that of the material matrix. This is due to the exfoliation and fragmentation of the grains on the surface of the material during HSM to form the cavities, coupled with the strong shear extrusion of the tool on the surface, which results in the appearance of outwardly extending microcracks around the cavities [4,42].
By increasing the cutting speed to 1000 m/min, the apophysis on the machined surface becomes smaller, while the phenomenon of chip adhesion becomes noticeable. The reason is that as the cutting speed increases, the metal removal rate per unit of time increases, and the cutting heat generated by the deformation resistance and friction of the material also increases [10,11], resulting in material softening and chip adhesion. Surface defects such as long strips, granular adhesion and scratches can be observed, and irregular scratches appear on the surface due to the rotation of slender chips with the spindle and the rapid movement on the surface of the material [4,43]. The chemical composition of the adhesive material contains higher C and O elements, which are oxidized by the chip adhering to the machined surface and the temperature rise caused by the cutting heat.
As the cutting speed is further increased to 1500 m/min, the apophysis becomes less pronounced and a large number of adherent chips and visible scratches become apparent. In addition, there are obvious smeared metals and tearing on the surface, broken small material with the tool on the machining surface movement and extrusion on the surface of the workpiece to produce tearing [44]. The analysis indicates that the high cutting speed generates more cutting heat, resulting in increased adherent chips, and the impact extrusion of the tool is also enhanced, resulting in the surface residue being ironed on the machining surface [7,43], and chips or shedding particles following the movement of the tool cause scratches [4,22].

3.1.2. Effect of Feed Rate on Surface Morphology and Defects

The effect of feed rate on surface morphology and defects is illustrated in Figure 4. At fz = 0.025 mm/z, the surface is smooth, resulting in good overall surface quality. Burrs and small particles adhere to the machined surface, and the main elements in the chemical composition of the burr are the same as the matrix material, which is the result of plastic deformation caused by high-speed impact and compression of the tool on the surface [43,45].
As the feed rate increases, scratches, apophysis, cavities and large amounts of adherent chips emerge, leading to a gradual deterioration in surface morphology. For the surface defects with fz =0.1 mm/z, the content of C and O elements in the chemical composition of the adhesive chip is higher. The microcracks are distributed around the adherent material and extending into the interior of the substrate at fz = 0.125 mm/z, and the chemical composition of the microcracks is the same as the material matrix composition, so the microcracks are caused by the extrusion effect and thermal stress [42,43,46].

3.1.3. Effect of Cutting Parameter on Surface Roughness

The roughness parameter Ra is employed to further quantitatively characterize the effect of cutting parameters on surface quality, as illustrated in Figure 5. The surface roughness rises and subsequently falls with increasing cutting speed, as depicted in Figure 5a. The increase in cutting speed results in drastic plastic deformation of the material, an increase in cutting force and easy formation of a built-up edge [22,41], resulting in maximum surface roughness at Vc = 750 m/min. A further increase in cutting speed gradually decreases the surface roughness due to the effect of thermal softening and the inhibition of high speed on the built-up edge [4,41], and Δ = 0.027 μm is the difference between the minimum and greatest surface roughness. With the increase in feed rate, apophysis, cavities and large amounts of adherent chips emerge on the surface, increasing surface roughness [41,45].

3.2. Characterization of Microstructure Evolution in HSM

The microstructure of the machined subsurface at Vc = 1000 m/min is illustrated in Figure 6. The grains in the subsurface have been deformed into small-sized grains with different orientations by the force/thermal coupling effect of the cutting process [41], and the grains are mainly dominated by red and green colors, shown in Figure 6a, with the red color representing the <001> crystal orientation and the green color representing the <101> crystal orientation. As exhibited in Figure 6b, the proportions of recrystallization grains, substructured grains and deformation grains on the machined subsurface are 12%, 60% and 28%, respectively. Due to the high stacking fault energy of aluminum alloy, the material in the HSM process is dominated by dynamic recovery [47], and the strong dynamic recovery inhibits the recrystallization process, so that the processed subsurface has more substructured grains and fewer recrystallization grains. Figure 6c shows the misorientation angle (MA) distribution, in which the largest percentage of grain boundaries with MA ≤ 2° is about 54%, of low-angle grain boundaries (LAGBs) (2° < MA ≤ 15°) is about 9% and of high-angle grain boundaries (HAGBs) (MA > 15°) is about 37%. Due to the high mismatch of HAGBs, the grains near the grain boundaries will be limited, and the formation of recrystallized structures may be inhibited. The crystallographic textures in the machined subsurface at Vc = 1000 m/min are illustrated in Figure 6d,e, where the orientation distribution function (ODF) maps take φ2 to be 0°, 45° and 90°, respectively. The main crystallographic textures are {001}<100> Cube texture, {110}<112> Brass texture, {338}<344> D texture, {123}<634> S texture and {124}<112> R texture. Among them, the {338}<344> D texture intensity is higher, and the other texture intensity is weaker. In the HSM process, there is grain deformation and individual grains along the slip direction on the slip surface [19], thus making the original orientation of the polycrystalline grains in the spatial distribution of different grains in the spatial distribution of a certain intensity of the texture.
The microstructure of the machined subsurface at Vc = 1500 m/min is illustrated in Figure 7. The grains are mainly dominated by green and blue colors, exhibited in Figure 7a, and the crystal orientation is different from Figure 6a. Compared with Figure 6b, the percentages of recrystallization grains, substructured grains and deformation grains on the machined subsurface shown in Figure 7b increased by 5%, decreased by 10% and increased by 5%, respectively. Due to the high cutting speed, the workpiece undergoes severe plastic deformation under the action of force/thermal coupling and affects the dislocation proliferation and annihilation to promote recrystallization and deformation behavior. Compared with Figure 6c, the proportions of MA ≤ 2°, LAGBs and HAGBs presented in Figure 7c increased by 3.5%, increased by 3.1% and decreased by 6.6%, respectively. The high cutting forces and cutting heat during HSM cause plastic deformation of the material, leading to grain deformation, a lot of dislocations and grain boundary migration, increasing LAGBs. As shown in Figure 7d,e, the main crystallographic textures are {110}<112> Brass texture, {112}<110> R-Copper texture, {123}<634> S texture and {124}<112> R texture, all of which have high texture strength. Compared with Figure 6d,e, there is {112}<110> R-Copper texture, no {338}<344> D texture, and the texture intensity of {110}<112> Brass texture, {123}<634> S texture and {124}<112> R texture are enhanced. The high cutting speed increases the cutting temperature and strain rate and further activates the internal movement of the crystal, with increased crystal rotation and distortion [17], resulting in increased texture strength.
The microstructure of the machined subsurface at fz = 0.125 mm/z is illustrated in Figure 8. The crystal orientation shown in Figure 8a is significantly different from that shown in Figure 6a. As depicted in Figure 8b, the proportions of recrystallization grains, substructured grains and deformation grains on the machined subsurface are 19%, 56% and 25%, respectively. Compared with Figure 6b, the percentage of recrystallization grains, substructured grains and deformation grains increased by 7%, decreased by 4% and decreased by 3%, respectively. Due to the large amount of cutting heat generated by a large feed rate, high cutting temperature promotes recrystallization behavior. As shown in Figure 8c, the proportions of MA ≤ 2°, LAGBs and HAGBs are 54%, 8.6% and 37.4%, respectively. High temperatures can promote the migration and recrystallization of grain boundaries, thus forming HAGBs. The main crystallographic textures are the {110}<112> Brass texture, {123}<634> S texture, {124}<112> R texture, and {110}<100> Goss texture shown in Figure 8d,e. Compared with Figure 6d,e, there is a {110}<100>Goss texture present, and the texture intensity of {110}<112> Brass texture, {123}<634> S texture and {124}<112> R texture are enhanced. With the increase in feed rate, the material removal rate and friction effect are enhanced, resulting in increased cutting heat and higher temperature, and the dynamic recrystallization {124}<112> R-oriented recrystallized grains grow preferentially, which makes the {124}<112> R texture strength high. The change in feed rate leads to different shear deformation of the deformed layer, and the texture type and strength change [48].

3.3. Mechanism of Multi-Physical Field Coupling in HSM

3.3.1. Cutting Force

The effect of cutting parameter on cutting force in three directions is presented in Figure 9. With the increase in cutting speeds, the cutting force in three directions increases first and then decreases, shown in Figure 9a. The cutting force at Vc = 1000 m/min is the maximum for both the X and Z directions, while the cutting force in the Y direction first shows a decrease after Vc = 750 m/min. Analysis indicates that cutting speed gradually increases, so that the tool–chip friction becomes more intense [49], and the cutting force in three directions increases. The cutting speed keeps on increasing, and the cutting heat generated by the severe tool–chip friction increases, and the chip is unable to take away more heat. Excessive cutting heat causes thermal softening [50], and the cutting force in three directions decreases. In particular, the Y-direction force is more obviously affected, because the Y-direction force is directly related to the cutting strength and cutting temperature of the material, which leads to a decrease in the Y-direction force first [22,51]. The cutting force in three directions is positively correlated with the feed rate, as shown in Figure 9b. Analysis indicates that the increase in feed rate changes the volume of material removed per unit of time, produces large plastic deformation, increases the extrusion between tool and work and the tool–chip friction [10] and then makes the cutting force in three directions show an increasing trend.

3.3.2. Temperature Field

The temperature field cloud maps at different cutting parameters are depicted in Figure 10. In the HSM, the intense contact between chip–tool and workpiece–tool and the rapid elastic–plastic deformation of the workpiece generate a lot of heat. The cutting heat is mainly distributed in three regions: shear zone, tool–chip contact zone and tool–workpiece contact zone, where the cutting temperature of the tool–chip contact zone is the highest. Moreover, the good thermal conductivity of Al 7050 results in the removal of cutting heat by the chips. Cutting temperatures correlate positively with cutting speeds and feed rates. As the cutting speed goes up, the friction of the chip–tool and workpiece–tool increases, as does cutting heat. The faster the cutting speed is, the faster the chip is removed, and the chip cannot remove the heat in time, resulting in a higher local cutting temperature. The increase in feed rate causes an increase in extrusion and friction effects on the cutting zone, which results in a rise in cutting temperature.

3.3.3. Stress Field

The equivalent stress field cloud maps at different cutting parameters are illustrated in Figure 11. The first deformation region of Al 7050 is characterized by large and concentrated stresses, and the stresses diminish as the distance from the cutting region increases. The chip shape is relatively complete and curved banded, and the internal stress of the chip is small. During the HSM, the chip is moved away from the rake face by the tool’s continuous movement, releasing the stress that flows out of it, and the stress will be reduced. Mechanical/thermal loads are mainly applied to the surface layer of the workpiece, with minimal effect on the inner material. This extreme incongruity results in the formation of a thin stress layer on the surface after machining is completed, and the deeper the location from the surface, the smaller the residual stresses. As cutting speed and feed rate increase, the distribution range of high stress shows an expanding trend. High-speed cutting leads to severe plastic and thermal deformation of Al 7050, which alters the grain orientation and the characteristics of the grain boundaries. These deformations are mainly responsible for the increase in stress that is causing the distribution range of effective stress to expand.

3.3.4. Strain Field

The equivalent strain field cloud maps at different cutting parameters are illustrated in Figure 12, the plastic strain of the contact area between chip–tool and workpiece–tool is higher, and the plastic strain decreases with the increase in the distance from the tool. Although the stress of the outflow chip is reduced, the plastic strain is higher, which is mainly the result of its shear deformation and the friction and extrusion by the tool rake face [52]. The increase in cutting speed and feed results in greater shear forces at the interface between the grains, leading to grain slip and deformation. In addition, more heat is generated during high-speed cutting, which increases the temperature of the material, thus causing expansion and deformation of the material, and the combined effect of these factors leads to an increase in the equivalent strain.

3.3.5. Discussion

The improvement in surface quality and the control of microstructure evolution of difficult-to-cut materials is always a challenge in HSM. High-speed dry cutting is a difficult task; high temperature, strain and strain rate are generated during the HSM. Multi-physical fields (stress–strain field and thermo-mechanical coupling field) play an important role in the control of microstructure properties, which reflect the deformation behavior, mechanical properties, thermal conductivity and thermal stability of the material to achieve the required control effect on the microstructure [19,20,21,30].
The aluminum alloy is a face-centered cubic structure; due to the high stacking fault energy, the processing deformation is mainly completed by dislocation slip [47]. Different grain orientations lead to the activation of different numbers and types of slip systems, which directly affect the plastic deformation degree of the ally, which causes different flow stresses and then produces different cutting forces and cutting heat. The workpiece’s surface quality and microstructure evolution are greatly affected by this. The multi-physical field mechanism is presented in Figure 13.
High cutting force leads to a rise in temperature in the cutting area of Al 7050, while a high cutting temperature causes the softening and plastic deformation of the alloy, which further affects the cutting force. High-speed machining causes slip, dislocation and lattice deformation in the material, thus forming internal stress and strain fields. The size and distribution of cutting force and temperature cause significant stress concentrations in the cutting area [53]. The uneven distribution of stress and strain leads to uneven deformation of the material and offset or distortion of the texture, which affects the surface roughness, dimensional accuracy, shape accuracy and texture evolution after machining. The high cutting temperature promotes the movement, dynamic recovery and recrystallization of dislocations, and the recrystallized grains increase and the grains that are formed through recrystallization rotate, creating an increase in the misorientation angle [30,54]. Moreover, high-angle grain boundaries can induce grain boundary slip and recrystallization, while low-angle grain boundaries can act as a barrier for plastic slip dislocations. The local misorientation leads to the inhomogeneity of the texture, and the formation of the texture helps to reduce or eliminate the local misorientation.
Different cutting parameters lead to different tool–workpiece friction and extrusion during the HSM, resulting in different degrees of plastic deformation of the alloy, and differences in the evolution of grain size, lattice distortion and dislocation density. The multi-field coupling of cutting force, temperature field, stress field and strain field affects machined surface quality and evolution of subsurface microstructure.

4. Conclusions

Based on the study of surface integrity and machining mechanism of Al 7050 induced by multi-physical field coupling in high-speed machining, the following conclusions can be drawn:
(1)
The surface morphology and roughness of Al7050 during HSM are optimal at fz = 0.025 mm/z. Surface defects such as adherent chips, cavities, microcracks, material compression and tearing appear on the machined surface; the formation of surface defects in HSM is mainly affected by thermo-mechanical coupling.
(2)
The recrystallization behavior and deformation behavior of the microstructure of the machined subsurface are affected by the multi-physical field coupling, which is mainly manifested in the fact that the misorientation angles and the texture type and strength on the machined subsurface are different, and the recrystallization behavior is particularly affected by the temperature.
(3)
The crystallographic texture types on high-speed machined subsurface are mainly {110}<112> Brass texture, {001}<100> Cube texture, {123}<634> S texture and{124}<112> R texture. {338}<344> D texture appears at Vc = 1000 m/min, {112}<110> R-Copper texture appears at Vc = 1500 m/min, and {110}<100> Goss texture appears at fz = 0.125 mm/z. High cutting speeds and feed rates cause the texture intensity to be enhanced.
(4)
The elastic–plastic deformation and crystal texture evolution of the material during the HSM process are seriously affected by the coupling of multi-physical fields of heat, force, strain and strain rate. The main performance is that the multi-physical field coupling affects the plastic deformation, grain slip, rotation and recrystallization, and local misorientation, and then affects the surface quality and microstructure evolution of the material.
In the future, the introduction of advanced energy fields (laser energy field, ultrasonic energy field, low-temperature micro-lubrication energy field, etc.) assisted by green processing technology should be considered to further improve the machinability of difficult-to-cut materials and to reveal the multi-field coupling mechanism and realize the control of microstructure evolution.

Author Contributions

Conceptualization, W.L. and C.N.; methodology, W.L., C.N. and Y.W.; validation, W.L., C.N. and X.H.; formal analysis, W.L. and C.N.; investigation, C.Z., D.L. and X.H.; data curation, W.L., C.Z. and D.L.; writing—original draft preparation, W.L.; writing—review and editing, C.N. and Y.W.; supervision, C.N. and C.Z.; project administration, C.N. and Y.W.; funding acquisition, C.N. and Y.W. All authors have read and agreed to the published version of the manuscript.

Funding

The study was supported by the National Natural Science Foundation of China (Grant No. 52074161), the Shandong Provincial Natural Science Foundation of China (Grant No. ZR2023QE041), the China Postdoctoral Science Foundation (Grant No. 2023M731862) and the Special Foundation of Taishan Scholar Project (Grant No. tsqn202211177).

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

Nomenclature

HSMhigh-speed machiningυPoisson ratio
EBSDelectron backscatter diffractionρdensity (kg/m3)
DTCdifficult-to-cutVccutting speed (m/min)
FEfinite elementapcutting depth (mm)
VPSCvisco-plastic self-consistentfzfeed rate (mm/z)
J-CJohnson–Cook ε equivalent plastic strain
MAmisorientation angle ε ˙ equivalent plastic strain rate
LAGBslow-angle grain boundaries ε ˙ 0 reference plastic strain rate
HAGBshigh-angle grain boundaries T temperature of the workpiece
ODForientation distribution function T 0 initial temperature of the workpiece
IPFinverse pole figure T m melting point of the material
PFpole figure A initial yield stress (MPa)
σsyield strength (MPa) B hardening coefficients (MPa)
σbtensile strength (MPa) C strain rate coefficients
δelongation (%) n strain-hardening index
Eelastic modulus (GPa) m thermal softening index

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Figure 1. HSM experimental equipment.
Figure 1. HSM experimental equipment.
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Figure 2. The 3D FE simulation model for HSM.
Figure 2. The 3D FE simulation model for HSM.
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Figure 3. Effect of cutting speed on surface morphology and defects.
Figure 3. Effect of cutting speed on surface morphology and defects.
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Figure 4. Effect of feed rate on surface morphology and defects.
Figure 4. Effect of feed rate on surface morphology and defects.
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Figure 5. Effect of cutting parameter on surface roughness: (a) cutting speed, (b) feed rate.
Figure 5. Effect of cutting parameter on surface roughness: (a) cutting speed, (b) feed rate.
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Figure 6. Microstructure of the machined subsurface at Vc = 1000 m/min: (a) inverse pole figure (IPF), (b) recrystallization map, (c) misorientation angle distribution, (d) pole figure (PF), (e) ODF maps.
Figure 6. Microstructure of the machined subsurface at Vc = 1000 m/min: (a) inverse pole figure (IPF), (b) recrystallization map, (c) misorientation angle distribution, (d) pole figure (PF), (e) ODF maps.
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Figure 7. Microstructure of the machined subsurface at Vc = 1500 m/min: (a) IPF, (b) recrystallization map, (c) misorientation angle distribution, (d) PF, (e) ODF maps.
Figure 7. Microstructure of the machined subsurface at Vc = 1500 m/min: (a) IPF, (b) recrystallization map, (c) misorientation angle distribution, (d) PF, (e) ODF maps.
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Figure 8. Microstructure of the machined subsurface at fz = 0.125 mm/z: (a) IPF, (b) recrystallization map, (c) misorientation angle distribution, (d) PF, (e) ODF maps.
Figure 8. Microstructure of the machined subsurface at fz = 0.125 mm/z: (a) IPF, (b) recrystallization map, (c) misorientation angle distribution, (d) PF, (e) ODF maps.
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Figure 9. Effect of cutting parameter on cutting force in three directions: (a) cutting speed, (b) feed rate.
Figure 9. Effect of cutting parameter on cutting force in three directions: (a) cutting speed, (b) feed rate.
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Figure 10. Temperature field cloud maps at different cutting parameters: (a) Vc = 500 m/min, fz = 0.075 mm/z, (b) Vc = 1000 m/min, fz = 0.075 mm/z, (c) Vc = 1500 m/min, fz = 0.075 mm/z, (d) Vc = 1000 m/min, fz = 0.025 mm/z, (e) Vc = 1000 m/min, fz = 0.1 mm/z, (f) Vc = 1000 m/min, fz = 0.125 mm/z.
Figure 10. Temperature field cloud maps at different cutting parameters: (a) Vc = 500 m/min, fz = 0.075 mm/z, (b) Vc = 1000 m/min, fz = 0.075 mm/z, (c) Vc = 1500 m/min, fz = 0.075 mm/z, (d) Vc = 1000 m/min, fz = 0.025 mm/z, (e) Vc = 1000 m/min, fz = 0.1 mm/z, (f) Vc = 1000 m/min, fz = 0.125 mm/z.
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Figure 11. Equivalent stress field cloud maps at different cutting parameters: (a) Vc = 500 m/min, fz = 0.075 mm/z, (b) Vc = 1000 m/min, fz = 0.075 mm/z, (c) Vc = 1500 m/min, fz = 0.075 mm/z, (d) Vc = 1000 m/min, fz = 0.025 mm/z, (e) Vc = 1000 m/min, fz = 0.1 mm/z, (f) Vc = 1000 m/min, fz = 0.125 mm/z.
Figure 11. Equivalent stress field cloud maps at different cutting parameters: (a) Vc = 500 m/min, fz = 0.075 mm/z, (b) Vc = 1000 m/min, fz = 0.075 mm/z, (c) Vc = 1500 m/min, fz = 0.075 mm/z, (d) Vc = 1000 m/min, fz = 0.025 mm/z, (e) Vc = 1000 m/min, fz = 0.1 mm/z, (f) Vc = 1000 m/min, fz = 0.125 mm/z.
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Figure 12. Equivalent strain field cloud maps at different cutting parameters: (a) Vc = 500 m/min, fz = 0.075 mm/z, (b) Vc = 1000 m/min, fz = 0.075 mm/z, (c) Vc = 1500 m/min, fz = 0.075 mm/z, (d) Vc = 1000 m/min, fz = 0.025 mm/z, (e) Vc = 1000 m/min, fz = 0.1 mm/z, (f) Vc = 1000 m/min, fz = 0.125 mm/z.
Figure 12. Equivalent strain field cloud maps at different cutting parameters: (a) Vc = 500 m/min, fz = 0.075 mm/z, (b) Vc = 1000 m/min, fz = 0.075 mm/z, (c) Vc = 1500 m/min, fz = 0.075 mm/z, (d) Vc = 1000 m/min, fz = 0.025 mm/z, (e) Vc = 1000 m/min, fz = 0.1 mm/z, (f) Vc = 1000 m/min, fz = 0.125 mm/z.
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Figure 13. Multi-physical field mechanism.
Figure 13. Multi-physical field mechanism.
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Table 1. Al 7050 material composition (wt.%).
Table 1. Al 7050 material composition (wt.%).
ElementZnMgCuZrFeSiCrMnAl
Mass (%)5.7–6.71.9–2.61.9–2.60.08–0.15≤0.15≤0.12≤0.100.1Margin
Table 2. Mechanical properties of Al 7050.
Table 2. Mechanical properties of Al 7050.
Yield Strength
σs (MPa)		Tensile Strength
σb (MPa)		Hardness (HV)Elongation
Δ (%)		Elastic
Modulus
E (GPa)		Poisson Ratio
υ		Density
Ρ (kg/m3)
4555101351071.70.332830
Table 3. Specific experimental scheme of Al 7050.
Table 3. Specific experimental scheme of Al 7050.
No.Cutting Speed Vc
(m/min)		Cutting Depth ap
(mm)		Feed Rate fz
(mm/z)
1500, 750, 1000, 1250, 15001.50.075
210001.50.025, 0.05, 0.075, 0.1, 0.125
Table 4. J-C constitutive parameters of Al 7050 [40].
Table 4. J-C constitutive parameters of Al 7050 [40].
A (MPa)B (MPa)Cnm
463.4319.50.0270.320.099
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Lu, W.; Ni, C.; Wang, Y.; Zong, C.; Liu, D.; Huang, X. Surface Integrity and Machining Mechanism of Al 7050 Induced by Multi-Physical Field Coupling in High-Speed Machining. Lubricants 2025, 13, 47. https://doi.org/10.3390/lubricants13020047

AMA Style

Lu W, Ni C, Wang Y, Zong C, Liu D, Huang X. Surface Integrity and Machining Mechanism of Al 7050 Induced by Multi-Physical Field Coupling in High-Speed Machining. Lubricants. 2025; 13(2):47. https://doi.org/10.3390/lubricants13020047

Chicago/Turabian Style

Lu, Wei, Chenbing Ni, Youqiang Wang, Chengguo Zong, Dejian Liu, and Xingbao Huang. 2025. "Surface Integrity and Machining Mechanism of Al 7050 Induced by Multi-Physical Field Coupling in High-Speed Machining" Lubricants 13, no. 2: 47. https://doi.org/10.3390/lubricants13020047

APA Style

Lu, W., Ni, C., Wang, Y., Zong, C., Liu, D., & Huang, X. (2025). Surface Integrity and Machining Mechanism of Al 7050 Induced by Multi-Physical Field Coupling in High-Speed Machining. Lubricants, 13(2), 47. https://doi.org/10.3390/lubricants13020047

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