Study on the surface integrity of 7050 aluminum alloy with different crystal orientations during high-speed machining

This paper investigates the effect of crystal orientations and machining parameters on the surface integrity of 7050 aluminum alloy in high-speed machining. Three groups of pre-compression deformations (10%, 15%, and 20%) are performed on the aluminum alloy to fabricate specimens with different crystal orientations. A single-factor dry-cutting experiment was designed, and the surface roughness, surface morphology and defects, work hardening, and X-ray diffraction (XRD) were tested. The results show that the 15% pre-deformed 7050 aluminum alloy specimen had lower surface roughness and the best surface quality under the same cutting parameters. The surface roughness of 7050 aluminum alloy with different crystal orientations tended to first increase and then decrease with the increase of the cutting speed, and the surface roughness was positively correlated with the cutting depth and feed rate. Under different cutting parameters, the machined surfaces of 7050 aluminum alloy specimens with different crystal orientations exhibited different degrees of plowings, apophysis, sticky chips, and pits. Moreover, the degree of work hardening of the 15% pre-deformed 7050 aluminum alloy specimen was small, while that of the 20% pre-deformed specimen was more serious. XRD analysis demonstrated that the diffraction peak width of the 10% pre-deformed 7050 aluminum alloy specimen slightly increased with the increase of the cutting depth. Moreover, the (111) and (200) diffraction peak intensities of the 15% pre-deformed 7050 aluminum alloy specimen increased with the increase of the cutting speed, and there was no obvious Al2CuMg phase on the machined surface of the 20% pre-deformed specimen.


Introduction
7050 aluminum alloy has excellent overall performance and is widely used in aerospace and other fields [1][2][3], mostly as the main structural parts. Most of the main structural parts are machined by cutting, and the microstructure of aluminum alloys will significantly impact the cutting process [4], thereby affecting the integrity of the cutting surface [5,6], which in turn affects the mechanical properties and performance of the parts [7,8]. Therefore, it is of great significance to explore the cutting surface integrity of aluminum alloys with different crystal orientations.
The crystal orientation has a significant impact on the physical properties of the material, the deformation of the microstructure, and the related reflection on the macroscopic level [9,10]; thus, research on the crystal orientation of the material is favored by many experts and scholars. To et al. [11] explored the influence of the crystal orientation on the main cutting force, chip formation, and surface micromorphology of single-crystal aluminum and pointed out that understanding the influence of crystallographic factors will help to better design and select heat treatment and machining processes. Liu et al. [12] conducted cutting experiments on five different crystalline orientations of single-crystal copper. They also proposed an analytical model to predict the material accumulation and subsurface defect distribution on the workpiece surface, which can effectively explain the surface generation mechanism during the nano-cutting of single-crystal copper with arbitrary crystal orientation. Xu et al. [13] used molecular dynamics simulations to investigate the plastic deformation behavior and surface formation of single-crystal aluminum and considered the effects of the crystal orientation and cutting edge radius on the dislocation evolution, stacking layer dislocation evolution, shear surface evolution, cutting forces, surface morphology, and material removal mechanisms. Ding et al. [14] conducted cutting experiments on polycrystalline Al 6061 T6 and analyzed the changes in the cutting forces and machined surface finish; they found that the grain size and grain orientation have important effects on machinability. Zhang et al. [15,16] took 7055 aluminum alloy under different heat treatments as the research object and analyzed its microstructure and crystal orientation before and after cutting via transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) technology. Pang et al. [17] investigated the machined surface properties of TC4 titanium alloy, simulated the surface texture with the help of VPSC software, obtained its pole figure and orientation distribution function (ODF) map, and further analyzed the typical shear texture of the surface. In addition, work hardening is an important index with which to measure the integrity of machined surfaces. Fu et al. [18] investigated the effect of the cutting speed on work hardening during the cutting of 7050-T7451 aluminum alloy, and they found that high-speed cutting has a higher degree of work hardening than low-speed cutting and that work hardening has a positive impact on the wear resistance of workpieces. Rao et al. [19] conducted cutting experiments on 7075 aluminum alloy with TiB 2 -coated cemented carbide tools and found that the change of the microhardness with the cutting parameters can be explained by the effect of heat on the hardness of the machined surface. Zhao et al. [20] established a Taylor factor model with the polycrystalline material AA7075 to quantitatively analyze the effect of crystal orientation on microhardness and to improve the accuracy of microhardness prediction. During the cutting process, the machined surface is plastically deformed by the heat and force loads and its microstructure is changed. An et al. [21] investigated the microstructure of the machined surface of 7050-T7451 aluminum alloy via X-ray diffraction (XRD), designed a dry cutting experiment with the cutting speed as the variable, and analyzed the XRD spectra of the cutting surface at different cutting speeds. Imbrogno et al. [22] analyzed the XRD spectra of AA7075-T6 aluminum alloy under dry and low-temperature conditions, respectively, and the analysis indicated the changes in the precipitate phase, plastic deformation, lattice distortion, and grain refinement under different cutting parameters.
In this study, 7050 aluminum alloy with different crystal orientations was obtained by compression pre-deformation, and a single-factor experimental scheme was designed to study the influences of the cutting parameters on the cutting surface integrity. The findings are expected to enrich the current understanding of the high-speed cutting surface integrity of 7050 aluminum alloy with different crystal orientations and are of substantial importance for the further development of the use of 7050 aluminum alloy in green manufacturing and the aerospace manufacturing industry.

Preparation of experimental materials
To investigate the surface integrity of the high-speed cutting of 7050 aluminum alloy with different crystal orientations, the original material used in the experiment was 7050 aluminum alloy, the chemical composition of which is shown in Table 1. Three groups of workpiece materials with different crystal orientations were obtained by a WDW-100 microcomputercontrolled electronic universal testing machine with 10%, 15%, and 20% compression pre-deformation, respectively. The microstructures of the surfaces of the three 7050 aluminum alloy specimens were observed by the EBSD technique. The collected data were analyzed and processed by an Oxford HKL Channel 5 system, and the crystal orientation information of the three 7050 aluminum alloy specimens was obtained. Figure 1 presents the crystal orientations of the three predeformed 7050 aluminum alloy specimens, where different colors represent different crystal orientations. The closer the colors, the smaller the orientation difference, and the closer the orientation. The grains of the three 7050 aluminum alloy specimens were mostly strip-shaped, and there were some equiaxed grains. The crystal orientations of different samples were different, which affects the physical properties of the material and ultimately the cutting process of aluminum alloy. Figure 2 presents the ODF diagrams of the three predeformed 7050 aluminum alloy specimens. The highest orientation distribution density of the 10% pre-deformed 7050 aluminum alloy specimen was 15.6, and at φ 2 = 45°, the main textures were Goss texture and copper texture. The highest orientation distribution density of the 15% pre-deformed 7050 aluminum alloy specimen was 13.5, and at φ 2 = 45°, there were cube, Goss, and copper textures. The strength of the Goss texture was weakened relative to that of the 10% predeformed 7050 aluminum alloy specimen, and the strength of the copper texture was also changed relative to that of the 10% pre-deformed specimen. The highest orientation distribution density of the 20% pre-deformed 7050 aluminum alloy specimen was 17.7, and at φ 2 = 45°, there were Goss, copper, and brass R textures. The strength of the Goss texture was changed relative to those of the 10% and 15% pre-deformed specimens. Different compression pre-deformation will cause the texture distribution and strength of 7050 aluminum alloy to differ, which will have different effects on the machining process.

Experimental equipment and methods
The cutting speed, cutting depth, and feed rate were used as experimental variables, and the experimental scheme of high-speed machining was designed, as shown in Table 2.
High-speed cutting experiments were carried out on an MV820 CNC machining center with a maximum spindle speed of 8000 r/min and a tool diameter of 80 mm, and the selected tool was an APKT1604PDER-MA H01 carbide tool with a tool fillet radius of 0.2 mm. After cutting, the surface roughness was tested by a JD520 surface roughness instrument, and 5 points were randomly selected for each specimen to test the surface roughness and take the average value. The machined surface morphology and defects were tested by a MERLIN compact field emission scanning electron microscope, and the hardness value of the workpiece was tested by an HV-1000 (A) microhardness tester, and 4 points were selected for each specimen at different locations to measure the hardness value and take the average value.
Moreover, the samples were tested by XRD with a SHI-MADZU XRD-6100 multi-functional X-ray diffractometer, (a) 10% pre-deformation (b) 15% pre-deformation (c) 20% pre-deformation   and the diffraction pattern was analyzed to obtain the microstructure information of the aluminum alloy surface after high-speed cutting. Figure 3 shows the experimental and testing equipment.

Results and discussion
3.1 Analysis of the surface roughness 3.1.1 Effect of cutting speed on the surface roughness Figure 4 describes the effect of the cutting speed on the surface roughness of 7050 aluminum alloy. The surface roughness of 7050 aluminum alloy specimens with different crystal orientations exhibited a trend of first increasing and then decreasing with the increase of the cutting speed, and the surface roughness was increased to the maximum value at the cutting speed of 1200 m/min. The surface roughness of the 20% pre-deformed 7050 aluminum alloy specimen changed the most significant and was larger than those of the other specimens, and the surface roughness of the 15% pre-deformed specimen was smaller. As the cutting speed gradually increases, the plastic deformation of the material is intensified, and the cutting heat generated by cutting increases; thus, the chip in the front of the tool surface easily forms a bond, which produces an overcutting phenomenon on the processing surface and ultimately increases the surface roughness. With the further increase of the cutting speed, the cutting power increases and the cutting temperature rises, so the material produces a certain amount of thermal softening; moreover, high-speed cutting can better suppress the generation of built-up edges and scales [23], thus reducing the surface roughness. In addition, during the process of metal cutting, under the action of friction and extrusion between the tool and the workpiece, the workpiece material produces significant elastic-plastic deformation, and the elastic deformation will recover after the tool passes through. There are differences in the elastic-plastic recovery of polycrystalline materials after cutting [13], and, The effect of the cutting speed on the surface roughness of 7050 aluminum alloy due to the influence of different grain orientations, sizes, and grain boundaries, there are obvious differences in the microstructural deformation of the machined surface [24]; thus, in this work, the surface roughness of 7050 aluminum alloy specimens with different crystal orientations was significantly different. Figure 5 describes the effect of the cutting depth on the surface roughness of 7050 aluminum alloy. The surface roughness of the 7050 aluminum alloy specimens with different crystal orientations exhibited an increasing trend with the increase of the cutting depth, and the surface roughness of the 20% pre-deformed 7050 aluminum alloy specimen was larger at a smaller cutting depth. Under the same cutting parameters, the surface roughness of the 15% pre-deformed 7050 aluminum alloy specimens was smaller than those of the other two specimens. The reason for this is that with the gradual increase of the cutting depth, the volume of material removed by each tool increases correspondingly, the extrusion and friction of the tool intensify, the cutting force and cutting heat significantly increase, and the processing stability decreases, resulting in the increase of the surface roughness. In addition, additional plastic deformation occurs with a small cutting depth; this resulted in a larger surface roughness value for the 20% pre-deformed 7050 aluminum alloy specimen at the cutting depth of 0.5 mm. Figure 6 describes the effect of the feed rate on the surface roughness of 7050 aluminum alloy. The surface roughness of the 15% and 20% pre-deformed 7050 aluminum alloy specimens was positively correlated with the feed rate, and the surface roughness of the 10% pre-deformed specimen appeared to be larger at a lower feed rate. Under the same cutting parameters, the surface roughness of the 20% predeformed 7050 aluminum alloy specimen was overall larger and that of the 15% pre-deformed specimen was smaller. Further analysis revealed that with the increase of the feed rate, the surface residual height increased accordingly, and the increase of the unit material removal amount also increased the cutting force and reduced the cutting stability, which increased the surface roughness. In addition, at a too-low feed rate, the surface roughness appeared to increase rather than decrease due to the frequent extrusion of the tool and the plastic deformation of the material. Figure 7 shows the SEM and EDS images of the machined surface of the 10% pre-deformed 7050 aluminum alloy specimen at different cutting speeds. Figure 7a reveals that the machining surface had apparent surface breakage and sticky chips at the cutting speed of 600 m/min, and the EDS analysis shows that the main elements of the machining surface breakage were consistent with the elements of the 7050 aluminum alloy base material. This is because the workpiece material flaked and broke during high-speed cutting, and the broken material followed the high-speed rotating tool to move on the machining surface and rub and squeeze the The effect of the feed rate on the surface roughness of 7050 aluminum alloy machining surface, thus forming a breakage. The cutting speed was then increased to 1200 m/min; the residual cutting surface can be observed on the surface shown in Fig. 7b, and the surface smoothness was poor. This is because the material was not completely broken and separated during the high-speed cutting process and remained on the machined surface. As presented in Fig. 7c, with the increase of the cutting speed to 1800 m/min, the surface was found to have particles adhering to it, but the overall surface quality was good, and the chemical elements at the particle adhesion contained Si elements and a higher content of O elements. This was due to the material adhering to the tool during cutting, after which the material adhering to the tool came off and stayed on the machined surface, and more serious oxidation occurred due to the effect of the temperature increase. Figure 8 displays the SEM and EDS images of the machined surface of the 15% pre-deformed 7050 aluminum alloy specimen at different cutting speeds. Figure 8a and b reveals sticky chips and white bands visible on the machined surface, the main chemical elements of which were consistent with the main elements of the 7050 aluminum alloy material. Cutting speed increases were accompanied by an increase in surface defects, and the overall surface quality decreased to a certain extent. Sticky chips and tiny craters can be observed on the surface shown in Fig. 8c, and the overall surface quality was improved. After EDS analysis, the elements at the surface material adhesion and craters were mainly Al, Zn, Mg, and Cu elements, which were consistent with the 7050 aluminum alloy matrix. This was due to the stripping and breaking of grains during the high-speed cutting process, thus forming craters. In addition, under the same cutting parameters, the 15% pre-deformed 7050 aluminum alloy specimen had fewer surface defects and better surface quality than the 10% pre-deformed specimen. Figure 9 exhibits the SEM and EDS images of the machined surface of the 20% pre-deformed 7050 aluminum alloy specimen at different cutting speeds. As shown in Fig. 9a, surface damage, pits, and white bands were found on the surface at the cutting speed of 600 m/ min. As presented in Fig. 9b, with the increase of the cutting speed to 1200 m/min, sticky chips were found on the surface, and the surface quality was not high. After the cutting speed reached 1800 m/min, as shown in Fig. 9c, there were sticky chips on the surface, which was because the cutting speed increases were accompanied by an increase in the cutting heat, which caused the chips to adhere to the machined surface. The comparison of the surface morphologies of the 7050 aluminum alloy specimens with three different crystal orientations under the same cutting conditions reveals that the overall surface quality of the 15% pre-deformed 7050 aluminum alloy specimen was superior.  The SEM and EDS images of the machined surface of the 20% pre-deformed 7050 aluminum alloy at different cutting speeds had sticky chips and the plowing phenomenon, resulting in low surface quality. Figure 11 exhibits the SEM and EDS images of the machined surface of the 15% pre-deformed 7050 aluminum alloy specimen at different cutting depths. As shown in Fig. 11a, small pits and small sticky chips were found on the machined surface of the specimen with a cutting depth of 0.5 mm, which was due to the damage and peeling of the material during the high-speed cutting. With the gradual increase of the cutting depth, as shown in Fig. 11b and c, different degrees of chip sticking were observed on the surfaces, and there were micro-burrs on the surface of the specimen shown in Fig. 11c. This was due to the formation of micro-burrs when the material was not completely broken and remained on the machined surface as the highspeed rotating tool passed through the machined surface. In addition, the surface quality of the 15% pre-deformed 7050 aluminum alloy specimen was better than that of the 10% pre-deformed specimen under the same cutting parameters. Figure 12 displays the SEM and EDS images of the machined surface of the 20% pre-deformed 7050 aluminum alloy specimen at different cutting depths. As shown in Fig. 12a, at the cutting depth of 0.5, white bands and sticky chips were found on the surface. As shown in Fig. 12b, particle adhesion and plow grooves were found on the surface of the specimen at the cutting depth of 1.5 mm, and the particle adhesion contained C, O, and Si elements, which were due to the built-up edge breaking off and adhering to the surface. The plow grooves were due to the built-up edge moving with the tool to form longitudinal grooves on the surface. As shown in Fig. 12c, at the cutting depth of 2.5 mm, there were more sticky chips on the surface, because the cutting depth increase was accompanied by an increase in the cutting force and cutting heat. In addition, based on the comparison of the surface morphologies of the 7050 aluminum alloy specimens with three different crystal orientations, under the same cutting conditions, the surface quality of the 15% pre-deformed 7050 aluminum alloy specimen was superior overall.

Effect of feed rate on surface morphology
and machining defects Figure 13 shows the SEM and EDS images of the machined surface of the 10% pre-deformed 7050 aluminum alloy specimen at different feed rates. As shown in Fig. 13a, at the feed rate of 0.025 mm/z, the surface exhibited obvious surface residue and poor surface quality, which is consistent with the phenomenon of the large surface roughness of the 10% predeformed 7050 aluminum alloy specimen under a low feed rate. As shown in Fig. 13b, at the feed rate of 0.075 mm/z, white bands and sticky chips were observed on the surface. The sticky chip phenomenon became serious at the feed rate of 0.125 mm/z, as shown in Fig. 13c, which is because as the feed rate increased, the volume of material removed by each tool increased. The surface was affected by the increase of the cutting force and cutting heat, resulting in a large area of sticky chips in the material and ultimately caused the poor machined surface quality.   Figure 14 presents the SEM and EDS images of the machined surface of the 15% pre-deformed 7050 aluminum alloy specimen at different feed rates. At the feed rate of 0.025 mm/z, as shown in Fig. 14a, obvious grooves and sticky chips were observed on the surface. The grooves were formed by small chips moving on and scratching the surface, and small chips were embedded on the machined surface. More sticky chips and burrs were found on the surfaces of the specimen at the feed rates of 0.075 and 0.125 mm/z, as respectively shown in Fig. 14b and c, which were related to the increase of the feed rate. In addition, based on the comparison of Figs. 13 and 14, the surface quality of the 15% pre-deformed 7050 aluminum alloy specimen was better than that of the 10% pre-deformed 7 specimens under the same cutting conditions. Figure 15 displays the SEM and EDS images of the machined surface of the 20% pre-deformed 7050 aluminum alloy specimen at different feed rates. As shown in Fig. 15a, at the feed rate of 0.025 mm/z, there were irregular surface residues and sticky chips on the surface. At the feed rate of 0.075 mm/z, as shown in Fig. 15b, there was material accumulation on the surface, and the EDS analysis revealed that the chemical elements of the accumulation material were consistent with the main elements of the 7050 aluminum alloy matrix, which was formed by the friction and extrusion of the tool on the workpiece surface during high-speed cutting. As shown in Fig. 15c, at the feed rate of 0.125 mm/z, sticky chips and obvious furrows were found on the surface of the specimen, which affected the quality of the surface. In addition, based on a comparison with Figs. 14 and 15, the surface quality of the 15% pre-deformed 7050 aluminum alloy specimen was generally better under the same cutting conditions. Figure 16 describes the effect of the cutting speed on the work hardening of 7050 aluminum alloy. The degree of work hardening of the 7050 aluminum alloy specimens with different crystal orientations exhibited a trend of first increasing and then decreasing with the increase of the cutting speed, and the microhardness of the specimens reached the maximum value at the cutting speed of 1200 m/min. The degree of work hardening of the 20% pre-deformed 7050 aluminum alloy specimen was higher than those of the 10% and 15% pre-deformed specimens under the same cutting parameters. The reason for this is that the friction and extrusion between the tool and the workpiece are enhanced accordingly with the increase of the cutting speed, the material has severe elastoplastic deformation affected by the cutting force, and the regular state of the metal surface is destroyed. The grain deformation and dislocation movement are intensified, resulting in the increase of work hardening. With the further increase of the cutting speed, the interaction time between the tool and the workpiece becomes shorter, while the cutting heat generated increases and the softening effect of the surface metal is enhanced, resulting in the reduction of work hardening. The combined effect of heat and force loads impacts the processing surface, thus forming the final work hardening state. In addition, the different crystal orientations will affect the slip and deformation of the surface grains [24], and the elastic recovery of different crystal orientations of aluminum alloy is also different [13]. Thus, the work hardening of the 7050 aluminum alloy specimens with different crystal orientations exhibited differences. Figure 17 describes the effect of the cutting depth on the work hardening of 7050 aluminum alloy. The degree of work hardening of the 7050 aluminum alloy specimens with different crystal orientations exhibited a trend of first increasing and then decreasing with the gradual increase of the cutting depth, and the microhardness of the specimens reached the maximum at the cutting depth of 2 mm. Under the same cutting parameters, the 15% pre-deformed 7050 aluminum alloy specimen had a relatively small degree of work hardening as compared to the other specimens. The reason for this is that the unit removal of material increases with the cutting depth, the friction and extrusion of the machined surface by the tool increases, and the surface grain slip and deformation intensify. The dislocation movement also changes, and the chips take away some heat generated by cutting, resulting in an increase in work hardening. The cutting heat increases with the further increase of the cutting depth, so the thermal softening of the surface layer increases. This reduces the degree of work hardening, and the cutting depth increases, after which the workpiece material is subject to greater force and thermal load, resulting in a certain degree of increase in the depth of the hardened layer. Fig. 18 The effect of the feed rate on the work hardening of 7050 aluminum alloy (a) 10% pre-deformation (b) 15% pre-deformation (c) 20% pre-deformation Figure 18 describes the effect of the feed rate on the work hardening of 7050 aluminum alloy. The degree of work hardening of the 7050 aluminum alloy specimens with different crystal orientations was positively correlated with the feed rate, and the microhardness of the specimens reached the maximum value at 100 µm from the machined surface. Under the same cutting parameters, the degree of work hardening of different specimens appeared to be obviously different; the degree of work hardening of the 15% pre-deformed specimen was lower than those of the 10% and 20% predeformed specimens, and the work hardening of the 20% pre-deformed specimen was more serious. The reason for this is that with the gradual increase of the feed rate, the cutting force increases, the plastic deformation of the surface layer increases, and the grain slip and deformation of the surface layer of the material intensify. Meanwhile, the elastic recovery of the workpiece material will cause extrusion and friction between the tool and the machined surface, thereby further increasing the plastic deformation of the surface. Moreover, the chips take away some cutting heat, which will eventually increase the degree of work hardening and the depth of the hardened layer.

XRD analysis of the machined surface layer at different cutting speeds
After processing the obtained XRD data with Jade 6.5 software, the XRD spectra of the 7050 aluminum alloy specimens at different cutting speeds were obtained, as exhibited in Fig. 19. The XRD spectra present the detection results at the diffraction angle 2 from 20 to 90°. The characteristic diffraction peaks of 7050 aluminum alloy include (111),  Figure 19a shows the XRD spectrum of the 10% predeformed 7050 aluminum alloy specimen, from which it can be seen that the intensity of the diffraction peaks changed with the variation of the cutting speed. At the cutting speed of 900 m/min, the intensities of the (111), (200), and (311) diffraction peaks were the highest, and the relative content Fig. 19 The XRD spectra of 7050 aluminum alloy at different cutting speeds (a) 10% pre-deformation (b) 15% pre-deformation (c) 20% pre-deformation of MgZn 2 was higher. With the further increase of the cutting speed, the intensity of the diffraction peaks decreased and then increased, the crystallinity also changed, and the relative content of Al 2 CuMg increased. The intensity of the diffraction peaks is affected by the preferred orientation and crystallinity of the crystal, and the greater the intensity of the diffraction peak, the stronger the orientation of the corresponding crystal plane. Moreover, the crystal has better crystallinity, and the grain size is relatively large; on the contrary, the diffraction peak is wide and weak. The grain orientation will rotate with the plastic deformation process, and at a cutting speed of 900 m/min, the crystals were found to be concentrated in the (111) and (200) crystal planes, resulting in a large diffraction peak intensity. Moreover, the relative contents of MgZn 2 and Al 2 CuMg were changed by the influence of the thermoplastic deformation of high-speed cutting processing and precipitation. Figure 19b shows the XRD spectrum of the 15% pre-deformed 7050 aluminum alloy specimen. With the increase in the cutting speed, the intensities of the (111) and (200) diffraction peaks tended to increase, and the width of the diffraction peaks did not change significantly. The width of the diffraction peaks is mainly affected by the microstructure state of the material (grain size, dislocation density, and micro distortion), micro stress, etc., and the diffraction peaks are widened due to grain refinement. Figure 19c shows the XRD spectrum of the 20% pre-deformed 7050 aluminum alloy specimen. With the increase of the cutting speed, the intensities of the (111) and (200) diffraction peaks first increased and then decreased. Compared with the 10% and 15% pre-deformed 7050 aluminum alloy specimens, the 20% predeformed specimen did not exhibit a more obvious Al 2 CuMg phase. The diffraction peaks on the machined surface were shifted due to residual stress and microscopic distortion. Figure 20 displays the XRD spectra of 7050 aluminum alloy at different cutting depths, in which the main phases were the Al phase, the second phase MgZn 2 phase, and the Al 2 CuMg phase. Figure 20a shows the XRD spectrum of the 10% pre-deformed 7050 aluminum alloy specimen, and the changes of the (111), (200), and (311) diffraction peaks are obvious. With the increase of the cutting depth, the  Figure 20b shows the XRD spectrum of the 15% predeformed 7050 aluminum alloy specimen, and the overall intensity of each diffraction peak was higher than that of the 10% pre-deformed specimen. Figure 20c shows the XRD spectrum of the 20% predeformed 7050 aluminum alloy. There is no obvious Al 2 CuMg phase in the figure. The (111) and (200) diffraction peak intensities of the 20% pre-deformed 7050 aluminum alloy specimen were between those of the 10% and 15% pre-deformed specimens, and the (220) and (311) diffraction peak intensities were higher than those of the other two specimens. The grain orientation rotates under the influence of heat force load during machining, which enhances the orientation of the corresponding crystal surface, and the dislocation movement affected by the heat load leads to a high diffraction peak intensity. In addition, the annihilation of dislocations and the random growth of grains will weaken the orientation of the crystal planes. During the process of high-speed cutting, the plastic deformation of the machined surface is serious, the grains are elongated and broken, the original morphology is changed, and the surface lattice distortion and dislocation movement are also intensified. This causes the machined surface grains to become refined, internal stress is generated between the grains and microcrystals [25], and, finally the diffraction peak width increases.

XRD analysis of machined surface at layer different feed rates
The XRD spectra of 7050 aluminum alloy under different feed rates are shown in Fig. 21. Figure 21a presents the XRD spectrum of the 20% pre-deformed 7050 aluminum alloy specimen, in which the main phases were the Al phase, MgZn 2 phase, and Al 2 CuMg phase. The intensities of the (111) and (200) diffraction peaks changed with the increase of the feed rate, and the width of the diffraction peaks tended to widen slightly, which indicates the rotation of the grain orientation on the processed surface and the refinement of grains. Figure 21b exhibits the XRD spectrum of the 15% pre-deformed 7050 aluminum alloy specimen, in which the intensity of the diffraction peak of the Al 2 CuMg phase was higher than that of the 10% pre-deformed 7050 aluminum Fig. 21 The XRD spectra of 7050 aluminum alloy at different feed rates (a) 10% pre-deformation (b) 15% pre-deformation (c) 20% pre-deformation alloy. Moreover, the relative content of the Al 2 CuMg phase increased, and more Al 2 CuMg phase precipitated inside the material. Figure 21c is the XRD spectrum of a 20% predeformed 7050 aluminum alloy specimen, and there was no obvious diffraction peak of the Al 2 CuMg phase. Moreover, with the increase of the feed rate, the intensity of the (111) diffraction peak exhibited an overall increasing trend, and that of the (200) diffraction peak first increased and then decreased. The drastic plastic deformation of the machined surface causes the internal organization to change, while the orientation of the corresponding crystal plane increases or weakens, and the diffraction peak intensity also changes.

Conclusions
This study investigated the surface integrity of 7050 aluminum alloy with different crystal orientations during high-speed machining, and the surface roughness, surface morphology and defects, work hardening, and surface microstructure were studied by high-speed cutting experiments. The conclusions are as follows: (1) The surface roughness of the 20% pre-deformed 7050 aluminum alloy specimen was the largest and changed significantly, while the surface roughness of the 15% pre-deformed specimen was the smallest among the three specimens with different crystal orientations. The 20% and 10% pre-deformed 7050 aluminum alloy specimens had large surface roughness under a low cutting depth and low feed rate, respectively. (2) The formation of the cutting surface of 7050 aluminum alloy with different crystal orientations was also accompanied by the generation of surface defects, and the mechanism of defect formation was further elucidated by EDS elemental analysis. Under the same cutting conditions, the surface quality of 15% pre-deformed 7050 aluminum alloy specimens was better than 10% pre-deformed specimens than 20% pre-deformed specimens. (3) The degree of work hardening of 7050 aluminum alloy with different crystal orientations exhibited a trend of first increasing and then decreasing with the increase of the cutting speed and the cutting depth. With the increase in the feed rate, the degree of work hardening and the hardening layer depth exhibited an increasing trend. Under the same cutting parameters, the work hardening degree of 15% pre-deformed specimens in three different crystal orientations of 7050 aluminum alloy specimens was smaller. (4) XRD analysis of the cutting surface of 7050 aluminum alloy with different crystal orientations revealed that there was no obvious Al 2 CuMg phase on the machined surface of the 20% pre-deformed specimens. Under the same cutting depth, the (111) and (200) diffraction peak intensities of 20% pre-deformed specimen were between those of the 10% and 15% specimens, and the (220) and (311) diffraction peak intensities were overall higher than those of the other two specimens.
Author contribution Wei Lu: conceptualization, investigation, data curation, and writing. Chengguo Zong: supervision, project administration, and funding acquisition. Chenbing Ni: supervision, project administration, and funding acquisition. Xiao Yu: investigation and data curation. Dejian Liu: investigation and data curation.
Funding The work was supported by the Shandong Qingchuang Talent Attracting and the Nurturing Program Project and Shandong Qingchuang Science and Technology Project (Grant Number 2019KJB022).
Data availability Not applicable.

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Competing interests
The authors declare no competing interests.