Material Removal and Surface Integrity Analysis of Ti6Al4V Alloy after Polishing by Flexible Tools with Different Rigidity

Ti6Al4V alloy has been widely used in many fields, such as aerospace and medicine, due to its excellent biocompatibility and mechanical properties. Most high-performance components made of Ti6Al4V alloy usually need to be polished to produce their specific functional requirements. However, due to the material properties of Ti6Al4V, its polishing process still requires significant development. Therefore, this study aimed to investigate the performance of polishing Ti6Al4V by using tools with different rigidities. Two kinds of bonnet tool were used, namely a pure rubber (PR) bonnet and a semirigid (SR) bonnet. The characterization of material removal and surface integrity after polishing was conducted through a series of experiments on a 6-DOF robotic polishing device. The results demonstrate that both bonnet tools successfully produce nanometric level surface roughness. Moreover, the material removal rate of the SR bonnet tool is significantly higher than that of the PR bonnet, which is consistent with the material removal characteristics of glass polishing in previous research. In addition, the presented analysis on key polishing parameters and surface integrity lays the theoretical foundation for the polishing process of titanium alloy in different application fields.


Introduction
Ti6Al4V alloy is an α+β-type, dual-phase alloy with excellent material properties [1][2][3], including low density, high mechanical strength, good corrosion resistance, high biocompatibility, and other distinct mechanical and physical properties. It also offers the ability to adjust these material properties to a large extent by optimizing its microstructure and surface properties [4][5][6][7]. It has been widely used in the manufacture of turbine engine blades, compressor discs, jet engines in the aerospace industry, and armor steel used for bullet-proofing in the military industry [8][9][10]. Furthermore, Ti6Al4V is commonly used in the biomedical field due to its high biocompatibility [11,12], such as artificial hip joints, compression hip screws, dental implants body, bone plate, heart catheter, artificial heart valve for orthodontic surgery, etc. [13][14][15][16][17]. Furthermore, low surface roughness is usually required to obtain superior functionality for most of its applications, such as implants and turbine blades [18,19]. However, it is a typical hard-to-machine material like other  Figure 1 shows the metallographic structure observed by a metallographic microscope. As the titanium alloy formed by saw blade cutting had many scratches on its surface and the flatness was poor, it could not be directly polished. To ensure the unity of the experiment, a titanium alloy sheet with length × width × thickness = 100 mm × 100 mm × 10 mm was used as the sample for the mechanical characteristics experiment. The surface of the sample was ground by a 150-mesh grinding wheel to 3~4 λ (λ = 632.8 nm) peak-to-valley (PV), together with a surface average arithmetic roughness (Sa) of 0.2~0.3 µm.
Materials 2022, 15, x FOR PEER REVIEW

Material
The material of the workpiece used in this study was α+β type, two-phase alloy (BAOTi in China) with a density of 4.51 g/cm 3 and a hardness of 31HRC, fo a rolling process. In addition, the titanium alloy material used in this experimen went annealing heat treatment before leaving the factory. The chemical compositi plies with the GB/T3624-2007 standard, and its basic composition is shown in Tab  Figure 1 shows the metallographic structure observed by a metallographi scope. As the titanium alloy formed by saw blade cutting had many scratches on face and the flatness was poor, it could not be directly polished. To ensure the uni experiment, a titanium alloy sheet with length × width × thickness = 100 mm × 10 10 mm was used as the sample for the mechanical characteristics experiment. The of the sample was ground by a 150-mesh grinding wheel to 3~4 λ (λ = 632.8 nm) valley (PV), together with a surface average arithmetic roughness (Sa) of 0.2~0.3 µ Alumina polishing slurry with different grades of size was used as the p slurry. The samples were ultrasonically cleaned in alcohol for 10 min and dried on and ventilated laboratory bench before and after polishing.

Experimental Setup
The SR bonnet and PR bonnet with radii of 40 mm were used in the experi shown in Figure 2. The structure of SR bonnet is shown in Figure 2a [41]. It is co of three layers. The outermost and innermost layers are rubber membranes, and dle layer is a stainless-steel metal sheet with a thickness of 0.3 mm, which helps the tool rigidity and flexibility. The outermost rubber film is covered with a polish The flexible layer of the PR bonnet shown in Figure 2b is made of rubber with hardness of 80. Alumina polishing slurry with different grades of size was used as the polishing slurry. The samples were ultrasonically cleaned in alcohol for 10 min and dried on a clean and ventilated laboratory bench before and after polishing.

Experimental Setup
The SR bonnet and PR bonnet with radii of 40 mm were used in the experiment, as shown in Figure 2. The structure of SR bonnet is shown in Figure 2a [41]. It is composed of three layers. The outermost and innermost layers are rubber membranes, and the middle layer is a stainless-steel metal sheet with a thickness of 0.3 mm, which helps increase the tool rigidity and flexibility. The outermost rubber film is covered with a polishing pad. The flexible layer of the PR bonnet shown in Figure 2b is made of rubber with a shore hardness of 80.  When polishing with the SR bonnet or PR bonnet, the bonnet tool contacts the surface to be polished, squeezing the polishing abrasive. In the meantime, the rotation of the bonnet tool drives the movement of the abrasive to continuously scrape the surface, forming a small amount of material to remove. Figure 3 shows the self-developed, robot-assisted polishing device that was used in the experiment, which is equipped with a 6-DOF ABB robot arm (IRB4600-60/2.05). This 6-DOF industrial robot arm includes three rotation axes and three swing axes that can handle a payload of up to 60 kg. The farthest achievable distance of the arm is 2.05 m, and the repeated positioning accuracy is ±0.05 mm. The bonnet tool was fixed at the end of the H-axis, and the rotation of the bonnet tool was controlled through a PLC electric control cabinet. The polishing pad used in this experiment was UN1NAP5-40W damping cloth (Universal Photonics, Central Islip, NY, USA). It is a velvet-like polishing material with fine texture, soft surface, porous, elastic, and long service life. It can effectively impregnate the polishing liquid during polishing to improve the removal efficiency while avoiding scratches on the workpiece. It is a high-performance, optical-level processing material [43]. Figure 4 shows the geometric model of bonnet tool polishing. To observe the material removal characteristics, two groups of experiments were conducted to extract the tool influence functions (TIFs) of the SR and PR bonnet tools. Following the conditions in Table  2, the first group was obtained under the same polishing conditions except for dwell time, while different tool offsets were used in the other group. The polishing slurry was ~3 wt.% When polishing with the SR bonnet or PR bonnet, the bonnet tool contacts the surface to be polished, squeezing the polishing abrasive. In the meantime, the rotation of the bonnet tool drives the movement of the abrasive to continuously scrape the surface, forming a small amount of material to remove. Figure 3 shows the self-developed, robot-assisted polishing device that was used in the experiment, which is equipped with a 6-DOF ABB robot arm (IRB4600-60/2.05). This 6-DOF industrial robot arm includes three rotation axes and three swing axes that can handle a payload of up to 60 kg. The farthest achievable distance of the arm is 2.05 m, and the repeated positioning accuracy is ±0.05 mm. The bonnet tool was fixed at the end of the H-axis, and the rotation of the bonnet tool was controlled through a PLC electric control cabinet. When polishing with the SR bonnet or PR bonnet, the bonnet tool contacts the surface to be polished, squeezing the polishing abrasive. In the meantime, the rotation of the bonnet tool drives the movement of the abrasive to continuously scrape the surface, forming a small amount of material to remove. Figure 3 shows the self-developed, robot-assisted polishing device that was used in the experiment, which is equipped with a 6-DOF ABB robot arm (IRB4600-60/2.05). This 6-DOF industrial robot arm includes three rotation axes and three swing axes that can handle a payload of up to 60 kg. The farthest achievable distance of the arm is 2.05 m, and the repeated positioning accuracy is ±0.05 mm. The bonnet tool was fixed at the end of the H-axis, and the rotation of the bonnet tool was controlled through a PLC electric control cabinet. The polishing pad used in this experiment was UN1NAP5-40W damping cloth (Universal Photonics, Central Islip, NY, USA). It is a velvet-like polishing material with fine texture, soft surface, porous, elastic, and long service life. It can effectively impregnate the polishing liquid during polishing to improve the removal efficiency while avoiding scratches on the workpiece. It is a high-performance, optical-level processing material [43]. Figure 4 shows the geometric model of bonnet tool polishing. To observe the material removal characteristics, two groups of experiments were conducted to extract the tool influence functions (TIFs) of the SR and PR bonnet tools. Following the conditions in Table  2, the first group was obtained under the same polishing conditions except for dwell time, while different tool offsets were used in the other group. The polishing slurry was ~3 wt.% The polishing pad used in this experiment was UN1NAP5-40W damping cloth (Universal Photonics, Central Islip, NY, USA). It is a velvet-like polishing material with fine texture, soft surface, porous, elastic, and long service life. It can effectively impregnate the polishing liquid during polishing to improve the removal efficiency while avoiding scratches on the workpiece. It is a high-performance, optical-level processing material [43].   Table 2, the first group was obtained under the same polishing conditions except for dwell time, while different tool offsets were used in the other group. The polishing slurry was 3 wt.% alumina. The average diameter of the abrasive grain size was~5 µm. Finally, the SR bonnet and PR bonnet tools each obtained ten TIFs. alumina. The average diameter of the abrasive grain size was ~5 µm. Finally, the SR bonnet and PR bonnet tools each obtained ten TIFs.

Uniform Polishing Experiment
To compare and analyze the polishing characteristics of the SR and PR bonnets, a uniform polishing experiment using the extracted TIFs was conducted. The experiment was performed on a Ti6Al4V alloy sample with a size of 100 mm × 100 mm × 10 mm. The sample was firstly ground to produce a form error lower than 6 µm Sa. To avoid damaging the bonnet tool and discard the influence of the edge effect, the actual polishing area was 80 mm × 80 mm, and a raster tool path with a 1 mm scanning interval was used. The other experimental parameters are detailed in Table 3. In the experiment, ten polishing cycles were performed using the SR bonnet, followed by nine cycles with the PR bonnet on the same workpiece. The surface roughness of each cycle was measured and recorded.

Measurement Methods
The polished surface morphology, surface energy spectrum analysis, surface chemical composition, surface roughness (Sa, Sq, and Sz), and material volume removal rate (VRR) were measured in this study to evaluate the polishing performance of the two kinds of bonnet tools on Ti6Al4V alloy. The surface morphology was measured by a Scan Electron Microscope (SEM, ZEISS sigma 500, Oberkochen, Germany) and a super depth of field 3D Microscope System (KEYENCE, VHX-2000C, Osaka, Japan). The SEM detection

Uniform Polishing Experiment
To compare and analyze the polishing characteristics of the SR and PR bonnets, a uniform polishing experiment using the extracted TIFs was conducted. The experiment was performed on a Ti6Al4V alloy sample with a size of 100 mm × 100 mm × 10 mm. The sample was firstly ground to produce a form error lower than 6 µm Sa. To avoid damaging the bonnet tool and discard the influence of the edge effect, the actual polishing area was 80 mm × 80 mm, and a raster tool path with a 1 mm scanning interval was used. The other experimental parameters are detailed in Table 3. In the experiment, ten polishing cycles were performed using the SR bonnet, followed by nine cycles with the PR bonnet on the same workpiece. The surface roughness of each cycle was measured and recorded.

Measurement Methods
The polished surface morphology, surface energy spectrum analysis, surface chemical composition, surface roughness (Sa, Sq, and Sz), and material volume removal rate (VRR) were measured in this study to evaluate the polishing performance of the two kinds of bonnet tools on Ti6Al4V alloy. The surface morphology was measured by a Scan Electron Microscope (SEM, ZEISS sigma 500, Oberkochen, Germany) and a super depth of field 3D Microscope System (KEYENCE, VHX-2000C, Osaka, Japan). The SEM detection parameters were set at 15 kV acceleration voltage and 8.8 mm working distance. In the meantime, the surface chemical composition was analyzed through the energy-dispersive X-ray (EDX) mounted on the SEM. The crystallographic structure of the titanium alloy before and after polishing was analyzed through X-ray diffraction (XRD, Rigaku SmartLab SE, Tokyo, Japan) with the Bragg-Brentano method. A ZYGO NewView TM 9000 was used to measure the TIF and surface roughness. A 10-magnification objective lens and a 0.5-magnification eyepiece were used to detect the three-dimensional shape of the entire TIF, and a 50-magnification objective lens and a 2-magnification eyepiece were used in to measure the surface roughness. The experiment data were saved in a text format, and the data points in the text were extracted into a three-dimensional matrix to calculate the volume removal rate (VRR) of TIF by performing volumetric integration.

Material Removal Rate Analysis
The VRR's variation with respect to the dwell time and offset of the two bonnet tools is demonstrated in Figure 5. It was found that the SR bonnet has a higher VRR than the PR bonnet under the same polishing conditions, which coincides with the results of polishing BK7 optical glasses reported in [41,44]. However, it was noted that the VRR increment is not proportional to the increase of the dwell time, as shown in Figure 5a. The VRRs of both of the two bonnet tools gradually decrease with the increase in dwell time and converge to a constant value, which is quite different from the results reported in [41,44]. When the dwell time is short, the peaks on the surface are easier to effectively remove or smoothen, leading to a high material removal rate. With the longer dwell time due to the strong tenacity of titanium alloy materials, the difficulty of material removal increases. Both variation trends between VRRs and tool offsets increase gradually.
The VRR's variation with respect to the dwell time and offset of th is demonstrated in Figure 5. It was found that the SR bonnet has a hi PR bonnet under the same polishing conditions, which coincides with ishing BK7 optical glasses reported in [41,44]. However, it was noted ment is not proportional to the increase of the dwell time, as shown in F of both of the two bonnet tools gradually decrease with the increase converge to a constant value, which is quite different from the results When the dwell time is short, the peaks on the surface are easier to eff smoothen, leading to a high material removal rate. With the longer dw strong tenacity of titanium alloy materials, the difficulty of material Both variation trends between VRRs and tool offsets increase graduall Similarly to previous VRR-offset investigation experiments on B linear relationship was found between the VRR and the tool offset. Ta mensions of the cross-sectional profiles of the TIFs of the SR bonnet under different dwell times, and Figure 6 presents the correspondin sectional profiles of the SR bonnet and PR bonnet. Figure 7a-f show field morphology of the SR bonnet and the PR bonnet under different be seen from all these comparisons that under the same conditions, th of the cross-section of the SR bonnet polishing point are larger than tho which also indicates that increases in bonnet hardness lead to greate rates.   Similarly to previous VRR-offset investigation experiments on BK7 glass [41,44], a linear relationship was found between the VRR and the tool offset. Table 4 shows the dimensions of the cross-sectional profiles of the TIFs of the SR bonnet and the PR bonnet under different dwell times, and Figure 6 presents the corresponding measured crosssectional profiles of the SR bonnet and PR bonnet. Figure 7a-f show the super depth of field morphology of the SR bonnet and the PR bonnet under different dwell times. It can be seen from all these comparisons that under the same conditions, the radius and depth of the cross-section of the SR bonnet polishing point are larger than those of the PR bonnet, which also indicates that increases in bonnet hardness lead to greater material removal rates.    Figure 8a,c,e demonstrate the relationship between the roughness (in Sa, Sq, and Sz) and dwell time in the center of the TIF spot. The results show that both tools can greatly reduce the surface roughness in a short time. After polishing for 30 s, Sa can be reduced from 91 nm to ~36 nm. With the increase in time, the surface roughness converges to a value and the further increment is tiny. As shown in Figure 8b,d,f, the SR bonnet achieves the lowest surface roughness at the tool offset of 0.2 mm, while it is 0.4 mm for the PR bonnet. This is because a larger tool offset means a higher contact pressure, and the contact pressure of the SR bonnet is much larger than the PR bonnet. Furthermore, we found that under the same dwell time, the surface roughness achieved by the PR bonnet is slightly less than that obtained with the SR bonnet, which indicates that the SR bonnet can be used for the pre-polishing of titanium alloy, after which the PR bonnet can be introduced for a finer finish.     Figure 8a,c,e demonstrate the relationship between the roughness (in Sa, Sq, and Sz and dwell time in the center of the TIF spot. The results show that both tools can greatly reduce the surface roughness in a short time. After polishing for 30 s, Sa can be reduced from 91 nm to ~36 nm. With the increase in time, the surface roughness converges to a value and the further increment is tiny. As shown in Figure 8b,d,f, the SR bonnet achieve the lowest surface roughness at the tool offset of 0.2 mm, while it is 0.4 mm for the PR bonnet. This is because a larger tool offset means a higher contact pressure, and the contac pressure of the SR bonnet is much larger than the PR bonnet. Furthermore, we found tha under the same dwell time, the surface roughness achieved by the PR bonnet is slightly less than that obtained with the SR bonnet, which indicates that the SR bonnet can be used for the pre-polishing of titanium alloy, after which the PR bonnet can be introduced for a finer finish.  Figure 8a,c,e demonstrate the relationship between the roughness (in Sa, Sq, and Sz) and dwell time in the center of the TIF spot. The results show that both tools can greatly reduce the surface roughness in a short time. After polishing for 30 s, Sa can be reduced from 91 nm to~36 nm. With the increase in time, the surface roughness converges to a value and the further increment is tiny. As shown in Figure 8b,d,f, the SR bonnet achieves the lowest surface roughness at the tool offset of 0.2 mm, while it is 0.4 mm for the PR bonnet. This is because a larger tool offset means a higher contact pressure, and the contact pressure of the SR bonnet is much larger than the PR bonnet. Furthermore, we found that under the same dwell time, the surface roughness achieved by the PR bonnet is slightly less than that obtained with the SR bonnet, which indicates that the SR bonnet can be used for the pre-polishing of titanium alloy, after which the PR bonnet can be introduced for a finer finish.

Surface Integrity Analysis after Uniform Polishing
The uniform polishing of the Ti6Al4V surface after grinding was al Since the SR bonnet has higher VRR than the PR bonnet, the SR bonnet wa pre-polishing and the PR bonnet was used for the fine polishing in this exper 9 shows the surface roughness variations of the SR-bonnet-polished surfa uniform polishing experiment on Ti6Al4V alloy. The initial roughness of th loy used in the experiment was 172.1 nm Sa, and the PV of the surface for µm. The results reveal that with more polishing cycles, the surface roughne alloy decreases gradually and converges to 22.2 nm Sa after 10 cycles. The the surface roughness measurement results before and after 10 cycles poli demonstrated in Figure 9. It can be observed that the surface of Ti6Al4V allo grinding tool marks and deep grooves before polishing. After 10 cycles of surface scratches and grooves were effectively removed, which indicates th net is feasible for pre-polishing the surface of titanium alloy after grinding.

Surface Integrity Analysis after Uniform Polishing
The uniform polishing of the Ti6Al4V surface after grinding was also conducted. Since the SR bonnet has higher VRR than the PR bonnet, the SR bonnet was used for the pre-polishing and the PR bonnet was used for the fine polishing in this experiment. Figure 9 shows the surface roughness variations of the SR-bonnet-polished surfaces under the uniform polishing experiment on Ti6Al4V alloy. The initial roughness of the Ti6Al4V alloy used in the experiment was 172.1 nm Sa, and the PV of the surface form exceeded 3 µm. The results reveal that with more polishing cycles, the surface roughness of Ti6Al4V alloy decreases gradually and converges to 22.2 nm Sa after 10 cycles. The 3D contour of the surface roughness measurement results before and after 10 cycles polishing are also demonstrated in Figure 9. It can be observed that the surface of Ti6Al4V alloy has obvious grinding tool marks and deep grooves before polishing. After 10 cycles of polishing, the surface scratches and grooves were effectively removed, which indicates that the SR bonnet is feasible for pre-polishing the surface of titanium alloy after grinding. According to the above results, the use of the SR bonnet to polish the ab alloy samples reached the convergence value of surface roughness. Hence, fin was subsequently conducted using the PR bonnet. Figure 10 shows the meas roughness after each polishing cycle using the PR bonnet. It can be seen tha roughness of the Ti6Al4V titanium alloy was further reduced to 17 nm Sa aft polishing. Polishing slurry with a smaller abrasive size of 0.5 µm was used two cycles of polishing, and the surface roughness of 10 nm Sa was then obta a polishing liquid with an average particle size of 0.05 µm was used for furth ment of the surface. After five cycles of polishing, the surface roughness of alloy converged to 6.1 nm Sa. Figure 11 shows the comparison of the surfac alloy before and after polishing with the SR bonnet and PR bonnet, respectiv smooth, mirror-like surface was successfully produced.  According to the above results, the use of the SR bonnet to polish the above titanium alloy samples reached the convergence value of surface roughness. Hence, finer polishing was subsequently conducted using the PR bonnet. Figure 10 shows the measured surface roughness after each polishing cycle using the PR bonnet. It can be seen that the surface roughness of the Ti6Al4V titanium alloy was further reduced to 17 nm Sa after 2 cycles of polishing. Polishing slurry with a smaller abrasive size of 0.5 µm was used for the next two cycles of polishing, and the surface roughness of 10 nm Sa was then obtained. Finally, a polishing liquid with an average particle size of 0.05 µm was used for further improvement of the surface. After five cycles of polishing, the surface roughness of the Ti6Al4V alloy converged to 6.1 nm Sa. Figure 11 shows the comparison of the surface of titanium alloy before and after polishing with the SR bonnet and PR bonnet, respectively; a highly smooth, mirror-like surface was successfully produced. According to the above results, the use of the SR bonnet to polish the ab alloy samples reached the convergence value of surface roughness. Hence, fin was subsequently conducted using the PR bonnet. Figure 10 shows the meas roughness after each polishing cycle using the PR bonnet. It can be seen tha roughness of the Ti6Al4V titanium alloy was further reduced to 17 nm Sa aft polishing. Polishing slurry with a smaller abrasive size of 0.5 µm was used two cycles of polishing, and the surface roughness of 10 nm Sa was then obta a polishing liquid with an average particle size of 0.05 µm was used for furth ment of the surface. After five cycles of polishing, the surface roughness of alloy converged to 6.1 nm Sa. Figure 11 shows the comparison of the surfac alloy before and after polishing with the SR bonnet and PR bonnet, respectiv smooth, mirror-like surface was successfully produced.

Surface Topography Analysis of Ti6Al4V before and after Polishing
The surface micro-topography of the Ti6Al4V surface before polishing is shown in Figure 12. It can be seen that, due to the extrusion of the abrasive grains of the grinding wheel, plastic grooves and a large number of scratches formed on the surface of the titanium alloy. In addition, the adherent material can be clearly observed.

Surface Topography Analysis of Ti6Al4V before and after Polishing
The surface micro-topography of the Ti6Al4V surface before polishing is shown in Figure 12. It can be seen that, due to the extrusion of the abrasive grains of the grinding wheel, plastic grooves and a large number of scratches formed on the surface of the titanium alloy. In addition, the adherent material can be clearly observed.  Figure 13 shows the SEM photograph at the center region of the TIF spot under different dwell times. Most of the grinding tool marks were removed and replaced by much smaller abrasive scratches. Specifically, only dense scratches along one direction were observed on the surface when the dwell time was 30 s, as shown in Figure 13a,c. Since the tool was not moving during the generation of the TIF spot, all of the scratches were along the same direction. When the dwell time was 300 s, some deep scratches were found on both spots generated by the SR and PR bonnet, as shown in Figure 13b,d. Although a long dwell time can remove the grinding marks thoroughly, extra deep scratches can be easily generated by some larger abrasives. This phenomenon also explains why the surface roughness increases when the dwell time is longer than a certain time, as shown in Figure  8a,c. Figure 14 shows the SEM topography of the titanium alloy surface after uniform polishing. A highly smooth surface was obtained, and the grinding marks were thoroughly removed. Moreover, much shallower and disorganized scratches were found on the polished surface compared to the results in Figure 13, which indicates that it is effective to use the SR bonnet tool for the rough polishing of Ti6Al4V, followed by the PR bonnet for a finer finish. The super depth of field morphology of Ti6Al4V alloy before and after uniform polishing in Figure 15 also indicates how the grinding tool marks were removed to obtain the ultra-smooth surface.  Figure 13 shows the SEM photograph at the center region of the TIF spot under different dwell times. Most of the grinding tool marks were removed and replaced by much smaller abrasive scratches. Specifically, only dense scratches along one direction were observed on the surface when the dwell time was 30 s, as shown in Figure 13a,c. Since the tool was not moving during the generation of the TIF spot, all of the scratches were along the same direction. When the dwell time was 300 s, some deep scratches were found on both spots generated by the SR and PR bonnet, as shown in Figure 13b,d. Although a long dwell time can remove the grinding marks thoroughly, extra deep scratches can be easily generated by some larger abrasives. This phenomenon also explains why the surface roughness increases when the dwell time is longer than a certain time, as shown in Figure 8a,c. Figure 14 shows the SEM topography of the titanium alloy surface after uniform polishing. A highly smooth surface was obtained, and the grinding marks were thoroughly removed. Moreover, much shallower and disorganized scratches were found on the polished surface compared to the results in Figure 13, which indicates that it is effective to use the SR bonnet tool for the rough polishing of Ti6Al4V, followed by the PR bonnet for a finer finish. The super depth of field morphology of Ti6Al4V alloy before and after uniform polishing in Figure 15 also indicates how the grinding tool marks were removed to obtain the ultra-smooth surface.

Surface Material Composition Analysis before and after Polishing
To investigate whether chemical reactions or material changes took place before and after polishing, EDX and XRD analysis were carried out. The EDX analysis results are shown in Figure 16, demonstrating that the surface chemical composition of the Ti6Al4V was basically unchanged. This reveals that the adhesion of titanium alloy during polishing is not due to a chemical reaction. At the same time, the phase detection of the titanium

Surface Material Composition Analysis before and after Polishing
To investigate whether chemical reactions or material changes took place before and after polishing, EDX and XRD analysis were carried out. The EDX analysis results are shown in Figure 16, demonstrating that the surface chemical composition of the Ti6Al4V was basically unchanged. This reveals that the adhesion of titanium alloy during polishing is not due to a chemical reaction. At the same time, the phase detection of the titanium

Surface Material Composition Analysis before and after Polishing
To investigate whether chemical reactions or material changes took place before and after polishing, EDX and XRD analysis were carried out. The EDX analysis results are shown in Figure 16, demonstrating that the surface chemical composition of the Ti6Al4V was basically unchanged. This reveals that the adhesion of titanium alloy during polishing is not due to a chemical reaction. At the same time, the phase detection of the titanium

Surface Material Composition Analysis before and after Polishing
To investigate whether chemical reactions or material changes took place before and after polishing, EDX and XRD analysis were carried out. The EDX analysis results are shown in Figure 16, demonstrating that the surface chemical composition of the Ti6Al4V was basically unchanged. This reveals that the adhesion of titanium alloy during polishing is not due to a chemical reaction. At the same time, the phase detection of the titanium alloy before and after final polishing was carried out by XRD. As shown in Figure 17, the XRD curves are quite similar before and after polishing. Hence, no phase change occurred after polishing. Even though there are some variations at certain peaks, these may be due to the differences in surface roughness. alloy before and after final polishing was carried out by XRD. As shown in Figure 17, the XRD curves are quite similar before and after polishing. Hence, no phase change occurred after polishing. Even though there are some variations at certain peaks, these may be due to the differences in surface roughness.

Comparison of SR Bonnet Polishing to Other Polishing Methods
Several different polishing methods have been developed for the polishing of Titanium alloys. Table 5 presents a general comparison between different polishing methods, including laser polishing, belt polishing, abrasive flow machining, chemical mechanical polishing, and SR bonnet polishing. The effects of polishing performance on surface accuracy, polishing efficiency, stability, polishing cost, and adaptability to complicated surfaces are evaluated. However, we must state that some of these effects may not fit this general comparison for specific tools. Generally, the comparison indicates the superior performance of SR bonnet polishing considering these factors. Hence, it has the potential to be used as an attractive polishing tool to enhance the efficiency of titanium polishing. alloy before and after final polishing was carried out by XRD. As shown in F XRD curves are quite similar before and after polishing. Hence, no phase cha after polishing. Even though there are some variations at certain peaks, these to the differences in surface roughness.

Comparison of SR Bonnet Polishing to Other Polishing Methods
Several different polishing methods have been developed for the polis nium alloys. Table 5 presents a general comparison between different polish including laser polishing, belt polishing, abrasive flow machining, chemica polishing, and SR bonnet polishing. The effects of polishing performance on racy, polishing efficiency, stability, polishing cost, and adaptability to com faces are evaluated. However, we must state that some of these effects ma general comparison for specific tools. Generally, the comparison indicates performance of SR bonnet polishing considering these factors. Hence, it has to be used as an attractive polishing tool to enhance the efficiency of titanium

Comparison of SR Bonnet Polishing to Other Polishing Methods
Several different polishing methods have been developed for the polishing of Titanium alloys. Table 5 presents a general comparison between different polishing methods, including laser polishing, belt polishing, abrasive flow machining, chemical mechanical polishing, and SR bonnet polishing. The effects of polishing performance on surface accuracy, polishing efficiency, stability, polishing cost, and adaptability to complicated surfaces are evaluated. However, we must state that some of these effects may not fit this general comparison for specific tools. Generally, the comparison indicates the superior performance of SR bonnet polishing considering these factors. Hence, it has the potential to be used as an attractive polishing tool to enhance the efficiency of titanium polishing.