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JoVE Journal Medicine

Medicine

Plasma Polishing as a New Polishing Option to Reduce the Surface Roughness of Porous Titanium Alloy for 3D Printing

Published: April 28, 2023 doi: 10.3791/65108
Automatic Translation
*1,2, *3, *4, 4,5, 4, 6, 1, 1,2,4
1Orthopedic Center, Affiliated Hospital of Guangdong Medical University, 2The School of Basic Medical Sciences, Guangdong Medical University, 3The School of Basic Medical Sciences, Fujian Medical University, 4Guangdong Engineering Research Center for Translation of Medical 3D Printing Application, Guangdong Provincial Key Laboratory of Digital Medicine and Biomechanics, National Key Discipline of Human Anatomy, School of Basic Medical Sciences, Southern Medical University, 5Integrated Hospital of Traditional Chinese Medicine, Southern Medical University, 6Shenzhen Hospital of Guangzhou University of Chinese Medicine (Futian)
* These authors contributed equally

Summary

Plasma polishing is a promising surface processing technology, especially suitable for 3D printing of porous titanium alloy workpieces. It can remove semi-molten powders and ablative oxide layers, thereby effectively reducing surface roughness and improving surface quality.

Abstract

Porous titanium alloy implants with simulated trabecular bone fabricated by 3D printing technology have broad prospects. However, due to the fact that some powder adheres to the surface of the workpiece during the manufacturing process, the surface roughness in direct printing pieces is relatively high. At the same time, since the internal pores of the porous structure cannot be polished by conventional mechanical polishing, an alternative method needs to be found. As a surface technology, plasma polishing technology is especially suitable for parts with complex shapes that are difficult to polish mechanically. It can effectively remove particles and fine splash residues attached to the surface of 3D printed porous titanium alloy workpieces. Therefore, it can reduce surface roughness. Firstly, titanium alloy powder is used to print the porous structure of the simulated trabecular bone with a metal 3D printer. After printing, heat treatment, removal of the supporting structure, and ultrasonic cleaning is carried out. Then, plasma polishing is performed, consisting of adding a polishing electrolyte with the pH set to 5.7, preheating the machine to 101.6 °C, fixing the workpiece on the polishing fixture, and setting the voltage (313 V), current (59 A), and polishing time (3 min). After polishing, the surface of the porous titanium alloy workpiece is analyzed by a confocal microscope, and the surface roughness is measured. Scanning electron microscopy is used to characterize the surface condition of porous titanium. The results show that the surface roughness of the whole porous titanium alloy workpiece changed from Ra (average roughness) = 126.9 µm to Ra = 56.28 µm, and the surface roughness of the trabecular structure changed from Ra = 42.61 µm to Ra = 26.25 µm. Meanwhile, semi-molten powders and ablative oxide layers are removed, and surface quality is improved.

Introduction

Titanium and titanium alloy materials have been widely used as dental and orthopedic implant materials because of their good biocompatibility, corrosion resistance, and mechanical strength1,2,3. However, due to the high elastic modulus of the compact titanium alloy produced by traditional processing methods, these plates are not suitable for bone repair, since close proximity to the bone surface for long periods can result in stress shielding and bone embrittlement4,5 . Therefore, the porous microstructure of simulated bone trabeculae should be used in titanium alloy implants in order to reduce its elastic modulus to the level matching the bone6,7. Many scaffolds have been used in the field of orthopedics to improve cell viability, attachment, proliferation and homing, osteogenic differentiation, angiogenesis, host integration, and weight bearing4,8,9. Traditional fabrication methods of porous metal structures include the structural template method, defect formation method, compression or supercritical carbon dioxide method, electro-deposition technique10,11, etc. Although these production techniques are highly traditional, they occasionally squander raw materials and have substantial preparatory costs when compared to 3D printing12,13. 3D printing is a technology that uses metal or plastic powder and other adhesive materials to build solid 3D objects from computer aided design (CAD) models via the deposition of overlying layers14,15 . 3D printing shows great potential in directly customizing metallic cellular scaffolds for orthopedic implants and opens up new possibilities for manufacturing customizable complex designs with highly interconnected pores. Among them, selective laser melting (SLM) is one of the most representative 3D printing and manufacturing technologies for porous titanium implant structures16 .

The SLM process uses titanium alloy powder as the raw material, essentially powder melting and forming the structure. Therefore, a large number of semi-molten powders and ablative oxide layers often adhere to the surface of titanium alloy implants, which leads to high surface roughness17. Poor surface quality of porous titanium orthopedic implants leads to inflammation, decreased fatigue performance, and even new biological risks18 . Since the internal pores of porous structures cannot be polished by conventional mechanical polishing, an alternative method needs to be found. Plasma polishing is a new green polishing method for metal workpieces that can efficiently polish workpieces with complex shapes without pollution19 . It has great development potential in the field of titanium alloy implant post-processing.

As a kind of surface technology, plasma polishing technology is particularly suitable for metal workpieces with complex shapes that are not easy to be mechanically polished. The overall goal of this polishing option is to obtain a porous titanium alloy surface with low roughness. The technology can effectively remove particles and fine splash residues attached to the surface of porous titanium orthopedic implants fabricated by 3D printing and reduce surface roughness20. The principle of plasma polishing is a composite reaction process based on a combination of current-induced chemical and physical removal21; the entire circuit forms a transient short circuit, forming a vapor plasma-surrounding layer on the workpiece surface20. This process breaks through the gas layer to form a discharge channel, impacting the workpiece surface. The higher current impacts the convex part of the workpiece surface, leading to the faster removal of semi-molten powder and the burnt oxide layer. The concavity and convexity are constantly changing, and the rough surface becomes gradually smoothed, improving the surface roughness of the workpiece to achieve the purpose of polishing.

At the same time, this technology is a green processing technology, causing no pollution to the environment, and has great advantages compared with other polishing methods. Conventional mechanical polishing techniques mainly include mechanical polishing, chemical polishing, and electrochemical polishing22. Mechanical polishing is the most widely used conventional polishing process; it has the disadvantages of low polishing efficiency, higher demand for manual labor, and inability to polish parts with complex geometries. The potential for employee injury and the likelihood of exceeding tolerances due to human factors are frequent drawbacks of mechanical polishing23. In contrast to chemical polishing, which is based on utilizing a chemical solution to remove parts of a workpiece's material, electrochemical polishing utilizes an electric current and chemical solution to obtain the same result. Unfortunately, both these processes produce hazardous gases and liquids as by-products of use, the composition of which being dependent on the strength of the acid or alkaline chemical reagent being used. As a result, not only are the workers present deemed to be at risk due to exposure, but there is also the potential for severe damage to the environment24. Aliakseyeu et al.25 proposed utilizing plasma polishing for polishing titanium alloy workpieces with simple electrolyte composition. They found that, after polishing titanium sample surface scratches are removed and the surface gloss is significantly improved. Smyslova et al.26 deliberated upon the prospects of applying plasma polishing technology to treat the surfaces of medical implants.

Theoretically, plasma polishing technology can be utilized to polish the structure of any metal part. It has been widely applied for coating, in metal finishing industries, and in 3C electronics, among others22,27,28. However, the present study has some limitations. First of all, the manuscript only focuses on the surface quality and surface roughness of 3D printing porous titanium alloy before and after plasma polishing; the remaining changes are not involved. Secondly, we didn't measure and record the results after heat treatment. Jinyoung Kim et al.29 compared titanium surface modification strategies for osseointegration enhancement. Another study shows that the target-ion induced plasma sputtering (TIPS) technique can impart excellent biological functions to the surface of metallic bio-implants30. In order to further investigate the polishing efficacy and safety of porous titanium alloy for 3D printing, the next step will be to further study SLM part's other properties, such as fatigue performance and osteogenic differentiation. These issues need further refinement. This work differs from earlier plasma polishing studies in that it focuses on 3D printing porous titanium alloy rather than compact titanium alloy. As a result, different manufacturing processes should adopt different polishing parameters. The purpose of this manuscript is to introduce the plasma polishing scheme of 3D printing porous titanium alloy in detail, so as to reduce the surface roughness of workpieces.

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Protocol

1. Printing and preparation of a titanium alloy workpiece

  1. Prepare a workpiece made of porous titanium alloy using the SLM printing technique. Import STL format files into the metal printer, add Ti-6Al-4V powder, install the build substrate, set up the wiper blade, set the laser spot size to 70 µm, and set the layer thickness to 30 µm (Figure 1).
  2. Grade 23 Ti-6Al-4V powder with chemical composition as shown in Table 1 and a powder particle size of 15-53 µm.
  3. Design the porous titanium alloy structure with simulated trabecular bone based on Tyson polygon anisotropy using parametric modeling, with an aperture size of 400-600 µm, small beam diameter of 100-300 µm, and porosity of 70%31 .
  4. Ensure the porous titanium alloy workpiece is in the shape of the medical lumbar cage32. For the porous structure and the lumbar cage, use Boolean operations to obtain the porous workpiece structure.

2. Heat treatment

  1. A high temperature gradient during SLM printing will cause residual stress in the workpiece. Use heat treatment to eliminate the residual stress inside the workpiece and maintain the toughness, plasticity, tensile strength, and other physical properties of the workpiece.
  2. Separate the porous titanium alloy workpiece from the printing substrate after printing using a medium speed wire-cutting machine. Install the titanium plate on the medium speed wire-cutting machine, in order to make the plate perpendicular to the ground, and ensure that the wire just contacts the support surface. Then, cut along the support and titanium plate to separate the porous titanium alloy workpiece from the printing substrate.
  3. Place the porous titanium alloy workpiece into the ultrasonic cleaning machine with deionized water for 15 min and the temperature controlled at 30 °C. Keep the ultrasonic frequency at 40,000 Hz. The ultrasonic cleaning aims to remove the titanium alloy powder remaining in the porous structure.
  4. Repeat the aforementioned ultrasonic cleaning procedure four times to remove residual titanium alloy powder and deionized water from the porous structure. After that, aim high-pressure air at the porous structure for 20 s to blow away the residual powder and liquid. The pressure of the high-pressure air is 0.71 MPa, which is generated by an air compressor and air dryer.
  5. Put the titanium basket into the heat treatment furnace at room temperature. The titanium basket is equipped with titanium alloy workpieces separated from the substrate. Keep different workpieces from touching each other and close the furnace door.
  6. Open the gas valve, take out the air, and keep the vacuum degree at 3.9 x 10-3 Pa.
  7. Set the heat treatment process. First, heat the furnace to 800 °C for 1.5 h, maintain the temperature for 2 h, and then cool the workpiece inside the furnace. This process ensures that the vacuum pressure remains unchanged.
  8. After the heat treatment, cool the furnace to room temperature and fill the furnace with air. After returning to atmospheric pressure, as seen on the panel, take out the porous titanium alloy workpiece.

3. Removing the support

  1. After heat treatment, the porous titanium alloy workpieces have no internal residual stress, so the workpiece surface will not crack and/or fracture when removing the support.
  2. Measure the support thickness using a vernier caliper, fix the workpiece on the low-speed wire-cutting electrical discharge machining (EDM) machine, and ensure that the copper wire just contacts the support surface.
  3. Set the cutting depth equal to the support thickness. It's inevitable that removing the support by the wire-cutting EDM machine will form an ablation oxide layer. When removing the support, ensure that the workpiece is immersed in deionized water to minimize burns to the workpiece surface.
  4. A reasonable support design ensures accuracy when removing the support. If there are still some support residues, polish the workpiece with sandpaper.

4. Ultrasonic cleaning

  1. Since the workpiece is immersed in deionized water during support removal, perform ultrasonic cleaning before plasma polishing to remove other impurities.
  2. Put the porous titanium alloy workpiece into the ultrasonic cleaning machine with deionized water, set the water temperature to 30 °C, and clean it for 5 min. After 5 min, take out the workpiece and blow out residual liquid with high-pressure air.

5. First characterization

  1. Scanning electron microscope (SEM): Image the surfaces with a SEM at 15 and 20 kV accelerating voltage, after ultrasonic cleaning and before plasma polishing.
  2. Take images at 30x, 100x, and 500x visual fields. Observe the general surface morphology, particle adhesion, and pore size of the porous titanium alloy workpiece, and qualitatively evaluate the plasma polishing effect.
  3. Confocal microscope: Image the surfaces using a confocal microscope.
  4. Place the workpiece on the storage platform horizontally. Measure the surface arithmetic average roughness (Ra) parameter. Use ZEN core v3.0 and ConfoMap ST 8.0 software.
    1. Select 2.5x magnification, choose Wide for live mode, click Auto intensity, and then go to 5x magnification to observe the overall situation. Click Auto intensity and set the live mode to Comp. Select the area of interest, click Set first at the lowest point and Set last at the highest point, and then set the acquisition to Normal.
    2. After about 5 min, import the results into a new document in ConfoMap ST 8.0. The Ra is easy to obtain in the parameters table in ConfoMap ST.
  5. Observe the overall condition of the workpiece with a fivefold mirror, then switch to a high-power mirror and focus the field of vision on a trabecula. Evaluate the plasma polishing effect quantitatively by describing the Ra of the porous titanium alloy workpiece before plasma polishing.

6. Plasma polishing

  1. For this, use an electrolytic cell to immerse the workpiece in an electrolyte connected as an anode20. Use 4% ammonium sulfate solution [(NH4)2SO4], pH between 5.7-6.1, as the electrolyte. Preheat the polishing electrolyte to 80 °C before plasma polishing.
  2. Set the polishing current to 59 A, the voltage to 313 V, and the polishing electrolyte temperature to 101.6 °C (Figure 2A). Conduct plasma polishing according to these parameters.
  3. Place the surface of the porous titanium alloy workpiece to be polished horizontally and fix it on the fixture, and then put the fixture into the plasma polishing machine (Figure 2B). Conduct plasma polishing for 90 s, and then take the fixture out of the plasma polishing machine.
  4. Since the porous titanium alloy workpiece is fixed on the fixture through the clamping point, the clamping point is not in contact with the polishing solution, and the corresponding electrochemical reaction does not occur at the clamping point. Therefore, change the position of the clamping point slightly after the fixture is taken out.
  5. Conduct plasma polishing again for 90 s and take the fixture out of the plasma polishing machine. Remove the porous titanium alloy workpiece from the fixture and then put it into the ultrasonic cleaning machine with deionized water.
  6. Set the water temperature at 30 °C and clean the workpiece for 2 min. After 2 min, take out the workpiece and blow out the residual liquid with high-pressure air.

7. Second characterization

  1. After the completion of plasma polishing, image the surfaces using a SEM and a confocal microscope in the same way as in step 5. Assess the influence of plasma polishing on the surface roughness and surface quality of 3D printing porous titanium alloy by comparing the above two shooting results.

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Representative Results

Surface morphology
Figure 3 shows the SEM result of the surface morphology of the porous titanium alloy workpiece before and after plasma polishing. We observed that at 30x and 100x magnification, the surface of the porous titanium alloy workpiece before plasma polishing seems to be rougher (Figure 3A,B). When magnified to 500x, we found that a large amount of semi-molten powders and ablative oxide layers could be observed on the surface of the porous titanium alloy (Figure 3C). However, most of the semi-molten powders and ablative oxide layers on the surface of the porous titanium alloy were removed after plasma polishing (Figure 3F). At the same time, the pore size and trabecular diameter were consistent with the design, which was not damaged (Figure 3D,E). This shows that plasma polishing can improve the surface quality of 3D printing porous titanium alloy workpieces and does not damage the original design pore structure.

Surface roughness measurement
The whole and part of the porous titanium alloy workpiece were imaged using the fast rotary confocal microscope, as shown in Figure 4, and the surface roughness was measured. The surface roughness is high, whether it's the whole surface of the porous titanium alloy or a small beam forming a porous structure, before plasma polishing (Figure 4A,B). The surface roughness of porous structure is significantly reduced; the Ra of the overall surface is 56.28 µm (Figure 4C), While the Ra of part of the porous titanium alloy workpiece is 26.65 µm (Figure 4D).

Figure 1
Figure 1: SLM metal 3D printing. SLM printing technology is used and 23 Ti-6Al-4V powder is graded to prepare a porous titanium alloy workpiece. Please click here to view a larger version of this figure.

Figure 2
Figure 2: Plasma polishing machine and polishing fixture. (A) Plasma polishing machine setting parameters: the polishing current is set as 59 A, the voltage is set as 313 V, and the polishing electrolyte temperature is set as 101.6 °C, after preheating the polishing electrolyte. (B) Polishing fixture. The surface of the porous titanium alloy workpiece to be polished is placed horizontally and fixed on the fixture, ensuring that the fixture is immersed in the polishing electrolyte. Please click here to view a larger version of this figure.

Figure 3
Figure 3: The images of the 3D printed porous titanium alloy workpiece using an SEM. Before plasma polishing, (A) at 30x, the whole porous structure can be observed. (B) At 100x, the pore structure can be observed. The surface of the porous titanium alloy workpiece before plasma polishing seems to be rougher. (C) At 500x, a large amount of semi-molten powders and ablative oxide layers can be observed on the surface of the trabecular structure. After plasma polishing, (D) at 30x, the whole porous structure can be observed. (E) At 100x, the pore structure can be observed. The pore size and trabecular diameter were consistent with the design, which was not damaged. (F) At 500x, most of the semi-molten powders and ablative oxide layers on the surface of the porous titanium alloy were removed. Please click here to view a larger version of this figure.

Figure 4
Figure 4: The images of the 3D printed porous titanium alloy workpiece using a confocal microscope. The image shows the surface morphology of the porous titanium alloy, with the coordinate axis representing the length. After plasma polishing, the surface of porous titanium alloy exhibits a shiny metallic appearance. (A) The whole porous titanium alloy workpiece was imaged before plasma polishing, Ra = 126.9 µm. (B) A part of the porous titanium alloy workpiece was imaged before plasma polishing, Ra = 42.61 µm. (C) The whole porous titanium alloy workpiece was imaged after plasma polishing, Ra = 56.28 µm. The overall surface roughness can be reduced by plasma polishing. (D) A part of the porous titanium alloy workpiece was imaged after plasma polishing, Ra = 26.65 µm. The surface roughness of the trabecular structure can be reduced by plasma polishing. Please click here to view a larger version of this figure.

Element Mass(%)
Titanium Balance
Aluminium 5.50 to 6.50
Vanadium 3.50 to 4.50
Iron < 0.25
Oxygen < 0.13
Carbon < 0.08
Nitrogen < 0.05
Hydrogen < 0.012
Residual < 0.10 each, 0.40 total

Table 1: Chemical composition of Ti-6Al-4V alloy powder.

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Discussion

Surface roughness is used to describe the amount of undulation and unevenness of micro geometric shapes on workpiece surfaces within a small spacing range. A number of previous studies have reported how to polish metal surfaces using different procedures, such as mechanical polishing, chemical polishing, electrochemical polishing, and more22,33,34,35. Although numerous studies have showed prospective polishing effects based on these conventional mechanical polishing techniques, the polishing method for 3D printing porous titanium alloy is crucial in order to reduce the surface roughness. Plasma polishing can efficiently polish workpieces with complex shapes without pollution. Therefore, surface roughness can measure the surface quality of 3D printed porous titanium alloy. The surface roughness of metallic orthopedic implants can not only optimize implant-bone interactions, but simultaneously minimize implant-bacteria interactions36 . Metallic cellular scaffolds can provide a place for cells and blood vessels to grow into, while osteoblasts seem to prefer rougher surfaces37. In this experiment, the surface roughness of 3D printed porous titanium alloy is maintained at 26.65 µm after plasma polishing, which meets the basic requirements of promoting the growth of cells and blood vessels.

It is essential to conduct ultrasonic cleaning before heat treatment, so as to prevent the porous structure from being blocked by molten titanium powder. The porous titanium alloy workpiece is put into the ultrasonic machine with deionized water for 15 min for cleaning. Residual titanium alloy powder is blown off with high-pressure air after cleaning, and the ultrasonic cleaning and blowing off of residual powder is repeated three more times. In other words, 1 h of ultrasonic cleaning and four instances of high-pressure air blowing are carried out to remove residual titanium alloy powder.

During plasma polishing, the workpiece should be gently fixed on the fixture to protect the trabecula of the porous structure from damage, since a bit of the polishing fixtures become sharper after freuqent polishing. The fixture is taken out of the plasma polishing machine, the position of the clamping point is changed slightly after polishing for 90 s, and then plasma polishing is conducted for the remaining 90 s. If plasma polishing lasts for 180 s at one time without changing the position of the clamping point, the polishing around the clamping point will be successful, but the clamping point covered by the fixture of porous titanium alloy will present an unpolished surface state.

However, this polishing technology also has some limitations, such as high energy consumption. Due to the bath size limitation, plasma polishing equipment cannot process large parts. This technology can also be further studied. It is recommended to utilize more modeling and simulation studies to accurately predict optimal process parameter values, with the intention of achieving predicted workpiece improvements while minimizing the time and expense required for experimentation. We can conduct further studies to determine the optimal parameters for plasma polishing of porous titanium alloy workpieces22.

From a microscopic perspective, plasma polishing is a process in which the surface of a metal is melted by heat that is generated by high-speed electron impact. It is a new development trend in the field of green manufacturing and precision machining and is very suitable for 3D printed porous titanium alloy. In conclusion, this protocol for polishing 3D printing porous titanium alloy workpieces will be a new option to reduce surface roughness and improve surface quality.

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Disclosures

The authors have nothing to disclose.

Acknowledgments

I would like to thank my supervisor, Wenhua Huang, for providing support conditions and guidance for this experiment. This research was funded by the Discipline construction project of Guangdong Medical University (4SG22260G), Young Innovative Talents Project of Guangdong Higher Education Institutions (2021KQNCX023), National Natural Science Foundation of China (82205301), and Futian Healthcare Research Project (FTWS2022051).

Materials

Name Company Catalog Number Comments
Confocal microscope: Smartproof-5 ZEISS 4702000198
ConfoMap ST 8.0 ZEISS 4702000198
Electrical discharge machining (EDM) machine: MV1200S Mitsubishi Electric Automation (China) Ltd. 92U3038
Heat treatment furnace: HSQ1-644 Jiangsu Huasu Industrial Furnace Manufacturing CO., LTD. HSD20190812403
Metal 3D printer: Renishaw AM400 Renishaw plc 1HGW89
Middle speed wire-cut machine: HQ-400EZ Suzhou Hanqi CNC Equipment CO., LTD. W40ES20005
Permanent magnet frequency conversion screw air compressor M7-Y75AZ KUNJI MACHINERY(SHANGHAI) MANUFACTURING CO.,LTD.  19055065
Refrigeration compressed air dryer SY-230FG Shanghai TaiLin Compressor Co., Ltd. S190826698
Scanning electron microscope (SEM): JSM-IT100 JEOL (BEIJING) CO., LTD. MP1030004260426
Titanium alloy powder Renishaw plc H-5800-1086-01-A
Ultrasonic cleaning machine: AK-030S Shenzhen Yujie Cleaning Equipment Co., Ltd 30820004
ZEN core v3.0 ZEISS 4702000198

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Tags

Plasma Polishing Surface Roughness Reduction Porous Titanium Alloy 3D Printing Alternative Method Environmentally Friendly Processing Complex Shapes Titanium Basket Heat Treatment Furnace Vacuum Temperature Control Confocal Microscope Surface Arithmetic Average Roughness (RA) Magnification
Plasma Polishing as a New Polishing Option to Reduce the Surface Roughness of Porous Titanium Alloy for 3D Printing
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Cite this Article

Lin, Z., Luo, L., Lin, D., Deng, Y., … More

Lin, Z., Luo, L., Lin, D., Deng, Y., Yang, Y., Huang, X., Wu, T., Huang, W. Plasma Polishing as a New Polishing Option to Reduce the Surface Roughness of Porous Titanium Alloy for 3D Printing. J. Vis. Exp. (194), e65108, doi:10.3791/65108 (2023).

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