Name: plasma polishing as a new polishing option to reduce the surface roughness of porous titanium alloy for 3d printing
Uploaded: 2023-04-28T12:50:00
Description: Watch this Scientific Journal Video about Plasma Polishing as a New Polishing Option to Reduce the Surface Roughness of Porous Titanium Alloy for 3D Printing at JoVE.com

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).

1. Printing and preparation of a titanium alloy workpiece
<ol>
	<li>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 &#181;m, and set the layer thickness to 30 &#181;m (Figure 1).</li>
	<li>Grade 23 Ti-6Al-4V powder with chemical composition as shown in Table 1 and a powd...

<ol>
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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 prosp...

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&#39;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&#39;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&#39;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.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Confocal microscope: Smartproof-5</td>
			<td>ZEISS</td>
			<td>4702000198</td>
			<td></td>
		</tr>
		<tr>
			<td>ConfoMap ST 8.0</td>
			<td>ZEISS</td>
			<td>4702000198</td>
			<td></td>
		</tr>
		<tr>
			<td>Electrical discharge machining (EDM) machine: MV1200S</td>
			<td>Mitsubishi Electric Automation (China) Ltd.</td>
			<td>92U3038</td>
			<td></td>
		</tr>
		<tr>
			<td>Heat treatment furnace: HSQ1-644</td>
			<td>Jiangsu Huasu Industrial Furnace Manufacturing CO., LTD.</td>
			<td>HSD20190812403</td>
			<td></td>
		</tr>
		<tr>
			<td>Metal 3D printer: Renishaw AM400</td>
			<td>Renishaw plc</td>
			<td>1HGW89</td>
			<td></td>
		</tr>
		<tr>
			<td>Middle speed wire-cut machine: HQ-400EZ</td>
			<td>Suzhou Hanqi CNC Equipment CO., LTD.</td>
			<td>W40ES20005</td>
			<td></td>
		</tr>
		<tr>
			<td>Permanent magnet frequency conversion screw air compressor M7-Y75AZ</td>
			<td>KUNJI MACHINERY(SHANGHAI) MANUFACTURING CO.,LTD.&#160;</td>
			<td>19055065</td>
			<td></td>
		</tr>
		<tr>
			<td>Refrigeration compressed air dryer SY-230FG</td>
			<td>Shanghai TaiLin Compressor Co., Ltd.</td>
			<td>S190826698</td>
			<td></td>
		</tr>
		<tr>
			<td>Scanning electron microscope (SEM): JSM-IT100</td>
			<td>JEOL (BEIJING) CO., LTD.</td>
			<td>MP1030004260426</td>
			<td></td>
		</tr>
		<tr>
			<td>Titanium alloy powder</td>
			<td>Renishaw plc</td>
			<td>H-5800-1086-01-A</td>
			<td></td>
		</tr>
		<tr>
			<td>Ultrasonic cleaning machine: AK-030S</td>
			<td>Shenzhen Yujie Cleaning Equipment Co., Ltd</td>
			<td>30820004</td>
			<td></td>
		</tr>
		<tr>
			<td>ZEN core v3.0</td>
			<td>ZEISS</td>
			<td>4702000198</td>
			<td></td>
		</tr>
	</tbody>
</table>

plasma polishing as a new polishing option to reduce the surface roughness of porous titanium alloy for 3d printing

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 &#176;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 &#181;m to Ra = 56.28 &#181;m, and the surface roughness of the trabecular structure changed from Ra = 42.61 &#181;m to Ra = 26.25 &#181;m. Meanwhile, semi-molten powders and ablative oxide layers are removed, and surface quality is improved.

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...

Watch this Scientific Journal Video about Plasma Polishing as a New Polishing Option to Reduce the Surface Roughness of Porous Titanium Alloy for 3D Printing at JoVE.com

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

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.

Porous titanium alloy implants with simulated trabecular bone fabricated by 3D printing technology have broad prospects. However, due to the fact that ...

Since the internal pulse of porous structures cannot be polished by conventional mechanical pollution, an alternative method needs to be found. Plasma pollution is an environmentally friendly processing method that is especially effective for mental work pieces with complex shapes. To begin, place the separated titanium alloy workpieces in the titanium basket without the different workpieces touching each other.Put the titanium basket into the heat treatment furnace at room temperature and close the furnace door. Open the gas valve to remove the air and maintain the appropriate degree of vacuum. To set the heat treatment process first, heat the furnace to 800 degrees Celsius for 1.5 hours, and then maintain the temperature for two hours before cooling.After the heat treatment, cool the furnace to room temperature and fill the furnace with air. Once the furnace returns to atmospheric pressure as seen on the panel, take out the porous titanium alloy workpiece. To image the surfaces of the workpiece using a confocal microscope, place the workpiece on the storage platform horizontally.Measure the surface arithmetic average roughness or RA parameter. Select 2.5x magnification and choose wide for live mode. To observe the overall situation click auto intensity and go to 5x magnification.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 set the acquisition to normal. After about five minutes, import the results into a new document in the ConfoMap ST8 software.The RA is easy to obtain in the parameters table in ConfoMap ST.Observe the overall condition of the workpiece with a five-fold mirror. Then, switch to a high power mirror and focus the field of vision on a trabecular. Quantify the plasma polishing effect by the RA of the workpiece before plasma polishing.For plasma polishing, use 4%ammonium sulfate solution with pH between 5.7 to 6.1 as the electrolyte. Place the surface of the porous titanium alloy workpiece to be polished horizontally and fix it on the fixture. Then, put the fixture into the plasma polishing machine.Set the polishing current to 59 amp pairs, the voltage to 313 volts and the polishing electrolyte temperature to 101.6 degrees Celsius, and conduct plasma polishing according to these parameters. After conducting plasma polishing for 90 seconds, take the fixture out of the plasma polishing machine. Then, slightly change the position of the clamping point where the workpiece is fixed to the fixture.No electrochemical reaction occurred there as it was not in contact with the polishing solution. Conduct plasma polishing again for 90 seconds, and take the fixture out of the plasma polishing machine. Remove the workpiece from the fixture and put it into the ultrasonic cleaning machine with deionized water.Set the water temperature at 30 degrees Celsius and clean the workpiece for two minutes. After two minutes, take out the workpiece and blow out the residual liquid with high pressure air. After the completion of plasma polishing, image the surfaces using scanning electron microscopy and confocal microscopy in the same way as demonstrated before.The scanning electron microscopy images revealed the difference in the surface morphologies of the porous titanium alloy workpiece before and after plasma polishing. At 30x and 100x magnification, the surface before plasma polishing appeared rougher. When magnified to 500x, the semi-molten powders and ablative oxide layers observed on the alloy surface before plasma polishing were mostly found to be absent after the polishing.Interestingly, the porous size and trabecular diameters were consistent with the design even after polishing. The whole and part of the porous titanium alloy workpiece were imaged using the fast rotary confocal microscope. The surface roughness in both cases was high before plasma polishing.The surface roughness of porous structures as revealed by the RA, significantly reduced after polishing. This technology has explained that porous titanium alloy workpieces can reduce surface roughness through plasma pollution technology. Further studies can be conducted to determine the optimal parameters.

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