E-Archive

VOL. 26 January ISSUE YEAR 2025

Science Update

in Vol. 26 - January Issue - Year 2025
Plasma Electrolytic Polishing of Additively Manufactured Ti-6Al-4V
Fig. 1: Typical PEP process at 300 V for 5 minutes using a direct current (DC) and constant voltage mode. (a) Increase and stabilization of the applied PEP voltage and (b) Evolutions of the current density and the electrolyte temperature.

Fig. 1: Typical PEP process at 300 V for 5 minutes using a direct current (DC) and constant voltage mode. (a) Increase and stabilization of the applied PEP voltage and (b) Evolutions of the current density and the electrolyte temperature.

Fig. 2: Topographies of individual AM Ti-6Al-4V coupons before and after PEP process for 5 minutes. (a) Before and (b) After PEP at 300 V, and (c) Before and (d) After PEP at 325 V. The circles marked the same locations before and after PEP, respectively. The arrow indicates the buildup direction of the AM coupons.

Fig. 2: Topographies of individual AM Ti-6Al-4V coupons before and after PEP process for 5 minutes. (a) Before and (b) After PEP at 300 V, and (c) Before and (d) After PEP at 325 V. The circles marked the same locations before and after PEP, respectively. The arrow indicates the buildup direction of the AM coupons.

Fig. 3: Average surface roughness (Ra) derived from the topographies of the individual AM Ti-6Al-4V coupons before and after PEP processes at 275, 300, 325, and 350 V, respectively.

Fig. 3: Average surface roughness (Ra) derived from the topographies of the individual AM Ti-6Al-4V coupons before and after PEP processes at 275, 300, 325, and 350 V, respectively.

Fig. 4: Optical microscopic (OM) images of the AM Ti-6Al-4V coupons before and after PEP process. (a) Before PEP process, (b) After PEP process at 300 V, and (c) After PEP process at 350 V.

Fig. 4: Optical microscopic (OM) images of the AM Ti-6Al-4V coupons before and after PEP process. (a) Before PEP process, (b) After PEP process at 300 V, and (c) After PEP process at 350 V.

Introduction                        

Plasma electrolytic polishing (PEP) is an advanced surface finishing method that combines electrochemical polishing and plasma effect in electrolyte to smoothen the surface of metallic alloy components. Although the detailed polishing mechanism of PEP process is still under debating, its effective surface finishing characteristics are commonly accepted. However, whether this technology can be developed for smoothening the surface with high roughness, e.g., with the average roughness (Ra) larger than 10 μm, is still an open question. A basic argument is that the PEP process occurred to the metallic component everywhere on the surface contacting the electrolyte, leading to rounding of the corners when extending the processing time to smoothen the rough surface with increase peak-to-valley heights [1]. Increasing research interests have been trigged by this argument in recent years, driven by potential application of PEP for surface finishing of additively manufactured (AM) metallic alloys, which usually have high surface roughness, i.e., Ra > 10 μm.

Ti-6Al-4V is an important aerospace alloy with high strength-to-weight ratio and great mechanical performance at high temperatures. The excellent corrosion resistance and biocompatibility of Ti-6Al-4V make it a candidate material for medical applications, e.g., manufacturing implant devices. Recent developments in AM technologies offered great advantages for rapid prototyping structural components based on metallic alloys, e.g., customized medical implants based on Ti-6Al-4V alloys. However, there is a great challenge to smoothen the surface of AM Ti-6Al-4V alloys by traditional mechanical grinding and polishing methods, especially for those with near-net-shape, due to the complex three-dimensional surface geometry [2].

PEP has been recently proposed for surface finishing of Ti-6Al-4V alloys, however, most of the earlier studies were carried out on conventional wrought alloys with the surface pre-treated by traditional mechanical methods [3]. In comparison, PEP process on AM Ti-6Al-4V alloys without pre-treatment, which is desired for surface finishing of near-net-shape components, has been rarely reported in the literature. In this regard, we carried out PEP process on AM Ti-6Al-4V coupons that were vertically built up by laser powder bed fusion (LPBF), addressing the surface chemical and structural evolutions. Here, we report the surface smoothening effect of PEP on AM Ti-6Al-4V coupons in terms of topographic and morphological characterizations as a function of processing voltages.

Materials and Methods

Ti-6Al-4V coupons of 100 mm × 10 mm × 3 mm were vertically built up on commercial Ti-6Al-4V substrate using LPBF. After heat treatment at 800 °C for 120 minutes in vacuum and quenched by gaseous nitrogen at 3.0 bar, the AM coupons were removed from the substrate by electrical discharge machining. An aqueous electrolyte consisted of NH4F (3.53 wt%) and KF (2.18 wt%) with the pH value of ~6.4 was used in the PEP process. The Ti-6Al-4V was used as the working electrode (i.e., the anode) while a stainless-steel plate (50 mm × 50 mm × 1 mm) was used as the cathode. The distance between the anode and the cathode electrodes was about 50 mm, and the temperature of the electrolyte was kept at 75 °C before starting the PEP process. The PEP process was carried out for 5 minute using a direct current and constant voltage mode, the applied voltage was ranged from 275 to 325 V. The working current and the electrolyte temperature were in-situ monitored during the PEP process. 

After PEP process, the coupons were carefully cleaned and characterized using optical microscopy (OM), step-profilometer, and X-ray fluorescence (XRF). The results show that the small nonmelted and/or semi-melted particles on the surface of the coupons were readily removed by the PEP process, regardless of the initial surface conditions and the PEP processing voltages, resulting in a limited effectiveness of surface smoothening. Further increase in the surface smoothening effect largely depends on oxidation and dissolution of oxide scales under the plasma electrochemical conditions. These observations shed new light on developing of PEP for surface finishing of metallic near-net-shape components manufactured via AM methods.

Results and Discussion

Figure 1(a) presents the increase and stabilization of the applied voltage, i.e., 300 V, as a function of time after starting the PEP process. The current density and the electrolyte temperature evolutions are presented in Fig. 1(b). It is seen that the applied voltage increased and stabilized at about 5.5 s after starting the PEP process. Likewise, the rapid drop in current density transited to a ‘switching mode’ at about 5.5 s. Within this period, plasma and vapor gaseous envelope (VEP) dynamically occurred on the surface of the coupon alongside the increase in the temperature of the electrolyte. This period, i.e., the ‘switching mode’ stage, stopped at about 85 s [see Fig. 1(b)] and the PEP process entered a steady-stable mode along with the temperature stabilization of the electrolyte. An increase in the PEP process voltage slightly increased the stabilized temperature (i.e., by 1~2 °C) of the electrolyte alongside a slight decrease in the current density (i.e., by ~20 mA/cm2). However, they did not change the trends of the temperature and current density profiles. These observations indicate that the surface reactions, i.e., oxidation and dissolution, within the steady-stable stage were influenced by the applied voltage.

Topographies were measured and compared by a step-profilometer from the AM Ti-6Al-4V coupons before and after PEP process. To provide better comparisons, individual coupons were measured before and after PEP process at the same locations. Figures 2(a) and 2(b) present the topographies before and after PEP at 300 V, respectively. Likewise, those measured before and after PEP at 350 V were presented in Figs. 2(c) and 2(d), respectively. They comparisons revealed the presence of small particles and large features on the AM coupons. The small particles are commonly known as nonmelted and/or semi-melted feedstock mechanically adhered to the surface while the large features originated, most likely, from the melt pools that terminated on the sidewall (i.e., the studied surface) of the vertically built up of the coupons. 

It is seen that the small particles were readily removed by the PEP process, regardless of the applied voltages. The removal of the surface particles can be associated with the micro arc discharge-induced localized heating combined with the pressure waves induced by rapid changes in the volume of the bubbles (i.e., cavitation effect) during the PEP process. However, this physical mechanism did not work for the large features since they were formed from melt pools and fused into the coupons surface. A comparison between Figs. 2(b) and 2(d) revealed that the leveling of the large features is quite sensitive to the applied voltages, which is attributable to varied surface oxidations and dissolutions in response to the applied PEP voltages.

Average surface roughness (Ra) was derived from the topographies of the Ti-6Al-4V coupons before and after PEP processes at 275, 300, 325, and 350 V, respectively. The results are presented in Fig. 3. They show that the initial roughness of Ra = 12.2 ± 0.6 μm has been apparently decreased to Ra = 8.3 ± 0.5 μm within the applied voltage ranges of 275-350 V. This surface smoothening effect, in terms of the PEP-induced topographical changes in Fig. 2, can be attributed to the removal of surface particles. Also seen in Fig. 3 that the surface roughness was significantly reduced to Ra = 4.9 μm by the PEP process at 300 V, which is much smoother than those resulted from the PEP processes at 275, 325, and 350 V. Combined with the topographical comparisons in Figs. 2(b) and 2(d), the much more effective surface smoothening of the PEP process at 300 V than those at other voltages can be attributed to the leveling of the large surface features. This observation confirms that the leveling of large surface features is sensitive to the PEP processing voltage through affecting the surface oxidation and dissolution within the plasma electrochemical conditions.

Figure 4 presents the OM images taken from the surface of the AM Ti-6Al-4V coupons before and after PEP processes at 300 and 350 V, respectively. They comparisons provide direct evidence that the small surface particles [see Fig. 4(a)] have been removed after the PEP process, regardless of the applied voltages [see Figs. 4(b) and 4(b)]. The large surface features as indicated by the arrows in Fig. 4(a) have also been leveled in Figs. 4(b) and 4(c). However, remarkable differences have been observed in Figs. 4(b) and 4(c). Melt pool-like features, having the similar dimensions as the large features in Fig. 2, are seen in Fig. 4(b) as indicated by the arrows, confirming their origination from the termination of the melt pools at the surface of the coupons. Instead, scale-like features appeared in Fig. 4(c) as indicated by the arrows. This comparison provides evidence for the presence of surface oxidation and scale dissolution during the PEP process. The oxidation was faster than the scale dissolution at 350 V, rather than 300 V, of the PEP process., shedding light on optimization of PEP process, including the electrolyte compositions and the processing voltage, towards surface finishing of metallic near-net-shape components manufactured by AM methods.

Finally, it is worth mentioning that XRF measurements, results unshown for the sake of brevity, indicate that Al and V contents increased while Ti content decreased in the near-surface regions of the AM Ti-6Al-4V coupons after the PEP process. The increment in Al content was more significant than that in V. These results imply atomic diffusions [4], especially outward diffusions of Al, within the near-surface regions driven by the surface oxidation and dissolution reactions during the PEP process.

Conclusion

Surface finishing by PEP process has been carried out on AM Ti-6Al-4V coupons vertically built up by LPBF. Surface morphology and topography were studied by varying the processing voltage within the range of 275 to 350 V. They revealed that the PEP-induced surface smoothening was contributed by two mechanisms. One is attributed to the removal of nonmelted and/or semi-melted particles upon a combination of micro arc discharge-induced localized heating and the pressure wave induced by suddenly volume changes of the gas bubbles (i.e., the cavitation effect) during the PEP process. The other is associated with the surface oxidation and dissolution within the plasma electrochemical conditions. The former has a wide PEP processing window, i.e., the surface particles can be readily removed by the PEP process regardless of the applied voltages. In comparison, the latter is much more sensitive to the processing parameters via affecting the surface reactions, i.e., the relative reaction rates of surface oxidation and dissolution. Atomic outward diffusions, much more significant for Al than V, have been observed in the near-surface regions of the Ti-6Al-4V coupons, driven by the surface oxidation and dissolution within the plasma electrochemical conditions.

Acknowledgement

This work is partly supported by Singapore Aerospace Programme (SAP Cycle 17, Grant no. M2315a0084, and Project no. SC25/23-834917). Dr Tan Xian Yi and Mr Ng Yee are appreciated for their help in PEP process and data collections.

References

[1] K Nestler, et al., “Plasma electrolytic polishing – an overview of applied technologies and current challenges to extend the polishable material range” Procedia CIRP, Vol 42, pp. 503-507, 2016.

[2] K. Navickaite, et al., “Efficient polishing of additive manufactured titanium alloys” Procedia CIRP, Vol 108, pp. 346-351, 2022.

[3] D. Yang, et al., “The formation and stripping mechanism of oxide film on Ti6Al4V alloy surface during electrolytic plasma polishing” Surf. Coat. Technol., Vol 478, pp. 130469, 2024.

[4] N. Gong, et al., “High-temperature oxidation and hot corrosion of Ni-based single crystal superalloy in the incubation stage” Corros. Sci., Vol 214, pp. 111026, 2023.


(Ph. D. in Physics)

Principal Scientist

 E-mail: liuhf@imre.a-star.edu.sg 

Group Leader of Surface Engineering and Protective Coating

Institute of Materials Research and Engineering (IMRE), A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Singapore 138634, Singapore