Review—Electropolishing of Additive Manufactured Metal Parts

High surface roughness is known to decrease the corrosion resistance of a part. ^56,57 For example, the study by Melchers et al. ⁵⁸ shows that the surface finish of a part strongly influences its corrosion rate during the first few weeks of exposure to the corrosive media. In particular, it was observed that higher surface roughness results in higher early corrosion rates. This effect was attributed to the fact that greater surface roughness provides a greater effective corrosion surface. In the case of AM parts, process-induced defects, such as the presence of partially melted particles on the surface or heterogeneous microstructures, can further increase local corrosion susceptibility. ⁴⁵ However, so far, the effect of surface finish on corrosion resistance of AM parts has been only sporadically studied. The study by Cabrini et al. ⁵⁹ focused on the effect of surface roughness on corrosion resistance of Al-Si10-Mg alloy fabricated by Direct Metal Laser Sintering and indicated that the corrosion resistance of an as-built rough part increases when applying shot peening and polishing successively. In 2017, Leon et al. ⁶⁰ investigated the effect of surface roughness on corrosion fatigue performance of Al-Si10-Mg alloy produced by SLM. Their studies indicated that the parts polished by grit paper, show greater corrosion resistance in comparison with unpolished samples. The negative impact of surface roughness on the corrosion behavior of AM parts was further confirmed by other studies. ^61–63

Fatigue properties are among the paramount aspects of part performance. Various factors such as high surface roughness, residual stress, and surface defects are known to affect the fatigue resistance of AM parts. ^26,64–66

The fact that surface roughness plays an important role in the fatigue behavior of AM parts is undebated in the literature. Typically, the endurance limit of an "as-built" laser additive manufactured Ti-6Al-4V part is around 210 MPa whereas the endurance limit of a polished part can increase to 500 MPa. ⁶⁶ This has been attributed to a smoother surface profile and removal of surface defects that act as crack initiation sites on the as-built part.

Some authors even consider the high surface roughness of the AM parts as the dominant factor causing its inferior fatigue behavior compared to the traditionally manufactured parts. ⁶⁷ This is further supported by the results of Chan et al. ⁶⁸ that reported that re-machined AM Ti-6Al-4V samples give a better fatigue performance than the 'as-built' samples.

However, as demonstrated by Edwards et al., ¹⁶ the fatigue behavior of AM parts does not solely depend on the surface roughness. Indeed, the removal of the rough surface layer by machining exposes the sub-surface porosities present in the structure. These porosities in turn can serve as stress risers and accelerate the premature failure of the part. Thus, it was concluded that although surface roughness is the main determinant in fatigue behavior of the 'as-built' samples, in the case of mechanically polished parts, porosities provide stress concentration sites for fatigue crack initiation. Therefore, it has been suggested that, in the absence of the porosities, the mechanically polished component will outperform the 'as-built' part in terms of fatigue.

Wycisk et al. ⁶⁹ reported similar results for SLM Ti-6Al-4V parts, noting that for the 'as-printed' components, cracks originate solely from the outer surface and progress inside, but for the polished samples, crack initiation occurs from both the surface and the interior pores.

Although the surface roughness (as well as the internal porosities that diminish the efficiency of the polishing process used to reduce crack initiation sites on the surface) plays a key role, some authors ^16,65 argue that the main factor promoting the crack propagation in AM parts is the residual stress imparted to the component due to the fast cooling rates and high temperature gradients.

An important field of applications for AM parts is the manufacturing of customized implants. Titanium alloys, stainless steel, Co-Cr alloys, and shape memory alloys are some of the metallic biomaterials that have been adopted in AM technology. Titanium alloys provide several advantages, including light weight and high strength, as well as excellent biocompatibility and corrosion resistance. As a result, they can be employed in the manufacturing of a variety of metallic implants, such as joints, cranium, and dental implants. ⁷⁰ Due to its availability, low fabrication cost, superior mechanical properties, and acceptable corrosion resistance, 316 l stainless steel is also commonly utilized in biomedical applications. Such properties have made it popular in the fabrication of surgical instruments, temporary dental implants, and temporary load-bearing implants like fracture fixation devices. ^70,71 The Co–Cr alloys are another well-known type of biocompatible materials. They exhibit a much higher wear resistance than titanium alloys and stainless steel, making them attractive candidates for extensively used orthopedic implants such as hip and knee replacements. ^70,71 Shape memory alloys, such as nitinol (nickel-titanium), can also remember and restore their initial shape when subjected to external stimulation. Nitinol can be used in the fabrication of cardiovascular stents, orthodontic wires, and dental braces. ⁷⁰

It is understood that generally speaking, the nature of an implant surface strongly affects its functionality and durability. The stakes are even higher in the case of AM parts designed to be used as human implants as such parts are known to show a high surface roughness and contain microstructural defects, such as porosities, that could adversely impact their mechanical performance. In fact, the surface condition of implants can affect their corrosion, friction coefficient, and wear rate. Surface finish can also accelerate the release of metal ions which may result in long-term health issues. ⁸

For example, the surface condition of SLM Ti-6Al-4V prosthetic implants determines its stable anchorage in the body as well as its fatigue behavior. ⁶ In the case of dental implants, the speed of the healing process and durability of the implant rely on the quality of the osteointegration, which is the process of living bone tissue growth into the implant through direct contact. In vitro and in vivo studies on titanium implants confirm that surface properties such as surface morphology, surface chemistry, surface energy, surface charge, and wettability control the primary body-biomaterial interactions at the body-implant interphase. ^72,73

It is also known that surface roughness has a double-edged effect on the performance of biomaterials. On one hand, it is demonstrated that high surface roughness provides a favorable stress distribution at the bone-implant interface by increasing the implant's contact and mechanical interlocking with the bones. It may also facilitate the formation of interfacial bone. ⁷³

On the other hand, a rough surface may have various adverse effects on the functionality of implants. Body areas with limited access of blood vessels are good environments for bacterial growth. For example, during the fabrication of a SLM Ti-6Al-4V mandible implant, it was observed that the mandibular area has a relatively low density of blood vessels and a rough implant surface promotes the development of a bacteria film. ⁷⁴ High surface roughness tends as well to increase the ion release from the implant due to increased surface area. ⁷⁵

In the case of stents, high surface roughness can also have a negative impact on body-cardiovascular stent interactions. Indeed, the short-term and long-term response of the vascular tissue to the stent as well as the stent's blood compatibility depend on the nature of the thin metal oxide layer formed on its surface during the fabrication and surface treatment processes. For instance, thrombosis occurs when a blood clot developed in a blood vessel obstructs the normal blood flow in the affected area. An important stage in thrombosis is the activation and aggregation of platelets to form a clot (thrombi), which is favored by high surface roughness and non-uniform and contaminated oxide layers. As such, the reduction of surface roughness of 316 l stainless steel cardiovascular stents contributes to decreasing the probability of thrombus formation. ^76,77

Not only can an adequate surface quality promote phenomena such as thrombosis, inflammation, metal toxicity, and carcinogenesis that all diminish the stent performance, but also it determines the fatigue behavior of the stent that is expected to endure the cyclic pulsations of the artery. ^76,78

Finally, let us note that considering the complications related to the removal, repair, and replacement of implants, the application of effective surface treatment is of utmost importance in biomedical applications. ⁸

Modification and optimization of the additive manufacturing process parameters such as scanning speed, laser power, powder layer thickness, or slope angle of the to-be-built surface, are known to affect the part's final surface quality. For example, Mohammadi et al. ⁸⁵ lowered the surface roughness and balling occurrence in SLM Al-Si10-Mg parts through a combination of increased laser power and lower scan speed. However, according to the authors, such process parameters are only applied to the few upskin layers. Therefore, this strategy is not useful to the side and internal surfaces of the AM part. Mumtaz and Hopkinson ⁸⁶ also employed lower scan speeds to reduce the top surface roughness of SLM Inconel 625 parts. However, it was revealed that smooth top and side surfaces can not be achieved together because decreasing the scan speed promotes balling mechanism and increases the side surface roughness.

Unfortunately, as illustrated by the examples of the work done on process parameter optimization, such optimizations in general do not offer sufficient control over the surface roughness of parts. In order to fulfill the morphological and mechanical requirements for AM parts, industries require further treatments that apply to AM parts with complex structures and render customized and controlled surface finish. Such treatments are applied either during or after the part manufacturing.

Laser re-melting is one treatment, among others, added during the build phase, in order to enhance the surface quality of SLM parts and remove their residual porosities. The process is applied in the building chamber either after scanning each layer of the part or only at its top surface where the laser beam scans the previously scanned layer again to completely melt it. As shown by Kruth et al. ³⁰ and Vaithilingam et al. ⁸⁷ using laser re-melting results in a significant decrease of the surface roughness and residual porosities of the SLM Ti-6Al-4V and 316 l stainless steel parts. However, some side effects remain. For example, during the treatment, the moving laser beam can drag the re-molten material to the edges of the part to form ridges upon solidification which can affect the part dimensions and surface flatness. ³⁰ Moreover, balling phenomenon or changes in the chemistry of the oxide layer might happen both of which can affect the wettability, corrosion resistance, biocompatibility, and mechanical performance of the parts. ⁸⁷ Note that considering the need for access of the laser beam to the newly built layer, it appears that the laser re-melting process is better suited to the treatment of top surfaces rather than the side or internal surfaces. ⁸⁸

In view of the limited efficiency of the corrective approaches employed during the fabrication process, the post-processing of AM parts to obtain the required surface finish and mechanical performance seems like an inevitable step.

Heat treatment. As already stated, heat treatment steps are applied after the fabrication of most metal AM parts to enhance their mechanical performance. For instance, Brandl et al. ³⁸ reported that peak hardening can be applied to enhance the microstructure and mechanical properties of SLM Al-Si10-Mg samples. Peak hardening homogenized the microstructure of the parts and increased their fatigue resistance. Song et al. ⁸⁹ improved the mechanical performance of SLM Iron parts by means of a vacuum annealing process. It was observed that the relief of the residual stress by the annealing treatment resulted in grain refinement and increased the yield strength and ultimate tensile strength of the parts. In addition, the hardness of the parts was reduced due to the removal of the defects and the residual stress.

Hot isostatic pressing (HIP) is another heat treatment that densifies the part under high temperature and pressure. It is a promising process for improving the fatigue behavior of AM parts, since it reduces the size of the pores and even closes them, relieves the residual stress, and enhances the microstructure. ^34,65,90 Several studies proved that hot isostatic pressing (HIP) has a positive effect on the fatigue response of AM parts. ^42,91 For instance, Wycisk et al. ⁹² successfully used HIP post-treatment for Ti alloy AM parts. Qiu. et al. ⁹³ proved that HIP treatment closed almost all of the porosities in the structure of Ti alloy laser-melted part and was able to completely transform the as-built martensitic structure of the part to a more ductile microstructure (composed of α and β phases).

Mechanical finishing. AM parts are characterized by their complex geometries and intricate design containing different planes, undercuts, and internal spaces. Thus, the improvement of such complicated structures can be very challenging for surface treatments due to limited access to the part surface. For example, traditional mechanical surface treatments, such as machining, extrude honing, or sandblasting can not uniformly polish different surface areas of such complex 3D parts because they require contact between their tool and the workpiece. ^24,54,94 There are other reasons why most conventional mechanical post-treatments may have a limited scope of use for AM parts. The direct contact between the tool and the part leaves behind a deformed surface layer with polishing marks and contaminates the oxide layer and substrate surface by incorporation of impurities. Mechanical polishing is also known to form residual stresses on the part surface. Therefore, the service life of the part is considerably affected. Some studies even reported that mechanical polishing has hardened the workpiece. ^95–97 In summary, it appears that despite their simplicity and general good result, mechanical polishing treatments do not appear as a suitable approach for additive manufactured parts. ^94,98 It is true that some mechanical polishing techniques like milling and blasting, are commonly used for the post-processing of AM parts, where blasting is more suited than milling for the treatment of parts with intricate design. ⁴¹ However, as for all mechanical polishing processes, both methods require direct contact and hence are not an optimum solution. An interesting alternative is abrasive-flow machining.

Abrasive-Flow machining (AFM) is a non-traditional mechanical polishing technique that uses the sliding action of a semi-solid media carrying abrasive grains to polish and deburr complex geometry parts. The abrasive material flows back and forth across a container that holds the workpiece by means of two extruding cylinders. ^99,100 Despite its ability to polish hard-to-reach areas, AFM has some drawbacks, including a low material removal rate, ¹⁰¹ limited minimum achievable surface roughness, ¹⁰² the appearance of abrasive marks on the surface at high abrasive grain concentrations, ¹⁰³ and difficulty polishing blind holes effectively. ¹⁰² AFM might also damage thin-walled structures due to the high pumping pressures, ¹⁰⁴ or subject interior channels to surface contamination from embedded abrasive particles. ^102,104 The flow properties of the abrasive media are another factor that influences the outcome of the AFM technique. Effective polishing of internal surfaces of complex parts, for instance, is limited to relatively large channels due to the high viscosity of the working media. ^101,105 Other impediments caused by the flow properties include the difficulty to apply uniform forces to complex internal geometries, as well as preferential flow of the abrasive media over more restricted locations, which might result in a non-uniform surface finish. ^102,104

Among the metalical polishing methods, hand finishing of AM parts can be considered. It brings however major issues. On one side, it requires highly skilled operators and is highly time-consuming. On the other side, the complex geometry of the AM part makes it challenging, from a practical point of view, and sometimes even impossible for the tool to reach the surface of the workpiece. ²⁴ Despite these major drawbacks, hand finishing of AM parts is currently often among the only available solutions in the industry.

Chemical polishing. Another method suggested for surface treatment of AM parts is chemical polishing. In this technique, a viscous oxide film is formed on the surface of the workpiece after being submerged in a corrosive solution. The protruding zones on the surface exhibit a higher dissolution rate because they are covered with a thinner oxide layer compared to surface hollows. As a result, the surface is smoothened. Surface roughness is also removed through the preferential dissolution of specific phases present in the alloy as well as local pitting corrosion. ⁹⁷ Clearly, chemical polishing offers better access to the challenging areas of the AM part like internal chambers and different planes. ^95,106 Also, since there is no physical contact involved, chemical polishing does not cause surface tensions. However, chemical polishing has a relatively low polishing rate. ⁹⁷ Moreover, given the fact that most polishing solutions used in chemical polishing are strong acids, like Hydrofluoric acid, special equipment and training are required for safe handling of such highly hazardous materials. ^95,106,107 It is also known that the level of surface brightness required for luxury applications, like in the watchmaking industry, specialized techniques such as tribofinishing are required.

Electropolishing. An interesting approach for post-treatment of additive manufactured parts is electropolishing. It is an electrochemical finishing process for the removal of surface impurities and irregularities from a metallic component. Here, the workpiece is polarized as the anode of an electrolytic cell (a two electrodes cell in general) with a suitable material as the cathode. Electrochemically dissolved cations diffuse through the electrolyte to the cathode, where reduction reactions generally yield hydrogen (Fig. 3). ^20,22,23

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The review articles by Landolt, ²⁰ Yang et al., ²² and Han et al. ²³ provide in depth details on the process, including significant insights into the theories proposed for the mechanism of material removal and surface roughness reduction, critical process parameters, and diverse applications of electropolishing. The following section, disucsses electropolishing in the context of metalic AM parts.

Aside from leveling the surface irregularities of complicated and delicate AM parts, electropolishing can greatly enhance their corrosion resistance by removing the corrosion-promoting defective surface oxides and non-metallic inclusions and replacing them with a dense, stable, and chemically homogenous protective passive layer. ^54,84,108 Also, by eliminating micro-cracks and surface defects that can serve as stress concentration sites, electropolishing can improve the fatigue behavior of AM parts.

The optimization of the numerous process parameters—such as the applied current or voltage, polishing duration, electrolyte flow and temperature, and inter-electrode space is generally challenging as they all have a significant effect on the surface quality of the AM parts after electropolishing. A good understanding of their role is required in order to select them correctly.

Electropolishing current. Overall, it is understood that the application of higher electropolishing currents can accelerate the metal dissolution at the expense of control over part geometry and probable pitting. Lower currents, on the other hand, deliver significantly lower polishing rates. Therefore, it was suggested that 316 l stainless steel miniature AM parts (which are more prone to change in their intricate details) could be successfully treated with a two-step low-current/high-current electropolishing procedure. ¹¹⁷ Here, a lower current electropolishing applied for a longer time is followed by a higher current electropolishing for a shorter time.

Electropolishing potential. Sufficient electric potential has to be applied before the polishing electrode reactions can occur at the required rate. For a given cell terminal-voltage, as the inter-electrode gap increases, so does the resistance between the electrodes, and hence the electrical current decreases. ²² The presence of a passive film on the anode surface can also increase the ohmic drop and decrease the current efficiency of an electropolishing system. ¹¹⁸

High voltages are well known for accelerating metal dissolution. However, while low voltages might not activate the polishing electrode reactions, applications of very high potentials are generally avoided. The reason is that very high voltages can potentially result in poor surface quality due to non-uniform metal dissolution and pitting attacks. ^119,120 Overpolishing due to high voltages can also diminish the part's dimensional accuracy. ¹¹⁹

It has been suggested that the high surface roughness of the as-printed 316 l stainless steel and Maraging steel AM parts can be better handled with a process termed 'overpotential electropolishing' which is performed by the application of a voltage slightly higher than voltages of the current plateau region. ^121,122 This process can selectively eliminate the partially melted particles from the surface with minor thickness reduction. An efficient surface treatment of the SLM 316 l stainless steel parts was observed in the work of Chang et. al ¹²¹ where an overpotential electropolishing process (20 min) was followed by a conventional electropolishing step (20 min) for further smoothening of the parts.

Electropolishing duration. It is known that the surface roughness of the workpiece exhibits a rapid drop during the early stages of the electropolishing. However, the decrease rate of the surface roughness is reduced with further prolonging of the electropolishing process until it reaches a limiting value. Figure 5, indicates the variation in the surface roughness in case of 316 l stainless steel samples with electropolishing duration as reported by Haïdopoulos et al. ¹⁰⁷ Although extending the polishing duration generally tends to improve the surface finish, it is known that not only prolonged conventional electropolishing cannot make a considerable improvement in the surface quality of complex AM structures, but also it can cause excessive dissolution and breaking of the lattice struts.

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Electropolishing current signals. Electropolishing does not have to be used exclusively in direct current. PC (pulsed current), and PPR (pulse/pulse reverse) waveforms are known to yield a higher rate of material removal which can be attributed to a larger average current density. It has been observed on SLM 316 l stainless steel samples ¹²³ that DC electropolishing yields a shiny surface with minor roughness reduction whereas PPR electropolishing is more effective in smoothening rather than brightening the surface. Pulsed current electropolishing with low duty cycles has also been reported to efficiently decrease the surface roughness of SLS titanium parts. ¹²⁴ However, compared to the applied potential, the solution temperature has been reported to have a more pronounced effect on the polishing rate of EBM Ti-6Al-4V parts. ¹²⁵

Electropolishing bath temperature. One of the highly influencing parameters of the electropolishing process is the temperature of the electrolyte bath. The current density increases at elevated bath temperatures due to accelerated electrochemical reaction rates, the higher solubility of the metal ions in the solution, accelerated ions diffusion and lower electrolyte viscosity. ²³ Electropolishing of AM components has also been reported to produce better results at higher temperatures. Improved polishing results at higher temperatures are also reported for electropolishing of 316 l stainless steel and Ti-6Al-4V AM parts. ^115,116,125 The Taguchi analysis performed by Brent et al. indicated a 40% contribution of the electrolyte temperature in improving the surface roughness of 316 l steel AM samples. ⁹⁸ However, it has been suggested that too high temperatures must be avoided because of excessive reduction of the viscosity of the diffusion layer and consequently, difficulty in keeping a viscous layer on the workpiece surface which may lead to formation of etching pits. ^22,23 Similar trends were observed in the electropolishing of titanium and Inconel 625 AM parts where electropolishing at elevated temperatures resulted in non-uniform metal dissolution and loss of geometry of the workpiece. ^118,124,125

Electropolishing bath agitation. Another important aspect to be considered in the design of the electropolishing setup is electrolyte agitation because a uniform electrolyte flow can yield a uniform surface finish. It has been argued that a turbulent flow of the electrolyte can impart normal and shear pressures to the different areas of the samples and lead to a nonuniform smoothening effect. ⁸⁰ The study of Yang et al. ¹²⁶ indicated that a split configuration can be employed to regulate the electrolyte flow. However, it appears that in order to benefit from the uniform electrolyte flow, higher voltages should be applied to accelerate metal dissolution and promote the onset of the mass-transport limited mechanism. Moreover, stirring of the electrolyte can render a better control of the temperature and avoid local overheating that would affect the surface film. ¹²⁷ Agitation is also suggested for breaking down the oxide layer on the surface of the titanium parts in the absence of strong acids such as hydrofluoric acid and perchloric acid. ¹²⁵

Inter-electrode distance. The inter-electrode distance is a further important electropolishing parameter that determines the final surface roughness of the parts ¹²⁷ and explains the difficulty of uniform electropolishing of parts with complex geometry. As of today, very few studies have been conducted on the qualitative effect of variation of the inter-electrode gap in the context of AM. This aspect will be further discussed in the section dedicated to challenges of electropolishing of AM parts- achieving a uniform electropolishing effect over complex geometries.

Workpiece material. Electropolishing is well established as a post-processing method for a large variety of metals, ferrous and non-ferrous alike. Surface finishing of stainless steels (mainly 200 and 300 series), titanium alloys (nearly all biomedical grade NiTi parts), and SRF (Superconducting Radio Frequency) niobium cavities are among the most common commercial applications for electropolishing. Furthermore, Inconel, high-quality aluminum alloys, homogenous brasses, and copper alloys electropolish efficiently.

As pointed out in the papers by Han et al. ²³ and Yang et al., ¹²⁸ most common metals and alloys can be electropolished, but the microstructure of the workpiece, and hence the alloying composition and fabrication procedure, has a tremendous impact on the electropolishing outcome. Cast metals, for example, are often porous and hard to electropolish, according to industrial experience. Furthermore, alloys containing carbon, silicon, sulfur, phosphorus, tin, zinc, or lead have been found to be difficult to electropolish. In such applications, the electropolishing treatment, if used, is limited to cleaning and preparing the workpiece surface to meet certain technical requirements.

Considering the high number of parameters that can be fine-tuned and the complexity on how they affect the process results, Taguchi ^54,98 or full factorial designs ¹¹⁹ of experiments are interesting tools to find the optimum combination of electropolishing factors to obtain the targeted result. This was shown in the work of Brent et al., ⁹⁸ where the application of the optimal level of time, temperature, agitation, and electrolyte composition decreased the surface roughness of 3D printed 316 l stainless steel parts by a factor of more than half. Similarly, in the study of Tyagi et al. ⁵⁴ optimum EP conditions obtained from a Taguchi design of experiments yielded an almost featureless AM steel surface with mirror-like luster.

As mentioned previously, usually electropolishing can not completely eliminate the high amount of partially melted particles fused to the surface of the printed part. However, if combined with another technique, such as a primary mechanical polishing method or a heat treatment step, electropolishing could deliver more favorable results. In the following such hybrid processes are described.

Electropolishing and mechanical polishing. Löber et al. ¹²⁹ and García-Blanco ¹³⁰ demonstrated that an extra 2% and 40% reduction of surface roughness could be obtained upon the application of a mechanical polishing step prior to electropolishing of SLM 316 l stainless steel and Ti-6Al-4V parts respectively.

AM parts can as well highly benefit from the high roughness surface reduction of the blasting process and the capacity of electropolishing in delivering bright surfaces free from blasting media impurities. This idea was adopted in the work of García-Blanco et al., ¹³¹ where a homogenous, bright, and clean finish for SLM Ti-6Al-4V parts was observed with a 92% decrease in surface roughness upon the application of this process. Furthermore, it has been reported that a combination of the electropolishing and grinding processes can decrease the surface roughness of the as-built EBM Ti-6Al-4V parts from about 20–30 μm to a value of below 1 μm. It is also noteworthy that the electropolishing step following the grinding process removes the deformed layer and tool marks left on the surface. ¹³² Lately, a new surface treatment combining sandblasting, abrasive polishing, and electropolishing has been devised for the post-treatment of SLM 316 l stainless steel parts. ⁸ Similar to previous cases, the mechanical polishing steps were employed for a significant reduction in the surface roughness while the subsequent electropolishing step was meant for removal of the remaining foreign particles from the surface.

Electropolishing and heat treatment. Some studies investigated how heat treatment and electropolishing processes can be beneficially combined. For instance, Demir et al. ⁷⁸ suggested that in addition to electropolishing of AM stents, stress-relieving heat treatment can be applied for lowering their hardness and enhancing their mechanical properties. The study justified that this would not necessarily induce longer manufacturing cycles for additive manufacturing since surface finishing operations and heat treatments are already part of the conventional manufacturing process of stents during laser micro-cutting of micro-tubes. Moreover, Zhao et al. ¹³³ stated that a pre-treatment composed of annealing and acid pickling is necessary for obtaining satisfactory surface quality by electropolishing. The authors argued that slags formed on the surface of the AM stents should be removed with acid pickling and the stents should then be softened by heat treatment before implantation. Wiesent et al. ¹³⁴ also utilized a combination of heat treatment and electropolishing for improving the surface roughness and mechanical performance of L-PBF 316 l stainless steel cardiovascular stents. The goal of the heat treatment was to lower the yield strength and enhance the ductility of the stents to facilitate stent expansion. According to the study, heat treatment reduced the compressive strength of the AM parts by relieving their residual stress as well as decreasing their dislocation density and their level of δ-ferrite phase which is known to be a strength-enhancing second phase. Also, not only the following electropolishing further enhanced the surface roughness of the stents it was also responsible for smaller strut diameter which led to decreased stent stiffness.

A recent study used a combination of electropolishing and hot isostatic pressure (HIP), among other post-sintering techniques, to improve the fatigue behavior of SLM Ti-6Al-4V parts. The results indicated that sole electropolishing enhances the fatigue behavior of the part only slightly because removing the surface layers exposes sub-surface porosity defects that can be the driver for crack initiation. HIP, however, could markedly enhance the fatigue properties by reduction of the porosities and probability of critical defects. ⁶ Thus, it can be concluded that a combination of HIP and EP processes for the treatment of AM parts provides an acceptable outcome in terms of surface quality and mechanical performance. The HIP process can remove or reduce the sub-surface porosities and modify the part's as-built structure to a microstructure more resistant to crack growth. Electropolishing on the other hand can effectively remove the surface roughness of the part. ¹³⁵

Electropolishing and chemical polishing. Combining electropolishing with chemical polishing process is another interesting option. As a matter of fact, the idea of combining chemical and electrochemical polishing methods for surface treatment of conventional parts has already been conducted by some studies. ^76,77

It has been shown that, since chemical etching can effectively remove the slag and oxide scales formed on the surface of the parts during previous manufacturing steps, the subsequent electropolishing step results in better surface quality. A similar principle is suggested to be applicable to AM parts.

For instance, having observed that the chemically polished parts exhibit a uniform surface roughness and minimal localized dissolution, Dong et al. ¹³⁶ proposed that chemical polishing can improve the result of electropolishing and increase the depth of polishing inside a Ti-6Al-4V AM part. The study of Wen et al. ¹³⁷ also illustrates the benefits to be gained from combining the two technologies. The authors compared the micrographs of an SLM Zn lattice part surface in the as-built and chemically polished conditions (Fig. 6). Evidently, sole chemical polishing can not uniformly remove the large number of particles of different sizes and shapes that are randomly attached to the surface of the struts. Moreover, prolonging the chemical etching procedure would lead to excessive local corrosion and drastic change in the dimension of the struts and structural integrity of the part. However, a combination of chemical polishing with electropolishing rendered smooth parts with a more homogenous topography.

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A further advantage of combining the two polishing technologies is addressing the issue of the limited effect of electropolishing on internal areas of the part, and therefore, this two-step process enables finishing the entire surface of the AM parts. ⁵⁵

It is worth noting that, the treatment combined from chemical and electrochemical polishing processes has also been shown—by micro-CT images—to significantly influence the in-vitro cell behavior of SLM Ti-6Al-4V scaffolds. ¹³⁸

A rather serious drawback of such a combination of surface treatments might be the excessive decrease in the thickness of the parts which in the case of lattice structure can result in increased pore size and consequently, significant damage to the part's strength and stiffness as demonstrated by Wennerberg et al. ⁶⁶ (Fig. 7).

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This issue can be addressed using empirical correlations to predict the final strut thickness as a function of chemical polishing and electrochemical polishing time and compensating for the dimension variations in the design stage. ^79,137 Similar approaches were used by Van Bael et al. ¹³⁹ for the prediction of the as-produced SLM Ti-6Al-4V part pore size and porosity as a function of the CAD design pore size.

It has also been understood that the order in which the electropolishing and chemical polishing processes are applied can affect the final surface quality of the AM parts. In fact, considering their different mechanisms, a significant difference can be expected between the outcomes of CH-EP (chemical polishing followed by electropolishing) and EP-CH (electropolishing followed by chemical polishing). This was shown by Dillard et al. ⁵⁵ where the AM 316 l stainless steel part treated with the EP-CH process exhibited a smooth surface with a topography similar to the one obtained after chemical polishing, whereas the CH-EP surface, while being quite smooth bore some crack-like features. The CH-EP process also delivered the lowest internal surface roughness value.

Electropolishing is known to enhance the corrosion resistance of AM parts by replacing their initial loose and thin surface film with a less defective, more homogenous and, corrosion-resistant oxide layer. This was shown by bio-corrosion analysis of AM Ti–6Al–4 V parts where their corrosion potential decreased during the early stages of electropolishing, due to removal of the initial oxide layer but was later increased due to a formation of a more compact and thicker surface film. In addition, the corrosion current density of the samples was also found to be decreased upon decreasing the surface roughness. ⁴¹

Figure 9 illustrates the mechanism of replacement of the naturally formed oxide film on AM Ti–6Al–4 V parts surface with a stable and tightly adhered TiO₂ layer. ¹⁴² First, the original oxide layer is removed, then Ti⁴⁺ ions of the surface react with Cl⁻ ions in the bath, and the viscous TiCl₄ layer is formed which later reacts with water to produce the TiO₂ layer. The formation of such a protective surface layer leads to a significant enhancement in the corrosion behavior.

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Similar results have been reported in the case of AM 316 l stainless steel ²⁴ and 17–4 PH steel ¹⁴³ parts where a Chromium-rich surface layer is formed on the surface upon electropolishing that protects the part from chemical corrosion. Figure 10 exhibits the dramatic improvement of the corrosion resistance of 316 l steel parts fabricated by SLS processes after electropolishing. ²⁴

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It has also been established that the composition and thickness of the new oxide layer depend on the duration of electropolishing and determine the corrosion resistance of the AM part. ¹⁴⁴ For example, in a study by Zhang et al. ¹⁴⁴ on SLM Ti–6Al–4 V parts, the extension of the electropolishing treatment beyond the optimum duration, resulted in a more irregular or porous oxide layer with small defects due to excessive dissolution. This was evidenced by a higher passive current density.

The local corrosion behavior of AM parts is also influenced by the electropolishing treatment. In the case of passive metals, process-induced features on the as-printed AM part surfaces can act as potential corrosion initiation sites and accelerate localized corrosion. In addition, the Galvele criterion, also known as the stability product, is a critical value that can anticipate the likelihood of stabilized pit growth and passive film breakdown. The stability product is equal to the product of the pit depth and the anodic current density at its bottom. According to this criterion, the presence of sharp features on rough surfaces increases the diffusion length for the ions, demanding a lower current density for the pits to shift from metastable to stable. ¹⁴⁵ Therefore, rougher surfaces exhibit a lower breakdown potential and are more susceptible to pitting corrosion. ⁴⁵ Consequently, by smoothening the AM part's surface and forming a more stable and protective passive layer, electropolishing increases its pitting potential and the length of its passivation plateau. ^142,144,146

The ability of electropolishing to produce a smooth surface finish and remove the stress and crack concentrators on AM parts can be exploited to enhance their mechanical properties. It has been confirmed by several studies that the tension-tension fatigue performance is among such mechanical properties that can be enhanced. For example, electropolishing was reported to improve the tension-tension fatigue behavior of EBM Ti-6Al-4V samples by approximately 300% which further attests to its potential for enhancing the performance of AM parts under cyclic loading conditions. ¹²⁰ Another example is a study conducted by Bagehorn et al. ¹⁴⁷ where the removal of the surface particles and roughness valleys had a considerable effect on the fatigue resistance of the Ti-6Al-4V parts manufactured by laser beam melting (174% improvement).

The secret behind the improved fatigue behavior of AM parts is believed to be the removal of the irregular microstructure, surface roughness, and the pits present on the as-built AM part, which promote stress concentration and crack initiation under cyclic loadings. ^41,84 In fact, after electropolishing, cracks leading to failure nucleate in the bulk of the material rather than the surface defects. ^91,147 This statement is supported by the experiments conducted by Reichelt et al. ⁹¹ on AM Ti-6Al-4V aircraft spoilers, where, prior to electropolishing, Hot Isostatic Pressing (HIP) was applied to the parts to alleviate the presence of internal defects. As a result, after electropolishing, a 93% percent reduction in the surface roughness of the specimens was achieved resulting in an increase of their fatigue limit from 300 MPa to 500 MPa after 3×10⁷ cycles. Crack initiation was found to be originated from the internal defects that were not eliminated by the HIP process. Moreover, it has been understood that the surface of AM parts undergoes compressive residual stress due to the AM process and plastic deformation during the surface treatments. Guo et al. ¹³² suggest that although compressive residual stress is known to have a positive effect on fatigue performance, electropolishing can be used to relieve such residual stress in case stress-free parts are required.

The compression and extension behavior of AM parts can also be enhanced by electropolishing. Wiesent et al. ¹³⁴ demonstrated that the compression and extension resistance of L-PBF 316 l stainless steel cardiovascular stents could be boosted by a heat treatment followed by an electropolishing procedure. Moreover, the results of the study by Chang et al. ¹²¹ on the effect of electropolishing on compression behavior of an AM 316 l stainless steel lattice, indicate that the specific energy absorption and compressive plateau stress of treated samples are almost two times higher than that of as-printed ones. Also, in the case of L-PBF 316 l stainless steel cardiovascular stents, electropolishing has been reported to increase the compression strength of polished parts by removing the surface roughness and sharp features and reducing their stiffness. As a result, the electropolished stents bore a lower risk of damaging the balloon during the crimping process which occurs by application of a radially compressive force to reduce the stent diameter and prevent its dislodgement from the balloon. ¹³⁴

It appears however that the inherent elastic deformation behavior of AM parts is only slightly altered by electropolishing. For example, in the study of Wu et al. ⁴¹ only a negligible change was observed in the average Young's modulus and the average yield stress of AM Ti-6Al-4V parts. This can be understood keeping in mind that a part's intrinsic elastic deformation behavior is independent of its surface roughness. However, Fig. 11 shows that the average ultimate tensile stress and the tensile elongation of the samples increased for lower levels of surface roughness acquired by higher polishing current densities. High surface roughness can promote stress concentration and crack propagation on the as-built surface of an AM part, which explains the reported improvement in plasticity and fracture behaviour for electropolished parts. As a result, during tensile testing on the as-built parts, the necking occurs earlier compared to smooth electropolished samples and the tensile elongation decreases.

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Other mechanical properties of AM parts can also be influenced by electropolishing. Namely, the surface hardness of SLM Inconel 718 parts were found to decrease upon electropolishing. Baicheng et al. ¹⁴⁸ showed that after electropolishing of SLM parts, the hardening phase γ, which obstructs the movement of dislocations under compression or tension, was dissolved and the residual stress in the part was relieved.

Notwithstanding its effective role in improving the mechanical performance of AM parts, excessive electropolishing can reverse its positive effect on the fatigue behavior of AM parts. As noted by Benedetti et al. ⁶ and Wiesent et al. ¹³⁴ on Ti-6Al-4V and 316 l stainless steel AM parts, by removing surface layers in order to obtain a significant decrease in the surface roughness, electropolishing exposes the sub-surface pores which can act as crack initiation sites (Fig. 12). Also, Tyagi et al. ⁸⁴ surmised that electropolishing could undermine the strength of the AM steel components through preferential etching of iron carbide compound that was formed along the grain boundaries in the laser sintering process. Furthermore, it has been noted that the residual compressive stress resulting from the AM process may be eliminated from the surface of EBM Ti-6Al-4V parts upon electropolishing, which can have a negative impact on the fatigue behavior of the parts. ¹³² Finally, in the case of very small and delicate parts, excessive electropolishing, like any other polishing method, will result in part failure. ¹³⁴

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Nickel alloys like Inconel 718 and Inconel 625, titanium alloys especially Ti-6Al-4V, aluminum alloys, and stainless steels are some of the alloys that are frequently used for fabrication of metallic AM parts. This section will highlight the most typical material-related challenges that arise during electropolishing of such alloys.

Titanium alloys have become an indispensable material in the aerospace, biomedical, and automobile industries due to their low density and remarkable strength, corrosion resistance, and biocompatibility. ¹³² Surface treatment of titanium alloys, however, has proven to be challenging due to their high hardness and chemical stability. It is known that instead of the expensive and time-consuming mechanical polishing processes, that might further introduce impurities to the surface, AM titanium parts can greatly benefit from the electropolishing process to obtain a bright surface finish free from inclusions (especially if electropolishing is preceded by a mechanical polishing step to remove the partially molten particles ¹³⁰ ).

However, the electropolishing of this alloy, is not without its challenges. First off, the spontaneous, thick, and compact passive oxide film to which titanium owes its chemical stability and excellent corrosion resistance, hinders electropolishing by decreasing the rate of material dissolution. ^155–158 Moreover, this naturally formed passive layer may contain inclusions and other inhomogeneities that can promote corrosion initiation and result in surface pitting rather than polishing. ¹⁵⁸ Therefore, in order to handle such oxide layers, titanium alloys are conventionally electropolished using strong acids such as HF and HClO₄ at low temperatures and high voltages. ^159–161

The situation becomes even more complicated in the case of titanium AM parts. On one hand, it has been reported that EP solutions with low chloride concentrations can not completely remove the particles on the AM part nor can they significantly reduce the surface roughness because of the small current density which results in non-uniform distribution of the thin TiCl₄ layer formed on the surface. ¹⁴² On the other hand, aside from the safety issues involved with using strong acids, it has been observed that high chloride concentrations accelerate the metal dissolution and lead to the formation of a thick TiCl₄ layer which is difficult to remove during electropolishing and causes pit formation making the surface rough and dark. ¹⁴² Moreover, although high voltages are employed in electropolishing of titanium parts to expedite the depassivation process, this may increase the temperature of the solution and worsen the quality of the polished surface. ¹⁵⁸ A certain range of solution agitation speed is also usually required to remove the passive viscous TiCl₄ film without risking uneven or excessive polishing or localized pitting.

The Ni-based superalloys offer superior mechanical properties, such as high tensile strength and wear resistance, as well as excellent corrosion resistance at high temperatures. To that end, alloys like Inconel 718 and Inconel 625 are widely used by the aerospace and oil and gas sectors in high-temperature applications such as gas turbine blades and combustion chambers. ^106,148 The oxidation behavior of Ni-based alloys is known to resemble that of titanium alloys. ¹²⁶ Therefore, one can expect similar challenges with electropolishing these alloys including the need for the application of high potentials/currents or the use of strong acids for the breakdown of the highly passive film formed on the surface. ^148,162

Also adopted by the metal AM technology, aluminum alloys are widely used for aerospace and automotive applications due to appealing properties such as light weight, high electrical and thermal conductivity, and strong corrosion resistance. Similar to other readily passivated alloys, effective electropolishing of aluminum alloys require highly oxidizing electrolytes which are often mixtures of toxic chemicals such as perchloric acid and chromic acid. ^163–165 To make the matters worse, the electropolishing of aluminum alloys in these electrolytes often requires high temperatures (50°C–90°C) which can damage the industrial equipment in addition to raising serious health and safety concerns. ¹⁶⁴

Given these limitations, it appears that the conventional electropolishing process needs to be modified for an effective surface improvement of the AM parts made of highly passive metals.

Another group of alloys employed by the AM processes are the stainless steels. AISI 316 l stainless steel is also commonly utilised in a variety of industrial applications, particularly in the biomedical and food processing sector. This alloy has excellent corrosion properties in a variety of environments, as well as high mechanical strength, machinability, and biocompatibility. ^46,116,146 To the best of our knowledge, no material related limitations has been reported for electropolishing of 316 l stainless steel.

Obtaining a uniform electropolishing effect over the entire surface of the AM parts is among the most highlighted challenges. For example, the study by Tyagi et al. ⁵⁴ indicated that although electropolishing rendered a reflective and mostly featureless exterior surface with sub-μm level roughness for cylindrical AM parts, the internal surface of the samples could not be treated.

Several studies attribute this issue to the non-uniform primary current distribution over the complex geometry and internal cavities of the AM part. ^20,166–168 Indeed, when charge transfer and mass transport overvoltage in an electrochemical cell are negligible, the primary current distribution prevails. Under these conditions, the local current density on the AM part depends on the cell geometry, shape, and position of the part. Primary current distribution is known to have a significant impact on the electropolishing of the part since it determines the difference between the dissolution rate of the peaks and valleys of the surface profile.

The limited access of counter electrodes to the workpiece surface is another reason suggested for the difficulty of electropolishing internal spaces. Considering that AM parts generally hold many intricate details and unreachable narrow spaces, this factor could further complicate the effective treatment of such parts and lead to a non-uniform polishing effect. ¹⁶⁹ Moreover, some studies have concluded that the excessive build-up of heat and polishing products are responsible for the inferior surface quality of the internal spaces after electropolishing. ^131,150,169

It has also been argued that the difficulty of achieving a uniform surface finish for AM parts by electropolishing stems from their heterogeneous microstructure. ¹⁷⁰ On one hand, during the AM fabrication process, each part of the microstructure may experience a different history of melting, solidification, and subsequent heating and cooling cycles. On the other hand, electropolishing of multi-phase materials is known to be challenging due to the different dissolution rate of each phase. ^171,172

All of these issues can render electropolishing for surface finishing of AM parts even more difficult to implement across the industry. It is well understood that there is no universal parameter set for polishing different parts and identifying the appropriate polishing parameters has already proven to be challenging. Furthermore, when AM technology is employed in the context of individualized parts, the geometry, thermal history, and microstructure are constantly changing, which makes the adjustment of the polishing settings for each part, all the more cost and time-intensive.

A major challenge in electropolishing of AM parts is the geometric and dimensional changes due to excessive material dissolution which can be particularly troublesome as AM parts are intended to have superior mechanical properties or bear delicate features. For example, Min et al. ¹⁵¹ observed that upon electropolishing of AM In718 microchannels a 47% decrease in the surface roughness resulted in an 8% increase in the size of the channels. However, a larger pitch (the distance between adjacent microchannels) was found to reduce the electrolyte penetration through the channel walls.

In summary, as was recommended by Wiesent et al. ¹³⁴ the choice between near-net-shape part design followed by limited electropolishing and designing parts with larger dimensions followed by excessive electropolishing should be made carefully.

AM parts are generally characterized by having a high surface roughness. The review study by Liu et al. stated that Ti-6AL-4V components fabricated through commonly used metal AM processes such as SLM and EBM exhibit a surface roughness in the ranges of 5–40 and 25–131 μm, respectively. ³⁶ If such components are to be utilized in biomedical applications, where the allowable Ra value for a medical implant is often less than 2 μm, ¹⁷³ then they require effective surface treatment.

Despite the fact that the current literature reports considerable improvements in the surface roughness of AM parts, ¹⁴⁷ electropolishing is known in the industry to deliver a 50% average surface roughness reduction. ^174–176 This implies that successful reduction of the high surface roughness of AM parts is possible but challenging for standard industrial setups. This is due to the fact that, for the electropolishing process to yield high surface quality AM parts, as discussed earlier, optimal polishing parameters must be used, and the time-consuming trial and error and adjustment process for determining the proper polishing parameters is costly. Since electropolishing of AM parts is not yet widely established in the industry, there is limited knowledge available concerning the proper operating conditions.

Possible solutions such as the application of higher voltages or extending the electropolishing time to increase the material removal are not always practical due to overheating of the system and rapid degradation of the electrolyte by the build-up of the polishing byproducts both of which can decrease the polished surface quality. Additionally, prolonged treatment times impose cost and sustainability issues such as higher energy consumption, longer use of the equipment, and disposal of highly contaminated waste to the environment. ¹⁶⁹ A better solution is the usage of low conductivity electrolytes to increase the voltage gradient between the peaks and valleys of a rough surface with large asperities and accelerate their selective dissolution. Likewise, highly viscous solutions can be used to focus the electric field on the small asperities and enhance their preferential removal during diffusion-controlled dissolution. However, in both cases, as the electrolyte resistance is greatly increased, the electrochemical cell tends to increase its temperature rapidly, impeding the electropolishing process. ¹⁷⁷

A more advanced solution is the use of pulsed or pulse reverse current electropolishing to deal with the issue of excessive heat and metal dissolution products. ^171,177 According to these studies, incorporation of off-current and/or cathodic current intervals in the process can help to increase the instantaneous anodic current density while maintaining the average current density lower compared to conventional electropolishing. Therefore, the process offers a high rate of material removal. Also, by providing enough time for cooling and refreshing the electrolyte and stopping the growth and build-up of gas bubbles on the part surface, this method can yield more uniform polishing results. However, compared with the conventional electropolishing process, this approach requires significantly longer polishing times and is not always practical from an industrial viewpoint due to the relatively short pulse times required.