Keywords
corrosion - electropolishing - laser powder bed fusion - surface roughness - Ti6Al4V
Introduction
Traditional manufacturing methods like casting, forging, and machining have been the
general methods for shaping metals into finished products. However, due to the limitations
of these conventional methods such as material waste, design limit, and production
time, additive manufacturing (AM) has drawn increased attention. The AM, also known
as direct digital manufacturing, autonomous manufacturing, or three-dimensional (3D)
printing, has been manufacturing many kinds of metals, including stainless steel,
cobalt-chromium (CoCr), or titanium (Ti), for biomedical, aerospace, defense, and
automotive industries. AM has rapidly risen as a popular technology because it enables
the creation of complex geometric parts from digital models by adding material layer
by layer. By providing on-demand production and reducing waste, expense, energy consumption,
and carbon dioxide emissions, AM technology has the potential to completely change
logistics and manufacturing operations.[1] One of the most widely utilized AM methods in dentistry is laser powder bed fusion
(L-PBF).
In dentistry, AM enables digital workflows that combine intraoral scanning and advanced
manufacturing processes to create metal prostheses for either fixed prostheses or
removable dentures and also to facilitate the fabrication of implant-supported prostheses
by integrating digital and conventional methods.[2] CoCr and Ti/ Ti alloys are two most frequently utilized metal alloys in AM technology
in dentistry.[2] Ti6Al4V ELI alloy, also known as Ti grade 23, is a form of Ti6Al4V alloy (Ti grade
5), which is appropriate for dental implants and prostheses due to its strong corrosion
resistance and constant biocompatibility, particularly in oral conditions. Furthermore,
its mechanical properties make it ideal for applications requiring a strong and lightweight
material.[3]
[4]
[5] L-PBF is commonly used for printing metal prostheses in dentistry. L-PBF operates
by employing a laser beam to fuse metal powder incrementally layer by layer until
the intended form is achieved.[6] Therefore, the surface finish of the printed parts is one of the primary concerns
of this method. However, the as-printed surface roughness is a significant barrier
to L-PBF applications. This is due to the surface roughness produced during L-PBF
process, which can be rougher than that in conventional manufacturing procedures depending
on the manufacturing parameters.[7]
[8]
To resolve this drawback of surface roughness caused by the printing process, several
methods have been introduced to improve the surface quality of 3D-printed parts, such
as mechanical, chemical, or electrochemical surface treatments, laser-based surface
treatments, and coatings.[9] These methods can compensate for technological deficiencies and mishaps, such as
the staircase effect related to layer-by-layer deposition, partially melted powders,
spatters, the balling effect, imprecise support removal, uncontrolled wetting, and
melt pool instability.[7]
[8] These are also effective methods to modify the surfaces of L-PBF components.[9]
[10]
[11] Electropolishing (EP) is a method that removes the surface layer of the metal, and
EP can improve the surface smoothness and generate a stable oxide film that enhances
the corrosion resistance of Ti.[12]
[13]
[14] The major advantages of EP include eliminating surface imperfections and working
with complex surface topography.[4]
[12]
[13] Zhang[14] investigated the electropolished Ti6Al4V alloy and found that the mean roughness
(Ra) decreased, resulting in a smoother surface and improved corrosion resistance.
Compared with other polishing methods, the EP is a promising polishing method for
improving the surface quality of L-PBF components because of its simplicity and ability
to polish complex structures effectively.[12]
However, the EP also has limitations, mainly due to the complexity of the process,
which is influenced by various parameters that can affect biophysical outcomes, such
as the composition of the electrolyte, the distance between the anode and cathode,
the shapes and areas of the anode and cathode, the agitation speed of the solution,
temperature, duration, and the voltage or electric current used.[15]
[16] Therefore, it is a challenge to determine the optimal parameters suitable for each
metal material. Although a previous study on the EP of L-PBF Ti6Al4V has focused on
varying polishing durations and electrolyte solutions, a relationship between surface
roughness after the EP and corrosion resistance remains unclear.[14] Therefore, this study aimed to investigate the effects of current density, voltage,
and distances between anode and cathode of the EP method on the surface roughness
and corrosion resistance of electropolished L-PBF Ti6Al4V ELI alloy.
Materials and Methods
Sample Preparation
Ti6Al4V ELI alloy plates sized 8 × 20 × 2 mm were fabricated using an L-PBF printing
machine (TruPrint 1000, TRUMPF, Germany) with the printing parameters listed in [Table 1]. Ti6Al4V ELI powder with a particle size distribution of 15 to 45 μm (AP&C GE Additive,
Canada) was used as the base material.
Table 1
Process parameters of L-PBF manufacturing used to fabricate the samples
|
Process parameters
|
Value
|
|
Laser power (W)
|
125
|
|
Scanning speed (mm/s)
|
1200
|
|
Layer thickness (μm)
|
20
|
|
Hatch spacing (μm)
|
110
|
|
Laser spot size (μm)
|
30
|
|
Scanning pattern
|
Chess
|
Abbreviation: L-PBF, laser powder bed fusion.
Procedure of EP
Before conducting the EP, all Ti6Al4V ELI alloy plates were ultrasonically washed
with acetone for 15 minutes in order to get rid of the remaining metal powder from
their surfaces but no chemical or mechanical surface polishing was applied to the
samples. The Ti6Al4V ELI alloy plates were electropolished in an electrolyte solution
of perchloric acid (HClO4), glacial acetic acid (CH3COOH), and distilled water (H2O) in a volume ratio of 1:10:1.2 for 15 minutes at room
temperature.[14] The cathode was stainless steel and the anode was a Ti6Al4V ELI printed plate ([Fig. 1]). Based on the parameters of EP, the Ti6Al4V ELI alloy samples were divided into
five groups (with group labels), as indicated in [Table 2]. Note that 0.3/2 and 0.3/4 EP groups were performed at a current density of 0.3
A/cm2 with 2 and 4 cm distances between anode–cathode distances. Note that 15/2 and 15/4
EP groups were conducted with a voltage of 15 V, and the distances between the anode
and cathode were 2 and 4 cm, respectively. During the EP, the electrolyte was stirred
at 500 revolutions per minute using a magnetic stirrer to remove the remaining reactants
from the surface. After EP, all specimens were cleaned with acetone for 15 minutes
using an ultrasonic cleaner.
Fig. 1 Schematic diagram of the electropolishing process.
Table 2
Setup parameters of electropolishing (EP) for each experimental group
|
Experimental group
|
Condition of the EP process
|
Anode–cathode distance (cm)
|
Group label
|
|
Control
|
N/A
|
N/A
|
Control
|
|
1
|
0.3 A/cm2
|
2
|
0.3/2
|
|
2
|
4
|
0.3/4
|
|
3
|
15 V
|
2
|
15/2
|
|
4
|
4
|
15/4
|
Abbreviation: N/A, not available.
Surface Roughness and Surface Morphology
The surface roughness of the samples was determined using a contact profilometer (form
Talysurf i-Series PRO, Taylor Hobson, United Kingdom). The surface roughness was measured
using a stylus gage, configured at 1.040-mm ranges, with a speed of 0.1 mm/s, and
length of 4 mm, three times per sample. The data were analyzed using Metrology 4.0
Software (Taylor Hobson, United Kingdom). The surface roughness values were represented
as the arithmetic mean deviation (Ra), and four samples from each group were measured.
The surface morphology was observed under a scanning electron microscope (Quanta250,
FEI, United States).
Corrosion Resistance Analysis
The corrosion resistance of the electropolished samples was assessed by potentiodynamic
polarization testing using a standard three-electrode system. The Ti6Al4V ELI alloy
plates with an area of 1 cm2 were mounted to a working electrode, where a platinum electrode was used as the counter
electrode and a saturated calomel electrode was used as the reference electrode. All
measurements were performed in 3.5% NaCl solution at 27 °C using the μAutolab electrochemical
workstation (μAutolab Type III, Eco Chemie, Utrecht, Netherlands) operated by Autolab
NOVA software version 1.11.2. The polarization curves were obtained from Tafel plots,
and the waiting time for the stable open circuit potential (OCP) was set to be 600 seconds.
The potential was then swept from –0.25 to +0.75 V versus the OCP at a scan rate of
1 mV/s while recording the corresponding current response. Tafel extrapolation was
used to determine the corrosion potential (E
corr) and corrosion current density (I
corr) by examining the linear regions of the anodic and cathodic branches of the polarization
curves. These parameters were then used to calculate the corrosion rate.
Statistical Analysis
The surface roughness data were analyzed using SPSS version 22.0 (SPSS Inc., Chicago,
Illinois, United States). The normality of data was determined using the Shapiro–Wilk
test. A one-way analysis of variance and Tukey's post hoc tests were used to compare the surface roughness at various current densities, voltages,
and anode–cathode distances. The significance level for all tests was set at 5%.
Results
Surface Roughness
The surface roughness values of the Ti6Al4V ELI printed samples for each group are
listed in [Table 3]. The surface roughness of all groups decreased after the EP. It was found that when
using 15 V and a 2-cm distance, the surface roughness of the printed sample was the
lowest (p < 0.05). However, there was no significant difference among the other groups.
Table 3
Mean (SD) values of surface roughness of the Ti6Al4V ELI alloy experimental groups
|
Group label
|
Current density (A/cm2)
|
Voltage (V)
|
Anode–cathode distance (cm)
|
Ra (μm)
|
|
Control
|
–
|
–
|
–
|
6.28 (±0.65)a
|
|
0.3/2
|
0.3
|
–
|
2
|
2.12 (±0.33)b
|
|
0.3/4
|
0.3
|
–
|
4
|
2.19 (±0.34)b
|
|
15/2
|
–
|
15
|
2
|
1.69 (±0.51)c
|
|
15/4
|
–
|
15
|
4
|
1.95 (±0.29)bc
|
Abbreviation: Ra, mean roughness; SD, standard deviation.
Note: Different superscript letters indicate significant differences between groups
(p < 0.05).
The surface morphologies of the specimens after the EP are shown in [Fig. 2]. The surface morphology of as-printed Ti typically exhibits partially melted or
unmelted powder particles adhering to the surface, and no powder particles were observed
in the specimens after EP. Although the surface morphologies of the 15/2 EP group
displayed lower surface roughness with comparatively smooth and homogeneous surface
qualities, all groups showed porosity on the electropolish surfaces, regardless of
EP condition.
Fig. 2 Scanning electron micrographs of the Ti6Al4V ELI printed samples: (A) as-printed, (B) 0.3/2, (C) 0.3/4, (D) 15/2, and (E) 15/4.
Corrosion Resistance
The potentiodynamic curves exhibiting corrosion properties of the control and experimental
groups are shown in [Fig. 3]. The curves of the postprocessing specimens were comparable. The control group (as-printed)
exhibited the highest E
corr, indicating superior corrosion resistance compared with the other electropolish groups.
The control group's I
corr was relatively low, implying a minimum corrosion rate. This baseline performance
highlights the inherent corrosion resistance of the Ti6Al4V ELI alloy in the absence
of EP treatment, providing a reference point for evaluating the effects of various
EP conditions.
Fig. 3 Representative potentiodynamic polarization curves.
The 15/2 EP sample presented an E
corr value closest to that of the control group. The I
corr value of this group was the lowest among all tested groups, implying the lowest corrosion
rate. In contrast, the other condition groups had lower E
corr and higher I
corr than the 15/2 EP group. These results implied slightly higher corrosion rates. The
increased porosity observed on the surfaces of these groups may have contributed to
their reduced corrosion resistance. Despite the improvements in surface finishing,
the porosity caused by the EP indicates that although the EP improved the surface
quality, the reverse effect of the EP compromised the corrosion resistance. Additionally,
the I
corr and corrosion rates of 0.3/2, 0.3/4, and 15/4 EP samples were similar to those of
the control group ([Table 4]).
Table 4
Corrosion parameter values of the Ti6Al4V ELI alloy groups
|
Sample group
|
E
corr [V]
|
I
corr [μA/cm2]
|
Corrosion rate [mm/y]
|
|
Control
|
–0.045
|
0.016
|
0.00019
|
|
0.3/2
|
–0.126
|
0.018
|
0.00021
|
|
0.3/4
|
–0.146
|
0.011
|
0.00013
|
|
15/2
|
–0.068
|
0.008
|
0.00010
|
|
15/4
|
–0.150
|
0.015
|
0.00018
|
Discussion
This study aimed to determine how various EP parameters, including current density,
applied voltage, and anode–cathode distance, affect the corrosion resistance and surface
roughness of L-PBF Ti6Al4V ELI alloy. Our findings showed that after the EP, the surface
roughness of the L-PBF Ti6Al4V ELI alloy samples was consistently lower in all groups
than in the control group. Notably, the group with voltage control exhibited lower
surface roughness than the group with controlled current density. When evaluating
the influence of the anode–cathode distance, no difference in surface roughness was
observed between the groups. The results show that a shorter anode–cathode distance
results in lower surface roughness. However, other factors would affect the electrochemical
mechanism.
The surface roughness of as-printed Ti6Al4V ELI alloy sample is influenced by partially
melted or unmelted powder particles adhering to the surface, resulting in a rough
texture. This inherent roughness can compromise the material's mechanical properties
and biocompatibility, especially in the field of medical applications.[17] The EP is a type of postprocessing treatment employed to improve the smoothness
of alloy surfaces. The results showed that all EP samples exhibited a marked reduction
in surface roughness compared with the as-printed samples, which is consistent with
a study by Tsoeunyane et al.[18] Controlling the current density and voltage had a positive impact on the resulting
surface roughness. A study by Urlea and Brailovski[19] demonstrated that controlling the current density significantly improved the surface
roughness across all tested orientations and reduced Ra. Similarly, studies by Guo
et al[20] on voltage control have shown a reduction in surface roughness.
The surface morphology revealed that all unmelted powder was completely removed by
the EP. However, pitting was observed on the sample surfaces. This finding is consistent
with the previous studies[18]
[21] reporting the use of an electrolyte containing perchloric acid (HClO4), as the chloride ions have considerable effect on the metal alloy. Moreover, Ti
tetrachloride (TiCl4) films can cause uneven surfaces if not quickly removed. Optimal polishing occurs
within a specific range of voltage but exceeds the limit of oxygen evolution and pitting.
Proper stirring is essential to dissolve the TiCl4 film. The insufficient stirring can cause passivation, whereas excessive stirring
results in uneven polishing and pitting.[22]
[23]
In general, the EP smoothens surfaces by eliminating imperfections, thereby improving
corrosion resistance by reducing the number of sites at which corrosion can start.
Consequently, this typically leads to elevated E
corr.[17] However, despite the improvements in surface smoothness, the electropolished samples
exhibited lower E
corr values. This phenomenon can be attributed to residual porosities that may remain
on the surface layer after the EP. These residual pores can act as initiation sites
for localized corrosion, leading to lower E
corr values when exposed during corrosion testing.[24]
[25] Similarly, Zhang et al[26] reported that the E
corr of Ti6Al4V ELI before polishing had a higher E
corr than after the EP. A decrease in E
corr represents lower thermodynamic stability, which indicates lower resistance of a material
to chemical reactions with its environment. However, this metric alone does not capture
all aspects of corrosion behavior, as other factors like kinetic barriers, protective
films, and environmental conditions also play significant roles in determining the
overall corrosion resistance.[27]
[28]
When examining the I
corr and corrosion rate, the study found that they were not different between the groups,
except for the 15/2 EP group, which had the lowest values. These results agree with
the measured surface roughness, showing that the EP conditions at 15 V with 2-cm distance
improved the surface roughness and enhanced the corrosion resistance. These findings
indicate that surface imperfections affect corrosion performance and that the absence
of porosity significantly enhances corrosion resistance. Similarly, Zhang[14] and Wu et al[29] evaluated the biocorrosion resistance of Ti6Al4V alloy after EP. Proper EP treatment
can reduce surface roughness, resulting in a flatter surface with no defects and improved
biocorrosion resistance.
Overall, this study emphasizes the importance of optimizing the EP parameters to maximize
the performance and durability of Ti6Al4V ELI alloys, where both surface quality and
corrosion resistance are crucial. The results showed that the EP with constant voltage
produced better surface quality than constant current. However, different types of
components or devices, such as 3D-printed dental prostheses or medical devices, may
require adjustment of the EP parameters because these parameters cannot be universally
applied. Additionally, this study identified the occurrence of pitting in some areas,
indicating that combining the EP with other postprocessing treatments is needed to
enhance surface performance, usability, and corrosion properties.
However, this study has the limitation that it focused on surface roughness as the
key parameter influencing surface quality and its subsequent effect on corrosion resistance.
While other surface properties such as surface hardness, wettability, or surface chemistry
also play significant roles on surface quality. These factors should therefore be
explored in future research to better understand their impact on corrosion resistance.
In addition, the sample used in this study was restricted to a simple shape, which
does not fully represent the complex geometries commonly found in dental or medical
applications. The complex geometries may yield different outcomes due to their intricate
surface geometry.
Conclusion
EP significantly improved the surface roughness and morphology of the Ti6Al4V alloy.
Interestingly, applying a constant voltage of 15 V with appropriate anode–cathode
distance produced the lowest surface roughness and corrosion resistance. This could
yield better results than employing the constant current. However, improper EP can
reduce corrosion resistance by increasing surface porosity, which leads to pitting
corrosion. These findings are essential for dental or medical applications, as surface
quality and high corrosion resistance are crucial for the performance and longevity
of prostheses. Consequently, appropriate EP can improve the L-PBF component's surface
quality. This method could also be beneficial for surface finishing in other 3D-printing
industries.
