Electropolishing of Ti6Al4V fabricated by powder bed fusion additive manufacturing

1. Electropolishing of Ti6Al4V
fabricated by powder bed fusion
additive manufacturing
By: Dr. Khuram Shahzad
Email: engineerkhuram@gmail.com
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2. Electropolishing fundamentals
• Electropolishing (EP) is also know as electrochemical polishing, anodic
polishing or electrolytic polishing.
• Definition: EP is a metal finishing process that removes material from a
metal or alloy by anodic dissolution, in which material is removed ion by
ion.
• Unlike other metal finishing mechanical process (cutting, grinding, milling
and buffing) EP is a non-contact damage free process.
• Polishing: Elimination of roughness, absence of crystallographic and grain
boundary attack, resulting in smooth and bright surfaces.
• EP is vastly applied to metal finishing industry due to its simplicity and
ability to polish complex shapes. For example: stainless steel cutlery,
cardiovascular and orthopadic body implants, cornory stents,
superconductive niobium cavaties etc.
Wei Han, Fengzhou Fang, Fundamental aspects and resent developments in electropolishing, International Journal of Machine Tools and Manufacture 139 (2019) 1-23
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3. • Typically, an EP process involves a work
piece immersed in a temperature
controlled bath of electrolyte and serves
as anode. The anode/work piece is
attached to positive polarity of a DC/AC
power supply. Whereas cathode is
connected to negative polarity.
• Metal on the surface of anode is
dissolved in electrolyte when current
passes from anode to cathode. At
cathode surface a reaction occurs which
normally produces hydrogen.
Electropolishing fundamentals
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Wei Han, Fengzhou Fang, Fundamental aspects and resent developments in electropolishing, International Journal of Machine Tools and Manufacture 139 (2019) 1-23

4. Material Removal mechanism in EP
The material dissolution during EP follows two laws of Friday
1. The amount of material dissolved or deposited is proportional to electricity
m Q , where m is mass of material dissolved or deposited, Q is amount of charge
passed
2. The amount of material dissolved or deposited by the same quantity of electricity
is proportional to their chemical equivalent weights
According to Faraday’s law, the necessary quality of electricity to dissolve 1 g work
piece can be expressed as nF/M coulumbs. n is atomic valency, F is Faraday’s constant
and M is atomic weight. Then, the dissolved worked piece material m (g) caused by I
(A) during EP time t can be expressed as
m = MIt/nF
Hence, the current I and EP time t can be controlled to realize specific material
removal. When the density of work piece material is ρ, the theoretical material
removal can be expressed as
V = MIt/nFρ (mm3)
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Wei Han, Fengzhou Fang, Fundamental aspects and resent developments in electropolishing, International Journal of Machine Tools and Manufacture 139 (2019) 1-23

5. • Typical current density-voltage curve
• Regions of Etching, passivating, EP (limiting current plateau)
and gas evolution can be identified
• In etching region, direct dissolution of workpiece occurs. Due to
mechanically worked surfaces some pitting generates on the
surface in this region
• In passivation region, an oxide layer forms on the anode surface
which leads to reduction of current density
• In limiting current plateau region, current remain almost
constant by increasing the voltage
• In gas evolution region, passive oxide layer breaks down with
increasing voltage, and anodic dissolution is accompanied by
the evolution of oxygen. Oxygen bubbles are trapped on
workpiece surface causing pitting on the surface. For which this
region is also called pitting region.
• Typically EP occurs in the region limiting current plateau, at high
anodic voltages where the metal surface is passivated and
metal surfaces tend to be smoother as the current increases.
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Voltage current density in EP
Wei Han, Fengzhou Fang, Fundamental aspects and resent developments in electropolishing, International Journal of Machine Tools and Manufacture 139 (2019) 1-23

6. • Jacquet proposed that a dissolution product form a viscous film on
the surface of anode.
• The film shows high electrical resistance and limits the current
density consequently material removal
• Viscous film is flat on the electrolyte side which means thickness of
the film is not consistent as the anode/work piece surface is rough
• In the valleys viscous film is thick leading to low material dissolution
due to high electrical resistance and low potential distribution
• On the contrary, viscous film is thin at the protruding position and the
material dissolution is fast due to high potential distribution
• The potential distribution difference at the valley and protruding
position generates the polishing effect
• Dubrovski et al. showed that a viscous film thickness can be around 1-
2 mm
• Hoar proposed that a very thin passive film forms between the
surface of anode and viscous layer. The material is dissolved in to the
oxide and then to the electrolyte
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EP mechanism: Viscous film theory
Wei Han, Fengzhou Fang, Fundamental aspects and resent developments in electropolishing, International Journal of Machine Tools and Manufacture 139 (2019) 1-23

7. • Mass transport limitation theory is generally believed to be
responsible for the anode dissolution during EP. It requires
formation of a salt film which limits the concentration of
metal cations, or a diffusion limitation for transport of an
acceptor molecule required for solvation. Two famous
transport mechanisms are proposed: the salt film model
and the adsorbate acceptor model
• Duplex salt film model was proposed by Grimm, fig (a). Two
films form on the surface of anode one is compact and one
is porous. The pores of the porous film are filled by
electrolyte (at the saturation concentration) and the mobile
charge carriers (anions and cations) transport the current
by migration in the pores. To explain high electrical
resistance of porous films are attributed to have rather low
porosity and are taken to be several micron thick. In the
compact film region a solid electric barrier forms through
which the cations are transported by solid state ionic
conduction in the presence of much higher electric field.
Compact films are generally considered to be 10 nm thick.
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EP mechanism: Mass transport limitation
theory
Wei Han, Fengzhou Fang, Fundamental aspects and resent developments in electropolishing, International Journal of Machine Tools and Manufacture 139 (2019) 1-23

8. • Adsorbate acceptor mechanism is proposed by
Matlosz, fig (b). According to this model dissolution
occurs by oxidation of metal to adsorbed cations
followed by the solvation of adsorbed ions by
acceptor species. The mass transport process are
limited by the diffusion of the acceptor specie
through the electrolyte diffusion layer. The limiting
current is reached when concentration of acceptor
species A drop to near zero at the
electrode/electrolyte interface.
• With the salt film model, it is the thickening of the
salt film which leads to an increase in potential drop
along the limiting current plateau region. With the
adsorbate acceptor model, it is the accumulation of
adsorbed ions on the electrode surface which leads to
blocking of the surface and a subsequent increase in
potential drop for workpiece dissolution.
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EP mechanism: Mass transport limitation
theory
Wei Han, Fengzhou Fang, Fundamental aspects and resent developments in electropolishing, International Journal of Machine Tools and Manufacture 139 (2019) 1-23

9. • Surface smoothening in EP process is assumed to be due to Anodic leveling
(macro smoothening) an brightening (micro smoothening).
• Anodic levelling refers to the elimination of the surface roughness heights
greater than 1 µm. It is achieved due to the potential difference between
protruding points and valleys.
• Anodic brightening refers to elimination of surface roughness height less
than 1 µm. It is resulted from the suppression of the influence of surface
defects and crystallographic orientation on the electrochemical dissolution.
• It is possible to achieve anodic levelling effect without brightening effect
and vice versa by controlling the process factors.
• EP is a complex process, there are no fix parameters for all work piece
materials. Electrolyte temperature and composition, polishing time, initial
surface roughness, current density etc. are important process parameters
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EP: Main process parameters
Wei Han, Fengzhou Fang, Fundamental aspects and resent developments in electropolishing, International Journal of Machine Tools and Manufacture 139 (2019) 1-23

10. • Decrease in electrolyte temperature slows diffusion of
dissolved metal ions away from the anode surface and
acceptor ions towards the surface. Lower temperature also
decreases solubility of metal ions in solution resulting in
additional effect on the reducing current density.
• Increase in electrolyte temperature makes EP process more
active because of low viscosity and continuous supply of fresh
electrolyte.
• Figure shows current density-voltage curve for EP of porous
austenetic stainlss steel in phosphoric-sulphuric mixed oxide
electrolyte. The current density increased with increasing
temperature, and higher current density helps to generate a
better surface effect due to bettre surface roughness
• However, at very high temperatures shoul be avoided as it0,1
can cause formation of etching pits. Ma et al, found during the
EP of aluminium samples that when electrolyte (perchloric
acid : ethanol, 1:8) temperature is between 10-30 C a limitting
current palteu can be observed. When electrolyte
temperature is increased to 40 C a continuous curve is
obtained in the absence of limitting current plateau.
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Effect of electrolyte temperature on EP
Wei Han, Fengzhou Fang, Fundamental aspects and resent developments in electropolishing, International Journal of Machine Tools and Manufacture 139 (2019) 1-23

11. • EP is carrier of current, heat and reaction products
• Organic, inorganic or organic-inorganic mixtures can be used
as electrolytes
• Electrolytes used for stainless steel are the most well known.
These electrolytes typically contain 50-70 wt.% acid, 5-15 wt.
% deionized water, remainder is inhibitor. The acid component
of electrolyte generally consists of phosphoric acid and
sulfuric acid mixed in a 1:1 or 2:1 ratio.
• Different metals may need different types of electrolyte an
properties of electrolyte has a direct influence on final EP
• The volume ratio of acid is of critical importance. In the case
of copper, Ra after EP decreases with inresing acid
concentration, and higher acid ration showed a higher
polishing effect. Whn 30Nb-Ti alloys were Eped with
methanolic sulfuric acid, anodic polarization behavior
recorded as shown in fig. It was found that magnitude of
limitting current density decreased by increasing acid
concentration. The decrease of current density due to increase
in sulfuric acid concentration shows that (SO4)-2 ions does not
play a critical rule in mass transport in limitting current
plateau region. The decrease in limitting current density and
broadening of limitting current plateau can be attributed to
decrease in metal ion solubility.
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Effect of electrolyte composition on EP
Wei Han, Fengzhou Fang, Fundamental aspects and resent developments in electropolishing, International Journal of Machine Tools and Manufacture 139 (2019) 1-23

12. 12
Effect of electrolyte composition on EP
Abd El Khalick, Danwei Wang, Electromechanical polishing technology: recent developments an future research and industrial needs, International Journal of advanced manufacturing technology (2016)
86: 1909-1924

13. • Th surface roughness of the workpiece
tends to improve as the polishing time is
increased. However, polishing rate
decrease with polishing time.
• Fig shows that mean Ra for
electropolished 316 stainless steel follows
a exponential decay for 20µm and 80µm
scale.
• At the begining of the EP process, the
potential difference between protruding
surfaces and valleys is high whih leads to
signifiacntly fast metal dissolution rate.
With increase in time surface of the anode
becomes smoother leading to lower
potential difference between protruding
surfaces and valleys and lower polishing
rate.
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Effect of electropolishing time on EP
Wei Han, Fengzhou Fang, Fundamental aspects and resent developments in electropolishing, International Journal of Machine Tools and Manufacture 139 (2019) 1-23

14. • It shall be noted that initial surface
roughness has a great influence on
surface finish of EPed part.
• Figure shows that nitinol surface with
Ra 1µm was improved to 0.5µm in less
than 50 sec, but roughness did not
significantly improve after 50 sec. With
initial Ra of 2µm, the surface
roughness was improved gradually to
Ra = 0.98 µm after 300 sec.
• Hence, the initial work piece surface
shall be considered carefully for the
fast and effective EP process because
the EP has limitations in finishing
quality.
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Effect of initial surface roughness on EP
Wei Han, Fengzhou Fang, Fundamental aspects and resent developments in electropolishing, International Journal of Machine Tools and Manufacture 139 (2019) 1-23

15. • Anodic dissolution of 420 stainless steel (Fe-13Cr) in
concentrated phosphoric acid, sulfuric acid, and
their mixtures shows that secondary effect is
inhibited due to secondary passivation, even at very
high anode potentials.
• However, EP effect is observed in trans-passive
potential region and beyond it.
• A wide range of limitting currnet density was
obtained with different rotation speeds and height
of the limiting current plateau increases with
rotation speed. This is because the increase in
rotation speed decreases thickness of diffusion
layer on the surface of workpiece, causing the
shorter path length for diffusion of ions.
• On the other hand, it is reported that there is an
optimum rotation speed for the minimum surface
roughness because the surface quality detriorates
with very high rotation speed by removing too muh
of the valley position of the surface.
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Effect of electrode rotation speed on EP
Wei Han, Fengzhou Fang, Fundamental aspects and resent developments in electropolishing, International Journal of Machine Tools and Manufacture 139 (2019) 1-23

16. 16
EP of Ti6Al4V as build parts fabricated by selective
laser melting and electron beam melting
Ref. AM Process Electrolyte EP current density
or EP voltage
EP time Initial Ra
(μm)
Ra after EP
(μm)
1 EBM Ethanol, isopropyl alcohol, zinc chloride,
aluminum chloride
80V + 38 °C 20 min 20 2.3
2 EBM 700 ml Ethanol, 300 ml isopropyl alcohol,
250g zinc chloride, 60g aluminum chloride
55V, 400 rpm 20 min 23 6
3 SLM Perchloric acid, glacial acetic acid and
distilled water, 1:10:1.2
0.3 A/cm2, 30 °C 15 min 6.33 1.132
4 EBM Acetic acid, 60% perchloric acid and 95%
ethanol. 16:5:4
442 mA/cm2, 4°C 20 min 24.1±2.6 4.5±0.5
5 EBM 700 ml Ethanol, 300 ml isopropyl alcohol,
250g zinc chloride, 60g aluminum chloride
60-80 V, 26-38 °C 20 min 30-40 5-15
6 SLM 60% perchloric acid and glacial acetic acid.
1:9 vol. ratio
240 mA/cm2 1600 sec 11.25 0.87
1- Li Yang, et al., Further study of the electropolishing of Ti6Al4V parts made via electron beam melting, Solid free form fabrication symposium, Aug 2015.
2- Li Yang, et al., Surface treatment of Ti6Al4V parts made by powder bed fusion additive manufacturing processes using electropolishing, Solid free form fabrication symposium, 2014.
3- Yifei Zhang, Electropolishing mechanism of Ti6Al4V alloy fabricated by selective laser melting, International journal of electrochemical science, 13 (2018) 4792-4807.
4- Yao-Cheng Wu, et al., Effects of electrochemical polishing on mechanical properties and bio-corrosion of Ti6Al4V by electron beam melting, Materials, 12 (2019) 1466.
5- Li Yang, et al., Electropolishing of Ti6Al4V parts fabricated by electron beam melting, Solid free form fabrication symposium, 2016
6- V. Urlea, et al., Electropolishing and electropolishing-related allowances for powder bed selective-laser-melted Ti-6Al-4V alloy components, Journal of Materials Processing Technology, 242 (2017) 1-11

17. • V. Urlea studied effect of voltage on
current density in 60% perchloric acid
and glacial acetic acid, 1:9 vol. ratio.
• No stable plateau was observed which
indicates that diffusion is not rate
determining process and current
density is the main process controlling
parameter. He used 160, 240 and 320
mA/cm2 current densities whereas 240
mA/cm2 showed the best results for EP
• Yao-Cheng Wu also observed the
absence of limiting current plateau. He
selected 300 mA/cm2 current density
to optimize polishing time.
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EP of AM Ti64: current density vs voltage
Yao-Cheng Wu, et al., Effects of electrochemical polishing on mechanical properties and bio-corrosion of Ti6Al4V by electron beam melting, Materials, 12 (2019) 1466.
V. Urlea, et al., Electropolishing and electropolishing-related allowances for powder bed selective-laser-melted Ti-6Al-4V alloy components, Journal of Materials Processing Technology, 242 (2017) 1-11

18. EP of the Ti64 alloy in the solution of perchloric acid and
glacial acetic acid system has three stages.
First stage: the oxidation of alloy on metal surface proceeds
according to reaction in below:
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EP of AM Ti64: EP mechanism
2nd stage: surface oxides react with the perchloric acid in the
electrolyte and the oxides dissolve simultaneously as EP
progresses according to following reactions:
3rd stage: the macro-/micro-smoothening of alloy surface. With
electropolishing in progress, the alloy surface is continuously
levelled and smoothened after the oxide desquamate in the form
of large particles.
Yifei Zhang, Electropolishing mechanism of Ti6Al4V alloy fabricated by selective laser melting, International journal of electrochemical science, 13 (2018) 4792-4807.

19. • Surface of parts produced by SLM
or EBM is highly dependent on the
building angle
• Initial surface has a great influence
on final surface after EP. This was
also observed by V. Urlea. Higher
the roughness of the part higher
the roughness after polishing.
• During application this fact can lead
to inhomogeneous surface
roughness of the parts as AM parts
show inhomogeneous surface
roughness due to difference in
building direction for each surface.
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EP of AM Ti64: Effect of building direction
V. Urlea, et al., Electropolishing and electropolishing-related allowances for powder bed selective-laser-melted Ti-6Al-4V alloy components, Journal of Materials Processing Technology, 242 (2017) 1-11

20. 20
EP of AM Ti64: Effect of current density
V. Urlea, et al., Electropolishing and electropolishing-related allowances for powder bed selective-laser-melted Ti-6Al-4V alloy components, Journal of Materials Processing Technology, 242 (2017) 1-11
• Optimum value of current density shall be used
• Low current density leads to yellow peel off, oxidation.
• High current density leads to pitting

21. • By increasing EP time surface roughness
decreases
• Too long EP time can lead to pitting which
can influence mechanical properties and
biocorrosion
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EP of AM Ti64: Effect of EP time
Yao-Cheng Wu, et al., Effects of electrochemical polishing on mechanical properties and bio-corrosion of Ti6Al4V by electron beam melting, Materials, 12 (2019) 1466.

22. • By increasing EP time thickness of the parts decreases
• By increasing EP time weight of the part decreases
22
EP of AM Ti64: Effect of EP time on thickness
and weight loss
V. Urlea, et al., Electropolishing and electropolishing-related allowances for powder bed selective-laser-melted Ti-6Al-4V alloy components, Journal of Materials Processing Technology, 242 (2017) 1-11

23. 23
EP of AM Ti64: Ra evolution and thickness
reduction vs EP time
V. Urlea, et al., Electropolishing and electropolishing-related allowances for powder bed selective-laser-melted Ti-6Al-4V alloy components, Journal of Materials Processing Technology, 242 (2017) 1-11

24. • EP is a non contact method so is suitable for polishing complex shaped parts.
• EP involves acids and hazardous chemicals, health and safety of operators shall be
considered.
• EP shows promising results however further research is needed to develop
optimum process for Ti6Al4V AM parts.
• It seems that surface roughness of the as build parts is very high which leads to
poor surface roughness after EP. The successful application of EP might require
pretreatment like grinding or milling which can lead to additional costs.
• Surface roughness of AM parts is inhomogeneous depending on the fabrication
angle. EP process is also sensitive to in homogeneity of surface roughness. This
might lead to the requirement of pretreatment to homogenize the surface before
application of EP.
• It can be very interesting to investigate EP in combination of different
pretreatments preferably automated like laser polishing and tumbling.
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Important notes