Abstract

The proposed novel polishing method, hybrid electrochemical magnetorheological (H-ECMR) finishing, combines electrochemical reactions and mechanical abrasion on the workpiece surface to reduce finishing time. Moreover, H-ECMR finishing on the biomaterial surface produces a uniform, thick passive oxide layer to improve corrosion resistance. Herein, the electrolytic solution facilitates the chemical reaction and acts as a carrier medium for carbonyl iron particles (CIPs) in magnetorheological (MR) fluid. The synergic action of the two processes reduces the surface finishing time, which takes longer in the case of the conventional magnetorheological Finishing (MRF) process, as observed experimentally. The developed H-ECMR finishing process employs an electromagnet, maneuvering in situ surface quality variation by altering the magnetic field during finishing. The magnetic shield material (i.e., mu-metal) confines the bottom of the electromagnet core to restrict the magnetic field's leakage and provide a uniform and concentrated magnetic field at the polishing spot. The effectiveness of the H-ECMR process is evaluated based on various surface roughness parameters (i.e., average surface roughness (Ra), skewness (Rsk), and kurtosis (Rku)) and compared with the MRF process. A 96.4% reduction in Ra value is attained in the H-ECMR polishing compared to 49.6% in MRF for identical polishing time. Furthermore, an analytical model is developed to evaluate the final Ra attained from the developed H-ECMR polishing process and agrees well with the experimental results. The impact of different process parameters on surface roughness values is also analyzed. The electrochemical reaction forms a thick and unvarying passive layer on the Ti–6Al–4V surface as layer thickness increases to 78 nm from 8 nm. A case study on the femoral head of the Total Hip Arthroplasty (THA) for enhancement in the surface roughness and biocompatibility is performed through the developed H-ECMR polishing. The Ra value is decreased to 21.3 nm from 326 nm on the femoral head surface through the contour-parallel radial toolpath strategy.

Graphical Abstract Figure
Graphical Abstract Figure
Close modal

1 Introduction

The surface characteristics of the biomaterials impact their functionality while operating inside the human body. A highly polished surface having surface roughness (Ra) in the nanometer range is essential for orthopedic implants to improve wear resistance during their tribological interaction [1]. The debris particles produced due to wear spread through blood circulation inside human beings and are responsible for the toxicity of different body parts. Moreover, aseptic loosening caused by mechanical failure of implants during their operation may lead to pseudotumors, metal hypersensitivity, and inflammatory masses [2]. The improved surface quality also mended the wettability of orthopedic implants facilitating hydrophilic surface and further decreasing coagulation and platelet adhesion [3]. Magnetorheological Finishing (MRF) uses an unbonded polishing tool for surface roughness reduction without disturbing the surface topography of the biomaterials. Magnetorheological (MR) fluid used during MRF gets stiffened under the external magnetic flux influence and acts as a polishing tool. However, the lower polishing rate of the MRF process is a challenge. Singh et al. [4] developed an MRF-based process and performed the finishing operation on a flat workpiece (50 mm × 10 mm) made of ferromagnetic material (i.e., iron). They achieved a final surface roughness (Ra) of 16.6 nm in 120 min. Nagdeve et al. [5] carried out the MRF technique on a knee implant made of Ti alloy with optimized process parameters achieving a final Ra value of 50 nm for 120 min finishing time. Similarly, Barman et al. performed a surface finishing operation for 390 min on a Ti alloy knee implant to achieve a surface roughness of 10 nm using a magnetic field-assisted finishing process. Several researchers have introduced different methods to reduce the polishing rate during the MRF process, including the design and development of an effective experimental setup [5], optimization of the process parameters [6], and new development of MR fluid [7].

This manuscript presents a novel Hybrid Electrochemical Magnetorheological (H-ECMR) polishing technique to reduce the polishing time. The combined effect of both mechanical action (i.e., MRF process) and chemical integration (i.e., electrochemical reaction) enhances the Ti–6Al–4V surface quality. Apart from reducing the finishing time, the electrochemical reaction helps to generate a thick and uniform oxide surface over Ti alloy to increase its biocompatibility. Also, the material removal and oxide layer formation mechanism during the H-ECMR polishing is discussed. The article analyzes the surface characteristics, including average surface roughness (Ra), kurtosis (Rku), and skewness (Rsk) on the surface before and after polishing. Furthermore, an analytical model is developed to evaluate the Ra from the developed finishing process. Different input parameters' influence on surface roughness output responses is further examined through the H-ECMR polishing to evaluate their best combinations to produce a uniform surface quality over the entire polished surface. Additionally, Scanning Electron Microscope (SEM), X-ray Photoelectron Spectroscopy (XPS), and Energy Dispersive X-ray (EDX) are used to determine the change in the passive (TiO2) layer over the Ti–6Al–4V biomedical surface. Furthermore, a case study is made on the femoral head of the hip implant for enhancement in the surface roughness and biocompatibility through the developed H-ECMR polishing.

2 H-ECMR Finishing Principle and Polishing Medium

The H-ECMR polishing utilizes the synchronized effect of mechanical action and electrochemical process for the roughness reduction of the biomaterial's surface. This section discusses the mechanism involved in the H-ECMR polishing and the chemical reactions between the polishing tool and workpiece due to the electrochemical reaction.

The MRF technique utilizes the Magnetorheological (MR) fluid's rheological characteristics during the surface finishing process. The ferromagnetic (carbonyl iron particles) and abrasive particles (diamond) are mixed in a carrier fluid to prepare the MR fluid. However, the chances of sedimentation are high due to the higher density of powder particles; hence, glycerol is used as the stabilizer in the MR fluid. The MR fluid is in the liquid phase in the ambient environment, as illustrated in Fig. 1. However, with the influence of the exterior magnetic flux, it gets solidified and generates the essential finishing forces for the surface roughness reduction of the targeted region. An electromagnet is used during the present work to deliver in situ control of the magnetic flux in the H-ECMR polishing, which is unattainable with a permanent magnet. The electromagnet design provides the desired magnetic flux at the tip of the conical core. However, the various aspects of electromagnet design are analyzed in the succeeding section. The stiffened MR fluid is squeezed between the electromagnet and the workpiece, and their relative motion produces the required forces (i.e., normal and tangential force) during the surface finishing. The normal force (Fn) is produced due to the alignment of abrasive particles in the magnetic flux direction generated through the electromagnet, whereas the tangential force (Ft) is produced from the polishing tool's feed and rotational motion, as illustrated in Fig. 1. The normal force indents the CIPs chain into the substrate, and the tangential force assists in the detachment of the CIPs chain from the targeted workspace. Hence, there is the joint effect of normal and tangential forces for the surface roughness reduction of the biomaterial.

Fig. 1
Magnetorheological fluid with and without the influence of external magnetic field and mechanism of material removal during magnetorheological finishing process
Fig. 1
Magnetorheological fluid with and without the influence of external magnetic field and mechanism of material removal during magnetorheological finishing process
Close modal

Ti–6Al–4V, a biomaterial with high mechanical strengths and biocompatibility compared to other biomaterials, makes it preferable in the biomedical industry. The Ti–6Al–4V reacts with the environment oxygen available in the environment and forms a passive oxide layer of TiO2. The pH value of the passive oxide layer is the same as the human body and helps to reduce corrosion during their interaction. However, the thickness of the oxide layer on the biomaterial surface is nonuniform, with a layer thickness in the range of a few nanometers (i.e., 5–10 nm) [8], which provides the potential sites for initiating corrosion. Different surface finishing methods, including the MRF, are incompetent to produce a thick and uniform oxide surface on the biomaterial. Hence, it is essential to synergically integrate an electrochemical reaction with the MRF process to achieve the desired objective. Magnetorheological Fluid-Assisted Finishing (MFAF) process and the workpiece material are listed in Table 1. Furthermore, the polishing time for the MFAF is very high, as shown in Table 1. Including an electromagnetic reaction with the MRF process reduces the finishing time. The synergic combination of the above-mentioned process contributes to the newly developed H-ECMR polishing to produce Ra value in nanometers with a thick and uniform oxide surface over the biomaterial.

Table 1

Different magnetorheological fluid-assisted finishing processes

Sl. No.AuthorsType of MFAF processWorkpiece Material/Polishing areaFinal surface roughness (Ra)Finishing timeReferences
1Kim et al. (2004)Disc-based MRFSilicon/100 mm211.1 nm5 min[9]
2Seok et al. (2007)Disc-based MRFNA/NA40 nm140 min[10]
3Jung et al. (2009)Disc-based MRFAl2O3–TiC/10 mm2Total 255.1 nm reduction in initial Ra value60 min[11]
4Jang et al. (2010)MFAFGlass carbon/100 mm211.8 nm10 min[12]
5Singh et al. (2011)BEMRFEN31 steela70 nma100 mina[13]
Copperb/1500 mm2102 nmb60 minb
6Singh et al. (2012)BEMRFIron/500 mm216.6 nm120 min[4]
7Jang et al. (2012)MFAFStainless steela147.1 nma4 min[14]
Brassb/NA14 nmb
8Sidpara and Jain (2012)MFAFTi alloy/NA28 nm64 h[15]
9Kumar et al. (2015)R-MRAFFStainless steel/NA36 nm24.8 h[16]
10Maan et al. (2017)BEMRFTool steel/NA30 nm120 min[17]
11Bedi and Singh (2018)MRAH*Mild steel/NA78% reduction in Ra value90 min[18]
12Barman and Das (2019)MFAFTi alloy/225 mm210 nm6.3 h[19]
13Yadav and Singh (2019)MFAFEN 24 steel/NA25.3 nm20 nm[20]
14Nagdeve al. (2020)R-MRAFFTi alloy/NA50 nm2 h[21]
15Zhang et al. (2020)MFAFStainless steel/4710 mm2308 nm20 min[22]
Sl. No.AuthorsType of MFAF processWorkpiece Material/Polishing areaFinal surface roughness (Ra)Finishing timeReferences
1Kim et al. (2004)Disc-based MRFSilicon/100 mm211.1 nm5 min[9]
2Seok et al. (2007)Disc-based MRFNA/NA40 nm140 min[10]
3Jung et al. (2009)Disc-based MRFAl2O3–TiC/10 mm2Total 255.1 nm reduction in initial Ra value60 min[11]
4Jang et al. (2010)MFAFGlass carbon/100 mm211.8 nm10 min[12]
5Singh et al. (2011)BEMRFEN31 steela70 nma100 mina[13]
Copperb/1500 mm2102 nmb60 minb
6Singh et al. (2012)BEMRFIron/500 mm216.6 nm120 min[4]
7Jang et al. (2012)MFAFStainless steela147.1 nma4 min[14]
Brassb/NA14 nmb
8Sidpara and Jain (2012)MFAFTi alloy/NA28 nm64 h[15]
9Kumar et al. (2015)R-MRAFFStainless steel/NA36 nm24.8 h[16]
10Maan et al. (2017)BEMRFTool steel/NA30 nm120 min[17]
11Bedi and Singh (2018)MRAH*Mild steel/NA78% reduction in Ra value90 min[18]
12Barman and Das (2019)MFAFTi alloy/225 mm210 nm6.3 h[19]
13Yadav and Singh (2019)MFAFEN 24 steel/NA25.3 nm20 nm[20]
14Nagdeve al. (2020)R-MRAFFTi alloy/NA50 nm2 h[21]
15Zhang et al. (2020)MFAFStainless steel/4710 mm2308 nm20 min[22]

Note: *MRAH, Magnetorheological abrasive honing; MRF, magnetorheological finishing; MFAF, magnetorheological fluid-assisted finishing; MRAFF, magnetorheological abrasive flow finishing: R-MRAFF, Rotational magnetorheological abrasive flow finishing. a,bFinal surface roughness and finishing time for materials stainless steel and brass respectively.

The electrochemical reaction produces the oxide surface over the biomaterial (Ti alloy) from the anodic oxidation. Electrolytes replace the transporter liquid (i.e., paraffin wax) of the MR fluid to permit the electrochemical process, as shown in Fig. 2(a). Magnesium chloride (MgCl2) is combined with ethylene glycol (C6H6O2) in distilled water to form an electrolyte that triggers the rate of electrochemical reaction [23,24]. Voltage is applied among the electromagnet's core and the Ti–6Al–4V (workpiece). Herein, Ti alloy acts as an anode, and the electromagnet's core a cathode. The applied differences in the potential between the biomaterial and core cause the discharge of electrons from the biomaterial's surface, and Ti4+ is formed. Due to a similar effect, the electrolytes break down in Mg2+ and the Cl, where the anion reacts with Ti4+ and forms TiCl4 as shown in Eq. (1) [25].
Ti4++4Cl=TiCl4
(1)
Fig. 2
(a) Chemical reactions at electrodes and (b) formation of unvarying and thick passive surface on Ti alloy during electrochemical reactions
Fig. 2
(a) Chemical reactions at electrodes and (b) formation of unvarying and thick passive surface on Ti alloy during electrochemical reactions
Close modal
The elevated viscosity of TiCl4 poses a formidable challenge to its removal from the surface of the Ti alloy. Consequently, a persistent layer of TiCl4 gradually accrues on the substrate. Furthermore, the electron imbalance between Ti4+ and Cl induces the thickening of the Cl layer. Conversely, the heightened polarity of water in comparison to ethylene glycol prompts the reaction of H2O with TiCl4, leading to the formation of TiO2, as depicted in Eq. (2).
TiCl4+2H2O=TiO2(s)+4H++4Cl
(2)
Moreover, the interaction of ethylene glycol with TiCl4 results in the formation of TiO2, elucidated by Eqs. (3), (4), (5), and (6).
TiCl4+2HO(CH2)2OH=Ti[O(CH2)2O]2+4H++4Cl
(3)
Ti[O(CH2)2O]2+2H2O=TiO2(s)+2HO(CH2)2OH
(4)
TiCl4+4HO(CH2)2OH=Ti[O(CH2)2O]4+4H++4Cl
(5)
Ti[O(CH2)2O]4+2H2O=TiO2(s)+4HO(CH2)2OH
(6)

The freshly generated TiO2 (solid) exhibits robust adhesion characteristics with the Ti alloy surface and demonstrates inertness toward reactions with other ions present in the electrolyte [26]. The sequential electrochemical processes yield a homogeneous and substantial passive surface on the Ti alloy, as illustrated in Fig. 2(b).

2.1 Experimental Setup.

A novel electromagnet-based H-ECMR technique is innovatively devised to enhance both the biocompatibility and surface quality of Ti alloy. The consistent magnetic field, facilitated by the electromagnet, significantly influences the efficiency of the proposed polishing method. The electromagnet-based H-ECMR method enables in situ modulation of the rheological characteristics of Magnetorheological (MR) fluid, catering to the diverse requirements of biomedical implants. A pure iron core is strategically employed to concentrate the magnetic flux precisely at the polishing zone, as depicted in Fig. 3. Additionally, cooling coils wrap around the electromagnet and help to mitigate heat generated due to resistive heating of the electromagnetic coil. The incorporation of four mounting plates not only houses the electromagnet but also ensures its stable position throughout the surface finishing operation. Threaded joints on each mounting plate allow for secure locking in their respective positions. The magnetic shield material (i.e., mu-metal) confines the tip of the electromagnet core to restrict the magnetic field's leakage and provide a uniform and focused magnetic flux at the finishing spot. Due to the higher permeability value for mu-metal, a low reluctance path for the magnetic field provides a shield against a static magnetic field. It further allows the passing of a magnetic field around the shield area, overpassing the protected space. However, due to its high permeability, direct contact of mu-metal with core material results in its magnetization. A thin insulating material tightly fitted over the core of the material is provided between the core and the mu-metal to prevent its magnetization. Figure 3 illustrates the experimental configuration for H-ECMR. Additionally, the H-ECMR finishing setup, based on the electromagnet, is seamlessly integrated into a five-axis computerised numerical control-operated machine to afford the requisite DOF crucial for optimal interaction between the workpiece and the polishing tool, as depicted in Fig. 3.

Fig. 3
H-ECMR finishing experimental setup
Fig. 3
H-ECMR finishing experimental setup
Close modal

An electrical potential is introduced across the tool and the polishing part, utilizing a carbon brush linked to the core of the electromagnet to facilitate meticulous control. Furthermore, the MR fluid's carrier medium is substituted with electrolytes to facilitate electrochemical reactions between the polishing tool and the workpiece. The distinct process parameters and MR fluid composition during the polishing process are detailed in Table 2. Ti–6Al–4V is a biomaterial used as the workpiece (50 mm × 50 mm) during H-ECMR finishing operation; the improved biocompatibility and mechanical strength make it more favorable than other biomaterials. The surface features of the finished and unpolished surfaces are examined with an optical profilometer. XPS, EDX, and SEM are used to explore the characteristics of the oxide layer developed on the biomaterial.

Table 2

MR Fluid's composition and process parameters of H-ECMR polishing

Parameters/ConstituentsMagnitude/Composition (Vol. %)
Diamond7%
CIPs40%
MgCl216.3%
C2H6O26.2%
Glycerol8%
H2O22.5%
Feed rate1 mm/min
Standoff distance0.5–2 mm
Polishing tool's rotational speed600–1600 mm
Applied current (continuous)1–2.4 A
Parameters/ConstituentsMagnitude/Composition (Vol. %)
Diamond7%
CIPs40%
MgCl216.3%
C2H6O26.2%
Glycerol8%
H2O22.5%
Feed rate1 mm/min
Standoff distance0.5–2 mm
Polishing tool's rotational speed600–1600 mm
Applied current (continuous)1–2.4 A

3 Analytical Model of H-ECMR Finishing

An analytical model is developed to determine the Ra achieved after the H-ECMR finishing process; a few assumptions are made prior to developing the analytical model. The direct current energizes the electromagnet for the generation of magnetic flux. Hence, the production of the magnetic field because of the time-dependent electric field in the ampere's law is eliminated (termed as the displacement current). The reduced Maxwell equation, as shown in Eq. (7), states that the total current (Jda) passes through a closed surface (S) is equal to magnetic field intensity (H) about the closed contour (C). Moreover, Eq. (8) states the magnetic field density (B) conservation over a closed contour [27].
CHdl=SJda
(7)
CBda=0
(8)
The second assumption states that a highly permeable core material has confined the magnetic flux path along its structure. Figure 4(a) illustrates the magnetic flux produced by the electromagnet during the surface finishing operation incorporating the above assumption. The permeability of the core material is higher than its surroundings (µ >> µo) is also a valid reason for the justification of the second assumption. Winding of the copper wire (a good conductor of electricity) is provided over the core material to generate the magnetic flux. The turn number (N) and the electric current (i) supplied to the winding are 5472 and 3 Amp, respectively, exciting core of the electromagnet. The high permeability of the core makes it possible to confine its magnetic field during the finishing operation. The product of ampere and the turns number (i.e., Ni) is also termed the magnetomotive force (F), which is used to generate the magnetic flux (ϕ) in the equivalent circuit diagram. Moreover, the magnetic flux (ϕ) is defined by the normal component of the B passing over the surface (S) as shown in Eq. (9); however, the total ϕ entering and leaving the closed surface is zero.
ϕ=SBda
(9)
Fig. 4
(a) Magnetic flux generated inside electromagnet core and (b) equivalent magnetic circuit
Fig. 4
(a) Magnetic flux generated inside electromagnet core and (b) equivalent magnetic circuit
Close modal
The conical shape at the bottom of the core generates a focused magnetic flux at the finishing spot. Hence, the core part is divided into sections C1 and C2, representing cylindrical and conical shapes. The length corresponds to the core sections denoted by LC1 and LC2, respectively. Similarly, the standoff distance and the workpiece's thickness are denoted as z and t in Fig. 4(a). Equation (10) shows the scalar representation of the ϕ at the different cross-sections of the core.
ϕC=BC1AC1+BC2AC2
(10)
where BC1 and BC2 signify the magnetic field density of sections C1 and C2, respectively. Similarly, the AC1 and AC2 represent the crosswise area of sections C1 and the bottom surface of cone C2, respectively. Moreover, Eq. (11) represents the relation between the magnetomotive force (F) (product of ampere (i) and the number of turns (N)) and the magnetic field intensity (H) of the circuit shown in Fig. 4(b).
F=Ni=CHdl
(11)
The correlation between the magnetic field density and intensity, as shown in Eq. (12), is material's permeability (μ) dependent function. The permeability of the material (μ) is a product of free space permeability ((μo=4π×107N/A2)) and its relative permeability (μr). The relative permeability of the materials used during the H-ECMR finishing process, including core, MR fluid, and biomaterial, is listed in Table 3. However, before selecting the core material, the pure iron's relative permeability is compared with the mild steel (conventionally used as the core material) through a vibrating sample magnetometer, as shown in Fig. 5(a). It is noticed that the permeability of almost 99.9% pure iron is higher than mild steel; further, it is used as the electromagnet's core material to produce high magnetic field density. Moreover, the comparison is made between the magnetic strength of the CIPs manufactured by Badische Anilin- und SodaFabrik (BASF) and electrolytic iron particles, as illustrated in Fig. 5(b). Furthermore, the strength of CIP particles is higher than the iron particle, with reduced hysteresis loss; hence CIPs have opted as ferromagnetic particles during the preparation of MR fluid [28].
B=μH
(12)
Fig. 5
M–H curve comparison between (a) pure iron and mild steel and (b) electrolytic iron and carbonyl iron (CIPs) particles
Fig. 5
M–H curve comparison between (a) pure iron and mild steel and (b) electrolytic iron and carbonyl iron (CIPs) particles
Close modal
Table 3

Magnitude of relative permeability

MaterialRelative permeability (μr)
Electromagnet core (99.9% pure iron)10,000
MR fluid5
Air1
Workpiece (Ti–6Al–4V)1
MaterialRelative permeability (μr)
Electromagnet core (99.9% pure iron)10,000
MR fluid5
Air1
Workpiece (Ti–6Al–4V)1
The magnetic flux (ϕ) is determined by Eq. (13). The equivalent reluctance of the circuit is further calculated through Eq. (14).
ϕ=FReqv
(13)
Reqv=Rc1+Rc2+RSD+RW+Ra2
(14)
where reluctance produced by the core C1 and C2 is represented by the RC1 and RC2, respectively. The reluctance caused by the Magnetorheological fluid, standoff distance, workpiece, and air is represented by RMR, RSD, RW, and Ra, respectively. However, reluctance (R) is a material property determined by Eq. (15) [29].
R=lμA
(15)
where l is the equivalent length of the material, and A is its crosswise area. The permeability of the material is represented by μ. After implementing the geometrical data and the permeability of the materials, Eq. (13) is deduced to Eq. (16). LC1, LC2, D1, D2, d, and t are145, 90, 30, 10, 4, and 10 mm, respectively. The equivalent reluctance of the circuit is determined through Eq. (15), and the magnetic flux (ϕ) generated from the electromagnet-based polishing tool is shown in Eq. (17). N and i represent the number of turns of the electromagnet and the current supplied.
ϕ=NiReqv
(16)
ϕ=Ni(5.35+20.3z)×109
(17)
The magnetic field primarily depends upon the standoff distance (z), and for an electric current of magnitude 3 Amp with 5472 turns in an electromagnet, the calculated magnetic field is shown in Eq. (18), where z is in mm. However, computational analysis for variation of the magnetic field over the workpiece at different stepovers is evaluated through finite element-based COMSOL software and illustrated in Fig. 6(a). The magnetic field variation is calculated experimentally through a Gauss meter to validate the analytical and computational results. The magnetic field is studied at a different standoff distance; its variation is plotted in Fig. 3(b). It is found that the analytical and computational results follow a similar trend as observed from the experimental data. However, the magnetic field also depends upon the electric current, and a higher value produces a strong magnetic flux, as illustrated in Fig. 6(b).
B=327(420+2694z)
(18)
Fig. 6
(a) Contour plot of simulated magnetic field at different standoff distances and (b) comparison of magnitude of magnetic field between simulation study, experimental measurement, and analytical model
Fig. 6
(a) Contour plot of simulated magnetic field at different standoff distances and (b) comparison of magnitude of magnetic field between simulation study, experimental measurement, and analytical model
Close modal

3.1 Normal Indentation Force (Fn).

The magnetic field generated from the electromagnet applied normal force (Fn) on the diamond particles and evaluated from Eq. (19) assists in penetrating diamond particles in the substrate [30].
Fn=nfn=B22μo(11μm)S
(19)
where n is the number of particles, fn is the normal force applied on the individual diamond particle, S is the surface area factor, and μm is MR fluid's permeability. The normal force deduced from the derived magnetic field for the specified electromagnet is shown in Eq. (20).
fn=193×107(420+269z)2
(20)
The number of abrasive particles (n) is calculated from the MR fluid composition, as listed in Table 2. The diameter of the CIPs (d) used during the experiment is 6 µm, and n is calculated from Eq. (21).
n=Vol.ofMRfluid×Vol.%ofabrasiveVol.ofasingleabrasiveparticle=0.1D2zd3
(21)
Where D is the diameter of the polishing tool (i.e., 10 mm) and z is the standoff distance (i.e., 1 mm). The Vol. % of the abrasive particle is 7% in the MR fluid. The number of the abrasive particles is 5.1 × 108, calculated from Eq. (21). Moreover, the normal force variation with standoff distance is calculated from Eq. (22) and is plotted in Fig. 7(a). It is observed that with an increase in the standoff distance, the normal force applied to the abrasive particle is decreased. Furthermore, the variation of the magnetic field with the applied current to the electromagnet is illustrated in Fig. 7(b), suggesting that a higher value of electric current leads to producing high-strength magnetic field density in the polishing region.
Fn=nfn=9822.7(420+269z)2
(22)
Fig. 7
(a) Variation of normal force with standoff distance at different applied electromagnet current and (b) magnetic field variation with applied current for different standoff distances from tooltip
Fig. 7
(a) Variation of normal force with standoff distance at different applied electromagnet current and (b) magnetic field variation with applied current for different standoff distances from tooltip
Close modal

3.2 Tangential Force (Ft).

The tangential force on the diamond particle takes out the indented material from the biomaterial. As shown in Eq. (23), the tangential force is the summation of the centrifugal force (Fcen) and shear force (Fshear) generated on the abrasive particles during finishing.
Ft=Fcen+Fshear
(23)
The centrifugal force developed because of the rotational motion of the finishing tool and is calculated as
Fcen=m(2πN60)2r
(24)
where N is the rotational speed of the polishing tool, m is the single abrasive particle's mass, and r is its radial distance. Similarly, the shear force on the abrasive particle is given by Eq. (25). Here, the projected area of the indented abrasive particle into the workpiece surface is represented by Ap and calculated from Fig. 8(a) and shown in Eq. (26). Where D is the diamond particle diameter, and t represents the indented thickness. τ is the shear stress in the polishing zone and is represented by the subquadratic power law as shown in Eq. (27) [31].
Fshear=τAp
(25)
Ap=π3t2(3D2t)
(26)
τ=6ϕμ0M12H32
(27)
where ϕ is the CIP volume fraction in MR fluid, μ0 is the free space permeability, H is magnetic field intensity, and M is saturation magnetization. The Vibrating Sample Magnetometer (VSM) reading of MR fluid, as shown in Fig. 8(c), is used to determine the correlation among the magnetic field density (B, Tesla) and magnetization (M, emu), as shown in Eq. (28).
M=5.7×1014B3+8.4×1010B23.9×106B+7.3×102(28)
(28)
Fig. 8
(a) Abrasive particle indented into workpiece surface, (b) diameter and depth of indented abrasive particle, and (c) M–H curve of MR fluid
Fig. 8
(a) Abrasive particle indented into workpiece surface, (b) diameter and depth of indented abrasive particle, and (c) M–H curve of MR fluid
Close modal

3.3 Surface Roughness Modeling.

The initial surface roughness profile is generated from the optical profilometer for the biomaterial surface. The forces developed during the polishing are used to calculate the reduction in the average surface roughness; abrasive particles get indented inside the workpiece under the normal force impact, and the indented thickness (t¯) is calculated from Eq. (29) and also shown in Fig. 8(b). The diameter of the indentation (d) is further calculated from the Brinell Hardness Number (BHN) of the workpiece material, as shown in Eq. (30).
t¯=D212D2d2
(29)
d=(D2(D2FnπD(BHN)))
(30)
The average width of the valley from the mean line is 0.68 µm on the surface of biomaterial before polishing; however, the abrasive diameter is 6 µm; hence, the narrow valleys are initially unattainable. Therefore, the abrasive particle will remove the higher peaks (max(Pi)) during the first stroke of the surface finishing process. The indented thickness (t¯) from the initial peaks is removed, and the new coordinate of the peak is defined by Eq. (31).
Pi,new=Pi,maxt¯
(31)
Figure 9(a) illustrates the material removal from the higher peaks; however, the valleys of the surface profile will remain the same. Furthermore, the area of valleys and crests below and above the mean line is equal. Material reduction from the biomaterial surface leads to a shift in the mean line and is calculated from Eq. (32). After every iteration, the mean line gets updated to calculate the average surface roughness, as illustrated in Eq. (33). Here, n is the number of data points. The iteration is repeated for the predefined number of cycles, and the flow diagram of the respective algorithm is illustrated in Fig. 9(b). Furthermore, the % decrease in Ra is calculated from Eq. (33).
Yi,new=YiΔY
(32)
Ra=i=1n|Yi|n
(33)
%reductioninRa=(Ra)beforepolishing(Ra)afterpolishing(Ra)beforepolishing×100
(34)
Fig. 9
Surface irregularities (a) before and (b) after polishing; (c) Flowchart showing algorithm to calculate final surface roughness profile
Fig. 9
Surface irregularities (a) before and (b) after polishing; (c) Flowchart showing algorithm to calculate final surface roughness profile
Close modal

4 Results and Discussion

The H-ECMR enhances biocompatibility and the surface quality of biomaterials; herein, the concurrent interplay of electrochemical reactions and mechanical abrasion synergistically minimizes finishing time while yielding a homogenous surface quality. Furthermore, average surface roughness (Ra), kurtosis (Rku), and skewness (Rsk) are further studied to regulate the change in surface characteristics after post-polishing. Moreover, the inclusion of an electrochemical reaction between the substrate and tool produces an unvarying and thick passive surface deposit on the substrate of Ti alloy. SEM, EDX, and XPS are used to analyze characteristics of the oxide layer formed on biomaterial before and after finishing and are discussed in the subsequent sections. Various input parameters' impact on the workpiece's surface quality is also analyzed.

4.1 Impact of Mu-Metal on Effectiveness of Polishing Tool.

The magnetic flux circulating through the core's tip is responsible for spreading the MR fluid at the polishing zone, generating nonvarying surface quality on the finished surface. Hence, concentrated magnetic field lines must be produced at the polishing spot to overcome limitations. A simulation study of the polishing tool with and without mu-metal is illustrated in Fig. 10. The magnetic shield material (i.e., mu-metal) confines the tip of the electromagnet core to restrict the magnetic field's leakage and provides a uniform and concentrated magnetic field at the polishing spot. Mu-metal, a magnetic shielding material is an alloy of nickel, molybdenum, and iron; its chemical composition is analyzed through the EDX and shown in Fig. 11(b). Due to the higher permeability value for mu-metal, a low reluctance path for the magnetic field provides a shield against a static magnetic field. It further allows the passing of a magnetic field around the shield area, overpassing the protected space. Furthermore, simulation analysis is performed to study the magnetic field line generated with the inclusion of mu-metal. It is observed that direct contact with the mu-metal initially leads to the magnetic flux flow through it, as illustrated in Fig. 10(a). Hence, a nonmagnetic material (i.e., Teflon) is required between the core material and mu-metal to discontinue the magnetization of the mu-metal. The computational results show that the inclusion of the mu-metal over the polishing tool's surface restricts the magnetic field's leakage and provides a uniform and concentrated magnetic field at the polishing spot, as shown in 10(b).

Fig. 10
Simulation study of polishing tool (a) with and (b) without mu-metal
Fig. 10
Simulation study of polishing tool (a) with and (b) without mu-metal
Close modal
Fig. 11
Variation of magnetic field (a) along the axial direction (i.e., Z-axis) from tooltip and (b) EDX study for mu-metal composition.
Fig. 11
Variation of magnetic field (a) along the axial direction (i.e., Z-axis) from tooltip and (b) EDX study for mu-metal composition.
Close modal

Moreover, concentrated MR fluid is achieved with an enclosed mu-metal over the surface of the electromagnet core, as shown in Fig. 11(a). The increase in the magnetic flux in the axial direction (red dashed line) of the polishing tool is also observed during experiments.

4.2 Impact of Process Parameters.

Analyzing process parameters' impact on the output responses during the H-ECMR finishing is crucial for determining their optimum combinations to achieve maximum surface roughness reduction with uniform surface quality. The effect of various process parameters, namely the polishing tool's rotational speed, the percentage composition of electrolyte in MR fluid, the current applied between the workpiece and polishing tool, and surface finishing time on surface quality, is examined and discussed in the subsequent section.

4.2.1 Rotational Speed of Polishing Tool.

The rotational motion of the polishing tool is provided to produce the centrifugal forces on the abrasives during the surface finishing. Figure 12(a) shows the influence of the polishing tool's rotation speed on the % reduction in Ra while keeping the electrolyte's Vol. concentration, applied current, and finishing time constant at 45%, 2 Amp, and 30 min, respectively. The polishing tool's rotational speed is varied between 400 and 1400 rpm. Initially, it is observed that with increased rotational speed, the % reduction in the Ra rises to 800 rpm. Afterward, the surface quality starts to deteriorate. The maximum surface finish improvement (i.e., % reduction in Ra of 96.2%) is obtained at 800 rpm, as shown in Fig. 12(a). The CIP chains are bonded because of the applied external magnetic field; it is observed that increased rotational speed beyond optimum value leads to the dominance of centrifugal force applied to the CIP chains over their holding capacity. This phenomenon leads to the detachment of CIP chains beyond optimum rotational speed, which is responsible for the deterioration in the surface quality of the polished surface.

Fig. 12
Variation of percentage reduction in % reduction in Ra with (a) rotational speed of polishing tool and (b) electrolyte concentration (Vol. %) in MR fluid.
Fig. 12
Variation of percentage reduction in % reduction in Ra with (a) rotational speed of polishing tool and (b) electrolyte concentration (Vol. %) in MR fluid.
Close modal

4.2.2 Electrolyte Concentration (Vol. %).

Magnesium chloride (MgCl2) is combined with ethylene glycol (C6H6O2) in distilled water to form an electrolyte to upsurge the rate of reaction during the electrochemical process. The electrolyte serves as the transport medium for the MR fluid, facilitating electrochemical reactions in the proposed polishing method. The concurrent engagement of mechanical polishing and electrochemical processes contributes to the attainment of a consistent surface quality. Variation of % reduction in Ra with the polishing tool's rpm is illustrated in Fig. 12(a). The percentage concentration of electrolyte in MR fluid varies between 35% and 55% while keeping other process parameters, i.e., the rotational speed, applied current, and polishing time constant at 800 rpm, 2 Amp, and 30 min, respectively. An increase in the electrolyte's concentration initially leads to an increased % reduction in Ra. However, after a certain point, the % reduction in Ra decreases with higher electrolyte concentration. The reaction rate of Ti–6Al–4V with MgCl2 and C2H6O2 in distilled water is low when the electrolyte's concentration in MR fluid is below 45%. However, with a further increase in the percentage of electrolyte, the reaction rate becomes vigorous, producing pits on the polished surface. Figure 12(b) shows the variation of % reduction in Ra with a change in electrolyte concentration. The utmost % decrease in Ra (i.e., 97.2%) is attained at 45% electrolyte concentration. Furthermore, the vigorous reactions between the substrate and electrolyte produce a steep downward slope of % reduction in Ra between 45 and 55% electrolyte concentration.

4.2.3 Finishing Time.

The effect of the polishing duration on the percentage reduction in surface roughness (Ra) is illustrated in Fig. 13(a). During experiments, the finishing time is varied between 20 and 35 min, keeping tool's rotational speed, applied current, and electrolyte concentration fixed at 800 rpm, 2 Amp, and 45%, respectively. Initially, the % reduction in Ra rises with an increased finishing time. After 30 min of finishing time, the utmost % decrease in Ra is observed (i.e., 97.1%). However, with a further upsurge in the finishing time, the reduction in average surface roughness decreases because diamond particles generate scratches over the finished part, as illustrated in Fig. 13(a).

Fig. 13
Variation of % reduction in Ra with (a) finishing time and (b) applied current
Fig. 13
Variation of % reduction in Ra with (a) finishing time and (b) applied current
Close modal

4.2.4 Applied Current.

The current supplied amongst the substrate and finishing tool triggers the electrochemical reaction, and the reaction rate primarily depends on it during H-ECMR finishing. The analysis of the variation of current applied to the % reduction in Ra was studied (Fig. 13(b)) by varying it between 1 and 2.4 Amp while keeping rotational speed, electrolyte concentration, and finishing time constant at 800 rpm, 45%, and 30 min, respectively. The increased applied current leads to a rise in the % reduction in the average surface up to the optimum point (i.e., 2 Amp); beyond that, the % reduction in Ra reduces as the reaction rate increases between electrolyte and substrate, which causes the production of pits, as illustrated in Fig. 13(b).

4.3 Characterization of Workpiece Surface.

The proposed finishing method produces an unvarying polished surface over the Ti–6Al–4V. The initial Ra value of 326 nm decreases to 12.9 nm on the polished workpiece surface through optimum parameters, as discussed in the previous section. A mirror-like finished surface is produced on the biomaterial, as shown in Fig. 14(a). Furthermore, the 2D surface irregularities profiles comparison among pre- and post-finishing of the Ti alloy is shown in Fig. 14(a). It is noticed that initially, different scratch marks produced on the biomaterial surface are significantly removed after H-ECMR finishing. Furthermore, a comparison between the linear Ra profiles before and after polishing is illustrated in Fig. 14(b).

Fig. 14
Comparison between (a) area and (b) linear surface roughness profiles before and after finishing
Fig. 14
Comparison between (a) area and (b) linear surface roughness profiles before and after finishing
Close modal

SEM imagery, depicted in Fig. 15, is employed for the examination of the Ti alloy's surface topography in both pre- and post-polishing operations. Initially, pits and scratch marks are easily visible on the Ti–6Al–4V surface (Fig. 15). However, a smooth and uniform surface is produced on the biomaterial surface after finishing. Furthermore, the Atomic Force Microscopic (AFM) image (Fig. 15) also shows a substantial enhancement in the surface topography of the Ti alloy surface. The assessment of Ra exclusively provides information concerning the average depth of surface valleys and the elevation of crests relative to the average line on surface irregularities. Hence, a study of kurtosis (Rku) and skewness (Rsk) is also performed to get details regarding the surface characteristics of Ti alloy during H-ECMR finishing. Kurtosis is a statistical tool used to calculate the flatness of the peaks produced over the substrate surface; However, skewness analyzes the biasness of valleys and crests of the workpiece surface. An observed kurtosis value exceeding three, specifically Rku = 3.4, is evident in the pre-polishing state, indicative of acute peaks discerned on the surface irregularities, as illustrated in Fig. 15.

Fig. 15
Kurtosis versus skewness mapping, and workpiece images from atomic force microscope and scanning electron microscope before and after polishing
Fig. 15
Kurtosis versus skewness mapping, and workpiece images from atomic force microscope and scanning electron microscope before and after polishing
Close modal

Moreover, the positive skewness value (i.e., Rsk = 1.1) is observed on the initial workpiece surface, denoting that valley numbers are lower than peaks. Following H-ECMR finishing, the value of kurtosis undergoes reduction to below three (i.e., Rku = 1.7), indicative of the emergence of planar peaks on the polished surface. Correspondingly, the negative skewness (i.e., Rsk = −0.5) signifies a reduction in the number of peaks compared to valleys after the H-ECMR process on the Ti alloy surface. The observed skewness with a negative value and a value of kurtosis below three collectively suggest a minimized likelihood of surface irregularity wear during tribological contact, as the generation of flat peaks is less than that of valleys.

The efficiency of the H-ECMR and conventional Magnetorheological Finishing (MRF) is compared by performing finishing operations at identical process parameters conditions on the substrates having the same mapping area (i.e., 50 mm × 50 mm). The % reduction in Ra is measured continuously with an interval of 5 min, as illustrated in Fig. 16(a). A higher % reduction in Ra value of 96.4% is achieved using H-ECMR compared to a lower value of 49.6% using MRF for 30 min of polishing time. The synergic operation of the electrochemical reaction and the polishing reduces the surface finishing time for the H-ECMR process.

Fig. 16
Comparison of Avg. surface roughness between (a) H-ECMR and MRF at different finishing times and (b) experimental measurement and analytical model at different standoff distances
Fig. 16
Comparison of Avg. surface roughness between (a) H-ECMR and MRF at different finishing times and (b) experimental measurement and analytical model at different standoff distances
Close modal

The variable manipulation of the standoff distance between the tool and workpiece induces fluctuations in the magnetic force exerted on the abrasive particles throughout the finishing process. At a 0.5 mm standoff distance, the final average Ra is decreased to 37.6 nm from 317 nm; however, a further surge in the standoff distance (till 1 mm) causes a reduction in the final Ra, as shown in Fig. 16(b). Subsequently, as the distance between the tool and the workpiece increases, the value of Ra also increases proportionately. Active abrasive particles number rise with a surge in the standoff distance, which causes a decrease in the Ra value till 1 mm of standoff distance. However, a further surge in the distance between the polishing tool and Ti alloy causes a significant decrease in the magnetic force generated on the diamond particles during polishing, leading to a higher Ra value of the polished surface.

The developed analytical model, as discussed in Sec. 3, for determining average surface roughness is compared with the experimental results (Fig. 16(b)). It is analyzed that with an increase in the standoff distance, the difference (i.e., % error) between the results obtained from the analytical model and experimental data increases. Initially, for a range of standoff distance between 0.5 and 1 mm, the % error varies from 2.3 to 3.7. However, at a 2 mm standoff distance, the % error is increased to 9.0. As the standoff distance increases, the number of active abrasive particles increases, though the magnetic field applied from the electromagnet is still constant for holding the CIPs chain. Hence, there is a higher chance of detachment of a few CIP chains from the polishing media, which leads to a lower number of active abrasive particles calculated analytically than from the experiment.

4.4 Oxide Layer Thickness.

The oxide layer development on the biomaterial's surface is crucial as it increases its corrosion resistance. The Ti–6Al–4V reacts with the environmental oxygen and forms a passive oxide layer of TiO2. The pH value of the passive oxide layer is the same as the human body and helps to reduce corrosion during their interaction. However, the passive layer thickness on the biomaterial surface is nonuniform, with a layer thickness in the range of a few nanometers (i.e., 5–10 nm); hence, the number of corrosion potential sites on the Ti alloy surface is higher. The proposed polishing method uses an electrolytic solution consisting of MgCl2 and C2H6O2 in distilled water that reacts with the Ti–6Al–4V and forms its oxide on the polished surface, as shown in Eq. (6). The polished surfaces of the substrate are investigated through the SEM images pre- and post-polishing. Initially, a nonuniform and thinner oxide layer is easily visible on the Ti alloy's surface (Fig. 17). However, after finishing with H-ECMR, an unvarying and thick passive surface is formed on the Ti alloy. EDX, XPS, and SEM studies are performed to examine the compositions on the biomaterial surface. The weight % of O is increased from its initial value of 0.7–8.2 on the polished Ti alloy surface, as shown in Fig. 17. Furthermore, it is observed that a uniform atomic concentration of O is increased from 8 nm to 78 nm on the Ti–6Al–4V surface after finishing, as observed from the depth profiling of XPS and shown in Fig. 18. It is because the reaction of TiCl4 (produced from the chemical reaction of MgCl2 with Ti) with C2H6O2 leads to the formation of TiO2. The genesis of TiO2 manifests robust adhesion characteristics with the Ti alloy surface, exhibiting chemical inertness by refraining from reactivity with the presence of different ions inside the electrolyte. This results in the creation of an unvarying and substantial passive oxide layer on the surface of the Ti alloy.

Fig. 17
Comparison between oxide layers by EDX, SEM, and XPS studies on Ti–6Al–4V biomaterial surface pre- and post-polishing
Fig. 17
Comparison between oxide layers by EDX, SEM, and XPS studies on Ti–6Al–4V biomaterial surface pre- and post-polishing
Close modal
Fig. 18
Depth profiles of different constituents on Ti–6Al–4V surface (a) before and (b) after finishing using XPS analysis
Fig. 18
Depth profiles of different constituents on Ti–6Al–4V surface (a) before and (b) after finishing using XPS analysis
Close modal

High-resolution XPS spectra of Ti–6Al–4V detected its constituents like Ti, Al, V, and C. The Al is available on the biomaterial surface in the form Al2O3 before and after polishing (i.e., 5.3 and 5.8 on the unpolished and polished surface, respectively); hence, a significantly low amount of Al is detected in spectra. Furthermore, the content of V in the Ti–6Al–4V remains constant (i.e., 4.2 and 4.1 on the unpolished and polished surfaces, respectively) and is much tougher to fit the V 2p region for additional information. Hence, the Ti 2p is the primal focus of the high-resolution XPS spectra. A comparatively higher signal is detected for Ti 2p on substrates before and after polishing, as shown in Fig. 17. Furthermore, it is also observed that the intensity of O 1s increases on the polished surface compared to the unpolished surface. Ti3+ 2p1/2, Ti3+ 2p3/2, and Ti4+ 2p3/2 are the three subsets of the Ti 2p spectra having a binding energy of 462, 459, and 456 eV, respectively. However, electrochemical reactions occur between the substrate and workpiece, which increases the intensity of the Ti4+ 2p3/2 on the polished surface. Furthermore, the binding energy of the subset mentioned earlier is close to TiO, Ti2O3, and TiO2, respectively. The unoxidized Ti (i.e., 462.1) was detected on the unpolished surface of the substrate, as shown in Fig. 17. Though it is unattainable after finishing, indicating that a subset of Ti available on the polished surface is in the form of its oxide, as shown in Fig. 17. TiO2 is the main subset of the passive layer forms on the substrate surface, showing stable properties with an octahedral structure. O 1s spectral region illustrates that the intensity of TiO2 increases on the Ti alloy surface after finishing, as illustrated in Fig. 17.

5 Case Study on Surface Enhancement of Femoral Head

The femoral head of the hip implant undergoes H-ECMR finishing to optimize its surface quality and bolster biocompatibility attributes. CAM package Unigraphics NX 10 (Siemens Digital Industries Software) is used to develop different toolpath strategies on the femoral head's surface, as illustrated in Fig. 19. The different toolpath strategies included Zig-Zag with and without tool retraction and contour-parallel toolpath in the radial and axial direction. The proposed polishing operation is performed while mapping the femoral head surface with different toolpath strategies. The surface roughness is measured at various points on the surface of the femoral head to determine the uniformity achieved while mapping the femoral head's surface with different toolpath strategies, as illustrated in Fig. 20(e).

Fig. 19
Different toolpath strategies during surface mapping of femoral head
Fig. 19
Different toolpath strategies during surface mapping of femoral head
Close modal
Fig. 20
Comparison between surface quality, 1D Ra profiles (a), (c) before and (b), (d) after H-ECMR finishing with contour-parallel (i.e., radial) toolpath, and (e) bar chart showing comparison between surface roughness values for different toolpath strategies
Fig. 20
Comparison between surface quality, 1D Ra profiles (a), (c) before and (b), (d) after H-ECMR finishing with contour-parallel (i.e., radial) toolpath, and (e) bar chart showing comparison between surface roughness values for different toolpath strategies
Close modal

The 93.4% reduction in Ra has been achieved through the contour-parallel (i.e., radial) toolpath higher than the other toolpath strategies; the initial Ra value of 326 nm is reduced to 21.4, with the least variation in the surface quality on the polished. A comparison between the femoral head surface quality pre- and post-finishing is illustrated in Figs. 20(a) and 20(b), respectively. The 1D surface roughness profiles on the femoral head pre- and post-finishing are compared in Figs. 20(c) and 20(d), respectively. It is observed that the tool retraction during the proposed polishing method reduces the stability of the MR fluid. Due to that, the maximum variation (i.e., ±2.6%) in the Ra is achieved when the femoral head surface is mapped to the Zig-Zag toolpath with lift. The 82.3% reduction in Ra, where the final Ra value is reduced to 56.1 nm from 324 nm. The increase in the average surface roughness value on the femoral surface of the total hip arthroplasty compared with a flat workpiece is due to its roundness, which leads to varying the standoff distance between the polishing tool and the femoral head during finishing.

6 Conclusions

This work proposes to design and develop a novel H-ECMR finishing to enhance the surface quality of the Ti–6Al–4V biomaterial with reduced polishing time. The polishing method combines electrochemical reactions and mechanical abrasion to reduce the surface roughness value of the polished surfaces in the few nanometers range. The electrolyte as a carrier medium in MR fluid enables the electrochemical reaction by providing the potential difference across the polishing tool and Ti–6Al–4V. The developed process improves the passive oxide layer thickness and uniformity Ti–6Al–4V surface after polishing. Moreover, the developed experimental setup consists of an electromagnet-based polishing tool that provides in situ variations of the magnetic field during the polishing. The key findings from the present study are summarized as follows:

  • A 96.4% reduction in surface roughness (% ΔRa) is achieved during H-ECMR finishing, compared to 49.6% using the conventional Magnetorheological Finishing (MRF) for 30 min of polishing time. Instigating an electrochemical reaction along with mechanical abrasion increases the polishing efficiency.

  • The results achieved from the developed analytical model to predict the surface roughness agree with the experimental observations; however, with an increase in standoff distance, the percentage error between analytical and experimental results increases (i.e., 2.3–9.0%) due to the lower holding capacity of the CIP chains by the polishing tool at a higher standoff distance.

  • The impact of different parameters is analyzed on the % reduction of Ra; the values of the optimized process parameters, i.e., the polishing tool's rotational speed, electrolyte concentration (Vol.%), finishing time, and applied current, are 800 rpm, 45%, 30 min, and 2 Amp, respectively.

  • The mirror-like polished surface is achieved on the Ti–6Al–4V surface after the H-ECMR finishing. The initial value of average surface roughness (Ra) of 326 nm is reduced to its final value of 12.9 nm. Moreover, the value of skewness (Rsk) and kurtosis (Rku) achieved on the polished surface is −0.5 and 1.7, respectively, indicating that the low probabilities of wearing out of the biomaterial surface on their tribological interactivity as flat peaks with a diminished count relative to the valleys are generated on the biomaterial surface.

  • The electrochemical reaction between the Ti–6Al–4V, MgCl2, and C2H6O2 produces an unvarying and thick passive surface (i.e., TiO2) on Ti alloy. The layer thickness increases to 78 nm from 8 nm, enhancing the biomaterial's corrosion resistance.

  • A study is conducted on the Total Hip Arthroplasty's femoral head for enhancement in the surface roughness and biocompatibility through the developed H-ECMR finishing process. The Ra value is reduced to 21.4 nm from 326 nm using the contour-parallel radial toolpath strategy.

Drawing upon empirical observations in this investigation, it is inferred that the established H-ECMR finishing method has the potential to abbreviate surface finishing durations while concurrently augmenting the thickness of the oxide layer on the Ti–6Al–4V biomaterial surface, thereby further advancing its biocompatibility.

Acknowledgment

We acknowledge the Science & Engineering Research Board (SERB), New Delhi, India, for financial support for project No. EEQ/2017/000597 entitled “Fabrication of Prosthetic Implants and further Nanofinishing Using Magnetic Field Assisted Finishing (MFAF) Process,” and Department of Science and Technology (DST), New Delhi, India, for financial support for project No. SR/FST/ET-II/2017/111 (C) under Fund for Improvement of S&T Infrastructure in Universities and Higher Educational Institutions (FIST) Program. We also acknowledge Technology Innovation Hub (TIH), IIT Guwahati to provide experimental facilties.

Conflict of Interest

There are no conflicts of interest.

Data Availability Statement

No data, models, or code were generated or used for this paper.

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