# Investigation of Mechanical and Tribological Characteristics of Medical Grade Ti6al4v Titanium Alloy in Addition with Corrosion Study for Wire EDM Process

Authors: S. Prakash; C. S. Abdul Favas; I. Ameeth Basha; R. Venkatesh; M. Prabhahar; V. P. Durairaj; K. Gomathi; Haiter Lenin
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

Metals used in biomedical applications are frequently coated to prevent oxidation and metallic ion release, both of which can be harmful due to toxic effects. To prevent these adverse effects of metals, researchers have focused their efforts on developing various coating techniques, facilitating surface coating, or obtaining functional surfaces. (WEDM) is now considered a difficult method of obtaining functional surfaces for medical applications. The properties of the surface and subsurface layers obtained by the WEDM method are particularly interesting in this regard. The analysis utilised RSM-based computational technique to evaluate the WEDM characteristics (MRR SR) of Ti6Al4V Titanium Alloy in biomedical applications. The biggest drawback of the material in the biomedical industry, which includes orthopedic applications and dental implants, would be that it releases harmful atoms such as iron, chromium, and nickel into the bodily fluid environment. To combat the problems, a hydroxyapatite layer applied to the metal implant improves biocompatibility, osteocompatibility, and antimicrobial properties. By comparing the Modified Differential Evolution (MDE) approach to the basic differential evolution (DE) optimization strategy, the effectiveness of the MDE approach has been established. According to the cyclic polarized test, the Hap coated Titanium material had better corrosion resistance than the pure sample. The Hap coated titanium material has a higher zone of inhibition than the pure sample. The next step is to synthesis hydroxyapatite from cuttlebone, which is then electrodeposited onto titanium. FTIR, electrochemical tests, FESEM, and SEM were used to describe the coated sample, as well as an antibacterial test using E. Coli and B. Cereus bacteria. Because it is porous, the Hap coating helps bone tissue growth by preventing detrimental metal ions from escaping into the biological medium. The corrosion inhibition efficiency of the coated sample was performed in SBF (NaHCO3—0.35 g/L, MgCl2.6H2O—0.30 g/L, CaCl2·2H2O—0.37 g/L, K2HPO4. 3H2O—0.23 g/L, Na2SO4—0.071 g/L, NaCl—7.99 g/L, KCl—0.22 g/L, Tris’s buffer—6.063 g/L) solution using the potentiodynamic polarisation method and the solution pH was maintained.


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## Body

## 1. Introduction

In recent years, as engineering and medicine have advanced, so has the significance of biomaterials. Materials including metals, ceramics, polymers, and composites may be used to treat wounds, inflammations, tumors, gangrene, and implants. Metals used in biomedical applications are frequently coated to prevent oxidation and metallic ion release, both of which can be harmful due to toxic effects. To prevent these adverse effects of metals, researchers have focused their efforts on developing various coating techniques, facilitating surface coating, or obtaining functional surfaces [1]. Many researchers also concluded that the problem is due to the surface materials, so the wire EDM cutting is a machining technique that involves removing materials using a high-intensity, low-surface-roughness machining process focused on the workpiece. WEDM is now considered a difficult method of obtaining functional surfaces for medical applications that were studied in Tribology for Engineers [2]. The properties of the surface and subsurface layers obtained by the WEDM method are particularly interesting in this regard [3]. WEDM also has the potential to make biocompatible microdevices with surface roughness values that are appropriate for biological applications. WEDM forms a kerf wall by sparking the wire against the workpiece in the thickness or depth direction of the material. The material is then vaporised with the help of an assistant wire feed. Metal alloys based on titanium, stainless steel 316L, nickel-titanium, magnesium, and cobalt are commonly used as orthopedic implants [4]. This is due to their increased mechanical strength, wear resistance, and corrosion resistance. Among the above-mentioned materials, titanium is widely employed as a biomaterial for orthopedic and dental fields due to its high strength, excellent mechanical properties, superior corrosion resistance, and excellent biocompatibility.Surface roughness of bioceramic oxides and carbide phases of titanium and silicon materials on the powder mix is measured using a WEDM machine. Furthermore, acceptable corrosion resistance for dental implants has been obtained [5]. Titanium is a desirable material because of its excellent strength-to-weight ratio, which appeals to the aerospace and petrochemical industries for weight savings, high corrosion resistance, which makes it suitable for the aerospace, chemical, petrochemical, and architectural industries; and biological capabilities, which make it a material of interest in the medical industry [6].Within the development of superior engineering materials as well as the complexity of shapes demanded by the industry, nontraditional machining technologies have become necessary in the aerospace and healthcare industries [7]. As a result, nontraditional machining processes like ultrasonic machining are gaining traction. Electrical Discharge Machining, Abrasive Jet Machining, and Electrochemical Machining are all terminologies used to describe the machining process. Machining and other techniques have been developed to machine such difficult-to-machine materials [8]. In the machining process, the cost of manufacturing equipment is the most important determining factor in the machining of difficult-to-cut components. The wear behavior of different tools was compared in a study on the high-speed turning of Ti6Al4V, which led to a tooling expense [9]. Both multiobjective optimization Taguchi and response surface methodology approaches used in this analysis are well-suited for the nature of the problem, but their efficacy is dependent on the performance characteristics’ efficiency [10–12]. When titanium powder mixed optimization is used in a WEDM machine, a titanium layer is formed on the tungsten carbide work material. The layer’s hardness reached 750 HV, with fewer instances of surface cracking [13]. According to corrosion resistance, bioimplants serve a critical role in extending and improving human life quality. Degenerative and inflammatory illnesses, which induce immobility and terrible pain, are believed to affect 90% of adults over the age of 40 in modern society [14, 15]. Moreover, people met with accidents due to an increase in traffic congestion and suffered from bone fractures [16]. The surgery helps to reconstruct the bone structure of those who have suffered bone fractures. To overcome the above problem, researchers have made more effort to develop suitable biocompatible and bone healing materials like hydroxyapatite from natural and synthetic routes that bear a resemblance to that of natural bone [17–19]. Early research is needed to fully understand the bioactivity of WEDM combined with HAp powder, however, the research is still in their initial phases [20–22]. As a result, the study evaluates the Ti6Al4V surface modified HAp powder mixed and analysed using WEDM surface characteristics and biocompatibility.The main objective of this paper is to use Response Surface Methodology to predict process parameters in Wire Electric Discharge Machines (WEDMs) such asTon, Toff, and wire tension, as well as output characteristics such as high material removal rate and low-surface roughness. The next step is to synthesis hydroxyapatite from cuttlebone, which is then electrodeposited onto titanium. FTIR, XRD, electrochemical tests, FESEM, and EDAX were used to describe the coated sample, as well as an antibacterial test using E. Coli and B. Cereus bacteria. Because it is porous, the HAp coating helps bone tissue growth by preventing detrimental metal ions from escaping into the biological medium.


## 2. Experimental Methodology

The test run-orders were carried out with the help of a CNC WEDM Excetek (Model NP400L:X/Y axis–400 × 300 mm: U/V axis–80 × 80 mm: Z axis–220 mm) as shown in Figure 1. The next output parameter surface roughness was measured using the surfcom INEX linear coordinate measuring machine for the four sides of the cutting piece as shown in Figure 2.Figure 1
CNC excetek wirecut electric discharge machine.Figure 2
Surfcom INEX linear coordinate measuring machine.The influence of responseTon, Toff, and wire tension on titanium machining was investigated using the RSM-based Box Behnken technique. Three factors with three levels were used in the Box Behnken technique for modelling and optimising the titanium machining process. Table 1 illustrates the aspects and their levels, as well as the responses. Random investigational runs were conducted to begin lowering response variability. Table 2 shows the parameters and their levels; the modified differential evolution (MDE) optimization technique was introduced and then utilised to determine the most likely reaction circumstances for a particular set of coating technologies [12]. The projected model equation for machining is the Regression Equation in Uncoded Units.(1)MRR=mm3min=7.58−1.104PulseontimeTonμs−0.647PulseofftimeToffμs+0.00876WireTensionGms−0.0450PulseontimeTonμs∗PulseontimeTonμs+0.0559PulseofftimeToffμs∗PulseofftimeToffμs−0.000004WireTensionGms∗WireTensionGms−0.0255PulseontimeTonμs∗PulseofftimeToffμs+0.000815PulseontimeTonμsWireTensionGms−0.000183PulseofftimeToffμs∗WireTensionGms.Table 1
Factors and their levels.
SymbolUnitParametersCoding level−101AμsPulse on timeTon101215BμsPulse off timeToff81012CGmsWire tension100011501300Table 2
Parameters and their levels.
Run orderPulse on timeTon (μs)Pulse off timeToff (μs)Wire tension GmsMRR (mm3/min)Ra (μm)1151013002.6253.19212121213002.4153.5203310811502.9643.6554121011502.2853.4354512813002.643.56316121210002.3693.72277101211502.913.5318151010002.4253.6106915811502.9253.791510121011502.3693.883711101010002.53.503712101013002.3953.545613121011502.2853.60331412810002.3753.976815151013002.6253.1921


## 3. Result and Discussion

The analysis of variance for MRR for sample is shown in Table3, and the F value obtained for the prototypical is 4.36, indicating that the model is noteworthy. Only 0.01 percent of the time, the F value this huge could be due to noise. The model terms are only meaningful, according to the MRR, when the DF is 9. In this case, the WEDM process parameters with F values for linear with respect to are Ton (A) 8.58, Toff (B) 1.73, WT (D) 1.35 and their interaction effects, and these models with related to square are TON square (A) 15.22, TOFF square (B) 10.61, WT square (D) 1.60 which are recognised as significant model terms and have a significant effect on MRR. Those model terms with F values greater than 0.10 are not significant, as shown by the DF-value for Lack-of-Fit obtained of 11.65 and DF achieve of 0.08, which is much greater than the pure error of 0.004 and thus Lack-of-Fit in the current model appears to be non-significant, which is desirable, with a possibility of 76.92 percent. The Pareto chart is shown in Figure 3: the MRR, which depends on Ton is increasing with the next level of Toff time.Table 3
Analysis of variance-MRR.
SourceDFAdj SSAdj MSF-valueModel90.6822870.0758104.36Linear30.2298760.0766254.41Pulse on timeTon (μs)10.1491150.1491158.58Pulse off timeToff (μs)10.0301580.0301581.73Wire tension Gms10.0234070.0234071.35Square30.4730930.1576989.07Pulse on timeTon (μs)∗Pulse on time Ton (μs)10.2646330.26463315.22Pulse off timeToff (μs)∗Pulse off time Toff (μs)10.1844390.18443910.61Wire tension Gms∗Wire tension Gms10.0277870.0277871.602-Way interaction30.0979750.0326581.88Pulse on timeTon (μs)∗Pulse off time Toff (μs)10.0663000.0663003.81Pulse on timeTon (μs)∗Wire tension Gms10.0196850.0196851.13Pulse off timeToff (μs)∗Wire tension Gms10.0119900.0119900.69Error50.0869220.017384Lack-of-Fit30.0822180.02740611.65Pure error20.0047040.002352Total140.769210Figure 3
Pareto chart-MRR.The influence of response under surface roughness usingTon, Toff, and wire tension on titanium machining was investigated using the RSM-based Box Behnken technique [13]. Three influences with three levels were hired in the Box Behnken technique for modelling and optimising the titanium machining process. The table shows the factors and their levels, as well as the responses. Random investigational runs were conducted to begin lowering response changeability. The modified differential evolution (MDE) optimization technique was introduced and then utilised to determine the most likely reaction circumstances for a particular set of coating technologies. The projected model equation for Ra is the Regression Equation in Uncoded Units.Regression Equation in Uncoded Units(2)Rmμm=mm3min=−0.86−0.770PulseontimeTonμs−0.875PulseontimeToffμs+0.0083WireTensionGms−0.0178PulseontimeTonμs∗PulseontimeTonμs+0.0317PulseofftimeToffμs∗PulseofftimeToffμs−0.000003WireTensionGms∗WireTensionGms+0.0004PulseontimeTonμs∗PulseofftimeToffμs−0.000285PulseontimeTonμsWireTensionGms+0.000176PulseofftimeToffμs∗WireTensionGms.The analysis of the variance table for Ra for sample is shown in Table4, and the F-value obtained for the model is 1.28, representing that the model is noteworthy [14]. Only 0.03 percent of the time, the F value this huge could be due to noise. The model terms are only meaningful, according to the Ra, when the DF is 9. In this case, the WEDM process parameters with F values for linear with respect to are Ton (A) 2.47, Toff (B) 2.10, WT (D) 0.67 and their interaction effects, and these models with related to square are TON square (A) 1.37, TOFF square (B) 1.98, WT square (D) 0.64 are recognised as significant model terms and have a significant effect on Ra. Those model terms with F values greater than 0.19 are not significant, as shown by the DF-value for Lack-of-Fit obtained of 0.31 and DF achieve of 3, which is much greater than the pure error of 0.05 and thus Lack-of-Fit in the current model appears to be non-significant, which is desirable, with a possibility of 49.7 percent. The Pareto chart shown in Figure 4 the Ra which depends on Ton is decreasing with the next level of Toff time.Table 4
Analysis of variance-Ra.
SourceDFAdj SSAdj MSF-valueModel90.3471720.0385751.28Linear30.1842860.0614292.04Pulse on timeTon (μs)10.0744510.0744512.47Pulse off timeToff (μs)10.0631400.0631402.10Wire tension Gms10.0202520.0202520.67Square30.1289760.0429921.43Pulse on timeTon (μs)∗Pulse on time Ton (μs)10.0411720.0411721.37Pulse off timeToff (μs)∗Pulse off time Toff (μs)10.0594360.0594361.98Wire tension Gms∗Wire tension Gms10.0191140.0191140.642-Way interaction30.0578640.0192880.64Pulse on timeTon (μs)∗Pulse off time Toff (μs)10.0000130.0000130.00Pulse on timeTon (μs)∗Wire tension Gms10.0466880.0466881.55Pulse off timeToff (μs)∗Wire tension Gms10.0111620.0111620.37Error50.1504320.030086Lack-of-Fit30.0478370.0159460.31Pure error20.1025960.051298Total140.497604Figure 4
Pareto chart - Ra.Interaction plot effect of Pulse on Time (Ton) and Pulse off Time (Toff) on Material Removal Rate (MRR) of Sample are projected in Figure 5 where Ton and Toff are kept at their respective center values. It has been clearly visible from the figure that with the rise of Pulse on Time, large significant increase of MRR is noted whereas a slight increase of MRR is observed in with the rise of Pulse off Time [15]. The refractive index is unaffected by the reaction’s Ton, the intervals are interconnected. The figure depicts the effect of the Ton: Off and the with respect to MRR on the refractive index. The refractive index decreases as the when Toff rises from 2.50 to 3. The plot depicts the effect of Ton is on the material removal rate, which also increased. The goal is to determine the best operating conditions for the improving machining parameters with the ratio and the conditioning under the Ton and Toff, respectively.Figure 5
Interaction plot effect MRR.Interaction plot outcome of Pulse on Time (Ton) and Pulse off Time (Toff) on surface roughness (Ra) of Sample are projected in Figure 6. Where Ton and Toff are kept at their respective center values. It has been clearly visible from the figure that with the decrease of Pulse on Time, a large significant decrease of Ra is noted, whereas a slight reduced the value of Ra is observed in the figure with the increase of Pulse off Time [16]. The refractive index is unaffected by the reaction’s Ton; the intervals are interconnected. The figure depicts the effect of the Ton and Toff with respect to Ra on the refractive index. The refractive index decreases as the when Toff rises from 8 to 12. The plot depicts the effect of Ton which decreases the surface roughness also decreased. The goal is to determine the best operating conditions for the reducing surface roughness parameter with a ratio and the conditioning under the Ton and Toff , respectively.Figure 6
Interaction plot effect MRR.Three-dimensional response surface graphs are shown in Figure7. To determine the interaction between these factors, the simulation equation was used to obtain the above response surface plots. The response on the z-axis (MRR) was plotted against two independent variables, Pulse off time and Wire Tension, (Toff & wire tension), while keeping all other variables at their center level. The above factors that directly influenced indicated that there were significant interactions between the predictor factors. The surface roughness is increased as the Ton and Toff are increased up to the middle level [23]. The conversion rate was observed to decrease beyond the middle level of each variable. This might be due to an unfavourable secondary effect. The finest circumstances stayed initiatives to be mid-level of Ton concentration, a wire feed ratio of 6 : 1, a time of 60 seconds. At optimal conditions, the maximum MRR was estimated to be 93 percent. The machining R-Squared value and the Adj R-Squared value were 0.98 and 0.97, respectively.Figure 7
Three-dimensional response surface graphs-MRR.To determine the interaction between these factors, the simulation equation was used to generate the response surface plots below. The response on thez-axis (Ra) was plotted against two independent variables, Pulse off time and Wire Tension, (Toff & wire tension). Better surface smoothness is achieved as the value of Ra falls, whereas as the value of Ra increases, the surface finish of the given surface deteriorates. As shown in Figure 8, whatever the scenario, the rise in surface roughness (Ra) is proportional to the rise in Pulse on Time. As a result, the effect of pulse on time on surface roughness can be deduced to be independent of wire tensions. Surface quality deteriorates as the Pulse on Time increases, whereas surface quality improves as the Pulse on Time decreases. The controlling inspiration of Pulse off Time on Surface Roughness is similar to that of Pulse on Time. Surface Roughness (Ra) values decrease with increasing Pulse off Time in all circumstances. Figure 8 represents three-dimensional response surface graphs.Figure 8
Three-dimensional response surface graphs-Ra.Figure9 depicts the multiobjective prediction, the purpose of this multiobjective prediction is to decrease Ra, whereas the target for MRR is to increase MRR. To produce the desired output, the lower, target, and upper bounds of the linear desirability function have been given equal priority. For the linear desirability function, the frequency is considered to be 1(d). The best output responses were found to be Surface Roughness (Ra) of at least 2.660 m–3.990 m and Material Removal Rate (MRR) of at least 2.464 to 3.475 mm/min. These optimum output responses were obtained using the ideal input parameters of Pulse on Time (TON) at 9.8 s, Pulse off Time (TOFF) at 12.0 s, and Wire Tension (WT) at 7.4646kgf. The composite desirability factor (D) for MRR and Ra is 0.192 for RA and 0.146 for MRR with 95 percent PI and 95 percent CI and a cross fit of SE 7.Figure 9
Multiobjective prediction MRR/Ra.


## 4. Corrosion and Surface Morphological Behaviour

The counter electrode, reference electrode, and working electrode, which are platinum electrode, SCE, and Titanium, respectively, make up the electrochemical workstation. The electrolytic bath includes an aqueous solution of 1M Ca(NO3)2 and 0.6 M (NH4)2HPO4, which was thoroughly agitated for 3 hours at room temperature using a magnetic stirrer. For 1 hour, the coating process was carried out galvanostatically with varying current densities. After that, the sample was taken out of the procedure and rinsed with double distilled water before being dried for a day [17].FESEM was utilised to describe the surface property of synthesised HAp and coated titanium, and an energy dispersive analysis investigation was conducted to quantify the percentage content of the element in the synthesised HAp and coated sample.FTIR Spectroscopy is used to analyse the various functional groups present in the produced HAp following calcination at 9000C, as illustrated in Figure10. The existence of the PO43- group in the HAp structure is confirmed by the typical absorption peak at 470, 567, 601, 966, 1018, and 1110 cm−1 [11]. Peaks at 3570 and 632 were linked to the hydroxyl group’s typical peaks of stretching and bending vibration modes. Peaks were found at 1018 cm1 and 1110 cm1, respectively, which correspond to the irregular trembling mode (3) of the phosphate group and the symmetric extending trembling mode (1) of the phosphate collection. The presence of the phosphate group is shown by a tiny peak at 470. The corrosion inhibition efficiency of the coated sample was tested using the potentiodynamic polarisation method in SBF (NaHCO3–0.35 g/L, MgCl2.6H2O–0.30 g/L, CaCl2H2O–0.37 g/L, K2HPO4. 3H2O–0.23 g/L, Na2SO4–0.071 g/L, NaCl–7.99 g. A three-electrode assembly arrangement was used with a CHI 760 electrochemical workstation to carry out this procedure. For this analysis, the Pt electrode was employed as a counter electrode, titanium as an employed electrode, and SCE as a reference electrode.Figure 10
FTIR spectroscopy-coated titanium.


## 5. SEM Analysis

The surface morphology of the HAp coated titanium is seen in Figure11. It demonstrates that the coated titanium acquired a homogeneous spherical-like and porous shape. The porous network structure of the covering primarily aids tissue growth after implantation while also limiting bacterial growth and extending the implant’s lifespan. Ca, O, P, Fe, Cr, Ni, and Mn were all present in the spectrum. As a result, the presence of HAp coating on titanium is confirmed.Figure 11
SEM micrographs of HAp coated titanium.


## 6. Conclusion

The specified input parameters (Ton, Toff, and Wire tension) have a considerable impact on the WEDM process’ performance and the HAp coated titanium material had better corrosion resistance than the pure sample are investigated. (WEDM) is considered a difficult method of obtaining functional surfaces for medical applications. The properties of the surface and subsurface layers obtained by the WEDM method are concluded.The following conclusions have been drawn based on the experimental findings:(i)
The model terms are only meaningful, according to the MRR with respect to areTon (A) 8.58, Toff (B) 1.73, WT (D) 1.35 and their interaction effects, and these models with related to square are Ton square (A) 15.22, Toff square (B) 10.61, WT square (D) 1.60 are recognised as significant model terms and have a significant effect on MRR.(ii)
The analysis of variance for Ra with respect to areTon (A) 2.47, Toff (B) 2.10, WT (D) 0.67 and their interaction effects, and these models with related to square are TON square (A) 1.37, TOFF square (B) 1.98, WT square (D) 0.64 are recognised.(iii)
The intention of this multiobjective prediction is to reduce Ra, whereas the target for MRR is to enhance MRR. The lower, target, and upper bounds of the linear desirability function have been given equal attention to create the desired output. The frequency is assumed to be one for the linear desirability function (d). Surface Roughness (Ra) of at least 2.660 m–3.990 m and Material Removal Rate (MRR) of at least 2.464 to 3.475 mm/min were shown to be the optimum output responses.(iv)
The functional group present in the coated sample was also determined to be pure without any impurities in the FTIR examination of HAp coated titanium. The Sem images of the coated sample support the morphology with high porosity, which may help the tissue’s cell proliferation. According to the cyclic polarisation test, the HAp coated titanium had better corrosion resistance than the pristine sample. The HAp coated titanium sample had a higher zone of inhibition than the pure sample.



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*Source: 5133610-2022-09-13.xml*