R.Kml Jyrj,S.Mlrvizhi,V.Blsurmnin,*

aCentre for Materials Joining and Research(CEMAJOR),Department of Manufacturing Engineering,Annamalai University,Annamalai Nagar,608 002,Tamil Nadu,India

bCentre for Materials Joining and Research(CEMAJOR),Department of Manufacturing Engineering,Annamalai University,Annamalai Nagar,608 002,Tamil Nadu,India

Optimizing the micro-arc oxidation(MAO)parameters to attain coatings with minimum porosity and maximum hardness on the friction stir welded AA6061 aluminium alloy welds

R.Kamal Jayaraja,S.Malarvizhib,V.Balasubramanianb,*

aCentre for Materials Joining and Research(CEMAJOR),Department of Manufacturing Engineering,Annamalai University,Annamalai Nagar,608 002,Tamil Nadu,India

bCentre for Materials Joining and Research(CEMAJOR),Department of Manufacturing Engineering,Annamalai University,Annamalai Nagar,608 002,Tamil Nadu,India

A R T I C L E I N F O

Article history:

Received 15 March 2017

Received in revised form

17 March 2017

Accepted 21 March 2017

Available online 23 March 2017

Micro-arc oxidation

Micro-arc oxidation(MAO)technique is capable of producing dense oxide films on the aluminium alloy surface.This oxide film protects the aluminium alloy from the corrosion attack for longer duration. Empirical relationships were derived to evaluate the MAO coating properties(porosity and hardness)by incorporating very important MAO parameters(current density,inter-electrode distance and oxidation time).MAO parameters were also optimized to achieve coatings with minimum porosity and maximum hardness.Further,the effect of MAO parameters on coating characteristics was analysed.From the results,it is found that the current density has greater in fluence on the responses than the other two parameters.

?2017 Published by Elsevier Ltd.This is an open access article under the CC BY-NC-ND license(http:// creativecommons.org/licenses/by-nc-nd/4.0/).

1.Introduction

Aluminium and its alloys are extensively joined by the friction stir welding(FSW)process due to the many advantages[1].The major problems like porosity,hot crack,etc.,occurred in fusion welding techniques are completely eliminated by the FSW process due to low heat input and solid state joining of aluminium alloys [2].The FSW joint consists of four different zones,namely unaffected parent metal,heat affected zone(HAZ),thermomechanically affected zone(TMAZ)and stir zone(SZ).Of these four regions,the stir zone consists of finer grains compared to the other zones[3,4].

Number of corrosion studies were carried out on aluminium alloys and its welds[5-8].The effects of chloride ion concentration and pH level of environment on pitting corrosion of aluminium alloys were studied using potentiodynamic anodic polarization test [9].It is reported by many researchers that the corrosion behaviour of aluminium alloy was dependent on chloride concentration and pH level of the solution.It is also reported that AA6061 aluminium alloy corroded caused by chemical dissolution of alkaline solutions by the presence of OH-and at a comparatively low amount in acidic solution.Few studies were carried out to compare the corrosion behaviour of the weld metal with parent metal.Farhad et al. investigated the corrosion behaviour of AA6061 aluminium alloy joint produced by the FSW process[10].In this the SZ of the FSW joint experienced more corrosion attacks than the parent metal. Also,few researchers reported that the corrosion behaviour of the FSW joint depends on the FSW parameters used to weld the joint [11,12].It is essential to enhance the corrosion resistance of FSW weldment to avoid corrosion failure of the joint.

Many surface treatment techniques are used to modify the surface of the aluminium alloys[13,14].Thermal spraying methods such as high velocity oxy-fuel spraying,plasma spraying,detonation spraying are extensively used in now-a-days,but these coatings are capable to enhance the corrosion resistance to some extent due to the mechanical bonding of the coating with substrate. However,micro-arc oxidation(MAO)technique provides a better bonding(metallurgical)between the component and coating.MAO coating improves the corrosion and wear resistance of the aluminium alloys compared to many other coatings[15].In thisinvestigation,an attempt has been made to identify the optimum MAO parameters to attain protective coatings with minimum porosity and maximum hardness on the stir zone of FSW joints of AA6061 aluminium alloy.

1.1.Experimental work

The parent metal used in this study was AA6061 aluminium alloy with a thickness of 6 mm.The alloy composed of 1%Mg,0.6% Si,0.2%Cr,0.25%Cu and remaining Al in weight percentage.The joints were fabricated with the taper cylindrical tool using optimised welding parameters.The dimensions of tool and optimised FSW parameters are listed in Table 1.

The major in fluencing MAO parameters were identi fied and selected from the literatures[16-18].Among the several independently controllable primary and secondary MAO parameters, which affects the coating property of the aluminium alloy,the parameters considered in this investigation are current density(I), inter-electrode distance(D)and oxidation time(T).

Many preliminary trial experiments were conducted using the specimen extracted from SZ of FSW joints(50 mm×20 mm)to identify the working limits of MAO parameters.Various combinations of MAO parameters were used during the trial experiments and the following inferences were made:

(i)If the current density was＜0.04 A/cm2,the micro-arc on the surface of the aluminium alloy did not formed due to insuf ficient of current.

(ii)If the current density was＞0.20 A/cm2,the surface of the coated material was composed of loose and pores oxide film due to excess current applied.

(iii)There is a constraint in the electrolytic bath so the minimum and maximum inter-electrode distance was kept 4 and 8 cm respectively.

(iv)If the oxidation time was＜10 min,the coating was formed with a very lower hardness.

(v)If the oxidation time was＞50 min,the micro-arc was extinguished.

Specially designed electrical controller MAO coating unit of 30 kVAwas used in the present study(Fig.1(a)).An alkali solution with a mixture of potassium hydroxide and sodium meta silicate was employed as electrolyte in this present work.The temperature of the electrolyte is maintained at 20-30°C during the process with a help of the chiller.The electrolyte gets cooled in chiller and it circulated throughout the process.Substrate(specimen)was connected to an AC power source(high-voltage)with a constant current density all over the coating process.The substrate immersed fully in the electrolyte bath in a non-conductive container (Fig.1(b)).The range of factors considered is given in Table 2 and the 20 sets of experimental conditions employed are given in Table 3.The photographs of uncoated and coated specimens are displayed in Fig.1(c-d)respectively.The morphology(top surface and cross-section)of the coatings were analysed using secondary electron microscopy(JEOL-JSM-5610LV)and then the percentage of porosity was measured with an image analysis software.X-Ray diffraction(XRD)analysis was carried out using an X-ray diffractometer(special attachment no.HTK 1200,Anton Paar,Austria, Europe)of XPert,to find out the phases present in the MAO coatings.

1.2.Developing empirical relationships

The coating porosity and coating hardness(Y)are the functions of MAO parameters such as current density(I),inter-electrode distance(D)and oxidation time(T),and it is given by following expression.

Second-order polynomial(regression)equation used to represent the response surface Y is given by

The response(Y)can be expressed in the form of MAO parameters as given below

Where,the average response is denoted by b0and b1,b2,b3,are linear responding terms;b12,b13,b23are the response of interaction terms;b11,b22,b33are the responses of squared terms.Analysis of Variance(ANOVA)test was conducted for both the responses and the results are shown in Tables 4 and 5.From the results,it is understood that the F-value for the developed porosity and hardness models is 324.82 and 86.80 respectively and suggest that the models are signi ficant.For these models,there is a 0.01%chance that a“Model F-Value”this large could occur due to noise.Values of "Prob＞F′′less than 0.0500 indicates the model terms are significant.In hardness I,T,DT,I2and T2are signi ficant model terms and in porosity I,D,T,ID,IT,DT,I2,C2and T2are the signi ficant model terms.The values of"Prob＞F′′greater than 0.1000 indicates that the model terms are not signi ficant.

For the porosity and hardness the“Lack of Fit F-value”of 3.39 and 3.20 respectively,indicates the“Lack of Fit”is not signi ficant relative to a pure error.There is a 10.34%and 11.38%chance that a“Lack of Fit F-value”this large could happen due to noise for porosity and hardness respectively.Non signi ficant lack of fit is good for both the model.For the porosity“Pred R-Squared”of 0.9772 is in reasonable conformity with the“Adj R-Squared”of 0.9935 and for the hardness the“Pred R-Squared”of 0.9207 is in reasonable conformity with the“Adj R-Squared”of 0.9760.“Adeq Precision”calculates the signal to noise ratio and the ratio larger than 4 is desirable.The ratio for porosity of 57.790 and hardness of 33.311 indicates an adequate signal.

The empirical relationships were developed using only with the signi ficant co-efficients and the constructed final empirical relationships were given below

Table 1FSW parameters and tool dimensions.

Fig.1.Experimental details.

Table 2Important MAO parameters and their levels.

To con firm the constructed empirical relationships,three tests were conducted with the MAO parameters selected randomly from the feasible working limits(Table 2).The actual response was calculated as the average of three measured results.From the validation results(Table 6),it is evident that the constructed empirical relationships were relatively accurate as the variation in prediction is±5%.

Table 3Design matrix and experimental results.

Table 4ANOVA test results for coating porosity.

Table 5ANOVA test results for coating hardness.

1.3.Relationship between coating porosity and coating hardness

The experimentally measured porosity and hardness values of the coatings(presented in Table 3)are plotted in a linear graph as shown in Fig.2.All the points,fitted and connected by a best fit and the equation for the best fit line is given by Coating Hardness（HV）=1489-58.68（Porosity，Vol.%）(6)

The slope of the best fit line(-58.68)is negative and it suggests that the porosity is having the opposite effect with hardness.The co-efficient of determination,R2 is found to be 92.6%of the above equation.The coefficient of determination,R2will provideinformation about the goodness of fit of the derived regression equation.The above derived equation can be utilized to determine the mean value of coating hardness for a given level of coating porosity.

Table 6Validation test results.

Fig.2.Relationship between porosity and hardness.

1.4.Optimizing MAO parameters

The response surface methodology(RSM)was employed to optimize the MAO parameters in this work.In the response plot the lowest point shows the minimum achievable responses(coating porosity)and the peak point shows the maximum achievable values(coating hardness).The aim of the optimization involves achieving maximum hardness with a minimum percentage of porosity.By analyzing the contour plots and response surface plots as shown in Figs.3 and 4,the minimum porosity recorded is found to be 1.76 vol%and the maximum achievable coating hardness is 1360 HV.The corresponding MAO parameters that yielded the minimum porosity with maximum hardness are current density of 0.11 A/cm2,inter-electrode distance of 6.33 cm and oxidation time of 26.61 min.

1.5.Characteristics of MAO coating

Fig.3.Response and contour graphs for porosity of MAO coat between current density and oxidation time.

Fig.4.Response and contour graphs for coating hardness of MAO coat between current density and oxidation time.

Top surface area and cross-sectional view of the MAO coated stir zone of FSW joint are characterised by scanning electronmicroscopy(SEM).MAO coat consists of porous coating with a typical discharge channels on the top surface(Fig.5(a-c))and the thickness were measured in the cross-section surface (Fig.5(d-f)).The corresponding porosity of the coating is shown in(Fig.5(g-i)).For the minimum condition,i.e.,lower porosity with higher hardness the coating thickness is to be 72μm and for the maximum condition,i.e.,higher percentage of porosity with lower hardness the coating thickness is to be 78μm.The MAO parameters are optimised to achieve lower porosity with higher hardness in this the coating thickness is 70μm.

Fig.6 illustrates the XRD analysis of the MAO coatings carried out in the optimised condition.In this some aluminium peaks was observed it may be an indication of substrate.A small amount ofα-Al2O3present on the top surface layers of the MAO coating and it mainly comprised ofγ-Al2O3phase[19].

Fig.5.SEM surface morphology of(a-c)top surface(d-f)cross-section and(g-i)porosity analysis for the minimum(a,d,g),maximum(b,e,h)and optimum conditions(c,f,i).

Fig.6.XRD patterns of MAO coatings deposited at optimised condition.

2.Conclusions

1)Empirical relationships were constructed to determine the porosity and hardness of MAO coatings deposited on the stirzone of FSW joints of AA6061 aluminium alloy at 95%con fidence level,incorporating important MAO parameters.

2)MAO parameters were optimized using response surface methodology to achieve coatings with minimum porosity and maximum hardness.The optimized values(1.76 vol%of porosity and 1360 HV of hardness)are closely matched with the experimentally determined values.

3)Among the three MAO parameters investigated,current density is found to be the predominant factor,followed by oxidation time and inter-electrode distance and this is con firmed by Analysis of Variance test(F-ratio).

Acknowledgements

The authors wish to record their heartfelt thanks to Council of Scienti fic and Industrial Research(G8/19901/2013),New Delhi for the financial support provided to conduct this investigation through sponsored project No.22(0615)/13/EMR-II dated 26.02.2013.

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*Corresponding author.

E-mail addresses:jayaraj_kamal@yahoo.co.in(R.Kamal Jayaraj),jeejoo@ rediffmail.com(S.Malarvizhi),visvabalu@yahoo.com(V.Balasubramanian).

Peer review under responsibility of China Ordnance Society

Aluminium alloy

Friction stir weld

Response surface methodology