The Effects of Anodization Treatment on the Microstructureand Fatigue Behavior of 7075-T73 Aluminum Alloy

Teng-Shih Shih+, Tin-Hou Lee and Ying-Jhe Jhou

Department of Mechanical Engineering, National Central University, Jhongli City, Taoyuan County 32001, Taiwan

The fatigue behaviors of 7075-T73 alloy with/without anodization/sealing treatment were investigated in this study. The microstructure ofdifferent samples was analyzed by electron microscopy. As the experiments, the microstructure were observed from the anodized/sealed samplesto indicate that precipitates trapped at subsurface near film/metal (f/m) interface would partly dissolved and the deformed matrix existed highfractions of high angle grain boundaries. The bare 7075-T73 alloy samples achieved fatigue strength of 225MPa and the anodized/sealedsamples enhanced their fatigue strength at 107 life cycles with 10­20µm film thickness. The fractured anodized/sealed sample showed finestriation spacing than bare sample after subjected to low stress amplitude. [doi:10.2320/matertrans.M2014121]

(Received April 9, 2014; Accepted May 26, 2014; Published July 11, 2014)

Keywords: 7075 aluminum alloy, fatigue life, anodization treatment, microstructure

1. Introduction

AA 7075 aluminum alloys contain high amounts of Zn,Mg and Cu. After T6 and/or T73 heat treatments, the alloywill develop excellent tensile strength along with properelongation. There are complex intermetallic compoundswithin the matrix of the 7075 alloy. The precipitates and/orintermetallic compounds acquire a specific morphology dueto the application of the heat treatment and the manufacturingprocess. These intermetallic compounds and the fineprecipitates existing in the aluminum matrix greatly influencethe anodizing performance and mechanical properties ofAA7075 aluminum alloys.1­4)

The coating of aluminum alloys with anodic aluminumoxide (AAO) films can protect the alloy from corrosion inseverely corrosive environments. However, localized dis-solution tends to occur around intermetallic particles andprecipitates during anodization. The oxidation rate is fasterwith large intermetallic particles such as the Mg2Si, Al2Cu,Al7Cr, ¢-AlMg and AlZnMg than in the aluminum matrix,meaning that the particles could dissolve into the electrolyteduring the anodizing treatment.5,6) On the other hand,immobile elements such as silicon will form siliconcontaining particles that will be trapped in the AAO film.The effects of intermetallic particles on the fatigue life cyclesof high strength 7075 alloys are of concern because thismaterial is commonly used in industrial applications.

Precipitates and intermetallic compound particles tend tobe trapped in AAO films during the anodization of thealuminum alloy. Pitting that preferentially occurs at theinterface between the silicon/iron particles and reduces thecorrosion resistance of the films. Effect of anodization on thefatigue life of 7050-T7451 alloys has been studied anddiscussed by Sharzad et al.7) They found that the inclusionparticles trapped in the AAO film had a significant effect onthe formation of corrosion pits, which accelerated fatiguecrack initiation. As a result, the fatigue life of the aluminumalloy was reduced. Fratila-Apachitei et al.8) utilized TEM toobserve the particles trapped in anodized alumina oxides on

an Al­Si alloy. The alumina film was encroached upon bysilicon particles in association with a non-uniform porosity.In addition, gas-filled voids above the silicon particles werealso observed to influence the morphology of the envelopingporous AAO film, pore termination above the particle andpore branching and deflection around and beneath theparticles. Mukhopadhyay et al. used electron probe micro-analysis (EPMA) to examine the constitution of intermetallicparticles in the matrix. They found particles containing aniron rich phase that could be trapped in hard anodic coatingfilms to locally inhibit the formation of anodic oxide films.9)

The behavior and mechanisms of sealing treatment, whichaffect the quality of AAO film have been examined in manystudies.10­14) The basic reaction in the sealing process is ahydrothermal reaction which turns amorphous alumina intocrystalline boehmite, AlO(OH). The four steps of the sealingprocess have been described by Lopez et al.,15) starting withthe filling of the pores by the sealing solution, plugging thepore mouths, the formation of acicular pseudoboehmitecrystals and a compact intermediate layer on the surface andfinally the growth of hydrated alumina crystals during thesealing process. The entrapped intermetallic compoundparticles and/or precipitates in the AAO films are affectedby the sealing process, which alters the quality of theresultant AAO films.

In this study, we examine the relationship of the number ofcycles to failure versus stress amplitude for 7075-T73 sampleswith/without anodization treatment. After sealing of thecoated AAO film, there was improvement in the corrosionresistance of the 7075-T73 alloy.16) However, there has beenno published study discussing the effects of the matrixstructure on the relation between the number of cycles tofailure versus the stress amplitude of an anodized 7075-T73alloy. We provide experimental results along with micro-structure observations to explain the effects of microstructureon changing fatigue behavior on a given stress amplitude.

2. Experimental Procedure

2.1 MaterialsThe chemical composition of the material used in this+Corresponding author, E-mail: t330001@cc.ncu.edu.tw

Materials Transactions, Vol. 55, No. 8 (2014) pp. 1280 to 1285©2014 The Japan Institute of Metals and Materials

study, 7075-T73 extruded bars (15mm in diameter and150mm in length) is listed in Table 1. After annealing at415°C for 120mm, all samples were removed for the purposeof carrying out the T73 treatment which involved solutiontreatment at 475°C for 90min, followed by rapid quenchingin water and finally two-step age hardening at 110 and 175°Cfor 480min.

2.2 Specimen preparationThe bar-shaped samples were machined to achieve the

tensile test with a gage diameter of 6mm, according to theASTM B557 specifications,17) and the rotating-bendingfatigue test was carried out following the JIS Z2274specifications,18) with a gage diameter of 8mm.

After the tensile test, the 7075-T73 alloy samples received591(9.8)MPa (ultimate tensile strength, UTS), 564(12)MPa(yield strength, YS) and the elongation1 is 13.4%. TheVicker’s hardness was 180 (2.3) and 177 (1.9) Hv for the baresample and anodized/sealed sample from an average of atleast ten tests. Deviations are listed in the parentheses. For theanodized/sealed samples the tested UTS is 590 (2.6)MPa,YS is 560 (6.2)MPa and the elongation which was 11.5 (0.4)% was calculated from the average of five tests. A strain rateof 1.87 © 10¹5 s¹1 was used in the tensile test.

2.3 Surface treatmentBefore anodization, all samples were polished to have

surface roughness about Ra 5 0:1µm, followed by beingdipped into methanol and ultrasonic vibration. The specimenswere initially degreased by immersed in an alkaline solution(5mass% NaOH) at 60°C for 30 s and then water rinsing for1­2min. For the pickling process, specimens were sub-merged in an aqueous solution of HNO3 (30 vol%) for 90 s atroom temperature followed by 1­2min of rinsing in water.The anodization was conducted at 15mAcm¹2 at 15°C for900 s in a 15mass% sulfuric acid solution. Furthermore, theanodized samples were sealed in hot water at 95°C for1200 s. After sealing, the thickness of the AAO film wasmeasured by SEM inspection.

2.4 Fatigue testThe fatigue tests were carried out in air at a frequency of

40Hz. All tests were conducted under load-controlledconditions using an Ono’s rotary bending fatigue testingmachine with a maximum momentum of 100N·m. The testswere stopped if the specimen did not fail after 1.0 © 107

cycles.

2.5 Specimen inspectionAfter the rotating-bending test, the fractured samples were

removed and cut in a longitudinal direction for opticalmicrograph (OM) observation. The cut sample was first

polished with #2000 abrasive sandpaper followed by furtherpolishing using an alumina powder slurry 1 µm and thencolloidal silica (0.03 µm) prior to OM observation andintermetallic particle count measurement.

In preparation for TEM observation, the anodized/sealedsample was milled with a Versa 3D FEI Dual-Beam FocusIon Beam (FIB) device equipped with a Ga+ beam source. A5 © 5µm and 100 nm thick piece was cross-sectioned fromthe tested sample using the FIB technique. The TEM (JEOL-2000 FX II) operated at 160 kV was used to observe themicrostructure of the sample.

Different fractured samples were also observed by FieldEmission Scanning Electron Microscopy (FE-SEM) to gettheir specific fractured surface. Prior to analysis, pieces of thespecimens were cut longitudinally from the fractured samplesthen polished with #2000 abrasive sandpapers followed byelectron polishing using an electrolyte of 730mL ethanol,78mL perchloric acid, 100mL 2-butoxy ethanol and 90mLdeionized water at 27V/1.5A for 10 s. The samples wereexamined with a SEM (JEOL 6330) to reveal crystallo-graphic contrast. Electron back scatter diffraction (EBSD)pattern examination was carried out at 20 keV with a probecurrent of 0.1 µA and a working distance of 15mm. Thediffraction data were acquired as an orientation map fromraster scans of 600 © 300 µm with step sizes of 1 µm.

Samples were prepared for hydrogen testing by cuttingpieces about 0.2­0.9 grams from the bare and anodized/sealed samples at locations close to the subsurface and thecore. Before testing, the samples were dry-polished withabrasive sandpaper then cleaned with acetone. The hydrogencontent was detected by using a hydrogen analyzer (Horiba,EMGA-521). This analyzer used inert gas (Ar) fusion andimpulse furnace-column separation-thermal conductivitydetection (for more details about this procedure please referto reference).19)

3. Results and Discussion

3.1 Microstructure observationAccording to the tensile test results, the anodized/sealed

samples gained a relatively low elongation compared to thebare samples. The mechanical properties of different sampleswere affected by the alteration of the microstructure by theanodization process, which will be discussed in more detail inlater sections.

The coarse intermetallic compound particles apparentlylocated at the grain boundaries were aligned along thelongitudinal (extrusion) direction of the bar sample, as shownin Fig. 1(a). Some subgrains are visible and likely formeddue to thermal-activated recrystallization in the solutiontreatment, which was affected by soluble Mg and Zn atoms inthe Al matrix.20,21)

The intermetallic compound particles could amount to363 countsmm¹2 according to reference.16) The particles thatranged in size from 6­10 µm mostly contained Mg­Si or Mg­Zn, while those that ranged in size from 2­5 µm were mainlycomposed of Al­Si, Al­Cu or Al­Cu­Fe. Figure 1(b) showsthe shape and size of precipitates that are located in thematrix of the bare sample. Some precipitates were apparentlylocated at the grain boundaries. The needle-like precipitates

Table 1 Chemical compositions for the 7075 aluminum alloy.

Elements (mass%)

Material Si Fe Cu Mn Mg Cr Zn Ti Pb Al

7075 0.08 0.1 1.62 0.03 2.5 0.23 5.78 <0.02 <0.01 Bal.

The Effects of Anodization Treatment on the Microstructure and Fatigue Behavior of 7075-T73 Aluminum Alloy 1281

in the matrix are likely from the Al2CuMg (S phase) whileother precipitates have been reported to be comprised ofAl2Cu (ªA-phase) and Al2Mg3Zn3 (T-phase).22­24)

3.2 S-N curves of different samplesAfter the rotating-bending fatigue test, the relationships of

the number of cycles to failure versus stress amplitudes (S-Ncurves) for three batches of test samples are constructed. TheS-N curves shown in the Fig. 2 clearly display the differencesin the 7075-T73 samples with/without anodization treatmentin terms of the changes in high-cycle fatigue life (>105

cycles). Yhar25) applied the following equation to get a bestfit on S-N curve of aluminum alloy.

S ¼ ðE=4ffiffiffiffi

Np

Þ lnð100=ð100� AÞÞ þ B . . . : ð1ÞWhere E is elastic modulus, S is stress amplitude, N is

number of cycles to failure, and the parameters A and B areconstants that are selected to fit the equation to match thedata. The value for B, as this is the endurance limit, whichapplied for this study is 225MPa for bare sample and240MPa for anodized/sealed samples. The A value was thepercentage of reduction in area from tensile test; 34 and27% were obtained from testing bare and anodized/sealedsamples, respectively. The fitted experimental results areshown in Fig. 2. They are applicable to predict the fatiguelife cycles at different stress amplitudes lower than about90% of yield stress.

Experimental results show that the anodized/sealedsamples achieved lower fatigue strength at 104­105 life

cycles than the bare sample. This is mainly due to a factthat crack initiation occurred relatively easily in theanodized/sealed samples. Therefore, it is important toidentify the origin of fatigue cracking for the differentspecimens.

Figures 3 and 4 revealed the fractured surface morpholo-gies for the bare and anodized/sealed samples, respectively.Cracks tended to be initiated in both samples at multiplesites when they were subjected to high stress amplitudesof 560MPa, as shown in Figs. 3(a) and 4(a). Lowering thestress amplitude led to a remarkable decrease in the numberof crack initiation sites on the fractured bare samples, asshown in Fig. 3(b) for 390MPa and Fig. 3(c) for 260MPa.The anodized/sealed samples showed notable crack initiationat multiple sites when the stress amplitude decreased to 390­400MPa; Figs. 4(b) and 4(d). This was due to that afteranodization, the emergence of Si-containing particles and/orAl­Fe particles at the film/metal (f/m) interface. The Si-containing particles trapped at the f/m interface wereaccompanied by a concave interface nearby. Cirik et al. alsoexplained the particle-induced irregularity was caused by thepreferential dissolution in anodization process.4) The irregu-larity (or notch effect) at the f/m interface offered morepotent sites for crack initiation, which acted to reduce numberof cycles to failure when the stress amplitude decreased toabout 300­400MPa. Such notch effects also led todeterioration in the tensile properties to reduce elongationfrom 13.4% (bare sample) to 11.5% (anodized/sealed samplewith 10 µm film thickness).

Fig. 1 (a) Optical microscopic observations showing top views of themicrostructure of the 7075-T73 bare sample; (b) TEM photo indicatingthe precipitate morphologies.

Fig. 2 S-N curves for the 7075-T73 alloy bare sample and anodized/sealedwith 10 and 20µm thick AAO film samples.

Fig. 3 Fractographs for the 7075-T73 alloy samples tested and fractured at: (a) 560MPa; (b) 390MPa; and (c) 260MPa.

T.-S. Shih, T.-H. Lee and Y.-J. Jhou1282

Different bar samples were prepared for cross-sectionalobservation in the longitudinal direction with EBSD analysis,as shown in Figs. 5(a)­5(c). The fractions of differentmisorientation angle grain boundaries were counted andrecorded from observations on different fractured samplesthat tested at 240 and 390­400MPa stress amplitudes, asshown in Fig. 6. There was an increase in the fractions ofhigh angle grain boundaries (HAGBs) in the fatigue-fracturedanodized/sealed samples and an increase in the fractions oflow angle grain boundaries (LAGBs) in the fatigue-fracturedbare sample.

During a rotating-bending test, the bare sample wassubjected to a repeated shear stress to increase the storageof energy in the matrix and to form high-density disloca-tions,26) which acted to preferentially increase LAGBs withinthe matrix.27) The dislocations blocked the movement ofCu and Mg atoms at the sites of the dislocation cells. Theaddition of copper and magnesium to the aluminum alloycould significantly reduce the alloy’s stacking fault energy(SFE). For the alloys with low SFE such as Al­Cu, Al­Mg

and Al­Zn alloys, planar slips readily occurred duringdeformation.28,29) Dislocations were finally blocked or pinnedat the intersection (sessile jogs) and/or grain boundaries.Dynamic recrystallization would occur at the intersectionand/or grain boundaries.

The fatigue-fractured bare samples tested at a stress levelof 240MPa were prepared for TEM observation. Figures 7(a)and 7(b) shows the complex structure with tanglingdislocations and fine grains 100­200 nm in size, which wereapparently driven by dynamic recrystallization caused fromtests. The selected area diffraction pattern shown in Fig. 7(c)confirms the polycrystalline structure of the matrix inFig. 7(b).

The microstructure of the fatigue-fractured anodized/sealed sample was apparently affected by anodization,especially at the subsurface region. We observed at locationabout 10 µm beneath the film/metal interface. This locationwas also selected to be close to fracture surface and is shownin Fig. 8. During anodization the Al2CuMg and Al2Mg3Zn3

Fig. 4 Fractographs of 7075-T73 alloy anodized samples with 10µm thick AAO film tested and fractured at (a) 560MPa, (b) 400MPaand (c) 260MPa; anodized samples with 20 µm thick AAO film tested and fractured at (d) 400MPa and (e) 240MPa.

Fig. 5 EBSD photos for the: (a) undeformed bare sample; (b) bare samplefractured at 390MPa; and (c) anodized/sealed sample fractured at400MPa where the colored lines show the degree of misorientation angle:black (> 15°), gray (< 15°).

Fig. 6 Fractions of different misorientation angle grain boundaries for thebare and anodized/sealed samples fractured at different stress amplitudes;(240 and 390MPa for bare sample, 240 and 400MPa for anodized/sealedwith 10 µm thick AAO film samples).

The Effects of Anodization Treatment on the Microstructure and Fatigue Behavior of 7075-T73 Aluminum Alloy 1283

precipitates displayed anodic electrochemical behavior withrespect to the matrix.30) Coarse precipitates along the grainboundaries are partly dissolved (Figs. 9(a) and 9(b)) togetherwith low amounts of fine precipitates remained within grains;the needle-like precipitates notably disappeared. As aconsequence, early-crack propagation would be influencedby the altered microstructure on the anodized/sealed samplesespecially at low stress amplitudes.

Aluminum alloys are inherently taking some amounts ofhydrogen in the matrix. The presence of hydrogen also playsan important role in influencing the structure of 7075-T73alloys. Soluble hydrogen in the aluminum matrix could

diffuse into AAO films during anodization to furtherintroduce the crystallization within AAO film. Hydrogencontent was slightly decreased at matrix close to f/minterface.31,32) Hydrogen content test that prepared fromsubsurface area and from core portion of anodized specimensrespectively as listed in Table 2. The data indeed show lowerhydrogen content on sample taking from subsurface than thatfrom the core portion. Decreasing the hydrogen content inthe aluminum matrix allowed the cross-slip process tocontinue.33) Therefore, dislocations are readily to move andaccumulate at the grain boundaries leading to preferentiallyform HAGBs in the deformed matrix of anodized/sealedsamples.

In summary, when sample was subjected to high stressamplitude, high stress induced dense dislocations movingreadily along shear plans. They would interact to initiateearly-crack at film/metal interface due to increasing irregu-larity. Therefore, fatigue life was decreased comparing withthat of bare sample. There are two factors significantly affectthe fatigue behavior of tested sample at low stress amplitudes.First, the anodized/sealed samples got an increase in fractionof HAGBs due to decreasing hydrogen content in matrix.Second, the precipitates (partly) dissolved in matrix andcoarse precipitates notable reduced their size at grainboundary near f/m interface. These factors would affect thefatigue behavior of tested sample when they were subjectedto low stress amplitudes. Lower peak stress could develop atthe grain boundaries with higher tilt angles than with low tiltangles.34) As a result, the matrix at the subsurface areawas toughened to resist the early-crack propagation in theanodized/sealed sample leading to increase fatigue life cyclesat low stress amplitudes; the fatigue limit was raised from225 to 240MPa at 107 life cycles. In contrast, increasingAAO film thickness from 10 to 20 µm degraded the integrity

Fig. 7 TEM images of bare sample that fractured at 260MPa (1.3 © 106

cycles) showing; (a) tangling dislocations; (b) complex structurescontaining dislocations cells and fine grains with sizes of 100­200 nm;(c) selected area diffraction pattern showing the poly-crystalline structure.

Fig. 8 FIB sampling position relative to the AAO film and fatiguefractured surface.

Fig. 9 TEM images of the anodized/sealed sample that fractured at260MPa (8.8 © 104 cycles): (a) matrix near subsurface; (b) partlydissolved precipitates along grain boundaries.

Table 2 Hydrogen contents of fractured bare and anodized/sealed samples.

Sample location Hydrogen content, ppm

Bare sampleCore 0.243

subsurface 0.240

Anodized/sealed sampleCore 0.211

subsurface 0.156

T.-S. Shih, T.-H. Lee and Y.-J. Jhou1284

of f/m interface to raise the effect of surface notch due todissolution of precipitates. The fatigue strength was thereforedecreased from 240 to 230MPa as the thickness of AAO filmincreased.

4. Conclusion

After anodization and sealing treatment, the coated AAOfilm brought irregularity at the f/m interface leading toa reduction in elongation of 13.4% (bare sample) to 11.5%.When sample was subjected to high stress amplitude, highstress induced dense dislocations moving readily along shearplans. They would interact to initiate early-crack at film/metal interface due to increasing irregularity. The stressamplitude at 107 life cycles for the bare samples was225MPa and those for anodized/sealed samples were 240and 230MPa for the samples with AAO film thickness of 10and 20 µm respectively. The fatigue strength of anodized/sealed samples was improved by an increase in the fractionsof HAGBs and partly dissolution of precipitates at subsurfacearea near f/m interface.

Acknowledgement

We gratefully acknowledge the financial support from theNational Science Council of the Republic of China (102-2221-E-008-027). Many thanks also to National CentralUniversity for providing the TEM equipment (JEOL-2000FX II), and to National Sun-Yat Sen University for the EBSDanalysis.

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