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Influence of heat treatments and anodization on fatigue life of 2017A alloy
Engineering Failure Analysis 35 (2013) 554–561
Contents lists available at SciVerse ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Influence of heat treatments and anodization on fatigue life of 2017A alloy L. Hemmouche a,⇑, C. Fares a,b, M.A. Belouchrani a a b
Laboratoire Génie des Matériaux, Ecole Militaire Polytechnique, BP17C Bordj El Bahri, 16111 Alger, Algeria Département de Génie des Procédés, Faculté de Technologie, Université Hassiba Ben Bouali, BP151 Hai Essalam, 02000 Chlef, Algeria
a r t i c l e
i n f o
Article history: Available online 27 May 2013 Keywords: Heat treatments Aluminium alloy 2017A Fatigue Anodization
a b s t r a c t In order to investigate the coupled effects of heat treatments and anodizing processes on fatigue life of aluminium alloy 2017A, a series of fatigue tests were conducted at 25 Hz. The effect of different tempers, naturally and artificially ageing (T4 and T6) and overageing (T7) conditions before sulphuric anodization were studied. Additionally, information on the microstructure of the anodic films was acquired by Scanning Electron Microscopy (SEM) analyses. The image analysis yielded qualitative information on the evolution of the surface morphology as a function of the substrate microstructure. Hence measured mechanical properties were directly related to the corresponding microstructure, the result of fatigue tests showed a decrease in fatigue life of anodized specimens as compared to untreated ones. This phenomenon became more pronounced in the T6 and T7 states. The decrease in the fatigue life could mainly be attributed to the brittle nature of oxide layer and to the heterogeneous microstructure of the film. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The 2000 series aluminium alloys are frequently used for making a wide variety of structural components for aircraft where damage tolerance and relatively high strength are paramount. The main requirements for the components include high tensile strength with high fatigue properties and wear resistance, are achieved by heat treatments and resistant coatings. The strength of the alloys can be defined with heat treatments which develops fine strengthening precipitates [1]. Conversely, the coatings are applied to increase the tribological properties and corrosion resistance of the alloys [2]. Anodic oxidation, which is an electrochemical process to form stable oxide films on the surface of metals, is one of the most used methods to improve surface performance such as corrosion and wear resistance [2]. However, the anodic coating has a detrimental effect on the fatigue performance [3–11]. Since the oxide coating is brittle and hard compared to the aluminium substrate, it easily cracks under cyclic stress. The oxide layer adheres extremely well to the substrate; any cracks developed in the film act as stress raisers and can thus contribute initiation sites for fatigue failure. This contribution depend on the brittle nature of oxide layer [3–5], the irregularities beneath the coating [8], the substrate material and its pre-treatment, the film thickness [6–8], the presence of residual stress [5,9], the type of anodizing process [7,10] and on surface pre-treatment before anodizing [11]. The influence of the film thickness has been investigated extensively. However, there is little work that has investigated the other parameters influencing fatigue failure. In the previous work, the effect of the substrate microstructure on the growth of the anodic oxide layers by sulphuric anodization were evaluated for the both artificial ageing and annealing heat treatment of the 2017A alloy [12]. For the both
⇑ Corresponding author. Tel.: +213 661810413. E-mail address: [email protected] (L. Hemmouche). 1350-6307/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2013.05.003
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heat treatments, alloying elements are incorporated to the oxide layer as intermetallic compounds or oxides with copper enrichment at the interface for ageing state. The present work has been conducted in order to accomplish two different purposes. Firstly, to study the effect of the substrate heat treatment on the sulphuric anodization (SA), then secondly, to demonstrate the coupled effects of heat treatments and anodization on the fatigue behavior of 2017A alloy. The combined effect of the substrate microstructure and the anodization treatment on the fatigue behavior was specifically investigated by anodizing the substrate obtained from different heat treatments: annealing, natural and artificial age hardening and the overage hardening. The fatigue performance of the anodized specimens with different heat treatments has been investigated by conducting a series of plane bending fatigue tests at 25 Hz. Also, a fractographic analysis of some samples has been carried out in order to assess the role of the anodic layer on the nucleation of fatigue cracks. 2. Experimental details 2.1. Materials preparation The present investigation was carried out with samples of a 2017A-T4 aluminium alloy with the chemical composition given in Table 1. The material was supplied as bars of approximately 4 mm in thickness and 4 m in length, from which a number of tensile and fatigue specimens were machined following the German standards DIN 50125 and DIN 50142 respectively. Fig. 1 illustrates the sketches of both the tensile (Fig. 1a) and fatigue specimens (Fig. 1b). In order to induce different metallurgical states in the substrate, samples were submitted to four (04) heat treatments (annealed O, solution heat treated and naturally aged T4, solution heat treated and artificially aged T6 and T7 solution heat treated and then stabilized or overaged). The remaining samples were solution-heat treated in a programmable electric furnace type ‘‘ELTI’’ with argon atmosphere according to the sample identification schema presented in Table 2. 2.2. Anodizing and sealing Heat-treated aluminium alloys were dipped for 5 min at 65 °C in an alkaline bath (15 g/l of NaOH). They were then rinsed thoroughly in deionised water and neutralized in a sulphuric/chromic mixture (H2SO4:180 ml/l, CrO3:65 g/l) for 5 min at
Table 1 Chemical composition of 2017A-T4 aluminium alloy (wt%). Element
Si
Cu
Fe
Zn
Mg
Mn
Cr
Ti
Al
Wt%
0.840
3.890
0.478
0.061
0.806
0.783
0.040
0.077
Bal.
Fig. 1. Test specimen’s geometry: (a) tensile specimen, (b) fatigue specimen.
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Table 2 Heat treatments conditions. Heat treatment process
Heat treatment conditions
Annealed O Naturally age hardened T4 Artificially age hardened T6 Overaged T7
Heated Heated Heated Heated
to to to to
(500 ± 5) °C (500 ± 5) °C (500 ± 5) °C (500 ± 5) °C
then slowly cooled for 50 min, water quenched <40°C then naturally aged for 4 days for 50 min, water quenched then and artificially aged at (165 ± 5) °C for 10 h for 50 min, water quenched then artificially aged at (200 ± 5) °C for 10 h
Fig. 2. Different metallurgical states of aluminium 2017A alloy: (a) annealing (O), (b) natural ageing (T4), artificial ageing (T6) and Overageing (T7).
65 °C. Finally, the panels were rinsed and dried in a warm air stream. The anodization was carried out in a thermostatically controlled electrochemical cell (±2 °C) using a lead cathode and the heat-treated aluminium alloy as the anode. All specimens were individually anodized at 20 °C in a sulphuric bath (180 g/lH2SO4) and at a constant current of 1.5 A/dm2 for 30 mn. After anodizing, specimens were washed in distilled water and sealed in boiling water for 30 mn at 97 °C. Following the sealing; the specimens were dried in a cool air stream. 2.3. Mechanical testing The static mechanical properties of both anodized and no anodized samples were determined by means of tensile testing, employing a computer-controlled servohydraulic machine (Zwick/Roell). The tensile load was applied at constant cross-head travel of 5 mm/min. The performance of all samples under dynamic loading was evaluated out under plane bending conditions (R = 1), employing a Fatigue Dynamics SHENK-PWS equipment, at a frequency of 25 Hz. The fatigue tests carried in air were conducted at maximum stresses of 100 MPa. The microhardnesses (Vickers) were determined using a 100 g load. 2.4. Metallographic characterization The Metallographic specimens for the samples (see Table 2) were prepared by metallographic grinding (with 800, 1000, 1200, and 1500 grit size silicon carbide emery papers) and polishing (using high alumina powder) followed by metallographic etching by use of Keller’s reagent (950 ml water, 35 ml HNO3, 15 ml HCl, and 10 ml HF). Microstructural characterization involved use of an optical microscope linked with a computerized imaging system equipped with Cyberlink software.
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L. Hemmouche et al. / Engineering Failure Analysis 35 (2013) 554–561 Table 3 Evolution of the size of the grains according to the nature of the heat treatment. Metallurgical states
O
T4
T6
T7
Size of the grains (lm)
–
12.5 ± 0.6
10.5 ± 0.5
12.7 ± 0.7
Table 4 Mechanicals characteristics of 2017A alloy at different tempers.
Hardness (HV0,1) Re (MPa) Rm(MPa) A (%) E(MPa) Number of cycle to failure
O
T4
T6
T7
65 ± 2 86 ± 2 203 ± 4 11.82 74000 62133
145 ± 2 275 ± 5 412 ± 8.2 14.23 74000 170200
172 ± 3 400 ± 8 466 ± 9 6.16 74000 257467
144 ± 2 338 ± 7 414 ± 8.2 8.73 74000 183700
Fig. 3. SEM–EDS micrographs for the precipitates identification.
Table 5 Evolution of the film thickness for the different tempers. Metallurgical states
O
T4
T6
T7
Thickness (lm)
6.5 ± 1.1
9.3 ± 1.2
13.5 ± 1.3
9.1 ± 1.2
The Substrate microstructure for different samples and the surface fracture of some selected samples (anodized and unanodized), tested at lowest alternating stresses were analyzed by means of SEM techniques (FEI QUANTA 600 (LMS)). The observations were conducted at a potential range of 25–30 kV. 3. Results and discussion Prior to discussing the effect of heat treatment on the investigated material (2017A aluminium alloy), we first analyze microstructure (Fig. 2). Fig. 2 illustrates a typical cross-section of the substrate alloy after heat treatment (see Table 2) taken to the axis of the sheet, observed by means Optical Microscopy, indicating that the structure is constituted of elongated grains and seconds
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Fig. 4. SEM micrograph for the T4 anodized sample.
Fig. 5. Fatigue test results for anodized and unanodized samples for different tempers.
phase particles. These observations are similar for the heat treatments T4, T6 and T7 with a difference in the grain size and in the precipitates sizes and their dispersion. The average grain size shown in Table 3 is estimated by counting the number of grains intercepted by one or more straight lines ‘‘line intercept method’’. For the annealing state (Fig. 2a), the average grain size is not identified but the precipitates have large size. These large particles give rise to a heterogeneous distribution of dislocations, during deformation with local lattice at particle–matrix interface. So, they offer to the substrate low mechanical characteristics (Table 4). The effect of age hardening heat treatment (natural and artificial) on the microstructure of the 2017A aluminium alloy is shown, respectively, in the micrograph in Fig. 2b and d. It is clearly observed that for T6 state there are finer and brought closer precipitates than in the T4 state. The fine particles impede the movement of dislocations. In addition, owing to the pinning effect of particles on the movement of boundaries, close interparticle spacing makes it difficult for local lattice curvature regions to grow into grains. Consequently, the average grain size reaches the minimal value for the T6 treatment (Fig. 2d). The mechanical characteristics are inversely proportional to the grains size. So, this state (T6), offer high hardness, ultimate tensile strength and fatigue strength. The effect of overheating tempering on the solution-treated and quenched alloy is shown in the micrograph in Fig. 2c. The microstructure of sample reveals coarse particle phase which is non-coherent to matrix [13]. This microstructural feature indicates loss in strength; which is confirmed by a low mechanical characteristic (see Table 4). It is, therefore, quite logical to conclude that the artificial ageing heat treatment given to the alloy the optimum precipitation strengthening parameters.
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L. Hemmouche et al. / Engineering Failure Analysis 35 (2013) 554–561 Table 6 Mechanicals characteristics of Anodized 2017A alloys at different tempers.
Hardness (HV0,1) Re (MPa) Rm (MPa) A (%) E (MPa) Number of cycle to failure
O
T4
T6
T7
135 64 ± 2 181 ± 3 12.00 73,600 50,067
188 253 ± 5 390 ± 8 11.11 73,600 142,300
205 378 ± 7 443 ± 9 3.86 73,600 143,500
200 392 ± 8 316 ± 6 5.33 73,600 99,367
Fig. 6. Fatigue fracture surface for unanodized samples at different tempers: (a) annealing (O), (b) natural ageing (T4), (c) artificial ageing (T6) and (d) Overageing (T7).
The SEM and EDS analysis that were conducted on such sections showed the presence of different types of intermetallic particles as Al2Cu, Al2CuMg and Al–Cu–Mn–Fe–Si (Mg) particles (Fig. 3). The SEM micrographs carried out on cross sections of the anodized samples demonstrate that the thickness of the alloy layer was different for the different tempers (Table 5). This difference is due to the size and the spacing of the precipitates [12]. Example for the thickness measurement, for T4 state, is shown in Fig. 4. The results of the tensile cracking experiments of anodized and no anodized samples were conducted using the 2017A aluminium tensile specimens described in Section 2.3. For each tempers, two to three experiments were conducted. The results from all of the specimens for each temper can be seen in Fig. 5 and resumed in Tables 4 and 6. According to the studies, the influence of particle density and spacing and solute content on tensile fracture of aluminium alloys were revealed [14]. Particle size and mean interparticle spacing influence ductility. As the particle density increases, there is a decrease in duc-
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Fig. 7. Fatigue fracture surface for anodized samples at different tempers: (a) annealing (O), (b) natural ageing (T4), (c) artificial ageing (T6) and (d) Overageing (T7).
tility. At large inclusions and precipitates, for annealing state, the plastic deformation begins to form randomly. Identically as unanodized samples, the results of tensile cracking of anodized samples reveal a substantially higher fracture-strength level for T6 state. The tensile properties of the corresponding specimens are shown in Table 6. Compared to the other states, the T6 heat treatment enhanced the strength but severely degraded the ductility. The elastic properties of anodized specimen closely approach those of unanodized samples. The results from the plane bending fatigue experiments for the anodized and unanodized samples at different tempers are shown on the histogram in Fig. 5. Recall that each point on this histogram represents the average of a series of three to five fatigue experiments at a given alternating stress amplitude. It is evident that there is a relationship between the fatigue life, anodization and heat treatment. First of all, when comparing the treated and untreated samples by sulphuric anodization, the untreated specimens have a significantly longer fatigue life. For T6 and T7 tempers, fatigue strength of anodized specimen decreased by 54–55% at low loads of 100 MPa as compared with that of the unanodized one. After completing the fatigue experiments, scanning electron microscopy (SEM) was used to examine the failure surfaces of the fatigue specimens. The failure surfaces were inspected to see if any microstructural features could be correlated to the results shown in the fatigue measurements and the heat treatments. The failure surfaces of unanodized specimens run at loads below 100 MPa were characterized by single crack initiation points at the precipitates as Al2Cu and AlCuFeMgSi (Fig. 6). For annealing (O) and overageing T7state, the precipitates located coarse so, they present a favorable site of nucleation of the fatigue cracks. For the anodized specimens, we found multiple crack nucleation sites at the anodic film (Fig. 7). Accommodate to the substrate metal the anodic film is too brittle any crack that develops in it acts like stress raiser and propagates to substrate [3–5]. The anodic film formed on 2017A alloys is heterogeneous; there are local heterogeneities or defects in the oxide layer, which will affect the mechanical behavior [8,13].
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4. Conclusion The present study focused on the coupled effect of substrate microstructure and anodizing on the mechanical properties. The microstructure was modified by means of an annealing (O), a natural ageing (T4), an artificial ageing (T6) and an over ageing (T7) heat treatments. These treatments show differences in the particles size, the interparticle spacing and in the grain size. So, they reveal different mechanical properties for both anodized and unanodized samples. For anodized sample, the elastic properties of anodized specimen closely approach those of unanodized samples, but the ductility decrease strongly for the anodized samples especially for T6 and T7 tempers. The results of hardness attributed to the brittleness of the oxide films, which readily crack when deformed. The effect of heat treatment and anodizing on fatigue properties of 2017A aluminium alloy was also investigated. It was found that anodic oxide layers decrease the fatigue strength of aluminium alloys. The decrease of fatigue life was principally caused by the brittleness of the oxide layer but also, to the heterogeneities, as intermetallic compounds and element alloys oxide, which act as stress concentrator and promoted the nucleation and the propagation of cracks. The fatigue tests indicate that the oxide layers developed on samples heat treated at T6 and T7 have weak fatigue resistance than those of T4 and O states. References [1] Gregson PJ, Harris SJ. Aluminium alloys: their physical and mechanical properties. In: Proceedings of the 8th International Conference. Cambridge, U.K; 2002. [2] ASM Handbook. Surface engineering, vol. 5. Materials Park, USA: ASM International; 1994. p. 1416. [3] Cree AM, Weidmann GW. Effect of anodised coatings on fatigue crack growth rates in aluminium alloy. Surf Eng 1997;13:51–5. [4] Cree AM, Weidmann GW. The fracture and fatigue properties of anodised aluminium alloy. Trans Inst Metal Finish 1997;75:199–202. [5] Cree AM, Hainsworth SV, Weidmann GW. The fracture and fatigue properties of anodised aluminium alloy. Trans Inst Metal Finish 2006;84:246–51. [6] Goetz JM. Investigation of coating cracking and fatigue strength of 7050-T74 aluminium alloy with different anodize coating thicknesses. Doctorate Thesis. The Ohio State University; 2005. [7] Lonyuk B, Apachitei I, Duszczyk J. The effect of oxide coatings on fatigue properties of 7475-T6 aluminium alloy. Surf Coat Technol 2007;201:8688–94. [8] Cirik E, Genel K. Effect of anodic oxidation on fatigue performance of 7075-T6 alloy. Surf Coat Technol 2008;202:5190–201. [9] Camargo JAM, Cornelis HJ, Cioffi VMOH, Costa MYP. Coating residual stress effects on fatigue performance of 7050-T7451 aluminium alloy. Surf Coat Technol 2007;201:9448–55. [10] Sadeler R. Effect of a commercial hard anodizing on the fatigue property of a 2014-T6 aluminium alloy. J Mater Sci 2006;41:5803–9. [11] Shahzad M, Chaussumier R, Chieragatti R, Mabru C, Rezai-Aria F. Influence of anodizing process on fatigue life of machined aluminium alloy. Procedia Eng 2010;2:1015–24. [12] Fares C, Belouchrani MA, Bellayer S, Boukharouba T, Britah A. Influence of intermetallic compounds and metallurgical state of the 2017A aluminium alloy on the morphology of alumina films developed by anodic oxidation. J Tribology Surf Eng 2011;2. [13] Huda Zainul. Precipitation strengthening and age-hardening in 2017 aluminium alloy for aerospace application. Eur J Sci Res 2009;26(4):558–64. [14] ASM Handbook. Fractography, vol. 12. Materials Park, USA: ASM International; 1987. p. 193.
Contents lists available at SciVerse ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Influence of heat treatments and anodization on fatigue life of 2017A alloy L. Hemmouche a,⇑, C. Fares a,b, M.A. Belouchrani a a b
Laboratoire Génie des Matériaux, Ecole Militaire Polytechnique, BP17C Bordj El Bahri, 16111 Alger, Algeria Département de Génie des Procédés, Faculté de Technologie, Université Hassiba Ben Bouali, BP151 Hai Essalam, 02000 Chlef, Algeria
a r t i c l e
i n f o
Article history: Available online 27 May 2013 Keywords: Heat treatments Aluminium alloy 2017A Fatigue Anodization
a b s t r a c t In order to investigate the coupled effects of heat treatments and anodizing processes on fatigue life of aluminium alloy 2017A, a series of fatigue tests were conducted at 25 Hz. The effect of different tempers, naturally and artificially ageing (T4 and T6) and overageing (T7) conditions before sulphuric anodization were studied. Additionally, information on the microstructure of the anodic films was acquired by Scanning Electron Microscopy (SEM) analyses. The image analysis yielded qualitative information on the evolution of the surface morphology as a function of the substrate microstructure. Hence measured mechanical properties were directly related to the corresponding microstructure, the result of fatigue tests showed a decrease in fatigue life of anodized specimens as compared to untreated ones. This phenomenon became more pronounced in the T6 and T7 states. The decrease in the fatigue life could mainly be attributed to the brittle nature of oxide layer and to the heterogeneous microstructure of the film. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The 2000 series aluminium alloys are frequently used for making a wide variety of structural components for aircraft where damage tolerance and relatively high strength are paramount. The main requirements for the components include high tensile strength with high fatigue properties and wear resistance, are achieved by heat treatments and resistant coatings. The strength of the alloys can be defined with heat treatments which develops fine strengthening precipitates [1]. Conversely, the coatings are applied to increase the tribological properties and corrosion resistance of the alloys [2]. Anodic oxidation, which is an electrochemical process to form stable oxide films on the surface of metals, is one of the most used methods to improve surface performance such as corrosion and wear resistance [2]. However, the anodic coating has a detrimental effect on the fatigue performance [3–11]. Since the oxide coating is brittle and hard compared to the aluminium substrate, it easily cracks under cyclic stress. The oxide layer adheres extremely well to the substrate; any cracks developed in the film act as stress raisers and can thus contribute initiation sites for fatigue failure. This contribution depend on the brittle nature of oxide layer [3–5], the irregularities beneath the coating [8], the substrate material and its pre-treatment, the film thickness [6–8], the presence of residual stress [5,9], the type of anodizing process [7,10] and on surface pre-treatment before anodizing [11]. The influence of the film thickness has been investigated extensively. However, there is little work that has investigated the other parameters influencing fatigue failure. In the previous work, the effect of the substrate microstructure on the growth of the anodic oxide layers by sulphuric anodization were evaluated for the both artificial ageing and annealing heat treatment of the 2017A alloy [12]. For the both
⇑ Corresponding author. Tel.: +213 661810413. E-mail address: [email protected] (L. Hemmouche). 1350-6307/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2013.05.003
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heat treatments, alloying elements are incorporated to the oxide layer as intermetallic compounds or oxides with copper enrichment at the interface for ageing state. The present work has been conducted in order to accomplish two different purposes. Firstly, to study the effect of the substrate heat treatment on the sulphuric anodization (SA), then secondly, to demonstrate the coupled effects of heat treatments and anodization on the fatigue behavior of 2017A alloy. The combined effect of the substrate microstructure and the anodization treatment on the fatigue behavior was specifically investigated by anodizing the substrate obtained from different heat treatments: annealing, natural and artificial age hardening and the overage hardening. The fatigue performance of the anodized specimens with different heat treatments has been investigated by conducting a series of plane bending fatigue tests at 25 Hz. Also, a fractographic analysis of some samples has been carried out in order to assess the role of the anodic layer on the nucleation of fatigue cracks. 2. Experimental details 2.1. Materials preparation The present investigation was carried out with samples of a 2017A-T4 aluminium alloy with the chemical composition given in Table 1. The material was supplied as bars of approximately 4 mm in thickness and 4 m in length, from which a number of tensile and fatigue specimens were machined following the German standards DIN 50125 and DIN 50142 respectively. Fig. 1 illustrates the sketches of both the tensile (Fig. 1a) and fatigue specimens (Fig. 1b). In order to induce different metallurgical states in the substrate, samples were submitted to four (04) heat treatments (annealed O, solution heat treated and naturally aged T4, solution heat treated and artificially aged T6 and T7 solution heat treated and then stabilized or overaged). The remaining samples were solution-heat treated in a programmable electric furnace type ‘‘ELTI’’ with argon atmosphere according to the sample identification schema presented in Table 2. 2.2. Anodizing and sealing Heat-treated aluminium alloys were dipped for 5 min at 65 °C in an alkaline bath (15 g/l of NaOH). They were then rinsed thoroughly in deionised water and neutralized in a sulphuric/chromic mixture (H2SO4:180 ml/l, CrO3:65 g/l) for 5 min at
Table 1 Chemical composition of 2017A-T4 aluminium alloy (wt%). Element
Si
Cu
Fe
Zn
Mg
Mn
Cr
Ti
Al
Wt%
0.840
3.890
0.478
0.061
0.806
0.783
0.040
0.077
Bal.
Fig. 1. Test specimen’s geometry: (a) tensile specimen, (b) fatigue specimen.
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Table 2 Heat treatments conditions. Heat treatment process
Heat treatment conditions
Annealed O Naturally age hardened T4 Artificially age hardened T6 Overaged T7
Heated Heated Heated Heated
to to to to
(500 ± 5) °C (500 ± 5) °C (500 ± 5) °C (500 ± 5) °C
then slowly cooled for 50 min, water quenched <40°C then naturally aged for 4 days for 50 min, water quenched then and artificially aged at (165 ± 5) °C for 10 h for 50 min, water quenched then artificially aged at (200 ± 5) °C for 10 h
Fig. 2. Different metallurgical states of aluminium 2017A alloy: (a) annealing (O), (b) natural ageing (T4), artificial ageing (T6) and Overageing (T7).
65 °C. Finally, the panels were rinsed and dried in a warm air stream. The anodization was carried out in a thermostatically controlled electrochemical cell (±2 °C) using a lead cathode and the heat-treated aluminium alloy as the anode. All specimens were individually anodized at 20 °C in a sulphuric bath (180 g/lH2SO4) and at a constant current of 1.5 A/dm2 for 30 mn. After anodizing, specimens were washed in distilled water and sealed in boiling water for 30 mn at 97 °C. Following the sealing; the specimens were dried in a cool air stream. 2.3. Mechanical testing The static mechanical properties of both anodized and no anodized samples were determined by means of tensile testing, employing a computer-controlled servohydraulic machine (Zwick/Roell). The tensile load was applied at constant cross-head travel of 5 mm/min. The performance of all samples under dynamic loading was evaluated out under plane bending conditions (R = 1), employing a Fatigue Dynamics SHENK-PWS equipment, at a frequency of 25 Hz. The fatigue tests carried in air were conducted at maximum stresses of 100 MPa. The microhardnesses (Vickers) were determined using a 100 g load. 2.4. Metallographic characterization The Metallographic specimens for the samples (see Table 2) were prepared by metallographic grinding (with 800, 1000, 1200, and 1500 grit size silicon carbide emery papers) and polishing (using high alumina powder) followed by metallographic etching by use of Keller’s reagent (950 ml water, 35 ml HNO3, 15 ml HCl, and 10 ml HF). Microstructural characterization involved use of an optical microscope linked with a computerized imaging system equipped with Cyberlink software.
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L. Hemmouche et al. / Engineering Failure Analysis 35 (2013) 554–561 Table 3 Evolution of the size of the grains according to the nature of the heat treatment. Metallurgical states
O
T4
T6
T7
Size of the grains (lm)
–
12.5 ± 0.6
10.5 ± 0.5
12.7 ± 0.7
Table 4 Mechanicals characteristics of 2017A alloy at different tempers.
Hardness (HV0,1) Re (MPa) Rm(MPa) A (%) E(MPa) Number of cycle to failure
O
T4
T6
T7
65 ± 2 86 ± 2 203 ± 4 11.82 74000 62133
145 ± 2 275 ± 5 412 ± 8.2 14.23 74000 170200
172 ± 3 400 ± 8 466 ± 9 6.16 74000 257467
144 ± 2 338 ± 7 414 ± 8.2 8.73 74000 183700
Fig. 3. SEM–EDS micrographs for the precipitates identification.
Table 5 Evolution of the film thickness for the different tempers. Metallurgical states
O
T4
T6
T7
Thickness (lm)
6.5 ± 1.1
9.3 ± 1.2
13.5 ± 1.3
9.1 ± 1.2
The Substrate microstructure for different samples and the surface fracture of some selected samples (anodized and unanodized), tested at lowest alternating stresses were analyzed by means of SEM techniques (FEI QUANTA 600 (LMS)). The observations were conducted at a potential range of 25–30 kV. 3. Results and discussion Prior to discussing the effect of heat treatment on the investigated material (2017A aluminium alloy), we first analyze microstructure (Fig. 2). Fig. 2 illustrates a typical cross-section of the substrate alloy after heat treatment (see Table 2) taken to the axis of the sheet, observed by means Optical Microscopy, indicating that the structure is constituted of elongated grains and seconds
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Fig. 4. SEM micrograph for the T4 anodized sample.
Fig. 5. Fatigue test results for anodized and unanodized samples for different tempers.
phase particles. These observations are similar for the heat treatments T4, T6 and T7 with a difference in the grain size and in the precipitates sizes and their dispersion. The average grain size shown in Table 3 is estimated by counting the number of grains intercepted by one or more straight lines ‘‘line intercept method’’. For the annealing state (Fig. 2a), the average grain size is not identified but the precipitates have large size. These large particles give rise to a heterogeneous distribution of dislocations, during deformation with local lattice at particle–matrix interface. So, they offer to the substrate low mechanical characteristics (Table 4). The effect of age hardening heat treatment (natural and artificial) on the microstructure of the 2017A aluminium alloy is shown, respectively, in the micrograph in Fig. 2b and d. It is clearly observed that for T6 state there are finer and brought closer precipitates than in the T4 state. The fine particles impede the movement of dislocations. In addition, owing to the pinning effect of particles on the movement of boundaries, close interparticle spacing makes it difficult for local lattice curvature regions to grow into grains. Consequently, the average grain size reaches the minimal value for the T6 treatment (Fig. 2d). The mechanical characteristics are inversely proportional to the grains size. So, this state (T6), offer high hardness, ultimate tensile strength and fatigue strength. The effect of overheating tempering on the solution-treated and quenched alloy is shown in the micrograph in Fig. 2c. The microstructure of sample reveals coarse particle phase which is non-coherent to matrix [13]. This microstructural feature indicates loss in strength; which is confirmed by a low mechanical characteristic (see Table 4). It is, therefore, quite logical to conclude that the artificial ageing heat treatment given to the alloy the optimum precipitation strengthening parameters.
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L. Hemmouche et al. / Engineering Failure Analysis 35 (2013) 554–561 Table 6 Mechanicals characteristics of Anodized 2017A alloys at different tempers.
Hardness (HV0,1) Re (MPa) Rm (MPa) A (%) E (MPa) Number of cycle to failure
O
T4
T6
T7
135 64 ± 2 181 ± 3 12.00 73,600 50,067
188 253 ± 5 390 ± 8 11.11 73,600 142,300
205 378 ± 7 443 ± 9 3.86 73,600 143,500
200 392 ± 8 316 ± 6 5.33 73,600 99,367
Fig. 6. Fatigue fracture surface for unanodized samples at different tempers: (a) annealing (O), (b) natural ageing (T4), (c) artificial ageing (T6) and (d) Overageing (T7).
The SEM and EDS analysis that were conducted on such sections showed the presence of different types of intermetallic particles as Al2Cu, Al2CuMg and Al–Cu–Mn–Fe–Si (Mg) particles (Fig. 3). The SEM micrographs carried out on cross sections of the anodized samples demonstrate that the thickness of the alloy layer was different for the different tempers (Table 5). This difference is due to the size and the spacing of the precipitates [12]. Example for the thickness measurement, for T4 state, is shown in Fig. 4. The results of the tensile cracking experiments of anodized and no anodized samples were conducted using the 2017A aluminium tensile specimens described in Section 2.3. For each tempers, two to three experiments were conducted. The results from all of the specimens for each temper can be seen in Fig. 5 and resumed in Tables 4 and 6. According to the studies, the influence of particle density and spacing and solute content on tensile fracture of aluminium alloys were revealed [14]. Particle size and mean interparticle spacing influence ductility. As the particle density increases, there is a decrease in duc-
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Fig. 7. Fatigue fracture surface for anodized samples at different tempers: (a) annealing (O), (b) natural ageing (T4), (c) artificial ageing (T6) and (d) Overageing (T7).
tility. At large inclusions and precipitates, for annealing state, the plastic deformation begins to form randomly. Identically as unanodized samples, the results of tensile cracking of anodized samples reveal a substantially higher fracture-strength level for T6 state. The tensile properties of the corresponding specimens are shown in Table 6. Compared to the other states, the T6 heat treatment enhanced the strength but severely degraded the ductility. The elastic properties of anodized specimen closely approach those of unanodized samples. The results from the plane bending fatigue experiments for the anodized and unanodized samples at different tempers are shown on the histogram in Fig. 5. Recall that each point on this histogram represents the average of a series of three to five fatigue experiments at a given alternating stress amplitude. It is evident that there is a relationship between the fatigue life, anodization and heat treatment. First of all, when comparing the treated and untreated samples by sulphuric anodization, the untreated specimens have a significantly longer fatigue life. For T6 and T7 tempers, fatigue strength of anodized specimen decreased by 54–55% at low loads of 100 MPa as compared with that of the unanodized one. After completing the fatigue experiments, scanning electron microscopy (SEM) was used to examine the failure surfaces of the fatigue specimens. The failure surfaces were inspected to see if any microstructural features could be correlated to the results shown in the fatigue measurements and the heat treatments. The failure surfaces of unanodized specimens run at loads below 100 MPa were characterized by single crack initiation points at the precipitates as Al2Cu and AlCuFeMgSi (Fig. 6). For annealing (O) and overageing T7state, the precipitates located coarse so, they present a favorable site of nucleation of the fatigue cracks. For the anodized specimens, we found multiple crack nucleation sites at the anodic film (Fig. 7). Accommodate to the substrate metal the anodic film is too brittle any crack that develops in it acts like stress raiser and propagates to substrate [3–5]. The anodic film formed on 2017A alloys is heterogeneous; there are local heterogeneities or defects in the oxide layer, which will affect the mechanical behavior [8,13].
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4. Conclusion The present study focused on the coupled effect of substrate microstructure and anodizing on the mechanical properties. The microstructure was modified by means of an annealing (O), a natural ageing (T4), an artificial ageing (T6) and an over ageing (T7) heat treatments. These treatments show differences in the particles size, the interparticle spacing and in the grain size. So, they reveal different mechanical properties for both anodized and unanodized samples. For anodized sample, the elastic properties of anodized specimen closely approach those of unanodized samples, but the ductility decrease strongly for the anodized samples especially for T6 and T7 tempers. The results of hardness attributed to the brittleness of the oxide films, which readily crack when deformed. The effect of heat treatment and anodizing on fatigue properties of 2017A aluminium alloy was also investigated. It was found that anodic oxide layers decrease the fatigue strength of aluminium alloys. The decrease of fatigue life was principally caused by the brittleness of the oxide layer but also, to the heterogeneities, as intermetallic compounds and element alloys oxide, which act as stress concentrator and promoted the nucleation and the propagation of cracks. The fatigue tests indicate that the oxide layers developed on samples heat treated at T6 and T7 have weak fatigue resistance than those of T4 and O states. References [1] Gregson PJ, Harris SJ. Aluminium alloys: their physical and mechanical properties. In: Proceedings of the 8th International Conference. Cambridge, U.K; 2002. [2] ASM Handbook. Surface engineering, vol. 5. Materials Park, USA: ASM International; 1994. p. 1416. [3] Cree AM, Weidmann GW. Effect of anodised coatings on fatigue crack growth rates in aluminium alloy. Surf Eng 1997;13:51–5. [4] Cree AM, Weidmann GW. The fracture and fatigue properties of anodised aluminium alloy. Trans Inst Metal Finish 1997;75:199–202. [5] Cree AM, Hainsworth SV, Weidmann GW. The fracture and fatigue properties of anodised aluminium alloy. Trans Inst Metal Finish 2006;84:246–51. [6] Goetz JM. Investigation of coating cracking and fatigue strength of 7050-T74 aluminium alloy with different anodize coating thicknesses. Doctorate Thesis. The Ohio State University; 2005. [7] Lonyuk B, Apachitei I, Duszczyk J. The effect of oxide coatings on fatigue properties of 7475-T6 aluminium alloy. Surf Coat Technol 2007;201:8688–94. [8] Cirik E, Genel K. Effect of anodic oxidation on fatigue performance of 7075-T6 alloy. Surf Coat Technol 2008;202:5190–201. [9] Camargo JAM, Cornelis HJ, Cioffi VMOH, Costa MYP. Coating residual stress effects on fatigue performance of 7050-T7451 aluminium alloy. Surf Coat Technol 2007;201:9448–55. [10] Sadeler R. Effect of a commercial hard anodizing on the fatigue property of a 2014-T6 aluminium alloy. J Mater Sci 2006;41:5803–9. [11] Shahzad M, Chaussumier R, Chieragatti R, Mabru C, Rezai-Aria F. Influence of anodizing process on fatigue life of machined aluminium alloy. Procedia Eng 2010;2:1015–24. [12] Fares C, Belouchrani MA, Bellayer S, Boukharouba T, Britah A. Influence of intermetallic compounds and metallurgical state of the 2017A aluminium alloy on the morphology of alumina films developed by anodic oxidation. J Tribology Surf Eng 2011;2. [13] Huda Zainul. Precipitation strengthening and age-hardening in 2017 aluminium alloy for aerospace application. Eur J Sci Res 2009;26(4):558–64. [14] ASM Handbook. Fractography, vol. 12. Materials Park, USA: ASM International; 1987. p. 193.