Materials Science & Engineering A 580 (2013) 202–208

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Materials Science & Engineering A

0921-50http://d

n CorrE-m

journal homepage: www.elsevier.com/locate/msea

On the natural aging behavior of Aluminum 6061 alloy after severeplastic deformation

M.H. Farshidi a,b, M. Kazeminezhad a,n, H. Miyamoto b

a Department of Materials Science and Engineering, Sharif University of Technology, Azadi Avenue, Tehran, Iranb Department of Mechanical Engineering, Doshisha University, Kyotanabe city, Kyoto, Japan

a r t i c l e i n f o

Article history:Received 29 April 2013Received in revised form13 May 2013Accepted 15 May 2013Available online 23 May 2013

Keywords:Aluminum 6061 alloySevere plastic deformationNatural agingMicrohardnessMicrostructure evolution

93/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.msea.2013.05.051

esponding author. Tel.: +98 21 66165227; faxail address: mkazemi@sharif.edu (M. Kazemin

a b s t r a c t

Natural aging behavior of the aluminum 6061 alloy after a novel Severe Plastic Deformation processcalled Tube Channel Pressing (TCP) was studied. For this purpose, Vickers microhardness test was used toinvestigate the changes of mechanical properties while TEM and XRD observations were utilized in orderto characterize microstructural evolution during natural aging. Results show that Si-enriched precipitatesappear and coarsen rapidly in the first few days of natural aging of TCPed aluminum 6061 alloy whichcauses consecutive increase and decrease of Vickers microhardness, respectively. Similarly, other alloyingelements such as Cu and Fe lead to formation of coarse precipitates during natural aging of TCPedaluminum 6061 alloy. Moderated rate recovery is also observed during natural aging after SPD whichresults to progression of cell microstructure.

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1. Introduction

Severe Plastic Deformation (SPD) processes are well known dueto their effects on improvement of mechanical properties andgrain refinement of materials. In addition, these processes canaffect other properties of materials such as precipitation and agingbehavior. Thus, numerous studies have been focused on theeffect of SPD processes on aging behavior of materials [1–5].The attractive point in aging of SPDed alloys is that the aging canalso affect other phenomena such as recovery and recrystallization[6–8].

On the other hand, Al–Mg–Si alloys are widely used in indus-trial applications due to attractive characteristics such as highductility, high strength and age hardening capability. As a result,multiple studies have been concentrated on behavior of thesealloys which are focused not only on grain refinement andmechanical properties improvement, but also on aging behaviorof these alloys after SPD [9–13]. Despite so, although differentworks have been focused on artificial aging of these alloys afterSPD processes [4,7,9], few studies have been concentrated onnatural aging of these alloys after SPD processes. For example,appearance of precipitates after 7 days of natural aging in warmlyEqual Channel Angular Pressed (ECAPed) aluminum 6082 alloywas reported by Kashyap et al. [14]. However, precipitationbehavior was less attended in that work. Also, natural aging

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behavior of 6061 alloy after Friction Stir Process (FSP) was studiedby Woo et al. [15]. Despite so, some concerns restrict extension ofthe results of that work to SPDed materials. At first, the materialused in that work was in T6 treatment which has precipitates andis too brittle to subject under usual SPD process. Secondly, FSPproduces significant deformation heat which changes the micro-structure and aging behavior [15].

Regarding those mentioned above, this work is carried out tostudy mechanical and microstructural evolution of SPDed alumi-num 6061 alloy during natural aging. For this purpose, the SPDprocess named “Tube Channel Pressing” (TCP) is used.

2. Process, material and experiment

Fig. 1 shows the principles of TCP. As shown, tube is passedthrough a channel which has a neck zone in the middle. Thiscauses consecutive decrease and increase of inner and outerdiameters of tube accompanying with shear strain in tube wall[16]. After finishing each pass of TCP, the die and specimen wererotated by 1801 to repeat the process from other side as can beseen in Fig. 1. The TCP process was carried out at room tempera-ture with strain rate of about 0.01 s−1 caused negligible heat ofdeformation in specimen. Average true strain of about 1 can beobtained in each pass of TCP as shown before [17]. The inner andouter diameters of the used tube were 19 and 26 mm, respectively.

Chemical analysis of used material was obtained by spectro-metry as Al–1.01Mg–0.49Si–0.31Cu–0.24Fe–0.06Cr wt%. Tube wasreceived in wrought form and cut in 65 mm length pieces. Then,

Fig. 1. Principles of TCP [16].

Fig. 2. Microhardness variations of non-TCPed, 1 pass and 3 passes TCPed speci-mens during natural aging.

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specimens were solid solution treated at 530 1C for 1 h andsubjected to 1 and 3 passes of TCP. After TCP, specimens werenaturally aged at room temperature for up to 504 h (21 days).

In order to study variations of mechanical properties of speci-mens, Vickers microhardness was measured by load of 1.96 N indifferent times of natural aging. For all specimens, the microhard-ness measurement was carried out in the midthickness of tubewall in longitudinal direction. Additionally, microhardness testswere repeated six times for any specimen in order to obtain moreaccurate results.

X-Ray Diffraction (XRD) analysis was utilized to investigate theeffect of natural aging on dislocation density. To do so, XRDpatterns were obtained for 1 pass and 3 passes TCPed specimenbefore and after natural aging between diffraction angle of 15–451by step angle of 0.021. Williamson–Hall method was used forevaluation of XRD results.

SEM studies were carried out for 3 passes TCPed specimenbefore and after 504 h (21 days) of natural aging. To do so, SEMspecimens were cut from wall plane and were prepared bymechanical polishing and chemical etch in 5 ml HF, 10 ml H2SO4

and 85 ml H2O solution for 30 s. JEOL-2100F TEM with accelerationvoltage of 200 kv was utilized in order to investigate microstruc-tural changes during natural aging and to study chemical compo-sition of formed precipitates. TEM samples were cut from planeperpendicular to tube axis and were polished to thickness of0.15 mm by mechanical polishing and then subjected to jetpolishing in 25% HNO3 and 75% CH3OH solution at −30 1C for 5–15 min.

3. XRD evaluation

Williamson–Hall is a well known approach for estimation ofmicrostructural characteristics such as crystallite size and disloca-tion density. This approach is based on the relation between FullWidth in Half Maximum (FWHM) of XRD peaks, diffraction angle,crystallite size and lattice microstrain as presented below [18,19]:

BCOS ðθÞ ¼ Kλtþ 4ε Sin ðθÞ ð1Þ

where, θ is the diffraction angle, K is a constant, λ is the wavelength of X-ray, t is the crystallite size, ε is the lattice microstrainand B is the normalized FWHM obtained from Gaussian relation as

shown below:

B2 ¼ B2exp−B

2ins ð2Þ

Here, Bexp is the experimentally measured FWHM and Bins is theinstrumental correction of FWHM obtained using a referencematerial.

Concerning linear essence of Eq. (1), plotting BCOS ðθÞ againstSin(θ) can give t and ε. Then, dislocation density can be obtainedusing these relations [20,21]:

ρ¼ ðρt � ρsÞ1=2 ð3Þ

ρt ¼3t2

ð4Þ

ρs ¼6πε2

b2ð5Þ

where ρ is the dislocation density, ρt is the dislocation densityrelated to crystallite size, ρs is the dislocation density related tolattice microstrain and b is the Burgers vector.

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4. Results and discussion

Fig. 2 compares variation of Vickers microhardness of 1 and3 passes TCPed specimens with non-TCPed [22] one's during naturalaging. As can be seen, initial hardness of TCPed specimens issignificantly higher than that of non-TCPed one which is due tothe effect of plastic strain. Additionally, although microhardness ofnon-TCPed specimen is increased continuously during naturalaging, microhardness values of 1 and 3 passes TCPed specimensreach to maximum amounts after 24 h (1 day) and 6 h, respectively,and then fall. Similar behavior was reported up to 504 h (21 days)natural aging of FSPed aluminum 6061 alloy [15]. Also, it can beconcluded that the time needed to reach maximum microhardnessis decreased with increasing pass number. On the other hand, after

Fig. 3. Evolution of Si-enriched precipitates during aging of 1 pass TCPed specim

rapid decreasing of hardness, it slows down in both TCPed speci-mens. Finally, it must be mentioned that increscent rate of hardnessfor non-TCPed alloy is much more than those of TCPed specimens.

Fig. 3 shows evolution of Si-enriched precipitates in 1 passTCPed specimen during natural aging. As shown in Fig. 3(a), fewten nanometers sized Si-enriched precipitates appear after 24 h(1 day) of natural aging. The distribution of these precipitates isrelatively homogenous as can be seen in Fig. 3(a). These precipi-tates have been grown to about one hundred nanometers after48 h (2 days) of natural aging which is shown in Fig. 3(b). The sizeof Si-enriched precipitates is increased to about 200 nm after168 h (7 days) of natural aging as shown in Fig. 3(c).

Fig. 4 shows evolution of Si-enriched precipitates in 3 passesTCPed specimen during natural aging. The Si-enriched precipitates

en: (a) after 24 h (1 day), (b) after 48 h (2 days), and (c) after 168 h (7 days).

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have been grown from few ten nanometers to few hundrednanometers with increasing natural aging duration from 6 h to24 h (1 day) as illustrated in Fig. 4(a) and (b). On the other hand,

Fig. 4. Evolution of Si-enriched precipitates during aging of 3 passes TCPed specimen: (a)

after coarsening to few hundred nanometers size, the Si-enrichedprecipitates have been grown very slowly as shown in Fig. 4(c) and (d).This implies that the Si-enriched precipitates do not grow after

after 6 h, (b) after 24 h (1 day), (c) after 168 h (7 days) and (d) after 504 h (21 days).

Fig. 5. Microstructure of 3 pass TCPed specimen obtained by SEM; (a) before naturalaging, (b) and (c) after 504 h (21 days) natural aging in different magnifications.

Fig. 6. Precipitation of secondary phases enriched from transition metals during natural a—(c) 6 h and (d) 504 h (21 days).

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reaching to a definite size. On the other hand, Si-enrichedprecipitates growth is more rapid in 3 passes TCPed specimencompared with that in 1 pass TCPed specimen. This can be relatedto higher diffusion rate in 3 passes TCPed specimen due to higherconcentration of vacancies as a result of higher plastic strain [6].Appearance of Si-enriched precipitates was reported during nat-ural aging of other aluminum alloys subjected to SPD [14,23].

Fig. 5 compares the SEM-microstructures of 3 passes TCPedspecimen before and after 504 h (21 days) of natural aging. As canbe seen in Fig. 5(a) and (b), naturally aged specimen has muchhigher concentration of precipitates compared with as deformedspecimen. This is due to precipitation during natural aging asdiscussed before. The size of precipitates is about one micrometerwhich is in agreement with TEM studies. The distribution ofprecipitates is relatively homogenous as shown in Fig. 5(c).

Comparing the results of microhardness changes with micro-structural evolutions during natural aging of TCPed 6061 alumi-num alloy, one can guess that microhardness changes are heavilyrelated to the Si-enriched precipitates. For example, the micro-hardness of 1 pass TCPed specimen reaches to a maximum after24 h (1 day) of natural aging where few ten nanometers sized Si-enriched precipitates can be traced in the microstructure. Addi-tionally, the microhardness falls through more natural aging timewhich can be interpreted by the coarsening of Si-enriched pre-cipitates. Similar results have been reported for artificial aging ofSPDed aluminum alloys. For example, an impressive decrease inthe strength of Al–7Si was reported due to coarsening of Siprecipitates during aging [24]. More aging time causes not onlycoarsening of precipitates, but also increscent of average freedistance between precipitates which results to decrease in hard-ness and strength [24,25].

Additionally, hardness decrease rate is reduced after 168 h (7days) of natural aging where Si-enriched precipitates reach to fewhundred nanometers size. This can be interpreted by decrease inprecipitate coarsening rate after reaching to definite size which ismentioned before. Regarding so, it can be concluded that pre-cipitation and growth of Si-enriched precipitates is the main effectof natural aging of TCPed aluminum 6061 alloy in first 504 h (21days). Precipitation of Si-clusters is also the first stage of naturalaging of non-TCPed Al–Mg–Si alloys as shown before [22,26].

In addition, other phases precipitate during natural aging ofTCPed aluminum 6061 alloy which are mainly consist of transition

ging of 1 pass TCPed after—(a) 12 h and (b) 168 h (7 days); and 3 passes TCPed after

Fig. 8. Dislocation structures for (a) 1 pass TCPed specimen before natural aging, (b) 3natural aging, (d) 3 passes TCPed specimen after 24 h natural aging, (e) 1 pass TCPed saging, (g) 1 pass TCPed specimen after 504 h natural aging, and (h) 3 passes TCPed spe

Fig. 7. Comparison of dislocation density measured by XRD method before andafter natural aging of 1 pass and 3 passes TCPed specimens.

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metals such as Cu and Fe as shown in Fig. 6(a)–(d). The growth rateof these precipitates are very rapid similar to Si-enriched pre-cipitates. As shown in Fig. 6(b) and (d), these precipitates tend toaccumulate near each others. This results to inhomogeneousdistribution of these precipitates in matrix. Regarding this phe-nomenon and low volume fraction of these precipitates, it can beconcluded that these precipitates have little contribution inmechanical properties of specimens.

Although Mg has highest concentration between alloyingelements, little evidence is available showing the presence of Mgin precipitates. Comparing solubility limits of Si, Cu and Fe in Alwith Mg one's, it is clear that Mg has much higher solubility inaluminum rather than others. This can explain little precipitationtendency of Mg in TCPed aluminum 6061 alloy.

As mentioned before, the hardness increase of non-TCPedspecimen is much higher than that of TCPed specimen duringnatural aging. On the other hand, although mechanical propertiesimprovement was reported during natural aging of non-SPDed Al–Mg–Si alloys [22], little instances are available showing significant

passes TCPed specimen before natural aging, (c) 1 pass TCPed specimen after 24 hpecimen after 168 h natural aging, (f) 3 passes TCPed specimen after 168 h naturalcimen after 504 h natural aging.

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microstructural changes. In fact, appearance of few nanometersized clusters detected by “Atom Probe” after long term of naturalaging has been reported [22]. Despite so, formation of precipitatescan be observed in TEM image only after aging of non-SPDedAl–Mg–Si alloys at high temperatures as reported in previousstudies [22,26,27]. On the other hand, relatively coarse precipitatesmainly enriched by Si appear during natural aging of TCPedaluminum 6061 alloy as shown in Figs. 3 and 4. Comparing thisphenomenon with behavior of non-TCPed 6061 alloy, it can beconcluded that accelerated growth of precipitates can be classifiedas the main effect of SPD processes on the natural aging behaviorof aluminum 6061 alloy. As mentioned before, this phenomenoncan be related to higher diffusion rate in TCPed material whichcauses the rapid growth of precipitates [6,28]. Regarding so, onecan guess that the hardness increase is higher during natural agingof non-TCPed specimen due to existence of the nano-scaledclusters compared with the coarse precipitates appear duringnatural aging of TCPed specimens.

Fig. 7 shows the dislocation density of TCPed specimensmeasured by XRD before and after natural aging. As can be seen,the dislocation density of naturally aged TCPed specimens iscomparatively lower than that of TCPed one's. This can be inter-preted as activation of dislocation annihilation mechanism duringnatural aging of TCPed aluminum 6061 alloy. Fig. 8 shows theevolution of cell microstructures in 1 pass and 3 passes TCPedspecimens during natural aging. As shown in Fig. 8(a) and (b), asdeformed microstructures have high density of dislocations whichare distributed disorderly. Cell microstructure initiates to formafter 24 h (1 day) of natural aging as shown in Fig. 8(c) and (d).Fig. 8(e) and (f) shows appearance of thick cell wall which isformed after 168 h (7 days) of natural aging. This implies progres-sion of cell microstructures. Finally, sharp cells with dense wallsare formed after 504 h (21 days) of natural aging as shown in Fig. 8(g) and (h). Comparing these results with variations of dislocationdensity after natural aging shown in Fig. 7, it can be concludedthat dislocation annihilation during natural aging of TCPed 6061alloy can be occurred by moderated rate recovery. This can alsoexplain the moderated hardness decrease during second and thirdweeks of natural aging shown in Fig. 2. It must be mentioned thatthe recovery during natural aging of TCPed aluminum 6061 alloycan be affected by accelerated precipitation which is discussedbefore. Note that rapid precipitation during natural aging after SPDresults to decrease of solute elements in matrix. This can result anincrease in dislocation activity [29–30] which causes increscent ofdislocation annihilation.

5. Conclusions

The results of this work show that TCP has an impressivecontribution on the natural aging behavior of aluminum 6061alloy. Moderated rate dislocation annihilation can be activatedduring natural aging of this alloy after TCP. The brief conclusions ofthis work are presented below:

1

TCPed aluminum 6061 alloy shows a maximum hardness pointduring natural aging. The time needed to reach maximumhardness is decreased with increasing pass number of SPD.

2

Appearance and rapid growth of Si-enriched precipitates areobserved during natural aging of TCPed aluminum 6061 alloywhich can describe changes in microhardness.

3

Natural aging after SPD process results to appearance of coarseprecipitates which are mainly consist of Si, Cu and Fe. The sizeof these precipitates is much higher than clusters which appearduring natural aging of non-TCPed alloy.

4

Cell structure progression and dislocation density decreaseindicate activation of recovery during natural aging of TCPed6061 aluminum alloy. This can describe moderated hardnessdecrease during second and third weeks of natural aging.

Acknowledgments

The authors wish to thank the research board of Sharif Universityof Technology and Doshisha University for the financial support andthe provision of the research facilities used in this work.

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