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Lowering the temperature for high strain rate superplasticity in an Al–Mg–Zn–Cu alloy via cooled friction stir processing
Materials Chemistry and Physics 142 (2013) 182e185
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Lowering the temperature for high strain rate superplasticity in an AleMgeZneCu alloy via cooled friction stir processing A. Orozco-Caballero a, *, C.M. Cepeda-Jiménez a, P. Hidalgo-Manrique a, P. Rey b, D. Gesto b, D. Verdera b, O.A. Ruano a, F. Carreño a a b
Department of Physical Metallurgy, CENIM-CSIC, Avda. Gregorio del Amo 8, Madrid 28040, Spain Technological Centre AIMEN, Relva 27A-Torneiros, Porriño, Pontevedra, 36410 Spain
h i g h l i g h t s The Al 7075-T6 alloy was friction stir processed over two kinds of backing anvils. The FSPed grain sizes depend on the refrigeration temperature for each backing anvil. Large refinement down to 0.5 (in air) and 0.3 (N2 refrigerated) mm allows GBS. Low temperature high strain rate superplastic window at 250e350 C was obtained. A eF ¼ 270% record at 250 C and 102 s1is obtained for the finest FSPed material.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 January 2013 Received in revised form 22 May 2013 Accepted 25 June 2013
An AleZneMgeCu alloy was friction stir processed over two kinds of backing anvils, at two different cooling rates. A finer grain size, 0.3 vs 0.5 mm, was obtained by processing at the highest cooling rate. Both materials showed superplastic behavior with a maximum elongation to fracture of about 510%. Grain boundary sliding was the operative deformation mechanism. Furthermore, the finer grain size material showed high strain rate superplasticity, at 102 s1, at lower temperatures, as low as 250 C. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Alloys Hot working Electron microscopy Mechanical testing Creep Ductility
1. Introduction Superplastic forming (SPF) is an attractive method for the fabrication of complex parts from sheet metals [1]. Nevertheless, the generalization of its use is limited by the low strain rates and high temperatures for optimum superplasticity [2]. The great majority of metals require an appropriated thermo-mechanical processing in order to achieve a microstructure which leads to superplastic deformation [3]. During the last decade friction stir processing (FSP) has developed as one of the most promising techniques to obtain this appropriated microstructure in extensive
* Corresponding author. Tel.: þ34 91 5538900x217; fax: þ34 91 5347425. E-mail addresses: [email protected], [email protected] (A. Orozco-Caballero). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.06.055
dimensions sheets [4e7]. FSP is based on the basic principles of friction stir welding (FSW), which is a solid state joining process widely used [8e10], developed initially for aluminum alloys by The Welding Institute (TWI-UK), in 1991. FSP involves traversing a rotation tool, formed by a pin and shoulder inserted in the material, producing intense plastic deformation through the friction between the tool and the worksheet. This processing results, generally, in a stirred zone with a fully recrystallized, equiaxed and very fine grain size [11e13]. In this way, commercial aluminum alloys processed by FSP have been studied since early this century [14,15]. In general, high strength alloys present dislocation slip creep at constant substructure as the dominant high-temperature deformation mechanism. During deformation at high temperature alloys reinforced by a homogeneous dispersion of particles, such as-rolled Al 7075-T6 alloy, generate a (sub)grain size determined by the
A. Orozco-Caballero et al. / Materials Chemistry and Physics 142 (2013) 182e185
interparticle distance (lp). This (sub)grain size is independent of the applied stress and remains constant during deformation [16]. The constitutive equation for this mechanism is the following [16]:
3 n lp s Q ε_ ¼ A exp L b E RT
(1)
where ε_ is the strain rate, s is the stress, A is a constant, b is Burgers vector, E is Young’s modulus, R is the gas constant, T is the temperature, QL is the activation energy for lattice self-diffusion, and the value of the stress exponent, n, is usually 8. On the other side, materials presenting fine (<15 mm), equiaxed and highly misoriented grains, can deform by grain boundary sliding (GBS) [17]. This high-temperature mechanism has an explicit dependence on the grain size (L). The constitutive equation for this mechanism is the following [17]:
p n s b Q ε_ ¼ A exp L RT E
(2)
where n is 2, and p is 2 (if Q ¼ QL) or 3 (if Q ¼ QGB, the activation energy for self-diffusion along grain boundaries). In this alloy, as long as the microstructure is stable at high temperature, remaining fine and highly misoriented, it is expected the operation of GBS, and thus superplasticity [18]. AleZneMgeCu alloys, such as Al 7075 alloy, combine high strength-density ratio with excellent mechanical properties, due to their homogeneous distribution of hardening precipitates, but have limited formability during conventional forging at elevated temperature. Ma et al. [19] demonstrated that appropriate FSP parameters applied to this commercial alloy lead to a significant improvement of its superplastic properties. They processed by FSP Al 7075 obtaining a grain size of 3.8 mm that allows elongations >1250% at strain rates ranging from 3 103 to 3 102 s1 at 480 C. It should be noted that a decrease in the grain size results in an increase in strain rate and a decrease in the temperature at which superplasticity appears [15,17,20]. Up to now, high strain rate superplasticity (HSRS) has been observed in FSPed Al 7075 at relatively high temperatures, over 450 C [19,21,22]. Therefore, the next step for the superplastic forming in Al 7075 alloy to become widespread is to reduce the temperature for which HSRS can be obtained, making SPF a cost-effective process for commercial applications.
183
traverse speed of 500 mm min1 along the rolling direction. A steel tool with scrolled shoulder 9.5 mm in diameter and a concentric threaded pin with flutes 3 mm in diameter and 2 mm in length was used. Under these conditions, two backing anvils were selected in order to determinate two different cooling rates. The first one is a conventional martensitic stainless steel anvil where the sheet cools at room temperature. The second one consists in a copper anvil refrigerated by liquid nitrogen, generating a much faster cooling rate. The microstructure of processed samples was analyzed by means of a microscope equipped with a focus ion beam (SEM/FIB FEI Helios NanoLab 600) were the grain boundaries have been revealed through a focus ion beam, FIB, attack. Grain size was determined by the mean linear intercept method. Planar dog-bone tensile samples with 6 mm 2 mm 1.8 mm gage dimensions were electro-discharge machined and tested using a universal testing machine to evaluate the superplastic behavior. Tensile samples were machined along the traverse direction, in such a way that the gauge section coincided with the nugget zone. The samples were then grounded and polished to a 1 mm finish. A set of tensile tests was performed at elevated temperatures and at constant strain rate of 102 s1. Additionally, another set was performed to determinate the apparent stress exponent, nap, using strain-rate-change (SRC) tests. Tensile samples were heated by a four lamp ellipsoidal furnace at temperatures in the range 200e 450 C. Finally, the surfaces of the deformed tensile samples were observed in a Hitachi S4800 cold-FEG scanning electron microscope (SEM). 3. Results and discussion 3.1. Microstructure after processing Fig. 1(b) and (c) shows SEM micrographs of the microstructure in the nugget zone of the as-processed materials. Both processing conditions result in materials with an equiaxed fine grain structure and grain sizes under 1 mm. No processing defects were detected at any FSP condition. The measured grain size is 0.5 mm for the material processed over the conventional anvil (uncooled material, UC), and 0.3 mm for the material processed over the cooper anvil refrigerated by liquid nitrogen (cooled material, CM). 3.2. High temperature tensile test at a constant strain rate of 102 s1
2. Materials and experimental procedures In this study, commercial 3 mm Al 7075-T6 (nominal composition in wt pct 5.68Zn-2.51Mg-1.59Cu-0.19Cr-bal Al) rolled plates were subjected to FSP using a rotation rate of 1000 rpm and a
Stressestrain curves at a strain-rate of 102 s1 and different temperatures of the as-rolled Al 7075-T6 plates and the FSP nugget for both processing conditions are also shown in Fig. 1. The as-rolled Al 7075-T6 showed the usual mechanical behavior of metals,
Fig. 1. Stressestrain curves obtained from tensile tests at constant strain-rate (102 s1) at different temperatures for (a) the as-start Al 7075-T6 alloy, (b) friction stir processed Al 7075-T6 alloy over conventional backing anvil (UC) and (c) friction stir processed Al 7075-T6 alloy over the copper backing anvil refrigerated by liquid nitrogen (CM). SEM-FIB micrographs showing the microstructure after processing in the nugget zone are also presented in (b) and (c) for each material.
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decreasing smax and increasing elongation (eF) with temperature. At 300 C an elongation to failure of 70% is obtained while the maximum elongation achieved was 110% at 350 C. Fig. 1b and c shows that lower flow stresses and higher elongations to failure are obtained for the FSPed samples than for the as rolled material. On the other hand, at 300 C, elongations of 145% for the uncooled material (Lini ¼ 0.5 mm) vs. eF ¼ 450% for the cooled material (Lini ¼ 0.3 mm) are found. Additionally, elongation to failure values well over 200%, in a determined range of temperatures are found for both FSPed conditions. This temperature range for the uncooled material is 350e400 C while for the cooled material the range is wider and starts at lower temperatures, 250e350 C. Similar maximum elongations to failure of about 510% are observed for the UC (at 400 C) and CM (at 350 C) samples. Furthermore, the cooled material showed an elongation of 270% at a temperature as low as 250 C. Those exceptional elongations together with the low flow stress values observed point to grain boundary sliding (GBS) as the operating deformation mechanism. It should be noted that a ductility drop is observed at temperatures over 400 C for UC and 350 C for CM, that should be attributed to a change in the operative deformation mechanism.
shown in Fig. 2) indicating microstructural instability at such high temperatures. This excessive grain growth in both FSPed materials is related to the ductility drop observed in Fig. 1b and c at temperatures 400 C for UC and 350 C for CM samples. The mechanical data show low stresses, high elongations to fracture and equiaxic microstructures after deformation, demonstrating superplastic behavior and GBS as the operative deformation mechanism. In fact, the strong influence of the small grain size on the increase of elongation to failure confirms the dependence on grain size of the GBS equation. While grain growth is moderate (Fig. 2aee), GBS controls deformation. However, for conditions where grain growth is strong (for example, Fig. 2f), GBS is no longer operative (Fig. 1c, tested 400 C) and a slip creep deformation mechanism is rate controlling.
3.3. Topography of deformed specimens
nap ¼
SEM micrographs of the topography of the deformed region after constant strain-rate tests of the processed materials are shown in Fig. 2. Both materials revealed the typical surface aspect of samples deformed by grain boundary sliding. Equiaxed grains, that emerged from the sample interior, were observed. Fig. 2aec shows increasing grain sizes ranging between 2 and 6 mm for the UC material when tested at increasing temperatures from 300 to 400 C. Similarly, Fig. 2d, e shows increasing grain sizes ranging between 1 and 2 mm for the CM material when tested at increasing temperatures from 300 to 350 C. In contrast, for the CM sample tested at 400 C and 102 s1 (Fig. 2f) grain sizes in excess of 60 mm have been observed. This extreme grain growth is observed also for the UC material tested at 450 C and at the same strain-rate (not
Sigmoidal curves are found for both materials, in which the strain rate markedly increases with increasing temperature in the range 250e400 C for the uncooled material and 250e350 C for the cooled material. All curves show a low stress exponent region where nap is about 2, which is associated unequivocally to GBS. The dashed lines in Fig. 3c show the minimum values of nap for each temperature and for each processing condition. Absolute minimum values of nap were obtained at 400 C and 3$103 s1 for the uncooled material and at 350 C and 102 s1 for the cooled material. Those low values of nap together with elongations well over 200% and low flow-stress are evidences of superplasticity [17]. Solid lines in Fig. 3c show values of elongation to failure (eF) extracted from the constant strain rate tests at 102 s1. The higher
3.4. Determination of the deformation mechanism Fig. 3a and b shows strain rateestress curves at various temperatures (200e450 C) for the uncooled and cooled materials, respectively. The apparent stress exponent (nap) was obtained from the slope of the curves using the relation:
vLn_ε vLns T
(3)
Fig. 2. SEM micrographs showing the topography of the deformed region after constant strain-rate testing at 102 s1 at (a) 300 C, (b) 350 C and (c) 400 C for the uncooled material and at (d) 300 C, (e) 350 C and (f) 400 C for the cooled material.
A. Orozco-Caballero et al. / Materials Chemistry and Physics 142 (2013) 182e185
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Fig. 3. Strain rateestress curves at different temperatures for (a) the uncooled material (UC) and (b) the cooled material (CM). (c) Minimum nap values (dashed line) and eF (solid line) vs testing temperature for both processed materials at a constant strain rate of 102 s1.
values of eF are obtained inside the temperature window where the lower values of nap were reported, corroborating the operation of GBS. At the highest temperatures (450 C for the uncooled material and 400 C for the cooled material) grain coarsening occurs and GBS is no longer operative. The flow stresses and apparent stress exponents increase substantially towards values in line to the asreceived material, for which nap z 8 are found, associated to the operation of a constant substructure slip creep mechanism, given by Equation (1). High strain rate superplasticity (HSRS) is conventionally referred to the achievement of elongations over 200% at strain rates 102 s1 [23]. The uncooled material presented HSRS in the range of temperatures between 350 and 400 C. On the other hand, the cooled material presented HSRS in a wider range of temperatures and at lower values, 250e350 C. This is a strong improvement respect to the usual values in conventionally processed AleZne MgeCu alloys, such as Al7475, where superplasticity is obtained at 400e500 C and at strain rates between 104 s1 and 103 s1 [24], and also respect to the previously FSPed 7075 alloy [19], where HSRS is obtained at around 480 C. This again corroborates the application of the constitutive equation for GBS (Equation (2)) showing how a finer grain size favors GBS at higher strain-rates and lower temperatures [17]. 4. Conclusions In summary, friction stir processing at 1000 rpm and 500 mm min1 conducted over an uncooled backing anvil allows to obtain a grain size of 0.5 mm, while 0.3 mm is obtained using the liquid nitrogen refrigerated copper backing anvil. Mechanical testing of these materials revealed that: a) The uncooled material, with a grain size of 0.5 mm, reached a maximum elongation to failure eF ¼ 511% at 400 C and 102 s1. At this strain-rate, elongations over 200% were obtained at the temperature range of 350e400 C. The minimum apparent stress exponent value of about 2 was reached at 400 C and 3$103 s1. b) The cooled-material, with the finest grain size of 0.3 mm, reached a maximum elongation similar to the uncooled material but at a lower temperature, 350 C. Elongations to failure over 200% at 102 s1 were obtained at the temperature range of 250e350 C. This range is wider and situated at lower temperatures than for the uncooled material. The minimum apparent stress exponent value of about 2 was reached at a
lower temperature and at a higher strain-rate than for the uncooled material, 350 C and 102 s1. c) Grain boundary sliding is the deformation mechanism that allows reaching those exceptional elongations at such low temperatures and high strain-rates. Out of this range, the material deforms by dislocation slip creep at constant substructure with n ¼ 8. d) The decrease in the temperature for obtaining high strain rate superplasticity down to 250 C is a clear improvement to use superplastic forming as a profitable forming technique for this alloy. Acknowledgments Financial support from MICINN (Project MAT2009-14452 and MAT2012-38962) is gratefully acknowledged. A. Orozco-Caballero also thanks CSIC for a JAE-Pre fellowship. References [1] T.Y.M. Al-Naib, J.L. Duncan, Int. J. Mech. Sci. 12 (1970) 463e470. [2] J. Schroers, T.M. Hodges, G. Kumar, H. Raman, A.J. Barnes, Q. Pham, T.A. Waniuk, Mater. Today 14 (2011) 14e19. [3] Z. Horita, M. Furukawa, M. Nemoto, A.J. Barnes, T.G. Langdon, Acta Mater. 48 (2000) 3633e3640. [4] R.S. Mishra, M.W. Mahoney, S.X. McFadden, N.A. Mara, A.K. Mukherjee, Scr. Mater. 42 (2000) 163e168. [5] J.Q. Su, S. Swaminathan, S.K. Menon, T.R. McNelley, Metall. Mater. Trans. A 42 (2011) 2420e2430. [6] X. Feng, H. Liua, S.S. Babu, Scr. Mater. 65 (2011) 1057e1060. [7] P. Rey, D. Gesto, J.A. Del Valle, D. Verdera, O.A. Ruano, Mater. Sci. Forum 706e 709 (2012) 1002e1007. [8] W.M. Thomas, E.D. Nicholas, Mater. Des. 18 (1997) 269e273. [9] R. Nandan, T. DebRoy, H.K.D.H. Bhadeshia, Prog. Mater. Sci. 53 (2008) 980e1023. [10] Y.C. Chen, A. Gholinia, P.B. Prangnell, Mater. Chem. Phys. 134 (2012) 459e463. [11] T.R. McNelley, S. Swaminathan, J.Q. Su, Scr. Mater. 58 (2008) 349e354. [12] Q. Yang, B.L. Xiao, Z.Y. Ma, R.S. Chen, Scr. Mater. 65 (2011) 335e338. [13] Q. Zhang, B.L. Xiao, P. Xue, Z.Y. Ma, Mater. Chem. Phys. 134 (2012) 294e301. [14] R.S. Mishra, JOM 53 (2001) 23e26. [15] Y.J. Kwon, I. Shigematsu, N. Saito, Scr. Mater. 49 (2003) 785e789. [16] O.D. Sherby, R.H. Klundt, A.K. Miller, Metall. Trans. A 8 (1977) 843e850. [17] O.A. Ruano, O.D. Sherby, Rev. Phys. Appl. 23 (1988) 625e637. [18] C.M. Cepeda-Jiménez, J.M. García-Infanta, O.A. Ruano, F. Carreño, J. Alloy. Compd. 509 (2011) 9589e9597. [19] Z.Y. Ma, R.S. Mishra, M.W. Mahoney, Acta Mater. 50 (2002) 4419e4430. [20] R.Z. Valiev, D.A. Salimonenko, N.K. Tsenev, P.B. Berbon, T.G. Langdon, Scr. Mater. 37 (1997) 1945e1950. [21] F.C. Liu, Z.Y. Ma, J. Mater. Sci. 44 (2009) 2647e2655. [22] K. Wang, F.C. Liu, Z.Y. Ma, F.C. Zhang, Scr. Mater. 64 (2011) 572e575. [23] JIS H 7007 Glossary of Terms Used in Metallic Superplastic Materials, Japanese Standards Association, Tokyo, 1995. [24] H.E. Adabbo, G. González-Doncel, O.A. Ruano, J.M. Belzunce, O.D. Sherby, J. Mater. Res. 4 (1989) 587e594.
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Lowering the temperature for high strain rate superplasticity in an AleMgeZneCu alloy via cooled friction stir processing A. Orozco-Caballero a, *, C.M. Cepeda-Jiménez a, P. Hidalgo-Manrique a, P. Rey b, D. Gesto b, D. Verdera b, O.A. Ruano a, F. Carreño a a b
Department of Physical Metallurgy, CENIM-CSIC, Avda. Gregorio del Amo 8, Madrid 28040, Spain Technological Centre AIMEN, Relva 27A-Torneiros, Porriño, Pontevedra, 36410 Spain
h i g h l i g h t s The Al 7075-T6 alloy was friction stir processed over two kinds of backing anvils. The FSPed grain sizes depend on the refrigeration temperature for each backing anvil. Large refinement down to 0.5 (in air) and 0.3 (N2 refrigerated) mm allows GBS. Low temperature high strain rate superplastic window at 250e350 C was obtained. A eF ¼ 270% record at 250 C and 102 s1is obtained for the finest FSPed material.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 January 2013 Received in revised form 22 May 2013 Accepted 25 June 2013
An AleZneMgeCu alloy was friction stir processed over two kinds of backing anvils, at two different cooling rates. A finer grain size, 0.3 vs 0.5 mm, was obtained by processing at the highest cooling rate. Both materials showed superplastic behavior with a maximum elongation to fracture of about 510%. Grain boundary sliding was the operative deformation mechanism. Furthermore, the finer grain size material showed high strain rate superplasticity, at 102 s1, at lower temperatures, as low as 250 C. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Alloys Hot working Electron microscopy Mechanical testing Creep Ductility
1. Introduction Superplastic forming (SPF) is an attractive method for the fabrication of complex parts from sheet metals [1]. Nevertheless, the generalization of its use is limited by the low strain rates and high temperatures for optimum superplasticity [2]. The great majority of metals require an appropriated thermo-mechanical processing in order to achieve a microstructure which leads to superplastic deformation [3]. During the last decade friction stir processing (FSP) has developed as one of the most promising techniques to obtain this appropriated microstructure in extensive
* Corresponding author. Tel.: þ34 91 5538900x217; fax: þ34 91 5347425. E-mail addresses: [email protected], [email protected] (A. Orozco-Caballero). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.06.055
dimensions sheets [4e7]. FSP is based on the basic principles of friction stir welding (FSW), which is a solid state joining process widely used [8e10], developed initially for aluminum alloys by The Welding Institute (TWI-UK), in 1991. FSP involves traversing a rotation tool, formed by a pin and shoulder inserted in the material, producing intense plastic deformation through the friction between the tool and the worksheet. This processing results, generally, in a stirred zone with a fully recrystallized, equiaxed and very fine grain size [11e13]. In this way, commercial aluminum alloys processed by FSP have been studied since early this century [14,15]. In general, high strength alloys present dislocation slip creep at constant substructure as the dominant high-temperature deformation mechanism. During deformation at high temperature alloys reinforced by a homogeneous dispersion of particles, such as-rolled Al 7075-T6 alloy, generate a (sub)grain size determined by the
A. Orozco-Caballero et al. / Materials Chemistry and Physics 142 (2013) 182e185
interparticle distance (lp). This (sub)grain size is independent of the applied stress and remains constant during deformation [16]. The constitutive equation for this mechanism is the following [16]:
3 n lp s Q ε_ ¼ A exp L b E RT
(1)
where ε_ is the strain rate, s is the stress, A is a constant, b is Burgers vector, E is Young’s modulus, R is the gas constant, T is the temperature, QL is the activation energy for lattice self-diffusion, and the value of the stress exponent, n, is usually 8. On the other side, materials presenting fine (<15 mm), equiaxed and highly misoriented grains, can deform by grain boundary sliding (GBS) [17]. This high-temperature mechanism has an explicit dependence on the grain size (L). The constitutive equation for this mechanism is the following [17]:
p n s b Q ε_ ¼ A exp L RT E
(2)
where n is 2, and p is 2 (if Q ¼ QL) or 3 (if Q ¼ QGB, the activation energy for self-diffusion along grain boundaries). In this alloy, as long as the microstructure is stable at high temperature, remaining fine and highly misoriented, it is expected the operation of GBS, and thus superplasticity [18]. AleZneMgeCu alloys, such as Al 7075 alloy, combine high strength-density ratio with excellent mechanical properties, due to their homogeneous distribution of hardening precipitates, but have limited formability during conventional forging at elevated temperature. Ma et al. [19] demonstrated that appropriate FSP parameters applied to this commercial alloy lead to a significant improvement of its superplastic properties. They processed by FSP Al 7075 obtaining a grain size of 3.8 mm that allows elongations >1250% at strain rates ranging from 3 103 to 3 102 s1 at 480 C. It should be noted that a decrease in the grain size results in an increase in strain rate and a decrease in the temperature at which superplasticity appears [15,17,20]. Up to now, high strain rate superplasticity (HSRS) has been observed in FSPed Al 7075 at relatively high temperatures, over 450 C [19,21,22]. Therefore, the next step for the superplastic forming in Al 7075 alloy to become widespread is to reduce the temperature for which HSRS can be obtained, making SPF a cost-effective process for commercial applications.
183
traverse speed of 500 mm min1 along the rolling direction. A steel tool with scrolled shoulder 9.5 mm in diameter and a concentric threaded pin with flutes 3 mm in diameter and 2 mm in length was used. Under these conditions, two backing anvils were selected in order to determinate two different cooling rates. The first one is a conventional martensitic stainless steel anvil where the sheet cools at room temperature. The second one consists in a copper anvil refrigerated by liquid nitrogen, generating a much faster cooling rate. The microstructure of processed samples was analyzed by means of a microscope equipped with a focus ion beam (SEM/FIB FEI Helios NanoLab 600) were the grain boundaries have been revealed through a focus ion beam, FIB, attack. Grain size was determined by the mean linear intercept method. Planar dog-bone tensile samples with 6 mm 2 mm 1.8 mm gage dimensions were electro-discharge machined and tested using a universal testing machine to evaluate the superplastic behavior. Tensile samples were machined along the traverse direction, in such a way that the gauge section coincided with the nugget zone. The samples were then grounded and polished to a 1 mm finish. A set of tensile tests was performed at elevated temperatures and at constant strain rate of 102 s1. Additionally, another set was performed to determinate the apparent stress exponent, nap, using strain-rate-change (SRC) tests. Tensile samples were heated by a four lamp ellipsoidal furnace at temperatures in the range 200e 450 C. Finally, the surfaces of the deformed tensile samples were observed in a Hitachi S4800 cold-FEG scanning electron microscope (SEM). 3. Results and discussion 3.1. Microstructure after processing Fig. 1(b) and (c) shows SEM micrographs of the microstructure in the nugget zone of the as-processed materials. Both processing conditions result in materials with an equiaxed fine grain structure and grain sizes under 1 mm. No processing defects were detected at any FSP condition. The measured grain size is 0.5 mm for the material processed over the conventional anvil (uncooled material, UC), and 0.3 mm for the material processed over the cooper anvil refrigerated by liquid nitrogen (cooled material, CM). 3.2. High temperature tensile test at a constant strain rate of 102 s1
2. Materials and experimental procedures In this study, commercial 3 mm Al 7075-T6 (nominal composition in wt pct 5.68Zn-2.51Mg-1.59Cu-0.19Cr-bal Al) rolled plates were subjected to FSP using a rotation rate of 1000 rpm and a
Stressestrain curves at a strain-rate of 102 s1 and different temperatures of the as-rolled Al 7075-T6 plates and the FSP nugget for both processing conditions are also shown in Fig. 1. The as-rolled Al 7075-T6 showed the usual mechanical behavior of metals,
Fig. 1. Stressestrain curves obtained from tensile tests at constant strain-rate (102 s1) at different temperatures for (a) the as-start Al 7075-T6 alloy, (b) friction stir processed Al 7075-T6 alloy over conventional backing anvil (UC) and (c) friction stir processed Al 7075-T6 alloy over the copper backing anvil refrigerated by liquid nitrogen (CM). SEM-FIB micrographs showing the microstructure after processing in the nugget zone are also presented in (b) and (c) for each material.
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decreasing smax and increasing elongation (eF) with temperature. At 300 C an elongation to failure of 70% is obtained while the maximum elongation achieved was 110% at 350 C. Fig. 1b and c shows that lower flow stresses and higher elongations to failure are obtained for the FSPed samples than for the as rolled material. On the other hand, at 300 C, elongations of 145% for the uncooled material (Lini ¼ 0.5 mm) vs. eF ¼ 450% for the cooled material (Lini ¼ 0.3 mm) are found. Additionally, elongation to failure values well over 200%, in a determined range of temperatures are found for both FSPed conditions. This temperature range for the uncooled material is 350e400 C while for the cooled material the range is wider and starts at lower temperatures, 250e350 C. Similar maximum elongations to failure of about 510% are observed for the UC (at 400 C) and CM (at 350 C) samples. Furthermore, the cooled material showed an elongation of 270% at a temperature as low as 250 C. Those exceptional elongations together with the low flow stress values observed point to grain boundary sliding (GBS) as the operating deformation mechanism. It should be noted that a ductility drop is observed at temperatures over 400 C for UC and 350 C for CM, that should be attributed to a change in the operative deformation mechanism.
shown in Fig. 2) indicating microstructural instability at such high temperatures. This excessive grain growth in both FSPed materials is related to the ductility drop observed in Fig. 1b and c at temperatures 400 C for UC and 350 C for CM samples. The mechanical data show low stresses, high elongations to fracture and equiaxic microstructures after deformation, demonstrating superplastic behavior and GBS as the operative deformation mechanism. In fact, the strong influence of the small grain size on the increase of elongation to failure confirms the dependence on grain size of the GBS equation. While grain growth is moderate (Fig. 2aee), GBS controls deformation. However, for conditions where grain growth is strong (for example, Fig. 2f), GBS is no longer operative (Fig. 1c, tested 400 C) and a slip creep deformation mechanism is rate controlling.
3.3. Topography of deformed specimens
nap ¼
SEM micrographs of the topography of the deformed region after constant strain-rate tests of the processed materials are shown in Fig. 2. Both materials revealed the typical surface aspect of samples deformed by grain boundary sliding. Equiaxed grains, that emerged from the sample interior, were observed. Fig. 2aec shows increasing grain sizes ranging between 2 and 6 mm for the UC material when tested at increasing temperatures from 300 to 400 C. Similarly, Fig. 2d, e shows increasing grain sizes ranging between 1 and 2 mm for the CM material when tested at increasing temperatures from 300 to 350 C. In contrast, for the CM sample tested at 400 C and 102 s1 (Fig. 2f) grain sizes in excess of 60 mm have been observed. This extreme grain growth is observed also for the UC material tested at 450 C and at the same strain-rate (not
Sigmoidal curves are found for both materials, in which the strain rate markedly increases with increasing temperature in the range 250e400 C for the uncooled material and 250e350 C for the cooled material. All curves show a low stress exponent region where nap is about 2, which is associated unequivocally to GBS. The dashed lines in Fig. 3c show the minimum values of nap for each temperature and for each processing condition. Absolute minimum values of nap were obtained at 400 C and 3$103 s1 for the uncooled material and at 350 C and 102 s1 for the cooled material. Those low values of nap together with elongations well over 200% and low flow-stress are evidences of superplasticity [17]. Solid lines in Fig. 3c show values of elongation to failure (eF) extracted from the constant strain rate tests at 102 s1. The higher
3.4. Determination of the deformation mechanism Fig. 3a and b shows strain rateestress curves at various temperatures (200e450 C) for the uncooled and cooled materials, respectively. The apparent stress exponent (nap) was obtained from the slope of the curves using the relation:
vLn_ε vLns T
(3)
Fig. 2. SEM micrographs showing the topography of the deformed region after constant strain-rate testing at 102 s1 at (a) 300 C, (b) 350 C and (c) 400 C for the uncooled material and at (d) 300 C, (e) 350 C and (f) 400 C for the cooled material.
A. Orozco-Caballero et al. / Materials Chemistry and Physics 142 (2013) 182e185
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Fig. 3. Strain rateestress curves at different temperatures for (a) the uncooled material (UC) and (b) the cooled material (CM). (c) Minimum nap values (dashed line) and eF (solid line) vs testing temperature for both processed materials at a constant strain rate of 102 s1.
values of eF are obtained inside the temperature window where the lower values of nap were reported, corroborating the operation of GBS. At the highest temperatures (450 C for the uncooled material and 400 C for the cooled material) grain coarsening occurs and GBS is no longer operative. The flow stresses and apparent stress exponents increase substantially towards values in line to the asreceived material, for which nap z 8 are found, associated to the operation of a constant substructure slip creep mechanism, given by Equation (1). High strain rate superplasticity (HSRS) is conventionally referred to the achievement of elongations over 200% at strain rates 102 s1 [23]. The uncooled material presented HSRS in the range of temperatures between 350 and 400 C. On the other hand, the cooled material presented HSRS in a wider range of temperatures and at lower values, 250e350 C. This is a strong improvement respect to the usual values in conventionally processed AleZne MgeCu alloys, such as Al7475, where superplasticity is obtained at 400e500 C and at strain rates between 104 s1 and 103 s1 [24], and also respect to the previously FSPed 7075 alloy [19], where HSRS is obtained at around 480 C. This again corroborates the application of the constitutive equation for GBS (Equation (2)) showing how a finer grain size favors GBS at higher strain-rates and lower temperatures [17]. 4. Conclusions In summary, friction stir processing at 1000 rpm and 500 mm min1 conducted over an uncooled backing anvil allows to obtain a grain size of 0.5 mm, while 0.3 mm is obtained using the liquid nitrogen refrigerated copper backing anvil. Mechanical testing of these materials revealed that: a) The uncooled material, with a grain size of 0.5 mm, reached a maximum elongation to failure eF ¼ 511% at 400 C and 102 s1. At this strain-rate, elongations over 200% were obtained at the temperature range of 350e400 C. The minimum apparent stress exponent value of about 2 was reached at 400 C and 3$103 s1. b) The cooled-material, with the finest grain size of 0.3 mm, reached a maximum elongation similar to the uncooled material but at a lower temperature, 350 C. Elongations to failure over 200% at 102 s1 were obtained at the temperature range of 250e350 C. This range is wider and situated at lower temperatures than for the uncooled material. The minimum apparent stress exponent value of about 2 was reached at a
lower temperature and at a higher strain-rate than for the uncooled material, 350 C and 102 s1. c) Grain boundary sliding is the deformation mechanism that allows reaching those exceptional elongations at such low temperatures and high strain-rates. Out of this range, the material deforms by dislocation slip creep at constant substructure with n ¼ 8. d) The decrease in the temperature for obtaining high strain rate superplasticity down to 250 C is a clear improvement to use superplastic forming as a profitable forming technique for this alloy. Acknowledgments Financial support from MICINN (Project MAT2009-14452 and MAT2012-38962) is gratefully acknowledged. A. Orozco-Caballero also thanks CSIC for a JAE-Pre fellowship. References [1] T.Y.M. Al-Naib, J.L. Duncan, Int. J. Mech. Sci. 12 (1970) 463e470. [2] J. Schroers, T.M. Hodges, G. Kumar, H. Raman, A.J. Barnes, Q. Pham, T.A. Waniuk, Mater. Today 14 (2011) 14e19. [3] Z. Horita, M. Furukawa, M. Nemoto, A.J. Barnes, T.G. Langdon, Acta Mater. 48 (2000) 3633e3640. [4] R.S. Mishra, M.W. Mahoney, S.X. McFadden, N.A. Mara, A.K. Mukherjee, Scr. Mater. 42 (2000) 163e168. [5] J.Q. Su, S. Swaminathan, S.K. Menon, T.R. McNelley, Metall. Mater. Trans. A 42 (2011) 2420e2430. [6] X. Feng, H. Liua, S.S. Babu, Scr. Mater. 65 (2011) 1057e1060. [7] P. Rey, D. Gesto, J.A. Del Valle, D. Verdera, O.A. Ruano, Mater. Sci. Forum 706e 709 (2012) 1002e1007. [8] W.M. Thomas, E.D. Nicholas, Mater. Des. 18 (1997) 269e273. [9] R. Nandan, T. DebRoy, H.K.D.H. Bhadeshia, Prog. Mater. Sci. 53 (2008) 980e1023. [10] Y.C. Chen, A. Gholinia, P.B. Prangnell, Mater. Chem. Phys. 134 (2012) 459e463. [11] T.R. McNelley, S. Swaminathan, J.Q. Su, Scr. Mater. 58 (2008) 349e354. [12] Q. Yang, B.L. Xiao, Z.Y. Ma, R.S. Chen, Scr. Mater. 65 (2011) 335e338. [13] Q. Zhang, B.L. Xiao, P. Xue, Z.Y. Ma, Mater. Chem. Phys. 134 (2012) 294e301. [14] R.S. Mishra, JOM 53 (2001) 23e26. [15] Y.J. Kwon, I. Shigematsu, N. Saito, Scr. Mater. 49 (2003) 785e789. [16] O.D. Sherby, R.H. Klundt, A.K. Miller, Metall. Trans. A 8 (1977) 843e850. [17] O.A. Ruano, O.D. Sherby, Rev. Phys. Appl. 23 (1988) 625e637. [18] C.M. Cepeda-Jiménez, J.M. García-Infanta, O.A. Ruano, F. Carreño, J. Alloy. Compd. 509 (2011) 9589e9597. [19] Z.Y. Ma, R.S. Mishra, M.W. Mahoney, Acta Mater. 50 (2002) 4419e4430. [20] R.Z. Valiev, D.A. Salimonenko, N.K. Tsenev, P.B. Berbon, T.G. Langdon, Scr. Mater. 37 (1997) 1945e1950. [21] F.C. Liu, Z.Y. Ma, J. Mater. Sci. 44 (2009) 2647e2655. [22] K. Wang, F.C. Liu, Z.Y. Ma, F.C. Zhang, Scr. Mater. 64 (2011) 572e575. [23] JIS H 7007 Glossary of Terms Used in Metallic Superplastic Materials, Japanese Standards Association, Tokyo, 1995. [24] H.E. Adabbo, G. González-Doncel, O.A. Ruano, J.M. Belzunce, O.D. Sherby, J. Mater. Res. 4 (1989) 587e594.