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Achieving ultrafine grain size in Mg–Al–Zn alloy by friction stir processing
Scripta Materialia 57 (2007) 209–212 www.elsevier.com/locate/scriptamat
Achieving ultrafine grain size in Mg–Al–Zn alloy by friction stir processing C.I. Chang,a X.H. Dua,b and J.C. Huanga,* a
Institute of Materials Science and Engineering, Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung 804, Taiwan, ROC b Department of Materials Engineering, Shenyang Institute of Aeronautical Engineering, Shenyang 110034, China Received 27 February 2007; revised 6 April 2007; accepted 9 April 2007 Available online 10 May 2007
Ultrafine-grained (UFG) microstructures with an average grain size of 100–300 nm are achieved in solution-hardened AZ31 Mg– Al–Zn alloy prepared by friction stir processing equipped with a rapid heat sink. The mean hardness of the UFG region reaches 120Hv, which is more than twice as high as that of the AZ31 matrix. The grain refinement kinetics are analyzed and the results are self-consistent. 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Friction stir processing; Magnesium alloy; Ultrafine-grained microstructure; Grain refinement
Magnesium alloys are attractive for lightweight structural applications in the transportation industry because of their low density and high specific strength and stiffness [1]. However, the symmetry of the hexagonal close-packed crystal structure has the limited number of independent slip systems, resulting in poor formability and ductility near room temperature [2]. Fortunately, this can be resolved by the preparation of an ultrafine-grained (UFG) structure which can bring about sufficient room temperature ductility and even superplasticity at high strain rates and low temperatures [3–5]. Of the many techniques used for achieving UFG microstructures, severe plastic deformation (SPD) has been considered to be a promising route [6]. Matsubara et al. [7] and Lin et al. [8] developed a two-stage extrusion plus equal channel angular pressing (ECAP) to fabricate the UFG Mg alloys. The original coarse grain size can be reduced to less than 10 lm after extrusion at 300 C and is further reduced to around 0.7 lm after subsequent 8-pass ECAP at 200 C. However, the processing is time consuming and the resulting materials are in a rod shape of finite dimensions. Recently, another trial was undertaken employing friction stir processing (FSP) in which the localized heating was produced by the friction generated between the rotating * Corresponding author. Tel.: +886 7 525 2000; fax: +886 7 525 4099; e-mail: [email protected]
tool and the workpiece. During this process, the material undergoes intense plastic deformation at elevated temperatures, resulting in significant grain refinement via repeated dynamic recrystallization [9–13]. Successful FSP of Mg-based alloys refining the microstructure of the alloys down to 1–5 lm have been widely reported recently [14–23]. A UFG structure is more easily achieved in precipitate-hardened Mg alloys or Mg-based composites due to the effective pining effect from the precipitates or added ceramic particles on the grain boundaries [16–19]. For pure Mg or solutesolution hardened Mg alloys (such as AZ31) with a low content of alloying elements, it is difficult to achieve a UFG microstructure due to the rapid growth kinetics of the single-phase grains. In fact, FSP has so far failed to refine the grain size of AZ31 to less than 0.5 lm [20–23]. In this study, FSP combining rapid heat sink is used to prepare UFG AZ31 Mg alloys. With just one single FSP pass under effective cooling, the mean grain size of the obtained specimens can be refined to an ultrafine scale (100–300 nm) which is much finer than previous SPD results [8,24–26] and is also the finest microstructure obtained by FSP in pure AZ31 alloys to date. The material for this study was commercial AZ31 Mg alloy with the chemical composition Mg–3.02Al– 1.01Zn–0.30Mn (in mass%). The as-received billet, 178 mm in diameter and 300 mm in length, possessed nearly equiaxed grains around 75 lm in size. The
1359-6462/$ - see front matter 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2007.04.007
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C. I. Chang et al. / Scripta Materialia 57 (2007) 209–212
average hardness value of the as-received AZ31 billet is about 50Hv. A tool with shoulder diameter, pin tool diameter and length of 10, 3 and 3 mm, respectively, was used to perform the FSP. A constant tool rotation rate of 1200 rpm was adopted and the advancing speed was 28–33 mm min1. A tool tilt angle of 1.5 was used. In order to obtain rapid heat sinking during FSP, a newly efficient cooling system was designed, as presented schematically in Figure 1. Based on our previous experience, we concluded that the primary heat loss during FSP is made from the bottom of the pin to the back plate beneath the sample. In order to transfer the heat generated between the tool and the sample during FSP as quickly as possible, a thin copper mould and liquid nitrogen were used. Two tunnels were machined beneath the surface of the copper mould, and the liquid nitrogen can be immerged and flowed through. Vickers hardness tests were conducted on the crosssectional plane using a Vickers indenter with a 200 gf load for 10 s. The grain structures on cross-sectional planes of the etched specimens were examined by optical microscopy (OM) and scanning electron microscopy (SEM). Microstructural and hardness characterizations in this study are focused on the central and bottom parts of the nugget zone, where the cooling rate is the highest. The grain size was measured by Optimas image analysis software on the SEM micrographs. The grain size near the top surface layer contacting the tool shoulder is typically slightly larger due to the higher and longer heat exposure and the inevitably slower cooling rate. Figure 2 shows a cross-sectional view of the AZ31 FSP specimen containing the entire nugget zone after one single FSP pass with an advancing velocity of 28 mm min1. The degree of refinement and the extent
of homogeneity of the final FSP microstructure are the two aspects that are most noticeable. In Figure 3, the grain structures in the AZ31 billets after one-pass FSP with an advancing velocity of 28 mm min1, viewed at low magnification, show a well-defined equaixed and highly homogeneous nature. Figure 4 shows the microstructures with an advancing velocity of 33 mm min1 at higher magnifications. All of these figures show that the recrystallized fine grains are distributed homogeneously in the nugget region. Compared with other SPD processes, such as extrusion, accumulated roll bonding (ARB) or ECAP [8,24–26], the UFG microstructure obtained in the present study has clearer grain boundaries and more uniform ultrafine grain sizes without abnormal local grain growth. Such fine grains appear to be fully recrystallized and do not belong to subgrain structures with a ‘‘diffused’’ boundary nature. Figure 5 shows the grain size distribution of the 33 mm min1 specimen, which is summarized from numerous SEM micrographs. It shows that the grain sizes are mostly scattered from less than 100 to 500 nm, and more than 80% of the grains are refined to less than 300 nm. The average grain size is typically around 200–280 nm. The ultrafine grains lead to pronounced hardening, as demonstrated by the microhardness tests. The typical microhardness values, Hv, in the UFG zone of the FSP specimens are depicted in Figure 6. The highest Hv reaches 128, with the mean hardness values around 120Hv, which is even higher than those observed in the FSP AZ31 based composites [15,16,19]. In the AZ31 matrix away from the nugget, Hv remains at around 50, indicating that the ultrafine grain structure in the
ND
Pure AZ31 plate
Copper mould
Liquid nitrogen Figure 1. Schematic drawing of the newly designed cooling system.
Figure 3. SEM micrograph at low magnification showing the uniform UFG structure in the AZ31 alloy after one-pass FSP at 28 mm min1 with liquid N2 cooling.
Figure 2. Photograph of the cross-section of the AZ31 FSP specimen after a single FSP pass with an advancing speed of 28 mm min1.
Figure 4. SEM micrographs for the FSP AZ31 alloy at an advancing speed of 33 mm min1.
C. I. Chang et al. / Scripta Materialia 57 (2007) 209–212 14 80%
Percentage, %
12 10 8 6 4 2 0
200
300
400
500
600
700
800
900
Average grain size, nm Figure 5. Grain size distribution chart of the UFG microstructure in FSP AZ31 alloys. 140
33 mm/min 28 mm/min
Vickers hardness, /Hv
Retreating Side 120 100 80 Nugget (UFG region)
60 40 -4
-2
0
2
4
Distance from weld center, d/mm Figure 6. Microhardness (Hv) profile measured on cross-sectional planes for the FSP AZ31 alloy.
FSP nugget has strengthened the alloy 2.4-fold. Since there is no twin observed in such fine grains, the hardening is postulated to be a result of the UFG microstructure plus the retained matrix dislocations. Some previous works have demonstrated that the inability in preparing UFG microstructures in pure Mg or Mg alloys with low content of alloying elements by SPD, such as ECAP [27]. This is reasonable from the physical point of view. Since the lattice and grain boundary diffusion rates of Mg at working temperatures, e.g. 300 C, are 4.7 · 1017 m2 s1 and 2 · 1020d m3 s1 (where d is the grain boundary width), respectively [28], both are much higher than the values of 1.8 · 1017 m2 s1 and 1.1 · 1021d m3 s1 for Al counterparts [28]. The achievement of ultrafine and uniform grain structures is accomplished by the combination of a high degree of SPD and a sufficiently rapid heat release. To clarify the mechanism for the formation of UFG, the relationship between the Zener–Hollomon parameter, Z, and the average recrystallized grain size, d, in lm, can be used because recrystallization proceeds during the course of FSP. First, the strain rate and working temperature of the nugget region experienced during FSP need to be determined. The material flow strain rate, e_ , during FSP may be estimated by the torsion-type deformation as [22]: e_ ¼
Rm 2pce Le
ð1Þ
where Rm is the average material flow rate (assumed to be about half of the pin rotational speed, namely 1200/
211
2 rpm), and re and Le are the effective (or average) radius and depth of the dynamically recrystallized zone. An effective radius, re, that can represent the average radius for all parts of the materials inside this zone is assumed to be equal to about 0.78 (or p/4 [29]) of the observed zone boundary radius (1.7 mm in the current case). A similar argument can also be applied to Le (0.78 · 3 mm). Thus, e_ can be calculated to be 36 s1. From the previous work [22], the relationship between Z and d in lm for the AZ31 alloy during FSP can be estimated as: ln d = 9.0–0.27 ln Z, where Z ¼ e_ expðQ=RT Þ, Q is the activation energy for lattice diffusion (135 kJ mol1 [28]) and RT has its usual meaning. In this case, with an average grain size of 0.3 lm and a strain rate of 36 s1, the working temperature can be calculated to be 200 C. Note that the heating history during FSP in Mg alloys without rapid cooling design is typically a heating stage from room temperature to 400 C over 30 s, followed by a cooling stage to room temperature over 100s, as monitored by the inserted thermal couples [22]. In the present FSP case under effective rapid cooling, the heat generated during FSP can be conducted away quickly, as reflected by the low calculated working temperature of 200 C. Watanabe et al. [30] reported that the grain size after dynamic recrystallization, drec, was dependent on the initial grain size, dinit. They proposed that the initial grain size and the Z-parameter of Mg alloys could be related by the following empirical equation: ðd rec =d init Þ ¼ 103 Z 1=3 ¼ 103 fe_ expðQ=RT Þg1=3 ð2Þ Using the initial grain size of 75 lm and the estimated working temperature of 200 C, the achievable grain size after dynamic recrystallization based on Eq. (2) will be about 250 nm, which is consistent with the current experimental result. Here, precipitates that are often helpful to stabilize the microstructure (the Zener pining) are not available because the Mg17Al12 precipitates in the AZ31 alloy dissolve into the matrix above 200 C [31]. Therefore, the low working temperature in the current case is critical in achieving the UFG microstructure for the AZ31 alloy. It is known that one-pass FSP can only produce UFG structures along the weld line. In order to achieve a wider area with such microstructures, it is necessary to use multiple overlapping FSP using robot control. Experimental trials in the laboratory using multiple FSP passes under liquid N2 cooling ensure the same fine grain structures. It is possible to scale up the current processing route for possible engineering applications. In short, the ultrafine grain size in solid solutionhardened AZ31 Mg alloy is successfully achieved by one-pass FSP coupled with rapid heat sink. The results can be summarized as follows: (1) With proper control of the working temperature history, an ultrafine and uniform grained structure can be achieved. The grain boundaries are well defined and the mean grain size can be refined to 100– 300 nm from the initial 75 lm by a single FSP pass. (2) The ultrafine-grained structure can drastically increase the microhardness from an initial 50 up to 120Hv, or an increment factor of 2.4.
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(3) The estimated high strain rate and low working temperature during FSP with rapid heat sink agree self-consistently with the achieved ultrafine grains. The authors gratefully acknowledge the sponsorship from National Science Council of ROC under the project NSC 94-2216-E-110-010. X.H.D. is grateful for the post-doctoral sponsorship from NSC under contract NSC 95-2816-E-110-001. [1] K.U. Kainer, F. von Buch, in: K.U. Kainer (Ed.), Magnesium Alloys and Technologies, DGM, Weinheim, 2003, p. 1. [2] P.G. Patridge, Met. Rev. 118 (1967) 169. [3] C.C. Koch, D.G. Morris, K. Lu, A. Inoue, Mater. Res. Soc. Bull. 24 (1999) 54. [4] J.R. Weertman, D. Farkas, K. Hemker, H. Kung, M. Mayo, R. Mitra, Mater. Res. Soc. Bull. 24 (1999) 44. [5] S.X. McFadden, R.S. Mishar, R.Z. Valiev, A.P. Zhilyaev, A.K. Mukherjee, Nature 398 (1999) 684. [6] T.C. Lowe, R.Z. Valiev (Eds.), Investigations and Applications of Severe Plastic Deformation, Kluwer, Dordrecht, 2000. [7] K. Matsubara, Y. Miyahara, Z. Horita, T.G. Langon, Acta Mater. 51 (2003) 3073. [8] H.K. Lin, J.C. Huang, T.G. Landon, Mater. Sci. Eng. A 402 (2005) 250. [9] R.S. Mishra, M.W. Mahoney, Mater. Sci. Forum 507 (2001) 357. [10] R.S. Mishra, M.W. Mahoney, S.X. McFadden, N.A. Mara, A.K. Mukherjee, Scripta Mater. 42 (2000) 163. [11] J.Q. Su, T.W. Nelson, C.J. Sterling, Scripta Mater. 52 (2005) 135. [12] Y.S. Sato, Y. Kurihara, S.H.C. Park, H. Kokawa, N. Tsuji, Scripta Mater. 50 (2004) 57.
[13] Y.J. Kwon, I. Shigematsu, N. Saito, Scripta Mater. 49 (2003) 785. [14] A.H. Feng, Z.Y. Ma, Scripta Mater. 56 (2007) 397. [15] Y. Morisada, H. Fujii, T. Nagaoka, M. Fukusumi, Mater. Sci. Eng. A 419 (2006) 344. [16] Y. Morisada, H. Fujii, T. Nagaoka, M. Fukusumi, Scripta Mater. 55 (2006) 1067. [17] C.J. Lee, J.C. Huang, Mater. Trans. 47 (2006) 2773. [18] C.J. Lee, J.C. Huang, P.J. Hsieh, Scripta Mater. 54 (2006) 1415. [19] C.I. Chang, Y.N. Wang, H.R. Pei, C.J. Lee, J.C. Huang, Mater. Trans. 47 (2006) 2942. [20] F.Y. Hung, C.C. Shih, L.H. Chen, T.S. Lui, J. Alloys Compd. 428 (2007) 106. [21] J.A. Esparza, W.C. Davis, E.A. Trillo, L.E. Murr, J. Mater. Sci. Lett. 21 (2002) 917. [22] C.I. Chang, C.J. Lee, J.C. Huang, Scripta Mater. 51 (2004) 509. [23] W. Woo, H. Choo, D.W. Brown, P.K. Liaw, Z. Feng, Scripta Mater. 54 (2006) 1859. [24] T.C. Chang, J.Y. Wang, C.M. O, S. Lee, J. Mater. Proc. Techn. 140 (2003) 588. [25] M.T. Perez-Prado, J.A. del Valle, O.A. Ruano, Scripta Mater. 51 (2004) 1093. [26] M. Eddahbi, J.A. del Valle, M.T. Perez-Prado, O.A. Ruano, Mater. Sci. Eng. A 410 (2005) 308. [27] A. Yamashita, Z. Horita, T.G. Langdon, Mater. Sci. Eng. A 300 (2001) 142. [28] H.J. Forst, M.F. Ashby, Deformation-Mechanism Maps, Pergamon Press, Oxford, 1982, p. 21 and p. 44. [29] A.J. Ardell, Metall. Trans. 16 A (1985) 2131. [30] H. Watanabe, H. Tsutsui, T. Mukai, H. Ishikawa, Y. Okanda, M. Kohzu, K. Higashi, Mater. Trans. 42 (2001) 1200. [31] T.B. Massalski, H. Okamoto, P.R. Subramanian, L. Kacprzak (Eds.), Binary Alloy Phase Diagrams, ASM International, Materials Park, OH, 1990, p. 2444.
Achieving ultrafine grain size in Mg–Al–Zn alloy by friction stir processing C.I. Chang,a X.H. Dua,b and J.C. Huanga,* a
Institute of Materials Science and Engineering, Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung 804, Taiwan, ROC b Department of Materials Engineering, Shenyang Institute of Aeronautical Engineering, Shenyang 110034, China Received 27 February 2007; revised 6 April 2007; accepted 9 April 2007 Available online 10 May 2007
Ultrafine-grained (UFG) microstructures with an average grain size of 100–300 nm are achieved in solution-hardened AZ31 Mg– Al–Zn alloy prepared by friction stir processing equipped with a rapid heat sink. The mean hardness of the UFG region reaches 120Hv, which is more than twice as high as that of the AZ31 matrix. The grain refinement kinetics are analyzed and the results are self-consistent. 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Friction stir processing; Magnesium alloy; Ultrafine-grained microstructure; Grain refinement
Magnesium alloys are attractive for lightweight structural applications in the transportation industry because of their low density and high specific strength and stiffness [1]. However, the symmetry of the hexagonal close-packed crystal structure has the limited number of independent slip systems, resulting in poor formability and ductility near room temperature [2]. Fortunately, this can be resolved by the preparation of an ultrafine-grained (UFG) structure which can bring about sufficient room temperature ductility and even superplasticity at high strain rates and low temperatures [3–5]. Of the many techniques used for achieving UFG microstructures, severe plastic deformation (SPD) has been considered to be a promising route [6]. Matsubara et al. [7] and Lin et al. [8] developed a two-stage extrusion plus equal channel angular pressing (ECAP) to fabricate the UFG Mg alloys. The original coarse grain size can be reduced to less than 10 lm after extrusion at 300 C and is further reduced to around 0.7 lm after subsequent 8-pass ECAP at 200 C. However, the processing is time consuming and the resulting materials are in a rod shape of finite dimensions. Recently, another trial was undertaken employing friction stir processing (FSP) in which the localized heating was produced by the friction generated between the rotating * Corresponding author. Tel.: +886 7 525 2000; fax: +886 7 525 4099; e-mail: [email protected]
tool and the workpiece. During this process, the material undergoes intense plastic deformation at elevated temperatures, resulting in significant grain refinement via repeated dynamic recrystallization [9–13]. Successful FSP of Mg-based alloys refining the microstructure of the alloys down to 1–5 lm have been widely reported recently [14–23]. A UFG structure is more easily achieved in precipitate-hardened Mg alloys or Mg-based composites due to the effective pining effect from the precipitates or added ceramic particles on the grain boundaries [16–19]. For pure Mg or solutesolution hardened Mg alloys (such as AZ31) with a low content of alloying elements, it is difficult to achieve a UFG microstructure due to the rapid growth kinetics of the single-phase grains. In fact, FSP has so far failed to refine the grain size of AZ31 to less than 0.5 lm [20–23]. In this study, FSP combining rapid heat sink is used to prepare UFG AZ31 Mg alloys. With just one single FSP pass under effective cooling, the mean grain size of the obtained specimens can be refined to an ultrafine scale (100–300 nm) which is much finer than previous SPD results [8,24–26] and is also the finest microstructure obtained by FSP in pure AZ31 alloys to date. The material for this study was commercial AZ31 Mg alloy with the chemical composition Mg–3.02Al– 1.01Zn–0.30Mn (in mass%). The as-received billet, 178 mm in diameter and 300 mm in length, possessed nearly equiaxed grains around 75 lm in size. The
1359-6462/$ - see front matter 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2007.04.007
210
C. I. Chang et al. / Scripta Materialia 57 (2007) 209–212
average hardness value of the as-received AZ31 billet is about 50Hv. A tool with shoulder diameter, pin tool diameter and length of 10, 3 and 3 mm, respectively, was used to perform the FSP. A constant tool rotation rate of 1200 rpm was adopted and the advancing speed was 28–33 mm min1. A tool tilt angle of 1.5 was used. In order to obtain rapid heat sinking during FSP, a newly efficient cooling system was designed, as presented schematically in Figure 1. Based on our previous experience, we concluded that the primary heat loss during FSP is made from the bottom of the pin to the back plate beneath the sample. In order to transfer the heat generated between the tool and the sample during FSP as quickly as possible, a thin copper mould and liquid nitrogen were used. Two tunnels were machined beneath the surface of the copper mould, and the liquid nitrogen can be immerged and flowed through. Vickers hardness tests were conducted on the crosssectional plane using a Vickers indenter with a 200 gf load for 10 s. The grain structures on cross-sectional planes of the etched specimens were examined by optical microscopy (OM) and scanning electron microscopy (SEM). Microstructural and hardness characterizations in this study are focused on the central and bottom parts of the nugget zone, where the cooling rate is the highest. The grain size was measured by Optimas image analysis software on the SEM micrographs. The grain size near the top surface layer contacting the tool shoulder is typically slightly larger due to the higher and longer heat exposure and the inevitably slower cooling rate. Figure 2 shows a cross-sectional view of the AZ31 FSP specimen containing the entire nugget zone after one single FSP pass with an advancing velocity of 28 mm min1. The degree of refinement and the extent
of homogeneity of the final FSP microstructure are the two aspects that are most noticeable. In Figure 3, the grain structures in the AZ31 billets after one-pass FSP with an advancing velocity of 28 mm min1, viewed at low magnification, show a well-defined equaixed and highly homogeneous nature. Figure 4 shows the microstructures with an advancing velocity of 33 mm min1 at higher magnifications. All of these figures show that the recrystallized fine grains are distributed homogeneously in the nugget region. Compared with other SPD processes, such as extrusion, accumulated roll bonding (ARB) or ECAP [8,24–26], the UFG microstructure obtained in the present study has clearer grain boundaries and more uniform ultrafine grain sizes without abnormal local grain growth. Such fine grains appear to be fully recrystallized and do not belong to subgrain structures with a ‘‘diffused’’ boundary nature. Figure 5 shows the grain size distribution of the 33 mm min1 specimen, which is summarized from numerous SEM micrographs. It shows that the grain sizes are mostly scattered from less than 100 to 500 nm, and more than 80% of the grains are refined to less than 300 nm. The average grain size is typically around 200–280 nm. The ultrafine grains lead to pronounced hardening, as demonstrated by the microhardness tests. The typical microhardness values, Hv, in the UFG zone of the FSP specimens are depicted in Figure 6. The highest Hv reaches 128, with the mean hardness values around 120Hv, which is even higher than those observed in the FSP AZ31 based composites [15,16,19]. In the AZ31 matrix away from the nugget, Hv remains at around 50, indicating that the ultrafine grain structure in the
ND
Pure AZ31 plate
Copper mould
Liquid nitrogen Figure 1. Schematic drawing of the newly designed cooling system.
Figure 3. SEM micrograph at low magnification showing the uniform UFG structure in the AZ31 alloy after one-pass FSP at 28 mm min1 with liquid N2 cooling.
Figure 2. Photograph of the cross-section of the AZ31 FSP specimen after a single FSP pass with an advancing speed of 28 mm min1.
Figure 4. SEM micrographs for the FSP AZ31 alloy at an advancing speed of 33 mm min1.
C. I. Chang et al. / Scripta Materialia 57 (2007) 209–212 14 80%
Percentage, %
12 10 8 6 4 2 0
200
300
400
500
600
700
800
900
Average grain size, nm Figure 5. Grain size distribution chart of the UFG microstructure in FSP AZ31 alloys. 140
33 mm/min 28 mm/min
Vickers hardness, /Hv
Retreating Side 120 100 80 Nugget (UFG region)
60 40 -4
-2
0
2
4
Distance from weld center, d/mm Figure 6. Microhardness (Hv) profile measured on cross-sectional planes for the FSP AZ31 alloy.
FSP nugget has strengthened the alloy 2.4-fold. Since there is no twin observed in such fine grains, the hardening is postulated to be a result of the UFG microstructure plus the retained matrix dislocations. Some previous works have demonstrated that the inability in preparing UFG microstructures in pure Mg or Mg alloys with low content of alloying elements by SPD, such as ECAP [27]. This is reasonable from the physical point of view. Since the lattice and grain boundary diffusion rates of Mg at working temperatures, e.g. 300 C, are 4.7 · 1017 m2 s1 and 2 · 1020d m3 s1 (where d is the grain boundary width), respectively [28], both are much higher than the values of 1.8 · 1017 m2 s1 and 1.1 · 1021d m3 s1 for Al counterparts [28]. The achievement of ultrafine and uniform grain structures is accomplished by the combination of a high degree of SPD and a sufficiently rapid heat release. To clarify the mechanism for the formation of UFG, the relationship between the Zener–Hollomon parameter, Z, and the average recrystallized grain size, d, in lm, can be used because recrystallization proceeds during the course of FSP. First, the strain rate and working temperature of the nugget region experienced during FSP need to be determined. The material flow strain rate, e_ , during FSP may be estimated by the torsion-type deformation as [22]: e_ ¼
Rm 2pce Le
ð1Þ
where Rm is the average material flow rate (assumed to be about half of the pin rotational speed, namely 1200/
211
2 rpm), and re and Le are the effective (or average) radius and depth of the dynamically recrystallized zone. An effective radius, re, that can represent the average radius for all parts of the materials inside this zone is assumed to be equal to about 0.78 (or p/4 [29]) of the observed zone boundary radius (1.7 mm in the current case). A similar argument can also be applied to Le (0.78 · 3 mm). Thus, e_ can be calculated to be 36 s1. From the previous work [22], the relationship between Z and d in lm for the AZ31 alloy during FSP can be estimated as: ln d = 9.0–0.27 ln Z, where Z ¼ e_ expðQ=RT Þ, Q is the activation energy for lattice diffusion (135 kJ mol1 [28]) and RT has its usual meaning. In this case, with an average grain size of 0.3 lm and a strain rate of 36 s1, the working temperature can be calculated to be 200 C. Note that the heating history during FSP in Mg alloys without rapid cooling design is typically a heating stage from room temperature to 400 C over 30 s, followed by a cooling stage to room temperature over 100s, as monitored by the inserted thermal couples [22]. In the present FSP case under effective rapid cooling, the heat generated during FSP can be conducted away quickly, as reflected by the low calculated working temperature of 200 C. Watanabe et al. [30] reported that the grain size after dynamic recrystallization, drec, was dependent on the initial grain size, dinit. They proposed that the initial grain size and the Z-parameter of Mg alloys could be related by the following empirical equation: ðd rec =d init Þ ¼ 103 Z 1=3 ¼ 103 fe_ expðQ=RT Þg1=3 ð2Þ Using the initial grain size of 75 lm and the estimated working temperature of 200 C, the achievable grain size after dynamic recrystallization based on Eq. (2) will be about 250 nm, which is consistent with the current experimental result. Here, precipitates that are often helpful to stabilize the microstructure (the Zener pining) are not available because the Mg17Al12 precipitates in the AZ31 alloy dissolve into the matrix above 200 C [31]. Therefore, the low working temperature in the current case is critical in achieving the UFG microstructure for the AZ31 alloy. It is known that one-pass FSP can only produce UFG structures along the weld line. In order to achieve a wider area with such microstructures, it is necessary to use multiple overlapping FSP using robot control. Experimental trials in the laboratory using multiple FSP passes under liquid N2 cooling ensure the same fine grain structures. It is possible to scale up the current processing route for possible engineering applications. In short, the ultrafine grain size in solid solutionhardened AZ31 Mg alloy is successfully achieved by one-pass FSP coupled with rapid heat sink. The results can be summarized as follows: (1) With proper control of the working temperature history, an ultrafine and uniform grained structure can be achieved. The grain boundaries are well defined and the mean grain size can be refined to 100– 300 nm from the initial 75 lm by a single FSP pass. (2) The ultrafine-grained structure can drastically increase the microhardness from an initial 50 up to 120Hv, or an increment factor of 2.4.
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