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Flow stress of wrought magnesium alloys during hot compression deformation at medium and high temperatures
Materials Science and Engineering A 499 (2009) 238–241
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Flow stress of wrought magnesium alloys during hot compression deformation at medium and high temperatures Y.Q. Yang a,∗ , B.C. Li a,b , Z.M. Zhang a,b a b
College of Materials Science and Engineering, North University of China, Taiyuan 030051, China Engineering Center for Precision Forming of Shanxi Province, Taiyuan 030051, China
a r t i c l e
i n f o
Article history: Received 26 May 2007 Received in revised form 30 July 2007 Accepted 10 November 2007 Keywords: AZ31 alloy ZK60 alloy Hot compression deformation Flow stress
a b s t r a c t The plasticity of AZ31 and ZK60 magnesium alloys was studied by means of hot compression deformation tests on Gleeble 1500D machines at different temperatures and strain rates. The results indicate that the thermal simulation curves of AZ31 and ZK60 have different forms under the same deformation condition. The general curves of AZ31 have the character of dynamic recrystallization that the flow stress increases to a peak and then decreases to a steady state. Most deformation curves of ZK60 have the obvious character that around 0.2 in strain the stress reaches the peak and declines rapidly to the lowest, while the other curves have the character of dynamic recrystallization. From the analysis, a deformation temperature should reasonably be selected from 250 ◦ C to 400 ◦ C for AZ31, while it was concluded to be 200 ◦ C or 400 ◦ C for ZK60. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
At present, the volume of wrought magnesium products is considerably less than that of casting products. However, wrought magnesium alloys may have more development potential with the higher strength, improved ductility and other mechanical property advantages [1]. Due to the intrinsic characteristics of the h.c.p. structure, magnesium alloys have poor formability and limited ductility at room temperature [2,3], so their forming technology should be proceeded at the medium and high temperature. The absence of the related data of the forming technological factors is still the first cause of limiting the widespread use of the wrought magnesium alloys. Therefore, it is necessary to investigate on the basic forming data of magnesium alloys which have different brands. AZ31 is one of the Mg–Al–Zn alloys and ZK60 is one of the Mg–Zn–Zr alloys, both of them are the most widely used wrought alloys. In the present study, we conducted hot deformation experiments on AZ31 and ZK60 alloys and recorded the corresponding flow stress curves. By investigating the properties of the curves and the deformation parameters, the optimized deformation parameters have been concluded which can provide the theoretical support to formulate the deformation technology of magnesium alloys.
Hot compression tests were performed on the Gleeble 1500D machine. The test samples were cylindrical in shape having a diameter of 10 mm and a length of 15 mm. Prior to the compression, the specimens were heated to the deformation temperature in 5 min. The deformation temperature was measured by thermocouples welded onto the center of a specimen surface. The deformation strain, temperature and strain rate were automatically controlled and recorded. Compression was conducted in a temperature ranging from 150 ◦ C to 400 ◦ C. The strain rates were varied from 0.01 s−1 to 30 s−1 . After the hot compression, the specimens were watercooled. The compositions of the AZ31 and ZK60 magnesium alloys are given in Table 1.
∗ Corresponding author. Tel.: +86 351 392 3956; fax: +86 351 355 7519. E-mail address: [email protected] (Y.Q. Yang). 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.11.106
3. Results and discussion 3.1. Comparison of flow curves of AZ31 and ZK60 Fig. 1 shows the stress–strain curves of the AZ31 alloy under different deformation conditions. The general characteristics of the flow stress curves are similar under all deformation conditions. The flow stress increases to a peak (initial strain hardening) and then decreases to a steady state. Such flow stress behaviors are typical characteristics of hot working that is accompanied by dynamic recrystallization [4,5].
Y.Q. Yang et al. / Materials Science and Engineering A 499 (2009) 238–241 Table 1 The chemical composition of the AZ31 and ZK60 alloys (wt.%) Alloy
Al
Zn
Zr
Mn
Si
Fe
Cu
Mg
AZ31 ZK60
3.20 ≤0.05
1.11 5.6
0.54
0.30 0.10
0.0143 ≤0.05
0.0015 ≤0.05
0.0021 ≤0.05
Balance Balance
Figs. 2 and 3 shows that the thermal simulation curves of ZK60 have the different shapes under various deformation conditions. For example the curves have the character of dynamic recrystallization at a strain rate of 1 s−1 at 200 ◦ C or 400 ◦ C and the material can be deformed successively, while most curves of ZK60 have the other obvious character that around 0.2 in strain the stress reaches the peak and declines rapidly afterwards and lands the lowest. The declining of the curve illustrates that the test specimen has been destroyed and crackle can be found in the test specimens correspondingly. The results from these tests demonstrate that AZ31 and ZK60 alloys have different forms of thermal simulation curves and deformation ability owing to their different eutectic microstructures. The grain boundaries are the weaker part of the material. The frac-
239
ture is formed more easily near the boundaries and spreads along them [6]. There are more grain boundaries in ZK60 magnesium alloy because of the generous eutectic element in ZK60 casting microstructure, which mainly includes rough branch crystal and more secondary Mg–Zn phases which have unfixed shapes [7]. As there are not enough independent slip systems to harmonize the plastic deformation between the boundaries, dislocations always gather around the boundaries and produce stress concentration, and these will give rise to cracks. More serious stress concentration will be produced with higher strain rate (or the lower deformation temperature) and the cracks will expand unsteadily when the stress exceeds the critical value. In AZ31 there is little effect of the second-phase on dynamic recrystallization not only as there is less second-phase material due to the lower content of the alloy metal, but also and especially as the second-phase material dissolves into the matrix material in the deformation process at high temperature. 3.2. Effect of processing parameters on the flow stress of AZ31 Generally, such flow stress behavior is typical of hot working that is accompanied by dynamic recrystallization, which can be
Fig. 1. Flow stress–strain curves of AZ31 alloy in compression at various conditions (a) 400 ◦ C and (b) 1 s−1 .
Fig. 2. Flow stress–strain curves of ZK60 alloy in compression at various temperatures. (a) 0.01 s−1 , (b) 0.1 s−1 , (c) 1 s−1 and (d) 10 s−1
240
Y.Q. Yang et al. / Materials Science and Engineering A 499 (2009) 238–241
Fig. 3. Flow stress–strain curves of ZK60 alloy in compression at various strain rates (a) 150 ◦ C, (b) 200 ◦ C, (c) 250 ◦ C, (d) 300 ◦ C, (e) 350 ◦ C and (f) 400 ◦ C.
described by the thermally activated stored energy developed during deformation-controlled softening mechanisms. With decreasing strain rate or increasing temperature, strain hardening becomes weaker, while strain softening becomes notable (e.g. 400 ◦ C). As a result, the peak stress varies according to processing parameters, so does the peak strain [8]. Under a constant strain rate, the peak stress and the peak strain increase with decreasing temperature. At the same temperature, the peak stress and the peak strain increase with increasing stain rate. As the thermal simulation curves show a flow softening whose mechanism is dynamic recrystallization (except for some curves that show a fluctuation mode especially at 200 ◦ C and 1 s−1 ), it can generally be concluded that dynamic recrystallization is responsible for the high-temperature deformation features of AZ31 alloy [9]. Therefore, the deformation features of AZ31 should be selected from 250 ◦ C to 400 ◦ C and the deformation rates from 0.01 s−1 to 10 s−1 .
3.3. Effect of processing parameters on the flow stress of ZK60 Fig. 2 shows that under a constant strain rate, the peak stress increases with decreasing temperature. Fig. 3 shows that at the same temperature, the peak stress increases with increasing strain rate [10]. It can be concluded from Fig. 2 that the curves have complex properties at the various deformation temperatures as the secondary phase and the base material have various influences under the various conditions. The secondary-phase fraction is small, so there is a small number of grain boundaries in ZK60 at lower temperatures and more secondary phase is formed with increasing deformation temperature, so reduces strength with the adding of grain boundaries. Though the grains separated out grows obviously and they have bad effect on the plasticity, ZK60 alloy still has better plasticity at high temperature since non-basal slip systems of the alloy can be activated at high temperature (higher than the recrystallization temperature) [11,12].
Y.Q. Yang et al. / Materials Science and Engineering A 499 (2009) 238–241 Table 2 True strains of ZK60 under various deformation conditions Train rate (s−1 )
150 ◦ C
200 ◦ C
250 ◦ C
300 ◦ C
350 ◦ C
400 ◦ C
0.01 0.1 1 10
× × × ×
× ≤1.8 ≤1.8 ≤2.0
× × × ≤2.0
≤0.8 × × ×
≤1.8 ≤1.3 × ×
≤1.0 ≤1.8 ≤1.6 ×
Symbol “×” means that ZK60 is fractured easily under the deformation condition.
It can be concluded from Fig. 3 that not only the flow stress rises, but also the decrease of the peak stress becomes more notable with increasing strain rate for ZK60 magnesium alloy. This is mainly because there are more grain boundaries in ZK60, and the dislocations gather more easily around the boundaries with increasing strain rate, which produces the critical stress concentration. So the material will have ruptured as there is not enough time to obtain copious dynamic recrystallization at the higher strain rates. According to the above-mentioned analyses, matching deformation parameters of ZK60 can be stated as in Table 2. 4. Conclusion (i) The thermal simulation curves of AZ31 and ZK60 have different forms under the same deformation condition, as their eutectic
241
microstructures are different. The general curves of AZ31 and few of ZK60 have the character of dynamic recrystallization, while most curves of ZK60 have the obvious fluctuation. (ii) The deformation temperatures of AZ31 should be selected from 250 ◦ C to 400 ◦ C and the deformation rates from 0.01 s−1 to 10 s−1 . (iii) The optimized set of deformation parameters of ZK60 alloy can be stated as in Table 2, which shows that there are two suitable deformation temperature ranges, one is the low temperature (such as 200 ◦ C) and the other is the high temperature (such as 400 ◦ C). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
E. Aghion, B. Bronfin, Mater. Sci. Forum 350–351 (2000) 19–28. B.L. Mordike, T. Ebert, Mater. Sci. Eng. A 302 (2001) 37–45. A. Yamashita, Z. Horita, T.G. Langdon, Mater. Sci. Eng. A 300 (2001) 142–147. W.M. Mo, X.B. Zhao, Dynamic Recrystallization, Metall. Ind. Press, Beijing, 1994. A. Mwembela, E. Konopleva, H.J. Mcqueen, Scr. Mater. 37 (1997) 1789–1795. C.H. Caceres, D.M. Rovera, Light Met. 21 (2001) 151–156. C.J. Ma, M.P. Liu, G.H. Wu, W.J. Ding, Y.P. Zhu, Mater. Sci. Eng. A 349 (2003) 207–212. J.C. Tan, M.J. Tan, Mater. Sci. Eng. A 339 (2003) 124–132. K.N.S. Govind, M.C. Mittal, K. Lal, R.K. Mahanti, C.S. Sivaramakrishnan, Mater. Sci. Eng. A 304–306 (2001) 520–523. E. Cerri, S. Barbagallo, Mater. Lett. 56 (2002) 716–720. M.Y. Gu, Z.G. Wu, Y.P. Jin, M. Kocak, Mater. Sci. Eng. A 272 (1999) 257–263. A. Galiyev, R. Kaibyshev, G. Gottstein, Acta Metall. Mater. 49 (2001) 1199–1207.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Flow stress of wrought magnesium alloys during hot compression deformation at medium and high temperatures Y.Q. Yang a,∗ , B.C. Li a,b , Z.M. Zhang a,b a b
College of Materials Science and Engineering, North University of China, Taiyuan 030051, China Engineering Center for Precision Forming of Shanxi Province, Taiyuan 030051, China
a r t i c l e
i n f o
Article history: Received 26 May 2007 Received in revised form 30 July 2007 Accepted 10 November 2007 Keywords: AZ31 alloy ZK60 alloy Hot compression deformation Flow stress
a b s t r a c t The plasticity of AZ31 and ZK60 magnesium alloys was studied by means of hot compression deformation tests on Gleeble 1500D machines at different temperatures and strain rates. The results indicate that the thermal simulation curves of AZ31 and ZK60 have different forms under the same deformation condition. The general curves of AZ31 have the character of dynamic recrystallization that the flow stress increases to a peak and then decreases to a steady state. Most deformation curves of ZK60 have the obvious character that around 0.2 in strain the stress reaches the peak and declines rapidly to the lowest, while the other curves have the character of dynamic recrystallization. From the analysis, a deformation temperature should reasonably be selected from 250 ◦ C to 400 ◦ C for AZ31, while it was concluded to be 200 ◦ C or 400 ◦ C for ZK60. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
At present, the volume of wrought magnesium products is considerably less than that of casting products. However, wrought magnesium alloys may have more development potential with the higher strength, improved ductility and other mechanical property advantages [1]. Due to the intrinsic characteristics of the h.c.p. structure, magnesium alloys have poor formability and limited ductility at room temperature [2,3], so their forming technology should be proceeded at the medium and high temperature. The absence of the related data of the forming technological factors is still the first cause of limiting the widespread use of the wrought magnesium alloys. Therefore, it is necessary to investigate on the basic forming data of magnesium alloys which have different brands. AZ31 is one of the Mg–Al–Zn alloys and ZK60 is one of the Mg–Zn–Zr alloys, both of them are the most widely used wrought alloys. In the present study, we conducted hot deformation experiments on AZ31 and ZK60 alloys and recorded the corresponding flow stress curves. By investigating the properties of the curves and the deformation parameters, the optimized deformation parameters have been concluded which can provide the theoretical support to formulate the deformation technology of magnesium alloys.
Hot compression tests were performed on the Gleeble 1500D machine. The test samples were cylindrical in shape having a diameter of 10 mm and a length of 15 mm. Prior to the compression, the specimens were heated to the deformation temperature in 5 min. The deformation temperature was measured by thermocouples welded onto the center of a specimen surface. The deformation strain, temperature and strain rate were automatically controlled and recorded. Compression was conducted in a temperature ranging from 150 ◦ C to 400 ◦ C. The strain rates were varied from 0.01 s−1 to 30 s−1 . After the hot compression, the specimens were watercooled. The compositions of the AZ31 and ZK60 magnesium alloys are given in Table 1.
∗ Corresponding author. Tel.: +86 351 392 3956; fax: +86 351 355 7519. E-mail address: [email protected] (Y.Q. Yang). 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.11.106
3. Results and discussion 3.1. Comparison of flow curves of AZ31 and ZK60 Fig. 1 shows the stress–strain curves of the AZ31 alloy under different deformation conditions. The general characteristics of the flow stress curves are similar under all deformation conditions. The flow stress increases to a peak (initial strain hardening) and then decreases to a steady state. Such flow stress behaviors are typical characteristics of hot working that is accompanied by dynamic recrystallization [4,5].
Y.Q. Yang et al. / Materials Science and Engineering A 499 (2009) 238–241 Table 1 The chemical composition of the AZ31 and ZK60 alloys (wt.%) Alloy
Al
Zn
Zr
Mn
Si
Fe
Cu
Mg
AZ31 ZK60
3.20 ≤0.05
1.11 5.6
0.54
0.30 0.10
0.0143 ≤0.05
0.0015 ≤0.05
0.0021 ≤0.05
Balance Balance
Figs. 2 and 3 shows that the thermal simulation curves of ZK60 have the different shapes under various deformation conditions. For example the curves have the character of dynamic recrystallization at a strain rate of 1 s−1 at 200 ◦ C or 400 ◦ C and the material can be deformed successively, while most curves of ZK60 have the other obvious character that around 0.2 in strain the stress reaches the peak and declines rapidly afterwards and lands the lowest. The declining of the curve illustrates that the test specimen has been destroyed and crackle can be found in the test specimens correspondingly. The results from these tests demonstrate that AZ31 and ZK60 alloys have different forms of thermal simulation curves and deformation ability owing to their different eutectic microstructures. The grain boundaries are the weaker part of the material. The frac-
239
ture is formed more easily near the boundaries and spreads along them [6]. There are more grain boundaries in ZK60 magnesium alloy because of the generous eutectic element in ZK60 casting microstructure, which mainly includes rough branch crystal and more secondary Mg–Zn phases which have unfixed shapes [7]. As there are not enough independent slip systems to harmonize the plastic deformation between the boundaries, dislocations always gather around the boundaries and produce stress concentration, and these will give rise to cracks. More serious stress concentration will be produced with higher strain rate (or the lower deformation temperature) and the cracks will expand unsteadily when the stress exceeds the critical value. In AZ31 there is little effect of the second-phase on dynamic recrystallization not only as there is less second-phase material due to the lower content of the alloy metal, but also and especially as the second-phase material dissolves into the matrix material in the deformation process at high temperature. 3.2. Effect of processing parameters on the flow stress of AZ31 Generally, such flow stress behavior is typical of hot working that is accompanied by dynamic recrystallization, which can be
Fig. 1. Flow stress–strain curves of AZ31 alloy in compression at various conditions (a) 400 ◦ C and (b) 1 s−1 .
Fig. 2. Flow stress–strain curves of ZK60 alloy in compression at various temperatures. (a) 0.01 s−1 , (b) 0.1 s−1 , (c) 1 s−1 and (d) 10 s−1
240
Y.Q. Yang et al. / Materials Science and Engineering A 499 (2009) 238–241
Fig. 3. Flow stress–strain curves of ZK60 alloy in compression at various strain rates (a) 150 ◦ C, (b) 200 ◦ C, (c) 250 ◦ C, (d) 300 ◦ C, (e) 350 ◦ C and (f) 400 ◦ C.
described by the thermally activated stored energy developed during deformation-controlled softening mechanisms. With decreasing strain rate or increasing temperature, strain hardening becomes weaker, while strain softening becomes notable (e.g. 400 ◦ C). As a result, the peak stress varies according to processing parameters, so does the peak strain [8]. Under a constant strain rate, the peak stress and the peak strain increase with decreasing temperature. At the same temperature, the peak stress and the peak strain increase with increasing stain rate. As the thermal simulation curves show a flow softening whose mechanism is dynamic recrystallization (except for some curves that show a fluctuation mode especially at 200 ◦ C and 1 s−1 ), it can generally be concluded that dynamic recrystallization is responsible for the high-temperature deformation features of AZ31 alloy [9]. Therefore, the deformation features of AZ31 should be selected from 250 ◦ C to 400 ◦ C and the deformation rates from 0.01 s−1 to 10 s−1 .
3.3. Effect of processing parameters on the flow stress of ZK60 Fig. 2 shows that under a constant strain rate, the peak stress increases with decreasing temperature. Fig. 3 shows that at the same temperature, the peak stress increases with increasing strain rate [10]. It can be concluded from Fig. 2 that the curves have complex properties at the various deformation temperatures as the secondary phase and the base material have various influences under the various conditions. The secondary-phase fraction is small, so there is a small number of grain boundaries in ZK60 at lower temperatures and more secondary phase is formed with increasing deformation temperature, so reduces strength with the adding of grain boundaries. Though the grains separated out grows obviously and they have bad effect on the plasticity, ZK60 alloy still has better plasticity at high temperature since non-basal slip systems of the alloy can be activated at high temperature (higher than the recrystallization temperature) [11,12].
Y.Q. Yang et al. / Materials Science and Engineering A 499 (2009) 238–241 Table 2 True strains of ZK60 under various deformation conditions Train rate (s−1 )
150 ◦ C
200 ◦ C
250 ◦ C
300 ◦ C
350 ◦ C
400 ◦ C
0.01 0.1 1 10
× × × ×
× ≤1.8 ≤1.8 ≤2.0
× × × ≤2.0
≤0.8 × × ×
≤1.8 ≤1.3 × ×
≤1.0 ≤1.8 ≤1.6 ×
Symbol “×” means that ZK60 is fractured easily under the deformation condition.
It can be concluded from Fig. 3 that not only the flow stress rises, but also the decrease of the peak stress becomes more notable with increasing strain rate for ZK60 magnesium alloy. This is mainly because there are more grain boundaries in ZK60, and the dislocations gather more easily around the boundaries with increasing strain rate, which produces the critical stress concentration. So the material will have ruptured as there is not enough time to obtain copious dynamic recrystallization at the higher strain rates. According to the above-mentioned analyses, matching deformation parameters of ZK60 can be stated as in Table 2. 4. Conclusion (i) The thermal simulation curves of AZ31 and ZK60 have different forms under the same deformation condition, as their eutectic
241
microstructures are different. The general curves of AZ31 and few of ZK60 have the character of dynamic recrystallization, while most curves of ZK60 have the obvious fluctuation. (ii) The deformation temperatures of AZ31 should be selected from 250 ◦ C to 400 ◦ C and the deformation rates from 0.01 s−1 to 10 s−1 . (iii) The optimized set of deformation parameters of ZK60 alloy can be stated as in Table 2, which shows that there are two suitable deformation temperature ranges, one is the low temperature (such as 200 ◦ C) and the other is the high temperature (such as 400 ◦ C). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
E. Aghion, B. Bronfin, Mater. Sci. Forum 350–351 (2000) 19–28. B.L. Mordike, T. Ebert, Mater. Sci. Eng. A 302 (2001) 37–45. A. Yamashita, Z. Horita, T.G. Langdon, Mater. Sci. Eng. A 300 (2001) 142–147. W.M. Mo, X.B. Zhao, Dynamic Recrystallization, Metall. Ind. Press, Beijing, 1994. A. Mwembela, E. Konopleva, H.J. Mcqueen, Scr. Mater. 37 (1997) 1789–1795. C.H. Caceres, D.M. Rovera, Light Met. 21 (2001) 151–156. C.J. Ma, M.P. Liu, G.H. Wu, W.J. Ding, Y.P. Zhu, Mater. Sci. Eng. A 349 (2003) 207–212. J.C. Tan, M.J. Tan, Mater. Sci. Eng. A 339 (2003) 124–132. K.N.S. Govind, M.C. Mittal, K. Lal, R.K. Mahanti, C.S. Sivaramakrishnan, Mater. Sci. Eng. A 304–306 (2001) 520–523. E. Cerri, S. Barbagallo, Mater. Lett. 56 (2002) 716–720. M.Y. Gu, Z.G. Wu, Y.P. Jin, M. Kocak, Mater. Sci. Eng. A 272 (1999) 257–263. A. Galiyev, R. Kaibyshev, G. Gottstein, Acta Metall. Mater. 49 (2001) 1199–1207.