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High temperature deformation behavior of extruded AZ31B magnesium alloy
Journal of Materials Processing Tech. 251 (2018) 360–368
Contents lists available at ScienceDirect
Journal of Materials Processing Tech. journal homepage: www.elsevier.com/locate/jmatprotec
Research Paper
High temperature deformation behavior of extruded AZ31B magnesium alloy
MARK
⁎
Tsz Wun Wong , Amir Hadadzadeh, Mary A. Wells University of Waterloo, 200 University Ave. West, Waterloo, N2L 3G1, Canada
A R T I C L E I N F O
A B S T R A C T
Keywords: Hot deformation Magnesium Extruded AZ31B Hot compression Dynamic recrystallization
Uniaxial isothermal compression tests were conducted on cylindrical specimens extracted from an extruded AZ31B rod along both the extrusion and radial directions over the temperature range of 300 °C–500 °C, at strain rates of 10−3 s−1–1.0 s−1. The anisotropy of flow observed at high strain rates in specimens oriented along the extrusion direction can likely be attributed to the dominance of tensile twinning at temperatures below 400 °C, and pyramidal slip for temperatures above 400 °C. The degree of anisotropy increased with deformation strain rate, with specimens deformed at the lowest strain rate of 10−3 s−1 exhibiting little to no anisotropy. Samples oriented along the radial direction exhibited strong anisotropy of flow at low deformation temperatures, and the extent of anisotropy decreased with increasing deformation temperature. It was determined that the occurrence of basal slip at select locations away from the compression axis likely resulted in preferential elongation of the sample along the circumferential direction of the extruded rod. Moreover, cracking was observed at 300 °C and macroscopic shear bands were observed at deformation temperatures below 450 °C, which suggested that low temperature deformation was less favorable. It was observed that finer dynamically recrystallized (DRX) grains could be obtained by processing at lower temperatures and/or higher strain rates. However, greater microstructural homogeneity was achieved through processing at high temperatures and low strain rates. Furthermore, it was evident that the area fraction of DRX grains increased with strain. Grain growth after deformation occurs rapidly within the first 60 s, and little to no grain growth was observed beyond that.
1. Introduction With increasingly stringent emissions regulations coming into effect, automotive manufacturers have been investigating methods to reduce fuel consumption in their vehicles. Friedrich and Schumann (2001) reported that a reduction in overall vehicle weight is one of the most effective methods of lowering fuel consumption, and subsequent exhaust emissions. As one of the lightest engineering materials available, magnesium (Mg) has attracted attention as a potential material substitute for steels and aluminum alloys currently used in automotive applications. Although Mg has been used in select automotive applications, such as intake manifolds, seat structures, and mounting brackets, the applications have mainly been limited to die-cast components, as shown by Friedrich and Mordike (2004). The reluctance to apply Mg to structural components is due to the relatively poor mechanical properties exhibited by castings. On the other hand, Mordike and Ebert (2001) reported that the limited applications of wrought Mg alloys have mainly been due to poor formability of Mg at ambient temperature, as a result of its HCP crystal structure, and strong texture
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evolution during plastic deformation (Agnew et al., 2005). In general, it can be said that wrought Mg alloys exhibit more desirable mechanical properties for fatigue-critical structural components than the cast alloys (Friedrich and Mordike, 2004), and the most direct method of producing a near net shape wrought component with a nonuniform cross section is by forging. The hesitation to move to wrought alloys can be attributed to limited knowledge in the forming of Mg alloys. However, the poor formability of Mg at low temperatures is well documented, and has been attributed to the lack of contribution from non-basal slip systems (Doege and Dröder, 2001). Luckily, the critical resolved shear stresses (CRSS) required for slip on non-basal systems has shown strong dependence on deformation temperature, and these systems can be readily activated at elevated forming temperatures, as reported by Chapuis and Driver (2010). The CRSS for the various slip systems, the orientation of the grains, and the direction of loading all influence the mechanism(s) activated during deformation. Furthermore, it is well documented that Mg alloys develop strong textures from processing; for example, Al-Samman and Gottstein (2008) reported that a hot extruded Mg rod exhibits a strong axisymmetric fiber texture
Corresponding author. E-mail address: [email protected] (T.W. Wong).
http://dx.doi.org/10.1016/j.jmatprotec.2017.09.006 Received 1 March 2017; Received in revised form 28 August 2017; Accepted 2 September 2017 Available online 05 September 2017 0924-0136/ © 2017 Elsevier B.V. All rights reserved.
Journal of Materials Processing Tech. 251 (2018) 360–368
T.W. Wong et al.
Metallographic examination was conducted along the compression axis, at the mid-plane of each specimen, using an Olympus BH2-UMA optical microscope. Chemical etching with an acetal-picral solution followed mechanical grinding and polishing. The offset compression specimen consisted primarily of large grains, with several finer grains coexisting at the grain boundaries; the average grain size was 55 μm. The centerline specimen consisted of fine, equiaxed grains averaging 12 μm in size. A narrow band of elongated grains was observed to exist at Ø32 mm but the overall volume fraction occupied by the region is miniscule by comparison. Furthermore, the initial axisymmetric fiber texture was measured using a Bruker D8 Discover x-ray diffraction (XRD) system. The initial microstructures are shown in Fig. 2, and the starting texture is shown in Fig. 3. The starting texture of the material is a typical fiber texture, where the majority of grains have their basal planes oriented parallel to the extrusion axis, as observed by Tang et al. (2011).
where the c-axes are oriented in the radial direction. In hot working, the processing conditions under which dynamic recrystallization (DRX) occurs are of utmost importance, as the process results in the formation of strain-free grains and allows the microstructure to recover from prior straining (Humphreys and Hatherly, 2004). Yukutake et al. (2003) observed significant mechanical anisotropy in a rolled AZ31 sheet. Compression samples oriented along the transverse and rolling directions of the sheet exhibited elliptical cross-sections, and compression tests conducted at temperatures below 300 °C revealed low yield points followed by rapid strain hardening. The observed stress-strain behavior was attributed to tensile twinning. Lee et al. (2011) attributed the twinning behavior and subsequent anisotropy in material flow to the initial texture, and Dai et al. (2011) also reached a similar conclusion. Rao et al. (2012) conducted forging studies and proposed that the dominance of pyramidal slip at high temperature could negate the effects of initial texture and restore the symmetry of flow. The majority of studies investigating the hot deformation behavior of Mg alloys to date had been conducted using a hot rolled plate, however using an extruded rod as the initial material may be more favorable for certain component geometries in terms of material flow and die filling. To add to the existing knowledge on Mg alloys, the objective of the current study is to investigate the anisotropic hot deformation behavior of extruded AZ31B Mg alloy under compression, and to identify viable conditions for an industrial forging operation. To do so, hot compression tests were conducted on extruded AZ31B in both the extrusion and transverse directions using a Gleeble® 3500 thermal-mechanical simulation system. Flow anisotropy and microstructural evolution during hot deformation were studied and correlated to the deformation conditions.
2.2. Compression tests The uniaxial compression tests were conducted in the temperature range of 300 °C–500 °C, at constant true strain rates of 10−3 s−1–1.0 s−1, using a Gleeble 3500® thermal-mechanical simulation system. Prior to compression, a nickel-graphite-based lubricant was applied to the specimen end faces to minimize the effects of friction on the contact surfaces. A thermocouple welder was used to instrument each specimen with a k-type thermocouple at the axial center to monitor and control the deformation temperature. Specimens were resistively heated to the specified deformation temperature at a rate of 10 °C per second, and held for five seconds to minimize any thermal gradients in the specimen. Specimens were deformed to various levels of strain and each was water quenched immediately after deformation to preserve the microstructure.
2. Materials and experimental methods 2.1. Test material
3. Results and discussion Cylindrical specimens, Ø10 mm by 15 mm, for uniaxial compression were extracted from a Ø63.5 mm AZ31B extruded rod purchased from Magnesium Elektron. The specimens were extracted in two orientations along a Ø44 mm reference circle; those with the compression axis oriented along the extrusion direction were termed ED, and those with the compression axis aligned along the transverse direction were termed TD. A schematic illustrating the locations and orientations of the compression specimens is provided in Fig. 1. The chemical composition of the radially-offset compression specimens was obtained using inductively coupled plasma optical emission spectroscopy (ICP-OES), and each was shown to consist of 3.81% aluminum (Al), 1.28% zinc (Zn), and 0.70% manganese (Mn), by weight.
3.1. Anisotropy in ED specimens To ensure proper die filling in Mg forgings, it is vital to understand the role of texture in the material flow behavior. With consideration to the initial axisymmetric texture of the extruded rod, it was evident that the c-axes of the grains in the radially-offset ED compression specimen were oriented along one direction. With a strong and relatively uniform texture in the compression sample, anisotropy of flow was to be expected. The geometries of the deformed ED specimens are shown in Fig. 4. Specimens deformed at 300 °C and high strain rate experienced Fig. 1. Locations and orientations of compression specimens.
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Fig. 2. Initial microstructure a) along the centerline of the extrusion, and b) in the radially-offset compression sample.
Fig. 3. Starting texture of extruded rod obtained using XRD.
dependence on deformation temperature, as shown by the minor to major axis ratios in Fig. 5. The result was not in agreement with those of Dai et al. (2011), where anisotropy of flow decreased with strain rate but also with increasing temperature. Scribe lines oriented
fractures. Furthermore, it was evident that the specimens deformed at the lowest strain rate of 10−3 s−1 exhibited minimal anisotropy of flow, and the anisotropy increased with deformation strain rate. Omitting the fractured specimens, the degree of anisotropy showed no clear
Fig. 4. Top-view images of ED compression specimens under various processing conditions, at a true strain of 0.5.
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longer observed in the microstructure or in the stress-strain behavior at temperatures beyond 400 °C, even at high strain rates. Therefore, it is likely that the observed anisotropy was the result of another mechanism. It was suspected that the dominant deformation mechanism had transitioned to pyramidal slip beyond temperatures of 400 °C, but had resulted in a similar major strain component along the same < c > direction. The presence of anisotropy after deformation at high temperature and high strain rate suggested that the partitioning of prismatic slip and pyramidal slip had resulted in a major strain component along the < c > direction. For deformation temperatures below 400 °C, a decrease in twinning activity at lower strain rates resulted in a partitioning of tensile twinning and prismatic slip which resulted in symmetric material flow. A similar conclusion was proposed for deformation temperatures beyond 400 °C, but between prismatic slip and pyramidal slip. To fully understand the flow behavior, grain rotation to orientations of hard slip, incurred from twinning (Tenckhoff, 1988) must be considered. Although twinning activity was greater at high strain rates (Dai et al., 2011), Meng et al. (2005) observed accelerated grain rotation with an increase in deformation strain rate. XRD measurements obtained at various strains showed deviation from the initial fiber texture and the partial development of a basal plane texture, as shown in Fig. 7. Although the anisotropy was observed to decrease at high strain (i.e. beyond a true strain of 1.0), the major axis was still observed to be oriented in the radial direction of the extrusion after deformation at 500 °C and 1.0 s−1. The observed decrease in anisotropy may be related to grain rotation during deformation, which resulted in basal slip during grain rotation, and pyramidal slip after the basal plane texture had been developed. The dominance of pyramidal slip had resulted in symmetric material flow at high strains. To confirm the role of initial texture on the observed material flow behavior, a centerline compression specimen was deformed to a true strain of 0.5, at 500 °C and 1.0 s−1. Unlike the radially-offset specimen, the centerline specimen exhibited symmetric material flow as a result of its axisymmetric initial texture.
Fig. 5. Minor to major axis ratios for ED specimens deformed to a final strain of 0.5, under various conditions.
perpendicular to the radial direction on the specimen end face revealed that the major axis was aligned with the c-axes of the grains (i.e. radial direction of the extrusion); this result was consistent with those of Dai et al. (2011). The forging study by Rao et al. (2012) observed the minor axis to be oriented along the normal direction (i.e. c-axes of the grains) when forging was conducted along the rolling or transverse directions. The relative strength of the initial textures and the stress states of the samples in each study may have contributed to the differences in observed flow behavior. With consideration to the initial texture of the specimen, the compressive load upon deformation was applied perpendicular to two parallel prismatic faces of the HCP structure. With such an orientation, it was evident that the grains were oriented favorably for tensile twinning, prismatic slip, and pyramidal slip (Chapuis and Driver, 2010). An increase in anisotropy, combined with the presence of twins in the microstructure, after high strain rate deformation was due to an increase in tensile twinning activity under high strain rate conditions (Dai et al., 2011). It was proposed by Dai et al. (2011) that the anisotropy observed in the hot compression of rolled AZ31B was due to tensile twinning activity, which had resulted in a major strain component along the < c > direction. Examination of the pyramidal system revealed a similar result; however prismatic slip resulted in a strain component along the < a > direction. To verify the dominance of tensile twinning, an interrupted compression test was conducted to a final strain of 0.1, where the stress-strain curve showed softening prior to hardening. The resultant microstructure was observed to contain numerous twins, as shown in Fig. 6. However, tensile twinning was no
3.2. Anisotropy in TD specimens Considering the initial texture in the extruded rod, it can be said that the c-axis compression experienced by the TD specimens is similar to compression along the normal direction of a hot rolled plate. The geometries of the deformed TD specimens, at a final strain of 0.5, are shown in Fig. 8. As with the ED specimens, fractures were observed under low temperature and high strain rate processing conditions. Furthermore, it was evident that the anisotropy of flow showed strong dependence on deformation temperature, and the specimens deformed at high temperature and high strain rate experienced the least anisotropy. With the initial texture of the specimen resisting basal slip and prismatic slip, it was evident that only compressive twinning and pyramidal slip were oriented favorably for activation. However, the studies by Dai et al. (2011) and Rao et al. (2012) discussed earlier, conducted using hot rolled plates, observed no anisotropy of flow under those circumstances. Scribe lines oriented perpendicular to the extrusion direction on the specimen end face revealed that the minor axis was oriented along the extrusion direction. With careful consideration to the initial axisymmetric texture of the extrusion, it was deduced that the initial texture in the specimen was not as uniform as that in a hot rolled plate. Grains located away from the specimen centerline, but along the scribe line, were oriented favorably for basal slip, as illustrated schematically in Fig. 9. However the activation of basal slip at the select locations on the specimen resulted in preferential material flow along the direction of the scribe line. The CRSS required for pyramidal slip decreased rapidly with increasing deformation temperature (Chapuis and Driver, 2010), and the activation of basal slip became less favorable as the
Fig. 6. Microstructure of ED specimen after deformation at 300 °C and 1.0 s−1, to a final strain of 0.1. (See Fig. 11 for corresponding flow curve).
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Fig. 7. Texture of ED specimen after deformation at 500 °C and 1.0 s−1, to a final strain of 1.0.
anisotropy of flow. To further verify the influence of basal slip on the observed flow stress, small scale TD specimens (Ø4 mm by 6 mm) with more uniform initial texture were extracted around the same Ø44 mm reference circle of the extrusion. Although the minor/major axis ratios were similar to that of the full size TD specimens, the observed flow stress had increased, suggesting the partial suppression of basal slip. With non-basal slip readily activated at higher temperatures, the discrepancy in flow stress between the ED and TD specimens diminishes beyond 400 °C, as shown in Fig. 12.
temperature increased. Subsequently, the specimen deformed at 500 °C and 1.0 s−1 resulted in the least anisotropy, as grain rotation was accelerated at high strain rate (Meng et al., 2005); grains which were initially oriented favorably for basal slip had more rapidly (i.e. lower strain) transitioned to the basal plane deformation texture and suppressed the activation of basal slip. The final basal plane texture after deformation is shown in Fig. 10. In addition to the specimen geometry, the stress-strain curves revealed that the TD specimen exhibited lower yield and peak stresses compared to the ED specimens under low temperature processing conditions, as shown in Fig. 11. The softening and hardening behavior in the ED specimen that was noted earlier can also be seen. This result was not in agreement with those by Lee et al. (2011), where c-axis compression along the normal direction resulted in higher yield and peak stresses than other directions. The behavior observed in the flow stress was attributed to the activation of basal slip at low strains. As demonstrated by the specimens deformed at high temperature, it was deduced that the suppression of basal slip would result in less
3.3. Microstructural development Closer examination of the specimens revealed that deformation at temperatures below 450 °C for strain rates of 1.0 s−1 and 10−1 s−1, and below 400 °C for strain rates of 10−2 s−1 and 10−3 s−1, resulted in the formation of shear bands on the surface. Sun et al. (2010) observed that shear bands consisted of fine DRX grains, and Kim et al. (2012) proposed that the shear bands were formed when coarse grains were split
Fig. 8. Top-view images of TD specimens deformed under various processing conditions, at a true strain of 0.5.
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Fig. 11. Flow stress curves of ED and TD specimens at 300 °C and 1.0 s−1.
Fig. 9. Schematic of initial grain and scribe line orientations in TD specimens.
by twins at low strains, with subsequent DRX occurring within the high energy region, and strain localization upon further deformation. Therefore, processing conditions that resulted in the formation of shear bands are less favorable for bulk deformation. As shown in Figs. 13 and 14, shear bands were evident on the specimen surface and in the microstructure after deformation at 300 °C and 1.0 s−1, but were not observed at 500 °C and 1.0 s−1. With preference given to high strain rate conditions for increased production rates, it would be beneficial to conduct forging operations in the temperature range of 450 °C–500 °C. Examining the microstructures of the ED specimens at a strain of 0.5 under various processing conditions, it was evident that the DRX grain size increased with deformation temperature and also with decreasing strain rate. The observations were in agreement with those by Beer and Barnett (2007). The corresponding microstructures are shown in Fig. 15. As a result of the larger DRX grain size, the high temperature and low strain rate deformation conditions resulted in the highest level of microstructural homogeneity and would require fewer cycles of DRX to produce a given volume fraction of DRX grains. To study the effect of strain on the microstructure, ED specimens were deformed at 500 °C and 1.0 s−1 to various levels of strain, as shown in Fig. 16. The results supported those by Yang et al. (2003), who had observed that the DRX
Fig. 12. Flow stress curves for ED and TD specimens at 450 °C and 500 °C, at 1.0 s−1.
grain size changes very little with strain. Consistent with the observations made by Fatemi-Varzaneh et al. (2007), the DRX volume fraction was observed to increase with deformation strain, however it was evident that a fully recrystallized microstructure was not obtained at a strain of 1.0. In addition to the processing condition, the initial grain size variation in the extruded rod played a role on the deformation behavior. The centerline specimen deformed at 500 °C and 1.0 s−1, resulted in lower flow stresses than the radially-offset specimen. The result was consistent with those observed by Barnett et al. (2004), where the finegrained AZ31 exhibited lower flow stresses. In the current investigation, the lower Al content at the centerline may also have contributed to the lower flow stress. Furthermore, Miao et al. (2010) had observed rapid grain growth in a fine-grained AZ31 rolled plate, it was important to examine the influence of the finer grain size observed along the centerline of the extruded rod. To study the static grain growth kinetics of fine-grained AZ31, a centerline ED specimen was heated to 500 °C at 10 °C per second and water quenched immediately. Although both Fig. 10. Texture of TD specimen after deformation at 500 °C and 1.0 s−1, to a final strain of 1.0.
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Fig. 13. Images of TD specimens after deformation at a) 300 °C and 1.0 s−1 and b) 500 °C and 1.0 s−1, at a true strain of 0.5. (Images not to scale).
Fig. 14. Microstructures of TD specimens after deformation at a) 300 °C and 1.0 s−1 and b) 500 °C and 1.0 s−1, at a true strain of 0.5.
processing plays a vital role in determining the mechanical properties. To study the effect of cooling rate on the final microstructure, TD specimens were deformed at 500 °C and 1.0 s−1 to a strain of 1.0, and subjected to various cooling rates. Three cooling rates were studied: a five minute hold at the deformation temperature, die cooling (i.e.
normal and abnormal grain growth were observed after heating, as shown in Fig. 17, the final microstructures after deformation to a strain of 0.5 were very similar for both the centerline and radially-offset specimens. For a structural component, the final microstructure after
Fig. 15. Microstructures of ED specimens deformed under various processing conditions, at a true strain of 0.5.
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Fig. 16. Microstructures of ED specimens after deformation at 500 °C and 1.0 s−1, at various levels of strain.
The other two cooling cases resulted in larger, but equiaxed, grains and a homogeneous microstructure. The similarity between the two cases suggested that static grain growth had occurred rapidly after deformation, and little to no further grain growth had occurred after the first 60 s. 4. Conclusions The high temperature deformation behavior of extruded AZ31B in both the extrusion and transverse directions was studied through isothermal uniaxial compression tests over the temperature range of 300 °C–500 °C, at strain rates of 10−3 s−1–1.0s−1. The key findings can be summarized as follows: 1. Anisotropy of flow observed in the ED specimens increased with deformation strain rate for all investigated temperatures. Anisotropy was likely to be attributed to the dominance of tensile twinning for low deformation temperatures (i.e. below 400 °C), and pyramidal slip for high deformation temperatures (i.e. above 400 °C). Symmetric material flow was observed at the deformation strain rate of 10−3 s−1 for all temperatures. Deformation of a centerline specimen resulted in no anisotropy. 2. Anisotropy of flow in the TD specimens was reduced at high temperature and high strain rate. The observed anisotropy was attributed to the occurrence of basal slip at select locations away from the specimen centerline. The accelerated rate of grain rotation at high
Fig. 17. Microstructure of centerline ED specimen after heating.
specimen remained between cooling anvils with no further heat input), and a water quench. Temperature readings showed that the die cooled specimen had cooled to below 200 °C after 60 s, despite resting between two hot anvils. The water quenched specimen exhibited a partially recrystallized microstructure with fine DRX grains, as shown in Fig. 18.
Fig. 18. Microstructures of TD specimens after deformation at 500 °C and 1.0 s−1 to a final strain of 1.0, followed by a) a water quench and b) die cooling.
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alloy: examining the role of texture on the deformation mechanisms. Mater. Sci. Eng. A 488, 406–414. Barnett, M., Beer, A., Atwell, D., Oudin, A., 2004. Influence of grain size on hot working stresses and microstructures in Mg-3Al-1Zn. Scr. Mater. 51, 19–24. Beer, A., Barnett, M., 2007. Microstructural development during hot working of Mg-3Al1Zn. Metall. Mater. Trans. A 38, 1856–1867. Chapuis, A., Driver, J., 2010. Temperature dependency of slip and twinning in plane strain compressed magnesium single crystals. Acta Mater. 59, 1986–1994. Dai, Q., Zhang, D., Chen, X., 2011. On the anisotropic deformation of AZ31 Mg alloy under compression. Mater. Des. 32, 5004–5009. Doege, E., Dröder, K., 2001. Sheet metal forming of magnesium wrought alloys − formability. J. Mater. Process Technol. 115 (1), 14–19. Fatemi-Varzaneh, S., Zarei-Hanzaki, A., Beladi, H., 2007. Dynamic recrystallization in AZ31 magnesium alloy. Mater. Sci. Eng. A 456, 52–57. Friedrich, H., Mordike, B. (Eds.), 2004. Magnesium Technology: Metallurgy, Design Data, Applications. Springer, Berlin. Friedrich, H., Schumann, S., 2001. Research for a new age of magnesium in the automotive industry. J. Mater. Process. Technol. 117 (3), 276–281. Humphreys, F., Hatherly, M., 2004. Recrystallization and Related Annealing Phenomena. Elsevier Ltd., Kidlington. Kim, H., Lee, J., Lee, C., Bang, W., Ahn, S., Chang, Y., 2012. Shear band formation during hot compression of AZ31 Mg alloy sheets. Mater. Sci. Eng. A 558, 431–438. Lee, B., Park, S., Hong, S., Park, K., Lee, C., 2011. Role of initial texture on the plastic anisotropy of Mg-3Al-1Zn alloy at various temperatures. Mater. Sci. Eng. A 528, 1162–1172. Meng, L., Yang, P., Zhao, Z., Mao, W., 2005. An OIM analysis on the deformation mechanism in hot compressed AZ31 magnesium alloy. Mater. Sci. Forum 488–489, 633–636. Miao, Q., Hu, L., Wang, X., Wang, E., 2010. Grain growth kinetics of a fine-grained AZ31 magnesium alloy produced by hot rolling. J. Alloys Compd. 493, 87–90. Mordike, B., Ebert, T., 2001. Magnesium: properties − applications − potential. Mater. Sci. Eng. A 302, 37–45. Rao, K., Prasad, Y., Suresh, K., 2012. Anisotropy of flow during isothermal forging of rolled AZ31B magnesium alloy rolled plate in three orthogonal directions: correlation with processing maps. Mater. Sci. Eng. A 558, 30–38. Sun, D., Chang, C., Kao, P., 2010. Microstructural study of strain localization in hot compressed Mg-3Al-1Zn. Mater. Sci. Eng. A 527, 7050–7056. Tang, W., Huang, S.Z., Li, D., Peng, Y., 2011. Influence of extrusion parameters on grain size and texture distributions of AZ31 alloy. J. Mater. Process. Technol. 211, 1203–1209. Tenckhoff, E., 1988. Deformation Mechanisms, Texture, and Anisotropy in Zirconium and Zircaloy. ASTM International, Philadelphia. Yang, X., Miura, H., Sakai, T., 2003. Dynamic evolution of new grains in magnesium alloy AZ31 during hot deformation. Mater. Trans. 44, 197–203. Yukutake, E., Kaneko, J., Sugamata, M., 2003. Anisotropy and non-uniformity in plastic behavior of AZ31 magnesium alloy plates. Mater. Trans. 44 (4), 452–457.
strain rates, combined with low CRSS for pyramidal slip at high temperatures, resulted in the suppression of basal slip and the restoration of symmetric material flow. 3. Shear bands were observed in specimens deformed at temperatures below 450 °C for strain rates of 1.0 s−1 and 10−1 s−1, and below 400 °C for strain rates of 10−2 s−1 and 10−3 s−1. The shear bands lead to strain localization, making these processing conditions less favorable. 4. After deformation at 500 °C and 1.0 s−1, the volume fraction of DRX grains increased with strain, but with a stable DRX grain size. A fully recrystallized microstructure was not obtained at a final strain of 1.0. 5. Slow cooling of the specimen (i.e. sample temperature had fallen from 500 °C to below 200 °C in 60 s) was conducted after deformation at 500 °C and 1.0 s−1, and static grain growth was observed to occur rapidly, resulting in larger, equiaxed grains and a more uniform microstructure. Longer times spent at high temperature resulted in little to no further grain growth after the first 60 s. Acknowledgements This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and Automotive Partnership Canada (APC) program under the APCPJ 459269-13 grant, with contributions from Multimatic Inc., Ford Motor Company, University of Waterloo, Canada Foundation for Innovation (CFI), CanmetMATERIALS, and Centerline Windsor. Furthermore, the authors wish to acknowledge Dr. Hamid Jaded and Dr. Sugrib Shaha for their help in obtaining the XRD textures, and Mr. Mark Whitney for his assistance in performing the compression tests. References Agnew, S., Mehrotra, P.L., Stoica, G., Liaw, P., 2005. Texture evolution of five wrought magnesium alloys during route A equal channel angular extrusion: Experiments and simulations. Acta Mater. 53, 3135–3146. Al-Samman, T., Gottstein, G., 2008. Room temperature formability of a magnesium AZ31
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Contents lists available at ScienceDirect
Journal of Materials Processing Tech. journal homepage: www.elsevier.com/locate/jmatprotec
Research Paper
High temperature deformation behavior of extruded AZ31B magnesium alloy
MARK
⁎
Tsz Wun Wong , Amir Hadadzadeh, Mary A. Wells University of Waterloo, 200 University Ave. West, Waterloo, N2L 3G1, Canada
A R T I C L E I N F O
A B S T R A C T
Keywords: Hot deformation Magnesium Extruded AZ31B Hot compression Dynamic recrystallization
Uniaxial isothermal compression tests were conducted on cylindrical specimens extracted from an extruded AZ31B rod along both the extrusion and radial directions over the temperature range of 300 °C–500 °C, at strain rates of 10−3 s−1–1.0 s−1. The anisotropy of flow observed at high strain rates in specimens oriented along the extrusion direction can likely be attributed to the dominance of tensile twinning at temperatures below 400 °C, and pyramidal slip for temperatures above 400 °C. The degree of anisotropy increased with deformation strain rate, with specimens deformed at the lowest strain rate of 10−3 s−1 exhibiting little to no anisotropy. Samples oriented along the radial direction exhibited strong anisotropy of flow at low deformation temperatures, and the extent of anisotropy decreased with increasing deformation temperature. It was determined that the occurrence of basal slip at select locations away from the compression axis likely resulted in preferential elongation of the sample along the circumferential direction of the extruded rod. Moreover, cracking was observed at 300 °C and macroscopic shear bands were observed at deformation temperatures below 450 °C, which suggested that low temperature deformation was less favorable. It was observed that finer dynamically recrystallized (DRX) grains could be obtained by processing at lower temperatures and/or higher strain rates. However, greater microstructural homogeneity was achieved through processing at high temperatures and low strain rates. Furthermore, it was evident that the area fraction of DRX grains increased with strain. Grain growth after deformation occurs rapidly within the first 60 s, and little to no grain growth was observed beyond that.
1. Introduction With increasingly stringent emissions regulations coming into effect, automotive manufacturers have been investigating methods to reduce fuel consumption in their vehicles. Friedrich and Schumann (2001) reported that a reduction in overall vehicle weight is one of the most effective methods of lowering fuel consumption, and subsequent exhaust emissions. As one of the lightest engineering materials available, magnesium (Mg) has attracted attention as a potential material substitute for steels and aluminum alloys currently used in automotive applications. Although Mg has been used in select automotive applications, such as intake manifolds, seat structures, and mounting brackets, the applications have mainly been limited to die-cast components, as shown by Friedrich and Mordike (2004). The reluctance to apply Mg to structural components is due to the relatively poor mechanical properties exhibited by castings. On the other hand, Mordike and Ebert (2001) reported that the limited applications of wrought Mg alloys have mainly been due to poor formability of Mg at ambient temperature, as a result of its HCP crystal structure, and strong texture
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evolution during plastic deformation (Agnew et al., 2005). In general, it can be said that wrought Mg alloys exhibit more desirable mechanical properties for fatigue-critical structural components than the cast alloys (Friedrich and Mordike, 2004), and the most direct method of producing a near net shape wrought component with a nonuniform cross section is by forging. The hesitation to move to wrought alloys can be attributed to limited knowledge in the forming of Mg alloys. However, the poor formability of Mg at low temperatures is well documented, and has been attributed to the lack of contribution from non-basal slip systems (Doege and Dröder, 2001). Luckily, the critical resolved shear stresses (CRSS) required for slip on non-basal systems has shown strong dependence on deformation temperature, and these systems can be readily activated at elevated forming temperatures, as reported by Chapuis and Driver (2010). The CRSS for the various slip systems, the orientation of the grains, and the direction of loading all influence the mechanism(s) activated during deformation. Furthermore, it is well documented that Mg alloys develop strong textures from processing; for example, Al-Samman and Gottstein (2008) reported that a hot extruded Mg rod exhibits a strong axisymmetric fiber texture
Corresponding author. E-mail address: [email protected] (T.W. Wong).
http://dx.doi.org/10.1016/j.jmatprotec.2017.09.006 Received 1 March 2017; Received in revised form 28 August 2017; Accepted 2 September 2017 Available online 05 September 2017 0924-0136/ © 2017 Elsevier B.V. All rights reserved.
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Metallographic examination was conducted along the compression axis, at the mid-plane of each specimen, using an Olympus BH2-UMA optical microscope. Chemical etching with an acetal-picral solution followed mechanical grinding and polishing. The offset compression specimen consisted primarily of large grains, with several finer grains coexisting at the grain boundaries; the average grain size was 55 μm. The centerline specimen consisted of fine, equiaxed grains averaging 12 μm in size. A narrow band of elongated grains was observed to exist at Ø32 mm but the overall volume fraction occupied by the region is miniscule by comparison. Furthermore, the initial axisymmetric fiber texture was measured using a Bruker D8 Discover x-ray diffraction (XRD) system. The initial microstructures are shown in Fig. 2, and the starting texture is shown in Fig. 3. The starting texture of the material is a typical fiber texture, where the majority of grains have their basal planes oriented parallel to the extrusion axis, as observed by Tang et al. (2011).
where the c-axes are oriented in the radial direction. In hot working, the processing conditions under which dynamic recrystallization (DRX) occurs are of utmost importance, as the process results in the formation of strain-free grains and allows the microstructure to recover from prior straining (Humphreys and Hatherly, 2004). Yukutake et al. (2003) observed significant mechanical anisotropy in a rolled AZ31 sheet. Compression samples oriented along the transverse and rolling directions of the sheet exhibited elliptical cross-sections, and compression tests conducted at temperatures below 300 °C revealed low yield points followed by rapid strain hardening. The observed stress-strain behavior was attributed to tensile twinning. Lee et al. (2011) attributed the twinning behavior and subsequent anisotropy in material flow to the initial texture, and Dai et al. (2011) also reached a similar conclusion. Rao et al. (2012) conducted forging studies and proposed that the dominance of pyramidal slip at high temperature could negate the effects of initial texture and restore the symmetry of flow. The majority of studies investigating the hot deformation behavior of Mg alloys to date had been conducted using a hot rolled plate, however using an extruded rod as the initial material may be more favorable for certain component geometries in terms of material flow and die filling. To add to the existing knowledge on Mg alloys, the objective of the current study is to investigate the anisotropic hot deformation behavior of extruded AZ31B Mg alloy under compression, and to identify viable conditions for an industrial forging operation. To do so, hot compression tests were conducted on extruded AZ31B in both the extrusion and transverse directions using a Gleeble® 3500 thermal-mechanical simulation system. Flow anisotropy and microstructural evolution during hot deformation were studied and correlated to the deformation conditions.
2.2. Compression tests The uniaxial compression tests were conducted in the temperature range of 300 °C–500 °C, at constant true strain rates of 10−3 s−1–1.0 s−1, using a Gleeble 3500® thermal-mechanical simulation system. Prior to compression, a nickel-graphite-based lubricant was applied to the specimen end faces to minimize the effects of friction on the contact surfaces. A thermocouple welder was used to instrument each specimen with a k-type thermocouple at the axial center to monitor and control the deformation temperature. Specimens were resistively heated to the specified deformation temperature at a rate of 10 °C per second, and held for five seconds to minimize any thermal gradients in the specimen. Specimens were deformed to various levels of strain and each was water quenched immediately after deformation to preserve the microstructure.
2. Materials and experimental methods 2.1. Test material
3. Results and discussion Cylindrical specimens, Ø10 mm by 15 mm, for uniaxial compression were extracted from a Ø63.5 mm AZ31B extruded rod purchased from Magnesium Elektron. The specimens were extracted in two orientations along a Ø44 mm reference circle; those with the compression axis oriented along the extrusion direction were termed ED, and those with the compression axis aligned along the transverse direction were termed TD. A schematic illustrating the locations and orientations of the compression specimens is provided in Fig. 1. The chemical composition of the radially-offset compression specimens was obtained using inductively coupled plasma optical emission spectroscopy (ICP-OES), and each was shown to consist of 3.81% aluminum (Al), 1.28% zinc (Zn), and 0.70% manganese (Mn), by weight.
3.1. Anisotropy in ED specimens To ensure proper die filling in Mg forgings, it is vital to understand the role of texture in the material flow behavior. With consideration to the initial axisymmetric texture of the extruded rod, it was evident that the c-axes of the grains in the radially-offset ED compression specimen were oriented along one direction. With a strong and relatively uniform texture in the compression sample, anisotropy of flow was to be expected. The geometries of the deformed ED specimens are shown in Fig. 4. Specimens deformed at 300 °C and high strain rate experienced Fig. 1. Locations and orientations of compression specimens.
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Fig. 2. Initial microstructure a) along the centerline of the extrusion, and b) in the radially-offset compression sample.
Fig. 3. Starting texture of extruded rod obtained using XRD.
dependence on deformation temperature, as shown by the minor to major axis ratios in Fig. 5. The result was not in agreement with those of Dai et al. (2011), where anisotropy of flow decreased with strain rate but also with increasing temperature. Scribe lines oriented
fractures. Furthermore, it was evident that the specimens deformed at the lowest strain rate of 10−3 s−1 exhibited minimal anisotropy of flow, and the anisotropy increased with deformation strain rate. Omitting the fractured specimens, the degree of anisotropy showed no clear
Fig. 4. Top-view images of ED compression specimens under various processing conditions, at a true strain of 0.5.
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longer observed in the microstructure or in the stress-strain behavior at temperatures beyond 400 °C, even at high strain rates. Therefore, it is likely that the observed anisotropy was the result of another mechanism. It was suspected that the dominant deformation mechanism had transitioned to pyramidal slip beyond temperatures of 400 °C, but had resulted in a similar major strain component along the same < c > direction. The presence of anisotropy after deformation at high temperature and high strain rate suggested that the partitioning of prismatic slip and pyramidal slip had resulted in a major strain component along the < c > direction. For deformation temperatures below 400 °C, a decrease in twinning activity at lower strain rates resulted in a partitioning of tensile twinning and prismatic slip which resulted in symmetric material flow. A similar conclusion was proposed for deformation temperatures beyond 400 °C, but between prismatic slip and pyramidal slip. To fully understand the flow behavior, grain rotation to orientations of hard slip, incurred from twinning (Tenckhoff, 1988) must be considered. Although twinning activity was greater at high strain rates (Dai et al., 2011), Meng et al. (2005) observed accelerated grain rotation with an increase in deformation strain rate. XRD measurements obtained at various strains showed deviation from the initial fiber texture and the partial development of a basal plane texture, as shown in Fig. 7. Although the anisotropy was observed to decrease at high strain (i.e. beyond a true strain of 1.0), the major axis was still observed to be oriented in the radial direction of the extrusion after deformation at 500 °C and 1.0 s−1. The observed decrease in anisotropy may be related to grain rotation during deformation, which resulted in basal slip during grain rotation, and pyramidal slip after the basal plane texture had been developed. The dominance of pyramidal slip had resulted in symmetric material flow at high strains. To confirm the role of initial texture on the observed material flow behavior, a centerline compression specimen was deformed to a true strain of 0.5, at 500 °C and 1.0 s−1. Unlike the radially-offset specimen, the centerline specimen exhibited symmetric material flow as a result of its axisymmetric initial texture.
Fig. 5. Minor to major axis ratios for ED specimens deformed to a final strain of 0.5, under various conditions.
perpendicular to the radial direction on the specimen end face revealed that the major axis was aligned with the c-axes of the grains (i.e. radial direction of the extrusion); this result was consistent with those of Dai et al. (2011). The forging study by Rao et al. (2012) observed the minor axis to be oriented along the normal direction (i.e. c-axes of the grains) when forging was conducted along the rolling or transverse directions. The relative strength of the initial textures and the stress states of the samples in each study may have contributed to the differences in observed flow behavior. With consideration to the initial texture of the specimen, the compressive load upon deformation was applied perpendicular to two parallel prismatic faces of the HCP structure. With such an orientation, it was evident that the grains were oriented favorably for tensile twinning, prismatic slip, and pyramidal slip (Chapuis and Driver, 2010). An increase in anisotropy, combined with the presence of twins in the microstructure, after high strain rate deformation was due to an increase in tensile twinning activity under high strain rate conditions (Dai et al., 2011). It was proposed by Dai et al. (2011) that the anisotropy observed in the hot compression of rolled AZ31B was due to tensile twinning activity, which had resulted in a major strain component along the < c > direction. Examination of the pyramidal system revealed a similar result; however prismatic slip resulted in a strain component along the < a > direction. To verify the dominance of tensile twinning, an interrupted compression test was conducted to a final strain of 0.1, where the stress-strain curve showed softening prior to hardening. The resultant microstructure was observed to contain numerous twins, as shown in Fig. 6. However, tensile twinning was no
3.2. Anisotropy in TD specimens Considering the initial texture in the extruded rod, it can be said that the c-axis compression experienced by the TD specimens is similar to compression along the normal direction of a hot rolled plate. The geometries of the deformed TD specimens, at a final strain of 0.5, are shown in Fig. 8. As with the ED specimens, fractures were observed under low temperature and high strain rate processing conditions. Furthermore, it was evident that the anisotropy of flow showed strong dependence on deformation temperature, and the specimens deformed at high temperature and high strain rate experienced the least anisotropy. With the initial texture of the specimen resisting basal slip and prismatic slip, it was evident that only compressive twinning and pyramidal slip were oriented favorably for activation. However, the studies by Dai et al. (2011) and Rao et al. (2012) discussed earlier, conducted using hot rolled plates, observed no anisotropy of flow under those circumstances. Scribe lines oriented perpendicular to the extrusion direction on the specimen end face revealed that the minor axis was oriented along the extrusion direction. With careful consideration to the initial axisymmetric texture of the extrusion, it was deduced that the initial texture in the specimen was not as uniform as that in a hot rolled plate. Grains located away from the specimen centerline, but along the scribe line, were oriented favorably for basal slip, as illustrated schematically in Fig. 9. However the activation of basal slip at the select locations on the specimen resulted in preferential material flow along the direction of the scribe line. The CRSS required for pyramidal slip decreased rapidly with increasing deformation temperature (Chapuis and Driver, 2010), and the activation of basal slip became less favorable as the
Fig. 6. Microstructure of ED specimen after deformation at 300 °C and 1.0 s−1, to a final strain of 0.1. (See Fig. 11 for corresponding flow curve).
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Fig. 7. Texture of ED specimen after deformation at 500 °C and 1.0 s−1, to a final strain of 1.0.
anisotropy of flow. To further verify the influence of basal slip on the observed flow stress, small scale TD specimens (Ø4 mm by 6 mm) with more uniform initial texture were extracted around the same Ø44 mm reference circle of the extrusion. Although the minor/major axis ratios were similar to that of the full size TD specimens, the observed flow stress had increased, suggesting the partial suppression of basal slip. With non-basal slip readily activated at higher temperatures, the discrepancy in flow stress between the ED and TD specimens diminishes beyond 400 °C, as shown in Fig. 12.
temperature increased. Subsequently, the specimen deformed at 500 °C and 1.0 s−1 resulted in the least anisotropy, as grain rotation was accelerated at high strain rate (Meng et al., 2005); grains which were initially oriented favorably for basal slip had more rapidly (i.e. lower strain) transitioned to the basal plane deformation texture and suppressed the activation of basal slip. The final basal plane texture after deformation is shown in Fig. 10. In addition to the specimen geometry, the stress-strain curves revealed that the TD specimen exhibited lower yield and peak stresses compared to the ED specimens under low temperature processing conditions, as shown in Fig. 11. The softening and hardening behavior in the ED specimen that was noted earlier can also be seen. This result was not in agreement with those by Lee et al. (2011), where c-axis compression along the normal direction resulted in higher yield and peak stresses than other directions. The behavior observed in the flow stress was attributed to the activation of basal slip at low strains. As demonstrated by the specimens deformed at high temperature, it was deduced that the suppression of basal slip would result in less
3.3. Microstructural development Closer examination of the specimens revealed that deformation at temperatures below 450 °C for strain rates of 1.0 s−1 and 10−1 s−1, and below 400 °C for strain rates of 10−2 s−1 and 10−3 s−1, resulted in the formation of shear bands on the surface. Sun et al. (2010) observed that shear bands consisted of fine DRX grains, and Kim et al. (2012) proposed that the shear bands were formed when coarse grains were split
Fig. 8. Top-view images of TD specimens deformed under various processing conditions, at a true strain of 0.5.
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Fig. 11. Flow stress curves of ED and TD specimens at 300 °C and 1.0 s−1.
Fig. 9. Schematic of initial grain and scribe line orientations in TD specimens.
by twins at low strains, with subsequent DRX occurring within the high energy region, and strain localization upon further deformation. Therefore, processing conditions that resulted in the formation of shear bands are less favorable for bulk deformation. As shown in Figs. 13 and 14, shear bands were evident on the specimen surface and in the microstructure after deformation at 300 °C and 1.0 s−1, but were not observed at 500 °C and 1.0 s−1. With preference given to high strain rate conditions for increased production rates, it would be beneficial to conduct forging operations in the temperature range of 450 °C–500 °C. Examining the microstructures of the ED specimens at a strain of 0.5 under various processing conditions, it was evident that the DRX grain size increased with deformation temperature and also with decreasing strain rate. The observations were in agreement with those by Beer and Barnett (2007). The corresponding microstructures are shown in Fig. 15. As a result of the larger DRX grain size, the high temperature and low strain rate deformation conditions resulted in the highest level of microstructural homogeneity and would require fewer cycles of DRX to produce a given volume fraction of DRX grains. To study the effect of strain on the microstructure, ED specimens were deformed at 500 °C and 1.0 s−1 to various levels of strain, as shown in Fig. 16. The results supported those by Yang et al. (2003), who had observed that the DRX
Fig. 12. Flow stress curves for ED and TD specimens at 450 °C and 500 °C, at 1.0 s−1.
grain size changes very little with strain. Consistent with the observations made by Fatemi-Varzaneh et al. (2007), the DRX volume fraction was observed to increase with deformation strain, however it was evident that a fully recrystallized microstructure was not obtained at a strain of 1.0. In addition to the processing condition, the initial grain size variation in the extruded rod played a role on the deformation behavior. The centerline specimen deformed at 500 °C and 1.0 s−1, resulted in lower flow stresses than the radially-offset specimen. The result was consistent with those observed by Barnett et al. (2004), where the finegrained AZ31 exhibited lower flow stresses. In the current investigation, the lower Al content at the centerline may also have contributed to the lower flow stress. Furthermore, Miao et al. (2010) had observed rapid grain growth in a fine-grained AZ31 rolled plate, it was important to examine the influence of the finer grain size observed along the centerline of the extruded rod. To study the static grain growth kinetics of fine-grained AZ31, a centerline ED specimen was heated to 500 °C at 10 °C per second and water quenched immediately. Although both Fig. 10. Texture of TD specimen after deformation at 500 °C and 1.0 s−1, to a final strain of 1.0.
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Fig. 13. Images of TD specimens after deformation at a) 300 °C and 1.0 s−1 and b) 500 °C and 1.0 s−1, at a true strain of 0.5. (Images not to scale).
Fig. 14. Microstructures of TD specimens after deformation at a) 300 °C and 1.0 s−1 and b) 500 °C and 1.0 s−1, at a true strain of 0.5.
processing plays a vital role in determining the mechanical properties. To study the effect of cooling rate on the final microstructure, TD specimens were deformed at 500 °C and 1.0 s−1 to a strain of 1.0, and subjected to various cooling rates. Three cooling rates were studied: a five minute hold at the deformation temperature, die cooling (i.e.
normal and abnormal grain growth were observed after heating, as shown in Fig. 17, the final microstructures after deformation to a strain of 0.5 were very similar for both the centerline and radially-offset specimens. For a structural component, the final microstructure after
Fig. 15. Microstructures of ED specimens deformed under various processing conditions, at a true strain of 0.5.
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Fig. 16. Microstructures of ED specimens after deformation at 500 °C and 1.0 s−1, at various levels of strain.
The other two cooling cases resulted in larger, but equiaxed, grains and a homogeneous microstructure. The similarity between the two cases suggested that static grain growth had occurred rapidly after deformation, and little to no further grain growth had occurred after the first 60 s. 4. Conclusions The high temperature deformation behavior of extruded AZ31B in both the extrusion and transverse directions was studied through isothermal uniaxial compression tests over the temperature range of 300 °C–500 °C, at strain rates of 10−3 s−1–1.0s−1. The key findings can be summarized as follows: 1. Anisotropy of flow observed in the ED specimens increased with deformation strain rate for all investigated temperatures. Anisotropy was likely to be attributed to the dominance of tensile twinning for low deformation temperatures (i.e. below 400 °C), and pyramidal slip for high deformation temperatures (i.e. above 400 °C). Symmetric material flow was observed at the deformation strain rate of 10−3 s−1 for all temperatures. Deformation of a centerline specimen resulted in no anisotropy. 2. Anisotropy of flow in the TD specimens was reduced at high temperature and high strain rate. The observed anisotropy was attributed to the occurrence of basal slip at select locations away from the specimen centerline. The accelerated rate of grain rotation at high
Fig. 17. Microstructure of centerline ED specimen after heating.
specimen remained between cooling anvils with no further heat input), and a water quench. Temperature readings showed that the die cooled specimen had cooled to below 200 °C after 60 s, despite resting between two hot anvils. The water quenched specimen exhibited a partially recrystallized microstructure with fine DRX grains, as shown in Fig. 18.
Fig. 18. Microstructures of TD specimens after deformation at 500 °C and 1.0 s−1 to a final strain of 1.0, followed by a) a water quench and b) die cooling.
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alloy: examining the role of texture on the deformation mechanisms. Mater. Sci. Eng. A 488, 406–414. Barnett, M., Beer, A., Atwell, D., Oudin, A., 2004. Influence of grain size on hot working stresses and microstructures in Mg-3Al-1Zn. Scr. Mater. 51, 19–24. Beer, A., Barnett, M., 2007. Microstructural development during hot working of Mg-3Al1Zn. Metall. Mater. Trans. A 38, 1856–1867. Chapuis, A., Driver, J., 2010. Temperature dependency of slip and twinning in plane strain compressed magnesium single crystals. Acta Mater. 59, 1986–1994. Dai, Q., Zhang, D., Chen, X., 2011. On the anisotropic deformation of AZ31 Mg alloy under compression. Mater. Des. 32, 5004–5009. Doege, E., Dröder, K., 2001. Sheet metal forming of magnesium wrought alloys − formability. J. Mater. Process Technol. 115 (1), 14–19. Fatemi-Varzaneh, S., Zarei-Hanzaki, A., Beladi, H., 2007. Dynamic recrystallization in AZ31 magnesium alloy. Mater. Sci. Eng. A 456, 52–57. Friedrich, H., Mordike, B. (Eds.), 2004. Magnesium Technology: Metallurgy, Design Data, Applications. Springer, Berlin. Friedrich, H., Schumann, S., 2001. Research for a new age of magnesium in the automotive industry. J. Mater. Process. Technol. 117 (3), 276–281. Humphreys, F., Hatherly, M., 2004. Recrystallization and Related Annealing Phenomena. Elsevier Ltd., Kidlington. Kim, H., Lee, J., Lee, C., Bang, W., Ahn, S., Chang, Y., 2012. Shear band formation during hot compression of AZ31 Mg alloy sheets. Mater. Sci. Eng. A 558, 431–438. Lee, B., Park, S., Hong, S., Park, K., Lee, C., 2011. Role of initial texture on the plastic anisotropy of Mg-3Al-1Zn alloy at various temperatures. Mater. Sci. Eng. A 528, 1162–1172. Meng, L., Yang, P., Zhao, Z., Mao, W., 2005. An OIM analysis on the deformation mechanism in hot compressed AZ31 magnesium alloy. Mater. Sci. Forum 488–489, 633–636. Miao, Q., Hu, L., Wang, X., Wang, E., 2010. Grain growth kinetics of a fine-grained AZ31 magnesium alloy produced by hot rolling. J. Alloys Compd. 493, 87–90. Mordike, B., Ebert, T., 2001. Magnesium: properties − applications − potential. Mater. Sci. Eng. A 302, 37–45. Rao, K., Prasad, Y., Suresh, K., 2012. Anisotropy of flow during isothermal forging of rolled AZ31B magnesium alloy rolled plate in three orthogonal directions: correlation with processing maps. Mater. Sci. Eng. A 558, 30–38. Sun, D., Chang, C., Kao, P., 2010. Microstructural study of strain localization in hot compressed Mg-3Al-1Zn. Mater. Sci. Eng. A 527, 7050–7056. Tang, W., Huang, S.Z., Li, D., Peng, Y., 2011. Influence of extrusion parameters on grain size and texture distributions of AZ31 alloy. J. Mater. Process. Technol. 211, 1203–1209. Tenckhoff, E., 1988. Deformation Mechanisms, Texture, and Anisotropy in Zirconium and Zircaloy. ASTM International, Philadelphia. Yang, X., Miura, H., Sakai, T., 2003. Dynamic evolution of new grains in magnesium alloy AZ31 during hot deformation. Mater. Trans. 44, 197–203. Yukutake, E., Kaneko, J., Sugamata, M., 2003. Anisotropy and non-uniformity in plastic behavior of AZ31 magnesium alloy plates. Mater. Trans. 44 (4), 452–457.
strain rates, combined with low CRSS for pyramidal slip at high temperatures, resulted in the suppression of basal slip and the restoration of symmetric material flow. 3. Shear bands were observed in specimens deformed at temperatures below 450 °C for strain rates of 1.0 s−1 and 10−1 s−1, and below 400 °C for strain rates of 10−2 s−1 and 10−3 s−1. The shear bands lead to strain localization, making these processing conditions less favorable. 4. After deformation at 500 °C and 1.0 s−1, the volume fraction of DRX grains increased with strain, but with a stable DRX grain size. A fully recrystallized microstructure was not obtained at a final strain of 1.0. 5. Slow cooling of the specimen (i.e. sample temperature had fallen from 500 °C to below 200 °C in 60 s) was conducted after deformation at 500 °C and 1.0 s−1, and static grain growth was observed to occur rapidly, resulting in larger, equiaxed grains and a more uniform microstructure. Longer times spent at high temperature resulted in little to no further grain growth after the first 60 s. Acknowledgements This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and Automotive Partnership Canada (APC) program under the APCPJ 459269-13 grant, with contributions from Multimatic Inc., Ford Motor Company, University of Waterloo, Canada Foundation for Innovation (CFI), CanmetMATERIALS, and Centerline Windsor. Furthermore, the authors wish to acknowledge Dr. Hamid Jaded and Dr. Sugrib Shaha for their help in obtaining the XRD textures, and Mr. Mark Whitney for his assistance in performing the compression tests. References Agnew, S., Mehrotra, P.L., Stoica, G., Liaw, P., 2005. Texture evolution of five wrought magnesium alloys during route A equal channel angular extrusion: Experiments and simulations. Acta Mater. 53, 3135–3146. Al-Samman, T., Gottstein, G., 2008. Room temperature formability of a magnesium AZ31
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