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Modification of grain refinement and texture in AZ31 Mg alloy by a new plastic deformation method
Accepted Manuscript Letter Modification of Grain Refinement and Texture in AZ31 Mg Alloy by a New Plastic Deformation Method Liwei Lu, Chuming Liu, Jun Zhao, Wenbing Zeng, Zhongchang Wang PII: DOI: Reference:
S0925-8388(14)03124-7 http://dx.doi.org/10.1016/j.jallcom.2014.12.196 JALCOM 32985
To appear in:
Journal of Alloys and Compounds
Received Date: Revised Date: Accepted Date:
30 October 2014 18 December 2014 19 December 2014
Please cite this article as: L. Lu, C. Liu, J. Zhao, W. Zeng, Z. Wang, Modification of Grain Refinement and Texture in AZ31 Mg Alloy by a New Plastic Deformation Method, Journal of Alloys and Compounds (2014), doi: http:// dx.doi.org/10.1016/j.jallcom.2014.12.196
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Modification of Grain Refinement and Texture in AZ31 Mg Alloy by a New Plastic Deformation Method Liwei Lua,b, Chuming Liua,*, Jun Zhaob, Wenbing Zengb, Zhongchang Wang c a
School of Materials Science and Engineering, Central South University, Changsha 410083, Hunan, P. R. China
b
College of Mechanical and Electrical Engineering, Hunan University of Science and Technology, Xiangtan 411201, Hunan, P. R. China
c
Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
ABSTRACT：We have designed a novel severe plastic deformation technique by integrating forward extrusion and torsion deformation into a single processing step and tested its feasibility to the Mg alloys AZ31. We find that this technique can be successfully applied to the AZ31 Mg alloys by showing no cracks. The grains of the alloys are refined and the basal texture is dramatically improved due to the dynamic recrystallization induced by the heavily accumulated strains. This new deformation technique could become a promising method for further tailoring microstructure and texture of Mg-based alloys for a wide range of engineering applications. Keywords: Magnesium alloys; Extrusion; Torsion deformation; Grain refinement; Texture evolution
1
1.
Introduction As the lightest metallic structural material, magnesium alloys have received a
great deal of attentions in the aerospace and automotive fields [1,2]. However, due to their HCP structure and low stacking fault energy, they show poor ductility and strength at ambient temperature, thereby limiting their wide range of applications as engineering materials. One of the most effective ways to improve their properties is to refine their grains [3,4]. To date, many conventional forming processes such as rolling [5], forging [6] and extrusion [7] have been employed to refine grains. Nevertheless, these conventional processes often introduce strong crystallographic basal textures, which in turn restricts the magnitude of shear strain in all slip systems during subsequent deformation [8,9]. The texture weakening and randomization are in principle useful for rotating grains away from the basal orientation, a direction that benefits the generation of non-basal texture, thereby improving the isotropy and formability [10]. To fabricate Mg alloys with fine grains and favorable texture, a large number of approaches have been attempted, in which the intense shear deformation method has captured many attentions. To date, several methods have been employed successfully to process Mg alloys, including the equal channel angular pressing (ECAP) [11], multi-axial forging (MAF) [12], cyclic extrusion compression (CEC) [13], and accumulative back extrusion (ABE) [14]. However, these methods generally require several passes before fine grains and favorable texture can be obtained. On the other hand, it is difficult to increase the process efficiency, thereby posing a hurdle to the industrial mass processing of Mg alloys. In this work, we propose a new technique, the integrated forward extrusion and torsion deformation method, by introducing the torsional shearing deformation after
2
the conventional extrusion to further refine grains and weaken basal texture. We demonstrate that this method would improve greatly efficiency of processing of Mg alloys and promote the up-scaling processing. We tested this technique on AZ31 Mg alloys, aim to investigate the distribution and inhomogeneity of equivalent strain and the microstructure and texture evolution. We find that this technique can produce remarkably refined grains and favorable texture in the AZ31 Mg alloys.
2.
Experimental Figure 1 showed sketch of the integrated extrusion die. To clearly introduce the
die, the characteristic zones of the extrusion deformation were labeled as I, II, III and IV, which represented the conical part, the forward extruded rod, the torsional part, and the eventual forming stage, respectively. The integrated extrusion die consisted of two equal sectional dies that were clamped by screws, each of which had a half torsion structure after forward extrusion. All parts of the die were made of the H13 steel. The channel diameter was reduced from 30 to 10 mm after forward extrusion, followed by subsequent torsion deformation with a length of 15 mm and a torsion angle of 30°. It should be noted that the ellipse design was adopted in order to realize the torsion deformation. The major and minor axes of the ellipse were 10 mm and 8 mm, respectively. The torsional shearing strain can thereby be imposed on forward extruded AZ31 Mg alloys to further modify microstructure and texture. Finite element simulation was first performed on the AZ31 Mg alloys processed at 573K. A friction coefficient of 0.3 and a velocity of 1 mm/s were adopted. The billet, die and ram were assumed to be elastic, plastic and rigid, respectively. The distribution and inhomogeneity of equivalent strain could be calculated by analyzing the simulation data.
3
The as-cast AZ31 bars were machined to specimens with a diameter of 30 mm in and a length of 65 mm. Firstly, the bars were fully lubricated, heated to the target temperature and held for ~10 min. The heated samples were then placed in the die to conduct the extrusion process. It is worth noting that the die was preheated to and stabilized at the target temperature before an already heated sample was inserted into the entrance channel. Finally, the extrusion products were cut to rectangle shape with a dimension of 6 × 6 × 8 mm along the longitudinal plane, as shown in Fig.1. The central part of the extrusion product was used to conduct optical microscopy (OM) and X-ray diffraction (XRD) analyses. The average grain size was measured by the linear intercept process.
3.
Results and discussion
3.1. Equivalent strain Since dynamic recrystallization is only initiated if strain reaches an equivalent state, the distribution and inhomogeneity of equivalent strain shall play an important role in grain refinement. From Fig. 2a, one can note that the equivalent strain varies noticeably with the change of deformation zone. The maximum of equivalent strain can reach four at the surface, and the central part of IV zone also has an appreciable strain value of 2.8. Moreover, further increase in extrusion ratio and torsional angle can reinforce the equivalent strain in progress. In the II zone, there occurs a gradient from 3.5 on the edge to 2.0 at the center, which indicates that forward extrusion is not effective to produce homogeneous strain. However, homogeneity of equivalent strain becomes more pronounced as the sample slides along the III zone due to the
4
shearing effect. The center strain reaches 2.9, although the strain on the edge still has the maximum value. With disappearance of the torsional shearing effect, a relative homogeneous value of 2.8 can be achieved eventually in the IV zone. These further indicate that torsional shearing deformation plays an important role in enhancing the homogeneity of equivalent strain and can therefore provide energy especially in the central part, which is necessary for initiating fully dynamic recrystallization (DRX). As a deformation method, torsion can generate plastic deformation using high torsion angle and small torsion length. However, the strain in this case is generally non-uniform [15]. To obtain uniform strain, we try a low angle 30° and a long length of 15 mm, thereby weakening the shearing effect. It should be noted that the value of extrusion ratio is relatively low (9), and the strain in the central part is much smaller than that at the surface. The addition of torsion deformation can dramatically change the flow direction of the surface metal. Since the metal in the central part can also be driven by the surface metal flow, the strain at the central part is therefore increased and the strain gradient is decreased. In this way, the torsion deformation can improve the homogeneity of the strain distribution. To further offer a quantitative analysis of the inhomogeneity, we draw five lines on the transverse section of the IV zone, each of which has 20 nodes, as shown in Fig. 2b. The inhomogeneity index (Ci) can be employed to measure the degree of the inhomogeneity [16]: Ci
max min Avg
,
(1)
Where the max 、 min and Avg represent the maximum, minimum, and average equivalent strain, respectively. Moreover, the average equivalent strain can be
5
expressed as: n
Avg
i 1
n
i
,
(2)
Where the n is the number of nodes in the lines 1~5 in the transverse section, and the
i
is the equivalent strain value at the node i. Based on these formulas, the inhomogeneity parameter can be calculated using
the simulation data, as listed in Table 1. The average equivalent strain can arrive at 2.7995, and the inhomogeneity index is relatively low at 0.191. Obviously, the average equivalent strain is much larger than 2.44 and the Ci is much lower than 6.5 in the sample processed by compound deformation and ECAE [17,18]. These results demonstrate that this integrated process can show high strain and homogeneity, thus refining grains and improving significantly homogeneity.
3.2. Optical microstructure and texture evolution The high and homogeneous equivalent strain produced by the forward extrusion and torsional shearing deformation during integrated processing may have a strong influence on grain refinement and texture evolution. To test this scenario, we first investigate optical microstructure of the AZ31 Mg alloys before and after extrusion, as shown in Fig. 3. The average grain size of the as-cast billet is estimated to be more than 300 μm. In addition, the grains have a heterogeneous distribution with grain size ranging from 50 μm to 400 μm (Fig. 3a). Evidently, the originally coarse grains are refined markedly by even a single pass of the integrated processing, thus demonstrating the efficiency of this method in refining grains due to the DRX (Fig.
6
3b−3d). During the integrated processing, we also identify many tinny grains of ~6 μm along the extrusion direction (ED) and some large elongated grains of 25~125 μm (Fig. 3b), suggesting that the DRX does not take place completely. This can be attributed to the major inhomogeneity of its equivalent strain distribution in the II zone, especially to the lower strain at the central part. However, for the III zone, the local grains have an angle of ~18° from the ED due to the shearing effect, which can be explained by the above simulation results showing that the central metal can be also driven by the surface metal flow. These, together with the disappearance of the large elongated grains, indicate that the DRX for all the elongated grains take place fully. The grains are homogeneously refined to 3.5 μm (Fig. 3c), which can also be explained by the above simulation results showing that the equivalent strain in the III zone is so high and homogeneously that induces a full DRX for the elongated grains. It is noteworthy that the average grain size of 3.5 μm is comparable to the value of 3.6 μm for the AZ31 alloys pressed by the four repetitive C shape equal channel reciprocating extrusions at the same temperature, and is even less than that of 5.5 μm after eight-pass ECAE at 523 K [19,20]. With the increase of extrusion temperature to 673K, many tiny grains within shearing bands are observed with a mean grain size of 6 μm. However, some grains are coarse because of the grain growth at elevated temperature, (Fig. 3d), which is common for the extruded Mg alloys [21]. Figure 4 shows the {0002} and {10 1 0} pole figures measured for the samples from the I, II, and IV zones. From Fig. 4a, one can see that the majority of basal pole
7
is approximately parallel to the ED, and the I zone shows a relatively low intensity of 8.32. However, the texture intensity increases suddenly to 16.17 in the II zone (Fig. 4b), which can be ascribed to the oriented formation of the DRX grains. This result is consistent with the report by Li et al. [22], which shows a limited number of available plastic deformation modes in the conventionally extruded Mg alloy sheets. However, in the IV zone, the texture is much weaker and more dispersive compared with that in the case of the II zone. The maximum value dramatically reduces to 8.51. We therefore conclude that the torsional shearing deformation could rotate the basal plane from the ED toward the imposed shearing direction, which produces increased number of grains that are more favorable for non-basal slip than basal slip, thereby improving the ductility at ambient temperature.
4. Conclusions We have developed an integrated extrusion process and applied it to AZ31 Mg alloys, aimed to investigate the equivalent strain distribution, inhomogeneity, grain refinement, and texture evolution of the processed alloys. We find that the processed alloys show a relatively high and uniform strain distribution, and the grain size is reduced significantly from 300 to 3.5 μm after extrusion only once at 573 K due to dynamic recrystallization. The torsional shearing deformation is found to be able to further refine the microstructure and even improve the homogeneity of the grains. Moreover, we also find that some local grains are rotated to a certain angle with the extrusion direction due to torsional shearing force. The texture evolution manifests
8
that the strong basal texture formed in the forward extrusion can be further weakened by torsional shearing deformation. This approach can serve as an efficient alternative to modify grain refinement and texture of the important engineering materials Mg alloys AZ31.
Acknowledgments This work was supported in part by the China Postdoctoral Science Foundation (grant no. 2014M562128), the Postdoctoral Scientific Research Foundation of Central South University (grant no. 134383), the National 973 Major Project of China (grant no. 2013CB632202), and the Hunan Provincial Natural Science Foundation of China (grant no. 14JJ3111). L.L. appreciates financial supports from the Scientific Research Fund of Hunan Provincial Education Department (grant no. 14C0455) and Aid Program for College Students’ Inquiry Learning and Innovative Experiment Plan of Hunan Province. Z.C.W. thanks the financial support from the Grant-in-Aid for Young Scientists (A) (grant no. 24686069), the NSFC (grant no. 11332013), the JSPS and CAS under the Japan-China Scientific Cooperation Program, and the Murata Science Foundation.
*E-mail: [email protected] (CM. Liu)
References [1] X.P. Chen, L.X. Wang, R. Xiao, X.Y. Zhong, G.J. Huang, Q. Liu, Comparison of annealing on microstructure and anisotropy of magnesium alloy AZ31 sheets
9
processed by three different routes, J. Alloys. Compd. 604 (2014) 112–116. [2] M. Horynová, J. Zapletal, P. Doležal, P. Gejdoš, Evaluation of fatigue life of AZ31 magnesium alloy fabricated by squeeze casting, Mater. Des. 45 (2014) 253–264. [3] K. Hamad, B.K. Chung, Y.G. Ko, Microstructure and mechanical properties of severely deformed Mg–3%Al–1%Zn alloy via isothermal differential speed rolling at 453K, J. Alloys. Compd. 615 (2014) S590–S594. [4] Q.H. Huo, X.Y. Yang, H. Sun, B. Li, J. Qin, J. Wang, J.J. Ma, Enhancement of tensile ductility and stretch formability of AZ31 magnesium alloy sheet processed by cross-wavy bending, J. Alloys. Compd. 581 (2013) 230–235. [5] X.F. Wang, J.Z. Zhao, J. He, Z.Q. Hu, Hot rolling characteristics of spray-formed AZ91 magnesium alloy, Trans Nonferr Met Soc China. 17 (2007) 238–243. [6] H.T. Kang, T. Ostrom, Mechanical behavior of cast and forged magnesium alloys and their microstructures, Mater. Sci. Eng., A 490 (2008) 52–56. [7] T.J. Lee, W.J. Kim, The significant effect of adding trace amounts of Ti on the high-temperature deformation behavior of fine-grained Mg–6Al–1Zn magnesium alloys, J. Alloys. Compd. 617 (2014) 352–358. [8] M.H. Maghsoudi, A. Zarei-Hanzaki, H.R. Abedi, Modification of the grain structure, γ phase morphology and texture in AZ81 Mg alloy through accumulative back extrusion, Mater. Sci. Eng., A 595 (2014) 99–108. [9] J. Hirsch, T. Al-Samman, Superior light metals by texture engineering: Optimized aluminum and magnesium alloys for automotive applications, Acta. Mater. 61 (2013) 818–843.
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[10] Q.H. Huo, X.Y. Yang, J.J. Ma, H. Sun, J .Wang, L. Zhang, Texture weakening of AZ31 magnesium alloy sheet obtained by a combination of bidirectional cyclic bending at low temperature and static recrystallization, J. Mater. Sci. 48 (2013) 913–919. [11] G.D. Fan, M.Y. Zheng, X.S. Hu, C. Xu, K. Wu, I.S. Golovin, Effect of heat treatment on internal friction in ECAP processed commercial pure Mg, J. Alloys. Compd. 549 (2013) 38–45. [12] K.B. Nie, X.J. Wang, K.K. Deng, F.J. Xu, K. Wu, M.Y. Zheng, Microstructures and
mechanical properties of AZ91
magnesium alloy processed
by
multidirectional forging under decreasing temperature conditions, J. Alloys. Compd. 617 (2014) 979–987. [13] W. Guo, Q.D. Wang, B. Ye, X.C. Li, X.T. Liu, H. Zhou, Microstructural refinement and homogenization of Mg–SiC nanocomposites by cyclic extrusion compression, Mater. Sci. Eng., A 556 (2012) 267–670. [14] S.M. Fatemi-Varzaneh, A. Zarei-Hanzaki, R. Vaghar, J.M. Cabrer, The origin of microstructure inhomogeneity in Mg–3Al–1Zn processed by severe plastic deformation, Mater. Sci. Eng., A 551 (2012) 128–132. [15] C.P. Wang, F.G. Li, Q.H. Li, J. Li, L. Wang, J.Z. Dong, A novel severe plastic deformation method for fabricating ultrafine grained pure copper, Mater. Des. 43 (2013) 492–498. [16] V. Patil Basavaraj, U. Chakkingal, T.S. Prasanna Kumar, Study of channel angle influence on material flow and strain inhomogeneity in equal channel angular
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pressing using 3D finite element simulation, J. Mater. Process. Technol. 209 (2009) 89–95. [17] H.J. Hu, D.F. Zang, M.B. Yang, M. Deng, Grain refinement in AZ31 magnesium alloy rod fabricated by extrusion-shearing severe plastic deformation process, Trans. Nonferr. Met. Soc. China. 21 (2011) 243–249. [18] H.J. Hu, Simulations of isothermal ECAE for magnesium alloy using FEM software and experimental validations, J. Manuf. Processes. 14 (2012) 181–187. [19] Q.D. Wang, Y.J. Chen, J.B. Lin, L.J. Zhang, C.Q. Zhai, Microstructure and properties of magnesium alloy processed by a new severe plastic deformation method, Mater. lett. 61 (2007) 4599–4602. [20] S. Seipp, M.F.X. Wagner, K. Hockauf, I. Schneider, L.W. Meyer, M.Hockauf, Microstructure, crystallographic texture and mechanical properties of the magnesium alloy AZ31B after different routes of thermo-mechanical processing, Int. J. Plasticity. 35 (2012) 155–166. [21] H. Borkar, R. Gauvin, M. Pekguleryuz, Effect of extrusion temperature on texture evolution and recrystallization in extruded Mg–1% Mn and Mg–1% Mn–1.6%Sr alloys, J. Alloys. Compd. 555 (2013) 219–224. [22] R.H. Li, F.S. Pan, B. Jiang, Q.S. Yang, A.T. Tang, Effects of combined additions of Li and Al–5Ti–1B on the mechanical anisotropy of AZ31 magnesium alloy, Mater. Des. 46 (2013) 922–927.
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Table 1 Inhomogeneity parameters of equivalent strain on the transverse section.
Figure Captions
Fig. 1 Sketch of integrated extrusion die illustrating the die structure and extrusion fashion, and definition of external orientations. The ED, TD, and ND denote the extrusion, transverse, and normal direction.
Fig. 2 Analysis of the equivalent strain: (a) distribution and (b) inhomogeneity.
Fig. 3 Optical micrographs of the AZ31 alloys: (a) as-cast, and extruded (b) at 573 K in the II zone, (c) at 573 K in the III zone, and (d) at 673 K in the III zone.
Fig. 4 The {0002} and {101 0} pole figures for the AZ31 alloys in (a) I zone, (b) II zone, and (c) IV zone during the integrated extrusion at 573 K.
13
Figure 1
Ram Ⅱ zone Ⅲ zone I zone
Ⅳ zone
Die ND TD 8 ED6 6
14
Figure 2
Equivalent strain
4.0
1
2 3
3.5 5
4
3.0 Line-1 Line-2 Line-3 Line-4 Line-5
2.5
(b)
(a) 2.0 0
1
15
2 3 4 Radial distance/mm
5
Figure 3
(a)
(b)
200μm
25μm
(d)
(c)
25μm
16
25μm
Figure 4
{0002}
(a)
{10 1 0}
(b)
(c)
ED
ND
17
TD
Table 1
max
min
Avg
Ci
3.1858
2.6506
2.7995
0.191
18
S0925-8388(14)03124-7 http://dx.doi.org/10.1016/j.jallcom.2014.12.196 JALCOM 32985
To appear in:
Journal of Alloys and Compounds
Received Date: Revised Date: Accepted Date:
30 October 2014 18 December 2014 19 December 2014
Please cite this article as: L. Lu, C. Liu, J. Zhao, W. Zeng, Z. Wang, Modification of Grain Refinement and Texture in AZ31 Mg Alloy by a New Plastic Deformation Method, Journal of Alloys and Compounds (2014), doi: http:// dx.doi.org/10.1016/j.jallcom.2014.12.196
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Modification of Grain Refinement and Texture in AZ31 Mg Alloy by a New Plastic Deformation Method Liwei Lua,b, Chuming Liua,*, Jun Zhaob, Wenbing Zengb, Zhongchang Wang c a
School of Materials Science and Engineering, Central South University, Changsha 410083, Hunan, P. R. China
b
College of Mechanical and Electrical Engineering, Hunan University of Science and Technology, Xiangtan 411201, Hunan, P. R. China
c
Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
ABSTRACT：We have designed a novel severe plastic deformation technique by integrating forward extrusion and torsion deformation into a single processing step and tested its feasibility to the Mg alloys AZ31. We find that this technique can be successfully applied to the AZ31 Mg alloys by showing no cracks. The grains of the alloys are refined and the basal texture is dramatically improved due to the dynamic recrystallization induced by the heavily accumulated strains. This new deformation technique could become a promising method for further tailoring microstructure and texture of Mg-based alloys for a wide range of engineering applications. Keywords: Magnesium alloys; Extrusion; Torsion deformation; Grain refinement; Texture evolution
1
1.
Introduction As the lightest metallic structural material, magnesium alloys have received a
great deal of attentions in the aerospace and automotive fields [1,2]. However, due to their HCP structure and low stacking fault energy, they show poor ductility and strength at ambient temperature, thereby limiting their wide range of applications as engineering materials. One of the most effective ways to improve their properties is to refine their grains [3,4]. To date, many conventional forming processes such as rolling [5], forging [6] and extrusion [7] have been employed to refine grains. Nevertheless, these conventional processes often introduce strong crystallographic basal textures, which in turn restricts the magnitude of shear strain in all slip systems during subsequent deformation [8,9]. The texture weakening and randomization are in principle useful for rotating grains away from the basal orientation, a direction that benefits the generation of non-basal texture, thereby improving the isotropy and formability [10]. To fabricate Mg alloys with fine grains and favorable texture, a large number of approaches have been attempted, in which the intense shear deformation method has captured many attentions. To date, several methods have been employed successfully to process Mg alloys, including the equal channel angular pressing (ECAP) [11], multi-axial forging (MAF) [12], cyclic extrusion compression (CEC) [13], and accumulative back extrusion (ABE) [14]. However, these methods generally require several passes before fine grains and favorable texture can be obtained. On the other hand, it is difficult to increase the process efficiency, thereby posing a hurdle to the industrial mass processing of Mg alloys. In this work, we propose a new technique, the integrated forward extrusion and torsion deformation method, by introducing the torsional shearing deformation after
2
the conventional extrusion to further refine grains and weaken basal texture. We demonstrate that this method would improve greatly efficiency of processing of Mg alloys and promote the up-scaling processing. We tested this technique on AZ31 Mg alloys, aim to investigate the distribution and inhomogeneity of equivalent strain and the microstructure and texture evolution. We find that this technique can produce remarkably refined grains and favorable texture in the AZ31 Mg alloys.
2.
Experimental Figure 1 showed sketch of the integrated extrusion die. To clearly introduce the
die, the characteristic zones of the extrusion deformation were labeled as I, II, III and IV, which represented the conical part, the forward extruded rod, the torsional part, and the eventual forming stage, respectively. The integrated extrusion die consisted of two equal sectional dies that were clamped by screws, each of which had a half torsion structure after forward extrusion. All parts of the die were made of the H13 steel. The channel diameter was reduced from 30 to 10 mm after forward extrusion, followed by subsequent torsion deformation with a length of 15 mm and a torsion angle of 30°. It should be noted that the ellipse design was adopted in order to realize the torsion deformation. The major and minor axes of the ellipse were 10 mm and 8 mm, respectively. The torsional shearing strain can thereby be imposed on forward extruded AZ31 Mg alloys to further modify microstructure and texture. Finite element simulation was first performed on the AZ31 Mg alloys processed at 573K. A friction coefficient of 0.3 and a velocity of 1 mm/s were adopted. The billet, die and ram were assumed to be elastic, plastic and rigid, respectively. The distribution and inhomogeneity of equivalent strain could be calculated by analyzing the simulation data.
3
The as-cast AZ31 bars were machined to specimens with a diameter of 30 mm in and a length of 65 mm. Firstly, the bars were fully lubricated, heated to the target temperature and held for ~10 min. The heated samples were then placed in the die to conduct the extrusion process. It is worth noting that the die was preheated to and stabilized at the target temperature before an already heated sample was inserted into the entrance channel. Finally, the extrusion products were cut to rectangle shape with a dimension of 6 × 6 × 8 mm along the longitudinal plane, as shown in Fig.1. The central part of the extrusion product was used to conduct optical microscopy (OM) and X-ray diffraction (XRD) analyses. The average grain size was measured by the linear intercept process.
3.
Results and discussion
3.1. Equivalent strain Since dynamic recrystallization is only initiated if strain reaches an equivalent state, the distribution and inhomogeneity of equivalent strain shall play an important role in grain refinement. From Fig. 2a, one can note that the equivalent strain varies noticeably with the change of deformation zone. The maximum of equivalent strain can reach four at the surface, and the central part of IV zone also has an appreciable strain value of 2.8. Moreover, further increase in extrusion ratio and torsional angle can reinforce the equivalent strain in progress. In the II zone, there occurs a gradient from 3.5 on the edge to 2.0 at the center, which indicates that forward extrusion is not effective to produce homogeneous strain. However, homogeneity of equivalent strain becomes more pronounced as the sample slides along the III zone due to the
4
shearing effect. The center strain reaches 2.9, although the strain on the edge still has the maximum value. With disappearance of the torsional shearing effect, a relative homogeneous value of 2.8 can be achieved eventually in the IV zone. These further indicate that torsional shearing deformation plays an important role in enhancing the homogeneity of equivalent strain and can therefore provide energy especially in the central part, which is necessary for initiating fully dynamic recrystallization (DRX). As a deformation method, torsion can generate plastic deformation using high torsion angle and small torsion length. However, the strain in this case is generally non-uniform [15]. To obtain uniform strain, we try a low angle 30° and a long length of 15 mm, thereby weakening the shearing effect. It should be noted that the value of extrusion ratio is relatively low (9), and the strain in the central part is much smaller than that at the surface. The addition of torsion deformation can dramatically change the flow direction of the surface metal. Since the metal in the central part can also be driven by the surface metal flow, the strain at the central part is therefore increased and the strain gradient is decreased. In this way, the torsion deformation can improve the homogeneity of the strain distribution. To further offer a quantitative analysis of the inhomogeneity, we draw five lines on the transverse section of the IV zone, each of which has 20 nodes, as shown in Fig. 2b. The inhomogeneity index (Ci) can be employed to measure the degree of the inhomogeneity [16]: Ci
max min Avg
,
(1)
Where the max 、 min and Avg represent the maximum, minimum, and average equivalent strain, respectively. Moreover, the average equivalent strain can be
5
expressed as: n
Avg
i 1
n
i
,
(2)
Where the n is the number of nodes in the lines 1~5 in the transverse section, and the
i
is the equivalent strain value at the node i. Based on these formulas, the inhomogeneity parameter can be calculated using
the simulation data, as listed in Table 1. The average equivalent strain can arrive at 2.7995, and the inhomogeneity index is relatively low at 0.191. Obviously, the average equivalent strain is much larger than 2.44 and the Ci is much lower than 6.5 in the sample processed by compound deformation and ECAE [17,18]. These results demonstrate that this integrated process can show high strain and homogeneity, thus refining grains and improving significantly homogeneity.
3.2. Optical microstructure and texture evolution The high and homogeneous equivalent strain produced by the forward extrusion and torsional shearing deformation during integrated processing may have a strong influence on grain refinement and texture evolution. To test this scenario, we first investigate optical microstructure of the AZ31 Mg alloys before and after extrusion, as shown in Fig. 3. The average grain size of the as-cast billet is estimated to be more than 300 μm. In addition, the grains have a heterogeneous distribution with grain size ranging from 50 μm to 400 μm (Fig. 3a). Evidently, the originally coarse grains are refined markedly by even a single pass of the integrated processing, thus demonstrating the efficiency of this method in refining grains due to the DRX (Fig.
6
3b−3d). During the integrated processing, we also identify many tinny grains of ~6 μm along the extrusion direction (ED) and some large elongated grains of 25~125 μm (Fig. 3b), suggesting that the DRX does not take place completely. This can be attributed to the major inhomogeneity of its equivalent strain distribution in the II zone, especially to the lower strain at the central part. However, for the III zone, the local grains have an angle of ~18° from the ED due to the shearing effect, which can be explained by the above simulation results showing that the central metal can be also driven by the surface metal flow. These, together with the disappearance of the large elongated grains, indicate that the DRX for all the elongated grains take place fully. The grains are homogeneously refined to 3.5 μm (Fig. 3c), which can also be explained by the above simulation results showing that the equivalent strain in the III zone is so high and homogeneously that induces a full DRX for the elongated grains. It is noteworthy that the average grain size of 3.5 μm is comparable to the value of 3.6 μm for the AZ31 alloys pressed by the four repetitive C shape equal channel reciprocating extrusions at the same temperature, and is even less than that of 5.5 μm after eight-pass ECAE at 523 K [19,20]. With the increase of extrusion temperature to 673K, many tiny grains within shearing bands are observed with a mean grain size of 6 μm. However, some grains are coarse because of the grain growth at elevated temperature, (Fig. 3d), which is common for the extruded Mg alloys [21]. Figure 4 shows the {0002} and {10 1 0} pole figures measured for the samples from the I, II, and IV zones. From Fig. 4a, one can see that the majority of basal pole
7
is approximately parallel to the ED, and the I zone shows a relatively low intensity of 8.32. However, the texture intensity increases suddenly to 16.17 in the II zone (Fig. 4b), which can be ascribed to the oriented formation of the DRX grains. This result is consistent with the report by Li et al. [22], which shows a limited number of available plastic deformation modes in the conventionally extruded Mg alloy sheets. However, in the IV zone, the texture is much weaker and more dispersive compared with that in the case of the II zone. The maximum value dramatically reduces to 8.51. We therefore conclude that the torsional shearing deformation could rotate the basal plane from the ED toward the imposed shearing direction, which produces increased number of grains that are more favorable for non-basal slip than basal slip, thereby improving the ductility at ambient temperature.
4. Conclusions We have developed an integrated extrusion process and applied it to AZ31 Mg alloys, aimed to investigate the equivalent strain distribution, inhomogeneity, grain refinement, and texture evolution of the processed alloys. We find that the processed alloys show a relatively high and uniform strain distribution, and the grain size is reduced significantly from 300 to 3.5 μm after extrusion only once at 573 K due to dynamic recrystallization. The torsional shearing deformation is found to be able to further refine the microstructure and even improve the homogeneity of the grains. Moreover, we also find that some local grains are rotated to a certain angle with the extrusion direction due to torsional shearing force. The texture evolution manifests
8
that the strong basal texture formed in the forward extrusion can be further weakened by torsional shearing deformation. This approach can serve as an efficient alternative to modify grain refinement and texture of the important engineering materials Mg alloys AZ31.
Acknowledgments This work was supported in part by the China Postdoctoral Science Foundation (grant no. 2014M562128), the Postdoctoral Scientific Research Foundation of Central South University (grant no. 134383), the National 973 Major Project of China (grant no. 2013CB632202), and the Hunan Provincial Natural Science Foundation of China (grant no. 14JJ3111). L.L. appreciates financial supports from the Scientific Research Fund of Hunan Provincial Education Department (grant no. 14C0455) and Aid Program for College Students’ Inquiry Learning and Innovative Experiment Plan of Hunan Province. Z.C.W. thanks the financial support from the Grant-in-Aid for Young Scientists (A) (grant no. 24686069), the NSFC (grant no. 11332013), the JSPS and CAS under the Japan-China Scientific Cooperation Program, and the Murata Science Foundation.
*E-mail: [email protected] (CM. Liu)
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Table 1 Inhomogeneity parameters of equivalent strain on the transverse section.
Figure Captions
Fig. 1 Sketch of integrated extrusion die illustrating the die structure and extrusion fashion, and definition of external orientations. The ED, TD, and ND denote the extrusion, transverse, and normal direction.
Fig. 2 Analysis of the equivalent strain: (a) distribution and (b) inhomogeneity.
Fig. 3 Optical micrographs of the AZ31 alloys: (a) as-cast, and extruded (b) at 573 K in the II zone, (c) at 573 K in the III zone, and (d) at 673 K in the III zone.
Fig. 4 The {0002} and {101 0} pole figures for the AZ31 alloys in (a) I zone, (b) II zone, and (c) IV zone during the integrated extrusion at 573 K.
13
Figure 1
Ram Ⅱ zone Ⅲ zone I zone
Ⅳ zone
Die ND TD 8 ED6 6
14
Figure 2
Equivalent strain
4.0
1
2 3
3.5 5
4
3.0 Line-1 Line-2 Line-3 Line-4 Line-5
2.5
(b)
(a) 2.0 0
1
15
2 3 4 Radial distance/mm
5
Figure 3
(a)
(b)
200μm
25μm
(d)
(c)
25μm
16
25μm
Figure 4
{0002}
(a)
{10 1 0}
(b)
(c)
ED
ND
17
TD
Table 1
max
min
Avg
Ci
3.1858
2.6506
2.7995
0.191
18