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Improvements in microstructure and mechanical properties of AZ80 magnesium alloy by means of an efficient, novel severe plastic deformation process
Journal of Manufacturing Processes 24 (2016) 71–77
Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Technical Paper
Improvements in microstructure and mechanical properties of AZ80 magnesium alloy by means of an efficient, novel severe plastic deformation process S. Sepahi-Boroujeni a,∗ , A. Sepahi-Boroujeni b a b
Department of Mechanical Engineering, Faculty of Engineering, Bu-Ali Sina University, Hamedan 69178, Iran Faculty of Mechanical Engineering, Iran University of Science and Technology, Tehran 16846-13114, Iran
a r t i c l e
i n f o
Article history: Received 19 January 2016 Received in revised form 23 April 2016 Accepted 31 July 2016 Keywords: H-tube pressing Severe plastic deformation Magnesium alloy Mechanical properties Grain refinement
a b s t r a c t Present paper includes efforts to develop dual equal channel lateral extrusion (DECLE) into an efficient SPD technique for processing tubular samples. In modified technique, entitled H-tube pressing (HTP), specimen accumulates outstanding amount of plastic strain in a couple of half cycles. Theoretically imposed plastic strain by means of the HTP operation exceeds that is attainable via a wide range of SPD methods. In this research work, the HTP process was experimentally conducted up to 2 passes (4 half cycles) on as-cast AZ80 magnesium alloy at elevated temperature. The microstructural and mechanical improvements follow the HTP process were investigated. Findings revealed that by conducting the first half cycle of HTP process, the average grain size reduced by 73%. Moreover, the yield stress and the elongation were increased by 90% compared to as-cast condition while the ultimate tensile strength and the microhardness were improved by 100 and 58%, respectively. Stability of mechanical properties of AZ80 alloy was observed at the end of the first half cycle. These results imply that HTP can be employed as an efficient, single-pass operation. © 2016 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
1. Introduction Reduction in the grain size dramatically affects both the microstructural and mechanical features of metals. Grain refinement not only leads to the production of materials with interesting mechanical and physical characteristics, but it can eliminate various drawbacks of coarse-grained metals such as paradox of strength and ductility as well. Among existing procedures for achieving ultrafine-grained (UFG) materials, induction of severe plastic strain has efficiently led to grain refinement and products with considerable mechanical capabilities. Accordingly, severe plastic deformation (SPD) techniques are widely utilized for producing UFG parts [1]. In the last decade, number of SPD methods has grown continuously. Most of recently proposed techniques are inspired by well-established methods such as equal channel angular extrusion (ECAE) [2,3], cyclic extrusion–compression (CEC) [4], and high pressure torsion (HPT) [5]. Accumulating larger amounts of plastic strain, decreasing the forming load, processing specimens with various forms and geometries, and facilitating the practice of
∗ Corresponding author. Tel.: +98 9365955144; fax: +98 3834222793. E-mail address: saeid [email protected] (S. Sepahi-Boroujeni).
conducting further passes can be the main incentives that motivate researchers to develop primary SPD operations into more efficient methods. For example, to improve the ECAE operation with the aims of declining the forming load and increasing the imposed plastic strain, ECAE with movable die walls [6] and expansion equal channel angular extrusion (Exp-ECAE) [7] have been proposed, respectively. Several SPD processes have been recently intended for tubular billets. To illustrate, HPT was developed into high-pressure tube twisting (HPTT) [8] and some new operations such as tube channel pressing (TCP) [9], parallel tubular channel angular pressing (PTCAP) [10], and tubular channel angular pressing (TCAP) [11] were proposed in order to modify the ECAE method for processing cylindrical tubes. All these ECAE-based methods possess the same drawback of punch buckling which has been resolved with the advent of repetitive tube expansion and shrinking (RTES) [12]. Today, due to the crucial role of components’ strength to weight ratio in design and manufacture, few can dispute the contributions of hollow-cross-section parts into various fields of industry. Among them, tubular products, especially those made from light alloys, are widely used in the aerospace industry. For instance, titanium tubes effectively work in airplanes’ high pressure hydraulic systems while considerably reduce these vehicles’ weights rather than traditional stainless steel tubes. In addition, magnesium (the lightest
http://dx.doi.org/10.1016/j.jmapro.2016.07.007 1526-6125/© 2016 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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Fig. 1. Schematic illustrations of (a–f) sequences of the HTP process and (g) geometric parameters of the billet in the HTP operation.
structural metal) is an appealing material for engineering applications due to its unique properties such as great specific strength and recycling capabilities. Thus, it is clear that making high-strength, hollow-cross-section components of such light metals can widen the range of their applications. In this paper, dual equal channel lateral extrusion (DECLE) [13] was adapted for cylindrical specimens. The proposed method, HTube pressing (HTP), was experimentally carried out on as-cast AZ80 magnesium alloy up to 2 passes. Both the microstructural and mechanical developments of the alloy then were measured. Findings showed that the HTP process was able to effectively improve the strength, ductility, and the microhardness of the alloy. This process also managed to dramatically reduce the average grain size (AGS) of AZ80 merely after a half cycle. To recover the initial shape of the specimen, however, it is inevitable to complete the cycle by performing the second half cycle. 2. Principles of HTP As Fig. 1 illustrates, an HTP die involves a couple of parallel tubular channels, i.e. inner channel and outer channel, with equal width and coincident centerlines. These channels are connected at the middle of their length with a radial channel (Fig. 1). First of all, as Fig. 1a depicts, a tubular billet is inserted into the inner channel. A couple of synchronized tubular punches (inner punches), which are represented by arrows in Fig. 1, are utilized to move against
each other and toward the billet. As inner punches touch the billet, the specimen is entirely constrained by the die walls and the inner punches except at the entrance of the radial channel. By continuation of the opposite movements of punches, according to Fig. 1b, the billet is forced to radially flow outward. By this lateral (radial) extrusion, material reaches the wall of outer channel (Fig. 1b). Now, the specimen undergoes dual lateral extrusion and, in turn, laterally (longitudinally) extrudes within the outer channel in both upward and downward directions (Fig. 1c). As Fig. 1d presents, the inner punches almost meet each other at the end of the first half cycle. At the beginning of the second half cycle, inner punches are pulled out and another couple of tubular punches (outer punches) are inserted into the outer channel. Simultaneous and opposite movements of outer punches reverse the material flow established during the first half cycle (Fig. 1e). Eventually, as Fig. 1f displays, as the material recovers its initial shape inside the inner channel, the second half cycle of HTP operation is finalized. As Fig. 2a displays, by considering the axisymmetric view of specimen during the process, the flow direction evokes an axisymmetric, H-shaped path in which the material experiences DECLE deformation in the radial direction. Through this path, the material is exposed to the lateral extrusion which can lead to accumulation of notable plastic strain inside the billet. Total plastic strain induced via the first half cycle of HTP can be resolved into a couple of components. The first component of total strain (¯ε1 ) is imposed as the material radially extrudes
Fig. 2. (a) Axisymmetric schemas of HTP and RTES [12] processes and (b) schematic illustrations of DECLE and ECAE operations [13].
S. Sepahi-Boroujeni, A. Sepahi-Boroujeni / Journal of Manufacturing Processes 24 (2016) 71–77
outward (Fig. 1b) and, afterwards, longitudinally shears upward and downward within the outer channel (Fig. 1c). Based on the relation proposed by Lee [14], the shear strain created via DECLE (see Fig. 2b) can be estimated by: DECLE = cot ˛ + cot ˇ
(1)
Then the von Mises plastic strain can be formulated as: ε¯ DECLE =
cot ˛ + cot ˇ √ 3
(2)
This amount of plastic strain is accumulated inside the billet as it radially extrudes and reaches the wall of outer channel (Fig. 1b). Herein, according to the equal width of inner, outer and radial channels (Fig. 1g), ˛ = 90 − ˇ = tan−1 1/2. As a result, Eq. (1) yields a plastic strain 25% greater than that is attainable by means of the ECAE with perpendicular channels (Fig. 2b) [13]. According to Fig. 1c, as the billet longitudinally extrudes within the outer channel, the same amount of plastic strain as that is estimated by Eq. 2 is imposed to the specimen. Then the first component of total strain (¯ε1 ) can be expresses as: ε¯ 1 =
2(cot ˛ + cot ˇ) √ 3
(3)
During the first half cycle, change in the tube diameter accounts for the second component of imposed plastic strain (¯ε2 ). By assuming homogeneous plastic deformation, ε¯ 2 can be expressed as follows: ε¯ 2 = 2 ln
(D1 + D2 )/(d1 + d2 ) √ 3
(4)
Parameters d1 , d2 , D1 , and D2 are presented in Fig. 1g. Therefore, the total effective strain stored after the first half cycle of HTP process can be estimated as: ε¯ half = ε¯ 1 + ε¯ 2 = 2
cot ˛ + cot ˇ + ln((D1 + D2 )/(d1 + d2 )) √ 3
(5)
By conducting the second half cycle, the same amount of plastic strain is accumulated in the billet. Thus a single-pass HTP operation,
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Table 1 Chemical composition of as-cast AZ80 alloy. Element Weight percent
Mg Bal.
Al 8.500
Zn 0.500
Mn 0.300
Si 0.05
Cu 0.001
Fe 0.000
Ni 0.001
including a couple of half cycles, results in the effective plastic strain of: ε¯ total = 2(¯ε1 + ε¯ 2 ) = 4
(cot ˛ + cot ˇ + ln((D1 + D2 )/(d1 + d2 ))) √ 3
(6)
Based on the geometries of fabricated HTP die, i.e. d1 = 15 mm, d2 = 21 mm, D1 = 27 mm, and D2 = 33 mm, after a single-pass HTP operation, the imposed effective strain was calculated to be 6.9. 3. Experiments Cylindrical AZ80 billets composed of elements listed in Table 1 were firstly casted with nominal height and diameter of 90 and 25 mm, respectively. Billets then were machined and longitudinally drilled to prepare tubular specimens with 3 mm thickness, 15 mm inner diameter, and 60 mm height. Die and punches were manufactured using VCN steel (DIN 1.6565) and, then, hardened to 50 HRC. Both the exploded and assembled views of designed tools together with the fabricated equipment are illustrated in Fig. 3. By utilizing electrical heaters, the temperatures of die and specimen rose to 250 ◦ C. HTP experiments then were carried out up to 4 half cycles (2 passes) with a punch velocity of 5 mm/min. After completing each half cycle, the corresponding punches were pulled off the die and by employing a specific fixture, other punches were inserted into the die. By exploiting a thermometer in the middle of the die and close enough to the deformation zone, the temperature was monitored and controlled to vary within ±5 ◦ C. All contact surfaces were fully lubricated using MoS2 . The products were cut in the longitudinal direction for conducting microstructural studies as well as microhardness tests. After polishing, the cross sections of products were etched for 25 s via a solution composed of 10% acetic acid, 10% picric acid, 10% distilled water, and 70% ethanol and, afterwards, were microstructurally characterized utilizing an optical microscope. The microhardness of each sample was also measured
Fig. 3. Schemas of (a) exploded and (b) assembled views of designed equipment, together with (c) fabricated HTP die.
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Fig. 4. (a) Preparation steps of tension test specimen after the second half cycle (the first pass) and (b) the drawing of tension test sample.
by applying a 200 gf load for 20 s and was calculated as the average of measurements for 3 uniformly distributed points along the thickness. As Fig. 4 illustrates, based on the ASTM E8M standard, longitudinal tension test specimens were machined from the products. Tension tests were performed at the ambient temperature and with a deformation rate of 1 mm/min. 4. Results and discussion Fig. 5a depicts the initial shape of AZ80 tubular sample. By conducting the first half cycle of HTP process, the billet laterally flows outward and by reaching the wall of outer channel, longitudinally extrudes while its outer diameter increases from 21 to 33 mm. Fig. 5b illustrates the specimen shape in the middle of the first half cycle. At the end of the first half cycle, the heights of material in the inner and outer channels reach their minimum and maximum values, respectively (Fig. 5c). As Eq. 6 estimates, a single-pass HTP operation creates an effective plastic strain of 6.9. This amount is comparable to the plastic strains attainable by means of other SPD techniques such as RTES. An RTES die, which is geometrically equivalent to the fabricated HTP tool, theoretically induces a plastic strain of 5.8 [12], denoting about 16% smaller than that is applied by the HTP process. As mentioned before, the HTP method is a kind of tube-adapted DECLE process (Fig. 2). The DECLE operation can be considered as a couple of back to back channel angular deformations (Fig. 2b) [13]. Even though it is believed that the same mechanism plays role in the plastic deformation of both ECAE and DECLE methods, required
Fig. 5. Billet shapes (a) at the beginning, (b) in the middle, and (c) at the end of the first half cycle of HTP process.
forming load in DECLE is notably lower than that in ECAE [15]. During the conventional ECAE operation, a pressure imposed by part of the wall of inlet channel acts against the material flow while this resistance does not exist during the DECLE process (see Fig. 2b). By drawing the same comparison between HTP and RTES processes, as Fig. 2a illustrates, the HTP can be considered as a couple of back to back RTES deformations. Then one can conclude that the HTP method will be capable of processing materials via a lower forming load rather than the RTES technique. It is worth mentioning that although the RTES delivers smaller amount of plastic strain, this operation benefits from simpler method and tools rather than the HTP process. Thus, compared to the HTP operation, it is expected that the RTES will receive more attention, particularly from the industrial point of view. According to Fig. 6, the AGS of as-cast billet was measured to be 79 m. Relatively homogeneous microstructure observed for the as-cast billet could be formed due to gradual solidification during the casting procedure. As this figure implies, at the grain boundaries the original magnesium grains are surrounded by an interconnected network of Mg17 Al12 phase (exhibits dark color). Fig. 7 displays the AGS of products after the first and the second half cycles along with the initial (as-cast) AGS. It can be seen from Fig. 6b that applying a plastic strain of 3.5 that goes with conducting the first half cycle of HTP operation has reduced the AGS from 79 to 21 m, showing a reduction of 73% compared to the as-cast alloy. Moreover, the consequent microstructure is more homogeneous rather than the initial one. By conducting the second half cycle, as the stored plastic strain reaches 6.9, the AGS also decreases to 11 m (Fig. 6c). It should be mentioned that at the end of each HTP test and before the billet was detached from the die, the tools were left to be air-cooled below 100 ◦ C which could cause grain growth and microstructural alteration. Nevertheless, studies indicate that procedures which provide the same amount of plastic strain as HTP are potentially capable of decreasing the AGS to Nano dimensions [16]. Fig. 8a illustrates the stress–strain curves obtained from tension tests conducted on as-cast AZ80 as well as on samples processed through various cycles. Based on these curves, the yield stress (YS), the ultimate tensile strength (UTS), and the elongation (El) of each product are also presented in Fig. 8b. As this figure shows, after the first half cycle, the YS and the UTS are increased to 191 and 245 MPa, denoting almost 90 and 100% improvements compared
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Fig. 6. Micrographs of (a) as-cast AZ80 alloy as well as the products processed via (b) a half-cycle and (c) a one-pass HTP.
phase could confine dislocation movements. This mechanism can cause dislocation pile-up and, consequently, enhances the lattice strength [17]. According to Eqs. (5) and (6), the plastic strain applied during the second half cycle is twice as much as that of the first half cycle. The AGS has also decreased from 21 m at the end of first half cycle to 11 m after the first pass (after the second half cycle), denoting 47% refinement (Fig. 9). However, the fluctuations in UTS, YS, and El have been recorded to be less than 4% (see Fig. 8). Moreover, Fig. 7 demonstrates that by performing the second half cycle, the microhardness also varies insignificantly. As Figs. 7 and 8 display, despite the accumulation of outstanding amount of plastic strain through each half cycle of HTP, the mechanical properties of AZ80 alloy are
300 250 200 150 as-cast 1 half cycle 1 pass 3 half cycles 2 passes
100 50 0 2
0
79
AGS
106
109
108
107
80
100
60 80 40
67
60
21
20
11
40
0
1 0.5 1.5 Number of cycles
2
0
Fig. 7. The average grain size (AGS) together with the Vickers microhardness of AZ80 alloy processed via the HTP process through various cycles.
b
350
Stress (MPa)
HV
100 Average grain size (µm)
Microhardness (HV)
120
250
4 6 8 Engineering strain (%)
11.1
10.8
247
253
191
198
300
200 150 100
5.8
12
12
11.2
10.9
261
256
8
201
6
10
188
4
124 101
YS UTS El
50 0
10
0.5 1 1.5 Number of cycles
Elongation (%)
a Engineering Stress (MPa)
with the as-cast alloy, respectively. This considerable increase in the strength is observed while the workability of as-cast alloy is also notably improved merely after doing a half-cycle HTP. After the first half cycle of HTP, as Fig. 8 shows, the ductility reaches 11.1, indicating more than 90% enhancement in comparison with the as-cast state. Microhardness measurements which are plotted in Fig. 7 reveal that this parameter rose by 58% from 67 HV in the initial state to 106 HV at the end of the first half cycle. There is agreement that the Mg17 Al12 precipitates play a pivotal role in blocking dislocation movements during plastic deformation of Mg alloys. As AZ80 alloy undergoes an SPD operation, especially at elevated temperatures, the Mg17 Al12 phase dissolves in the matrix while fine Mg17 Al12 particles will re-precipitate during the following cooling process [17]. By comparing Fig. 6a and 6b, one can observe that after conducting the first half cycle the thick Mg17 Al12 particles are replaced by the fine ones, which could considerably reduce local stress concentration and, consequently, postpone the formation of micro-cracks. Therefore, as Fig. 8 indicates, ductility of the alloy is effectively improved after the first half cycle of HTP. In addition, apart from activation of more slip systems during hot-forming processes, reduction in the influence of Mg17 Al12 phase usually couples with the dynamic recrystallization and, as a result, significantly improves the ductility. Considering the Hall–Petch relation, it is also expected that decrease in the AGS during the first half cycle results in mechanical strengthening. Furthermore, re-dissolved Mg17 Al12
2 0
2
Fig. 8. (a) Tensile stress–strain curves along with (b) the yield stress (YS), the ultimate tensile strength (UTS), and the elongation (El) of AZ80 alloy processed through various cycles via the HTP process.
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depicts, a microcrack has been appeared in region A. Nevertheless, the material passes through region B without any consequent defects or cracks (Fig. 9). Imposing the plastic strain, especially at higher temperatures [21], can active more slip systems in Mg lattice. Moreover, texture development, microstructural homogeneity, and dynamic recovery could improve the workability of Mg alloys [22]. According to Eq. (2), it can be asserted that accumulating an effective strain of 1.44 at a temperature of 250 ◦ C while the material passes through region A could effectively improve the ductility of as-cast AZ80 alloy. Accordingly, as Fig. 9 illustrates, the material successfully shears through the outer corner of HTP die (region B) and no microcrack appears in the billet. 5. Summary and conclusions Fig. 9. Micrographs of AZ80 alloy passing through the inner (region A) and the outer (region B) shear zones during the first half cycle of HTP process.
less likely to be affected by further half cycles. This phenomenon evidences a kind of saturation and stable equilibrium in the microstructure of AZ80 alloy. Mechanical behavior of metals is generally affected by numerous hardening and softening mechanisms which play roles during plastic deformation. By strain accumulation during successive passes of SPD operations, some mechanisms such as grain refinement, precipitation of strengthening phases, and dislocation distribution increase the material’s strength. On the other hand, some mechanisms such as microstructural recovery, dynamic recrystallization, and texture softening weaken the metal’s structure. Saturation in mechanical properties occurs as a balance between the above-mentioned hardening and softening mechanisms is established. Due to large amount of plastic strain attainable by means of SPD techniques, especially methods intended for tubular billets such as RTES and HTP, it is expected that the saturation happens merely over a single pass or even after a half cycle of the operation. Previous studies on AZ80 alloy demonstrated that saturation in mechanical properties is significantly affected by imposed plastic strain [18]. Thereby, the HTP process can be utilized as a very efficient, single-pass SPD method. Even though the results show that AZ80 alloy reaches a kind of mechanical stability at the end of the first half cycle of HTP, it might be needed to conduct the second half cycle in order to recover the initial shape of specimen. It should be kept in mind that saturation occurrence depends on material behavior and the mechanism of plastic deformation as well as on the initial microstructure. In other words, saturation in mechanical properties might occur at different levels of induced plastic strain. Previous findings regarding the effects of Exp-ECAE process on ascast AZ80 and AA6063 alloys revealed that although AZ80 reached the mechanical stability after the first pass (¯ε ≈ 2.7) [18], considerable variations in mechanical properties were observed for AA6063 alloy by doing further passes of Exp-ECAE operation [7]. Fig. 9 presents microscopic illustrations of the AZ80 billet passing through shear zones during the first half cycle of HTP process. In this figure, the inner and the outer shear zones are indicated by region A and region B, respectively. Marked with a white arrow in this figure, a microcrack has been appeared at the inner shear zone (region A). This phenomenon could be explained by hcp crystal structure of coarse-grained Mg alloys and their limited number of slip systems which account for poor workability of such metals [19]. In addition, during the ECAE process, a tensile stress state occurs immediately after the shear zone which initiates growth in microcracks and results in fracture and segmentation [20]. In the first half cycle of HTP process the required load for material deformation at the outer shear zone provides the inner shear zone with an in-process back pressure. Although it was expected that crack occurrence would be prevented by such a back pressure, as Fig. 9
This paper introduces H-tube pressing (HTP) as a novel SPD method for processing tubular billets. Through an HTP operation, dual lateral extrusion is performed on tubular specimens during a couple of half cycles. This process was experimentally carried out on as-cast AZ80 tubes. Experiments were performed up to 2 passes at 250 ◦ C and with a ram velocity of 5 mm/min. Complementary studies including microstructural analysis, tension test, and microhardness measurement were conducted to investigate microstructural and mechanical improvements after the HTP process. The yield stress and the elongation were increased by 90% after doing a half-cycle HTP while the ultimate tensile strength and the microhardness were improved by 100 and 58%, respectively. A single-pass HTP was also able to effectively decrease the average grain size of AZ80 alloy from 79 to 21 m. Saturation in mechanical properties observed after the first half cycle reveals substantial efficiency of HTP process which can be employed as a single-pass or even as a half-cycle SPD operation. The HTP die fabricated in this research work possesses the ability to accumulate a remarkable plastic strain of 6.9 over a single pass. Microstructural studies also detected no significant structural defect inside the processed billets. Acknowledgments The authors gratefully acknowledge all technical and financial supports provided by Mohammad Sadegh Alpanah-Soltanian and Mohammad Ghorbani in Qetesazan-e-Hekmatan Co., Hamedan, Iran. References [1] Zhu YT, Lowe TC, Langdon TG. Performance and applications of nanostructured materials produced by severe plastic deformation. Scripta Mater 2004;51:825–30. [2] Nagasekhar AV, Chakkingal U, Venugopal P. Equal channel angular extrusion of tubular aluminum alloy specimens – analysis of extrusion pressures and mechanical properties. J Manuf Process 2006;8:112–20. [3] Patil BV, Chakkingal U, Prasanna Kumar TS. Effect of geometric parameters on strain, strain inhomogeneity and peak pressure in equal channel angular pressing – a study based on 3D finite element analysis. J Manuf Process 2015;17:88–97. [4] Lin J, Wang Q, Peng L, Roven HJ. Study on deformation behavior and strain homogeneity during cyclic extrusion and compression. J Mater Sci 2008;43:6920–4. [5] Xu J, Wang X, Shirooyeh M, Xing G, Shan D, Guo B, Langdon TG. Microhardness, microstructure and tensile behavior of an AZ31 magnesium alloy processed by high-pressure torsion. J Mater Sci 2015;50:7424–36. [6] Segal VM. Engineering and commercialization of equal channel angular extrusion (ECAE). Mater Sci Eng A 2004;386:269–76. [7] Sepahi-Boroujeni S, Fereshteh-Saniee F. Expansion equal channel angular extrusion, as a novel severe plastic deformation technique. J Mater Sci 2015;50:3908–19. [8] Tóth LS, Arzaghi M, Fundenberger JJ, Beausir B, Bouaziz O, Arruffat-Massion R. Severe plastic deformation of metals by high-pressure tube twisting. Scripta Mater 2009;60:175–7.
S. Sepahi-Boroujeni, A. Sepahi-Boroujeni / Journal of Manufacturing Processes 24 (2016) 71–77 [9] Zangiabadi A, Kazeminezhad M. Development of a novel severe plastic deformation method for tubular materials: tube channel pressing (TCP). Mater Sci Eng A 2011;528:5066–72. [10] Faraji G, Babaei A, Mashhadi MM, Abrinia K. Parallel tubular channel angular pressing (PTCAP) as a new severe plastic deformation method for cylindrical tubes. Mater Lett 2012;77:82–5. [11] Faraji G, Mashhadi MM, Kim HS. Tubular channel angular pressing (TCAP) as a novel severe plastic deformation method for cylindrical tubes. Mater Lett 2011;65:3009–12. [12] Jafarzadeh H, Abrinia K. Fabrication of ultra-fine grained aluminium tubes by RTES technique. Mater Charact 2015;102:1–8. [13] Talebanpour B, Ebrahimi R, Janghorban K. Microstructural and mechanical properties of commercially pure aluminum subjected to dual equal channel lateral extrusion. Mater Sci Eng A 2009;527:141–5. [14] Lee DN. An upper-bound solution of channel angular deformation. Scripta Mater 2000;43:115–8. [15] Talebanpour B, Ebrahimi R. Upper-bound analysis of dual equal channel lateral extrusion. Mater Des 2009;30:1484–9.
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[16] Valiev RZ, Islamgaliev RK, Alexandrov IV. Bulk nanostructured materials from severe plastic deformation. Prog Mater Sci 2000;45:103–89. [17] Wang Z, Yang Y, Li B, Zhang Y, Zhang Z. Effect of hot-deformation on microstructure and mechanical properties of AZ80 magnesium alloy. Mater Sci Eng A 2013;582:36–40. [18] Sepahi-Boroujeni S, Fereshteh-Saniee F. The influences of the expansion equal channel angular extrusion operation on the strength and ductility of AZ80 magnesium alloy. Mater Sci Eng A 2015;636:249–53. [19] Su CW, Chua BW, Lu L, Lai MO. Properties of severe plastically deformed Mg alloys. Mater Sci Eng A 2005;402:163–9. [20] Lapovok RY. The role of back-pressure in equal channel angular extrusion. J Mater Sci 2005;40:341–6. [21] Staroselsky A, Anand L. A constitutive model for hcp materials deforming by slip and twinning: application to magnesium alloy AZ31B. Int J Plast 2003;19:1843–64. [22] Gehrmann R, Frommert MM, Gottstein G. Texture effects on plastic deformation of magnesium. Mater Sci Eng A 2005;395:338–49.
Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Technical Paper
Improvements in microstructure and mechanical properties of AZ80 magnesium alloy by means of an efficient, novel severe plastic deformation process S. Sepahi-Boroujeni a,∗ , A. Sepahi-Boroujeni b a b
Department of Mechanical Engineering, Faculty of Engineering, Bu-Ali Sina University, Hamedan 69178, Iran Faculty of Mechanical Engineering, Iran University of Science and Technology, Tehran 16846-13114, Iran
a r t i c l e
i n f o
Article history: Received 19 January 2016 Received in revised form 23 April 2016 Accepted 31 July 2016 Keywords: H-tube pressing Severe plastic deformation Magnesium alloy Mechanical properties Grain refinement
a b s t r a c t Present paper includes efforts to develop dual equal channel lateral extrusion (DECLE) into an efficient SPD technique for processing tubular samples. In modified technique, entitled H-tube pressing (HTP), specimen accumulates outstanding amount of plastic strain in a couple of half cycles. Theoretically imposed plastic strain by means of the HTP operation exceeds that is attainable via a wide range of SPD methods. In this research work, the HTP process was experimentally conducted up to 2 passes (4 half cycles) on as-cast AZ80 magnesium alloy at elevated temperature. The microstructural and mechanical improvements follow the HTP process were investigated. Findings revealed that by conducting the first half cycle of HTP process, the average grain size reduced by 73%. Moreover, the yield stress and the elongation were increased by 90% compared to as-cast condition while the ultimate tensile strength and the microhardness were improved by 100 and 58%, respectively. Stability of mechanical properties of AZ80 alloy was observed at the end of the first half cycle. These results imply that HTP can be employed as an efficient, single-pass operation. © 2016 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
1. Introduction Reduction in the grain size dramatically affects both the microstructural and mechanical features of metals. Grain refinement not only leads to the production of materials with interesting mechanical and physical characteristics, but it can eliminate various drawbacks of coarse-grained metals such as paradox of strength and ductility as well. Among existing procedures for achieving ultrafine-grained (UFG) materials, induction of severe plastic strain has efficiently led to grain refinement and products with considerable mechanical capabilities. Accordingly, severe plastic deformation (SPD) techniques are widely utilized for producing UFG parts [1]. In the last decade, number of SPD methods has grown continuously. Most of recently proposed techniques are inspired by well-established methods such as equal channel angular extrusion (ECAE) [2,3], cyclic extrusion–compression (CEC) [4], and high pressure torsion (HPT) [5]. Accumulating larger amounts of plastic strain, decreasing the forming load, processing specimens with various forms and geometries, and facilitating the practice of
∗ Corresponding author. Tel.: +98 9365955144; fax: +98 3834222793. E-mail address: saeid [email protected] (S. Sepahi-Boroujeni).
conducting further passes can be the main incentives that motivate researchers to develop primary SPD operations into more efficient methods. For example, to improve the ECAE operation with the aims of declining the forming load and increasing the imposed plastic strain, ECAE with movable die walls [6] and expansion equal channel angular extrusion (Exp-ECAE) [7] have been proposed, respectively. Several SPD processes have been recently intended for tubular billets. To illustrate, HPT was developed into high-pressure tube twisting (HPTT) [8] and some new operations such as tube channel pressing (TCP) [9], parallel tubular channel angular pressing (PTCAP) [10], and tubular channel angular pressing (TCAP) [11] were proposed in order to modify the ECAE method for processing cylindrical tubes. All these ECAE-based methods possess the same drawback of punch buckling which has been resolved with the advent of repetitive tube expansion and shrinking (RTES) [12]. Today, due to the crucial role of components’ strength to weight ratio in design and manufacture, few can dispute the contributions of hollow-cross-section parts into various fields of industry. Among them, tubular products, especially those made from light alloys, are widely used in the aerospace industry. For instance, titanium tubes effectively work in airplanes’ high pressure hydraulic systems while considerably reduce these vehicles’ weights rather than traditional stainless steel tubes. In addition, magnesium (the lightest
http://dx.doi.org/10.1016/j.jmapro.2016.07.007 1526-6125/© 2016 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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Fig. 1. Schematic illustrations of (a–f) sequences of the HTP process and (g) geometric parameters of the billet in the HTP operation.
structural metal) is an appealing material for engineering applications due to its unique properties such as great specific strength and recycling capabilities. Thus, it is clear that making high-strength, hollow-cross-section components of such light metals can widen the range of their applications. In this paper, dual equal channel lateral extrusion (DECLE) [13] was adapted for cylindrical specimens. The proposed method, HTube pressing (HTP), was experimentally carried out on as-cast AZ80 magnesium alloy up to 2 passes. Both the microstructural and mechanical developments of the alloy then were measured. Findings showed that the HTP process was able to effectively improve the strength, ductility, and the microhardness of the alloy. This process also managed to dramatically reduce the average grain size (AGS) of AZ80 merely after a half cycle. To recover the initial shape of the specimen, however, it is inevitable to complete the cycle by performing the second half cycle. 2. Principles of HTP As Fig. 1 illustrates, an HTP die involves a couple of parallel tubular channels, i.e. inner channel and outer channel, with equal width and coincident centerlines. These channels are connected at the middle of their length with a radial channel (Fig. 1). First of all, as Fig. 1a depicts, a tubular billet is inserted into the inner channel. A couple of synchronized tubular punches (inner punches), which are represented by arrows in Fig. 1, are utilized to move against
each other and toward the billet. As inner punches touch the billet, the specimen is entirely constrained by the die walls and the inner punches except at the entrance of the radial channel. By continuation of the opposite movements of punches, according to Fig. 1b, the billet is forced to radially flow outward. By this lateral (radial) extrusion, material reaches the wall of outer channel (Fig. 1b). Now, the specimen undergoes dual lateral extrusion and, in turn, laterally (longitudinally) extrudes within the outer channel in both upward and downward directions (Fig. 1c). As Fig. 1d presents, the inner punches almost meet each other at the end of the first half cycle. At the beginning of the second half cycle, inner punches are pulled out and another couple of tubular punches (outer punches) are inserted into the outer channel. Simultaneous and opposite movements of outer punches reverse the material flow established during the first half cycle (Fig. 1e). Eventually, as Fig. 1f displays, as the material recovers its initial shape inside the inner channel, the second half cycle of HTP operation is finalized. As Fig. 2a displays, by considering the axisymmetric view of specimen during the process, the flow direction evokes an axisymmetric, H-shaped path in which the material experiences DECLE deformation in the radial direction. Through this path, the material is exposed to the lateral extrusion which can lead to accumulation of notable plastic strain inside the billet. Total plastic strain induced via the first half cycle of HTP can be resolved into a couple of components. The first component of total strain (¯ε1 ) is imposed as the material radially extrudes
Fig. 2. (a) Axisymmetric schemas of HTP and RTES [12] processes and (b) schematic illustrations of DECLE and ECAE operations [13].
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outward (Fig. 1b) and, afterwards, longitudinally shears upward and downward within the outer channel (Fig. 1c). Based on the relation proposed by Lee [14], the shear strain created via DECLE (see Fig. 2b) can be estimated by: DECLE = cot ˛ + cot ˇ
(1)
Then the von Mises plastic strain can be formulated as: ε¯ DECLE =
cot ˛ + cot ˇ √ 3
(2)
This amount of plastic strain is accumulated inside the billet as it radially extrudes and reaches the wall of outer channel (Fig. 1b). Herein, according to the equal width of inner, outer and radial channels (Fig. 1g), ˛ = 90 − ˇ = tan−1 1/2. As a result, Eq. (1) yields a plastic strain 25% greater than that is attainable by means of the ECAE with perpendicular channels (Fig. 2b) [13]. According to Fig. 1c, as the billet longitudinally extrudes within the outer channel, the same amount of plastic strain as that is estimated by Eq. 2 is imposed to the specimen. Then the first component of total strain (¯ε1 ) can be expresses as: ε¯ 1 =
2(cot ˛ + cot ˇ) √ 3
(3)
During the first half cycle, change in the tube diameter accounts for the second component of imposed plastic strain (¯ε2 ). By assuming homogeneous plastic deformation, ε¯ 2 can be expressed as follows: ε¯ 2 = 2 ln
(D1 + D2 )/(d1 + d2 ) √ 3
(4)
Parameters d1 , d2 , D1 , and D2 are presented in Fig. 1g. Therefore, the total effective strain stored after the first half cycle of HTP process can be estimated as: ε¯ half = ε¯ 1 + ε¯ 2 = 2
cot ˛ + cot ˇ + ln((D1 + D2 )/(d1 + d2 )) √ 3
(5)
By conducting the second half cycle, the same amount of plastic strain is accumulated in the billet. Thus a single-pass HTP operation,
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Table 1 Chemical composition of as-cast AZ80 alloy. Element Weight percent
Mg Bal.
Al 8.500
Zn 0.500
Mn 0.300
Si 0.05
Cu 0.001
Fe 0.000
Ni 0.001
including a couple of half cycles, results in the effective plastic strain of: ε¯ total = 2(¯ε1 + ε¯ 2 ) = 4
(cot ˛ + cot ˇ + ln((D1 + D2 )/(d1 + d2 ))) √ 3
(6)
Based on the geometries of fabricated HTP die, i.e. d1 = 15 mm, d2 = 21 mm, D1 = 27 mm, and D2 = 33 mm, after a single-pass HTP operation, the imposed effective strain was calculated to be 6.9. 3. Experiments Cylindrical AZ80 billets composed of elements listed in Table 1 were firstly casted with nominal height and diameter of 90 and 25 mm, respectively. Billets then were machined and longitudinally drilled to prepare tubular specimens with 3 mm thickness, 15 mm inner diameter, and 60 mm height. Die and punches were manufactured using VCN steel (DIN 1.6565) and, then, hardened to 50 HRC. Both the exploded and assembled views of designed tools together with the fabricated equipment are illustrated in Fig. 3. By utilizing electrical heaters, the temperatures of die and specimen rose to 250 ◦ C. HTP experiments then were carried out up to 4 half cycles (2 passes) with a punch velocity of 5 mm/min. After completing each half cycle, the corresponding punches were pulled off the die and by employing a specific fixture, other punches were inserted into the die. By exploiting a thermometer in the middle of the die and close enough to the deformation zone, the temperature was monitored and controlled to vary within ±5 ◦ C. All contact surfaces were fully lubricated using MoS2 . The products were cut in the longitudinal direction for conducting microstructural studies as well as microhardness tests. After polishing, the cross sections of products were etched for 25 s via a solution composed of 10% acetic acid, 10% picric acid, 10% distilled water, and 70% ethanol and, afterwards, were microstructurally characterized utilizing an optical microscope. The microhardness of each sample was also measured
Fig. 3. Schemas of (a) exploded and (b) assembled views of designed equipment, together with (c) fabricated HTP die.
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Fig. 4. (a) Preparation steps of tension test specimen after the second half cycle (the first pass) and (b) the drawing of tension test sample.
by applying a 200 gf load for 20 s and was calculated as the average of measurements for 3 uniformly distributed points along the thickness. As Fig. 4 illustrates, based on the ASTM E8M standard, longitudinal tension test specimens were machined from the products. Tension tests were performed at the ambient temperature and with a deformation rate of 1 mm/min. 4. Results and discussion Fig. 5a depicts the initial shape of AZ80 tubular sample. By conducting the first half cycle of HTP process, the billet laterally flows outward and by reaching the wall of outer channel, longitudinally extrudes while its outer diameter increases from 21 to 33 mm. Fig. 5b illustrates the specimen shape in the middle of the first half cycle. At the end of the first half cycle, the heights of material in the inner and outer channels reach their minimum and maximum values, respectively (Fig. 5c). As Eq. 6 estimates, a single-pass HTP operation creates an effective plastic strain of 6.9. This amount is comparable to the plastic strains attainable by means of other SPD techniques such as RTES. An RTES die, which is geometrically equivalent to the fabricated HTP tool, theoretically induces a plastic strain of 5.8 [12], denoting about 16% smaller than that is applied by the HTP process. As mentioned before, the HTP method is a kind of tube-adapted DECLE process (Fig. 2). The DECLE operation can be considered as a couple of back to back channel angular deformations (Fig. 2b) [13]. Even though it is believed that the same mechanism plays role in the plastic deformation of both ECAE and DECLE methods, required
Fig. 5. Billet shapes (a) at the beginning, (b) in the middle, and (c) at the end of the first half cycle of HTP process.
forming load in DECLE is notably lower than that in ECAE [15]. During the conventional ECAE operation, a pressure imposed by part of the wall of inlet channel acts against the material flow while this resistance does not exist during the DECLE process (see Fig. 2b). By drawing the same comparison between HTP and RTES processes, as Fig. 2a illustrates, the HTP can be considered as a couple of back to back RTES deformations. Then one can conclude that the HTP method will be capable of processing materials via a lower forming load rather than the RTES technique. It is worth mentioning that although the RTES delivers smaller amount of plastic strain, this operation benefits from simpler method and tools rather than the HTP process. Thus, compared to the HTP operation, it is expected that the RTES will receive more attention, particularly from the industrial point of view. According to Fig. 6, the AGS of as-cast billet was measured to be 79 m. Relatively homogeneous microstructure observed for the as-cast billet could be formed due to gradual solidification during the casting procedure. As this figure implies, at the grain boundaries the original magnesium grains are surrounded by an interconnected network of Mg17 Al12 phase (exhibits dark color). Fig. 7 displays the AGS of products after the first and the second half cycles along with the initial (as-cast) AGS. It can be seen from Fig. 6b that applying a plastic strain of 3.5 that goes with conducting the first half cycle of HTP operation has reduced the AGS from 79 to 21 m, showing a reduction of 73% compared to the as-cast alloy. Moreover, the consequent microstructure is more homogeneous rather than the initial one. By conducting the second half cycle, as the stored plastic strain reaches 6.9, the AGS also decreases to 11 m (Fig. 6c). It should be mentioned that at the end of each HTP test and before the billet was detached from the die, the tools were left to be air-cooled below 100 ◦ C which could cause grain growth and microstructural alteration. Nevertheless, studies indicate that procedures which provide the same amount of plastic strain as HTP are potentially capable of decreasing the AGS to Nano dimensions [16]. Fig. 8a illustrates the stress–strain curves obtained from tension tests conducted on as-cast AZ80 as well as on samples processed through various cycles. Based on these curves, the yield stress (YS), the ultimate tensile strength (UTS), and the elongation (El) of each product are also presented in Fig. 8b. As this figure shows, after the first half cycle, the YS and the UTS are increased to 191 and 245 MPa, denoting almost 90 and 100% improvements compared
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Fig. 6. Micrographs of (a) as-cast AZ80 alloy as well as the products processed via (b) a half-cycle and (c) a one-pass HTP.
phase could confine dislocation movements. This mechanism can cause dislocation pile-up and, consequently, enhances the lattice strength [17]. According to Eqs. (5) and (6), the plastic strain applied during the second half cycle is twice as much as that of the first half cycle. The AGS has also decreased from 21 m at the end of first half cycle to 11 m after the first pass (after the second half cycle), denoting 47% refinement (Fig. 9). However, the fluctuations in UTS, YS, and El have been recorded to be less than 4% (see Fig. 8). Moreover, Fig. 7 demonstrates that by performing the second half cycle, the microhardness also varies insignificantly. As Figs. 7 and 8 display, despite the accumulation of outstanding amount of plastic strain through each half cycle of HTP, the mechanical properties of AZ80 alloy are
300 250 200 150 as-cast 1 half cycle 1 pass 3 half cycles 2 passes
100 50 0 2
0
79
AGS
106
109
108
107
80
100
60 80 40
67
60
21
20
11
40
0
1 0.5 1.5 Number of cycles
2
0
Fig. 7. The average grain size (AGS) together with the Vickers microhardness of AZ80 alloy processed via the HTP process through various cycles.
b
350
Stress (MPa)
HV
100 Average grain size (µm)
Microhardness (HV)
120
250
4 6 8 Engineering strain (%)
11.1
10.8
247
253
191
198
300
200 150 100
5.8
12
12
11.2
10.9
261
256
8
201
6
10
188
4
124 101
YS UTS El
50 0
10
0.5 1 1.5 Number of cycles
Elongation (%)
a Engineering Stress (MPa)
with the as-cast alloy, respectively. This considerable increase in the strength is observed while the workability of as-cast alloy is also notably improved merely after doing a half-cycle HTP. After the first half cycle of HTP, as Fig. 8 shows, the ductility reaches 11.1, indicating more than 90% enhancement in comparison with the as-cast state. Microhardness measurements which are plotted in Fig. 7 reveal that this parameter rose by 58% from 67 HV in the initial state to 106 HV at the end of the first half cycle. There is agreement that the Mg17 Al12 precipitates play a pivotal role in blocking dislocation movements during plastic deformation of Mg alloys. As AZ80 alloy undergoes an SPD operation, especially at elevated temperatures, the Mg17 Al12 phase dissolves in the matrix while fine Mg17 Al12 particles will re-precipitate during the following cooling process [17]. By comparing Fig. 6a and 6b, one can observe that after conducting the first half cycle the thick Mg17 Al12 particles are replaced by the fine ones, which could considerably reduce local stress concentration and, consequently, postpone the formation of micro-cracks. Therefore, as Fig. 8 indicates, ductility of the alloy is effectively improved after the first half cycle of HTP. In addition, apart from activation of more slip systems during hot-forming processes, reduction in the influence of Mg17 Al12 phase usually couples with the dynamic recrystallization and, as a result, significantly improves the ductility. Considering the Hall–Petch relation, it is also expected that decrease in the AGS during the first half cycle results in mechanical strengthening. Furthermore, re-dissolved Mg17 Al12
2 0
2
Fig. 8. (a) Tensile stress–strain curves along with (b) the yield stress (YS), the ultimate tensile strength (UTS), and the elongation (El) of AZ80 alloy processed through various cycles via the HTP process.
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depicts, a microcrack has been appeared in region A. Nevertheless, the material passes through region B without any consequent defects or cracks (Fig. 9). Imposing the plastic strain, especially at higher temperatures [21], can active more slip systems in Mg lattice. Moreover, texture development, microstructural homogeneity, and dynamic recovery could improve the workability of Mg alloys [22]. According to Eq. (2), it can be asserted that accumulating an effective strain of 1.44 at a temperature of 250 ◦ C while the material passes through region A could effectively improve the ductility of as-cast AZ80 alloy. Accordingly, as Fig. 9 illustrates, the material successfully shears through the outer corner of HTP die (region B) and no microcrack appears in the billet. 5. Summary and conclusions Fig. 9. Micrographs of AZ80 alloy passing through the inner (region A) and the outer (region B) shear zones during the first half cycle of HTP process.
less likely to be affected by further half cycles. This phenomenon evidences a kind of saturation and stable equilibrium in the microstructure of AZ80 alloy. Mechanical behavior of metals is generally affected by numerous hardening and softening mechanisms which play roles during plastic deformation. By strain accumulation during successive passes of SPD operations, some mechanisms such as grain refinement, precipitation of strengthening phases, and dislocation distribution increase the material’s strength. On the other hand, some mechanisms such as microstructural recovery, dynamic recrystallization, and texture softening weaken the metal’s structure. Saturation in mechanical properties occurs as a balance between the above-mentioned hardening and softening mechanisms is established. Due to large amount of plastic strain attainable by means of SPD techniques, especially methods intended for tubular billets such as RTES and HTP, it is expected that the saturation happens merely over a single pass or even after a half cycle of the operation. Previous studies on AZ80 alloy demonstrated that saturation in mechanical properties is significantly affected by imposed plastic strain [18]. Thereby, the HTP process can be utilized as a very efficient, single-pass SPD method. Even though the results show that AZ80 alloy reaches a kind of mechanical stability at the end of the first half cycle of HTP, it might be needed to conduct the second half cycle in order to recover the initial shape of specimen. It should be kept in mind that saturation occurrence depends on material behavior and the mechanism of plastic deformation as well as on the initial microstructure. In other words, saturation in mechanical properties might occur at different levels of induced plastic strain. Previous findings regarding the effects of Exp-ECAE process on ascast AZ80 and AA6063 alloys revealed that although AZ80 reached the mechanical stability after the first pass (¯ε ≈ 2.7) [18], considerable variations in mechanical properties were observed for AA6063 alloy by doing further passes of Exp-ECAE operation [7]. Fig. 9 presents microscopic illustrations of the AZ80 billet passing through shear zones during the first half cycle of HTP process. In this figure, the inner and the outer shear zones are indicated by region A and region B, respectively. Marked with a white arrow in this figure, a microcrack has been appeared at the inner shear zone (region A). This phenomenon could be explained by hcp crystal structure of coarse-grained Mg alloys and their limited number of slip systems which account for poor workability of such metals [19]. In addition, during the ECAE process, a tensile stress state occurs immediately after the shear zone which initiates growth in microcracks and results in fracture and segmentation [20]. In the first half cycle of HTP process the required load for material deformation at the outer shear zone provides the inner shear zone with an in-process back pressure. Although it was expected that crack occurrence would be prevented by such a back pressure, as Fig. 9
This paper introduces H-tube pressing (HTP) as a novel SPD method for processing tubular billets. Through an HTP operation, dual lateral extrusion is performed on tubular specimens during a couple of half cycles. This process was experimentally carried out on as-cast AZ80 tubes. Experiments were performed up to 2 passes at 250 ◦ C and with a ram velocity of 5 mm/min. Complementary studies including microstructural analysis, tension test, and microhardness measurement were conducted to investigate microstructural and mechanical improvements after the HTP process. The yield stress and the elongation were increased by 90% after doing a half-cycle HTP while the ultimate tensile strength and the microhardness were improved by 100 and 58%, respectively. A single-pass HTP was also able to effectively decrease the average grain size of AZ80 alloy from 79 to 21 m. Saturation in mechanical properties observed after the first half cycle reveals substantial efficiency of HTP process which can be employed as a single-pass or even as a half-cycle SPD operation. The HTP die fabricated in this research work possesses the ability to accumulate a remarkable plastic strain of 6.9 over a single pass. Microstructural studies also detected no significant structural defect inside the processed billets. Acknowledgments The authors gratefully acknowledge all technical and financial supports provided by Mohammad Sadegh Alpanah-Soltanian and Mohammad Ghorbani in Qetesazan-e-Hekmatan Co., Hamedan, Iran. References [1] Zhu YT, Lowe TC, Langdon TG. Performance and applications of nanostructured materials produced by severe plastic deformation. Scripta Mater 2004;51:825–30. [2] Nagasekhar AV, Chakkingal U, Venugopal P. Equal channel angular extrusion of tubular aluminum alloy specimens – analysis of extrusion pressures and mechanical properties. J Manuf Process 2006;8:112–20. [3] Patil BV, Chakkingal U, Prasanna Kumar TS. Effect of geometric parameters on strain, strain inhomogeneity and peak pressure in equal channel angular pressing – a study based on 3D finite element analysis. J Manuf Process 2015;17:88–97. [4] Lin J, Wang Q, Peng L, Roven HJ. Study on deformation behavior and strain homogeneity during cyclic extrusion and compression. J Mater Sci 2008;43:6920–4. [5] Xu J, Wang X, Shirooyeh M, Xing G, Shan D, Guo B, Langdon TG. Microhardness, microstructure and tensile behavior of an AZ31 magnesium alloy processed by high-pressure torsion. J Mater Sci 2015;50:7424–36. [6] Segal VM. Engineering and commercialization of equal channel angular extrusion (ECAE). Mater Sci Eng A 2004;386:269–76. [7] Sepahi-Boroujeni S, Fereshteh-Saniee F. Expansion equal channel angular extrusion, as a novel severe plastic deformation technique. J Mater Sci 2015;50:3908–19. [8] Tóth LS, Arzaghi M, Fundenberger JJ, Beausir B, Bouaziz O, Arruffat-Massion R. Severe plastic deformation of metals by high-pressure tube twisting. Scripta Mater 2009;60:175–7.
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