Magnesium is one of the lightest materials among all alkaline earth metals. They are more attractive as structural material used in many applications such as Aerospace industries, automotive industries, material's handling, transportation industries and electronic industries (B.L. Mordike et.al. (2001), R.Ye. Lapovok et.al. (2004), Mustafa Kemal Kulekci (2008)). Because of many advantages of magnesium, that is low density, excellent damping capacities, very good recycling capacities, good machinability (T.S. Srivatsan et.al. (2008), Yuichi Miyahara et.al. (2006)), and also characterized by high specific strength, stiffness, high electromagnetic shielding and good thermal or electrical conductivity at elevated temperatures (JIANG Ju-fu et.al. (2010)). Even though magnesium alloys have many advantages, they have few limitations in industrial applications at atmospheric temperatures, because of low strength, poor formability, and hexagonal closed pack (HCP) structure with the minimum slip system, which leads to poor ductility (Manuel Marya et.al. (2006)). In order to overcome these limitations few decades ago many researchers have developed conventional and multi-pass extrusion processes for imposing the large strain in the materials but these processes did not achieve up to the levels which are used in the industrial applications (A. Azushima et.al. (2008)). After extensive investigations, researchers used severe plastic deformation (SPD) techniques to impose the large plastic strain in the material by fabricating the bulk metals to create into ultrafine grained metals (R.Z. Valiev et.al. (2000), Y. Iwahashi et.al. (1997, 1998)).
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So to provide the best results, one of the interesting SPD techniques is Equal Channel Angular Pressing and also known as Equal Channel Angular Extrusion (V.M. Segal (1995, 2002), L. Olejnik et.al. (2005)). It was developed among all the techniques such as Accumulative Back Extrusion (S.M. Fatemi-Varzaneh et.al. (2009, 2010)), Cyclic Extrusion Compression (Y.J. Chen et.al. (2008), Qudong Wanga et.al. (2010)), Accumulative roll Bonding (J.A. del Valle et.al. (2005), ZHAN Mei-yan et.al. (2008)), Friction stir processing (B.M. Darras et.al. (2007), DU Xing-hao et.al. (2008), ZHANG Da-tong et.al. (2011)), High Pressure Torsion (Genki Sakai et.al. (2005)), Twist extrusion (Y. Beygelzimer et.al. (2009a, 2009b), M.I. Latypov et.al. (2012)), Reciprocating extrusion (Shih-Wei Lee et,al. (2007), Jien-Wei Yeh et.al. (1998), YANG Wen-peng et.al. (2012)), Repetitive corrugation and strengthening (V. Rajinikanth et.al. (2008), S.C. Pandey et.al. (2012)), Severe torsion straining (Katsuaki Nakamura et.al. (2004)), Cylinder covered compression (X. Zhao et.al. (2004), ZHAO Xin et.al. (2007)), Submerged friction stir processing (Douglas C. Hofmann et.al. (2005)), Constrained groove pressing (Dong Hyuk Shin et.al. (2002), F. Khakbaz et.al. (2012)) and Repetitive upsetting (W. Guo et.al. (2012)).The aim of ECAP technique is to obtain effective mechanical properties and microstructures in nanometer scale of the material. By refining the bulk materials in ECAP at abrupt die angle, without any changes in shape of the material, large plastic strain was imposed and concluded the average grain size was less than a micrometer. It was clearly explained by the following Hall-Petch equation (Ruslan Z. Valiev et.al. (2006)):
Where k is a constant value, σo is a friction stress, σ is a yield stress and d is a grain size of the material.
Mainly, ECAP is developed for few advantages as compared to other techniques. First, it has a simple procedure to impose the plastic strain in the material at a shear zone where the shear deformation is occurred. Second, the size of the grains in the material is decreased by increasing the number of passes so that yield strength of the material can improve. Third, it is a best method to improve the mechanical properties of the material at any conditions (hot working and cold working processes). Fourth, the flexibility (crystal structure) of the material improves effectively in the ECAP process. Overall, the ECAP process is a valuable technique among all the SPD techniques.
Wrought magnesium alloys
Wrought alloys are available in various shapes as bars, sheets and ingots as compared to cast magnesium alloys (ingots only), as illustrated in Fig.1(a) (Horst E. et.al. (2006)). In this, the major percentage of magnesium material combined with minor percentage of other metals such as an Aluminum, Zinc and Manganese for changing their properties, normally called as alloys. Especially AZ (Aluminum and Zinc) wrought magnesium alloys have more industrial application as compared to aluminum alloys. Therefore, AZ wrought magnesium alloys are used to augment the strength required for material who is increase with the increase in Al's content.
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According to ASM handbook (1992), the compositions and their mechanical properties of the AZ wrought magnesium alloys are illustrated in the Table.1. The AZ31B alloys are mainly used in cathodic protection due to their medium strength with formability, but in case of AZ31C is a lower-purity commercial variation of AZ31B used where the minimum corrosion resistance in light weight applications. The AZ61 alloy has high strength than AZ31 alloy, and it has a high corrosion resistance as high-purity alloy. The highest strength alloy in the AZ alloy is AZ80 alloy, and it can be artificially aged for additional strength.
This review paper has explained detailed overview of various principles, which are developed in ECAP processes. Moreover, it has described the current status of the ECAP process was used as a processing tool for the fabrication of bulk metals to create into ultrafine grained structures in the material by minimizing the number of passes. This paper reviewed on factors, which are influencing in ECAP process at different parameters and also this paper reviewed the properties (yield strength, Ultimate tensile strength, elongation, hardness) of this wrought magnesium alloy at different conditions.
Methodologies of ECAP
Many researchers with new innovative ideas in the overall world have done numerous modifications and modernizations of different die-set design for ECAP process for improving the shear deformation and properties of the materials as illustrated in table. 2. Therefore, the basic information about the different dies for ECAP, which was developed by researchers explained clearly as follows.
2.1. Conventional Side Extrusion for ECAP
First, the Segal was proposed ECAP process to impose the large plastic strain in the specimen by refined bulk materials. Moreover, the grain size of the material has to be reduced extensively for improving the mechanical properties. The principle of ECAP is schematically outlined in Fig. 2. In this process, a sample was pressed through ECAP die to impose the large plastic strain in bulk materials to create an ultra-fine grain micro-structure or sub micrometer structures. The ECAP die channel was bent at regular angle called as die angle (Φ) normally taken as 90 deg. (see in table 5) and the point at which channels are intersected is called as either outer arc of curvature (Ψ) or radius (r). Hence the total strain imposed on the specimen is calculated by using the following equation (F. Djavanroodi et.al. (2010), Seung Chae Yoon et.al. (2008), I. Balasundar et.al. (2009), C.J. Luis Perez (2004)). At a single pass, the total strain of the specimen for this conventional ECAP was acquired by equation (2), and the accumulated total strain is nε after 'n' number of passes.
ε = …………….. (2)
Where ε is total strain, is die angle, is outer arc curvature.
On other hands, the ECAP has four basic fundamental routes to material flow processes during the operation. They are route A, route, route, and route C (Minoru Furukawa et.al. (1998), P. Venkatachalam et.al. (2010)). Where in route A, the samples were pressed without any rotations; in route , the specimen rotated by 90 deg. with clock wise direction between consecutive passes; in route , the specimen rotated by 90 deg. counter clock wise between consecutive passes, and specimen rotated by 180 deg. between passes in Route C. From the Table 5, it can be observed that maximum number of researchers had been used route and they reported that the micro structure of the specimen in Route after 4 passes was same in Route A after 10 passes. But in this die in every pass of ECAP process the sample has to be removed, with or without rotations (Ruslan Z. Valiev et.al. (2006)). In order order to avoid these limitations, many researchers have been developed different procedures are like rotary-die ECAP, multi pass for ECAP, side extrusion process for ECAP.
2.2 Rotary-die ECAP
It has a simple principle to increase the plastic strain without removing samples from die in every consecutive pass by using the rotational method of ECAP process. The principle of rotary die ECAP is schematically outlined in fig.3 (Aibin Ma, et.al. (2005), Y. Nishida, et.al. (2002)). In this, the channels intersect each other with an angle of 90 degrees, and the material was inserted in die for press by plunger in fig. 3(a). In fig. 3(b), the specimen was pressed by plunger it means one pass has to be finished, and in fig. 3(c) the sample was pressed again after rotated by 90 deg. By using this processes for a single sample can be pressed up to maximum 32 passes. By observation of fig. 3(a), Route A was suitable for this process without any rotation. However, the sample is inhomogeneous due to aspect ratios are small (Ruslan Z. Valiev et.al. (2006)).
2.3 Side Extrusion with back pressure for ECAP
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The principle of repetitive side extrusion for ECAP is schematically outlined in Fig. 4 (Akira Azushima, et.al. (2002)). It seems to be a rotary-die ECAP process but have a different methodology. It consists of four punches, i.e. punch A, punch B, punch C, and punch D. Punch A and punch B is movable with a constant speed and constant back pressure respectively. Where, Punch C and Punch D are fixed as shown in fig. 4.
The sample was pressed through the die in between Punch A and Punch B to impose the large plastic strain in the specimen obtained 1.15 after one pass. So Route A is suitable for these processes as same as rotary-die ECAP but the number of passes increased up to 10passes effectively without removal of the specimen (Ruslan Z. Valiev et.al. (2006)).
2.4 Multi-Pass Extrusion for ECAP
In order to reduce the number of passes like rotary-die ECAP process and repetitive side extrusion for ECAP process researchers have been developed another processes by increasing the number of channel angles are schematically outlined in fig. 5 (Kiyotaka Nakashima, et.al. (2000)). However it has multiple turns to press the material through the channel. Hence it is a very effective method to improve mechanical properties by reducing the number of passes, and they reported the total strain of the material reveal no difference between multi-pass ECAP and standard repetitive ECAP process in either micro hardness or micro structure evaluation.
2.5 Parallel channel Extrusion for ECAP
The principle of the Parallel channel extrusion for ECAP process is schematically illustrated in fig. 6 (G.I. Raab, et.al. (2005)). In this the shear deformation takes place two times by single operation when a sample was pressed through a parallel channels for obtaining the ultrafine grained metals. Where 'Φ' is the angle which intersects at the parallel channel, 'N' is the shear direction and 'K' is the displacement between the channels it was approximately equal to channel diameter 'dc' as shown in fig 6. The advantage of this process is to impose the total strain in the material is homogeneous with the minimum number of passes.
Equal channel angular rolling
In order to produce continues long metal strips, the Equal channel angular rolling (ECAR) have been introduced. It is also called as continuous confined strip shearing (S2C2) or Dissimilar-channel angular pressing (DCAP). The principle of ECAR/DCAP/S2C2 is schematically outlined in fig. 7 (Yong Qi Cheng, et.al. (2007, 2008)). In this process, the thin sheet material entered in between specially designed rollers (feeding roll and Guide roll) which gives power to the specimen to get into the ECAP channels (Channels are in dissimilar to entry and exit) for forming as shown in fig. 7. Here the entry channel at rolling operation and outlet channel of ECAP thickness was decreased and 'H' is a channel height, 'θ' is an oblique angle and 'r' is an oblique radius of the ECAR channel.
Con shearing ECAP
The principle of con shearing ECAP is schematically outlined in fig. 8 (H. Utsunomiya, et.al. (2004), Y. Saito, et.al. (2000)). This method was developed for continues and long metallic strip, which is similar to ECAR processes but here the specimen is fed into the satellite rolls and the central roll which is rotated at the same speeds to get a high extrusion force then after the specimen entered into the ECAP die from the roll mill. In this, the total strain imposed effectively because of continuous shear deformation taken place.
Another alternative method for producing the ultrafine grained long metallic bars is ECAP-conform process. The principle of conform ECAP is schematically outlined in fig. 9 (Ruslan Z. Valiev et.al. (2006)). It has a rotating shaft with a channel at the center of conform facility, and a stationary constraints die. So that the frictional forces are generated at these three contact interfaces (rotating shaft, stationary constrained die, channel) to give a movement of the work material into ECAP where it was arranged at exit channel of this conform facility. The constrained stationary die is used to control the work material and also forces to displacement the direction to regular ECAP process. Therefore, this process is using for continues processing of metals (Cheng Xu, et.al. (2010)).
2.9 Change channel angular pressing (CCAP)
Tianmo Liu et al (2009), LIU Yu et al (2011), were proposed new technique to create an ultrafine grain in a nanometer range in the material. The principle of Change channel angular Extrusion (CCAE) was schematically outlined in fig. 10. It is a combination of ECAE process and High extrusion ratio process. In this, the total strain imposed in bulk material by pressed the sample into two inequalities cross-sectional area intersecting channels to create ultrafine grain materials and also to improve mechanical properties of metals. The main objective of CCAE process is to reduce the grain size by reduces the number of passes. AZ31 magnesium alloys have been examined in CCAE process, obtained the grains to 15 µm from 500 µm at 523 K (LIU Yu et al (2011).
2.10 Double change channel angular pressing (DCCAP)
Liwei Lu et al (2012), proposed a new extrusion technique is a double change channel angular pressing (DCCAP) to refine the grains significantly without any cracks during the shear deformation of AZ31 alloys. The principle of Double change channel angular pressing (DCCAP) is as shown in fig. 11. It has a vertical channel same as in the ECAP process and also have two smaller diameter horizontal channels for produce the flexibility of the material. The main advantage of this process is to impose the large strain for creating the UFG micro structures in the bulk metals at even corner points of the sample. It is used as a simple technique and effectively to change the micro structures at the ambient temperatures for HCP metals like magnesium alloys.
2.11 Tubular Channel angular pressing TCAP
Ghader Faraji et.al. (2011, 2012), introduced a new SPD novel technique is Tubular channel angular pressing (TCAP). As illustrated in fig.12, the principle of TCAP process has constrained by the inner and outer dies. It has a four flat region (a, b, c and d) and three shear deformation zones for impose the total strain in the tubular work piece with additional radial and circumferential tensile and compressive strain in a region b and c respectively pressed by punch as shown in fig. 12(a). R and Ri are the radii of the tube in the channel region and final tube respectively as shown in fig. 12(b).
Researchers have been obtained the equivalent strain of a tubular work pieces was calculated as 2.67, and it was higher than the three passes of ECAP process is 1.863. This technique was applied to a commercial AZ91 magnesium alloy, and a significant grain refinement was achieved even after single cycle TCAP. The micro hardness of the tube was increased to 78Hv from an initial value of 51 Hv. This new novel SPD process is promising for future industrial application's, particularly cylindrical tubes. The channels are made by either triangular or semicircular shapes.
2.12 Parallel tubular channel angular pressing (PTCAP)
Another new novel SPD process is a parallel tubular channel angular pressing (PTCAP) was introduced by Ghader Faraji et al (2012), for the producing ultrafine grained and nanostructure tubes. It is a new design of TCAP process, and the principle of this process is schematically represented as shown in fig. 13.
It consists of two half cycles. The tube material is pressed through two shear zones to reach a larger size, and then the material is pressed back through the same shear zones to reach its initial dimensions at first half and second half cycles of PTCAP respectively. The equivalent plastic strain was calculated in PTCAP is about 3±0.05. The grain size of the tube is refined from ~59 μm to 150-300 nm and the micro-hardness increased from 61 Hv to about 117 Hv for pure Cu material (Ghader Faraji, et.al. (2012)).
On the other hand, the Equal channel angular drawing (ECAD) process (A. Azushima et.al. (2008), (Ruslan Z. Valiev et.al. (2006)), I-shaped equal channel angular pressing (I-ECAP) (A. Azushima et.al. (2008)), and friction reduced ECAP (A. Azushima et.al. (2008)) were proposed by many researchers to create ultrafine grained metals by impose the large plastic deformation in the material.
3. Factors of ECAP
The ECAP process is a metal forming process in which; the sample was pressed through the die by using a plunger, to impose the large plastic strain by simple shear deformation on bulk metals to evaluate the ultrafine grained structures. However, there are several factors are influenced on ultra fine grained micro structural characteristics of the as-pressed specimens. Therefore, this paper reviewed the factors which are influenced mainly while doing the operation of ECAP process. They are, values of the angles (die angle (Φ) and angle of curvature (Ψ)) of the die in ECAP process, the slip systems and shearing patterns took place while processing the routes (A, , and C), the ram speed of pressing, the temperature of the pressing operation, back pressure (with or without).
3.1 Die geometry
In order to improve the ultrafine grain metals and its micro structures, the large plastic strain is imposed in metals by pressing the specimen into the die of ECAP. It means that, the values of the angles within the die acts as a key role for impose the large strain in the specimen. In this, the die angle (Φ) influenced directly to the total strain imposed on the specimen was calculated from equation (2) and reported the results in fig. 14. Many researchers have been developed and reported that, the minimum die angles and corner angles are produced large equivalent strains for significant UFG micro structures in the material (Ruslan Z. Valiev et.al. (2006), Kiyotaka Nakashima et.al. (1998)).
From fig. 14, researchers have been evaluated the total strain which was decreased when the channel angle is increased from 45 deg. to 180 deg. with the arc of curvature ranges from zero deg. to 90 deg. for a single pass at N=1 (Ruslan Z. Valiev et.al. (2006)). However, the corner angles are not affected much as compared to the Φ angles except below 90 deg. So that lower angles are given always better strains for a material as compared to higher angles. From table 5, researchers were suggested the minimum die angle with minimum arc of curvature to obtain the UFG structure in the material.
However, till the date, there is a little or no attempt to create any significant comparison between the different channel angles for wrought magnesium alloys as reported like pure aluminum at different angles as shown in fig. 15. In this they were examined the total strain values of the specimen at different channel angles by the minimum of arc of curvature for Al's material (Ruslan Z. Valiev et.al. (2006), Kiyotaka Nakashima et.al. (1998)). Unlike from the above figure, there is a little data described on AZ wrought magnesium alloys by compare the materials at different channel angles with different arc of curvatures for optimizing their results (see table 5).
3.2 Processing Routes in ECAP
There are four basic fundamental processing routes in ECAP process was already described in this paper as shown in fig. 16. Researchers have been improved the different micro structures in various wrought magnesium alloys by using these routes. They obtained significant UFG micro structures in as-pressed samples with route than route A (see table 5). Moreover, they have examined the combination of routes at different consecutive stages and addressed there is no additional profit in micro structures.
While processing these routes in ECAP process, they are associated with the different slip systems in different passes at X, Y and Z planes as represented schematically in fig. 17. So that the total strain in a sample was evaluated in every pass through the process. Further, when evaluating the influence of processing routes consider the shearing system implications in corresponding four routes, which are described by took an example of three-dimensional element. So the distortions of routes are introduced into a three-dimensional element at X, Y and Z planes for eight passes to evaluate the UFG microstructure as represented in fig. 18 (Ruslan Z. Valiev et.al. (2006)).
However, from the above observations, researchers have concluded in the following ways. First, there is no deformation occurred in route A at Z plane. Second, the element has restored its original shape in route and route C at Y plane is in every 2passes, and everyone pass respectively but in the X plane, the element restored its original shape in every 4passes and every consecutive pass in the same two routes respectively. Moreover, the element was deformed effectively in the route A and route (Ruslan Z. Valiev et.al. (2006)).
The specimen pressed through the die in the ECAP process by using ram for refining the grain size of the specimens. Therefore, the ram speed also to be considered as one of the factors influencing in ECAP process. The hydraulic presses usually used ram speed is up to 20 mm/sec. In last few decades researchers have been examined on the ram speeds from mm/sec to 10 mm/sec. They reported that there is no significant influence on the equilibrium size of UFG metals during ECAP because it was influenced by increasing the number of passes (Ruslan Z. Valiev et.al. (2006)).
Chin-Sung Chung et.al. (2005), proposed the yield strength of the AZ31 alloy at ram speed was 5 mm/sec, The values vary 126 MPa, 126 MPa, 152 MPa and 180 MPa for 1, 2, 3and four passes correspondingly. Furthermore, for the same ram speed was 5mm/sec, the yield strength of the AZ61 alloy varies 193 MPa, 202 MPa, 208 MPa and 191 MPa for 1, 2, 3 and four passes respectively.
So the ram speed has a minor significant influenced factor for grain refinement in the specimen. A similar observation was also made from others, and it was presented in table.5. They reported that the ram speed may have increased when the specimen was heated at higher temperatures as well as lower speeds are used for lower temperatures.
Temperature of the specimen is another important factor influencing in the ECAP process while processing the wrought magnesium alloys because of HCP structure. Yuichi Miyahara et.al. (2006), selected the temperatures of 473 K and 523 K for AZ61 alloy in Ex-ECAP process for refine the grain size of the sample. They improved the size of the grains for these temperatures are around ~0.6 µm and ~1.3 µm respectively. M. Janecek et.al. (2007), proposed the yield stress (MPa) decreases when the specimen temperature increased as shown in fig. 19.
At room temperatures, the yield strength of the AZ31 alloy varies from 65 MPa at zero passes to 210 MPa after four passes, but the yield stress decreased to less than 65 MPa at 573 K. So that the yield stress of the specimen gives an effective result at room temperature than the elevated temperatures. K. Xia et.al. (2005), improved the grain size of the AZ31 alloy by processing the ECAP at lower temperatures. They reported the grain size was decreased from ~15-22 µm to ~2 µm at 473 K after eight passes but the grain size significantly reduced to ~1 µm at 423 K. The size of the grains at 373 K reduced between 0.2 µm and 0.5 µm after four passes. Therefore, the lower angle grain boundaries improved by increasing the temperature, but whereas at lower temperatures, the smallest possible equilibrium grain size and the highest fraction of high-angle boundaries have been achieved (see table 5).
Back pressure is one of the important factors in the ECAP process for obtaining the UFG structured materials. Fig. 20 (V.V. Stolyarov et.al. (2003)), shows a set-up of ECAP process with controlled back pressure at typical loading curves. The main advantage of back pressure is used to improve the workability of the material. To enhance the visibility in the uniformity of the metal flow during ECAP operation. And also to improve the grain refinement by reducing the number of passes in the pressed material (K. Xia et.al. (2005), Feng Kang et.al. (2009), Cheng Xu et.al. (2009), R. Lapovok et.al. (2008)).
Majid Al-Maharbi et.al. (2011), was reported the results to enhance the uniformity shear deformation in the AZ31 alloy, with an extrusion ratio of 0.075mm/sec at 473 K with the back pressure of 30 MPa. X.N. Gu et.al. (2011), used multi pass ECAP process to improve the mechanical properties of the AZ31 magnesium alloy with back pressure and without back pressure. They reported that it had been effectively improved the mechanical properties of magnesium alloys with back pressure whereas without back pressure. In this, the grain size decreased from 28 µm to 8.5 µm after 4passes, also the yield strength, ultimate tensile strength and elongation vary about 150MPa, 230 MPa and 16.5% respectively whereas without back pressure the YS, UTS and elongation vary about 105 MPa, 280 MPa and 31% respectively. Finally, they have decreased the average grain size about 1.7MPa and also the YS, UTS, Elongation values was improved about 285 MPa, 430 MPa, and 31% respectively.
Jizhong Li et.al. (2011), proposed the results with same multi-pass ECAP process with the back pressure was the average grain size (0.8 µm) by refining the coarse grained (980 µm) for pure magnesium ingot at room temperature. Furthermore, it is a reasonable to anticipate from earlier results that the presence of back pressure will lead to significant additional grain refinement but very few results are available to date for evaluating the effect of back pressure on the ability to achieve homogeneity in the magnesium AZ wrought alloys.