An Overview of Deformation Path Shapes on Equal Channel Angular Pressing

Abstract

In recent years, research on ultra-fine grain materials has gained attention. While attempts have been made to improve the properties of the material, it has also become increasingly important to decrease the costs. Studies on improving material properties have revealed new production methods or have required the revision of existing production methods. In this direction, severe plastic deformation methods have come to the fore as a good alternative, and by improving the methods with new variations, materials with grain sizes below 1 µm have been obtained. In addition, this method positively affects the mechanical properties of the material. In this study, the Equal Channel Angular Pressing (ECAP) method, one of the severe plastic deformation methods, which has attracted great attention among researchers, was examined and the development stages of the method were investigated according to recent studies. The effective parameters in the method were examined and the effects of these parameters on the grain structure and mechanical properties of the material were discussed. Channel shapes, which are open to innovation and increase the efficiency of the ECAP method, were kept in the foreground among the prominent parameters in the ECAP process, and the results of the design changes made with new variations were examined.

1. Introduction

Rapid technological developments in industry require the production of new materials and the improvement of the properties of existing materials. In addition, the problems experienced in terms of energy resources around the world have revealed the necessity to economize by making new productions lighter. For these reasons, the need for materials with high performance and reduced weight, such as Al and Mg alloys, as an alternative to steel and its alloys, which play an important role in the industry, is increasing day by day [1,2,3]. While studies on the development of lightweight materials continue, the usability of material production methods, different heat treatments applied after production and shaping processes to improve the mechanical properties of materials have been investigated [4,5]. Methods such as extrusion, rolling and forging are known as mechanical or thermo-mechanical methods used to improve the mechanical properties of materials. However, it is difficult to produce grain sizes smaller than 10 µm with these processes [6] and the grain size to be achieved as a result of these methods remains at a limited level due to the applied deformation, ductility and changes in the cross-sectional area of the material. The application of heat treatment after refining the grain sizes with mechanical or thermo-mechanical processes also increases the cost. For these reasons, it is required to develop new processes other than the conventional methods to achieve an ultra-fine grain (UFG) structure below 1 µm. Severe Plastic Deformation (SPD), which has come to the fore in recent years, is the application of high deformation without any change in material dimensions. It is seen that, by the SPD methods given in Figure 1, an ultra-fine grain size below 1 µm has been achieved.

The SPD method has attracted great attention since it has achieved success in increasing the strength and toughness of Al alloys, especially by obtaining a UFG internal structure with its excellent grain refinement ability [8,9,10,11,12,13]. High pressure torsion (HPT), accumulative roll-bonding (ARB), multi-directional forging (MDF), twist extrusion (TE), cyclic extrusion and compression (CEC) and equal channel angular pressing (ECAP) are some of the methods widely used in SPD [14,15,16,17]. Among these methods, the ECAP method is an effective method for producing UFG granular structures such as enhanced super-plasticity with high strength and toughness without volume change [18,19,20]. Although the method is simple, does not require specific equipment costs and is repetitive, the drawbacks are that the die used become unusable in a short time and the process is complicated. It is extremely important to know and determine the optimum values of process parameters (such as die design, speed, temperature, friction and preform design) in ECAP in terms of the plastic deformation behavior of the material. The ECAP method consists of a channel die, a hydraulic press that can provide the push and, if necessary, a furnace. The sample is pushed to the other side of the die by the punch. When the sample passes through the angled region of the die channel, it is subjected to a high amount of shear strain. As a type of deformation, shearing causes the formation of a small-grained structure, unlike other conventional methods such as forging, extrusion, rolling, etc. [21,22,23].

With the introduction of ECAP as an ultra-grain refinement method, researchers have focused on its effects on different materials and the optimization of process parameters. The mechanical properties of many materials, especially titanium alloys used in medicine, including Al and Mg alloys used in engineering, have been improved by the ECAP method. Horita et al. [21] obtained submicron grain sizes on different aluminum alloys (AA1100, AA2024, AA3004, AA5083, AA6061, AA7075) by using equal-channel angular pressing method. In the study, the channel angle was selected as 90° and the processing route was determined as the B_C route. Llorca et al. [22] subjected the AA8090 alloy in route A to 4 and nine passes at 150 °C die temperature by determining the channel angle as 90°, and compared the microstructure and mechanical properties of the samples before and after processing. Jafarlou et al. [23] subjected an AA6061 alloy tube to heat treatment with an equal channel angular pressing process and showed improvement in mechanical properties by recording a 60% reduction in grain size. Howeyze et al. [24] applied the ECAP process up to six passes using the AA5052 alloy. In the mechanical properties obtained in this study, it was concluded that the hardness and tensile strength increased, especially as the number of passes increased, and the yield strength tended to decrease in the 6th pass after increasing in the 2nd and 4th passes. Abioye et al. [25] applied ECAP up to six passes using the AA6063 alloy. By selecting the channel angle as 90°, the mechanical properties obtained in the application using route B_C were compared, and the highest modulus of elasticity was obtained especially after the first pass. Cavaliere and Cabibbo [26] applied the ECAP process by adding Zr and Zr + Sc to the AA6106 alloy. In this application, the number of passes was determined as four and the channel angle was selected as 90°. The true stress–strain curves of the obtained samples were obtained and compared. Kim et al. [27] investigated the tensile and fatigue strength of pure titanium after ECAP. The channel angle of the die used in the study was determined as 110°, the corner angle was 20°, and six passes were provided by using route B_C. After the ECAP process, the tensile property improved by 60% and the fatigue property by 67%. In addition, in the literature there are studies conducted on composite materials [28,29,30], titanium [31,32,33], brass alloy [34], gold [35], Mg–Zn–Zr alloy [36], Mg [37,38,39,40], and austenitic stainless steel [41].

In this study, the process parameters used in the ECAP method are discussed. In addition, with the increase in the applications of the SPD method, improvements have been made continuously to increase its efficiency. Therefore, there are many variations of the method. In this study, the findings were examined by including different die designs used in the SPD method.

2. Materials and Methods

2.1. Ecap Process

In the ECAP process, there are several parameters that affect the mechanical and microstructure properties of the materials (billet). These parameters can be divided into different categories within themselves. First of all, the channel angle (Φ) and corner angle (Ψ), which are related to the die, are the parameters that directly affect the material flow [10]. Secondly, the different routes (A, B_A, B_C and C) are used for the repetitive ECAP process and the number of passes. The determined route and number of passes are important since they directly affect the microstructure and mechanical properties of the billet [42]. Lastly, pressing speed, process temperature and back pressure are process-related parameters [13]. Studies are carried out on these parameters to increase the efficiency of the ECAP process. The effects of these parameters are discussed below.

2.2. Channel Angle (Φ) and Corner Angle (Ψ)

In the ECAP method, there are two channels that intersect at certain angles. These angles, called the channel angle (Φ) and the corner angle (Ψ), intersect two channels with the same cross-section (Figure 2). The sample moves along the channel by being pushed with the help of a stamper and is plastically deformed by simple shear at the intersection of the channels.

In the studies, the plastic deformation of the material varies depending on the channel angle and corner angle. According to the researchers, the optimum channel angle should be Φ = 90° and the corner angle should be Ψ ≃ 20°. In designs where Φ < 90°, a high-pressure requirement arises, and this pressure value causes stamp or die breakage. Since Ψ = 0° cannot completely fill the channel intersection, especially in samples with low ductility, it causes a dead zone. The equivalent strain, which varies depending on the applied channel angles, is shown in Figure 3.

2.3. Ecap Routes

There are basically four routes in the ECAP process, namely Route A, Route B_A, Route B_C and Route C. Binary combinations of routes have also been tried, but researchers focused on these four routes [44,45,46]. In the studies determined as Route A, the desired number of passes is provided without any change in direction after the first pass. In Route B_A, the sample is rotated by 90° from the first pass regardless of direction. In each following pass, the sample is rotated by 90° in the opposite direction of the rotation. In Route B_C, after the initial pass, the sample is rotated by 90° in the same direction between each pass. Lastly, in Route C, after the initial pass, the sample is rotated in the same direction between each pass by 180° to complete the number of passes. The most used ECAP routes (A, B_C, C, B_A) are given in Figure 4.

The preferred routes have a direct effect on the grain size. In Figure 5, the average grain sizes varying depending on the routes are shown after two ECAP passes at a temperature of 489 K. Accordingly, the ECAP route, which has coaxial grain boundaries and is superior in microhardness and percent elongation, is Route B_C and therefore it is preferred over the other routes [48,49,50].

2.4. Temperature in the Ecap Process

The temperature at which the process is performed, which is one of the important variables in the ECAP process, varies according to the ductility and brittleness of the material. According to the studies, the process temperature is determined as the smallest temperature value that will not create cracks during plastic deformation. Therefore, it can be said that this temperature is room temperature for ductile materials and higher processing temperatures for brittle materials. Increasing the ECAP die temperature increases the number of passes of ductile materials such as aluminum and pure metals which can be processed at room temperature by ECAP. Another advantage of heating the die is that it enables ECAP treatment of metals and their alloys such as magnesium [51,52], titanium [53], steel [54] and tungsten [55], where ECAP treatment is not possible or difficult at room temperature. In the ECAP process, the die temperature can be controlled by placing the die in a temperature-controlled furnace (Figure 6a,b). As an alternative to this method, the temperature of the ECAP die is increased with the resistances placed in the die and the die temperature is controlled by thermocouples placed in the die or the resistance (Figure 6c,d).

In the ECAP method, various changes occur in the microstructural and mechanical properties of the sample as a result of increasing the die temperature. The general opinion on this subject is that recrystallization and grain coarsening occur in the ECAP sample with increasing die temperature, and this situation, as expected, causes a decrease in its mechanical properties compared to the ECAP process performed at room temperature for the same sample [60]. Gautam and Biswas [61], in their study on the ECAP process of pure magnesium at different temperatures, performed the ECAP process in eight passes at 27, 150, 200 °C temperatures. As a result of the study, the average grain size was measured as ~2.6, ~3.7 and ~4.8 µm, respectively. In a similar study, Yan et al. [62] reported an increase in grain size with increasing die temperature as a result of the ECAP treatment performed on Mg6Zn alloy in different passes at 160, 200 and 240 °C. In the study conducted by Gupta et al. [63] on the Al 6063 alloy, the effect of 1-3-6 passes ECAP treatment performed at room temperature and 250 °C on microstructure and hardness was investigated. The obtained results were compared with the commercially used 6063 alloy. Figure 7 shows the measured hardness values of their study. The hardness values increased rapidly in the initial pass of the ECAP process performed at room temperature, and then continued to increase at a lower rate in the 3rd and 6th passes. On the other hand, in the same study, it was seen that the hardness values were in a certain range in the ECAP process performed at 250 °C. In this study, the researchers stated that after the initial pass, almost all grains were equiaxed or elongated and accumulation occurred at the grain boundaries, recrystallization started when the die temperature increased to 250 °C and there was a decrease in the hardness value.

Studies show that increasing die temperatures in the ECAP process cause microstructural changes in the microstructure such as grain growth and equiaxiality of the grains. In addition, this may seem contrary to the nature of the ECAP process, because it is aimed to obtain a reduction in the grain size at maximum level as a result of the ECAP process. For this reason, performing the process at room temperature is effective in achieving minimum grain size by preventing recrystallization compared to high ECAP temperatures. However, although the process can be performed at room temperature, heavy deformation damage caused by ECAP is eliminated with increasing die temperatures, but a decrease in mechanical properties occurs. Therefore, heating ECAP die becomes necessary to prevent tearing and cracking that may occur because of extreme cold deformation. For example, it was stated by Tan et al. [64] that because of ECAP process performed at 300 °C using magnesium alloyed with rare earth elements resulted in cracks on the surface and deterioration of the surface topography. By increasing the die temperature to 360 °C, the damage on the surface was eliminated and a smoother surface topography was obtained. Studies show that heating ECAP die influences the number of passes. In a study conducted by Shaeri et al. [65] on the Al 7075 alloy, the ECAP process was performed in four passes with three different die temperatures as 120, 150 and 180 °C. As a result of the study, significant cracking and fragmentation were observed on the sample surface after the three passes in the ECAP process performed at 120 °C die temperature. On the other hand, in the ECAP process performed at 150 and 180 °C, no damage was observed in the sample even after four passes. In the literature, there are studies in which the surface deformation process is successfully performed even in 50 passes by increasing the ECAP die temperature [66]. Another issue that should be considered is that the type of alloy and the pre-treatment temperature can change the ECAP process temperature, and it should be taken into account that pre-heating processes, such as homogenization and solution, in general facilitate the ECAP process [67]. Due to the above-mentioned reasons, ECAP makes the process temperature inevitable in metals and alloys where active sliding planes are limited, especially in cases where the channel angle is 90° [68]. Studies on obtaining maximum properties from the samples produced by the ECAP process by eliminating the die temperature make the ECAP process performed at room temperature possible for many metals and alloys by changing the ECAP channel angle [69,70,71,72]. In order to optimize the channel angle, methods such as the finite element method and artificial neural networks are frequently used [72,73]. This shows that the properties of the ECAP sample can be optimized with various changes in the parameters such as die temperature, channel angle and deformation path shapes in the ECAP process. It is anticipated that these parameters, on which there is currently limited literature, may attract more attention in terms of the development of the properties of ECAP samples in the future.

2.5. Number of Passes

A decrease in grain size occurs after each pass in the ECAP process. Grain size influences mechanical properties. This effect varies according to the sample component. However, the researchers stated that the mechanical properties increased with increased number of passes. In the study conducted for copper, the best values were obtained after the 4th pass in the sample examined up to 10 passes (Figure 8a) [74]. In the study conducted for Al–1%Mg, 0.28 µm grain size was achieved after the 7th pass [75]. For the magnesium alloy, the highest yield and tensile values were obtained after the 2nd pass in the examination made up to the 6th pass [76]. In another study conducted for magnesium, samples were examined up to four, eight and sixteen passes and the highest yield and tensile strength values were obtained after the 8th pass [77]. The LAE442 magnesium alloy was examined for 12 passes, and it was found that the yield and tensile strength values after the 4th pass were approximately close, while the grain size was measured as ≃1.5 µm after 12 passes (Figure 8b) [78]. Similarly, in the study conducted on pure aluminum and copper, it was found that the yield and shrinkage values increased depending on the number of passes. Especially after the 1st and 2nd passes, the increase was higher [79]. In the examination made for Al-6061 up to 10 passes, the highest tensile strength was obtained after the 4th pass [80].

2.6. Die Shapes

In the ECAP method, the effect of the design changes made in the channel path by going beyond the conventional channel angles has increased the effect on the mechanical properties of the material. While these studies, which are different from the ECAP process, come to the fore with their different designs, they have taken their place in the literature by providing new names that express the change in the method. In this study, studies showing superior features than the ECAP method, which has taken its place in the literature, are included. Twist channel angular pressing (TCAP) (Figure 9) is an extreme plastic deformation technique presented as an alternative to the ECAP method. In this technique, unlike the ECAP method, the sample is first twisted in the die path and then passes through the channel intersection and completes the plastic deformation transition. The mechanical properties obtained with this new method are superior to the mechanical properties obtained with ECAP [81]. After the first pass in TCAP, 70% of the grain size is smaller than 5 μm in billet. In the ECAP method, on the other hand, after two passes, the same grain size between 40–50% of the grains could be obtained. In addition, while higher yield strength was obtained, microhardness increased more than two times.

The cyclic extrusion compression angular pressing (CECAP) (Figure 10) method also includes a design change in the channel path. Basically, this method is a combination of the cyclic extrusion compression (CEC) and ECAP methods. The sample is first subjected to CEC deformation in the die path and then it completes the transition process by making a corner turn. It has been understood that the single pass study conducted using this new method shows higher strength when compared to ECAP [37]. Higher strength and ductility values are obtained for a single pass. However, it requires higher pushing force compared to the ECAP method.

In the half-channel angular extrusion (HCAE) (Figure 11) method, unlike other methods, the sample exit width is half of the input width after the channel intersection. Due to this design, which is a combination of ECAP and traditional extrusion, significant changes have occurred in elongation and hardness values with the increase in processing efficiency for one pass [38]. When the studies on Mg alloys were examined, grain sizes of 11.2 μm in four passes at 563 K and 8.4 μm in eight passes at 548 K were obtained with ECAP. Grain sizes of 5.4 μm at 523 K and 7.3 μm at 573 K were obtained by HCAE. Thus, the method offers faster grain refinement and better mechanical properties.

In half-channel angular extrusion (HCAE) (Figure 11) and the equal channel angular expansion extrusion with spherical cavity (ECAEE-SC) (Figure 12) methods, the sample inlet is the same as in the conventional ECAP, but it has taken its place in the literature with a different design as a transitional channel, an expanded channel and an outlet channel in the continuation of a spherical path at the channel intersection. While the method provides effective and highly mechanical properties to create ultra-fine grains, it shows high deformation efficiency compared to the ECAP method [12]. In ECAEE, approximately 80% of the grain size is smaller than 20 μm after a single pass, and sub-grains with 400 nm grain size were obtained. The microhardness value and material properties obtained as a result of the method are better than those obtained with ECAP.

Another method—equal channel reciprocating extrusion (ECRE) (Figure 13)—is a new SPD method that will reduce two passes to a single pass in the ECAP method. The feature that distinguishes the ECRE method from the ECAP method is that the angles in the channel intersection area are completely different. Mechanical properties in the sample outer channel are higher than those in the first return zone [82]. In line with the data obtained in the study, the mechanical properties of the sample passing from two angles of the channel are better than those passing through a single channel angle.

Equal-channel angular hydro extrusion (ECAH) (Figure 14) is another SPD method offered as an alternative to the ECAP method. The difference from the ECAP method is that the driving force is provided with the help of liquid. The sample die friction, which is the limitation of the ECAP method, has been reduced and the short sample length has been eliminated by this developed method [83]. The purpose of the ECAH method is to pass the long pieces that are difficult to pass through the channel in the ECAP process with liquid pressure. The average grain size was obtained as 16 μm.

Another proposed method to improve the mechanical properties of structural beams especially with a U profile is called thin-walled open channel angular pressing (TWO-CAP) (Figure 15). In the method, the sample remains the same as its initial size after expansion and contraction. This newly developed method for SPD has taken its place in the literature as applicable [84]. The TWOCAP method is an especially designed ECAP die for U profiles. This method shows that it is possible to improve the mechanical properties of differently shaped samples with ECAP.

In another study, a new method called twin parallel channel angular extrusion (TPCAE) based on the ECAP method has been proposed. Unlike ECAP, the process is performed by placing two billet back-to-back into the die at the same time (Figure 16). As a result of the study, it was stated that a more homogeneous stress was applied to the billet. It was also emphasized that more material could be processed in each cycle, the energy consumption per volume would decrease, and the stability and buckling resistance would increase as the punch size increased. While more product is obtained within the same duration as ECAP in the TPCAE method, buckling resistance is increased by increasing the cross section of the punch. In addition, energy consumption per volume has been reduced.

In the next study, ECAP and torsional deformation were combined. This technique is called torsional-equal channel angular pressing (T-ECAP) (Figure 17). With this modification, the billet was rotated around its axis at the exit part of the ECAP mold. Thus, extra shear stresses were applied to the billet. When examined hardness resulted in the T-ECAP process, a more uniform hardness distribution was obtained compared to ECAP. On the other hand, the push force required during the procedure was compared, and it has been stated that a lower value is needed for T-ECAP. The reason for this phenomenon was stated to be due to the change in friction with the torsion deformation [86]. According to ECAP, as the number of passes increased, better grain structure, microhardness, yield strength and ductility values were obtained.

Mani et al. [87] showed that the T-ECAP method was more effective in terms of the elimination porosities of commercial pure aluminum, which has an average particle size of 50 μm and an irregular shape. As a result of the T-ECAP process, the density and microhardness values improved more than with the ECAP process as the number of passes increased. Density and microhardness tests have shown superior values for the samples produced by the T-ECAP process than those of the samples produced using the ECAP process, which can be improved by increasing the number of passes. Figure 18 shows metallographic images of the relative intensities of T-ECAP and ECAP billets after each pass. It is clearly seen that the porosities are lower due to the higher shear strains applied in T-ECAP.

Xia and Wu [88] conducted an experimental study on pure Al powder and solid Al ingot wrapped in Al foil in the back-pressure ECAP mold they designed (Figure 19). In the study, the die was heated up to 100 C. The samples were passed through the mold. As a result, it was determined that PM Al particles had better bonding, full density and excellent mechanical properties.

In addition, there are studies in the literature combining ECAP with different SPD processes [89]. Paydar et al. [90,91] planned to obtain fully dense billets by designing the second channel of the ECAP die according to the extrusion process—this method was named ECAP-FE (Figure 20a). A similar design is seen in Figure 20b. In this method, the extrusion is made in the inlet channel and therefore the method is called FE-ECAP [91]. The ECAP-FE method is the most striking feature of combining the two methods and eliminating additional equipment. The extrusion process applies a back pressure for the ECAP process, which removes surface cracks and improves the microstructure and mechanical properties. It was stated that this was related to the increase in the self-diffusion coefficient of Al due to the induced shear deformation and applied back pressure during ECAP. In Figure 20c, porosities are observed after FE, while in Figure 20d, there are no porosities after ECAP-FE. It has been proposed as a method that combines extrusion and ECAP in one process. In FE-ECAP and ECAP-FE methods, it was applied to Al powders with an average particle size of 45 μm. As a result, a 4 μm grain size was obtained, while positive improvements were achieved in hardness and yield strength values. The biggest advantage of the method is that it eliminates surface cracks and porosities of microstructure.

In order to increase efficiency in the SPD process, different die shapes and combined SPD techniques were combined in a single die and their effects were examined. While the strain value applied on the sample is increased with different die shapes, different deformation techniques can be applied at the same time in the combined techniques. Every new technique developed has a common purpose. This aim is to obtain a micro-structure with smaller grain size and more homogeneous grain size distribution. Below are examples of different dies designed to increase the strain on the billet for improved mechanical properties and obtain UFG (Table 1).

Some studies demonstrate that excellent bonding is achieved between particles when subjected to severe shear deformation at low temperatures under an average pressure. The reason for this is that the cut surfaces of the particles are clean and fresh and they provide good contact between them due to the compressive stress. Under these conditions, bonding between particles occurs spontaneously without the need for high temperature and pressure. In contrast to the long time required for diffusion bonding in sintering, the particles bond quickly as they pass through the shear zone. In addition, thanks to the back pressure, full density is reached in the first pass [88,90,91,92,93].

Table 1. Comparison of ECAP and ECAP-derived methods.

Method	Schematic Illustration	Comparison
Twisted channel multi-angular pressing (TCMAP) [94].		After a single pass at TCMAP, the average grain size was obtained 2.4 μm and an increase in microhardness values was observed. This method is an improved version of TCAP.
Planar twist channel angular extrusion (PTCAE) [95].		In this study, a canal structure given the buckling angle was considered. According to FEA analysis, the method predicts to reduce the number of passes and increase efficiency compared to ECAP.
Symmetrical channels angular pressing (SCAP) [96].		It was put forward as a new method by combining two methods (ECAP + TE = SCAP). With the method, a more homogeneous sample structure can be obtained and the hardness values increase.
Expansion equal channel angular extrusion (Exp-ECAE) [97].		With a single pass, the yield strength increases by 300%. In the method, a spherical design was preferred at the intersection of the channels. As a result, the design changes brought about important developments in the material.
Spiral equal channel angular extrusion (Sp-ECAE) [98].		In the Sp-ECAE method, an average grain size of 17 μm was obtained after a single pass. In the comparison made for Mg, it was found to be more efficient than 3-pass ECAP.
High Pressure Torsion Extrusion (HPTE) [99].		A grain size of 350 nm was obtained with a single pass. This technique, which is presented as a new method, has contributed to the literature with its effect on grain size.

Table 1. Comparison of ECAP and ECAP-derived methods.

Method	Schematic Illustration	Comparison
Twisted channel multi-angular pressing (TCMAP) [94].		After a single pass at TCMAP, the average grain size was obtained 2.4 μm and an increase in microhardness values was observed. This method is an improved version of TCAP.
Planar twist channel angular extrusion (PTCAE) [95].		In this study, a canal structure given the buckling angle was considered. According to FEA analysis, the method predicts to reduce the number of passes and increase efficiency compared to ECAP.
Symmetrical channels angular pressing (SCAP) [96].		It was put forward as a new method by combining two methods (ECAP + TE = SCAP). With the method, a more homogeneous sample structure can be obtained and the hardness values increase.
Expansion equal channel angular extrusion (Exp-ECAE) [97].		With a single pass, the yield strength increases by 300%. In the method, a spherical design was preferred at the intersection of the channels. As a result, the design changes brought about important developments in the material.
Spiral equal channel angular extrusion (Sp-ECAE) [98].		In the Sp-ECAE method, an average grain size of 17 μm was obtained after a single pass. In the comparison made for Mg, it was found to be more efficient than 3-pass ECAP.
High Pressure Torsion Extrusion (HPTE) [99].		A grain size of 350 nm was obtained with a single pass. This technique, which is presented as a new method, has contributed to the literature with its effect on grain size.

The common purpose of the presented methods is to improve the microstructure and improve the mechanical properties by subjecting the material irreversibly to excessive plastic deformation. After different number of passes in the SPD process, grain refinement is associated with phase transformations [100]. During phase transformations, the formation of nano-clusters, segregations, nanotwins and dislocation substrate occur, which provides the hardening mechanism of the material. Studies show that UFG can control the deformation mechanism, which increases the strain-hardening rate. Thus, the material can show good properties in terms of both strength and ductility [10,101,102,103].

3. Summary and Conclusions

In this study, the effect of the ECAP process, which makes it possible to achieve a submicron ultra-fine grain (UFG) grain structure, on the process parameters and material properties was investigated. In addition, the ECAP method was compared with other SPD methods.

• This study shows that ECAP stands out compared to similar processes, especially in terms of the simplicity of the system and the initial investment cost;
• While submicron grain sizes can be easily reduced with ECAP, the grain size of the final product is highly dependent on the process parameters. For this reason, ECAP process parameters such as channel angle (Φ), corner angle (Ψ), routes, ECAP process temperature, number of passes and die shapes were examined and discussed in detail;
• On the other hand, ECAP process temperature is one of the parameters that needs to be studied comprehensively. The ECAP processing temperature is preferred as the room temperature for ductile materials. However, in materials that are difficult to shape, the temperature is increased, making the ECAP process applicable;
• The fact that the ECAP process is so dependent on process parameters causes changes in the grain size and other properties of the product by changing the die shapes. For this reason, the process is still up-to-date and new die designs are tried and research on this issue continues. In addition, this situation has led to the emergence of many variations of the ECAP method, which is known by different names. It has been determined that better results can be obtained while each channel path change brings along numerous possibilities in the ECAP method. The ability of the ECAP method to be combined with other SPD methods plays an important role in improving mechanical properties and obtaining UFG;
• Two methods were used in mold design to increase the efficiency of the ECAP process. The first is carried out by changing the shape of the path inside the die, while the second is done by combining one of the ECAP and SPD methods.
• The aim of ECAP and ECAP-derived methods is to obtain UFG microstructure, to provide homogeneous particle size distribution and to improve mechanical properties. All these methods reduce the need to use expensive materials, especially since they improve the mechanical properties of existing materials;
• When the studies conducted on different materials using the ECAP process are evaluated, it is seen that recycled materials and materials produced by powder metallurgy can be used to improve their mechanical properties after the ECAP process;
• This study brings together different ECAP designs and creates a roadmap for researchers who want to work on the subject. The findings obtained in the study show that ECAP is a process that will continue to maintain its popularity, and it can allow the production of parts with very high mechanical properties by changing the process parameters;
• In future studies, the effects of ECAP treatment on materials produced with different manufacturing methods should be examined. In particular, studies on the changes in the mechanical properties and grain structure of metal and non-metal materials produced with 3D printer technologies after ECAP and the adaptation of ECAP to new developments can be studied.