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A new method for severe plastic deformation of the copper sheets
Author’s Accepted Manuscript A new method for severe plastic deformation of the copper sheets A. Torkestani, M.R. Dashtbayazi
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S0921-5093(18)31250-4 https://doi.org/10.1016/j.msea.2018.09.054 MSA36935
To appear in: Materials Science & Engineering A Received date: 15 July 2018 Revised date: 11 September 2018 Accepted date: 15 September 2018 Cite this article as: A. Torkestani and M.R. Dashtbayazi, A new method for severe plastic deformation of the copper sheets, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2018.09.054 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A new method for severe plastic deformation of the copper sheets
A. Torkestani, M. R. Dashtbayazi* Department of Mechanical Engineering, Faculty of Engineering, Shahid Bahonar University of Kerman, Kerman, Iran *
Corresponding author. Postal address: Department of Mechanical Engineering, Faculty of Engineering, Shahid Bahonar University of Kerman, Jomohori Boulevard, P.O. Box 76175-133, Kerman, Iran. Tel.: +98 34 32111763; fax: +98 34 32120964. [email protected] (M. R. Dashtbayazi)
Abstract A new severe plastic deformation technique, named "constrained studded pressing" (CSP), was developed for the production of plate-shaped ultrafine grain metals without changing their initial dimensions. In the CSP method, the material is subjected to the repetitive shear deformation by dies with two orthogonal grooves then becomes flat. The repetitive shear deformation and flattening done by constrained-blocks. Calculations showed that the effective strain for the CSP method is more than the CGP (constrained groove pressing) method. The microstructure and the mechanical properties of the CSPed samples investigated by scanning electron microscopy (SEM) and tensile test, respectively. SEM observations showed that the CSP method as the other repetitive corrugation and straightening (RCS) methods is a useful method to refine the grain size. Mechanical properties investigations indicate that the ductility of the samples produced by the CSP method is more than CGPed on, while the ultimate tensile strength of them is approximately the same.
Keywords: Severe plastic deformation; Constrained Studded Pressing; Ductility; Tensile Strength; Toughness; Copper.
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1. Introduction Metals and alloys with superior mechanical properties are desirable because they have a wide range of applications [1]. In polycrystalline metals, a substantial increase in some mechanical properties, like super-plasticity, fracture toughness, strength, and hardness, can be obtained by reducing the grain size to sub-micrometer (ultra-fine) or nanometer scale [2]. Severe plastic deformation (SPD) is one of the most efficient ways to produce nanostructure (NS) or ultra-fine grain (UFG) materials [3]. UFG or NS materials processed by the SPD methods have appealed to an increasing number of specialists in material sciences [4]. The SPD processes enhance the tensile strength and improve some mechanical properties like super-plasticity, fatigue strength and the fracture toughness [5], but the main limitation of the SPDed metals is low ductility [6]. Several methods have been proposed to apply the SPD for various types of materials which can produce 100% dense and defect-free fine grain metals [7]. The SPD methods can apply to different types of metals. Some of them like EqualChannel Angular Pressing (ECAP) [8], High-Pressure Torsion (HPT) [9], Cyclic Closed-die Forging (CCF) [10], Simple Shear Extrusion (SSE) [11], are proper for bulk products. Methods like Accumulative Roll Bonding (ARB) [12], Repetitive Corrugation and Straightening (RCS) [13], Multi-pass Coins Forging (MCF) [14] are proper for plate shape structures. Furthermore, High-Pressure Tube Twisting (HPTT) [15] is suitable for tubes. The ECAP method is a kind of double-axis extrusion or side extrusion [16]. The ECAP method was proposed by Segal [17] and developed by many researchers like Valiev [18], Langdon [19]. In the ECAP, the sample passed through two equal angular channel, so pure shear deformation occurs [20]. The ECAP method was used to improve the properties of various metals. The ECAP method has many disadvantages, such as a large extrusion force, heterogeneous grain size, poor thermal stability [21].
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The HPT method develops by Valiev [22]. In the HPT, the conical sample subjected to torsional shear strain under high hydrostatic pressure. It concluded from recent studies that the HPT method was more effective than the ECAP method in producing exceptionally small grain size [9]. But, the HPT method has the disadvantage that it utilizes specimens in the form of relatively small discs and not available for the production of large materials. The ARB method is the only SPD method using rolling deformation itself [23]. The rolling is the most advantageous process for continuous production of plates. In the ARB method, 50% rolled sheet is cut into two same parts and stacked to be the initial dimension, then rolled again., The ARB rolling must be a bonding process to get a single body [24]. The existence of the poor interface may degrade the mechanical properties if a perfect bounding not achieved through the ARB method [12], But this method is very useful for bulk sheet production. The RCS is another method which seems helpful for the continuous production of bulky ultra-fine grain materials [13]. This method involves a repetitive corrugating and straightening processes. In the first RCS step, the sample is pressed by grooved dies so, the sample subjected to the in-plane shear strain, and in the second step, it flattened by flat dies and the reverse shear strain occurs [25]. Since a large strain cannot apply to the most samples at once, the RCS steps should repeat, so this method has low speed in the application of plastic strain. Comparatively, in the RCS method, there is no bonding between sheets, so there is no importance to sample cleanliness and surface quality [26]. In this method, usually the length of samples increases after many passes, so some methods like Constrained Groove Pressing (CGP) [27] and Semi-Constrained Groove Pressing (SCGP) [28] proposed, which in them the sample constrained and its dimensions do not change. The principle of the CGP and the SCGP methods are similar to the RCS. The RCS and the CGP methods are the best methods for mass production of fine-grain metal sheets because, 3
these methods do not require any special equipment, and they need less pressure than the other methods like the ARB, HPT or ECAP. In this study, a new SPD method presented which is suitable for the production of the ultra-fine grain metal sheets. The new method is based on the CGP method and called "Constrained Studded Pressing" (CSP). It claimed that the CSP method is better than the other RCS's methods like the CGP, because, the CSP method can apply more strain than the CGP method in one pass, so in fewer numbers of pressing passes it produces favorable severe plastic deformation in the material. The performance of the CSP and CGP methods compared using experimental tests on pure copper sheets, the mechanical properties and the microstructures of the samples in the same passes of each method investigated.
2. Illustration of the new SPD process The SPD methods are efficient methods to produce the UFG or NS materials. One of the best SPD methods for producing of the bulk plate-shape metals are the RCS methods. In the RCS methods repetitive corrugating and straightening processes are applied to the sample. A schematic illustration of the RCS method shown in Fig. 1. The primary geometry of the RCS dies shown in Fig. 1a. As shown, the groove angle of the die, Ɵ , is 45 and its width and height are both equal to the sample thickness, t. In the first step, as shown in Fig. 1b, a sample is corrugated and subjected to the shear deformation by locating between a pair grooved dies. In the second step, the corrugated sheet flattens between two flat dies and the previously deformed region is subjected to the reverse shear deformation, as shown in Fig. 1c, but previously undeformed regions remain undeformed [29]. To create a homogeneous deformation in the sample, in the third step, as shown in Fig. 1d-1e, the previously flattened sheet is pressed again between two grooved-dies, like the first step, but at this time the sample is shifted to the right/left about a length of a groove. Finally, in the fourth step, as shown in Fig. 1f, the corrugated sample becomes flat again. Since it requires more strain to 4
achieve the severe plastic deformation, it is necessary to repeat the corrugating and the straightening processes. The principle of the CGP method is similar to the RCS method [26]. Fig. 2 depicts the schematic of the CGP method. In the CGP, as shown in Fig. 2, a plate-shaped sample is located between a pair of the asymmetrically grooved dies tightly constrained. In the CGP method, constrained deformation guarantees to keep constant across the sheet length during the deformation. In the current study, a new SPD method for producing the UFG metal sheet introduced. This new method is called Constrained Studded Pressing (CSP). The CSP method found in the CGP method. Fig. 3 shows schematic configurations of the CGP and CSP dies, comparatively. Fig. 3a shows the CGP dies which grooved in one direction, but as shown in Fig. 3b dies in the CSP method have studs. In the CSP method, like the CGP method, the sample is pressed between two conjugated dies, and then the deformed sheet becomes flat between two flat dies. The studded dies can apply shear strain in two directions of the sample plate and make more strain in comparison with one direction grooves, which can create strain only in one direction, so in the CSP method, the severing plastic deformation can achieve in fewer numbers of passes. Fig. 4 shows the geometry of the CSP dies. The width and height of the studs and their roots space in two directions are equal to t, which is the same as the sample thickness, and the angle of the studs (imperfect pyramids) is
.
In the CSP method, the inclined regions of the sample in around the studs are subjected to the in shear deformation, while the flat regions of the sample at the top of the studs and in the gap between two of them remain undeformed. In the flattening step, the corrugated sheet deforms in a reverse direction, and the previously deformed region subjected to the reverse shear strain. As mentioned, the CGP or RCS one pass contains four pressing steps [30]. In 5
comparison, in the CSP method due to the applying shear strain in two direction, simultaneously, the sample is subjected to more homogeneous shear strain, and only two pressing steps have completed a pass. Fig. 5 provided a Two/three-dimensional comparison of the deformed region for the CSPed and the CGPed samples. The white zones represent the undeformed zone, and the light/dark gray zones represent the shear deformed zone. As is clear, the strain of the white zone is zero. According to the Shirdel et al. [29], the effective strain for a point in the deformed zone of the CGPed sample is 0.58, which shown in Figs. 5a and 5c by light gray zone. In this regions, the shear strain exists only in one direction. In the CSP method, as shown in Figs. 5b and 5d, the different zones of deformation can be distinguished which marked by dark and light gray. The deformation of the light gray areas includes shear deformation in one direction and is similar to the deformation of the grooved sample, which can easily understand by cutting the sample from the plane A, but the deformation of the dark gray areas includes shear deformation in two directions. The deformation is not uniform in this area and on the edge of the pyramid is higher. The applied effective strain for a point on the edge of these pyramids in the deformed zone for the first step of the CSP calculated as follows [26]: (1)
γ √ [(
)
(
)
(
) ]
[
]
(2) (3) (4)
√ [
where γ is the engineering shear strain,
]
√
(5)
is the engineering shear strain in the z-x plane,
is the engineering shear strain in the z-y plane, t is the width and height of deformation 6
shear zone, and z,
,
is the effective strain, ,
,
,
, are the normal strain in direction of the x, y
are the shear strain in x-y, x-z and y-z planes, respectively.
As can be seen in the Figs 5c and 5d, the strain in the CSP method is higher than the CGP which can illustrate by using the average strain of the specified area. It's easy to calculate that the average strain of the CSPed sample is 20.7% more than the CGPed one. To calculate the average strain, the cells of the Fig. 5d are assumed to be as points.
3. Experimental procedure Dies which made for the CGP and the CSP processes shown in Fig. 6. The dies made from the structural steel. Fig. 6a Reveals that the CGP die contains one-directional groove. Fig. 6b shows the CSP die contains studs, which is made by two perpendicular grooves. Fig. 7 shows the complete set of the CSP dies with constrained-blocks. Fig. 7 shows two studded dies, which tightly constrained by two constrained-blocks. Constrained-blocks so designed that length and thickness of the sheet were constant during deformation. Commercial pure copper plate (99.9%) was selected as a raw material for processing by the CSP method. The dimension of the plates is 40×40×1 mm. At first, the copper sheet, which has elongated grains caused by rolling, was annealed at 650℃ for 2 hours to achieve non-elongated microstructures. The CSP process was conducted for different passes, on a 20ton hydraulic press machine at room temperature. The samples coated with a lubricant before pressing to reduce the frictional effects and the pressing was carried out at a constant press speed of 0.1 mm/s. To compare the amount of plastic deformation per passes and its effect on the mechanical properties and the microstructure of the samples, the CSP processes considered for 5, 10, 15 and 20 passes. The microstructure and mechanical properties enhancement of the CSP and CGP methods compared, more precisely. The mechanical properties extracted by tensile tests. The tensile tests conducted on the CSPed and the CGPed standard (ASTM E8-M) specimen with a gage length of 25mm using an STM-20 universal 7
testing machine at 0.5 mm/min constant speed, also all tests have been performed twice to verify the validity of the results. The microstructure of the samples characterized by Scanning Electron Microscopy (SEM). Specimens for the SEM were cut from the center of the sample and mechanically polished to a mirror-like surface by mechanical grinding (using P800, P1200, P2000 and P3000 silicon carbide sandpaper, respectively) and etched by 50% HNO3 to reveal the corresponding microstructure. The SEM illustrations were carried out by VEGA TS5130MM device with SE detector, high vacuum conditions and at an acceleration voltage of 20.0 kV.
4. Results and discussion 4-1. Microstructural observations Fig. 8 shows the copper sheets after one pass of the CGP and the CSP processes. Fig. 8a shows, in the CGP process the sample deformed along the grooves in one direction, but as shown in Fig. 8b, the sample was deformed in two directions in the plane of the sheet by the studs. The sheet sample is severe plastic-deformed in both of the CGP and the CSP processes, but as mentioned already, the induced strain in the CSP process is higher than the CGP process. Also, theoretically, it is desirable the strain induced on the sheet sample is homogeneous in every part of the sheet but, results are showing the strain induced in the severe plastic deformed sheet is inhomogeneous, practically [26]. As is known, the dislocation is one of the linear crystalline defects which is a factor for atom slip in the lattice and the plastic deformation caused by dislocation movement which occurs due to shear stress [31]. In the plastic deformation, the density of dislocations increases. But, if the number of the dislocations is high, in addition to the positive effect on the ease of deformation, it can have a negative effect. As a result of increasing the density of dislocations, there are obstacles in the way of the movement of other dislocations which can increase the strength of the material, in other words, deformation is not easily possible. The 8
accumulation of dislocations leads to the formation of sub-cells, which at their boundaries the density of dislocations is higher. More dislocations accumulated on the new boundaries with the continuation of deformation and finally, the new grains created [32]. In other words, plastic deformation can reduce grain size. Fig. 9 shows SEM micrographs of the samples after different passes of the CSP and the CGP processes. The grain size measured by Heyn intercept method [33]. Fig. 9a shows SEM micrograph of the initial annealed sample with a mean grain size of 15 μm, also as is clear the grain size distribution was heterogeneous, as reported by other researchers [32-34], and some of the grains showed a short elongated morphology. Figs. 9b to 9e show the microstructures of the copper samples after 5, 10, 15 and 20 CSP passes, respectively. Also, the SEM micrographs show structures that have undergone large amounts of plastic deformation, since some well-developed equiaxed grains can be detected, which were probably formed by the fragmentation of elongated grain. Figs. 9b to 9e also show the grain reduction during CSP passes, so that, the mean grain size of the initial sample is about 15 15th and 20th pass reduced to 13.33
, 11.26
10.43
, and 7.86
and after 5th, 10th, , respectively. Fig.
9f shows the microstructures of the copper sample after 10 passes of the CGP. The mean grain size of the CGPed sample is about 11.34
, which is roughly similar to the grain size
of 10 passes of the CSPed sample (Fig. 9c). Comparing Figs. 9c and 9f shows the grains morphology are also approximately same, but in CGPed sample (Fig. 9f) the elongated grains are a little bit more, which is due to the grooved dies. It should be noted that the CGP pass consist of four pressing step while the CSP pass is complete only by two pressing step. The dependency of the grain refinement and the strain induced can explain by the dislocation theory [34]. The dislocations theory represents microstructural evolution during plastic deformation for metallic materials. The plastic deformation performs based on the motion of the dislocations. When the motion of the dislocations restricts, it makes the 9
material stronger then the greater strength is required to continue the plastic deformation. Work-hardening based on a principle of hindering the dislocation motion. During the CSP and the CGP processes, the work-hardening occurs, and it causes to increase the dislocation density due to the dislocation multiplication or formation of new dislocations. Increasing the dislocation density reveals that the distance of the dislocations decreases. Interactions between the dislocations result in the motion of dislocations constrained due to the presence of other dislocations. The dislocation multiplication or formation of new dislocations increase in dislocation density and thus, higher stresses are required to deform the material. Fig. 10 shows the grain size variation against the number of the CSP passes. The zero pass represented the initial annealed sample. As shown in Fig. 10, increasing the plastic strain will always reduce the mean grain size, as reported by other researchers [26, 35, 36]. The relation between the strain rate and dislocation evolution is defined as follows [34]: (6) where
is the strain rate, b is the Burger’s vector, and
is the rate of dislocation evolution,
L is the dislocation mean free path. The grain size is determining by [37]: (7) where is a constant,
is the dislocation density.
According to Equations 6 and 7, the grain reduction occurs when the dislocation density increases during plastic deformation. Also, dynamic recovery occurs during the plastic deformation. The dynamic recovery prevents the steady rate of the grain reduction. The dynamic recovery causes to annihilate the dislocations, and the rate of dynamic recovery is proportional to the dislocation density. With continuing the plastic deformation, dislocation density increases, thereby increasing the rate of dynamic recovery which annihilates the dislocations, so the rate of grain refinement at higher strains will decrease [38].
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4-2. Mechanical properties As the metals are plastically deformed and the strain induced in them, the microstructure and the mechanical properties change. The microstructural refinement is an important phenomenon that takes place during the plastic deformation of the metals, where the grain size refined and the properties of the metals improve [26]. Strength and ductility are important mechanical properties for different materials. As well known, in the conventional metal forming processes, the strength and ductility change, conversely [39]. In other words, the conventional materials may be strong but may not be ductile and vice versa. Fig. 11 shows stress-strain curves of the samples after different CSP passes, as is clear, the severe plastic deformation greatly changes the mechanical properties. Fig. 11 shows that the initial sample has the highest breaking strain (ductility) and the lowest amount of ultimate tensile stress (UTS) and yield stress (YS), actually, the toughness of the initial sheet sample before plastic deformation is low. The toughness defined as the ability of a material to absorb energy and plastically deform without fracturing (the area under the stress-strain curve). Some investigations of the CGP process for the aluminum and the copper showed that the UTS and the YS increased and the ductility decreased when the SPD pass number increased [26]. As Fig. 11 shows, the toughness reduces until 10th CSP passes, then increases for 15th and 20th CSP passes, respectively. In other words, after 10th CSP pass, the ductility begins to increase, while the UTS remains approximately constant. Similar results also are given by Hajizadeh et al. [40], which they applied different CGP passes on the aluminum samples and showed that the ductility of them decreased by increasing the CGP pass number, and after 4 passes the ductility increases while UTS is always increasing. As also shown in Fig. 11, the modulus of elasticity for all samples are same, since, the modulus of elasticity is a structure-insensitive [41].
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Fig. 12 shows the variation of the UTS and the elongation for the sheet samples for different CSP passes. It observed that the UTS always increases but with the increase in accumulated strain, the rate of increment gets decrease (after the 10th pass). Increasing of the UTS of the samples can be explained by an increase in the amount of flow stress results from the grain refinement phenomenon that occurs due to severe plastic deformation and also due to the increase in the dislocation density which require higher applied stress for dislocation motion by slip (strain hardening) [26, 29]. The decrease in the rate of UTS increment implies that dislocation annihilation by the mechanism of cross-slip or climb of dislocations associated with dynamic recovery [42]. In other words, the strain hardening mechanism is dominant at small values of strain, and the dislocation density increases drastically, and with an increase in dislocation density, the driving force for dynamic recovery gets increased [26, 43]. Decreasing in the rate of UTS increment was reported for the copper samples which processed by ECAP method [44, 45]. As Fig. 12 shows the initial sample has an elongation of 33.6%, which is decreased to 27.2% and 16.6% after 5th and 10th passes, respectively. This reduction in elongation can be due to the strain hardening which is explained, but after the 10th pass, elongation begins to increase, so that, after 20th pass, the elongation is 24.1%. This increment in elongation caused by dynamic recovery. Similar results are also shown in [40] which indicated that CGPed samples exhibited relatively high magnitudes of elongation (16.6 – 19%). These results show that though the material subjected to a high amount strain, it still highly appropriate elongation. As already mentioned, after the 10th CSP pass, with increasing dislocation density, dynamic recovery will be the dominant mechanism of dislocation annihilation. Fig. 13 shows the variation of the UTS and the grain size for different CSP passes. As Fig. 13 shows the grain size decreased and the UTS increased with the increase in the number of CSP passes. These results in Fig. 13 are agreement with the Hall-Petch effect [46]. 12
Fig. 14 shows the stress-strain curve of the samples after different CGP passes, as expected, the severe plastic deformation changes the mechanical properties. As others showed, the CGP process for the copper caused that the UTS increased and the ductility decreased [26]. As is clear, unlike the CSP method increasing the CGP passes almost increased the strength and decreased the elongation. Fig. 15 illustrate this manner better, which shows the variation of the ultimate tensile strength (UTS) and the elongation for the samples after different CGP passes. Fig. 12 illustrates that after 10 CSP passes the elongation increase, while for CGP method it is not. Fig. 16 shows the stress-strain curves of the CSPed and CGPed samples, after 10th pass, which compared to the initial sample curve. As Fig. 16 shows, the UTS of the CSPed and CGPed sample are approximately the same, but the CSPed sample elongation is more. As already mentioned, the amount of strain in the CSP is more than CGP, which it seems that the dynamic recovery in the CSP method activated sooner than the CGP [26]. Thus, more elongation for the CSPed sample is not unpredictable.
Conclusion: In this study, a new method for severe plastic deformation of the copper sheets introduced. The new method was called constrained studded pressing (CSP). In the CSP method, a copper sheet was subjected to the repetitive in-plane shear deformation and then straightened by a set of conjunctival studded dies and flat dies, respectively, which are constrained tightly by constrained-blocks. The grain size reduction and the mechanical properties variation of the severe plastic deformed copper sheet were studied. The results show the grain size of the copper sheet refined by the CSP method and its mechanical properties have changed dramatically. The characteristics of the CSP method were compared to the CGP method
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theoretically, experimentally and microstructurally, respectively. The main results are following as: 1. The average strain of the CSP method is about 21% more than the CGP method for one pass pressing. 2. In-plane plastic deformation in the CSP and the CGP methods achieves in two and one directions in each pressing step, respectively. As a result, the plastic deformation achieved in two steps CSPed sample (deformation and straightening) is similar to deformation of four steps CGPed sample. So the CSP method is more competitive and faster than the CGP method. 3. The CSP method is an economical method to refine the grain size of the metal sheets. Results show that the average grain size of the copper sheet reduced by ~ 48% after 20 pressing passes of the CSP. 4. The ultimate tensile strength of the copper sheets increased about 34.6% after 20 passes of the CSP. 5. Results show that the ductility of copper samples decreases after zero to 10 passes, and increases after 10 to 20 passes of the CSP, while the UTS of them always increases after 0 to 20 CSP passes. Consequently, the toughness decreases when CSP passes increase up to 10, then the toughness increases when passes increase from 10 to 20. 6. Unlike the CSP method, increasing the CGP passes almost increased the strength and decreased the elongation.
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Fig 1: The schematic representation of the RCS method, a) RCS dies, b) first corrugation, c) first flattening, d) sample shifted to the right or left, e) second corrugation, f) second flattening.
17
Fig 2: The schematic illustration of constrained groove pressing (CGP) method a) grooving, b) flattening pass.
18
Fig 3: The die configuration of a) CGP and b) Constrained Studded Pressing (CSP) methods.
19
Fig 4: Geometry of the CSP dies, t is width and height of studs and the angle of the imperfect pyramid is 45o.
20
(a)
(b)
(c)
(d)
Fig 5: Schematic of the compressed sample in CSP and CGP methods, a) 3-dimensional groove region for the CGP method, b) 3-dimensional schematic of stud region for the CSP method, c) 2-dimensional schematic of the CGPed sample, d) 2-dimensional of the CSPed sample.
21
Fig 6: Dies for a) the CGP and b) the CSP methods.
22
Fig 7: The CSP dies setup.
23
Fig 8: The corrugated sample after one pass for the a) CGP and b) CSP methods.
24
(a)
(b)
(c)
(d)
(e)
(f)
Fig 9: SEM micrograph of the samples, a) initial annealed, b) 5 SCP pass, c) 10 CSP pass, d) 15 CSP pass, e) 20 CSP pass and f) 10 CGP pass.
25
Mean grain size (μm)
16 14 12 10 8 6 0
5
10 Pass Number
15
20
Fig 10: Variation of the grain size against the number of CSP passes.
26
300
Initial sample 5 pass CSP 10 pass CSP 15 pass CSP 20 pass CSP
Stress (MPa)
250 200 150 100 50 0 0
0.05
0.1
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Strain Fig 11: The stress-strain curves of samples for different CSP passes.
27
0.35
40
300
30
200 150
20
100
Elongation
UTS (MPa)
Elongation (%)
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50
UTS 10
0 0
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Pass Number Fig 12: Variation of the ultimate tensile strength (UTS) and the elongation for the samples after different CSP passes.
28
300 250
12
200 150
8
Grain Size UTS
UTS (MPa)
Mean grain size (μm)
16
100
4
50 0
5
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15
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Pass Number
Fig 13: The variation of the grain size and the UTS of samples against CSP passes.
29
Fig 14: The stress-strain curves of samples for different CGP passes
30
38
300
Elongation 28
250
18
200
8
150 0
5
10
UTS (MPa)
Elongation (%)
UTS
15
Passb Number Fig 15: Variation of the ultimate tensile strength (UTS) and the elongation for the samples after different CGP passes.
31
Initial sample
250
10 pass CGP
Stress (MPa)
200
10 pass CSP
150 100 50 0 0
0.05
0.1
0.15 0.2 Strain
0.25
0.3
0.35
Fig 16: The stress-strain curves of the initial sample and the samples after 10 passes of the CSP and the CGP methods.
32
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PII: DOI: Reference:
S0921-5093(18)31250-4 https://doi.org/10.1016/j.msea.2018.09.054 MSA36935
To appear in: Materials Science & Engineering A Received date: 15 July 2018 Revised date: 11 September 2018 Accepted date: 15 September 2018 Cite this article as: A. Torkestani and M.R. Dashtbayazi, A new method for severe plastic deformation of the copper sheets, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2018.09.054 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A new method for severe plastic deformation of the copper sheets
A. Torkestani, M. R. Dashtbayazi* Department of Mechanical Engineering, Faculty of Engineering, Shahid Bahonar University of Kerman, Kerman, Iran *
Corresponding author. Postal address: Department of Mechanical Engineering, Faculty of Engineering, Shahid Bahonar University of Kerman, Jomohori Boulevard, P.O. Box 76175-133, Kerman, Iran. Tel.: +98 34 32111763; fax: +98 34 32120964. [email protected] (M. R. Dashtbayazi)
Abstract A new severe plastic deformation technique, named "constrained studded pressing" (CSP), was developed for the production of plate-shaped ultrafine grain metals without changing their initial dimensions. In the CSP method, the material is subjected to the repetitive shear deformation by dies with two orthogonal grooves then becomes flat. The repetitive shear deformation and flattening done by constrained-blocks. Calculations showed that the effective strain for the CSP method is more than the CGP (constrained groove pressing) method. The microstructure and the mechanical properties of the CSPed samples investigated by scanning electron microscopy (SEM) and tensile test, respectively. SEM observations showed that the CSP method as the other repetitive corrugation and straightening (RCS) methods is a useful method to refine the grain size. Mechanical properties investigations indicate that the ductility of the samples produced by the CSP method is more than CGPed on, while the ultimate tensile strength of them is approximately the same.
Keywords: Severe plastic deformation; Constrained Studded Pressing; Ductility; Tensile Strength; Toughness; Copper.
1
1. Introduction Metals and alloys with superior mechanical properties are desirable because they have a wide range of applications [1]. In polycrystalline metals, a substantial increase in some mechanical properties, like super-plasticity, fracture toughness, strength, and hardness, can be obtained by reducing the grain size to sub-micrometer (ultra-fine) or nanometer scale [2]. Severe plastic deformation (SPD) is one of the most efficient ways to produce nanostructure (NS) or ultra-fine grain (UFG) materials [3]. UFG or NS materials processed by the SPD methods have appealed to an increasing number of specialists in material sciences [4]. The SPD processes enhance the tensile strength and improve some mechanical properties like super-plasticity, fatigue strength and the fracture toughness [5], but the main limitation of the SPDed metals is low ductility [6]. Several methods have been proposed to apply the SPD for various types of materials which can produce 100% dense and defect-free fine grain metals [7]. The SPD methods can apply to different types of metals. Some of them like EqualChannel Angular Pressing (ECAP) [8], High-Pressure Torsion (HPT) [9], Cyclic Closed-die Forging (CCF) [10], Simple Shear Extrusion (SSE) [11], are proper for bulk products. Methods like Accumulative Roll Bonding (ARB) [12], Repetitive Corrugation and Straightening (RCS) [13], Multi-pass Coins Forging (MCF) [14] are proper for plate shape structures. Furthermore, High-Pressure Tube Twisting (HPTT) [15] is suitable for tubes. The ECAP method is a kind of double-axis extrusion or side extrusion [16]. The ECAP method was proposed by Segal [17] and developed by many researchers like Valiev [18], Langdon [19]. In the ECAP, the sample passed through two equal angular channel, so pure shear deformation occurs [20]. The ECAP method was used to improve the properties of various metals. The ECAP method has many disadvantages, such as a large extrusion force, heterogeneous grain size, poor thermal stability [21].
2
The HPT method develops by Valiev [22]. In the HPT, the conical sample subjected to torsional shear strain under high hydrostatic pressure. It concluded from recent studies that the HPT method was more effective than the ECAP method in producing exceptionally small grain size [9]. But, the HPT method has the disadvantage that it utilizes specimens in the form of relatively small discs and not available for the production of large materials. The ARB method is the only SPD method using rolling deformation itself [23]. The rolling is the most advantageous process for continuous production of plates. In the ARB method, 50% rolled sheet is cut into two same parts and stacked to be the initial dimension, then rolled again., The ARB rolling must be a bonding process to get a single body [24]. The existence of the poor interface may degrade the mechanical properties if a perfect bounding not achieved through the ARB method [12], But this method is very useful for bulk sheet production. The RCS is another method which seems helpful for the continuous production of bulky ultra-fine grain materials [13]. This method involves a repetitive corrugating and straightening processes. In the first RCS step, the sample is pressed by grooved dies so, the sample subjected to the in-plane shear strain, and in the second step, it flattened by flat dies and the reverse shear strain occurs [25]. Since a large strain cannot apply to the most samples at once, the RCS steps should repeat, so this method has low speed in the application of plastic strain. Comparatively, in the RCS method, there is no bonding between sheets, so there is no importance to sample cleanliness and surface quality [26]. In this method, usually the length of samples increases after many passes, so some methods like Constrained Groove Pressing (CGP) [27] and Semi-Constrained Groove Pressing (SCGP) [28] proposed, which in them the sample constrained and its dimensions do not change. The principle of the CGP and the SCGP methods are similar to the RCS. The RCS and the CGP methods are the best methods for mass production of fine-grain metal sheets because, 3
these methods do not require any special equipment, and they need less pressure than the other methods like the ARB, HPT or ECAP. In this study, a new SPD method presented which is suitable for the production of the ultra-fine grain metal sheets. The new method is based on the CGP method and called "Constrained Studded Pressing" (CSP). It claimed that the CSP method is better than the other RCS's methods like the CGP, because, the CSP method can apply more strain than the CGP method in one pass, so in fewer numbers of pressing passes it produces favorable severe plastic deformation in the material. The performance of the CSP and CGP methods compared using experimental tests on pure copper sheets, the mechanical properties and the microstructures of the samples in the same passes of each method investigated.
2. Illustration of the new SPD process The SPD methods are efficient methods to produce the UFG or NS materials. One of the best SPD methods for producing of the bulk plate-shape metals are the RCS methods. In the RCS methods repetitive corrugating and straightening processes are applied to the sample. A schematic illustration of the RCS method shown in Fig. 1. The primary geometry of the RCS dies shown in Fig. 1a. As shown, the groove angle of the die, Ɵ , is 45 and its width and height are both equal to the sample thickness, t. In the first step, as shown in Fig. 1b, a sample is corrugated and subjected to the shear deformation by locating between a pair grooved dies. In the second step, the corrugated sheet flattens between two flat dies and the previously deformed region is subjected to the reverse shear deformation, as shown in Fig. 1c, but previously undeformed regions remain undeformed [29]. To create a homogeneous deformation in the sample, in the third step, as shown in Fig. 1d-1e, the previously flattened sheet is pressed again between two grooved-dies, like the first step, but at this time the sample is shifted to the right/left about a length of a groove. Finally, in the fourth step, as shown in Fig. 1f, the corrugated sample becomes flat again. Since it requires more strain to 4
achieve the severe plastic deformation, it is necessary to repeat the corrugating and the straightening processes. The principle of the CGP method is similar to the RCS method [26]. Fig. 2 depicts the schematic of the CGP method. In the CGP, as shown in Fig. 2, a plate-shaped sample is located between a pair of the asymmetrically grooved dies tightly constrained. In the CGP method, constrained deformation guarantees to keep constant across the sheet length during the deformation. In the current study, a new SPD method for producing the UFG metal sheet introduced. This new method is called Constrained Studded Pressing (CSP). The CSP method found in the CGP method. Fig. 3 shows schematic configurations of the CGP and CSP dies, comparatively. Fig. 3a shows the CGP dies which grooved in one direction, but as shown in Fig. 3b dies in the CSP method have studs. In the CSP method, like the CGP method, the sample is pressed between two conjugated dies, and then the deformed sheet becomes flat between two flat dies. The studded dies can apply shear strain in two directions of the sample plate and make more strain in comparison with one direction grooves, which can create strain only in one direction, so in the CSP method, the severing plastic deformation can achieve in fewer numbers of passes. Fig. 4 shows the geometry of the CSP dies. The width and height of the studs and their roots space in two directions are equal to t, which is the same as the sample thickness, and the angle of the studs (imperfect pyramids) is
.
In the CSP method, the inclined regions of the sample in around the studs are subjected to the in shear deformation, while the flat regions of the sample at the top of the studs and in the gap between two of them remain undeformed. In the flattening step, the corrugated sheet deforms in a reverse direction, and the previously deformed region subjected to the reverse shear strain. As mentioned, the CGP or RCS one pass contains four pressing steps [30]. In 5
comparison, in the CSP method due to the applying shear strain in two direction, simultaneously, the sample is subjected to more homogeneous shear strain, and only two pressing steps have completed a pass. Fig. 5 provided a Two/three-dimensional comparison of the deformed region for the CSPed and the CGPed samples. The white zones represent the undeformed zone, and the light/dark gray zones represent the shear deformed zone. As is clear, the strain of the white zone is zero. According to the Shirdel et al. [29], the effective strain for a point in the deformed zone of the CGPed sample is 0.58, which shown in Figs. 5a and 5c by light gray zone. In this regions, the shear strain exists only in one direction. In the CSP method, as shown in Figs. 5b and 5d, the different zones of deformation can be distinguished which marked by dark and light gray. The deformation of the light gray areas includes shear deformation in one direction and is similar to the deformation of the grooved sample, which can easily understand by cutting the sample from the plane A, but the deformation of the dark gray areas includes shear deformation in two directions. The deformation is not uniform in this area and on the edge of the pyramid is higher. The applied effective strain for a point on the edge of these pyramids in the deformed zone for the first step of the CSP calculated as follows [26]: (1)
γ √ [(
)
(
)
(
) ]
[
]
(2) (3) (4)
√ [
where γ is the engineering shear strain,
]
√
(5)
is the engineering shear strain in the z-x plane,
is the engineering shear strain in the z-y plane, t is the width and height of deformation 6
shear zone, and z,
,
is the effective strain, ,
,
,
, are the normal strain in direction of the x, y
are the shear strain in x-y, x-z and y-z planes, respectively.
As can be seen in the Figs 5c and 5d, the strain in the CSP method is higher than the CGP which can illustrate by using the average strain of the specified area. It's easy to calculate that the average strain of the CSPed sample is 20.7% more than the CGPed one. To calculate the average strain, the cells of the Fig. 5d are assumed to be as points.
3. Experimental procedure Dies which made for the CGP and the CSP processes shown in Fig. 6. The dies made from the structural steel. Fig. 6a Reveals that the CGP die contains one-directional groove. Fig. 6b shows the CSP die contains studs, which is made by two perpendicular grooves. Fig. 7 shows the complete set of the CSP dies with constrained-blocks. Fig. 7 shows two studded dies, which tightly constrained by two constrained-blocks. Constrained-blocks so designed that length and thickness of the sheet were constant during deformation. Commercial pure copper plate (99.9%) was selected as a raw material for processing by the CSP method. The dimension of the plates is 40×40×1 mm. At first, the copper sheet, which has elongated grains caused by rolling, was annealed at 650℃ for 2 hours to achieve non-elongated microstructures. The CSP process was conducted for different passes, on a 20ton hydraulic press machine at room temperature. The samples coated with a lubricant before pressing to reduce the frictional effects and the pressing was carried out at a constant press speed of 0.1 mm/s. To compare the amount of plastic deformation per passes and its effect on the mechanical properties and the microstructure of the samples, the CSP processes considered for 5, 10, 15 and 20 passes. The microstructure and mechanical properties enhancement of the CSP and CGP methods compared, more precisely. The mechanical properties extracted by tensile tests. The tensile tests conducted on the CSPed and the CGPed standard (ASTM E8-M) specimen with a gage length of 25mm using an STM-20 universal 7
testing machine at 0.5 mm/min constant speed, also all tests have been performed twice to verify the validity of the results. The microstructure of the samples characterized by Scanning Electron Microscopy (SEM). Specimens for the SEM were cut from the center of the sample and mechanically polished to a mirror-like surface by mechanical grinding (using P800, P1200, P2000 and P3000 silicon carbide sandpaper, respectively) and etched by 50% HNO3 to reveal the corresponding microstructure. The SEM illustrations were carried out by VEGA TS5130MM device with SE detector, high vacuum conditions and at an acceleration voltage of 20.0 kV.
4. Results and discussion 4-1. Microstructural observations Fig. 8 shows the copper sheets after one pass of the CGP and the CSP processes. Fig. 8a shows, in the CGP process the sample deformed along the grooves in one direction, but as shown in Fig. 8b, the sample was deformed in two directions in the plane of the sheet by the studs. The sheet sample is severe plastic-deformed in both of the CGP and the CSP processes, but as mentioned already, the induced strain in the CSP process is higher than the CGP process. Also, theoretically, it is desirable the strain induced on the sheet sample is homogeneous in every part of the sheet but, results are showing the strain induced in the severe plastic deformed sheet is inhomogeneous, practically [26]. As is known, the dislocation is one of the linear crystalline defects which is a factor for atom slip in the lattice and the plastic deformation caused by dislocation movement which occurs due to shear stress [31]. In the plastic deformation, the density of dislocations increases. But, if the number of the dislocations is high, in addition to the positive effect on the ease of deformation, it can have a negative effect. As a result of increasing the density of dislocations, there are obstacles in the way of the movement of other dislocations which can increase the strength of the material, in other words, deformation is not easily possible. The 8
accumulation of dislocations leads to the formation of sub-cells, which at their boundaries the density of dislocations is higher. More dislocations accumulated on the new boundaries with the continuation of deformation and finally, the new grains created [32]. In other words, plastic deformation can reduce grain size. Fig. 9 shows SEM micrographs of the samples after different passes of the CSP and the CGP processes. The grain size measured by Heyn intercept method [33]. Fig. 9a shows SEM micrograph of the initial annealed sample with a mean grain size of 15 μm, also as is clear the grain size distribution was heterogeneous, as reported by other researchers [32-34], and some of the grains showed a short elongated morphology. Figs. 9b to 9e show the microstructures of the copper samples after 5, 10, 15 and 20 CSP passes, respectively. Also, the SEM micrographs show structures that have undergone large amounts of plastic deformation, since some well-developed equiaxed grains can be detected, which were probably formed by the fragmentation of elongated grain. Figs. 9b to 9e also show the grain reduction during CSP passes, so that, the mean grain size of the initial sample is about 15 15th and 20th pass reduced to 13.33
, 11.26
10.43
, and 7.86
and after 5th, 10th, , respectively. Fig.
9f shows the microstructures of the copper sample after 10 passes of the CGP. The mean grain size of the CGPed sample is about 11.34
, which is roughly similar to the grain size
of 10 passes of the CSPed sample (Fig. 9c). Comparing Figs. 9c and 9f shows the grains morphology are also approximately same, but in CGPed sample (Fig. 9f) the elongated grains are a little bit more, which is due to the grooved dies. It should be noted that the CGP pass consist of four pressing step while the CSP pass is complete only by two pressing step. The dependency of the grain refinement and the strain induced can explain by the dislocation theory [34]. The dislocations theory represents microstructural evolution during plastic deformation for metallic materials. The plastic deformation performs based on the motion of the dislocations. When the motion of the dislocations restricts, it makes the 9
material stronger then the greater strength is required to continue the plastic deformation. Work-hardening based on a principle of hindering the dislocation motion. During the CSP and the CGP processes, the work-hardening occurs, and it causes to increase the dislocation density due to the dislocation multiplication or formation of new dislocations. Increasing the dislocation density reveals that the distance of the dislocations decreases. Interactions between the dislocations result in the motion of dislocations constrained due to the presence of other dislocations. The dislocation multiplication or formation of new dislocations increase in dislocation density and thus, higher stresses are required to deform the material. Fig. 10 shows the grain size variation against the number of the CSP passes. The zero pass represented the initial annealed sample. As shown in Fig. 10, increasing the plastic strain will always reduce the mean grain size, as reported by other researchers [26, 35, 36]. The relation between the strain rate and dislocation evolution is defined as follows [34]: (6) where
is the strain rate, b is the Burger’s vector, and
is the rate of dislocation evolution,
L is the dislocation mean free path. The grain size is determining by [37]: (7) where is a constant,
is the dislocation density.
According to Equations 6 and 7, the grain reduction occurs when the dislocation density increases during plastic deformation. Also, dynamic recovery occurs during the plastic deformation. The dynamic recovery prevents the steady rate of the grain reduction. The dynamic recovery causes to annihilate the dislocations, and the rate of dynamic recovery is proportional to the dislocation density. With continuing the plastic deformation, dislocation density increases, thereby increasing the rate of dynamic recovery which annihilates the dislocations, so the rate of grain refinement at higher strains will decrease [38].
10
4-2. Mechanical properties As the metals are plastically deformed and the strain induced in them, the microstructure and the mechanical properties change. The microstructural refinement is an important phenomenon that takes place during the plastic deformation of the metals, where the grain size refined and the properties of the metals improve [26]. Strength and ductility are important mechanical properties for different materials. As well known, in the conventional metal forming processes, the strength and ductility change, conversely [39]. In other words, the conventional materials may be strong but may not be ductile and vice versa. Fig. 11 shows stress-strain curves of the samples after different CSP passes, as is clear, the severe plastic deformation greatly changes the mechanical properties. Fig. 11 shows that the initial sample has the highest breaking strain (ductility) and the lowest amount of ultimate tensile stress (UTS) and yield stress (YS), actually, the toughness of the initial sheet sample before plastic deformation is low. The toughness defined as the ability of a material to absorb energy and plastically deform without fracturing (the area under the stress-strain curve). Some investigations of the CGP process for the aluminum and the copper showed that the UTS and the YS increased and the ductility decreased when the SPD pass number increased [26]. As Fig. 11 shows, the toughness reduces until 10th CSP passes, then increases for 15th and 20th CSP passes, respectively. In other words, after 10th CSP pass, the ductility begins to increase, while the UTS remains approximately constant. Similar results also are given by Hajizadeh et al. [40], which they applied different CGP passes on the aluminum samples and showed that the ductility of them decreased by increasing the CGP pass number, and after 4 passes the ductility increases while UTS is always increasing. As also shown in Fig. 11, the modulus of elasticity for all samples are same, since, the modulus of elasticity is a structure-insensitive [41].
11
Fig. 12 shows the variation of the UTS and the elongation for the sheet samples for different CSP passes. It observed that the UTS always increases but with the increase in accumulated strain, the rate of increment gets decrease (after the 10th pass). Increasing of the UTS of the samples can be explained by an increase in the amount of flow stress results from the grain refinement phenomenon that occurs due to severe plastic deformation and also due to the increase in the dislocation density which require higher applied stress for dislocation motion by slip (strain hardening) [26, 29]. The decrease in the rate of UTS increment implies that dislocation annihilation by the mechanism of cross-slip or climb of dislocations associated with dynamic recovery [42]. In other words, the strain hardening mechanism is dominant at small values of strain, and the dislocation density increases drastically, and with an increase in dislocation density, the driving force for dynamic recovery gets increased [26, 43]. Decreasing in the rate of UTS increment was reported for the copper samples which processed by ECAP method [44, 45]. As Fig. 12 shows the initial sample has an elongation of 33.6%, which is decreased to 27.2% and 16.6% after 5th and 10th passes, respectively. This reduction in elongation can be due to the strain hardening which is explained, but after the 10th pass, elongation begins to increase, so that, after 20th pass, the elongation is 24.1%. This increment in elongation caused by dynamic recovery. Similar results are also shown in [40] which indicated that CGPed samples exhibited relatively high magnitudes of elongation (16.6 – 19%). These results show that though the material subjected to a high amount strain, it still highly appropriate elongation. As already mentioned, after the 10th CSP pass, with increasing dislocation density, dynamic recovery will be the dominant mechanism of dislocation annihilation. Fig. 13 shows the variation of the UTS and the grain size for different CSP passes. As Fig. 13 shows the grain size decreased and the UTS increased with the increase in the number of CSP passes. These results in Fig. 13 are agreement with the Hall-Petch effect [46]. 12
Fig. 14 shows the stress-strain curve of the samples after different CGP passes, as expected, the severe plastic deformation changes the mechanical properties. As others showed, the CGP process for the copper caused that the UTS increased and the ductility decreased [26]. As is clear, unlike the CSP method increasing the CGP passes almost increased the strength and decreased the elongation. Fig. 15 illustrate this manner better, which shows the variation of the ultimate tensile strength (UTS) and the elongation for the samples after different CGP passes. Fig. 12 illustrates that after 10 CSP passes the elongation increase, while for CGP method it is not. Fig. 16 shows the stress-strain curves of the CSPed and CGPed samples, after 10th pass, which compared to the initial sample curve. As Fig. 16 shows, the UTS of the CSPed and CGPed sample are approximately the same, but the CSPed sample elongation is more. As already mentioned, the amount of strain in the CSP is more than CGP, which it seems that the dynamic recovery in the CSP method activated sooner than the CGP [26]. Thus, more elongation for the CSPed sample is not unpredictable.
Conclusion: In this study, a new method for severe plastic deformation of the copper sheets introduced. The new method was called constrained studded pressing (CSP). In the CSP method, a copper sheet was subjected to the repetitive in-plane shear deformation and then straightened by a set of conjunctival studded dies and flat dies, respectively, which are constrained tightly by constrained-blocks. The grain size reduction and the mechanical properties variation of the severe plastic deformed copper sheet were studied. The results show the grain size of the copper sheet refined by the CSP method and its mechanical properties have changed dramatically. The characteristics of the CSP method were compared to the CGP method
13
theoretically, experimentally and microstructurally, respectively. The main results are following as: 1. The average strain of the CSP method is about 21% more than the CGP method for one pass pressing. 2. In-plane plastic deformation in the CSP and the CGP methods achieves in two and one directions in each pressing step, respectively. As a result, the plastic deformation achieved in two steps CSPed sample (deformation and straightening) is similar to deformation of four steps CGPed sample. So the CSP method is more competitive and faster than the CGP method. 3. The CSP method is an economical method to refine the grain size of the metal sheets. Results show that the average grain size of the copper sheet reduced by ~ 48% after 20 pressing passes of the CSP. 4. The ultimate tensile strength of the copper sheets increased about 34.6% after 20 passes of the CSP. 5. Results show that the ductility of copper samples decreases after zero to 10 passes, and increases after 10 to 20 passes of the CSP, while the UTS of them always increases after 0 to 20 CSP passes. Consequently, the toughness decreases when CSP passes increase up to 10, then the toughness increases when passes increase from 10 to 20. 6. Unlike the CSP method, increasing the CGP passes almost increased the strength and decreased the elongation.
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Fig 1: The schematic representation of the RCS method, a) RCS dies, b) first corrugation, c) first flattening, d) sample shifted to the right or left, e) second corrugation, f) second flattening.
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Fig 2: The schematic illustration of constrained groove pressing (CGP) method a) grooving, b) flattening pass.
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Fig 3: The die configuration of a) CGP and b) Constrained Studded Pressing (CSP) methods.
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Fig 4: Geometry of the CSP dies, t is width and height of studs and the angle of the imperfect pyramid is 45o.
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(a)
(b)
(c)
(d)
Fig 5: Schematic of the compressed sample in CSP and CGP methods, a) 3-dimensional groove region for the CGP method, b) 3-dimensional schematic of stud region for the CSP method, c) 2-dimensional schematic of the CGPed sample, d) 2-dimensional of the CSPed sample.
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Fig 6: Dies for a) the CGP and b) the CSP methods.
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Fig 7: The CSP dies setup.
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Fig 8: The corrugated sample after one pass for the a) CGP and b) CSP methods.
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(a)
(b)
(c)
(d)
(e)
(f)
Fig 9: SEM micrograph of the samples, a) initial annealed, b) 5 SCP pass, c) 10 CSP pass, d) 15 CSP pass, e) 20 CSP pass and f) 10 CGP pass.
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Mean grain size (μm)
16 14 12 10 8 6 0
5
10 Pass Number
15
20
Fig 10: Variation of the grain size against the number of CSP passes.
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300
Initial sample 5 pass CSP 10 pass CSP 15 pass CSP 20 pass CSP
Stress (MPa)
250 200 150 100 50 0 0
0.05
0.1
0.15
0.2
0.25
0.3
Strain Fig 11: The stress-strain curves of samples for different CSP passes.
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0.35
40
300
30
200 150
20
100
Elongation
UTS (MPa)
Elongation (%)
250
50
UTS 10
0 0
5
10
15
20
Pass Number Fig 12: Variation of the ultimate tensile strength (UTS) and the elongation for the samples after different CSP passes.
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300 250
12
200 150
8
Grain Size UTS
UTS (MPa)
Mean grain size (μm)
16
100
4
50 0
5
10
15
20
Pass Number
Fig 13: The variation of the grain size and the UTS of samples against CSP passes.
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Fig 14: The stress-strain curves of samples for different CGP passes
30
38
300
Elongation 28
250
18
200
8
150 0
5
10
UTS (MPa)
Elongation (%)
UTS
15
Passb Number Fig 15: Variation of the ultimate tensile strength (UTS) and the elongation for the samples after different CGP passes.
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Initial sample
250
10 pass CGP
Stress (MPa)
200
10 pass CSP
150 100 50 0 0
0.05
0.1
0.15 0.2 Strain
0.25
0.3
0.35
Fig 16: The stress-strain curves of the initial sample and the samples after 10 passes of the CSP and the CGP methods.
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