- Email: [email protected]
Production of nanostructure copper by planar twist channel angular extrusion process
Journal of Alloys and Compounds 682 (2016) 552e556
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Production of nanostructure copper by planar twist channel angular extrusion process M. Shamsborhan a, *, M. Ebrahimi b, ** a b
Department of Engineering, Mahabad Branch, Islamic Azad University, Mahabad, Iran Department of Mechanical Engineering, University of Maragheh, Maragheh, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 January 2016 Received in revised form 20 April 2016 Accepted 2 May 2016 Available online 3 May 2016
This study challenges experimentally, the feasibility of new severe plastic deformation method entitling planar twist channel angular extrusion (PTCAE) for fabrication of nanostructure pure copper. The results indicated, deformed copper has 71%, 69% and 113% higher yield strength, ultimate tensile strength and hardness magnitude as compared to the as-received condition. Additionally, the fractography of tensile sample showed the mechanism alteration from multiple dimples to cleavage planes, associating the production of brittle type material. Moreover, the reduction of bulk formability index of the processed copper in comparison with the initial condition confirms the decrease of material ductility due to the strain hardening and grain refinement phenomena. The grain size of about 270 nm is also attained after the single pass processing. Eventually, a grain size correction coefficient is proposed to the strength to hardness ratio as a rule of thumb for estimation of strength by means of hardness irrespective of the grain size of the pure copper material. © 2016 Elsevier B.V. All rights reserved.
Keywords: Planar twist channel angular extrusion Nanostructure copper Mechanical properties Strength to hardness ratio
1. Introduction Ultrafine grain (UFG) and nanostructure (NS) metals and alloys are recently well established because of their significant and unique properties in comparison with conventional coarse grain (CG) counterparts [1,2]. It is widely accepted that one of the most prosperous methods of top-down approach for achievement of the UFG and even NS materials is severe plastic deformation (SPD) method in which very intense plastic strain is imposed to a specimen without any considerable change at the cross-sectional area [1e4]. A comprehensive study on the various existing severe plastic deformation literature reveals that there are some concerns about their capability on the various industrial and medical applications. These worries return to the problems, including the inability of the method to scale-up, complexity of die geometry and discontinuity of the material production, especially in the form of sheet and strip [5e8]. Accordingly, several SPD techniques and methods have been proposed, developed and experimented for elimination of these
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] [email protected] (M. Ebrahimi). http://dx.doi.org/10.1016/j.jallcom.2016.05.012 0925-8388/© 2016 Elsevier B.V. All rights reserved.
(M.
Shamsborhan),
limitations such as equal channel angular pressing (ECAP) [9], elliptical cross-section spiral equal-channel extrusion (ECSEE) [10], equal channel forward extrusion (ECFE) [11], accumulative back extrusion (ABE) [12], multi axial forging (MAF) [13], repetitive upsetting (RU) [14], planar twist extrusion (PTE) [15], simple shear extrusion (SSE) [16], tubular channel angular pressing (TCAP) [17], constrained groove pressing (CGP) [18] and accumulative roll bonding (ARB) [19]. Recently, the authors have offered a new SPD method entitling planar twist channel angular extrusion process in order to conquer the as-mentioned SPD constraints as far as possible [20e22]. In other words, this method has been introduced by this duty. The schematic representation of PTCAE method and efficacious processing parameters have been shown in Fig. 1 beside the both PTE and ECAP methods. During this process which combines two ECAP and PTE methods in one deformation zone, intensive shear strain is simultaneously imposed to the three orthogonal faces of the work-piece. According to the previous investigation of PTCAE method, the main advantages of this process in comparison with the other SPD methods are simultaneous imposing of plastic shear strain in three perpendicular planes in one single deformation zone, small volume of plastic deformation zone, less required pass numbers to attain material with the saturated grain size, low required extrusion load due to the reduction of contact surfaces
M. Shamsborhan, M. Ebrahimi / Journal of Alloys and Compounds 682 (2016) 552e556
Fig. 1. Schematic representation of a) equal channel angular pressing, b) planar twist extrusion and c) planar twist channel angular extrusion.
between the sample and the die and eventually, processing efficiency. The method details have been found elsewhere [20e22]. In this research, PTCAE die which is suitable for induction of extremely large strain to the sample in the single deformation zone has been designed and manufactured. Afterwards, mechanical properties and microstructural evolution of one pass processed copper have been experimentally investigated. 2. Experimental procedures All experimentations were performed on annealed commercial pure (CP) copper with a length of 70 mm and square cross-section of 10 mm 10 mm. PTCAE process was conducted using a split die with channel angle, outer corner angle and planar twist angle of 90 , 90 and 35 , respectively. This process was carried out at ambient temperature with a ram speed of about 1 mm s1 and application of molybdenum disulfide as lubricant [23]. Fig. 2 represents the used PTCAE die and the copper sample after the first pass. The effective plastic strain during one pass of PTCAE process with the die parameters used for this study is equal to 1.95 [20e22], which is higher pffiffiffithan from the conventional equal channel angular pressing (ð1= 3Þ½2 cotðF þ J=2Þ þ cosecðF þ J=2Þ with the die
553
configurationspof ffiffiffi F ¼ 90 , J ¼ 90 ) [9] and also, planar twist extrusion (ð2= 3ÞtanðaÞ with the die configuration of a ¼ 35 ) [24] processes. It is better to mention that the magnitudes of strain for each pass of ECAP and PTE processes are equal to 0.577 and 0.808, respectively. After the process, various mechanical and microstructural examinations were carried out on the CP copper sample to investigate the capability of the method in comparison with the other SPD processes. Therefore, Vickers microhardness (HV) measurements with a load of 300 gf and dwell time of 10 s were accomplished on a plane normal to the extrusion direction (Newage MS-HMVG20SV microhardness testing system). It should be noted that 16 HV tests with the step of 2 mm were recorded and the average magnitude was reported. In addition, room temperature tensile test by means of Instron 5582 universal tester in accordance with the ASTM E8 was done to evaluate tensile strength and elongation of the extruded copper sample as compared to the annealed one. It is better to know that the tensile testing sample was prepared at the middle part of the billet by means of CNC wire cut electrical discharge machining with its tensile axis parallel to the extrusion direction. Two tensile tests were carried out for each circumstance at the constant strain rate of 3 103 s1 and the average magnitudes were recorded. Afterwards, scanning electron microscopy (SEM) using Jeol JSM-IT100 instrument was utilized to size up breakage surface morphology of the tensile samples. Finally, the samples were respectively subjected to SEM and Jeol JEMARM200F transmission electron microscopy (TEM) at the crosssectional section for the samples of the as-received and the deformed conditions to confirm the grain refinement of pure copper material after the PTCAE process. To attain TEM image, thin foil was prepared by use of mechanical grinding and polishing and also, twin jet electropolishing machine at a voltage of 60 V and a temperature of 35 C.
3. Results and discussion The engineering stress e strain curve of the pure copper sample before and after the PTCAE process has been represented in Fig. 3. By considering that there is not any well determined yield occurrence at the tension behavior of Cu samples before and after the process, the yield strength of the 0.2% offset was utilized. It is apparent that imposing severe plastic deformation by planar twist channel angular extrusion leads to the improvement of both yield strength and ultimate tensile strength and to the reduction of elongation to failure. It means that PTCAE process has significant influence on the mechanical properties of pure copper. For the deformed material, the strength increases dramatically with strain up to the peak point and then, it decreases gradually with strain increment till to the breaking. The quantitative analyses of the results reveal that 71% and 69% higher yield strength and ultimate tensile strength and a 48% lower elongation to failure are achieved after the first pass of PTCAE process in comparison with the initial condition. It should be useful to define a formability index for the
Fig. 2. The utilized PTCAE die and deformed copper sample after the first pass.
554
M. Shamsborhan, M. Ebrahimi / Journal of Alloys and Compounds 682 (2016) 552e556
Fig. 3. The engineering stress e strain curves of pure copper sample before and after the PTCAE process.
bulk materials beside the forming limit diagram (FLD) for evaluation of sheet form samples. Therefore, bulk formability index (BFI) which is defined as the multiplication of ultimate tensile strength and elongation to failure has been applied to estimate the material ability to endure plastic deformation before damage taking place [25]. BFI is reduced from 180 MPa% to 159 MPa% which is equal to 12%. This gentle reduction of BFI magnitude may be related to the work hardening phenomenon. In addition to the strain hardening happening of copper sample through the process, there is another strength mechanism, including grain refinement which has considerable effect during material strength enhancement by performing the PTCAE process. These two strength mechanisms have been widely investigated before. Accordingly, shear force performs a considerable role on the grain refinement of material during SPD processes. It is worth mentioning that mechanical shearing and initial coarse grain subdivision are the main refining mechanism. During imposing intense shear strain by means of PTCAE method, the dislocation boundaries develop in two determined patterns entitling geometrically necessary boundaries (GNBs) and incidental dislocation boundaries (IDBs). The GNDs density increases during the process to arrange and form the geometrically necessary high dislocation density regions and to make cell blocks. Additionally, IDBs lead to the formation of ordinary cell boundaries. As known, boundary spacing and misorientation angle distribution of both GNBs and IDBs are simultaneously developed in different manners through imposing of plastic strain. They denote various morphologies up to the medium strain, but similar ones at the intense strain. It should be mentioned that IDBs misorientation axes are of random distribution, while GNBs show preferred axes. Afterwards, dynamic recovery converts the GNBs and IDBs into more
equilibrium grain boundaries through increasing their misorientation. On the other hand, dynamic recrystallization (DRX) as another grain refinement mechanism is a renewal of the grain structure including continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX). In CDRX, grain boundaries do not move but new ones are continuously created from GNBs, while in DDRX, new grains can grow from nuclei or the existing grain boundaries move [25e28]. Tensile fracture morphology of the pure copper sample before and after the PTCAE process has been exhibited in Fig. 4. It is apparent that the surface mechanism of the annealed pure copper sample contains multiple dimples and voids with different depths and sizes, whereas most of the fracture surface of the first pass PTCAE copper includes cleavage planes. Accordingly, the surface fracture mechanism type is changed from ductile to brittle by performing PTCAE process. Thus, it seems that grain refinement by imposing plastic strain using the presented new severe plastic deformation method leads to the alteration of fracture mechanism type. The same behavior has been previously reported. As known, PTCAE processing performance leads to the increment of dislocations density. The creation of high density dislocations increases the probability of dislocations trapping and annihilation. Also, softening mechanism results in the localized plastic deformation which forms a necking behavior after the uniform elongation. It is worth to note that the necking degree is lessened at the intense plastic strain, because dislocations which dispensed uniformly in more grains can postpone their trapping and annihilation due to the grain refinement. Hence, the shear characteristic of fracture will have been the main role and therefore, brittle type of fracture mechanism will be appeared [29e31]. Vickers microhardness measurements indicate a considerable increment of average hardness magnitude of the pure copper sample by application of PTCAE process. The findings show that an approximate 113% improvement has been obtained after the single pass as contrasted to the annealed condition. These strength and hardness enhancement and also, ductility reduction may be attributed to the grain refinement of processed material. Hence, bright field TEM image of copper after the first pass has been attained and shown in Fig. 5, next to the SEM observation of sample at the annealed condition. It is observed that the average grain size of 62 mm at the annealed condition reaches equiaxed-elongated structure with the mean grain size of about 270 nm after the PTCAE process. Strength to hardness ratio (SHR) which correlate two main mechanical parameters of material has been computed for pure copper sample before and after the PTCAE process [32,33]. The calculations indicate that SHR magnitude decreases from 4.08 to 3.25 during one pass of process which is equal to 20% reduction. Therefore, it seems that grain size has a sizeable effect on this ratio.
Fig. 4. The morphology of the fracture surface of the tensile test for pure copper specimens (a) before and (b) after the PTCAE process.
M. Shamsborhan, M. Ebrahimi / Journal of Alloys and Compounds 682 (2016) 552e556
555
Fig. 5. The microstructure observation of pure copper sample (a) before and (b) after the PTCAE process.
If a correction coefficient due to the grain size difference has been imposed to SHR, this ratio will be constant for each material and by this way, it seems that the estimation of strength by use of hardness can be possible. It is worth to mention that this ratio leads to the estimation of material strength through nondestructive and quick method. Of course, more studies are needed for this matter. Finally, in a word, these findings reveal that the planar twist channel angular extrusion process is a promising severe plastic deformation method in order to produce UFG and even NS metals and alloys with higher mechanical properties next to the other SPD techniques. According to the achieved results, the main advantages of the investigated new method in comparison with the other SPD ones can be drawn as follows: Imposing of higher plastic strain and better strain distribution homogeneity as compared to the MAF and RU methods [13,14], Fabrication of relatively large sample size as compared to the MAF and CGP processes [13,18], Lower deformed sample production cost in contrast to the ECSEE and ARB processes because of simple manufacturing process of PTCAE die and set-up [10,19], and No need to back pressure punch in comparison with the SSE method [16].
4. Conclusion This study has been allotted to the capability, mechanical and microstructural characterization of commercial pure copper processed by planar twist channel angular extrusion. Accordingly, the following conclusions can be listed from the present research: PTCAE process leads to the increment of both yield and ultimate tensile strengths as equal as 71% and 69%, respectively. In addition, elongation to failure is decreased from 71% to 37% after the one pass deformation. Furthermore, bulk formability index of processed copper sample shows the reduction of 12% in comparison with the initial condition due to the strain hardening and grain refinement occurrences. The tensile fracture morphology is gradually altered from the ductile mechanism (dimples and voids morphology) to the brittle counterpart (cleavage planes) by performing single pass of PTCAE process. The improvement of tensile strength of the deformed copper sample is accompanied with its hardness magnitude, indicating an about of 113% increase at the Vickers hardness value. The application of PTCAE process causes fabrication of copper
sample with sizeable grain refinement, pointing out the final grain size of about 270 nm as compared to the as-received condition which is equal to 62 mm. A grain size correction coefficient has been also imposed to the strength to hardness ratio as a rule of thumb for the estimation of copper strength by use of hardness irrespective of the Cu grain size. All above results imply the promising of this new SPD method for future industrial and medical applications. Acknowledgments The authors would like to thank the Iran Nanotechnology Initiative Council (INIC) and Mahabad Branch of Islamic Azad University for the financial support of this research. References [1] B. Cherukuri, T.S. Nedkova, R. Srinivasan, A comparison of the properties of SPD-processed AA-6061 by equal-channel angular pressing, multi-axial compressions/forgings and accumulative roll bonding, Mater. Sci. Eng. A 410e411 (2005) 394e397, http://dx.doi.org/10.1016/j.msea.2005.08.024. [2] R.Z. Valiev, T.G. Langdon, Principles of equal-channel angular pressing as a processing tool for grain refinement, Prog. Mater. Sci. 51 (2006) 881e981, http://dx.doi.org/10.1016/j.pmatsci.2006.02.003. [3] F. Khodabakhshi, M. Kazeminezhad, The effect of constrained groove pressing on grain size, dislocation density and electrical resistivity of low carbon steel, Mater. Des. 32 (2011) 3280e3286, http://dx.doi.org/10.1016/ j.matdes.2011.02.032. [4] J.H. Kim, S.K. Hwang, Y.-T. Im, I.-H. Son, C.M. Bae, High-strength bolt-forming of fine-grained aluminum alloy 6061 with a continuous hybrid process, Mater. Sci. Eng. A 552 (2012) 316e322, http://dx.doi.org/10.1016/ j.msea.2012.05.046. [5] K. Edalati, Z. Horita, Continuous high-pressure torsion, J. Mater. Sci. 45 (2010) 4578e4582, http://dx.doi.org/10.1007/s10853-010-4381-z. [6] J. Huang, Y.T. Zhu, D.J. Alexander, X. Liao, T.C. Lowe, R.J. Asaro, Development of repetitive corrugation and straightening, Mater. Sci. Eng. A 371 (2004) 35e39, http://dx.doi.org/10.1016/S0921-5093(03)00114-X. [7] Q. Chen, Z. Zhao, D. Shu, Z. Zhao, Microstructure and mechanical properties of AZ91D magnesium alloy prepared by compound extrusion, Mater. Sci. Eng. A 528 (2011) 3930e3934, http://dx.doi.org/10.1016/j.msea.2011.01.028. [8] M. Shahbaz, N. Pardis, R. Ebrahimi, B. Talebanpour, A novel single pass severe plastic deformation technique: vortex extrusion, Mater. Sci. Eng. A 530 (2011) 469e472, http://dx.doi.org/10.1016/j.msea.2011.09.114. [9] F. Djavanroodi, M. Ebrahimi, Effect of die channel angle, friction and back pressure in the equal channel angular pressing using 3D finite element simulation, Mater. Sci. Eng. A 527 (2010) 1230e1235, http://dx.doi.org/ 10.1016/j.msea.2009.09.052. [10] C. Wang, F. Li, Q. Li, J. Li, L. Wang, J. Dong, A novel severe plastic deformation method for fabricating ultrafine grained pure copper, Mater. Des. 43 (2013) 492e498, http://dx.doi.org/10.1016/j.matdes.2012.07.047. [11] M. Ebrahimi, H. Gholipour, F. Djavanroodi, A study on the capability of equal channel forward extrusion process, Mater. Sci. Eng. A 650 (2015) 1e7, http:// dx.doi.org/10.1016/j.msea.2015.10.014. [12] H. Alihosseini, G. Faraji, A.F. Dizaji, K. Dehghani, Characterization of ultra-fine
556
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
M. Shamsborhan, M. Ebrahimi / Journal of Alloys and Compounds 682 (2016) 552e556 grained aluminum produced by accumulative back extrusion (ABE), Mater. Charact. 68 (2012) 14e21, http://dx.doi.org/10.1016/j.matchar.2012.03.004. Q. Chen, D. Shu, C. Hu, Z. Zhao, B. Yuan, Grain refinement in an as-cast AZ61 magnesium alloy processed by multi-axial forging under the multitemperature processing procedure, Mater. Sci. Eng. A 541 (2012) 98e104, http://dx.doi.org/10.1016/j.msea.2012.02.009. W. Guo, Q.D. Wang, B. Ye, M.P. Liu, T. Peng, X.T. Liu, et al., Enhanced microstructure homogeneity and mechanical properties of AZ31 magnesium alloy by repetitive upsetting, Mater. Sci. Eng. A 540 (2012) 115e122, http:// dx.doi.org/10.1016/j.msea.2012.01.111. Y. Beygelzimer, D. Prilepo, R. Kulagin, V. Grishaev, O. Abramova, V. Varyukhin, et al., Planar twist extrusion versus twist extrusion, J. Mater. Process. Technol. 211 (2011) 522e529, http://dx.doi.org/10.1016/j.jmatprotec.2010.11.006. N. Pardis, R. Ebrahimi, Deformation behavior in Simple Shear Extrusion (SSE) as a new severe plastic deformation technique, Mater. Sci. Eng. A 527 (2009) 355e360, http://dx.doi.org/10.1016/j.msea.2009.08.051. G. Faraji, M.M. Mashhadi, H.S. Kim, Tubular channel angular pressing (TCAP) as a novel severe plastic deformation method for cylindrical tubes, Mater. Lett. 65 (2011) 3009e3012, http://dx.doi.org/10.1016/j.matlet.2011.06.039. A. Sajadi, M. Ebrahimi, F. Djavanroodi, Experimental and numerical investigation of Al properties fabricated by CGP process, Mater. Sci. Eng. A 552 (2012) 97e103, http://dx.doi.org/10.1016/j.msea.2012.04.121. M. Shaarbaf, M.R. Toroghinejad, Nano-grained copper strip produced by accumulative roll bonding process, Mater. Sci. Eng. A 473 (2008) 28e33, http://dx.doi.org/10.1016/j.msea.2007.03.065. A. Shokuhfar, M. Shamsborhan, A planar twist channel angular extrusion (PTCAE) as a novel severe plastic deformation method based on equal channel angular extrusion (ECAE) method, J. Mech. Sci. Technol. 28 (2014) 1753e1757, http://dx.doi.org/10.1007/s12206-014-0321-1. M. Shamsborhan, A. Shokuhfar, O. Nejadseyfi, J. Kakemam, M. Moradi, Experimental and numerical comparison of equal channel angular extrusion (ECAE) with planar twist channel angular extrusion (PTCAE), Proc. Inst. Mech. Eng. C J. Mech. Eng. Sci. 229 (2015) 3059e3067, http://dx.doi.org/10.1177/ 0954406214566035. A. Shokuhfar, M. Shamsborhan, Finite element analysis of planar twist channel angular extrusion (PTCAE) as a novel severe plastic deformation method, J. Mech. Sci. Technol. 28 (2014) 1753e1757, http://dx.doi.org/10.1007/ s12206-014-0321-1.
[23] F. Djavanroodi, M. Ebrahimi, Effect of die parameters and material properties in ECAP with parallel channels, Mater. Sci. Eng. A 527 (2010) 7593e7599, http://dx.doi.org/10.1016/j.msea.2010.08.022. [24] E. Bagherpour, M. Reihanian, R. Ebrahimi, Processing twining induced plasticity steel through simple shear extrusion, Mater. Des. 40 (2012) 262e267, http://dx.doi.org/10.1016/j.matdes.2012.03.055. [25] O.V. Mishin, D. Juul Jensen, N. Hansen, Microstructures and boundary populations in materials produced by equal channel angular extrusion, Mater. Sci. Eng. A 342 (2003) 320e328, http://dx.doi.org/10.1016/S0921-5093(02) 00311-8. [26] S. Malopheyev, R. Kaibyshev, Strengthening mechanisms in a Zr-modified 5083 alloy deformed to high strains, Mater. Sci. Eng. A 620 (2015) 246e252, http://dx.doi.org/10.1016/j.msea.2014.10.030. [27] N. Haghdadi, A. Zarei-Hanzaki, D. Abou-Ras, Microstructure and mechanical properties of commercially pure aluminum processed by accumulative back extrusion, Mater. Sci. Eng. A 584 (2013) 73e81, http://dx.doi.org/10.1016/ j.msea.2013.06.060. [28] M. Cabibbo, Microstructure strengthening mechanisms in different equal channel angular pressed aluminum alloys, Mater. Sci. Eng. A 560 (2013) 413e432, http://dx.doi.org/10.1016/j.msea.2012.09.086. [29] Y. Xiong, T. He, Z. Guo, H. He, F. Ren, A.A. Volinsky, Mechanical properties and fracture characteristics of high carbon steel after equal channel angular pressing, Mater. Sci. Eng. A 563 (2013) 163e167, http://dx.doi.org/10.1016/ j.msea.2012.11.068. [30] P. Zhang, S. Qu, M.X. Yang, G. Yang, S.D. Wu, S.X. Li, et al., Varying tensile fracture mechanisms of Cu and Cu-Zn alloys with reduced grain size: from necking to shearing instability, Mater. Sci. Eng. A 594 (2014) 309e320, http:// dx.doi.org/10.1016/j.msea.2013.11.079. [31] C. Kammerhofer, A. Hohenwarter, S. Scheriau, H.P. Brantner, R. Pippan, Influence of morphology and structural size on the fracture behavior of a nanostructured pearlitic steel, Mater. Sci. Eng. A 585 (2013) 190e196, http:// dx.doi.org/10.1016/j.msea.2013.07.032. [32] M. Gasko, G. Rosenberg, Correlation between hardness and tensile properties in ultra-high strength dual phase steelseshort communication, Mater. Eng. 18 (2011) 155e159. http://fstroj.uniza.sk/journal-mi/PDF/2011/27-2011.pdf. [33] P. Zhang, S.X. Li, Z.F. Zhang, General relationship between strength and hardness, Mater. Sci. Eng. A 529 (2011) 62e73, http://dx.doi.org/10.1016/ j.msea.2011.08.061.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Production of nanostructure copper by planar twist channel angular extrusion process M. Shamsborhan a, *, M. Ebrahimi b, ** a b
Department of Engineering, Mahabad Branch, Islamic Azad University, Mahabad, Iran Department of Mechanical Engineering, University of Maragheh, Maragheh, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 January 2016 Received in revised form 20 April 2016 Accepted 2 May 2016 Available online 3 May 2016
This study challenges experimentally, the feasibility of new severe plastic deformation method entitling planar twist channel angular extrusion (PTCAE) for fabrication of nanostructure pure copper. The results indicated, deformed copper has 71%, 69% and 113% higher yield strength, ultimate tensile strength and hardness magnitude as compared to the as-received condition. Additionally, the fractography of tensile sample showed the mechanism alteration from multiple dimples to cleavage planes, associating the production of brittle type material. Moreover, the reduction of bulk formability index of the processed copper in comparison with the initial condition confirms the decrease of material ductility due to the strain hardening and grain refinement phenomena. The grain size of about 270 nm is also attained after the single pass processing. Eventually, a grain size correction coefficient is proposed to the strength to hardness ratio as a rule of thumb for estimation of strength by means of hardness irrespective of the grain size of the pure copper material. © 2016 Elsevier B.V. All rights reserved.
Keywords: Planar twist channel angular extrusion Nanostructure copper Mechanical properties Strength to hardness ratio
1. Introduction Ultrafine grain (UFG) and nanostructure (NS) metals and alloys are recently well established because of their significant and unique properties in comparison with conventional coarse grain (CG) counterparts [1,2]. It is widely accepted that one of the most prosperous methods of top-down approach for achievement of the UFG and even NS materials is severe plastic deformation (SPD) method in which very intense plastic strain is imposed to a specimen without any considerable change at the cross-sectional area [1e4]. A comprehensive study on the various existing severe plastic deformation literature reveals that there are some concerns about their capability on the various industrial and medical applications. These worries return to the problems, including the inability of the method to scale-up, complexity of die geometry and discontinuity of the material production, especially in the form of sheet and strip [5e8]. Accordingly, several SPD techniques and methods have been proposed, developed and experimented for elimination of these
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] [email protected] (M. Ebrahimi). http://dx.doi.org/10.1016/j.jallcom.2016.05.012 0925-8388/© 2016 Elsevier B.V. All rights reserved.
(M.
Shamsborhan),
limitations such as equal channel angular pressing (ECAP) [9], elliptical cross-section spiral equal-channel extrusion (ECSEE) [10], equal channel forward extrusion (ECFE) [11], accumulative back extrusion (ABE) [12], multi axial forging (MAF) [13], repetitive upsetting (RU) [14], planar twist extrusion (PTE) [15], simple shear extrusion (SSE) [16], tubular channel angular pressing (TCAP) [17], constrained groove pressing (CGP) [18] and accumulative roll bonding (ARB) [19]. Recently, the authors have offered a new SPD method entitling planar twist channel angular extrusion process in order to conquer the as-mentioned SPD constraints as far as possible [20e22]. In other words, this method has been introduced by this duty. The schematic representation of PTCAE method and efficacious processing parameters have been shown in Fig. 1 beside the both PTE and ECAP methods. During this process which combines two ECAP and PTE methods in one deformation zone, intensive shear strain is simultaneously imposed to the three orthogonal faces of the work-piece. According to the previous investigation of PTCAE method, the main advantages of this process in comparison with the other SPD methods are simultaneous imposing of plastic shear strain in three perpendicular planes in one single deformation zone, small volume of plastic deformation zone, less required pass numbers to attain material with the saturated grain size, low required extrusion load due to the reduction of contact surfaces
M. Shamsborhan, M. Ebrahimi / Journal of Alloys and Compounds 682 (2016) 552e556
Fig. 1. Schematic representation of a) equal channel angular pressing, b) planar twist extrusion and c) planar twist channel angular extrusion.
between the sample and the die and eventually, processing efficiency. The method details have been found elsewhere [20e22]. In this research, PTCAE die which is suitable for induction of extremely large strain to the sample in the single deformation zone has been designed and manufactured. Afterwards, mechanical properties and microstructural evolution of one pass processed copper have been experimentally investigated. 2. Experimental procedures All experimentations were performed on annealed commercial pure (CP) copper with a length of 70 mm and square cross-section of 10 mm 10 mm. PTCAE process was conducted using a split die with channel angle, outer corner angle and planar twist angle of 90 , 90 and 35 , respectively. This process was carried out at ambient temperature with a ram speed of about 1 mm s1 and application of molybdenum disulfide as lubricant [23]. Fig. 2 represents the used PTCAE die and the copper sample after the first pass. The effective plastic strain during one pass of PTCAE process with the die parameters used for this study is equal to 1.95 [20e22], which is higher pffiffiffithan from the conventional equal channel angular pressing (ð1= 3Þ½2 cotðF þ J=2Þ þ cosecðF þ J=2Þ with the die
553
configurationspof ffiffiffi F ¼ 90 , J ¼ 90 ) [9] and also, planar twist extrusion (ð2= 3ÞtanðaÞ with the die configuration of a ¼ 35 ) [24] processes. It is better to mention that the magnitudes of strain for each pass of ECAP and PTE processes are equal to 0.577 and 0.808, respectively. After the process, various mechanical and microstructural examinations were carried out on the CP copper sample to investigate the capability of the method in comparison with the other SPD processes. Therefore, Vickers microhardness (HV) measurements with a load of 300 gf and dwell time of 10 s were accomplished on a plane normal to the extrusion direction (Newage MS-HMVG20SV microhardness testing system). It should be noted that 16 HV tests with the step of 2 mm were recorded and the average magnitude was reported. In addition, room temperature tensile test by means of Instron 5582 universal tester in accordance with the ASTM E8 was done to evaluate tensile strength and elongation of the extruded copper sample as compared to the annealed one. It is better to know that the tensile testing sample was prepared at the middle part of the billet by means of CNC wire cut electrical discharge machining with its tensile axis parallel to the extrusion direction. Two tensile tests were carried out for each circumstance at the constant strain rate of 3 103 s1 and the average magnitudes were recorded. Afterwards, scanning electron microscopy (SEM) using Jeol JSM-IT100 instrument was utilized to size up breakage surface morphology of the tensile samples. Finally, the samples were respectively subjected to SEM and Jeol JEMARM200F transmission electron microscopy (TEM) at the crosssectional section for the samples of the as-received and the deformed conditions to confirm the grain refinement of pure copper material after the PTCAE process. To attain TEM image, thin foil was prepared by use of mechanical grinding and polishing and also, twin jet electropolishing machine at a voltage of 60 V and a temperature of 35 C.
3. Results and discussion The engineering stress e strain curve of the pure copper sample before and after the PTCAE process has been represented in Fig. 3. By considering that there is not any well determined yield occurrence at the tension behavior of Cu samples before and after the process, the yield strength of the 0.2% offset was utilized. It is apparent that imposing severe plastic deformation by planar twist channel angular extrusion leads to the improvement of both yield strength and ultimate tensile strength and to the reduction of elongation to failure. It means that PTCAE process has significant influence on the mechanical properties of pure copper. For the deformed material, the strength increases dramatically with strain up to the peak point and then, it decreases gradually with strain increment till to the breaking. The quantitative analyses of the results reveal that 71% and 69% higher yield strength and ultimate tensile strength and a 48% lower elongation to failure are achieved after the first pass of PTCAE process in comparison with the initial condition. It should be useful to define a formability index for the
Fig. 2. The utilized PTCAE die and deformed copper sample after the first pass.
554
M. Shamsborhan, M. Ebrahimi / Journal of Alloys and Compounds 682 (2016) 552e556
Fig. 3. The engineering stress e strain curves of pure copper sample before and after the PTCAE process.
bulk materials beside the forming limit diagram (FLD) for evaluation of sheet form samples. Therefore, bulk formability index (BFI) which is defined as the multiplication of ultimate tensile strength and elongation to failure has been applied to estimate the material ability to endure plastic deformation before damage taking place [25]. BFI is reduced from 180 MPa% to 159 MPa% which is equal to 12%. This gentle reduction of BFI magnitude may be related to the work hardening phenomenon. In addition to the strain hardening happening of copper sample through the process, there is another strength mechanism, including grain refinement which has considerable effect during material strength enhancement by performing the PTCAE process. These two strength mechanisms have been widely investigated before. Accordingly, shear force performs a considerable role on the grain refinement of material during SPD processes. It is worth mentioning that mechanical shearing and initial coarse grain subdivision are the main refining mechanism. During imposing intense shear strain by means of PTCAE method, the dislocation boundaries develop in two determined patterns entitling geometrically necessary boundaries (GNBs) and incidental dislocation boundaries (IDBs). The GNDs density increases during the process to arrange and form the geometrically necessary high dislocation density regions and to make cell blocks. Additionally, IDBs lead to the formation of ordinary cell boundaries. As known, boundary spacing and misorientation angle distribution of both GNBs and IDBs are simultaneously developed in different manners through imposing of plastic strain. They denote various morphologies up to the medium strain, but similar ones at the intense strain. It should be mentioned that IDBs misorientation axes are of random distribution, while GNBs show preferred axes. Afterwards, dynamic recovery converts the GNBs and IDBs into more
equilibrium grain boundaries through increasing their misorientation. On the other hand, dynamic recrystallization (DRX) as another grain refinement mechanism is a renewal of the grain structure including continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX). In CDRX, grain boundaries do not move but new ones are continuously created from GNBs, while in DDRX, new grains can grow from nuclei or the existing grain boundaries move [25e28]. Tensile fracture morphology of the pure copper sample before and after the PTCAE process has been exhibited in Fig. 4. It is apparent that the surface mechanism of the annealed pure copper sample contains multiple dimples and voids with different depths and sizes, whereas most of the fracture surface of the first pass PTCAE copper includes cleavage planes. Accordingly, the surface fracture mechanism type is changed from ductile to brittle by performing PTCAE process. Thus, it seems that grain refinement by imposing plastic strain using the presented new severe plastic deformation method leads to the alteration of fracture mechanism type. The same behavior has been previously reported. As known, PTCAE processing performance leads to the increment of dislocations density. The creation of high density dislocations increases the probability of dislocations trapping and annihilation. Also, softening mechanism results in the localized plastic deformation which forms a necking behavior after the uniform elongation. It is worth to note that the necking degree is lessened at the intense plastic strain, because dislocations which dispensed uniformly in more grains can postpone their trapping and annihilation due to the grain refinement. Hence, the shear characteristic of fracture will have been the main role and therefore, brittle type of fracture mechanism will be appeared [29e31]. Vickers microhardness measurements indicate a considerable increment of average hardness magnitude of the pure copper sample by application of PTCAE process. The findings show that an approximate 113% improvement has been obtained after the single pass as contrasted to the annealed condition. These strength and hardness enhancement and also, ductility reduction may be attributed to the grain refinement of processed material. Hence, bright field TEM image of copper after the first pass has been attained and shown in Fig. 5, next to the SEM observation of sample at the annealed condition. It is observed that the average grain size of 62 mm at the annealed condition reaches equiaxed-elongated structure with the mean grain size of about 270 nm after the PTCAE process. Strength to hardness ratio (SHR) which correlate two main mechanical parameters of material has been computed for pure copper sample before and after the PTCAE process [32,33]. The calculations indicate that SHR magnitude decreases from 4.08 to 3.25 during one pass of process which is equal to 20% reduction. Therefore, it seems that grain size has a sizeable effect on this ratio.
Fig. 4. The morphology of the fracture surface of the tensile test for pure copper specimens (a) before and (b) after the PTCAE process.
M. Shamsborhan, M. Ebrahimi / Journal of Alloys and Compounds 682 (2016) 552e556
555
Fig. 5. The microstructure observation of pure copper sample (a) before and (b) after the PTCAE process.
If a correction coefficient due to the grain size difference has been imposed to SHR, this ratio will be constant for each material and by this way, it seems that the estimation of strength by use of hardness can be possible. It is worth to mention that this ratio leads to the estimation of material strength through nondestructive and quick method. Of course, more studies are needed for this matter. Finally, in a word, these findings reveal that the planar twist channel angular extrusion process is a promising severe plastic deformation method in order to produce UFG and even NS metals and alloys with higher mechanical properties next to the other SPD techniques. According to the achieved results, the main advantages of the investigated new method in comparison with the other SPD ones can be drawn as follows: Imposing of higher plastic strain and better strain distribution homogeneity as compared to the MAF and RU methods [13,14], Fabrication of relatively large sample size as compared to the MAF and CGP processes [13,18], Lower deformed sample production cost in contrast to the ECSEE and ARB processes because of simple manufacturing process of PTCAE die and set-up [10,19], and No need to back pressure punch in comparison with the SSE method [16].
4. Conclusion This study has been allotted to the capability, mechanical and microstructural characterization of commercial pure copper processed by planar twist channel angular extrusion. Accordingly, the following conclusions can be listed from the present research: PTCAE process leads to the increment of both yield and ultimate tensile strengths as equal as 71% and 69%, respectively. In addition, elongation to failure is decreased from 71% to 37% after the one pass deformation. Furthermore, bulk formability index of processed copper sample shows the reduction of 12% in comparison with the initial condition due to the strain hardening and grain refinement occurrences. The tensile fracture morphology is gradually altered from the ductile mechanism (dimples and voids morphology) to the brittle counterpart (cleavage planes) by performing single pass of PTCAE process. The improvement of tensile strength of the deformed copper sample is accompanied with its hardness magnitude, indicating an about of 113% increase at the Vickers hardness value. The application of PTCAE process causes fabrication of copper
sample with sizeable grain refinement, pointing out the final grain size of about 270 nm as compared to the as-received condition which is equal to 62 mm. A grain size correction coefficient has been also imposed to the strength to hardness ratio as a rule of thumb for the estimation of copper strength by use of hardness irrespective of the Cu grain size. All above results imply the promising of this new SPD method for future industrial and medical applications. Acknowledgments The authors would like to thank the Iran Nanotechnology Initiative Council (INIC) and Mahabad Branch of Islamic Azad University for the financial support of this research. References [1] B. Cherukuri, T.S. Nedkova, R. Srinivasan, A comparison of the properties of SPD-processed AA-6061 by equal-channel angular pressing, multi-axial compressions/forgings and accumulative roll bonding, Mater. Sci. Eng. A 410e411 (2005) 394e397, http://dx.doi.org/10.1016/j.msea.2005.08.024. [2] R.Z. Valiev, T.G. Langdon, Principles of equal-channel angular pressing as a processing tool for grain refinement, Prog. Mater. Sci. 51 (2006) 881e981, http://dx.doi.org/10.1016/j.pmatsci.2006.02.003. [3] F. Khodabakhshi, M. Kazeminezhad, The effect of constrained groove pressing on grain size, dislocation density and electrical resistivity of low carbon steel, Mater. Des. 32 (2011) 3280e3286, http://dx.doi.org/10.1016/ j.matdes.2011.02.032. [4] J.H. Kim, S.K. Hwang, Y.-T. Im, I.-H. Son, C.M. Bae, High-strength bolt-forming of fine-grained aluminum alloy 6061 with a continuous hybrid process, Mater. Sci. Eng. A 552 (2012) 316e322, http://dx.doi.org/10.1016/ j.msea.2012.05.046. [5] K. Edalati, Z. Horita, Continuous high-pressure torsion, J. Mater. Sci. 45 (2010) 4578e4582, http://dx.doi.org/10.1007/s10853-010-4381-z. [6] J. Huang, Y.T. Zhu, D.J. Alexander, X. Liao, T.C. Lowe, R.J. Asaro, Development of repetitive corrugation and straightening, Mater. Sci. Eng. A 371 (2004) 35e39, http://dx.doi.org/10.1016/S0921-5093(03)00114-X. [7] Q. Chen, Z. Zhao, D. Shu, Z. Zhao, Microstructure and mechanical properties of AZ91D magnesium alloy prepared by compound extrusion, Mater. Sci. Eng. A 528 (2011) 3930e3934, http://dx.doi.org/10.1016/j.msea.2011.01.028. [8] M. Shahbaz, N. Pardis, R. Ebrahimi, B. Talebanpour, A novel single pass severe plastic deformation technique: vortex extrusion, Mater. Sci. Eng. A 530 (2011) 469e472, http://dx.doi.org/10.1016/j.msea.2011.09.114. [9] F. Djavanroodi, M. Ebrahimi, Effect of die channel angle, friction and back pressure in the equal channel angular pressing using 3D finite element simulation, Mater. Sci. Eng. A 527 (2010) 1230e1235, http://dx.doi.org/ 10.1016/j.msea.2009.09.052. [10] C. Wang, F. Li, Q. Li, J. Li, L. Wang, J. Dong, A novel severe plastic deformation method for fabricating ultrafine grained pure copper, Mater. Des. 43 (2013) 492e498, http://dx.doi.org/10.1016/j.matdes.2012.07.047. [11] M. Ebrahimi, H. Gholipour, F. Djavanroodi, A study on the capability of equal channel forward extrusion process, Mater. Sci. Eng. A 650 (2015) 1e7, http:// dx.doi.org/10.1016/j.msea.2015.10.014. [12] H. Alihosseini, G. Faraji, A.F. Dizaji, K. Dehghani, Characterization of ultra-fine
556
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
M. Shamsborhan, M. Ebrahimi / Journal of Alloys and Compounds 682 (2016) 552e556 grained aluminum produced by accumulative back extrusion (ABE), Mater. Charact. 68 (2012) 14e21, http://dx.doi.org/10.1016/j.matchar.2012.03.004. Q. Chen, D. Shu, C. Hu, Z. Zhao, B. Yuan, Grain refinement in an as-cast AZ61 magnesium alloy processed by multi-axial forging under the multitemperature processing procedure, Mater. Sci. Eng. A 541 (2012) 98e104, http://dx.doi.org/10.1016/j.msea.2012.02.009. W. Guo, Q.D. Wang, B. Ye, M.P. Liu, T. Peng, X.T. Liu, et al., Enhanced microstructure homogeneity and mechanical properties of AZ31 magnesium alloy by repetitive upsetting, Mater. Sci. Eng. A 540 (2012) 115e122, http:// dx.doi.org/10.1016/j.msea.2012.01.111. Y. Beygelzimer, D. Prilepo, R. Kulagin, V. Grishaev, O. Abramova, V. Varyukhin, et al., Planar twist extrusion versus twist extrusion, J. Mater. Process. Technol. 211 (2011) 522e529, http://dx.doi.org/10.1016/j.jmatprotec.2010.11.006. N. Pardis, R. Ebrahimi, Deformation behavior in Simple Shear Extrusion (SSE) as a new severe plastic deformation technique, Mater. Sci. Eng. A 527 (2009) 355e360, http://dx.doi.org/10.1016/j.msea.2009.08.051. G. Faraji, M.M. Mashhadi, H.S. Kim, Tubular channel angular pressing (TCAP) as a novel severe plastic deformation method for cylindrical tubes, Mater. Lett. 65 (2011) 3009e3012, http://dx.doi.org/10.1016/j.matlet.2011.06.039. A. Sajadi, M. Ebrahimi, F. Djavanroodi, Experimental and numerical investigation of Al properties fabricated by CGP process, Mater. Sci. Eng. A 552 (2012) 97e103, http://dx.doi.org/10.1016/j.msea.2012.04.121. M. Shaarbaf, M.R. Toroghinejad, Nano-grained copper strip produced by accumulative roll bonding process, Mater. Sci. Eng. A 473 (2008) 28e33, http://dx.doi.org/10.1016/j.msea.2007.03.065. A. Shokuhfar, M. Shamsborhan, A planar twist channel angular extrusion (PTCAE) as a novel severe plastic deformation method based on equal channel angular extrusion (ECAE) method, J. Mech. Sci. Technol. 28 (2014) 1753e1757, http://dx.doi.org/10.1007/s12206-014-0321-1. M. Shamsborhan, A. Shokuhfar, O. Nejadseyfi, J. Kakemam, M. Moradi, Experimental and numerical comparison of equal channel angular extrusion (ECAE) with planar twist channel angular extrusion (PTCAE), Proc. Inst. Mech. Eng. C J. Mech. Eng. Sci. 229 (2015) 3059e3067, http://dx.doi.org/10.1177/ 0954406214566035. A. Shokuhfar, M. Shamsborhan, Finite element analysis of planar twist channel angular extrusion (PTCAE) as a novel severe plastic deformation method, J. Mech. Sci. Technol. 28 (2014) 1753e1757, http://dx.doi.org/10.1007/ s12206-014-0321-1.
[23] F. Djavanroodi, M. Ebrahimi, Effect of die parameters and material properties in ECAP with parallel channels, Mater. Sci. Eng. A 527 (2010) 7593e7599, http://dx.doi.org/10.1016/j.msea.2010.08.022. [24] E. Bagherpour, M. Reihanian, R. Ebrahimi, Processing twining induced plasticity steel through simple shear extrusion, Mater. Des. 40 (2012) 262e267, http://dx.doi.org/10.1016/j.matdes.2012.03.055. [25] O.V. Mishin, D. Juul Jensen, N. Hansen, Microstructures and boundary populations in materials produced by equal channel angular extrusion, Mater. Sci. Eng. A 342 (2003) 320e328, http://dx.doi.org/10.1016/S0921-5093(02) 00311-8. [26] S. Malopheyev, R. Kaibyshev, Strengthening mechanisms in a Zr-modified 5083 alloy deformed to high strains, Mater. Sci. Eng. A 620 (2015) 246e252, http://dx.doi.org/10.1016/j.msea.2014.10.030. [27] N. Haghdadi, A. Zarei-Hanzaki, D. Abou-Ras, Microstructure and mechanical properties of commercially pure aluminum processed by accumulative back extrusion, Mater. Sci. Eng. A 584 (2013) 73e81, http://dx.doi.org/10.1016/ j.msea.2013.06.060. [28] M. Cabibbo, Microstructure strengthening mechanisms in different equal channel angular pressed aluminum alloys, Mater. Sci. Eng. A 560 (2013) 413e432, http://dx.doi.org/10.1016/j.msea.2012.09.086. [29] Y. Xiong, T. He, Z. Guo, H. He, F. Ren, A.A. Volinsky, Mechanical properties and fracture characteristics of high carbon steel after equal channel angular pressing, Mater. Sci. Eng. A 563 (2013) 163e167, http://dx.doi.org/10.1016/ j.msea.2012.11.068. [30] P. Zhang, S. Qu, M.X. Yang, G. Yang, S.D. Wu, S.X. Li, et al., Varying tensile fracture mechanisms of Cu and Cu-Zn alloys with reduced grain size: from necking to shearing instability, Mater. Sci. Eng. A 594 (2014) 309e320, http:// dx.doi.org/10.1016/j.msea.2013.11.079. [31] C. Kammerhofer, A. Hohenwarter, S. Scheriau, H.P. Brantner, R. Pippan, Influence of morphology and structural size on the fracture behavior of a nanostructured pearlitic steel, Mater. Sci. Eng. A 585 (2013) 190e196, http:// dx.doi.org/10.1016/j.msea.2013.07.032. [32] M. Gasko, G. Rosenberg, Correlation between hardness and tensile properties in ultra-high strength dual phase steelseshort communication, Mater. Eng. 18 (2011) 155e159. http://fstroj.uniza.sk/journal-mi/PDF/2011/27-2011.pdf. [33] P. Zhang, S.X. Li, Z.F. Zhang, General relationship between strength and hardness, Mater. Sci. Eng. A 529 (2011) 62e73, http://dx.doi.org/10.1016/ j.msea.2011.08.061.