- Email: [email protected]
A novel combined severe plastic deformation method for producing thin-walled ultrafine grained cylindrical tubes
Author's Accepted Manuscript
A novel combined severe plastic deformation method for producing thin-walled ultrafine grained cylindrical tubes H. Abdolvnd, H. Sohrabi, G. Faraji, F. Yusof
www.elsevier.com/locate/matlet
PII: DOI: Reference:
S0167-577X(14)02273-3 http://dx.doi.org/10.1016/j.matlet.2014.12.107 MLBLUE18258
To appear in:
Materials Letters
Received date: 28 November 2014 Accepted date: 20 December 2014 Cite this article as: H. Abdolvnd, H. Sohrabi, G. Faraji, F. Yusof, A novel combined severe plastic deformation method for producing thin-walled ultrafine grained cylindrical tubes, Materials Letters, http://dx.doi.org/10.1016/j. matlet.2014.12.107 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 novel combined severe plastic deformation method for producing thin-walled ultrafine grained cylindrical tubes H. Abdolvnda, H. Sohrabia, G. Farajia*, F. Yusofb a b
School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, 11155-4563, Iran.
Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia. Corresponding author, email: [email protected]; Tel. / Fax: +982161114012.
Abstract A novel severe plastic deformation (SPD) process entitled combined parallel tubular channel angular pressing (PTCAP) and tube backward extrusion (TBE) is proposed for producing thin-walled ultrafinegrained (UFG) tubes. In this new combined SPD approach, the PTCAP and TBE processes are consequently applied to the tube material in which a severe plastic strain is applied to produce a UFG thin-walled tube. This technique was performed on an AZ31 magnesium tube, and a remarkable grain refinement was achieved. The results showed that this method could easily produce a high strength thin walled tube. The microhardness increased significantly to 70 HV after the process from an initial value of 38 HV. Keywords: Severe plastic deformation; Thin-walled tube; AZ31; Microstructure; Grain boundaries; Nanocrystalline materials.
1- Introduction Various severe plastic deformation (SPD) techniques have received much attention in recent years due to their efficient in improving properties of metallic materials [1]. In all SPD processes, the intense shear plastic strain is applied to the specimen and results ultrafine-grained (UFG) materials. Due to the HallPetch equation, materials with finer grain sizes exhibits higher yield strength. Equal channel angular pressing (ECAP) [2], accumulative roll bonding (ARB) [3], high pressure torsion (HPT) [4], and cyclic extrusion compression (CEC) [5] are successful SPD methods suitable for deforming bulk materials. Despite the need of high strength tubes for a wide range of industrial application, few SPD methods have been proposed for deforming tubular components. Many studies were done in recent years. Mohebbi and 1
Akbarzadeh [6] developed accumulative spin bonding (ASB) inspired from ARB process to manufacture UFG tubes. Toth et al. [7] proposed a high pressure tube twisting (HPTT) method. This method applies a high hydrostatic stress, but there is a large strain inhomogeneity through the radial direction. Faraji et al. [8, 9] proposed tubular channel angular pressing (TCAP) as an effective method. Recently, they developed parallel tubular channel angular pressing (PTCAP) based on TCAP for producing UFG and nanostructure tubes [10, 11]. Among these processes, the PTCAP has several advantages compared to other methods. It needs lower process load, in addition, there are a superior strain and hardness homogeneity through the thickness and length direction [10]. In all SPD methods for tubular components, there is a limitation that cannot be used for thin-walled tubes. Because, as the thickness of the tube is reduced, the most of the processing load is the friction force and so the friction is the main obstacle, and the SPD process is technically difficult to perform. In order to facilitate the SPD process for thin-walled tubes, the present work introduces a combined tube backward extrusion (TBE) and PTCAP method as a suitable process for producing nanostructured and UFG thinwalled cylindrical tubes. To investigate the applicability of this new combined SPD approach, an AZ31 magnesium tube is processed.
2- Principles of the process This new combined method consists of two stages of PTCAP and TBE processes. First, the PTCAP process is applied to the thick tube and then the TBE process is consequently applied to reduce the thickness. The PTCAP process consists of two half cycles shown schematically in Fig. 1. In the first half cycle, the first punch presses the tube material into the gap between mandrel and die including two shear zones to increase the tube diameter (Fig. 1 (a)). Then the tube is pressed back using the second punch in the second half cycle, decreasing the tube diameter to its initial value (Fig. 1 (b)). In the next step, the TBE process shown schematically in Fig. 1 (c) is applied to the UFG PTCAP processed tube. In this stage, the punch presses the tube to reduce its thickness. The equivalent strain achieved from the N passes of PTCAP stage can be estimated via the following equation [10]:
2 cot (φi / 2 + ψ i / 2) + ψ i cos ec(φi / 2 + ψ i / 2) 3 i =1 2
ε PTCAP = 2 N ∑
R 2 ln 2 + 3 R1
(1)
Where R1 , R2 , ϕ and ψ were shown in Fig. 1 (d). With assuming the uniform deformation, the following equation can be used for the equivalent strain in the the TBE stage of the combined process:
2
ε TBE
A0 R02 − r02 = ln = ln 2 2 A R0 − rf
(2)
Where t1 and t 2 are shown in Fig. 1 (d). The total equivalent strain at the end of the combined process is equal to the sum of the equations (1) and (2):
ε tot
2 2cot (φi / 2 + ψ i / 2) + ψ i cos ec(φi / 2 + ψ i / 2) = 2 N ∑ 3 i =1
R02 − r02 R2 2 ln + ln 2 2 + R0 − rf 3 R1
(3)
The total equivalent accumulative plastic strain after PTCAP (1.6) and TBE (1.2) considering the parameter used in this study is about 2.8.
First Punch Die Die Punch Tube Tube Mandrel
(a)
(b)
Second Punch
(c)
І ІІ
(d)
(e)
Fig. 1 Schematic of the combined process; (a) the first and (b) the second half cycles of PTCAP and (c) TBE and die parameters of (d) PTCAP and (e) TBE stages. 3
3- Experimental procedures The material used in this study was an AZ31 magnesium alloy. Tubular samples of 20 mm in outer diameter, 2.5 mm in thickness and length of 35 mm were prepared. The PTCAP and TBE dies were manufactured from hot-worked tool steel and hardened to 55 HRC. Die parameters for PTCAP and TBE stages were shown in Fig. 1(d) and (e), respectively. Die parameters are as following: the channel angles
ϕ1 = ϕ2 = 150° , the angle of the curvature ψ 1 = ψ 2 = 0° , r0 =15 mm, R0 = 15 mm, rf = 9.25 mm r0 = 15 mm, R0 = 15 mm and rf = 9.25 mm. In TBE process, the thickness of the tube is reduced from the initial value 2.5 mm to 0.75 mm (extrusion ratio is 70%). The PTCAP and TBE processes were performed at the ram speed of 10 mm/min at 250 °C. The MoS2 lubricant was sparyed on the specimens and dies to reduce the friction. All the samples were cut along the axial direction and microstructural and microhardness investigations were done in this cross section at the point near the middle of the thickness. The samples were mechanically polished and then etched for 5 sec. using a solution of 140 ml ethanol, 4.2 gr picric acid, 10 ml acetic acid and 10 ml distilled water. Microstructure evolution was conducted by optical microscopy (OM). The HV microhardness testing was performed using with a load of 100 gr applied for 10 sec.
4- Results and discussion Fig. 2 shows OM micrographs of the microstructure and the grain size distribution of the as-received and processed samples via different routes. The average grain size of as-received tube (Fig. 2 (a)) is ~33 µm, as it can be seen from the histogram the grains are not uniformly distributed. The grain size is varied from 6 µm to 76 µm. After PTCAP process ( ε = 1.6 ) (Fig. 2(b)), the average grain size is reduced to ~14.5 µm, and the microstructure is relatively more uniform. The grain size was affected by accumulated strain. Faraji et al. reported that the grain size of the material could be refined and completely homogeneous by increasing the number of passes [10]. The mean grain sizes were dramatically affected by severe strain in TBE process in which a high extrusion ratio (70%) is applied ( ε = 1.2 ). Consequently the mean grain size is significantly refined to ~ 4.5 µm for TBE processed tube (Fig. 2(c)). After combined process of PTCAP+TBE process ( ε = 2.8 ) the grain size refined to below ~3 µm (Fig. 2(d)). It is obvious that after combined process, the fine grained material was achieved. The HCP structure of magnesium needs elevated forming temperatures to allow better formability. Thus, temperature plays an important role in a variation of the grain size during plastic deformation of magnesium alloys [12, 13]. Tan et al. reported that the optimum temperature to achieve finer grains and homogeneous microstructure is 250 ºC [14] which is identical the current experiment temperature. During plastic deformation, grain refinement is caused by dynamic recrystallization (DRX) at high temperature [12]. As depicted in Fig. 2(c) and (d), by increasing the strain, finer dynamic recrystallized grains develop to cover all the original microstructure. Subgrains are first formed in the adjacency of grain boundaries as deformation progresses during DRX. Then, Subgrain structure form over the whole volume and also subgrain boundary misorientation angle increases and then, low angle grain boundaries transform to high angle countorparts [14]. As depicted in Fig. 3, the microhardness of as-received tube increases from ~38 HV to ~50 HV after just PTCAP 4
process. For directly TBEed of as-received tube, it is ~58 HV. After combine PTCAP+TBE process, it remarkably increases to ~70 HV. Magnesium alloy exhibits a high dependency of hardness to the grain size due to the limited number of slip system [15]. Thus, a superior increase in hardness of the PTCAP+TBEed tube is due to the severe grain refinement. True stress-strain curves of the samples at room temperature were shown in Fig. 4. As shown, after PTCAP process the ductility was slightly increased, and the strength was decreased. This result is in good agreement with the work done on AZ31 alloy in ECAP by Xia et al. [16]. After PTCAP+TBE process, an intense plastic deformation is imposed to the tube and results in a remarkable increase of the strength. As discussed in the previous section, UFG material is achieved after PTCAP+TBE process. Thus, a high dislocation density and a large fraction of fine grain cause a significant increase in yield and ultimate strength. Chen et al. [17] reported that as the extrusion ratio is increased, the mechanical properties of magnesium alloy improved effectively. As depicted, because of grain refinement and more homogeneous structure, the strength for PTCAP+TBE processed sample is higher than the TBE sample directly from asreceived tube.
25
Fraction (%)
20
(a)
As-recieved GS= 33 µm
15 10 5 0
0
5
10
20 Grain 30 Size 40 (µm) 50
60
70
Fraction (%)
15 PTCAP GS = 14.5 µm
10
(b)
5
0
Fraction (%)
0
(c)
4
8
40 35 30 25 20 15 10 5 0
12 16 20 Grain size (µm)
24
28
TBE GS= 4.5 µm
0
2
4
6Grain 8 size 10 (µm) 12 14 16 18
35 Fraction (%)
30
(d)
PTCAP+TBE GS=3 µm
25 20 15 10 5 0 0
2
4
6
8 10 12 14 16 18 Grain size
Fig. 2 OM photographs showing the microstructural change and the grain size distribution of (a) Asreceived, (b) PTCAP, (c) TBE, and (d) PTCAP+TBE processed tube.
90
Microhardness (Hv)
80 70 60 50 40 30 20 10 0 As-received
PTCAP
6
TBE
PTCAP+TBE
Fig. 3 Microhardness values of the samples processed via different routes.
400000
1.2
350000
1
True Stress
250000 200000
101170
150000
0.8
Elongation (%)
Stress (Mpa)
300000
0.6 0.4
100000 0.2
50000 0
0 0
50
100
True Strain
150
0
200
(a)
(b)
Fig. 4 (a) True stress versus true strain curves at room temperature for different samples. (b) UTS, YS and elongation of different samples.
5- Conclusions In this study, a combined SPD process is introduced suitable for producing thin-walled UFG tubes. The effect of this process on grain refinement, mechanical properties and microhardness of AZ31 was successfully investigated, and following conclusions could be made: •
A fine grained thin-walled tube with the thickness of 0.75 mm was produced.
•
A formula is proposed for a total equivalent strain, and it is 2.8 at the end of PTCAP + TBE process.
•
A remarkable grain refinement could achieve from ~33 µm for as-received tube to below ~3 µm after the combined process.
•
The Vickers microhardness was significantly increased from the initial value 38 HV to 70 HV due to the grain refinement after the combined process.
•
Yield and ultimate strengths were remarkably increased.
Acknowledgements 7
The authors would like to acknowledge the University of Malaya for providing the necessary facilities and resources for this research. This research was fully funded by the Ministry of Higher Education, Malaysia with the high impact research (HIR) grant number of HIR-MOHE-16001-00-D000001. This work was financially supported by Iran National Science Foundation (INSF). References [1] R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Progress in Materials Science, 45 (2000) 103-189. [2] R.Z. Valiev, T.G. Langdon, Progress in Materials Science, 51 (2006) 881-981. [3] Y. Saito, H. Utsunomiya, N. Tsuji, T. Sakai, Acta Materialia, 47 (1999) 579-583. [4] A.P. Zhilyaev, T.G. Langdon, Progress in Materials Science, 53 (2008) 893-979. [5] Y.J. Chen, Q.D. Wang, H.J. Roven, M. Karlsen, Y.D. Yu, M.P. Liu, J. Hjelen, Journal of Alloys and Compounds, 462 (2008) 192-200. [6] M.S. Mohebbi, A. Akbarzadeh, - 528 (2010) - 188. [7] L.S. Tóth, M. Arzaghi, J.J. Fundenberger, B. Beausir, O. Bouaziz, R. Arruffat-Massion, Scripta Materialia, 60 (2009) 175-177. [8] G. Faraji, M.M. Mashhadi, H.S. Kim, Materials Letters, 65 (2011) 3009-3012. [9] G. Faraji, M.M. Mashhadi, H.S. Kim, Materials Transactions 53 (01), 8-12. [10] G. Faraji, A. Babaei, M.M. Mashhadi, K. Abrinia, Materials Letters, 77 (2012) 82-85. [11] G. Faraji, M.M. Mashhadi, A.R. Bushroa, A. Babaei, Materials Science and Engineering: A, 563 (2013) 193-198. [12] S.M. Fatemi-Varzaneh, A. Zarei-Hanzaki, H. Beladi, Materials Science and Engineering: A, 456 (2007) 52-57. [13] X.Y. J.xing , H.Miura, T.Sakai, Materials Transactions, 49 (2008) 69-75. [14] J.C. Tan, M.J. Tan, Materials Science and Engineering: A, 339 (2003) 124-132. [15] S. Kleiner, O. Beffort, P.J. Uggowitzer, Scripta Materialia, 51 (2004) 405-410. [16] K. Xia, J.T. Wang, X. Wu, G. Chen, M. Gurvan, Materials Science and Engineering: A, 410–411 (2005) 324-327. [17] Y. Chen, Q. Wang, J. Peng, C. Zhai, W. Ding, Journal of Materials Processing Technology, 182 (2007) 281-285.
8
Highlights
•
A novel combined SPD process was developed.
•
An ultrafine grained thin-walled tube with the thickness of 0.75 mm was produced.
•
A remarkable grain refinement could achieve from ~33 µm for as-received tube to below ~3 µm after the combined process.
•
The Vickers microhardness was significantly increased from the 38 HV to 70 HV after the combined process.
•
Yield and ultimate strengths were remarkably increased.
9
A novel combined severe plastic deformation method for producing thin-walled ultrafine grained cylindrical tubes H. Abdolvnd, H. Sohrabi, G. Faraji, F. Yusof
www.elsevier.com/locate/matlet
PII: DOI: Reference:
S0167-577X(14)02273-3 http://dx.doi.org/10.1016/j.matlet.2014.12.107 MLBLUE18258
To appear in:
Materials Letters
Received date: 28 November 2014 Accepted date: 20 December 2014 Cite this article as: H. Abdolvnd, H. Sohrabi, G. Faraji, F. Yusof, A novel combined severe plastic deformation method for producing thin-walled ultrafine grained cylindrical tubes, Materials Letters, http://dx.doi.org/10.1016/j. matlet.2014.12.107 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 novel combined severe plastic deformation method for producing thin-walled ultrafine grained cylindrical tubes H. Abdolvnda, H. Sohrabia, G. Farajia*, F. Yusofb a b
School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, 11155-4563, Iran.
Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia. Corresponding author, email: [email protected]; Tel. / Fax: +982161114012.
Abstract A novel severe plastic deformation (SPD) process entitled combined parallel tubular channel angular pressing (PTCAP) and tube backward extrusion (TBE) is proposed for producing thin-walled ultrafinegrained (UFG) tubes. In this new combined SPD approach, the PTCAP and TBE processes are consequently applied to the tube material in which a severe plastic strain is applied to produce a UFG thin-walled tube. This technique was performed on an AZ31 magnesium tube, and a remarkable grain refinement was achieved. The results showed that this method could easily produce a high strength thin walled tube. The microhardness increased significantly to 70 HV after the process from an initial value of 38 HV. Keywords: Severe plastic deformation; Thin-walled tube; AZ31; Microstructure; Grain boundaries; Nanocrystalline materials.
1- Introduction Various severe plastic deformation (SPD) techniques have received much attention in recent years due to their efficient in improving properties of metallic materials [1]. In all SPD processes, the intense shear plastic strain is applied to the specimen and results ultrafine-grained (UFG) materials. Due to the HallPetch equation, materials with finer grain sizes exhibits higher yield strength. Equal channel angular pressing (ECAP) [2], accumulative roll bonding (ARB) [3], high pressure torsion (HPT) [4], and cyclic extrusion compression (CEC) [5] are successful SPD methods suitable for deforming bulk materials. Despite the need of high strength tubes for a wide range of industrial application, few SPD methods have been proposed for deforming tubular components. Many studies were done in recent years. Mohebbi and 1
Akbarzadeh [6] developed accumulative spin bonding (ASB) inspired from ARB process to manufacture UFG tubes. Toth et al. [7] proposed a high pressure tube twisting (HPTT) method. This method applies a high hydrostatic stress, but there is a large strain inhomogeneity through the radial direction. Faraji et al. [8, 9] proposed tubular channel angular pressing (TCAP) as an effective method. Recently, they developed parallel tubular channel angular pressing (PTCAP) based on TCAP for producing UFG and nanostructure tubes [10, 11]. Among these processes, the PTCAP has several advantages compared to other methods. It needs lower process load, in addition, there are a superior strain and hardness homogeneity through the thickness and length direction [10]. In all SPD methods for tubular components, there is a limitation that cannot be used for thin-walled tubes. Because, as the thickness of the tube is reduced, the most of the processing load is the friction force and so the friction is the main obstacle, and the SPD process is technically difficult to perform. In order to facilitate the SPD process for thin-walled tubes, the present work introduces a combined tube backward extrusion (TBE) and PTCAP method as a suitable process for producing nanostructured and UFG thinwalled cylindrical tubes. To investigate the applicability of this new combined SPD approach, an AZ31 magnesium tube is processed.
2- Principles of the process This new combined method consists of two stages of PTCAP and TBE processes. First, the PTCAP process is applied to the thick tube and then the TBE process is consequently applied to reduce the thickness. The PTCAP process consists of two half cycles shown schematically in Fig. 1. In the first half cycle, the first punch presses the tube material into the gap between mandrel and die including two shear zones to increase the tube diameter (Fig. 1 (a)). Then the tube is pressed back using the second punch in the second half cycle, decreasing the tube diameter to its initial value (Fig. 1 (b)). In the next step, the TBE process shown schematically in Fig. 1 (c) is applied to the UFG PTCAP processed tube. In this stage, the punch presses the tube to reduce its thickness. The equivalent strain achieved from the N passes of PTCAP stage can be estimated via the following equation [10]:
2 cot (φi / 2 + ψ i / 2) + ψ i cos ec(φi / 2 + ψ i / 2) 3 i =1 2
ε PTCAP = 2 N ∑
R 2 ln 2 + 3 R1
(1)
Where R1 , R2 , ϕ and ψ were shown in Fig. 1 (d). With assuming the uniform deformation, the following equation can be used for the equivalent strain in the the TBE stage of the combined process:
2
ε TBE
A0 R02 − r02 = ln = ln 2 2 A R0 − rf
(2)
Where t1 and t 2 are shown in Fig. 1 (d). The total equivalent strain at the end of the combined process is equal to the sum of the equations (1) and (2):
ε tot
2 2cot (φi / 2 + ψ i / 2) + ψ i cos ec(φi / 2 + ψ i / 2) = 2 N ∑ 3 i =1
R02 − r02 R2 2 ln + ln 2 2 + R0 − rf 3 R1
(3)
The total equivalent accumulative plastic strain after PTCAP (1.6) and TBE (1.2) considering the parameter used in this study is about 2.8.
First Punch Die Die Punch Tube Tube Mandrel
(a)
(b)
Second Punch
(c)
І ІІ
(d)
(e)
Fig. 1 Schematic of the combined process; (a) the first and (b) the second half cycles of PTCAP and (c) TBE and die parameters of (d) PTCAP and (e) TBE stages. 3
3- Experimental procedures The material used in this study was an AZ31 magnesium alloy. Tubular samples of 20 mm in outer diameter, 2.5 mm in thickness and length of 35 mm were prepared. The PTCAP and TBE dies were manufactured from hot-worked tool steel and hardened to 55 HRC. Die parameters for PTCAP and TBE stages were shown in Fig. 1(d) and (e), respectively. Die parameters are as following: the channel angles
ϕ1 = ϕ2 = 150° , the angle of the curvature ψ 1 = ψ 2 = 0° , r0 =15 mm, R0 = 15 mm, rf = 9.25 mm r0 = 15 mm, R0 = 15 mm and rf = 9.25 mm. In TBE process, the thickness of the tube is reduced from the initial value 2.5 mm to 0.75 mm (extrusion ratio is 70%). The PTCAP and TBE processes were performed at the ram speed of 10 mm/min at 250 °C. The MoS2 lubricant was sparyed on the specimens and dies to reduce the friction. All the samples were cut along the axial direction and microstructural and microhardness investigations were done in this cross section at the point near the middle of the thickness. The samples were mechanically polished and then etched for 5 sec. using a solution of 140 ml ethanol, 4.2 gr picric acid, 10 ml acetic acid and 10 ml distilled water. Microstructure evolution was conducted by optical microscopy (OM). The HV microhardness testing was performed using with a load of 100 gr applied for 10 sec.
4- Results and discussion Fig. 2 shows OM micrographs of the microstructure and the grain size distribution of the as-received and processed samples via different routes. The average grain size of as-received tube (Fig. 2 (a)) is ~33 µm, as it can be seen from the histogram the grains are not uniformly distributed. The grain size is varied from 6 µm to 76 µm. After PTCAP process ( ε = 1.6 ) (Fig. 2(b)), the average grain size is reduced to ~14.5 µm, and the microstructure is relatively more uniform. The grain size was affected by accumulated strain. Faraji et al. reported that the grain size of the material could be refined and completely homogeneous by increasing the number of passes [10]. The mean grain sizes were dramatically affected by severe strain in TBE process in which a high extrusion ratio (70%) is applied ( ε = 1.2 ). Consequently the mean grain size is significantly refined to ~ 4.5 µm for TBE processed tube (Fig. 2(c)). After combined process of PTCAP+TBE process ( ε = 2.8 ) the grain size refined to below ~3 µm (Fig. 2(d)). It is obvious that after combined process, the fine grained material was achieved. The HCP structure of magnesium needs elevated forming temperatures to allow better formability. Thus, temperature plays an important role in a variation of the grain size during plastic deformation of magnesium alloys [12, 13]. Tan et al. reported that the optimum temperature to achieve finer grains and homogeneous microstructure is 250 ºC [14] which is identical the current experiment temperature. During plastic deformation, grain refinement is caused by dynamic recrystallization (DRX) at high temperature [12]. As depicted in Fig. 2(c) and (d), by increasing the strain, finer dynamic recrystallized grains develop to cover all the original microstructure. Subgrains are first formed in the adjacency of grain boundaries as deformation progresses during DRX. Then, Subgrain structure form over the whole volume and also subgrain boundary misorientation angle increases and then, low angle grain boundaries transform to high angle countorparts [14]. As depicted in Fig. 3, the microhardness of as-received tube increases from ~38 HV to ~50 HV after just PTCAP 4
process. For directly TBEed of as-received tube, it is ~58 HV. After combine PTCAP+TBE process, it remarkably increases to ~70 HV. Magnesium alloy exhibits a high dependency of hardness to the grain size due to the limited number of slip system [15]. Thus, a superior increase in hardness of the PTCAP+TBEed tube is due to the severe grain refinement. True stress-strain curves of the samples at room temperature were shown in Fig. 4. As shown, after PTCAP process the ductility was slightly increased, and the strength was decreased. This result is in good agreement with the work done on AZ31 alloy in ECAP by Xia et al. [16]. After PTCAP+TBE process, an intense plastic deformation is imposed to the tube and results in a remarkable increase of the strength. As discussed in the previous section, UFG material is achieved after PTCAP+TBE process. Thus, a high dislocation density and a large fraction of fine grain cause a significant increase in yield and ultimate strength. Chen et al. [17] reported that as the extrusion ratio is increased, the mechanical properties of magnesium alloy improved effectively. As depicted, because of grain refinement and more homogeneous structure, the strength for PTCAP+TBE processed sample is higher than the TBE sample directly from asreceived tube.
25
Fraction (%)
20
(a)
As-recieved GS= 33 µm
15 10 5 0
0
5
10
20 Grain 30 Size 40 (µm) 50
60
70
Fraction (%)
15 PTCAP GS = 14.5 µm
10
(b)
5
0
Fraction (%)
0
(c)
4
8
40 35 30 25 20 15 10 5 0
12 16 20 Grain size (µm)
24
28
TBE GS= 4.5 µm
0
2
4
6Grain 8 size 10 (µm) 12 14 16 18
35 Fraction (%)
30
(d)
PTCAP+TBE GS=3 µm
25 20 15 10 5 0 0
2
4
6
8 10 12 14 16 18 Grain size
Fig. 2 OM photographs showing the microstructural change and the grain size distribution of (a) Asreceived, (b) PTCAP, (c) TBE, and (d) PTCAP+TBE processed tube.
90
Microhardness (Hv)
80 70 60 50 40 30 20 10 0 As-received
PTCAP
6
TBE
PTCAP+TBE
Fig. 3 Microhardness values of the samples processed via different routes.
400000
1.2
350000
1
True Stress
250000 200000
101170
150000
0.8
Elongation (%)
Stress (Mpa)
300000
0.6 0.4
100000 0.2
50000 0
0 0
50
100
True Strain
150
0
200
(a)
(b)
Fig. 4 (a) True stress versus true strain curves at room temperature for different samples. (b) UTS, YS and elongation of different samples.
5- Conclusions In this study, a combined SPD process is introduced suitable for producing thin-walled UFG tubes. The effect of this process on grain refinement, mechanical properties and microhardness of AZ31 was successfully investigated, and following conclusions could be made: •
A fine grained thin-walled tube with the thickness of 0.75 mm was produced.
•
A formula is proposed for a total equivalent strain, and it is 2.8 at the end of PTCAP + TBE process.
•
A remarkable grain refinement could achieve from ~33 µm for as-received tube to below ~3 µm after the combined process.
•
The Vickers microhardness was significantly increased from the initial value 38 HV to 70 HV due to the grain refinement after the combined process.
•
Yield and ultimate strengths were remarkably increased.
Acknowledgements 7
The authors would like to acknowledge the University of Malaya for providing the necessary facilities and resources for this research. This research was fully funded by the Ministry of Higher Education, Malaysia with the high impact research (HIR) grant number of HIR-MOHE-16001-00-D000001. This work was financially supported by Iran National Science Foundation (INSF). References [1] R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Progress in Materials Science, 45 (2000) 103-189. [2] R.Z. Valiev, T.G. Langdon, Progress in Materials Science, 51 (2006) 881-981. [3] Y. Saito, H. Utsunomiya, N. Tsuji, T. Sakai, Acta Materialia, 47 (1999) 579-583. [4] A.P. Zhilyaev, T.G. Langdon, Progress in Materials Science, 53 (2008) 893-979. [5] Y.J. Chen, Q.D. Wang, H.J. Roven, M. Karlsen, Y.D. Yu, M.P. Liu, J. Hjelen, Journal of Alloys and Compounds, 462 (2008) 192-200. [6] M.S. Mohebbi, A. Akbarzadeh, - 528 (2010) - 188. [7] L.S. Tóth, M. Arzaghi, J.J. Fundenberger, B. Beausir, O. Bouaziz, R. Arruffat-Massion, Scripta Materialia, 60 (2009) 175-177. [8] G. Faraji, M.M. Mashhadi, H.S. Kim, Materials Letters, 65 (2011) 3009-3012. [9] G. Faraji, M.M. Mashhadi, H.S. Kim, Materials Transactions 53 (01), 8-12. [10] G. Faraji, A. Babaei, M.M. Mashhadi, K. Abrinia, Materials Letters, 77 (2012) 82-85. [11] G. Faraji, M.M. Mashhadi, A.R. Bushroa, A. Babaei, Materials Science and Engineering: A, 563 (2013) 193-198. [12] S.M. Fatemi-Varzaneh, A. Zarei-Hanzaki, H. Beladi, Materials Science and Engineering: A, 456 (2007) 52-57. [13] X.Y. J.xing , H.Miura, T.Sakai, Materials Transactions, 49 (2008) 69-75. [14] J.C. Tan, M.J. Tan, Materials Science and Engineering: A, 339 (2003) 124-132. [15] S. Kleiner, O. Beffort, P.J. Uggowitzer, Scripta Materialia, 51 (2004) 405-410. [16] K. Xia, J.T. Wang, X. Wu, G. Chen, M. Gurvan, Materials Science and Engineering: A, 410–411 (2005) 324-327. [17] Y. Chen, Q. Wang, J. Peng, C. Zhai, W. Ding, Journal of Materials Processing Technology, 182 (2007) 281-285.
8
Highlights
•
A novel combined SPD process was developed.
•
An ultrafine grained thin-walled tube with the thickness of 0.75 mm was produced.
•
A remarkable grain refinement could achieve from ~33 µm for as-received tube to below ~3 µm after the combined process.
•
The Vickers microhardness was significantly increased from the 38 HV to 70 HV after the combined process.
•
Yield and ultimate strengths were remarkably increased.
9