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Equal channel angular pressing with converging billets—Experiment
Materials Science & Engineering A 560 (2013) 358–364
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Equal channel angular pressing with converging billets—Experiment A. Rosochowski a,n, L. Olejnik b, J. Richert c, M. Rosochowska d, M. Richert c a
Design, Manufacture and Engineering Management, University of Strathclyde, Glasgow, UK Institute of Manufacturing Processes, Warsaw University of Technology, Warsaw, Poland Faculty of Non-Ferrous Metals, AGH University of Science and Technology, Krakow, Poland d Advanced Forming Research Centre, University of Strathclyde, Glasgow, UK b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 August 2012 Received in revised form 19 September 2012 Accepted 21 September 2012 Available online 27 September 2012
A new concept of equal channel angular pressing (ECAP) with converging billets is proposed and tested experimentally. In its basic configuration, the new ECAP process uses two equal square input channels converging into a single output channel, which is twice as wide as the input channels so that it can accept two converging billets. The contact surface between converging billets plays the same role as a movable die wall in the output channel of classical ECAP and thus reduces friction and the process force. The process productivity is doubled and material pickup, especially problematic in the output channel, avoided. This paper presents results of experimental trials of the new process using purposely designed tooling incorporated in a horizontal press with three hydraulic cylinders. One pass of ECAP with converging Al 1070 billets has been carried out and the resulting hardness distribution and microstructure examined. It is concluded that the new process is a feasible version of ECAP both in the engineering and the micro-structural terms, with the added benefit of doubled productivity as well as friction and force reduction. & 2012 Elsevier B.V. All rights reserved.
Keywords: Severe plastic deformation Equal channel angular pressing Ultrafine grained structure
1. Introduction Bulk metals with ultrafine grained (UFG) structure, characterised by the average grain size o1 mm, draw substantial attention due to their unique mechanical and physical properties such as improved strength described by the Hall–Petch relationship [1], enhanced superplastic forming [2], better biocompatibility [3] or particular suitability for microforming [4]. The preferable method of producing bulk UFG metals, which avoids the health hazards associated with nanopowders, is severe plastic deformation (SPD) [5]. In this method, a very large plastic deformation (true strain 3–10 depending on the metal) ‘‘subdivides’’ coarse metal grains into sub-micrometre size grains. Unlike traditional metal forming processes, SPD processes retain the shape of the workpiece. Equal channel angular pressing (ECAP), originally proposed by Segal et al. under the name of equal channel angular extrusion [6], is the most popular SPD process used to produce UFG metals. In this process (Fig. 1), a square or cylindrical billet is pushed by a punch through an input constant profile channel to an output channel of the same profile orientated at an angle Z901 to the previous one. Plastic deformation of the material is caused by simple shear in a thin layer along the diagonal plane at the
n
Corresponding author. Tel.: þ44 141 548 4353. E-mail address: [email protected] (A. Rosochowski).
0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.09.079
channel crossing. The process is usually repeated several times with the billet being rotated about its axis between consecutive passes. The process is simple in terms of tooling and machines used, however, it suffers from friction present in the die channels, which increases the process force and tool contact pressure, limits the length of billets processed and causes material pickup. Good illustration of this last problem is a die segment used in threeturn 3D-ECAP [7], which shows a layer of aluminium sticking to the bottom surface of die channel (Fig. 2). To reduce friction, a concept of movable die walls both in the input and the output channel of the die was proposed [8]. Less complicated seems to be a movable wall representing the bottom part of the output channel; it was realised in practice in industrial ECAP of large plates for sputtering targets [9]. But even in this case the introduction of a movable die wall leads to a more complex and expensive ECAP device. Even if friction between a billet and the bottom part of the output channel is eliminated, it does not disappear completely; it is just moved to a new location between the stationary and movable device elements. Thus friction reduction in the output channel still remains a challenge. A new solution to the problem of friction in the output channel of an ECAP die has been proposed recently. It is based on the idea of ECAP with converging billets, which was explained and modelled using finite element (FE) simulation [10]. The current paper focuses on checking the engineering feasibility of this idea by carrying out a laboratory experiment and investigating changes in properties and microstructure of the processed billets.
A. Rosochowski et al. / Materials Science & Engineering A 560 (2013) 358–364
Punch
359
ECAP discussed above. It reduces friction and the process force. This effect is achieved without using a complex die with movable parts. Instead, two punches are used to push two billets synchronously from the opposite sides. The system doubles productivity compared to the case of processing a single billet. Fig. 4 shows comparison of classical ECAP of a single billet (without moving die walls) and ECAP with two converging billets in terms of strain distribution [10]. Friction coefficient in both processes is the same and equal m ¼0.1. Strain appears to be similar, except the bottom part of the billet, where it is smaller for ECAP with converging billets. This is related to the fact that, in the absence of friction on the bottom part of the billet, the die corner at the channel intersection is filled less; the same effect would be observed in ECAP with a movable bottom wall. A remedy might be back pressure, which improves filling of the die and makes strain distribution more homogeneous [10]. ECAP with two converging billets reduces friction in the output channel, which leads to a lower value of the maximum process force [10]. For friction coefficient m ¼ 0.1, the force reduction predicted by plane strain FE simulation was 23% (Fig. 5).
Die
Billet
Fig. 1. Process configuration for classical ECAP.
3. Machine
Fig. 2. Material pickup on the bottom surface of a 3D-ECAP channel.
Die
Punch
Punch
Billets
Fig. 3. Process configuration for ECAP with two converging billets.
2. Concept of ECAP with converging billets In its basic configuration (Fig. 3), the new ECAP system uses two substantially equal square or rectangular input channels converging into a single output channel, which is twice as wide as the input channels so that it can accept two converging billets. The contact surface between converging billets plays the same role as a movable bottom wall in the output channel of classical
The machine chosen for experimental trials (Fig. 6) was originally used at AGH University of Science and Technology, Krakow, Poland [11] for cyclic extrusion compression (CEC), which is another SPD process. The CEC machine was equipped with two hydraulic double action horizontal actuators opposing each other. Their work cycle involved fixing the die in the horizontal direction with external actuators and then using internal actuators to move the extrusion punch at a controlled speed while the compression punch provided only back pressure. Using the CEC machine for ECAP with converging billets also involved fixing the die in the horizontal direction but required a different type of punch control to allow the punches to move in the opposite directions and at the same speed. Another feature of the original CEC machine was a horizontally split die, whose halves were kept together during the process by a vertical hydraulic actuator. On process completion, the actuator lifted the upper half of the die to enable billet removal. In ECAP with converging billets, the billets could leave the die using a lateral hole opposite the vertical actuator so the die did not have to be split in the horizontal plane. Thus the clamping action of the vertical actuator was used only to keep the die in the right position in the vertical direction.
4. Tooling The die set shown in Fig. 7 was designed to fulfil the functional and geometrical requirements of the machine and to provide adequate performance as an ECAP die. The former was realised by incorporating intermediate tool elements between the die with punches and the hydraulic actuators. These were the pushers driven by the internal actuators whose role was to push punches and limit their stroke and the guides pressed by the external actuators whose role was to guide pushers and transmit a horizontal fixing force. The latter was realised by using a segmented die insert prestressed with outer rings. There were five die segments in total. Two short ones on one side of the die, which created a gap for the billet exit, and one long on the opposite side of the die, which featured a profiled spike to facilitate material flow from the horizontal channel into the die exit. The remaining two segments served as the side walls for the channel defined by the above three segments. The outer rings were forced on the
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Fig. 4. Distribution of equivalent plastic strain in single-billet ECAP (a) and ECAP with two converging billets (b) [10].
assembly of five segments to create an interference fit. The centring of die set on the machine was provided by properly shaped blocks, guided by four pillars (Fig. 6). The lower of these blocks was fixed to the machine bed while the upper one was attached to the vertical actuator. The lower block had a hole, going further down through the machine bed, to extract the processed billets.
30
Force, kN
One billet
20
Converging billets
5. Experiment
10
0 0
0.01
0.02
0.03
0.04
Stroke, m Fig. 5. FE simulation results for punch force vs. punch stroke in single-billet ECAP and in ECAP with two converging billets; friction coefficient m ¼ 0.1 [10].
Die set
Fig. 6. CEC machine used for ECAP with two converging billets (arrows indicate position and flow of material).
The 8 8 46 Al 1070 billets were machined and treated chemically to produce a coating of calcium aluminate (Fig. 8). This served as conversion coating for the MoS2 lubricant, which was applied to the billets just before they were placed in the die channel. Then the die insert was clamped in the horizontal direction using the outer horizontal actuators and clamped in the vertical direction using the vertical actuator. This was followed by the synchronised movement of the inner horizontal actuators which, through the pushers and the punches, caused material flow from both sides of the horizontal channel to the die exit in the middle of this channel. After reaching the limit position, all actuators were withdrawn, which enabled a new pair of billets to be inserted and the whole cycle repeated. This caused the previous billets to be pushed out of the die. Some of the actuator forces as well as punch displacements were recorded during the process. The clamping forces in the horizontal direction were approximately 130 kN while the forming forces were only 8.5 kN. The velocity of both punches was 3.5 mm/min and was kept relatively constant during the process (Fig. 9). This resulted in a fairly symmetrical flow of the material, which produced similar billets in each processed pair (Fig. 8). Since the purpose of this experiment was to prove the concept, only one pass of ECAP with converging billets was performed. However, for further passes, details of the channel geometry, e.g. the length of the output channel, would have to be changed to avoid billet bending visible in Fig. 8.
A. Rosochowski et al. / Materials Science & Engineering A 560 (2013) 358–364
361
Prestressing rings
Guide Pusher Punch Segments of die insert Fig. 7. Die set for ECAP with two converging billets.
RP
Stroke, mm
LP
Time, s
Fig. 9. Displacement of the left punch, LP, and the right punch, RP, during ECAP with converging billets.
Fig. 8. Aluminium billets before and after ECAP with converging billets.
6. Properties In order to improve mechanical properties of metals processed by ECAP, several passes of ECAP are usually required to achieve a minimum grain size and high values of grain boundary misorientation angles. On the other hand it is known that one or two passes of ECAP already have a substantial effect on both the properties and the microstructure of the material. To examine the change in material properties caused by one pass of ECAP with converging billets, one of the billets was polished on its side and microhardness measurements were carried out using the Vickers indenter loaded with 0.1 kg (Fig. 10). The microhardness results along a longitudinal path ‘‘a’’ in Fig. 10 confirm the existence of so-called end effects in the ECAPed billets as well as a nonuniform distribution of strain in the transverse direction along paths ‘‘b’’, ‘‘c’’ and ‘‘d’’. This is what FE simulation results presented in Fig. 4b suggested except the top surface of the billet in Fig. 10, where hardness is smaller than in the centre of the billet, contrary to FE strain distribution. Generally the level of hardness was increased from approximately HV0.1 ¼32 for the initial Al 1070 to HV0.1¼52 after one pass of ECAP with converging billets.
developing in the early stages of classical ECAP. The observations were made in three planes, X, perpendicular to billet axis, Y, parallel to the billet side and, Z, perpendicular to X and Y. All images have been obtained on a Hitachi ultra-high resolution FESEM, SU-70, microscope equipped with a STEM detector. Thin foils have been produced by mechanical polishing down to 0.6 mm and then electro-polishing down to 0.1 mm to obtain perforation. Fig. 11 shows the grain structure in the X plane. Since the initial grain size of Al 1070 was approximately 200 mm, it is clear that there has been a significant reduction in the average grain/ subgrain size to about 2 mm after first pass of ECAP with converging billets. It is interesting that some boundaries of larger cells are very thin and well defined while smaller cells, which develop within some of those larger ones, are thicker and blurred. High resolution images presented in Figs. 12 and 13 enabled identification of those inner boundaries as originating from dense dislocation tangles. Grains in the Y plane, which are shown in Fig. 14, seem to be smaller and elongated. This may result from shear bands crossing this plane as illustrated in Fig. 15. Fig. 16 shows this in detail suggesting that the width of these bands is 0.1– 0.2 mm and that the adjacent strings of elongated grains are 0.5– 1 mm wide. Figs. 17–19 present one area in the Z plane using the increasing magnification. The microstructure in this plane looks like a combination of the equiaxed grain structure in the X plane and more directional and elongated grain structure in the Y plane. Some grains are relatively free of dislocations and having well defined and thin boundaries while others feature polygonal dislocation loops, both indicating a possibility of recovery at room temperature.
7. Microstructure Increased hardness can be caused by the increased average dislocation density but equally it can be attributed to reduction in grain size. Despite performing only one pass of ECAP with converging billets it was decided to check the material microstructure in order to confirm its similarity to the microstructure
8. Conclusions Taking into account many different aspects addressed in this paper, a relatively complete picture of the new process of ECAP
A. Rosochowski et al. / Materials Science & Engineering A 560 (2013) 358–364
Vickers hardness HV 0.1
362
Distance, mm
8
8
8
0 b
0 c
0 d
a
Vickers hardness HV 0.1
d
Distance, mm Fig. 10. Results of microhardness measurements performed on a side of Al 1070 billet subjected to one pass of ECAP with converging billets along a longitudinal path ‘‘a’’ and three transverse paths ‘‘b’’, ‘‘c’’ and ‘‘d’’.
Fig. 11. Grain structure in X plane (magnification 10K).
Fig. 12. Grain structure in X plane (magnification 20K) showing the central region from Fig. 11.
with converging billets can be presented. The following statements summarise the main results obtained:
FE simulation results of classical ECAP and ECAP with conver-
ECAP with converging billets is a new original process, which reduces friction between die and billet by introducing another symmetrical billet processed at the same time in a way, which makes both billets converge in the output channel of the die. This leads to forming force reduction and avoidance of material pickup on the die surface.
ging billets show that strain distribution in both processes is similar except slightly lower strain generated in the contact area of both billets. Adaptation of CEC machine for ECAP with converging billets required changing the punch movement only. State of the art tool system, with a prestressed segmented die, was designed and manufactured to realise the new process on the CEC machine.
A. Rosochowski et al. / Materials Science & Engineering A 560 (2013) 358–364
363
Fig. 13. Grain structure in X plane (magnification 50K) showing a detail from Fig. 12.
Fig. 16. Grain structure in Y plane (magnification 50K) showing one of the shear bands from Fig. 15.
Fig. 14. Grain structure in Y plane (magnification 10K).
Fig. 17. Grain structure in Z plane (magnification 20K).
Fig. 15. Grain structure in Y plane (magnification 20K) showing the left bottom corner from Fig. 14 with possible shear bands.
Fig. 18. Grain structure in Z plane (magnification 50K) showing the central area from Fig. 17.
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with converging billets is similar to classical ECAP with the added advantage of doubling productivity, reducing friction and material pickup and lowering the process force. It can be considered as a legitimate alternative to other SPD processes used for grain refinement in metals. Since material handling and utilisation are very important factors in industrial applications, a new version of ECAP with converging billets will be proposed to enable processing of long billets.
Acknowledgements Part of this research was supported by The Engineering and Physical Sciences Research Council [Grant no. EP/G03477X/1].
References Fig. 19. Grain structure in Z plane (magnification 100K) showing the central area from Fig. 18.
Successful operation of the machine and the tooling enabled,
for the first time, performing one pass of ECAP with two converging Al 1070 billets. Microhardness measurements on the billet side revealed a substantial increase in hardness whose distribution corresponded well with the predicted strain field. After one pass of ECAP with converging billets, grains/subgrains became much smaller, e.g. 1–2 mm, however, details of the new microstructure depended on the orientation of the plane examined. The observed microstructures confirmed the main mechanisms of grain refinement to be formation of dislocation cells and shear banding. There were signs of recovery for Al 1070 processed at room temperature such as thin, well defined grain boundaries, within which no dislocations existed or polygonal dislocation loops were observed. Generally, taking into account FE simulation results, microhardness distribution and the microstructure produced, ECAP
[1] D.H. Shin, K.-T. Park, in: M. Zehetbauer, R.Z. Valiev (Eds.), Proceedings of the Conference on Nanomaterials by Severe Plastic Deformation—NANOSPD2, Vienna, Austria, 9–13 December 2002, Wiley-VCH, pp. 616–622. [2] Y.G. Ko, C.S. Lee, D.H. Sheen, S.L. Semiatin, Metall. Mater. Trans. 37A (2006) 381–391. [3] R.Z. Valiev, I.P. Semenova, E. Jakushina, V.V. Latysh, H. Rack, T.C. Lowe, J. Petruzˇelka, L. Dluhoˇs, D. Hruˇsa´k, J. Sochova´, Mater. Sci. Forum 584–586 (2008) 49–54. [4] W. Presz, A. Rosochowski, in: N. Juster, A. Rosochowski (Eds.), Proceedings of the 9th International Conference on Material Forming, ESAFORM 2006, Glasgow, United Kingdom, April 26–28, 2006, Publishing House Akapit, Krakow, pp. 587–590. [5] A. Azushima, R. Kopp, A. Korhonen, D.Y. Yang, F. Micari, G.D. Lahoti, P. Groche, J. Yanagimoto, N. Tsuji, A. Rosochowski, A. Yanagida, CIRP Ann.—Manuf. Technol. 57 (2008) 716–735. [6] V.M. Segal, V.A. Reznikov, A.E. Drobyshevskiy, V.I. Kopylov, Russ. Metall. 1 (1981) 99–105. [7] A. Rosochowski, L. Olejnik, M. Richert, in: D. Banabic (Ed.), Advanced Methods in Material Forming, Springer, Berlin, Heidelberg, New York, 2007, pp. 215–232. [8] V.M. Segal, Mater. Sci. Eng. A 386 (2004) 269–276. [9] S. Ferrasse, V.M. Segal, F. Alford, J. Kardokus, S. Strothers, Mater. Sci. Eng. A 493 (2008) 130–140. [10] A. Rosochowski, L. Olejnik, in: G. Hirt, A.E. Tekkaya (Eds.), Proceedings of the 10th International Conference on Technology of Plasticity, ICTP 2011, 25–30 September 2011, Aachen, Germany, pp. 235–240. [11] J. Richert, Arch. Metall. Mater. 55/2 (2010) 391–408.
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Equal channel angular pressing with converging billets—Experiment A. Rosochowski a,n, L. Olejnik b, J. Richert c, M. Rosochowska d, M. Richert c a
Design, Manufacture and Engineering Management, University of Strathclyde, Glasgow, UK Institute of Manufacturing Processes, Warsaw University of Technology, Warsaw, Poland Faculty of Non-Ferrous Metals, AGH University of Science and Technology, Krakow, Poland d Advanced Forming Research Centre, University of Strathclyde, Glasgow, UK b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 August 2012 Received in revised form 19 September 2012 Accepted 21 September 2012 Available online 27 September 2012
A new concept of equal channel angular pressing (ECAP) with converging billets is proposed and tested experimentally. In its basic configuration, the new ECAP process uses two equal square input channels converging into a single output channel, which is twice as wide as the input channels so that it can accept two converging billets. The contact surface between converging billets plays the same role as a movable die wall in the output channel of classical ECAP and thus reduces friction and the process force. The process productivity is doubled and material pickup, especially problematic in the output channel, avoided. This paper presents results of experimental trials of the new process using purposely designed tooling incorporated in a horizontal press with three hydraulic cylinders. One pass of ECAP with converging Al 1070 billets has been carried out and the resulting hardness distribution and microstructure examined. It is concluded that the new process is a feasible version of ECAP both in the engineering and the micro-structural terms, with the added benefit of doubled productivity as well as friction and force reduction. & 2012 Elsevier B.V. All rights reserved.
Keywords: Severe plastic deformation Equal channel angular pressing Ultrafine grained structure
1. Introduction Bulk metals with ultrafine grained (UFG) structure, characterised by the average grain size o1 mm, draw substantial attention due to their unique mechanical and physical properties such as improved strength described by the Hall–Petch relationship [1], enhanced superplastic forming [2], better biocompatibility [3] or particular suitability for microforming [4]. The preferable method of producing bulk UFG metals, which avoids the health hazards associated with nanopowders, is severe plastic deformation (SPD) [5]. In this method, a very large plastic deformation (true strain 3–10 depending on the metal) ‘‘subdivides’’ coarse metal grains into sub-micrometre size grains. Unlike traditional metal forming processes, SPD processes retain the shape of the workpiece. Equal channel angular pressing (ECAP), originally proposed by Segal et al. under the name of equal channel angular extrusion [6], is the most popular SPD process used to produce UFG metals. In this process (Fig. 1), a square or cylindrical billet is pushed by a punch through an input constant profile channel to an output channel of the same profile orientated at an angle Z901 to the previous one. Plastic deformation of the material is caused by simple shear in a thin layer along the diagonal plane at the
n
Corresponding author. Tel.: þ44 141 548 4353. E-mail address: [email protected] (A. Rosochowski).
0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.09.079
channel crossing. The process is usually repeated several times with the billet being rotated about its axis between consecutive passes. The process is simple in terms of tooling and machines used, however, it suffers from friction present in the die channels, which increases the process force and tool contact pressure, limits the length of billets processed and causes material pickup. Good illustration of this last problem is a die segment used in threeturn 3D-ECAP [7], which shows a layer of aluminium sticking to the bottom surface of die channel (Fig. 2). To reduce friction, a concept of movable die walls both in the input and the output channel of the die was proposed [8]. Less complicated seems to be a movable wall representing the bottom part of the output channel; it was realised in practice in industrial ECAP of large plates for sputtering targets [9]. But even in this case the introduction of a movable die wall leads to a more complex and expensive ECAP device. Even if friction between a billet and the bottom part of the output channel is eliminated, it does not disappear completely; it is just moved to a new location between the stationary and movable device elements. Thus friction reduction in the output channel still remains a challenge. A new solution to the problem of friction in the output channel of an ECAP die has been proposed recently. It is based on the idea of ECAP with converging billets, which was explained and modelled using finite element (FE) simulation [10]. The current paper focuses on checking the engineering feasibility of this idea by carrying out a laboratory experiment and investigating changes in properties and microstructure of the processed billets.
A. Rosochowski et al. / Materials Science & Engineering A 560 (2013) 358–364
Punch
359
ECAP discussed above. It reduces friction and the process force. This effect is achieved without using a complex die with movable parts. Instead, two punches are used to push two billets synchronously from the opposite sides. The system doubles productivity compared to the case of processing a single billet. Fig. 4 shows comparison of classical ECAP of a single billet (without moving die walls) and ECAP with two converging billets in terms of strain distribution [10]. Friction coefficient in both processes is the same and equal m ¼0.1. Strain appears to be similar, except the bottom part of the billet, where it is smaller for ECAP with converging billets. This is related to the fact that, in the absence of friction on the bottom part of the billet, the die corner at the channel intersection is filled less; the same effect would be observed in ECAP with a movable bottom wall. A remedy might be back pressure, which improves filling of the die and makes strain distribution more homogeneous [10]. ECAP with two converging billets reduces friction in the output channel, which leads to a lower value of the maximum process force [10]. For friction coefficient m ¼ 0.1, the force reduction predicted by plane strain FE simulation was 23% (Fig. 5).
Die
Billet
Fig. 1. Process configuration for classical ECAP.
3. Machine
Fig. 2. Material pickup on the bottom surface of a 3D-ECAP channel.
Die
Punch
Punch
Billets
Fig. 3. Process configuration for ECAP with two converging billets.
2. Concept of ECAP with converging billets In its basic configuration (Fig. 3), the new ECAP system uses two substantially equal square or rectangular input channels converging into a single output channel, which is twice as wide as the input channels so that it can accept two converging billets. The contact surface between converging billets plays the same role as a movable bottom wall in the output channel of classical
The machine chosen for experimental trials (Fig. 6) was originally used at AGH University of Science and Technology, Krakow, Poland [11] for cyclic extrusion compression (CEC), which is another SPD process. The CEC machine was equipped with two hydraulic double action horizontal actuators opposing each other. Their work cycle involved fixing the die in the horizontal direction with external actuators and then using internal actuators to move the extrusion punch at a controlled speed while the compression punch provided only back pressure. Using the CEC machine for ECAP with converging billets also involved fixing the die in the horizontal direction but required a different type of punch control to allow the punches to move in the opposite directions and at the same speed. Another feature of the original CEC machine was a horizontally split die, whose halves were kept together during the process by a vertical hydraulic actuator. On process completion, the actuator lifted the upper half of the die to enable billet removal. In ECAP with converging billets, the billets could leave the die using a lateral hole opposite the vertical actuator so the die did not have to be split in the horizontal plane. Thus the clamping action of the vertical actuator was used only to keep the die in the right position in the vertical direction.
4. Tooling The die set shown in Fig. 7 was designed to fulfil the functional and geometrical requirements of the machine and to provide adequate performance as an ECAP die. The former was realised by incorporating intermediate tool elements between the die with punches and the hydraulic actuators. These were the pushers driven by the internal actuators whose role was to push punches and limit their stroke and the guides pressed by the external actuators whose role was to guide pushers and transmit a horizontal fixing force. The latter was realised by using a segmented die insert prestressed with outer rings. There were five die segments in total. Two short ones on one side of the die, which created a gap for the billet exit, and one long on the opposite side of the die, which featured a profiled spike to facilitate material flow from the horizontal channel into the die exit. The remaining two segments served as the side walls for the channel defined by the above three segments. The outer rings were forced on the
360
A. Rosochowski et al. / Materials Science & Engineering A 560 (2013) 358–364
Fig. 4. Distribution of equivalent plastic strain in single-billet ECAP (a) and ECAP with two converging billets (b) [10].
assembly of five segments to create an interference fit. The centring of die set on the machine was provided by properly shaped blocks, guided by four pillars (Fig. 6). The lower of these blocks was fixed to the machine bed while the upper one was attached to the vertical actuator. The lower block had a hole, going further down through the machine bed, to extract the processed billets.
30
Force, kN
One billet
20
Converging billets
5. Experiment
10
0 0
0.01
0.02
0.03
0.04
Stroke, m Fig. 5. FE simulation results for punch force vs. punch stroke in single-billet ECAP and in ECAP with two converging billets; friction coefficient m ¼ 0.1 [10].
Die set
Fig. 6. CEC machine used for ECAP with two converging billets (arrows indicate position and flow of material).
The 8 8 46 Al 1070 billets were machined and treated chemically to produce a coating of calcium aluminate (Fig. 8). This served as conversion coating for the MoS2 lubricant, which was applied to the billets just before they were placed in the die channel. Then the die insert was clamped in the horizontal direction using the outer horizontal actuators and clamped in the vertical direction using the vertical actuator. This was followed by the synchronised movement of the inner horizontal actuators which, through the pushers and the punches, caused material flow from both sides of the horizontal channel to the die exit in the middle of this channel. After reaching the limit position, all actuators were withdrawn, which enabled a new pair of billets to be inserted and the whole cycle repeated. This caused the previous billets to be pushed out of the die. Some of the actuator forces as well as punch displacements were recorded during the process. The clamping forces in the horizontal direction were approximately 130 kN while the forming forces were only 8.5 kN. The velocity of both punches was 3.5 mm/min and was kept relatively constant during the process (Fig. 9). This resulted in a fairly symmetrical flow of the material, which produced similar billets in each processed pair (Fig. 8). Since the purpose of this experiment was to prove the concept, only one pass of ECAP with converging billets was performed. However, for further passes, details of the channel geometry, e.g. the length of the output channel, would have to be changed to avoid billet bending visible in Fig. 8.
A. Rosochowski et al. / Materials Science & Engineering A 560 (2013) 358–364
361
Prestressing rings
Guide Pusher Punch Segments of die insert Fig. 7. Die set for ECAP with two converging billets.
RP
Stroke, mm
LP
Time, s
Fig. 9. Displacement of the left punch, LP, and the right punch, RP, during ECAP with converging billets.
Fig. 8. Aluminium billets before and after ECAP with converging billets.
6. Properties In order to improve mechanical properties of metals processed by ECAP, several passes of ECAP are usually required to achieve a minimum grain size and high values of grain boundary misorientation angles. On the other hand it is known that one or two passes of ECAP already have a substantial effect on both the properties and the microstructure of the material. To examine the change in material properties caused by one pass of ECAP with converging billets, one of the billets was polished on its side and microhardness measurements were carried out using the Vickers indenter loaded with 0.1 kg (Fig. 10). The microhardness results along a longitudinal path ‘‘a’’ in Fig. 10 confirm the existence of so-called end effects in the ECAPed billets as well as a nonuniform distribution of strain in the transverse direction along paths ‘‘b’’, ‘‘c’’ and ‘‘d’’. This is what FE simulation results presented in Fig. 4b suggested except the top surface of the billet in Fig. 10, where hardness is smaller than in the centre of the billet, contrary to FE strain distribution. Generally the level of hardness was increased from approximately HV0.1 ¼32 for the initial Al 1070 to HV0.1¼52 after one pass of ECAP with converging billets.
developing in the early stages of classical ECAP. The observations were made in three planes, X, perpendicular to billet axis, Y, parallel to the billet side and, Z, perpendicular to X and Y. All images have been obtained on a Hitachi ultra-high resolution FESEM, SU-70, microscope equipped with a STEM detector. Thin foils have been produced by mechanical polishing down to 0.6 mm and then electro-polishing down to 0.1 mm to obtain perforation. Fig. 11 shows the grain structure in the X plane. Since the initial grain size of Al 1070 was approximately 200 mm, it is clear that there has been a significant reduction in the average grain/ subgrain size to about 2 mm after first pass of ECAP with converging billets. It is interesting that some boundaries of larger cells are very thin and well defined while smaller cells, which develop within some of those larger ones, are thicker and blurred. High resolution images presented in Figs. 12 and 13 enabled identification of those inner boundaries as originating from dense dislocation tangles. Grains in the Y plane, which are shown in Fig. 14, seem to be smaller and elongated. This may result from shear bands crossing this plane as illustrated in Fig. 15. Fig. 16 shows this in detail suggesting that the width of these bands is 0.1– 0.2 mm and that the adjacent strings of elongated grains are 0.5– 1 mm wide. Figs. 17–19 present one area in the Z plane using the increasing magnification. The microstructure in this plane looks like a combination of the equiaxed grain structure in the X plane and more directional and elongated grain structure in the Y plane. Some grains are relatively free of dislocations and having well defined and thin boundaries while others feature polygonal dislocation loops, both indicating a possibility of recovery at room temperature.
7. Microstructure Increased hardness can be caused by the increased average dislocation density but equally it can be attributed to reduction in grain size. Despite performing only one pass of ECAP with converging billets it was decided to check the material microstructure in order to confirm its similarity to the microstructure
8. Conclusions Taking into account many different aspects addressed in this paper, a relatively complete picture of the new process of ECAP
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Vickers hardness HV 0.1
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Distance, mm
8
8
8
0 b
0 c
0 d
a
Vickers hardness HV 0.1
d
Distance, mm Fig. 10. Results of microhardness measurements performed on a side of Al 1070 billet subjected to one pass of ECAP with converging billets along a longitudinal path ‘‘a’’ and three transverse paths ‘‘b’’, ‘‘c’’ and ‘‘d’’.
Fig. 11. Grain structure in X plane (magnification 10K).
Fig. 12. Grain structure in X plane (magnification 20K) showing the central region from Fig. 11.
with converging billets can be presented. The following statements summarise the main results obtained:
FE simulation results of classical ECAP and ECAP with conver-
ECAP with converging billets is a new original process, which reduces friction between die and billet by introducing another symmetrical billet processed at the same time in a way, which makes both billets converge in the output channel of the die. This leads to forming force reduction and avoidance of material pickup on the die surface.
ging billets show that strain distribution in both processes is similar except slightly lower strain generated in the contact area of both billets. Adaptation of CEC machine for ECAP with converging billets required changing the punch movement only. State of the art tool system, with a prestressed segmented die, was designed and manufactured to realise the new process on the CEC machine.
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Fig. 13. Grain structure in X plane (magnification 50K) showing a detail from Fig. 12.
Fig. 16. Grain structure in Y plane (magnification 50K) showing one of the shear bands from Fig. 15.
Fig. 14. Grain structure in Y plane (magnification 10K).
Fig. 17. Grain structure in Z plane (magnification 20K).
Fig. 15. Grain structure in Y plane (magnification 20K) showing the left bottom corner from Fig. 14 with possible shear bands.
Fig. 18. Grain structure in Z plane (magnification 50K) showing the central area from Fig. 17.
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with converging billets is similar to classical ECAP with the added advantage of doubling productivity, reducing friction and material pickup and lowering the process force. It can be considered as a legitimate alternative to other SPD processes used for grain refinement in metals. Since material handling and utilisation are very important factors in industrial applications, a new version of ECAP with converging billets will be proposed to enable processing of long billets.
Acknowledgements Part of this research was supported by The Engineering and Physical Sciences Research Council [Grant no. EP/G03477X/1].
References Fig. 19. Grain structure in Z plane (magnification 100K) showing the central area from Fig. 18.
Successful operation of the machine and the tooling enabled,
for the first time, performing one pass of ECAP with two converging Al 1070 billets. Microhardness measurements on the billet side revealed a substantial increase in hardness whose distribution corresponded well with the predicted strain field. After one pass of ECAP with converging billets, grains/subgrains became much smaller, e.g. 1–2 mm, however, details of the new microstructure depended on the orientation of the plane examined. The observed microstructures confirmed the main mechanisms of grain refinement to be formation of dislocation cells and shear banding. There were signs of recovery for Al 1070 processed at room temperature such as thin, well defined grain boundaries, within which no dislocations existed or polygonal dislocation loops were observed. Generally, taking into account FE simulation results, microhardness distribution and the microstructure produced, ECAP
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