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Experimental study on multi-pass dieless drawing process of superplastic Zn–22%Al alloy microtubes
Journal of Materials Processing Technology 187–188 (2007) 236–240
Experimental study on multi-pass dieless drawing process of superplastic Zn–22%Al alloy microtubes T. Furushima ∗ , K. Manabe Department of Mechanical Engineering, Tokyo Metropolitan University, 1-1 Minami-ohsawa, Hachioji, Tokyo 192-0397, Japan
Abstract The fabrication process of superplastic microtubes using multi-pass dieless drawing is studied experimentally. The superplastic material used in this experiment is a Zn–22%Al alloy tube with an outer diameter of 2 mm and wall thickness of 0.5 mm. A high-frequency induction heating apparatus with an air-cooling nozzle is used for the dieless drawing. As a result, after four-pass dieless drawing, a microtube with outer and inner diameters of 190 and 91 m, respectively, is fabricated successfully. It is confirmed that the ratio of inner to outer tube diameters remains a certain constant value in multi-pass dieless drawing. In addition, microtubes fabricated by this process are evaluated by their microstructure and surface roughness. Consequently, it is found that the manufacture of initial tubes with fine grains and high accuracy is essential for fabricating superior microtubes. © 2006 Elsevier B.V. All rights reserved. Keywords: Multi-pass dieless drawing; Microtube; Superplasticity; Zn–22%Al alloy; Surface roughness
1. Introduction Microforming is an important technology for fabricating very small metal components that are required in many industrial products [1]. Recently, miniaturization is being demanded in technical applications such as medical equipment, sensor technology and optoelectronics because of the development of micro electro-mechanical systems (MEMS). In particular, the microtube is commonly required for micro components, for example, micro nozzles, painless needles and micro exchangers. The manufacture of microtubes using die drawing techniques with tools such as dies, plugs and mandrels has advanced markedly. However, it is not easy to scale down the conventional process to the micro-scale. It is increasingly difficult to fabricate a micro tool with high accuracy, and to insert a plug and a mandrel into a microtube. In addition, the increase in frictional resistance between the tool and the microtube due to the size effect with miniaturization causes a decrease in the forming limit. Thus, the development of new drawing technologies to fabricate microtubes without tools such as dies, plugs and mandrels has become necessary. From this background, a multi-pass dieless drawing process utilizing the superplastic characteristic was initiated by the
∗
Corresponding author. Tel.: +81 426 77 2941; fax: +81 426 77 2701. E-mail address: [email protected] (T. Furushima).
0924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.11.204
authors to fabricate microtubes. Tools are unnecessary in this process. Thus, there is no need to consider the technical issues of such tools in this process in contrast with the conventional die drawing process. Additionally, this process with high flexibility can be realized. In our previous study, we carried out finite element analysis (FEA) of superplastic dieless drawing [2]. The effectiveness of this as a process for fabrication of microtubes was demonstrated numerically. However, its effectiveness was not confirmed experimentally. In this study, multi-pass superplastic dieless drawing was conducted to fabricate microtubes of Zn–22%Al alloy experimentally. In addition, the effect of the number of drawing passes on the surface roughness and the microstructure of superplastic microtubes fabricated by multi-pass dieless drawing was investigated, and the effectiveness of the drawing process was demonstrated. 2. Multi-pass superplastic dieless drawing Conventional dieless drawing techniques have been investigated since the 1970s [3,4], because of the advantage of a larger reduction in area compared with the die drawing process. Dieless drawing is a unique deformation process without the need for conventional dies. A tube is fixed at one end, and is heated and cooled at another part. It is then pulled at the other end with tensile speed V1 , while the heater and cooler are moved in the
T. Furushima, K. Manabe / Journal of Materials Processing Technology 187–188 (2007) 236–240
237
opposite direction at V2 . Since the heated part of the tube has low flow stress, necking occurs only in this region. Necking is diffused out by the continuous motion of the heater and cooler, achieving a large reduction in tube size in a single pass drawing. The relationship between the reduction in area r and the speed ratio V1 /V2 is formulated as [3] r=
V1 . V1 + V 2
(1)
The difference in flow stress between heated and cooled parts is closely related to the forming limit in the process. The relationship between the limiting reduction in area rc and the flow stresses at heating and cooling σ h and σ c , respectively, can be formulated as [3] rc = 1 −
σh . σc
(2)
This means that by applying superplastic materials, which exhibit extremely low flow stress at the elevated temperatures in this process, further improvement of the forming limit can be achieved. Thus, a great reduction in area to the micro-scale is realized in the superplastic dieless drawing process. In this study, a multi-pass dieless drawing process, i.e., repetition of superplastic dieless drawing, was performed to fabricate finer microtubes. The equivalent plastic strain εeq in multi-pass dieless drawing is calculated using Eq. (3) if the stress state in dieless drawing is uniaxial and the material is considered to be isotropic: εeq = −N ln(1 − r)
Fig. 1. Experimental apparatus for dieless drawing process: (a) schematic illustration of dieless drawing apparatus and (b) photographs of dieless drawing apparatus.
4. Fabrication of microtube
(3)
where N is the drawing pass number. 3. Materials and experimental procedure 3.1. Materials The superplastic material used in the experiments was a Zn–22%Al alloy. First, a sheet with a thickness of 5 mm was machined into hollow billets with outer and inner diameters of 4.4 and 1.1 mm, respectively. The billet was extruded to an outer diameter of 2 mm and wall thickness of 0.5 mm. The extrusion process was carried out at 523 K. Finally, the solution heat treatment for 2 h at 653 K was conducted for the tubes.
4.1. Deformation behavior of tubes in single pass drawing The deformation behavior of tubes with increasing r was investigated in a single pass drawing. Fig. 2 shows photographs of deformation profiles of tubes after a single pass drawing at various tensile speeds, V1 . Instable deformations are observed at early drawing stages at r = 75 and 80%, as shown in Fig. 5(c and d). Actually, the fracture of tubes may occur due to instable deformation at early drawing stages. The limiting reduction in area was 80%, and tubes were fractured at greater reductions in
3.2. Experimental apparatus and procedure Fig. 1 shows a schematic illustration and photographs of the superplastic dieless drawing apparatus, respectively. A high-frequency induction heating device with a maximum power of 2 kW and a frequency of 2.2 MHz was used. The heating coil was moved with a high positioning accuracy using a z-axis table. A cooling coil using the Joule–Thomson effect was attached above the heating coil. The experiments were conducted under tensile speeds of V1 = 2.5–20.4 mm/min and z-axis table speed of V2 = 3.6 mm/min. Cooling was carried out except only during the four-pass drawing, since the tube was vibrated by the cooling air. The heating temperature was set to be 523 K by adjusting the power of the highfrequency induction heating device on the basis of the results of preliminary experiments. The microstructure of the tubes was observed using scanning electron microscopy (SEM) after mirror polishing the test piece. Grain size was calculated by the intercept method. Surface roughness Ra on the outside and inside of the tube was measured using a laser microscope.
Fig. 2. Photographs of deformation profile of tube drawn by a single pass drawing at various tensile speeds V1 .
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Fig. 3. Photograph of superplastic microtubes fabricated by multi-pass dieless drawing.
Fig. 4. Variation in diameter ratio, d/D of superplastic microtubes fabricated by multi-pass dieless drawing.
area in the experiment. A tube with outer and inner diameters of D = 894 m and d = 496 m, respectively, was fabricated at r = 80% in a single pass drawing. Instable deformations occurred if finer tubes are fabricated by a single pass drawing. Therefore, multi-pass dieless drawing is essential for fabricating finer microtubes.
5. Evaluation of fabricated microtube
4.2. Fabrication of microtubes using multi-pass dieless drawing The reduction in area of 66.7%, under which the deformation profile was stable at early drawing stages and a large reduction in area was obtained easily, was selected not only for a single pass drawing but also for multi-pass dieless drawing. After a four-pass dieless drawing, a microtube with outer and inner diameters of D = 190 m and d = 91 m, respectively, was fabricated successfully, as shown in Fig. 3. In addition, Fig. 4 shows the relationship between the diameter ratio d/D and outer diameter D from experimental results. It is confirmed that d/D remains a constant value in this process. In other words, the geometrical similarity law in the cross section is satisfied when the tube is miniaturized to the micro-scale. Microtubes can be fabricated successfully without the closing phenomenon of a hollow tube and without any tools. Therefore, the geometrical similarity law in the cross section is an essential characteristic in this process. The experimental results also demonstrated that the multi-pass dieless drawing process is effective for the fabrication of fine microtubes.
5.1. Microstructure The effect of the number of drawing passes on microstructures of microtubes fabricated by the multi-pass dieless drawing process was investigated. Observations by SEM were carried out to show the microstructure evolution of the tubes during the process. Fig. 5 shows the microstructures of microtubes fabricated by dieless drawing. The initial tube average grain size of 0.5 m varies to 0.7, 0.9, 1.1 and 1.6 m during multi-pass dieless drawing. However, marked grain growth cannot be observed. The grains are still equiaxial and the microstructures are essentially homogeneous. Thus, it is seen that superplasticity is attained during the process. The average grain size increases significantly after only the fourth pass drawing. This may be due to insufficient cooling at only the fourth pass drawing. Therefore, this grain growth can be attributed to maintaining a long heating time. In contrast, if we had carried out sufficient cooling in the process, the attainment of superplasticity during this process would have occurred after four-passes. This is an important characteristic of superplastic dieless drawing. 5.2. Surface roughness Roughening phenomenon on the free surface and surface smoothing mechanism are typical behaviors on surfaces in conventional forming. For the multi-pass dieless drawing pro-
Fig. 5. Microstructures of microtubes fabricated by multi-pass dieless drawing process (a) 0-pass (0.5 m); (b) 1-pass (0.7 m); (c) 2-pass (0.9 m); (d) 3-pass (1.1 m) and (e) 4-pass (1.6 m).
T. Furushima, K. Manabe / Journal of Materials Processing Technology 187–188 (2007) 236–240
239
Fig. 6. Effect of equivalent strain εeq on surface roughness Ra on outside and inside of tube in multi-pass dieless drawing process: (a) outside; (b) inside and (c) change in convex part of surface roughness due to decrease and the increase in surface area.
cess, however, roughening phenomenon on the free surface is the overwhelmingly dominant factor. Furthermore, the surface smoothing mechanism caused by sliding between the tube and the tool cannot be expected in the process. Therefore, investigation of roughening phenomenon on the free surface during the process is necessary. Fig. 6 shows the effect of equivalent strain on surface roughness Ra on the inside and outside of the tube along the drawing direction. Ra on the inside and outside of the tube increases after the one-pass drawing. However, Ra decreases gradually and converges to a value after the two-pass drawing. The results imply that roughening phenomenon on the free surface is restrained in multi-pass dieless drawing. Two broken lines in the figure show the theoretical curves of the relationship between Ra and equivalent strain εeq given by Yamaguchi et al. [5] and Osakada et al. [6]. It is known that theoretical surface roughness generally increases linearly with the increase in equivalent strain [5] Ra = cdg εeq + R0 ,
(4)
where c is an experimental constant, dg is the average grain size of the initial tube and R0 is the surface roughness of the initial tube. However, theoretical results obtained using Eq. (4) are in large disagreement with the experimental results of multi-pass dieless drawing. This is because, as reported by Yamaguchi et
al. [5] the equation is satisfied for εeq < 0.6. The roughening phenomenon on the free surface at εeq > 0.6 cannot be observed by an ordinary tensile test at room temperature because of the occurrence of fracture. As another approach to the roughening mechanism, Yamaguchi et al. carried out a deep drawing experiment with rubber tools to investigate the free surface roughening behavior for larger strains up to εeq = 1.6 [7]. The results showed that surface roughness did not increase linearly for εeq > 0.6, and the increment of surface roughness decreased with the increase in εeq . This phenomenon can be explained by Eq. (5) considering the ratio of change in surface area S0 /S1 expressed by Osakada and Oyane [6]. The convex part of surface roughness decreases due to increasing surface area in the case of tensile deformation as shown in Fig. 6(c). On the other hand, in the case of compressive deformation, the reverse roughening takes place owing to decreasing surface roughness. Ra = cdg
S0 εeq + R0 S1
(5)
S0 /S1 represents exp(−εeq /2) in the case of tensile deformation. The theoretical results obtained using Eq. (5) are in agreement with the experimental results of multi-pass dieless drawing, as shown in Fig. 6. It is considered that surface roughness is dominated by surface area change rather than the free
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surface roughening phenomenon in the case of extremely large strain, which is attained by superplastic deformation. By this mechanism, the surface roughness decreases and converges to a certain value with increasing εeq . Consequently, it is concluded that initial tubes with fine grains and high surface accuracy is an important requirement for fabricating superior fine microtubes. However, the measurement of surface roughness has not yet been conducted at the range of extremely large strains, such as the superplastic deformations in the previous study. Further detailed experimental and analytical investigations are necessary to clarify the fundamental mechanism. 6. Conclusions In this study, a multi-pass dieless drawing process utilizing the superplastic characteristic was performed for Zn–22%Al alloys to experimentally fabricate a microtube. Also, the microtubes fabricated by this process was evaluated from the viewpoints of microstructure and surface roughness. (1) A microtube with outer and inner diameters of D = 190 m and d = 91 m, respectively, was fabricated successfully by a four-passes drawing. The diameter ratio d/D of the fabricated microtubes maintains a constant value in this process. (2) The grains of fabricated microtubes are equiaxial and the microstructures are essentially homogeneous. Thus, it is confirmed that superplasticity was attained during the process. (3) The surface roughness of the fabricated microtube does not increase with increasing equivalent strain. The surface
roughness decreases and converges to a certain value with increasing εeq after the two-pass drawing in the experiment. Consequently, it is concluded that singular initial tubes with fine grains and high surface accuracy is an important requirement for fabricating superior fine microtubes. From these results, it is demonstrated that the multi-pass dieless drawing utilizing superplasticity is a valid way of fabricating microtubes. References [1] F. Vollertsen, Z. Hu, H.S. Niehoff, C. Theiler, State of the art in micro forming and investigations into micro deep drawing, J. Mater. Process. Technol. 151 (2004) 70–79. [2] T. Furushima, T. Sakai, K. Manabe, Finite element modeling of dieless tube drawing of strain rate sensitive material with coupled thermo-mechanical analysis, in: Proceedings of the 8th International Conference on NUMIFORM, 2004, pp. 522–527. [3] H. Sekiguchi, K. Kobatake, K. Osakada, A Fundamental study on dieless drawing, in: Proceedings of the 15th International MTDR Conference, 1974, pp. 539–544. [4] Y. Li, N.R. Quick, A. Kar, Structural evolution and drawability in laser dieless drawing of fine nickel wires, Mater. Sci. Eng. A 358 (2003) 59–70. [5] K. Yamaguchi, P.B. Mellor, Thickness and grain size dependence of limit strains in sheet metal stretching, Int. J. Mech. Sci. 18 (1976) 85–90. [6] K. Osakada, M. Oyane, On the roughening phenomenon of free surface in deformation process, Trans. Jpn. Soc. Mech. Eng. 36–286 (1970) 1017–1022 (in Japanese). [7] M. Fukuda, K. Yamaguchi, N. Takakura, Y. Sakano, Roughening phenomenon on free surface of products in sheet metal forming, J. Jpn. Soc. Technol. Plast. 15–167 (1974) 994–1002 (in Japanese).
Experimental study on multi-pass dieless drawing process of superplastic Zn–22%Al alloy microtubes T. Furushima ∗ , K. Manabe Department of Mechanical Engineering, Tokyo Metropolitan University, 1-1 Minami-ohsawa, Hachioji, Tokyo 192-0397, Japan
Abstract The fabrication process of superplastic microtubes using multi-pass dieless drawing is studied experimentally. The superplastic material used in this experiment is a Zn–22%Al alloy tube with an outer diameter of 2 mm and wall thickness of 0.5 mm. A high-frequency induction heating apparatus with an air-cooling nozzle is used for the dieless drawing. As a result, after four-pass dieless drawing, a microtube with outer and inner diameters of 190 and 91 m, respectively, is fabricated successfully. It is confirmed that the ratio of inner to outer tube diameters remains a certain constant value in multi-pass dieless drawing. In addition, microtubes fabricated by this process are evaluated by their microstructure and surface roughness. Consequently, it is found that the manufacture of initial tubes with fine grains and high accuracy is essential for fabricating superior microtubes. © 2006 Elsevier B.V. All rights reserved. Keywords: Multi-pass dieless drawing; Microtube; Superplasticity; Zn–22%Al alloy; Surface roughness
1. Introduction Microforming is an important technology for fabricating very small metal components that are required in many industrial products [1]. Recently, miniaturization is being demanded in technical applications such as medical equipment, sensor technology and optoelectronics because of the development of micro electro-mechanical systems (MEMS). In particular, the microtube is commonly required for micro components, for example, micro nozzles, painless needles and micro exchangers. The manufacture of microtubes using die drawing techniques with tools such as dies, plugs and mandrels has advanced markedly. However, it is not easy to scale down the conventional process to the micro-scale. It is increasingly difficult to fabricate a micro tool with high accuracy, and to insert a plug and a mandrel into a microtube. In addition, the increase in frictional resistance between the tool and the microtube due to the size effect with miniaturization causes a decrease in the forming limit. Thus, the development of new drawing technologies to fabricate microtubes without tools such as dies, plugs and mandrels has become necessary. From this background, a multi-pass dieless drawing process utilizing the superplastic characteristic was initiated by the
∗
Corresponding author. Tel.: +81 426 77 2941; fax: +81 426 77 2701. E-mail address: [email protected] (T. Furushima).
0924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.11.204
authors to fabricate microtubes. Tools are unnecessary in this process. Thus, there is no need to consider the technical issues of such tools in this process in contrast with the conventional die drawing process. Additionally, this process with high flexibility can be realized. In our previous study, we carried out finite element analysis (FEA) of superplastic dieless drawing [2]. The effectiveness of this as a process for fabrication of microtubes was demonstrated numerically. However, its effectiveness was not confirmed experimentally. In this study, multi-pass superplastic dieless drawing was conducted to fabricate microtubes of Zn–22%Al alloy experimentally. In addition, the effect of the number of drawing passes on the surface roughness and the microstructure of superplastic microtubes fabricated by multi-pass dieless drawing was investigated, and the effectiveness of the drawing process was demonstrated. 2. Multi-pass superplastic dieless drawing Conventional dieless drawing techniques have been investigated since the 1970s [3,4], because of the advantage of a larger reduction in area compared with the die drawing process. Dieless drawing is a unique deformation process without the need for conventional dies. A tube is fixed at one end, and is heated and cooled at another part. It is then pulled at the other end with tensile speed V1 , while the heater and cooler are moved in the
T. Furushima, K. Manabe / Journal of Materials Processing Technology 187–188 (2007) 236–240
237
opposite direction at V2 . Since the heated part of the tube has low flow stress, necking occurs only in this region. Necking is diffused out by the continuous motion of the heater and cooler, achieving a large reduction in tube size in a single pass drawing. The relationship between the reduction in area r and the speed ratio V1 /V2 is formulated as [3] r=
V1 . V1 + V 2
(1)
The difference in flow stress between heated and cooled parts is closely related to the forming limit in the process. The relationship between the limiting reduction in area rc and the flow stresses at heating and cooling σ h and σ c , respectively, can be formulated as [3] rc = 1 −
σh . σc
(2)
This means that by applying superplastic materials, which exhibit extremely low flow stress at the elevated temperatures in this process, further improvement of the forming limit can be achieved. Thus, a great reduction in area to the micro-scale is realized in the superplastic dieless drawing process. In this study, a multi-pass dieless drawing process, i.e., repetition of superplastic dieless drawing, was performed to fabricate finer microtubes. The equivalent plastic strain εeq in multi-pass dieless drawing is calculated using Eq. (3) if the stress state in dieless drawing is uniaxial and the material is considered to be isotropic: εeq = −N ln(1 − r)
Fig. 1. Experimental apparatus for dieless drawing process: (a) schematic illustration of dieless drawing apparatus and (b) photographs of dieless drawing apparatus.
4. Fabrication of microtube
(3)
where N is the drawing pass number. 3. Materials and experimental procedure 3.1. Materials The superplastic material used in the experiments was a Zn–22%Al alloy. First, a sheet with a thickness of 5 mm was machined into hollow billets with outer and inner diameters of 4.4 and 1.1 mm, respectively. The billet was extruded to an outer diameter of 2 mm and wall thickness of 0.5 mm. The extrusion process was carried out at 523 K. Finally, the solution heat treatment for 2 h at 653 K was conducted for the tubes.
4.1. Deformation behavior of tubes in single pass drawing The deformation behavior of tubes with increasing r was investigated in a single pass drawing. Fig. 2 shows photographs of deformation profiles of tubes after a single pass drawing at various tensile speeds, V1 . Instable deformations are observed at early drawing stages at r = 75 and 80%, as shown in Fig. 5(c and d). Actually, the fracture of tubes may occur due to instable deformation at early drawing stages. The limiting reduction in area was 80%, and tubes were fractured at greater reductions in
3.2. Experimental apparatus and procedure Fig. 1 shows a schematic illustration and photographs of the superplastic dieless drawing apparatus, respectively. A high-frequency induction heating device with a maximum power of 2 kW and a frequency of 2.2 MHz was used. The heating coil was moved with a high positioning accuracy using a z-axis table. A cooling coil using the Joule–Thomson effect was attached above the heating coil. The experiments were conducted under tensile speeds of V1 = 2.5–20.4 mm/min and z-axis table speed of V2 = 3.6 mm/min. Cooling was carried out except only during the four-pass drawing, since the tube was vibrated by the cooling air. The heating temperature was set to be 523 K by adjusting the power of the highfrequency induction heating device on the basis of the results of preliminary experiments. The microstructure of the tubes was observed using scanning electron microscopy (SEM) after mirror polishing the test piece. Grain size was calculated by the intercept method. Surface roughness Ra on the outside and inside of the tube was measured using a laser microscope.
Fig. 2. Photographs of deformation profile of tube drawn by a single pass drawing at various tensile speeds V1 .
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Fig. 3. Photograph of superplastic microtubes fabricated by multi-pass dieless drawing.
Fig. 4. Variation in diameter ratio, d/D of superplastic microtubes fabricated by multi-pass dieless drawing.
area in the experiment. A tube with outer and inner diameters of D = 894 m and d = 496 m, respectively, was fabricated at r = 80% in a single pass drawing. Instable deformations occurred if finer tubes are fabricated by a single pass drawing. Therefore, multi-pass dieless drawing is essential for fabricating finer microtubes.
5. Evaluation of fabricated microtube
4.2. Fabrication of microtubes using multi-pass dieless drawing The reduction in area of 66.7%, under which the deformation profile was stable at early drawing stages and a large reduction in area was obtained easily, was selected not only for a single pass drawing but also for multi-pass dieless drawing. After a four-pass dieless drawing, a microtube with outer and inner diameters of D = 190 m and d = 91 m, respectively, was fabricated successfully, as shown in Fig. 3. In addition, Fig. 4 shows the relationship between the diameter ratio d/D and outer diameter D from experimental results. It is confirmed that d/D remains a constant value in this process. In other words, the geometrical similarity law in the cross section is satisfied when the tube is miniaturized to the micro-scale. Microtubes can be fabricated successfully without the closing phenomenon of a hollow tube and without any tools. Therefore, the geometrical similarity law in the cross section is an essential characteristic in this process. The experimental results also demonstrated that the multi-pass dieless drawing process is effective for the fabrication of fine microtubes.
5.1. Microstructure The effect of the number of drawing passes on microstructures of microtubes fabricated by the multi-pass dieless drawing process was investigated. Observations by SEM were carried out to show the microstructure evolution of the tubes during the process. Fig. 5 shows the microstructures of microtubes fabricated by dieless drawing. The initial tube average grain size of 0.5 m varies to 0.7, 0.9, 1.1 and 1.6 m during multi-pass dieless drawing. However, marked grain growth cannot be observed. The grains are still equiaxial and the microstructures are essentially homogeneous. Thus, it is seen that superplasticity is attained during the process. The average grain size increases significantly after only the fourth pass drawing. This may be due to insufficient cooling at only the fourth pass drawing. Therefore, this grain growth can be attributed to maintaining a long heating time. In contrast, if we had carried out sufficient cooling in the process, the attainment of superplasticity during this process would have occurred after four-passes. This is an important characteristic of superplastic dieless drawing. 5.2. Surface roughness Roughening phenomenon on the free surface and surface smoothing mechanism are typical behaviors on surfaces in conventional forming. For the multi-pass dieless drawing pro-
Fig. 5. Microstructures of microtubes fabricated by multi-pass dieless drawing process (a) 0-pass (0.5 m); (b) 1-pass (0.7 m); (c) 2-pass (0.9 m); (d) 3-pass (1.1 m) and (e) 4-pass (1.6 m).
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239
Fig. 6. Effect of equivalent strain εeq on surface roughness Ra on outside and inside of tube in multi-pass dieless drawing process: (a) outside; (b) inside and (c) change in convex part of surface roughness due to decrease and the increase in surface area.
cess, however, roughening phenomenon on the free surface is the overwhelmingly dominant factor. Furthermore, the surface smoothing mechanism caused by sliding between the tube and the tool cannot be expected in the process. Therefore, investigation of roughening phenomenon on the free surface during the process is necessary. Fig. 6 shows the effect of equivalent strain on surface roughness Ra on the inside and outside of the tube along the drawing direction. Ra on the inside and outside of the tube increases after the one-pass drawing. However, Ra decreases gradually and converges to a value after the two-pass drawing. The results imply that roughening phenomenon on the free surface is restrained in multi-pass dieless drawing. Two broken lines in the figure show the theoretical curves of the relationship between Ra and equivalent strain εeq given by Yamaguchi et al. [5] and Osakada et al. [6]. It is known that theoretical surface roughness generally increases linearly with the increase in equivalent strain [5] Ra = cdg εeq + R0 ,
(4)
where c is an experimental constant, dg is the average grain size of the initial tube and R0 is the surface roughness of the initial tube. However, theoretical results obtained using Eq. (4) are in large disagreement with the experimental results of multi-pass dieless drawing. This is because, as reported by Yamaguchi et
al. [5] the equation is satisfied for εeq < 0.6. The roughening phenomenon on the free surface at εeq > 0.6 cannot be observed by an ordinary tensile test at room temperature because of the occurrence of fracture. As another approach to the roughening mechanism, Yamaguchi et al. carried out a deep drawing experiment with rubber tools to investigate the free surface roughening behavior for larger strains up to εeq = 1.6 [7]. The results showed that surface roughness did not increase linearly for εeq > 0.6, and the increment of surface roughness decreased with the increase in εeq . This phenomenon can be explained by Eq. (5) considering the ratio of change in surface area S0 /S1 expressed by Osakada and Oyane [6]. The convex part of surface roughness decreases due to increasing surface area in the case of tensile deformation as shown in Fig. 6(c). On the other hand, in the case of compressive deformation, the reverse roughening takes place owing to decreasing surface roughness. Ra = cdg
S0 εeq + R0 S1
(5)
S0 /S1 represents exp(−εeq /2) in the case of tensile deformation. The theoretical results obtained using Eq. (5) are in agreement with the experimental results of multi-pass dieless drawing, as shown in Fig. 6. It is considered that surface roughness is dominated by surface area change rather than the free
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surface roughening phenomenon in the case of extremely large strain, which is attained by superplastic deformation. By this mechanism, the surface roughness decreases and converges to a certain value with increasing εeq . Consequently, it is concluded that initial tubes with fine grains and high surface accuracy is an important requirement for fabricating superior fine microtubes. However, the measurement of surface roughness has not yet been conducted at the range of extremely large strains, such as the superplastic deformations in the previous study. Further detailed experimental and analytical investigations are necessary to clarify the fundamental mechanism. 6. Conclusions In this study, a multi-pass dieless drawing process utilizing the superplastic characteristic was performed for Zn–22%Al alloys to experimentally fabricate a microtube. Also, the microtubes fabricated by this process was evaluated from the viewpoints of microstructure and surface roughness. (1) A microtube with outer and inner diameters of D = 190 m and d = 91 m, respectively, was fabricated successfully by a four-passes drawing. The diameter ratio d/D of the fabricated microtubes maintains a constant value in this process. (2) The grains of fabricated microtubes are equiaxial and the microstructures are essentially homogeneous. Thus, it is confirmed that superplasticity was attained during the process. (3) The surface roughness of the fabricated microtube does not increase with increasing equivalent strain. The surface
roughness decreases and converges to a certain value with increasing εeq after the two-pass drawing in the experiment. Consequently, it is concluded that singular initial tubes with fine grains and high surface accuracy is an important requirement for fabricating superior fine microtubes. From these results, it is demonstrated that the multi-pass dieless drawing utilizing superplasticity is a valid way of fabricating microtubes. References [1] F. Vollertsen, Z. Hu, H.S. Niehoff, C. Theiler, State of the art in micro forming and investigations into micro deep drawing, J. Mater. Process. Technol. 151 (2004) 70–79. [2] T. Furushima, T. Sakai, K. Manabe, Finite element modeling of dieless tube drawing of strain rate sensitive material with coupled thermo-mechanical analysis, in: Proceedings of the 8th International Conference on NUMIFORM, 2004, pp. 522–527. [3] H. Sekiguchi, K. Kobatake, K. Osakada, A Fundamental study on dieless drawing, in: Proceedings of the 15th International MTDR Conference, 1974, pp. 539–544. [4] Y. Li, N.R. Quick, A. Kar, Structural evolution and drawability in laser dieless drawing of fine nickel wires, Mater. Sci. Eng. A 358 (2003) 59–70. [5] K. Yamaguchi, P.B. Mellor, Thickness and grain size dependence of limit strains in sheet metal stretching, Int. J. Mech. Sci. 18 (1976) 85–90. [6] K. Osakada, M. Oyane, On the roughening phenomenon of free surface in deformation process, Trans. Jpn. Soc. Mech. Eng. 36–286 (1970) 1017–1022 (in Japanese). [7] M. Fukuda, K. Yamaguchi, N. Takakura, Y. Sakano, Roughening phenomenon on free surface of products in sheet metal forming, J. Jpn. Soc. Technol. Plast. 15–167 (1974) 994–1002 (in Japanese).