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Fabrication of noncircular multicore microtubes by superplastic dieless drawing process
Journal of Materials Processing Technology 214 (2014) 29–35
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Fabrication of noncircular multicore microtubes by superplastic dieless drawing process Tsuyoshi Furushima ∗ , Atsushi Shirasaki 1 , Ken-ichi Manabe Department of Mechanical Engineering, Tokyo Metropolitan University, 1-1 Minami-ohsawa, Hachioji, Tokyo 192-0397, Japan
a r t i c l e
i n f o
Article history: Received 12 February 2013 Accepted 7 July 2013 Available online xxx Keywords: Dieless drawing Superplasticity Microtube Noncircular Multicore tube
a b s t r a c t A superplastic dieless drawing process that requires no dies or tools is applied to the drawing of a Zn–22Al superplastic alloy for noncircular microtubes such as square, rectangular and noncircular multi core tubes having square inner and rectangular outer cross-sections. In this study, the effects of heating condition, such as heating length and the use or nonuse of cooling device, on deformation behavior are investigated. As a result, a square microtube with 0.58 mm side and a rectangular microtube of 0.75 mm × 1.3 mm were fabricated after 3-pass superplastic dieless drawing. In addition, the fundamental deformation behavior of noncircular tubes combined with square and rectangular tubes during the dieless drawing process has been clarified experimentally. The cross-sectional shape of the noncircular tubes after the superplastic dieless drawing process tends to be maintained on the basis of the similarity law in case of a wide heating length compared with a narrow heating length. Furthermore, a noncircular microtube, which has inner square tubes with a 335 m side, and an outer rectangular tube of 533 m × 923 m were fabricated successfully after a 4-pass superplastic dieless drawing process. Consequently, it was found that the superplastic dieless drawing is effective for the fabrication of noncircular multicore microtubes. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Recently, the miniaturization of components has been demanded to improve the performance of products in various fields such as medicine, chemistry, biology, mechanical engineering, and information technologies. In particular, metallic microtubes have a hollow structure and can be widely used in equipment requiring the passage of gases, liquids, and light through the tubes. Microtubes with a circular cross-section, which is the most basic cross-sectional shape of hollow tubes, are expected to be used for painless injection needles (Tsuchiya et al., 2010) and micronozzles (Chena et al., 2008). Moreover, noncircular tubes have a complicated cross-sectional shape, and complex or multicore tubes can be obtained by assembling or integrating multiple tubes and thereby have many fine holes; these tubes are expected to be used in heat exchangers, commutators, the major components of fuel cells, and electrode tubes for electric discharge machining. For example, the efficiency of heat exchangers with multiple channels in the axial direction can be increased by reducing the size of the channels to increase the surface area (Hallmark et al., 2008).
∗ Corresponding author. Tel.: +81 42 677 2941; fax: +81 42 677 2701. E-mail address: [email protected] (T. Furushima). 1 Present address: Suzuki Motor Corporation, 300 Takatsuka, Minami, Hamamatsu 432-8611, Japan. 0924-0136/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmatprotec.2013.07.005
In the miniaturization of noncircular and complex tubes, there is a problem related to the dies and tools required for fabricating the tubes. In general, the extrusion process is used to fabricate noncircular and complex tubes with a complicated cross-sectional shape (Kojima et al., 2001). However, when the cross-sectional shape of the tube is more complicated, the shape of a mandrel that will be inserted into the tube must also be complicated and the fabrication and handling of such a mandrel becomes more difficult. Moreover, Mitsukawa (2003) reported that decrease in the rigidity of mandrels with decreasing size remains a major problem. Even when noncircular and complex tubes fabricated by extrusion are miniaturized by drawing, problems related to mandrels, plugs, and other tools to be inserted into dies inevitably arise because the tubes are hollow. In short, the fabrication of hollow components with a micro complicated cross-sectional shape accompanies the problem of dies and tools. For this problem of dies and tools, the dieless drawing for circular tubes by the use of local heating and cooling devices of metal wires, bars and tubes has been proposed by Weiss and Kot (1969). This technique can achieve a great reduction of materials in a single pass by the local heating and cooling approach compared with conventional die drawing. We have focused on superplastic dieless drawing as a means of fabricating microtubes without using dies or tools. In practice, we successfully fabricated very thin microtubes with an outer diameter of 190 m (Furushima and Manabe, 2007a,b). In this method, no dies are required and
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T. Furushima et al. / Journal of Materials Processing Technology 214 (2014) 29–35 Table 1 Superplastic material properties of Zn–22Al alloy. Material properties
K value m value
Temperature (◦ C) 20
250
572 0.13
68 0.49
Fig. 1. Schematic illustration of superplastic dieless drawing.
the geometrical similarity law in the cross-section holds; these are great advantages. During the dieless drawing process, the geometrical similarity law in cross-section which the tube is drawn while maintaining its initial shape can be satisfied (Furushima and Manabe, 2007a,b). We actually confirmed that noncircular hollow tubes with a complicated cross-sectional shape were miniaturized while maintaining the same cross-sectional shape as before the drawing (Furushima and Manabe, 2010). However, the miniaturization of noncircular tubes as well as multicore tubes, in which multiple single tubes are placed in another single tube, to a micro scale has not yet been examined, to the best of our knowledge. The purpose of this study is to fabricate noncircular multicore microtubes by miniaturizing multicore tubes that consist of square and rectangular single tubes through superplastic dieless drawing. Specifically, we focused on the heating and cooling conditions as factors that affect the basic deformation behavior of the multicore tubes during dieless drawing. Moreover, we examined whether the geometrical similarity law in cross-section, which holds for circular tubes in dieless drawing, also holds for square, rectangular, and noncircular multicore tubes. The possibility and effectiveness of applying superplastic dieless drawing to noncircular and multicore tubes were also discussed. 2. Superplastic dieless drawing process Superplastic dieless drawing is a method of miniaturizing tubes by locally heating a tube with superplasticity and inducing tensile deformation to the tube without using any dies or tools. Fig. 1 shows the principle of continuous dieless drawing process for a circular tube. With the drawing and feeding speeds denoted as V1 and V2 , respectively, the reduction in area r of the tube expressed by crosssectional area A1 and A2 before and after drawing, respectively is given by Eq. (1) following the volume constant law (Sekiguchi et al., 1974). r =1−
V2 V1
(1)
The cross-sectional shapes of a tube before and after dieless drawing are similar, thus we can say that the geometrical similarity law in cross-section holds (Furushima and Manabe, 2007a,b) Although this law holds for not only simple circular tubes but also noncircular tubes, the applicability of the law to multicore tubes, in which multiple single tubes are placed in another single tube and which are of interest in this study, has not yet been examined, to the best of our knowledge. 3. Fabrication of initial multicore tubes and experimental methods 3.1. Fabrication of tubes 3.1.1. Fabrication of inner square tubes A 10 mm diameter rod made of Zn–22Al alloy was used as the material of specimens. The Zn–22Al alloy contains 78% Zn and 22% Al, which shows good superplastic characteristics. A jump tensile
Fig. 2. Fabrication process for inner square tubes.
test was carried out to determine the superplastic characteristics of the material. The test temperatures were room temperature and 250 ◦ C, and the strain rate was in the range of 4.17 × 10−4 to 10−2 s−1 . The strength coefficient K and the strain rate sensitivity m were evaluated using the equation of the relationship between the flow stress and the strain rate ε˙ during superplastic deformation as shown in Eq. (2) = K ε˙ m .
(2)
Table 1 shows the values of K and m at each test temperature. m is approximately 0.5 at 250 ◦ C, where good superplastic characteristics are obtained. The grain size was 0.5 m. We fabricated the two types of tubes that constitute a noncircular multicore tube, i.e., the outer tube and the inner tubes that are inserted into the outer tube, by combining the extrusion and drawing processes as follows. First, a hole with a diameter of 4 mm (corresponding to the inner diameter, d1 ) was formed in a rod with an outer diameter of d2 = 10 mm to obtain a hollow billet that was then subjected to the extrusion process with a fixed mandrel at 250 ◦ C. Thus, a circular tube with d2 = 5 mm and d1 = 4 mm was fabricated. This mother tube was drawn to form a square tube with a side of 3 mm, which was used as the inner tube. Fig. 2 shows the fabrication process for the inner square tube. Drawing was carried out without using plugs or mandrels, meaning that hollow sinking was performed. 3.1.2. Fabrication of outer rectangular tubes The outer rectangular tube was also fabricated by similar procedures as for the inner tube. A hollow billet with d2 = 10 mm and d1 = 7 mm, obtained by machining, was extruded to form a circular tube with d2 = 8 mm and d1 = 7 mm and then drawn to form a rectangular tube with a size of 5.39 mm × 7.76 mm. This rectangular tube was used as the outer tube. Fig. 3 shows the fabrication process for the outer rectangular tube. In addition, the above rectangular tube was further drawn to obtain a rectangular tube with a size of 4 mm × 7 mm to examine the behavior of the single rectangular tube during dieless drawing. 3.1.3. Fabrication of multicore tubes In this experiment, a multicore tube was fabricated by the following procedures to increase the adhesion between the outer and inner tubes. Two inner square tubes with sides of 3 mm were inserted into an outer rectangular tube with a size of 5.39 mm × 7.76 mm. The obtained tube was forced to pass through a 4 mm × 7 mm die during drawing, thus completing the initial
T. Furushima et al. / Journal of Materials Processing Technology 214 (2014) 29–35
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Fig. 7. Photographs of cross-sectional shape of micro square tube fabricated by superplastic dieless drawing process. Fig. 3. Fabrication process for outer rectangular tubes.
multicore tube. Fig. 4 shows a cross-sectional photograph of the initial multicore tube. 3.2. Experimental procedure of superplastic dieless drawing
Fig. 4. Fabrication process for non-circular multi-core tube.
Fig. 5 shows the schematic and photograph of the experimental setup of superplastic dieless drawing process used in this study. Tensile speed V1 given to the tube and feeding speed V2 can be independently controlled following Eq. (1) using two servo motors. As the heat source for the specimens, a high-frequency induction heating device with an power of 2 kW and a frequency of 2.2 MHz was used. A heating coil was fixed at an output transformer. The power of the high-frequency induction heating device was adjusted by feedback control so that the temperature of the specimen, which was measured using a noncontact pyrometer, was 260 ◦ C, at which Zn–22Al alloy exhibits good superplasticity. Here, the specimen was sprayed with graphite to make the emissivity 90% for the temperature measurement. The feeding speed V2 was fixed at 0.1 mm/s and the tensile speed V1 was controlled to adjust reduction in area r following Eq. (1). As the cooling device, a cooling water tank was fixed next to the heating coil. In the experiment, we focused on the heating and cooling conditions as factors that affect the deformation behavior, and used two types of high-frequency heating coil with different numbers of turns, i.e., coils made of two turns with a heating length of hl = 6 mm and of three turns with a heating length of hl = 16 mm. The cooling water tank was used with the two-turn coil but not with the three-turn coil so that the effect of heating length hl could be clearly observed. 4. Results and discussion 4.1. Superplastic dieless drawing of noncircular single tubes
Fig. 5. Schematic illustration and photograph of superplastic dieless drawing apparatus in the experiment.
We examined the miniaturization of noncircular single tubes, i.e., square and rectangular tubes, before performing superplastic dieless drawing for the multicore tube. In the experimental fabrication of noncircular microtubes, multiple passes of superplastic dieless drawing were carried out with reduction in area r of 60% in each pass of dieless drawing. Dieless drawing of the square tube was performed using the two-turn coil with the cooling device and hl = 6 mm. Fig. 6 shows photographs of the square tube before and after drawing. The tube was drawn to become long and very thin as a result of threepass superplastic dieless drawing. Fig. 7 shows cross-sectional
Fig. 6. Photographs of micro square tube fabricated superplastic dieless drawing process.
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T. Furushima et al. / Journal of Materials Processing Technology 214 (2014) 29–35
Fig. 8. Photographs of micro rectangular tube fabricated by superplastic dieless drawing process.
photographs of the square tube after each pass of drawing. The geometrical similarity law in cross-section essentially holds during the process of thinning, meaning that the tube was successfully miniaturized without being closed inside of tube. Each side of the square tube was initially 3 mm and was reduced to 335 m after three passes of dieless drawing. Thus, we succeeded in fabricating a square microtube. Dieless drawing of the rectangular tube was performed using the three-turn coil with hl = 16 mm without the cooling device. Fig. 8 shows photographs of the rectangular tube before and after drawing. Similar to the square tube, the rectangular tube became very small as a result of three-pass superplastic dieless drawing. Fig. 9 shows cross-sectional photographs of the rectangular tube after each pass of drawing. Similar to the square tube, the rectangular tube was also miniaturized without being closed inside of tube. The dimensions of the rectangular tube were initially 4 mm × 7 mm and were reduced to 533 m × 923 m by three-pass dieless drawing, indicating the successful fabrication of a rectangular microtube. As described above, it was demonstrated that superplastic dieless drawing is effective for fabricating single metallic noncircular microtubes, such as square and rectangular tubes.
device. Temperature distribution was measured by spot-welding a thermocouple to the tube and moving it through each coil at V1 = V2 = 0.1 mm/s so as not to deform the tube. Fig. 11 shows temperature distributions on the surface of the multicore tube in the two cases. The tube was locally heated when using the two-turn coil with hl = 6 mm (with the cooling device). In contrast, when using the three-turn coil with hl = 16 mm (without the cooling device), the tube was not cooled even after exiting the heating coil because of the absence of the cooling device. Thus, the temperature distribution depends on the heating and cooling conditions. Next, we examined the effects of the above difference in heating and cooling conditions on the basic deformation behavior of the multicore tube during dieless drawing. Fig. 12 shows photographs of the deformed area of the tube in the steady state after drawing. The length of the deformed area when using the two-turn coil with hl = 6 mm (with the cooling device), with which a local temperature distribution was obtained, is shorter than that when using the three-turn coil. Moreover, the cross-section of the drawn tubes was observed. As shown in Fig. 13, a gap between the outer and
4.2. Basic deformation behavior of multicore tube during superplastic dieless drawing 4.2.1. Effect of preset value of reduction in area r We carried out an experiment with varying reduction in area r (60% and 70%) and V2 fixed at 0.1 mm/s to examine the basic deformation behavior of the noncircular multicore tube during superplastic dieless drawing. Fig. 10 shows a photograph of the site of the drawn tube at the beginning of drawing. The tube was relatively stably drawn when r = 60%. When r = 70%, however, undesired deformation occurred, that is, local deformation occurred under the unsteady state at the beginning of drawing (Furushima and Manabe, 2009). On the basis of these results, the subsequent experiments of multipass superplastic dieless drawing for the noncircular multicore tube were performed at r = 60%, at which a stable shape can be obtained.
Fig. 10. Effect of reduction in area on deformation behavior at non-steady state in superplastic dieless drawing for non-circular tubes combined with square and rectangular tubes.
4.2.2. Effects of heating and cooling conditions on temperature distribution and deformation profile We examined the effects of the heating and cooling conditions on the temperature distribution and deformation profile of drawn tubes by experiments under different conditions, i.e., the case using the two-turn coil with hl = 6 mm and the cooling device and the case using the three-turn coil with hl = 16 mm without the cooling
Fig. 9. Photographs of cross-sectional shape of micro rectangular tube fabricated superplastic dieless drawing process.
Fig. 11. Effect of heating and cooling condition on temperature distribution in superplastic dieless drawing with reduction in area of 0%.
T. Furushima et al. / Journal of Materials Processing Technology 214 (2014) 29–35
Fig. 12. Effect of heating and cooling condition on longitudinal deformation behavior at steady state in superplastc dieless drawing (a) hl = 6 mm with cooling device and (b) hl = 16 mm without cooling device.
Fig. 13. Effect of heating and cooling condition on cross-sectional deformation behavior in superplastic dieless drawing (a) hl = 6 mm with cooling device and (b) hl = 16 mm without cooling device.
inner tubes was observed after drawing when using the two-turn coil with hl = 6 mm (with the cooling device). In contrast, such a gap was not observed when using the three-turn coil with hl = 16 mm (without the cooling device), indicating that the multicore tube was drawn while maintaining its initial shape. The width of the heated area during drawing, i.e., the length of the deformed area in the steady state, affects whether the geometrical similarity law in cross-section holds. In this experiment, the length of the deformed area in the steady state increased with the width of the heated area, allowing the tube to attain the uniaxial tensile stress state. Therefore, it is considered that the tube was drawn while maintaining its initial shape following the geometrical similarity law (Furushima and Manabe, 2010). 4.3. Fabrication of noncircular multicore microtubes 4.3.1. Multipass superplastic dieless drawing A noncircular multicore microtube was fabricated by the multipass superplastic dieless drawing. The two-turn coil with hl = 6 mm (with the cooling device) was used with r = 60% and four passes. Fig. 14 shows photographs of the tube obtained after each drawing pass. The initial tube consisting of a 4 mm × 7 mm outer tube and 3 mm × 3 mm inner tubes. Upon four-pass dieless drawing, a noncircular multicore microtube consisting of a 533 m × 923 m
33
Fig. 15. Definition of dimensions and similarity ratios of initial non-circular multicore tube combined with inner square tubes and outer rectangular tube (a) dimensions of initial specimen and (b) definition of initial similarity ratios.
outer tube and 335 m × 335 m inner tubes was successfully fabricated. When using the three-turn coil with hl = 16 mm (without the cooling device), the tube fractured during the fourth pass of superplastic dieless drawing at r = 60%. This may be because the length of the heated area relative to the dimensions of the initial noncircular multicore tube increased with the progress of drawing and undesired deformation occurred, resulting in the breakage of the tube (Furushima et al., 2009). It is necessary to set r to a lower value to accomplish successful four-pass drawing. 4.3.2. Geometrical similarity law in cross-section Table 2 shows the cross-sectional photographs of the noncircular multicore tube obtained after each pass of drawing. As described in Section 3.2, the gap between the outer and inner tubes generated during the first pass also remained after the second and subsequent passes when using the two-turn coil with hl = 6 mm. This indicates that the geometrical similarity law in cross-section did not hold during the deformation due to dieless drawing. This is considered to be because the deformed area, as shown in Fig. 12, became narrow for a smaller heating length hl and the uniaxial tensile stress state was unsatisfied (Furushima and Manabe, 2010). In contrast, when using the three-turn coil with hl = 16 mm (without the cooling device), no gap was generated between the outer and inner tubes even in the second and subsequent passes, indicating that the geometrical similarity law in cross-section appeared to hold. Here, we measured the dimensions of the tube in the cross-section to quantitatively evaluate whether the geometrical similarity law in cross-section holds for the drawn tube. The ratios of each dimension to the long side, a, of the outer tube were defined as the similarity ratios. Fig. 15 shows the dimensions of each side and the similarity ratios for the initial tube. Fig. 16 shows the values of b/a and d/a, which are the similarity ratios of a to the dimensions in the vertical direction of the tube for a particularly large difference
Fig. 14. Photographs of micro non-circular multi-core tubes combined with inner square tubes with 335 m of a side and outer rectangular tube with 533 m × 923 m of each side after superplastic dieless drawing.
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T. Furushima et al. / Journal of Materials Processing Technology 214 (2014) 29–35
Table 2 Photographs of cross-sectional shape of non-circular multi-core tubes combined with inner square tubes outer rectangular tube after superplastic dieless drawing. Pass number
Heating and cooling condition hl = 6 mm with cooling device
hl = 16 mm without cooling device
Initial tubes
1
2
3
4
Breaking
in deformation behavior. From these results, the similarity ratios of hl = 16 mm are found to be closer to the initial similarity ratios than those of hl = 6 mm, indicating that the shape of the tube before deformation is maintained following the geometrical similarity law of hl = 16 mm. As above, it is considered that, similar to the rectangular tube, the multicore tube also deforms while maintaining its initial shape during dieless drawing when hl is greater. The possibility of the fabrication of noncircular multicore microtubes
Similarity ratio b/a and d/a
hl=16mm
hl=6mm
hl=16mm
hl=6mm 0.4 b/a(HL=6mm) b/a (h l=6mm) (h l=16mm) b/a b/a(HL=16 mm)
d/a(HL=6mm) d/a (hl=6mm) (hl=16mm) d/a d/a(HL=16mm)
0.3 0
1
2
5. Conclusions In this study, superplastic dieless drawing was performed for single square and rectangular tubes as well as for a noncircular multicore microtube consisting of these single tubes. We examined the possibility and effectiveness of fabricating noncircular single and multicore microtubes from fundamental aspects and obtained the following results.
0.6
0.5
by superplastic dieless drawing was demonstrated. However, the geometrical similarity law in cross-section does not rigorously hold for any of the tubes examined in this study. We must examine the conditions under which the geometrical similarity law more accurately holds in greater detail.
3
Pass number Fig. 16. Effect of heating and cooling condition on similarity ratio for non-circular multi-core tube combined with inner square tubes and outer rectangular tube during superplastic dieless drawing process.
(1) A noncircular multicore tube was successfully fabricated by four-pass superplastic dieless drawing with r = 60%, forming a noncircular multicore microtube consisting of a 533 m × 923 m outer tube and 335 m × 335 m inner tubes. (2) Square and rectangular single tubes were subjected to three-pass superplastic dieless drawing. As a result, a 0.58 mm × 0.58 mm square microtube and a 0.75 mm × 1.3 mm rectangular microtube were successfully fabricated. (3) With increasing length of the heated area, the deformed area expands to allow the tube to be drawn while maintaining its initial cross-sectional shape. When the width of the heated area is reduced, in contrast, a gap was generated between the outer and inner tubes.
T. Furushima et al. / Journal of Materials Processing Technology 214 (2014) 29–35
The above results are considered to be important in the application of dieless drawing to not only conventional single tubes but also noncircular multicore tubes. In the future, we will examine the effects of heating and cooling conditions and the drawing speed in wider ranges, accumulate more data, and analyze the deformation behavior of tubes during drawing. Acknowledgement This study was supported by a Grant-in-Aid for Scientific Research (B) in Japan Society for the Promotion of Science (JSPS). References Chena, Y.T., Kang, S.W., Wu, L.C., Lee, S.H., 2008. Fabrication and investigation of PDMS micro-diffuser/nozzle. Journal of Materials Processing Technology 198, 478–484. Furushima, T., Manabe, K., 2007a. Experimental study on multi-pass dieless drawing process of superplastic Zn–22%Al alloy microtubes. Journal of Materials Processing Technology 187–188, 236–240. Furushima, T., Manabe, K., 2007b. Experimental and numerical study on deformation behavior in dieless drawing process of superplastic microtubes. Journal of Materials Processing Technology 191, 59–63.
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Furushima, T., Manabe, K., 2010. Experimental and numerical study on dieless drawing process of aluminum alloy extruded tubes with double hollow section. Journal of the Chinese Society of Mechanical Engineers 31, 99–105. Furushima, T., Manabe, K., 2009. Finite element simulation with coupled thermomechanical analysis of superplastic dieless tube drawing considering strain rate sensitivity. Materials Transactions 50, 161–166. Furushima, T., Hirose, S., Manabe, K., 2009. Effective temperature distribution and drawing speed control for stable dieless drawing process of metal tubes. Journal of Solid Mechanics and Materials Engineering 3, 236–246. Hallmark, B., Hornung, C.H., Broady, D., Price-Kuehne, C., Mackley, M.R., 2008. The application of plastic microcapillary films for fast transient micro-heat exchange. International Journal of Heat and Mass Transfer 51, 5344–5358. Kojima, F., Takenouchi, S., Kobayashi, H., 2001. Recent trend of plastic working technology for heat exchanger. Journal of the Japan Society for Technology of Plasticity 42, 872–876 (in Japanese). Mitsukawa, K., 2003. Parts working technology contributing to weight reduction and performance upgrading in automobile heat exchanger. Journal of the Japan Society for Technology of Plasticity 44, 1184–1188 (in Japanese). Sekiguchi, H., Kobatake, K., Osakada, K., 1974. A fundamental study on dieless drawing. In: Proceedings of the Int. M. T. D. R. Conference: Advance in Machine Tool Design and Research, Vol. 15, pp. 539–544. Tsuchiya, K., Jinnin, S., Yamamoto, H., Uetsuji, Y., Nakayama, E., 2010. Design and development of a biocompatible painless microneedle by the ion sputtering deposition method. Precision Engineering 34, 461–466. Weiss, V., Kot, R.A., 1969. Dieless wire drawing with transformation plasticity. Wire Journal 9, 182–189.
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Fabrication of noncircular multicore microtubes by superplastic dieless drawing process Tsuyoshi Furushima ∗ , Atsushi Shirasaki 1 , Ken-ichi Manabe Department of Mechanical Engineering, Tokyo Metropolitan University, 1-1 Minami-ohsawa, Hachioji, Tokyo 192-0397, Japan
a r t i c l e
i n f o
Article history: Received 12 February 2013 Accepted 7 July 2013 Available online xxx Keywords: Dieless drawing Superplasticity Microtube Noncircular Multicore tube
a b s t r a c t A superplastic dieless drawing process that requires no dies or tools is applied to the drawing of a Zn–22Al superplastic alloy for noncircular microtubes such as square, rectangular and noncircular multi core tubes having square inner and rectangular outer cross-sections. In this study, the effects of heating condition, such as heating length and the use or nonuse of cooling device, on deformation behavior are investigated. As a result, a square microtube with 0.58 mm side and a rectangular microtube of 0.75 mm × 1.3 mm were fabricated after 3-pass superplastic dieless drawing. In addition, the fundamental deformation behavior of noncircular tubes combined with square and rectangular tubes during the dieless drawing process has been clarified experimentally. The cross-sectional shape of the noncircular tubes after the superplastic dieless drawing process tends to be maintained on the basis of the similarity law in case of a wide heating length compared with a narrow heating length. Furthermore, a noncircular microtube, which has inner square tubes with a 335 m side, and an outer rectangular tube of 533 m × 923 m were fabricated successfully after a 4-pass superplastic dieless drawing process. Consequently, it was found that the superplastic dieless drawing is effective for the fabrication of noncircular multicore microtubes. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Recently, the miniaturization of components has been demanded to improve the performance of products in various fields such as medicine, chemistry, biology, mechanical engineering, and information technologies. In particular, metallic microtubes have a hollow structure and can be widely used in equipment requiring the passage of gases, liquids, and light through the tubes. Microtubes with a circular cross-section, which is the most basic cross-sectional shape of hollow tubes, are expected to be used for painless injection needles (Tsuchiya et al., 2010) and micronozzles (Chena et al., 2008). Moreover, noncircular tubes have a complicated cross-sectional shape, and complex or multicore tubes can be obtained by assembling or integrating multiple tubes and thereby have many fine holes; these tubes are expected to be used in heat exchangers, commutators, the major components of fuel cells, and electrode tubes for electric discharge machining. For example, the efficiency of heat exchangers with multiple channels in the axial direction can be increased by reducing the size of the channels to increase the surface area (Hallmark et al., 2008).
∗ Corresponding author. Tel.: +81 42 677 2941; fax: +81 42 677 2701. E-mail address: [email protected] (T. Furushima). 1 Present address: Suzuki Motor Corporation, 300 Takatsuka, Minami, Hamamatsu 432-8611, Japan. 0924-0136/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmatprotec.2013.07.005
In the miniaturization of noncircular and complex tubes, there is a problem related to the dies and tools required for fabricating the tubes. In general, the extrusion process is used to fabricate noncircular and complex tubes with a complicated cross-sectional shape (Kojima et al., 2001). However, when the cross-sectional shape of the tube is more complicated, the shape of a mandrel that will be inserted into the tube must also be complicated and the fabrication and handling of such a mandrel becomes more difficult. Moreover, Mitsukawa (2003) reported that decrease in the rigidity of mandrels with decreasing size remains a major problem. Even when noncircular and complex tubes fabricated by extrusion are miniaturized by drawing, problems related to mandrels, plugs, and other tools to be inserted into dies inevitably arise because the tubes are hollow. In short, the fabrication of hollow components with a micro complicated cross-sectional shape accompanies the problem of dies and tools. For this problem of dies and tools, the dieless drawing for circular tubes by the use of local heating and cooling devices of metal wires, bars and tubes has been proposed by Weiss and Kot (1969). This technique can achieve a great reduction of materials in a single pass by the local heating and cooling approach compared with conventional die drawing. We have focused on superplastic dieless drawing as a means of fabricating microtubes without using dies or tools. In practice, we successfully fabricated very thin microtubes with an outer diameter of 190 m (Furushima and Manabe, 2007a,b). In this method, no dies are required and
30
T. Furushima et al. / Journal of Materials Processing Technology 214 (2014) 29–35 Table 1 Superplastic material properties of Zn–22Al alloy. Material properties
K value m value
Temperature (◦ C) 20
250
572 0.13
68 0.49
Fig. 1. Schematic illustration of superplastic dieless drawing.
the geometrical similarity law in the cross-section holds; these are great advantages. During the dieless drawing process, the geometrical similarity law in cross-section which the tube is drawn while maintaining its initial shape can be satisfied (Furushima and Manabe, 2007a,b). We actually confirmed that noncircular hollow tubes with a complicated cross-sectional shape were miniaturized while maintaining the same cross-sectional shape as before the drawing (Furushima and Manabe, 2010). However, the miniaturization of noncircular tubes as well as multicore tubes, in which multiple single tubes are placed in another single tube, to a micro scale has not yet been examined, to the best of our knowledge. The purpose of this study is to fabricate noncircular multicore microtubes by miniaturizing multicore tubes that consist of square and rectangular single tubes through superplastic dieless drawing. Specifically, we focused on the heating and cooling conditions as factors that affect the basic deformation behavior of the multicore tubes during dieless drawing. Moreover, we examined whether the geometrical similarity law in cross-section, which holds for circular tubes in dieless drawing, also holds for square, rectangular, and noncircular multicore tubes. The possibility and effectiveness of applying superplastic dieless drawing to noncircular and multicore tubes were also discussed. 2. Superplastic dieless drawing process Superplastic dieless drawing is a method of miniaturizing tubes by locally heating a tube with superplasticity and inducing tensile deformation to the tube without using any dies or tools. Fig. 1 shows the principle of continuous dieless drawing process for a circular tube. With the drawing and feeding speeds denoted as V1 and V2 , respectively, the reduction in area r of the tube expressed by crosssectional area A1 and A2 before and after drawing, respectively is given by Eq. (1) following the volume constant law (Sekiguchi et al., 1974). r =1−
V2 V1
(1)
The cross-sectional shapes of a tube before and after dieless drawing are similar, thus we can say that the geometrical similarity law in cross-section holds (Furushima and Manabe, 2007a,b) Although this law holds for not only simple circular tubes but also noncircular tubes, the applicability of the law to multicore tubes, in which multiple single tubes are placed in another single tube and which are of interest in this study, has not yet been examined, to the best of our knowledge. 3. Fabrication of initial multicore tubes and experimental methods 3.1. Fabrication of tubes 3.1.1. Fabrication of inner square tubes A 10 mm diameter rod made of Zn–22Al alloy was used as the material of specimens. The Zn–22Al alloy contains 78% Zn and 22% Al, which shows good superplastic characteristics. A jump tensile
Fig. 2. Fabrication process for inner square tubes.
test was carried out to determine the superplastic characteristics of the material. The test temperatures were room temperature and 250 ◦ C, and the strain rate was in the range of 4.17 × 10−4 to 10−2 s−1 . The strength coefficient K and the strain rate sensitivity m were evaluated using the equation of the relationship between the flow stress and the strain rate ε˙ during superplastic deformation as shown in Eq. (2) = K ε˙ m .
(2)
Table 1 shows the values of K and m at each test temperature. m is approximately 0.5 at 250 ◦ C, where good superplastic characteristics are obtained. The grain size was 0.5 m. We fabricated the two types of tubes that constitute a noncircular multicore tube, i.e., the outer tube and the inner tubes that are inserted into the outer tube, by combining the extrusion and drawing processes as follows. First, a hole with a diameter of 4 mm (corresponding to the inner diameter, d1 ) was formed in a rod with an outer diameter of d2 = 10 mm to obtain a hollow billet that was then subjected to the extrusion process with a fixed mandrel at 250 ◦ C. Thus, a circular tube with d2 = 5 mm and d1 = 4 mm was fabricated. This mother tube was drawn to form a square tube with a side of 3 mm, which was used as the inner tube. Fig. 2 shows the fabrication process for the inner square tube. Drawing was carried out without using plugs or mandrels, meaning that hollow sinking was performed. 3.1.2. Fabrication of outer rectangular tubes The outer rectangular tube was also fabricated by similar procedures as for the inner tube. A hollow billet with d2 = 10 mm and d1 = 7 mm, obtained by machining, was extruded to form a circular tube with d2 = 8 mm and d1 = 7 mm and then drawn to form a rectangular tube with a size of 5.39 mm × 7.76 mm. This rectangular tube was used as the outer tube. Fig. 3 shows the fabrication process for the outer rectangular tube. In addition, the above rectangular tube was further drawn to obtain a rectangular tube with a size of 4 mm × 7 mm to examine the behavior of the single rectangular tube during dieless drawing. 3.1.3. Fabrication of multicore tubes In this experiment, a multicore tube was fabricated by the following procedures to increase the adhesion between the outer and inner tubes. Two inner square tubes with sides of 3 mm were inserted into an outer rectangular tube with a size of 5.39 mm × 7.76 mm. The obtained tube was forced to pass through a 4 mm × 7 mm die during drawing, thus completing the initial
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Fig. 7. Photographs of cross-sectional shape of micro square tube fabricated by superplastic dieless drawing process. Fig. 3. Fabrication process for outer rectangular tubes.
multicore tube. Fig. 4 shows a cross-sectional photograph of the initial multicore tube. 3.2. Experimental procedure of superplastic dieless drawing
Fig. 4. Fabrication process for non-circular multi-core tube.
Fig. 5 shows the schematic and photograph of the experimental setup of superplastic dieless drawing process used in this study. Tensile speed V1 given to the tube and feeding speed V2 can be independently controlled following Eq. (1) using two servo motors. As the heat source for the specimens, a high-frequency induction heating device with an power of 2 kW and a frequency of 2.2 MHz was used. A heating coil was fixed at an output transformer. The power of the high-frequency induction heating device was adjusted by feedback control so that the temperature of the specimen, which was measured using a noncontact pyrometer, was 260 ◦ C, at which Zn–22Al alloy exhibits good superplasticity. Here, the specimen was sprayed with graphite to make the emissivity 90% for the temperature measurement. The feeding speed V2 was fixed at 0.1 mm/s and the tensile speed V1 was controlled to adjust reduction in area r following Eq. (1). As the cooling device, a cooling water tank was fixed next to the heating coil. In the experiment, we focused on the heating and cooling conditions as factors that affect the deformation behavior, and used two types of high-frequency heating coil with different numbers of turns, i.e., coils made of two turns with a heating length of hl = 6 mm and of three turns with a heating length of hl = 16 mm. The cooling water tank was used with the two-turn coil but not with the three-turn coil so that the effect of heating length hl could be clearly observed. 4. Results and discussion 4.1. Superplastic dieless drawing of noncircular single tubes
Fig. 5. Schematic illustration and photograph of superplastic dieless drawing apparatus in the experiment.
We examined the miniaturization of noncircular single tubes, i.e., square and rectangular tubes, before performing superplastic dieless drawing for the multicore tube. In the experimental fabrication of noncircular microtubes, multiple passes of superplastic dieless drawing were carried out with reduction in area r of 60% in each pass of dieless drawing. Dieless drawing of the square tube was performed using the two-turn coil with the cooling device and hl = 6 mm. Fig. 6 shows photographs of the square tube before and after drawing. The tube was drawn to become long and very thin as a result of threepass superplastic dieless drawing. Fig. 7 shows cross-sectional
Fig. 6. Photographs of micro square tube fabricated superplastic dieless drawing process.
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Fig. 8. Photographs of micro rectangular tube fabricated by superplastic dieless drawing process.
photographs of the square tube after each pass of drawing. The geometrical similarity law in cross-section essentially holds during the process of thinning, meaning that the tube was successfully miniaturized without being closed inside of tube. Each side of the square tube was initially 3 mm and was reduced to 335 m after three passes of dieless drawing. Thus, we succeeded in fabricating a square microtube. Dieless drawing of the rectangular tube was performed using the three-turn coil with hl = 16 mm without the cooling device. Fig. 8 shows photographs of the rectangular tube before and after drawing. Similar to the square tube, the rectangular tube became very small as a result of three-pass superplastic dieless drawing. Fig. 9 shows cross-sectional photographs of the rectangular tube after each pass of drawing. Similar to the square tube, the rectangular tube was also miniaturized without being closed inside of tube. The dimensions of the rectangular tube were initially 4 mm × 7 mm and were reduced to 533 m × 923 m by three-pass dieless drawing, indicating the successful fabrication of a rectangular microtube. As described above, it was demonstrated that superplastic dieless drawing is effective for fabricating single metallic noncircular microtubes, such as square and rectangular tubes.
device. Temperature distribution was measured by spot-welding a thermocouple to the tube and moving it through each coil at V1 = V2 = 0.1 mm/s so as not to deform the tube. Fig. 11 shows temperature distributions on the surface of the multicore tube in the two cases. The tube was locally heated when using the two-turn coil with hl = 6 mm (with the cooling device). In contrast, when using the three-turn coil with hl = 16 mm (without the cooling device), the tube was not cooled even after exiting the heating coil because of the absence of the cooling device. Thus, the temperature distribution depends on the heating and cooling conditions. Next, we examined the effects of the above difference in heating and cooling conditions on the basic deformation behavior of the multicore tube during dieless drawing. Fig. 12 shows photographs of the deformed area of the tube in the steady state after drawing. The length of the deformed area when using the two-turn coil with hl = 6 mm (with the cooling device), with which a local temperature distribution was obtained, is shorter than that when using the three-turn coil. Moreover, the cross-section of the drawn tubes was observed. As shown in Fig. 13, a gap between the outer and
4.2. Basic deformation behavior of multicore tube during superplastic dieless drawing 4.2.1. Effect of preset value of reduction in area r We carried out an experiment with varying reduction in area r (60% and 70%) and V2 fixed at 0.1 mm/s to examine the basic deformation behavior of the noncircular multicore tube during superplastic dieless drawing. Fig. 10 shows a photograph of the site of the drawn tube at the beginning of drawing. The tube was relatively stably drawn when r = 60%. When r = 70%, however, undesired deformation occurred, that is, local deformation occurred under the unsteady state at the beginning of drawing (Furushima and Manabe, 2009). On the basis of these results, the subsequent experiments of multipass superplastic dieless drawing for the noncircular multicore tube were performed at r = 60%, at which a stable shape can be obtained.
Fig. 10. Effect of reduction in area on deformation behavior at non-steady state in superplastic dieless drawing for non-circular tubes combined with square and rectangular tubes.
4.2.2. Effects of heating and cooling conditions on temperature distribution and deformation profile We examined the effects of the heating and cooling conditions on the temperature distribution and deformation profile of drawn tubes by experiments under different conditions, i.e., the case using the two-turn coil with hl = 6 mm and the cooling device and the case using the three-turn coil with hl = 16 mm without the cooling
Fig. 9. Photographs of cross-sectional shape of micro rectangular tube fabricated superplastic dieless drawing process.
Fig. 11. Effect of heating and cooling condition on temperature distribution in superplastic dieless drawing with reduction in area of 0%.
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Fig. 12. Effect of heating and cooling condition on longitudinal deformation behavior at steady state in superplastc dieless drawing (a) hl = 6 mm with cooling device and (b) hl = 16 mm without cooling device.
Fig. 13. Effect of heating and cooling condition on cross-sectional deformation behavior in superplastic dieless drawing (a) hl = 6 mm with cooling device and (b) hl = 16 mm without cooling device.
inner tubes was observed after drawing when using the two-turn coil with hl = 6 mm (with the cooling device). In contrast, such a gap was not observed when using the three-turn coil with hl = 16 mm (without the cooling device), indicating that the multicore tube was drawn while maintaining its initial shape. The width of the heated area during drawing, i.e., the length of the deformed area in the steady state, affects whether the geometrical similarity law in cross-section holds. In this experiment, the length of the deformed area in the steady state increased with the width of the heated area, allowing the tube to attain the uniaxial tensile stress state. Therefore, it is considered that the tube was drawn while maintaining its initial shape following the geometrical similarity law (Furushima and Manabe, 2010). 4.3. Fabrication of noncircular multicore microtubes 4.3.1. Multipass superplastic dieless drawing A noncircular multicore microtube was fabricated by the multipass superplastic dieless drawing. The two-turn coil with hl = 6 mm (with the cooling device) was used with r = 60% and four passes. Fig. 14 shows photographs of the tube obtained after each drawing pass. The initial tube consisting of a 4 mm × 7 mm outer tube and 3 mm × 3 mm inner tubes. Upon four-pass dieless drawing, a noncircular multicore microtube consisting of a 533 m × 923 m
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Fig. 15. Definition of dimensions and similarity ratios of initial non-circular multicore tube combined with inner square tubes and outer rectangular tube (a) dimensions of initial specimen and (b) definition of initial similarity ratios.
outer tube and 335 m × 335 m inner tubes was successfully fabricated. When using the three-turn coil with hl = 16 mm (without the cooling device), the tube fractured during the fourth pass of superplastic dieless drawing at r = 60%. This may be because the length of the heated area relative to the dimensions of the initial noncircular multicore tube increased with the progress of drawing and undesired deformation occurred, resulting in the breakage of the tube (Furushima et al., 2009). It is necessary to set r to a lower value to accomplish successful four-pass drawing. 4.3.2. Geometrical similarity law in cross-section Table 2 shows the cross-sectional photographs of the noncircular multicore tube obtained after each pass of drawing. As described in Section 3.2, the gap between the outer and inner tubes generated during the first pass also remained after the second and subsequent passes when using the two-turn coil with hl = 6 mm. This indicates that the geometrical similarity law in cross-section did not hold during the deformation due to dieless drawing. This is considered to be because the deformed area, as shown in Fig. 12, became narrow for a smaller heating length hl and the uniaxial tensile stress state was unsatisfied (Furushima and Manabe, 2010). In contrast, when using the three-turn coil with hl = 16 mm (without the cooling device), no gap was generated between the outer and inner tubes even in the second and subsequent passes, indicating that the geometrical similarity law in cross-section appeared to hold. Here, we measured the dimensions of the tube in the cross-section to quantitatively evaluate whether the geometrical similarity law in cross-section holds for the drawn tube. The ratios of each dimension to the long side, a, of the outer tube were defined as the similarity ratios. Fig. 15 shows the dimensions of each side and the similarity ratios for the initial tube. Fig. 16 shows the values of b/a and d/a, which are the similarity ratios of a to the dimensions in the vertical direction of the tube for a particularly large difference
Fig. 14. Photographs of micro non-circular multi-core tubes combined with inner square tubes with 335 m of a side and outer rectangular tube with 533 m × 923 m of each side after superplastic dieless drawing.
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Table 2 Photographs of cross-sectional shape of non-circular multi-core tubes combined with inner square tubes outer rectangular tube after superplastic dieless drawing. Pass number
Heating and cooling condition hl = 6 mm with cooling device
hl = 16 mm without cooling device
Initial tubes
1
2
3
4
Breaking
in deformation behavior. From these results, the similarity ratios of hl = 16 mm are found to be closer to the initial similarity ratios than those of hl = 6 mm, indicating that the shape of the tube before deformation is maintained following the geometrical similarity law of hl = 16 mm. As above, it is considered that, similar to the rectangular tube, the multicore tube also deforms while maintaining its initial shape during dieless drawing when hl is greater. The possibility of the fabrication of noncircular multicore microtubes
Similarity ratio b/a and d/a
hl=16mm
hl=6mm
hl=16mm
hl=6mm 0.4 b/a(HL=6mm) b/a (h l=6mm) (h l=16mm) b/a b/a(HL=16 mm)
d/a(HL=6mm) d/a (hl=6mm) (hl=16mm) d/a d/a(HL=16mm)
0.3 0
1
2
5. Conclusions In this study, superplastic dieless drawing was performed for single square and rectangular tubes as well as for a noncircular multicore microtube consisting of these single tubes. We examined the possibility and effectiveness of fabricating noncircular single and multicore microtubes from fundamental aspects and obtained the following results.
0.6
0.5
by superplastic dieless drawing was demonstrated. However, the geometrical similarity law in cross-section does not rigorously hold for any of the tubes examined in this study. We must examine the conditions under which the geometrical similarity law more accurately holds in greater detail.
3
Pass number Fig. 16. Effect of heating and cooling condition on similarity ratio for non-circular multi-core tube combined with inner square tubes and outer rectangular tube during superplastic dieless drawing process.
(1) A noncircular multicore tube was successfully fabricated by four-pass superplastic dieless drawing with r = 60%, forming a noncircular multicore microtube consisting of a 533 m × 923 m outer tube and 335 m × 335 m inner tubes. (2) Square and rectangular single tubes were subjected to three-pass superplastic dieless drawing. As a result, a 0.58 mm × 0.58 mm square microtube and a 0.75 mm × 1.3 mm rectangular microtube were successfully fabricated. (3) With increasing length of the heated area, the deformed area expands to allow the tube to be drawn while maintaining its initial cross-sectional shape. When the width of the heated area is reduced, in contrast, a gap was generated between the outer and inner tubes.
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The above results are considered to be important in the application of dieless drawing to not only conventional single tubes but also noncircular multicore tubes. In the future, we will examine the effects of heating and cooling conditions and the drawing speed in wider ranges, accumulate more data, and analyze the deformation behavior of tubes during drawing. Acknowledgement This study was supported by a Grant-in-Aid for Scientific Research (B) in Japan Society for the Promotion of Science (JSPS). References Chena, Y.T., Kang, S.W., Wu, L.C., Lee, S.H., 2008. Fabrication and investigation of PDMS micro-diffuser/nozzle. Journal of Materials Processing Technology 198, 478–484. Furushima, T., Manabe, K., 2007a. Experimental study on multi-pass dieless drawing process of superplastic Zn–22%Al alloy microtubes. Journal of Materials Processing Technology 187–188, 236–240. Furushima, T., Manabe, K., 2007b. Experimental and numerical study on deformation behavior in dieless drawing process of superplastic microtubes. Journal of Materials Processing Technology 191, 59–63.
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