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Experimental and FEM study on sinking of miniature inner grooved copper tube
Journal of Materials Processing Technology 209 (2009) 5333–5340
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Experimental and FEM study on sinking of miniature inner grooved copper tube Yong Tang a , Long-sheng Lu a,∗ , Dong Yuan a , Qing-hui Wang a , Xiao-lin Zhao a,b a b
School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, China Department of Mechanical and Energy Engineering, Shaoyang University, Shaoyang 422004, China
a r t i c l e
i n f o
Article history: Received 13 June 2008 Received in revised form 19 March 2009 Accepted 1 April 2009 Keywords: Copper tubes Inner grooved tube Drawing Sinking Plastic deformation Heat pipes
a b s t r a c t Miniature inner grooved copper tubes (mIGCT) which have an outer diameter less than 6 mm are in demand for the production of heat pipes. In this work, it is proposed to manufacture such tubes by a multi-stage tube sinking process with an initial mIGCT having an outer diameter of 6 mm. A FEM simulation approach is used to analyze stress, strain and damage distribution for the proposed process. For comparison, a smooth copper tube is also used in the simulation study. Furthermore, experiments are conducted to investigate plastic deformation of the grooves and teeth of the tube. Results show that the maximum stress and strain are occurred at the grooves area, while the maximum damage is located at the top of the teeth. The ratio of groove width to tooth width (ˇ) is reduced after each drawing pass. Bonding, folding and segmenting, which represent potential flaws, have also been observed in the multi-stage tube sinking process, and are discussed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Heat pipes are widely applied for heat transfer in electronic products. Xie et al. (2008) and Lan et al. (2007) have evaluated the heat sink performance with integrated heat pipe for high flux chip and LED array system, respectively. Vasiliev (2008) has also presented a short review covering thermal performance in micro and miniature heat pipes. They all reported that heat pipes are useful on high flux spreading in a limited space. Generally, the performance of a heat pipe depends on its wick structure. Due to cost-effective and lightweight, the inner grooved wick structure represents one of the most important wick types. Kyu et al. (2008) developed a mathematical model for predicting the thermal performance of a heat pipe with a rectangular inner grooved wick, and found that heat pipe performance could be improved by 20% with optimized groove width and depth geometry. Hopkins et al. (1999) investigated the grooves geometry effect on flat miniature heat pipe performance. Results show that heat pipes with a micro-capillary inner grooved wick demonstrate excellent performance characteristics. Jiao et al. (2007) suggested that a heat pipe with a thin tube wall has better thermal performance than one with a thick wall. Therefore, the manufacturing process for controlling the grooves geometry needs to be studied to improve or optimize heat pipe production. Generally, an inner grooved structure is used to enhance heat transfer in air-conditioner industry. A grooved structure is nor-
∗ Corresponding author. Tel.: +86 020 8711 4634; fax: +86 020 8711 4634. E-mail address: [email protected] (L.-s. Lu). 0924-0136/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2009.04.003
mally manufactured by either fixed plug or floating plug drawing. Firstly, teeth are fabricated on the surface of the plug, and then the reverse grooves are generated on the inner surface of the tube via the plastic deformation during drawing. These methods could also be applied to manufacture miniature inner grooved copper tubes (mIGCT) with an outer diameter larger than 6 mm, but it is difficult to satisfy the groove geometry requirements needed for heat pipes. Hence, ball spinning with a plug has been applied in manufacturing mIGCT. This process was theoretically analyzed by Rotarescu (1995), and results showed that it usually consumed much less power when compared to the conventional die drawing. Zhang et al. (2007) focused on the folding defect on the inner surface of the inner grooved copper tube during the ball spinning process, and several effective solutions were proposed to prevent the folding defects. However, it has proven hard to manufacture mIGCT having an outer diameter less than 6 mm (especially less than 4 mm) even with an optimized ball spinning method, due to the difficulty of fabricating high accuracy micro-tools and pulling the plug through the tube, as indicated by Furushima and Manabe (2008). Furthermore, Kazunari and Hiroaki (2004) concluded that the fracture frequency of a tube dramatically increased because the drawing force might overflow the mIGCT’s plastic limit. However, inner grooved heat pipes having an outer diameter less than 6 mm is in high demand to deal with the demands of miniaturization development of electronic products as analyzed by Suman (2007). So the manufacturing technology of mIGCT needs to be studied to develop effective manufacturing processes for mIGCT. Sinking is one of the most important drawing methods to manufacture tubes. Compared with plug drawing, sinking is a process
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Fig. 1. Characterization of mIGCT. h—Teeth height (grooves depth), D—outer diameter of copper tube, ˛—grooves angular, l1 —grooves width in average, l2 —teeth width in average, t—wall thickness of mIGCT.
Fig. 2. The sketch of drawing process and die. (a) Drawing machine and (b) drawing die.
that can reduce dimensions of the tubes without an inside mandrel, but with a smaller drawing force and a higher productivity. Generally, sinking is applied for the drawing of smooth tube without any internal grooves or any other structures. Without a mandrel, the thickness and shape of the tube wall are determined by its initial dimensions and drawing dies. Therefore, the tube sinking can be used to draw micro-tubes with outer diameters even less than 1 mm. In this work, multi-stage tube sinking is designed to manufacture mIGCT having an outer diameter less than 6 mm. Compared with the conventional tube sinking utilized smooth copper tube as workpiece, the multi-stage tube sinking is processed on the mIGCT having an outer diameter of 6 mm. So it is a further processing method of mIGCT fabricated by the ball spinning method. The grooves deformation during tube sinking is analyzed by both finite element simulation and experiments.
formation mechanism of the mIGCT from copper tube by oil-filled ball spinning method was analyzed, and optimal manufacturing parameters were suggested. In another paper (Li et al., 2008), the design of plug was analyzed, and some methods were proposed to avoid tube defects. So the analysis of the grooves deformation by oilfilled ball spinning method will not be included in this paper. Before mIGCT sinking, further processing methods, including tempering and sizing, are conducted to ensure the copper tubes in straight state and 68 MPa in yield strength ( 0.2 ). As shown in Table 2, three kinds of copper tubes with different ratio of groove width to tooth width (ˇ = l1 /l2 ) are used in the experiments. 2.2. Experimental procedure Multi-stage tube sinking is operated on mIGCT having an outer diameter of 6 mm. The sinking passes follow as 4 mm, 3.5 mm,
2. Experimental method 2.1. Materials The workpiece material, as shown in Fig. 1, is mIGCT. At first, mIGCT is manufactured from copper T2 by oil-filled ball spinning method. In one of authors’ previous papers (Tang et al., 2007), the Table 1 Parameters of drawing dies. Drawing die no.
D/mm
D /mm
d/mm
1 2 3 4 5
6 4 3.5 2.6 1.9
6.2 4.2 3.7 2.8 2.1
4 3.5 2.6 1.9 1.6
Fig. 3. FEM simulation model.
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Fig. 4. Stress comparison between No. 2 mIGCT and smooth copper tube. (a) Radial stress r , (b) hoop stress , (c) axial stress l and (d) effective stress.
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Table 2 Parameters of inner grooves copper tubes during sinking. mIGCT no.
Tube state
Grooves number
D/mm
˛/◦
l1 /m
1
Initiation Simulation Experiment
18
6 4 4
60 40 43
2
Initiation Simulation Experiment
12
6 4 4
3
Initiation Simulation Experiment
12
6 4 4
l2 /m
t/m
h/m
ˇ
420 136 142
420 280 290
480 475 470
168 147 150
1 0.49 0.49
60 12 52
925 490 506
350 251 260
480 476 476
168 146 152
2.64 1.95 1.95
10 5 8
1080 552 560
190 145 160
480 475 476
168 136 140
5.68 3.81 3.5
Fig. 5. Strain comparison between No. 2 mIGCT and smooth copper tube.
2.6 mm, 1.9 mm, and 1.6 mm. All the experiments are done on the same drawing machine but with different drawing dies. The drawing machine consists of a drawing die, a die holder and a chuck jaw, as shown in Fig. 2(a). The sketch of the drawing dies is shown in Fig. 2(b). It could be divided into four zones: I—guard, II—reducing, III—calibrating and IV—receding. The drawing angular in sizing reduction area is 12◦ ; the length of calibrating area is 4 mm; the receding angular in receding area is also 12◦ . The dimensions of D and d are as shown in Table 1. All the experiments are done at a drawing speed of 10 m/min under good lubrication condition. 2.3. FEM simulation method It is difficult to observe and clarify the deformation behavior of inner grooves and teeth during the sinking process. In order to
investigate the stress and strain, A FEM analysis with rigid-plastic principle is performed, as shown in Fig. 3. It is carried out by using DEFORM ver.2001. In the FEM model, only one groove is separated from the mIGCT copper tube because the workpiece is axisymmetric. During simulations, the workpiece (plastic body) is set as static with its length of 15 mm, and it is divided into 100,000 tetrahedral elements in average. The drawing dies (rigid body) move at a relative speed (10 m/min). The friction factor between workpiece and drawing die is set as 0.06 because oil film lubrication is used. Three kinds of mIGCT which have the same dimensions in cross-section as in experiment are used, as shown in Table 2. Besides, a smooth copper tube, which has the same dimensions with mIGCT except the teeth, is used for comparison. The dimensions of tube sinking die are also the same as that used in the experiment.
Fig. 6. Damage comparison between No. 2 mIGCT and smooth copper tube.
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Fig. 7. SEM images of No. 2 mIGCT in cross-section during multi-stage tube sinking.
3. Results and discussions 3.1. Stress analysis with FEM model Fig. 4 shows stress comparison between No. 2 mIGCT and smooth copper tube during the first tube sinking pass in the analyzing plane. The tube sinking is a three-dimensional stress process, which includes axial stress 1, radial stress r, and hoop stress . For smooth copper tube, r is the compressive stress, r = 0 at the inner surface, and grows to its maximum value at the outer surface. For mIGCT, however, r may changes into tensile stress in the tooth region. of both smooth copper tube and mIGCT tube wall region is also the compressive stress, and its absolute value increases in radial from outer to inner surface, but decreases rapidly from the tooth root to top for mIGCT. Furthermore, 1 is tensile stress for both smooth copper tube and mIGCT. It is almost the same as in the tube wall region of mIGCT and in the smooth copper tube, but it grows
quickly in the tooth region for mIGCT. Consequently, the effective stress of smooth copper tube grows gradually in radial from outer to inner surface. The effective stress of mIGCT is different between the tube wall and tooth region, it increases in radial from outer to inner surface in the tube wall, and decreases from tooth root to top in the tooth region. 3.2. FEM and experimental analysis on grooves deformation Fig. 5 shows the effective strain comparison between smooth copper tube and No. 2 mIGCT. Results indicate that the effective strain distribution is similar as effective stress. The strain reaches its maximum on the blending region between groove and tooth. The average strain in groove region is larger than that in tooth. So the groove deformation is more intensive than that of the tooth. Table 2 shows mIGCT dimensions changes after tube sinking by experiments and simulations. Results indicate that ˛, l1 , l2 dimen-
Fig. 8. SEM images of No. 2 mIGCT in axial during multi-stage tube sinking.
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sions are all reduced after drawing. The reduced amount of l1 is larger than that of l2 . So the plastic deformation of the groove is more intensive than that of the tooth. However, for a single tooth, its strain distribution differs from the root to top. The strain in the root region is larger than that of the top. So the deformation of the tooth root is more intensive than that of the tooth top. The angle of the tooth root changes into negative, while the angle of the tooth top remains positive. The comparison of damage distribution between mIGCT and smooth copper tube is shown in Fig. 6. The damage of the smooth copper tube ranges from 0.12 to 0.15 with an identical growth rate from outer to inner surface, and its maximum value is located at the inner surface. The damage of mIGCT ranges from 0.15 to 0.34, and its maximum value is located at the tooth top. So the damage of mIGCT is larger than that of the smooth copper tube. Furthermore, the average damage value of the tooth region is about three times larger than that of the tube wall region. So the tooth is a fragile region during the tube sinking process of mIGCT. It is due to the cyclic stress and weak rigidity in tooth area. 3.3. Multi-stage tube sinking analysis mIGCT having an outer diameter of 1.6 mm is manufactured by multi-stage tube sinking on No. 2 mIGCT. In view of the demand of thin wall tube for heat pipes and affordable drawing force of copper tube, experiments show that five drawing passes are suitable, following as 4 mm, 3.5 mm, 2.6 mm, 1.9 mm, and 1.6 mm. Fig. 7 shows the SEM images in cross-section during the multi-stage tube sinking process, and Fig. 8 shows the SEM images in axial section. Results show that both the grooves and teeth reduce gradually during sinking. The changes include mIGCT dimensions and inner surface roughness. The inner surfaces of grooves and teeth turn to be rougher than that of before after processing. And grooves gradually grow closed after multistage tube sinking. Table 3 presents detail information about mIGCT after each drawing pass in experiments. After five drawing passes, outer diameter (D) reduces by 73.3%, grooves width (l1 ) reduces by 91.4%, teeth width (l2 ) reduces by 48.6%, wall thickness (t) reduces by 51.5%, and teeth height (h) reduces by 36.3%. It finds
Table 3 Dimensions of mIGCT during multi-stage tube sinking. Drawing passes
D/mm
˛/◦
l1 /m
l2 /m
t/m
h/m
ˇ
0 1 2 3 4 5
6 4 3.5 2.6 1.9 1.6
60 52 32 −5 −10 −26
925 506 300 180 92 80
350 260 235 205 190 180
480 476 386 300 260 233
168 152 153 150 131 107
2.64 1.95 1.28 0.88 0.48 0.44
that the reduction rate is l1 > t > l2 > h. Hence, the experimental results agree with the responding stress and strain distribution of mIGCT in simulation very well. Results also show that ˇ is reduced after each drawing pass. It may be due to the work-hardening of sinking. 3.4. Flaw analysis during multi-drawing 3.4.1. Teeth bonding Fig. 9 shows SEM images of mIGCT while ˇ = 1 in radial during multi-drawing. Compared with Fig. 8 (with ˇ = 2.64), grooves gradually disappear during the multi-stage tube sinking process. While D = 3.5 mm, tooth root contacts each other. Then after a further drawing pass, the grooves in mIGCT bond with each other gradually at the direction from tooth root to top while D = 2.6 mm and 1.9 mm, respectively, as shown in Fig. 9. While D = 1.6 mm, the grooves vanish completely, and no grooves are available anymore except the rough inner surface. So, ˇ value should be carefully designed while multi-stage tube sinking process is utilized to manufacture mIGCT. Normally, ˇ of originally mIGCT is above 1. For a detail mIGCT manufacturing design, a reversal design method may be useful. Based on the final mIGCT dimensions, the corresponding initial mIGCT is determined. 3.4.2. Folding grooves The roundness of mIGCT is easily deteriorated while ˇ increasing. The cross-section of mIGCT can be described as a polygon while ˇ increasing to a certain extent. Fig. 10 shows SEM images of
Fig. 9. SEM images of No. 1 mIGCT in cross-section during multi-stage tube sinking.
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their dimensions may be not all equal, and their distribution may be not uniform in a circle. As lugs grew to a certain extent, they would squeeze teeth, and the shapes of teeth become not regular, as shown in Fig. 11. 3.4.3. Segmental teeth For the weakness of tooth, the straightness of tooth is very important during mIGCT tube sinking. If the tooth of initial mIGCT twisted, it may lead to segmental teeth after several tube sinking passes, as shown in Fig. 12. 4. Conclusions Multi-stage tube sinking process is used to manufacture mIGCT having an outer diameter less than 6 mm. The results are summarized as follows:
Fig. 10. SEM images of No. 3 mIGCT in cross-section during multi-stage tube sinking.
Fig. 11. SEM axial images of No. 3 mIGCT during multi-stage tube sinking.
No. 3 mIGCT during multi-stage tube sinking. While at the second drawing pass, drawn from 4 mm to 3.5 mm, several folding grooves appear. There is a flute mark generated in the middle of a groove outside of the mIGCT, and a lug generated on the back side of the flute mark. The folding grooves seldom generate at other place except in the middle of grooves, and their dimensions will increase after further tube sinking going on. Larger pipe reduction will results in a larger folding groove than that of minor one. Furthermore, several folding grooves can appear at the same time around the circle. But
Fig. 12. SEM images of segmental teeth. (a) Twisted teeth and (b) segmental teeth.
(1) The tube sinking is a three-dimensional stress process, which has axial stress 1 , radial stress r , and hoop stress . r is the compressive stress in tube wall zone, but it may change into tensile stress in the tooth area. is the compressive stress, its absolute value grows in radial in the tube wall region, but reduces dramatically in the tooth region. 1 is the tensile stress, which is almost the same as the value in the tube wall area, but grows quickly in the tooth area. (2) Effective strain reaches its maximum value on the blending region between groove and tooth. The tooth zone is fragile during the sinking, and its average damage value is about three times larger than that of the tube wall region. (3) During multi-stage tube sinking of mIGCT, both grooves and teeth change a lot, including the mIGCT dimensions and the inner surface. The plastic deformation of groove is more intensive than that of tooth, and more intensive at the tooth root than at the tooth top. Furthermore, groove shape grows closed, groove surface grows rough. The ratio of groove width to tooth width (ˇ) reduces after each drawing pass. (4) There are three kinds of flaws easily generated during multistage tube sinking, including teeth bonding, folding grooves and segmental teeth. Acknowledgements This work is financially supported by the National Natural Science Foundation of China, Project Nos. 50705031 and U0834002 and Guangdong Natural Science Foundation, Project Nos. 07118064 and 8151064101000058. References Furushima, T., Manabe, K., 2008. FE analysis of size effect on deformation and heat transfer behavior in microtube dieless drawing. J. Mater. Process. Technol. 201, 123–127. Hopkins, R., Faghri, A., Khrustalev, D., 1999. Flat miniature heat pipes with micro capillary grooves. J. Heat Trans-T ASME 121, 102–109. Jiao, A.J., Ma, H.B., Critse, J.K., 2007. Evaporation heat transfer characteristics of a grooved heat pipe with micro-trapezoidal grooves. Int. J. Heat Mass Transfer 50, 2905–2911. Kazunari, Y., Hiroaki, F., 2004. Mandrel drawing and plug drawing of shape-memoryalloy fine tubes used in catheters and stents. J. Mater. Process. Technol. 153–154, 145–150. Kyu, H.D., Sung, J.K., Suresh, V.G., 2008. A mathematical model of analyzing the thermal characteristics of a flat micro heat pipe with a grooved wick. Int. J. Heat Mass Transfer 51, 4637–4650. Lan, K., Jong, H.C., Sun, H.J., Moo, W.S., 2007. Thermal analysis of LED array system with heat pipe. Thermochim. Acta 455, 21–25. Li, Y., Tang, Y., Li, X.B., et al., 2008. Study on inner micro-grooves of heat pipe spinning process and multi-tooth mandrel. Key Eng. Mater. 375-376, 358–363. Rotarescu, M.I., 1995. A theoretical analysis of tube spinning using balls. J. Mater. Process. Technol. 54, 224–229.
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Suman, B., 2007. Modeling, experiment, and fabrication of micro-grooved heat pipes: an update. Appl. Mech. Rev. 60, 107–119. Tang, Y., Chi, Y., Chen, J.C., et al., 2007. Experimental study of oil-filled high-speed spin forming mic-groove fin-inside tubes. Int. J. Mach. Tool Manuf. 47, 1059–1068. Vasiliev, L.L., 2008. Micro and miniature heat pipes—electronic component coolers. Appl. Therm. Eng. 28, 266–273.
Xie, X.L., He, Y.L., Tao, W.Q., Yang, H.W., 2008. An experimental investigation on a novel high-performance integrated heat pipe–heat sink for high-flux chip cooling. Appl. Therm. Eng. 28, 433–439. Zhang, G.L., Zhang, S.H., Li, B., Zhang, H.Q., 2007. Analysis on folding defects of inner grooved copper tubes during ball spin forming. J. Mater. Process. Technol. 184, 393–400.
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Experimental and FEM study on sinking of miniature inner grooved copper tube Yong Tang a , Long-sheng Lu a,∗ , Dong Yuan a , Qing-hui Wang a , Xiao-lin Zhao a,b a b
School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, China Department of Mechanical and Energy Engineering, Shaoyang University, Shaoyang 422004, China
a r t i c l e
i n f o
Article history: Received 13 June 2008 Received in revised form 19 March 2009 Accepted 1 April 2009 Keywords: Copper tubes Inner grooved tube Drawing Sinking Plastic deformation Heat pipes
a b s t r a c t Miniature inner grooved copper tubes (mIGCT) which have an outer diameter less than 6 mm are in demand for the production of heat pipes. In this work, it is proposed to manufacture such tubes by a multi-stage tube sinking process with an initial mIGCT having an outer diameter of 6 mm. A FEM simulation approach is used to analyze stress, strain and damage distribution for the proposed process. For comparison, a smooth copper tube is also used in the simulation study. Furthermore, experiments are conducted to investigate plastic deformation of the grooves and teeth of the tube. Results show that the maximum stress and strain are occurred at the grooves area, while the maximum damage is located at the top of the teeth. The ratio of groove width to tooth width (ˇ) is reduced after each drawing pass. Bonding, folding and segmenting, which represent potential flaws, have also been observed in the multi-stage tube sinking process, and are discussed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Heat pipes are widely applied for heat transfer in electronic products. Xie et al. (2008) and Lan et al. (2007) have evaluated the heat sink performance with integrated heat pipe for high flux chip and LED array system, respectively. Vasiliev (2008) has also presented a short review covering thermal performance in micro and miniature heat pipes. They all reported that heat pipes are useful on high flux spreading in a limited space. Generally, the performance of a heat pipe depends on its wick structure. Due to cost-effective and lightweight, the inner grooved wick structure represents one of the most important wick types. Kyu et al. (2008) developed a mathematical model for predicting the thermal performance of a heat pipe with a rectangular inner grooved wick, and found that heat pipe performance could be improved by 20% with optimized groove width and depth geometry. Hopkins et al. (1999) investigated the grooves geometry effect on flat miniature heat pipe performance. Results show that heat pipes with a micro-capillary inner grooved wick demonstrate excellent performance characteristics. Jiao et al. (2007) suggested that a heat pipe with a thin tube wall has better thermal performance than one with a thick wall. Therefore, the manufacturing process for controlling the grooves geometry needs to be studied to improve or optimize heat pipe production. Generally, an inner grooved structure is used to enhance heat transfer in air-conditioner industry. A grooved structure is nor-
∗ Corresponding author. Tel.: +86 020 8711 4634; fax: +86 020 8711 4634. E-mail address: [email protected] (L.-s. Lu). 0924-0136/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2009.04.003
mally manufactured by either fixed plug or floating plug drawing. Firstly, teeth are fabricated on the surface of the plug, and then the reverse grooves are generated on the inner surface of the tube via the plastic deformation during drawing. These methods could also be applied to manufacture miniature inner grooved copper tubes (mIGCT) with an outer diameter larger than 6 mm, but it is difficult to satisfy the groove geometry requirements needed for heat pipes. Hence, ball spinning with a plug has been applied in manufacturing mIGCT. This process was theoretically analyzed by Rotarescu (1995), and results showed that it usually consumed much less power when compared to the conventional die drawing. Zhang et al. (2007) focused on the folding defect on the inner surface of the inner grooved copper tube during the ball spinning process, and several effective solutions were proposed to prevent the folding defects. However, it has proven hard to manufacture mIGCT having an outer diameter less than 6 mm (especially less than 4 mm) even with an optimized ball spinning method, due to the difficulty of fabricating high accuracy micro-tools and pulling the plug through the tube, as indicated by Furushima and Manabe (2008). Furthermore, Kazunari and Hiroaki (2004) concluded that the fracture frequency of a tube dramatically increased because the drawing force might overflow the mIGCT’s plastic limit. However, inner grooved heat pipes having an outer diameter less than 6 mm is in high demand to deal with the demands of miniaturization development of electronic products as analyzed by Suman (2007). So the manufacturing technology of mIGCT needs to be studied to develop effective manufacturing processes for mIGCT. Sinking is one of the most important drawing methods to manufacture tubes. Compared with plug drawing, sinking is a process
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Fig. 1. Characterization of mIGCT. h—Teeth height (grooves depth), D—outer diameter of copper tube, ˛—grooves angular, l1 —grooves width in average, l2 —teeth width in average, t—wall thickness of mIGCT.
Fig. 2. The sketch of drawing process and die. (a) Drawing machine and (b) drawing die.
that can reduce dimensions of the tubes without an inside mandrel, but with a smaller drawing force and a higher productivity. Generally, sinking is applied for the drawing of smooth tube without any internal grooves or any other structures. Without a mandrel, the thickness and shape of the tube wall are determined by its initial dimensions and drawing dies. Therefore, the tube sinking can be used to draw micro-tubes with outer diameters even less than 1 mm. In this work, multi-stage tube sinking is designed to manufacture mIGCT having an outer diameter less than 6 mm. Compared with the conventional tube sinking utilized smooth copper tube as workpiece, the multi-stage tube sinking is processed on the mIGCT having an outer diameter of 6 mm. So it is a further processing method of mIGCT fabricated by the ball spinning method. The grooves deformation during tube sinking is analyzed by both finite element simulation and experiments.
formation mechanism of the mIGCT from copper tube by oil-filled ball spinning method was analyzed, and optimal manufacturing parameters were suggested. In another paper (Li et al., 2008), the design of plug was analyzed, and some methods were proposed to avoid tube defects. So the analysis of the grooves deformation by oilfilled ball spinning method will not be included in this paper. Before mIGCT sinking, further processing methods, including tempering and sizing, are conducted to ensure the copper tubes in straight state and 68 MPa in yield strength ( 0.2 ). As shown in Table 2, three kinds of copper tubes with different ratio of groove width to tooth width (ˇ = l1 /l2 ) are used in the experiments. 2.2. Experimental procedure Multi-stage tube sinking is operated on mIGCT having an outer diameter of 6 mm. The sinking passes follow as 4 mm, 3.5 mm,
2. Experimental method 2.1. Materials The workpiece material, as shown in Fig. 1, is mIGCT. At first, mIGCT is manufactured from copper T2 by oil-filled ball spinning method. In one of authors’ previous papers (Tang et al., 2007), the Table 1 Parameters of drawing dies. Drawing die no.
D/mm
D /mm
d/mm
1 2 3 4 5
6 4 3.5 2.6 1.9
6.2 4.2 3.7 2.8 2.1
4 3.5 2.6 1.9 1.6
Fig. 3. FEM simulation model.
Y. Tang et al. / Journal of Materials Processing Technology 209 (2009) 5333–5340
Fig. 4. Stress comparison between No. 2 mIGCT and smooth copper tube. (a) Radial stress r , (b) hoop stress , (c) axial stress l and (d) effective stress.
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Table 2 Parameters of inner grooves copper tubes during sinking. mIGCT no.
Tube state
Grooves number
D/mm
˛/◦
l1 /m
1
Initiation Simulation Experiment
18
6 4 4
60 40 43
2
Initiation Simulation Experiment
12
6 4 4
3
Initiation Simulation Experiment
12
6 4 4
l2 /m
t/m
h/m
ˇ
420 136 142
420 280 290
480 475 470
168 147 150
1 0.49 0.49
60 12 52
925 490 506
350 251 260
480 476 476
168 146 152
2.64 1.95 1.95
10 5 8
1080 552 560
190 145 160
480 475 476
168 136 140
5.68 3.81 3.5
Fig. 5. Strain comparison between No. 2 mIGCT and smooth copper tube.
2.6 mm, 1.9 mm, and 1.6 mm. All the experiments are done on the same drawing machine but with different drawing dies. The drawing machine consists of a drawing die, a die holder and a chuck jaw, as shown in Fig. 2(a). The sketch of the drawing dies is shown in Fig. 2(b). It could be divided into four zones: I—guard, II—reducing, III—calibrating and IV—receding. The drawing angular in sizing reduction area is 12◦ ; the length of calibrating area is 4 mm; the receding angular in receding area is also 12◦ . The dimensions of D and d are as shown in Table 1. All the experiments are done at a drawing speed of 10 m/min under good lubrication condition. 2.3. FEM simulation method It is difficult to observe and clarify the deformation behavior of inner grooves and teeth during the sinking process. In order to
investigate the stress and strain, A FEM analysis with rigid-plastic principle is performed, as shown in Fig. 3. It is carried out by using DEFORM ver.2001. In the FEM model, only one groove is separated from the mIGCT copper tube because the workpiece is axisymmetric. During simulations, the workpiece (plastic body) is set as static with its length of 15 mm, and it is divided into 100,000 tetrahedral elements in average. The drawing dies (rigid body) move at a relative speed (10 m/min). The friction factor between workpiece and drawing die is set as 0.06 because oil film lubrication is used. Three kinds of mIGCT which have the same dimensions in cross-section as in experiment are used, as shown in Table 2. Besides, a smooth copper tube, which has the same dimensions with mIGCT except the teeth, is used for comparison. The dimensions of tube sinking die are also the same as that used in the experiment.
Fig. 6. Damage comparison between No. 2 mIGCT and smooth copper tube.
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Fig. 7. SEM images of No. 2 mIGCT in cross-section during multi-stage tube sinking.
3. Results and discussions 3.1. Stress analysis with FEM model Fig. 4 shows stress comparison between No. 2 mIGCT and smooth copper tube during the first tube sinking pass in the analyzing plane. The tube sinking is a three-dimensional stress process, which includes axial stress 1, radial stress r, and hoop stress . For smooth copper tube, r is the compressive stress, r = 0 at the inner surface, and grows to its maximum value at the outer surface. For mIGCT, however, r may changes into tensile stress in the tooth region. of both smooth copper tube and mIGCT tube wall region is also the compressive stress, and its absolute value increases in radial from outer to inner surface, but decreases rapidly from the tooth root to top for mIGCT. Furthermore, 1 is tensile stress for both smooth copper tube and mIGCT. It is almost the same as in the tube wall region of mIGCT and in the smooth copper tube, but it grows
quickly in the tooth region for mIGCT. Consequently, the effective stress of smooth copper tube grows gradually in radial from outer to inner surface. The effective stress of mIGCT is different between the tube wall and tooth region, it increases in radial from outer to inner surface in the tube wall, and decreases from tooth root to top in the tooth region. 3.2. FEM and experimental analysis on grooves deformation Fig. 5 shows the effective strain comparison between smooth copper tube and No. 2 mIGCT. Results indicate that the effective strain distribution is similar as effective stress. The strain reaches its maximum on the blending region between groove and tooth. The average strain in groove region is larger than that in tooth. So the groove deformation is more intensive than that of the tooth. Table 2 shows mIGCT dimensions changes after tube sinking by experiments and simulations. Results indicate that ˛, l1 , l2 dimen-
Fig. 8. SEM images of No. 2 mIGCT in axial during multi-stage tube sinking.
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sions are all reduced after drawing. The reduced amount of l1 is larger than that of l2 . So the plastic deformation of the groove is more intensive than that of the tooth. However, for a single tooth, its strain distribution differs from the root to top. The strain in the root region is larger than that of the top. So the deformation of the tooth root is more intensive than that of the tooth top. The angle of the tooth root changes into negative, while the angle of the tooth top remains positive. The comparison of damage distribution between mIGCT and smooth copper tube is shown in Fig. 6. The damage of the smooth copper tube ranges from 0.12 to 0.15 with an identical growth rate from outer to inner surface, and its maximum value is located at the inner surface. The damage of mIGCT ranges from 0.15 to 0.34, and its maximum value is located at the tooth top. So the damage of mIGCT is larger than that of the smooth copper tube. Furthermore, the average damage value of the tooth region is about three times larger than that of the tube wall region. So the tooth is a fragile region during the tube sinking process of mIGCT. It is due to the cyclic stress and weak rigidity in tooth area. 3.3. Multi-stage tube sinking analysis mIGCT having an outer diameter of 1.6 mm is manufactured by multi-stage tube sinking on No. 2 mIGCT. In view of the demand of thin wall tube for heat pipes and affordable drawing force of copper tube, experiments show that five drawing passes are suitable, following as 4 mm, 3.5 mm, 2.6 mm, 1.9 mm, and 1.6 mm. Fig. 7 shows the SEM images in cross-section during the multi-stage tube sinking process, and Fig. 8 shows the SEM images in axial section. Results show that both the grooves and teeth reduce gradually during sinking. The changes include mIGCT dimensions and inner surface roughness. The inner surfaces of grooves and teeth turn to be rougher than that of before after processing. And grooves gradually grow closed after multistage tube sinking. Table 3 presents detail information about mIGCT after each drawing pass in experiments. After five drawing passes, outer diameter (D) reduces by 73.3%, grooves width (l1 ) reduces by 91.4%, teeth width (l2 ) reduces by 48.6%, wall thickness (t) reduces by 51.5%, and teeth height (h) reduces by 36.3%. It finds
Table 3 Dimensions of mIGCT during multi-stage tube sinking. Drawing passes
D/mm
˛/◦
l1 /m
l2 /m
t/m
h/m
ˇ
0 1 2 3 4 5
6 4 3.5 2.6 1.9 1.6
60 52 32 −5 −10 −26
925 506 300 180 92 80
350 260 235 205 190 180
480 476 386 300 260 233
168 152 153 150 131 107
2.64 1.95 1.28 0.88 0.48 0.44
that the reduction rate is l1 > t > l2 > h. Hence, the experimental results agree with the responding stress and strain distribution of mIGCT in simulation very well. Results also show that ˇ is reduced after each drawing pass. It may be due to the work-hardening of sinking. 3.4. Flaw analysis during multi-drawing 3.4.1. Teeth bonding Fig. 9 shows SEM images of mIGCT while ˇ = 1 in radial during multi-drawing. Compared with Fig. 8 (with ˇ = 2.64), grooves gradually disappear during the multi-stage tube sinking process. While D = 3.5 mm, tooth root contacts each other. Then after a further drawing pass, the grooves in mIGCT bond with each other gradually at the direction from tooth root to top while D = 2.6 mm and 1.9 mm, respectively, as shown in Fig. 9. While D = 1.6 mm, the grooves vanish completely, and no grooves are available anymore except the rough inner surface. So, ˇ value should be carefully designed while multi-stage tube sinking process is utilized to manufacture mIGCT. Normally, ˇ of originally mIGCT is above 1. For a detail mIGCT manufacturing design, a reversal design method may be useful. Based on the final mIGCT dimensions, the corresponding initial mIGCT is determined. 3.4.2. Folding grooves The roundness of mIGCT is easily deteriorated while ˇ increasing. The cross-section of mIGCT can be described as a polygon while ˇ increasing to a certain extent. Fig. 10 shows SEM images of
Fig. 9. SEM images of No. 1 mIGCT in cross-section during multi-stage tube sinking.
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their dimensions may be not all equal, and their distribution may be not uniform in a circle. As lugs grew to a certain extent, they would squeeze teeth, and the shapes of teeth become not regular, as shown in Fig. 11. 3.4.3. Segmental teeth For the weakness of tooth, the straightness of tooth is very important during mIGCT tube sinking. If the tooth of initial mIGCT twisted, it may lead to segmental teeth after several tube sinking passes, as shown in Fig. 12. 4. Conclusions Multi-stage tube sinking process is used to manufacture mIGCT having an outer diameter less than 6 mm. The results are summarized as follows:
Fig. 10. SEM images of No. 3 mIGCT in cross-section during multi-stage tube sinking.
Fig. 11. SEM axial images of No. 3 mIGCT during multi-stage tube sinking.
No. 3 mIGCT during multi-stage tube sinking. While at the second drawing pass, drawn from 4 mm to 3.5 mm, several folding grooves appear. There is a flute mark generated in the middle of a groove outside of the mIGCT, and a lug generated on the back side of the flute mark. The folding grooves seldom generate at other place except in the middle of grooves, and their dimensions will increase after further tube sinking going on. Larger pipe reduction will results in a larger folding groove than that of minor one. Furthermore, several folding grooves can appear at the same time around the circle. But
Fig. 12. SEM images of segmental teeth. (a) Twisted teeth and (b) segmental teeth.
(1) The tube sinking is a three-dimensional stress process, which has axial stress 1 , radial stress r , and hoop stress . r is the compressive stress in tube wall zone, but it may change into tensile stress in the tooth area. is the compressive stress, its absolute value grows in radial in the tube wall region, but reduces dramatically in the tooth region. 1 is the tensile stress, which is almost the same as the value in the tube wall area, but grows quickly in the tooth area. (2) Effective strain reaches its maximum value on the blending region between groove and tooth. The tooth zone is fragile during the sinking, and its average damage value is about three times larger than that of the tube wall region. (3) During multi-stage tube sinking of mIGCT, both grooves and teeth change a lot, including the mIGCT dimensions and the inner surface. The plastic deformation of groove is more intensive than that of tooth, and more intensive at the tooth root than at the tooth top. Furthermore, groove shape grows closed, groove surface grows rough. The ratio of groove width to tooth width (ˇ) reduces after each drawing pass. (4) There are three kinds of flaws easily generated during multistage tube sinking, including teeth bonding, folding grooves and segmental teeth. Acknowledgements This work is financially supported by the National Natural Science Foundation of China, Project Nos. 50705031 and U0834002 and Guangdong Natural Science Foundation, Project Nos. 07118064 and 8151064101000058. References Furushima, T., Manabe, K., 2008. FE analysis of size effect on deformation and heat transfer behavior in microtube dieless drawing. J. Mater. Process. Technol. 201, 123–127. Hopkins, R., Faghri, A., Khrustalev, D., 1999. Flat miniature heat pipes with micro capillary grooves. J. Heat Trans-T ASME 121, 102–109. Jiao, A.J., Ma, H.B., Critse, J.K., 2007. Evaporation heat transfer characteristics of a grooved heat pipe with micro-trapezoidal grooves. Int. J. Heat Mass Transfer 50, 2905–2911. Kazunari, Y., Hiroaki, F., 2004. Mandrel drawing and plug drawing of shape-memoryalloy fine tubes used in catheters and stents. J. Mater. Process. Technol. 153–154, 145–150. Kyu, H.D., Sung, J.K., Suresh, V.G., 2008. A mathematical model of analyzing the thermal characteristics of a flat micro heat pipe with a grooved wick. Int. J. Heat Mass Transfer 51, 4637–4650. Lan, K., Jong, H.C., Sun, H.J., Moo, W.S., 2007. Thermal analysis of LED array system with heat pipe. Thermochim. Acta 455, 21–25. Li, Y., Tang, Y., Li, X.B., et al., 2008. Study on inner micro-grooves of heat pipe spinning process and multi-tooth mandrel. Key Eng. Mater. 375-376, 358–363. Rotarescu, M.I., 1995. A theoretical analysis of tube spinning using balls. J. Mater. Process. Technol. 54, 224–229.
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Suman, B., 2007. Modeling, experiment, and fabrication of micro-grooved heat pipes: an update. Appl. Mech. Rev. 60, 107–119. Tang, Y., Chi, Y., Chen, J.C., et al., 2007. Experimental study of oil-filled high-speed spin forming mic-groove fin-inside tubes. Int. J. Mach. Tool Manuf. 47, 1059–1068. Vasiliev, L.L., 2008. Micro and miniature heat pipes—electronic component coolers. Appl. Therm. Eng. 28, 266–273.
Xie, X.L., He, Y.L., Tao, W.Q., Yang, H.W., 2008. An experimental investigation on a novel high-performance integrated heat pipe–heat sink for high-flux chip cooling. Appl. Therm. Eng. 28, 433–439. Zhang, G.L., Zhang, S.H., Li, B., Zhang, H.Q., 2007. Analysis on folding defects of inner grooved copper tubes during ball spin forming. J. Mater. Process. Technol. 184, 393–400.