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Influences of lubrication conditions and blank holder force on micro deep drawing of C1100 micro conical–cylindrical cup
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Influences of lubrication conditions and blank holder force on micro deep drawing of C1100 micro conical–cylindrical cup Feng Gong a,∗ , Zhi Yang a , Qiang Chen b , Zhiwen Xie c , Dayu Shu b , Jiali Yang a a b c
Shenzhen Key Laboratory of Advanced Manufacturing Technology for Mold and Die, Shenzhen University, Shenzhen 518060, China Southwest Technology and Engineering Research Institute, Chongqing 400039, China School of Mechanical Engineering and Automation, University of Science and Technology Liaoning, Anshan 114051, China
a r t i c l e
i n f o
Article history: Received 19 June 2014 Received in revised form 25 March 2015 Accepted 12 May 2015 Available online xxx Keywords: Micro deep drawing Micro conical–cylindrical cup Lubrication condition Blank holder force Limiting drawing ratio
a b s t r a c t Lubrication conditions and blank holder force (BHF) are two key processing parameters in deep drawing. This is more obvious in micro forming because of the miniaturization of the specimen size. Micro conical–cylindrical cups with internal conical bottom diameter of only 0.4 mm were well formed. The influences of lubrication conditions and BHF on micro deep drawing of micro conical–cylindrical cups were investigated using a micro blanking–deep drawing compound mold. Pure copper C1100 with a thickness of 50 m, which was annealed at 450 ◦ C for 2 h in vacuum condition, was chosen as the specimen material. The experiments were conducted on a universal testing machine with a forming velocity of 0.05 mm/s under 4 kinds of lubrication conditions and BHF. The experimental results showed that a micro conical–cylindrical cup with internal conical bottom diameter of only 0.4 mm was well formed, and the limiting drawing ratio (LDR) reached 2.1. The polyethylene (PE) film, which decreased the drawing force and increased the drawing ratio (DR), was superior to castor oil, petroleum jelly and dry friction, and can be chosen as a proper lubricant for micro deep drawing. The rim of the micro cup seriously wrinkled when BHF was less than 4.2 N. The bottom of the micro cup cracked when the BHF was larger than 5.6 N. © 2015 Elsevier Inc. All rights reserved.
1. Introduction Micro forming is considered to be a promising process to fabricate metallic micro parts for micro electro mechanical system, microelectronic technology, and precision machinery due to its high efficiency, low cost, low duration, and less pollution [1,2]. Micro deep drawing, which is a fundamental process of micro forming, has potential application in forming hollow, thin walled, cup or box like micro parts. If integrated with other micro forming processes, many kinds of micro parts can be produced [3]. Thus, the investigation of micro deep drawing is more comprehensive than other forming methods and becomes a research focus in micro forming [4]. Erhardt et al. [5] applied a local laser heating method to improve the formability of the drawing blank in micro deep drawing. Saotome et al. [6] investigated the influences of punch diameter to sheet thickness on micro deep drawing by using a special apparatus. Vollertsen et al. [7] studied micro deep drawing according to similarity theory with different materials and found that friction coefficient in microforming was much higher than that in
∗ Corresponding author. Tel.: +86 755 22673522; fax: +86 755 26557471. E-mail address: [email protected] (F. Gong).
macroforming. Justinger et al. [8] formed 1 mm micro cup with CuZn37 and revealed that the cup geometry was influenced by microstructure. Manabe et al. [9] developed a sequential blanking and drawing setup, and fabricated a micro cup of only 0.5 mm in diameter using SUS304 ultra-thin foil. Chen et al. [10] reported that thickness, grain size and number of grains throughout thickness greatly influenced the LDR in micro deep drawing. Shimizu et al. [11] reported that the tool surface greatly affected the formability and accuracy of the micro parts in micro deep drawing. Behrens et al. [12] formed a micro cup of 0.75 mm inner diameter and studied the forming limit by using magnetron sputtered Al-Zr foils. Fu et al. [13] investigated the size effects in micro blanking and deep drawing process by experiments and simulation. Gau et al. [14] studied micro deep drawing with two ironing stages and successfully formed a micro cup of 3.039 mm in height and 2.218 mm in outer diameter using SUS304 with grain size of 45 m, which was annealed at 1050 ◦ C. Compared with macro deep drawing, micro deep drawing is more difficult because the material property, lubrication conditions and BHF change with the miniaturization of the specimen size [15]. Lubrication conditions and BHF, which are two key factors affecting the whole process of deep drawing, including drawing force, DR, surface quality, tool life and others, should be systemically investigated. Although micro deep drawing was widely investigated in
http://dx.doi.org/10.1016/j.precisioneng.2015.05.004 0141-6359/© 2015 Elsevier Inc. All rights reserved.
Please cite this article in press as: Gong F, et al. Influences of lubrication conditions and blank holder force on micro deep drawing of C1100 micro conical–cylindrical cup. Precis Eng (2015), http://dx.doi.org/10.1016/j.precisioneng.2015.05.004
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Fig. 1. Micro blanking–deep drawing mold (a) schematic and (b) photograph.
Fig. 2. Schematic of micro blanking–deep drawing process (a) initial state, (b) blanking, and (c) deep drawing.
the past few years, the research on lubrication conditions and BHF in micro deep drawing is limited. Liquid lubricants, which result in friction size effects in micro forming, are not proper for micro deep drawing. Thus, some researchers focused on solid lubrication films because of their excellent frictional property. Gong et al. [16] formed a 0.95 mm micro cup and studied the effects of solid lubrication films on SKD11 mold and die material. Hu et al. [17] indicated that the diamond like carbon (DLC) coated forming tools can decrease the forming force in micro deep drawing. Although micro cups were well formed, the cost was so expensive compared with normal lubricants and was not appropriate for mass production. With the decrease of the specimen size, BHF is much more difficult to apply than macro deep drawing and cannot be directly computed according to similarity theory (Vollertsen et al. [4]). In micro deep drawing process, micro cylinder cups, micro rectangular cups, micro conical cups, and micro spherical cups are typical parts. However, most existing researches were concentrated on micro cylindrical cups, and few studies were reported on other shaped parts. The purpose of this work is to form micro conical–cylindrical cups with internal conical bottom diameter of 0.4 mm and choose
proper lubricant and BHF for micro deep drawing. Besides, the influences of lubrication conditions and BHF on deep drawing force, LDR, forming quality were also investigated. 2. Experimental setup 2.1. Mold and principle Because the diameter of the conical–cylindrical cup to be formed was too small, the localization is difficult if a simple operation mold was used. To solve this problem, a micro blanking–deep drawing compound mold was developed, as shown in Fig. 1. A micro load cell, which was used to measure the drawing force, was installed in the upper mold. The range of the micro load cell was 500 N, and the measurement accuracy was 0.03% of the full scale. The schematic of micro blanking–deep drawing process can be seen in Fig. 2. Firstly, a blank was placed on the die. Secondly, a circular workpiece with a diameter of D0 was blanked out with the move downward of the upper mold. Thirdly, a proper BHF was generated by the spring and was applied to the workpiece
Please cite this article in press as: Gong F, et al. Influences of lubrication conditions and blank holder force on micro deep drawing of C1100 micro conical–cylindrical cup. Precis Eng (2015), http://dx.doi.org/10.1016/j.precisioneng.2015.05.004
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Fig. 3. Dimensions of the micro deep drawing punch.
Fig. 4. Grain images of C1100 thin sheets.
with the upper mold move downward continuously. Lastly, a conical–cylindrical cup with a top diameter of d1 and bottom diameter of d2 was formed when the drawing punch downward to a specific distance. To increase the strength of the micro deep drawing punch, a stepped construction was used in this study, as shown in Fig. 3. The micro deep drawing punch was fabricated with a highly dimensional accuracy of ±1 m by micro grinding. To study the LDR in micro deep drawing of micro conical–cylindrical cups, a series of compound punch-dies, blanking dies, die inserts, and blank holders were manufactured. The DR is given by the following equation: K=
C0 Cp
(1)
where K is the DR, C0 = D0 is the circumference of the deep drawing workpiece, and Cp = d1 is the circumference of the middle section of the terminal face of the part. 2.2. Specimen and die material Pure copper C1100, which was widely used in micro electronic industry, micro electro mechanical system and micro system technologies, was chosen as testing material. The raw material was hard rolled sheet, the thickness of which was 50 m. The chemical compositions were (wt%): Cu ≥ 99.90, O ≤ 0.06, S ≤ 0.005, Pb ≤ 0.005, Sb ≤ 0.002, Bi ≤ 0.002, As ≤ 0.002. In order to increase the plasticity of the thin sheet, the raw materials were annealed at 450 ◦ C for 2 h in vacuum condition. The annealing sheet was etched using a solution of 1 g of Fe(NO3 )3 and 25 mL of C2 H5 OH for 3–5 s. Grain images of the sheet were captured by an optical microscope. The average grain size was about 20 m, as shown in Fig. 4. Tensile tests were conducted to obtain the true strain–true stress curves of the sheet by using a Zwick/Roell Z050 universal test machine, as shown in Fig. 5. The yield stress and tensile stress were about 80 MPa and 200 MPa, respectively.
Fig. 5. True stress–true strain curve of 50 m thick C1100 sheet.
Tool steel alloy SKD11, which was widely used as punch and die material in microforming, was chosen as mold material. The hardness of the mold materials was 60 HRC after quenching. The specific chemical compositions were (wt%): Cr: 11–13, C: 1.4–1.6, Mo: 0.5–1.2, V: 0.2–0.5, Mn ≤ 0.6, Ni ≤ 0.5, Si ≤ 0.4, S ≤ 0.03, P ≤ 0.03, Fe: Balance. 2.3. Experimental parameters Micro blanking–deep drawing of conical–cylindrical micro cup experiments were performed on a SANS CMT5504 electronic universal testing machine at room temperature. The drawing velocity was 0.05 mm/s. The drawing punch stroke was acquired directly by this machine, and the drawing force can be acquired by the micro load cell. The diameters of the micro blanking punches were 1.8, 1.9, 2.0, 2.1 and 2.2 mm. The drawing clearance between the micro drawing punch and die was 50 m, which was the same to the workpiece. The radius of the micro deep drawing die was 0.2 mm. Besides dry friction, three kinds of lubricants were used. One was castor oil, whose dynamic viscosity was 0.61 Pa s. Another was petroleum jelly with dynamic viscosity of which was 1.08 Pa s. The third was PE film, whose thickness was 7 m. The BHF were 2.8, 4.2, 5.6 and 7.0 N. 3. Results and discussion 3.1. Micro conical–cylindrical cups SEM images of micro conical–cylindrical cups with different DR are shown in Fig. 6. The lubricant was PE film, and the maximum BHF was 4.2 N. It is clear that the micro conical–cylindrical cup with a LDR of 2.1 was well formed. There were only a few wrinkles in the rim of the cups when the DR was large. This is because the BHF for these micro cups was constant. On one hand, the degree of deformation increased with increasing the DR, which resulted the
Please cite this article in press as: Gong F, et al. Influences of lubrication conditions and blank holder force on micro deep drawing of C1100 micro conical–cylindrical cup. Precis Eng (2015), http://dx.doi.org/10.1016/j.precisioneng.2015.05.004
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Fig. 6. SEM images of micro conical–cylindrical cups with different DR (a) K = 1.9; (b) K = 2.0; (c) K = 2.1 and (d) K = 2.2.
Fig. 7. Deep drawing force–deep drawing punch stroke curves with different DR.
Fig. 8. Drawing force–displacement curves under different lubrication conditions.
increase of tangential compressive stress and wrinkles. On the other hand, the width of deformation area increased with increasing the DR, which resulted in the decrease of resistance ability of buckling and wrinkles. There were clear wrinkles in micro deep drawing of cylindrical cups with large DR in the previous studies [4,8,18].
the increase of deformation resistance and decrease of deformation area in the deep drawing process, the deep drawing force first increased and then decreased, and this resulted in the first peak. Then the flange wrinkled because of the inadequate BHF. However, the drawing punch compelled the wrinkled blank into the drawing clearance, which resulted in the second peak. With the increase of the DR, the wall of the micro cup was cracked, and the deep drawing process was terminated. The deep drawing force increased with the increase of the DR, and the maximum drawing force increased 12.1% with the DR increasing from 1.8 to 2.1. This is because the deformation degree and deformation resistance of the flange increase with the increase of the DR.
3.2. Deep drawing force Fig. 7 shows the deep drawing force–deep drawing punch stroke curves with different DR. The same trend with two peaks in each curve can be seen from the figure. This was similar to micro deep drawing of micro cylindrical cups [11,18]. Due to the influence of
Please cite this article in press as: Gong F, et al. Influences of lubrication conditions and blank holder force on micro deep drawing of C1100 micro conical–cylindrical cup. Precis Eng (2015), http://dx.doi.org/10.1016/j.precisioneng.2015.05.004
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Fig. 9. Drawing force–displacement curves with different BHF.
The deep drawing force–deep drawing punch stroke curves under different lubrication conditions are shown in Fig. 8. The deep drawing force was much lower than those of castor oil and petroleum jelly when a PE film was used. However, the friction coefficient of castor oil, petroleum jelly and PE film was all most the same in macro sheet forming. This is due to occurrence of friction size effects in micro sheet forming. Most of the castor oil and petroleum jelly were squeezed out with the miniaturization of the workpiece, which increased the friction coefficient. However, the PE film was always between the workpiece and drawing die interfaces in the whole micro deep drawing process. Thus, the PE film was much better than castor oil and petroleum jelly, which can be chosen as a proper lubricant for micro sheet forming.
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Fig. 10. Effect of lubrication condition on LDR.
Fig. 9 shows the deep drawing force–deep drawing punch stroke curves with different maximum BHF. The maximum deep drawing force increased with increasing BHF. However, when the maximum BHF was larger than 5.6 N, the micro cup was cracked and the forming process was terminated. It also can be seen from the figure that the second peak of the deep drawing force–deep drawing punch stroke curves diminished with the increase of the BHF, because the wrinkles diminished with the increase of the BHF. 3.3. LDR LDR, which depends on the maximum tension stress on the wall of the cup and tensile strength of the dangerous cross-section, is a quite important factor in industry in order to make full use
Fig. 11. SEM images of micro conical–cylindrical cups under different lubrication conditions (a) castor oil, (b) petroleum jelly, (c) PE film, (d) dry friction.
Please cite this article in press as: Gong F, et al. Influences of lubrication conditions and blank holder force on micro deep drawing of C1100 micro conical–cylindrical cup. Precis Eng (2015), http://dx.doi.org/10.1016/j.precisioneng.2015.05.004
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Fig. 12. SEM images of micro conical–cylindrical cups with different BHF (a) 2.8 N; (b) 4.2 N; (c) 5.6 N and (d) 7.0 N.
of the forming ability of raw material and to reduce the cost of manufacture. The mechanical property of material, punch radius, dies radius, drawing clearance, lubrication condition and other parameters have great influences on LDR. The effects of lubrication conditions on LDR in micro deep drawing were shown in Fig. 10. It is clear from the figure that LDR increased significantly using lubricant compared to dry friction. The minimum LDR was only 1.7 under dry friction. However, the maximum LDR reached 2.1 under the lubrication of PE film, which was much better than the existing researches on micro deep drawing of cylindrical cups [4,8]. The LDR were 1.8 and 1.9 under the lubrication of castor oil and petroleum jelly, respectively, which were only a little higher than that under dry friction, because of the friction size effect in micro deep drawing.
3.4. Forming quality SEM images of micro conical–cylindrical cups formed under different lubrication conditions are shown in Fig. 11. The maximum BHF was 4.2 N, and the DR was 1.8. It is clear from the figure that the micro cup was cracked under dry friction. Although the micro cups were successfully formed under the lubrication of castor oil and petroleum jelly, there were more scratches on the rim of the cups compared to those under the lubrication of PE film. This is because most of the lubricant was squeezed out in micro sheet forming, and the workpiece and deep drawing die contacted directly, scratches were inevitable under those serious large deformation and friction. The SEM images of micro conical–cylindrical cups formed with different BHF are shown in Fig. 12. The lubricant was PE film, and the DR was 1.8. The BHF have great influence on the surface quality of the micro cups. When there was no BHF or the BHF was small, the material could flow freely, which resulted in wrinkles, as shown in Fig. 12(a). When there was appropriate BHF on the blank, the material could generate sufficient stretching, neither wrinkles nor cracks occurred, as shown in Fig. 12(b) and (c). When the blank
holder force was too large, the material cannot flow. However, the deep drawing punch compels the material to stretching, which results in crack, as shown in Fig. 12(d). It is quite difficult to determine the proper BHF with the miniaturization of the specimen size. There were serious wrinkles in the rim of the cup in the existing researches of which improper BHF were used [4,5,9]. 4. Conclusions Based on the micro deep drawing experiments of C1100 thin sheet with 4 kinds of different lubrication conditions and BHF, the following results can be concluded: 1. Micro conical–cylindrical cup with an internal conical bottom diameter of only 0.4 mm was successfully formed using a micro blanking–deep drawing compound mold, and the LDR reached to 2.1. 2. PE film, which reduced the drawing force and increased the LDR, was superior to liquid lubricants, and was suitable for micro forming as a lubricant. 3. When the BHF was less than 4.2 N, the rim of the micro cup was wrinkled. When the BHF was larger than 5.6 N, the bottom of the micro cup was cracked. Acknowledgments The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (No. 51205258), the Educational Commission of Guangdong Province (No. 2012LYM 0113), and the Scientific Research Foundation of Shenzhen University (No. 201141). References [1] Geiger M, Kleiner M, Eckstein R, Tiesler N, Engel U, Microforming. CIRP Ann Manuf Technol 2001;52(2):445–62.
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[2] Schubert A, Jahn SF, Müller B. Modular tool concept and process design for micro impact extrusion. Precis Eng 2014;38(1):57–63. [3] Shan DB, Guo B, Zhou J. Development in micro forming of thin metal sheet. J Plast Eng 2007;14(3):93–9. [4] Vollertsen F, Hu Z, Stock H, Koehler B. State of the art in micro forming and investigations into micro deep drawing. J Mater Process Technol 2004;151(1–3):70–9. [5] Erhardt R, Schepp F, Schmoeckel D. Micro forming with local part heating by laser irradiation in transparent tools. Proc 7th SHEMET 1999:497–504. [6] Saotome Y, Yasuda K, Kaga H. Microdeep drawability of very thin sheet steels. J Mater Process Technol 2001;113:641–7. [7] Vollertsen F, Hu Z. Analysis of punch velocity dependent process window in micro deep drawing. Prod Eng 2010;4(6):553–9. [8] Justinger H, Hirt G, Witulski N. Analysis of cup geometry and temperature condition in the miniaturized deep drawing. Proc 8th ICTP 2005. [9] Manabe K, Shimizu T, Koyama H, Yang M, Ito K. Validation of FE simulation based on surface roughness model in micro-deep drawing. J Mater Process Technol 2008;204(1-3):89–93. [10] Chen CH, Gau JT, Lee RS. An experimental and analytical study on the limit drawing ratio of stainless steel 304 foils for microsheet forming. Mater Manuf Process 2009;24(12):1256–65.
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[11] Shimizu T, Murashige Y, Ito K, Manabe K. Influence of surface topographical interaction between tool and material in micro-deep drawing. J Solid Mech Mater Eng 2009;3(2):397–408. [12] Behrens G, Kovac J, Köhler B, Vollertsen F, Stock H. Drawability of thin magnetron sputtered Al-Zr foils in micro deep drawing. Trans Nonferr Metals Soc China 2012;22(S2):s268–74. [13] Fu MW, Yang B, Chan WL. Experimental and simulation studies of micro blanking and deep drawing compound process using copper sheet. J Mater Process Technol 2013;213(1):101–10. [14] Gau JT, Teegala S, Huang KM, Hsiao TJ, Lin BT. Using micro deep drawing with ironing stages to form stainless steel 304 micro cups. J Manuf Process 2013;15(2):298–305. [15] Engel U. Tribology in microforming. Wear 2006;260(3):265–73. [16] Gong F, Guo B, Wang CJ, Shan DB. Micro deep drawing of micro cups by using DLC film coated blank holders and dies. Diamond Related Mater 2011;20(2):196–200. [17] Hu Z, Schubnov A, Vollertsen F. Tribological behaviour of DLC-films and their application in micro deep drawing. J Mater Process Technol 2012;212(3):647–52. [18] Gong F, Guo B, Wang CJ, Shan DB. Effects of lubrication conditions on micro deep drawing. Microsyst Technol 2010;16(10):1741–7.
Please cite this article in press as: Gong F, et al. Influences of lubrication conditions and blank holder force on micro deep drawing of C1100 micro conical–cylindrical cup. Precis Eng (2015), http://dx.doi.org/10.1016/j.precisioneng.2015.05.004
ARTICLE IN PRESS
PRE-6240; No. of Pages 7
Precision Engineering xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Precision Engineering journal homepage: www.elsevier.com/locate/precision
Influences of lubrication conditions and blank holder force on micro deep drawing of C1100 micro conical–cylindrical cup Feng Gong a,∗ , Zhi Yang a , Qiang Chen b , Zhiwen Xie c , Dayu Shu b , Jiali Yang a a b c
Shenzhen Key Laboratory of Advanced Manufacturing Technology for Mold and Die, Shenzhen University, Shenzhen 518060, China Southwest Technology and Engineering Research Institute, Chongqing 400039, China School of Mechanical Engineering and Automation, University of Science and Technology Liaoning, Anshan 114051, China
a r t i c l e
i n f o
Article history: Received 19 June 2014 Received in revised form 25 March 2015 Accepted 12 May 2015 Available online xxx Keywords: Micro deep drawing Micro conical–cylindrical cup Lubrication condition Blank holder force Limiting drawing ratio
a b s t r a c t Lubrication conditions and blank holder force (BHF) are two key processing parameters in deep drawing. This is more obvious in micro forming because of the miniaturization of the specimen size. Micro conical–cylindrical cups with internal conical bottom diameter of only 0.4 mm were well formed. The influences of lubrication conditions and BHF on micro deep drawing of micro conical–cylindrical cups were investigated using a micro blanking–deep drawing compound mold. Pure copper C1100 with a thickness of 50 m, which was annealed at 450 ◦ C for 2 h in vacuum condition, was chosen as the specimen material. The experiments were conducted on a universal testing machine with a forming velocity of 0.05 mm/s under 4 kinds of lubrication conditions and BHF. The experimental results showed that a micro conical–cylindrical cup with internal conical bottom diameter of only 0.4 mm was well formed, and the limiting drawing ratio (LDR) reached 2.1. The polyethylene (PE) film, which decreased the drawing force and increased the drawing ratio (DR), was superior to castor oil, petroleum jelly and dry friction, and can be chosen as a proper lubricant for micro deep drawing. The rim of the micro cup seriously wrinkled when BHF was less than 4.2 N. The bottom of the micro cup cracked when the BHF was larger than 5.6 N. © 2015 Elsevier Inc. All rights reserved.
1. Introduction Micro forming is considered to be a promising process to fabricate metallic micro parts for micro electro mechanical system, microelectronic technology, and precision machinery due to its high efficiency, low cost, low duration, and less pollution [1,2]. Micro deep drawing, which is a fundamental process of micro forming, has potential application in forming hollow, thin walled, cup or box like micro parts. If integrated with other micro forming processes, many kinds of micro parts can be produced [3]. Thus, the investigation of micro deep drawing is more comprehensive than other forming methods and becomes a research focus in micro forming [4]. Erhardt et al. [5] applied a local laser heating method to improve the formability of the drawing blank in micro deep drawing. Saotome et al. [6] investigated the influences of punch diameter to sheet thickness on micro deep drawing by using a special apparatus. Vollertsen et al. [7] studied micro deep drawing according to similarity theory with different materials and found that friction coefficient in microforming was much higher than that in
∗ Corresponding author. Tel.: +86 755 22673522; fax: +86 755 26557471. E-mail address: [email protected] (F. Gong).
macroforming. Justinger et al. [8] formed 1 mm micro cup with CuZn37 and revealed that the cup geometry was influenced by microstructure. Manabe et al. [9] developed a sequential blanking and drawing setup, and fabricated a micro cup of only 0.5 mm in diameter using SUS304 ultra-thin foil. Chen et al. [10] reported that thickness, grain size and number of grains throughout thickness greatly influenced the LDR in micro deep drawing. Shimizu et al. [11] reported that the tool surface greatly affected the formability and accuracy of the micro parts in micro deep drawing. Behrens et al. [12] formed a micro cup of 0.75 mm inner diameter and studied the forming limit by using magnetron sputtered Al-Zr foils. Fu et al. [13] investigated the size effects in micro blanking and deep drawing process by experiments and simulation. Gau et al. [14] studied micro deep drawing with two ironing stages and successfully formed a micro cup of 3.039 mm in height and 2.218 mm in outer diameter using SUS304 with grain size of 45 m, which was annealed at 1050 ◦ C. Compared with macro deep drawing, micro deep drawing is more difficult because the material property, lubrication conditions and BHF change with the miniaturization of the specimen size [15]. Lubrication conditions and BHF, which are two key factors affecting the whole process of deep drawing, including drawing force, DR, surface quality, tool life and others, should be systemically investigated. Although micro deep drawing was widely investigated in
http://dx.doi.org/10.1016/j.precisioneng.2015.05.004 0141-6359/© 2015 Elsevier Inc. All rights reserved.
Please cite this article in press as: Gong F, et al. Influences of lubrication conditions and blank holder force on micro deep drawing of C1100 micro conical–cylindrical cup. Precis Eng (2015), http://dx.doi.org/10.1016/j.precisioneng.2015.05.004
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Fig. 1. Micro blanking–deep drawing mold (a) schematic and (b) photograph.
Fig. 2. Schematic of micro blanking–deep drawing process (a) initial state, (b) blanking, and (c) deep drawing.
the past few years, the research on lubrication conditions and BHF in micro deep drawing is limited. Liquid lubricants, which result in friction size effects in micro forming, are not proper for micro deep drawing. Thus, some researchers focused on solid lubrication films because of their excellent frictional property. Gong et al. [16] formed a 0.95 mm micro cup and studied the effects of solid lubrication films on SKD11 mold and die material. Hu et al. [17] indicated that the diamond like carbon (DLC) coated forming tools can decrease the forming force in micro deep drawing. Although micro cups were well formed, the cost was so expensive compared with normal lubricants and was not appropriate for mass production. With the decrease of the specimen size, BHF is much more difficult to apply than macro deep drawing and cannot be directly computed according to similarity theory (Vollertsen et al. [4]). In micro deep drawing process, micro cylinder cups, micro rectangular cups, micro conical cups, and micro spherical cups are typical parts. However, most existing researches were concentrated on micro cylindrical cups, and few studies were reported on other shaped parts. The purpose of this work is to form micro conical–cylindrical cups with internal conical bottom diameter of 0.4 mm and choose
proper lubricant and BHF for micro deep drawing. Besides, the influences of lubrication conditions and BHF on deep drawing force, LDR, forming quality were also investigated. 2. Experimental setup 2.1. Mold and principle Because the diameter of the conical–cylindrical cup to be formed was too small, the localization is difficult if a simple operation mold was used. To solve this problem, a micro blanking–deep drawing compound mold was developed, as shown in Fig. 1. A micro load cell, which was used to measure the drawing force, was installed in the upper mold. The range of the micro load cell was 500 N, and the measurement accuracy was 0.03% of the full scale. The schematic of micro blanking–deep drawing process can be seen in Fig. 2. Firstly, a blank was placed on the die. Secondly, a circular workpiece with a diameter of D0 was blanked out with the move downward of the upper mold. Thirdly, a proper BHF was generated by the spring and was applied to the workpiece
Please cite this article in press as: Gong F, et al. Influences of lubrication conditions and blank holder force on micro deep drawing of C1100 micro conical–cylindrical cup. Precis Eng (2015), http://dx.doi.org/10.1016/j.precisioneng.2015.05.004
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Fig. 3. Dimensions of the micro deep drawing punch.
Fig. 4. Grain images of C1100 thin sheets.
with the upper mold move downward continuously. Lastly, a conical–cylindrical cup with a top diameter of d1 and bottom diameter of d2 was formed when the drawing punch downward to a specific distance. To increase the strength of the micro deep drawing punch, a stepped construction was used in this study, as shown in Fig. 3. The micro deep drawing punch was fabricated with a highly dimensional accuracy of ±1 m by micro grinding. To study the LDR in micro deep drawing of micro conical–cylindrical cups, a series of compound punch-dies, blanking dies, die inserts, and blank holders were manufactured. The DR is given by the following equation: K=
C0 Cp
(1)
where K is the DR, C0 = D0 is the circumference of the deep drawing workpiece, and Cp = d1 is the circumference of the middle section of the terminal face of the part. 2.2. Specimen and die material Pure copper C1100, which was widely used in micro electronic industry, micro electro mechanical system and micro system technologies, was chosen as testing material. The raw material was hard rolled sheet, the thickness of which was 50 m. The chemical compositions were (wt%): Cu ≥ 99.90, O ≤ 0.06, S ≤ 0.005, Pb ≤ 0.005, Sb ≤ 0.002, Bi ≤ 0.002, As ≤ 0.002. In order to increase the plasticity of the thin sheet, the raw materials were annealed at 450 ◦ C for 2 h in vacuum condition. The annealing sheet was etched using a solution of 1 g of Fe(NO3 )3 and 25 mL of C2 H5 OH for 3–5 s. Grain images of the sheet were captured by an optical microscope. The average grain size was about 20 m, as shown in Fig. 4. Tensile tests were conducted to obtain the true strain–true stress curves of the sheet by using a Zwick/Roell Z050 universal test machine, as shown in Fig. 5. The yield stress and tensile stress were about 80 MPa and 200 MPa, respectively.
Fig. 5. True stress–true strain curve of 50 m thick C1100 sheet.
Tool steel alloy SKD11, which was widely used as punch and die material in microforming, was chosen as mold material. The hardness of the mold materials was 60 HRC after quenching. The specific chemical compositions were (wt%): Cr: 11–13, C: 1.4–1.6, Mo: 0.5–1.2, V: 0.2–0.5, Mn ≤ 0.6, Ni ≤ 0.5, Si ≤ 0.4, S ≤ 0.03, P ≤ 0.03, Fe: Balance. 2.3. Experimental parameters Micro blanking–deep drawing of conical–cylindrical micro cup experiments were performed on a SANS CMT5504 electronic universal testing machine at room temperature. The drawing velocity was 0.05 mm/s. The drawing punch stroke was acquired directly by this machine, and the drawing force can be acquired by the micro load cell. The diameters of the micro blanking punches were 1.8, 1.9, 2.0, 2.1 and 2.2 mm. The drawing clearance between the micro drawing punch and die was 50 m, which was the same to the workpiece. The radius of the micro deep drawing die was 0.2 mm. Besides dry friction, three kinds of lubricants were used. One was castor oil, whose dynamic viscosity was 0.61 Pa s. Another was petroleum jelly with dynamic viscosity of which was 1.08 Pa s. The third was PE film, whose thickness was 7 m. The BHF were 2.8, 4.2, 5.6 and 7.0 N. 3. Results and discussion 3.1. Micro conical–cylindrical cups SEM images of micro conical–cylindrical cups with different DR are shown in Fig. 6. The lubricant was PE film, and the maximum BHF was 4.2 N. It is clear that the micro conical–cylindrical cup with a LDR of 2.1 was well formed. There were only a few wrinkles in the rim of the cups when the DR was large. This is because the BHF for these micro cups was constant. On one hand, the degree of deformation increased with increasing the DR, which resulted the
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Fig. 6. SEM images of micro conical–cylindrical cups with different DR (a) K = 1.9; (b) K = 2.0; (c) K = 2.1 and (d) K = 2.2.
Fig. 7. Deep drawing force–deep drawing punch stroke curves with different DR.
Fig. 8. Drawing force–displacement curves under different lubrication conditions.
increase of tangential compressive stress and wrinkles. On the other hand, the width of deformation area increased with increasing the DR, which resulted in the decrease of resistance ability of buckling and wrinkles. There were clear wrinkles in micro deep drawing of cylindrical cups with large DR in the previous studies [4,8,18].
the increase of deformation resistance and decrease of deformation area in the deep drawing process, the deep drawing force first increased and then decreased, and this resulted in the first peak. Then the flange wrinkled because of the inadequate BHF. However, the drawing punch compelled the wrinkled blank into the drawing clearance, which resulted in the second peak. With the increase of the DR, the wall of the micro cup was cracked, and the deep drawing process was terminated. The deep drawing force increased with the increase of the DR, and the maximum drawing force increased 12.1% with the DR increasing from 1.8 to 2.1. This is because the deformation degree and deformation resistance of the flange increase with the increase of the DR.
3.2. Deep drawing force Fig. 7 shows the deep drawing force–deep drawing punch stroke curves with different DR. The same trend with two peaks in each curve can be seen from the figure. This was similar to micro deep drawing of micro cylindrical cups [11,18]. Due to the influence of
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Fig. 9. Drawing force–displacement curves with different BHF.
The deep drawing force–deep drawing punch stroke curves under different lubrication conditions are shown in Fig. 8. The deep drawing force was much lower than those of castor oil and petroleum jelly when a PE film was used. However, the friction coefficient of castor oil, petroleum jelly and PE film was all most the same in macro sheet forming. This is due to occurrence of friction size effects in micro sheet forming. Most of the castor oil and petroleum jelly were squeezed out with the miniaturization of the workpiece, which increased the friction coefficient. However, the PE film was always between the workpiece and drawing die interfaces in the whole micro deep drawing process. Thus, the PE film was much better than castor oil and petroleum jelly, which can be chosen as a proper lubricant for micro sheet forming.
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Fig. 10. Effect of lubrication condition on LDR.
Fig. 9 shows the deep drawing force–deep drawing punch stroke curves with different maximum BHF. The maximum deep drawing force increased with increasing BHF. However, when the maximum BHF was larger than 5.6 N, the micro cup was cracked and the forming process was terminated. It also can be seen from the figure that the second peak of the deep drawing force–deep drawing punch stroke curves diminished with the increase of the BHF, because the wrinkles diminished with the increase of the BHF. 3.3. LDR LDR, which depends on the maximum tension stress on the wall of the cup and tensile strength of the dangerous cross-section, is a quite important factor in industry in order to make full use
Fig. 11. SEM images of micro conical–cylindrical cups under different lubrication conditions (a) castor oil, (b) petroleum jelly, (c) PE film, (d) dry friction.
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Fig. 12. SEM images of micro conical–cylindrical cups with different BHF (a) 2.8 N; (b) 4.2 N; (c) 5.6 N and (d) 7.0 N.
of the forming ability of raw material and to reduce the cost of manufacture. The mechanical property of material, punch radius, dies radius, drawing clearance, lubrication condition and other parameters have great influences on LDR. The effects of lubrication conditions on LDR in micro deep drawing were shown in Fig. 10. It is clear from the figure that LDR increased significantly using lubricant compared to dry friction. The minimum LDR was only 1.7 under dry friction. However, the maximum LDR reached 2.1 under the lubrication of PE film, which was much better than the existing researches on micro deep drawing of cylindrical cups [4,8]. The LDR were 1.8 and 1.9 under the lubrication of castor oil and petroleum jelly, respectively, which were only a little higher than that under dry friction, because of the friction size effect in micro deep drawing.
3.4. Forming quality SEM images of micro conical–cylindrical cups formed under different lubrication conditions are shown in Fig. 11. The maximum BHF was 4.2 N, and the DR was 1.8. It is clear from the figure that the micro cup was cracked under dry friction. Although the micro cups were successfully formed under the lubrication of castor oil and petroleum jelly, there were more scratches on the rim of the cups compared to those under the lubrication of PE film. This is because most of the lubricant was squeezed out in micro sheet forming, and the workpiece and deep drawing die contacted directly, scratches were inevitable under those serious large deformation and friction. The SEM images of micro conical–cylindrical cups formed with different BHF are shown in Fig. 12. The lubricant was PE film, and the DR was 1.8. The BHF have great influence on the surface quality of the micro cups. When there was no BHF or the BHF was small, the material could flow freely, which resulted in wrinkles, as shown in Fig. 12(a). When there was appropriate BHF on the blank, the material could generate sufficient stretching, neither wrinkles nor cracks occurred, as shown in Fig. 12(b) and (c). When the blank
holder force was too large, the material cannot flow. However, the deep drawing punch compels the material to stretching, which results in crack, as shown in Fig. 12(d). It is quite difficult to determine the proper BHF with the miniaturization of the specimen size. There were serious wrinkles in the rim of the cup in the existing researches of which improper BHF were used [4,5,9]. 4. Conclusions Based on the micro deep drawing experiments of C1100 thin sheet with 4 kinds of different lubrication conditions and BHF, the following results can be concluded: 1. Micro conical–cylindrical cup with an internal conical bottom diameter of only 0.4 mm was successfully formed using a micro blanking–deep drawing compound mold, and the LDR reached to 2.1. 2. PE film, which reduced the drawing force and increased the LDR, was superior to liquid lubricants, and was suitable for micro forming as a lubricant. 3. When the BHF was less than 4.2 N, the rim of the micro cup was wrinkled. When the BHF was larger than 5.6 N, the bottom of the micro cup was cracked. Acknowledgments The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (No. 51205258), the Educational Commission of Guangdong Province (No. 2012LYM 0113), and the Scientific Research Foundation of Shenzhen University (No. 201141). References [1] Geiger M, Kleiner M, Eckstein R, Tiesler N, Engel U, Microforming. CIRP Ann Manuf Technol 2001;52(2):445–62.
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Please cite this article in press as: Gong F, et al. Influences of lubrication conditions and blank holder force on micro deep drawing of C1100 micro conical–cylindrical cup. Precis Eng (2015), http://dx.doi.org/10.1016/j.precisioneng.2015.05.004