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Electroconductive ceramic tooling for dry deep drawing
Journal of Materials Processing Technology 210 (2010) 48–53
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Electroconductive ceramic tooling for dry deep drawing Kenji Tamaoki a,∗ , Ken-ichi Manabe b , Seiji Kataoka c , Tatsuhiko Aizawa d a
Tokyo Metropolitan Industrial Technology Research Institute, 3-13-10, Nishigaoka, Kita-ku, Tokyo 115-8586, Japan Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji-shi, Tokyo 192-0397, Japan c Shonan Institute of Technology, 1-1-25, Nisikaigan, Tujido, Fujisawa-shi, Kanagawa 251-8511, Japan d Shibaura Institute of Technology, 3-9-14, Shibaura, Minato-ku, Tokyo 108-8548, Japan b
a r t i c l e
i n f o
Article history: Received 20 May 2009 Received in revised form 13 August 2009 Accepted 17 August 2009
Keywords: Ceramic tooling Electroconductive ceramic composites Electric discharge machining Dry deep drawing Environmental friendliness
a b s t r a c t Large use and waste of lubricating oil has become a very serious environmental issue. Hence, oil-free metal forming is one of the most promising solutions to this issue. Among them, ceramic tooling is proposed because of its good tribological properties. Its feasibility for dry deep drawing has been studied in laboratory; ceramic tooling is thought to be applicable to dry deep drawing. However, the workability of ceramics in tooling preparation is inferior to that of alloy tool steel. In the present paper, electroconductive ceramics are proposed as an alternative tool material. The electroconductive ceramic tooling is shaped by electric discharge machining such as wire electric discharge machining, die sinking electric discharge, and so forth; any shape of tools can be easily accommodated by these methods. The workability of electroconductive ceramics by electric discharge machining as well as the deep drawability with electroconductive ceramics tooling is experimentally evaluated. Selection of electric conditions for electric discharge machining influences the surface finish quality of electroconductive ceramics. Zirconia-based electroconductive ceramic die has an equivalent deep drawability in dry to conventional SKD11 die with lubricant. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Lubricant is an essential key in the conventional metal forming to reduce the friction between tool and work material, increase the forming limit, shorten the forming process, and to prolong the tool life by prevention of galling and seizure. Hence, adaptive selection of lubricants to each metal forming process is necessary for successful operation and reduction in energy consumption. This above traditional way inevitably be able to waste of huge amount of lubricating oils, and induces a serious environmental issue. Kataoka (2005) reported a fatal influence of lubricants on the environmental deterioration. It was reported that environmental condition would fall into a critical situation without effective policy to reduce the waste of lubricating oils. Many efforts have been explored to gradually decrease the amount of lubricants, step by step. McClure and Gugger (2002) proposed micro-lubrication in metal machining operations. Takeuchi et al. (1999) proposed an environmentally friendly lubricant for cold forging. Goto (2002) studied a non-graphite dispersion type forging oil. Starting from the chlorine-free, amine-free lubricants and bonderite-free lubrication, through the closed system of lubrication and cleaning, and simplification or reduction of cleaning
∗ Corresponding author. Tel.: +81 3 3909 2151; fax: +81 3 3909 2590. E-mail address: [email protected] (K. Tamaoki). 0924-0136/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2009.08.020
after lubrication, semidry or dry metal forming are practically discussed as a feasible alternative policy. Although zero emission of lubricants via dry stamping is attractive, many difficulties must be solved in its application to metal sheet forming in general. As one of the most promising methods without using the lubricant, several ceramic toolings have been proposed because of their special tribological properties. Buckley and Miyoshi (1984) studied friction and wear of ceramics. Both friction and wear of ceramics become anisotropic by alone relation to crystal structure. In addition, they reported that surface contaminants on ceramics affect friction and adhesive wear. Czichos et al. (1989) made various sliding wear tests with ceramics and steel. Hiratsuka and Sugahara (1989) reported a role of atmospheric oxygen in friction and wear between dissimilar pure metals. Atmospheric oxygen could lower friction according to proper combination of materials. Enomoto et al. (1991) reported an effect of oxidation activity of metal on the friction and wear between metal and alumina. This frictional and wear behavior between metal and alumina is the same as that between dissimilar metals. Hisakado et al. (1992) surveyed the friction and wear mechanisms between ceramics and metals. In practice, the ceramic dies were applied in trial to wiredrawing by Sato and Tada (1987). Feasibility of zirconia die was also investigated in the deep drawing of stainless steel sheets by Sato et al. (1989). Zirconia die resulted in relatively positive results under some dry conditions. Through the continuous dry deep drawing
K. Tamaoki et al. / Journal of Materials Processing Technology 210 (2010) 48–53 Table 3 Electric conditions of EDM.
Table 1 Content design of electroconductive ceramic composites. Content ZrO2 –WC Al2 O3 –TiC
50 vol% ZrO2 70 mass% Al2 O3
Condition
Density (g/cm3 )
Hardness (HRA)
Transverse rupture strength (MPa)
Young’s modulus (GPa)
10.70 4.24
92.5 94.0
2000 835
330 392
test up to a hundred blanks of galvanized Zn-alloyed steel sheets, Motoi et al. (2001) proved that the ceramic dies had higher deep drawability than the SKD11 die. In addition, the deep drawability with the zirconia die was reported to be the highest among several conditions. Furthermore, Kataoka et al. (2004) systematically studied the compatibility between monolithic ceramic tools and metallic sheet materials. Oxide ceramics like ZrO2 and Al2 O3 are effective to improve the dry formability of cold rolled mild steel (SPCC) and copper sheets. In addition, these monolithic ceramics work well as die materials in simple shaping. Since they are made by sintering, their application to complex shaping is strictly limited in practice even via high cost processing. In the present paper, new ceramic tooling for dry forming is proposed by using the electroconductive ceramic composites. Two types of composites are prepared to investigate their feasibility in electric discharge machining (EDM) and dry deep drawing. Owing to electric discharge machinability, various die geometries as designed can be fabricated by using the wire electric discharge machining and die sinking electric discharge machining. This dry deep drawability is compared to those by monolithic ZrO2 and SKD11 dies to demonstrate their superiority to monolithic ceramics. 2. Experimental procedure 2.1. Die materials Two types of electroconductive ceramics were prepared for a die material: ZrO2 –WC and Al2 O3 –TiC composites. Owing to the percolation theory for electronic composites (Taya, 2005), electroconductive phase of ceramics, or, WC and TiC must have a volume fraction over 25 vol%.
Fig. 1. SEM micrograph of ZrO2 –WC composite.
Polarity
50 vol% WC 30 mass% TiC
Table 2 Physical characteristic of die materials used.
ZrO2 –WC Al2 O3 –TiC
49
A B C D E F G H
+ + + + − − − −
Electric condition Current peak (A)
Pulse width (s)
10 5 1 1.5 1.5 1 1 1
97.01 29.86 4.92 1.74 1.74 1.32 1.15 1.07
Working time (min)
5 5 10 10 30 30 30 30
Fig. 2. Schematic figure of the ceramic tooling.
In the similar design, matrix phase of ceramics, or, ZrO2 and Al2 O3 , must have a volume ratio over 25 vol% in order to keep the toughness and strength of composites. Hence, the content of the ceramics composites is designed to be listed in Table 1. Table 2 summarizes the physical properties of these ceramic composites. Fig. 1 shows the SEM micrograph of ZrO2 –WC composite. WC uniformly distributes in the matrix of ZrO2 . Al2 O3 –TiC composite also has homogeneous microstructure. In addition, partially stabilized zirconia (PSZ) and alloy tool steel (SKD11) were also prepared for reference. This PSZ is composed of 5.2 mass% Y2 O3 and 94 mass% ZrO2 . The grade of this SKD11 is equivalent to AISI D2 class. 2.2. Sheet materials The cold rolled mild steel (SPCC) sheet was employed as a work material for deep drawing. Its thickness was constant, 0.6 mm.
Fig. 3. Relationship between removal rate and surface roughness by EDM conditions.
50
K. Tamaoki et al. / Journal of Materials Processing Technology 210 (2010) 48–53
Fig. 5. LDR in dry deep drawing of cold rolled mild steel against various dies.
0.05 mRz. Pure copper (C1100) was used for the electrode material. 2.4. Dry deep drawing test The present ceramic tooling was schematically shown in Fig. 2: the die, the punch, and the blank holder. To be free from rupture or chipping of ceramic dies in tensile, the whole ceramic die was fitted into a die holder with the interference of 1.0% to keep it in compressive stress state.
Fig. 4. Micrographs of ceramic surfaces after EDM: (a) condition B, (b) condition E and (c) condition G.
2.3. EDM test ZrO2 –WC composite was used to investigate the workability in EDM for various electric conditions. The diemilling electric discharge was varied in eight conditions as listed in Table 3. The removal rate and the surface roughness were measured for evaluation. The surface roughness of ZrO2 –WC before EDM was
Fig. 6. Micrographs of the deep drawn cup surfaces after degreasing: (a) SKD11 die and (b) ZrO2 –WC die.
K. Tamaoki et al. / Journal of Materials Processing Technology 210 (2010) 48–53
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Fig. 7. Micrographs of the deep drawn cup surfaces with lubricant: (a) SKD11 die and (b) ZrO2 –WC die.
SKD11 was used for the flat-headed punch. The punch diameter is 28.4 mm and the punch profile radius is 3 mm. The diameter of the die cavity is 30.0 mm and the die profile radius is 3 mm. This surface roughness was finished below 0.2 mRz in all the cases excluding ceramics. The ZrO2 –WC die was finished at 1.2 mRz and the Al2 O3 –TiC die was finished at 3.9 mRz. Deep drawing tests were performed with three different lubrication conditions as follows: (a) complete degreasing by ultrasonic cleaning of blank with acetone, (b) as received condition without lubricant, and (c) lubricating with G-3173 (Nihon Kohsakuyu Co., Ltd.), mineral oil including the sulfur additive with the kinematic viscosity of 25 mm2 /s at 40 ◦ C. Deep drawability was evaluated by the limiting drawing ratio (LDR), which is defined by the ratio of maximum drawable blank diameter (Rm ), to the punch diameter (Rp ): LDR =
Rm Rp
(1) Fig. 8. Micrograph and surface profile of ceramics: (a) PSZ and (b) ZrO2 –WC.
where Rm was determined in maximum when five blanks are successfully deep drawn without failure. The punch load was also measured in unlubricating without any lubricant spread on blank, in case of (b). ZrO2 –WC and SKD11 were Table 4 Maximum punch load in dry deep drawing. Material
Maximum punch load (kN)
ZrO2 –WC die SKD11 die
15.8 20.1
used for the die material. The punch load is determined by the mean value of punch loads in deep drawing five blanks with 50 mm. 3. Experimental results 3.1. EDM workability Fig. 3 shows the relationship between the removal rate and the surface roughness by EDM condition. The removal rate has positive
52
K. Tamaoki et al. / Journal of Materials Processing Technology 210 (2010) 48–53
Fig. 9. Photograph and micrograph of dry deep drawn cup: (a) SKD11 die. (b) ZrO2 –WC die.
correlation with the surface roughness: the higher removal rate results in the higher surface roughness. In cases of A and B in Table 3, the surface roughness after EDM was enhanced by high removal rate: e.g. the surface roughness was 16 mRz in condition A. This roughing leads to a defect for chipping. In the six conditions from C to H, the surface roughness after EDM was reduced by lowering removal rate: e.g. the surface roughness 2 mRz or less in the conditions G and H. Moreover, in the condition G, the surface roughness became smallest. This condition G was adopted for final finish machining. That is, for EDM of ZrO2 –WC, the condition B is selected for rough machining, and the condition G for finish machining. To be noted, to reduce the finish machining time, the condition E is also used for semi-finishing machining between the condition B and G. Fig. 4 shows micrographs of ceramic surface after EDM: (a) condition B, (b) condition E and (c) condition G. The surface roughness reduced from (a) to (c): (a) 11.11 mRz, (b) 2.80 mRz and (c) 1.84 mRz. 3.2. Dry deep drawing tests 3.2.1. LDR Fig. 5 compares LDR in deep drawing the cold rolled mild steel sheet for four different die materials in three conditions: (a) degrease, (b) without lubricant, (c) with lubricant. In case of degreasing, LDR (ZrO2 –WC) = 2.18, LDR (Al2 O3 –TiC) = 2.01, LDR (PSZ) = 1.90, and LDR (SKD11) = 1.73. Even in case of unlubricating, or, dry deep drawing, nearly the
same LDRs were measured as those in degreasing. In deep drawing with lubricant, high LDR was shown with all dies regardless of the die materials. To be noted, LDR in dry deep drawing by ZrO2 –WC die is equal to LDR by lubricated SKD11: LDR (ZrO2 –WC) in dry = 2.22 and LDR (SKD11) in lubricating = 2.25. 3.2.2. Surface observation of deep drawn cups Fig. 6 shows the micrographs of deep drawn cup surface after degreasing: (a) SKD11 die and (b) ZrO2 –WC die. As shown in Fig. 6(a), severe scratches were seen along the drawing direction on the most of drawn cup surfaces when dry deep drawing by the SKD11 die. The surface roughness was 5.84 mRz. On the other hand, scratches were hardly detected in using the ZrO2 –WC die (Fig. 6(b)). The surface roughness was 4.72 mRz. Fig. 7 shows micrographs of deep drawn cup surface with lubricant: (a) SKD11 die and (b) ZrO2 –WC die. Scratches on deep drawn cup surface by the SKD11 die were much reduced by lubricating as shown in Fig. 7(a). The surface roughness was 4.24 mRz. On the other hand, a deep drawn cup surface by the ZrO2 –WC die was roughened by micropool formed in lubricating as shown in Fig. 7(b). The surface roughness was 5.78 mRz. 3.2.3. Deep drawability of ceramic composite Among four die materials, ZrO2 –WC die has the highest LDR both in degreasing and unlubricating: LDR (ZrO2 –WC) degreasing = 2.18 and LDR (ZrO2 –WC) unlubricating = 2.22. Since both LDRs are over 2.0, sufficient deep drawability is attained even in dry only by using ZrO2 –WC die.
K. Tamaoki et al. / Journal of Materials Processing Technology 210 (2010) 48–53
Between ZrO2 –WC and Al2 O3 –TiC, the former ceramic composite has superior LDR in the whole conditions to the latter. This might be because ZrO2 matrix has sufficient wear toughness with and without lubricants. 3.2.4. Superiority of ZrO2 –WC in dry deep drawing Between ZrO2 –WC and PSZ, the former has higher LDR both in degreasing and unlubricating. This difference reflects on the wearing behavior of die materials. Fig. 8 shows the micrograph and surface profiles both for PSZ and ZrO2 –WC die materials. The surface was lapped by the sand paper #1000. Many pits were observed on the surface of PSZ in Fig. 8(a), while ZrO2 –WC die preserved smooth surface in Fig. 8(b). These pits might be due to abrasive wearing of ZrO2 in dry. Increase of hardness by WC-addition into ZrO2 –WC composite is preferable to significant reduction of abrasive wear rate. 3.2.5. Reduction of punch load in dry deep drawing Table 4 compares the maximum punch load in dry deep drawing between ZrO2 –WC and SKD11 dies. Reduction of punch load by 25% in case of ZrO2 –WC is attributed to decreasing friction and adhesion between die and SPCC blank sheet. Fig. 9 also compares the deep drawn cup outlook, micrograph and surface roughness between ZrO2 –WC and SKD11 dies. In dry deep drawing by SKD11 die, the surface of deep drawn cups became dull and many scratches were seen on it. This is a proof that metallic adhesions occurred on the SKD11 die wall during dry deep drawing. Little scratches in Fig. 9(b) and significant reduction of roughness from 8.59 mRz to 3.41 mRz demonstrate that ZrO2 –WC die should be preferable to dry deep drawing. 4. Conclusions Aiming at the environmentally benign metal forming that is free from lubricants, a new ceramic tooling is proposed for dry deep drawing process. In particular, electroconductive ceramic tooling by ZrO2 –WC and Al2 O3 –TiC is found to be adaptive to die fabrication and dry deep drawing. 1. Optimum selection of electric conditions for electrical discharge machining improves the surface finish quality of electroconductive ceramic dies and shortens the machining time.
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2. Both zirconia-based and alumina-based electroconductive ceramic dies have high LDR in dry deep drawing. Especially, the zirconia-based electroconductive ceramic like ZrO2 –WC has an equivalent deep drawability in dry to conventional SKD11 die with lubricant. Acknowledgement Authors would like to express their gratitude to Mr. K. Minamoto, Nippon Tungsten Co., Ltd, for his help in preparation for electroconductive ceramic materials. References Buckley, D., Miyoshi, K., 1984. Friction and wear of ceramics. Wear 100, 333– 353. Czichos, H., Becker, S., Lexow, J., 1989. International multilaboratory sliding wear tests with ceramics and steel. Wear 135, 171–191. Enomoto, A., Hiratsuka, K., Sasada, T., 1991. Effect of oxidation activity of metal and of atmospheric oxygen on the friction and wear between metal and alumina. J. Jpn. Soc. Tribologists 36 (1), 51–56. Goto, K., 2002. The study of non-graphite dispersion type forging oil and re-used effect of actual machine. In: Kiuchi, M. (Ed.), Proceedings of the Seventh ICTP. Yokohama, Japan, pp. 769–775. Hiratsuka, K., Sugahara, A., 1989. Role of atmospheric oxygen in friction & wear between dissimilar pure metals. J. Jpn. Soc. Tribologists 34 (11), 799– 806. Hisakado, T., Suda, H., Watanabe, H., 1992. The friction and wear mechanism between ceramics and metals. Wear 155, 251–268. Kataoka, S., 2005. Influence of lubricants on global environment. J. Jpn. Soc. Technol. Plast. 46 (528), 4–10. Kataoka, S., Murakawa, M., Aizawa, T., Ike, H., 2004. Tribology of dry deep-drawing of various metal sheets with use of ceramics tools. Surf. Coat. Technol. 177–178, 582–590. McClure, T.F., Gugger, M.D., 2002. Microlubrication in metal machining operations. J. Soc. Tribologists Lubrication Eng. 12, 15–21. Motoi, A., Kataoka, S., Sasaki, T., Kato, K., 2001. Tribo-characteristics test of zinccoated steel sheets during deep drawing with use of ceramic tool under dry condition. J. Mater. Testing Res. Assoc. 46 (4), 234–238. Sato, T., Tada, Y., 1987. Actual test of ceramic die for wire drawing of SUS304. J. Jpn. Soc. Technol. Plast. 28 (323), 1289–1295. Sato, T., Tada, Y., Maekawa, O., Besshi, T., 1989. Deep drawing of stainless steel with zirconia die. J. Jpn. Soc. Technol. Plast. 30 (340), 671–676. Takeuchi, M., Ikesue, F., Kashimura, N., 1999. Development of environmentally friendly lubricant with high performance and simple treatment for cold forging. In: Geiger, M. (Ed.), Proceedings of the Sixth ICTP. Berlin, Germany, pp. 383– 390. Taya, M., 2005. Electronic Composites. Cambridge University Press, pp. 173– 206.
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Electroconductive ceramic tooling for dry deep drawing Kenji Tamaoki a,∗ , Ken-ichi Manabe b , Seiji Kataoka c , Tatsuhiko Aizawa d a
Tokyo Metropolitan Industrial Technology Research Institute, 3-13-10, Nishigaoka, Kita-ku, Tokyo 115-8586, Japan Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji-shi, Tokyo 192-0397, Japan c Shonan Institute of Technology, 1-1-25, Nisikaigan, Tujido, Fujisawa-shi, Kanagawa 251-8511, Japan d Shibaura Institute of Technology, 3-9-14, Shibaura, Minato-ku, Tokyo 108-8548, Japan b
a r t i c l e
i n f o
Article history: Received 20 May 2009 Received in revised form 13 August 2009 Accepted 17 August 2009
Keywords: Ceramic tooling Electroconductive ceramic composites Electric discharge machining Dry deep drawing Environmental friendliness
a b s t r a c t Large use and waste of lubricating oil has become a very serious environmental issue. Hence, oil-free metal forming is one of the most promising solutions to this issue. Among them, ceramic tooling is proposed because of its good tribological properties. Its feasibility for dry deep drawing has been studied in laboratory; ceramic tooling is thought to be applicable to dry deep drawing. However, the workability of ceramics in tooling preparation is inferior to that of alloy tool steel. In the present paper, electroconductive ceramics are proposed as an alternative tool material. The electroconductive ceramic tooling is shaped by electric discharge machining such as wire electric discharge machining, die sinking electric discharge, and so forth; any shape of tools can be easily accommodated by these methods. The workability of electroconductive ceramics by electric discharge machining as well as the deep drawability with electroconductive ceramics tooling is experimentally evaluated. Selection of electric conditions for electric discharge machining influences the surface finish quality of electroconductive ceramics. Zirconia-based electroconductive ceramic die has an equivalent deep drawability in dry to conventional SKD11 die with lubricant. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Lubricant is an essential key in the conventional metal forming to reduce the friction between tool and work material, increase the forming limit, shorten the forming process, and to prolong the tool life by prevention of galling and seizure. Hence, adaptive selection of lubricants to each metal forming process is necessary for successful operation and reduction in energy consumption. This above traditional way inevitably be able to waste of huge amount of lubricating oils, and induces a serious environmental issue. Kataoka (2005) reported a fatal influence of lubricants on the environmental deterioration. It was reported that environmental condition would fall into a critical situation without effective policy to reduce the waste of lubricating oils. Many efforts have been explored to gradually decrease the amount of lubricants, step by step. McClure and Gugger (2002) proposed micro-lubrication in metal machining operations. Takeuchi et al. (1999) proposed an environmentally friendly lubricant for cold forging. Goto (2002) studied a non-graphite dispersion type forging oil. Starting from the chlorine-free, amine-free lubricants and bonderite-free lubrication, through the closed system of lubrication and cleaning, and simplification or reduction of cleaning
∗ Corresponding author. Tel.: +81 3 3909 2151; fax: +81 3 3909 2590. E-mail address: [email protected] (K. Tamaoki). 0924-0136/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2009.08.020
after lubrication, semidry or dry metal forming are practically discussed as a feasible alternative policy. Although zero emission of lubricants via dry stamping is attractive, many difficulties must be solved in its application to metal sheet forming in general. As one of the most promising methods without using the lubricant, several ceramic toolings have been proposed because of their special tribological properties. Buckley and Miyoshi (1984) studied friction and wear of ceramics. Both friction and wear of ceramics become anisotropic by alone relation to crystal structure. In addition, they reported that surface contaminants on ceramics affect friction and adhesive wear. Czichos et al. (1989) made various sliding wear tests with ceramics and steel. Hiratsuka and Sugahara (1989) reported a role of atmospheric oxygen in friction and wear between dissimilar pure metals. Atmospheric oxygen could lower friction according to proper combination of materials. Enomoto et al. (1991) reported an effect of oxidation activity of metal on the friction and wear between metal and alumina. This frictional and wear behavior between metal and alumina is the same as that between dissimilar metals. Hisakado et al. (1992) surveyed the friction and wear mechanisms between ceramics and metals. In practice, the ceramic dies were applied in trial to wiredrawing by Sato and Tada (1987). Feasibility of zirconia die was also investigated in the deep drawing of stainless steel sheets by Sato et al. (1989). Zirconia die resulted in relatively positive results under some dry conditions. Through the continuous dry deep drawing
K. Tamaoki et al. / Journal of Materials Processing Technology 210 (2010) 48–53 Table 3 Electric conditions of EDM.
Table 1 Content design of electroconductive ceramic composites. Content ZrO2 –WC Al2 O3 –TiC
50 vol% ZrO2 70 mass% Al2 O3
Condition
Density (g/cm3 )
Hardness (HRA)
Transverse rupture strength (MPa)
Young’s modulus (GPa)
10.70 4.24
92.5 94.0
2000 835
330 392
test up to a hundred blanks of galvanized Zn-alloyed steel sheets, Motoi et al. (2001) proved that the ceramic dies had higher deep drawability than the SKD11 die. In addition, the deep drawability with the zirconia die was reported to be the highest among several conditions. Furthermore, Kataoka et al. (2004) systematically studied the compatibility between monolithic ceramic tools and metallic sheet materials. Oxide ceramics like ZrO2 and Al2 O3 are effective to improve the dry formability of cold rolled mild steel (SPCC) and copper sheets. In addition, these monolithic ceramics work well as die materials in simple shaping. Since they are made by sintering, their application to complex shaping is strictly limited in practice even via high cost processing. In the present paper, new ceramic tooling for dry forming is proposed by using the electroconductive ceramic composites. Two types of composites are prepared to investigate their feasibility in electric discharge machining (EDM) and dry deep drawing. Owing to electric discharge machinability, various die geometries as designed can be fabricated by using the wire electric discharge machining and die sinking electric discharge machining. This dry deep drawability is compared to those by monolithic ZrO2 and SKD11 dies to demonstrate their superiority to monolithic ceramics. 2. Experimental procedure 2.1. Die materials Two types of electroconductive ceramics were prepared for a die material: ZrO2 –WC and Al2 O3 –TiC composites. Owing to the percolation theory for electronic composites (Taya, 2005), electroconductive phase of ceramics, or, WC and TiC must have a volume fraction over 25 vol%.
Fig. 1. SEM micrograph of ZrO2 –WC composite.
Polarity
50 vol% WC 30 mass% TiC
Table 2 Physical characteristic of die materials used.
ZrO2 –WC Al2 O3 –TiC
49
A B C D E F G H
+ + + + − − − −
Electric condition Current peak (A)
Pulse width (s)
10 5 1 1.5 1.5 1 1 1
97.01 29.86 4.92 1.74 1.74 1.32 1.15 1.07
Working time (min)
5 5 10 10 30 30 30 30
Fig. 2. Schematic figure of the ceramic tooling.
In the similar design, matrix phase of ceramics, or, ZrO2 and Al2 O3 , must have a volume ratio over 25 vol% in order to keep the toughness and strength of composites. Hence, the content of the ceramics composites is designed to be listed in Table 1. Table 2 summarizes the physical properties of these ceramic composites. Fig. 1 shows the SEM micrograph of ZrO2 –WC composite. WC uniformly distributes in the matrix of ZrO2 . Al2 O3 –TiC composite also has homogeneous microstructure. In addition, partially stabilized zirconia (PSZ) and alloy tool steel (SKD11) were also prepared for reference. This PSZ is composed of 5.2 mass% Y2 O3 and 94 mass% ZrO2 . The grade of this SKD11 is equivalent to AISI D2 class. 2.2. Sheet materials The cold rolled mild steel (SPCC) sheet was employed as a work material for deep drawing. Its thickness was constant, 0.6 mm.
Fig. 3. Relationship between removal rate and surface roughness by EDM conditions.
50
K. Tamaoki et al. / Journal of Materials Processing Technology 210 (2010) 48–53
Fig. 5. LDR in dry deep drawing of cold rolled mild steel against various dies.
0.05 mRz. Pure copper (C1100) was used for the electrode material. 2.4. Dry deep drawing test The present ceramic tooling was schematically shown in Fig. 2: the die, the punch, and the blank holder. To be free from rupture or chipping of ceramic dies in tensile, the whole ceramic die was fitted into a die holder with the interference of 1.0% to keep it in compressive stress state.
Fig. 4. Micrographs of ceramic surfaces after EDM: (a) condition B, (b) condition E and (c) condition G.
2.3. EDM test ZrO2 –WC composite was used to investigate the workability in EDM for various electric conditions. The diemilling electric discharge was varied in eight conditions as listed in Table 3. The removal rate and the surface roughness were measured for evaluation. The surface roughness of ZrO2 –WC before EDM was
Fig. 6. Micrographs of the deep drawn cup surfaces after degreasing: (a) SKD11 die and (b) ZrO2 –WC die.
K. Tamaoki et al. / Journal of Materials Processing Technology 210 (2010) 48–53
51
Fig. 7. Micrographs of the deep drawn cup surfaces with lubricant: (a) SKD11 die and (b) ZrO2 –WC die.
SKD11 was used for the flat-headed punch. The punch diameter is 28.4 mm and the punch profile radius is 3 mm. The diameter of the die cavity is 30.0 mm and the die profile radius is 3 mm. This surface roughness was finished below 0.2 mRz in all the cases excluding ceramics. The ZrO2 –WC die was finished at 1.2 mRz and the Al2 O3 –TiC die was finished at 3.9 mRz. Deep drawing tests were performed with three different lubrication conditions as follows: (a) complete degreasing by ultrasonic cleaning of blank with acetone, (b) as received condition without lubricant, and (c) lubricating with G-3173 (Nihon Kohsakuyu Co., Ltd.), mineral oil including the sulfur additive with the kinematic viscosity of 25 mm2 /s at 40 ◦ C. Deep drawability was evaluated by the limiting drawing ratio (LDR), which is defined by the ratio of maximum drawable blank diameter (Rm ), to the punch diameter (Rp ): LDR =
Rm Rp
(1) Fig. 8. Micrograph and surface profile of ceramics: (a) PSZ and (b) ZrO2 –WC.
where Rm was determined in maximum when five blanks are successfully deep drawn without failure. The punch load was also measured in unlubricating without any lubricant spread on blank, in case of (b). ZrO2 –WC and SKD11 were Table 4 Maximum punch load in dry deep drawing. Material
Maximum punch load (kN)
ZrO2 –WC die SKD11 die
15.8 20.1
used for the die material. The punch load is determined by the mean value of punch loads in deep drawing five blanks with 50 mm. 3. Experimental results 3.1. EDM workability Fig. 3 shows the relationship between the removal rate and the surface roughness by EDM condition. The removal rate has positive
52
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Fig. 9. Photograph and micrograph of dry deep drawn cup: (a) SKD11 die. (b) ZrO2 –WC die.
correlation with the surface roughness: the higher removal rate results in the higher surface roughness. In cases of A and B in Table 3, the surface roughness after EDM was enhanced by high removal rate: e.g. the surface roughness was 16 mRz in condition A. This roughing leads to a defect for chipping. In the six conditions from C to H, the surface roughness after EDM was reduced by lowering removal rate: e.g. the surface roughness 2 mRz or less in the conditions G and H. Moreover, in the condition G, the surface roughness became smallest. This condition G was adopted for final finish machining. That is, for EDM of ZrO2 –WC, the condition B is selected for rough machining, and the condition G for finish machining. To be noted, to reduce the finish machining time, the condition E is also used for semi-finishing machining between the condition B and G. Fig. 4 shows micrographs of ceramic surface after EDM: (a) condition B, (b) condition E and (c) condition G. The surface roughness reduced from (a) to (c): (a) 11.11 mRz, (b) 2.80 mRz and (c) 1.84 mRz. 3.2. Dry deep drawing tests 3.2.1. LDR Fig. 5 compares LDR in deep drawing the cold rolled mild steel sheet for four different die materials in three conditions: (a) degrease, (b) without lubricant, (c) with lubricant. In case of degreasing, LDR (ZrO2 –WC) = 2.18, LDR (Al2 O3 –TiC) = 2.01, LDR (PSZ) = 1.90, and LDR (SKD11) = 1.73. Even in case of unlubricating, or, dry deep drawing, nearly the
same LDRs were measured as those in degreasing. In deep drawing with lubricant, high LDR was shown with all dies regardless of the die materials. To be noted, LDR in dry deep drawing by ZrO2 –WC die is equal to LDR by lubricated SKD11: LDR (ZrO2 –WC) in dry = 2.22 and LDR (SKD11) in lubricating = 2.25. 3.2.2. Surface observation of deep drawn cups Fig. 6 shows the micrographs of deep drawn cup surface after degreasing: (a) SKD11 die and (b) ZrO2 –WC die. As shown in Fig. 6(a), severe scratches were seen along the drawing direction on the most of drawn cup surfaces when dry deep drawing by the SKD11 die. The surface roughness was 5.84 mRz. On the other hand, scratches were hardly detected in using the ZrO2 –WC die (Fig. 6(b)). The surface roughness was 4.72 mRz. Fig. 7 shows micrographs of deep drawn cup surface with lubricant: (a) SKD11 die and (b) ZrO2 –WC die. Scratches on deep drawn cup surface by the SKD11 die were much reduced by lubricating as shown in Fig. 7(a). The surface roughness was 4.24 mRz. On the other hand, a deep drawn cup surface by the ZrO2 –WC die was roughened by micropool formed in lubricating as shown in Fig. 7(b). The surface roughness was 5.78 mRz. 3.2.3. Deep drawability of ceramic composite Among four die materials, ZrO2 –WC die has the highest LDR both in degreasing and unlubricating: LDR (ZrO2 –WC) degreasing = 2.18 and LDR (ZrO2 –WC) unlubricating = 2.22. Since both LDRs are over 2.0, sufficient deep drawability is attained even in dry only by using ZrO2 –WC die.
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Between ZrO2 –WC and Al2 O3 –TiC, the former ceramic composite has superior LDR in the whole conditions to the latter. This might be because ZrO2 matrix has sufficient wear toughness with and without lubricants. 3.2.4. Superiority of ZrO2 –WC in dry deep drawing Between ZrO2 –WC and PSZ, the former has higher LDR both in degreasing and unlubricating. This difference reflects on the wearing behavior of die materials. Fig. 8 shows the micrograph and surface profiles both for PSZ and ZrO2 –WC die materials. The surface was lapped by the sand paper #1000. Many pits were observed on the surface of PSZ in Fig. 8(a), while ZrO2 –WC die preserved smooth surface in Fig. 8(b). These pits might be due to abrasive wearing of ZrO2 in dry. Increase of hardness by WC-addition into ZrO2 –WC composite is preferable to significant reduction of abrasive wear rate. 3.2.5. Reduction of punch load in dry deep drawing Table 4 compares the maximum punch load in dry deep drawing between ZrO2 –WC and SKD11 dies. Reduction of punch load by 25% in case of ZrO2 –WC is attributed to decreasing friction and adhesion between die and SPCC blank sheet. Fig. 9 also compares the deep drawn cup outlook, micrograph and surface roughness between ZrO2 –WC and SKD11 dies. In dry deep drawing by SKD11 die, the surface of deep drawn cups became dull and many scratches were seen on it. This is a proof that metallic adhesions occurred on the SKD11 die wall during dry deep drawing. Little scratches in Fig. 9(b) and significant reduction of roughness from 8.59 mRz to 3.41 mRz demonstrate that ZrO2 –WC die should be preferable to dry deep drawing. 4. Conclusions Aiming at the environmentally benign metal forming that is free from lubricants, a new ceramic tooling is proposed for dry deep drawing process. In particular, electroconductive ceramic tooling by ZrO2 –WC and Al2 O3 –TiC is found to be adaptive to die fabrication and dry deep drawing. 1. Optimum selection of electric conditions for electrical discharge machining improves the surface finish quality of electroconductive ceramic dies and shortens the machining time.
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2. Both zirconia-based and alumina-based electroconductive ceramic dies have high LDR in dry deep drawing. Especially, the zirconia-based electroconductive ceramic like ZrO2 –WC has an equivalent deep drawability in dry to conventional SKD11 die with lubricant. Acknowledgement Authors would like to express their gratitude to Mr. K. Minamoto, Nippon Tungsten Co., Ltd, for his help in preparation for electroconductive ceramic materials. References Buckley, D., Miyoshi, K., 1984. Friction and wear of ceramics. Wear 100, 333– 353. Czichos, H., Becker, S., Lexow, J., 1989. International multilaboratory sliding wear tests with ceramics and steel. Wear 135, 171–191. Enomoto, A., Hiratsuka, K., Sasada, T., 1991. Effect of oxidation activity of metal and of atmospheric oxygen on the friction and wear between metal and alumina. J. Jpn. Soc. Tribologists 36 (1), 51–56. Goto, K., 2002. The study of non-graphite dispersion type forging oil and re-used effect of actual machine. In: Kiuchi, M. (Ed.), Proceedings of the Seventh ICTP. Yokohama, Japan, pp. 769–775. Hiratsuka, K., Sugahara, A., 1989. Role of atmospheric oxygen in friction & wear between dissimilar pure metals. J. Jpn. Soc. Tribologists 34 (11), 799– 806. Hisakado, T., Suda, H., Watanabe, H., 1992. The friction and wear mechanism between ceramics and metals. Wear 155, 251–268. Kataoka, S., 2005. Influence of lubricants on global environment. J. Jpn. Soc. Technol. Plast. 46 (528), 4–10. Kataoka, S., Murakawa, M., Aizawa, T., Ike, H., 2004. Tribology of dry deep-drawing of various metal sheets with use of ceramics tools. Surf. Coat. Technol. 177–178, 582–590. McClure, T.F., Gugger, M.D., 2002. Microlubrication in metal machining operations. J. Soc. Tribologists Lubrication Eng. 12, 15–21. Motoi, A., Kataoka, S., Sasaki, T., Kato, K., 2001. Tribo-characteristics test of zinccoated steel sheets during deep drawing with use of ceramic tool under dry condition. J. Mater. Testing Res. Assoc. 46 (4), 234–238. Sato, T., Tada, Y., 1987. Actual test of ceramic die for wire drawing of SUS304. J. Jpn. Soc. Technol. Plast. 28 (323), 1289–1295. Sato, T., Tada, Y., Maekawa, O., Besshi, T., 1989. Deep drawing of stainless steel with zirconia die. J. Jpn. Soc. Technol. Plast. 30 (340), 671–676. Takeuchi, M., Ikesue, F., Kashimura, N., 1999. Development of environmentally friendly lubricant with high performance and simple treatment for cold forging. In: Geiger, M. (Ed.), Proceedings of the Sixth ICTP. Berlin, Germany, pp. 383– 390. Taya, M., 2005. Electronic Composites. Cambridge University Press, pp. 173– 206.