- Email: [email protected]
A research on electroplastic effects in wire-drawing process of an austenitic stainless steel
Scripta Materialia 45 (2001) 533±539
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A research on electroplastic eects in wire-drawing process of an austenitic stainless steel Ke-Fu Yao*, Jun Wang, Mingxin Zheng, Peng Yu, and Huatang Zhang Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China
Received 5 May 2000; accepted 24 April 2001
Keywords: Electroplastic eect; Cold working; Stress; Mechanical properties; Steel
Introduction Moving electrons in metal crystals may interact with dislocations therein, as was ®rst reported by Troitskii and Lichtman in 1963 [1]. Since then, Troitskii and many other researchers in former Soviet Union and in the US [2±7] have extensively studied the eect of high density electric current pulses on plastic deformation and other mechanical properties of metals and alloys. That the ¯ow stress of metals can be reduced by applying high-density current pulses in deformation process is presently, and generally accepted. And this phenomenon is termed the electroplastic eect. Experimental results have revealed that the concurrent application of high-density current pulses during deformation process can reduce deformation stress and brittleness of the metals, improving mechanical properties and changing microstructure and texture [3±5,8±11]. Since deformation stresses required for metalworking, such as rolling or drawing, can be reduced by deforming with current pulses, electroplastic eect can be applied in metal processing. In former Soviet Union, wire-drawing experiments of the stainless steel of Fe±0.12%C±18%Cr±10%Ni±1%Ti (in wt.%) with current pulses were studied [12±14]. It was found that the drawing stress was reduced and c ! a phase transformation was suppressed. However, the relationship between drawing stress drop and accumulated drawing strain and the drawing deformation behavior have not been carefully investigated. In the present work, cold wire drawing of an austenite stainless steel both with and without employing high density current pulses has been carried out with dierent drawing dies. The in¯uence of high-density current pulses on the drawing deformation behavior has been discussed.
*
Corresponding author. Fax: +86-10-6277-0190. E-mail address: [email protected] (K.-F. Yao).
1359-6462/01/$ - see front matter Ó 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 6 4 6 2 ( 0 1 ) 0 1 0 5 4 - 5
534
K.-F. Yao et al./Scripta Materialia 45 (2001) 533±539
Experimental procedures Cold wire-drawing experiments have been carried out on a specially designed cold wire-drawing machine system which involves a dc electric pulse power supply, a drawing machine and a concurrent supply system. The drawing process was lubricated and cooled by circulating oil. The pulse current is supplied by sliding contact with the wire. The electrical negative contact is following exit of the wire from the die and the positive prior to entrance. In present research, the pulse frequency was 100 Hz and the wiredrawing speed was 6 m/min. The pulse duration time is 80 ls. The maximum output of the pulsed current is 1000 A. The wire-drawing force exerted by the drawing machine is measured by a sensor with an accuracy of 0.1 N. The experimental material is an austenite stainless steel with the chemical composition of Fe±0.1%C±18%Cr±9%Ni (in wt.%). Its original form is /1.5 mm wire and is in softened state with ultimate tensile stress rb 710 MPa and tensile elongation d 45%. Mechanical properties of as-drawn wires deformed both with and without applying current pulses were measured by use of a normal testing machine. The surface quality of as-drawn wires has been examined by use of a Hitatchi S-450 scanning electron microscope. Results Cold wire-drawing experiments with dierent drawing dies have been carried out in turn both with applying high density dc electric pulses in deformation process and without current (that is with traditional cold wire-drawing process). Fig. 1(a) shows the 2 relationship between drawing force and true drawing strain e(e ln D0 =Di , where D0 is the original diameter of the wire and Di is the diameter of the wire after i times drawings in turn). It can be found that wire-drawing forces are signi®cantly reduced by
Fig. 1. (a) Drawing forces versus drawing strains for wires deformed both with and without employing current pulses; (b) The relative drawing force drop varies with drawing strain.
K.-F. Yao et al./Scripta Materialia 45 (2001) 533±539
535
employing current pulses in the deformation process. The relative drop of wire-drawing force is determined as: DP Pn Pe =Pn , where Pe and Pn are drawing forces of wires deformed with and without current pulses, respectively. Fig. 1(b) shows that the relative drop of wire-drawing force DP increases with the increment of drawing strain. The maximum value is about 50%, larger than the reported results in other stainless steel [12]. As shown in Fig. 1(a) and (b), when e < 60%, the value of drawing force drop increases rapidly and almost linearly with the increment of drawing strain, while when e > 60%, it increases much slowly. This may be due to that as e increases, the work hardening rate dr=de decreases. Plots of the true drawing stress versus drawing strain are shown in Fig. 2. When wires deformed with current pulses, the drawing stress increases much slowly with the increment of drawing strain, while those deformed without current the drawing stress increases quickly. That is the work hardening rate of the wires is reduced by applying current pulses in drawing process. According to Hollomon's empirical formula, the relationship between the true stress and the true strain can be expressed as: r ken , where k is the work hardening coecient and n is the work hardening exponent of the deformed material. For wire-drawing deformation, Rack and Cohen's work con®rmed that work hardening behavior of multi-drawing was similar to that of single tensile deformation [15]. The relationship between the true drawing stress and true drawing strain can be described by Hollomon's empirical formula, which are indicated by solid lines shown in Fig. 2. The work hardening exponents for both processes are derived by a numerical iteration method. They are 0.5201 and 0.2905, corresponding to wiredrawing processes without and with employing current pulses, respectively. The result
Fig. 2. The true drawing stress as a function of drawing strain.
536
K.-F. Yao et al./Scripta Materialia 45 (2001) 533±539
Fig. 3. The ultimate tensile strength (a) and the tensile elongation (b) of as-drawn wires vary with drawing strain.
shows that the work hardening exponent of the wire in drawing deformation is decreased 44.1% by applying current pulses. Mechanical properties of as-drawn wires, which experienced dierent drawing strain, have been measured and the results are shown in Fig. 3. It can be found that the ultimate tensile stress rb of as-drawn wires is signi®cantly reduced by applying current pulses in drawing process (Fig. 3(a)). As drawing strain increases, the dierence of rb between two drawing processes increases. The maximum decrement of rb is about 50%. The tensile elongation of as-drawn wires decreases rapidly with the increment of drawing strain, especially when the drawing strain is small (Fig. 3(b)). When drawing strain is greater than 40%, the tensile elongation of wires drawn without current is less than 1%, while that of wires drawn with employing current pulses decreases much slowly (Fig. 3(b)). These results indicate that the ductility of as-drawn wires has been much improved by drawing with current pulses. It would result in an increment of drawing deformation ability. In present research, wires with original diameter of /1:50 mm have been drawn to ®ne wires of /0:124 mm without employing any intermediate heat treatment process. The accumulated drawing strain is about 500%. However, in traditional wire-drawing process it is necessary to anneal wires several times to soften them for further deformation. In present work, close attention was paid to the temperature rise DT of the wire resulted from the pulsing current. The temperature rise was measured by using a touching thermocouple recorder in the drawing process without cooling oil. The measuring position is about 30 mm away from the exit of the die. By comparing the temperature rise obtained in drawing with and without current, DT resulted from Joule heating is obtained and its average value for /1:16 mm wire is about 43°C, similar to the reported results [16±18]. However, the temperature rise near the deformation zone might be dierent. The precise measurement is still in trying. The details of the measurement will be reported elsewhere. In present work, the polarity eect has been
K.-F. Yao et al./Scripta Materialia 45 (2001) 533±539
537
Fig. 4. Surface morphologies of as-drawn wires deformed without (a) and with (b) applying current pulses, respectively. The drawing strain is e 118%.
studied. It has been found that the drawing stress drop is reduced 30±40% by changing the current polarity oppositely, agreed with the reported results [4]. The surface quality of as-drawn wires has been carefully examined by use of scanning electron microscope. Fig. 4(a) and (b) shows surface morphologies of as-drawn wires deformed without and with employing current pulses, respectively. These as-drawn wires have experienced eight drawing courses and the accumulated drawing strain is e 118%. The drawing direction is indicated by the arrowed line shown in Fig. 4. As shown in Fig. 4(a) and (b), surface morphologies of wires drawn without and with current pulses are quite dierent. Many small microcracks and pits, resulted from wearing with inner surface of drawing dies, are observed on the surface of the as-drawn wire deformed without current (Fig. 4(a)). While those drawn with employing current pulses the surface is much smooth and almost no microcrack is observed on the surface, except that some groove-like wear trace lines, which are parallel to the drawing direction, are observed (Fig. 4(b)). The surface quality of as-drawn wires has been much improved by drawing with employing current pulses. Discussion During wire-drawing deformation process, the wire is plastically deformed in the deformation zone of the drawing die. The drawing stress mainly depends on deforming resistance, that is the resistance to dislocation movement. By applying current pulses in
538
K.-F. Yao et al./Scripta Materialia 45 (2001) 533±539
deformation process, drawing forces are signi®cantly decreased for all drawing courses (Fig. 1(a) and (b)). These indicate that there should exist an extra force acting on moving dislocations, which should be related to current pulses. Troitskii [1] and other researchers [4,7] suggested that there would exist a force acting on dislocations induced by drift electrons. So drift electrons would help dislocations to overcome resistance from obstacles and lattice resistance, and result in load drop. In addition, drift electrons could aect the thermally activated motion of dislocations [4,5,16]. With the promotion of drift electrons many dislocations would activate more easily, resulting in the decrement of drawing stress and work hardening rate, agreed with experimental results (Fig. 2). The results shown in Fig. 3(a) and (b) indicate that the ductility of as-drawn wires is much improved and the ultimate tensile strength of as-drawn wires is signi®cantly decreased by applying current pulses in drawing process. It implies that the as-drawn wire is softened by deforming with current pulses. This would result in the decrement of drawing stress in next drawing course even though the next drawing is done without current [19]. So when next drawing course is done with current pulses, drawing force drop is contributed both by current pulses during deformation and by the softening eects from previous drawing with current. However, it has not yet been made clear why the work hardening exponent of the wire is reduced 44% by applying current pulses in wire-drawing deformation. During deforming process with employing current pulses, Joule heating is another important factor in¯uencing deformation behavior [4]. In present research, the measured average temperature rise resulted from pulsing current is about 43°C. For the experimental steel, this temperature rise may result in the decrement of drawing stress by 3±5% according to the relationship of rs ±T and rb ±T of the steel [20], indicating the existence of Joule heating eects. The Joule heating eect, especially local heating, would promote the activation and the movement of dislocations and result in the decrement of the drawing stress and the work hardening rate. It has reported that Joule heating may contribute 50±60% of ¯ow stress drop in Ti alloys [17,18]. In present work, the fraction of drawing stress drop contributed by Joule heating is not clear. But polarity-change experiments show that there exists signi®cant dierence in drawing stress drop when the polarity of pulsing current is changed, con®rming the existence of nonthermal eect's contribution to the drawing stress drop [4,17,18]. During drawing deformation process, the wire surface will interact with the inner surface of the drawing die. It would result in occurrence of wearing on the wire surface. When the wire is drawn without current, surface wear of the wire is heavier, resulted in formation of many micro-cracks and detached pits, indicating the mechanism of adhesive wear. When the wire is drawn employing current pulses, the surface wear behavior and the surface quality of as-drawn wires have been much improved (Fig. 4(b)). According to the observed groove-like wear traces, the wear behavior is related to abrasive wear. Although how current pulses in¯uence the wear behavior and wear mechanism of the wire is not clear, pulsing current has really improved the surface quality of the as-drawn wire and changed the wear mechanism. It may be related to the change of stress states in deformation zone, the softening eects and the in¯uence of drift electrons on deformation behavior.
K.-F. Yao et al./Scripta Materialia 45 (2001) 533±539
539
Conclusions (1) The wire-drawing force is signi®cantly reduced by applying current pulses in drawing deformation process. The decrement of the drawing force is closely related to the accumulated drawing strain and the largest value is about 50%. When e < 60% the drawing force drop increases rapidly and almost linearly with the increment of drawing strain, while when e > 60% it increases much slowly. (2) The work hardening of the wires in drawing deformation is signi®cantly decreased by applying current pulses in the drawing process and the work hardening exponent is reduced by 44%. (3) The ultimate tensile strength of as-drawn wires is signi®cantly decreased and the tensile ductility of as-drawn wires is much improved by drawing with applying current pulses. The as-drawn wire is softened by drawing with employing current pulses. (4) The surface quality of as-drawn wires is much improved and the wear mechanism of the wires is changed by employing current pulses in the drawing process.
Acknowledgements This work is supported by Beijing Nature Science Foundation. Supports from Analysis Center of Tsinghua University and the ``985'' Research Fund of Tsinghua University are greatly appreciated.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Troitskii, O. A., & Likhtman, V. I. (1963). Dokl Akad Nauk SSSR 148, 332. Troitskii, O. A. (1969). Zh ETF Pis Red 10, 18. Troitskii, O. A. (1985). Mater Sci Eng 75A, 37. Sprecher, A. F., Mannan, S. L., & Conrad, H. (1986). Acta Metall 34, 1145. Conrad, H., Sprecher, A. F., Cao, W. D., & Lu, X. P. (1990). JOM 42(9), 28. Yang, D., & Conrad, H. (1997). J Am Ceram Soc 80, 1389. Kravchenko, V. Y. (1966). J Exp Theoret Phys USSR 51, 1676. Conrad, H., Karam, N., & Mannan, S. (1983). Scripta Metall 17, 411. Conrad, H., Karam, N., & Mannan, S. (1984). Scripta Metall 18, 275. Conrad, H., Cao, W. D., Lu, X. P., & Sprecher, A. F. (1987). Scripta Metall 23, 697. Li, S., & Conrad, H. (1998). Scripta Mater 39, 847. Troitskiy, O. A., Baldokhin, Y. V., Kir'yancher, N. Y., Ryzhkov, V. G., Kalugin, V. D., Sokolov, N. V., Klekovkin, A. A., & Klevtsur, S. A. (1983). Phys Met Metall 55, 108. Troitskii, O. A., Spitsyn, A. V. I., Sokolov, N. V., & Ryzhkov, V. G. (1977). Sov Phys Dokl 22, 769. Troitskii, O. A., Spitsyn, A. V. I., Sokolov, N. V., & Ryzhkov, V. G. (1979). Phys Stat Sol (a) 52, 85. Rack, H. J., & Cohen, M. (1972). Mater Sci Eng 9, 175. Sprecher, A. F., Mannan, S. L., & Conrad, H. (1983). Scripta Metall 17, 769. Cao, W. D., Sprecher, A. F., & Conrad, H. (1989). J Phys E: Sci Instrum 22, 1026. Okazaki, K., Kagawa, M., & Conrad, H. (1980). Mater Sci Eng 25, 109. Yao, K.-F., Wang, J., Zheng, M., Yu, P., & Zhang, H. in press. Peckner, D., & Bernstein, I. M. (1977). Handbook of stainless steels. New York, USA: McGraw-Hill.
www.elsevier.com/locate/scriptamat
A research on electroplastic eects in wire-drawing process of an austenitic stainless steel Ke-Fu Yao*, Jun Wang, Mingxin Zheng, Peng Yu, and Huatang Zhang Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China
Received 5 May 2000; accepted 24 April 2001
Keywords: Electroplastic eect; Cold working; Stress; Mechanical properties; Steel
Introduction Moving electrons in metal crystals may interact with dislocations therein, as was ®rst reported by Troitskii and Lichtman in 1963 [1]. Since then, Troitskii and many other researchers in former Soviet Union and in the US [2±7] have extensively studied the eect of high density electric current pulses on plastic deformation and other mechanical properties of metals and alloys. That the ¯ow stress of metals can be reduced by applying high-density current pulses in deformation process is presently, and generally accepted. And this phenomenon is termed the electroplastic eect. Experimental results have revealed that the concurrent application of high-density current pulses during deformation process can reduce deformation stress and brittleness of the metals, improving mechanical properties and changing microstructure and texture [3±5,8±11]. Since deformation stresses required for metalworking, such as rolling or drawing, can be reduced by deforming with current pulses, electroplastic eect can be applied in metal processing. In former Soviet Union, wire-drawing experiments of the stainless steel of Fe±0.12%C±18%Cr±10%Ni±1%Ti (in wt.%) with current pulses were studied [12±14]. It was found that the drawing stress was reduced and c ! a phase transformation was suppressed. However, the relationship between drawing stress drop and accumulated drawing strain and the drawing deformation behavior have not been carefully investigated. In the present work, cold wire drawing of an austenite stainless steel both with and without employing high density current pulses has been carried out with dierent drawing dies. The in¯uence of high-density current pulses on the drawing deformation behavior has been discussed.
*
Corresponding author. Fax: +86-10-6277-0190. E-mail address: [email protected] (K.-F. Yao).
1359-6462/01/$ - see front matter Ó 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 6 4 6 2 ( 0 1 ) 0 1 0 5 4 - 5
534
K.-F. Yao et al./Scripta Materialia 45 (2001) 533±539
Experimental procedures Cold wire-drawing experiments have been carried out on a specially designed cold wire-drawing machine system which involves a dc electric pulse power supply, a drawing machine and a concurrent supply system. The drawing process was lubricated and cooled by circulating oil. The pulse current is supplied by sliding contact with the wire. The electrical negative contact is following exit of the wire from the die and the positive prior to entrance. In present research, the pulse frequency was 100 Hz and the wiredrawing speed was 6 m/min. The pulse duration time is 80 ls. The maximum output of the pulsed current is 1000 A. The wire-drawing force exerted by the drawing machine is measured by a sensor with an accuracy of 0.1 N. The experimental material is an austenite stainless steel with the chemical composition of Fe±0.1%C±18%Cr±9%Ni (in wt.%). Its original form is /1.5 mm wire and is in softened state with ultimate tensile stress rb 710 MPa and tensile elongation d 45%. Mechanical properties of as-drawn wires deformed both with and without applying current pulses were measured by use of a normal testing machine. The surface quality of as-drawn wires has been examined by use of a Hitatchi S-450 scanning electron microscope. Results Cold wire-drawing experiments with dierent drawing dies have been carried out in turn both with applying high density dc electric pulses in deformation process and without current (that is with traditional cold wire-drawing process). Fig. 1(a) shows the 2 relationship between drawing force and true drawing strain e(e ln D0 =Di , where D0 is the original diameter of the wire and Di is the diameter of the wire after i times drawings in turn). It can be found that wire-drawing forces are signi®cantly reduced by
Fig. 1. (a) Drawing forces versus drawing strains for wires deformed both with and without employing current pulses; (b) The relative drawing force drop varies with drawing strain.
K.-F. Yao et al./Scripta Materialia 45 (2001) 533±539
535
employing current pulses in the deformation process. The relative drop of wire-drawing force is determined as: DP Pn Pe =Pn , where Pe and Pn are drawing forces of wires deformed with and without current pulses, respectively. Fig. 1(b) shows that the relative drop of wire-drawing force DP increases with the increment of drawing strain. The maximum value is about 50%, larger than the reported results in other stainless steel [12]. As shown in Fig. 1(a) and (b), when e < 60%, the value of drawing force drop increases rapidly and almost linearly with the increment of drawing strain, while when e > 60%, it increases much slowly. This may be due to that as e increases, the work hardening rate dr=de decreases. Plots of the true drawing stress versus drawing strain are shown in Fig. 2. When wires deformed with current pulses, the drawing stress increases much slowly with the increment of drawing strain, while those deformed without current the drawing stress increases quickly. That is the work hardening rate of the wires is reduced by applying current pulses in drawing process. According to Hollomon's empirical formula, the relationship between the true stress and the true strain can be expressed as: r ken , where k is the work hardening coecient and n is the work hardening exponent of the deformed material. For wire-drawing deformation, Rack and Cohen's work con®rmed that work hardening behavior of multi-drawing was similar to that of single tensile deformation [15]. The relationship between the true drawing stress and true drawing strain can be described by Hollomon's empirical formula, which are indicated by solid lines shown in Fig. 2. The work hardening exponents for both processes are derived by a numerical iteration method. They are 0.5201 and 0.2905, corresponding to wiredrawing processes without and with employing current pulses, respectively. The result
Fig. 2. The true drawing stress as a function of drawing strain.
536
K.-F. Yao et al./Scripta Materialia 45 (2001) 533±539
Fig. 3. The ultimate tensile strength (a) and the tensile elongation (b) of as-drawn wires vary with drawing strain.
shows that the work hardening exponent of the wire in drawing deformation is decreased 44.1% by applying current pulses. Mechanical properties of as-drawn wires, which experienced dierent drawing strain, have been measured and the results are shown in Fig. 3. It can be found that the ultimate tensile stress rb of as-drawn wires is signi®cantly reduced by applying current pulses in drawing process (Fig. 3(a)). As drawing strain increases, the dierence of rb between two drawing processes increases. The maximum decrement of rb is about 50%. The tensile elongation of as-drawn wires decreases rapidly with the increment of drawing strain, especially when the drawing strain is small (Fig. 3(b)). When drawing strain is greater than 40%, the tensile elongation of wires drawn without current is less than 1%, while that of wires drawn with employing current pulses decreases much slowly (Fig. 3(b)). These results indicate that the ductility of as-drawn wires has been much improved by drawing with current pulses. It would result in an increment of drawing deformation ability. In present research, wires with original diameter of /1:50 mm have been drawn to ®ne wires of /0:124 mm without employing any intermediate heat treatment process. The accumulated drawing strain is about 500%. However, in traditional wire-drawing process it is necessary to anneal wires several times to soften them for further deformation. In present work, close attention was paid to the temperature rise DT of the wire resulted from the pulsing current. The temperature rise was measured by using a touching thermocouple recorder in the drawing process without cooling oil. The measuring position is about 30 mm away from the exit of the die. By comparing the temperature rise obtained in drawing with and without current, DT resulted from Joule heating is obtained and its average value for /1:16 mm wire is about 43°C, similar to the reported results [16±18]. However, the temperature rise near the deformation zone might be dierent. The precise measurement is still in trying. The details of the measurement will be reported elsewhere. In present work, the polarity eect has been
K.-F. Yao et al./Scripta Materialia 45 (2001) 533±539
537
Fig. 4. Surface morphologies of as-drawn wires deformed without (a) and with (b) applying current pulses, respectively. The drawing strain is e 118%.
studied. It has been found that the drawing stress drop is reduced 30±40% by changing the current polarity oppositely, agreed with the reported results [4]. The surface quality of as-drawn wires has been carefully examined by use of scanning electron microscope. Fig. 4(a) and (b) shows surface morphologies of as-drawn wires deformed without and with employing current pulses, respectively. These as-drawn wires have experienced eight drawing courses and the accumulated drawing strain is e 118%. The drawing direction is indicated by the arrowed line shown in Fig. 4. As shown in Fig. 4(a) and (b), surface morphologies of wires drawn without and with current pulses are quite dierent. Many small microcracks and pits, resulted from wearing with inner surface of drawing dies, are observed on the surface of the as-drawn wire deformed without current (Fig. 4(a)). While those drawn with employing current pulses the surface is much smooth and almost no microcrack is observed on the surface, except that some groove-like wear trace lines, which are parallel to the drawing direction, are observed (Fig. 4(b)). The surface quality of as-drawn wires has been much improved by drawing with employing current pulses. Discussion During wire-drawing deformation process, the wire is plastically deformed in the deformation zone of the drawing die. The drawing stress mainly depends on deforming resistance, that is the resistance to dislocation movement. By applying current pulses in
538
K.-F. Yao et al./Scripta Materialia 45 (2001) 533±539
deformation process, drawing forces are signi®cantly decreased for all drawing courses (Fig. 1(a) and (b)). These indicate that there should exist an extra force acting on moving dislocations, which should be related to current pulses. Troitskii [1] and other researchers [4,7] suggested that there would exist a force acting on dislocations induced by drift electrons. So drift electrons would help dislocations to overcome resistance from obstacles and lattice resistance, and result in load drop. In addition, drift electrons could aect the thermally activated motion of dislocations [4,5,16]. With the promotion of drift electrons many dislocations would activate more easily, resulting in the decrement of drawing stress and work hardening rate, agreed with experimental results (Fig. 2). The results shown in Fig. 3(a) and (b) indicate that the ductility of as-drawn wires is much improved and the ultimate tensile strength of as-drawn wires is signi®cantly decreased by applying current pulses in drawing process. It implies that the as-drawn wire is softened by deforming with current pulses. This would result in the decrement of drawing stress in next drawing course even though the next drawing is done without current [19]. So when next drawing course is done with current pulses, drawing force drop is contributed both by current pulses during deformation and by the softening eects from previous drawing with current. However, it has not yet been made clear why the work hardening exponent of the wire is reduced 44% by applying current pulses in wire-drawing deformation. During deforming process with employing current pulses, Joule heating is another important factor in¯uencing deformation behavior [4]. In present research, the measured average temperature rise resulted from pulsing current is about 43°C. For the experimental steel, this temperature rise may result in the decrement of drawing stress by 3±5% according to the relationship of rs ±T and rb ±T of the steel [20], indicating the existence of Joule heating eects. The Joule heating eect, especially local heating, would promote the activation and the movement of dislocations and result in the decrement of the drawing stress and the work hardening rate. It has reported that Joule heating may contribute 50±60% of ¯ow stress drop in Ti alloys [17,18]. In present work, the fraction of drawing stress drop contributed by Joule heating is not clear. But polarity-change experiments show that there exists signi®cant dierence in drawing stress drop when the polarity of pulsing current is changed, con®rming the existence of nonthermal eect's contribution to the drawing stress drop [4,17,18]. During drawing deformation process, the wire surface will interact with the inner surface of the drawing die. It would result in occurrence of wearing on the wire surface. When the wire is drawn without current, surface wear of the wire is heavier, resulted in formation of many micro-cracks and detached pits, indicating the mechanism of adhesive wear. When the wire is drawn employing current pulses, the surface wear behavior and the surface quality of as-drawn wires have been much improved (Fig. 4(b)). According to the observed groove-like wear traces, the wear behavior is related to abrasive wear. Although how current pulses in¯uence the wear behavior and wear mechanism of the wire is not clear, pulsing current has really improved the surface quality of the as-drawn wire and changed the wear mechanism. It may be related to the change of stress states in deformation zone, the softening eects and the in¯uence of drift electrons on deformation behavior.
K.-F. Yao et al./Scripta Materialia 45 (2001) 533±539
539
Conclusions (1) The wire-drawing force is signi®cantly reduced by applying current pulses in drawing deformation process. The decrement of the drawing force is closely related to the accumulated drawing strain and the largest value is about 50%. When e < 60% the drawing force drop increases rapidly and almost linearly with the increment of drawing strain, while when e > 60% it increases much slowly. (2) The work hardening of the wires in drawing deformation is signi®cantly decreased by applying current pulses in the drawing process and the work hardening exponent is reduced by 44%. (3) The ultimate tensile strength of as-drawn wires is signi®cantly decreased and the tensile ductility of as-drawn wires is much improved by drawing with applying current pulses. The as-drawn wire is softened by drawing with employing current pulses. (4) The surface quality of as-drawn wires is much improved and the wear mechanism of the wires is changed by employing current pulses in the drawing process.
Acknowledgements This work is supported by Beijing Nature Science Foundation. Supports from Analysis Center of Tsinghua University and the ``985'' Research Fund of Tsinghua University are greatly appreciated.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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