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Lubricity of volatile lubricants in sheet metal rolling
Journal of Materials Processing Technology 140 (2003) 548–554
Lubricity of volatile lubricants in sheet metal rolling Zhrgang Wang a , Kuniaki Dohda a , Young-Hoon Jeong b,∗ a
Department of Mechanical Systems Engineering, Gifu University, Gifu, Japan b Graduate School of Engineering, Gifu University, Gifu, Japan
Abstract A series of experiments is carried out by using a rolling type tribometer to investigate the lubricity of the volatile lubricants at high speed forming. The roll material is the die steel alloy SKD11, and the workpiece material is the mild steel SPCE with a rough surface and the aluminum alloy A3004 with a smooth surface. Experimental results show that the friction coefficient decreases with increasing working velocity for both SPCE and A3004, in any lubricant. With an increase of reduction in thickness, the friction coefficient decreases for SPCE, but increases for A3004. Some volatile lubricants have the same lubricity as the generally used mineral oil with low-viscosity by judging from the value of friction coefficient, the surface appearance of rolled workpiece and the roll surface damage. © 2003 Elsevier B.V. All rights reserved. Keywords: Plastic forming; Tribology; Lubrication; Volatile lubricant; Rolling; Friction coefficient
1. Introduction In metal forming, the role of lubricant is to reduce the friction coefficient, guarantee the surface quality of products, and increase the tool life. However, the lubricant stuck to the products must be cleaned after forming. The agent of degreasing and cleaning for products damages the environment of the earth. Recently, the environment-friendly lubrication technologies have been actively developed to decrease the environmental contamination [1]. The volatile lubricant can be considered as one choice of the substitute lubricant. The volatile lubricant consists of the volatilization ingredients with low boiling point, it does not remain on the product surface at all after several hours. Furthermore, the volatile lubricant can be collected by cooling the volatilized lubricant. At present, the volatile lubricants are practically used in the shearing process of the parts of household electrical machinery [1]. A few studies have been reported on the lubricity of volatile lubricant in the deep drawing process [2,3]. However, no application to the plastic forming processes except for shearing and deep drawing process can be found. In the previous paper [4], we have carried out a series of experiments by using a strip-ironing type tribometer to investigate the possibility of the volatile lubricant as the lubricant for the bulk metal forming processes. The results
∗ Corresponding author. Tel./fax: +81-58-293-2516. E-mail address: [email protected] (Y.-H. Jeong).
0924-0136/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0924-0136(03)00833-1
show that the volatile lubricants have the same lubricity as the generally used mineral oil with low-viscosity by judging from the value of friction coefficient, the surface appearance of ironed workpiece and the surface damage of the die. The volatile lubricants can be applied to the ironing process with 40% reduction in thickness when the workpiece surface texture and the compatibility between the die and workpiece are properly chosen. In this present paper, we will investigate the lubricity of the volatile lubricants at high speed forming by using a rolling type tribometer.
2. Experimental conditions 2.1. Principle of tribometer The principle of the present tribometer is shown in Fig. 1 [5]. The tribometer consists of two rolls with different radii (upper roll: traction roll; lower roll: friction roll). A workpiece is drawn into the gap between rolls by the friction force on the traction roll. The surface of the traction roll is roughened to 40 m Rz by EDM to increase the friction force. The reduction in the workpiece thickness can be changed freely through moving the traction roll up and down. The normal force ND can be measured by two load cells installed between the columns, and the friction force FD can be detected by a torque detector installed on the axis of the friction roll. The value of friction coefficient can be calculated by using the measured values of normal and friction force.
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Table 2 Experimental conditions Reduction in thickness, Re (%) Working velocity, VT (mm/s) Sliding ratio, δ Oil film thickness, f0 (m) Working travel, h (mm) Temperature (◦ C)
2.4. Experimental conditions
Fig. 1. Principle of rolling type tribometer. Table 1 Properties of volatile lubricant Lubricant A Viscosity (10−6 m2 /s at 40 ◦ C) Speed of volatileness (h) Component (%) Hydrocarbon Corrosion inhibitor
B 0.8 0.5
≥99.0 ≤1.0
C 1.33 1
100 –
5, 20, 30 500, 1000 0.95 10 500 25 ± 1
1.7 3 100 –
2.2. Workpiece The workpiece materials used in the present paper were dull finished mild steel sheet (SPCE by JIS) and commercially aluminum alloy sheet (A3004-O by JIS). The surface texture of A3004 sheet was finished to a smooth surface by polishing (Ra = 0.01 m). The workpiece was 10 mm in width and 500 mm in length with keeping the rolling direction as its length direction. Its surface appearance is shown in Fig. 2.
Experimental conditions are given in Table 2. The friction roll is made of die steel alloy (SKD11 by JIS) and its surface was finished by polishing (Ra = 0.01 m). The working reduction in thickness was 5, 20 and 30%. The working velocity VT was defined as the velocity of the traction roll and set to 500 and 1000 mm/s. The sliding ratio δ was defined as the ratio of relative sliding speed between the workpiece and friction roll to the average speed of the workpiece. The sliding ratio δ was set to 0.95. This value is approximately equal to that for ironing process (δ = 1.0). All tests were performed at the oil film thickness of 10 m by controlling the time after the volatile lubricant applied to the workpiece. An experiment was conducted by the following procedures. The workpiece and rolls were degreased with acetone, dried with cold air, and lubricant was applied to workpiece surface on the friction roll side. After the controlled time to make oil film thickness 10 m, the workpiece was inserted to the gap between rotating rolls through a guide and was rolled. The friction roll was polished and cleaned thoroughly after each experiment to keep the same surface conditions.
3. Results and discussion 2.3. Volatile lubricants 3.1. Variation in friction coefficient µ Three kinds of volatile lubricants were used and their properties shown in Table 1. A paraffinic mineral oil P10 (9.87 × 10−6 m2 /s at 40 ◦ C) was used as the comparison lubricant.
Fig. 3 shows the variations in the friction coefficient µ with increasing working velocity VT for Re = 20%. The friction coefficient at 28 mm/s in this figure was the result
Fig. 2. Surface appearance of workpiece.
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Fig. 3. Variations in friction coefficient µ with working velocity VT .
Fig. 4. Variations in friction coefficient µ with reduction in thickness Re (VT = 500 mm/s).
of previous paper [4]. The friction coefficient µ decreases with increasing VT , in any case. The same trend was also recognized at Re = 5 and 30%. Figs. 4 and 5 show the variations in the friction coefficient µ with increasing reduction in thickness Re. In the case of SPCE, with an increase of reduction in thickness Re, the friction coefficient µ decreases for any lubricant and no large difference in friction coefficient µ due to the difference of lubricant can be recognized. In the case of A3004, with an increase of reduction in thickness Re, the friction coefficient
µ increases for any lubricant. The difference in friction coefficient µ due to the difference of lubricant cannot be largely recognized until Re = 20%, but appears at Re = 30%. 3.2. Surface appearance of rolled workpiece and roll surface damage Figs. 6 and 7 show the surface appearance of rolled workpiece for Re = 20%. In the case of SPCE, the oil pits exist in striped state along the working direction and a lightly
Fig. 5. Variations in friction coefficient µ with reduction in thickness Re (VT = 1000 mm/s).
Z. Wang et al. / Journal of Materials Processing Technology 140 (2003) 548–554
Fig. 6. Surface appearance of rolled workpiece (SPCE, Re = 20%).
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Fig. 7. Surface appearance of rolled workpiece (A3004, Re = 20%).
Z. Wang et al. / Journal of Materials Processing Technology 140 (2003) 548–554
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Fig. 8. Surface appearance of roll after rolling (Re = 30%).
scratched surface appears at rolled surface, for any lubricant. Compared with the volatile lubricants, the outflow trace of lubricant becomes more marked and the scratched portion decreases for lubricant P10. With an increase of VT , the number of oil pits increases markedly. Specially, when the volatile lubricant C is used, the scratched portion decreases compared with volatile lubricant A and B, and its surface appearance has the almost same as with lubricant P10. In the case of A3004, only extremely light scratch can be found at rolled surface, in any case. Fig. 8 shows the surface appearance of roll after rolling for Re = 30%. Observation position is the starting point of the contact area where the roll surface damage is the severest. When the lubricant A is used the roll surface is largely covered with adhered workmetal (the white portion) than other lubricants. 3.3. Discussion In this experiment, judging from the rolled surface appearance, the decrease in the friction coefficient µ with increasing working velocity VT may be due to the increase of the lubricant amount with an increase in the working velocity VT . Furthermore, another interesting result is the varying tendency of friction coefficient µ with increasing reduction in thickness Re, namely, the friction coefficient
decreases for SPCE, but increases for A3004 with increasing reduction in thickness Re. Generally, speaking, with an increase of reduction in thickness Re, the friction coefficient µ becomes larger because the lubricating state becomes severe due to the increase of the surface extension and bite angle. For this reason, in the case of A3004, the friction coefficient µ increases with increasing reduction in thickness Re. However, in the case of SPCE, with increasing reduction in thickness Re, the lubricant trapped in oil pits flows out easily and improves the lubricating state and thus the friction coefficient decreases [5]. Judging synthetically from the friction coefficient, the surface appearance of rolled workpiece and the roll surface damages, the volatile lubricants B and C have almost the same lubricity as the lubricant P10 and can be applied to the high speed forming.
4. Conclusion The present research is carried out by using a rolling type tribometer to investigate the lubricity of the volatile lubricants at high speed forming. (1) With an increase of working velocity VT , the friction coefficient µ decreases for both SPCE and A3004, in any lubricant. However, with an increase of reduction in
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thickness, the friction coefficient µ decreases for SPCE with a rough surface and increases for A3004 with a smooth surface. (2) The surface appearance of rolled workpiece is a lightly scratched surface and roll surface damage is extremely small. References [1] S. Kimura, Elementary knowledge of non-degreasing oil for application with skill, Press Work. 33 (8) (1995) 18–24 (in Japanese).
[2] S. Kataoka, Speed effect on deep drawing of aluminum alloy sheet with use of volatile lubricants, J. Jpn. Inst. Light Met. 48 (1) (1998) 25–29 (in Japanese). [3] S. Kataoka, Deep drawing process of aluminum alloy sheet with hydraulic counter pressure and vibration to tool system with use of volatile lubricants, J. Jpn. Inst. Light Met. 48 (2) (1998) 78–82 (in Japanese). [4] Z. Wang, K. Dohda, Y.-H. Jeong, Lubricity of volatile lubricants in ironing process, Mater. Sci. Forum, in press. [5] K. Dohda, Z. Wang, Effect of average lubricant velocity and sliding velocity on friction behavior in mild steel sheet forming, ASME J. Tribol. 120 (1998) 724–728.
Lubricity of volatile lubricants in sheet metal rolling Zhrgang Wang a , Kuniaki Dohda a , Young-Hoon Jeong b,∗ a
Department of Mechanical Systems Engineering, Gifu University, Gifu, Japan b Graduate School of Engineering, Gifu University, Gifu, Japan
Abstract A series of experiments is carried out by using a rolling type tribometer to investigate the lubricity of the volatile lubricants at high speed forming. The roll material is the die steel alloy SKD11, and the workpiece material is the mild steel SPCE with a rough surface and the aluminum alloy A3004 with a smooth surface. Experimental results show that the friction coefficient decreases with increasing working velocity for both SPCE and A3004, in any lubricant. With an increase of reduction in thickness, the friction coefficient decreases for SPCE, but increases for A3004. Some volatile lubricants have the same lubricity as the generally used mineral oil with low-viscosity by judging from the value of friction coefficient, the surface appearance of rolled workpiece and the roll surface damage. © 2003 Elsevier B.V. All rights reserved. Keywords: Plastic forming; Tribology; Lubrication; Volatile lubricant; Rolling; Friction coefficient
1. Introduction In metal forming, the role of lubricant is to reduce the friction coefficient, guarantee the surface quality of products, and increase the tool life. However, the lubricant stuck to the products must be cleaned after forming. The agent of degreasing and cleaning for products damages the environment of the earth. Recently, the environment-friendly lubrication technologies have been actively developed to decrease the environmental contamination [1]. The volatile lubricant can be considered as one choice of the substitute lubricant. The volatile lubricant consists of the volatilization ingredients with low boiling point, it does not remain on the product surface at all after several hours. Furthermore, the volatile lubricant can be collected by cooling the volatilized lubricant. At present, the volatile lubricants are practically used in the shearing process of the parts of household electrical machinery [1]. A few studies have been reported on the lubricity of volatile lubricant in the deep drawing process [2,3]. However, no application to the plastic forming processes except for shearing and deep drawing process can be found. In the previous paper [4], we have carried out a series of experiments by using a strip-ironing type tribometer to investigate the possibility of the volatile lubricant as the lubricant for the bulk metal forming processes. The results
∗ Corresponding author. Tel./fax: +81-58-293-2516. E-mail address: [email protected] (Y.-H. Jeong).
0924-0136/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0924-0136(03)00833-1
show that the volatile lubricants have the same lubricity as the generally used mineral oil with low-viscosity by judging from the value of friction coefficient, the surface appearance of ironed workpiece and the surface damage of the die. The volatile lubricants can be applied to the ironing process with 40% reduction in thickness when the workpiece surface texture and the compatibility between the die and workpiece are properly chosen. In this present paper, we will investigate the lubricity of the volatile lubricants at high speed forming by using a rolling type tribometer.
2. Experimental conditions 2.1. Principle of tribometer The principle of the present tribometer is shown in Fig. 1 [5]. The tribometer consists of two rolls with different radii (upper roll: traction roll; lower roll: friction roll). A workpiece is drawn into the gap between rolls by the friction force on the traction roll. The surface of the traction roll is roughened to 40 m Rz by EDM to increase the friction force. The reduction in the workpiece thickness can be changed freely through moving the traction roll up and down. The normal force ND can be measured by two load cells installed between the columns, and the friction force FD can be detected by a torque detector installed on the axis of the friction roll. The value of friction coefficient can be calculated by using the measured values of normal and friction force.
Z. Wang et al. / Journal of Materials Processing Technology 140 (2003) 548–554
549
Table 2 Experimental conditions Reduction in thickness, Re (%) Working velocity, VT (mm/s) Sliding ratio, δ Oil film thickness, f0 (m) Working travel, h (mm) Temperature (◦ C)
2.4. Experimental conditions
Fig. 1. Principle of rolling type tribometer. Table 1 Properties of volatile lubricant Lubricant A Viscosity (10−6 m2 /s at 40 ◦ C) Speed of volatileness (h) Component (%) Hydrocarbon Corrosion inhibitor
B 0.8 0.5
≥99.0 ≤1.0
C 1.33 1
100 –
5, 20, 30 500, 1000 0.95 10 500 25 ± 1
1.7 3 100 –
2.2. Workpiece The workpiece materials used in the present paper were dull finished mild steel sheet (SPCE by JIS) and commercially aluminum alloy sheet (A3004-O by JIS). The surface texture of A3004 sheet was finished to a smooth surface by polishing (Ra = 0.01 m). The workpiece was 10 mm in width and 500 mm in length with keeping the rolling direction as its length direction. Its surface appearance is shown in Fig. 2.
Experimental conditions are given in Table 2. The friction roll is made of die steel alloy (SKD11 by JIS) and its surface was finished by polishing (Ra = 0.01 m). The working reduction in thickness was 5, 20 and 30%. The working velocity VT was defined as the velocity of the traction roll and set to 500 and 1000 mm/s. The sliding ratio δ was defined as the ratio of relative sliding speed between the workpiece and friction roll to the average speed of the workpiece. The sliding ratio δ was set to 0.95. This value is approximately equal to that for ironing process (δ = 1.0). All tests were performed at the oil film thickness of 10 m by controlling the time after the volatile lubricant applied to the workpiece. An experiment was conducted by the following procedures. The workpiece and rolls were degreased with acetone, dried with cold air, and lubricant was applied to workpiece surface on the friction roll side. After the controlled time to make oil film thickness 10 m, the workpiece was inserted to the gap between rotating rolls through a guide and was rolled. The friction roll was polished and cleaned thoroughly after each experiment to keep the same surface conditions.
3. Results and discussion 2.3. Volatile lubricants 3.1. Variation in friction coefficient µ Three kinds of volatile lubricants were used and their properties shown in Table 1. A paraffinic mineral oil P10 (9.87 × 10−6 m2 /s at 40 ◦ C) was used as the comparison lubricant.
Fig. 3 shows the variations in the friction coefficient µ with increasing working velocity VT for Re = 20%. The friction coefficient at 28 mm/s in this figure was the result
Fig. 2. Surface appearance of workpiece.
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Fig. 3. Variations in friction coefficient µ with working velocity VT .
Fig. 4. Variations in friction coefficient µ with reduction in thickness Re (VT = 500 mm/s).
of previous paper [4]. The friction coefficient µ decreases with increasing VT , in any case. The same trend was also recognized at Re = 5 and 30%. Figs. 4 and 5 show the variations in the friction coefficient µ with increasing reduction in thickness Re. In the case of SPCE, with an increase of reduction in thickness Re, the friction coefficient µ decreases for any lubricant and no large difference in friction coefficient µ due to the difference of lubricant can be recognized. In the case of A3004, with an increase of reduction in thickness Re, the friction coefficient
µ increases for any lubricant. The difference in friction coefficient µ due to the difference of lubricant cannot be largely recognized until Re = 20%, but appears at Re = 30%. 3.2. Surface appearance of rolled workpiece and roll surface damage Figs. 6 and 7 show the surface appearance of rolled workpiece for Re = 20%. In the case of SPCE, the oil pits exist in striped state along the working direction and a lightly
Fig. 5. Variations in friction coefficient µ with reduction in thickness Re (VT = 1000 mm/s).
Z. Wang et al. / Journal of Materials Processing Technology 140 (2003) 548–554
Fig. 6. Surface appearance of rolled workpiece (SPCE, Re = 20%).
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Fig. 7. Surface appearance of rolled workpiece (A3004, Re = 20%).
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Fig. 8. Surface appearance of roll after rolling (Re = 30%).
scratched surface appears at rolled surface, for any lubricant. Compared with the volatile lubricants, the outflow trace of lubricant becomes more marked and the scratched portion decreases for lubricant P10. With an increase of VT , the number of oil pits increases markedly. Specially, when the volatile lubricant C is used, the scratched portion decreases compared with volatile lubricant A and B, and its surface appearance has the almost same as with lubricant P10. In the case of A3004, only extremely light scratch can be found at rolled surface, in any case. Fig. 8 shows the surface appearance of roll after rolling for Re = 30%. Observation position is the starting point of the contact area where the roll surface damage is the severest. When the lubricant A is used the roll surface is largely covered with adhered workmetal (the white portion) than other lubricants. 3.3. Discussion In this experiment, judging from the rolled surface appearance, the decrease in the friction coefficient µ with increasing working velocity VT may be due to the increase of the lubricant amount with an increase in the working velocity VT . Furthermore, another interesting result is the varying tendency of friction coefficient µ with increasing reduction in thickness Re, namely, the friction coefficient
decreases for SPCE, but increases for A3004 with increasing reduction in thickness Re. Generally, speaking, with an increase of reduction in thickness Re, the friction coefficient µ becomes larger because the lubricating state becomes severe due to the increase of the surface extension and bite angle. For this reason, in the case of A3004, the friction coefficient µ increases with increasing reduction in thickness Re. However, in the case of SPCE, with increasing reduction in thickness Re, the lubricant trapped in oil pits flows out easily and improves the lubricating state and thus the friction coefficient decreases [5]. Judging synthetically from the friction coefficient, the surface appearance of rolled workpiece and the roll surface damages, the volatile lubricants B and C have almost the same lubricity as the lubricant P10 and can be applied to the high speed forming.
4. Conclusion The present research is carried out by using a rolling type tribometer to investigate the lubricity of the volatile lubricants at high speed forming. (1) With an increase of working velocity VT , the friction coefficient µ decreases for both SPCE and A3004, in any lubricant. However, with an increase of reduction in
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Z. Wang et al. / Journal of Materials Processing Technology 140 (2003) 548–554
thickness, the friction coefficient µ decreases for SPCE with a rough surface and increases for A3004 with a smooth surface. (2) The surface appearance of rolled workpiece is a lightly scratched surface and roll surface damage is extremely small. References [1] S. Kimura, Elementary knowledge of non-degreasing oil for application with skill, Press Work. 33 (8) (1995) 18–24 (in Japanese).
[2] S. Kataoka, Speed effect on deep drawing of aluminum alloy sheet with use of volatile lubricants, J. Jpn. Inst. Light Met. 48 (1) (1998) 25–29 (in Japanese). [3] S. Kataoka, Deep drawing process of aluminum alloy sheet with hydraulic counter pressure and vibration to tool system with use of volatile lubricants, J. Jpn. Inst. Light Met. 48 (2) (1998) 78–82 (in Japanese). [4] Z. Wang, K. Dohda, Y.-H. Jeong, Lubricity of volatile lubricants in ironing process, Mater. Sci. Forum, in press. [5] K. Dohda, Z. Wang, Effect of average lubricant velocity and sliding velocity on friction behavior in mild steel sheet forming, ASME J. Tribol. 120 (1998) 724–728.