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Performance of a new dry lubricant in the forming of aluminum alloy sheets
Wear 249 (2001) 86–93
Performance of a new dry lubricant in the forming of aluminum alloy sheets K.P. Rao∗ , J.J. Wei Department of Manufacturing Engineering and Engineering Management, City University of Hong Kong, Kowloon, Hong Kong Received 27 April 2000; received in revised form 3 January 2001; accepted 17 January 2001
Abstract In metal forming processes, lubricants are necessary to prevent adhesion, scratching, galling and material transfer. With increasing concerns about environment and health, development and application of green lubricants became very important. In this paper, the usefulness and lubricity of boric acid dry films for cold forming of aluminum alloy sheets has been investigated. Drawing and stretching test results revealed that, regardless of types of alloys, the lubricity of boric acid films is comparable with that of some commonly used commercial solid and liquid lubricants over a wide range of forming speeds. Since boric acid is non-toxic and water soluble, the post-cleaning process is relatively simple and safe. Thus, it can be expected that the type of lubrication adopted in this study is not only cost-effective and highly efficient for aluminum forming, but also is environmentally safe. © 2001 Published by Elsevier Science B.V. Keywords: Sheet metal forming; Dry-film lubrication; Boric acid lubricant
1. Introduction Lubrication is critical during the forming of sheet metals. Inadequate lubrication or breakdown of lubricant films develops direct contact of workpiece and die, and can lead to transfer of the softer workpiece material on to tool surface. The build-up of particles on the tool, i.e. galling, is particularly significant in the forming of aluminum alloys [1]. Galling is a common problem in many metal forming processes that may cause changes to the tool geometry and increase in the forming force. These may result in premature tearing and/or scratching of workpiece in severely strained areas [1,2]. Many lubricants have been developed for metal forming operations. For light pressworking, low-viscosity mineral oil, synthetic oils, or water-based lubricants are suitable. For severe draw-stretch type operations, extreme pressure additives [3] and low-friction coatings such as phosphate [4] are often employed. It was reported that chlorine [5] and phosphate or phosphite [6] are very effective as additives of lubricating oils for anti-seizure or anti-galling during forming of steel. Their excellent lubricities are due to the formation of lubricious reaction layers at the interface of workpiece and die by tribochemical reactions between lubricants and
∗ Corresponding author. Tel.: +852-2788-8409; fax: +852-2788-8423. E-mail address: [email protected] (K.P. Rao).
0043-1648/01/$ – see front matter © 2001 Published by Elsevier Science B.V. PII: S 0 0 4 3 - 1 6 4 8 ( 0 1 ) 0 0 5 2 6 - 9
contact surfaces. Their deployment can significantly delay or prevent the occurrence of metal transfer and galling [7]. The global drive towards the use of lubricants benign to health and environment poses challenges for traditional lubricants. The commonly used lubricants are often flammable; contain active elements such as chlorine, sulfur, and phosphorous, which are potentially hazardous; and often require the use of volatile organic solvents for their removal from formed surfaces. Post-cleaning and disposal of these lubricants and solvents is difficult and costly [3,8,9]. Therefore, it is necessary that lubricants more suitable for metal forming process be developed. Recently, efforts have been made on the development of new lubricant formulations and lubrication technology. Some researchers modified the conventional lubricants by addition of other compounds to reduce the concentrations of active elements or even replace them. Miller et al. [10] used complex polymetric esters to replace chlorinated additives, fatty acid soaps and sulfurized materials in metal working fluids, and found that the reformulated product has better performance. Balulescu et al. [11] found that neat or water-soluble derivatives of vegetable oils show very good performances in cutting operations, equaling or even surpassing the performance of conventional fluids containing sulfur and chlorine additives. Schey [12,13] studied the interactions between lubricants and zinc coatings on steel sheets using a drawbead simulation tester. The results indicated that oleic acid can destroy the stability of the zinc transfer
K.P. Rao, J.J. Wei / Wear 249 (2001) 86–93
layer when sheets have a significant basal component (Zn) in the coating, resulting in junction growth, plowing and high friction, although it reduces friction on bare steel. However, a boron compound with 7.6% B seems to prevent junction growth and keep friction low. In addition, some works indicated [14,15] that boric acid could be used as a good additive in cutting and grinding fluids to control friction and corrosion. Liang et al. [16] found that addition of boric acid into distilled water increases the drilling rate of polycrystalline alumina. More recently, the works of Erdemir, Fenske and Wei [17,18] found that boric acid appears to be firmly adhered to aluminum surfaces and provides very low friction coefficient (about 0.04). Boric acid dry film alone and in combination with mineral oil can yield strains larger than commercial liquid and solid lubricants in a out-of-plane stretching test, indicating that they could be very good lubricants for aluminum cold forming. The lubricity of boric acid is related to its layered structure, in which the bonds between the layers are much weaker than the intralayer bonding [19]. In this study, simple brushing process was used to produce boric acid dry films on the three aluminum alloy blanks, and the lubrication performance in deep drawing operations has been investigated. The effect of drawing operation conditions on the performance of boric acid dry films has also been investigated. The lubrication performance has been compared with those achieved by typical commercial solid and liquid lubricants. Furthermore, the acting mechanism of boric acid films has been briefly described.
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9.5 g in 100 ml ethanol, and was about 5 g in 100 ml water at room temperature, and the solubility increased with increasing temperature. Saturated ethanol solution with boric acid was prepared by stirring them at room temperature. The solution was brushed onto both sides of the cleaned aluminum sheets. Dry films were obtained within a couple of minutes after the evaporation of ethanol. The brushing process was repeated two more times so as to build a sufficiently thick layer of lubricant film. The other selected lubricants were chemically pure oleic acid, commercial solid lubricants MoS2 (molybdenum disulfide or molykote) and Teflon or PTFE (polytetrafluoroethylene), and a commercial metal forming lubricant (mixture of graphite and lubricating oil). Dry films of PTFE and MoS2 were prepared by spraying their suspensions, and like in the case of boric acid, the spraying process had to be repeated one more time to obtain a good coverage of the lubricant over the sheet metals. The films of oleic acid and the mixture of graphite and oil were obtained by simple brushing on the sheet surfaces. 2.2. Test procedure Erichsen sheet metal drawing machine was used to measure the relative friction of dry and liquid films and to establish their lubrication ranking as metal forming lubricants. Schematic illustrations of the tooling used for forming a cylindrical cup and a hemispherical dome are shown in Fig. 1. In the drawing process, the sheet metal is drawn into a die cavity with the use of a flat punch. Sheet metal blank is held in place by a blankholder ring, and then the punch
2. Experimental details 2.1. Materials and lubricant application Three aluminum alloys (1100, 5052, and 6061) were used to obtain lubrication effectiveness of various lubricants through some metal forming tests. The chemical compositions of the three alloys are presented in Table 1. The thickness of Alloy 1100 was 1.0 mm and the other two materials were of 1.2 mm. Blanks of these aluminum alloy sheets were ground to ≈0.2 m center-line-average (CLA). Before the sheets were coated with lubricants, they were cleaned with commonly used dishwashing detergent and dried in air. It has been found that the solubility of boric acid is significantly higher in ethanol than in water. The solubility is about Table 1 Typical compositions of the aluminum alloys used in this study Trade code
1100 5052 6061
Grade
H12 H32 T6
Composition (wt.%) Mg
Si + Fe
Mn
Cu
Cr
0.05 2.2–2.8 1.0
0.95 0.45 0.4–0.8
0.05 0.10 0.15
0.05–0.2 0.10 0.30
0.05 0.15–0.35 0.2
Fig. 1. Schematic illustration of the sheet metal test tools: (a) flat-end punch for drawing; and (b) spherical-end punch for stretching.
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moves up to a preset stroke. For ideal deep drawing, it is required that the workpiece be drawn into the gap between punch and die to form a cup-shape part without over-all thinning. However, in dry drawing (unlubricated) or under insufficient lubrication, thinning and fracture of sheets may occur. The diameter of the punch (D1 ) is 50 mm and that of aluminum sheet blanks are 85 mm. Except for the tests used to study the effect of forming speed, all of the tests were performed at a punch speed of 0.067 mm/s and a blankholding pressure of 0.5 kN. A stroke of 40 mm was preset so that the workpiece can be fully drawn and a full cup shape can be achieved in most of the situations. The maximum drawing force required was recorded automatically. At least two tests were performed under each condition to check the reproducibility of the test results. The results indicated that the maximum test error was below 7%. On the other hand, in order to measure the maximum dome height of aluminum alloy under various lubrication conditions, a punch with spherical end (Fig. 1(b)) was used in stretch forming mode. A larger workpiece (diameter of 120 mm) was clamped using a holding down ring having 90 mm bead diameter (D2 ). The clamping force was set at 50 kN. The stretching tests proceeded until the workpiece was fractured. The maximum dome height was recorded. Optical and scanning electron microscopy (SEM) were used to examine the surface morphology of sheet surfaces before and after the forming tests.
3. Results and discussion With the application techniques employed in this study, it was noticed that different amounts of lubricants were needed to produce reasonably good coatings on the sheet materials tested. While excessive lubricant would easily drop off from the sheet surfaces, insufficient amounts exposed the parent sheet metal that is easily noticeable. Because of the varied nature of the coatings involved, these were quantified in terms of amount of lubricant retained per unit surface area of sheet material. In fact, this is a convenient measure if one needs to control the level of coating in actual production processes. Table 2 shows the amounts of lubricants employed in this study, which were established after some trials. It may be noted that least amount of lubricant used was boric acid, closely followed by PTFE. The amounts used in the case of oleic acid and MoS2 were about three times higher than
boric acid and PTFE, where as graphite in oil was about 10 times to provide a visibly good spread of the lubricant. Fig. 2 presents the maximum drawing forces for Alloy 1100 under different lubrication conditions. During dry drawing operation (unlubricated or uncoated), the blank could not be fully drawn up to the preset stroke of 40 mm, because of high friction between workpiece and blankholding ring, workpiece and die, workpiece and punch. It had fractured along the bottom corner (correspond to punch corner/radius) of the cup. However, introduction of lubricants on both sides of the workpiece significantly reduced the friction at contact interfaces, and thus the workpiece could be fully drawn without fracture. It has been found, as is normally expected, that the maximum drawing force for unlubricated condition is higher than with lubrication and, as can be seen from the error bars indicated in the figure, the difference between successive trials under similar conditions is also somewhat significant. Under lubricated conditions, lower drawing force means lower friction and better lubricity. It can be interpreted from Fig. 2 that the lubricity of MoS2 is lower than that of PTFE, mixture of graphite and oil, oleic acid and boric acid. It is interesting to note that the lubricity of boric acid dry film is similar to that obtained with PTFE or mixture of graphite with oil, and is better than that of MoS2 . This indicates that boric acid dry film is very effective for cold forming of Alloy 1100. In general, better lubricants yielded minimum variation between different experiments under similar drawing conditions. Fig. 3 shows the maximum drawing forces for Alloy 5052 under various lubrication conditions. Trend similar to that observed for Alloy 1100 has been found in terms of lubricant ranking. Once again, the sheet metal had fractured under unlubricated conditions and was fully drawn under other lubricated conditions. The lubricity of boric acid film is equivalent to those of PTFE, mixture of graphite and oil and oleic acid, and is better than that of MoS2 , showing that the dry films of boric acid provide good lubricity for cold forming of Alloy 5052. Fig. 4 shows the maximum drawing forces recorded for Alloy 6111 under various lubrication conditions. Trends of lubricities are similar to those seen in Fig. 2 and Fig. 3. Relative to commercial solid and liquid lubricants (PTFE, MoS2 and graphite), boric acid lubrication creates equivalent or lower drawing force, indicating that boric acid films are very good lubricants for cold forming of Alloy 6111 as well.
Table 2 Amount of lubricants applied to sheet materials Lubricant
Application method
Amount of lubricant (gms/m2 )
Remarks
Graphite in oil MoS2 PTFE Oleic acid Boric acid
Spread with a spatula Sprayed from aerosol can Sprayed from aerosol can Applied with a swab Applied with a swab
85.0–98.6 27.0–33.0 7.6–10.2 22.5–25.4 6.1–7.5
Coating appeared to be uniform over the entire sheet metal surface Applied in two coats with natural air drying between successive coats Applied in two coats with natural air drying between successive coats Lubricant accumulation or pooling in the form of spots was observed Applied in three coats with natural air drying between successive coats
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Fig. 2. Maximum forces obtained in drawing of Alloy 1100 under various lubrication conditions.
Fig. 5 presents the maximum dome heights (stretch depths) obtained with Alloy 1100 under various lubrication conditions. These tests were performed using a punch with a spherical end (tooling as shown in Fig. 1(b)). When compared with drawing under dry condition, addition of lubricants increased the draw depth to significant extent. The dome height with boric acid film is similar to or higher than the other solid and liquid lubricants (MoS2 , graphite
plus oil and PTFE), indicating that its lubricity is among the best. The trend for lubricant ranking is similar to that seen in Fig. 2 based on drawing force, indicating that different contact geometry of the two tests did not affect the lubricity of boric acid film. In order to identify the effect of drawing speed on the lubricity of boric acid film, maximum drawing forces (with flat punch) and maximum dome heights (with spherical punch)
Fig. 3. Maximum forces obtained in drawing of Alloy 5052 under various lubrication conditions.
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Fig. 4. Maximum forces obtained in drawing of Alloy 6111 under various lubrication conditions.
were studied on Alloy 1100 coated with boric acid films. The results are shown in Fig. 6. The maximum drawing force slightly increased with speed, and tends to attain a stable value of 14.1 kN. Moreover, the maximum dome height decreased slightly and then increased for higher forming speeds. In general, the lubricity of boric acid film did not
change appreciably with forming speed, even under the two different forming operations. Compared to other solid lubricants, indicated in Fig. 2, it has been found that boric acid film can keep its good lubricity over a wide range of speed. Based on the above test results, boric acid dry film shows excellent lubricity for different aluminum alloys in drawing
Fig. 5. Maximum dome heights obtained using hemispherical punch for Alloy 1100 under various lubrication conditions.
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Fig. 6. The effect of drawing speed on maximum drawing forces and dome heights obtained for Alloy 1100 under boric acid lubrication.
operations. In a previous study [18] involving out-of-plane stretching tests, similar results were found in which the lubricities of boric acid dry films are equivalent to or better than that of commercial liquid and solid lubricants for forming of Alloys 6111, 6061 and 5754. Since boric acid is non-toxic and can be removed by water, the post-cleaning process with boric acid lubrication becomes much simpler and safe. It appears that boric acid facilitates easy application and removal, is environmentally safe, cost-effective and highly efficient for cold forming of aluminum alloys. It is noted that oleic acid, which is a pure component of vegetable oils, also shows very good lubricity for forming of aluminum. Although, post-cleaning of oleic acid requires organic solvents, it is environmentally safe and biodegradable. Therefore, oleic acid and high oleic vegetable oils may also be considered as promising lubricants for metal forming. It may be noted that the lubricity and accessibility of vegetable oils as metal-working fluids have recently generated good interest [20]. Fig. 7 presents the SEM photographs of Alloy 1100 original surface and its deformed surfaces under boric acid lubrication. The sharp features of the surface scratches that can be seen in Fig. 7(a) clearly indicate that the original surface is somewhat rough and is relatively clean. The as-deposited boric acid films on the surface (Fig. 7(b)) appear to be thin since the original surface scratch texture is still apparent even after the coating process. It was noticed that this deposited layer is sufficiently firm and would not be removed by normal handling operations involved before forming. After the forming tests the deformed surface around the punch radius was still covered with the boric acid
film (Fig. 7(c)) and appears to be similar to that prevailing before forming, except for some thinning, i.e. the original porous layer is somewhat compressed. No exposed substrate surface was found on the deformed area. The appearance of the deformed substrate surface after removing these films (with water) is shown in Fig. 7(d). It is found to contain no obvious new scratches or damage of the substrate surface, although its texture is slightly different from the original surface due to deformation. The sharp scratch texture features that were observed on the original surface are evident and, once again, the cleaning process virtually removed all the extraneous materials from the surface. The surface measurements indicated that CLA was well below the original 0.2 m roughness, and was even much smaller (as low as 0.05 m) in those areas where there was movement of sheet metal over the punch. This indicates that bonded boric acid film, acting as boundary lubricant, effectively separates the workpiece and tool, and helps preventing the occurrence of aluminum transfer, scratches and even fracture. The effectiveness of boric acid films in forming of aluminum may be due to several reasons. Among them, good affinity between boric acid and clean aluminum surface is important. The test results indicated that, before and after drawing operations, as-deposited and deformed boric acid dry films are firmly adhered to aluminum surfaces. After forming, no scratches or transfer of aluminum were observed on the workpiece and tool surfaces. Since aluminum alloy surface is naturally oxidized and also contains many alloying elements such as magnesium, the bonding nature of boric acid on aluminum surface is very complicated. In addition, during sheet forming, there may be tribochemical
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Fig. 7. SEM photographs of Alloy 1100 under different conditions: (a) original surface; (b) with as-deposited boric acid film; (c) deformed surface with boric acid film; and (d) deformed substrate surface after removing the boric acid film.
interactions between boric acid and aluminum oxide, boric acid and alloying elements, producing new lubricious compounds. Another aspect is the layer structure of boric acid itself. Erdemir studied [19] crystal chemistry of boric acid, and proposed its self-lubrication mechanism. Boric acid crystallizes in a layered triclinic crystal structure. The atoms in each layer are closely packed and strongly bonded to each other. The atomic layers are 0.318 nm apart from each other and held together by weak van der Waals forces. In a sense, the layered crystal structure of boric acid is similar to the structures of MoS2 and graphite. Therefore, the very-slippery quality of boric acid is controlled by its layered crystalline structure. Under shear stress of sliding contact, the crystalline layers align themselves parallel to the direction of relative motion, and slide over one another relatively easily.
4. Conclusions The lubrication effectiveness of boric acid dry films for forming aluminum alloys was investigated using deep drawing and stretching type operations at room temperature. The lubricities were compared with those of commercial lubricants (PTFE, MoS2 and mixture of graphite and oil) under the same operation conditions. Based on the above tests, the following conclusions can be drawn: 1. The dry films of boric acid can be prepared by simply brushing its saturated ethanol solution on aluminum surfaces. The resultant film upon drying appears to be firmly adhered to clean aluminum surfaces. 2. The deep drawing tests indicated that, for three aluminum alloys (1100, 5052 and 6111), the lubricity of boric acid
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film is comparable with that of commonly used commercial lubricants, and its lubricity is good over a wide range of forming speeds. In addition, SEM photographs indicate that there are no obvious scratches on the deformed surfaces under boric acid lubrication. 3. As boric acid films are considered to be less toxic and water soluble, post-cleaning of formed components should be relatively simple and safe. 4. The results of this study indicated the beneficial effects of boric acid dry films in sheet metal forming operations, lending further credence to similar observations made by other researchers who attributed such good performance to its layer structure and good affinity with aluminum surfaces. Higher sheet metal drawability and lower forming forces are the direct benefits that can be realized.
Acknowledgements Financial assistance to carry out this research was provided by a Central Earmarked Research Grant from the Research Grants Council of the Hong Kong Special Administrative Region (RGC Ref. no. CityU 1023/00E). The authors would like to thank Mr. C.H. Yuen and Mr. W.K. Lee for their technical assistance and Dr. L.K.Y. Li of the Department of Manufacturing Engineering and Engineering Management for valuable discussion. References [1] J.M. Story, G.W. Jarvis, H.R. Zonker, S.J. Murtha, Issues and trends in automotive aluminum sheet forming, SAE Publication no. SP-944 (1993) 1–25. [2] W.R.D. Wilson, Tribology in cold metal forming, J. Manufac. Sci. Eng. 119 (1997) 695–698. [3] E.S. Nachtman, S. Kalpakjian, Lubricants and lubrication in metalworking operations, Marcel Dekker, New York, 1985.
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[4] H. Saiki, G. Ngaile, L. Ruan, Characterization of adhesive strength of phosphate coatings in cold metal forming, in: Proceedings of the 1996 ASME/STLE Joint Tribology Conference, San Francisco, CA, USA, 1996, pp. 1013–1017. [5] C.G. Wall, The laboratory evaluation of sheet metal forming lubricants, Lubric. Eng. 40 (1984) 139–147. [6] S. Komatsuzaki, T. Uematsu, T. Narahara, Cold forming of steel with lubricating oils, Lubric. Eng. 52 (1996) 259–266. [7] J.A. Schey, Friction in sheet metalworking, SAE Publication no. SP-1221 (1997) 87–106. [8] J.A. Schey, Tribology in Metalworking: Friction, Lubrication and Wear, American Society for Metals, Metals Park, OH, 1983. [9] L. Lazzarotto, L. Dubar, A. Dubois, P. Ravassard, J.P. Bricout, J. Oudin, A selection methodology for lubricating oils in cold metal forming processes, Wear 215 (1998) 1–9. [10] P.R. Miller, H. Patel, Using complex polymeric esters as multifunctional replacements for chlorine and other additives in metalworking, Lubric. Eng. 53 (1997) 31–33. [11] M. Balulescu, L. Cira, J. Herdan, Environmentally friendly metalworking fluids, Ecological and Economical Aspects of Tribology, in: Proceedings of the 9th International Colloqium, Technische Akademie Esslinggen, 11–13 January, 1994, pp. 7.11-1–7.11-5. [12] J.A. Schey, Friction in sheet forming with soft and hard tooling, Steel Res. 69 (1998) 148–153. [13] J. A Schey, Lubricant effects in drawing coated sheets over nitride die surfaces, Lubric. Eng. 52 (1996) 630–636. [14] W.T. Branneen, G.D. Burt, R.A. McDonald, Phosphite amine lubricant for metal working and machining, US Patent no. 4965002 (1990). [15] E.D. Rekow, G.M. Zhang, V.P. Thompson, S. Jahanmir, Factorial design technique to investigate the effect of machine tool parameters and machining environment on surface finish, J. Dent. Res., IADR Abstract 570 (1993). [16] H. Liang, S. Jahanmir, Boric acid as an additive for core-drilling of alumina, J. Tribol. 117 (1995) 65–73. [17] A. Erdemir and G.R. Fenske, Clean and cost-effective dry boundary lubricants for aluminium forming, SAE Special Publication no. SP-1350 (1998) 9–17. [18] J. Wei, A. Erdemir, G. Fenske, Dry lubricant films for aluminum forming, Tribol. Trans., in press. [19] A. Erdemir, Tribological properties of boric acid and boric-acidforming surfaces. Part I: Crystal chemistry and mechanism of self-lubrication of boric acid, Lubric. Eng. 47 (1991) 168–173. [20] D. Margaroni, High performance biofluids — natural or synthetic? Ind. Lubric. Tribol. 51 (1999) 139–143.
Performance of a new dry lubricant in the forming of aluminum alloy sheets K.P. Rao∗ , J.J. Wei Department of Manufacturing Engineering and Engineering Management, City University of Hong Kong, Kowloon, Hong Kong Received 27 April 2000; received in revised form 3 January 2001; accepted 17 January 2001
Abstract In metal forming processes, lubricants are necessary to prevent adhesion, scratching, galling and material transfer. With increasing concerns about environment and health, development and application of green lubricants became very important. In this paper, the usefulness and lubricity of boric acid dry films for cold forming of aluminum alloy sheets has been investigated. Drawing and stretching test results revealed that, regardless of types of alloys, the lubricity of boric acid films is comparable with that of some commonly used commercial solid and liquid lubricants over a wide range of forming speeds. Since boric acid is non-toxic and water soluble, the post-cleaning process is relatively simple and safe. Thus, it can be expected that the type of lubrication adopted in this study is not only cost-effective and highly efficient for aluminum forming, but also is environmentally safe. © 2001 Published by Elsevier Science B.V. Keywords: Sheet metal forming; Dry-film lubrication; Boric acid lubricant
1. Introduction Lubrication is critical during the forming of sheet metals. Inadequate lubrication or breakdown of lubricant films develops direct contact of workpiece and die, and can lead to transfer of the softer workpiece material on to tool surface. The build-up of particles on the tool, i.e. galling, is particularly significant in the forming of aluminum alloys [1]. Galling is a common problem in many metal forming processes that may cause changes to the tool geometry and increase in the forming force. These may result in premature tearing and/or scratching of workpiece in severely strained areas [1,2]. Many lubricants have been developed for metal forming operations. For light pressworking, low-viscosity mineral oil, synthetic oils, or water-based lubricants are suitable. For severe draw-stretch type operations, extreme pressure additives [3] and low-friction coatings such as phosphate [4] are often employed. It was reported that chlorine [5] and phosphate or phosphite [6] are very effective as additives of lubricating oils for anti-seizure or anti-galling during forming of steel. Their excellent lubricities are due to the formation of lubricious reaction layers at the interface of workpiece and die by tribochemical reactions between lubricants and
∗ Corresponding author. Tel.: +852-2788-8409; fax: +852-2788-8423. E-mail address: [email protected] (K.P. Rao).
0043-1648/01/$ – see front matter © 2001 Published by Elsevier Science B.V. PII: S 0 0 4 3 - 1 6 4 8 ( 0 1 ) 0 0 5 2 6 - 9
contact surfaces. Their deployment can significantly delay or prevent the occurrence of metal transfer and galling [7]. The global drive towards the use of lubricants benign to health and environment poses challenges for traditional lubricants. The commonly used lubricants are often flammable; contain active elements such as chlorine, sulfur, and phosphorous, which are potentially hazardous; and often require the use of volatile organic solvents for their removal from formed surfaces. Post-cleaning and disposal of these lubricants and solvents is difficult and costly [3,8,9]. Therefore, it is necessary that lubricants more suitable for metal forming process be developed. Recently, efforts have been made on the development of new lubricant formulations and lubrication technology. Some researchers modified the conventional lubricants by addition of other compounds to reduce the concentrations of active elements or even replace them. Miller et al. [10] used complex polymetric esters to replace chlorinated additives, fatty acid soaps and sulfurized materials in metal working fluids, and found that the reformulated product has better performance. Balulescu et al. [11] found that neat or water-soluble derivatives of vegetable oils show very good performances in cutting operations, equaling or even surpassing the performance of conventional fluids containing sulfur and chlorine additives. Schey [12,13] studied the interactions between lubricants and zinc coatings on steel sheets using a drawbead simulation tester. The results indicated that oleic acid can destroy the stability of the zinc transfer
K.P. Rao, J.J. Wei / Wear 249 (2001) 86–93
layer when sheets have a significant basal component (Zn) in the coating, resulting in junction growth, plowing and high friction, although it reduces friction on bare steel. However, a boron compound with 7.6% B seems to prevent junction growth and keep friction low. In addition, some works indicated [14,15] that boric acid could be used as a good additive in cutting and grinding fluids to control friction and corrosion. Liang et al. [16] found that addition of boric acid into distilled water increases the drilling rate of polycrystalline alumina. More recently, the works of Erdemir, Fenske and Wei [17,18] found that boric acid appears to be firmly adhered to aluminum surfaces and provides very low friction coefficient (about 0.04). Boric acid dry film alone and in combination with mineral oil can yield strains larger than commercial liquid and solid lubricants in a out-of-plane stretching test, indicating that they could be very good lubricants for aluminum cold forming. The lubricity of boric acid is related to its layered structure, in which the bonds between the layers are much weaker than the intralayer bonding [19]. In this study, simple brushing process was used to produce boric acid dry films on the three aluminum alloy blanks, and the lubrication performance in deep drawing operations has been investigated. The effect of drawing operation conditions on the performance of boric acid dry films has also been investigated. The lubrication performance has been compared with those achieved by typical commercial solid and liquid lubricants. Furthermore, the acting mechanism of boric acid films has been briefly described.
87
9.5 g in 100 ml ethanol, and was about 5 g in 100 ml water at room temperature, and the solubility increased with increasing temperature. Saturated ethanol solution with boric acid was prepared by stirring them at room temperature. The solution was brushed onto both sides of the cleaned aluminum sheets. Dry films were obtained within a couple of minutes after the evaporation of ethanol. The brushing process was repeated two more times so as to build a sufficiently thick layer of lubricant film. The other selected lubricants were chemically pure oleic acid, commercial solid lubricants MoS2 (molybdenum disulfide or molykote) and Teflon or PTFE (polytetrafluoroethylene), and a commercial metal forming lubricant (mixture of graphite and lubricating oil). Dry films of PTFE and MoS2 were prepared by spraying their suspensions, and like in the case of boric acid, the spraying process had to be repeated one more time to obtain a good coverage of the lubricant over the sheet metals. The films of oleic acid and the mixture of graphite and oil were obtained by simple brushing on the sheet surfaces. 2.2. Test procedure Erichsen sheet metal drawing machine was used to measure the relative friction of dry and liquid films and to establish their lubrication ranking as metal forming lubricants. Schematic illustrations of the tooling used for forming a cylindrical cup and a hemispherical dome are shown in Fig. 1. In the drawing process, the sheet metal is drawn into a die cavity with the use of a flat punch. Sheet metal blank is held in place by a blankholder ring, and then the punch
2. Experimental details 2.1. Materials and lubricant application Three aluminum alloys (1100, 5052, and 6061) were used to obtain lubrication effectiveness of various lubricants through some metal forming tests. The chemical compositions of the three alloys are presented in Table 1. The thickness of Alloy 1100 was 1.0 mm and the other two materials were of 1.2 mm. Blanks of these aluminum alloy sheets were ground to ≈0.2 m center-line-average (CLA). Before the sheets were coated with lubricants, they were cleaned with commonly used dishwashing detergent and dried in air. It has been found that the solubility of boric acid is significantly higher in ethanol than in water. The solubility is about Table 1 Typical compositions of the aluminum alloys used in this study Trade code
1100 5052 6061
Grade
H12 H32 T6
Composition (wt.%) Mg
Si + Fe
Mn
Cu
Cr
0.05 2.2–2.8 1.0
0.95 0.45 0.4–0.8
0.05 0.10 0.15
0.05–0.2 0.10 0.30
0.05 0.15–0.35 0.2
Fig. 1. Schematic illustration of the sheet metal test tools: (a) flat-end punch for drawing; and (b) spherical-end punch for stretching.
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moves up to a preset stroke. For ideal deep drawing, it is required that the workpiece be drawn into the gap between punch and die to form a cup-shape part without over-all thinning. However, in dry drawing (unlubricated) or under insufficient lubrication, thinning and fracture of sheets may occur. The diameter of the punch (D1 ) is 50 mm and that of aluminum sheet blanks are 85 mm. Except for the tests used to study the effect of forming speed, all of the tests were performed at a punch speed of 0.067 mm/s and a blankholding pressure of 0.5 kN. A stroke of 40 mm was preset so that the workpiece can be fully drawn and a full cup shape can be achieved in most of the situations. The maximum drawing force required was recorded automatically. At least two tests were performed under each condition to check the reproducibility of the test results. The results indicated that the maximum test error was below 7%. On the other hand, in order to measure the maximum dome height of aluminum alloy under various lubrication conditions, a punch with spherical end (Fig. 1(b)) was used in stretch forming mode. A larger workpiece (diameter of 120 mm) was clamped using a holding down ring having 90 mm bead diameter (D2 ). The clamping force was set at 50 kN. The stretching tests proceeded until the workpiece was fractured. The maximum dome height was recorded. Optical and scanning electron microscopy (SEM) were used to examine the surface morphology of sheet surfaces before and after the forming tests.
3. Results and discussion With the application techniques employed in this study, it was noticed that different amounts of lubricants were needed to produce reasonably good coatings on the sheet materials tested. While excessive lubricant would easily drop off from the sheet surfaces, insufficient amounts exposed the parent sheet metal that is easily noticeable. Because of the varied nature of the coatings involved, these were quantified in terms of amount of lubricant retained per unit surface area of sheet material. In fact, this is a convenient measure if one needs to control the level of coating in actual production processes. Table 2 shows the amounts of lubricants employed in this study, which were established after some trials. It may be noted that least amount of lubricant used was boric acid, closely followed by PTFE. The amounts used in the case of oleic acid and MoS2 were about three times higher than
boric acid and PTFE, where as graphite in oil was about 10 times to provide a visibly good spread of the lubricant. Fig. 2 presents the maximum drawing forces for Alloy 1100 under different lubrication conditions. During dry drawing operation (unlubricated or uncoated), the blank could not be fully drawn up to the preset stroke of 40 mm, because of high friction between workpiece and blankholding ring, workpiece and die, workpiece and punch. It had fractured along the bottom corner (correspond to punch corner/radius) of the cup. However, introduction of lubricants on both sides of the workpiece significantly reduced the friction at contact interfaces, and thus the workpiece could be fully drawn without fracture. It has been found, as is normally expected, that the maximum drawing force for unlubricated condition is higher than with lubrication and, as can be seen from the error bars indicated in the figure, the difference between successive trials under similar conditions is also somewhat significant. Under lubricated conditions, lower drawing force means lower friction and better lubricity. It can be interpreted from Fig. 2 that the lubricity of MoS2 is lower than that of PTFE, mixture of graphite and oil, oleic acid and boric acid. It is interesting to note that the lubricity of boric acid dry film is similar to that obtained with PTFE or mixture of graphite with oil, and is better than that of MoS2 . This indicates that boric acid dry film is very effective for cold forming of Alloy 1100. In general, better lubricants yielded minimum variation between different experiments under similar drawing conditions. Fig. 3 shows the maximum drawing forces for Alloy 5052 under various lubrication conditions. Trend similar to that observed for Alloy 1100 has been found in terms of lubricant ranking. Once again, the sheet metal had fractured under unlubricated conditions and was fully drawn under other lubricated conditions. The lubricity of boric acid film is equivalent to those of PTFE, mixture of graphite and oil and oleic acid, and is better than that of MoS2 , showing that the dry films of boric acid provide good lubricity for cold forming of Alloy 5052. Fig. 4 shows the maximum drawing forces recorded for Alloy 6111 under various lubrication conditions. Trends of lubricities are similar to those seen in Fig. 2 and Fig. 3. Relative to commercial solid and liquid lubricants (PTFE, MoS2 and graphite), boric acid lubrication creates equivalent or lower drawing force, indicating that boric acid films are very good lubricants for cold forming of Alloy 6111 as well.
Table 2 Amount of lubricants applied to sheet materials Lubricant
Application method
Amount of lubricant (gms/m2 )
Remarks
Graphite in oil MoS2 PTFE Oleic acid Boric acid
Spread with a spatula Sprayed from aerosol can Sprayed from aerosol can Applied with a swab Applied with a swab
85.0–98.6 27.0–33.0 7.6–10.2 22.5–25.4 6.1–7.5
Coating appeared to be uniform over the entire sheet metal surface Applied in two coats with natural air drying between successive coats Applied in two coats with natural air drying between successive coats Lubricant accumulation or pooling in the form of spots was observed Applied in three coats with natural air drying between successive coats
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Fig. 2. Maximum forces obtained in drawing of Alloy 1100 under various lubrication conditions.
Fig. 5 presents the maximum dome heights (stretch depths) obtained with Alloy 1100 under various lubrication conditions. These tests were performed using a punch with a spherical end (tooling as shown in Fig. 1(b)). When compared with drawing under dry condition, addition of lubricants increased the draw depth to significant extent. The dome height with boric acid film is similar to or higher than the other solid and liquid lubricants (MoS2 , graphite
plus oil and PTFE), indicating that its lubricity is among the best. The trend for lubricant ranking is similar to that seen in Fig. 2 based on drawing force, indicating that different contact geometry of the two tests did not affect the lubricity of boric acid film. In order to identify the effect of drawing speed on the lubricity of boric acid film, maximum drawing forces (with flat punch) and maximum dome heights (with spherical punch)
Fig. 3. Maximum forces obtained in drawing of Alloy 5052 under various lubrication conditions.
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Fig. 4. Maximum forces obtained in drawing of Alloy 6111 under various lubrication conditions.
were studied on Alloy 1100 coated with boric acid films. The results are shown in Fig. 6. The maximum drawing force slightly increased with speed, and tends to attain a stable value of 14.1 kN. Moreover, the maximum dome height decreased slightly and then increased for higher forming speeds. In general, the lubricity of boric acid film did not
change appreciably with forming speed, even under the two different forming operations. Compared to other solid lubricants, indicated in Fig. 2, it has been found that boric acid film can keep its good lubricity over a wide range of speed. Based on the above test results, boric acid dry film shows excellent lubricity for different aluminum alloys in drawing
Fig. 5. Maximum dome heights obtained using hemispherical punch for Alloy 1100 under various lubrication conditions.
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Fig. 6. The effect of drawing speed on maximum drawing forces and dome heights obtained for Alloy 1100 under boric acid lubrication.
operations. In a previous study [18] involving out-of-plane stretching tests, similar results were found in which the lubricities of boric acid dry films are equivalent to or better than that of commercial liquid and solid lubricants for forming of Alloys 6111, 6061 and 5754. Since boric acid is non-toxic and can be removed by water, the post-cleaning process with boric acid lubrication becomes much simpler and safe. It appears that boric acid facilitates easy application and removal, is environmentally safe, cost-effective and highly efficient for cold forming of aluminum alloys. It is noted that oleic acid, which is a pure component of vegetable oils, also shows very good lubricity for forming of aluminum. Although, post-cleaning of oleic acid requires organic solvents, it is environmentally safe and biodegradable. Therefore, oleic acid and high oleic vegetable oils may also be considered as promising lubricants for metal forming. It may be noted that the lubricity and accessibility of vegetable oils as metal-working fluids have recently generated good interest [20]. Fig. 7 presents the SEM photographs of Alloy 1100 original surface and its deformed surfaces under boric acid lubrication. The sharp features of the surface scratches that can be seen in Fig. 7(a) clearly indicate that the original surface is somewhat rough and is relatively clean. The as-deposited boric acid films on the surface (Fig. 7(b)) appear to be thin since the original surface scratch texture is still apparent even after the coating process. It was noticed that this deposited layer is sufficiently firm and would not be removed by normal handling operations involved before forming. After the forming tests the deformed surface around the punch radius was still covered with the boric acid
film (Fig. 7(c)) and appears to be similar to that prevailing before forming, except for some thinning, i.e. the original porous layer is somewhat compressed. No exposed substrate surface was found on the deformed area. The appearance of the deformed substrate surface after removing these films (with water) is shown in Fig. 7(d). It is found to contain no obvious new scratches or damage of the substrate surface, although its texture is slightly different from the original surface due to deformation. The sharp scratch texture features that were observed on the original surface are evident and, once again, the cleaning process virtually removed all the extraneous materials from the surface. The surface measurements indicated that CLA was well below the original 0.2 m roughness, and was even much smaller (as low as 0.05 m) in those areas where there was movement of sheet metal over the punch. This indicates that bonded boric acid film, acting as boundary lubricant, effectively separates the workpiece and tool, and helps preventing the occurrence of aluminum transfer, scratches and even fracture. The effectiveness of boric acid films in forming of aluminum may be due to several reasons. Among them, good affinity between boric acid and clean aluminum surface is important. The test results indicated that, before and after drawing operations, as-deposited and deformed boric acid dry films are firmly adhered to aluminum surfaces. After forming, no scratches or transfer of aluminum were observed on the workpiece and tool surfaces. Since aluminum alloy surface is naturally oxidized and also contains many alloying elements such as magnesium, the bonding nature of boric acid on aluminum surface is very complicated. In addition, during sheet forming, there may be tribochemical
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Fig. 7. SEM photographs of Alloy 1100 under different conditions: (a) original surface; (b) with as-deposited boric acid film; (c) deformed surface with boric acid film; and (d) deformed substrate surface after removing the boric acid film.
interactions between boric acid and aluminum oxide, boric acid and alloying elements, producing new lubricious compounds. Another aspect is the layer structure of boric acid itself. Erdemir studied [19] crystal chemistry of boric acid, and proposed its self-lubrication mechanism. Boric acid crystallizes in a layered triclinic crystal structure. The atoms in each layer are closely packed and strongly bonded to each other. The atomic layers are 0.318 nm apart from each other and held together by weak van der Waals forces. In a sense, the layered crystal structure of boric acid is similar to the structures of MoS2 and graphite. Therefore, the very-slippery quality of boric acid is controlled by its layered crystalline structure. Under shear stress of sliding contact, the crystalline layers align themselves parallel to the direction of relative motion, and slide over one another relatively easily.
4. Conclusions The lubrication effectiveness of boric acid dry films for forming aluminum alloys was investigated using deep drawing and stretching type operations at room temperature. The lubricities were compared with those of commercial lubricants (PTFE, MoS2 and mixture of graphite and oil) under the same operation conditions. Based on the above tests, the following conclusions can be drawn: 1. The dry films of boric acid can be prepared by simply brushing its saturated ethanol solution on aluminum surfaces. The resultant film upon drying appears to be firmly adhered to clean aluminum surfaces. 2. The deep drawing tests indicated that, for three aluminum alloys (1100, 5052 and 6111), the lubricity of boric acid
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film is comparable with that of commonly used commercial lubricants, and its lubricity is good over a wide range of forming speeds. In addition, SEM photographs indicate that there are no obvious scratches on the deformed surfaces under boric acid lubrication. 3. As boric acid films are considered to be less toxic and water soluble, post-cleaning of formed components should be relatively simple and safe. 4. The results of this study indicated the beneficial effects of boric acid dry films in sheet metal forming operations, lending further credence to similar observations made by other researchers who attributed such good performance to its layer structure and good affinity with aluminum surfaces. Higher sheet metal drawability and lower forming forces are the direct benefits that can be realized.
Acknowledgements Financial assistance to carry out this research was provided by a Central Earmarked Research Grant from the Research Grants Council of the Hong Kong Special Administrative Region (RGC Ref. no. CityU 1023/00E). The authors would like to thank Mr. C.H. Yuen and Mr. W.K. Lee for their technical assistance and Dr. L.K.Y. Li of the Department of Manufacturing Engineering and Engineering Management for valuable discussion. References [1] J.M. Story, G.W. Jarvis, H.R. Zonker, S.J. Murtha, Issues and trends in automotive aluminum sheet forming, SAE Publication no. SP-944 (1993) 1–25. [2] W.R.D. Wilson, Tribology in cold metal forming, J. Manufac. Sci. Eng. 119 (1997) 695–698. [3] E.S. Nachtman, S. Kalpakjian, Lubricants and lubrication in metalworking operations, Marcel Dekker, New York, 1985.
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