- Email: [email protected]
Evaluation of vegetable oils as pre-lube oils for stamping
Materials & Design Materials and Design 26 (2005) 587–593 www.elsevier.com/locate/matdes
Evaluation of vegetable oils as pre-lube oils for stamping A.C. Carcel *, D. Palomares, E. Rodilla, M.A. Pe´rez Puig Department of Mechanical and Materials Engineering, Universidad Polite´cnica de Valencia, Campus de Vera, 46022 Valencia, Spain Received 27 July 2004; accepted 18 August 2004 Available online 13 October 2004
Abstract Vegetable oils can offer a valuable alternative to mineral or synthetic oils as a source for manufacturing lubricant oils for stamping of sheet metal parts. They are biodegradable and have better intrinsic boundary lubricant (BL) properties, due to the presence of long chain fatty acids in their composition. This work evaluates the performance of some of these vegetable oils under the typical BL conditions found during stamping of car body parts. The values found for the friction coefficient under BL conditions are low and remain stable, with values between 0.11 and 0.13 for steel sheet, 0.09–0.12 for galvannealed sheets and 0.10–0.13 for zinc coated sheet, giving a similar or even better performance than mineral based compounded oils. 2004 Elsevier Ltd. All rights reserved. Keywords: Film and sheet (B); Drawing (C)
1. Introduction The success of stamping operations, in terms of part integrity and surface quality, is strongly dependent on the tribological behaviour during the stamping process. This behaviour is known to be the result of a complex set of factors like tools design, punch speed, blankholder forces, surface composition and topography of both dies and sheet materials, and also lubricant properties. Fig. 1 depicts a schematic diagram of the effects of these factors. It is current practice today in the car industry to feed the stamping press lines without adding any special press lubricant oils to the metal sheets. The same rustpreventive oil applied at the steel mill prior to coiling must provide lubrication during the stamping of the car body parts. The antirust protective oils are primarily applied onto steel sheets and zinc coated steel sheets in order to protect the surfaces from corrosion during *
Corresponding author. Tel.: +34 963 877 624; fax: +34 963 877 629. E-mail address: [email protected] (A.C. Carcel). 0261-3069/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2004.08.010
transport, storage and processing. When they must also provide lubrication during stamping, they are usually known as ‘‘prelube oils’’. These anti-rust or ‘‘prelube’’ oils have usually a very low viscosity (less than 100 cSt) and the amount of oil applied is also very low, 1–2 g/m2. Although most oil lubricated systems show a full Stribeck curve behaviour, the low viscosity and the typical operational conditions in press forming of sheet metal for car body panels create usually a regime of boundary lubrication. Under boundary lubrication (BL) conditions there is physical contact between the surfaces and the load must be carried entirely by the surface peaks which are in physical contact with the tools. Friction values are in this case not greatly influenced by viscosity properties, being mainly dependent on the properties and stability of the boundary layers adhered to the surfaces. The consumption of prelube oils in EU for these purposes surpasses 10,000 tons per year. These oils must be eliminated before painting, generating contaminant wastes, or go with non-used metal sheet as scrap, being lost.
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A.C. Carcel et al. / Materials and Design 26 (2005) 587–593
Pressure distribution. Contact area size
CONTACT/ GEOMETRY
Flattening of surface asperities
Rolling-Sliding Geometry: Plane, line, Point Surface topography (texture)
DIE AND SHEET MATERIALS
PROCESS CONDITIONS Temperature Pressure Velocity
TRIBOLOGY OF SHEET METAL FORMING
Mechanical properties;Hardness Adhesion-Compatibility Surface layers Surface contaminants
Sliding distances
LUBRICANTS Viscosity; Aditives
Lubricant film heigth
Rheology
Boundary layers from EP aditives
Compatibility after processing
THE FOUR MAIN COMPONENTS AND SOME INTERACTIONS AFFECTING THE TRIBOLOGICAL BEHAVIOUR DURING PRESS FORMING OF METAL SHEETS Fig. 1. Factors and interactions controlling friction during sheet metal forming.
Vegetable oils can offer a valuable alternative to mineral or synthetic oils as a source for manufacturing prelube oils for stamping. They are biodegradable and have better intrinsic BL lubricant properties, due to the presence of long chain fatty acids in their composition. The scope of this work was thus to evaluate the performance of some of these vegetable oils under the typical BL conditions that are found during stamping of car body parts.
2. Experimental details 2.1. Oils tested Five vegetable oils (pure olive oil from press extraction processes, olive oil from solvent extraction processes, sunflower, corn, and soybean oil) were tested to evaluate their lubricant properties for stamping operations. Soybean, sunflower and corn oils are being used as biodegradable lubricant sources for other purposes in Europe. We included also in this study two types of olive oils. Press oil is the typical oil used for alimentary purposes in Mediterranean countries. Until recently, also solvent-obtained olive oils were marketed for human consumption, but a recent prohibition for these purposes, due to the presence of some minor carcinogenic compounds in their composition, has obliged to close many factories that produced this type of oil as a secondary process of the olive oil industry. These solvent obtained oils are, like conventional olive oils, very rich in monounsaturated oleic acid, giving to these oils a good balance between low temperature flow properties
and oxidation resistance at high temperatures, and are cheaper than conventional olive oils. Usually, the lubricant performance of vegetable oils can be enhanced by compounding them with additives or by further chemical process modification, including changes in triglyceride structures or production of fatty acids and sterification. In this study we used the oils as available for alimentary purposes, with no special additives or treatments. For comparative and reference purposes, the same tests were carried out using a widely used prelube oil in the steel sheet metal industry, Ferrocote 6130 de Quaker Chemical B.V. It is a mineral-based oil with aliphatic esters and organometalic soaps as boundary lubrication additives. This oil has been approved as prelube oil by most of the car manufacturers in Europe: Volvo, Ford, VW, Audi, Renault, etc. The Table 1 shows the viscosity values measured in the oils tested at 50 C according to ASTM D 445. 2.2. Steel sheets Bare steel sheet, zinc electrocoated sheet and Zn–Fe alloy coated sheet (galvannealed) have been used in this work, in order to analyse the tribological performance of different coating-oil combinations. All the samples were obtained from EDDQ steels, with thickness between 0.76 and 0.78 mm. Samples 250 · 50 mm are carefully degreased and then both sides oiled prior to testing by a roller system that allows to obtain a uniform oil coating between 1.5 and 2 g/m2. The surface texture of the sheets was evaluated by 2D topography measurement by a profiling skidless instru-
A.C. Carcel et al. / Materials and Design 26 (2005) 587–593
589
Table 1 Oils viscosity and density Oil
Olive
Solvent olive
Sunflower
Corn
Soybean
Reference
E q (g/cm3) Viscosity (cSt)
2.615 0.906 19.61
2.744 0.909 20.57
2.333 0.909 17.50
2.590 0.925 19.42
2.256 0.921 16.92
2.231 0.905 16.73
ment Mhar Perthometer M2, according to ASME B 45.1-1995 and ISO 3274. 2.3. Friction test and evaluation criteria One important aspect in the evaluation of the tribological behaviour under BL conditions is the appropriated selection of the test conditions. Many types of friction tests have been proposed in the literature. These tests can be roughly classified in two groups: pure friction tests and stamping simulative tests, like Erichsen, Fukui, cup drawing, etc. The formers have the advantage that the results are almost independent on the mechanical properties of the material. Some of the most widely accepted pure friction tests are the Inland test [1], the Renault multifriction test [2] or the Draw bead simulator test [3,4]. All these are quite simple tests: the sheet previously oiled is forced to pass between two dies clamped with a normal force N. The pulling force T is continuously measured until a fixed travel length is reached. The coefficient of friction is usually defined as COF = T/2N, being the factor 2 included in order to account for the two sides of the sheet being in contact. The friction test should reproduce the real BL conditions found in real stamping operations. It is also important to have into account that the real tribological conditions, for a given point on the sheet surface, change continuously during the stamping process. When a point of the sheet surface reaches the die entry, for instance, its surface topography has been already flattened by the previous pass under the blankholder. A simple way to evaluate in a friction test the influence of the distance under pressure is to perform multiple runs on the same sheet, as in the Renault multifriction test [2] used in this study. After the first run, dies are opened, the pulling system returns to its initial position, dies are clamped again and a second pass is performed. This procedure can be repeated until a specified number of runs are completed. Samples are not reoiled between the runs. In the test, the dies reproduce a ‘‘plane line contact’’ by the use of a cylindrical tool with a radius of 10 mm and a plane tool, which are clamped with a normal force Fn = 10 dN/mm of width, controlled by and hydraulic unit. The pulling force Ft necessary to make the sheet move between the tools at a velocity of 1 mm/s is continuously measured by a testing machine Instron 4204.
Dies are obtained from tool steel Q + T at 60 HRC and finished with 600-paper grit transverse to the movement direction, giving a final surface finish with Ra = 0.15 lm. The width of the apparent contact zone was experimentally measured and has a value around 0.7 mm, higher than predicted by Hertz theory due to plastic deformation. Mean pressure on the contact zone was calculated as 140 MPa. Under these conditions, the Emmens factor [5] for the process has a value around 10 4, which is a clear indication of the BL conditions during the test. The results from this type of test give an excellent correlation with real stamping behaviour under BL conditions of car body parts [6,7]. It was found that the most relevant factor to allow a correct lubrication during stamping depends on the ability of the system lubricant + surface to give a stable friction behaviour. In the case of the test used in this work, the stability implies that the values of friction should remain stable when the number of successive runs are increased. In these previous studies, performed on zinc coated sheets, the maximum values of the dynamic friction coefficient grew up to 50% for coils that failed in real stamping when the travel length went from 50 to 100 mm (differences between the first and the second 50 mm runs). However, for the group of coils that allowed correct stamping, the maximum values of friction coefficient remained almost constant or even were slightly reduced. Based on this previous work, the COF values stability or uniformity and the absolute values obtained for each vegetable oil, as compared with the results obtained with the reference mineral based oil have been used as evaluation criteria. The relevance of friction stability has been also recognised by others [8–10]. 2.4. Corrosion protection The primary property required for any ‘‘antirust lubricant oil’’ is corrosion prevention. Oils should protect sheet metal components during long term storage and transport. The antirust performance can be evaluated by several tests: outdoor weathering test; indoor exposure; accelerated SST exposure DIN 50021 or ASTM B 117; accelerated 100%humidity exposure DIN 50014.
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In this work we have used the indoor exposure test, with an extended duration of 16 weeks, for sheets of bare steel coated with 1–2 g/m2 of oil applied on the surface. Temperatures during the test period varied between 17 and 22 C and relative humidity between 45% and 65%. Other important properties for the qualification of prelube oils have not been evaluated in this work. It is worthy to note out that these aspects should be also tested in order to have a complete assessment of the ability of vegetable oils as prelube oils. Most of these additional requirements are related to aspects that assure the compatibility of the oil with the habitual manufacturing processes: Compatibility with adhesives and joining processes (epoxi, mastics, sealants, etc. now widely used in the car body assembly process). Easy elimination in alkaline bath degreasing systems. Compatibility with cataphoretic paint baths (some oils cause pinholes in the paint coating).
3. Results and discussion 3.1. Friction behaviour The typical evolution of the friction forces in the test can be observed in Fig. 2, which depicts the results obtained with olive oil and the reference prelube oil when applied on steel sheet and zinc coated sheets. The friction force measured at 50 mm length travel in each run (series in the pictures) was taken to calculate the dynamic value of COF. As depicted in Fig. 2, the values of this coefficient of friction are lower and quasi stable for steel sheets, thus indicating a favourable frictional behaviour for stamping. The opposite situation occurs with zinc electrocoated sheets, which often give problems of galling and surface damage during stamping of difficult car body parts. The results from the tests reveal that, from the first run, the friction values are unstable and have a clear tendency to grow when the sheets are subjected to further runs without re-oiling. This behaviour is attributed either to the
2
2
1.5
1.5 Friction force (KN)
Friction force (KN)
Reference Prelube Oil- Steel Sheet
1 Serie1 Serie2 Serie3 Serie4 Serie5
0.5
1
Ref. prelube oil - Zn electrocoated sheet Serie1 Serie2 Serie3 Serie4 Serie5
0.5
0
0 0
20
40
0
60
20
40
60
Length (mm)
Length (mm) 2
2 Press Olive Oil- Zn electrocoated sheet
Press Olive Oil- Steel Friction force (KN)
Friction force (KN)
1.5
1.5
1 Serie1 Serie2 Serie3 Serie4 Serie5
0.5
1
Serie1 Serie2 Serie3 Serie4 Serie5
0.5
0
0 0
20
40
Length (mm)
60
0
20
40
60
Length (mm)
Fig. 2. Evolution of the friction forces in the friction tests under BL conditions for the five successive runs (series) without re-oiling.
A.C. Carcel et al. / Materials and Design 26 (2005) 587–593 0.25
0.25
Steel sheet
Galvannealed sheet- Test I 0.2
Dynamic COF
Dynamic COF
0.2
0.15
0.1
Press Olive Sunflower Corn Solvent olive Soybean Ref. prelube
0.05
0
50
100
150
200
250
0.15
0.1 Press Olive Sunflower Corn Solvent Olive Soybean Ref. prelube
0.05
0
0
300
0
Travel length in BL friction test
50
100
150
200
250
300
Travel length in BL friction test 0.25
0.25
Zn electrocoated sheet
0.15
0.1
Press Olive Sunflower Corn Solvent olive Soybean Ref. prelube
0.2
Dynamic COF
0.2
Dynamic COF
591
Press Olive Sunflower
0.15
0.1
Corn
0.05
Solvent Olive
Galvannalealed sheet Test II
0.05
Soybean Ref. prelube
0 0
50
100
150
200
250
300
Travel length in BL friction test
0 0
50
100
150
200
250
300
Travel length in BL friction test (mm) Fig. 3. Evolution of the coefficient of friction COF in the friction tests with steel and zinc electrocoated sheets.
absence of effective boundary layers or with the onset of lubricant breakdown, thus allowing metal to metal contact and the welding of surface asperities from the zinc coating to the steel tools. The results of dynamic COF for the other oils and sheets can be observed in the Figs. 3 and 4. Comparing the results obtained with olive oil and those obtained with the reference oil, it can be observed that all the vegetable oils allow to obtain lower COF values than the compounded mineral oil when applied on bare steel sheet. Differences between the vegetable oils are not relevant, although the best results are obtained with olive oils. In the case of zinc electrocoated sheets it should be noted that all the oils, including the reference mineral oil, showed clear limitations to guarantee a constant lubrication behaviour. Some vegetable oils (olive, corn and sunflower) give lower COF values than the reference oil in the two first runs, but their COF values grow respect to the values obtained in the first run, thus suggesting a progressive lack of effectiveness of the boundary layers formed. Globally, however, and having into ac-
Fig. 4. Evolution of the coefficient of friction COF in the friction tests with galvannealed (Zn–Fe) coated sheets.
count that the vegetable oils are not compounded, their behaviour can be considered acceptable and comparable to the results expected from the mineral based oils. 3.2. Surface topography effects The first batch of tests carried out with Zn–Fe coated steel sheets (Galvannealed sheets-Test I) were performed on samples obtained from two different sheet coils. No attention was paid at this stage of the work on the surface topography, which was supposed to be similar for any of the metal sheets. After performing this first batch of friction tests, it was found that the frictional behaviour for Zn–Fe coated sheet was unexpectedly worst than the frictional behaviour found with pure zinc coating, as shown in Fig. 4. A second batch of tests was then performed, but using all samples from the same XX009 coil, whose surface topography had been previously measured and controlled. Fig. 5 gives information about the differences in the surface topography of the sheets. These differences in the surface texture can be also detected by other
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Material ratio %
100 80 60
Coil XX237 ZnFe test I
40
Coil XX181ZnFe test I-Olive S
20
Coil XX009 ZnFe test II
0 0
-1
-2
-3
-4
-5
-6
-7
-8
-9 -10
Distance from surface (microns) Fig. 5. Bearing material ratio curve (Abbot–Firestone Curve) for three different Galvannealed coils used in the friction tests. Cut-off 2.5 mm.
Table 2 Topography parameters of the sheets (cut-off 2.5 mm) Coil coating
Ra (lm)
Rp (lm)
Rpk (lm)
Mr1 %
Steel Zn electrocoated Zn–Fe Tests I Zn–Fe Tests I + Olive S oil Zn–Fe Tests II
1.32 1.50 1.34 1.32
4.09 4.18 3.5 4.04
1.43 1.35 0.98 1.31
9.3 9.1 5.2 10
1.33
4.38
1.55
9.77
parameters, particularly from those related to de size of the surface peaks, like Rp, Rpk or Mr1, defined in ISO 13565-2 and ISO 4287, as shown in Table 2. The differences in the COF values observed in the first test batch between the oil Olive S (Olive obtained by solvent extraction) and the other oils, included the reference oil, are indeed caused by a different surface topography. When the tests were performed on galvannealed samples with an appropriated surface texture, the results changed as shown in the bottom chart depicted in Fig. 4. Again, like in the case of the bare steel sheets, the vegetable oils give a lubricant performance similar or even better (lower values of COF) than the reference mineral oil. These results confirm the great relevance of the surface topography in the tribological behaviour under BL conditions. Sheets with similar values of the mean roughness Ra can have a very different behaviour, even when using the same lubricant, thus introducing an error source in the results obtained from these studies. The relevance of the surface topography features have been recognised in real stamping operations of car body parts. These features affect the tribological behaviour because under BL conditions, the supply of lubricant to the contact zones relies mainly on the ability of the surface to carry oil in the remaining valleys or pockets of the surface, thus avoiding lubricant breakdown and the onset of galling or adhesion between the tools and the sheets. Also the surface aspect after painting of car body parts is greatly affected by the surface topography or texture.
Several anti-galling profiles have been proposed [11–13] that have proven to be effective in drawing bare steel sheets. The requirements from steel and car manufacturers are usually based on measures of Ra and Pc, but the range of acceptable values can vary from one manufacturer to another. For the american car industry, acceptable texture parameters can be roughly defined by a range of Ra values from 20 to 50 linch (0.5–1.30 lm) and a peak density Pc above 82 peaks per inch (35 peaks/cm), measured with cutoff 0.8 mm and a peak count level of 50 linch (1.27 lm). It is also usual to use the same requirements for different types of coatings: zinc electrocoated, hot dip, galvannealed sheet, etc. The values of several topography parameters of the coils used in this study are presented in the Table 2. The results of this study suggest that the definition of antigalling textures for galvannealed coatings requires the use of additional parameters, apart from the conventional mean roughness Ra and peak count Pc, in order to give an appropriated definition of the surface topography. Some useful parameters could be obtained from from the Abbot–Firestone curve. Control of these parameters can give a better definition of optimum texture for reducing friction and improving the stamping behaviour and should be controlled when performing any study on the lubricant properties of oils under BL conditions. The same effects of the surface topography were indeed observed in a previous work carried out with pure zinc electrocoated sheets [14]. 3.3. Corrosion performance Samples of bare steel 200 · 200 mm were oiled and then exposed at 60 inclination in the laboratory for a period of 16 weeks. Usually, the car manufacturing specifications require only 12-week exposure. After this period, all the samples showed complete absence of any red rust corrosion stain.
4. Conclusions The lubricant performance of all the vegetable oils tested, without additives, is comparable or even better than the lubricant performance afforded by one of the most widely used mineral – synthetic based compounded prelube oil for stamping of car body parts, even when pure zinc or galvannealed coated sheets are tested. The values of the COF remain stable after five successive runs using any of the vegetable oils applied on steel sheets or galvannealed sheets. The typical values of COF are 0.11 and 0.13 for steel sheet and between 0.09 and 0.12 for galvannealed sheets. The values of COF for zinc electrocoated sheet remain more stable also with the vegetable oils than with the mineral based prelube oil. In the first run the COF values for vegetable oils vary
A.C. Carcel et al. / Materials and Design 26 (2005) 587–593
from 0.10 to 0.13 and have a value inferior to the value of COF for the mineral prelube: 0.14. The corrosion protection afforded by any of the vegetable oils tested, without additives, is also correct in long term indoor exposure tests: 16 weeks without red rust when applied onto bare steel sheets. Other factors, like the shape and asperity distribution in the surface topography of the sheets have also a great effect on the tribological behaviour under BL conditions. Sheets with surface textures characterised by high material ratio curves (low values of Rpk and Mr1 according to ISO) cannot be correctly lubricated by any kind of the oils tested. Even the mineral-synthetic prelube oil failed in this case. This suggests that more attention should be paid to these aspects of the problem. Better control of topography would allow the use of less additivated vegetable oils or eliminate the use of special high viscosity press oils. Acknowledgement This work was developed thanks to the financial support of the Spanish National Plan of R + D + I (2000– 2003) to the Research Project TRIBOSOLVE. References [1] Bernick LM, Hilsen RR, Wandrei CL. Development of a quantitative sheet galling test. Wear 1978;48. p. 323, 346.
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[2] Specification Renault ‘‘Test Multifrottement Plan’’ RNUR D-31 1738. [3] Nine HD. Draw bead forces in sheet metal forming. In: Mechanics of sheet metal-forming. New York: Plenum Press; 1978. p. 179–211. [4] Nine HD. The applicability of CoulombÕs friction law to drawbeads in sheet metalforming. J Appl Metal work 1982;2(3): 200–210. [5] Emmens WC. The influence of surface roughness on friction. In: Proceedings of the 15th IDDRG biennial conference, Dearnborn; 1988. p. 63–70. [6] Carcel AC et al.. The role of friction in fractures of coated steel sheets during press forming. In: Advances in mechanical behaviour, plasticity and damage. Amsterdam: Elsevier; 2000. p. 685, 689. [7] Carcel A, Ferrer C, Perez Puig MA. Surface texture evolution of zinc coatings under BL conditions. In: Proceedings of the 22nd biennial congress IDDRG, Nagoya, May; 2002. [8] Ludema KC. Friction, Wear, Lubrication. Boca Raton: CRC Press; 1996. p. 10. [9] Schey J, Watts SW. Transient tribological phenomena in the drawbead simulation SAE paper 9200634; 1992. [10] Dalton G. New friction model for sheet metalforming. SAE paper 2001010081; 2001. [11] Rault D, Etringer M. Sheet metal forming and energy conservation. ASM 1976:97–114. [12] Hilsen RR, Bernick LM, Relationship between surface characteristics and galling index of steel sheet ASTM Special Tech. Publ. No. 647; 1978. [13] ASP Program Technical Report. Uniformity of surface texture of steel sheet for automotive applications. AISI – USA, January; 1993. [14] Carcel A, Ferrer C, Perez Puig MA. Alternative 2D surface topography parameters for optimising the friction behaviour of zinc coated sheets. In: Proceedings of the 22th Biennial Congress IDDRG, Nagoya, Japan, May; 2002.
Evaluation of vegetable oils as pre-lube oils for stamping A.C. Carcel *, D. Palomares, E. Rodilla, M.A. Pe´rez Puig Department of Mechanical and Materials Engineering, Universidad Polite´cnica de Valencia, Campus de Vera, 46022 Valencia, Spain Received 27 July 2004; accepted 18 August 2004 Available online 13 October 2004
Abstract Vegetable oils can offer a valuable alternative to mineral or synthetic oils as a source for manufacturing lubricant oils for stamping of sheet metal parts. They are biodegradable and have better intrinsic boundary lubricant (BL) properties, due to the presence of long chain fatty acids in their composition. This work evaluates the performance of some of these vegetable oils under the typical BL conditions found during stamping of car body parts. The values found for the friction coefficient under BL conditions are low and remain stable, with values between 0.11 and 0.13 for steel sheet, 0.09–0.12 for galvannealed sheets and 0.10–0.13 for zinc coated sheet, giving a similar or even better performance than mineral based compounded oils. 2004 Elsevier Ltd. All rights reserved. Keywords: Film and sheet (B); Drawing (C)
1. Introduction The success of stamping operations, in terms of part integrity and surface quality, is strongly dependent on the tribological behaviour during the stamping process. This behaviour is known to be the result of a complex set of factors like tools design, punch speed, blankholder forces, surface composition and topography of both dies and sheet materials, and also lubricant properties. Fig. 1 depicts a schematic diagram of the effects of these factors. It is current practice today in the car industry to feed the stamping press lines without adding any special press lubricant oils to the metal sheets. The same rustpreventive oil applied at the steel mill prior to coiling must provide lubrication during the stamping of the car body parts. The antirust protective oils are primarily applied onto steel sheets and zinc coated steel sheets in order to protect the surfaces from corrosion during *
Corresponding author. Tel.: +34 963 877 624; fax: +34 963 877 629. E-mail address: [email protected] (A.C. Carcel). 0261-3069/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2004.08.010
transport, storage and processing. When they must also provide lubrication during stamping, they are usually known as ‘‘prelube oils’’. These anti-rust or ‘‘prelube’’ oils have usually a very low viscosity (less than 100 cSt) and the amount of oil applied is also very low, 1–2 g/m2. Although most oil lubricated systems show a full Stribeck curve behaviour, the low viscosity and the typical operational conditions in press forming of sheet metal for car body panels create usually a regime of boundary lubrication. Under boundary lubrication (BL) conditions there is physical contact between the surfaces and the load must be carried entirely by the surface peaks which are in physical contact with the tools. Friction values are in this case not greatly influenced by viscosity properties, being mainly dependent on the properties and stability of the boundary layers adhered to the surfaces. The consumption of prelube oils in EU for these purposes surpasses 10,000 tons per year. These oils must be eliminated before painting, generating contaminant wastes, or go with non-used metal sheet as scrap, being lost.
588
A.C. Carcel et al. / Materials and Design 26 (2005) 587–593
Pressure distribution. Contact area size
CONTACT/ GEOMETRY
Flattening of surface asperities
Rolling-Sliding Geometry: Plane, line, Point Surface topography (texture)
DIE AND SHEET MATERIALS
PROCESS CONDITIONS Temperature Pressure Velocity
TRIBOLOGY OF SHEET METAL FORMING
Mechanical properties;Hardness Adhesion-Compatibility Surface layers Surface contaminants
Sliding distances
LUBRICANTS Viscosity; Aditives
Lubricant film heigth
Rheology
Boundary layers from EP aditives
Compatibility after processing
THE FOUR MAIN COMPONENTS AND SOME INTERACTIONS AFFECTING THE TRIBOLOGICAL BEHAVIOUR DURING PRESS FORMING OF METAL SHEETS Fig. 1. Factors and interactions controlling friction during sheet metal forming.
Vegetable oils can offer a valuable alternative to mineral or synthetic oils as a source for manufacturing prelube oils for stamping. They are biodegradable and have better intrinsic BL lubricant properties, due to the presence of long chain fatty acids in their composition. The scope of this work was thus to evaluate the performance of some of these vegetable oils under the typical BL conditions that are found during stamping of car body parts.
2. Experimental details 2.1. Oils tested Five vegetable oils (pure olive oil from press extraction processes, olive oil from solvent extraction processes, sunflower, corn, and soybean oil) were tested to evaluate their lubricant properties for stamping operations. Soybean, sunflower and corn oils are being used as biodegradable lubricant sources for other purposes in Europe. We included also in this study two types of olive oils. Press oil is the typical oil used for alimentary purposes in Mediterranean countries. Until recently, also solvent-obtained olive oils were marketed for human consumption, but a recent prohibition for these purposes, due to the presence of some minor carcinogenic compounds in their composition, has obliged to close many factories that produced this type of oil as a secondary process of the olive oil industry. These solvent obtained oils are, like conventional olive oils, very rich in monounsaturated oleic acid, giving to these oils a good balance between low temperature flow properties
and oxidation resistance at high temperatures, and are cheaper than conventional olive oils. Usually, the lubricant performance of vegetable oils can be enhanced by compounding them with additives or by further chemical process modification, including changes in triglyceride structures or production of fatty acids and sterification. In this study we used the oils as available for alimentary purposes, with no special additives or treatments. For comparative and reference purposes, the same tests were carried out using a widely used prelube oil in the steel sheet metal industry, Ferrocote 6130 de Quaker Chemical B.V. It is a mineral-based oil with aliphatic esters and organometalic soaps as boundary lubrication additives. This oil has been approved as prelube oil by most of the car manufacturers in Europe: Volvo, Ford, VW, Audi, Renault, etc. The Table 1 shows the viscosity values measured in the oils tested at 50 C according to ASTM D 445. 2.2. Steel sheets Bare steel sheet, zinc electrocoated sheet and Zn–Fe alloy coated sheet (galvannealed) have been used in this work, in order to analyse the tribological performance of different coating-oil combinations. All the samples were obtained from EDDQ steels, with thickness between 0.76 and 0.78 mm. Samples 250 · 50 mm are carefully degreased and then both sides oiled prior to testing by a roller system that allows to obtain a uniform oil coating between 1.5 and 2 g/m2. The surface texture of the sheets was evaluated by 2D topography measurement by a profiling skidless instru-
A.C. Carcel et al. / Materials and Design 26 (2005) 587–593
589
Table 1 Oils viscosity and density Oil
Olive
Solvent olive
Sunflower
Corn
Soybean
Reference
E q (g/cm3) Viscosity (cSt)
2.615 0.906 19.61
2.744 0.909 20.57
2.333 0.909 17.50
2.590 0.925 19.42
2.256 0.921 16.92
2.231 0.905 16.73
ment Mhar Perthometer M2, according to ASME B 45.1-1995 and ISO 3274. 2.3. Friction test and evaluation criteria One important aspect in the evaluation of the tribological behaviour under BL conditions is the appropriated selection of the test conditions. Many types of friction tests have been proposed in the literature. These tests can be roughly classified in two groups: pure friction tests and stamping simulative tests, like Erichsen, Fukui, cup drawing, etc. The formers have the advantage that the results are almost independent on the mechanical properties of the material. Some of the most widely accepted pure friction tests are the Inland test [1], the Renault multifriction test [2] or the Draw bead simulator test [3,4]. All these are quite simple tests: the sheet previously oiled is forced to pass between two dies clamped with a normal force N. The pulling force T is continuously measured until a fixed travel length is reached. The coefficient of friction is usually defined as COF = T/2N, being the factor 2 included in order to account for the two sides of the sheet being in contact. The friction test should reproduce the real BL conditions found in real stamping operations. It is also important to have into account that the real tribological conditions, for a given point on the sheet surface, change continuously during the stamping process. When a point of the sheet surface reaches the die entry, for instance, its surface topography has been already flattened by the previous pass under the blankholder. A simple way to evaluate in a friction test the influence of the distance under pressure is to perform multiple runs on the same sheet, as in the Renault multifriction test [2] used in this study. After the first run, dies are opened, the pulling system returns to its initial position, dies are clamped again and a second pass is performed. This procedure can be repeated until a specified number of runs are completed. Samples are not reoiled between the runs. In the test, the dies reproduce a ‘‘plane line contact’’ by the use of a cylindrical tool with a radius of 10 mm and a plane tool, which are clamped with a normal force Fn = 10 dN/mm of width, controlled by and hydraulic unit. The pulling force Ft necessary to make the sheet move between the tools at a velocity of 1 mm/s is continuously measured by a testing machine Instron 4204.
Dies are obtained from tool steel Q + T at 60 HRC and finished with 600-paper grit transverse to the movement direction, giving a final surface finish with Ra = 0.15 lm. The width of the apparent contact zone was experimentally measured and has a value around 0.7 mm, higher than predicted by Hertz theory due to plastic deformation. Mean pressure on the contact zone was calculated as 140 MPa. Under these conditions, the Emmens factor [5] for the process has a value around 10 4, which is a clear indication of the BL conditions during the test. The results from this type of test give an excellent correlation with real stamping behaviour under BL conditions of car body parts [6,7]. It was found that the most relevant factor to allow a correct lubrication during stamping depends on the ability of the system lubricant + surface to give a stable friction behaviour. In the case of the test used in this work, the stability implies that the values of friction should remain stable when the number of successive runs are increased. In these previous studies, performed on zinc coated sheets, the maximum values of the dynamic friction coefficient grew up to 50% for coils that failed in real stamping when the travel length went from 50 to 100 mm (differences between the first and the second 50 mm runs). However, for the group of coils that allowed correct stamping, the maximum values of friction coefficient remained almost constant or even were slightly reduced. Based on this previous work, the COF values stability or uniformity and the absolute values obtained for each vegetable oil, as compared with the results obtained with the reference mineral based oil have been used as evaluation criteria. The relevance of friction stability has been also recognised by others [8–10]. 2.4. Corrosion protection The primary property required for any ‘‘antirust lubricant oil’’ is corrosion prevention. Oils should protect sheet metal components during long term storage and transport. The antirust performance can be evaluated by several tests: outdoor weathering test; indoor exposure; accelerated SST exposure DIN 50021 or ASTM B 117; accelerated 100%humidity exposure DIN 50014.
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In this work we have used the indoor exposure test, with an extended duration of 16 weeks, for sheets of bare steel coated with 1–2 g/m2 of oil applied on the surface. Temperatures during the test period varied between 17 and 22 C and relative humidity between 45% and 65%. Other important properties for the qualification of prelube oils have not been evaluated in this work. It is worthy to note out that these aspects should be also tested in order to have a complete assessment of the ability of vegetable oils as prelube oils. Most of these additional requirements are related to aspects that assure the compatibility of the oil with the habitual manufacturing processes: Compatibility with adhesives and joining processes (epoxi, mastics, sealants, etc. now widely used in the car body assembly process). Easy elimination in alkaline bath degreasing systems. Compatibility with cataphoretic paint baths (some oils cause pinholes in the paint coating).
3. Results and discussion 3.1. Friction behaviour The typical evolution of the friction forces in the test can be observed in Fig. 2, which depicts the results obtained with olive oil and the reference prelube oil when applied on steel sheet and zinc coated sheets. The friction force measured at 50 mm length travel in each run (series in the pictures) was taken to calculate the dynamic value of COF. As depicted in Fig. 2, the values of this coefficient of friction are lower and quasi stable for steel sheets, thus indicating a favourable frictional behaviour for stamping. The opposite situation occurs with zinc electrocoated sheets, which often give problems of galling and surface damage during stamping of difficult car body parts. The results from the tests reveal that, from the first run, the friction values are unstable and have a clear tendency to grow when the sheets are subjected to further runs without re-oiling. This behaviour is attributed either to the
2
2
1.5
1.5 Friction force (KN)
Friction force (KN)
Reference Prelube Oil- Steel Sheet
1 Serie1 Serie2 Serie3 Serie4 Serie5
0.5
1
Ref. prelube oil - Zn electrocoated sheet Serie1 Serie2 Serie3 Serie4 Serie5
0.5
0
0 0
20
40
0
60
20
40
60
Length (mm)
Length (mm) 2
2 Press Olive Oil- Zn electrocoated sheet
Press Olive Oil- Steel Friction force (KN)
Friction force (KN)
1.5
1.5
1 Serie1 Serie2 Serie3 Serie4 Serie5
0.5
1
Serie1 Serie2 Serie3 Serie4 Serie5
0.5
0
0 0
20
40
Length (mm)
60
0
20
40
60
Length (mm)
Fig. 2. Evolution of the friction forces in the friction tests under BL conditions for the five successive runs (series) without re-oiling.
A.C. Carcel et al. / Materials and Design 26 (2005) 587–593 0.25
0.25
Steel sheet
Galvannealed sheet- Test I 0.2
Dynamic COF
Dynamic COF
0.2
0.15
0.1
Press Olive Sunflower Corn Solvent olive Soybean Ref. prelube
0.05
0
50
100
150
200
250
0.15
0.1 Press Olive Sunflower Corn Solvent Olive Soybean Ref. prelube
0.05
0
0
300
0
Travel length in BL friction test
50
100
150
200
250
300
Travel length in BL friction test 0.25
0.25
Zn electrocoated sheet
0.15
0.1
Press Olive Sunflower Corn Solvent olive Soybean Ref. prelube
0.2
Dynamic COF
0.2
Dynamic COF
591
Press Olive Sunflower
0.15
0.1
Corn
0.05
Solvent Olive
Galvannalealed sheet Test II
0.05
Soybean Ref. prelube
0 0
50
100
150
200
250
300
Travel length in BL friction test
0 0
50
100
150
200
250
300
Travel length in BL friction test (mm) Fig. 3. Evolution of the coefficient of friction COF in the friction tests with steel and zinc electrocoated sheets.
absence of effective boundary layers or with the onset of lubricant breakdown, thus allowing metal to metal contact and the welding of surface asperities from the zinc coating to the steel tools. The results of dynamic COF for the other oils and sheets can be observed in the Figs. 3 and 4. Comparing the results obtained with olive oil and those obtained with the reference oil, it can be observed that all the vegetable oils allow to obtain lower COF values than the compounded mineral oil when applied on bare steel sheet. Differences between the vegetable oils are not relevant, although the best results are obtained with olive oils. In the case of zinc electrocoated sheets it should be noted that all the oils, including the reference mineral oil, showed clear limitations to guarantee a constant lubrication behaviour. Some vegetable oils (olive, corn and sunflower) give lower COF values than the reference oil in the two first runs, but their COF values grow respect to the values obtained in the first run, thus suggesting a progressive lack of effectiveness of the boundary layers formed. Globally, however, and having into ac-
Fig. 4. Evolution of the coefficient of friction COF in the friction tests with galvannealed (Zn–Fe) coated sheets.
count that the vegetable oils are not compounded, their behaviour can be considered acceptable and comparable to the results expected from the mineral based oils. 3.2. Surface topography effects The first batch of tests carried out with Zn–Fe coated steel sheets (Galvannealed sheets-Test I) were performed on samples obtained from two different sheet coils. No attention was paid at this stage of the work on the surface topography, which was supposed to be similar for any of the metal sheets. After performing this first batch of friction tests, it was found that the frictional behaviour for Zn–Fe coated sheet was unexpectedly worst than the frictional behaviour found with pure zinc coating, as shown in Fig. 4. A second batch of tests was then performed, but using all samples from the same XX009 coil, whose surface topography had been previously measured and controlled. Fig. 5 gives information about the differences in the surface topography of the sheets. These differences in the surface texture can be also detected by other
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Material ratio %
100 80 60
Coil XX237 ZnFe test I
40
Coil XX181ZnFe test I-Olive S
20
Coil XX009 ZnFe test II
0 0
-1
-2
-3
-4
-5
-6
-7
-8
-9 -10
Distance from surface (microns) Fig. 5. Bearing material ratio curve (Abbot–Firestone Curve) for three different Galvannealed coils used in the friction tests. Cut-off 2.5 mm.
Table 2 Topography parameters of the sheets (cut-off 2.5 mm) Coil coating
Ra (lm)
Rp (lm)
Rpk (lm)
Mr1 %
Steel Zn electrocoated Zn–Fe Tests I Zn–Fe Tests I + Olive S oil Zn–Fe Tests II
1.32 1.50 1.34 1.32
4.09 4.18 3.5 4.04
1.43 1.35 0.98 1.31
9.3 9.1 5.2 10
1.33
4.38
1.55
9.77
parameters, particularly from those related to de size of the surface peaks, like Rp, Rpk or Mr1, defined in ISO 13565-2 and ISO 4287, as shown in Table 2. The differences in the COF values observed in the first test batch between the oil Olive S (Olive obtained by solvent extraction) and the other oils, included the reference oil, are indeed caused by a different surface topography. When the tests were performed on galvannealed samples with an appropriated surface texture, the results changed as shown in the bottom chart depicted in Fig. 4. Again, like in the case of the bare steel sheets, the vegetable oils give a lubricant performance similar or even better (lower values of COF) than the reference mineral oil. These results confirm the great relevance of the surface topography in the tribological behaviour under BL conditions. Sheets with similar values of the mean roughness Ra can have a very different behaviour, even when using the same lubricant, thus introducing an error source in the results obtained from these studies. The relevance of the surface topography features have been recognised in real stamping operations of car body parts. These features affect the tribological behaviour because under BL conditions, the supply of lubricant to the contact zones relies mainly on the ability of the surface to carry oil in the remaining valleys or pockets of the surface, thus avoiding lubricant breakdown and the onset of galling or adhesion between the tools and the sheets. Also the surface aspect after painting of car body parts is greatly affected by the surface topography or texture.
Several anti-galling profiles have been proposed [11–13] that have proven to be effective in drawing bare steel sheets. The requirements from steel and car manufacturers are usually based on measures of Ra and Pc, but the range of acceptable values can vary from one manufacturer to another. For the american car industry, acceptable texture parameters can be roughly defined by a range of Ra values from 20 to 50 linch (0.5–1.30 lm) and a peak density Pc above 82 peaks per inch (35 peaks/cm), measured with cutoff 0.8 mm and a peak count level of 50 linch (1.27 lm). It is also usual to use the same requirements for different types of coatings: zinc electrocoated, hot dip, galvannealed sheet, etc. The values of several topography parameters of the coils used in this study are presented in the Table 2. The results of this study suggest that the definition of antigalling textures for galvannealed coatings requires the use of additional parameters, apart from the conventional mean roughness Ra and peak count Pc, in order to give an appropriated definition of the surface topography. Some useful parameters could be obtained from from the Abbot–Firestone curve. Control of these parameters can give a better definition of optimum texture for reducing friction and improving the stamping behaviour and should be controlled when performing any study on the lubricant properties of oils under BL conditions. The same effects of the surface topography were indeed observed in a previous work carried out with pure zinc electrocoated sheets [14]. 3.3. Corrosion performance Samples of bare steel 200 · 200 mm were oiled and then exposed at 60 inclination in the laboratory for a period of 16 weeks. Usually, the car manufacturing specifications require only 12-week exposure. After this period, all the samples showed complete absence of any red rust corrosion stain.
4. Conclusions The lubricant performance of all the vegetable oils tested, without additives, is comparable or even better than the lubricant performance afforded by one of the most widely used mineral – synthetic based compounded prelube oil for stamping of car body parts, even when pure zinc or galvannealed coated sheets are tested. The values of the COF remain stable after five successive runs using any of the vegetable oils applied on steel sheets or galvannealed sheets. The typical values of COF are 0.11 and 0.13 for steel sheet and between 0.09 and 0.12 for galvannealed sheets. The values of COF for zinc electrocoated sheet remain more stable also with the vegetable oils than with the mineral based prelube oil. In the first run the COF values for vegetable oils vary
A.C. Carcel et al. / Materials and Design 26 (2005) 587–593
from 0.10 to 0.13 and have a value inferior to the value of COF for the mineral prelube: 0.14. The corrosion protection afforded by any of the vegetable oils tested, without additives, is also correct in long term indoor exposure tests: 16 weeks without red rust when applied onto bare steel sheets. Other factors, like the shape and asperity distribution in the surface topography of the sheets have also a great effect on the tribological behaviour under BL conditions. Sheets with surface textures characterised by high material ratio curves (low values of Rpk and Mr1 according to ISO) cannot be correctly lubricated by any kind of the oils tested. Even the mineral-synthetic prelube oil failed in this case. This suggests that more attention should be paid to these aspects of the problem. Better control of topography would allow the use of less additivated vegetable oils or eliminate the use of special high viscosity press oils. Acknowledgement This work was developed thanks to the financial support of the Spanish National Plan of R + D + I (2000– 2003) to the Research Project TRIBOSOLVE. References [1] Bernick LM, Hilsen RR, Wandrei CL. Development of a quantitative sheet galling test. Wear 1978;48. p. 323, 346.
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[2] Specification Renault ‘‘Test Multifrottement Plan’’ RNUR D-31 1738. [3] Nine HD. Draw bead forces in sheet metal forming. In: Mechanics of sheet metal-forming. New York: Plenum Press; 1978. p. 179–211. [4] Nine HD. The applicability of CoulombÕs friction law to drawbeads in sheet metalforming. J Appl Metal work 1982;2(3): 200–210. [5] Emmens WC. The influence of surface roughness on friction. In: Proceedings of the 15th IDDRG biennial conference, Dearnborn; 1988. p. 63–70. [6] Carcel AC et al.. The role of friction in fractures of coated steel sheets during press forming. In: Advances in mechanical behaviour, plasticity and damage. Amsterdam: Elsevier; 2000. p. 685, 689. [7] Carcel A, Ferrer C, Perez Puig MA. Surface texture evolution of zinc coatings under BL conditions. In: Proceedings of the 22nd biennial congress IDDRG, Nagoya, May; 2002. [8] Ludema KC. Friction, Wear, Lubrication. Boca Raton: CRC Press; 1996. p. 10. [9] Schey J, Watts SW. Transient tribological phenomena in the drawbead simulation SAE paper 9200634; 1992. [10] Dalton G. New friction model for sheet metalforming. SAE paper 2001010081; 2001. [11] Rault D, Etringer M. Sheet metal forming and energy conservation. ASM 1976:97–114. [12] Hilsen RR, Bernick LM, Relationship between surface characteristics and galling index of steel sheet ASTM Special Tech. Publ. No. 647; 1978. [13] ASP Program Technical Report. Uniformity of surface texture of steel sheet for automotive applications. AISI – USA, January; 1993. [14] Carcel A, Ferrer C, Perez Puig MA. Alternative 2D surface topography parameters for optimising the friction behaviour of zinc coated sheets. In: Proceedings of the 22th Biennial Congress IDDRG, Nagoya, Japan, May; 2002.