- Email: [email protected]
Characterising frictional behaviour in sheet metal forming
Journal of Materials Processing Technology 80 – 81 (1998) 251 – 256
Characterising frictional behaviour in sheet metal forming J.M. Lanzon a, M.J. Cardew-Hall b, P.D. Hodgson a,* a
School of Engineering and Technology, Deakin Uni6ersity, Waurn Ponds, Geelong, Victoria 3217, Australia b The Australian National Uni6ersity, Canberra, Australia
Abstract A two-level multi-variable design of experiment (DoE) approach was used to investigate the influence and interaction of lubricant type, die surface finish, contact pressure, sheet metal coating and draw speed on friction. For this DoE, the flat face friction test (FFF) and the draw bead simulator (DBS) were used to measure the coefficient of friction. The experiments were run in random order with at least three replicates. The Yates algorithm was employed to determine the significance of the main effects and their interactions. It was found that the Zincanneal coating lowers friction and reduces variation within a specific test condition. The DBS results illustrated an insensitivity of the variables investigated to the friction coefficient. However, the rough DBS die was sensitive to blank coating. The polished surface of the FFF test illustrated a similar insensitivity. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Coated steels; Friction coefficient; Lubrication; Surface roughness
1. Introduction Low friction is beneficial, in most forming operations, as it reduces stress on the tooling and the forming loads. In sheet metal forming, however, friction is vital to control metal flow, influencing product quality (both geometry and surface finish) and tool wear. The importance of friction increases in large, deep drawing operations. In complex parts draw beads and soakers are introduced to restrict metal flow into the die cavity by increasing friction, hence preventing wrinkles [1]. Nevertheless, lubricants are applied to assist metal flow and reduce wear, adding another variable to the already complicated stamping process. Ideally, the lubricant used would be insensitive to process parameters, allowing it to be used in the stamping of any part [2]. A further complication can be the use of coated steels. In the literature to date, coated blanks have presented inconsistent results. Zeng and Overby [3] observed an increase in friction with the use of galvanised sheet steel. On the other hand, Keeler and Dwyer report that the effect of the coated steel depends on the coating process and interactions with other process variables [4]. This highlights the importance of
* Corresponding author. 0924-0136/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0924-0136(98)00110-1
establishing the interaction of such parameters with the lubricant system to help design an insensitive drawing compound.
2. Test procedure In the present work, a two-level multi-variable design of experiments (DoE) approach was used to investigate the influence and interaction of lubricant type, die surface finish, contact pressure, sheet metal coating and draw speed on friction. The experiments were run in a random order with at least three replicates. The Yates algorithm was employed to determine the significance of the main effects and their interactions. Several test procedures have been developed to investigate the friction/lubricant system. Traditional tests can be separated into two categories: those that slide strips; and cup forming tests. For the current DoE, the flat face friction test (FFF) and draw bead simulator (DBS) were used to elucidate friction behaviour.
2.1. Flat face friction test (FFF) The FFF was designed to simulate the conditions existing in the blank holder area during a deep drawing
J.M. Lanzon et al. / Journal of Materials Processing Technology 80–81 (1998) 251–256
252
Table 1 Material properties of the strips Steel type
Ra (mm)
Yield stress (MPa)
UTS (MPa)
Uniform elongation (mm)
r-value
n-value
Zincanneal S.D.
1.35 0.12
146.1 2.5
298.2 1.0
26.34 2.1
1.61 0.04
0.25 0.04
Bare steel S.D.
1.29 0.06
128.5 1.7
290.5 1.6
26.55 0.90
1.41 0.00
0.27 0.00
operation. The test measures the force required to draw a lubricated strip through two flat die blocks of a known hardness and roughness. The ratio of the clamping force to half the dragging force gives the coefficient of friction [5].
2.2. Draw bead simulator (DBS) The DBS tests the lubricant’s ability to lower friction and prevent galling during sliding over the draw beads. The test therefore simulates metal flow over the hold down areas. The test does not simulate the action of stretching over the die shoulder [6].
2.3. Strip material properties After the test piece had been cut to size, it was visually inspected for corrosion, scratches and burrs. The material chosen for this experiment was typical of that used by The Ford Motor Company of Australia and is a non-ageing steel. Table 1 provides the average material properties of the strips used. Five samples were tested for each type of steel. Roughness measurements were taken 0° to the rolling direction over 0.8 mm. The r-value was measured at 15% engineering strain and the work-hardening exponent (n) determined between 5 and 15% engineering strain.
2.4. Die properties The dies for both test procedures were made from through hardened tool steel. Shot blasting was used to obtain a uniform average Ra (over 0.8 mm) of 3.690.5 mm for the FFF dies and 4.69 0.3 mm for the DBS dies. The smooth dies were polished to an average Ra value of 0.7 90.2 mm for the FFF dies and 0.4 9 0.1 mm for the DBS dies. Roughness measurements after the tests showed little variation in surface profiles, a maximum drop in Ra of 0.3 mm for the rough dies and a maximum increase of 0.1 mm for the smooth dies.
2.5. Lubrication Mill oil (PL101) was chosen to represent the unlubricated state as dry conditions introduce too many
variables. In a dry state, results are difficult to control and wear of the dies is common; reproducing exact surface profiles is expensive. In addition, the oxide layer on the die surface will lower friction and, finally, temperature and humidity are not precisely regulated during the experiment. The drawing compound (PS306) used is a water-based Castrol Australia Pty. product with good drawing characteristics.
3. Results and discussion Yates analysis identified lubricant type, contact pressure, die surface roughness and blank coating as having significant influence on friction for the FFF test (see Table 2). However for the DBS, surface finish of the die, lubricant type and the interaction of the die surface with blank coating were only found to be influential (see Table 3). This indicates that the blank coating behaves differently when in contact with different die surfaces. Analysis of the individual die surfaces for the DBS illustrated that with regard to friction, the smooth die was insensitive to this variable and that the blank coating influenced friction only when using a rough die.
3.1. Lubricant type Lubricant type was found to have the most influence on frictional stresses. This is understandable as the poor lubricant (mill oil) was chosen to represent an unlubricated system. The mill oil used has no additives to reduce friction during drawing and therefore it can be assumed that the fluid’s viscosity is the only property that assists in lowering the coefficient of friction. During the experiment, noises at a frequency representative of the stick slip action were heard [2]. Instantaneous plots of friction coefficient versus displacement demonstrated lubricant breakdown. The stick slip action suggests the fluid film strength of the mill oil could not support the contact pressure present during the run. However, with the inclusion of special-purpose additives to improve drawability at extreme pressures, the lubrication system would stabilise, reducing variation, potentially making it an adequate prelude.
+ − + − + − + − + − + − + − + − + − + − + − + − + − + − + − + −
5 21 23 28 6 4 27 26 7 3 22 2 8 1 24 25 14 11 32 20 16 9 17 30 13 12 31 18 15 10 29 19
UTS, ultimate tensile strength.
Lubricant +PS306, −PL101
Run no.
Table 2 Flat face friction (FFF) results
+ + − − + + − − + + − − + + − − + + − − + + − − + + − − + + − −
Surface finish +Polished, −Rough + + + + − − − − + + + + − − − − + + + + − − − − + + + + − − − −
Pressure +5 MPa −1 MPa + + + + + + + + − − − − − − − − + + + + + + + + − − − − − − − −
Draw speed +50 mm s−1 −10 mm s−1 + + + + + + + + + + + + + + + + − − − − − − − − − − − − − − − −
Metal coating +Zincanneal −Bare 0.10 0.17 0.13 0.18 0.12 0.18 0.21 0.24 0.09 0.19 0.14 0.19 0.12 0.23 0.20 0.25 0.14 0.19 0.15 0.20 0.18 0.23 0.24 0.30 0.15 0.20 0.15 0.21 0.17 0.24 0.25 0.29
Friction coefficient 0.01 0.00 0.01 0.00 0.01 0.00 0.01 0.01 0.00 0.01 0.00 0.00 0.01 0.01 0.01 0.01 0.01 0.00 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.01 0.01 0.01 0.00
S.D.
3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3
No. of replicates
J.M. Lanzon et al. / Journal of Materials Processing Technology 80–81 (1998) 251–256 253
Lubricant +PS306 − PL101
+ − + − + − + − + − + − + − + −
Run No.
8 1 13 16 7 2 14 15 5 4 10 12 6 3 9 11
Table 3 Draw bead simulator (DBS) results
+ + − − + + − − + + − − + + − −
Die surface +Polish − Rough + + + + − − − − + + + + − − − −
Draw speed +50 mm s−1 −10 mm s−1 + + + + + + + + − − − − − − − −
Metal coating +Zincanneal − Bare 0.08 0.11 0.16 0.17 0.09 0.12 0.14 0.19 0.09 0.10 0.18 0.19 0.08 0.10 0.19 0.20
0.00 0.01 0.01 0.01 0.01 0.01 0.00 0.00 0.01 0.01 0.01 0.00 0.00 0.01 0.01 0.01
Friction coefficient S.D.
4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4
No. of replicates
254 J.M. Lanzon et al. / Journal of Materials Processing Technology 80–81 (1998) 251–256
J.M. Lanzon et al. / Journal of Materials Processing Technology 80–81 (1998) 251–256
255
Fig. 1. The effect of contact pressure on: (a) bare steel; and (b) coated steel.
3.2. Blank coating As mentioned previously, the effect of blank coating depends on the manufacturing process. In the current DoE, it was found that the Zincanneal coating reduces friction for both test procedures. This was expected, as any powdering of the coating can act as a mechanical extreme pressure agent. In addition, the zinc coating is porous, offering an improved lubricant entrapment surface. This assists with the success of formation both lubricant regimes and load distributions. Finally, since zinc has a high potential to form strong boundary lubricant bonds, it is believed that the boundary lubricant regime formed with the zinc thicker than that formed with bare steel. The reason there is a less noticeable difference when mill oil is used can be explained by the absence of any boundary lubricant regime forming additives.
3.3. Contact pressure A drop in frictional resistance was observed as the contact pressure increased. Surprisingly, the coefficient of friction decreased by similar amounts for both lubricant types (Fig. 1(a) and (b)). This may be due to the decrease in viscosity with increasing pressure. Elevated pressures reduce film strength and therefore lower resistance to sliding. However, when the contact pressure is sufficient, the lubricant film strength breaks down, resulting in a sudden increase in friction. This does not occur with the drawing compound due to the inclusion of extreme pressure additives, both passive and active.
that, by ensuring the surface finish of the draw beads is smooth, the area is desensitised to lubricant type and blank coating. Examination of the results for the polished FFF dies demonstrated a similar insensitivity to variables investigated, and again the average values were lower for the smooth dies than the rough ones. It is believed that the polished dies have a greater ability to distribute load due to the concentration of asperity peaks, and are thereby insensitive to lubricant type and amount. The ability to distribute loads is a desired characteristic as it assists in reducing severity. This agrees with the findings of Sniekers [7].
3.5. Speed The effect of speed was not highlighted in either of the experiments, which agrees with the work of Kotchman et al. [8]. In that case, the author uses the effect of masking or confounding associated with fractionating of experiments to explain the absence of draw speed as an influential main effect. However, the current DoE was not fractionated and it is known that the draw speed influences work zone temperatures, as the same amount of heat is generated with less time for it to diffuse. This increase in temperature affects lubricant performance by activating, or breaking down, lubricant components. Therefore, it may be possible that since the tests were not performed continuously, the time between runs was sufficient to allow adequate heat dissipation such that this did not affect the lubricant properties. In addition, there is little to no forming in the test procedures, which reduces the amount of heat generated.
3.4. Die surface 3.6. Variation in strip surface properties As mentioned previously, investigations into the sensitivity of the smooth DBS dies to process variables illustrated little to no fluctuations in friction coefficient. However, the average values for the rough beads are higher than for the smooth beads. This would suggest
The rolling direction of the blank has little effect on the friction coefficient. Even though it has been established that surface profiles affect friction characteristics, previous test results illustrated that roughness varia-
256
J.M. Lanzon et al. / Journal of Materials Processing Technology 80–81 (1998) 251–256
tions in the blank due to rolling direction play a negligible role in the overall system (J.M. Lanzon, unpublished data). Evaluation of roughness contours of the blank before and after testing illustrated a smoothing of the asperity peaks. Examination of the strips after the tests revealed some scoring and powdering in the coated material, suggesting that the tool was hard enough to plough through the blank asperities without a significant increase in friction.
Acknowledgements The author would like to thank The Ford Motor Company of Australia and BHP Research for their assistance during this research project, and in particular David Haynes (BHP) and Jack Anskaitis (Ford).
References 4. Conclusions It has previously been established that even though there is a large variation in the coefficient of friction results between laboratories, the general ranking of the lubricants does not vary. This would suggest adequate consistency of the experimental procedure to evaluate lubricant performance [9]. Hence, not surprisingly, the results of the current DoE agree with those of others, with lubricant type and pressure being of greatest influence for the FFF test. However, considering the ever increasing number of drawing compounds available, all of which have varied performance under different forming conditions, it is clear that the stamping process needs to be desensitised to lubricant types, or that a standard lubricant that is insensitive to variations in the stamping process needs to be developed. Even though the basic fundamentals of the behaviour of friction during forming can be examined by laboratory experiments, the true meaning of the magnitudes of the values and their relationship to the final stamped product is unknown. As highlighted by Ref. [9], the difficulties encountered with reproducing friction results implies that the true value of the coefficient of friction is not as beneficial as the correlation between these results and a real stamping process.
.
[1] W.R.D. Wilson, Friction models for metal forming in the boundary lubricant regime, J. Eng. Mater. Technol. 113 (1991) 60 – 68. [2] S.P. Keeler, H.D. Nine, J.F. Sikrik, Formability criteria for selecting sheet metal lubricants, SAE Technical Paper Series, No. 880366, 1996. [3] B. Zeng, D. Overby, Strip experimental study on galvanised steel sheet, 15th Biennial Congr. International Deep Drawing Research Group, Dearborn, MI, USA, 16 – 18 May, 1988, pp. 85 – 90. [4] S.P. Keeler, T.E. Dwyer, Frictional characteristics of galvanised steels evaluated with a draw bead simulator, SAE Technical Paper Series, No. 860433, 1986. [5] G. Dalton, N. Oulton, Lubrication evaluation test methods, Ref. No. REP 001, Industrial Research and Development Industry, Midland, Ont., 1997. [6] H.D. Nine, Testing lubricant for sheet metal forming, in: Waggoner (Ed.), Novel Techniques in Metal Deformation Testing, The Metallurgical Society of AIME, Warrendale, PA, 1983, pp. 31 – 46. [7] R.J.J.M. Sniekers, Friction in Deep Drawing, Ph.D. Thesis, Technical University of Eindhoven, The Netherlands, 1996, pp. 1 – 114. [8] D.P. Kotchman, I. Kim, D. Lee et al., Determination of friction behaviour in sheet metals using orthogonal arrays, J. Mater. Eng. Performance 1 (4) (1992) 555-564. [9] J.A Schey, M.K. Smith, Report of NADDRG Friction Committee on Reproducibility of Friction Tests within and between Laboratories, SAE Technical Paper Series, No. 930811, Int. Congr. Expo., Detroit, MI, 1 – 5 March, 1993, reprinted from Sheet Metal and Stamping Symp. SP-944.
.
Characterising frictional behaviour in sheet metal forming J.M. Lanzon a, M.J. Cardew-Hall b, P.D. Hodgson a,* a
School of Engineering and Technology, Deakin Uni6ersity, Waurn Ponds, Geelong, Victoria 3217, Australia b The Australian National Uni6ersity, Canberra, Australia
Abstract A two-level multi-variable design of experiment (DoE) approach was used to investigate the influence and interaction of lubricant type, die surface finish, contact pressure, sheet metal coating and draw speed on friction. For this DoE, the flat face friction test (FFF) and the draw bead simulator (DBS) were used to measure the coefficient of friction. The experiments were run in random order with at least three replicates. The Yates algorithm was employed to determine the significance of the main effects and their interactions. It was found that the Zincanneal coating lowers friction and reduces variation within a specific test condition. The DBS results illustrated an insensitivity of the variables investigated to the friction coefficient. However, the rough DBS die was sensitive to blank coating. The polished surface of the FFF test illustrated a similar insensitivity. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Coated steels; Friction coefficient; Lubrication; Surface roughness
1. Introduction Low friction is beneficial, in most forming operations, as it reduces stress on the tooling and the forming loads. In sheet metal forming, however, friction is vital to control metal flow, influencing product quality (both geometry and surface finish) and tool wear. The importance of friction increases in large, deep drawing operations. In complex parts draw beads and soakers are introduced to restrict metal flow into the die cavity by increasing friction, hence preventing wrinkles [1]. Nevertheless, lubricants are applied to assist metal flow and reduce wear, adding another variable to the already complicated stamping process. Ideally, the lubricant used would be insensitive to process parameters, allowing it to be used in the stamping of any part [2]. A further complication can be the use of coated steels. In the literature to date, coated blanks have presented inconsistent results. Zeng and Overby [3] observed an increase in friction with the use of galvanised sheet steel. On the other hand, Keeler and Dwyer report that the effect of the coated steel depends on the coating process and interactions with other process variables [4]. This highlights the importance of
* Corresponding author. 0924-0136/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0924-0136(98)00110-1
establishing the interaction of such parameters with the lubricant system to help design an insensitive drawing compound.
2. Test procedure In the present work, a two-level multi-variable design of experiments (DoE) approach was used to investigate the influence and interaction of lubricant type, die surface finish, contact pressure, sheet metal coating and draw speed on friction. The experiments were run in a random order with at least three replicates. The Yates algorithm was employed to determine the significance of the main effects and their interactions. Several test procedures have been developed to investigate the friction/lubricant system. Traditional tests can be separated into two categories: those that slide strips; and cup forming tests. For the current DoE, the flat face friction test (FFF) and draw bead simulator (DBS) were used to elucidate friction behaviour.
2.1. Flat face friction test (FFF) The FFF was designed to simulate the conditions existing in the blank holder area during a deep drawing
J.M. Lanzon et al. / Journal of Materials Processing Technology 80–81 (1998) 251–256
252
Table 1 Material properties of the strips Steel type
Ra (mm)
Yield stress (MPa)
UTS (MPa)
Uniform elongation (mm)
r-value
n-value
Zincanneal S.D.
1.35 0.12
146.1 2.5
298.2 1.0
26.34 2.1
1.61 0.04
0.25 0.04
Bare steel S.D.
1.29 0.06
128.5 1.7
290.5 1.6
26.55 0.90
1.41 0.00
0.27 0.00
operation. The test measures the force required to draw a lubricated strip through two flat die blocks of a known hardness and roughness. The ratio of the clamping force to half the dragging force gives the coefficient of friction [5].
2.2. Draw bead simulator (DBS) The DBS tests the lubricant’s ability to lower friction and prevent galling during sliding over the draw beads. The test therefore simulates metal flow over the hold down areas. The test does not simulate the action of stretching over the die shoulder [6].
2.3. Strip material properties After the test piece had been cut to size, it was visually inspected for corrosion, scratches and burrs. The material chosen for this experiment was typical of that used by The Ford Motor Company of Australia and is a non-ageing steel. Table 1 provides the average material properties of the strips used. Five samples were tested for each type of steel. Roughness measurements were taken 0° to the rolling direction over 0.8 mm. The r-value was measured at 15% engineering strain and the work-hardening exponent (n) determined between 5 and 15% engineering strain.
2.4. Die properties The dies for both test procedures were made from through hardened tool steel. Shot blasting was used to obtain a uniform average Ra (over 0.8 mm) of 3.690.5 mm for the FFF dies and 4.69 0.3 mm for the DBS dies. The smooth dies were polished to an average Ra value of 0.7 90.2 mm for the FFF dies and 0.4 9 0.1 mm for the DBS dies. Roughness measurements after the tests showed little variation in surface profiles, a maximum drop in Ra of 0.3 mm for the rough dies and a maximum increase of 0.1 mm for the smooth dies.
2.5. Lubrication Mill oil (PL101) was chosen to represent the unlubricated state as dry conditions introduce too many
variables. In a dry state, results are difficult to control and wear of the dies is common; reproducing exact surface profiles is expensive. In addition, the oxide layer on the die surface will lower friction and, finally, temperature and humidity are not precisely regulated during the experiment. The drawing compound (PS306) used is a water-based Castrol Australia Pty. product with good drawing characteristics.
3. Results and discussion Yates analysis identified lubricant type, contact pressure, die surface roughness and blank coating as having significant influence on friction for the FFF test (see Table 2). However for the DBS, surface finish of the die, lubricant type and the interaction of the die surface with blank coating were only found to be influential (see Table 3). This indicates that the blank coating behaves differently when in contact with different die surfaces. Analysis of the individual die surfaces for the DBS illustrated that with regard to friction, the smooth die was insensitive to this variable and that the blank coating influenced friction only when using a rough die.
3.1. Lubricant type Lubricant type was found to have the most influence on frictional stresses. This is understandable as the poor lubricant (mill oil) was chosen to represent an unlubricated system. The mill oil used has no additives to reduce friction during drawing and therefore it can be assumed that the fluid’s viscosity is the only property that assists in lowering the coefficient of friction. During the experiment, noises at a frequency representative of the stick slip action were heard [2]. Instantaneous plots of friction coefficient versus displacement demonstrated lubricant breakdown. The stick slip action suggests the fluid film strength of the mill oil could not support the contact pressure present during the run. However, with the inclusion of special-purpose additives to improve drawability at extreme pressures, the lubrication system would stabilise, reducing variation, potentially making it an adequate prelude.
+ − + − + − + − + − + − + − + − + − + − + − + − + − + − + − + −
5 21 23 28 6 4 27 26 7 3 22 2 8 1 24 25 14 11 32 20 16 9 17 30 13 12 31 18 15 10 29 19
UTS, ultimate tensile strength.
Lubricant +PS306, −PL101
Run no.
Table 2 Flat face friction (FFF) results
+ + − − + + − − + + − − + + − − + + − − + + − − + + − − + + − −
Surface finish +Polished, −Rough + + + + − − − − + + + + − − − − + + + + − − − − + + + + − − − −
Pressure +5 MPa −1 MPa + + + + + + + + − − − − − − − − + + + + + + + + − − − − − − − −
Draw speed +50 mm s−1 −10 mm s−1 + + + + + + + + + + + + + + + + − − − − − − − − − − − − − − − −
Metal coating +Zincanneal −Bare 0.10 0.17 0.13 0.18 0.12 0.18 0.21 0.24 0.09 0.19 0.14 0.19 0.12 0.23 0.20 0.25 0.14 0.19 0.15 0.20 0.18 0.23 0.24 0.30 0.15 0.20 0.15 0.21 0.17 0.24 0.25 0.29
Friction coefficient 0.01 0.00 0.01 0.00 0.01 0.00 0.01 0.01 0.00 0.01 0.00 0.00 0.01 0.01 0.01 0.01 0.01 0.00 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.01 0.01 0.01 0.00
S.D.
3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3
No. of replicates
J.M. Lanzon et al. / Journal of Materials Processing Technology 80–81 (1998) 251–256 253
Lubricant +PS306 − PL101
+ − + − + − + − + − + − + − + −
Run No.
8 1 13 16 7 2 14 15 5 4 10 12 6 3 9 11
Table 3 Draw bead simulator (DBS) results
+ + − − + + − − + + − − + + − −
Die surface +Polish − Rough + + + + − − − − + + + + − − − −
Draw speed +50 mm s−1 −10 mm s−1 + + + + + + + + − − − − − − − −
Metal coating +Zincanneal − Bare 0.08 0.11 0.16 0.17 0.09 0.12 0.14 0.19 0.09 0.10 0.18 0.19 0.08 0.10 0.19 0.20
0.00 0.01 0.01 0.01 0.01 0.01 0.00 0.00 0.01 0.01 0.01 0.00 0.00 0.01 0.01 0.01
Friction coefficient S.D.
4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4
No. of replicates
254 J.M. Lanzon et al. / Journal of Materials Processing Technology 80–81 (1998) 251–256
J.M. Lanzon et al. / Journal of Materials Processing Technology 80–81 (1998) 251–256
255
Fig. 1. The effect of contact pressure on: (a) bare steel; and (b) coated steel.
3.2. Blank coating As mentioned previously, the effect of blank coating depends on the manufacturing process. In the current DoE, it was found that the Zincanneal coating reduces friction for both test procedures. This was expected, as any powdering of the coating can act as a mechanical extreme pressure agent. In addition, the zinc coating is porous, offering an improved lubricant entrapment surface. This assists with the success of formation both lubricant regimes and load distributions. Finally, since zinc has a high potential to form strong boundary lubricant bonds, it is believed that the boundary lubricant regime formed with the zinc thicker than that formed with bare steel. The reason there is a less noticeable difference when mill oil is used can be explained by the absence of any boundary lubricant regime forming additives.
3.3. Contact pressure A drop in frictional resistance was observed as the contact pressure increased. Surprisingly, the coefficient of friction decreased by similar amounts for both lubricant types (Fig. 1(a) and (b)). This may be due to the decrease in viscosity with increasing pressure. Elevated pressures reduce film strength and therefore lower resistance to sliding. However, when the contact pressure is sufficient, the lubricant film strength breaks down, resulting in a sudden increase in friction. This does not occur with the drawing compound due to the inclusion of extreme pressure additives, both passive and active.
that, by ensuring the surface finish of the draw beads is smooth, the area is desensitised to lubricant type and blank coating. Examination of the results for the polished FFF dies demonstrated a similar insensitivity to variables investigated, and again the average values were lower for the smooth dies than the rough ones. It is believed that the polished dies have a greater ability to distribute load due to the concentration of asperity peaks, and are thereby insensitive to lubricant type and amount. The ability to distribute loads is a desired characteristic as it assists in reducing severity. This agrees with the findings of Sniekers [7].
3.5. Speed The effect of speed was not highlighted in either of the experiments, which agrees with the work of Kotchman et al. [8]. In that case, the author uses the effect of masking or confounding associated with fractionating of experiments to explain the absence of draw speed as an influential main effect. However, the current DoE was not fractionated and it is known that the draw speed influences work zone temperatures, as the same amount of heat is generated with less time for it to diffuse. This increase in temperature affects lubricant performance by activating, or breaking down, lubricant components. Therefore, it may be possible that since the tests were not performed continuously, the time between runs was sufficient to allow adequate heat dissipation such that this did not affect the lubricant properties. In addition, there is little to no forming in the test procedures, which reduces the amount of heat generated.
3.4. Die surface 3.6. Variation in strip surface properties As mentioned previously, investigations into the sensitivity of the smooth DBS dies to process variables illustrated little to no fluctuations in friction coefficient. However, the average values for the rough beads are higher than for the smooth beads. This would suggest
The rolling direction of the blank has little effect on the friction coefficient. Even though it has been established that surface profiles affect friction characteristics, previous test results illustrated that roughness varia-
256
J.M. Lanzon et al. / Journal of Materials Processing Technology 80–81 (1998) 251–256
tions in the blank due to rolling direction play a negligible role in the overall system (J.M. Lanzon, unpublished data). Evaluation of roughness contours of the blank before and after testing illustrated a smoothing of the asperity peaks. Examination of the strips after the tests revealed some scoring and powdering in the coated material, suggesting that the tool was hard enough to plough through the blank asperities without a significant increase in friction.
Acknowledgements The author would like to thank The Ford Motor Company of Australia and BHP Research for their assistance during this research project, and in particular David Haynes (BHP) and Jack Anskaitis (Ford).
References 4. Conclusions It has previously been established that even though there is a large variation in the coefficient of friction results between laboratories, the general ranking of the lubricants does not vary. This would suggest adequate consistency of the experimental procedure to evaluate lubricant performance [9]. Hence, not surprisingly, the results of the current DoE agree with those of others, with lubricant type and pressure being of greatest influence for the FFF test. However, considering the ever increasing number of drawing compounds available, all of which have varied performance under different forming conditions, it is clear that the stamping process needs to be desensitised to lubricant types, or that a standard lubricant that is insensitive to variations in the stamping process needs to be developed. Even though the basic fundamentals of the behaviour of friction during forming can be examined by laboratory experiments, the true meaning of the magnitudes of the values and their relationship to the final stamped product is unknown. As highlighted by Ref. [9], the difficulties encountered with reproducing friction results implies that the true value of the coefficient of friction is not as beneficial as the correlation between these results and a real stamping process.
.
[1] W.R.D. Wilson, Friction models for metal forming in the boundary lubricant regime, J. Eng. Mater. Technol. 113 (1991) 60 – 68. [2] S.P. Keeler, H.D. Nine, J.F. Sikrik, Formability criteria for selecting sheet metal lubricants, SAE Technical Paper Series, No. 880366, 1996. [3] B. Zeng, D. Overby, Strip experimental study on galvanised steel sheet, 15th Biennial Congr. International Deep Drawing Research Group, Dearborn, MI, USA, 16 – 18 May, 1988, pp. 85 – 90. [4] S.P. Keeler, T.E. Dwyer, Frictional characteristics of galvanised steels evaluated with a draw bead simulator, SAE Technical Paper Series, No. 860433, 1986. [5] G. Dalton, N. Oulton, Lubrication evaluation test methods, Ref. No. REP 001, Industrial Research and Development Industry, Midland, Ont., 1997. [6] H.D. Nine, Testing lubricant for sheet metal forming, in: Waggoner (Ed.), Novel Techniques in Metal Deformation Testing, The Metallurgical Society of AIME, Warrendale, PA, 1983, pp. 31 – 46. [7] R.J.J.M. Sniekers, Friction in Deep Drawing, Ph.D. Thesis, Technical University of Eindhoven, The Netherlands, 1996, pp. 1 – 114. [8] D.P. Kotchman, I. Kim, D. Lee et al., Determination of friction behaviour in sheet metals using orthogonal arrays, J. Mater. Eng. Performance 1 (4) (1992) 555-564. [9] J.A Schey, M.K. Smith, Report of NADDRG Friction Committee on Reproducibility of Friction Tests within and between Laboratories, SAE Technical Paper Series, No. 930811, Int. Congr. Expo., Detroit, MI, 1 – 5 March, 1993, reprinted from Sheet Metal and Stamping Symp. SP-944.
.