Wear 244 (2000) 41–51
Effects of lubrication and die radius on the friction
behavior of Pb-coated sheet steels
Zhi Deng, M.R. Lovell∗
Department of Mechanical Engineering, University of Pittsburgh, Pittsburgh, PA 15261, USA
Received 11 January 2000; received in revised form 19 April 2000; accepted 19 May 2000
Abstract
Using a strip tensile friction simulator, the influence of lubricant properties (viscosity and extreme pressure) and pin radius on the
interfacial friction and the surface quality of Pb-coated sheet steels is investigated. Nine distinct testing conditions are examined by varying
the lubricants (three oils and two greases) and pin radii (10 and 20 mm). Friction coefficient curves are generated as a function of sliding
distance for each condition examined and the surface quality of the sheets are evaluated from surface roughness and micrographs taken
before and after testing. From the experimental results, tendencies for the interfacial friction and surface finish of Pb-coated sheet steels are
established with respect to lubricant properties and pin radius. Specifically, the results indicated that the friction coefficient increases with
both the viscosity and extreme pressure (oils only) of the lubricants. In addition, it was determined that the lubricant properties had little
influence on the final surface roughness of the deformed sheet. Considering the influence of pin size, the results showed that the friction
coefficient increased with decreasing pin radius values. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Lubrication; Pb-coated sheet; Friction
1. Introduction
Due to their anti-corrosive properties, Pb-coated sheet
steels have been prevalently used throughout the automotive industry in the manufacturing of oil pans and fuel
tanks. Despite the recent introduction of plastics and other
specialized coatings, Pb-coated sheets are still the primary
material used in anti-corrosion automotive applications,
particularly in eastern Europe and Asia. One common characteristic of the parts produced using Pb-coated sheets is
that their geometries are fairly complex in nature. This
makes the formation of Pb-coated parts difficult, as most
geometries must be produced under lubrication using a
single or multiple stage deep-drawing process. Due to the
large strain deformations that develop during deep-drawing,
failure within the sheet commonly occurs in areas of high
strain. As shown in Fig. 1, the primary failure mechanisms
within the sheet are characterized by (1) wrinkling along
the flange and die lip regions, and (2) wall fracture along
the punch lip region. Both of these failures are directly
dependent on the interfacial friction that develops between
∗ Corresponding author. Present address: Department of Mechanical Engineering, University of Pittsburgh, Pittsburgh, PA 15261, USA. Tel.:
+1-412-624-9601; fax: +1-412-624-4846.
E-mail address: [email protected] (M.R. Lovell).
the forming tools and the sheet, especially in the punch and
die lip regions [1]. In order to reduce friction and minimize
sheet failure, lubricants are typically applied to portions
of the workpiece that undergo severe contact with dies. In
fact, lubrication is still the most economical and effective
method for reducing the harmful effects of large interfacial
friction forces that can develop in stamping operations.
During the stamping of automobile components, the type
of applied lubricant is a critical parameter in determining
the overall quality of the final part. When lubricants, such as
oils and greases are applied to the workpiece, the frictional
resistance of the sheet material decreases and the strain uniformity of the sheet increases. This ultimately improves the
overall formability and surface quality of the workpiece. Due
to differences in material properties and deformation behavior, however, the effect of lubrication distinctly varies with
sheet metal coating material. In particular, Pb-coated sheet
steels are known to deform much differently than Zn-coated
sheet steels under identical stamping conditions. This is particularly important when one considers that almost all of the
published frictional studies of stamping operations have investigated galvanized sheet steels [2–7]. For this reason, it
is essential to characterize the nature of interfacial friction
in stamping processes that utilize Pb-coated sheet steels. By
conducting the frictional investigations for both galvanized
and Pb-coated sheet steels under identical conditions, the
0043-1648/00/$ – see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S 0 0 4 3 - 1 6 4 8 ( 0 0 ) 0 0 4 3 0 - 0
42
Z. Deng, M.R. Lovell / Wear 244 (2000) 41–51
Fig. 1. Failures in the workpiece during deep-drawing process.
difference in lubrication between Zn- and Pb-coated sheet
steels can be ascertained [8]. Since lubricants are currently
being selected using trial and error techniques, developing
correlations between lubricant properties, interfacial friction
and surface finish quality of Pb-coated sheet steels would
greatly benefit designers and automobile manufactures. With
such information, the effects of lubricant on the friction and
formability of terne-coated sheet steels could be more accurately predicted and controlled.
2. Methods of testing
2.1. Tensile strip friction tests
Previous investigations on sheet metals [9,10] have found
that frictional forces in stamping processes are a complicated
function of material properties, process parameters and contacting conditions so that it is difficult to construct a single
experimental test that fully represents the frictional behavior
of a sheet metal forming process. Therefore, in order to assess the interfacial friction properly, it is necessary to carry
out experiments under conditions that closely represent an
actual production process. In forming complex parts, such
as automotive oil pans, many convex–concave shapes (lips)
are subject to a complicated stress state that includes tension,
compression, bending and shear. These shapes have large
strains and often develop faults during the forming process.
As a result, lips typically become an emphasis in the design and optimization of automotive forming processes [1].
For this reason, a strip tensile friction simulator (see Fig. 2)
was utilized in the current investigation. Similar to that developed by Duncan et al. [11], the apparatus used was a
‘plane-strain bend/unbend tension’ device that stretches and
bends sheet metal strip specimens during testing so that the
frictional effects can be simulated near the lip regions of
Fig. 2. Tensile strip friction testing apparatus.
a forming process. By means of the testing apparatus, the
friction coefficient between the die and the workpiece, as
well as the surface roughness of the deformed sheets, could
be measured over a wide range of operating conditions. It
is important to note that such an apparatus does not exactly
capture the conditions of a classical deep drawing process
where high compressive stresses and strains develop in the
lips.
Friction tests were carried out on sheet metal strips using a MTS 318.10-type material tensile testing machine
that was modified to include a specially designed friction
measurement system. Fig. 2 shows a schematic view of the
friction measurement system and its overall dimensions. As
depicted, the system was rigidly attached to the loading and
clamping heads of a tensile testing machine. Prior to each
test, a strip specimen was bent around the two pins to simulate the lip region of the dies in a sheet metal forming process. Both ends of the strip specimen were tightly held by
two catchers. Then, loading of the tensile machine produced
significant plastic deformation in the strip. As the load was
increased, the plastic deformation in the strip continuously
increased until a neck or a crack developed at some point
along the vertical portions of the strip. During the loading
process, two extensometers that are mounted on the vertical
and horizontal portions of the strip continuously measure
extensions E1 and E2 as shown in Fig. 2. The forming load,
2P1 , is simultaneously recorded from the tensile machine so
that load-extension curves similar to that depicted in Fig. 3
can be produced. Since the metal strip consists of a uniform
material, the recorded load versus strain curves are valid for
all portions of the strip. For this reason, the force P2 can
be accurately estimated and the friction coefficient between
the specimen and pins, µ, as well as the sliding distance, S,
Z. Deng, M.R. Lovell / Wear 244 (2000) 41–51
43
Fig. 3. The experimental tensile force-extension curves.
along the pin can be determined for each condition tested.
The procedure for determining µ and S is outlined in the
following section.
The forces acting on the portion of a sheet strip contacting the pin are illustrated in Fig. 4. Assuming that there is
a constant friction coefficient in the contact region, µ, consider a section of the strip along the wrap angle, θ , at some
instant during deformation. From Fig. 4, and according to
the equilibrium of all the forces acting on an elemental cut
of the strip, dθ, it can be shown that:
P + µqwR dθ − (P + dP ) = 0
dθ
=0
qwR dθ − (P + dP + P )sin
2
(1)
(2)
where q is unit normal pressure and w the width of the
strip. We will assume that dθ is very small so that
dθ
dθ
≈
(3)
sin
2
2
Fig. 4. Forces acting on an elemental cut of the strip.
and
dP P
(4)
Combining Eqs. (1)–(4) we find
dP
= µ dθ
P
(5)
Integrating Eq. (5), friction coefficient µ is determined to be
P1
2
(6)
µ = ln
π
P2
The overall sliding distance of the strip over one of the
pins can be calculated by the following equation due to the
symmetry of the apparatus:
S = (a − R)E1
(7)
The extension of the sheet, E1, is an important parameter
because it not only determines the amount of sliding of the
sheet over the pins but also represents the deforming limit of
a specimen. By recording the value of E1 during the test until
the specimen cracks, the formability of the deformed sheets
can be ascertained under specific lubrication conditions.
It is important to note that in the apparatus, only the
sliding bend and unbend effects are captured in the plastic deformation of the strip specimen. It is clear from the
testing procedure that the measured loading force, 2P1, and
tangential force, P2, include the forces that develop as the
sheet bends around the pins. The calculated friction coefficient (Eq. (6)), however, does not explicitly include a bending force term. Similar to the apparatus used by Hao et al.
[12], the actual bending force cannot be uncoupled and
used in the calculation of the friction coefficient. In other
types of testing systems, such as that introduced by Wilson
et al. [13], the bending force can be directly measured by
44
Z. Deng, M.R. Lovell / Wear 244 (2000) 41–51
replacing the pins by rollers. For this case, the bending
force, Pb , has been shown to be:
Table 2
Physical properties of lubricants
Items
σy t 2 w
,
Pb =
2r
(8)
where t is the thickness of the sheet, w the width of the
sheet, and r the radius of the pin. Then, using Eq. (8), expressions for the friction coefficient can be generated that
include the bending force directly.
2.2. Testing conditions
By means of the testing apparatus, the interfacial friction
coefficient between the die and the sheet could be measured
over a wide range of operating conditions by varying lubricant and die profile radius. In the tests conducted, the speed
of the tensile testing machine was fixed at 25.4 mm/min.
The gauge lengths of both extensometers E1 and E2 were
50 mm. The pin radii, R, used in the investigations were 10
and 20 mm, respectively. Both pins consisted of 45# mild
carbon steel and had a surface hardness of HB=240 and
an initial surface roughness, Ra , of 0.15 ␮m. Five different
lubricants, three oils and two greases were applied to the
sheet steel. A total of nine distinct testing conditions were
examined by varying pin radius and lubricant.
2.3. Sheet material composition and lubricant properties
The chemical composition and mechanical properties of
the Pb-coated sheet steel analyzed in the experiments are
listed in Table 1. In Table 1 YS, TS, EL and IE are respectively, the yield stress, tensile strength, total elongation and
Erichsen value of the studied material. For the material examined, specimens were cut from the sheet as parallel-sided
strips that were 500.0 mm long, 20.0 mm wide and 1.0 mm
thick. Both parallel sides of each specimen were carefully
polished and maintained a parallel error of <0.013 mm.
It has been determined that the most important indices
among lubricant properties related to sheet metal forming
are density, viscosity and extreme pressure [8]. The extreme
pressure is defined as the maximum contact pressure the lubricant film can withstand without allowing significant asperity contact to occur between surfaces. In order to examine
the effect of lubricant, three commercial oils and two commercial greases were selected. The density, viscosity and
extreme pressure of these lubricants were respectively, measured by means of an optoelectronic balance, an automatic
kinematic viscometer and a 4-ball EP tester using standard
Oil #1
Oil #2 Oil #3 Grease #1 Grease #2
20◦ C,
Density (at
g/ml)
0.85
0.87
0.93
1.01
1.12
Viscosity (at 40◦ C, Pa s)
0.046 0.277 0.074
0.740
1.330
Extreme pressure (N)
1078.0 441.0 981.0 <98.0
<98.0
Table 3
The measured surface roughness of the studied Pb-coated sheet steel
tested under different conditions
The initial surface
roughness
Lubricant
Ra (␮m) Rm (␮m)
The surface roughness
after deformation
R=20 mm
Ra
(␮m)
0.40
1.03
Dry
0.50
Oil #1
0.67
Oil #2
0.65
Oil #3
0.65
Grease #1 0.67
Grease #2 0.68
R=10 mm
Rm
(␮m)
Ra
(␮m)
Rm
(␮m)
3.32
5.47
7.25
4.70
4.55
5.53
NA
0.40
0.55
NA
NA
0.53
NA
3.58
7.10
NA
NA
3.80
testing methods. The average density, viscosity and extreme
pressure for each lubricant are listed in Table 2. Prior to
testing, the pins and the strip specimen were cleaned with
acetone and uniformly lubricated or kept dry to measure the
friction coefficient. After each test, the pins were polished
with fine emery cloth (600 grit) to remove possible lead
build-up.
2.4. Surface roughness measurement and microscopic
surface observation
The surface roughness values of the original and deformed
Pb-coated sheet specimens were obtained using a VIDEO
T20-type 3-D automatic profilometer. The mean, Ra , and
maximum, Rm , surface roughness, within each examined
segment were measured for three to five times on the surface which made contact with the pins, along the directions,
respectively, parallel and perpendicular to the sliding direction of the specimen in the frictional test. The final measured
surface roughness values are the averages of these measurements and listed in Table 3 for the lubricants and pin radii
tested.
For the purpose of evaluating the influence of lubricant
properties and pin radius on the surface quality of the
Table 1
Chemical composition and mechanical properties of the studied material
Material
Pb-coated sheet steel
Chemical composition (wt.%)
Mechanical properties
Coating thickness (␮m)
C
Si
Mn
P
S
Al
YS (MPa)
TS (MPa)
EL (%)
IE (mm)
0.0076
≤0.05
0.16
0.012
0.004
–
161.5
303.9
46.6
11.4
5.71/4.95
Z. Deng, M.R. Lovell / Wear 244 (2000) 41–51
deformed sheets, the surfaces of the sheets were observed
and photographed before and after testing using an optical
microscope. The samples evaluated were obtained by cutting portions of the deformed sheets that directly contacted
the pins. Prior to observation, the deformed sheet specimens
were straightened and cleaned using acetone. It is important
to note that both the surface roughness measurements and
the microscopic surface observations are imperative for understanding the relationship between the surface quality and
the interfacial friction during the stamping of terne-coated
sheet steels.
45
thickness equations can be used to determine the conditions
required for two surfaces to operate in the EHL regime
[17]. It is important to note, however, that Hamrock and
Dowson’s equations will only provide a relative estimate of
the conditions required for EHL in our apparatus. This is
due to the fact that the central film thickness expressions
were derived for a spherical contact condition, whereas the
contact in our apparatus is more cylindrical in nature.
From Hamrock and Dowson’s work, the following expression can be used to predict the critical velocity at which
two surfaces will be entirely separated by a film of lubricant:
G−0.791 W 0.1
V = 0.228E 0 RH1.493
C
×(1 − 0.61e−0.73 )−1.493 η−1
3. Discussion of results
3.1. Effect of lubricant
Before discussing the results of the tensile frictional tests,
it is important that a brief description is given on the nature
of lubrication in large strain stamping processes. Prior work
[13] has found that the contact pressure between the sheet
and the die in stamping is relative low so that the frictional
interaction between surfaces is carried out through mixed
lubrication asperity contact that is dominated by boundary
effects. Under such conditions, the total applied load is partially carried by the hydrodynamic action of the lubricant
film as well as the predominant asperity contacts. The total friction force is then a combination of viscous friction
and asperity interaction [14]. In stamping, sliding asperity
contact between the die and workpiece surfaces can cause
both abrasion and adhesion. Naturally, viscosity and extreme
pressure of the applied lubricants will play an important role
in defining the nature of the asperity interaction. Specifically, a lubricant with high viscosity can form a thicker film,
enlarging the distance between the interacting asperities and
reducing their abrasion. Furthermore, a lubricant with high
extreme pressure may keep an oil film under higher contact
pressure between two asperities and minimize the possibility
of asperity adhesion [14,15]. Therefore, the tribological interaction between contacting surfaces in sheet metal stamping must be considered a complex function of the properties
of the applied lubricants.
As demonstrated by the fundamental Stribeck curve, the
magnitude of friction between two surfaces dramatically
changes with lubrication regime. In boundary lubrication
(BL), for example, the friction coefficient is relatively high
because there is considerable asperity interaction between
the contacting surfaces. In elastohydrodynamic lubrication
(EHL), on the other hand, the friction coefficient is significantly lower than BL because a film of lubricant entirely
separates the asperities of the surfaces in contact. To gain
insight into the physical nature of friction between the pin
and sheets, it is important to establish the lubrication regime
for which our experiments were conducted. This can be
accomplished utilizing the central film thickness equations
developed by Hamrock and Dowson [16]. The central film
where
"
(1 − ν12 ) (1 − ν22 )
+
E0 = 2
E1
E2
(9)
#−1
(10)
H0
R
(11)
G=
E0
piv,as
(12)
W =
F
E0R
(13)
HC =
In the preceding equations, E0 is the effective elastic modulus, E1 , E2 , ν 1 and ν 2 are the respective elastic moduli and
Poisson’s ratios of the sheet and pins, HC the minimum film
thickness parameter, H0 the film thickness of the lubricant,
R the pin radius, G the dimensionless material parameter,
F the total force acting on the sheet from each pin, piv,as
the asymptotic isoviscous pressure, and W the dimensionless
load parameter. Table 4 lists several of the material parameters in our experimental system. Using the values in Table 4,
Eq. (9) can be used to predict the critical sliding speed for
which EHL develops in our apparatus once F and H0 are
determined at a given operating condition.
To determine the critical velocity, the force F can be established using the work of Hao et al. [12]. In Hao’s work,
detailed discussion was given for the pin/strip contact conditions found in the current work. For the geometry shown in
Fig. 5, Hao introduced the following governing (Reynolds’)
equation for the pressure distribution:
Pi = Pi−1 +
x 2 (xi − xi−1 )
3ηV
× i−1 2
R
(H0 + (xi−1 /2R))
(14)
Table 4
The related material parameters for determining the critical speed
Sheet elastic modulus (E1 )
Pin elastic modulus (E2 )
Sheet Poisson’s ratio (ν 1 )
Pin Poisson’s ratio (ν 2 )
Asymptotic isoviscous pressure (piv,as )
81.0 GPa
207.0 GPa
0.28
0.30
4.795×107 Pa
46
Z. Deng, M.R. Lovell / Wear 244 (2000) 41–51
Fig. 5. Geometry of the interface between the sheet and pin.
where P is the pressure of the fluid, H0 the film thickness
at x=0, V the sliding speed of the strip, and η the lubricant
viscosity. Assuming that the film thickness of the lubricant (H0 ) is equivalent to the combined effective surface
roughness of the Pb-coated sheets and pins (0.427 ␮m), the
pressure distributions can be determined for different lubricants and pin radii using the parameters given in Tables 2
and 3. Once the pressure distribution is known, the force F
can be determined by multiplying the average contact pressure by the known contact area between the pin and sheet.
As an example, consider the case of oil #2 at a pin radius
of 20 mm. For this case, the maximum pressure produced
between the pin and sheet will be 0.1407 Pa and the critical speed for EHL to develop is 44.39 mm/s. Comparing
this velocity to the velocity used during our experiments
(0.423 mm/s), it is very likely that our experiments were
dominated by boundary lubrication effects. It is important
to note that the determination of the critical velocity in this
section was an idealized solution to a complicated phenomenon, and was merely intended to provide a relative
comparison of lubrication in the current investigation.
3.1.1. Grease lubricants
Examining Table 2 we find that the grease #1 and #2 have
distinctly different viscosities, while their density and extreme pressures are nearly identical. Hence, by plotting the
experimentally measured friction coefficient as a function of
the sliding distance for both greases (as shown in Fig. 6),
several trends for the effect of grease lubricant viscosity on
the friction characteristics of the Pb-coated sheet steels can
be ascertained. The first trend shows that when compared to
the dry condition, both greases reduce the interfacial friction
coefficient by more than a factor of two. Such a tendency is
in good agreement with boundary lubrication theory where
the addition of a lubricant will decrease abrasive asperity
interaction between the sheet and the dies as the lubricant
‘fills’ the valleys of the contacting surfaces. A second notable trend in Fig. 6 is that the friction coefficient for grease
#2 is greater than that of grease #1 over the entire range
of sliding distance. This trend can be explained by the fact
Fig. 6. Comparison of lubricating effect of both grease lubricants for a
pin radius of 20 mm (grease #1 and #2).
that the grease #2 has a significantly higher viscosity than
the grease #1. Since the contact between the sheet and die
is dominated by boundary effects, the higher viscosity lubricant will have a larger internal shear resistance as the die
and sheet surfaces slide relative to one another and ‘push’
the lubricant through the pin-sheet interface. A final tendency found in Fig. 6 is that the friction coefficient of both
greases remains essentially constant with respect to sliding
distance. This is to be expected because at the speeds tested,
the boundary lubrication effects of the grease will remain
uniform when sufficient amounts are applied to the contacting surfaces.
3.1.2. Oil lubricants
Unlike the grease lubricants that only vary in viscosity,
Table 2 shows that the three oils evaluated distinctly vary
in both viscosity and extreme pressure. Then, by plotting
the measured friction coefficient versus the sliding distance
for the three oils, we can determine the combined influence
of lubricant viscosity and extreme pressure on the friction
characteristics of the Pb-coated sheet steels. Fig. 7 depicts
the measured friction coefficient for the three oils evaluated at a pin radius of 20 mm. Comparing the curves for
Fig. 7. Friction coefficient curves of the Pb-coated sheet steel for three
oils and a pin radius of 20 mm.
Z. Deng, M.R. Lovell / Wear 244 (2000) 41–51
all three oils in the figure, we observe that oil #2 has the
lowest friction coefficient while oil #1 has the highest one.
Considering the properties of each lubricant in Table 2, we
find that the friction coefficient increases with lower oil viscosity and with higher oil extreme pressure. Combined with
the relative variation of friction coefficient with grease viscosity and extreme pressure, it is deduced that the extreme
pressure plays a more significant role than viscosity in determining the overall friction coefficient of Pb-coated sheet
steels. As discussed above, when lubricant extreme pressure
is lower as in the grease, the interaction between the sheet
and die is characterized by a boundary lubrication regime
where the interfacial friction coefficient increases with increasing lubricant viscosity. When lubricant extreme pressure is high enough such that the oil film on the interface is
maintained during the entire contact process, the interaction
between the sheet and die will be dominated by a mixed
lubrication regime. Sometimes referred to as partial lubrication, a mixed lubrication regime exhibits both boundary and
fluid-film effects. In this regime, more viscosity oils will fill
and be trapped in the cavities between the asperities to a
great extent in comparison to less viscosity oils. This ‘filling’
increases the fluid pressure generated within the cavities,
causing a more substantial ‘separation’. As a result, the interfacial friction coefficient becomes lower. Therefore, the
47
influence of lubricant viscosity on the interfacial friction behavior in a Pb-coated sheet steel stamping operation should
be evaluated together with lubricant extreme pressure.
3.1.3. Surface roughness and surface quality
Table 3 lists the mean, Ra , and maximum, Rm , surface
roughness values of the Pb-coated sheet steels before and
after the friction tests were performed. As shown in the
table, the mean surface roughness of the Pb-coated sheets
increased by more than 60% after testing for each of the
lubricants examined. This increase of surface roughness
occurs from the abrasive interfacial asperity interaction that
occurs when the coated sheets slide and plastically deform
over the pin. Comparing the mean surface roughness, Ra ,
of different lubricants after testing, one finds that lubricant viscosity and extreme pressure appear to have little
effect on the average surface roughness. In fact, for each
of the five lubricants tested at R=20 mm, the mean surface
roughness variation was between 0.65 and 0.68 mm. Such
a small difference in roughness is insignificant compared
to the variation in lubricant viscosity and extreme pressure which show larger maximum differences in extreme
pressure (98.0–1078.0 N) and viscosity (0.046–1.330 Pa s).
In order to further evaluate the effect of lubricant properties on the surface quality of the deformed sheets, Figs. 8
Fig. 8. The surface micrographs of the deformed Pb-coated sheet steel for both greases and a pin radius of 20 mm.
48
Z. Deng, M.R. Lovell / Wear 244 (2000) 41–51
Fig. 9. The surface micrographs of the deformed Pb-coated sheet steel for three oils and a pin radius of 20 mm.
and 9 were created. Fig. 8 depicts micrographs of the undeformed and deformed sheet surfaces for the two greases analyzed, and Fig. 9 consists of micrographs of the deformed
sheets for the three oils examined. From Figs. 8 and 9 several
trends can be ascertained for the role of lubricant properties
on the overall surface quality of Pb-coated sheet steels. The
first notable trend is that all of the deformed sheet surfaces
in Figs. 8 and 9 have significantly different surface finishes
from the initial coated sheet surface depicted in Fig. 8(a). In
Fig. 8(a), it is found that the undeformed coated sheet has
a relatively smooth surface finish with a minor peaks and
valleys and no distinguishable markings. In each of the deformed specimens of Figs. 8 and 9, however, one finds that
the distinctive tracks or craters developed on the Pb-coated
sheet during the friction tests.
Another notable trend in Fig. 8 is that the viscosity of
the grease lubricants has a significant impact on the overall surface finish of the deformed sheets. In the micrograph
for the lower viscosity grease #1 of Fig. 8(b), there is more
severe roughening of the coating surface than in the micrograph for the higher viscosity grease #2 of Fig. 8(c). In fact,
the overall surface wear patterns are distinctly different for
the two greases analyzed. For the low viscosity grease, there
are pronounced uniform sliding tracks along the Pb-coated
sheet surface. These sliding tracks indicate that significant
asperity abrasion occurred along the entire contact region
between the pin and sheet during sliding. For the high viscosity grease, however, there are no obvious sliding tracks
but a series of micro-craters on the deformed sheet surface.
Such a wear pattern would indicate that only localized asperity abrasion took place between the pin and Pb-coated sheet.
As discussed previously, the higher viscosity lubricants are
better for reducing the abrasion along the contact interface.
Despite these benefits, the highest viscosity lubricants may
not be able to eliminate localized abrasion at between the
highest asperity peaks. The formation of the micro-craters
on the sheet surfaces would indicate that grease #2 did not
fully separate the pin and sheet surfaces during the sliding
tests.
A final tendency of the dependence of surface finish on
lubricant properties can be deduced by examining the oil
lubricant micrographs of Fig. 9. In the micrographs for the
three oils tested, the lower friction coefficient lubricants,
oil #2 and oil #3, show more pronounced wear tracks than
the oil #1. This fact occurred although the final surface
roughness values of the steels tested with the three lubricants are virtually the same. This can be explained by the
fact that a higher extreme pressure lubricant will minimize
asperity interaction by carrying a larger amount of the
load during mixed lubrication. Then, as the coated sheet
Z. Deng, M.R. Lovell / Wear 244 (2000) 41–51
steel slides relative to the pin, less abrasion will occur between the surfaces, thereby reducing the depth of the wear
tracks that develop. Because Pb-coated sheet steels are not
typically utilized in surface critical applications, however,
the frictional properties of the applied lubricant should be
considered the most important parameter when selecting a
lubricant in Pb-coated sheet steel applications.
3.2. Effect of pin radius
3.2.1. Friction coefficient
Along with determining the influence of lubricant properties, a second variable investigated during the friction
tests was the pin radius. In particular, tensile strip tests were
performed on the Pb-coated sheet steels at pin radii of 10
and 20 mm. Since the strip testing system was designed to
highlight the strain characteristics of the die lip region in
a stamping process, the smaller radius pins (10 mm) represent a stamping operation with a sharper lip. It is typically
difficult to design forming dies when stamping sharp lips
because failure often occurs (see Fig. 1) as the coated sheet
undergoes large strains. In order to establish the character
of the friction while varying the pin radius, Fig. 10 was
generated. Fig. 10 plots the measured friction coefficient
as a function of the sliding distance for oil #1, oil #2, and
grease #2 at both pin radii. As shown in the figure, we find
that for all three lubricants, the friction coefficient increases
at the smaller pin radius. This is to be expected because
the plastic bending effect in the material region contacting
the pins increases for smaller pin radius. The increased
bending force causes the inside material to have stronger
compressive stress, which further increase the interaction
between the contacting asperities. Such an increased asperity interaction will then increase the overall friction
coefficient between the sheet and the pins, as demonstrated
at the smaller pin radius (10 mm) in Fig. 10, because more
energy is dissipated for this additional component. Hence,
it can be deduced that the friction coefficient between the
Fig. 10. Influence of pin radius on the friction behavior of the Pb-coated
sheet steel for different lubrications (oil #1 and #2, grease #2).
49
forming dies and Pb-coated steel sheets will increase as
sharper lips are formed in a stamping operation.
Although there was minimal build-up of the lead coating
on the pins during most of the friction experiments, it is
important to note that there were two specific operating
conditions where substantial lead residue was found. The
first condition that produced build-up was for oil #1 at
R=10 mm. The lead residue in this case can be attributed
to the combined effects of the low viscosity of oil #1 and
the small contact area of the 10 mm pin. Under these conditions, the interfacial abrasion between the sheet and pins
was significantly more severe than the other lubricated
cases, which caused more of the softer lead material to be
deposited onto asperities of the pins. In fact, as indicated in
Fig. 10, the lead build-up at this condition caused unique
behavior in the friction coefficient curve. In the figure, the
friction coefficient initially increased during sliding before
a maximum value was attained. After reaching this maximum value, µ decreased in a manner identical to the other
lubricants in Fig. 10. One explanation for this phenomenon
is that as the sheet progressed along the pin, the amount of
coating material on the pins gradually accumulated until a
critical level was reached where most of the pin/sheet asperities were separated. Once attaining this critical coating
level, the abrasion between the sheet and pin became constant or decreased, as indicated by the lower friction coefficient values at greater sliding distances. A similar tendency
was also observed for the dry condition, where the high
level of abrasion also resulted in a significant Pb residue on
the pins. As shown in Fig. 6, however, the build-up does
not appear to attain a critical level as with oil #1. For the
dry case, the friction coefficient increased throughout the
testing after an initial ‘sticking period’ is overcome. This
sticking period occurs because there is initially significant
adhesion in the absence of lubricant and the sheet must
actually ‘break away’ from the pins at the onset of motion.
The later increase in friction coefficient would indicate that
significant separation between the pin/sheet asperities does
not occur in the absence of a lubricant and that the lead
continued to build-up on the pins over the entire sliding distance. Using the two specific conditions as a basis, it would
appear that higher viscosity lubricants would be beneficial
for eliminating build-up when stamping Pb-coated sheet
steels, especially when a die with a sharp lip is utilized.
3.2.2. Surface roughness and surface quality
In addition to quantifying the friction coefficient, determining the influence of pin radius on the overall surface
quality of the deformed sheet is also extremely important in
stamping processes. For this purpose, Fig. 11 was created
to show the final surface finish at the two pin radii tested.
Initially examining Table 3, we find that the average surface roughness of the deformed sheets is lower for a pin
radius of 10 mm than for a pin radius of 20 mm. In addition
to a smaller surface roughness, Fig. 11 also demonstrates
that the smaller radius pin produced a smoother surface than
50
Z. Deng, M.R. Lovell / Wear 244 (2000) 41–51
Fig. 11. Comparison between the surface micrographs of the deformed Pb-coated sheet steel for different pin radii and lubricants.
the larger radius pin for oil #1 and oil #2. In particular, the
larger radius deformed sheets have more pronounced wear
tracks than the smaller radius sheets. It is important to note
that an identical result was found in Figs. 8 and 9 where
the smoothest surface finish was obtained under the conditions that yielded the highest friction coefficient. This phenomenon can best be explained by considering the physical
interaction between the sheet and the pins at a higher surface
pressure. At a smaller pin radius, higher contact pressures
develop, which leads to more substantial plastic flow of both
the Pb-coated sheet and pin asperities acting along contact
interface. This increase in plastic flow along the interface
is marked by an increase in adhesion, rather than abrasion,
as shown in the micrographs of Fig. 11. The adhesive interaction between asperities causes a smoother surface finish
with micro-craters while abrasion is characterized by sliding wear tracks. Hence, despite having an increased friction coefficient, a sharp lip in a stamping process may yield
smoother surface finishes than a rounded lip.
4. Summary and conclusions
In this work the frictional characteristics of Pb-coated
sheet steels have been investigated using a tensile strip test-
ing system. From the experimental results, the following
conclusions were obtained:
1. For the grease lubricants analyzed, the friction coefficient
between the sheet and dies was found to increase with
increasing lubricant viscosity.
2. For the oil lubricants analyzed, the friction coefficient
increased with increasing lubricant extreme pressure
and with decreasing lubricant viscosity. In addition, the
extreme pressure of the oil was found to play a more
substantial role than the oil viscosity in determining the
overall friction coefficient.
3. The lubricant properties were found to have little influence on the final surface roughness of the deformed
sheets.
4. When considering surface quality, lubricant properties were found to play a critical role. In particular,
a smoother surface finish was attained for lubricants
with higher viscosity and extreme pressures values that
produced a larger friction coefficient.
5. At a smaller pin radius, which corresponds to the stamping of a sharper lip, the friction coefficient between the
contacting sheet and pin surfaces increased and the final surface roughness of the deformed sheets decreased.
Smaller pin radius was also found to be liable to the
occurrence of lead build-up.
Z. Deng, M.R. Lovell / Wear 244 (2000) 41–51
Since very little work has been performed on determining the tribological behavior of Pb-coated sheet steels, the
above results represent an important step in helping manufacturers effectively to select lubricants for large strain
stamping processes. It is strongly recommended that an
optimum lubricant should be selected by considering several aspects including its ability to decrease the friction
coefficient, increase the sheet formability, and decrease the
possible surface defects.
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