Long Ju
School of Mechanical Engineering,
University of Science and Technology Beijing,
30 Xueyuan Road,
Haidian, Beijing 100083, China
e-mail: [email protected]
1
Tingting Mao
2
Center for Precision Forming,
The Ohio State University,
339 Baker Systems,
1971 Neil Avenue,
Columbus, OH 43210
e-mail: [email protected]
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Julio Malpica
Honda Engineering of North America, Inc.,
24000 Honda Parkway,
Marysville, OH 43040
e-mail: [email protected]
Taylan Altan1
Center for Precision Forming,
The Ohio State University,
339 Baker Systems,
1971 Neil Avenue,
Columbus, OH 43210
e-mail: [email protected]
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Evaluation of Lubricants
for Stamping of Al 5182-O
Aluminum Sheet Using Cup
Drawing Test
Lubricants are necessary to avoid adhesion, galling, and scratching in aluminum stamping processes. In this study, various lubricants, including dry lubes and wet lubes, were
evaluated using cup drawing test (CDT) for stamping of Al 5182-O aluminum sheets. The
effects of surface texturing, with electro-discharge texturing (EDT) and mill ﬁnish (MF),
on the friction behavior were also investigated. Furthermore, the methodology to evaluate the performance of lubricants was established based on (a) maximum applicable
blank holder force (BHF) and (b) draw-in length in ﬂange or ﬂange perimeter of formed
cups. Finite element (FE) simulations were carried out to determine the coefﬁcient of
friction (CoF) at tool–workpiece interface during deep drawing under different lubrication conditions. Flow stress data of Al 5182-O material were obtained using viscous pressure bulge (VPB) and tensile tests. In this study, it was conﬁrmed that, in forming Al
5182-O, dry ﬁlm lubricants have better lubricity than wet lubricants. A better lubrication
condition was found with EDT surface texture. [DOI: 10.1115/1.4030750]
Keywords: aluminum stamping, lubrication, surface texturing, ﬁnite element analysis
surface ﬁnishes: MF and EDT [3]. By using 2D and 3D measurements, it is reported that EDT can provide a higher closed void
Lightweight materials, such as aluminum alloys, are increasarea ratio in the sheet surface to ensure a better lubricant distribuingly used in sheet metal forming of automotive parts. In forming
tion and transport than MF. As a result, a lower CoF could be
aluminum alloys, adhesive wear and galling, due to insufﬁcient
achieved by using EDT [9,10]. From a microscopic scale point of
lubricant, can greatly affect the quality of stamped parts [1]. The
view, the characterization of the asperities and valleys on the die
right lubricant can reduce or even avoid failures associated with
and workpiece surfaces could provide more reasonable expiations
wrinkling and premature fracture, thus increasing the process winfor many tribological phenomena in sheet metal forming [11]. For
dow [2]. There are mainly two types of lubricants used for alumia given tool, sheet material, and surface ﬁnish, the friction is also
num sheet metal forming: dry-ﬁlm lubricants and mineral oil
affected by rolling direction of the sheet, sliding direction of the
based lubricants. Lubricants used in stamping lines are mainly
contacts [9] and the contact pressure as well as relative velocity
based on mineral oils. They have good deep drawing performance and interface temperature [12]. The well-known Stribeck curve
and can be used in forming complex parts, such as inner door pan- was applied to describe the dependency of friction on contact
els with most metals [3]. In recent studies, dry ﬁlm lubricants
pressure, sliding velocity, and dynamic viscosity of the lubricant
have been used in forming to avoid scratching and galling of the
[13]. From the experimental study on the strip drawing of Al
tools [4,5]. Regarding the evaluation of various lubricants, twist
sheets, adhesive wear was found to occur as a result of local peak
compression test [6], deep drawing test (including CDT [7], and
stress and temperature rise [1].
strip drawing test [8]) are found to be most practical testing methIn the present study, a methodology was established to evaluate
ods. Using inverse analysis and the measured data from CDT
the performance of different lubricants for stamping of Al 5182-O
tests, the CoF can be estimated with FE simulation. At the Center
sheets. Selected lubricants were evaluated by using CDT in a
of Precision Forming (CPF), it was concluded that, compared with
hydraulic press. The effects of two different surface texturing, MF
other lubricant testing method, the CDT can emulate the “nearand EDT, were investigated in detail. The speciﬁc objectives are
production” conditions well [7]. It should be noted that the present
to: (a) select the lubricants that perform best in drawing of alumitesting method applies primarily for deep drawing operations.
num sheet 5182-O, (b) compare the friction behavior between MF
However, in conducting FE simulations of stamping operations,
surface and EDT surface, and (c) determine the CoF at the
for practical reasons, only one average value of CoF is used.
tool–workpiece interface under various lubrication conditions.
Thus, the CoF obtained from the CDT can be used in FE analysis
The obtained CoF for selected lubricants can be used as input into
of stamping.
FE simulations of stamping with complex geometries used in
The surface ﬁnish of the sheet also affects the lubrication durautomotive production, such as hoods, decklids, and doors.
ing forming. Aluminum sheet is commercially available in two
1
Introduction
1
Corresponding author.
Contributed by the Manufacturing Engineering Division of ASME for publication
in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript received
January 14, 2015; ﬁnal manuscript received May 19, 2015; published online xx xx,
xxxx. Assoc. Editor: Blair E. Carlson.
2
Experimental Procedures
2.1 Sheet Materials. In the present study, the sheet materials
were characterized by both standard tensile test and VPB test. The
tensile tests were conducted in rolling, diagonal, and transverse
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Table 1 Mechanical properties of Al 5182-O from tensile tests
(conducted by Honda R&D)
Fig. 1 Flow stress curves obtained from tensile test and VPB
test
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2.2 Lubricant Samples and Surface Textures. In this study,
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14 lubricants were evaluated. All the lubricants are used as
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received, except lubricant I, which had to be diluted with water in
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the ratio of 1:3 (lubricant/water). The coating weight applied on
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the blank was 1 g/m2 for all lubricants. The details of lubricant
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type and application method are summarized in Table 2. The time 81
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period, from the application of a wet lubricant until the test is
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completed, was approximately 5 min.
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Application of lubricants on the blank was done by using a pip85
ette and draw down bar #0. Each lubricant was spread all over the 86
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blank surface on both sides uniformly. Three drops of the lubri88
cant were applied on each side of blank to achieve a coating
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weight (1.0 g/m2 6 0.3 g/m2). Since it is difﬁcult to manually
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apply the dry ﬁlm lubricant in the laboratory scale, some of the
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dry ﬁlm lubricants were required to be precoated by the lubricant
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companies with the same amount as wet lubricants. The coating
weight/amount was veriﬁed by measuring a few samples using the 95
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laboratory balance before and after lube application. Additionally, 97
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the applied lubricant thickness was veriﬁed by using a portable
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tool (Microderm CMS). The coating weight measured by Micro100
derm CMS tool was around 1.24 g/m 2, which was within the range 101
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of the coating requirement, recommended by the lubricant
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suppliers.
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The 5182-O aluminum sheets were available with two different
surface texturing, MF and EDT. Three-dimensional view of EDT
surface texture measurement, conducted by Honda Engineering, is
shown in Fig. 2. In order to have better understanding about the
difference in the surface topography, the roughness parameters,
Ra and Rz, were compared in Table 3. It can be seen that the EDT
texture provide higher deviation values and the pocket structure
on the surface can deﬁnitely effect the lubricated contacts.
directions, respectively. Table 1 lists the material properties
obtained from the tensile tests. VPB test, developed by CPF, could
provide higher strain values under biaxial state of stress [14].
Figure 1 shows the ﬂow stress curves obtained from the two tests.
It can be seen that the ﬂow stress could be obtained at true strain
of about 0.5 with the VPB test.
2.3 Principles of CDT–CDT. The cup drawing process is
extensively used to evaluate the performance of lubricants. The
schematic of CDT is illustrated in Fig. 3. In cup drawing, the most
severe friction takes place at the ﬂange and die corners.
Sample
Ultimate tensile
stress (MPa)
Yield stress
(MPa)
R-value at
strain 15 (%)
291.1
291.4
279.8
279.8
285.8
284.9
128
125.8
120.9
121.7
123.7
124.1
0.69
0.72
1.08
1.1
0.79
0.88
RD1
RD2
DD1
DD2
TD1
TD2
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Table 2
Lubricants and their details as provided by lube companies
Lubricant code
A
B
C
D
E
F
G
Lubricant type
Petroleum
oil
Draw
down bar
#0
Petroleum
oil
Draw
down bar
#0
Petroleum oil,
additive blend
Draw down
bar #0
Petroleum oil,
additive blend
Draw down
bar #0
Emulsiﬁed oil
(semisynthetic)
Draw down bar #0
Dry ﬁlm
Mineral oil
based
Lube company
applies
Application method
Lubricant code
H
I
Lubricant type
Mineral oil
based
Draw
down bar
#0
Mineral oil
based
Draw
down bar
#0
Application method
J
K
L
Draw down bar #0
M
N
Water based
Water based
Water based
Dry ﬁlm
Dry ﬁlm
Draw down
bar #0
Draw down
bar #0
Draw down
bar #0
Lube company
applies
Lube company
applies
Fig. 2 3D measurements of surface texture (Al 5182-O, 1.5 mm): (a) EDT and (b) MF, provided by Honda Engineering
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Table 3 Measured roughness parameters in different areas on
the EDT and MF surfaces
Raa (lm)
Area 1
Area 2
Area 3
Area 4
Area 5
109
110
111
112
113
114
Rzb (lm)
EDT
MF
EDT
MF
1.719
1.414
1.111
1.463
1.197
1.574
1.282
1.121
1.485
1.238
16.991
14.546
12.022
14.3
13.505
14.03
11.145
11.135
12.96
10.5
The following evaluation criteria are used to determine the
“better” performance of the selected lubricants: (a) higher applicable BHF for the successfully drawn part and (b) shorter perimeter
in ﬂange area (the perimeter is used in evaluation of the test
results because the formed ﬂange is not exactly round and the
measurement of the perimeter avoids measuring and averaging
several draw-in lengths (Ld)).
2.4 Description of the Tooling and Test Procedure. The
CDTs were carried out in a 160 ton hydraulic press, which has a
maximum ram speed of 300 mm/s (12 in./s). The schematic of the
tooling is shown in Fig. 4. A draw die attached to the upper ram
a
Ra is the arithmetic average of the absolute values.
moves down and forms a cup sample with a stationary punch. The
b
Rz is average distance between the highest peak and lowest valley in each
preset constant BHF is provided by the CNC hydraulic cushion
sampling length.
pins. During the CDT, the punch force is measured by a load cell
and the die displacement is recorded by an infrared laser sensor.
The lubrication condition in this ﬂange area inﬂuences the thickness The ﬂange perimeter is measured using a ﬂexible tape.
The cup draw test is selected because it is relatively simple to
distribution in the side wall of drawn cup. It also affects the draw-in
conduct and it communicates to the practical stamping engineer
length, Ld. As the blank holder pressure (Pb) increases, the frichow lubrication affects the metal ﬂow and possible cup fracture.
tional stress (s) also increases based on Coulomb’s law (Eq. (1))
In this study, the draw ratio, the ratio of initial blank diameter to
cup diameter, was selected to be 2.0. In order to leave enough
s ¼ l Á Pb
(1)
ﬂange area for measuring, the draw depth 80 mm was selected.
Round blanks with 304.8 mm diameter were cut from Al 5182-O
where s ¼ frictional shear stress, l ¼ coefﬁcient of friction, and
sheet.
Pb ¼ blank holder pressure.
Fig. 3 Schematic of the cup drawing operation (CDT) and tooling
Fig. 4
Deep draw tooling used in CDT [7]
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AQ1
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Table 4 FE simulation parameters for obtaining BHF range for
CDT
Parameter
Description
Sheet material
Anisotropic values (average)
Flow stress curve
Yield criterion
Blank diameter
BHF
CoF
Punch stroke
Forming speed
Al 5182-O, 1.5 mm
R0 ¼ 0.71, R45 ¼ 1.09, R90 ¼ 0.84
VPB test data, as shown in Fig. 1
Yld 2000-2d
304.8 mm
16–24 ton
0.1, 0.11, 0.12
80 mm
40 mm/s
Table 5 Predicted results under various BHF and CoF
BHF
CoF
16
18
20
22
24
0.1
0.11
0.12
Formed
Formed
Formed
Formed
Formed
Fracture
Formed
Fracture
Fracture
Fracture
Fracture
Fracture
Fracture
Fracture
—
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In order to obtain a workable range of BHF for the tests, the FE
analysis, using the commercial code PAM-STAMP, was carried out to
simulate the CDT process. The FE model was built in commercial
software PAM-STAMP. The sheet was modeled with four node quadrilateral shell elements, and the tools were assumed to be rigid.
The ﬂow stress curve of Al 5182-O was considered to be independent of strain-rate in the deformation rate used in CDT. The
ﬂow stress curve obtained from VPB test was used in the FE
model. The yield criterion Yld 2000-2d, having a better description of the anisotropic behavior of Al alloys, was used in the FE
model. The required eight yield parameters were characterized in
Numisheet 2005 Benchmark for Al 5182-O [15]. All the simulations were conducted under constant BHF and punch speed.
Table 4 summarizes the input parameters in the FE simulation.
In the FE simulations, the critical thinning (maximum thinning)
value of 20%, selected to predict the failure in forming of Al
5182-O is suggested by industrial experience from some of the
companies that form Al alloy sheets. The cup is formed, when the
Fig. 6 Flange perimeter recorded for 13 lubricant tests at 16
ton BHF (B, D, H, and N failed at 16 ton BHF). Note: The y-axis
on the graph does not start from 0. The error bands show the
deviation between samples.
FE Simulation of the CDT
Fig. 7 Flange perimeter recorded for ﬁve lubricant tests at 17
ton BHF (A, C, E, I, and J failed at 17 ton BHF). Note: The y-axis
on the graph does not start from 0. The error bands show the
deviation between samples.
Fig. 5 Test procedure for the evaluation of lubricants
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Fig. 8 Comparison between MF and EDT at (a) 16 ton BHF and (b) 17 ton BHF
Table 6 Flange perimeters prediction by FE simulations
BHF (ton)
16
17
CoF0.06 0.08 0.1 0.12 0.06 0.08 0.1 0.12
Flange perimeter (mm) 735 740 745 750 736 741 746 752
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maximum thinning value is below 20%. Otherwise, the cup gets
fracture. As shown in Table 5, the drawability of 5182-O using
the present die set was predicted under different BHF and CoF. It
was determined that 16–18 ton is a reasonable range for evaluation of the performance of various lubricants in this study.
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4.1 Test Results. Based on the FE simulation results listed in
Table 5, the CDTs were conducted under BHF 16 ton, 17 ton, and
18 ton, respectively; 17 ton was found to be the maximum useful
BHF for the experiments, since all the cups drawn with all lubricants cracked when BHF was 18 ton. The consistency of
Lubrication Tests, Analysis, and Results
lubrication condition is determined by using three samples for
each test. The test procedure for the evaluation of all lubricants is
shown in Fig. 5.
In the tests, the measurements of Ld could be easily affected by
the concentricity deviation of the sample position on the die.
Thus, the performance of the lubricants was evaluated by measuring the ﬂange perimeter of the drawn cup using a tape. At 16
ton BHF, all lubricants listed in Table 2 were tested. Lubricants
that cracked at 16 ton (B, D, H, and N) were not considered for
next round of evaluation, using 17 ton BHF. As shown in Fig. 6,
lubricant F shows the minimum ﬂange perimeter among all the
tested lubricants, indicating a better performance at 16 ton BHF.
At 17 ton BHF, cups could only form with ﬁve lubricants, including three wet lubricants G, K, L and two dry ﬁlm lubricants F, M.
The results under 17 ton BHF are shown in Fig. 7. It can be concluded that, among all the wet lubricants, G, K, and L could be
determined as “better performing wet lubricants.” The dry ﬁlm
lubricant F performed “best” at both 16 and 17 ton BHFs.
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4.2 Effect of Surface Texturing. The friction behavior of Al
5182-O with EDT surface and MF surface are compared, as
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Fig. 9 Comparison of ﬂange perimeters obtained from simulation and experiment to predict
the CoF at 16 ton BHF for Al 5182O/MF
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Fig. 10 Comparison of ﬂange perimeters obtained from simulation and experiment to predict
the CoF at 17 ton BHF for Al 5182O/MF
Fig. 11 Thickness comparison between experiment and simulation of drawn cup with lubricant F: (a) rolling direction and (b) transverse direction
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Fig. 12 Thickness comparison between experiment and simulation drawn cup with lubricant I:
(a) rolling direction and (b) transverse direction
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shown in Fig. 8. Lubricants K, J, and L were ﬁrst tested at 16 ton
BHF. However, when BHF was increased from 16 ton to 17 ton,
the cups fractured when using lubricant J. As shown in Fig. 8, the
cups formed with EDT surface show smaller ﬂange perimeters. It
can be concluded that, comparing to MF, EDT provides a better
lubrication condition in the current experimental procedure.
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4.3 Prediction of CoF at Tool–Workpiece Interface. To
determine the CoF, the perimeter of the ﬂange obtained at the end
of stroke in the simulation is compared to the results obtained in
the experiment using PAM-STAMP. The parameters used as input to
FE simulation are the same as parameters listed in Table 4, except
that (1) BHFs used in the simulation are 16 ton and 17 ton and (2)
the CoFs are 0.06, 0.08, 0.1, and 0.12. The predicted ﬂange perimeters under different BHFs and CoFs are shown in Table 6.
As shown in Figs. 9 and 10, the ﬂange perimeters obtained
from FE simulations and experiments are compared to determine
the CoFs for different lubricants under 16 ton BHF and 17 ton
BHF with MF blanks. It can be concluded that (i) the CoFs for
lubricants G and K are in the range of 0.08–0.1, (ii) the CoF for
lubricant F is around 0.08, and (iii) the CoF for the lubricant I is
around 0.12.
4.4 Comparison of Cup Thickness Variation Obtained
From Simulations and Experiments. In order to verify the simulation results (CoF), the thickness distributions in the drawn cup
obtained from experiments and simulations are compared. The
test results for lubricants F and I are selected to verify the results
of simulations. As mentioned in Sec. 4.3, the CoF of F is around
0.08 and the CoF of I is around 0.12. The cup samples drawn with
lubricants F and I were cut in rolling and transverse directions.
The thickness measurements along the rolling direction and transverse direction were completed using Starrett micrometer and are
shown in Figs. 11 and 12. It is seen that FE simulations provided a
good match with experimental results and the maximum error is
around 5%.
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Summary and Conclusions
In this study, the methodology for evaluating the performance
of various commercial lubricants for forming aluminum alloy
sheets was established by using CDT and FE analysis. Two surface textures were investigated and compared with different lubricants. The principal conclusions from this study are summarized
as follows:
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(1) CDTs were utilized to evaluate various wet and dry lubricants under different BHFs for Al 5182-O sheets with EDT
and MF surfaces. The effect of deformation on surface ﬁnish, as discussed in Ref. [16], was considered. The criteria
used in this study were able distinguish the performance of
selected lubricants.
(2) One of the dry ﬁlm lubricants was found to perform better
than wet lubricants with the same amount of coating weight
(1 g/m2). In addition, EDT surfaces, with higher roughness,
could provide a better lubrication condition than MF surfaces for aluminum stamping.
(3) The FE model used in PAM-STAMP yields reasonable estimate
of CoFs and accurate thickness predictions when deep
drawing conditions prevail. The lowest CoF 0.08 was determined for one dry ﬁlm lubricant under the current experimental condition.
Transactions of
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Acknowledgment
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The authors would like to extend special thanks to the member
companies of the Center of Precision Forming (CPF) that funded
this study. Special thanks are due to Honda North American Engineering center (Milan Jurich, Doug Staats), Honda R&D (Jim
Dykeman) and Honda Engineering (Dennis O’Connor) for supporting this project. The authors also thank to the lubricant companies
that provided samples and the Edison Welding Institute (Hyunok
Kim) for assisting in conducting the experimental part of this study.
References
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ICTMP, Nyborg, Denmark, June 15–18, pp. 489–500.
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