Journal of
Journal
Manufacturing
of Manufacturing
ProcessesProcesses
Vol. 6/No.Vol.
2 6/No. 2
2004
2004
Formability Improvement in Aluminum TailorWelded Blanks via Material Combinations
Amit V. Bhagwan and Ghassan T. Kridli, Center for Lightweighting Automotive Materials and Processing,
University of Michigan-Dearborn, Dearborn, Michigan, USA
Peter A. Friedman, Ford Research Laboratory, Ford Motor Company, Dearborn, Michigan, USA
Abstract
on controlling material flow into the die by the use
of drawbeads. The study showed that numerical
analyses were able to predict the findings observed
experimentally. The study concluded that using a
segmented drawbead could be used to reduce the
difference in strain values between the thick and thin
sheets in the TWB.
Buste et al. (2000) also used the FEA code LSDYNA (shell element based models) to predict the
strain distribution in aluminum TWBs. The study
modeled the limiting dome height test for three
different levels of gage mismatch and two welding
methods. The study also considered the effect of
different weld line orientations relative to loading
direction. The findings showed that good agreement
between the numerical and experimental strain
distributions could be achieved for the cases where
failure occurred in the weld. However, when failure
occurred away from the weld zone (in the thin sheet),
the shell element based models were reported to be
“overly conservative” and predicted strain localization
in the row of elements immediately adjacent to the
weld line.
Experiments conducted by Saunders and Wagoner
(1996) showed that the performance of TWBs closely
depended on the movement of the weld line. Kinsey
and Cao (2001) also indicated that the movement of
the weld increases the likelihood of wrinkling and/or
premature tearing near the weld, leading to a
reduction in formability. Movement of the weld line
occurs in TWBs made of the same material with
blanks of dissimilar thickness. In this case, the thin
blank undergoes a larger amount of deformation than
the thick blank, thus leading to weld line movement
or shift into the thick material. Weld line movement
also occurs in TWBs made of similar gage but
different strength materials. In such a case, the
stronger material has a higher resistance to
deformation than the weaker material, and the weld
The use of tailor-welded blanks (TWBs) in automotive applications is increasing due to the potential of weight and cost
savings. These blanks are manufactured by seam welding two
or more sheets of dissimilar gauge, properties, or both, to
form a lighter and stiffer blank. This allows engineers to “tailor” the properties of the blank to meet the design requirements of a particular part. TWBs are used in such places as
door inner panels, lift gates, and floor pans. Initial applications of TWBs were for steel alloys, but investigating the potential of using aluminum TWBs is also of interest. One of the
problems encountered with stamping TWBs is the difference
in load-bearing capacities of the dissimilar sheets that make
up the TWB. This can result in a reduction in the formability of
the TWB and possibly a movement of the weld from its design-intended location. This paper presents the results of investigating the use of different material combinations to
manipulate this type of preferential straining in the TWB in an
effort to minimize the movement of the weld line.
Keywords: Tailor-Welded Blanks, Numerical Modeling, Weld
Shift, Aluminum
Introduction
The use of new manufacturing concepts and
advanced materials is of interest to the major
automotive manufacturers, who are continuously
seeking means to reduce weight and cost. One such
concept is the use of tailor-welded blanks (TWBs),
which are used to replace multiple stampings that are
formed separately, then assembled or joined to
produce the final product (Davies et al. 2000).
Aluminum TWBs are being considered for use in the
manufacture of different automotive components due
to their positive impact on weight and stiffness.
Finite element analyses (FEA) have been used to
predict the deformation characteristics of TWBs. Kim
et al. (2000) used the FEA code LS-DYNA to predict
the drawing behavior of TWBs. The study focused
This paper is an original work and has not been previously published
except in the Transactions of NAMRI/SME, Vol. 31, 2003.
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Vol. 6/No. 2
2004
line shifts toward the stronger side. Therefore,
investigating practical means of minimizing the
movement of the weld line is desirable to improve
the formability of the TWBs.
Kinsey, Liu, and Cao (2000) proposed the use of a
segmented die with local adaptive controllers to
restrain the weld line and reduce its movement. The
method included incorporating hydraulic cylinders
in the punch and die over the initial position of the
weld line. The cylinders acted as local adaptive
controllers, which were activated during the process
to reduce straining in the weaker side of the TWB.
Their work was successful in improving the
formability of Al 6111-T4 as measured by the depth
of draw of the tested TWB panels.
A study by Heo et al. (2001) investigated weld line
movement under deep drawing conditions. They
investigated TWBs formed into a square cup shape
with the use of drawbeads. The weld line was in the
middle of the TWB and perpendicular to the sidewalls
of the formed cup. A drawbead was placed parallel
to the weld line on the thin side of the TWB. The
study concluded that weld movement during forming
could be reduced by controlling the size and depth of
the drawbead, and that reduction in weld movement was
inversely proportional to the size of the drawbead.
Another approach for controlling weld line
movement and improving the formability of TWBs
is proposed in this study. The study aims at
investigating the use of different material
combinations to manipulate the preferential straining
in the TWB and minimize the movement of the weld
line during stamping. The proposed approach targets
applications where TWBs made of sheets with
dissimilar thickness are used for improving the
stiffness of the stamped part. The premise is that
preferential straining occurs in the TWB due to the
difference in load-carrying capacity between the thin/
weak and thick/strong sides of the TWB. This leads
to inhomogeneous straining of the TWB, with higher
strain levels observed on the thin (or weak) side. Thus,
having a thick side of lower strength alloy and a thin
side of a higher strength alloy may reduce the effect
of gage mismatch on strain distribution, leading to a
reduction in the amount of weld movement. For
example, at a given amount of gage mismatch, if the
TWB were made of 5xxx series aluminum, the
geometric stiffness of the stamped part would be the
same regardless of the alloy combination used to make
the TWB. Thus, different 5xxx aluminum alloys could
Table 1
Gage Mismatch Ratio
Thickness of Thin
Gage Sheet (mm), t1
Thickness of Thick
Sheet (mm), t2
Mismatch Ratio, t2/t1
1.0
1.25
1.50
1.75
2.00
1.25
1.50
1.75
2.00
1.25
1.50
1.75
2.00
1.20
1.40
1.60
1.5
1.75
2.00
1.17
1.33
1.75
2.00
1.14
be substituted for one another to improve the
formability of the TWB.
To assess the validity of this presumption, TWBs
made with 5xxx series aluminum alloys are
investigated. The following sections provide detailed
information on the investigated alloy combinations.
Investigated Parameters
Two parameters were investigated in this study,
namely, material and gage mismatch ratio.
Gage Mismatch Ratio
The gage mismatch ratio is defined in this study
as the thickness ratio of thick to thin sides of the TWB.
Ten different levels of gage mismatch ratio ranging
from 1.14 to 2.0 were considered in the analysis based
on the thickness combinations shown in Table 1. The
selection of these ratios was based on their feasibility
for actual TWB stamping applications.
Investigated Materials
Four different AA 5xxx aluminum alloys that have
been used in the automotive industry were selected
for this study. The test alloys along with their nominal
aluminum and magnesium contents are shown in
Table 2. The strength of the test alloys is directly
proportional to the amount of magnesium in the alloy,
and the alloys are listed in Table 2 from highest to
lowest strength. The costs as well as the welding
techniques employed for these alloys are similar, thus
enabling ease of implementation of this concept.
To compensate for the preferential straining
experienced by the TWB, alloy substitutions were
made such that the higher strength alloy was always
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Table 2
Composition of Investigated Al 5xxx Alloys
Alloy
Composition
AA 5182-O
AA 5454-O
AA 5754-O
AA 5052-O
Al 95.2%, Mg 4.5%
Al 96.4%, Mg 3%
Al 95.2%, Mg 3%
Al 97.5%, Mg 2.5%
Table 3
Investigated Thin-Thick Alloy Combinations
Combination Number
1
2
3
4
5
6
7
8
9
10
Alloys (Thin-Thick)
5182-5182
5182-5454
5182-5754
5182-5052
5454-5454
5454-5754
5454-5052
5754-5754
5754-5052
5052-5052
Figure 1
Schematic of Tooling for LDH Test (dimensions are in mm)
Table 4
Material Characteristics
placed on the thin side of the TWB. All possible
combinations of the selected alloys meeting this
criterion were investigated. The findings were
compared to those of TWBs made with sheets of
only the higher strength alloy having the same
gage mismatch ratio. Thus, 10 different material
combinations were investigated. Table 3 shows the
thin-thick alloy combinations to be analyzed in
this study.
Alloy
Strength
Coefficient,
K (MPa)
Hardening
Exponent, n
AA 5182-O
AA 5454-O
AA 5754-O
AA 5052-O
598
504
440
383
0.320
0.286
0.269
0.270
for all the alloys are shown in Table 5. The blank was
modeled using the isotropic material model (MATL
18) and was meshed using shell elements. The
Belytschko-Tsay element formulation option was
used for the shell sections. Ranking of the alloys based
on their strength can be seen in Table 4, which
presents the materials from strongest to weakest.
Numerical Model
Tooling
The tooling components were modeled as rigid
bodies (MATL 20 in LS-DYNA) and meshed with
shell elements (shell thickness = 1 mm). Tool steel
with a modulus of elasticity of 207 GPa and a
Poisson’s ratio of 0.28 was used as the material for
the tooling. Although, the tooling components were
defined as rigid bodies, the material properties were
introduced to avoid numerical problems with contact
when rigid bodies interact in a contact definition, as
recommended by the user manual of LS-DYNA
version 950.
Finite element analysis (FEA) was conducted using
LS-DYNA. A finite element model of the limiting
dome height (LDH) tooling was created based on the
dimensions shown in Figure 1. The figure shows the
hemispherical punch, the blankholder with a
triangular lock bead, and the die.
Blank
The modeled TWBs were 177.8 mm long and 101.6
mm wide with the weld line perpendicular to the
loading direction. The weld was located in the middle
of the TWB and passed over the apex of the punch.
The material properties of the different blank
materials were introduced using the power law
material model with the parameters shown in Table
4. The values of the parameters that were common
Contact and Friction
Contact between the TWB and the tooling (punch,
die, and binder) was modeled and a friction coefficient
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Table 5
Common Properties for All Investigated Blank Materials
Property
Value
Density
Modulus of elasticity
Poisson’s ratio
2.65 gm/cc
71 GPa
0.29
of 0.25 was used in the analysis to represent the case
of dry friction. The value of the coefficient of friction
was the same for each of the evaluated conditions.
Boundary Conditions
A blankholding force of 250 kN was used in the
model, and the punch was set to travel at a speed of
1000 mm/s for a total distance of 20 mm. The
introduced blankholding force was based on an
experimental value, and the preset punch travel
distance was based on an experimental value without
failure for TWBs made of AA 5182-O with a gage
mismatch ratio of 1.5. The blankholding load and the
punch travel distance were kept constant for all
investigated cases.
Figure 2
Maximum Weld Movement Comparison
for 1.75 mm to 2.00 mm TWB
combinations shown in Table 3. In all three figures,
weld displacement is considered positive when the
weld line moves into the thick side of the TWB, and
negative when it displaces into the thin side. The data
presented in these three figures represent three
different gage mismatch ratios.
Figure 2 represents the case of gage combinations
of 1.75 mm to 2.0 mm, which results in the lowest
gage mismatch ratio of 1.14. The figure shows that
for this gage mismatch ratio there is a strong direct
relationship between the weld movement and the
strength of the thick side of the TWB. Additionally,
for TWBs made of a single alloy, the figure indicates
that the amount of weld shift is inversely proportional
to the strength of the alloy.
Figure 2 also shows that for the combinations that
included the strongest two alloys (AA 5182-O or AA
5454-O) on the thin side of the TWB, and the weakest
alloy (AA 5052-O) on the thick side, the weld moved
into the thin side. This observation was also made
for all gage mismatch ratios less than or equal to 1.25,
with the 5182-5052 TWBs. Therefore, the effect of
gage mismatch for these conditions was
overcompensated for by the difference in strength.
Figure 3 represents a case in the mid range of the
gage mismatch ratio (ratio = 1.4). Compared to Figure
2, Figure 3 shows that, as the gage mismatch ratio
increases, the weld shifts only into the thick sheet
regardless of alloy combination. Figure 3 also shows
that the amount of weld shift increases as the gage
mismatch ratio increases. The strong relationship
Results
The maximum weld movement or weld shift was
calculated by monitoring the initial and final positions
of the nodes at the weld-to-base-material interface.
Due to symmetry in the loading conditions and the
geometry, the location of maximum weld movement
was observed to be in the middle of the TWB (at
50.8 mm from either of the free edges) for all cases.
The direction of weld movement at the maximum
movement location was also observed to be
perpendicular to the weld line.
To compare the results, a percent benefit was
calculated for each alloy combination and gage
mismatch ratio. The percent benefit represented the
amount of reduction in the weld movement based on
the maximum displacement, which was always
observed to be for the case of a TWB made of only
the higher strength alloy. The following equation is
used to calculate the benefit:
⎛ Weld shift in TWB made of X -Y alloys ⎞
Benefit = ⎜ 1 −
⎟
⎝ Weld shift in TWB made of X -X alloys ⎠
provided that alloy X is used in the thin blank and is
of higher strength than alloy Y.
Figures 2, 3, and 4 present the maximum weld
displacement for the 10 investigated alloy
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2004
Figure 3
Maximum Weld Movement Comparison
for 1.25 mm to 1.75 mm TWB
Figure 4
Maximum Weld Movement Comparison
for 1.00 mm to 2.00 mm TWB
between the amount of weld shift and the strength of
the thick side was also observed from the figure.
Figure 4 represents the highest investigated gage
mismatch ratio. The figure shows that the amount of
weld shift can still be reduced by the proposed
method, but that the investigated alloy do not contain
a combination that can effectively compensate for the
effect of gage mismatch in the TWB.
The percent benefit of the proposed approach is
shown in Figures 5, 6, and 7. A 100% benefit indicates
that the weld shift can be fully eliminated through
the use of proper alloy combination. A negative
percent benefit indicates that the load-carrying
capacity of the thick side is significantly reduced,
causing it to experience higher levels of strain than
the thin side, thus leading to excessive weld shift into
the thin side of the TWB. This case was only observed
for the 5182-5052 alloy combination with the lowest
gage mismatch ratio, as can be seen in Figure 5.
Figure 5 represents comparison of a TWB entirely
made with AA 5182-O with the alloy combination in
which the thin sheet is made of AA 5182-O and the
thick sheet is made of 5454-O, 5754-O, or 5052-O.
This figure shows that as the gage mismatch ratio
increases, the benefit of using this method decreases.
The figure also shows that for the alloy combinations
5182-5052 and 5182-5754, there exist gage mismatch
ratios for which the weld will not displace. These
ratios were both observed to be less than 1.4.
Figure 6 compares the alloy combinations with the
thin sheet made of AA 5454-O and the thick sheet
made of either AA 5754-O or AA5052-O, with the
Figure 5
Benefit in Weld Movement for TWBs Made of AA 5182-O Thin
Sheet and Thick Sheet as Indicated in the Figure
case where the TWB is made only of AA 5454-O.
Figure 7 compares a TWB made of AA 5754-O with
the thin-thick combination 5754-5052. Both figures
show the same trends observed in Figure 5. All figures
show that as the strength of the thin sheet approaches
that of the thick sheet, the maximum potential benefit
decreases, and for some combinations, full
compensation of the effect of gage mismatch cannot
be achieved.
The effective plastic strain distributions in the thin
and thick sides of the TWB gave an insight into how
the strains get redistributed as the strength of the thick
side is reduced. As a lower strength material is used
in the thick blank, the strain induced in the thin blank
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Figure 6
Benefit in Weld Movement for TWBs Made of AA 5454-O Thin
Sheet and Thick Sheet as Indicated in the Figure
Figure 7
Benefit in Weld Movement for TWBs Made of AA 5754-O Thin
Sheet and Thick Sheet as Indicated in the Figure
reduces and that in the thick blank increases, and the
ratio of strain in the thin to the thick reduces. It was
observed, for the investigated parameters, that beyond
a strain ratio of 0.75 the weld starts to shift in the
opposite direction (into the thin sheet). It should be
noted that this value of critical ratio would be
dependent on the forming conditions.
All of the investigated cases showed that the
formability of the TWB is affected by the gage
mismatch ratio rather than the thickness difference
between the sheets. The results also show that the
actual sheet thickness affects the forming load
requirement but does not play a role on the movement
in the weld line, which is mainly controlled by the
gage mismatch ratio and the strength of the alloys
used to make the TWB. The observations made in this
study indicate that the proposed method can lead to
improving the formability of TWBs made of 5xxx
aluminum alloy series, and the idea may be extended to
other materials as long as the thicker material is needed
for improving the stiffness of the stamped part.
It should be noted that even though the reported
weld shifts in the study appear to be small, they should
be considered with respect to the dimensions of the
test material. If the size of the TWB were scaled up
to that of a typical production panel, the maximum
weld shift would be expected to increase accordingly.
drawbead and blankholding force. However, most
applications involve stretching and drawing
conditions where the material flow into the die is
needed; therefore, the case of drawing also needs to
be evaluated. A study was performed on drawing of a
circular hemispherical bottom cup made of a TWB.
The following is a description of this investigation
and its findings.
Procedure
Numerical analysis of drawing was initially
conducted with a 160 mm diameter TWB made of
AA-5182 TWB with 0.8 mm to 1.3 mm gage
mismatch (1.625 gage mismatch ratio). The weld line
passed through the center of the TWB, and this case
was considered as the base. The analysis was then
repeated by replacing the material of the thick blank
with AA-5052, representing the case where the
maximum benefit may be achieved based on earlier
observations. The friction coefficient was kept the
same as in the stretch-forming study. The blankholder
was replaced with a flat holder and the blankholding
force was reduced to 200 kN. The punch travel was
changed to 30 mm to allow for a deeper draw. The
weld shifts obtained for the two investigated cases
were noted and compared with each other.
Table 6 shows a comparison of the values of the
maximum and the average weld line movement in
the two investigated cases. The average weld shift
was calculated by considering all of the nodes along
the weld-base material interface. The percent change
was calculated as the difference between the predicted
Forming Under Drawing Conditions
The results presented thus far are for TWBs
subjected to stretching conditions with restricted
material flow into the die by the use of the triangular
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2004
Table 6
Comparison of Maximum Weld Shift and Average Weld Shift
Parameter
Thin-Thick
5182-5182
Thin-Thick
5182-5052
Percent
Change
(Reduction)
Maximum weld shift (mm)
Average weld shift (mm)
4.033
1.356
1.38
0.446
65.79 %
67.09 %
Acknowledgments
The authors would like to thank the Ford Motor
Company for providing funding for this project. The
authors would also like to thank Engineering
Technology Associates for providing software
licenses (DYNAFORM and FEMB 27) for the
purpose of this research.
weld shifts for the two cases divided by the weld shift
of the TWB made of only AA 5182.
This indicates that a large reduction in weld
movement can be obtained by substituting the thick
blank with a lower strength material. In this case
study, a reduction of 65.79% was obtained during the
cup forming operation by substituting the thick sheet
with AA-5052. This implies that the proposed method
may also be used for reducing the weld line movement
in parts under forming conditions similar to those of
the cup forming process.
References
Buste, A.; Lalbin, X.; Worswick, M.J.; Clarke, J.A.; Altshuller, B.; Finn,
M.; and Jain, M. (2000). “Prediction of strain distribution in aluminum
tailor welded blanks for different welding techniques.” Canadian
Metallurgical Quarterly (v39, n4), pp493-502.
Davies, R.; Grant, G.; Smith, M.; and Oliver, E. (2000). “Formability and
fatigue of aluminum tailor-welded blanks.” SAE Paper No. 2000-012664. Warrendale, PA: Society of Automotive Engineers.
Heo, Y.M.; Wang, S.H.; Kim, H.Y.; and Seo, D.G. (2001). “The effect of
the drawbead dimensions on the weld-line movements in the deep
drawing of tailor-welded blanks.” Journal of Materials Processing
Technology (v113), pp686-691.
Kim, H.; Heo, Y.; Kim, N.; Kim, H.Y.; and Seo, D. (2000). “Forming and
drawing characteristics of tailor welded sheets in a circular drawbead.”
Journal of Materials Processing Technology (v105, n3), pp294-301.
Kinsey, B. and Cao, J. (2001). “Enhancement of sheet metal formability
via local adaptive controllers.” Trans. of NAMRI/SME (v29). Dearborn,
MI: Society of Manufacturing Engineers, pp81-88.
Kinsey, B.; Liu, Z.; and Cao, J. (2000). “A novel forming technology for
tailor welded blanks.” Journal of Materials Processing Technology
(v99), pp145-153.
Saunders, F.I. and Wagoner, R.H. (1996). “Forming of tailor-welded blanks.”
Metallurgical and Materials Trans. A (v27A), pp2605-2616.
Conclusions
The results indicate that, at lower levels of gage
mismatch ratio, the reduction in weld shift obtained
by replacing the thick blank by lower strength alloys
is substantial. Also, as the strength of the thicker blank
is reduced, the percent benefit increases. However,
as the gage mismatch ratio increases, the benefit
obtained decreases because the difference in strength
does not fully compensate for the gage mismatch.
The presented approach can be used for other
material combinations to determine if there is a
potential gain by changing the material of the thick
blank. This can be used as a basis for material
selection prior to performing detailed numerical
analysis of the forming process of the actual part.
The presented investigations provide a comparative
analysis of the effect of different parameters on weld
shift during the forming of TWBs. It is clear that
preferential straining due to gage mismatch can be
controlled by manipulating the material strength of
the thin and thick sheets. However, the effectiveness
of this method is also dependent on weld location,
geometry of the formed part, weld orientation, as well
as other factors. Therefore, a study of the effects of
these parameters is required. In addition, experimental
validation of the results is needed.
Authors’ Biographies
Mr. Amit Bhagwan is a graduate student at the University of Michigan-Dearborn working toward a master of science in engineering (automotive systems engineering). He completed his undergraduate studies in
mechanical engineering at the University of Bombay in Mumbai, India.
His MS thesis work is on the formability of aluminum tailor-welded blanks.
Dr. Ghassan Kridli is an associate professor of industrial and manufacturing systems engineering at the University of Michigan-Dearborn.
He received his PhD from the University of Missouri-Columbia, Dept. of
Mechanical and Aerospace Engineering, and his MS and BS in mechanical engineering from the University of Miami. His research interests include formability evaluation of lightweight materials, finite element
modeling and analysis of sheet metal forming processes, and materialprocess-product relationships. His work is published in refereed journals
and in national and international conference proceedings.
Dr. Peter Friedman is a technical specialist at Ford Research and Advanced Engineering. His work is primarily on the development of forming processes for aluminum sheet alloys. Most recently, he has been
working on developing superplastic forming processes suitable for the
automotive industry. Dr. Friedman has a PhD from the University of Michigan, Dept. of Mechanical Engineering, as well as a bachelor’s degree
from the University of Pennsylvania and a master’s degree from Columbia University. He has published extensively in referred journals, presented papers at technical conferences, and has applied for several patents.
140