V. GURUVU et al.: OPTIMIZATION OF FORMABILITY OF TAILOR-WELDED BLANKS
151–155
OPTIMIZATION OF FORMABILITY OF TAILOR-WELDED
BLANKS
OPTIMIZACIJA OBLIKOVANJA PRILAGOJENIH VARJENIH
SUROVCEV
Valarmathi Guruvu1, Ramanathan Kalimuthu1, Sathiya Narayanan Chinnaiyan2,
Kathiresan Marimuthu3
1Alagappa Chettiar College of Engineering & Technology, Department of Mechanical Engineering, Karaikudi – 630004, Tamilnadu, India
2National Institute of Technology, Department of Production Engineering, Tiruchirappalli – 620015, Tamilnadu, India
3Thiagarajar College of Engineering, Department of Mechanical Engineering, Madurai – 625015, Tamilnadu, India
umkathir@gmail.com
Prejem rokopisa – received: 2017-05-18; sprejem za objavo – accepted for publication: 2017-07-28
doi:10.17222/mit.2017.058
Nowadays tailor-welded blanks (TWBs) are used in the automotive industries to meet economic concerns, government
regulations and design of vehicles with reduced weight and allow cost reduction. This technique is also employed while
improving structural integrity and crash performance. Due to the raising environmental concerns about automotive emissions
and the scarcity of natural resources, we need to reduce vehicle weight and improve fuel economy. As a result, the automotive
industries employ TWBs. In this work, three grades of aluminum sheets (5052, 6061 and 8011) with a 2-mm thickness were
made thinner by cold rolling, obtaining three different thicknesses of (0.8, 1.0 and 1.2) mm. Sheets were then joined using the
cold-metal-transfer (CMT) welding process. The Taguchi design of experiment (DOE) was carried out for the L27 orthogonal
array with nine variables at three levels. Due to this optimization process, the welding decreased the formability of TWBs
compared with the parent material and this was reflected in the forming behavior, represented by the strain-distribution profiles.
Keywords: tailor-welded blanks, aluminum alloy, incremental forming, cold metal transfer
Dandanes se zaradi ekonomi~nosti, vladnih regulativ, dizajna vozil z zmanj{anjo te`o in zaradi zmanj{evanja stro{kov v
avtomobilski industriji uporabljajo prilagojeni varjeni surovci (angl. TWBs). Ta tehnika se uporablja tudi zaradi izbolj{anja
strukturne celovitosti in obna{anja pri nesre~ah. Zaradi pove~anja okoljskih vpra{anj glede emisij izpu{nih plinov in
pomanjkanja naravnih virov, je treba zmanj{ati te`o vozila in izbolj{ati ekonomi~nost porabe goriva. Posledi~no avtomobilska
industrija uporablja prilagojene varjene surovce (TWB). V prispevku so bili trije sloji plo~evine aluminija (5052, 6061 in 8011),
z debelino 2 mm, stanj{ani s postopkom hladnega valjanja na tri razli~ne debeline (0,8, 1,0 in 1,2) mm. Plasti aluminija so bile
nato zdru`ene s postopkom varjenja s kovinskim prenosom (angl. CMT). Taguchijeva metoda na~rtovanja eksperimentiranja
(angl. DOE) je bila opravljena za L27 ortogonalno polje z devetimi spremenljivkami na treh ravneh. Iz tega optimizacijskega
procesa varjenje zmanj{a sposobnost TWB v primerjavi z originalnim materialom, kar se odra`a v obna{anju formiranj, ki ga
predstavljajo profili razdeljevanja sevov.
Klju~ne besede: prilagojeni varjeni surovci, aluminijeva zlitina, postopno oblikovanje, hladen transfer kovine
1 INTRODUCTION
Aluminum and its alloys exhibit many attractive
characteristics including light weight, high specific
strength, high thermal and electrical conductivity.
Aluminum also exhibits poor weldability due to its high
reflectivity, low molten viscosity and the existence of
oxide layers. Therefore, much research work on alumi-
num tailor-welded blanks with different thicknesses and
various alloy combinations is in progress. Tailored
blanks are the collective for semi-finished sheet pro-
ducts, characterized by the local sheet thickness, sheet
material, coating or material properties. TWBs ensure
that the components are light, stronger and provide the
required functionality at a low cost. Incremental sheet-
metal forming (ISMF) is a process, in which a sheet is
formed incrementally through a progression of localized
deformation. For small, batch-size products, there is no
need for specialized dies in ISMF. Cold metal transfer
(CMT) is an automated dip-transfer welding, charac-
terized by controlled material deposition to the work-
piece during the short circuit of the wire electrode.
Single-point incremental forming (SPIF) is a flexible
sheet-metal-forming method adapted to form various
complex shapes using a CNC milling machine, a multi-
axis robot or a dedicated machine without the use of
specific and costly tools, such as a punch and die.1 SPIF
can be used to form small or large pieces, based upon the
machine size for the various components. Furthermore,
the SPIF process, which is based on the forming limit
diagram (FLD) used to achieve a higher formability
compared to the conventional forming process, can be
adapted to the materials such as the Al-Mg-Si alloy.2
SPIF is used to form TWBs and also to improve the
forming limit. However, the shape inaccuracy, which is
usually an important limiting factor for SPIF applica-
tions is to be solved. Several mechanisms used for the
enhanced formability of the SPIF process were already
presented.3 A further elaborate study on the incremental
sheet forming regarding individual sheet quality is
Materiali in tehnologije / Materials and technology 52 (2018) 2, 151–155 151
UDK 669.018.254:669.71:62-412 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(2)151(2018)
required. The mechanical properties of friction-stir-
welded dissimilar aluminum alloys AA5052 and
AA6061 using a cylindrical-pin tool are analyzed.4 The
cold metal transfer (CMT) is an innovative process based
on short-circuiting metal transfer, introducing a relatively
low heat into the weld seam and this reduces the
formation of intermetallics and thermal distortion.5 The
welding of such hybrid joints is a big challenge due to
large differences in the melting point and thermal
expansion of aluminum alloys. The formation of exces-
sive intermetallic compounds (IMCs) at elevated tempe-
ratures also reduces the joint strength.6
Cold metal transfer requires no spatter welding; it
uses a low heat input during the welding and provides for
a good performance of joining dissimilar metals, like
aluminum and zinc-coated steel.7 When joining three
types of aluminum alloy with mild steel, design of
experiments technique (DOE) is used to optimize the
welding parameters. The effect of the intermetallic-layer
thickness was also analyzed.8 An advanced CMT method
and optimized welding parameters were used to join
dissimilar metals. The effect of the filler material on the
wettability of the base material was also studied.9 In this
work, the single-point incremental forming of TWBs
consisting of aluminum-alloy sheets made with cold-
metal-transfer welding was studied and optimization of
the process parameters was carried out using the Taguchi
method.
2 EXPERIMENTAL PART
Three grades of aluminum (5052, 6061 and 8011)
with a 2-mm thickness were made into sheets with three
different thicknesses of (0.8, 1.0 and 1.2) mm, using
V. GURUVU et al.: OPTIMIZATION OF FORMABILITY OF TAILOR-WELDED BLANKS
152 Materiali in tehnologije / Materials and technology 52 (2018) 2, 151–155
Table 1: Variables and levels of L27 DOE
No. Variable Level 1 Level 2 Level 3
1 1st material 5052 6061 8011
2 Thickness of 1st material 0.8 mm 1.0 mm 1.2 mm
3 2nd material 5052 6061 8011
4 Thickness of 2nd material 0.8 mm 1.0 mm 1.2 mm
5 Spindle speed 300 min–1 450 min–1 600 min–1
6 Feed 300mm/min
600
mm/min
900
mm/min
7 VSD 0.2 mm 0.4 mm 0.6 mm
8 Weld speed 400mm/min
500
mm/min
600
mm/min
9 Lubricant dry coconutoil grease
Table 2: L27 DOE (design of experiment) table using Taguchi method
No.
1st material 2nd material
S
pi
nd
le
sp
ee
d
(m
in
–1
)
F
ee
d
(m
m
/m
in
)
V
S
D
m
m
W
el
d
sp
ee
d
(m
m
/m
in
)
L
ub
ri
ca
nt
S
er
ie
s
T
hi
ck
ne
ss
(m
m
)
S
er
ie
s
T
hi
ck
ne
ss
(m
m
)
1 5052 0.8 5052 0.8 300 300 0.2 400 dry
2 5052 0.8 5052 0.8 450 600 0.4 500 coconut
3 5052 0.8 5052 0.8 600 900 0.6 600 grease
4 5052 1.0 6061 1.0 300 300 0.2 500 coconut
5 5052 1.0 6061 1.0 450 600 0.4 600 grease
6 5052 1.0 6061 1.0 600 900 0.6 400 dry
7 5052 1.2 8011 1.2 300 300 0.2 600 grease
8 5052 1.2 8011 1.2 450 600 0.4 400 dry
9 5052 1.2 8011 1.2 600 900 0.6 500 coconut
10 6061 0.8 6061 1.2 300 600 0.6 400 coconut
11 6061 0.8 6061 1.2 450 900 0.2 500 grease
12 6061 0.8 6061 1.2 600 300 0.4 600 dry
13 6061 1.0 8011 0.8 300 600 0.6 500 grease
14 6061 1.0 8011 0.8 450 900 0.2 600 dry
15 6061 1.0 8011 0.8 600 300 0.4 400 coconut
16 6061 1.2 5052 1.0 300 600 0.6 600 dry
17 6061 1.2 5052 1.0 450 900 0.2 400 coconut
18 6061 1.2 5052 1.0 600 300 0.4 500 grease
19 8011 0.8 8011 1.0 300 900 0.4 400 grease
20 8011 0.8 8011 1.0 450 300 0.6 500 dry
21 8011 0.8 8011 1.0 600 600 0.2 600 coconut
22 8011 1.0 5052 1.2 300 900 0.4 500 dry
23 8011 1.0 5052 1.2 450 300 0.6 600 coconut
24 8011 1.0 5052 1.2 600 600 0.2 400 grease
25 8011 1.2 6061 0.8 300 900 0.4 600 coconut
26 8011 1.2 6061 0.8 450 300 0.6 400 grease
27 8011 1.2 6061 0.8 600 600 0.2 500 dry
Figure 1: Specimens of TWBs: a) top surface, b) bottom surface
cold-rolling processes. The sheets were welded using
cold-metal-transfer welding. Using the Taguchi method,
the L27 orthogonal array with 9 variables in 3 levels were
used for the design of experiments as shown in Table 1.
The data regarding the L27 design of experiments are
shown in Table 2. TWB specimens are shown in Fig-
ure 1. In this work, a 200W Nd-YAG laser is used for
grid marking. The diameters of grid circles are 2 mm, the
width of grid lines are 1 μm and the grid marking is
shown in Figure 2.
2.1 Selection of the incremental-forming tool
A hemispherical-end tool with a 10-mm diameter and
100-mm length was used for incremental sheet-metal
forming. With this tool, we can form the sheet metal,
which is rigidly fixed onto a blank holder. The sheet
forming is carried out by passing the feed incrementally
to the tool.
2.2 Incremental sheet metal and straight groove
Single-point-incremental-forming tests were carried
out using a 3-axis CNC machine as shown in Figure 3.
In all the tests, a punch with a hemispherical head was
used and grease/coconut oil was applied as the lubricant
to reduce friction. An experimental blank of 150 mm ×
150 mm was clamped to a properly designed framework.
Circular grids of a 2-mm diameter were marked on the
surfaces of aluminum sheets using a laser beam of
200 W, while the width of a grid line was 1 μm. The
punch movement was determined with pure-stretching-
deformation mechanics. Throughout the process, the
punch head had a 10-mm diameter, and it determined the
expansion of the blank, which underwent plastic
deformation due to the punch movement, while its tool
path was controlled using the part program. For each
test, the tool trajectory depended on the testing con-
ditions. Such trajectories included both the horizontal
and vertical movements of the tool. The present work
was focused on investigating the material formability.
The test continuously varied with the wall angle and the
wall angle of breakage depended upon the material, the
thickness of the sheet and the grain orientation. Due to
the localized action of the small hemispherical punch,
the blank underwent a local thinning. Then the closest
zone of the blank was deformed without a significant
effect on the already deformed one. Therefore, forming
limits must be identified, for any material, in terms of the
critical threshold of the allowable local thinning.
According to the above considerations, the strain path in
incremental forming is typically very close to the major
axis in the diagram and fracture strains are remarkably
larger than the conventional stamping. Much higher
strains can be achieved with incremental forming than
with the traditional processes. During the straight-groove
test, grooves were made in the direction of rolling. The
size of grids is measured using a digital USB micro-
scope.
3 RESULTS AND DISCUSSIONS
The experiments from 1 to 27 were carried out as per
the DOE table, as shown in Table 2. The welding image
of the first experiment condition is shown in Figure 4.
The formability achieved along the transverse of the
weld was 0.955. A failure occurred on the weld because
it exceeded the forming limit. The formability achieved
along the weld was 0.32 and a failure occurred due to a
V. GURUVU et al.: OPTIMIZATION OF FORMABILITY OF TAILOR-WELDED BLANKS
Materiali in tehnologije / Materials and technology 52 (2018) 2, 151–155 153
Figure 3: Experimental set-up
Figure 2: Grid marking on welds: TWB No. 1 (AA6061 (1.2 mm) + AA5052 (1.2 mm))
poor weld quality. A tearing failure occurred along the
weld direction. In the conventional forming, the šn’ value
for AA5052 (0.8 mm) is 0.1354, which is lower than the
achieved formability for TWB No. 1. Similarly, the same
process was repeated for all the conditions listed in
Table 2 and from the forming limit diagram, the
formability values achieved along šthe transverse of the
weld direction’ and šalong the weld direction’ are shown
in Table 3.
Table 3: Formability achieved during forming
Ex. No.
Trans-
verse of
the weld
Along the
weld Ex. No.
Trans-
verse of
the weld
Along the
weld
1 0.995 0.32 15 0.768 0.253
2 0.522 0.395 16 0.6 0.232
3 0.59 0.215 17 0.432 0.329
4 0.477 0.176 18 0.573 0.307
5 0.511 0.322 19 0.651 0.194
6 0.381 0.239 20 0.698 0.44
7 0.763 0.214 21 0.918 0.602
8 0.524 0.348 22 0.658 0.214
9 0.373 0.141 23 0.793 0.376
10 0.311 0.076 24 0.599 0.294
11 0.806 0.343 25 0.675 0.138
12 0.693 0.18 26 1.2 0.963
13 0.736 0.273 27 0.52 0.343
14 0.668 0.321
3.1 Taguchi analysis: forming along the transverse of
the weld direction versus variables
A response table for signal-to-noise ratios for the
largest is better is shown in Table 4. The formability
along the weldment and perpendicular to the weldment
should be maximum until a rupture takes place. Hence,
the larger the better condition is used. The formability
parameters – 1st material thickness of 1.2 mm, 2nd mate-
rial thickness of 0.8 mm, the spindle speed of 450 min–1,
the feed of 300 mm/min, VSD of 0.6 mm, the weld
speed of 400 mm/min – are represented in the output
graph based on the Taguchi analysis, which is same as
that of the experiment.
4 CONCLUSIONS
Based on the results and discussion, the following
conclusions are drawn:
• The S/N ratio for both directions is maximum for
experiment No. 26.
• This indicates that the incremental-forming behavior
during experiment No. 26 in optimum and its para-
V. GURUVU et al.: OPTIMIZATION OF FORMABILITY OF TAILOR-WELDED BLANKS
154 Materiali in tehnologije / Materials and technology 52 (2018) 2, 151–155
Figure 4: Images of formed TWB No. 1: a) top surface, b) bottom
surface
Table 4: S/N ratio: largest is better
1st spindle 2nd spindle
Level 1st series thickness 2nd series thickness speed feed VSD
1 -5.272 -3.675 -4.104 -2.916 -4.094 -2.521 -3.596
2 -4.470 -4.356 -4.817 -4.978 -3.701 -5.023 -4.243
3 -2.801 -4.511 -3.620 -4.648 -4.747 -4.997 -4.703
Delta 2.471 0.837 1.197 2.062 1.045 2.502 1.107
Rank 2 9 6 3 8 1 7
Level weld speed lubricant
1 -4.485 -4.178
2 -4.717 -5.171
3 -3.340 -3.193
Delta 1.377 1.979
Rank 5 4
meters are optimum for the formation of tailor-
welded blanks.
• During the groove formation in the transverse
direction, a failure occurs on the weld because it
exceeds the forming limit and tearing occurs along
the weld.
• Welding decreases the formability of the TWBs
compared with the parent material, which is reflected
in the forming behavior, represented by strain-
distribution profiles. This is due to the non-uniform
strain distributions.
• The S/N ratios for the largest is better are proved by
the statistics and the Taguchi analysis, which is same
as that of the experiment.
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Materiali in tehnologije / Materials and technology 52 (2018) 2, 151–155 155