Failure Mode Transition of Triple­Thin­Sheet Aluminum Alloy Resistance Spot Welds under Tensile­ Shear Loads

Abstract

This paper investigates the failure mode transition of triple-thin-sheet aluminum alloy resistance spot welds under tensile-shear loads. Two stack-ups, i.e., 1.0/1.0/1.0 mm and 1.5/1.0/2.0 mm, were examined. The tensile-shear tests were performed for four different joint designs for each stack-up. The failure process and failure mode transition of the four types of joints in the 1.0/1.0/1.0 mm stack-up were investigated through step-by-step experimental methods. An analytical model, which is suitable for the three-sheet aluminum alloy resistance spot weld, was proposed to ensure the pullout failure mode. The results showed that a type of columnar grain, which has large secondary dendrite arm spacing, was the weak area in the three-sheet aluminum alloy resistance spot welds. The critical weld button size required to ensure the pullout failure mode was obtained. The proposed analytical models can be used to predict the critical button size for three-sheet aluminum alloy resistance spot welds.

Full text

WELDING RESEARCH
DECEMBER 2016 / WELDING JOURNAL 479-s
Introduction
Resistance spot welding (RSW) has
been widely used in the manufacturing
of aerospace, electronics, and especial-
ly the automotive industry because of
its high productivity, flexibility, and
suitability. The automotive industry
makes extensive application of RSW
with typically between 2000 and 5000
spot welds in a motor vehicle (Ref. 1).
It can be argued that the mechanical
performance of the spot welds deter-
mines the vehicle crashworthiness,
which is defined as the capability of a
vehicular structure to provide ade-
quate protection to its passengers dur-
ing a crash (Ref. 2).
Nowadays, with the demand for
lightweight vehicular structures,
three-sheet RSW is increasingly exert-
ed in some complex structures, such as
front longitudinal rails, A-, B-, and C-
pillars, and the bulkhead to the inner
wing (Ref. 3). Compared to two-sheet
RSW, joining three sheets is more
complicated because of the extra inter-
face introduced and different material
and sheet thickness combinations.
Therefore, it is important to under-
stand the nugget growth and failure
behavior of the three-sheet spot weld
joint.
Some researchers have investigated
the nugget growth and mechanical be-
havior of three-sheet RSW. Harlin et
al. found that the position of the ini-
tial heat generation is independent of
material thickness and stack configu-
ration (Ref. 4). They also found that
increasing the electrode force leads to
a shift in the initial position of the
weld nugget formation from the
sheet/sheet interface to the center of
the middle sheet (Ref. 5). Nielsen et al.
studied the weldability of thin, low-
carbon steel to two thicker, high-
strength steels through factorial ex-
perimentation and statistical analysis.
They found that it is feasible to obtain
a good weld with acceptable weld
strengths (Ref. 6). Pouranvari and
Marashi investigated weld nugget de-
velopment during RSW of three steel
sheets of equal thickness. They found
a critical sheet thickness of 1.5 mm at
which the size of the fusion zone at
the sheet/sheet interface is nearly
equal to that of the fusion zone at the
geometrical center of the joint (Ref. 7).
They also investigated the failure
behavior of three-sheet low-carbon
steel under a different joint type and
pointed out that the joint design sig-
nificantly affects the mechanical prop-
erties and the tendency to fail in the
interfacial failure mode (Ref. 2).
Many other studies using finite ele-
ment (FE) simulation investigated the
nugget formation process of three-
sheet RSW. Shen et al. performed a
coupled electrical-thermal-mechanical
model to predict the weld nugget for-
mation process of RSW of three steel
sheets of unequal thicknesses (Ref. 3).
Lei et al. built a two-dimensional FE
model considering the thermal-electri-
cal coupling for the RSW process of
mild steel (Ref. 8). Ma and Murakawa
developed an FE program considering
Failure Mode Transition of TripleThinSheet
Aluminum Alloy Resistance Spot Welds under
TensileShear Loads
The failure mechanism of threesheet 6061T6 aluminum alloy
resistance spot welds was investigated
BY Y. LI, Y. ZHANG, Z. LUO, H. SHAN, Y. Q. FENG, AND Z. X. LING
ABSTRACT
This paper investigates the failure mode transition of triplethinsheet aluminum alloy
resistance spot welds under tensileshear loads. Two stackups, i.e., 1.0/1.0/1.0 mm and
1.5/1.0/2.0 mm, were examined. The tensileshear tests were performed for four differ
ent joint designs for each stackup. The failure process and failure mode transition of the
four types of joints in the 1.0/1.0/1.0 mm stackup were investigated through stepby
step experimental methods. An analytical model, which is suitable for the threesheet
aluminum alloy resistance spot weld, was proposed to ensure the pullout failure mode.
The results showed that a type of columnar grain, which has large secondary dendrite
arm spacing, was the weak area in the threesheet aluminum alloy resistance spot welds.
The critical weld button size required to ensure the pullout failure mode was obtained.
The proposed analytical models can be used to predict the critical button size for three
sheet aluminum alloy resistance spot welds.
KEYWORDS
• Resistance Spot Weld • ThreeSheet Spot Welding • Aluminum Alloy • Failure Mode
Y. LI, Y. ZHANG, Z. LUO, H. SHAN, Y. Q. FENG, and Z. X. LING are with the School of Materials Science and Engineering, Tianjin University,
Tianjin, China. Z. LUO is also with the Collaborative Innovation Center of Advanced Ship and Deep Sea Exploration, Shanghai, China.
Yang Li Dec 2015120-REV.qxp_Layout 1 11/9/16 5:23 PM Page 479

WELDING RESEARCH
WELDING JOURNAL / DECEMBER 2016, VOL. 95480-s
the coupling of electrical, thermal, and
mechanical fields to study the nugget
formation in RSW of high-strength
steels (Ref. 9).
Although many studies have been
performed on the weld growth process
of three-sheet spot welds, these re-
searchers have all focused on mild or
high-strength steels. The usage of alu-
minum alloys in the automotive indus-
try is gradually increasing due to its
light weight, good formability, and
high corrosion resistance (Ref. 10).
However, very little work in open liter-
ature has studied the RSW of multiple
aluminum alloy sheets. Although Li et
al. investigated the weld growth mech-
anism of three-sheet RSW for alu-
minum alloys (Ref. 11), the failure
behavior of the spot welds was not the
focus of the paper, especially the fail-
ure transition mode, which is consid-
ered an important characteristic of the
spot weld joint.
The present article investigates the
failure mechanism of three-sheet
6061-T6 aluminum alloy resistance
spot welds (RSWs), especially the fail-
ure mode transition behavior of the
spot welds. Four types of joints are de-
signed in this paper. The mechanical
properties of three-sheet RSWs are
also investigated.
Experimental Procedures
In this study, 6061-T6 aluminum
alloy sheets with thicknesses of 1, 1.5,
and 2.0 mm were used. Table 1 lists
the chemical composition of the mate-
rials, while Table 2 lists the mechanical
properties.
Two thickness combinations were
used in the experiments. From the up-
per electrode tip to the lower one, the
two thickness combinations were
1.0/1.0/1.0 mm and 1.5/1.0/2.0 mm,
respectively. Four types of three-sheet
joints for each thickness combination
were designed, as shown in Fig. 1. In
the Type I and II joints, only one inter-
face bore the tensile force during the
test. In the Type III and IV joints, both
of the two interfaces bore the tensile
force during the test. It is obvious that
the stiffness of these joint designs and
consequently the tendency of the sam-
AB
Fig. 1 — Joint designs of the threesheet AA6061T6 RSW: A — 1.0/1.0/1.0 mm stack;
B — 1.5/1.0/2.0 mm stack.
Table 1 — Chemical Composion of 6061T6 Aluminum Alloy (wt%)
Si Mg Zn Cu Mn Fe Cr Ti Al
0.56 1.10 0.25 0.25 0.15 0.70 0.18 0.15 Balanced
Table 2 — Mechanical Properes of 6061T6 Aluminum Alloy
Yield Strength Tensile Strength Elongaon
(MPa) (MPa) (%)
285 310 6
Fig. 2 — The exemplary microstructure
of the 6061T6 resistance spot weld
nugget in the 1.0/1.0/1.0 mm stack (18
kA, 200 ms).
A
B
C
D
E
Yang Li Dec 2015120-REV.qxp_Layout 1 11/9/16 5:23 PM Page 480

ples to rotate during a tensile-shear
test are different. The Type III joint
experienced the largest rotation while
the Type IV joint bore pure shear dur-
ing the tensile-shear test. These affect-
ed the failure behavior and the suscep-
tibility to fail in interfacial mode.
Spot welding was performed using
a 220-kW direct current (DC) inverter
RSW machine. The welding parame-
ters are shown in Table 3. For the
1.0/1.0/1.0 mm stack, ten sample
welds were performed per welding
condition including three samples for
the complete tensile-shear test and
seven samples for the step-by-step
tensile-shear test. For the 1.5/1.0/2.0
mm stack, three sample welds were
performed per welding condition. The
sample dimensions used in this study
were 100 × 25 mm with a 25-mm-wide
overlap area. The tensile-shear tests
were performed at a crosshead of 1
mm min−1 with a CSS-44100 material
test system. The maximum load of
the CSS-44100 material test system
is 200 kN and the initial distance be-
tween the crosshead was 125 mm
(the gripped zone on each sheet was
25 mm).
The peak load was evaluated using
the average value of the three com-
plete tensile-shear tests. Button size
was measured from the failure surface
of the welded joint. Here the “button
size” is used to evaluate the weld quali-
ty rather than the “nugget size” be-
cause the button size is easier to meas-
ure in industrial production (Ref. 12).
The seven step-by-step tensile-shear
tests were used to investigate the fail-
ure processes of the weld joints. Seven
specimens were obtained from differ-
ent stages (load raising stage, peak
load stage, load drop stage, and final
fracture stage) during the tensile-
shear test. Afterward, the seven speci-
mens were ground, polished, and
etched using standard metallography
procedures. The cross sections of the
welds were etched by Keller’s reagent
(1 mL hydrofluoric acid, 1.5 mL hy-
drochloric acid, 2.5 mL nitric acid, and
95 mL water). The Vickers microhard-
ness test was performed using an in-
denter load of 100 g for a period of
10 s.
Results and Discussion
Joint Microstructure
Figure 2 shows the exemplary
microstructure of the 6061-T6 resist-
ance spot weld nugget in the
1.0/1.0/1.0 mm stack. The microstruc-
ture of the resistance spot weld nugget
in the 1.5/1.0/2.0 mm stack was simi-
lar to that in the 1.0/1.0/1.0 mm
stack, and it will not be given here.
As shown in Fig. 2B, from nugget
edge to nugget center, the microstruc-
ture is partially melted zone (PMZ),
columnar grain zone (CGZ), and
equiaxed grain zone (EGZ). The PMZ
refers to the area where temperature
was between the solidus temperature
and the liquidus temperature during
welding (Ref. 13). The material be-
comes a solid-liquid mixture, that is, it
is partially melted. The formation of
the CGZ is due to the relatively high
temperature gradient and low consti-
tutional supercooling at the edge of
the weld nugget. In the center of the
weld nugget, a low-temperature gradi-
ent and high constitutional supercool-
ing contributes to the formation of the
EGZ. Note that the columnar grain has
two morphologies (Fig. 2B, 2D): the
columnar grain with large secondary
dendrite arm spacing (LCGZ) and the
columnar grain with small secondary
dendrite arm spacing (SCGZ). The for-
mation of LCGZ was due to recession
of the nugget perimeter during weld-
ing. The authors found that the LCGZ
was easier to form at the lower inter-
face (close to the negative electrode).
This is because the Peltier effect (Refs.
14, 15) leads to the coldest areas of the
liquid nugget being adjacent to the
negative electrode when the welding
current is switched off. This area con-
tains less alloy content and is prone to
cracking under stress. The creation of
a significant LCGZ is an indication of a
poorly designed weld schedule (rela-
tively low heat input). When the heat
input is high enough (a more suitable
weld schedule), less or no LCGZ will
form.
Figure 3 shows the microhardness
distribution of the weld nugget. The
WELDING RESEARCH
DECEMBER 2016 / WELDING JOURNAL 481-s
A
Fig. 3 — Microhardness profile of the three
sheet aluminum alloy resistance spot weld
joint in the 1.0/1.0/1.0 mm stack.
Fig. 4 — Photos of the failure surface in the 1.0/1.0/1.0 mm stack: A — Interfacial failure;
B — partial thicknesspartial pullout failure; C — pullout failure.
Table 3 — Experimental Parameters
StackUps Welding Current Welding Time Electrode Force Electrodes
(mm) (kA) (ms) (kN)
1.0/1.0/1.0 16, 18, 20, 22 200 3.6 Truncated cone
50–300 with electrodes with a
1.0/1.0/1.0 20 increments 3.6 6mmdiameter p
of 50
1.5/1.0/2.0 18–34 with 120 4.0 Domed electrodes
increments of 2 with a sphere radius
of 50 mm and face
diameter of 20 mm
BC
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WELDING RESEARCH
WELDING JOURNAL / DECEMBER 2016, VOL. 95482-s
lowest microhardness appears in the
LCGZ, which has a coarser structure
and less alloy content. This low hard-
ness zone has a detrimental effect on
the mechanical properties of the joint,
which will be discussed in the follow-
ing sections.
Failure Mode Transition in
Three Equal Thickness Stacks
Failure of Joint Types I and II
Three types of failure modes, inter-
facial (IF) failure, partial thickness-
partial pullout (PT-PP) failure (Ref.
16), and pullout (PO) failure, are ob-
served in joint Types I and II, as
shown in Fig. 4.
Only the load-displacement curves
of the Type II joint are given here due
to the similarity of the mechanical be-
havior of the Type I and II joints, as
shown in Fig. 5. The load smoothly
dropped to zero after it reached its
maximum value in the IF failure, while
a residual force remained after the force
began to drop in the PT-PP and PO fail-
ures (Ref. 17). More details about the
failure process are shown in Fig. 6.
Figures 6A and 6B show the
macro/microstructures of the fracture
surface cross section of welds that
failed in the IF mode. Figure 6A lo-
cates the section where the force
achieved its maximum value, and a
crack formed, explaining the subse-
quent load reduction. The crack initial-
ly formed between the PMZ and LCGZ
and then propagated through the inte-
rior of the
LCGZ, and fi-
nally failed as
an interfacial
characteriza-
tion — Fig. 6B.
The subopti-
mized welding
parameters (16
kA, inadequate
heat input)
contributed to
the formation
of the LCGZ,
which has a low
hardness and
strength to resist the crack propaga-
tion. Note that the weld size in Figs.
6A,B is inconsistent. This variation
may be caused by local differences in
contact resistances of workpiece/work-
piece and electrode/workpiece, which
will alter the heat input and affect the
nugget formation (Ref. 18).
Figures 6C and 6D show fracture
initiation location of the welds that
failed in the PT-PP and PO mode, re-
spectively. In the PT-PP mode, the fail-
ure location was the PMZ while the
failure of the PO mode was due to
necking of the base metal. This sug-
gested that the PT-PP mode is a sub-
optimized failure mode.
Figure 7 shows the effect of button
size on the peak load and energy ab-
sorption of joint designs I and II. Sim-
ple linear regression was applied to
both the data obtained from joints I
and II, and a best fit line with a coeffi-
cient of determination (R2) of 0.878,
was obtained. The relatively high value
of R2suggested that a linear relation-
ship exists between the peak load and
button size. This phenomenon is also
observed by Han et al. (Ref. 18) and
Sun et al. (Ref. 19). However, a two-
order-polynomial relation exists be-
tween the energy absorption and but-
ton size, indicating that a larger weld
nugget could not only improve the
peak load, but also the microstructure
(less LCGZ due to suitable heat input)
and relieve the stress concentration
around the weld nugget and, in turn,
improve the ductility of the weld joint.
Failure of Joint Type III
Similar to joint Types I and II, IF,
PT-PP, and PO failures were observed
in the Type III joint. Only the PO fail-
ure mode will be discussed here be-
cause the analysis of the PT-PP failure
mode is similar to that for joint Types
I and II.
Figure 8 shows the macro/mi-
AB
CB
Fig. 5 — Typical loaddisplacement curves of the Type II joint in
the 1.0/1.0/1.0 mm stack.
Fig. 6 — Macro/microstructures of Type II weld joints in the
1.0/1.0/1.0 mm stack that failed in A, B — IF mode (16 kA, 200 ms);
C — PTPP mode (20 kA, 200 ms); D — PO mode (20 kA, 200 ms).
Yang Li Dec 2015120-REV.qxp_Layout 1 11/9/16 5:25 PM Page 482

WELDING RESEARCH
DECEMBER 2016 / WELDING JOURNAL 483-s
crostructures of the fracture surface
cross section of welds that failed in IF
mode and PO mode. From Fig. 8B, it
can be seen that the crack began to
form at the interior of LCGZ and then
propagated through the interface be-
tween SCGZ and LCGZ. A crack was
also found on the other workpiece/
workpiece interface —Fig. 8C. This in-
ferred that there is a competition be-
tween the two interfaces in a three-
sheet spot weld joint, and that failure
will occur on the weaker one. As can be
seen with Fig. 8B, C, the weld failed at
the interface where LCGZ formed (Fig.
8B) while the crack propagation was
restrained at the interface where no
LCGZ formed — Fig. 8C. This again
verifies that the LCGZ is the weak area
in a spot weld. In the PO mode, the
weld joint failed in the PMZ — Fig. 8E.
There is also a crack on the other
workpiece/workpiece interface (Fig.
8F), indicating competition between
the two interfaces during the tensile-
shear test.
Figure 9 shows the effect of button
size on the peak load and energy ab-
sorption of joint design III. The mini-
mum button size that guarantees a PO
mode was 5.1 mm. The results are
similar to the case of joint Types I and
II, i.e., a linear relation and two-order-
polynomial relation exists between
the peak load and button size, and
energy absorption and button size,
respectively.
Failure of Joint Type IV
Since the Type IV joint experienced
a pure shear dur-
ing the tensile-
shear test, the
failure modes of
this type of joint
are a little differ-
ent from those
of joint Types I,
II, and III. When
the nugget size
was small, both
of the two inter-
faces failed
through the in-
terfacial mecha-
nism — Fig.
10A. This is
called a double
interfacial fail-
ure (DIF) in this
paper. Note that
the nugget devi-
ated from its
original posi-
tion. The de-
tailed process
will be discussed
in the following
text. When the
nugget size grew larger, one interface
failed through the interfacial mecha-
nism while the other one failed
through the pullout mechanism (Fig.
10B), and this is called a one interfa-
cial/one pullout (IF/PO) failure. When
the nugget size was large enough, the
base metal fractured during the tensile
process (Fig. 10C), and this is called a
base metal fracture (BMF) failure.
Figure 11A shows the typical load-
displacement curves of the Type IV
weld joint that interfacially failed. The
load-displacement curve has two
peaks. The first drop of the load corre-
sponds to the initial formation of a
crack, as shown in Fig. 11, B1. Figure
11C corresponds to the second peak. It
can be seen that the middle sheet was
pulled out along the tensile direction
— Fig. 11, C1. The nugget was
squeezed (Fig. 11, C2) by the middle
sheet because of the movement of
middle sheet. At the same time, cracks
DC=3tID
P
FL
WN
cosIF
cosPO
8
()
Fig. 7 — Effect of button size on the peak load and energy of
joint designs I and II in the 1.0/1.0/1.0 mm stack.
Fig. 8 — Macro/microstructures of Type III weld joints in the
1.0/1.0/1.0 mm stack that failed in A, B, C — IF mode; D, E, F — PO
mode.
Fig. 9 — Effect of button size on the peak load and energy absorp
tion of the Type III joint in the 1.0/1.0/1.0 mm stack.
A
B
C
D
E
F
Yang Li Dec 2015120-REV.qxp_Layout 1 11/9/16 5:25 PM Page 483

formed and propagated at both of the
two interfaces. Figure 11, C3 shows
the deformed EGZ. The nugget can be
considered as experiencing work hard-
ening. It can be seen that the micro-
hardness of EGZ increased with an in-
creasing deformation degree. This
causes the load to rise again. Figure
11D is the stage when only one inter-
face failed. Figure 11, D1 shows the
fracture occurred in the interior of the
LCGZ. It is interesting to note that on
the other interface, the deformed
grain induced the crack propagated
into the interior of weld nugget — Fig.
11, D2. As a consequence, a failed weld
joint in the final stage of the load-
displacement curve as shown in Fig.
11A is formed.
The load-displacement curve of the
Type IV weld joint that failed in the
IF/PO mode is similar to that in the
DIF mode. Figure 12 shows the load-
displacement curve and microstruc-
tures of the Type IV weld joint that
failed in the BMF mode. The curve has
a “platform,” which indicates that the
crack is propagating in the base metal
and, therefore, the load is relatively
stable. The weld nugget had very small
deformation during the tensile process
compared with those that failed in the
DIF and IF/PO modes. This indicates
that the weld nugget was large enough
to resist being squeezed by the middle
sheet and therefore the crack formed
around the edge of the weld nugget
and then propagated to the base metal.
Figure 13 shows the effect of weld-
ing time on the peak load and energy
absorption of joint design IV. The ener-
gy absorption in the Type IV joint is de-
fined by the area under the load-
displacement curve up to the second
peak. Although a welding time of 250
ms should be used to guarantee the
BMF mode, the peak load and energy
absorption were almost unchanged af-
ter a welding time of 200 ms. This is be-
cause the hardening of the weld nugget
compensated the relatively small size of
the weld nugget when the weld joint
failed in the DIF and IF/PO modes.
From the macroscopic photos of the
failed joint and the microstructure of
the failed weld joint, it can be seen that
the weld nugget either deformed (DIF
and IF/PO modes) or became invisible
(BMF mode) after the joint failed. As a
result, the accuracy of the button size
cannot be measured from the failed
weld joint, and the effect of button size
on the peak load and energy of joint
design IV cannot be obtained.
Failure Mode Transition in Three
Unequal Thickness Stacks
The overall failure rules of the
1.5/1.0/2.0 mm stack were similar to
that of 1.0/1.0/1.0 mm stack. Figure
14 shows the macrostructures of weld
joints in 1.5/1.0/2.0 mm stack. For all
four types of joints, the IF failure loca-
tion moved from LCGZ to EGZ. In
fact, no obvious LCGZ formed in the
1.5/1.0/2.0 mm stack. This is because
WELDING RESEARCH
WELDING JOURNAL / DECEMBER 2016, VOL. 95484-s
Fig. 10 — Photos of failure modes of the Type IV joint in the 1.0/1.0/1.0 mm stack: A —
Double interfacial failure; B — one interfacial/one pullout failure; C — base metal frac
ture failure.
Fig. 11 — Typical loaddisplacement curve and microstructures of Type IV weld joints in
the 1.0/1.0/1.0 mm stack which failed by the interfacial mechanism (18 kA, 200 ms).
A
A
A
B
B1
C1 C2 C3
D1 D2 D3
B2 B3
C
Yang Li Dec 2015120-REV.qxp_Layout 1 11/9/16 5:26 PM Page 484

the domed electrodes and hard norm
welding parameters lead to a more
concentrated heat generation and less
heat dissipation, which is beneficial to
inhibiting the formation of LCGZ, as
discussed in the section of “joint mi-
crostructure.”
For the Type I joint, the PO failure
location was located at the SCGZ. All
the Type II joints failed in IF mode, al-
though the button size reached to
about 10 mm. This will be discussed in
the following text. For the Type III
joint, the PO failure location was the
PMZ. For the Type IV joint, the BMF
failure location was also located at the
PMZ.
Figure 15 shows the effect of but-
ton size on the peak load of joint de-
signs I, II, and III. Similar to the
1.0/1.0/1.0 mm stack, good linear re-
lationships exist between the peak
load and button size. The critical but-
ton sizes for the Type I joint and Type
III joint were 9.1 and 8.2 mm, respec-
tively. However, all the Type II joints
failed in IF mode during the tensile-
shear test. The failure mode of joint
design IV in the 1.5/1.0/2.0 mm stack
was similar to that in the thickness
combination of 1.0 /1.0 /1.0 mm. The
critical button size was about 6.2 mm,
which is nearly the same as that in the
1.0/1.0/1.0 mm stack (6.25 mm). This
indicates that for the joint design of
pure shear, the critical weld nugget
size or button size may be controlled
by the thickness of the middle sheet.
Analytical Model to Predict
Failure of ThreeSheet
Aluminum Spot Welds
Pouranvari et al. proposed a simple
analytical failure model for the RSW of
steel (Refs. 21, 22). However, the weld
rotation was not considered in their
mode. In this paper, an analytical
model considering the weld rotation
was developed based on weld area
stress analysis.
VandenBossche analyzed the stress
distribution when a spot weld failed in
the IF and PO modes (Ref. 23). As
shown in Fig. 16B, once the weld ro-
tates, the load on the weld interface
can be decomposed to two compo-
nents: the force N normal to the faying
surface and the force S parallel to it.
They are related to F by
S=Fcos(1)
N=Fsin(2)
In the tensile-shear test, the driv-
ing force for the IF mode is the shear
stress at the sheet/sheet interface
(Ref. 24). The shear load S generates a
shear stress Sdistributed across the
interface. If the average value of the
shear stress is V/A, then the maximum
value is (Ref. 23)
where IF is the weld rotation angle
when the joint experiences IF failure.
The driving force for the PO mode
is the tensile stress around the nugget
(Ref. 24). As shown in Fig. 16C, the
tensile stress due to S is
τSMAX
IF =3S
2A=6FcosθIF
πd23
(
)
WELDING RESEARCH
DECEMBER 2016 / WELDING JOURNAL 485-s
Fig. 12 — Typical loaddisplacement curve of Type IV weld joints in the 1.0/1.0/1.0
mm stack that failed in BMF mode (22 kA, 200 ms).
Fig. 13 — Effect of welding time on the peak load and energy of joint design IV in the
1.0/1.0/1.0 mm stack (20 kA).
Yang Li Dec 2015120-REV.qxp_Layout 1 11/9/16 5:26 PM Page 485

WELDING RESEARCH
WELDING JOURNAL / DECEMBER 2016, VOL. 95486-s
where PO is the weld rotation angle
when joint experiences PO failure.
The rotation models of the four types
of joints are schematically shown in
Fig. 17. It is obvious that the above
equations can be applied to the joint
Types I and II directly. For joint Type
III, although both the two interfaces
bear the tensile-shear load during the
tensile-shear test, each interface bears
the total tensile-shear load not the
half of it. Accordingly, the above equa-
tions are also suitable for the Type III
joint. Joint IV experiences pure shear
during the tensile-shear test, i.e., the
joint will not rotate during the test.
The above model is not suitable for
joint Type IV.
When the maximum shear stress
exceeds the shear strength of the weld
nugget, a crack will form at the weld
interface. This moment corresponds
to where the tensile-shear force
reached its peak, as shown in Figs. 5,
6. Letting the maximum shear stress
equal to the shear strength of the weld
nugget, and then the failure load at
the IF mode FIF can be expressed as
where dis the weld nugget, and WN is
the shear strength of the weld nugget.
For a three-sheet RSW, dshould be re-
placed by dIN, which is the weld nugget
diameter at the failure interface. Con-
sidering that the aluminum spot welds
are more sensitive to porosity or voids,
a porosity factor Pcan be introduced
into Equation 5 (Ref. 19)
where P= (Atotal - Aporosity)/Atotal. Atotal is
the area of the fusion zone on the frac-
ture surface and Aporosity is the area of
the porosity on the fracture surface of
the weld.
Letting the shear stress equal the
tensile strength of the pullout failure
location, then the peak load for a weld
to fail in the pullout mode under the
tensile-shear test can be approximated
as
where tID is the local sheet thickness
around the nugget accounting for in-
dentation, and σFL is the shear
strength of the failure location.
In order to ensure pullout failure
for a spot weld, the failure load for a
PO failure should be less than that for
IF failure, i.e., FPO < FIF. Thus, the criti-
cal nugget diameter DCcan be ob-
tained from Equations 6 and 7.
Applying the linear relationship be-
tween the strength and hardness, and
the linear approximate between shear
strength and tensile strength, Equa-
tion 8 can be rewritten as
where HFL is the hardness of the failure
location, HWN is the hardness of the
weld nugget, and fis a constant coeffi-
cient. For aluminum alloys, fis about
0.6 (Ref. 25). In this study, HWN should
be replaced by HLCGZ because the fail-
ure location of the IF mode occurred in
the LCGZ. Therefore, Equation 9 can
be rewritten as a more widely applica-
ble form
FIF =πd2
6cosθIF
τWN 5
(
)
FIF =P
πdIN
2
(
)
6cosθIF
τWN 6
(
)
FPO =dINtID
2cosPO
FL 7
()
DC=3tID
P
FL
WN
cosIF
cosPO
8
()
DC=3tID
Pf
HFL
HWN
cosθIF
cosθPO
9
(
)
σS
PO =S
A=S
πdt / 2=2FcosθPO
πdt 4
(
)
Fig. 14 —Macrostructures of weld joints in 1.5/1.0/2.0 mm stack: A — IF failure in Type I
joint (18 kA); B — PO failure in Type I joint (32 kA); C — IF failure in Type II joint (18 kA); D —
IF failure in Type II joint (34 kA); E — IF failure in Type III joint (18 kA); F — PO failure in Type
III joint (26 kA); G — DIF failure in Type IV joint (18 kA); H — BMF in Type IV joint (22 kA).
BA
D
C
FE
HG
Yang Li Dec 2015120-REV.qxp_Layout 1 11/9/16 5:27 PM Page 486

WELDING RESEARCH
DECEMBER 2016 / WELDING JOURNAL 487-s
where HPO is the hardness of pullout
failure location, and HIF is the hard-
ness of interfacial failure location.
Apply Equation 10 to the Types I
and II joints of the 1.0/1.0/1.0 mm
stack, The failure location in the PO
mode was the BM. The average hard-
ness of the BM was 95 Hv. In the case
of Types I and II joints, the failure lo-
cation of the IF mode was the failure
location in the interior of the LCGZ,
where the porosity is hard to form.
Thus, the Aporosity equals 0 and Pis 1.
The average indentation was about
70% of the original sheet thickness,
i.e., tID was 0.7 mm. The rotation angle
was measured after the tensile-shear
test. It was nearly zero when the joint
failed in the IF mode (16 kA, 200 ms),
while it was 2 deg when the joint failed
in the PO mode (20 kA, 200 ms).
The critical nugget diameter for
Types I and II joints can be obtained as
follows:
It can be seen that the predicted
value is very close to the experimental
result of 5.9 mm.
In the case of a Type III joint, the
failure location in the IF mode was still
in the interior of LCGZ or along the in-
terface between the LCGZ and SCGZ.
Note that the failure location in the
PO mode changed to PMZ, as shown
in Fig. 9. The average indentation was
about 90% of the original sheet thick-
ness, i.e., tID was 0.9 mm. The average
rotation angle was about 2 deg when
the joint failed in the IF mode (16 kA,
200 ms), while it was 7 deg when the
joint failed in the PO mode (18 kA,
200 ms). Thus, the critical nugget di-
ameter for the Type III joint can be ob-
tained as follows:
The result is a little larger than the ex-
perimental value (5.1 mm).
Equations 1–11 are not suitable for
the Type IV joint because the failure
mode of the Type IV joint is different
from the other types of joints. This pa-
DC
()
Types I& II =3tID
Pf
HBM
HLCGZ
cosIF
cosPO
=30.7
0.6
95
55
1
cos2deg
6.0mm 11A
()
310
)
(
=θ
θ
Dt
Pf
H
H
cos
cos
CID PO
IF
IF
PO
DC
()
Type III =3tID
Pf
HPMZ
HLCGZ
cosIF
cosPO
=30.9
0.6
65
55
cos2deg
cos7deg
5.3 mm 11B
()
Fig. 15 — Effect of button size on the peak load of the Types I, II, and
III joints for the 1.5/1.0/2.0 mm stack.
Fig. 16 — Stress analysis in the weld area: A — Weld rotation; B
— IF failure; C — PO failure (Ref. 23).
ABC
Fig. 17 — Schematic of joint rotation in the 1.0/1.0/1.0 mm stack: A — Type I joint; B —
Type II joint; C— Type III joint; D — Type IV joint.
B
A
C
D
Yang Li Dec 2015120-REV.qxp_Layout 1 11/9/16 5:27 PM Page 487

per constructs a model for predicting
the failure mode for the Type IV joint.
From the above discussion, it can
be known that in the DIF failure of the
Type IV joint, one interface fails
through the LCGZ while the other in-
terface fails through the interior of the
weld nugget, i.e., through the EGZ.
For simplification, the deformation
and the work hardening of the weld
nugget were ignored. The maximum
shear stress at each interface is
Accordingly, the failure load at the
DIF mode can be expressed
where EGZ is the shear strength of the
equiaxed grain zone, and LCGZ is the
shear strength of the columnar grain
with a large secondary dendrite arm
spacing.
In order to determine the mathe-
matical equation of the failure load for
the BMF mode, Fig. 18 depicted the
failure analysis of the BMF in the Type
IV joint. LOis the length of overlap-
ping, which is equal to the width of
the workpiece W. The failure location
around the weld nugget of the BMF
mode was the PMZ as shown in Fig.
12. Thus, the failure load for the BMF
mode can be expressed as
Note that the thickness tin Equa-
tion 14 is the thickness of the middle
sheet, which was not influenced by the
indentation.
Combining Equations 13 and 14,
the following equation can be obtained
The solution for Equation 15 is
Applying the linear relationship be-
tween the strength and hardness, and
the linear approximation between the
shear strength and tensile strength,
Equation 16 can be rewritten as
Using W= 25 mm, t= 1 mm, f= 0.6,
HPMZ = 65 Hv, HBM = 95 Hv, HEGZ = 60
Hv, and HLCGZ = 55 Hv, the critical
nugget diameter of the Type IV joint is
Although the predicted value is small-
er than the experimental result (about
6.25 mm), based on the experimental
results, when the button size was
about 6 mm (the corresponding weld-
ing parameters were 20 kA and 200
ms), both the peak load and energy ab-
sorption of weld joints were similar to
the weld joints that failed in BMF
mode. Accordingly, the predicted re-
sult is also acceptable for the Type IV
joint.
For the Type I joint of the
1.5/1.0/2.0 mm stack, the failure loca-
tion in the PO mode was the SCGZ.
The average hardness of the SCGZ was
85 Hv. The increased hardness in the
SCGZ maybe due to the hard norm of
welding parameters. The failure loca-
tion in the IF mode was the EGZ. The
average hardness of the EGZ was 60
Hv. The average indentation was about
85% of the uppersheet i.e., the tIN was
about 1.3 mm. The rotation angle was
nearly zero when the joint failed in the
IF mode, while it was about 2 deg
when the joint failed in the PO mode.

3EGZ +
3LGGZ




d2
+1
2tBM 
2tPMZ




d
W
2tBM +3W
4tBM




=015
()
FBMF =3
2
W
2tBM +d
2tPMZ
+Wd
2tBM 14
()
DC
()
Type IV =
=1
2tBM 
2tPMZ




+
1
2tBM 
2tPMZ





2
+4+
3
EGZ +4+
3LCGZ





W
2tBM +3W
4tBM





2+
3
EGZ +2+
3
LCGZ
DC
()
Type IV =
3
4
tHPMZ tHBM
()
+3
2
1
2tHBM 
2tHPMZ





2
+4+
3fHEGZ +4+
3fHLCGZ





W
2tHBM +3W
4tfHBM





fHEGZ +fHLCGZ
DC
()
Type IV 6.0mm
τSIMAX =3F/2
2A=3F
πd212
(
)
FDIF =
πdID
2
(
)
3τLCGZ +
πdID
2
(
)
3τEGZ 13
(
)
WELDING RESEARCH
WELDING JOURNAL / DECEMBER 2016, VOL. 95488-s
Fig. 18 — Failure analysis of the BMF in the Type IV joint in the
1.0/1.0/1.0 mm stack. Fig. 19 — Effect of joint design on the failure mode transition.
(16)
(17)
Yang Li Dec 2015120-REV.qxp_Layout 1 11/9/16 5:28 PM Page 488

The critical nugget diameter for the
Type I joint can be obtained as follows
This predicted value is very close to
the experimental result of 9.1 mm.
For the Type II joint of the
1.5/1.0/2.0 mm stack, all the joints
failed in IF mode, assuming that the
PO failure location of Type II joint is
the PMZ and the rotation angle is the
same as Type I joint. Note that the IF
failure location was the EGZ rather
than the LCGZ because the nugget will
shift to the thicker sheet. The critical
nugget diameter for Type II should be
However, the maximum button size
obtained from experiments was about
10 mm. Therefore, the prediction for
the Type II joint is also reasonable.
For the Type III joint of the
1.5/1.0/2.0 mm stack, the IF failure
location was the LCGZ, while the PO
failure location was the PMZ. The av-
erage indentation was about 90% of
the original sheet thickness, i.e., tID
was 1.35 mm. The average rotation an-
gle was about 3 deg when the joint
failed in the IF mode, while it was 10
deg when the joint failed in the PO
mode. Thus, the critical nugget diame-
ter for the Type III joint can be ob-
tained as follows:
The predicted value is very close to the
experimental result of 8.2 mm.
For the Type IV joint of the
1.5/1.0/2.0 mm stack, it can be seen
that in the DIF failure (Fig. 14), both
of the two interfaces failed through
the EGZ. Accordingly, all the HLCGZ in
Equation 17 should be replaced by
HEGZ. Using W= 25 mm, t= 1 mm, f=
0.6, HPMZ = 75 Hv, HBM = 95 Hv, and
HEGZ = 60 Hv, the critical nugget diam-
eter of the Type IV joint is
The predicted value is close to the pre-
dicted result of the 1.0/1.0/1.0 mm
stack. This is reasonable because the
BMF failure is dependent on the prop-
erty of the middle sheet. Since the
middle sheets in the two thickness
combinations were the same, the ex-
perimental and predicted results
should be similar.
Effect of Joint Design on the
Failure Mode Transition
The effect of joint design on the
failure mode transition is shown in
Fig. 19. The data point for the Type II
joint in the 1.5/1.0/2.0 mm stack
comes from the predicted results.
For the 1.0/1.0/1.0 mm stack, the
tendency to fail in the IF mode is in-
creased in the order Type III, Types I
and II, and Type IV. This is consistent
with Pouranvari and Marashi’s work
(Ref. 2). The failure of the weld joint is
the competition between shear stress
at the sheet/sheet interface (i.e., IF
failure) and the tensile stress at the
nugget circumference (i.e., PO failure)
(Ref. 20). The higher the shear stress
at the sheet/sheet interface, the high-
er the tendency to fail in the IF mode.
The Type III joint has the maximum
rotation angle and the minimum shear
stress at the sheet/sheet interface.
Therefore, it has the minimum critical
diameter DCto fail in the PO mode. In
contrast, the sheet/sheet interfaces in
the Type IV joint experienced pure
shear. The weld joint has virtually no
rotation and therefore, it has the
largest critical diameter DCto fail in
the PO mode (BMF mode).
For the 1.5/1.0/2.0 mm stack, the
tendency to fail in the IF mode is in-
creased in the order of Type III, Type
IV, Type I, and Type II. Without con-
sidering the Type IV joint, i.e., the
pure shear condition, the failure rules
for the two thickness combinations
are similar. The Type III joint experi-
enced the maximum rotation while the
Type II joint has the minimum rota-
tion angle. However, although the
Type IV joint experienced pure shear,
the strength of the middle sheet was
lower than the shear strength of two
sheet/sheet interfaces. Therefore, for
the three unequal thickness stacks, the
thickness of the middle sheet should
control the critical weld nugget size of
pure shear joint.
Conclusions and Future
Work
In this paper, the failure mode tran-
sition of three-sheet aluminum alloy
resistance spot welds (RSWs) during
tensile-shear tests were investigated
through experiments and an analytical
model. Four types of joints were inves-
tigated. The following conclusions can
be drawn:
1) The microstructure in the three-
sheet 6061 aluminum alloy RSWs con-
sists of a partially melted zone (PMZ),
columnar grain zone (CGZ), and
equiaxed grain zone (EGZ), where the
columnar grain zone is divided into
the columnar grain with large second-
ary dendrite arm spacing (LCGZ) and
the columnar grain with small second-
ary dendrite arm spacing (SCGZ). The
hardness test indicates that the LCGZ
has the lowest hardness.
2) Three failure modes in Types I,
II, and III joints, named the interfacial
(IF) failure, partial thickness-partial
pullout (PT-PP) failure, and pullout
(PO) failure, were observed. There is
no critical welding parameter or
nugget diameter to separate the PT-PP
and PO failures. The formation of the
LCGZ in the weld nugget contributes
to the PT-PP failure. There is a compe-
tition between the two interfaces in
the Type III joint, and failure will occur
on the weaker one.
3) Three failure modes in the Type
IV joint, named the double interfacial
(DIF) failure, one interfacial/one pull-
out (IF/PO) failure, and the base metal
fracture (BMF) failure were identified.
In the case of the DIF and IF/PO fail-
ures, the nugget was squeezed and ex-
perienced work hardening. In the DIF
failure, one interface failed through the
LCGZ first, and then the other inter-
face failed through the interior of the
weld nugget. In the case of IF/PO fail-
ure, the weld nugget experienced less
deformation due to its larger nugget
size. In the case of BMF failure, the
weld nugget had a very small deforma-
tion and the crack formed around the
edge of the weld nugget and then prop-
agated to the base metal.
4) The LCGZ is the weak area in
DC
()
Type III =3tID
Pf
HPMZ
HEGZ
cosIF
cosPO
=31.35
0.6
75
60
cos 3deg
cos 10 deg
8.4mm
DC
()
Type IV 6.0mm
DC
()
Type II =3tID
Pf
HPMZ
HEGZ
cosIF
cosPO
=31.8
0.6
75
60
1
cos 2deg
11.6mm
DC
()
Type I =3tID
Pf
HSCGZ
HEGZ
cosIF
cosPO
=31.3
0.6
85
60
1
cos 2deg
9.2mm
WELDING RESEARCH
DECEMBER 2016 / WELDING JOURNAL 489-s
Yang Li Dec 2015120-REV.qxp_Layout 1 11/9/16 5:28 PM Page 489

three-sheet aluminum alloy RSWs.
Cracks will form and propagate in the
interior of the LCGZ or along the in-
terface of SCGZ and LCGZ during the
tensile-shear test.
5) The following equations are pro-
posed to predict the critical nugget di-
ameter required to ensure PO
failure mode during the tensile-shear
tests of three-sheet aluminum alloy
spot weld joints
where tis thickness of the middle
sheet, tID is the sheet thickness consid-
ering the indentation, Wis the width
of the sheet, Pis the porosity factor, f
is a constant coefficient, IFis the rota-
tion angle when the joint fails in the IF
mode, PO is the rotation angle when
the joint fails in the PO mode, HFL is
the hardness of the failure location,
HLCGZ is the hardness of the columnar
grain with a large secondary dendrite
arm spacing, HBM is the hardness of the
base metal, HPMZ is the hardness of the
partially melted zone, and HEGZ is the
hardness of the equiaxed grain zone.
6) The joint design has a significant
effect on the failure mode transition.
For three equal-thickness sheet RSWs,
the critical weld nugget diameter (DC)
required for obtaining a PO failure
mode during the tensile-shear test in-
creases in order of Type III, Types I
and II, and Type IV. For three unequal-
thickness sheet RSWs, the DCmay be
controlled by the thickness of the mid-
dle sheet for the joint design of pure
shear.
This paper preliminarily investi-
gates the failure behavior of triple-
thin-sheet aluminum alloy resistance
spot welds under tensile-shear loads.
There are many contents that need
further research. More thickness com-
binations should be tested to verify
the proposed analytical mode. The
failure behaviors of spot welds under
other loading conditions, such as
coach peel and cross tension, are im-
portant issues and need to be studied.
The failure behaviors of spot welds of
other materials, such as 5000 series al-
loys, or dissimilar materials, such as
5000 series alloys to 6000 series al-
loys, are valuable to pursue.
This research is supported by the
National Nature Science Foundation
of China (Grants 51405334 and
51275342).
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DC
()
Type IV =
3
4
tHPMZ tHBM
()
+3
2
1
2tHBM 
2tHPMZ





2
+4+
3fHEGZ +4+
3fHLCGZ





W
2tHBM +3W
4tfHBM





fHEGZ +fHLCGZ
DC
()
Types I&II &III =3tID
Pf
HFL
HLCGZ
cosIF
cosPO
WELDING RESEARCH
WELDING JOURNAL / DECEMBER 2016, VOL. 95490-s
References
Acknowledgments
Yang Li Dec 2015120-REV.qxp_Layout 1 11/9/16 5:29 PM Page 490