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WELDING RESEARCH
SUPPLEMENT TO THE WELDING JOURNAL, MARCH 2016
Sponsored by the American Welding Society and the Welding Research Council
Resistance Spot Weldability of Deformed
TRIP800 Steel
Various parameters were investigated for obtaining optimum tensile shear strength,
weld button geometry, and electrode indentation
BY H. E. EMRE AND R. KAÇAR
ABSTRACT
In order to simulate automotive manufacturing conditions, the spot weldability of
cold­deformed, in tension by 10%, TRIP800 steels was investigated in this study. Tensile
shear strength and failure mode associated with button geometry and electrode indenta­
tions were also evaluated, and weld lobe was drawn for resistance spot­welded de­
formed TRIP800 steel. The microstructure of the weldment was evaluated and the hard­
ness profile of the weldment was also determined. It was found that the properties of
the weldment are directly related to parameters used for the process and deformation of
base metal prior to welding. It was found that the button diameter and button size ratio
of deformed TRIP800 steel spot welds should be at least 4√t and 0.14–0.30, respectively,
for pullout failure mode, acceptable shear strength, and surface quality.
KEYWORDS
• Resistance Spot Weld • Cold Deformation • Welding Parameters
• Weld Lobe Diagram
Introduction
Transformation-induced plasticity
steel (TRIP) is an advanced highstrength steel (AHSS). It has been applied widely in the automotive industry to reduce weight and improve vehicle safety (Refs. 1–3). Its structure
usually consists of bainite, martensite,
and retained austenite embedded in a
continuous soft ferrite matrix (Refs.
4–8). Decomposition of the austenite
phase into martensite during plastic
deformation improves ductility (Refs.
3, 8–11). This phenomenon involves
formation of strain-induced martensite by deformation of metastable
austenite and leads to an increase of
strength, ductility, and toughness of
the steel (Refs. 12, 13).
Among the welding methods used in
the automotive industry, resistance
spot welding is the most widely used
joining technology for manufacturing
auto body structures. In practical applications, steels are frequently subjected
to deformation prior to welding. As
mentioned earlier, during plastic deformation and straining, the retained
austenite phase is also transformed into
martensite in TRIP steel. High elongation values of the TRIP steels are explained by the interactions of the deformation and phase transformation
mechanisms during straining of the material (Ref. 4). The effectiveness of the
deformation can be reduced significantly during welding in the heat-affected
zone (HAZ), where the peak temperatures are too low to cause melting but
high enough to cause a change in the
microstructure and properties of the
materials (Ref. 14). The mechanical
properties of the cold-deformed TRIP
steels can be also affected by the phase
transformations associated with the
thermal cycle during welding.
An extensive literature survey found
no study about spot weldability of colddeformed TRIP steel. In order to simulate automotive manufacturing conditions, the spot weldability of deformed
TRIP800 steel was investigated in detail. For this purpose, the tensile shear
strength, weld button geometry, and
electrode indentation were determined
for a quality assessment of the weldment and obtained weld lobe diagram.
The results were compared with the
weldability of as-received welded
TRIP800 samples that were published
elsewhere (Ref. 15). The hardness profile and structure of the weldment were
also evaluated for optimization of the
welding parameters.
Experimental Method
Materials and Welding
Processes
The chemical composition of
TRIP800 steel sheet in 1.5 mm thickness is given in Table 1. The deformation in tension by 10% was carried out
by using a Shimadzu-type tensile test
machine with a cross-head speed of 5
mm/min.
Test samples were spot welded in a
pneumatic, phase-shift controlled AC
spot welding machine with 60 kVA capacity in 50 Hz electrical circuit. Spherical tip electrodes (CuCrZr) having 5.5
mm diameter were used. The welding
schedule applied in the experiment is
H. E. EMRE (hayriyeertek@karabuk.edu.tr) and R. KAÇAR are with the Department of Manufacturing Engineering, Karabük University,
Karabük, Turkey.
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Table 1 — Chemical Composition of TRIP800 Steel (wt­%)
Elements
C
Si
Mn
P
S
Cr
Mo
Al
Fe
TRIP800
0.2
1.66
1.69
0.015
0.0002
0.006
0.011
0.43
Balance
Table 2 — Welding Parameters
Welding
Current
(kA)
Welding
Time
(cycles)
Force
(kN)
Squeeze
Time
(cycles)
Hold
Time
(cycles)
1
3
5
6
7
8
9
10
5
10
15
20
25
6
15
15
ton diameter (dn), button height (hn),
and electrode indentation depth (ei)
— was measured on the metallographic cross section of the weldment. The
common criterion for the average weld
button diameter should be equal to or
larger than 4√t (t: material thickness
in mm) for desired pullout failure (PF)
mode for steels. However, there is no
information on average weld button
diameter, weld button height, and
weld button size ratio (hn/dn) for colddeformed and welded samples for producing the desired PF mode.
Note: 1 cycle = 0.02 s
Results
A
B
Effect of Deformation on the
Structure of Weldment
C
D
E
Fig. 1 — Microstructure of TRIP800 steel. A — As­received base metal; B — cold­
deformed base metal; C — SEM microstructure of as­received TRIP800 steel; D — SEM
microstructure of cold­deformed base metal; E — SEM microstructure of cold­deformed
base metal at high magnification (M: martensite, A: austenite, : ferrite, b: bainite).
given in Table 2.
Mechanical Test and
Metallographic Evaluation
All the welded joints were exposed
to a tensile-shear test for determining
joint strength. For this purpose, five
test samples were prepared for each
weld variable. Samples were tested
with a crosshead speed of 10 mm/min.
A sample was cross sectioned
through the center of the button,
mounted, ground, polished, and then
etched for 5 s with 2% nital. The weldment structure was evaluated by optical and scanning electron microscope
(SEM). The hardness measurements
were carried out by using a Shimadzu
microhardness tester with a load of
500 g.
Weld Button Geometry
Evaluation
The weld button geometry — but-
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The microstructure of the asreceived TRIP800 steel contains bainite, martensite, and retained austenite phases embedded in a continuous
ferrite matrix, as seen in Fig. 1A and
C. The microstructure of the asreceived samples was taken from the
vertical cross section in the rolling direction. However, the cold-deformed
samples exhibited elongated grains
along the strain direction, as seen in
Fig. 1B. Most of the retained austenite
in the structure was transformed into
martensite (Fig. 1D) due to cold deformation process. A small amount of retained austenite grains were observed
in the structure, only under high magnification — Fig. 1E.
The as-received and cold-deformed
TRIP steels were also analyzed by using x-ray diffractometer (XRD) to clarify the present phases in the structure
— Fig. 2. As-received TRIP800 steel
analysis revealed peaks for austenite,
ferrite, and martensite, which are the
main constituents also observed using
metallographic examinations. However, deformed sample analysis revealed
peaks for ferrite and martensite. The
XRD result for cold-deformed
TRIP800 steel indicated no evidence of
austenite peaks, hence, it is thought
that the majority of retained austenite
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WELDING RESEARCH
A
A
B
C
Fig. 3 — Microstructure across the resistance spot weld of 10% cold­deformed
TRIP800 steel. A — Weld nugget; B — HAZ; C — grain growth in HAZ.
ments within the fusion zone
produces a sharp gradient in the
B
composition distribution
through microsegregation (Ref.
16). In TRIP steels, due to the
strong affinity for oxygen, the
added Al and Si readily form oxides during welding, while leaving the weld pool depleted of
these elements (Refs. 17, 18). It
is also known that the reaction
between the dissolved alloying
elements in the weld pool with
the available oxygen, nitrogen,
and carbon forms nonmetallic
inclusions (Ref. 18). Inclusions
were observed in the weld
nugget and HAZ.
The energy-dispersive spectroscopy (EDS) analysis of incluFig. 2 — XRD results. A — As­received; B — cold­
sions is presented in Fig. 4. The
deformed TRIP800 steel.
result indicates that inclusions
could be a mixture of aluminum
and silicon oxide — Fig. 4A and
transformed into martensite.
B. Similar oxide inclusions were obThe microstructure of the weldment
served in GTA welded high-Al and
was also evaluated and results are
high-Si TRIP steels (Ref. 18).
shown in Fig. 3. The structure of the
weld button contains the martensite
phase due to its high cooling rate — Fig.
Effect of Deformation and
3A. The structure of the HAZ transWelding Parameters on
formed to predominantly martensite
Properties and Failure Mode
with small areas of ferrite, lath bainite,
and retained austenite, depending on
The macrographs of the failure
the ultimate temperature reached and
mode of the tested samples are shown
cooling rate — Fig. 3B and C.
in Fig. 5. Three distinct failure modes
Results imply that an increase in
were observed (Refs. 19–21).
welding current and time increases heat
1) Interfacial failure (IF), in which
input, which leads to expansion of the
fracture propagating through the fuHAZ region. Since the deformation desion zone was observed for all welding
creases the recrystalization temperature
currents at 5 cycles and for all welding
of the steel, grain growth proceeds more
times at 1 kA.
rapidly. Briefly, the deformation process
2) Pullout failure (PF), in which failalso extends to the HAZ.
ure occurs by the effect of the withIn the case of a very rapid cooling of
drawal of the weld button from a sinthe spot weldment, an insufficient difgle sheet, was observed from 3 to 9 kA
fusion occurs in the precipitated solid
welding currents, at 15 to 25 cycles
crystals and the remaining liquid, so
welding time. The fracture of the weld
the inhomogeneity of the alloying ele-
made with high expulsion current is
through the full thickness of the base
metal. The nugget diameter exceeds
the electrode diameter and the fracture occurs outside the indented area
(PF mode) (Ref. 22).
3) Partial interfacial failure (PIF), in
which the fracture first propagates in
the fusion zone, was observed for 5 cycles from 5 to 10 kA and 10 kA at all
welding times. With higher welding
times at higher welding currents, such
as 10 kA, the amount of molten metal
increases and fused metal ejects and
the weld nugget diameter starts to decrease, so the fracture occurs at the
edge of the button through the fusion
zone and causes IF or PIF modes. No
satisfactory joint is expected due to excessive heat input over 11 kA. The
strength of the weldment varies with
failure types. It increases in the order
of IF, PIF, and PF modes.
In determining the effect of welding parameters on the strength of deformed steel, all welded samples were
tensile shear tested in order to evaluate the weld quality. Test results are
shown in Fig. 6A and B.
As seen in Fig. 6A and B, the maximum strength is obtained for 9 kA
welding current at 20 cycles welding
time. It was found that the strength
increased with increasing welding time
and current up to maximum level,
then it gradually decreased due to increased molten metal and spreading of
fused metal as well as uneven electrode indentation. The desired button
diameter can only be obtained by
proper adjustment of welding current
vs. welding time. When time is short,
the button diameter decreases. On the
contrary, when it is long, the amount
of molten metal increases and fused
metal spreads out and as a result the
strength of the welding joint decreases
(Ref. 25). Kimchi also reports that exMARCH 2016 / WELDING JOURNAL 79-s
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tic deformation. This is consistent with
the results obtained from the tensile
shear test. It is believed that increased
hardness in the weld button increases
the strength of the weldments. However, the hardness of the cold-deformed
sample decreased in the HAZ.
The HAZ of the as-received samples
(450–500 HV0.5) had slightly higher
hardness than the cold-deformed samples (424–475 HV0.5) for the same
welding parameters, as seen in Fig. 7.
It may be attributed to the severe deformation that decreases the recrystalization temperature; this, in turn,
leads to the coarsening of grain in the
HAZ during welding. In other words,
work hardening is mostly wasted in
the HAZ during recrystallization and
grain growth.
A
B
Effect of Deformation and
Welding Parameters on the
Weld Button Geometry
Fig. 4 — EDS analysis of nonmetallic inclusions. A — 8 kA and 20 cycles; B — 9 kA and 25
cycles parameters in the fusion zone.
A
B
C
Fig. 5 — Failure mode of welded samples. A — IF; B — PIF; C — PF.
pulsion due to high heat input causes
loss of surface quality as well as
strength in the resistance spot weldment (Ref. 22).
Hardness measurements of samples
were also performed and results are
shown in Fig. 7A–D.
It was found that deformed TRIP800
base metal hardness values were ap-
proximately 100 HV0.5 higher than
those of the as-received sample due to
transformation of retained austenite
into martensite during plastic deformation. The weld button hardness of colddeformed samples increased as compared to as-received samples due to increasing volume of martensite phase
and grain refining with much more plas-
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The quality and strength of the weldment can be estimated by measuring its
button diameter (Ref. 23). Sun et al.
(Ref. 12) reports that AHSS spot welds
with a fusion zone size of 4√t (t = thickness of sheet, mm) cannot produce a
button with a desired pullout failure
mode for both DP800 and TRIP800
spot welds under lap shear loading. Kumar-Pal and Bhowmick (Ref. 24) claim
that the average button diameter
should be equal to or larger than 4√t for
button pull out failure mode in dualphase steels for a sheet thickness less
than 1.5 mm. The relationship between
button size and welding parameters was
determined and results are shown in
Fig. 8.
Figure 9 clearly shows that it is possible to achieve a desired PF mode
through a constant button diameter of
4√t by using welding parameters of
more than 10 cycles welding time and 3
kA welding current. The button diameter of the spot weld increased with increasing welding currents up to maximum level (i.e., 7 kA for 10 cycles), then
it gradually decreased. This phenomenon may be attributed to high heat input resulting in undesired expulsion,
which results in the loss of liquid metal
from the button.
It was also found that the button diameter increased while the button
height decreased. Because a higher
heat input produces a larger fused vol-
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WELDING RESEARCH
A
B
Fig. 6 — Tensile shear load bearing capacity of weldment. A — Effect of welding current; B — effect of welding time.
ume, the size of the button naturally
increases with increasing electrode indentation; therefore, the button
height decreases. The button diameter, or button height, is not enough to
determine the strength of spot welded
joints. However, the button size ratio
can be used for this purpose (Ref. 25).
Therefore, the relationship between
button geometry and welding parameters was determined for all the weldments, as shown in Figs. 9–11.
It was found that the button height
of the welded specimen decreased with
increasing welding parameters, except
for a welding time of 5 cycles — Fig. 9.
Meanwhile, the electrode indentation
depth appeared to increase with increased heat input — Fig. 10.
The button size ratio diagram associated with the welding parameters was
evaluated from Fig. 11. The (0.14–0.3)
button size ratio was found to be sufficient to achieve the desired PF mode.
The PIF mode observed when the ratio
was lower than 0.14 appears to be associated with the expulsion of molten
metal from the weld area resulting from
the high heat input, while the IF and
PIF modes associated with a ratio higher than 0.3 could be the result of low
heat input that is not sufficient enough
to create the required button and button size ratio.
Effect of Button Geometry on
the Tensile Shear Strength of
Samples
The strength associated with button geometry and electrode indenta-
tion depth was also determined and
results are shown in Figs. 12–15,
respectively.
As a result of an increase in button
diameter, the strength of the deformed samples increased. However,
the strength began to decrease after a
critical button diameter (i.e., 8-mm
weld button for 20 cycles) — Fig. 12.
By increasing welding current and
welding time, the amount of the fused
metal increased, so the button diameter
increased, and the button height nearly
reached the sheet thickness but then
decreased with increasing electrode indentation. The strength also increased
up to a limited value by decreasing button height and increasing electrode indentation depth, and then it started to
decrease due to thinning of the cross
section of the button as a result of high
heat input — Figs. 13 and 14.
When the button size ratio vs. tensile shear strength diagram was investigated, it was seen that with increasing
button size ratio, strength of the deformed samples decreased — Fig. 15.
Determining the Weld Lobe
A graphical explanation of the ranges
of welding parameters over which acceptable spot welds are formed at a constant electrode force is known as a “spot
weld lobe curve” (Ref. 26). The spot
welds defining the lower limit of the
lobe curve are undersize, resulting in
weaker nuggets. On the other hand, the
spot welds at the upper limit of the lobe
curve are large, resulting in severe expulsion, which decreases the weld
strength. The selection of the lower limit of the weld lobe is determined by the
specific demand of automakers. The
weld lobe diagram generally is developed with regard to the welding parameters vs. button size. However, some researchers developed a weld lobe diagram based on electrode indentation
depth and strength in resistance spot
welded steels (Refs. 25, 27).
In the present study, weld strength
was used as a criterion to assess the
weld quality. It can be seen from the left
limit of the diagram, when indentation
is small, low strength and button size
were achieved. High welding current
and weld time increase electrode indentation, which may exceed an acceptable
limit, causing severe expulsion (Ref. 27).
Weld strength reduces due to expulsion.
The right limit of weld lobe was drawn
according to 80% and 100% of maximum strength, while the left limit of
the weld lobe was established according
to indentation depth equal to about
25% of sheet thickness. The weld lobe
of deformed TRIP800 steel based on
welding parameters is shown in Fig. 16.
If higher strength (100%) is desired, 9 kA welding current and 15 cycles welding time should be applied.
However, 7 kA welding current and 25
or 20 cycles welding times are advised.
The 25% sheet thickness limit is exceeded in these conditions.
When a high surface quality rather
than strength is desired, the welding
current range of 3 to 4 kA and 20 to 25
cycles welding times should be applied. When the 80% of strength and
acceptable surface quality are desired,
the welding current range of 5–6 kA
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increasing density of the dislocations in the deformation
and the formation of Cottrell
atmosphere on the HAZ–base
metal interface during welding. Khodabakhshi et al. (Ref.
29) reported that different
rates of deformation applied
to low-carbon steel used in the
automobile industry caused
an increase in electrical resistance.
The strength of the colddeformed samples was determined to be higher than
the as-received ones using
the same welding parameters. More heat input raises
the temperature at the
electrode-sheet interface,
and more plastic deformaD
C
tion is generated even under
a constant electrode force
(Refs. 29, 30), and for this
reason the electrode indentation depth is found to be
higher in the cold-deformed
TRIP800 steel weldment. It
is believed that an increase
in electrode indentation
caused much more contact
surface between the watercooled electrode and weld
button so the cooling rate
increased in the colddeformed weldment. This
caused the weld button to
have more fine-grained microstructure and a higher
Fig. 7 — Hardness profiles. A — As­received samples produced by using 8 kA welding current; B —
hardness. The grain refining
cold­deformed samples by using 3 kA; C — 8 kA; D — 10 kA welding current.
increased the strength of the
weldment.
The width of the weld lobe
and 20–25 cycles welding times should
austenite transforms into a much
gives an indication of good welding pabe applied.
harder martensite phase and dislocarameters and the tolerance of the weld
tion density is increased. Therefore,
schedule in a production environment.
electron flow is prevented due to high
Discussion
Compared to the as-received welded
dislocation density and disordered
samples, weld lobe diagram in previcrystal structure, thus electrical resisWeld button diameter and weld
ous work (Ref. 15), the cold-deformed
tivity is increased. So, heat generation
button size ratio are the most critical
and then welded samples, weld lobe diin prestrained TRIP800 spot welds (Q
factors in weld quality in terms of
2
agram became narrow. The weld lobe
=
I
R
)
increased
with
the
same
weldpeak tensile shear load and failure
t
curves set back to lower welding curing parameters as compared with the
mode for TRIP800 spot welds. As comrents with the same welding time.
as-received ones, and more heat generpared to the as-received welded
ation leads to the formation of a larger
TRIP800 samples in previous work
weld area in the weldment.
Conclusions
(Ref. 15), the critical button diameter
Mukhopadhyay et al. (Ref. 28) invessize (4√t) for desired strength of 10%
From the results given above, the
tigated as-received resistance spot welds
for cold-deformed TRIP800 samples
following conclusions can be drawn:
and 5% prestrained BH (bake hardenwas found to be lower than as-received
1) The tensile shear strength of
ing) steels. They reported that prestrain
samples’ button diameter size (4.5√t).
the samples increased with heat input
samples’ strength was higher than the
It could be attributed to the plastic dedue to increased welding time and
as-received samples depending on the
formation of TRIP800 steels in which
A
82-s WELDING JOURNAL / MARCH 2016, VOL. 95
B
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WELDING RESEARCH
Fig. 8 — An effect of welding parameters on weld nugget diameter.
Fig. 9 — An effect of welding parameters on the weld nugget
height.
Fig. 10 — An effect of welding parameters on the electrode inden­
tation depth.
Fig. 11 — An effect of welding parameters on the weld nugget
size ratio.
current. However, after the critical
welding parameters, the strength
starts to decrease because of the excessive heat input and expulsion.
Cold deformation before welding increases the strength of the resistance
spot welded TRIP800 steels.
2) The button diameter and button
size ratio of welds for producing PF
mode that results in acceptable strength
and good surface quality should be at
least 4√t and 0.14–0.30, respectively, in
cold-deformed TRIP800 weldments.
3) The strength increases with an
increase in electrode indentation
depth and button diameter before severe expulsion occurs.
4) Cold deformation of TRIP800
steels increases the hardness and the
strength of the weldment.
5) Three distinct failure modes
were observed and the strength of the
weldment increase in the order of IF,
PIF, and PF modes.
6) Deformed TRIP800 steel hardness is found approximately 100 HV0.5
higher than those of the as-received
sample. The button hardness increases
with increasing heat input due to
martensite transformation.
7) The weld lobe was produced to
determine the acceptable strength
and desired surface quality associated with welding parameters. Cold deformation of TRIP800 steel shrinks
the weld lobe area and sets back the
weld lobe curves for lower welding
currents at the same welding times.
Acknowledgment
The authors thank the Karabük
University project office for its kind
support.
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