F. LIU et al.: STUDY OF DROPLET TRANSFER CHARACTERISTICS AND WELD FORMATION WITH ER430LNb ...
241–248
STUDY OF DROPLET TRANSFER CHARACTERISTICS AND
WELD FORMATION WITH ER430LNb FERRITIC STAINLESS
STEEL DURING GAS METAL ARC WELDING
PREIZKUS LASTNOSTI PRENOSA KAPLJIC TALINE IN
NASTAJANJA ZVARA FERITNEGA JEKLA VRSTE ER430LNB
MED OBLO^NIM VARJENJEM V TOKU PLINSKE ME[ANICE
KISIKA IN ARGONA
Feng Liu
1
,Y uGu
2
, Fanghong Xu
2
, Han Zhang
1
, Shaowei Xu
1
, Zhihang Yan
1
,
Zhifeng Yan
1
, Wenxian Wang
1*
1
College of Materials Science and Engineering, Taiyuan University of Technology, 79 West Yingze Street, Taiyuan 030024,
Shanxi Province, China
2
Shanxi Taigang Stainless Steel Co., Ltd., Taiyuan 030003, China
Prejem rokopisa – received: 2022-11-26; sprejem za objavo – accepted for publication: 2023-03-16
doi:10.17222/mit.2022.697
An ER430LNb ferritic stainless steel wire was used for a gas metal arc welding (GMAW) test. The characteristics of droplet
transfer and its corresponding voltage and current waveforms were investigated using a high-speed camera and synchronous ac-
quisition of electrical signals. The weld formation, droplet transfer patterns and corresponding microstructures under different
welding parameters were observed, and the effect of the current level on the droplet transfer frequency was discussed. As the re-
sults show, there were three typical metal transfer patterns during the GMAW using the ER430LNb ferritic stainless steel wire,
namely short circuit transfer, mix transfer and spray transfer. With an increase in the welding current, the weld formation is
changing constantly, and the droplet transfer frequency increases exponentially. With the change in the transfer pattern, the co-
lumnar ferrite structure of the weld is continuously coarsening. In addition, hardness measurements were taken on joints welded
at different welding currents for comparison.
Keywords: ER430LNb ferritic stainless steel wire, droplet transfer, weld formation, hardness
Za preizkuse oblo~nega varjenja v toku plinske me{anice kisika in argona (GMAW; angl:: gas metal arc welding) so uporabili
`ico iz nerjavnega feritnega jekla ER430LNb. S pomo~jo uporabe sistema za opazovanje s hitro kamero in sinhronega zajemanja
elektri~nih signalov so raziskovali lastnosti prenosa kapljic taline ter odgovarjajo~e napetostno in tokovno obliko valov. Med
preizkusi so opazovali in nato obravnavali nastajanje zvara, vzorcev prenosa kapljic taline in nastanka odgovarjajo~e
mikrostrukture pri razli~nih izbranih parametrih varjenja. Rezultati preizkusov in opazovanj so pokazali, da potekajo trije tipi~ni
vzorci prenosa kapljic taline pri GMAW z uporabo `ice iz nerjavnega feritnega jekla ER430LNb in sicer: kratko sti~ni prenos,
me{ani prenos in prenos v obliki razpr{ila. Z nara{~ajo~im varilnim tokom se nastajanje zvara konstantno spreminja in
frekvenca prenosa kapljic taline nara{~a eksponentno. S spremembo vzorca prenosa taline stebri~asta feritna struktura zvara
kontinuirno postaja bolj groba. Dodatno so s pomo~jo meritev trdote zvarnih spojev medsebojno primerjali njihove lastnosti
glede na izbrane parametre varjenja.
Klju~ne besede: varilna `ica iz feritnega jekla vrste ER430LNb, prenos kapljic taline, nastajanje zvara, trdota
1 INTRODUCTION
Ferritic stainless steels are widely used in various ar-
eas, including automobile manufacturing, white goods
and nuclear power industries, as they exhibit a good
combination of ductility, thermal conductivity and corro-
sion resistance. Their cost is also relatively lower and
more stable than those of austenitic grades as they do not
contain any nickel.
1–3
As a relatively new type of engi-
neering material, ER430LNb ferritic stainless steel with
excellent comprehensive properties is produced by par-
tially or completely replacing nickel with niobium to re-
duce the alloying-element costs. Deardo et al.
4
pointed
out that the precipitation of NbC, NbN, Fe
2
Nb and other
intermetallic compounds formed in steel by Nb, a strong
carbide-forming element, significantly improved the me-
chanical properties of ER430LNb at high temperatures,
5,6
effectively prevented the intergranular chromium poverty
caused by the precipitation of carbon-chromium com-
pounds, and improved the resistance to intergranular cor-
rosion.
7
Different forms of droplet transfer have various con-
sequences for the stability of the welding arc, the depth
of fusion of the weld, spatter and the generation of de-
fects and they affect the quality of the weld-seam forma-
tion. Oscillography and high-speed camera technology
are two of the main experimental tools used for studying
droplet transfers.
8
Oscillography records the waveform
of the welding current and arc voltage during a welding
process and indirectly researches the droplet transfer
based on the waveform modification. A high-speed cam-
Materiali in tehnologije / Materials and technology 57 (2023) 3, 241–248 241
UDK 669.15-194.57:621.791.75 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(3)241(2023)
*Corresponding author's e-mail:
wangwenxian@tyut.edu.cn (Wenxian Wang)

era captures the welding arc zone with a high-speed
video; a droplet-transfer process is recorded, and through
an analysis of droplet transfer images we directly investi-
gate the characteristics of the droplet transfer. The com-
bination of oscillography and a high-speed camera al-
lows us to better investigate the characteristics of a
droplet transfer. The droplet transfer of GMAW has been
extensively studied by domestic and international schol-
ars. Luo et al.
9
discussed the relationship between the
droplet-transfer behavior and arc current-voltage signal
and found that the characteristics of the droplet-transfer
behavior depend on the arc current-voltage signal. Neyka
et al.
10
investigated the droplet-transfer behavior of
Tandam based on the high-speed camera technology.
Simpson et al.
11
showed theoretically that when the
welding current is less than 250 A, the shape of the drop-
let is approximately spherical. Praveen et al.
12
combined
a high-speed camera and oscillography to research drop-
let-transfer patterns during a pulsed-welding process. Hu
and Tsai et al.
13
investigated the droplet-transfer behavior
at constant and pulsed currents through numerical simu-
lations, indicating that the higher the welding current, the
greater is the electromagnetic force on the droplet and
the easier is the droplet transfer from the wire to the base
material. It was also found that the higher the welding
current, the smaller is the droplet diameter and the higher
is the droplet transfer frequency.
Therefore, this work investigates the droplet transfer
characteristics and weld formation of stainless steel in an
GMAW process. The droplet transfer frequency, micro-
structure and hardness of joints welded at different weld-
ing currents were studied for comparison. The results of
the study lay the foundation for the subsequent process
optimization of the gas-shielded welding of ER430LNb
ferritic stainless steel.
2 EXPERIMENTAL PROCEDURE
The experimental system for the research of the
GMAW using an ER430LNb ferritic stainless steel wire
is shown in Figure 1; it mainly includes a robotic
GMAW system and a synchronous acquisition of droplet
transfer and electrical parameters. The robotic GMAW
system includes a welding robot and its control cabinet,
welding power supply, gas supply system and working
platform to provide a stable droplet-transfer process. The
electrical-signal synchronous acquisition system includes
a high-speed camera, xenon light source and Hanover
analyzer to capture the images and electrical signals of
the droplet-transfer process in real time. A 6-axis OTC
AX-V6 welding robot and YD-500GL3 welding power
source are utilized to sustain stable arc burning. A
MIX-L25 gas-mixture cabinet adjusts the content of ar-
gon and oxygen. The CR3000×2 high-speed camera op-
erating at 3000 fps in the same line as the 1000 W xenon
lamp light source generator and the welding torch remain
relatively stationary. The AH19 Hanover analyzer ac-
quires the data from the characteristic signals of voltage
and current during the welding process.
The base material (BM) included 4.0 mm thick plates
of AISI 430 ferritic stainless sheets and the welding wire
was an ER430LNb ferritic stainless steel wire with a di-
ameter of  1.2 mm. Table 1 lists the main components
of the welding wire and the base metal. Meanwhile, the
welding parameters are listed in Table 2. The welding
current was the crucial research variable in this work.
The welding speed was selected in a range of
10–100 cm/min. The wire feed speed (WFS) was auto-
matically matched with the GMAW power supply ac-
cording to the welding current. But the welding current
and arc voltage can be adjusted independently. To sim-
plify the experiment, the droplet transfer was captured
F. LIU et al.: STUDY OF DROPLET TRANSFER CHARACTERISTICS AND WELD FORMATION WITH ER430LNb ...
242 Materiali in tehnologije / Materials and technology 57 (2023) 3, 241–248
Figure 1: Schematic diagram of the experimental system

using the method of overlay welding on the plate
workpiece.
For the post-weld analysis, samples were cut using
wire electro-discharge machining. For the optical-micro-
scope testing, the sample was polished using emery pa-
per with grit sizes of 600, 1000, 1500 and 2000. Finally,
cloth polishing was done using 1-μm diamond paste to
achieve a mirror finish of the weld cross-section. After
polishing, the weld interface was etched with a Marble
reactant etching solution (4 g CuSO
4
+20mLHCl+100
mL H
2
O) and the microstructure was observed with an
optical microscope (BH200M). The crystal structure was
characterized, using an X-ray diffractometer (XRD,
D/MAX-2400) with Cu K
 radiation. The scanning range
was from 30° to 100° in 2 at a scanning rate of 5°/min.
A nanoindentation test (Nano indenter G200) was used
to measure the hardness of welded joints, with a load of
80 mN.
3 RESULTS AND DISCUSSION
3.1 Three typical metal transfer patterns
There were three typical metal transfer patterns ob-
served during the GMAW using the ER430LNb ferritic
stainless steel wire, namely the short circuit transfer, mix
transfer and spray transfer.
Short circuit transfer
A droplet is short-circuited to the welding pool at low
current and voltage before completing its growth. Under
the effects of surface tension, gravity and electromag-
netic forces, the droplet transfers to the base material,
forming a short-circuit transfer.
During the short-circuit transfer, a lot of spatter is ob-
served (Figure 2e) as the arc voltage increases with the
increasing welding current. In general, at low wire feed
speeds, the droplet-transfer pattern in a GMAW process
is either a short-circuit transfer or a particle transfer, de-
pending on the arc voltage. Figure 2 shows the curves of
the electrical parameter signals during the ER430LNb
GMAW process within 1 s and the complete transfer of a
droplet from wire tip generation, growth and then trans-
fer into the welding pool. The I and U in the figures il-
lustrate the average welding current and average arc volt-
age. When the welding current is moderate, the short
circuit transfer can be found to include three stages of
short circuit, arc reignition, and droplet growth. The wire
feed speed and welding current are relevant, and, as can
be found in Figure 2, when the welding current in-
creases, the wire feed speed increases as well, while the
transfer period and arc initiation time for a single droplet
gradually decrease.
The droplet size affects the magnitude of gravity, arc
force, etc. during the droplet-transfer process, resulting
in different impacts of the liquid melt pool, thus affect-
ing the melt pool flow forming. The change in the drop-
let size during the droplet transfer from Figure 2d is
shown in Figure 2g. From t
0
stot
0
+ 0.152 s, the droplet
diameter and size are marked in the voltage-waveform
figure with the time variation. The droplet diameter is
2.0 mm at t
0
+ 0.032 s. It increases to 2.3 mm and
2.6 mm after 0.33 s and 0.73 s, respectively, and reaches
a maximum of 2.7 mm at t
0
+ 0.141 s. The droplet size
decreases after t
0
+ 0.148 s to a diameter of 2.6 mm,
which rapidly decreases to 2.0 mm after 0.04 s.
During several droplet transfers within1so nt h e
graph, the welding current and arc voltage patterns re-
main largely unchanged, indicating a relatively stable
welding process. The wire tip returns to 1.2 mm after
0.03 s and the melt droplet transfer process ends. In sum-
mary, the short-circuit transfer is stable at a low wire
feed speed and a suitable welding current. Figures 2 to 4
show that the electrical voltage signal does not go to zero
at the time of the short circuit. Additionally, to smoothen
the wave, the collected data of electrical signals are pro-
cessed by mean filtering. The measured short circuit
voltage increases after mean filtering, and it is not equal
to zero when the short circuit occurs. As the proportion
of short circuit transients is very small, it has little influ-
ence on the analysis of electrical parameters.
3.1.1 Mix transfer
The welding current of mix transfer is between those
of short circuit transfer and spray transfer. The metal
transfer patterns include both particle transfer and short
circuit transfer, but the proportions vary randomly. With
the increase in the welding current, the arc current-volt-
age signal becomes more disordered between the short
circuit transfer from Figure 3a and the mix transfer from
Figure 3b, with both short-circuit and particle transfer.
The welding current continues to increase and the drop-
F. LIU et al.: STUDY OF DROPLET TRANSFER CHARACTERISTICS AND WELD FORMATION WITH ER430LNb ...
Materiali in tehnologije / Materials and technology 57 (2023) 3, 241–248 243
Table 1: Main components of the welding wire and the base metal (w/%)
Element C Si Mn P S Cr Ni Mo N Cu Nb Fe
Welding wire 0.03 0.5 0.6 0.03 0.03 15.5–17.0 0.6 0.75 – 0.75 0.24–1.2 Balance
Base metal 0.10 0.5 0.6 0.03 0.02 15.5–17.0 0.6 0.75 – 0.75 – Balance
Table 2: Various welding parameters
1234567891 0
Welding speed (cm/min) 40
Welding current (A) 60 78 90 110 120 138 162 178 198 214
Arc voltage (V) 17.6 18 18.8 20.4 20.6 21.6 22.4 23.6 25.6 27

F. LIU et al.: STUDY OF DROPLET TRANSFER CHARACTERISTICS AND WELD FORMATION WITH ER430LNb ...
244 Materiali in tehnologije / Materials and technology 57 (2023) 3, 241–248
Figure 2: Droplet transfer and electric signals: a)I=60A,b)I=78A,c)I=90A,d)I=110A,e)I=120A,f)I=138A,g)change in droplet
size at 110 A

let for the complete particle transfer pattern is shown in
Figure 3c. The molten droplet size decreases close to the
wire diameter, and the molten droplet transfer occurs
along the wire axis. The welding current continues to in-
crease, and we see a fully particle-transfer pattern in Fig-
ure 3c, with droplets falling along the axis of the wire. In
addition, the droplet size decreases close to the wire di-
ameter.
Compared to the short circuit transfer, it can be seen
from the voltage-current signal diagram that the fre-
quency of the mix transfer increases significantly. Fig-
ure 3c shows that the frequency of the particle transfer is
very fast, and the actual number of transfers per second
can reach 50. However, the reason why the periodic fre-
quency of the electric signal is only 4–5 is that the insta-
bility of the particle transfer causes a periodic short cir-
cuit. As the welding current increases to 198 A, the
droplet-transfer pattern changes from short circuit and
particle transfer to a completely particle transfer.
3.1.2 Spray transfer
As shown in Figure 4, when the welding current in-
creases to 214 A, the droplet-transfer pattern changes
from mix transfer to spray transfer. During the process of
spray transfer, a droplet with a diameter smaller than that
of the welding wire is generated from the tip of the weld-
ing wire and it falls off. Before the previous droplet falls
into the molten pool, the next droplet has been generated,
starting to fall off. The transfer frequency of droplets is
very high, and the number of transfers per second can
reach hundreds of times.
3.2 Weld formation
The weld formation at different GMAW parameters
is shown in Figure 5. For a stable short circuit transfer, a
good weld surface is demonstrated in Figure 5, with
negligible welding spatter where the welding process is
relatively stable. When the current increases to 162 A,
the droplet impacts the surface of the liquid molten pool
at the same time as the short circuit transfer so that the
droplet transfer is unstable and the splash is large. When
the welding current increases to 210 A or more, the
width of the weld increases sharply, and the weld surface
is badly formed, generating a lot of spatter and fumes.
Different welding currents used for the weld formation
are indicated in Figure 5d. It is worth mentioning that
under each welding process parameter, the residual
height of the weld does not change significantly.
F. LIU et al.: STUDY OF DROPLET TRANSFER CHARACTERISTICS AND WELD FORMATION WITH ER430LNb ...
Materiali in tehnologije / Materials and technology 57 (2023) 3, 241–248 245
Figure 4: Droplet transfer and electric signals at 214 A
Figure 3: Droplet transfer and electric signals: a) I = 162 A, b) I = 178
A, c) I = 198 A

F. LIU et al.: STUDY OF DROPLET TRANSFER CHARACTERISTICS AND WELD FORMATION WITH ER430LNb ...
246 Materiali in tehnologije / Materials and technology 57 (2023) 3, 241–248
Figure 5: Weld formation: a) short circuit transfer, b) mix transfer, c) spray transfer, d) weld formation at different welding currents
Figure 6: Droplet transfer patterns and microstructure: a) droplet transfer patterns, b) microstructure for different droplet transfer patterns, c)
XRD pattern of weld joints

3.3 Droplet transfer patterns, microstructure and drop-
let transfer frequency
With an increase in the welding parameters, the drop-
let transfer pattern changes gradually. As shown in Fig-
ure 6a, the droplet transfer pattern can be divided into
three kinds: the short circuit transfer with small parame-
ters, the mix transfer and the spray transfer. A set of pa-
rameters, including 78 A, 178 A and 214 A, for each
transfer pattern was selected to observe the micro-
structure. It can be seen that the microstructure is coarse
columnar ferrite, and the ferrite grain is also continu-
ously coarse with the increasing welding current, as
shown in Figure 6b. According to the XRD results
shown in Figure 6c, the phase composition of the weld
is a 100 % ferrite structure.
The droplet transfer frequency is a very important pa-
rameter, closely related to the welding efficiency and
weld formation. A higher droplet transfer frequency
causes a higher welding efficiency. With the increasing
welding parameters, the droplet transfer frequency also
changes gradually. For a weld formed of ferritic stainless
steel, a small welding current with a moderate droplet
transfer frequency is preferred. The droplet transfer fre-
quency increases exponentially as shown in Figure 7.
The regression fitting of the welding current to the drop-
let transfer frequency was obtained using the Origin soft-
ware, and the equation is as follows:
(3-1)
3.4 Hardness test
Five welding parameters were selected for hardness
testing of three different droplet transfer patterns by a
nanoindentation test. Figure 8 shows typical microhard-
ness distributions with different welding currents for the
three main welding zones: the base metal, HAZ and weld
metal. A variation caused by nanoindentation was ob-
served in the microhardness values from the base mate-
rial to the weld metal.
The highest hardness values were measured on the
weld metal, between 3.5 GPa and 4.5 GPa. The hardness
values of the HAZ were found to be between 2.2 GPa
and 3.9 GPa. It should be underlined that the base metal
has softer regions (1.8–2.3 GPa) than the HAZ. The in-
crease in the hardness of the weld metal and base metal
may be attributed to the changes in the microstructure
through a relatively higher cooling rate due to the higher
heat input during welding.
The overall hardness of the welding zone (HAZ and
weld metal) decreased due to the increase in the welding
current. With the increasing welding current, a higher
heat input is generated during welding. Therefore, the
cooling takes place at a higher temperature. Thus, the
changes in the volume ratio of the phases formed in the
microstructure may cause an increase in the hardness.
4 CONCLUSIONS
In this study, ER430LNb ferritic stainless steel used
for gas metal arc welding was studied. The test results
are summarized as follows:
(1) During the GMAW using an ER430LNb ferritic
stainless steel wire, the droplet transfer is a stable short
circuit transfer at currents of less than 138 A. The drop-
let transfer pattern is a mixed transfer at currents of
around 162–198 A. And it is a spray transfer when the
current is over approximately 214 A.
(2) During the short circuit transfer stage, the droplet
size is approximately 2–3 times the diameter of the
welding wire. With an increase in the current, the droplet
transfer gradually changes to particle transfer, and the
droplet size decreases gradually. As the current increases
further, the droplet transfer pattern is spray transfer, and
the droplet size is smaller than the diameter of the weld-
ing wire.
(3) With the increase in the welding current, the cur-
rent-voltage signal becomes more disordered, resulting
F. LIU et al.: STUDY OF DROPLET TRANSFER CHARACTERISTICS AND WELD FORMATION WITH ER430LNb ...
Materiali in tehnologije / Materials and technology 57 (2023) 3, 241–248 247
Figure 8: Hardness measurement results
Figure 7: Non-linear fitted curves of droplet transfer frequency

in more welding splash and fumes. For gas metal arc
welding with ER430LNb ferritic stainless steel, a low
welding current with a moderate droplet transfer fre-
quency is preferred.
(4) The weld microstructure of ER430LNb ferritic
stainless steel indicates coarse columnar ferrite, and the
grain size increases with the increase in the welding cur-
rent. In addition, with a continuous increase in the weld-
ing current, the droplet transfer frequency increases rap-
idly and exponentially.
(5) The hardness distribution values for the weld
joints of the weld metal, HAZ and base metal are 3.5–4.5
GPa, 2.2–3.9 GPa and 1.8–2.3 GPa, respectively. The
hardness in the welding zone (HAZ and weld metal) de-
creases due to the welding current increase.
Compliance with Ethical Standards
Funding: This study was funded by the National Nat-
ural Science Foundation of China (grant numbers
52274390 and 52075360).
Conflicts of Interest
The authors declare that they have no conflict of in-
terest.
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F. LIU et al.: STUDY OF DROPLET TRANSFER CHARACTERISTICS AND WELD FORMATION WITH ER430LNb ...
248 Materiali in tehnologije / Materials and technology 57 (2023) 3, 241–248