Zong Paper 2015104 May 2016_Layout 1 4/14/16 4:20 PM Page 174
WELDING RESEARCH
Undercutting Formation Mechanism
in Gas Metal Arc Welding
A vision­based sensing system was used to acquire 2D images of the weld pool
and tracer particle movement, and help determine why undercutting occurs
during high­speed welding
BY R. ZONG, J. CHEN, C. S. WU, AND M. A. CHEN
ABSTRACT
A vision­based sensing system was improved to acquire 2D images of weld pool shape
and tracer particle movement simultaneously. The improved algorithm that transforms
2D image coordinates to 3D world coordinates was calibrated by laser lattice points and
its errors were analyzed. Subsequently, the algorithm was used to calculate and analyze
flow velocities of tracer particles, which indicate flow patterns of weld pool on top
surface for different welding speeds. Based on analysis of tracer particle movement,
roles of different associated forces, such as gravity, arc drag force, Maragoni force, mag­
netic force, droplet impact, and viscous force, were investigated. The results show that
liquid metal under the arc decreases to a thin liquid layer if the welding speed is more
than 1.0 m/min. This phenomenon makes viscous resistance increase in the thin liquid
layer, which impedes transverse spreading of liquid metal. Thus, it is difficult for the weld
toe to be filled by molten metal. Meanwhile, experiments revealed the amount of
forward molten metal can be ignored in the front and middle of the weld pool.
Therefore, it is difficult for the weld toe to be filled by the backward molten metal. As a
result, undercutting defects appear during high­speed welding. Governing equations of
the undercutting width were deduced based on the arc and weld pool shapes. Further
study has been carried out and the experimental results show that a low­current
tungsten argon arc can change molten metal flow and effectively suppress undercutting.
KEYWORDS
• GMAW • Undercutting • Vision Inspection • Fluid Flow
Introduction
Gas metal arc welding (GMAW) is
predominantly employed in manufacturing because of its advantages, such
as relatively lower cost, easier operation, and better adaptability compared
to other welding processes (Ref. 1).
Manufacturing productivity can be improved by simply increasing welding
speed and current in GMAW. However,
if the welding speed is beyond a certain limit, undercutting defects will occur (Ref. 2). Located in the weld metal
adjacent to the weld toe, an undercut-
ting defect is a narrow groove left unfilled by the weld metal during solidification. Undercutting can also induce
other defects, such as humping, stress
corrosion cracking, and so on, which
limits improvement of manufacturing
productivity (Ref. 3). More severely,
undercutting creates a mechanical
notch at the weld interface that lowers
the static, fatigue, and fracture
strengths of the welded assembly (Ref.
4). Therefore, understanding the
mechanism of undercutting formation
is of great significance to suppress
these defects and to achieve higher
welding productivity.
Various research has been carried
out to demonstrate the undercutting
formation mechanism qualitatively
based on experimental observations.
Ishizaki found the degree of undercutting is much more pronounced in
high-sulphur steels than in lowsulphur casts (Ref. 5). This phenomenon was explained by the change in direction of molten metal movement
due to variations in temperature coefficient of surface tension with different sulphur contents (Ref. 6). However, experiments conducted on lowsulphur steels showed the undercutting
defect also appears if the welding speed
is high (Ref. 7). Matsunawa studied undercutting defects by calculating the
surface profile of the weld bead based
on the Young-Laplace equation (Ref. 8);
however, the effect of arc force was ignored. Nishi and Ohara found undercutting defects appear during submerged
arc welding when the liquid metal is displaced more than the solidification
point (Refs. 9, 10). This distribution
feature of the liquid metal in the weld
pool is caused by the arc force along the
welding direction. Mendez found the
arc pressure pushes the molten metal to
the rear of the weld pool, creating a thin
layer of liquid metal under the arc. The
premature solidification of this thin layer prevents wetting of the side of the
weld bead and leads to undercutting defects (Ref. 11). Karlsson discussed the
influence of surface oxides on the weld
pool shape during laser-arc hybrid welding. It is reported that different lowdensity and high-melting-point metallic
R. ZONG, J. CHEN (chenji@sdu.edu.cn), C. S. WU, and M. A. CHEN are with MOE Key Laboratory for Liquid­Solid Structural Evolution and Material Processing,
and Institute of Material Joining, Shandong University, China.
174-s WELDING JOURNAL / MAY 2016, VOL. 95
Zong Paper 2015104 May 2016_Layout 1 4/14/16 4:20 PM Page 175
WELDING RESEARCH
Fig. 2 — Schematic of vision­based sensing system.
A
B
was found
that highFig. 1 — Two types of undercutting defects in GMAW.
speed
backward
oxides, such as MnO and SiO2, gather at
fluid flow, caused by arc pressure and
the weld toe. These metallic oxides float
droplet impact, is the main reason for
toward the edge of the weld pool and
undercutting formation (Ref. 15). Sevsolidify on the outer bank of the melt
eral techniques, such as magnetic conzone. This solidified oxide coating hintrol welding and hybrid arc welding,
ders the lateral spreading of liquid metal
have also been demonstrated to sup(Ref. 12).
press undercutting defects by slowing
Several three-dimensional numeridown the backward flow of the liquid
cal models have been developed to un(Refs. 16, 17). However, the simulated
derstand the undercutting formation.
results still lack experimental tests for
Sudnik analyzed the main factors of
the speed of backward fluid flow to
undercutting defects using a quasiverify the undercutting formation
stable-state model in which the balmechanism.
ance between the distributed forces
Based on the weld morphology in
and thermal conditions in the weld
the preparatory GMAW experiments,
pool are taken into consideration
undercutting defects can be divided
(Refs. 13, 14). The results showed uninto two types. The first type is disdercutting defects are caused by decontinuous undercutting defects,
pression of the liquid metal level in the
which have an unstable profile and
weld pool. However, the usable range
discontinuous distribution, as shown
of welding parameters in this model is
in Fig. 1A. The second type is continurelatively narrow. Recently, Wu studous undercutting defects, which have a
ied the GMAW process by numerical
stable profile and continuous distribusimulation and visual inspection. It
tion, as illustrated in Fig. 1B. Discon-
tinuous undercutting defects usually
appear in the short-circuit transfer
mode when an oxidizing shielding gas
is used or when the welding current is
lower than 240 A (Refs. 18, 19). The
welding process is unstable and causes
severe welding spatter that deteriorates the weld bead appearance (Ref.
20). Continuous undercutting defects
usually appear in spray transfer mode.
In this mode, the welding process is
stable and the welding spatter is low
(Ref. 21). However, undercutting defects still appear at higher welding
speeds. Therefore, the formation and
suppression mechanisms of continuous undercutting defects were studied
in this work to achieve good-quality
welds.
In this study, a special filter was selected based on spectrographic observation of the GMAW arc spectrum.
The vision system consisted of a highspeed color camera fixed with the filter to reduce arc light interference.
Some particles come into the weld
pool from the blind holes in the base
metal and other particles drop into the
Table 1 — Weld Pool Width and Reinforcement
Test
No.
Welding
Speed
(m/min)
Width
(cross section)
(mm)
Width
(image)
(mm)
Error
(%)
Reinforcement
(cross section)
(mm)
Reinforcement
(image)
(mm)
Error
(%)
1
2
3
4
5
6
7
8
9
0.5
0.5
0.5
1.0
1.0
1.0
1.5
1.5
1.5
11.01
11.03
11.04
6.27
6.25
6.23
5.40
5.42
5.40
11.06
11.04
11.08
6.32
6.30
6.26
5.46
5.44
5.42
0.5
0.1
0.4
0.8
0.8
0.5
1.1
0.4
0.4
3.18
3.16
3.16
2.23
2.24
2.21
1.79
1.78
1.76
3.21
3.19
3.18
2.25
2.25
2.23
1.80
1.81
1.79
0.9
0.9
0.6
0.9
0.4
0.9
0.6
1.7
1.7
MAY 2016 / WELDING JOURNAL 175-s
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WELDING RESEARCH
Fig. 4 — Response curve of the CMOS sensor.
Fig. 3 — GMAW arc spectrum.
Fig. 5 — High­resolution image of the weld pool (I = 250 A, U = 30 V, v = 1.0 m/min).
Fig. 6 — Scanning traces of laser lattice points (I = 250 A, U = 30 V, vw = 1.0 m/min).
Table 2 — Chemical Compositions of Welding Wire and Base Metal (wt­%)
Composition (%)
C
Si
Mn
P
S
Wire (H08Mn2Si)
Q235
0.11
0.17
0.65
0.15
1.80
0.30
0.030
0.015
0.030
0.035
weld pool from a rotating device. The
visual sensing system was used to capture particle trajectories in the weld
pool. With illumination of a low-power
laser, the fluid flow on the top surface
of the weld pool was observed to detect undercutting formation by analyzing the particle trajectories. Threedimensional data of the weld pool was
also obtained by an adaptive algorithm
and partially used to study the main
factors influencing undercutting defects and to analyze the mechanism of
undercutting formation. Based on the
analysis of the experimental data, the
176-s WELDING JOURNAL / MAY 2016, VOL. 95
assistant gas tungsten arc with low
current was used to change the fluid
flow in the weld pool of GMAW. The
results show it has great potential to
improve the GMAW process and the
welding productivity.
Experimentation
Visual Sensing System for Fluid
Flow of Weld Pool
In order to describe the fluid flow
in the weld pool quantitatively (Ref.
22), an improved vision-based sensing
system was set up, as illustrated in Fig.
2. This system utilized a scanning
laser to recover the 3D weld pool surface. The GMAW gun and camera are
stationary while the workpiece is moving at a speed equal to the welding
speed (vw). The origin of world coordinates (point O, shown in Fig. 2) is the
intersection point between the workpiece surface and the extrapolated line
of welding wire. Its X axis is opposite
to the welding direction, Y axis is perpendicular to the X axis on the workpiece surface, and its Z axis is vertically upward. The first pixel of the image
at the top-left corner is defined as the
origin of image coordinates (point
O*, shown in Fig. 2). Its X* axis is opposite to the welding direction, and Y*
axis is perpendicular to the X* axis
downward.
In this system, a DS-UN1401USB3.0 high-speed color camera was
used to observe the weld pool surface in
real time. Its framing rate was 100 Hz.
The principal optic axis of the camera
was fixed on the OYZ plane at a depression angle of 45 deg, as depicted in Fig.
2. A prime lens, which has a 35-mm focal length and a 17.5-mm aperture, was
used to ensure that accurate images
were recorded with little optical distortion. In order to observe the fluid flow
on the weld pool surface, silicon carbide
(SiC) particles were delivered into the
weld metal through a special device. Silicon carbide was selected as the tracer
particle because its melting point is
higher than 3000 K and its density is
lower than the liquid metal of the weld
pool. In addition, SiC has little impact
on the welding process compared with
refractory oxides (Ref. 22). Generally,
Zong Paper 2015104 May 2016_Layout 1 4/14/16 4:20 PM Page 177
WELDING RESEARCH
A
Fig. 7 — Flow trajectory of tracer particle (I = 250 A, U = 30 V).
Table 3 — GMAW Parameters
B
Welding
Parameter
Torch
Angle
Electrode
Connection
Shielding
Gas Ar
(L/min)
GMAW
90 deg
DCEP
20
SiC particles should have diameters between 0.5 and 1-mm to avoid melting
by the arc and to ensure free flow. In
this study, the tracer particles were
placed in blind holes with dimensions of
1 mm diameter and 0.5 mm depth to
deliver them safely into the weld pool.
Meanwhile, other tracer particles on the
rotating disk come into the tail of the
weld pool through a pipeline, as demonstrated in Fig. 2. A laser generator was
used to provide a precise lattice point
moving across the weld pool surface to
help recover all three dimensions of the
weld pool. All scanning traces are parallel to plane OXZ, and their internal
spacing is constant (1 mm) in the Y direction. The trace of the laser point is
also demonstrated in Fig. 2.
During the GMAW process, arc interference needs to be reduced to obtain a clear and precise image of the
entire weld pool by the CMOS sensor.
Figure 3 is the average GMAW arc
spectrum, which was analyzed by an
Avs desktop-USB2 optical fiber digital
spectrometer. The response curve of
the CMOS sensor is shown in Fig. 4.
Comparing Figs. 3 and 4, wavelengths
in the range 433–516 nm or 602–650
nm can be selected as the observation
window, where the arc interference is
relatively low. Based on the Planck
principle, the radiation intensity of
the weld pool becomes stronger as the
wavelength increases from 400 to 650
nm (Ref. 23). So the ideal range of the
observation window is 602–650 nm.
In this study, a narrow-band pass filter
with central wavelength 630 nm and
semibandwidth of 20 nm was
CTWD Welding
(mm) Current
(A)
20
240–280
Arc
Voltage
(V)
Welding
Speed
(m/min)
30
0.4–1.5
equipped at the bottom of the optical
lens. In this way, the weld pool radiation could pass through the lens, and
arc radiation could be effectively lowered by the lens. To further lower the
arc radiation, a neutral filter was used
to cover part of the lens to suppress
the arc radiation, as shown in Fig. 2.
As a narrow-band pass filter was used,
RGB signal gain was set at 1:2:5 to improve the resolution of the images.
This system could capture the whole
weld pool and arc in one image simultaneously, as shown in Fig. 5. The boundary at the rear of the arc was caused by
the edge of a neutral filter, as illustrated
in Fig. 2. The tracer particles and slags
can be classified clearly from the characteristics of their profiles and flow trajectories. For example, the slag seldom has
a sharp-edged corner and has a shape
much smoother than that of a tracer
particle. So the boundary extraction algorithm can be applied to detect tracer
particles as well as the weld pool boundary. With the help of a reference sheet,
the length, width, and reinforcement of
the weld pool could be calculated, as illustrated in Fig. 5 (Ref. 24).
Restoration of Three­
Dimensional Data of Weld Pool
from Two­Dimensional Images
To obtain the velocities of tracer
particles, the image coordinates
should be transformed to the world
coordinates. In the GMAW process,
the weld pool surface is curved, so an
improved stereo algorithm should be
C
Fig. 8 — Surface fluctuation and laser
point deviation.
deduced for the coordinate transformation. Though a linear laser is an
ideal tool to recover the 3D weld pool
surface (Ref. 24), its reflecting light
cannot be identified under the arc as
the laser equipment used in this work
does not have enough power. Therefore, a point laser moving at a high
speed had been assumed as a linear
laser and was employed in this study.
To simplify data processing, the fluctuation of the weld pool surface
caused by the droplet transfer was
neglected, and the variation of weld
pool shape was also neglected.
Figure 6 demonstrates a series of
captured laser points on the weld pool
surface, and their image coordinates
can be obtained conveniently using
digital image process technology (Ref.
25). Thus, the image coordinates of
tracer particle P (xP , y*P) can be transformed to its world coordinates (xP , yP ,
zP) by the following stereo algorithm:
( y*P − y*L )( y*P − y*M )
( y*N − y*L )( y*N − y*M )
y*P − y*L )( y*P − y*N )
(
⋅ yN +
( y*M − y*L )( y*M − y*N )
y*P − y*M )( y*P − y*N )
(
⋅y
⋅yM +
( y*L − y*M )( y*L − y*N ) L
yP =
(2)
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WELDING RESEARCH
conducted on mild steel (Q235) plates
to study the influences of welding
speed and welding current on undercutting defects. The dimensions of the
workpiece were 250 (length) × 70
(width) × 5 (thickness) mm and the diameter of the welding wire was 1.2
mm. The chemical compositions of
welding wire and base metal are shown
in Table 2. The main welding parameters used in this study are listed in
Table 3.
Results and Discussion
Fig. 9 — Influence of welding speed and welding current on undercutting area.
Influence of Welding Speed and
Welding Current on
Undercutting Defects
Table 4 — Parameters Used in Equations 5–11
Physical Quantities
Value
Other Parameters
Value
Density  (kg/m )
Environment temperature T0 (K)
Melting point Tl (K)
7860
293
1789
Welding current I (A)
Arc voltage U (V)
Arc length L (m)
250–280
30
5 × 10­3
3
(
zP = y P − k y y*P − y0*
)
( 3)
The (x*O , y*O), (x*P ,y*P), (x*L, y*L),
(x*M, y*M), and (x*N, y*N) are the image
coordinates of points O, P, L, M, and N,
respectively. The yM, yN, yL can be obtained from Fig. 6 directly. The coordinate transformation coefficients kx and
ky are calculated by putting a known
scale grid in front of the lens (Ref. 24).
In the X* direction, 42 pixels in the image coordinate system correspond to 1
mm in the world coordinate system,
and in the Y* direction, 30 pixels corresponds to 1 mm. The Y* coordinate of
the point L, located at the scanning
trace, can be calculated by linear interpolation using Equation 4.
(
)(
)
/ ( x *B − x *A ) + y*A
y*L = x *L − x *A y*B − y*A
(4)
The (x*A, y*A) and (x*B, y*B) are the image coordinates of points A and B. The
Y* coordinates of the points M and N
can be obtained in the same way. Then
the flow trajectory of the tracer particle
can be drawn out as shown in Fig. 7.
Calibration
The experimental errors can be divided into two types. The first type of
178-s WELDING JOURNAL / MAY 2016, VOL. 95
error was caused by the variation of
weld pool shape. Figure 8A, B illustrates
the surface fluctuation of the weld pool
observed by a high-speed (2000 Hz)
CCD camera (Ref. 16) while the laser
system was stable. Experimental results
show the fluctuation of the liquid metal
was smaller than 0.1 mm, though
droplets impact the weld pool periodically. In addition, the weld pool widths
and reinforcements were measured
from the cross section of the weld bead
and also from the image of the weld
pool. Table 1 shows the relative errors
in both weld pool width and reinforcement were smaller than 2% when the
welding speed was between 0.5 and 1.5
m/min. So, it is considerable to neglect
the errors caused by the variation of
weld pool shape in data processing. The
data in Table 1 also show good experimental repeatability. The second type of
error was caused by mechanical vibrations of the laser moving system. Figure
8C demonstrates that this type of error
was smaller than 0.1 mm, even if the
device moved at high speed. Therefore,
the results of error analysis demonstrate that accuracy of these experiments is acceptable.
Materials and Process
Parameters Used
Bead-on-plate welding tests were
In Fig. 9, the undercutting area S
(the groove area of the cross section,
such as S1 and S2) was selected as the
quantitative index of undercutting defects. The value of the undercutting
area was calculated by statistics of pixels using the binary image of the weld
cross section. The experimental results show the undercutting defects always appear when the welding speed is
higher than 0.6 m/min. The undercutting area decreased slowly as the welding current increased from 240 to 280
A. This is because the mass of wire
consumed per unit length of the weld
increased with increasing welding current and the grooves at the weld toes
could be partly filled by the filler metal. However, undercutting defects cannot be eliminated only by increasing
the welding current. On the other
hand, the undercutting area increased
very quickly when the welding speed
was higher than 0.6 m/min and
reached its peak value when the welding speed was about 1 m/min. Then
the undercutting area decreased slowly
as the welding speed increased from 1
to 1.5 m/min. To explain this phenomenon, it is necessary to study flow patterns of the weld pool at different
welding speeds.
The Analyses of Undercutting
Forming Mechanism through
the Flow Patterns
Figure 10A–H shows the generalized flow patterns of tracer particles
on the top surface of the weld pool at
Zong Paper 2015104 May 2016_Layout 1 4/14/16 4:21 PM Page 179
WELDING RESEARCH
A
B
C
D
E
F
Fig. 11 — The flow velocity at different welding speeds
(I = 250 A, U = 30 V).
H
average
flow velocity of tracer
particles for
their backward flow
(i.e., in the
Fig. 10 — Flow patterns of tracer particles on the top surface of the weld
front and
pool at different welding speeds (I = 250 A, U = 30 V).
middle of
the weld
different welding speeds. The generalpool, shown in Fig. 10E) at different
ized flow patterns were obtained by
welding speeds. In this figure, the
repeating the experiments several
transverse flow velocities decreased
times. In these figures, the backward
from 0.098 to 0.051 m/s when the
flow velocities of tracer particles first
welding speed increased from 0.4 to
increased in the front of the weld pool
1.0 m/min and only decreased from
and then decreased slowly in the mid0.051 to 0.046 m/s when welding
dle of the weld pool. Unlike the nuspeed further increased from 1.0 to
merical simulation results, no particle
1.5 m/min. Furthermore, experiments
circulation was observed on the weld
show the average longitudinal flow vepool surface by experimental tests
locities were between 0.16 and 0.18
(Refs. 26, 27). When the tracer partim/s when the welding speed increased
cles were located on the centerline of
from 0.4 to 1.5 m/min. Therefore,
the weld pool, their starting flow diwhen the welding speed is 0.4 m/min,
rections were highly random and the
the ratio of transverse flow velocities
particles could flow in both directions,
to longitudinal flow velocities is 0.63.
as illustrated in Fig. 10C. However,
And this ratio decreases to 0.30 if the
when the tracer particles deviated
welding speed is higher than 1.0
from the centerline, most flowed in
m/min. This change in ratio of flow
the direction of deviation to the
velocities leads the filler metal to
boundary of the weld pool, as shown
move to the tail of the weld pool (i.e.,
in Fig. 10D.
when the ratio is 0.30) instead of the
Figure 11 shows the variations in
weld pool boundary (i.e., when the raG
Table 5 — GTA and GMA Hybrid Welding Parameters
Welding
Parameter
Torch
Angle
Electrode
Connection
Shielding
Gas Ar
(L/min)
GTAW
GMAW
60 deg
90 deg
DCEN
DCEP
20
20
CTWD Welding
(mm) Current
(A)
5
20
50
250
Arc
Voltage
(V)
Electrode
Diameter
(mm)
7
30
2.4
1.2
tio is 0.63). Thus, the grooves at the
weld toes lack filler metal, and serious
undercutting defects appear with increasing welding speed.
As illustrated in Fig. 10A, F and Fig.
12, the variable Ls is defined as the distance between the arc center and
starting position of the tracer particles. From these figures, it is obvious
that Ls decreases with increasing welding speed and the starting positions of
tracer particles seem to be converging
toward the arc center. When the welding speed was 0.4 m/min, starting positions were about 4.5 mm ahead of
the arc; the tracer particles could reach
the widest boundary of the weld pool.
Therefore, sufficient liquid metal was
brought to the weld toe. When the
welding speed increased to 1.0 m/min,
the Ls decreased to 0.3 mm, and the
starting positions almost converged at
the center of the arc. So there is not
enough time for the tracer particles
and the molten metal to reach the
weld pool edge before they flow to the
tail of the weld pool, thereby leaving
the weld toe unfilled and ultimately
causing undercutting defects.
To explain the behaviors of molten
metal in the weld pool and explore the
mechanism of undercutting defects, the
various forces associated with the liquid
metal in the weld pool should be analyzed. During the GMAW process, different forces such as arc force, droplet
impact, viscous resistance, Maragoni
force, gravity, electromagnetic force,
etc., may affect the movement of liquid
metal in the weld pool (Ref. 28). In order to analyze the effect of the above
forces on behaviors of molten metal, the
weld pool was divided into three regions
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WELDING RESEARCH
A
B
C
D
Fig. 13 — The shape of the weld pool under the arc (I = 250 A, U = 30 V).
Fig. 12 — Starting positions of tracer particles (I = 250 A,
U = 30 V).
based on the characteristics of flow velocities, as shown in Fig. 10H. In this
figure, the three different regions are 1)
the accelerating backward flow region in
the front of the weld pool, 2) the decelerating backward flow region in the
middle of the weld pool, and 3) the forward flow region in the tail of the weld
pool.
Behaviors of Liquid Metal in the Front
of the Weld Pool
Figure 13 demonstrates the liquid
layer of molten metal under the arc is
very thin in the front of the weld pool,
so the effect of gravity on the fluid
flow can be ignored in this area. The
direction of the electromagnetic force
is always toward the center of the arc
and, thus, the backward movement of
the tracer particles is obstructed. But
its magnitude is relatively small compared to other forces (Ref. 29). To
study the effect of Maragoni force,
contrast experiments were conducted
using argon and mixed shielding gases
(argon+CO2), as shown in Fig. 14. It is
known that when the volume fraction
of CO2 in the shielding gas is 10%, the
oxygen content of the liquid metal in
the weld pool reaches 240 ppm (Ref.
30). In this situation, the direction of
Maragoni force changes from the outward to the inward (Refs. 31, 32).
However, there are no significant differences in the flow patterns and flow
velocities in contrast experiments,
which indicate the effect of Maragoni
force is also relatively small. Hence,
the arc force and droplet impact
180-s WELDING JOURNAL / MAY 2016, VOL. 95
should be the
A
main driving
forces in the
front part of the
weld pool, which
push the molten
metal to move
outward and
backward (Ref.
B
28).
Figure
Fig. 14 — Flow patterns of the weld pool using different shielding
15A–C shows
gases (I = 250 A, U = 30 V).
the change in
arc shape was
not too signifiarc (Ref. 11). And this energy transforcant with an increase in the welding
mation ratio increased with decreasing
speed from 0.4 to 1.5 m/min. Therethickness of the liquid layer. Therefore, the influence of arc force and
fore, in the present scenario, first,
droplet impact on the flow speed of
there was a thin layer obstructing the
tracer particles remained stable (Ref.
backward flow due to its higher vis33). This indicated the movement of
cous resistance and, second, the thin
molten metal in the weld pool would
layer also caused more energy transnot vary significantly with welding
formation that promoted the backspeed if only arc force and droplet imward flow. The net result was that the
pact were considered. So there must be
backward velocities would experience
some other force that invokes undersmall variations under different weldcutting defects.
ing speeds. Finally, the liquid metal
Figure 13 depicts that the liquid
did not have sufficient time to reach to
metal layer under the arc became thinthe widest boundary of the weld pool
ner when the welding speed increased
before its solidification, as illustrated
from 0.4 to 1.0 m/min. Hence, the visin Fig. 10C–F. These changes of liquid
cous resistance caused by this thin liqmetal behaviors seem to be responsiuid layer increased. This viscous force
ble for the undercutting formation
obstructed the transverse spreading of
when the welding speed was higher
the liquid metal and converged startthan 0.7 m/min. The thickness of the
ing positions of trace particles toward
liquid metal layer changed only marthe arc center (i.e., reduces Ls). Meanginally when the welding speed inwhile, the kinetic energy of the droplet
creased from 1.0 to 1.5 m/min, and
would be converted to kinetic energy
the viscous resistance caused by this
of backward molten metal after it imthin liquid layer almost remained conpacted the thin liquid layer under the
stant. The transverse velocities de-
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WELDING RESEARCH
A
B
C
Fig. 16 — Measured and calculated base metal melted
widths (U = 30 V).
D
Fig. 5. The
tracer particles
flowed backward along the inclined surface of the
weld pool, and they were decelerated
by the gravity along the welding direction. As illustrated in Fig. 14, Maragoni force drove the tracer particles flowing from the boundary to the center of
the weld pool. The transverse velocities of tracer particles when shielded
by mixed gases were about 0.05 m/s,
and when shielded by argon were
about 0.04 m/s. This means that
Maragoni force was the main driving
force for the transverse velocities in
the middle of the weld pool.
In the middle of the weld pool, the
longitudinal velocities decreased slowly. Meanwhile, the transverse flow was
toward the center of the weld pool.
Therefore, the liquid metal could not
reverse to fill the weld toe.
Fig. 15 — Arc shape and sketch of Gauss heat source model.
creased slowly and the starting positions of tracer particles remained unchanged (i.e., Ls remained almost constant), while the melted width of the
base metal still obviously decreased,
i.e., melted width of base metal was
narrow. So it became easier for the liquid metal to partly fill the grooves at
the weld toes. Therefore, one reason
for the decrease in the undercutting
area was the reduction of the melted
width of the base metal when the
welding speed increased from 1.0 to
1.5 m/min.
Behaviors of Liquid Metal
in the Middle of the Weld Pool
It can be seen from Fig. 10 and
from an earlier discussion that when
the tracer particles flowed backward,
they were away from the effect of the
arc force and droplet impact in the
middle of the weld pool. The effect of
the electromagnetic force could be ignored because the electric current density was very low (Ref. 34). As the filler
metal accumulated in the weld pool
during the course of welding, the
thickness of the liquid metal increased. Hence, its viscous resistance
became relatively smaller than that in
the thin layer (Ref. 35). However,
there was a height gradient of the weld
pool surface from the front to the middle, i.e., the height of the middle surface was more than that of the front
surface, which can be asserted from
Behaviors of Liquid Metal in the
Tail of the Weld Pool
The weld pool surface, enclosed between the middle and tail of the weld
pool, was almost horizontal and far
away from the arc, as can be seen in
Fig. 5. So, the influence of arc force,
droplet impact, electromagnetic force,
and gravity on the fluid flow was very
small. As the temperature of the liquid
metal decreased in the tail of the weld
pool, the viscous resistance increased
(Ref. 36), but its influence on the metal flow was still very small. The Maragoni force always drove tracer particles
to flow from the low-temperature re-
gion to the high-temperature region
(Ref. 31). The maximum flow velocity
only increased to 0.060 m/s even
though CO2 was added into the argon
shielding gas. These observations
demonstrated that the Maragoni force
was the main driving force of fluid
flow in the tail of the weld pool, and it
was very small in this region. So the
influence of the slower fluid flow in
the tail on undercutting was not significant.
The analyses on the behavior fluid
flow in the front, middle, and tail of
the weld pool show the thin liquid
metal layer under the arc resulted in
the decrease of the transverse flow velocity and the retreat of the starting
positions. The transverse spreading of
the liquid metal was suppressed. At
the same time, the high-speed backward fluid flow decreased slowly and
the transverse velocities were inward
in the middle of the weld pool. There
was no reverse metal to fill the weld
toe. All these phenomena obstructed
the liquid metal from filling the
grooves at the weld toes before solidifying, thus the undercutting defects
appeared under high welding speed.
Prediction of Undercutting
Width
Undercutting width (Wu) was chosen as a predictive index of the size of
undercutting. It was defined as half
the difference between the width of
melted base metal (WM) and the width
of liquid metal spreading (WS), i.e.,
MAY 2016 / WELDING JOURNAL 181-s
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WELDING RESEARCH
Fig. 17 — Measured and calculated filler metal spreading widths (U =
30 V).
Fig. 18 — Measured and calculated widths of undercutting de­
fects (U = 30 V).
a=
A
B
D
C
D
Fig. 19 — GTA and GMA hybrid welding.
2Wu = WM – WS, as illustrated in Figs.
16–18. A simple model that can be
used to predict the width of undercutting defect was proposed to optimize
the numerical simulation process.
Figure 15A–C shows the arc shapes
captured using the color camera at different welding speeds. From these figures, it is clear the arc remained upright when the welding speed was lower than 1.0 m/min and leaned back by
about a mere 3 deg when the welding
speed reached 1.5 m/min. Therefore,
the influence of welding speed on arc
shapes can be ignored and the heat
distribution in the arc can be assumed
as Gaussian, described by Equations
5–7 (Refs. 37, 38, 28),
q ( x ,y ) =
3Qarc
πrH2
⎛ 3x 2 + 3 y2 ⎞
exp ⎜ −
⎟
rH
⎝
⎠
Qarc = 0.75UI
( 5)
(6
182-s WELDING JOURNAL / MAY 2016, VOL. 95
rH = 6 × 3.77 × 10−3 I 0.2645 L0.3214 (7 )
where q(x, y) is the heat density of
point (x, y), Qarc is the heat power of
the arc, and rH is the distribution diameter of the arc heat. All the other
parameters in Equations 5–7 are listed
in Table 4.
When using the previous Gauss
heat source in calculation, the temperature of a point located at (x, y, z) at
time t can be calculated by Equations 8
and 9 (Ref. 39),
T ( x ,y,z,t ) =
2Qarc 1
c P 0 ( 4a )1/2
( x + v )2 + y 2 z2 w
d
exp
4a 4a + 2rH2
4a + 2rH2
1
(8)
c P
(9)
where vw is in the welding speed. The
specific heat capacity cp and thermal
conductivity  are the functions of
temperature (Ref. 39).
Then Wm can be calculated by Equations 5–9 and Fig. 16 shows the calculated data were in good agreement
with experimental results when the
welding speed was between 0.4 and
1.5 m/min.
To predict Wu, it was also necessary
to determine Ws precisely and quickly.
Ws mainly depended on the thickness
of the liquid metal layer under the arc.
Figures 13 and 17 show the variations
in the thickness of the liquid metal
layer and Ws followed similar trends
when the welding speed increased
from 0.4 to 1.5 m/min. In addition,
Ws varied directly with the mass of
filler metal or welding current. So the
thickness of the liquid metal layer and
the welding current were used to predict Ws using Equation 10,
Wf = 0.024 I hs
0.65( hs hu )
( 10)
where hs and hu are the depths of solidliquid and gas-liquid interfaces, respectively, as illustrated in Fig. 13. The
units of hs and hu are in millimeters. As
shown in Fig. 17, the calculated Ws
were in good agreement with the experimental results within the welding
speed range of 0.4–1.5 m/min.
The measured and calculated un-
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WELDING RESEARCH
dercutting widths are shown in Fig.
18. The variation trend of predicted
Wu was in good agreement with that of
the measured, and the errors were acceptable. Equation 11 can be used to
predict the undercutting width, Wu:
Wu =
(
)
1
1
Wm Wf = Wm
2
2
0.65( hs hu )
0.012 I hs
( 11)
Suppression of Undercutting by
Low­Current Gas Tungsten Arc
The above studies support the idea
that lack of liquid metal flow toward
the weld pool boundary induces formation of undercut. In this work, it is
thought that a gas tungsten arc would
adjust the fluid flow in the weld pool
and suppress undercutting defects. In
GTA and GMA hybrid welding processes, the welding speed was set as 1.0
m/min and the GMAW current was
250 A, in which case serious undercutting defects occurred, as shown in Fig.
9. The distance between wire and
tungsten electrode was 5.0 mm, which
can reduce the burning of the tungsten electrode by the gas metal arc and
utilize the synergic effect of two arcs
on the welding process. Experiments
have been done when the GTAW current was from 50 to 280 A, and
showed that 50 A was enough to suppress the undercutting defects when
the GMAW current was 250 A. The gas
tungsten arc was used either as the
leading arc or as the trailing arc. The
welding parameters used in the
process are listed in Table 5.
When the gas tungsten arc is used
as the leading arc, it is beneficial for
the stability of droplet transfer. The
hybrid welding process is extremely
stable without welding spatters. In
this case, tracer particles came into the
weld pool near the front boundary, as
illustrated in Fig. 19A. From Fig. 11, it
is known that at welding speed 1.0
m/min, the average longitudinal velocity of the backward flow in GMAW was
175 mm/s, which decreased in this
case to 150 mm/s, calculated from experiments. On the other hand, the average transverse velocity varied only
marginally between the case in Fig. 11
and the present case. So the flow of
liquid metal was extended wider to-
ward the weld pool boundary. This
supplied a sufficient amount of filler
metal at the weld toe. Figure 19C, D
shows that Wm decreased obviously
from 9.6 to 5.3 mm. The reduction of
Wm facilitated the liquid metal to fill
the grooves at the weld toes.
When the gas tungsten arc was
used as the trailing arc, it became unstable due to the liquid metal fluctuation in the weld pool. The hybrid welding process was affected by the unstable gas tungsten arc, resulting in slight
welding spatter. The average longitudinal velocity of the backward flow at
welding speed 1.0 m/min in GMAW
decreased considerably from 175 to
120 mm/s (for the present case) under
the action of forward drag force from
the gas tunsten arc. Meanwhile, the
average transverse velocity slightly increased from 50 to 68 mm/s. As
shown in Fig. 19B, tracer particles
flowed into the weld pool near the
front boundary, and their direction of
flow was less deviated, i.e., toward the
undercutting area (this direction is in
contrast to the flow direction of tracer
particles in Fig. 10F where particles
were more deviated toward the pool
boundary in the middle of the weld
pool). Also, the flow covered a wider
distance on the weld pool side boundaries on both sides. Wm decreased from
9.6 to 7.7 mm, as shown in Fig. 19C, E.
These changes promoted liquid metal
to fill the grooves at the weld toes.
These experiments demonstrated
that the gas tungsten arc with low current changes the behaviors of molten
metal in such a way that the liquid
metal is directed toward the grooves at
weld toes and fills them. This suppresses or eliminates the undercutting
defects. Further, it was found the
welding speed can be increased to 1.5
m/min without undercutting defects
using this hybrid welding process.
depth, and avoiding undercutting defects) in cooperation with an expert
welding system.
2. In high-speed welding, a thin layer of liquid metal under the arc impedes the transverse spreading of liquid metal. The backward flow of liquid
metal decreases slowly in the middle
part of the weld pool and cannot flow
back to fill the grooves at the weld
toes. Therefore, there is lack of molten
metal at the weld toes resulting in undercutting defects.
3. The viscous force, drag force of
the arc and droplet impacts in the
front of the weld pool, gravity and
Maragoni force at the middle of the
weld pool, and Maragoni force at the
tail of the weld pool were responsible
for the flow patterns of liquid metal in
the weld pool.
4. The predicted width of undercutting showed good agreement with the
measured data, but experimental evaluation of hs and hu was very complicated.
These may be calculated with improved
numerical models in the future.
5. Either a leading or a trailing gas
tungsten arc with low current can improve the fluid flow of the weld pool
and suppress the undercutting defect.
With a low-current gas tungsten arc in
a hybrid process, welding at speeds as
high as 1.5 m/min did not result in
any undercutting defect. This will increase productivity.
Acknowledgments
This work is sponsored by the National Natural Science Foundation of
China under Grant No. 51305235,
and Specialized Research Fund for
the Doctoral Program of Higher
Education of China under Grant No.
20130131120013. The authors also
acknowledge valuable technical discussions with Girish Kumar Padhy.
Conclusions
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