Optics and Laser Technology 134 (2021) 106589
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Optics and Laser Technology
journal homepage: www.elsevier.com/locate/optlastec
Full length article
Investigation of filler wire melting and transfer behaviors in laser welding
with filler wire
T
Wenhao Huang, Shujun Chen, Jun Xiao , Xiaoqing Jiang, Yazhou Jia
⁎
Engineering Research Center of Advanced Manufacturing Technology for Automotive Components, Ministry of Education, College of Mechanical Engineering and Applied
Electronics, Beijing University of Technology, Beijing 100124, China
HIGHLIGHTS
different characteristics of the wire melting and transfer behavior.
• The
best absorptivity can be achieved when the wire feed angle is 45°.
• The
the Wx is the key to obtain stable transfer mode for the liquid bridge.
• Control
• The droplet transfer behavior is directly determined by the resultant force.
ARTICLE INFO
ABSTRACT
Keywords:
Melting and transfer behaviors
Liquid bridge transfer
Droplet transfer
For laser welding with filler wire process, there exist numerous technical parameters about the position between
the laser beam and the filler wire, so it is difficult to guarantee the process stability and weld quality. In order to
obtain stable welding process and high quality weld, the wire melting and transfer behavior must be controlled
as the interaction between the laser beam and filler wire is extremely complicated. In this work, laser welding
with filler wire process was studied, with particular attention to the melting dynamics of filler wire, the wire
melting and transfer behavior. The high-speed camera system was used to observe the wire melting and transfer
behaviors. The direct observation of the welding process by the high-speed camera can help to uncover the
mechanism of the process stability of laser welding with filler wire. Systematic experiments were carried out in
this study for liquid bridge and droplet transfer mode. The results indicated that the relative distance (Wx)
between the filler wire and the laser beam and wire feed rate could influence the wire melting and transfer
behaviors even the whole welding stability significantly. Therefore, control the Wx and wire feed rate is the key
to obtain stable transfer mode for the liquid bridge. The research findings provided a clearer understanding for
improving process stability and weld quality.
1. Introduction
The laser welding is a high power density welding technology,
which has been increasingly utilized in many industries especially for
aerospace and automotive industries in recent years [1,2]. The major
characteristics of laser welding are high precision, minimum heat affected zone and concentrated heat input, which helps to minimize the
residual stress and distortion on the welded specimens. As a result, laser
welding has been widely used in manufacturing industries to produce
high-quality weld joints.
However, owing to the small focal spot diameter of laser beam, high
precision of the workpiece assembly is required to guarantee the weld
quality. Therefore, the gap normally should be reduced to a minimum
or even be set to a zero-gap to ensure the stable process [3]. In order to
make the process more reliable and enhance the industrial applications
of laser welding, it is necessary to develop appropriate techniques to
make the process simpler and more reliable. Previous studies [4,5] indicated that compared with traditional laser welding, laser welding
with filler wire (LWFW) could effectively improve the gap-bridging
capability and greatly reduce the requirements for joint fit-up. In addition, laser welding with filler wire can also can solve the problem of
solidification cracks [6].
For LWFW process, there exist numerous technical parameters
about the position (angle, distance) between the laser beam and the
Corresponding author.
E-mail addresses: huangwenhaojay@163.com (W. Huang), shujunchenbjut@163.com (S. Chen), junxiaobjut@163.com (J. Xiao), xjiang@bjut.edu.cn (X. Jiang),
jiayazhousx@163.com (Y. Jia).
⁎
https://doi.org/10.1016/j.optlastec.2020.106589
Received 17 January 2020; Received in revised form 18 May 2020; Accepted 31 August 2020
Available online 14 September 2020
0030-3992/ © 2020 Elsevier Ltd. All rights reserved.
Optics and Laser Technology 134 (2021) 106589
W. Huang, et al.
filler wire, so it is difficult to guarantee the process stability and weld
quality [7]. Therefore, the wire feed angle must be determined first.
Antti Salminen [8] indicated that an important beam parameter affecting the absorptivity is the angle of incidence between the laser beam
and workpiece surfaces. However, the reflection of the filler wire in
laser welding process is difficult to measure directly. Absorptivity
mainly depends on the laser wavelength and material properties. The
previous research results showed clearly that the filler wire may reflect
a considerable fraction of incoming laser beam. The reflection effects as
well on the welded quality as the energy lost during welding. Research
[9,10] have mainly concentrated on the influence of welding parameters on weld quality. Laser welding with filler wire was usually
considered to be a difficult process for industrial applications, it has too
many parameters and the positioning requirements of the wire are too
strict.
However, the wire melting and transfer behavior was also an important factor influencing laser welding stability and would be reflected
on the final weld quality [11-13]. With the development of high-speed
imaging technique, researchers started to pay attention to the wire
melting dynamics which was closely related to the process stability and
weld quality. Based on this technique, Yu et al [14] studied the melting
dynamics of LWFW with different feed positions and feed rates. The
melting dynamics of LWFW can be generally characterized by three
different forms: explosion, big droplet and molten metal bridge. The
results showed that the explosion or big droplet form would lead to
unstable welding process and undesirable weld quality while the
molten metal bridge form could result in a stable welding process and a
uniform weld bead. Wang et al [15] used CMOS high speed video
system to observe the wire melting behavior and the weld pool dynamics in real time during the welding process. With different wire feed
positions, the transfer mode can be characterized by three different
forms: liquid bridge transfer mode, droplet transfer mode, and
spreading transfer mode. It was reported that liquid bridge transfer
mode was superior to the other two transfer modes with more calm and
stable welding process which resulted in better weld appearance and
lower porosity defect. Ma et al [16] investigated the effects of dual
beam configurations on wire melting and transfer behaviors in different
transfer modes. The results showed that side-by-side configuration
could improve the process stability in droplet transfer mode. Through
the force analysis, they found that the differences of droplet transfer
behaviors in different beam configurations and the action direction and
stability of the resultant force determined the entire transfer process. Su
et al [17] studied the effects of welding parameters on the laser-wire
coupling behavior, the characteristics of deposition layer and the
transition behavior of molten metal by high-speed imaging technique. It
was also reported that the transition behaviors of liquid metal were
decomposed into three transition behaviors: globular, liquid-bridge and
spreading. The results showed that the welding speed and the distance
between the laser and the filler wire had a great influence on the liquid
metal transition, the stability of welding process and the solidification
rate of molten metal.
Based on the above analysis, it can be summarized that there are
mainly two types of wire transfer modes in LWFW process: droplet
transfer mode and liquid bridge transfer mode. The liquid bridge
transfer mode could lead to a high stable welding process, uniform and
smooth appearance and good weld quality, while the droplet transfer
mode can strongly disturb the weld pool flow, causing fluctuations in
welding process, and resulting in undesirable weld bead [18–21].
However, in the practical production process, droplet transfer mode
often occurs and is unavoidable due to limitation of assembly accuracy.
Besides, the relative position of filler wire and laser beam could also
significantly affect the wire melting and transfer behaviors, and even
affect the whole welding stability. Therefore, it is necessary to study
how to determine the welding parameters range of the liquid bridge
transfer mode in LWFW process. A systematic study about the interaction between laser beam and filler wire is still missing. Therefore, the
principles governing the correlations between the wire melting and
transfer behavior, the wire melting dynamics and weld quality need to
be studied.
In this work, the mechanism of the wire melting and transfer behavior and the wire melting dynamics were studied, with particular
attention to the interaction mechanisms between laser beam and filler
wire and the establishment of stable processing conditions of liquid
bridge transfer mode. Therefore, the aim of the current research project
is to determine the influence of key LWFW parameters on wire transfer
modes, the formation of welding defects in droplet transfer mode, and
the range of welding parameters for liquid bridge transfer mode. The
research findings provided more clear understanding for improving
laser welding with filler wire process stability and weld quality.
2. Materials and experimental procedure
2.1. Materials
The bead-on-plate welding experiments are performed on the sheet
of 4043 aluminum alloys with a thickness of 5 mm and the focal point
of laser beam is just on the top surface of the base metal sheets. The
commercial ER5A06 aluminum alloy filler wire with the diameter of
1.2 mm was utilized. Their chemical compositions were listed in
Table 1. Before welding, the surface of aluminum alloy sheets was
polished by the abrasive paper for removing the oxides, and then wiped
with industrial alcohol.
2.2. Experimental setup
Fig. 1(a) shows the experimental setup of LWFW including a high
speed image acquisition system, a laser welding system, a robot system
and a wire feed system. The welding experiments were performed using
a continuous wave solid-state Ytterbium fiber laser (YLS-4000, IPG
Laser GmbH, U.S.A) with a maximum power of 4 kW. The wavelength
of fiber laser is 1.07 µm. The diameter of optical fiber is 600 μm. A
focusing lens of 200 mm and a collimating lens of 150 mm were used to
focus the laser beam. The spot diameter of single laser beam at the focal
length is 0.26 mm, and the focusing plane was positioned at the surface
of workpiece. A six-axis high precision industrial robot (KR 60-3, KUKA
Roboter GmbH, Germany) was used to control welding movement on
which the laser head was assembled. A wire feeder (KD-7000, Fronius
International GmbH, Austria) was used for wire feed. Ar as the shielding
gas was supplied to the surface of molten pool with a flow rate of 15 L/
min. To stabilize the welding process, the filler wire and shielding gas
were set on the same plane with the centerline of laser beam. The
shielding gas nozzle was in the welding direction with an angle ( ) 45°
with respect to the horizontal surface of the workpiece, and the vertical
height (h) with the workpiece was 20 mm, as shown in Fig. 1(b). The
shielding gas nozzle is made of ceramic material with a diameter of
40 mm. The wire melting and transfer behaviors during the welding
process was observed using a high-speed digital camera (i-speed 7–716
high-speed camera, iX Cameras company, England) with a normal
magnifying lens (SP AF 180 mm F/3.5 Tamron Lens, Tamron CO., Ltd,
Japan). The Tamron 180 mm F/3.5 Di LD (Model B01N) is a highly
portable, ultra-telephoto zoom lens with AF precision for shooting inthe-moment action clearly and easily. The images were recorded at a
frame rate of 3000 frames per sec. A backlight source and the high
speed camera are arranged on the two sides of the welding head. Due to
Table 1
Chemical composition of 4043 and ER5A06aluminum alloy (wt.%).
2
Alloy
Si
Fe
Cu
Mn
Mg
Zn
Ti
4043
5A06
4.5–16.0
< 0.40
< 0.80
0.40
< 0.30
< 0.10
< 0.05
0.50
< 0.05
6.8
< 0.10
< 0.20
< 0.20
0.1
Optics and Laser Technology 134 (2021) 106589
W. Huang, et al.
Fig. 1. Experimental setup for LWFW.
the internal resistance of the wire feeder, the data displayed by the
machine and the actual wire feed rate are different, so the wire feed rate
needs to be calibrated before the experiment. The wire feed rates displayed by the wire feeder and actual wire feed rates as shown in
Table 2. The welding parameters used in LWFW process are shown in
Table 3.
Table 2
The calibration of wire feed rates.
Wire feed rates displayed by wire feeder (m/
min)
Actual wire feed rates (m/min)
1
2
3
4
5
6
7
8
9
10
0.51
1.02
1.53
1.88
2.46
2.88
3.51
4.05
4.48
5.12
2.3. The absorptivity experiment
The absorptivity experiments were performed to examine the wire
feed angle –absorption interaction. The presence of base material hinders the measurement of the absorption intensity of the wire by the
laser beam. Therefore, these experiments were carried out without base
material. The schematic illustration of the experimental set-up for
measuring the absorptivity is shown in Fig. 2. Before the experiment,
the mass of wire sent out by the wire feeder was measured by analytical
balance within 1 min. Then the laser beam was focused on the surface
of the filler wire and the wire was fed into the focal cylinder in the open
air. Similarly, the mass of the melted wire was measured by an analytical balance.
Table 3
Welding parameters used in LWFW process.
Welding parameters
Values
Laser power (P)
Welding speed (VS)
Wire feed rates (VF)
Wire feed angle ( )
Wire feed direction
Shielding gas flow rate
2600 W
1 m/min
2–10 m/min
45°
leading
15 L/min
2.4. Definition of wire feed position
The distance between the filler wire and laser beam can be defined
as the distance between their axes. In the welding direction, the
Fig. 2. Schematic illustration of the experimental set-up for measuring absorptivity.
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Optics and Laser Technology 134 (2021) 106589
W. Huang, et al.
Fig. 3. Definition of wire feed position in LWFW.
distance between the laser beam and filler wire is defined as Wx. In the
direction perpendicular to the welding direction, the distance between
the laser beam and filler wire is defined as WY. The schematic of wire
feed position in LWFW is shown in Fig. 3. The distance WY is always
kept constant as 0 mm in order to guarantee that the weld is accurately
located on the welding line L1.
3. Results and discussion
3.1. Determination of wire feed angle
When the filler wire is fed into the focus of laser beam, it absorbs
part of the power and reflects rest part of the power. According to the
principle of conservation of energy, the total absorptivity Ak is given
by:
2.5. Weld formation analysis
After welding, in order to reveal the micro-structure of the welded,
the samples are cut in the size of 150 mm × 75 mm using wire-cut
Electric Discharge Machine (EDM). Cross sections of the welded are
polished by 800,1000,1200,1500 SiC grades, followed by a 3 diamond
suspension, to a mirror-like surface aspect. Then, etching with the
Keller’s reagent (100 ml distilled + 5 ml HNO3 + 3 ml HCL + 2 ml
HF) for 5–10 s was used to reveal the microstructure. OLYMPUS LEXTOLS4000 3D optical micro-scopy is used to observe the depth and width
of welded.
Ak = Pa/P
(1)
Pa = PF + PEV + PO
(2)
Ak = (PF + PEV + PO )/ P
(3)
where Pa is the power of the filler wire to absorb the laser beam, P is the
total power of the laser beam, PF is the power used in melting the filler,
PEV is the power of the filler wire evaporation, PO is the power consumed by overheating filler wire.
The absorptivity can be obtained as follow:
PF = vF Aw s [cs (Tm
4
T0) + HF ]
(4)
Optics and Laser Technology 134 (2021) 106589
W. Huang, et al.
Fig. 4. High-speed photographs of droplet
transfer at different
levels (P = 2600 w,
VF = 1 m/min).
(a) High-speed photographs of droplet transfer at
=0°
(b) High-speed photographs of droplet transfer at
=30°
(c) High-speed photographs of droplet transfer at
=45°
Tm)
(5)
PEV
m
m m1
= EV =
P
m
m
(6)
PO = vF Aw l cl (T1
optimum way to the process. Therefore, it is necessary to find the best
angle for wire feed to maximize the absorptivity of the filler wire. The
photography of the phenomena during laser beam melting the filler
wire in various wire feed angle resulted in acceptable quality photographs. The laser beam, droplet and filler wire interaction zone are all
clearly visible.
Fig. 4 describes the droplet transfer at different levels. The absorption reaches its maximum when the wire feed angle ( ) is 0°, but
decreases with increasing the wire feed angle, as shown in Fig. 5. Low
wire feed angle ( ) must usually be avoided in practice applications
because the dimensional problems of wire feeder equipment, which is
not typically available during practical welding. Increased in the wire
feed angle ( ) was a disadvantage for increasing the absorptivity of the
filler wire. With the increase of wire feed angle, the impact action of the
molten metal on the vertical direction of the molten pool becomes
where vF is wire feed rate, Aw is cross section area of filler wire, s is
average density of filler wire from room temperature to melting point,
cs is average specific heat of filler wire from room temperature to
melting point, Tm is melting point of the filler wire, T0 is the room
temperature, HF is latent heat of fusion, l is average density of liquid
filler wire, cl is average specific heat of liquid filler wire, Tm is average
temperature of liquid filler wire, mEV is the lose mass of the filler wire
by evaporation, m1 is the mass of filler wire by laser beam melting, m is
total mass of the filler wire.
Wire feed angle ( ) is an important parameter affecting the absorptivity. Usually, in the LWFW process, the filler wire is fed in non5
Optics and Laser Technology 134 (2021) 106589
W. Huang, et al.
3.2. Two typical wire transfer modes
The different characteristics of the wire melting and transfer behavior would lead to the different stability of the LWFW process. The
melting and transfer behavior of filler wire can be divided into two
modes, named as “liquid bridge transfer mode” and “droplet transfer
mode”. The liquid bridge transfer mode is that the filler wire is transfer
into the welding pool through a molten metal bridge. This transfer
mode can be used for high stability and good welded quality. It is an
ideal stable transfer mode, and the appearance of welded is uniform
and smooth. The welded appearance of liquid bridge transfer mode is
shown in Fig. 7(a). The droplet transfer mode can strongly disturb the
weld pool flow, causing fluctuations in welding process, and resulting
in undesirable weld bead. The welded appearance of droplet transfer
mode is shown in Fig. 7(b).
The interaction between the laser beam and the filler wire makes
the filler wire transfer behaviors complicated and unstable. The laser
beam heated the filler wire, resulting in different transfer mode. The
mechanism of filler wire transfer behaviors was altered with changing
welding parameters. LWLW parameters such as laser power (P), the
relative distance (Wx) between laser beam and filler wire, wire feed
rate (VF), and wire feed angle( ) determine which type of transfer mode
is achieved.
Fig. 5. The effect of the wire feed angle on absorptivity.
3.3. Analysis of welding stability in liquid bridge transfer mode
The filler wire melting and transfer behaviors in LWFW process in
liquid bridge transfer mode is shown in Fig. 8. As shown in Fig. 8(a), the
laser beam heated and melted the wire tip. Fig. 8(b) shows that the
melted wire grew up to a droplet and then the laser beam continued to
heat on the base material to form a molten pool. When the droplet
grows to a certain size, it would contact with the molten pool and a
“liquid bridge” was established between the wire tip and the molten
pool. Meanwhile, the melted wire would flow smoothly into the molten
pool through the “bridge”, as shown in Fig. 8(c, d). “liquid bridge”
assured that both the wire melting and transfer processes was stable in
LWFW. This wire transfer mode was defined as “liquid bridge transfer
mode”. After that, the molten droplets continuously enter the molten
pool through the “liquid bridge”, as shown in Fig. 8(e).
In laser welding with filler wire, the heating mechanism of the filler
wire affected the welding process stability and welded quality. In liquid
bridge transfer mode, laser beam, filler wire and molten pool have direct interaction between each other and therefore, the filler wire
melting and transfer behaviors was mainly influenced by the laser irradiation (PL), the plasma and metallic vapor radiation (PM) and weld
pool radiation (PW), as shown in Fig. 9.
Fig. 6. The effect of the wire feed angle on the weld appearance.
3.4. Analysis of welding stability in droplet transfer mode
Liquid bridge transfer mode is an optimal transfer mode, which
often gives smooth welded with a stable welding process, but this is a
highly delicate procedure with a small process window. During the
practical welding process, liquid bridge transfer mode is often difficult
to obtain due to inaccurate wire feed position. Due to the mismatch of
the welding parameters, the transfer mode is often the droplet transfer
mode. Droplet transfer mode is a very common but unacceptable mode
during LWFW process, which would result in bad weld appearance and
unstable welding process. In order to improve clearer understanding for
the droplet transfer mode, the filler wire melting and transfer behaviors
were analyzed in detail.
For the droplet transfer mode, the droplet transfer behavior is determined by the forces applied on the droplet. The droplet is affected
mainly by two types forces during LWFW process: one type of force is to
promote droplet transfer force which includes the gravity FG and the
evaporation recoil force FR, and the other type of force is to prevent
droplet transfer force which includes the surface tension FS and the
Fig. 7. The appearance of the welded.
stronger, making the molten pool surface fluctuations in the process
more intense. The best absorptivity can be achieved when the wire feed
angle is 45°.
Finally, the effect of the wire feed angle on the weld appearance was
investigated, as shown in Fig. 6. Considering the absorption rate and
the weld appearance comprehensively, the optimized wire feed angle
was determined to be 45 degrees.
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Optics and Laser Technology 134 (2021) 106589
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Fig. 8. The filler wire melting and transfer behaviors in liquid bridge transfer mode (P = 2600 W, VS = 1 m/min, VF = 2 m/min, WX = 0 mm).
metal vapor/plasma ejecting force FV. The characteristic of each force is
analyzed firstly.
The gravity FG is defined as follows:
FG = mg
m=
4 3
r
3
FR = 0.54P0 exp( HLV
T TLV
) rk2
RTTLV
(9)
where P0 is the atmospheric pressure, HLV is the latent heat of evaporation, T is the liquid temperature of droplet, TLV is the boiling point
of the base metal, R is the universal gas constant, and rk is radius of the
acting area taken as the radius of keyhole.
The surface tension FS is given as follows:
(7)
(8)
where r is the droplet radius, is the density of the droplet metal, g is
the gravity accelation.
It was observed by high-speed imaging technology that when the
laser beam interacts with the droplet, a small keyhole is formed in the
droplet. So a reaction force applied on the droplet was also generated
on the opposite direction of vapor/plasma ejection, which is the evaporation recoil force FR. FR is calculated as follows [19]:
FS = 2
(10)
rw
where is the surface tension coefficient, rw is the radius of filler wire.
FV is given as follows [13]:
FV = CD AP (
7
2
V V
2
)
(11)
Optics and Laser Technology 134 (2021) 106589
W. Huang, et al.
direction of resultant force F is oblique downward, which resulted in
the deviation of the position of the droplet and the laser beam, and then
the metal vapor/plasma ejecting force FV was generated from the high
speed metal vapor acting on the droplet which is ejected from the
keyhole. At this time, the metal vapor/plasma ejecting force FV and
surface tension FS acted as the restraining force together to prevent the
droplet transfer. The size of droplet increased mainly by absorbing the
plasma and metallic vapor radiation (PM) energy. However, due to the
periodic appearance and disappearance of the metal vapor/plasma
ejecting force FV and the evaporation recoil force FR, which affected the
change of action direction of the resultant force and led to the droplet
oscillation phenomenon. With the increase of absorbing energy, the
droplet size increased. With increasing droplet size to a certain extent,
the resultant force mostly pointed to the workpiece surface, and then it
contacted with the molten pool, as shown in Fig. 11. After the liquid
metal transferring to the molten pool completely, a transfer period was
finished.
In droplet transfer mode, laser beam heated the filler wire and base
metal to generate the plasma and metallic vapor. Therefore, the filler
wire melting and transfer behavior is mainly influenced by the laser
irradiation (PL) and the plasma and metallic vapor radiation (PM), as
shown in Fig. 12.
Because the melted wire was transferred to the molten pool in the
form of droplet, the transfer period is an important factor in evaluating
the transfer behavior. In this study, the transfer period is defined as the
time from the beginning of the laser beam heated the filler wire to form
a droplet to the droplet transfer into the molten pool completely. The
effect of the wire feed rate on the transfer period is shown in Fig. 13. It
indicated that increasing the wire feed rate could significantly shorten
the transfer period, while reducing the laser power prolonged the
period.
Fig. 9. The heating mechanism of the filler wire in liquid bridge transfer mode.
AP = RP2
(12)
where CD is the drag coefficient of metal vapor, AP is the projected area
on the plan perpendicular to the flow direction of metal vapor, V is the
density of metal vapor and V is the ejecting speed of the metal vapor.
RP is the radius of projected area if the droplet is spherical. Due to the
force is proportional to the projected area Ap, so the force is also related
to the size of droplet.
Fig. 10(b) shows the forces applied on the droplet in droplet transfer
mode. It is seen that the droplet is affected by four forces. F is the resultant force, and the droplet transfer behavior was directly determined
by the magnitude and action direction of the resultant force F. It can be
seen from Eqs. (7), (8), (11), (12) that the gravity FG and the metal
vapor/plasma ejecting force FV are related to the size of the droplet.
With the increasing size of the droplet, the gravity FG and the metal
vapor/plasma ejecting force FV increase. Therefore, the droplet transfer
behavior was mainly determined by the metal vapor/plasma ejecting
force FV and the gravity FG. Firstly, laser beam directly heated the filler
wire to form droplet, and then the melted wire retracted and grew up to
a spherical droplet. At the same time, a small keyhole was generated on
the droplet, so the droplet was subjected the evaporation recoil force
FR. The size of droplet increased mainly by absorbing the laser irradiation (PL) energy. With continuous wire feed, the droplet size and
gravity FG increase gradually. At this time, only the surface tension FS
acted as a restraining force to prevent the droplet transfer. The
3.5. The wire melting dynamics with different WX
According to the characteristic interaction between laser beam and
the filler wire, the relative distance (Wx) between laser beam and filler
wire can be classified into three different cases. (1) laser beam and filler
wire fully detach, WX > +1.5 mm; (2) laser beam and filler wire
partially overlap, −1.5 mm < WX < +1.5 mm; (3) laser beam and
filler wire fully overlap or laser beam is fully blocked by filler wire,
WX < −1.5 mm.
(1) When the distance Wx is larger than 1.5 mm, the laser beam and
filler wire are fully detached. It was observed that the filler wire
Fig. 10. The forces analysis in droplet transfer mode.
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Fig. 11. The filler wire melting and transfer behaviors in droplet transfer mode (P = 2600 w, VS = 1 m/min, VF = 1 m/min, WX = −1.5 mm).
was melted to form droplets and transfer into the molten pool. The
melting and transfer behavior of filler wire is the droplet transfer
mode. In this case, there was no direct laser beam heating the filler
or liquid droplet, so the melting dynamics in this case are mainly
influenced by the plasma and metallic vapor radiation (PM) and
weld pool radiation (PW) is shown in Fig. 14.
Based on physic mechanics, the wire melting dynamics was determined by the forces applied on the droplet. The droplet is affected
mainly by three forces which include the gravity FG, the surface tension
FS and the metal vapor/plasma ejecting force FV.
Fig. 15 shows the forces analysis in the case of WX > +1.5 mm. F
was the resultant force, and the vector sum or resultant force F can be
obtained as: F = FG + FS + FV .
In this case, there was no direct laser beam to heat the droplet, so
the evaporation recoil force FR was neglected. The wire melting dynamics was directly determiner by the magnitude and action direction
of the resultant force F. Since there was no direct laser beam acting on
the droplet, the size of droplet increased mainly by absorbing the
plasma and metallic vapor radiation (PM) and weld pool radiation (PW).
At the beginning, the filler wire absorbed less energy, the droplet size is
smaller, and the gravity FG is smaller. The direction of the resultant
force F is oblique upward, which makes it difficult for the droplet to
transfer into the molten pool. When the size of the droplet is big enough
to be able to contact with the molten pool, a transfer period was finished. As the filler wire is away from the laser beam-base metal interaction point, the droplet transfer period becomes longer in the case of
WX > +1.5 mm.
Based on the above analysis, it can be summarized that when the
relative distance (Wx) between laser beam and filler wire is too large
(WX > +1.5 mm), the droplet transfer behavior is not stable. As the
energy of melting the filler wire is small, the droplet transfer period
becomes longer and the longer transfer period would lead to a bad
welded appearance even with different wire feed rate. Therefore, the
large distance Wx between laser beam and filler wire should be
avoided.
Fig. 12. The heating mechanism of the filler wire in droplet transfer mode.
Fig. 13. Average transfer periods in droplet transfer mode with different wire
feed rate (P = 2600 w, VS = 1 m/min, WX = −1.5 mm).
(2) When the distance Wx is in the range of −1.5 mm to 1.5 mm, laser
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Optics and Laser Technology 134 (2021) 106589
W. Huang, et al.
Fig. 14. The heating mechanism of the filler wire in the case of WX > +1.5 mm.
continuous wire feed, the droplet size and gravity FG increase gradually.
When the droplet grows to a certain size, it would contact with the
molten pool. (III) Finally, more filler wire melts into liquid metal under
the combined energy from the laser irradiation (PL), the plasma and
metallic vapor radiation (PM) and weld pool radiation (PW). A “liquid
bridge” was established between wire tip and molten pool and the
melted filler wire would flow smoothly into the molten pool through
the “liquid bridge” under the combined force of gravity FG and surface
tension FS.
(3) When the distance Wx is smaller than −1.5 mm, laser beam and
filler wire fully overlap or the laser beam is fully blocked by the
filler wire. It was observed that the filler wire was melted to form
droplets and transfer into the workpiece surface quickly. The
melting and transfer behavior of filler wire is the droplet transfer
mode.
Fig. 15. The forces analysis in the case of WX > +1.5 mm.
Fig. 17 shows process of the filler wire melting and transfer behaviors in the case of WX < −1.5 mm. The process can be divided into
three phases: (I) Firstly, the laser beam directly heated and melted the
filler wire to form droplet, and then the melted wire retracted and grew
up to a spherical droplet. The droplet stuck to the end of the filler wire.
(II) Secondly, the size of droplet increased mainly by absorbing the
laser irradiation (PL) energy and the plasma and metallic vapor radiation (PM) energy. With continuous wire feed, the droplet size and
gravity FG increase gradually. (III) Finally, with increasing the droplet
size to some extent, the droplet contacted with the workpiece surface
and detached from the filler wire due to the gravity FG and surface
tension FS. After the droplet transferring to the workpiece surface
completely, a transfer period was finished. Different from the heating
mechanism of the filler wire of the case when Wx > +1.5 mm, the
energy of melting the filler wire to form the droplet mainly depends on
direct laser irradiation (PL) energy. Therefore, the speed of the filler
wire turning to droplet is faster than the case of Wx > +1.5 mm. By
analyzing the high speed imaging to record the time of droplet transfer,
the transfer period was also significantly shortened, only half of the
transfer period when Wx > +1.5 mm
Fig. 18 shows the forces analysis in the case of WX < −1.5 mm. F
was the resultant force, and the vector sum or resultant force F can be
obtained as: F = FG + FS + FR .
Then the wire melting dynamics was directly determined by the
magnitude and action direction of the resultant force F. As the laser
beam directly heated and melted the filler wire, the droplet grew up
quickly, the direction of the resultant force F is oblique downward,
which easily promotes the droplet transfer into the workpiece surface.
Finally, the droplet is in contact with the workpiece surface, and the
beam and filler wire partially overlap. Part of the laser beam is used
to heat and melt the wire tip and another part of the laser beam is
used to heat the base material to form a molten pool. The melting
and transfer behavior of filler wire is the liquid bridge transfer
mode in this case. When the laser beam and filler wire partially
overlap, the laser beam, filler wire and molten pool have direct
interaction between each other, the filler wire melting dynamics is
mainly influenced by the laser irradiation (PL), the plasma and
metallic vapor radiation (PM) and weld pool radiation (PW).
Therefore, the filler wire can be melted and contact with the molten
pool to establish a “liquid bridge” between wire tip and molten pool
as long as the laser power and wire feed rate are adequate. When
the laser power is constant and the distance Wx is in range
−1.5 mm to 1.5 mm (−1.5 mm < WX < +1.5 mm), the laser
energy used to melt the filler wire is determined by the wire feed
rate (VF). In other words, the wire feed rate can be adjusted in a
wide range while keeping both the wire melting and transfer processes stable in LWFW process. Due to the adjustability of the wire
feed rate to freely adjust the energy to melt the filler wire and form
molten pool, the welding process then became more stable and
more adaptable using different welding parameters.
Fig. 16 shows the schematic of melting mechanism in the case of
−1.5 mm < WX < +1.5 mm. The process can be divided into three
stages: (I) Firstly, the filler wire is fed into the heating zone. Part of the
laser irradiation energy PL1 directly heated the filler wire to form
droplet. At the same time, the other part of the laser irradiation energy
PL2 heated the base material to form a molten pool. (II) Secondly, with
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Optics and Laser Technology 134 (2021) 106589
W. Huang, et al.
Fig. 16. The heating mechanism of the filler wire in the case of −1.5 mm < WX < +1.5 mm.
Fig. 17. The heating mechanism of the filler wire in the case of WX < −1.5 mm.
filler wire moved at a certain speed to separate the droplets from the
filler wire tip.
Based on the above analysis, it can be summarized that when the
relative distance (Wx) is smaller than −1.5 mm, laser beam and filler
wire fully overlap or the laser beam is fully blocked by the filler wire,
the melting dynamics is similar to the case of Wx > +1.5 mm, the
droplet transfer is likely to form and the weld bead is characterized by
periodically present humpings. Although the heating mechanism of the
case of Wx > +1.5 mm and the case of Wx < -1.5 mm are different
and the case of Wx < −1.5 mm has a shorter transition period, droplet
transfer mode was an unacceptable mode, and it would lead to an unstable welding process and a bad welded appearance even with different welding parameters. Therefore, the distance Wx < −1.5 mm
should be avoided.
4. Conclusions
In this work, the mechanism of the wire melting and transfer behavior and the wire melting dynamics was studied, with particular attention to the interaction mechanisms between laser beam and filler
wire and the establishment of stable processing conditions of liquid
bridge transfer mode. The main conclusions of the study were summarized as follows:
(1) The wire feed angle ( ) is an important parameter affecting the
absorptivity. The large wire feed angle is a disadvantage for increasing the absorptivity of the filler wire, but low wire feed angle
( ) must usually be avoided in practice applications because of the
dimensional problems of wire feeder equipment. The best
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Fig. 18. The forces analysis in the case of WX < −1.5 mm.
absorptivity can be achieved when the wire feed angle is 45°.
(2) The different characteristics of the wire melting and transfer behavior cause of the different stability of the LWFW process. The
liquid bridge transfer mode would lead to high stability and good
weld quality. It is an ideal stable transfer mode, and the appearance
of welded is uniform and smooth. The droplet transfer mode can
strongly disturb the weld pool flow, causing fluctuations in the
welding process, and resulting in undesirable weld bead.
(3) The relative distance (Wx) between filler wire and laser beam could
influence the wire melting and transfer behaviors even the whole
welding stability significantly. Therefore, controlling the Wx and
wire feed rate are the key to obtain the stable transfer mode for
liquid bridge. The best WX ranged from −1.5 mm to +1.5 mm.
(4) The differences of droplet transfer behaviors in different WX were
explained by force analyse. The droplet transfer behavior was directly determined by the magnitude and action direction of the
resultant force.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors are grateful for the support from the Natural Science
Foundation of China under grant no. 51505009, 51475009, and
51575133.
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