IIW Doc IV- 922 - 07

FIBER LASER-MAG HYBRID WELDING OF 780MPA HIGH STRENGTHSTEELZhongjie LIU
Muneharu KUTSUNA
Graduate School of Engineering, Nagoya University
1 Furo-Cho, Chikusa-ku, Nagoya, 464-8603, Japanxyliuzhj8@hotmail.com, Kutsuna@numse.nagoya-u.ac.jp

1. Introduction
Joining process and procedures need to keep pace with new application and the developmentof materials. CO2 laser is normally used laser in laser welding [1, 2] and laser-arc hybrid
welding [3, 4] on the benefit of the low equipment cost. In laser few years, fiber laser has
developed very quickly. The output power of fiber laser has increased from 100 W tomulti-kilowatt in the laser six years [5]. Now, the 50 kW ytterbium-fiber laser is available [6].
Fiber lasers are a new generation of solid state lasers. In a fiber laser, the laser cavity (resonator)
is contained within optical fiber and the active medium is a clad doped gain fiber. The active
fiber is doped with rare earth elements such as Neodymium, Ytterbium, Erbium and Thulium.
The fibers are pumped (excited) by high brightness semiconductor diodes. For IPG, fiber lasers
are pumped out with ultra-long life multimode single emitter diodes that have estimatedlifetimes over 100,000 hours. Single emitter diodes have longer lifetimes than diode bars, diodearrays or lamps uses in other types of lasers.
Fiber lasers has many advantages overconventional laser technologies, including
high processing performance (low beam
divergence, no warm up and no variation inspot size with power), high beam quality,
low cost of ownership, maintenance free
operation, no diodes or consumables toreplace, and high wall plug efficiency, and acompact and mobile package.
The inherent efficiency of fiber lasers is
unrivalled when compared to conventionallaser technologies. Ytterbium fiber lasers
Fig. 1 Comparison of the most popular industrial high power
offer 30% wall plug efficiency, compared tolasers.
2-3% for a lamp pumped YAG and 10%
Diode laser Fiber laser Nd:YAG laser
CO laserfor a CO2 laser. Fig.1 shows the
0.30
comparison of the most popular industrial
high power laser in several aspects [7].
0.25
CuHigh power lasers can be used for deep
Steel
0.20
Ag
Au
penetration welding in a diversity ofmaterials since the low wave length that
0.15
Fecharacterizes these lasers allows its
0.10
absorption by almost all metals and alloys,
Al
Al
0.05
Mo
Au
and the fiber delivery system provides thenecessary flexibility on the positioning of0
0.1 0.2 0.3 0.5
12
4 6 8 1020
the beam. The absorption of number ofWavelength l, mm
metals as a function of laser radiation
Fig. 2 Absorption of a number of metals as a function of
wavelength is shown in Fig. 2.
Absorption2

laser radiation wavelength.

1Some of researches have paid their attention
on the fiber laser welding and fiber laser-arc
hybrid welding. The fiber laser and fiber
laser-arc are used for welding of X100 highstrength steel in the research works [8, 9].
In this chapter, the effect of leading heatsource of arc or laser on the bead formation wastested. The effects of laser-arc distance andMAG power on the bead formation were alsostudied. The phenomena of fiber laser-MAG
hybrid welding were observed using a highspeed video camera. The welding phenomena infiber laser-MAG hybrid welding and CO2
laser-MAG hybrid welding were compared.

6-AxialRobot

Shielding gas

Fiber
laser
opticalsystemTorch

Fig. 3 Fiber laser-arc hybrid welding system.

2. Experimental procedure

2.1 Laser equipment used for hybrid welding
An ytterbium doped fiber laser equipment,
IPG YLR-2000, with a maximum output power of 2 kW was used in the present work. The
characteristics of the fiber laser are as follows;
Wavelength : 1070 nm
Beam parameter product (BPP) : 2.2 mm mrad
Output fiber core diameter : 150 m
Minimum spot diameter : 152 m ( when focal length of lens is 100mm)
2.2 Experimental setup
Since the fiber laser with a wavelength of 1070 nm, it can be transferred by a fiber, whichmakes the welding system is more convenience. Fig.3 shows the fiber laser-arc hybrid welding
system developed by the authors. The laser optical system and the arc torch are fixed withspecial brackets and make up a hybrid welding head. The parameters concerning to the welding
head, such as laser-arc distance and electrode wire extension of arc torch, can be adjusted
according to the welding parameters. This laser-arc hybrid
welding head is fixed to a 6-axial robot to form the welding
system. It is also a fiber laser welding system when taking
off the arc torch from the hybrid welding head. During thelaser welding and laser-arc hybrid welding, the laser beam
does not perpendicular to the surface of specimen. There is
6 degree angle between laser beam and the perpendicularline of the surface of specimen for decreasing the porosity
formation [10].
2.3 Observe the phenomena of laser welding and hybrid
welding
A high speed video camera shown in Fig.4 was used forobserving the plasma formed by laser and metal transfer andarc during laser welding and laser-arc hybrid welding. The
digital, high-speed video system records color images at the
speed of 4,000 pictures per second (pps). The used highspeed video camera is NAC’s Memrecam fx6000 highspeed video camera. NAC’s Memrecam fx6000 is a fully
self-contained, digital, high-speed video system that records

2Fig. 4 Photograph of high speed video
camera.

Laser beam

ra

pie

ce

La

se

r le

e

p
hs

d
ee

vid

a
oc

Montering computer

Hig

Df

ad
ing
We
hy
ldin
bri
gd
dp
ire
roc
ctio
es
n
s

me

MA
G to
rch

Wo
rk

Arc

lea
din
gh
We
yb
ldin
rid
gd
pro
ire
ce
ctio
ss
n

Data collecter

Back lighter

Fig. 5 Schematic of setup for observing welding phenomena during laser-arc hybrid welding.

Table 1 Chemical Compositions and tensile property of the High Strength Steel Plate and filler Wire

Tensile properties(N/mm2)Elements (mass%)

Materials
C

Si

Mn P

S

Ni

Cr

Base metal

0.09

0.30

1.67

0.012 0.004

0.05Filler wire (DS80)

0.05

0.93

2.04 11
0.0 0.008

2.05

0.24

Mo Ti

0.07
0.45

Cu

0.01
0.16

F
e

0.16
*

YP

0.10

0.23
Bal.

TS

Bal.
-

743
-

*: Cu of electrode wire cover is also included.

brilliant color, or crisp monochrome images, with resolutions up to 512 X 512 pixels at 4,000
pps. Using an advanced CMOS sensor, the Memrecam fx6000 captures images at frame rates inexcess of 200,000 pps. The extremely rugged Memrecam fx6000 is easily networked andsynchronized using NAC’s multi-camera control system and it is completely compatible with
NAC’s complete line of accessory items including 2D and 3D Motion Analysis Systems. As
always, NAC allows you to obtain the ultimate performance and flexibility from these new
products by providing you with unparalleled service and support, including systems integrationand engineering. The setup of observation system is shown in Fig. 5.
2.4 Materials used in the experiment
Materials used in these welding were 6 mm thick high strength steel plate with tensile
strength of 800 MPa. Filler wires used in laser-MAG hybrid welding were 1 mm in diameter
solid wire for welding 800 MPa class high strength steel. The compositions of the steel plates
and solid wires are shown in Table 1.
3. Result and discussion
3.1 Effect of leading heat source in fiber laser-MAG hybrid welding
The laser-leading and arc-leading fiber laser-MAG hybrid welding can be realized bychanging the moving direction of the robot. When the laser-arc hybrid welding performs andheat sources perform in the sequence of arc-laser, the welding process is arc-leading hybrid
welding process. Otherwise, the hybrid welding process is laser leading hybrid welding process.
The fiber laser-MAG hybrid welding performed under the conditions of laser output power of 2
kW, defocusing distance of -2 mm, MAG current of 100 A, arc voltage of 14 V and welding
speed of 15 mm/s. Fig.6 shows the bead appearance and bead profiles at laser-arc distance of 0
3

839

DLA

Arc-leading laser-arc hybrid welding
Bead appearance

Laser-leading laser-arc hybrid welding
ProfileProfileBead appearance

0mm

2mm

10mm

3mm

10mm

3mm

10mm

3mm

10mm

3mm

3mm

3mm

Bead profile of arc leading
hybrid weld

Bead profile of laser leading
hybrid weld

Fig. 6 Effect of leading heat source on bead appearance and bead profile in fiber laser-MAG hybrid welding. (laser power of 2 kW,
defocusing distance of 0 mm, MAG current of 150 A, arc voltage of 17 V, laser-arc distance of 0 mm, welding speed of 15 mm/s.)

mm and 2 mm. It can be found that both of the welds achieved by laser-leading hybrid welding
and arc-leading hybrid welding had a good bead appearance. The weld bead of laser-leading
weld looks like a MAG weld bead because this weld bead formed finally by the MAG arc.
However, the weld bead of MAG-leading hybrid welding formed finally by the laser.
As for the bead profiles, arc-leading laser-arc hybrid welding achieved a ‘V’ shape weld
bead and laser-leading hybrid welding achieved a ‘Y’ shape weld bead. It can be decided that
arc-leading hybrid welding achieved a better bead profile than that achieved by the laser-leading
hybrid welding. One of the reasons is that the MAG arc had an action of preheating for the
successive laser welding in the arc-leading hybrid welding. Another reason is the electrodeorientation in laser-arc hybrid welding. In MAG welding, electrode orientation affects the weld
bead shape and penetration to a great extent than arc voltage or welding speed. The MAG
welding with backhand technique achieves a narrow and deep weld bead. However, the MAG
welding with forehand technique achieves a wide and shallow weld bead [11].
Fig.7 shows the photographs of plasma in laser-leading hybrid welding and arc-leading

Fig. 7 Photographs of plasma induced by laser and MAG in arc-leading and laser-leading hybrid welding. (laser power of 2 kW,
defocusing distance of -2 mm, MAG current of 200 A, arc voltage of 19 V, welding speed of 8 mm/s.)

4

hybrid welding with the laser output power of 2 kW and MAG current of 200 A and arc voltage
of 19A at the welding speed of 8 mm/s. One of the phenomena is that plasmas induced by the
MAG arc were absorbed by the laser beam in all of the conditions of laser-arc distance from 0
mm to 5 mm. The second phenomenon is that the volume of plasma induced in laser-leading
hybrid welding was bigger than that in arc-leading hybrid welding. The reason is that the weld
pool formed by laser firstly in laser-leading hybrid welding and there was some metal vapor
above the molten pool. The third phenomenon is that that the plasma induced by the fiber laser
was almost not found from the photos of laser-arc distance of 5 mm because of the short wave
length and high absorption of the fiber laser.

Bead width, mm

Penetration, mm

3.2 Effect of laser-arc distance in fiber laser-MAG hybrid welding
Like the laser-arc distance in CO2 laser-MAG hybrid welding, it also affects the weld beadshape and penetration in fiber laser-MAG hybrid welding. In this section, the effect of laser-arc
distance on the weld bead was investigated using the laser power of 2 kW and MAG current
was of 100 A, 150 A and 200 A. The laser arc distance of 0 mm, 1 mm, 2 mm, 3 mm and 5 mm
was investigated.
Fig.8 shows the laser-arc distance and MAG power (MAG current and arc voltage) on thepenetration. It can be found that the penetration decreased with the increase of laser-arc distancewith different MAG power, for example the MAG current of 100 A, 150 A and 200 A. The
penetration decreased from 4.5 mm to 3.5 mm when the laser-arc distance increased from 0 mm
to 5mm. The reason is that the interaction between laser and arc became weaker and weakerwith the increase of laser-arc distance. There was not interaction between laser and MAG arc
when laser-arc distance was 5 mm as shown in Fig.7. Arc augmented laser welding is a welding
process by adding an arc heat source to the laser beam to increase welding speed andpenetration. Arc augmented laser welding canbe divided into two kinds of welding process
5.0
4.5
that are laser-arc hybrid welding process (laser
4.0
and arc coupled as one process) and laser-arc
3.5
combined welding process (without any
3.0
mutual interaction between laser and arc) [12].
2.5
2.0Actually, the laser-arc welding process can not
1.5
2 kW+100A
be called laser-arc hybrid welding process
1.0
2 kW+150A
according to the definition of U. Dilthey
0.5
2 kW+200A
0because there is no interaction between laser
0
1
2
3
4
5
6
beam and MAG arc when the laser-arc
Laser-arc distance, mdistance was larger than 3 mm as shown in
Fig.7. It also can be found that there was little
Fig. 8 Effect of laser-arc distance and MAG power on
penetration. (Welding speed of 8 mm/s.)
plasma induced by fiber laser beam. Whenlaser-arc distance was 0 mm, the interaction
9
between laser and MAG arc was very strong and8
the deepest penetration was available in the fiber
7
laser-MAG hybrid welding.
6
Fig.9 shows the effect of laser-arc distance on
5
the weld bead width in fiber laser-MAG hybrid
43
welding. Bead width of hybrid welds increased
2 kW+100A
2
with the increase of MAG power using the same
2 kW+150A
1
2 kW+200A
output laser power. The bead width increased
0from 4~5 mm to about 8 mm when the MAG
0
1
2
3
4
5
6
current increased from 100 A to 200 A. However,Laser-arc distance, mat the same laser output power and MAG power
9 Effect of laser-arc distance and MAG power on(MAG current times arc voltage), the bead width Fig.
bead width. (Welding speed of 8 mm/s.)

5

Defocusing
distance

0 mm

-1 mm

-2 mm

-3 mm

-5 mm

Bead
profile
2mm

2mm

2mm

2mm

2mm

Fig.10 Macrograph of weld bead profile when defocusing distance changed from 0 mm to -5 mm in fiber laser-MAG
hybrid welding.

changed within 1 mm when the laser-arc distance changed from 0 mm to 5 mm. That means thatMAG power affects hybrid weld bead heavily. It is though that the increased melted filler
affected the weld bead width when the MAG power increased in those experiments. As for thelaser-arc distance, it affected the bead width slightly comparing with the MAG power.
Penetration and bead width, mm

3.3 Effect of defocusing distance of laser on weld bead in hybrid welding
Although the depth of focus of fiber laser is5longer than that of CO2 laser using the same
4
focusing lens with 200 mm focal length. The
defocusing distance of laser beam affects the3
welds in laser welding. In this section, the effect of2
defocusing distance of laser beam on weld bead is
Bead width1
investigated.
Penetration
Figs.10 and 11 show the macrographs and0
penetration of the weld bead profiles achieved
-6
-5
-4
-3
-2
-1
0
under defocusing distance from 0 mm to -5 mm at
Defocusing distance, mm
the welding speed of 15 mm/s and MAG current
Fig. 11 Effect of defocusing distance on penetrationof 150 A, arc voltage of 17 V and laser output and bead width in fiber laser-MAG hybrid welding.
power of 2 kW. From this figure, it can be foundthat the defocusing distance affected the bead penetration strongly although the defocusing
distance affected penetration slightly
in fiber laser welding. When the
defocusing distance changed from 0
mm to -2 mm gradually, thepenetration increased from 2.9 mm to
3.6 mm. However, the penetration
decreased from 3.6 mm to 2.4 mm
when the defocusing distance changed
from -2 mm to -5 mm. It seems that
defocusing distance of -2 is thesuitable parameter for laser in fiber
laser-MAG hybrid welding when theother conditions are MAG current of
150 A, arc voltage of 17 V andlaser-arc distance 0 mm.
Fig.12 shows the schematic ofarc-leading fiber laser-MAG hybrid
welding process. Because the surfaceof molten pool is lower than the upper Fig. 12 Schematic of fiber laser-MAG hybrid welding process. (Arcsurface of specimen during the leading process, DLA equaling to 0 mm.)

6

M

AG

cu

rre
nt,

A

Penetration,mmlaser-arc
hybridwelding[13],
defocusing distance of minus is better5for getting deeper penetration. In otherwords, the focused point of the laser
bead in the base metal is better forgetting deeper penetration. On the other
hand, there is little plasma induced by4
the fiber laser beam during thelaser-MAG hybrid welding as shown in
Fig.7. The volume of plasma induced by
200
MAG arc and laser beam was not bigger
than that produced by MAG arc when3150
the laser-arc distance was 0 mm, 3 mm
1
2
and 5 mm. That means the lost laser
Lase
3
r-arc
4
dista
energy in the plasma did not increase
nce,
5 100
mm
when the laser-arc distance changed
from 3 mm to 0 mm. And the laser beam
Fig.13 Effect of MAG current and laser-arc distance on
can interact with MAG arc completely
the penetration in fiber laser-MAG hybrid welding
when the laser-arc distance is 0 mm.
That is the reason of the deepest penetration gotten when the laser-arc distance was 0 mm.

3.4 Effect of MAG power on the weld bead
The fiber laser-MAG hybrid welding performed keeping the laser output power constant, 2
kW, and changing the MAG current at 100 A, 150 A and 200 A at the welding speed of 8 mm/s.
The relationship among penetration, MAG current and laser-arc distance is shown in Fig.13. It
shows that the penetration increased with the increase of MAG current and with the decrease oflaser-arc distance. The deepest penetration was about 4.6 mm when the laser-arc distance was 0mm and MAG current was 200 A. The shallowest penetration was about 3.1 mm when thelaser-arc distance was 5 mm and MAG current was 100 A. The reason is that the MAG powerincreased with the increase of MAG current and the interaction between laser beam and MAG

t = 0 ms

0.5 ms

1.0 ms

1.5 ms2.0 msFig. 14 Photographs of metal transfer at the welding current of 150 A, 17 V in MAG welding.

Fig. 15 Metal transfer in fiber laser-MAG hybrid welding. (MAG current of 150 A, arc voltage of 17 V, fiber laser output
power of 2 kW, laser-arc distance of 0 mm.)

7

2.5 msarc is best when the laser-arc distance was 0 mm as shown in Fig.7.
3.5 Metal transfer in fiber laser-MAG hybrid welding
Ordinarily, the metal transfer in MAG welding is short circuiting transfer when the arc
current at the range of 100 A to 200 A. Fig.14 shows a example of short circuiting transfer at the
MAG current of 15 A and arc voltage of 17 V using the fx-6000 NAC high-speed video camera
at the speed of 4000 pictures per second.
In the fiber laser-MAG hybrid welding, the same MAG conditions including welding speed,
MAG current, arc voltage were used as in MAG welding. Fig.15 shows the photographs ofmetal transfer in fiber laser-MAG hybrid welding. It shows that the metal transfer in fiber
laser-MAG hybrid welding was also short circuiting transfer. However, the metal transferring
time increased. It was about 4 ms from the high speed video. And there was some spatterproduced in thephotographs (thelast photograph inFig.15).
Fig.16 shows
the metal transfer
infiber
laser-MAG
hybrid welding
when the laser-arc
distance changed
from 1 mm to 5
mm at the MAG
current of 100 A
and arc voltage of
15 V. It showsthat there was not
so much different
Fig. 16 Metal transfer in fiber laser-MAG hybrid welding of different laser-arc distance( MAG current: 100A, Arc voltage: 15V, Laser power: 2kW)
in metal transfer
with the laser-arc
distance changed
from 1 mm to 5
mm at the MAG
current of 100 A
and arc voltage of
15 V. Even the
MAG current andarcvoltageincreased to 150
A, 17 V and 200
A, 19 V, the metaltransfer was also
shortcircuitingtransfer. However,the stability ofmetal transfer wasdifferent, in otherwords, the spatter
Fig. 17 Comparison of the metal transfer in MAG welding and laser-MAG hybrid welding
produced underwith laser-arc distance of 1 mm and 5 mm.

8Penetration, mm

some welding conditions.
As shown in Fig. 16, there was no spatter produced at theend of metal transfer with the laser-arc distance of 1 mm, 2
mm and 5 mm. And the metal transfer was short circuitingtransfer. But, with the increase of MAG current, the spatterproduced. At the MAG current of 200 A, spatter produced at
the laser-arc distance of 0 mm and 1 mm. There was not
spatter produced when the laser-arc distance over 2 mm at
the MAG current of 200 A. The MAG welding had not
spatter when the MAG current was 200 A and 19 V asshown in Fig. 17. The reasons are that the overheating of theneck between the solid wire and the droplet or the molten
pool and forces acting on the neck.
Fig.18 shows the schematic of forces dropped on the neckbetween droplet (or molten pool) and electrode and the heatproduced by the electrical resistance. The forces acting on
the neck are the gravity of the droplet or the melted solidFig. 18 Schematic of forces on the neckbetween droplet and electrode inwire between the neck and molten pool, magnetic force
laser-MAG hybrid welding.
produced by the welding current and the surface tension.
Those forces make the diameter of the neck small and small.
At the same time, theelectrical resistance between
Laser-arc distance
0 mm
2 mm
m 6m
electrode wire and the molten
pool became bigger andbigger because the electricalFiberlaser-MA G
is inversely proportion withthe cross-sectional area as
2mm
2mm
2mm
Cross
formula (1). It is well know
sectionthat the cross-sectional area isin direct proportion with theCO2squared diameter of the necklaser-MA G
as formula (2). And the Joule
3mm
3mm
3mm
heat will increase quickly
because the joule heat is in Fig. 19 Cross section of fiber laser-MAG and CO2 laser-MAG hybrid welds.
direct proportion with theelectrical resistance. So, the Joule heat
will in inversely proportion with the
5.0
squared diameter.
4.5
On the other hand, the melted solid
wire was heated to a high temperature
4.0
by the laser because of the short
3.5
laser-arc distance. The metal of theneck between solid wire and the
3.0
droplet will be evaporated and thanFiber laser-MAG hybrid welding
the exploration occurred. The spatter
2.5
CO2 laser-MAG hybrid welding
produced at those conditions.
2.0

3.6 Comparison of fiber laser-MAG
hybrid welding and CO2 laser-MAG
hybrid welding

0

1

2

3

4

5

6

Laser-arc distance, mFig. 20 Effect of laser-arc distance on penetration in fiber
laser-MAG and CO2 laser-MAG hybrid welding.

97

3.6.1 Effect of welding
process on the weld beadshape
In this section, CO2
laser-MAG hybrid welding
and fiber laser-MAG hybrid
welding performed to weld
9 mm thick and 6 mm thickhigh strength steels with 2
kW laser power and 140A x
16.5V and 150A x 17V
MAG power, respectively.
MAG leading laser-MAG
hybrid welding process was

Fig.21 Photoes of plasma produced by laser and arc in fiber laser-MAG hybrid
welding (a) and CO2 laser-MAG hybrid welding (b) at laser-arc distance of 5 mm.

adopted in these performances. Fig.19 shows some of cross sections of CO2 laser-MAG andfiber laser-MAG hybrid welds. It can be found that the nail shape weld bead appeared in CO2
laser-MAG hybrid welds when laser-arc distance was 0 mm and 2 mm. When the laser-arc
distance was 6 mm, the nail head disappeared in CO2 laser-MAG hybrid welding. However, thebead shows ‘V’ shape cross section when the laser-arc distance was 0 mm in fiber laser-MAG
hybrid welding. Even when the laser-arc distance was 2 mm or 6 mm, the bead shape is
acceptable. The reason is that there was more energy arriving at the substrate materials in fiber
laser-MAG hybrid welding than that in CO2 laser-MAG hybrid welding.

10

The
relationshipbetweenpenetration and welding speed is
100 A, 14.5V
150 A, 17V
200 A, 20V
MAG powershown in Fig.20. With theincrease of laser-arc distance, theFiber laserMAG hybrid
penetration of welds decreased in
weldingfiber laser-MAG hybrid welding
and CO2 laser-MAG hybrid
2mm
2mm
2mm
welding. But the penetration
decreased quickly when theCO2 laserlaser-arc distance increased fromMAG hybrid
0 mm to 2 mm. However, thewelding
penetration showed a bit increase
3mm
3mm
3mm
when the laser-arc distanceincreased from 0 mm to 2 mm inFig. 22 Cross sections of welds under different MAG power in fiber
CO2 laser-MAG hybrid welding.
laser-MAG and CO2 laser-MAG hybrid welding processes.
The penetration decreased at thesame trend in fiber laser-MAG hybrid welding and CO2 laser-MAG hybrid welding as thelaser-arc distance larger than 2 mm. But, the penetration of fiber laser-MAG hybrid welds isdeeper than that of CO2 laser-MAG hybrid welds about 0.3 mm. This phenomenon can be
explained by the plasma produced by laser and arc.
3.6.2 Plasma formed in hybrid welding process
When the focused laser beam irradiates onto the surface of specimen, a part of the radiation isabsorbed by the material and a part is lost by reflection, radiation, conduction and plasma
absorption. In laser-arc hybrid welding, the laser energy absorbed by the plasma produced bylaser and arc in laser-arc hybrid welding is very important aspect. Metal vapors are transparent
to the laser beam at low vapor temperatures and begin to absorb the beam strongly as the vapour
temperature increases [14]. The Roles of both arc plasma and laser plasma are important forbead formation in hybrid welding process [15].
In laser-MAG hybrid welding processes, the plasmas produced by laser and arc wereobserved by a high speed video camera at the speed of 4000 pictures per second. Fig.21 shows
the pictures of fiber laser-MAG hybrid welding and CO2 laser-MAG hybrid welding in the caseof laser-arc distance of 5 mm in arc leading hybrid welding processes. The plasma produced byfiber laser was not found as Fig.21(a), however, that produced by CO2 laser has very large
volume and high brightness as Fig.21(b). The reason is the different wavelengths of fiber laserand CO2 laser. The role played by the wavelength is important because the characteristics oflaser induced plasma are heavily dependent on the processes which take place in the region
whose electron density is close to that for cut-off of the radiation [16]. The cut-off density
electron of laser radiation can be calculated by the sample formula (3):For CO2 laser, wavelength, , is 10.6 m, the cut-off electron density, nc, is 1019. For fiber laser,
wavelength, , of 1.07 nm, the cut-off electron density, nc, is 1021. Assume that the absorption oflaser radiation by the plasma is in direct corresponding to the electron density, the lost energy oflaser can be calculated as:
When the nelost has the same value, the lost energy of fiber laser in plasma is about 1% of thatof CO2 laser beam.
On the other hand, in the two laser-MAG hybrid welding processes, fiber laser produced very
little plasma during welding; however, there was a lot of plasma produced in the CO2
laser-MAG hybrid welding as shown in Fig.21. There will be more laser energy reached at thesubstrate in fiber laser-MAG hybrid welding than that in CO2 laser-MAG hybrid welding.
3.6.3 Effect of MAG power on bead formation in CO2 laser- and fiber laser-MAG
hybrid welding

11Penetration and bead width, mm

Keep the laser output power content, 2 kW, change the MAG current and then the arc voltage,
the bead cross section of welds are shown in Fig.22. It can be found that the fiber laser-MAG
hybrid welds had a better shape than that of CO2 laser-MAG welds. In the range of MAG powerchanged from 100A, 14.5V to 200A, 20V, the wine cup shape weld bead did not appeared infiber laser-MAG hybrid welding. However, the wine cup shape weld bead appeared when the
MAG power increased from 150A, 17V to 200A, 20V. Even at the low MAG power of 100A,
14.5V, the weld bead showed a little bit wine cup shape.
Fig.23 shows the relationship between penetration, bead width and MAG power in fiber
laser-MAG and CO2 laser-MAG hybrid welding. With the increase of MAG power in hybrid
welding, the penetration increased in fiber laser-MAG hybrid welding. However, the penetration
increased with the MAG current increased from 100A to 160A. And then the penetration
decreased with the increase of MAG current form 160A to 200A.
The bead width increased with the increase of MAG current from 100A to 200A in fiber
laser-MAG hybrid welding and CO2 laser-MAG hybrid welding. That means the laser typeaffects the bead shape and penetration in laser-MAG hybrid welding processes. Using shortwavelength laser can increases the penetration in laser-arc hybrid welding processes.

4. Conclusion

8
A

7
From the experimental result and discussion,
BC
6
it can be concluded as the following:D
1. A better weld bead is gained by arc-leading
5
fiber laser-MAG hybrid welding.
4
2. Laser-arc distance in fiber laser-MAG hybrid
3
welding affects not only the weld bead, butalso the metal transfer and spatter produced
20
during the welding.
50
100
150
200
250
3. MAG power affects the bead shape andMAG current, A
penetration. With the increase of MAG
Fig. 23 Effect of MAG power on penetration andpower, the penetration increased in the bead width in laser-MAG hybrid welding. A-beadwidth of fiber laser-MAG hybrid welds; B-bead widthexperimental range, 100 A ~ 200 A.
of CO2 laser-MAG hybrid welds, C-penetration of
4. Deeper penetration and narrower bead width fiber laser-MAG hybrid welds, D-penetration of CO2
welds can be available in fiber laser welding laser-MAG hybrid welds.
than that in CO2 laser welding according to
the shorter wavelength and higher absorptionof fiber laser.
5. Short wavelength fiber laser produces little plasma than the long wavelength CO2 laser inhybrid welding process.
6. The plasma produced by the laser in laser-arc hybrid welding affects the weld bead shapeand the suitable laser-arc distance. The suitable laser-arc distance is 0 mm in fiber
laser-MAG hybrid welding, however, it is 2 mm in CO2 laser-MAG hybrid welding.
7. Using high energy density fiber laser in laser-arc hybrid welding can get better bead shape
weld beads than using CO2 laser when the welding performed with different MAG power,such as from 100A to 200A.

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