Hybrid welding with fiber laser and CO2 gas shielded arc

Hybrid welding with fiber laser and CO2 gas shielded arc

Accepted Manuscript Title: Hybrid Welding with Fiber Laser and CO2 Gas Shielded Arc Author: M. Wahba M. Mizutani S. Katayama PII: DOI: Reference: S09...

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Accepted Manuscript Title: Hybrid Welding with Fiber Laser and CO2 Gas Shielded Arc Author: M. Wahba M. Mizutani S. Katayama PII: DOI: Reference:

S0924-0136(15)00047-3 http://dx.doi.org/doi:10.1016/j.jmatprotec.2015.02.004 PROTEC 14277

To appear in:

Journal of Materials Processing Technology

Received date: Revised date: Accepted date:

2-11-2014 2-2-2015 4-2-2015

Please cite this article as: Wahba, M., Mizutani, M., Katayama, S.,Hybrid Welding with Fiber Laser and CO2 Gas Shielded Arc, Journal of Materials Processing Technology (2015), http://dx.doi.org/10.1016/j.jmatprotec.2015.02.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Hybrid Welding with Fiber Laser and CO2 Gas Shielded Arc

M. Wahbaa, *, M. Mizutanib, S. Katayamab

Central Metallurgical Research and Development Institute, Helwan, Cairo, 11421, Egypt

b

Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047,

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a

us

*

cr

Japan

Corresponding author Tel.: +201026115758

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E-mail address: [email protected]

Postal address: Central Metallurgical Research and Development Institute, Helwan, Cairo, 11421, Egypt

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Abstract

Argon-rich shielding gas was replaced by 100% CO2 gas for cost reduction in fiber laser-GMA hybrid

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welding of double-side welded T-joints. The welding process using 100% CO2 gas was characterized by a

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large number of spatters, while the penetration depth of a weld was increased and porosity was reduced. With the objective of obtaining a buried-arc transfer for the reduction of spatter formation, the welding

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parameters were optimized by observation with a high-speed video camera. Reduced arc voltage, arc leading arrangement and shortened wire extension were necessary to achieve a buried-arc transfer. A significant reduction in spatter generation could only be obtained by the procedure that the relative distances between the two heat sources in the X and Y directions were controlled to produce a proper profile of the arc cavity that could trap any spatters generated. A regulating action of a keyhole was observed to remove the disturbances in the melt flows caused by the arc short-circuiting, and high quality joints with good appearances and very few spatters could be produced.

Keywords: Hybrid laser-arc welding; CO2 gas; Buried-arc transfer; Spatter; Thick plates

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1. Introduction The addition of an electric arc to a laser beam in a single hybrid process for welding applications was found to result in higher processing speed and deeper penetration, similar to the effect of using a more

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powerful laser individually, as demonstrated (Steen, 1980). This combination of the two heat sources in a single welding process synergistically benefits the advantages and overcomes the drawbacks of each

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individual heat source. On the other hand, the welding process becomes more complicated with a large

number of parameters to control (Bagger and Olsen, 2005). Therefore, many operating parameters have

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been studied extensively to better understand and gain more knowledge about their influences on the welding process and the resulting joint, the interactions among them and the relevant welding phenomena.

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The shielding gas is important among these influential parameters.

The concept of selecting a shielding gas for laser and hybrid laser-arc welding processes differs

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according to the employed laser source. For CO2 laser, plasma control is the major concern. Beck, et al. (1995) have indicated that, during high power CO2 laser welding, deep penetration could be completely

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interrupted due to the absorption, refraction and defocusing of the laser beam by the laser induced plasma.

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Consequently, the priority is given to a gas that can reduce the plasma formation. Gao and Zeng (2009) have stated that helium, because of its high ionization potential, must be used as shielding gas in hybrid

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CO2 laser-arc welding in order to obtain deeply penetrated welds. However, due to considerations such as cost, bead profile and process stability several compositions of helium and other gases including argon, carbon dioxide and oxygen have been investigated. Gao, et al. (2007) found that full penetration in hybrid CO2 laser-TIG welding of stainless steel could be obtained only when helium content in helium-argon shielding gas was greater than 50%. Fellmann and Kujanpää (2006) denoted that high quality joints might be produced in hybrid CO2 laser-GMA welding of carbon steel with various compositions of the shielding gas and the best results were obtained by the addition of 2-5% CO2 gas to helium-argon mixture. Moreover, a positive influence of oxygen addition to the shielding gas on the chemical homogeneity of the fusion zone was noted by Zhao, et al. (2009). Almost homogenous distribution of alloying elements could be obtained when the oxygen content in the shielding gas was higher than 2%. In addition to the composition, the influence of the shielding gas flow rate was investigated considerably. Choosing a proper range of

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shielding gas pressure is essential to the suppression of plasma and to the performance of stable hybrid welding process, as presented Chen, et al. (2006). Similar results were also reported by Gao and Zeng (2009) and Tani, et al. (2007).

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For solid-state lasers, on the other hand, plasma formation is not a problem due to the shorter wavelength of the laser beam. Therefore, shielding gases are optimized in order to enhance the process

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stability and improve the quality of the welded joint. Naito, at al. (2006) noted a slight increase in the

penetration depth when oxygen was added to argon shielding gas in hybrid Nd:YAG laser-TIG welding of

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stainless steel. They reported that this increase in penetration depth might be attributed to inward surface tension-induced melt flows. Similarly, Zhao, et al. (2011) indicated that the penetration depth increased

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with increasing the oxygen percentage of helium-oxygen mixed shielding gas in hybrid fiber laser-GMA welding of carbon steel. However, they referred this increase to the formation of carbon monoxide gas by

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reaction between dissolved carbon and oxygen, which in turn expanded the keyhole. Furthermore, in a recent study, Zhao, et al. (2014) added that the vapor pressure inside the keyhole increased due to the

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formation of carbon monoxide gas. This could stabilize the keyhole and prevent porosity formation in

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partial penetration hybrid laser-GMA welding of carbon steel. It is evident from previous studies that for both CO2 and solid-state lasers the prevalence is for inert

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gases such as helium and argon, while active gases such as carbon dioxide are merely used as minor additions. However, in some countries, carbon dioxide is much cheaper than helium or argon. Moreover, higher welding speeds and deeper joint penetration could be obtained with carbon dioxide shielding in arc welding process. Accordingly, the present study was carried out to replace argon-rich shielding gas with pure carbon dioxide gas for cost reduction in hybrid fiber laser-GMA welding with the objectives of elucidating welding phenomena and establishing hybrid welding with fiber laser and CO2 gas-shielded arc. 2. Experimental 2.1 Materials The base material used in this study was K36D shipbuilding steel of 14 mm in plate thickness. The filler wires used in welding experiments were MIX-50S and MG-50 of 1.2 mm in diameter, the standard

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filler wires specified for MAG shielded arc welding and CO2 shielded arc welding, respectively. The chemical compositions of the base material and the filler wires used are given in Table 1. A double-side welded T-joint configuration was investigated. Test pieces were prepared in dimensions of 200 mm × 300 mm and 100 mm × 300 mm. The plates of 100 mm and 200 mm in width were set as a web and as a flange,

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respectively.

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Chemical analysis, mass%

Specimen Si

Mn

P

S

Cu

Nb+V+Ti

Ti+Zr

Fe

K36D

0.16

0.29

1.04

0.013

0.002

0.01

0.02

----

Bal.

MIX-50S a

0.08

0.56

1.22

0.009

0.011

0.24

----

0.07

Bal.

MG-50 b

0.04

0.71

1.6

0.007

0.24

----

0.19

Bal.

Used with MAG shielding gas

b

Used with CO2 shielding gas

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te

a

0.022

d

Filler wires

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C

M

Base material

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Table 1 Chemical compositions of base material and filler wires used.

2.2 Methods

Hybrid laser-arc welding trials were performed using a 20 kW continuous wave IPG Photonics Yb-

fiber laser and Fronius TransPuls Synergic 5000 GMAW machine. The geometrical arrangement of the two heat sources with respect to each other and to the test piece is shown in Fig. 1. A laser beam was transmitted through an optical fiber of 300 μm diameter and focused on the test piece surface by a focusing lens of 300 mm focal length in a processing head of 1.5 imaging ratio. The laser spot size at the focal plane was 450 μm. A base-line welding experiment was carried out with MAG (80% Ar + 20% CO2) shielding gas. Then 100% CO2 gas was applied. Shielding gases were provided at a constant flow rate of 30 L/min through the arc torch nozzle of 12 mm diameter.

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Z

C

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an

ω

B

A

θ

y

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X

d

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Fig. 1 Schematic representation of the experimental arrangement of the arc and the laser beam.

Welding parameter

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Travel speed, m/min

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Table 2 Optimized hybrid laser-arc welding parameters Value 1

Laser power, kW

8

Defocusing distance, mm

+5

Wire feed rate, m/min Arc current, A Arc voltage, V

10 MAG: 304, CO2: 310 MAG: 26.8, CO2: 26.7

Wire extension, mm

20

Welding direction

Laser leading

A, mm

1

B, mm

3

C, mm

2

θ, °

10

ω, °

10

α, °

30

β, °

20

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Welding parameters were optimized beforehand with MAG shielding gas. The optimized welding parameters are given in Table 2. During welding, a high-speed video camera was used to monitor the molten pool, keyhole, droplet transfer and arc behavior at a framing rate of 5000 f/sec. After welding, welded joints were examined visually and photographed. The images were then analyzed using image

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analysis software (Image-Pro Plus) to measure the number and sizes of spatters. The sizes were represented by the maximum diameter in the projected area of spatters. The minimum measurement level was set at 0.3

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mm. Measurements were carried out on the middle 50 mm long portion of the welded joints. Specimens for

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macrostructural investigation were cut across the welded joints, polished and etched with a solution of 90%

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ethanol + 10% nitric acid.

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3. Results and discussion

Preliminary experiments, not included here, were conducted to optimize the welding parameters using MAG gas. The resulting welded joints were inspected visually and macroscopically. The acceptance

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criteria included bead appearances, fillet profiles and sizes, presence of undercuts or lack of fusion and

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throat depth. Since the joint was designed to be double-side welded, 60% of the web thickness was set as a

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minimum throat depth to insure complete fusion between the web and the flange. After the welding parameters given in Table 2 were obtained, a comparative experiment was carried out using CO2 as a shielding gas. In general, it was observed that the CO2 arc was harsher and less stable than that of the MAG gas. This resulted in a larger number of spatters, rougher bead surfaces and less smooth transition from the weld to the flange surface. On the other hand, deeper penetration and reduced tendency to porosity formation were remarked in the case of CO2 gas. Exemplary bead appearances and cross sectional macrostructures of joints welded under MAG and CO2 are shown in Fig. 2. For each case, three cross sections, marked in the figure as I, II and III, were cut at distances of 40, 70 and 100 mm from the welding start point, respectively.

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(a)

MAG

(b)

9 mm

MAG section I

(c)

9.5 mm

9.2 mm

MAG section II

9.8 mm

(d)

9.5 mm

10 mm

10 mm

spatter

9.9 mm

CO2 section I

(g)

9.5 mm

CO2 section II

10.8 mm

10.7 mm

9.7 mm

(h)

CO2 section III

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10 mm

(f)

10.8 mm

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CO2

porosity

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(e)

MAG section III

Fig. 2 Bead appearances and fusion zone macrostructures of joints welded under MAG gas: (a) – (d) and

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CO2 gas: (e) – (h).

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The high-speed video camera observations indicated that in the case of MAG gas, metal droplets were transferred to the weld pool smoothly and regularly across the open arc. The electrode tip was conical most

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of the time and the size of the metal droplets was smaller than the electrode diameter. At the employed

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current level, two types of spray transfer, according to the IIW nomenclature described by Liu and Siewart (1989), were observed. The first type was projected spray where the electrode had a short taper and the

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droplets were only slightly smaller than the electrode diameter. The second one was streaming spray where the electrode had a long taper and the droplets were much smaller than the electrode diameter. For the case of CO2 gas, on the other hand, metal droplets were transferred to the weld pool irregularly. It was observed that a metal droplet was pushed up away from the weld pool by the arc forces and moved in random directions as a result of the high mobility of the arc root. With continuous feeding and melting of the filler wire, the droplet grew in size and touched the weld pool making an explosive short-circuiting which, in turn, ejected the molten metal in the form of larger-sized spatters. According to the IIW classifications this mode is described as repelled globular transfer. Snapshots from the high-speed video camera observations are shown in Fig. 3, illustrating the metal transfer mode for both shielding gases. In the first snapshot for MAG case at time t1 a tapered electrode tip was observed. At time t1 + 0.0018 s a droplet was formed at the electrode tip with a size slightly smaller than the electrode diameter indicating projected spray transfer.

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After 0.0016 s (at time t1 + 0.0034 s) the metal transfer mode was changed to streaming spray where the taper of the electrode tip was elongated and the droplet size decreased. In the CO2 gas case, a hemispherical electrode tip was observed in the image at time t2. A droplet remained attached to the electrode for more than 0.0484 s and it continued to grow in size till it contacted the weld pool with an explosive short-

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droplet

t = t1 sec filler wire

streaming spray

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1 mm

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projected spray

arc plasma

MAG

filler wire keyhole

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circuiting, as seen in the image at time t2 + 0.0498 s.

t = t1 + 0.0018 sec arc plasma

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droplet

explosive short circuiting

1 mm

t = t2 sec

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d

CO2

keyhole

t = t1 + 0.0034 sec

t = t2 + 0.0484 sec

t = t2 + 0.0498 sec

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Fig. 3 Droplet transfer modes for MAG and CO2 shielding gases during respective hybrid welding.

The above results confirmed two additional advantages of using CO2 gas. Besides its low cost, deeper penetration and less porosity were observed. This might be attributed to the dissociation of CO2 gas in the arc plasma into CO and O2 gases. Then O2 recombines with carbon from the base metal generating CO gas which increases its partial pressure in the keyhole and consequently results in deeper penetration and less porosity formation, as explained in previously published reports of Zhao, et al. (2011) and Zhao, et al. (2014). In the mean time, it was also confirmed that spatter formation was a serious problem that might not be accepted in some applications even with the presence of the aforementioned merits. Therefore, further experiments were conducted to solve this problem. Based on the previous reports published regarding this

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issue in conventional CO2 arc welding process, it was found that current waveform control was the major approach to reduce spatter formation as explained by Mita (1989). However, this solution requires a special power source. A more simple approach that can be applied with conventional power sources is to weld with a reduced arc voltage or what is called buried-arc transfer. Jeffus (2012) indicated that with a reduced arc

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voltage the wire tip can be driven below the surface of the weld pool, droplet size is reduced, and any spatter produced by short-circuiting is trapped inside the arc cavity. Consequently, the next welding

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experiment was performed at the arc voltage of 22 V while keeping the other parameters in Table 2

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unchanged.

The consecutive images shown in Fig. 4 were taken from a high-speed video during hybrid welding at

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22 V arc voltages. As can be seen, the situation is not so different from that at normal arc voltage given in Fig. 3 for CO2 case. A metal droplet remains attached to the wire and grows in size, an explosive short-

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circuit between the droplet and the weld pool takes place then melt expulsion and many spatters are produced. Fig. 5 (a) shows the surface appearance of the corresponding joint. Compared with the joint

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welded at normal arc voltage, presented in Fig. 2 (c), the number of spatters increased while the size was reduced. This is clearly evident in Fig. 5 (b) and (c) where size distribution of the spatters is given for each

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case.

filler wire

keyhole

t = t3 sec

arc plasma

1 mm

explosive shortcircuiting

melt expulsion and spatter

droplet

t = t3 + 0.0363 sec

t = t3 + 0.0424 sec

t = t3 + 0.0491 sec

Fig. 4 Effect of reduced arc voltage on metal transfer and spatter formation.

Although only arc voltage reduction is effective for spatter mitigation in the case of arc welding, it seems not enough for hybrid laser-arc welding of T-joint and other parameters should be optimized to obtain buried-arc transfer. The first parameter to consider was the welding direction. To this point, welding

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ip t

(a)

10 mm (b) 15

(c)6 5

10

4

5

3

us

Counts

Counts

27 total counts

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42 total counts

2 1

0 1.2

2.1

3

3.9

4.8

5.7

6.6

Spatter size, mm

an

0

0.3

0.3

0.9

1.5

2.1

2.7

3.3

3.9

Spatter size, mm

te

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hybrid welding at 22 V (b) and 26.7 V (c).

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Fig. 5 Bead appearance of hybrid-welded joint at 22 V arc voltage (a), and size distribution of spatters in

was performed with a laser beam as a leading heat source, which implied a forehand technique regarding

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the arc torch. This was based on previous knowledge of arc welding where forehand technique is preferred to backhand technique for one pass fillet welding because of wide and flat bead profile, as described by Jeffus (2012). However, it was thought that it might be easier to obtain a buried-arc transfer if the arc stroke on a solid metal rather than a laser molten pool. Accordingly, the arc voltage was kept at 22 V and the welding direction was switched to arc leading. Figure 6 displays representative snapshots taken from the high-speed video of the arc leading welding experiment. Buried-arc transfer could be obtained but it was not stable and the arc could be seen above the work piece surface from time to time. For example, in Fig. 6 only images at time t = t4 + 0.065 s and t = t4 + 0.1224 s represent a buried-arc transfer while other images show normal arc. Also, it was observed that the number of spatters increased compared with the previous experiments as indicated in Fig. 7. This experiment shows that arc leading is better than laser leading if buried-arc transfer is desirable. Meanwhile, the instability of the transfer mode and the increased amount of spatters implied that other welding parameters should be considered.

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filler wire keyhole

spatter spatter

spatter buried arc

t = t4 sec

1 mm

t = t4 + 0.065 sec

t = t4 + 0.1224 sec

t = t4 + 0.2084 sec

ip t

buried arc

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Fig. 6 Effect of arc leading arrangement on metal transfer and spatter formation.

(a)

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(b) 20

10

an

Counts

15

51 total counts

5

10 mm

M

0

0.3

0.6

0.9

1.2

1.5

1.8

2.1

Spatter size, mm

te

d

Fig. 7 Bead appearance (a) and size distribution of spatter (b) of joint welded at arc leading arrangement.

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The next parameter to investigate its influence was the wire extension. Jeffus (2012) has pointed out that at the same wire feeding rate increasing the wire extension would increase the melting rate and reduce the weld penetration due to the increased joule heating effect. Subsequently, the wire extension was shortened to 15 mm and other parameters were kept the same as those of the last experiment. The highspeed video observations revealed that buried-arc transfer could be obtained constantly over the full welding time, as can be seen in Fig. 8. Nevertheless, spatters were also observed to be generated repeatedly. The first two images in Fig. 8 (at time t = t5 s and t = t5 + 0.006 s) indicate that some spatters were generated from the molten metal around the keyhole inlet. At time t = t5 s, a spherical part of the molten metal at the keyhole inlet (denoted by a white arrow on the image) was protruded. After 0.006 s this part was completely ejected from the weld pool in the form of spatter and another portion (denoted by a yellow arrow on the image) was protruded. This might be attributed to a short distance between the arc and

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melt expulsion spatter

buried arc

t = t5 sec

1 mm

melt ejection

buried arc

buried arc

t = t5 + 0.006 sec

t = t5 + 0.0318 sec

t = t5 + 0.0334 sec

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cr

Fig. 8 Effect of shortened wire extension on metal transfer and spatter formation.

ip t

keyhole

transfer of the melt flow disturbance behind the keyhole

connected keyhole and arc cavity

the laser beam that caused the keyhole to lie in the area of melt flow disturbance induced by the arc shortcircuiting. The next snapshot taken at time t = t5 + 0.0318 s shows that the keyhole was connected to the arc

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cavity. This opened up the arc cavity and made it easier for the spatter to escape instead of being trapped inside the cavity. The next snapshot taken at time t = t5 + 0.0334 s reveals the transfer of a shock wave

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induced by the arc short-circuiting from the inside of the arc cavity below the work piece surface up to the surface of the molten pool around the keyhole, which also facilitates the generation of the spatter. The

d

surface appearance of the joint and the size distribution of the spatter presented in Fig. 9 indicate that,

(b) 25

49 total counts

20

Counts

(a)

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were produced.

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compared with the last experiment, the size of the spatters was reduced while a similar number of spatters

15 10 5 0

10 mm

0.3

0.6

0.9

1.2

1.5

1.8

2.1

Spatter size, mm

Fig. 9 Bead appearance (a) and size distribution of spatter (b) of joint welded at shortened wire extension.

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Successively, the distance between the arc and the laser beam, referred to as B in Fig. 1 and Table 2, was increased to 5 mm. It was observed that the stability of the buried-arc transfer was improved; however, many spatters were generated as can be seen in Fig. 10. Similar to the previous results, some spatters were generated by melt ejection from the weld pool near the keyhole inlet (snapshot at time t = t6 +

ip t

0.0112 s) while the majority of the generated spatters escaped from the arc cavity (snapshots at time t = t6 + 0.0112 s, t = t6 + 0.0262 s and t = t6 + 0.0294 s). Up to this point, buried-arc transfer could be obtained

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constantly and continuously, which led to a reduction in the size and number of spatters as can be seen in

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Fig. 11. However, a further reduction in spatter generation seemed to be possible if the opening of the arc cavity could be slightly closed. It is apparent in Fig. 10 that the arc cavity was widely opened particularly in

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the X direction that facilitated the outflow of the melt. This might be attributed to a long distance between the arc torch and the laser-irradiated point on the joint web, in the X direction, referred to as C in Fig. 1 and Table 2. The next experiment was conducted at 22 V arc voltage, arc leading, 15 mm wire extension, B = 5

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mm and C = 1 mm. The results are shown in Fig. 12 and 13. A stable buried-arc transfer that could reduce spatter generation significantly and good weld pool geometry were obtained, (see the Supplementary video

Z buried arc

Y

X

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keyhole

te

d

1).

t = t6 sec

1 mm

melt ejection

t = t6 + 0.0112 sec

spatter

spatter

spatter

t = t6 + 0.0262 sec

t = t6 + 0.0294 sec

Fig. 10 Effect of elongated distance in the Y direction between the arc and the laser beam on metal transfer and spatter formation.

A final experiment was carried out to investigate the macrostructure of the joint produced with the optimized parameters of the previous experiment. Only laser power was reduced to 7 kW since the throat depth, in Fig. 2 (f) – (h), exceeded 70% of the web thickness. A 150 mm long joint was produced and then

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(b) 20

(a)

34 total counts

10 5 0 0.3

0.6

0.9

1.2

1.5

1.8

2.1

cr

10 mm

ip t

Counts

15

Spatter size, mm

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Fig. 11 Bead appearance (a) and size distribution of spatter (b) of joint welded at elongated distance in Y

buried arc

regulated melt flow

buried arc

disturbed melt flows

buried arc

t = t7 sec

1 mm

d

M

keyhole

an

direction between arc and laser beam.

t = t7 + 0.0064 sec

t = t7 + 0.0328 sec

t = t7 + 0.041 sec

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te

Fig. 12 Buried-arc transfer and melt flow regulation under optimized conditions during hybrid welding.

three cross-sections were cut at the distances of 50, 75 and 100 mm from the welding start point to check the macrostructure of the fusion zone. The results are shown in Fig. 14. A consistent fusion zone profile was obtained along the joint. An average throat depth corresponding to approximately 64% of the web thickness was produced. No defects such as porosity, undercut or lack of fusion were detected. Compared with the results shown in Fig. 2, the fillet size was reduced and the arc penetration was increased because of backhand technique. On the other hand, besides stabilizing the buried-arc transfer, arc leading was observed to have a positive influence on the bead profile. The presence of the keyhole behind the arc could regulate the melt flows by forcing them to flow around its inlet provided that the keyhole was located at an adequate distance. This can be seen in Fig. 12 in the image taken at time t = t7 + 0.0328 s. Some arcinduced disturbed flows tried to close the keyhole on its way to the weld pool. However, these flows were

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(b) 6

(a)

13 total counts

Counts

5 4 3 1 0 0.3

0.6

0.9 1.2 Spatter size, mm

1.5

cr

10 mm

ip t

2

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Fig. 13 Bead appearance (a) and size distribution of spatters (b) of joint hybrid-welded under optimized

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conditions.

9.3 mm

8.9 mm

cross-section at 50 mm

10.5 mm

9.1 mm

cross-section at 75 mm

cross-section at 100 mm

te

9.7 mm

d

M

8.9 mm

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Fig. 14 Cross-sectional fusion zone macrostructure of joint welded under optimized conditions.

forced to advance in paths, indicated by white arrows, around the keyhole inlet in a regulated pattern which resulted in a smooth bead appearance as displayed in Fig. 13 (a). To the contrary, in the case of laser leading any disturbance in the melt flows caused by the arc would result in defects such as undercut or, optimally, rough and irregular bead shape. These results, regarding joint appearance and fusion zone macrostructure, confirm the feasibility of using CO2 as a shielding gas in hybrid laser-arc welding process where advantages such as low cost, defects reduction and deeper penetration can be utilized. However, further investigations regarding the

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influence of CO2 gas on the chemical and mechanical properties of the joint should be performed. This will be addressed in future work.

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4. Conclusions

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A feasibility study of using 100% CO2 as a shielding gas instead of MAG gas in hybrid fiber laserGMA welding of T-joints was conducted. The main conclusions are as follows:

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1- The application of CO2 as a shielding gas was accompanied by an increase in spatter formation and bead surface irregularities. On the other hand, positive influences including deeper penetration

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and reduced content of porosity were observed.

quality joints could be produced.

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2- Buried-arc transfer could be successfully implemented to suppress the spatter formation, and high

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3- Unlike the GMA welding process, only buried-arc transfer is not sufficient to reduce spatter

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generation in the case of hybrid laser-GMA welding of T-joint and the geometry of the arc cavity should be controlled to prevent the outflow of the melt.

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4- Important parameters to control in order to obtain buried-arc transfer and suitable arc cavity include leading heat source, arc voltage, wire extension and relative distances between the two heat sources in the X and Y directions.

5- In addition to the stabilization of the buried-arc transfer, arc leading arrangement had a positive influence on the bead appearance. The presence of a laser keyhole behind the arc at an adequate distance could regulate the disturbances in the melt flows caused by arc short-circuiting.

Acknowledgements This research was conducted in the committee "Research about the Application of Laser Welding

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Technology to Shipbuilding in Japan" of Japan Ship Technology Research Association (JSTRA) sponsored by The Nippon Foundation. References Bagger, C., Olsen, F.O., 2005. Review of laser hybrid welding. J. Laser Appl. 17 (1), 2-14.

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Beck, M., Berger, P., Hugel, H., 1995. The effect of plasma formation on beam focusing in deep penetration welding. J. Phys. D: Appl. Phys. 28, 2430-2442.

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Chen, Y., Lei, Z., Li, L., Wu, L., 2006. Influence of shielding gas pressure on welding characteristics in CO2 laser-MIG hybrid welding process. Chinese Optics Letters. 4 (1), 33-35.

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Fellman, A., Kujanpää, V., 2006. The effect of shielding gas composition on welding performance and weld properties in hybrid CO2 laser-gas metal arc welding of carbon manganese steel. J. Laser Appl. 18 (1), 12-20.

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Gao, M., Zeng, X.Y., 2009. Effect of shielding gas on hybrid laser-arc welding. In: F. Olsen, (Ed.), Hybrid Laser-Arc Welding, Woodhead Publishing, Cambridge, pp.85-105. Gao, M., Zeng, X., Hu, Q., 2007. Effects of gas shielding parameters on weld penetration of CO2 laser-TIG hybrid welding. J. Mater. Proc. Technol. 184, 177-183.

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Jefuus, L., 2012. Welding Principles and Applications. 7th ed. Cengage Learning, Dlmar, pp. 234-261. Mita, T., 1989. Reducing spatter in CO2 gas-shielded arc welding - waveform control. Weld. Int., 3, 227232.

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Naito, Y., Mizutani, M., Katayama, S., 2006. Effect of oxygen in ambient atmosphere on penetration characteristics in single yttrium-aluminum-garnet laser and hybrid welding. J. Laser Appl. 18 (1), 21-27.

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Liu, S., Siewert, T.A., 1989. Metal transfer in gas metal arc welding: droplet rate. Weld. J. Res. Suppl. 2, 52-58.

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Steen, W.M., 1980. Arc augmented laser processing of materials. J. Appl. Phys. 51 (11), 5636-5641. Tani, G., Campana, G., Fortunato, A., Ascari, A., 2007. The influence of shielding gas in hybrid laser-MIG welding. Appl. Surf. Sci. 253, 8050-8053. Zhao, L., Sugino, T., Arkane, G., Tsukamoto, S., 2009. Influence of welding parameters on distribution of wire feeding elements in CO2 laser-GMA hybrid welding. Sci. Technol. Weld. Joining, 14 (5), 457-467. Zhao, L., Tsukamoto, S., Arakane, G., Sugino, T., DebRoy, T., 2011. Influence of oxygen on weld geometry in fiber laser and fiber laser-GMA hybrid welding. Sci. Technol. Weld. Joining, 16 (2), 166-173. Zhao, L., Tsukamoto, S., Arakane, G., Sugino, T., 2014. Prevention of porosity by oxygen addition in fiber laser and fiber laser-GMA hybrid welding. Sci. Technol. Weld. Joining, 19 (2), 91-97.

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List of figure captions Fig. 1 Schematic representation of the experimental arrangement of the arc and the laser beam.

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Fig. 2 Bead appearances and fusion zone macrostructures of joints welded under MAG gas: (a) – (d) and CO2 gas: (e) – (h).

Fig. 4 Effect of reduced arc voltage on metal transfer and spatter formation.

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Fig. 3 Droplet transfer modes for MAG and CO2 shielding gases during respective hybrid welding.

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Fig. 5 Bead appearance of hybrid-welded joint at 22 V arc voltage (a), and size distribution of spatters in hybrid welding at 22 V (b) and 26.7 V (c). Fig. 6 Effect of arc leading arrangement on metal transfer and spatter formation.

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Fig. 7 Bead appearance (a) and size distribution of spatter (b) of joint welded at arc leading arrangement. Fig. 8 Effect of shortened wire extension on metal transfer and spatter formation.

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Fig. 9 Bead appearance (a) and size distribution of spatter (b) of joint welded at shortened wire extension. Fig. 10 Effect of elongated distance in the Y direction between the arc and the laser beam on metal transfer and spatter formation.

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Fig. 11 Bead appearance (a) and size distribution of spatter (b) of joint welded at elongated distance in Y direction between arc and laser beam.

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Fig. 12 Buried-arc transfer and melt flow regulation under optimized conditions during hybrid welding.

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Fig. 13 Bead appearance (a) and size distribution of spatters (b) of joint hybrid-welded under optimized conditions. Fig. 14 Cross-sectional fusion zone macrostructure of joint welded under optimized conditions.

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Research Highlights  Argon-rich shielding gas was replaced with 100% CO2 gas in hybrid laser-GMA welding.

 Parameters were optimized to obtain buried-arc transfer.

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 Deeper penetration, less porosity, rougher bead and more spatters were produced.

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 Sound joints with good appearance and little spatter could be produced.

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