Numerical simulation of droplet shapes in laser-MIG hybrid welding

Optics & Laser Technology 88 (2017) 1–10

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Numerical simulation of droplet shapes in laser-MIG hybrid welding Zhenglong Lei n, Longchang Ni, Bingwei Li, Kezhao Zhang State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin, 150001 China

art ic l e i nf o

a b s t r a c t

Article history: Received 4 June 2016 Received in revised form 17 July 2016 Accepted 27 August 2016

A three-dimensional finite element model based on minimum energy principle is developed to simulate the droplet transfer process in laser-MIG hybrid welding. The energy manifestations of all forces that determine droplet shapes are considered in this model, and the model has been used to predict droplet shapes. Offset of droplet centroid and critical additional axial acceleration are adopted to characterize the stability of droplet transfer. The calculated droplet shapes and offset of droplet centroid agree well with experimental results. It is found that increasing laser power or decreasing welding current would destabilize droplet transfer. Additional mechanical forces contribute to stable droplet transfer, and the positive effects of increased shielding gas flow rate on the stability of welding processes are subsequently verified. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Laser-MIG hybrid welding Numerical simulation Droplet shapes Additional mechanical force Shielding gas

1. Introduction Hybrid laser-metal inert gas (MIG) welding has been intensely investigated and successfully applied [1–3]. Increased gap tolerances, less deformation, capability of welding highly reflective materials and high welding speed are the advantages of laser-MIG hybrid welding [4–8]. The benefits of its industrial applications include high efficiency and reduction in production cost [9–12]. However, these benefits can be achieved only if the stability of hybrid welding process is ensured, and droplet transfer plays a significant role in determining arc stability and weld qualities in this process [13]. Therefore, it is necessary to investigate droplet transfer behaviors to achieve stable, efficient hybrid welding processes. Some research has been done to study the complementarity of the two coupled techniques and complex droplet transfer behaviors in hybrid welding. Liu [14] reported that the addition of laser decreases the electrical resistance of arc plasma, and in this way arc is attracted and constricted, which stabilizes the arc cathode spots. Besides, the experimental results showed that the droplet transfer mode is changed from globular transfer to projected transfer with the increasing distance between laser and arc. Liu [15] also pointed out that the arc energy is one important factor that determines modes of droplet transfer. Gao [16] studied the arc stabilized mechanism in laser-MIG hybrid welding and found that the laser keyhole fixes the arc root and improves the igniting ability of the arc. Besides, the laser–arc interaction prevents the n

Corresponding author. E-mail address: [email protected] (Z. Lei).

http://dx.doi.org/10.1016/j.optlastec.2016.08.011 0030-3992/& 2016 Elsevier Ltd. All rights reserved.

overheating of the droplet and smoothes the droplet transfer process. Gu [17] focused on the coupling mechanism of and arc and the results demonstrated that the laser provides a conductive, stable plasma channel for the arcs, which can affect the arc shape, slow down droplet transfer, reduce resistivity and stabilize arcs. Zhou and Tsai [18] developed mathematical models to investigate the transport phenomenon in hybrid welding and calculated the complicated velocity and temperature distributions caused by the impingement of filler droplets. It was reported that weld pool dynamics, cooling rate, and final weld bead geometry are strongly affected by the impingement process of droplets. However, no experimental results were presented. Chen [19] studied effects of welding positions on droplet transfer in laser-MAG hybrid welding and found that the mean droplet diameters, transfer frequencies and transfer modes including globular and short-circuiting modes were different at flat, vertical and horizontal positions. Also, research on the buried-arc transfer mode was conducted by Wahba [20] and Pan [21]. It was found that by optimizing welding parameters, buried-arc transfer could be implemented, which would suppress spatter formation in hybrid welding using 100% CO2 as a shielding gas. Li [22] found that in narrow gap laser-GMAW hybrid welding conditions, the droplet transfer frequency decreased with decreasing groove angles. When the groove angle reached its limitation, it caused the droplet to transfer to the groove side walls, rendering the system unstable. In addition to welding positions and groove angles, laser power [23], the laser-arc distance [23, 24] and welding direction [25] are also considered as important process parameters affecting transfer behaviors. Former research mainly focused on the synergic effects of two welding methods and influence of welding conditions on droplet transfer modes. It is well known that, besides droplet transfer

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modes, droplet shapes also help us to optimize hybrid welding processes. Droplet shapes directly reflect transfer stability and axial transfer properties of droplets, and subsequently determine welding qualities. However, few research has been conducted on droplet shapes. In this investigation, a three-dimensional finite element model based on minimum energy principle was developed. It systematically predicted different droplet shapes in various welding conditions, investigated the influence of laser power and welding current on transfer stability and verified the positive effects of additional mechanical forces and increased shielding gas flow rate on the stability of droplet transfer.

2. Mathematical model Compared with arc welding, the addition of laser changes the shape and stability of arc in hybrid welding processes [26,27]. Axial transfer properties of droplets become poor and their volumes increase (Fig. 1), which would lead to welding instability and decreased penetration depth. The main reason is that laser changes the original force conditions of droplets in MIG welding. In addition to surface tension, gravitational force and plasma drag force in MIG welding [28,29], vaporization-induced recoil force and hybrid electromagnetic force caused by both laser plasma and arc plasma also act on droplets in hybrid welding [29]. All these forces generate energy, and droplet shapes are always evolving towards the equilibrium shape with minimum energy during transfer processes. The three-dimensional finite element model developed in this paper is based on the minimum energy principle. The finite element calculations are performed using Surface Evolver, an interactive program for the study of liquid surfaces shaped by various energies, including surface tension energy, gravitational energy and many others [30,31]. 2.1. Energy analysis of droplet

Eg =

The gravitational potential energy of a droplet Eg can be given by

Eg =

∮V ρm gzdV

Fig. 2. Initial meshing of metal droplet for FEM analysis.

(1)

where ρm is the density of liquid metal, g is the gravitational acceleration, z is the distance between the droplet and the zero potential energy surface (as shown in Fig. 2) and V is the volume of the droplet. According to Gauss Formula, Eg can also be expressed by surface integral as follows

1 2

→ →

∮S ρm gz2 k dS

(2)

→ where S is the surface of the droplet and k is the unit vector along the z axis. The surface potential energy Eγ generated by surface tension is given by

Eγ =

∮s γ⋅dS

where γ is the surface tension coefficient.

Fig. 1. High speed video images of droplet transfer: (a) MIG welding, and (b) laser-MIG hybrid welding.

(3)

Z. Lei et al. / Optics & Laser Technology 88 (2017) 1–10

In laser-MIG hybrid welding, plasma drag force Fp promotes droplet transfer, and it can be given by [29]

⎛ ρ v2 ⎞ f f ⎟ FP = CDAP ⎜⎜ ⎟ ⎝ 2 ⎠

(

(4)

)

(5)

where Rd is the radius of the droplet and Rw is the radius of the necking part of the droplet. The potential energy generated by plasma drag force Ep can be calculated by

Ep =

∮V ( apyy + apzz)ρm dV

∮S

(

)

2 3CD Rd2 − R w ρf v2f ⎛ y2→ z2 →⎞ → ⎜ sin θ⋅ j + cos θ⋅ k ⎟⋅dS ⎝ 8R d 3 2 2 ⎠

(7)

where θ is the angle between wire axis and z axis (as shown in → Fig. 2) and j is the unit vector along the y axis. The vaporization-induced recoil force FRL can be calculated by [29,32,33] ⎧ ⎛ N k T 3/2 ⎞ 1 ⎪ ⎟⎟exp( − MaLv /Nak BTs ) C Aρ m2V02⎜⎜ a B s 2 D ⎪ 4 π R ⎝ MaB 0 ⎠ ⎪ h FRL = ⎨ 2 /2R 2 ⎪ ×exp − DLA h ⎪ ⎪ 0 ⎩

(

1 2

)

→ →

∮S ρm FmRL z2 k ⋅dS

⎛

.2→

∮S 4π3Rd3 ⎜⎝ sin( θ + α ) y2 j

+ cos( θ + α )

z2 →⎞ → k ⎟⋅dS 2 ⎠

(12)

In order to add critical additional axial acceleration to the model, it is assumed that critical additional axial acceleration is equal to n times the gravitational acceleration, that is, ac = n⋅g . Similarly, the potential energy Ec generated by additional mechanical forces can be calculated by

Ec =

⎛

2→

∮S nρm g ⎜⎝ sin θ y2 j

+ cos θ

z2 →⎞ → k ⎟⋅dS 2 ⎠

(13)

( DLA > Rh)

=−

3CDρm2 V02 32πRh2R d

⎛

(8)

3/2 ⎞

∮S ⎜⎝ NaMkBaTBs0

→ → × exp( −MaL v/NakBTs )exp( −DLA2 /2Rh2)z2 k ⋅dS

⎟ ⎠

Fig. 2 shows the initial meshing of the droplet in hybrid welding for finite element modelling analysis. For the convenience of meshing, the three-dimensional coordinate system is rotated θ degrees around X axis and the wire axis is the Z0 axis of the new coordinate system. The initial mesh consists of nodes (V1–V12), edges (E1–E20) and surfaces (F1–F11). Boundary conditions for the welding wire and droplet are discussed as follows. The welding wire is a cylinder, so the shape constraint for the wire is given as 2 x′ 2 + y′ 2 = R w

(14)

z′ = H1

(15)

where H1 is the distance between the upper surface of the wire and the zero potential energy surface. The height constraint for the lower surface of the wire is given as

z′ = H2

(16)

where H2 is the distance between the lower surface of the wire and the zero potential energy surface. In order to prevent droplets from sinking into the wire in iterations, the constraint for the interface between the droplet and the wire is given as

Nonpositive: z′ = H2 (9)

Hybrid electromagnetic force Fhem and its action angle α which refers to the angle between its direction and wire axis can be calculated by [29]

Fhem = − 152.6308 + 2.0481I + 128.4058P − 0.4618I⋅P − 0.0011I 2 − 5.196P 2

2.2. Boundary conditions

where Rw is the radius of the wire. The height constraint for the upper surface of the wire is given as

( DLA ≤ Rh)

where Rh is the largest radial dimension of metal vapor, A is the area of droplet projection image which is vertical to flow direction, V0 is a constant whose value is of the order of the speed of sound in the condensed phase, Na is Avogadro’s number, kB is Boltzmann’s constant, Ts is the melt's surface temperature, Ma is the molecular weight of metal vapor, B0 is a vaporization constant, L v is the latent heat of evaporation and DLA is the laser-wire distance. The direction of vaporization-induced recoil force is opposite to that of gravity in hybrid welding. Similarly, the potential energy ERL generated by vaporization-induced recoil force is calculated by

ERL = −

×

(6)

where apy and apz are the components of droplet acceleration generated by plasma drag force along the y and z axis, respectively. Then, according to Gauss Formula and Eq. (4) and (5), Ep can be eventually calculated by

Ep =

Em = ( −152.6308 + 2.0481I + 128.4058P − 0.4618I⋅P − 0.0011I 2 − 5.196P 2)

where CD is the Reynold number, ρf is the density of plasma fluid, vf is the velocity of plasma fluid and AP is the action area of plasma fluid which can be given by [29] 2 AP = π Rd2 − R w

3

(10)

(17)

The droplet is assumed to be incompressible and its volume is constant. The volume constraint for the droplet is given as

V=

∮V 1⋅dV

(18)

This can also be expressed by surface integral as follows

V=

→

∮S 13 ( x′ i

→ → → + y′ j + z′ k ⋅dS

)

(19)

→ where i is the unit vector along the x axis.

α = 36.52 − 0.2145I + 7.4152P − 0.029I⋅P + 0.0004I 2 − 0.106P 2

(11)

where P is laser power and I is welding current. Similar to the potential energy generated by plasma drag force, the potential energy Em generated by hybrid electromagnetic force can be calculated by

3. Experimental procedure Laser-MIG hybrid welding experiments were performed to validate numerical calculations. The base metal used for laser-MIG hybrid welding process was LF6 aluminum alloy with the

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dimension of 250  50  5 mm. 5356 Al-Mg alloy of 1.2 mm in diameter was used as filler wire. Their chemical compositions are shown in Table 1. Fig. 3 shows the experimental set-up for CO2 laser-MIG hybrid welding. A diffusion cooling carbon dioxide laser (Rofin-Sinar) with the maximum power of 3.0 kW and an arc welding machine (YD-500AG) with the maximum welding current of 500 A were used in this experiment. Shielding gas consisting of pure argon was supplied through the MIG torch located at the side of laser beam. The welding parameters are shown in Table 2. The droplet transfer phenomena in hybrid welding processes were observed and investigated using a high speed video camera (CA-D6-0256W) with the maximum recording speed of 1000 frames per second.

4. Results and discussion Fig. 3. Experimental set-up for laser-MIG hybrid welding.

In order to characterize the stability of droplet transfer, offset of droplet centroid L m and critical additional axial acceleration ac are adopted in this paper. Offset of droplet centroid L m , which means the vertical distance between droplet centroid and the extension line of the wire axis (Fig. 4), is used to characterize the axial transfer properties of droplets. Generally, less offset of droplet centroid indicates better axial transfer properties of droplets and more stable droplet transfer. Critical additional axial acceleration is used to characterize transfer frequency. The additional axial acceleration is produced by an assumed additional mechanical force that acts on droplets and points to the molten pool along the wire axis. This additional force generates mechanical potential energy and the total energy of droplets increases. Droplet shapes keep evolving to keep minimum energy and stable droplet transfer occurs once the additional axial acceleration reaches the critical value ac . Therefore, greater critical additional axial acceleration indicates more unstable droplet transfer and lower transfer frequency.

Table 2 Welding parameters. Laser power (kW) 0.8  2.5 MIG current (A) 80  300 MIG voltage (V) 15.0  32.0 Shielding gas flow rate (l/min) 10 80 Welding speed (m/min) 0.6  1.5 Distance between laser and arc (mm) 0  8.0 Laser-arc angle (deg.) 35 40

4.1. Effects of laser power on droplet transfer behaviors Fig. 5 shows high speed video images of droplets and calculated critical droplet shapes for a sequence of laser power with the same welding current of 150 A and shielding gas flow rate of 22 l/min. It is clear that the addition of laser causes poorer axial transfer properties of droplets. Fig. 6 shows high speed video images of droplets and calculated critical droplet shapes for the same sequence of laser power of 1000 W, 1500 W, 2000 W and 2400 W with a different welding current of 100 A. It can be also seen that with laser power increasing, axial transfer properties of droplets become poorer and the droplet volume increases gradually, indicating lower transfer frequency. Actually, the stability of droplet transfer can be quantitatively described by offset of droplet centroid. As shown in Fig. 7, offset of droplet centroid increases with increasing laser power. As laser power increases from 1000 W to 2400 W, offset of droplet centroid is 4.5 and 2 times the initial value for the current of 100 A and 150 A, respectively. Calculated droplet shapes and offset of droplet centroid are in good agreement with experimental results, thus verifying the validity of the model. One possible reason for the effects of laser power on transfer behaviors is that laser power affects laser-induced plasma

Fig. 4. Schematic of offset of droplet centroid.

and vaporization-induced recoil force. As shown in Fig. 8, the forces acting on the droplet include gravitational force Fg, plasma drag force Fp, hybrid electromagnetic force Fhem, surface tension Fγ and vaporization-induced recoil force FRL. The laser-induced plasma formed after the addition of laser contains a large number of charge particles and its electrical resistivity is lower than that of the arc plasma. When the laser-induced plasma and arc plasma encounter in hybrid welding, the interaction between the two plasmas occurs and results in the formation of an electric channel, through which the charge carriers of laser-induced plasma can move into arc plasma [14]. In this way, laser-induced plasma

Table 1 Chemical compositions of base metal and filler wires (wt%).

LF6 5356

Mg

Mn

Si

Fe

Cu

Cr

Ti

Zn

Al

5.84 4.5–5.6

0.64 0.1–0.2

0.10 r0.25

r 0.40 r 0.40

0.39 r 0.05

– 0.1–0.3

0.065 0.07–0.15

0.08 r 0.10

Balanced Balanced

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5

Fig. 5. Effects of laser power on critical droplet shapes with the welding current of 150 A (L ¼ 22 l/min): (a) P¼ 1000 W, (b) P¼ 1500 W, (c) P¼ 2000 W, and (d) P¼ 2400 W.

supplies the arc with additional charge particles, contributing to the electricity conduction of the electric arc [34]. This interaction generates a new conductive path with a lower electrical resistivity for the arc plasma. The arc always follows the conductive path with the least electrical resistivity, so the arc is attracted by laser and the arc root is mounted on the keyhole opening. Consequently, the direction of the electromagnetic force is changed, resulting in a deviation angle α between the direction of the changed hybrid electromagnetic force Fhem and that of the original electromagnetic force Fem, as shown in Fig. 8. Because of the changed direction of Fhem, the off-axis droplet transfer occurs. As laser power increases, the amount of laser-induced plasma increases, the attraction of laser to the arc becomes more intense, and the off-axis transfer phenomenon becomes increasingly obvious. Besides, the vaporization-induced recoil force generated by metal vapor from the laser keyhole inhibits droplet transfer. When laser power becomes higher, the amount of metal vapor increases and the size of this force subsequently increases. The retention effects of the vaporization-induced recoil force on droplets becomes increasingly obvious with ascending laser power. Therefore, the growing period of droplet increases, the droplet volume becomes larger and the transfer frequency becomes lower. Then, an additional mechanical force is assumed to act on droplets to explore its effects on transfer stability and the value of critical additional axial acceleration. Fig. 9 shows calculated critical droplet shapes for different laser power and corresponding critical additional axial acceleration. Compared with Fig. 5, the radial size of droplet necking decreases and axial transfer properties of droplets are markedly enhanced. Therefore, it is proved that additional mechanical forces can significantly promote droplet transfer and improve transfer stability. It can also be seen from Fig. 10 that offset of droplet centroid declines substantially due to the addition of the mechanical force. Specifically, offset of droplet centroid decreases to only 44% and 36% of that before the addition of the mechanical force for the laser power of 1000 W and 2400 W, respectively. In addition, it should be noted that critical additional axial acceleration required by stable droplet transfer is not constant for different laser power. Critical additional axial acceleration ascends from 5.47 g with the laser power of 1000 W to 10.81 g with the laser power of 2400 W, as shown in Fig. 9.

4.2. Effects of welding current on droplet transfer behaviors Fig. 11 shows high speed video images of droplets and calculated droplet shapes for different welding current with the same laser power of 2000 W and shielding gas flow rate of 22 l/min. It can be seen that increasing welding current leads to better axial transfer properties of droplets. Besides, the volume of the droplet decreases steadily with welding current increasing, which indicates higher transfer frequency. Fig. 12 shows high speed video images of droplets and calculated droplet shapes for the same sequence of welding current with a different laser power of 1500 W. Similar effects of increasing welding current on axial transfer properties and transfer stability of droplets can be also observed. As shown in Fig. 13, offset of droplet centroid gradually drops as welding current ascends. As welding current increases from 100 A to 200 A, offset of droplet centroid obtained in experimental processes decreases to only 23% and 21% of the initial value for the laser power of 1500 W and 2000 W, respectively, while the calculated percentage is 36% and 29%, respectively. This is probably because welding current affects the plasma jet formation, which makes the plasmatic matter move at high speed towards the welded plate and defines the arc trajectory. The plasma jet influence becomes stronger as the welding current level is raised [35]. The increasingly strong plasma jet effect can provide a stable conductive path for the MIG arc and force the arc straight down to the plate. Meanwhile, the electromagnetic force and plasma drag force generated by the MIG arc itself becomes stronger as welding current increases. As a result, the attraction influence of laser-induced plasma on the arc and the retention influence of the vaporization-induced recoil force on droplets become relatively weak. Therefore, the direction deviation of the hybrid electromagnetic force is less evident and transfer frequency is raised. An additional mechanical force is assumed to act on droplets again but welding current varies with constant laser power. Similarly, compared with Fig. 12, the radial size of droplet necking decreases significantly with the addition of mechanical forces, and droplets show excellent axial transfer properties, as shown in Fig. 14. The increase in welding current leads to higher transfer frequency and more stable droplet transfer, so the critical

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Fig. 6. Effects of laser power on critical droplet shapes with the welding current of 100 A (L ¼ 22 l/min): (a) P¼ 1000 W, (b) P¼ 1500 W, (c) P ¼2000 W, and (d) P¼ 2400 W.

Fig. 8. Schematic view of forces acting on the droplet. Fig. 7. Effects of laser power on offset of droplet centroid.

Z. Lei et al. / Optics & Laser Technology 88 (2017) 1–10

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Fig. 9. Critical droplet shapes for different laser power (I¼ 150 A; L ¼ 22 l/min): (a) P¼ 1000 W, ac ¼ 5.47 g, (b) P¼ 1500 W, ac ¼7.82 g, (c) P¼ 2000 W, ac ¼ 9.78 g, and d) P¼ 2400 W, ac ¼ 10.81 g.

droplet centroid with and without additional mechanical forces gradually declines as welding current increases, the former is consistently lower than the latter. After adding mechanical forces, offset of droplet centroid drops to 52% and 10% of the initial value for the welding current of 100 A and 200 A, respectively. Therefore, additional mechanical forces help achieve more stable droplet transfer and hybrid welding processes. 4.3. Effects of shielding gas flow rate on droplet transfer behaviors

Fig. 10. Comparison of offset of droplet centroid with and without additional mechanical forces.

additional axial acceleration required by stable droplet transfer experiences a downward trend from 13.92 g with the welding current of 100 A to 4.46 g with the welding current of 200 A. Fig. 15 shows that, although the difference between offset of

As is shown above, additional mechanical forces can increase transfer frequency and stabilize droplet transfer. Actually, increasing shielding gas flow rate can be equivalently regarded as an approach to adding axial mechanical forces. Increasing shielding gas flow rate serves to cool the arc, and according to the principle of minimum voltage, the arc volume would be contracted compared to that with lower shielding gas flow rate. Therefore, the density of plasma fluid is increased and plasma drag force is consequently increased. And plasma drag force can promote droplet transfer, so increasing shielding gas flow rate would improve axial transfer properties and stabilize transfer behaviors. Fig. 16 shows calculated droplet shapes when shielding gas flow rate varies with the laser power of 2000 W and the welding current of 150 A. It can be clearly seen that axial transfer properties of

Fig. 11. Effects of welding current on critical droplet shapes with the laser power of 2000 W (L ¼ 22 l/min): (a) I¼ 100 A, (b) I ¼130 A, (c) I¼ 150 A, and (d) I¼ 200 A.

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Fig. 12. Effects of welding current on critical droplet shapes with the laser power of 1500 W (L ¼ 22 l/min): (a) I¼ 100 A, (b) I ¼130 A, (c) I¼ 150 A, and (d) I ¼200 A.

Fig. 13. Effects of welding current on offset of droplet centroid.

Fig. 15. Comparison of offset of droplet centroid with and without additional mechanical forces.

droplets become better and the radial size of droplet necking decreases as shielding gas flow rate increases. The results above also demonstrate that as laser power

increases or welding current decreases, droplet transfer becomes more unstable and subsequently the critical additional axial acceleration increases. Increasing shielding gas flow rate positively

Fig. 14. Critical droplet shapes for different welding current (P¼ 1500 W, L ¼22 l/min): (a) I¼ 100 A; ac ¼13.92 g, (b) I ¼130 A; ac ¼ 10.05 g, (c) I ¼150 A; ac ¼ 7.82 g, and (d) I¼ 200 A; ac ¼4.46 g.

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Fig. 16. Variations of droplet shapes with different shielding gas flow rate (P¼ 2000 W, I ¼150 A): (a) L ¼22 l/min, (b) L ¼39 l/min, (c) L ¼53 l/min, and (d) L ¼ 57.5 l/min.

contributes to transfer stability, but the critical shielding gas flow rate required by stable transfer should be increased with laser power increasing or welding current decreasing, as shown in Fig. 17. The droplet shape shown in Fig. 16(d) corresponds to the point A in Fig. 17. The corresponding optimal shielding gas flow rate is 57.5 l/min with the laser power of 2000 W and welding current of 150 A. It is clear that with the optimal parameters, the axial transfer property of the droplet is excellent. Besides, the droplet volume of Fig. 16(d) is the smallest, indicating the droplet growing period is the shortest and the transfer frequency is the highest with optimal welding parameters. Fig. 18 shows a threedimensional image which demonstrates the relationship between the critical shielding gas flow rate, laser power and welding current. The critical shielding gas flow rate can be obtained from this image as laser power and welding current continuously change to provide guidance for achieving stable hybrid welding processes.

Fig. 17. Effects of laser power and welding current on the critical shielding gas flow rate.

5. Conclusions A three-dimensional finite element model for droplet transfer in laser-MIG hybrid welding has been developed and validated by experimental results. Droplet shapes, offset of droplet centroid and critical additional axial acceleration with different welding parameters are calculated using Surface Evolver. It is found that the addition of laser in hybrid welding destabilizes droplet transfer while increasing welding current can enhance transfer stability. The calculated results show that offset of droplet centroid increases as laser power increases or welding current decreases. Additional mechanical forces lead to decrease in radial necking size and offset of droplet centroid and improve transfer stability. Increasing shielding gas flow rate can increase plasma drag force and subsequently reduce offset of droplet centroid and stabilize transfer behaviors. Critical shielding gas flow rate increases as laser power increases or welding current decreases.

Fig. 18. The three-dimensional relationship between the critical shielding gas flow rate, laser power and welding current in stable laser-MIG hybrid welding processes.

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Acknowledgements The authors wish to thank prof. Yanbin Chen from the School of Materials Science and Engineering of Harbin Institute of Technology for offering helpful suggestions on carrying out the experiment and analyzing experimental results.

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