3. Simulation results for SAW process

#### 3.1. Single wire SAW process

#### 3.1.1. Spray mode of metal transfer

• Single DC

FrL ¼ JzLBθ<sup>L</sup> (36)

FzL ¼ JrLBθ<sup>L</sup> (37)

dλ (38)

sinh ð Þ λc

JzT <sup>¼</sup> <sup>I</sup> 2π ð ∞

0

Figure 9. Droplet flights due to the arc interaction effect in two wire SAW process [18].

Figure 8. Arc interaction effect of the two wire tandem SAW [17, 28].

16 Heat and Mass Transfer - Advances in Modelling and Experimental Study for Industrial Applications

<sup>λ</sup>J0ð Þ <sup>λ</sup>ra exp �λ<sup>2</sup>

σ2 AT=4da � � sinh ½ � λð Þ c � z Cho et al. [15] simulated the molten pool behaviors for single DC SAW process which compared the molten pool behaviors for different electrode angles as shown in Figure 10. They found that electrode angle plays on important role to form the bead shapes such as penetration and bead width.

When the negative electrode angle is applied, the penetration of weld bead increases deeper because the droplet impingement direction is very similar to the molten poo circulation. Thus the momentum can be transferred sufficiently to the weld pool. Specifically, the molten pool flows downward and backward in the dotted box between droplet generations (Figure 11(a) and (b)) and then forms a sharp and deep penetration on a transverse cross-section by convection heat transfer as shown in Figure 12(a). However, the positive electrode angle induces

Figure 10. Electrode angle used in the simulation [28].

somewhat different flow patterns because the droplet impingement direction does not match the molten pool direction as shown in Figure 13(a) and (b). Therefore, less momentum from the droplet impingement can be transferred in positive electrode angle compared to negative electrode angle.

AC welding signals can bring the different simulation results. Cho et al. [15] simulated the molten pool simulation with sinusoidal AC waveform with a negative electrode angle. As the arc shape and signals vary with the welding time, it could induce more dynamic molten pool flows than DC welding. Normally, the frequency of droplet impingement and welding signals cannot be the same so the molten pool under the arc flows forward and backward repeatedly

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Figure 13. Calculated temperature profiles and flow patterns on a longitudinal cross section for positive electrode angle

Figure 14. Calculated temperature profiles and flow patterns on a longitudinal cross section for sinusoidal AC waveform

(current, voltage) with a negative electrode angle [15] (a) 0.528 s (b) 0.538 s.

with a welding time as shown in Figure 14.

(+20�

CÞ [15] (a) 0.528 s (b) 0.538 s.

Figure 11. Calculated temperature profiles and flow patterns on a longitudinal cross section for negative electrode angle (�20� CÞ [15] (a) 0.528 s (b) 0.538 s.

Figure 12. Calculated temperature profiles and flow patterns on a transverse cross section at 0.598 s for negative and positive electrode angle [15] (a) negative angle (�20�) (b) positive angle (+20�).

AC welding signals can bring the different simulation results. Cho et al. [15] simulated the molten pool simulation with sinusoidal AC waveform with a negative electrode angle. As the arc shape and signals vary with the welding time, it could induce more dynamic molten pool flows than DC welding. Normally, the frequency of droplet impingement and welding signals cannot be the same so the molten pool under the arc flows forward and backward repeatedly with a welding time as shown in Figure 14.

somewhat different flow patterns because the droplet impingement direction does not match the molten pool direction as shown in Figure 13(a) and (b). Therefore, less momentum from the droplet impingement can be transferred in positive electrode angle compared to negative

18 Heat and Mass Transfer - Advances in Modelling and Experimental Study for Industrial Applications

Figure 11. Calculated temperature profiles and flow patterns on a longitudinal cross section for negative electrode angle

Figure 12. Calculated temperature profiles and flow patterns on a transverse cross section at 0.598 s for negative and

positive electrode angle [15] (a) negative angle (�20�) (b) positive angle (+20�).

electrode angle.

(�20�

CÞ [15] (a) 0.528 s (b) 0.538 s.

Figure 13. Calculated temperature profiles and flow patterns on a longitudinal cross section for positive electrode angle (+20� CÞ [15] (a) 0.528 s (b) 0.538 s.

Figure 14. Calculated temperature profiles and flow patterns on a longitudinal cross section for sinusoidal AC waveform (current, voltage) with a negative electrode angle [15] (a) 0.528 s (b) 0.538 s.

When the welding current value is not enough to form the spray metal transfer, it is possible to expect FWG metal transfer mode. Cho et al. [16] simulated the molten pool behavior of FWG metal transfer in V-groove SAW process. The molten droplet impinges to the inner wall boundary and then moves to the V-groove joint sequentially as shown in Figure 15. The Vgroove joint hardly melts because the arc heat and arc forces (arc pressure & EMF) are not focused on the V-groove joint and. Therefore, it is expected that the inclined side surface melts while the molten pool behavior induces the void in the V-groove joint.

Using this principle, Cho et al. [18] performed CFD simulations and analyzed the results. For higher current in the leading electrode, the arc center displacement of the leading electrode is very small; moreover, the arc heat, arc force and droplet impingement can be focused under the leading electrode. Therefore, the volume of the molten pool ahead of the leading electrode is very small because droplets do not fly ahead of the leading electrode as shown in Figure 16(a).

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Figure 16. Temperature profiles and streamlines on the longitudinal cross section for two wire tandem SAW process [18]

Figure 17. Temperature profiles and streamlines on the transverse cross section for two wire tandem SAW process [18] (a)

(a) higher current in the leading electrode (b) higher current in the trailing electrode.

higher current in the leading electrode (b) higher current in the trailing electrode.

#### 3.2. Multi-wire SAW process

### 3.2.1. Two wire tandem SAW

Kiran et al. [17] found that when the absolute current value was higher, the arc stiffness increased; thus, the arc tended to be fixed. However, when the absolute current value of the opposite electrode is higher, the arc stiffness decreases, so the arc tends to move backward or forward by the Lorentz force. Finally, the combination of current values from each electrode affects the arc center locations and droplet free flights [18]. For instance, the higher absolute current value can result in an increase in the wire feed rate, which can induce a frequent droplet impingement and a concentration of the arc heat and arc forces.

Figure 15. Temperature profiles and flow distributions on the transverse cross section in FWG mode [16].

Using this principle, Cho et al. [18] performed CFD simulations and analyzed the results. For higher current in the leading electrode, the arc center displacement of the leading electrode is very small; moreover, the arc heat, arc force and droplet impingement can be focused under the leading electrode. Therefore, the volume of the molten pool ahead of the leading electrode is very small because droplets do not fly ahead of the leading electrode as shown in Figure 16(a).

When the welding current value is not enough to form the spray metal transfer, it is possible to expect FWG metal transfer mode. Cho et al. [16] simulated the molten pool behavior of FWG metal transfer in V-groove SAW process. The molten droplet impinges to the inner wall boundary and then moves to the V-groove joint sequentially as shown in Figure 15. The Vgroove joint hardly melts because the arc heat and arc forces (arc pressure & EMF) are not focused on the V-groove joint and. Therefore, it is expected that the inclined side surface melts

Kiran et al. [17] found that when the absolute current value was higher, the arc stiffness increased; thus, the arc tended to be fixed. However, when the absolute current value of the opposite electrode is higher, the arc stiffness decreases, so the arc tends to move backward or forward by the Lorentz force. Finally, the combination of current values from each electrode affects the arc center locations and droplet free flights [18]. For instance, the higher absolute current value can result in an increase in the wire feed rate, which can induce a frequent

while the molten pool behavior induces the void in the V-groove joint.

20 Heat and Mass Transfer - Advances in Modelling and Experimental Study for Industrial Applications

droplet impingement and a concentration of the arc heat and arc forces.

Figure 15. Temperature profiles and flow distributions on the transverse cross section in FWG mode [16].

3.2. Multi-wire SAW process

3.2.1. Two wire tandem SAW

Figure 16. Temperature profiles and streamlines on the longitudinal cross section for two wire tandem SAW process [18] (a) higher current in the leading electrode (b) higher current in the trailing electrode.

Figure 17. Temperature profiles and streamlines on the transverse cross section for two wire tandem SAW process [18] (a) higher current in the leading electrode (b) higher current in the trailing electrode.

In the transverse section, droplets from the leading electrode impinged on the weld pool whose height is lower than the initial V-grove point; therefore, deep penetration can be from as shown in Figure 17(a). Moreover, the weld pool flows long after droplet impingement in the longitudinal section so this can be another reason to make the deep penetration due to the dynamic convection heat transfer. On the contrary, when the higher current welding signal in the trailing electrode is applied, the arc center displacement of the leading electrode due to the arc

interaction is much bigger than that of the trailing electrode. Therefore, the droplets, arc heat and arc forces from the leading electrode cannot be focused on a similar weld pool spot, but the droplets disperse forward or backward of the welding direction and the form the volume of the molten pool ahead of the leading electrode as shown in Figure 16(b). With these fluid behaviors, the molten pool can fill in the V-groove point; therefore, the molten pool penetrates to a lesser

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Kiran et al. [21] modeled and simulated the molten pool flow behavior for three wire tandem SAW in V-groove. They firstly measured welding signals and the arc interaction position of three wire SAW process as shown in Figure 18 and they found that the molten pool behavior from the arc interaction played an important role to increase the penetration of V-groove. It is evident that the middle and trailing arcs are closely concentrated during the attraction (dotted box 'a') compared to that of the same between leading and middle arcs (dotted box 'b'). When the distances of middle and trailing arcs are short (dotted box 'a'), the focused arc heat and arc forces activate the molten pool behavior more dynamic and these increase penetration in the

Figure 20. Temperature profiles and streamlines on the transverse cross section for three wire tandem SAW process [21].

degree than in the higher current in the leading electrode (Figure 17(b)).

3.2.2. Three wire tandem SAW

Figure 18. Current waveforms and the corresponding arc center displacement for three wire tandem SAW process [21].

Figure 19. Temperature profiles and streamlines on the longitudinal cross section for three wire tandem SAW process [21].

interaction is much bigger than that of the trailing electrode. Therefore, the droplets, arc heat and arc forces from the leading electrode cannot be focused on a similar weld pool spot, but the droplets disperse forward or backward of the welding direction and the form the volume of the molten pool ahead of the leading electrode as shown in Figure 16(b). With these fluid behaviors, the molten pool can fill in the V-groove point; therefore, the molten pool penetrates to a lesser degree than in the higher current in the leading electrode (Figure 17(b)).
