# Perspective on Double Pulsed Gas Metal Arc Welding
## 2 Process and Weld Characteristic of DP-GMAW
### Weld Pool and Bead Characteristic  
[PERSON] et al. [17] discovered that the weld pool stir frequency, \(f_{y}\), coincided with the TF, \(f_{s}\), for frequencies less than 30 Hz. Once \(f_{s}\) exceeded 30 Hz, \(f_{v}\) reached its maximum, and \(f_{v}\) decreased with a greater \(f_{s}\), instead of following \(f_{s}\). [PERSON] et al. [20] found that the weld pool stir amplitude, \(A_{v}\), increased sharply, with \(f_{s}\) values ranging from 0 to 10 Hz. \(A_{v}\) reached its maximum and remained almost constant, with values of \(f_{s}\) ranging from 10 to 30 Hz. \(A_{v}\) decreased gradually with \(f_{s}\), whereas \(f_{s}\) exceeded 30 Hz. DP-GMAW with \(f_{s}=0\) Hz is equivalent to CP-GMAW. Therefore, DP-GMAW generates a fiercer weld pool stir effect than does CP-GMAW. [PERSON] et al. [27] and [PERSON] et al. [28] observed palpable weld ripples generated by DP-GMAW. The weld crest is produced at the end of the TP phase, whereas the weld trough is generated at the end of the TB phase. Alternating weld crests and weld troughs produce weld ripples. Another outcome is that the ripple wave length reduced with higher TF. Figure 2 illustrates the weld penetration versus time of CP-GMAW and DP-GMAW [29]. The results indicate that the weld penetration of the CP-GMAW lies between the maximum weld penetration and minimum weld penetration of the DP-GMAW for the same heat input. Weld penetration increases with time during the TP phase due to high-level heat input. Maximum weld penetration occurs at the end of TP phase. Weld penetration decreases with time during the TB phase, due to low-level heat input. Minimum weld penetration is generated at the end of the TB phase. Therefore, the weld pool shape and weld bead dimension of DP-GMAW are different from CP-GMAW. With a high thermal frequency, the oscillation of penetration depth decreases due to the short acting time of TP and TB phase. Main driving forces for molten pool convection are buoyancy, electromagnetic, and Marangoni force [30]. [PERSON] et al. [29] developed a well-tested, transient, and three-dimensional heat transfer and fluid flow model of DP-GMAW, by solving the equations of conservation of mass, momentum, and energy. The results indicate that the molten pool expands during the TP phase, due to high-level heat input, and subsequently shrinks during the TB phase, due to low-level heat input. The calculated results exhibit suitable agreement with experimentally observed results [31]. This expansion of the molten pool causes unusual remelting and resolidification of the previously solidified metal. To examine this phenomenon, Figure 3 records the weld pool boundaries of DP-GMAW along the longitudinal section during the TP phase. Although the heat source is moving along the welding direction, the molten pool trailing edge moves in opposite to the welding direction, from 1.24 s to 1.3 s. It is strikingly different from CP-GMAW. As the weld pool expansion speed is higher than the welding speed from 1.24 s to 1.3 s, remelting of the previously solidified fusion metal occurs at the trailing edge  
Figure 2: Weld penetration versus time of CP-GMAW and DP-GMAW (reproduced with permission from [29], Copyright Elsevier, 2017). of the weld pool. This remelting and resolidification phenomenon produces a change in the mean solidification growth rate. Figure 4 demonstrates the temporal variation of the solidification growth rate (\(R\)) of CP-GMAW and DP-GMAW [29]. The value \(R\) was calculated at molten pool tail on the top surface using the following relation  
\[R=\frac{\Delta d_{s}}{\Delta t} \tag{3}\]  
where \(\Delta d_{s}\) is the displacement of the trailing edge during a short time interval of \(\Delta t\). The cooling rate was calculated at weld pool tail on the top surface for the microstructure evaluation. Unlike CP-GMAW, the solidification growth rate of DP-GMAW is time-dependent.