# Perspective on Double Pulsed Gas Metal Arc Welding
## 2 Process and Weld Characteristic of DP-GMAW
### Solidification Structure  
Two types of physical processes regarding grain refinement by DP-GMAW are revealed: (1) the weld pool stir effect originated from the oscillation of arc and droplet pressure, and (2) the increased cooling rate originated from the remelting and resolidification phenomenon. The mechanisms of grain refinement by DP-GMAW are revealed. Grain refining of welds is attractive because finer grains can improve mechanical properties and solidification cracking susceptibility [34; 35]. Figure 5 illustrates a typical solidification structure of 1060 aluminum welds by CP-GMAW and DP-GMAW with same heat input. The metallography is obtained from the central longitudinal plane of welds using a polarized microscope. The results indicate that DP-GMAW significantly reduces the mean grain dimension compared with CP-GMAW. Table 1 shows the microstructural refinement of aluminum alloy welds via DP-GMAW [16; 18; 19; 20; 22; 29]. The results indicate that DP-GMAW produces finer grain and dendrite than CP-GMAW with the same heat input; these experimentally observed results are powerful evidenc  
Figure 5: Solidification structure of 1070 aluminum welds by CP-GMAW and DP-GMAW with same heat input. refinement can be achieved using DP-GMAW. Grain dimension is very important for controlling the welding quality; finer grain indicates better mechanical properties [36]. This finding reveals the new concept that variations of the microstructure can be achieved by changing the pulsing parameters instead of changing the heat input. Among the various welding parameters of DP-GMAW, the thermal parameters, i.e., thermal frequency and thermal amplitude have a significant influence on the weld microstructures. Figure 6 demonstrates the mean grain dimension versus thermal frequency of CP-GMAW and DP-GMAW [17]. The results indicate that the grain dimension produced by DP-GMAW is smaller than that of CP-GMAW, regardless of the changes in the thermal frequency. The grain dimension decreases with TF less than 30 Hz, and subsequently increases with higher TF, once the TF exceeds 30 Hz. The optimized TF for grain refinement ranges from 10-30 Hz. The weld pool has a resonance frequency of about 15 Hz. When the TF ranges from 10-30 Hz, the most forceful weld pool stir and weld pool convection are achieved, and the greatest level of grain refinement is produced. The beneficial effect of an increase in the thermal amplitude is a decrease in grain dimension. It is possible to achieve grain refinement of fusion zone by increasing the thermal amplitude in a large section, assuming that the welding process stability is not considered [37]. Although the mean welding current and welding speed of CP-GMAW and DP-GMAW are the same, DP-GMAW produces unique microstructures. The diversity of grain orientation by DP-GMAW is observed in the experiments. Figure 7a shows the experimentally observed grains that are generated during the TB phase, some columnar grains appear bent and exhibit an obtuse angle with the welding direction. The calculated longitudinal view of the weld pool boundaries during 1.3 s to 1.5 s are shown in Figure 7b. The results indicate that weld pool dimension significantly decreases during the beginning period of the TB phase. Therefore, solidification occurs near the front edge of the weld \begin{table}
\begin{tabular}{c c c c c} \hline
**Materials** & **Object** & **CP-GMAW** & **DP-GMAW** & **References** \\ \hline
5052 & grain & coarse & refined & [16] \\
5754 & dendrite & coarse & fine & [18] \\ A7N01 & grain & 140 μm & \(\leq\)135 μm & [20] \\
6061 & dendrite & fine & finer & [19] \\
6061 & dendrite & coarse & fine & [22] \\
6061 & SDAS & 13.4 \(\pm\) 2.3 μm & 10.2 \(\pm\) 0.8 μm & [29] \\ \hline \end{tabular}
\end{table} 
Table 1: Microstructural refinement of aluminum alloy welds by DP-GMAW. Figure 6: Mean grain dimension versus thermal frequency of CP-GMAW and DP-GMAW (reproduced with permission from [17], Copyright Taylor & Francis, 1993). pool. For CP-GMAW (steady-state), this phenomenon does not occur.