# Perspective on Double Pulsed Gas Metal Arc Welding
## 2 Process and Weld Characteristic of DP-GMAW
### Arc and Droplet Transfer Characteristic  
As shown in Figure 1, the average current of the TP phase is significantly greater than the mean current of the TB phase. Greater welding current indicates a higher melting rate of filler wire. Therefore, droplet transfer from filler wire to base metal occurs primarily during the TP phase. The best situation  
Figure 1: Schematic diagram current waveform of double pulsed gas metal arc welding (DP-GMAW). is that the wire feeding rate is synergistic with the welding current. Unfortunately, it is infeasible in practice. Due to the mechanical inertia, the wire feeding system cannot catch up with the variation of the welding current with sufficient speed. The delay of wire feeding usually causes an unstable arc. Most welding machines utilize a constant speed wire feeding system. The TP phase has a higher wire melting rate than the TB phase. The shortest arc length and the longest electrode extension are achieved when the TB phase reaches its end. Conversely, the longest arc length and the shortest electrode extension are obtained when the TP phase reaches its end [21]. The welding current sharply decreases to a small value at the starting time of the TB phase; this little value current is occasionally insufficient to maintain the longest arc. Therefore, the actual welding operation suffers the high chance of arc quenching [22]. This \"arc quenching drawback\" reduces the welding process stability. Mitigation of this \"arc quenching drawback\" by adding two transition current pulses is discussed in the subsequent paragraph. Both welding current and arc dimension are relatively small during the stage of base current. Because arc force of the base current stage is significantly lower than the peak current stage, it is not considered. Only the arc force during the peak current stage is calculated and discussed. The relation between the arc force (\(F_{\text{A}}\)) and the welding current (\(I\)) is given as [23]:  
\[F_{A}=\frac{\mu_{0}I^{2}}{8\pi}\left(1+2\ln\left(\frac{R_{2}}{R_{1}}\right)\right) \tag{1}\]  
where \(\mu_{0}\) is the magnetic permeability of the vacuum, \(R_{1}\) and \(R_{2}\) are the radius of the arc at the welding electrode and base metal, respectively. Based on Equation (1), [PERSON] et al. [24] discussed the variation in the arc force during a thermal period. The results indicate that the arc force of CP-GMAW lies between the largest and smallest values of arc force generated by DP-GMAW. Another outcome is that the arc force of the TP phase is significantly greater than that of the TB phase. Therefore, an oscillation of arc force originated from pulsation in the welding current appears during DP-GMAW. The main driving force for the metal transfer of CP-GMAW and DP-GMAW is the high electromagnetic force generated by a current pulse. After detachment from the welding wire tip, the droplets are accelerated by the electromagnetic force, and impact into the weld pool with high speed. The droplet pressure is generated in this manner. [PERSON] et al. [24] generalized a method for calculation of the droplet pressure (\(P_{\text{D}}\)) during welding of 5754 aluminum alloy:  
\[P_{D}=1.8492f\left[\left(35.55{I_{b}}^{2}+19.6\right)h_{D}+0.2662{I_{p}}^{2} \right]^{\frac{1}{2}} \tag{2}\]  
where \(f\) is the frequency of the droplet transfer, \(h_{D}\) is the trajectory height of a overheated droplet, \(I_{b}\) is the base current, and \(I_{p}\) is the peak current. Based on Equation (2), [PERSON] et al. [24] calculated the droplet pressure on the weld pool of CP-GMAW and DP-GMAW. For CP-GMAW, the peak current, base current, and current frequency maintain the same value with time. The metal transfer frequency and trajectory distance versus time are the same, which ensures a constant droplet pressure. However, the frequency of droplet transfer, and the base current (\(I_{b}\)) during TP phase are greater than the TB phase during DP-GMAW. Thus, the droplet pressure of the TP phase is more than twice the droplet pressure of the TB phase.