# Perspective on Double Pulsed Gas Metal Arc Welding
## 1 Introduction  
Lightweight engineering alloys serve an important role in aerospace, automotive, and naval industries, for energy-saving and improved maneuverability [1; 2]. Steels are progressively substituted by lightweight engineering alloys in these applications [3; 4]. Among various lightweight engineering alloys, aluminum alloys have attracted considerable attention, due to their exceptional machinability, acceptable strength, and excellent anti-corrosion properties [5; 6]. However, aluminum alloy welding suffers from problems such as solidification cracking and hydrogen-induced porosity, which restrict the potential applications of aluminum alloys [7]. Researchers have made numerous attempts to mitigate these defects. Double pulsed gas metal arc welding (DP-GMAW) is a novel welding method in the manufacturing process [8]. Compared with conventional pulsed GMAW (CP-GMAW), DP-GMAW features many advantages, such as reduced porosity incidence, and improved solidification cracking susceptibility [9]. DP-GMAW has several aliases, e.g., pulsed GMAW with thermal pulsation, impulses, Alu-Plus, pulse/pulse, and low-frequency pulsed GMAW [10; 11; 12]. Figure 1 illustrates the schematic diagram current waveform of DP-GMAW [13]. The current waveform of DP-GMAW is comprised of the rhythmic thermal base (TB) and thermal pulse (TP) phase, which are employed to modulate the heat distribution without changing the heat input [14]. A thermal period equals the sum of times during a TB phase and TP phase, and thermal frequency (TF) is defined as the reciprocal of the thermal period. A thermal amplitude equals half of the subtraction value between the mean current of TP phase and the average current of TB phase. Both the TB phase and TP phase contain several current pulses (peak current and base current), which are used to generate a high electromagnetic force forprojected spray transfer [15; 16]. The alternating TB phase and TP phase produce a pulsation in the welding current. Therefore, the heat input is periodical and time-dependent during a thermal period.  
DP-GMAW achieves grain and dendrite refinement of aluminum alloy welds due to periodical heat input. [PERSON] et al. [17] found that the mean grain dimension could be reduced using DP-GMAW. In addition, the average grain dimension changed with an increase in the thermal frequency. [PERSON] et al. [18] observed that weld microstructures by CP-GMAW consisted primarily of coarse dendrite grains; coarse dendrite grains significantly decreased and fine dendrite grains increased using DP-GMAW. The dendrite refinement extent was relevant to the thermal frequency.  
Mitigated porosity incidence and solidification cracking are observed in aluminum welds using double pulsed gas metal arc welding. [PERSON] et al. [19] conducted experiments on 6061 aluminum alloys produced by CP-GMAW and DP-GMAW. The results indicated that both welds contained some amount of pores, but use of DP-GMAW minimized the porosity incidence. [PERSON] et al. [20] found that the weld bead cracking ratio was reduced by DP-GMAW during welding of the A7N01 aluminum alloy.  
Previous investigations indicate that the grain dimension, dendrite dimension, porosity incidence, and solidification cracking susceptibility of DP-GMAW and CP-GMAW differ. Although it is important, a comprehensive review of the mechanism of reduced porosity incidence and improved solidification cracking susceptibility by DP-GMAW is absent from the literature. Because previous investigations are sporadic, a summary of the thermal parameters, i.e., thermal frequency and thermal amplitude on weld profiles, microstructure, porosity incidence, and solidification cracking susceptibility is essential. At last, the advantages, previous evolution process, and future trend of DP-GMAW are summarized and discussed.