## 1 Introduction  
Aluminum alloys have been extensively applied in automobiles, high-speed trains, naval vessels, and the aerospace industry due to the advantages of lightweight, excellent corrosion resistance, splendid impact resistance, and convenient to recycle [1; 2; 3]. Along with the increasing demand of improving the processing quality of aluminum alloys, new welding methods continuously appear, such as double pulsed gas metal arc welding (DP-GMAW), cold arc welding, friction stir welding, cold metal transfer welding, and laser hybrid welding [4; 5; 6; 7; 8]. Among all of these methods, DP-GMAW has garnered increasing attentions due to its low cost, high productivity, and perfect welding quality [9].  
DP-GMAW is a variant of the conventional pulsed GMAW, which is now used in a extensive range of applications [10; 11]. The current waveform of DP-GMAW is shown in Figure 1. The main characteristic of DP-GMAW is the coexistence of high frequency and thermal frequency [12]. The thermal pulse consists of an array of thermal strong pulses and an array of thermal weak pulse. The role of high frequency (\(1/(t_{\mathrm{p}}+t_{\mathrm{b}})\)) is keeping the one droplet per pulse mode throughout the DP-GMAW process, and the role of thermal frequency is modulating the peak value and the time of the high frequency pulse to stir the weld pool [13]. During the DP-GMAW process, the arc force and heat input are varied with the thermal frequency. This can effectively control the amount of heat input and improve the overall performance of welded joints with the formation of continuous and aesthetic welding ripples [14].  
When compared with the traditional GMAW and pulsed GMAW, the merits of DP-GMAW can be summarized as follows: a wider adjusted welding current range, a wider joint gap configuration, less heat input, less porosity incidence, and crack sensitivity [15]. [PERSON] et al. [13] made a systematic study on porosity formation mechanism and crack sensitivity in the P-GMAW and DP-GMAW process. It was concluded that DP-GMAW could decrease the porosity susceptibility and reduce welding crack sensitivity. [PERSON] et al. [16] revealed the influence of aging treatment on AA7075-T651 aluminum alloy joints by DP-GMAW. It was found that the microstructure of weld was improved obviously after the heat treatment of welding joints, meanwhile both the microhardness and tensile properties of welded joints were enhanced. [PERSON] et al. [17] investigated the temperature field and the stress-strain field of the 6061-T6 aluminum alloy sheet T-joints by DP-GMAW. The results showed that the residual stress of T-joint was mainly concentrated in the welding seams. The molten pool temperature in weak pulse group period was relatively low and the volume was smaller, while the stress of the molten pool and the surrounding metal was relatively higher. [PERSON] et al. [18] reported the effect of average current, high frequency, thermal frequency, and welding velocity on the microstructure of aluminum alloy joints using the DP-GMAW process. [PERSON] et al. [19] made a comparison between DP-GMAW and pulsed GMAW on mechanical and metallurgical properties. The results showed that the joints of DP-GMAW had superior microhardness and tensile properties.  
The above-mentioned investigations about the DP-GMAW are mainly focused on the effect of welding parameters on the microstructures or mechanical properties of the aluminum alloy welded joints. However, the impact mechanism of thermal frequency on the aluminum alloy joints using DP-GMAW is still not thorough. The optimum thermal frequency must be found to provide acceptable weld bead shape, suitable microstructures, and appropriate mechanical and corrosion resistance for the welding process. Hence, in the current paper, the effect of thermal frequency on the microstructural evolution (porosity and grain size), microhardness, the mechanical properties (tensile, impact, and bend), and corrosion behavior of DP-GMA welded AA6061-T6 aluminum alloy joints are investigated.