Effect of Low-Frequency Modulation on the Penetration Profile and Microstructure in Double-Pulsed GMAW of AA6061 Aluminum Alloy

Double-pulsed gas metal arc welding (DP-GMAW), in which strong and weak pulse groups alternate periodically at a low frequency, is promising for improving the weld quality of aluminum alloys. However, how the low-frequency modulation shapes the bead profile and microstructure remains unclear. Bead-on-plate DP-GMAW of AA6061 aluminum alloy was therefore performed, in which the numbers of pulses in the strong and weak groups were varied from 1:1 to 30:30 (low frequency 41.67–1.39 Hz). Fish-scale ripples appeared on the bead surface at all tested frequencies, whereas the penetration fluctuated periodically only when the number of pulses per group reached 7 or more, so that the onset of the periodic fluctuation is bracketed between 8.33 Hz and 5.95 Hz. Both the penetration and the reinforcement peak/valley differences increased as the low frequency decreased; the largest penetration peak/valley difference reached 664.33 μm at 2.78 Hz, and the ratio of the penetration to the reinforcement peak/valley difference was largest at 3.79 Hz, where the penetration difference (627.83 μm) amounted to 532.06% of the reinforcement difference, suggesting that the droplet-impingement-driven flow affects the pool bottom far more strongly than the bead surface. At 3.79 Hz, the valley and peak angles reached 91.52° and 158.33°, respectively. It is proposed that the droplet-driven melt erodes the solid front wall at weak-to-strong transitions and forms sharp valleys, while the unconstrained flow preserves blunt peaks. Columnar grains formed along the downslope edges, whereas the grains were refined along the upslope edges. These results provide a quantitative basis for tailoring the weld profile and microstructure via low-frequency current modulation.