Abstract: “One droplet per pulse”is widely regarded as the best droplet transfer mode for pulsed metal inert gas welding． A peak pulse followed by median pulse in a square wave pulse can enhance the controllability of droplet transfer effectively and improve welding stability as well as seam appearance． In this paper，firstly，instantaneous current and voltage signals were acquired by using a wavelet analyzer． Secondly，current-voltage distribution proba-bility and U-I graphs were obtained by analysis，which were then combined with seam appearance to assess the welding efficiency． Experimental results on 1． 2-mm EＲ4043 aluminum alloy welding wire show that (1) at the same average current，the adoption of median waveform helps improve the welding quality significantly and form uniform scaly ripples on seam surface; (2) too small median current has no obvious effect on droplet transfer，and only an increase of base current is observed; (3) too large median current results in instability of welding process，and only an increase of peak current is observed; (4) if the median time is too short，the accumulated energy is not enough to control droplet transfer; if the median time is too long，irregular droplet transfer appears and scaly ripples disappear; and (5) a backward median time ranging from 6 to 10ms as well as a current value varying around the mean of peak current and base current may result in stable welding process and bright scaly ripples with excellent regularity and good appearance．

Key words: aluminum alloys, pulsed metal inert gas welding, backward median waveform, current waveform control