Effect of the current waveform on the droplet transfer in CMT welding high-nitrogen steel

Abstract

During the welding process of high-nitrogen steel, droplet bursts often occurs, which is caused by the escape of nitrogen. To analyze the influence of the current waveform on the cold metal transfer (CMT) welding process of high-nitrogen steel, a high-speed camera and current and voltage sensors were used to establish a simultaneous acquisition system for droplet transfer visualization and electrical parameter waveform monitoring. In this way, the characteristics of the droplet transfer behavior of the high-nitrogen steel CMT welding process were analyzed. The results showed that the solubility of nitrogen in the droplet decreased as the energy input rose, causing nitrogen to escape in the form of bubbles. When the pressure difference inside and outside of the droplet was greater than the surface tension, droplet cracking occurred. In the CMT welding process of high-nitrogen steel, the peak stage of the current waveform had a relatively high influence on droplet transfer. The condition of stable droplet transfer was that the peak phase current was 100–150 A, the time was 4.55–6 ms, and the maximum energy input in the peak stage was 15.5 J per CMT cycle. The current in the waiting phase, which was under 100 A, did not affect the droplet transfer behavior but did affect the shape of the molten pool. The increase in the peak wire feed speed reduced the CMT cycle time, but had little effect on droplet transfer.