Three-dimensional numerical models are established to investigate the convection in normal and high speed GMAW processes. A high speed photography system is used to capture the transient images of the weld pool. Based on the simulation and experimental results, the differences of weld pool formation, convection and stability are researched. The humping formation mechanism in high speed GMAW process, and humping inhibition mechanism in the twin wire GMAW process are also discussed. The results show that in normal speed GMAW process, a clockwise circulation and a backward fluid flow pattern exist in the weld pool behind the arc. While in high speed GMAW process, three main factors are responsible for the humping formation: the high momentum of the backward fluid flow, the large variation of the capillary pressure of the liquid channel in the welding direction, and the capillary instability. The first two factors impede the backfilling of molten metal, and make the liquid channel susceptible to premature solidification, the final factor makes the weld pool unstable and susceptible to collapse. In the twin wire GMAW process, these factors are suppressed, in order to obtain sound weld bead, the trailing wire current should not be larger than the leading wire current.
In order to study arc interference behavior during twin wire gas metal arc welding process, the synchronous acquisition system has been established to acquire instantaneous information of arc profile including dynamic arc length variation as well as relative voltage and current signals. The results show that after trailing arc (T-arc) is added to the middle arc (M-arc) in a stable welding process, the current of M arc remains unchanged while the agitation increases; the voltage of M arc has an obvious increase; the shape of M arc changes, with increasing width, length, and area; the transfer frequency of M arc droplet increases and the droplet itself becomes smaller. The wire extension length of twin arc turns out to be shorter than that of single arc welding.
In order to better understand the arc interference behavior and welding stability in twin wire GMA welding, the synchronous acquisition system has been established to acquire instantaneous information of electric signals, arc profile, and droplet transfer including dynamic arc length variation. The results showed that after Trailing arc (T-arc) was added to the Leading arc (L-arc) in a stable welding process, the current of L-arc decreased and the voltage kept constant. The stable globular transfer of L-arc was destroyed and deflected, turned into spray transfer due to the effect of Tarc. Compared to the single wire welding, the arc length of L-arc increased and the welding operating point changed in twin wire welding.
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