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Multi-pass gas metal arc welding was carried out for Al 5083 alloy plate with thickness 30 mm. During multi-pass welding of thick plate, the former welding pass experienced multiple thermal cycles that came from subsequent welding passes. This work focused on the effect of thermal cycle on metallurgical characteristics and mechanical properties. Different from what had long been recognized that welding thermal cycle was harmful, the results indicated that multiple thermal cycles caused the partial transition from columnar grains to equiaxed grains, which finally contributed to the refinement of microstructure. Increased precipitated particles resulted in the improvement of strengthening effect. These beneficial effects of microstructure and precipitated phase led to the increase of micro-hardness and the improvements of tensile strength and ductility. As the progress of welding process, overall properties of the welded joint improved. Conclusion could be made that appropriate thermal cycles were beneficial during multi-pass welding of Al 5083 alloy. [ABSTRACT FROM AUTHOR]

Thick plates of 5083 aluminum alloy are welded with GMAW and GMAW-TW processes; the influence of weld pool convection on porosity formation, microstructure, and mechanical property of the welding joints is investigated. The results showed that due to the appearance of outward flow and 'push-pull' patterns, and intense mixing and stirring effects in GMAW-TW process, a sound weld seam with less porosity can be obtained. The less probability of bubble formation, the shorter escaping distance of the bubble, the easy splitting of bubble, the higher bubble escape velocity, and lower solidification rate are responsible for porosity suppression. Even though the grain size in GMAW-TW process is larger than that in GMAW process, the ultimate tensile strength of the joints is larger, which is caused by less porosity formation, larger number, and uniform distribution of second-phase particles in the welds. Compared with GMAW process, the GMAW-TW process possesses the advantages of higher efficiency, less weld defects, and higher mechanical property in the welding of 5083 aluminum alloy thick plates. [ABSTRACT FROM AUTHOR]

A three-dimensional numerical model was established to improve the accuracy of simulation with stationary three-wire GMAW method. High-speed photography was utilized to detect the details of welding process. The effective radius for arcs was calculated by Abel inversion method. Besides, due to arc interference, transverse velocities of droplets were added by a theoretical calculation method. The simulated longitudinal cross-sectional welded shape indicated good corresponding to the optical result. Moreover, the flow pattern of simulated results accorded well with the reality. According to the simulation, there existed upward mobility in the leading and trailing pools which can be attributed to the droplet impact. This mobility can cause the accumulation of molten metal at the side near middle pool. Therefore, it will accelerate the process to form common weld pool. [ABSTRACT FROM AUTHOR]

A numerical model of a three-dimensional transient weld pool was conducted to analyze the hump formation mechanism in high-speed single-wire gas metal arc welding (GMAW) and the hump suppression in triple-wire GMAW. The effect of heat convection, radiation, and vaporization on temperature field and the impact of arc pressure, surface tension, Lorentz force, buoyancy force, and droplet impinging force on the fluid flow field were considered during simulations. In good agreement with the experimental beads and images recorded by high-speed camera, the simulated results are shown: during the hump formation in single-wire GMAW, the liquid channel was formed, elongated, pinched, and solidified due to the strong backward molten metal flow and the normal surface tension. However, in the same line energy, humps were suppressed in triple-wire GMAW. The 'pull-push' flow in the triple-wire GMAW weld pool could dramatically slow down the backward trend, which reduced the solidification rate. Consequently, the liquid channel disappears and the humps could not be formed any more. [ABSTRACT FROM AUTHOR]

In order to improve the understanding on arc interference behavior and verify the welding operating point change during twin wire MIG 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. It showed that the current and voltage signals changed significantly after trailing arc (T-arc) was added to the leading arc (L-arc) in a stable welding process. Moreover, the arc agitation of L-arc increased with increasing the current of T-arc but decreased with increasing the inter-wire distance. The results also showed that the stable spray transfer of L-arc was destroyed and deflected due to the effect of T-arc. With respect to a single wire welding, the arc length of L-arc increased and the welding operating point changed during the twin wire welding. [ABSTRACT FROM AUTHOR]

A three-dimensional numerical model is established to study the temperature and fluid flow fields in the twin-wire gas metal arc welding (GMAW) process. The high-speed photography system is used to capture the images of the weld pool during the welding. Based on simulation and experimental results, the weld pool formation, convection, and stability in twin-wire GMAW process are investigated. Both 'push-pull' and outward flow patterns exist in the twin-wire GMAW weld pool, which can contribute to decreasing the height of the bulge and increasing the width of the pool. The convection in the weld pool can proceed adequately, the arc force between the leading and trailing arcs is relatively balanced, surface tension normal force is uniform along the liquid channel, and the liquid channel is capillary stable, all of those contribute to the stability of the weld bead. The simulation results are in good agreement with those in the experiment. [ABSTRACT FROM AUTHOR]

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 T-arc. Compared to the single wire welding, the arc length of L-arc increased and the welding operating point changed in twin wire welding. [ABSTRACT FROM AUTHOR]