Preliminary Analysis Of Bead Defects Formation Mechanism In High-speed GMAW

In GMAW welding process, when the welding speed reaches a certain threshold, there will be onset of weld bead undercut defects. The further increasing of welding speed will cause humping bead. The occurrence of these weld bead defects limits the range of usable welding speed in most fusion welding processes and prevents further increase in welding productivity. Through analyzing weld bead defects formation mechanism in high-speed GMAW, technical measures to prevent or eliminate undercut and humping weld can be proposed and taken which is of great significance to improve the welding speed and welding productivity.The previous undercut models are based on the oversimplification of the pool shape, which leads to not only a large difference from the actual situations, but also a disability of connecting the onset of bead undercut with the welding parameters. Up to now, there is no any well accepted model to describe the physical mechanism of humping formation. It is unable to conduct detailed quantitative analysis, for the essential reason why humping bead is formed.In this paper, based on the actual characteristics of high-speed GMAW, the quasi-steady state mathematical model of weld pool was established, and the weld pool shapes under different welding parameters were calculated. To establish such a model, the keys are to select the appropriate heat source mode, consider the influence of arc movement to the arc heat flux and arc pressure distribution, and deal with the effect of metal transfer on the welding process reasonably.Through combining the actual weld pool geometry and dimension calculated from the numerical model with the hydrostatic model, the onset of bead undercut was predicted and linked with the welding process parameters. The effects of welding process parameters on the undercut depth were studied. Based on the observation of high-speed GMAW, the forces acting on the accumulating liquid metal at the tail of the pool were analyzed. The force-balance conditions at transition line which is the junction of wall jet in front of weld pool and the accumulation at pool tail were checked out. Preliminary analysis of humping bead formation was carried out. The accumulating liquid metal at the tail of pool grows up until balance is researched under the interactions of the gravity, arc pressure and droplet impinging, which is the necessary condition for humping formation. This phenomenon has been verified by GMAW experiment.