Liquid Metal Oscillation and Arc Behaviour during Welding

The purpose of this research is to obtain insight into the oscillation behaviour of the liquid metal and the arc behaviour during GMA welding. Observations of the weld pool and the arc were undertaken by visual means using a high-speed video and by analysis of the voltage. To deal with the complex phenomena that take place, the research is performed in two steps; firstly, with cold-wire GTA welding on stainless steel AISI 316L tubes where a filler wire is introduced into the weld pool without being preheated, and secondly with pulsed-current GMA (P-GMA) welding on mild steel S235JR plate. The results of the cold-wire GTA experiments showed that for the same penetration state the oscillation modes of the cold-wire and the autogenous GTA weld pools are similar. The partially and the fully penetrated welds oscillate in a sloshy mode and frequencies can be predicted from relatively simple analytical expressions based on weld pool length rather than an equivalent weld pool diameter. When cold-wire GTA welding is performed in the uninterrupted bridging transfer mode the metal transfer does not disturb the voltage signals. The oscillation frequency can be determined from the analysis of voltage signals with the same accuracy as in the case of autogenous GTA welding. Penetration sensing based on pool oscillation can be applied successfully with this welding mode. The pendant droplet in the case of interrupted metal transfer can oscillate due to the action of the plasma jet. This oscillation may disturb the voltage signals associated with weld pool oscillations. The interaction between the transferred droplets and the weld pool triggers the pool into oscillation. Accordingly the results of voltage analysis to determine the weld pool oscillation frequency can be less accurate than those obtained in the case of autogenous GTA welding or the uninterrupted cold-wire GTA welding. The results of the P-GMA welding experiments showed that the current pulses cannot be used to trigger weld pool oscillation effectively, but the impact of the transferred metal droplets can generate travelling liquid waves on the pool. It is also found that the pendant droplet oscillates in an up-and-down motion. In P-GMA welding the dynamics of the weld pool are not reflected clearly on the voltage. In contrast, the oscillation of the pendant droplet is indicated on the voltage signals. The experiments showed that the arc can be divided into different regions based on its intensities. The extent of the different arc regions changes due to the growth and the oscillation of the droplet, which influences the voltage and leads to a nonlinear relationship between the arc voltage and the arc length. There is strong evidence that the modification of the arc structure is caused by the alteration of metal vapour concentration in the plasma. A proposed conceptual model describes the relationship between the arc voltage and the extent of the different arc regions. The model indicates that the average electric field strength of the bright arc region close to the electrode is higher than that of the normal arc. This result can be of importance for improving welding process control and through the arc sensing, for example for optimising droplet detachment to minimise spatter and reduce fume generation The physical phenomena taking place within the different regions of the arc however require further research. This should include experimental measurements of arc composition and temperature, and the development of a mathematical model of the plasma, including energy exchange due to radiative transport and multi-component diffusion.Materials Science & EngineeringMechanical, Maritime and Materials Engineerin