F. Briand, AIR LIQUIDE, Saint Ouen l'Aumone, France
In the gas metal arc welding (GMAW) welding process, metal transfer can occur by short-circuits, globular and spray-arc transfer modes, depending mainly on the current and the shielding gas used.
In particular, increase of the percentage of carbon dioxide in argon induces increase of the transition current value from the globular to spray metal transfer mode. Previously [1], we showed that this effect could be linked to the microstructural modifications of the anode tip during the GMAW welding process as a function of the gas composition.
In this work, we report on investigations of plasma operated in pure argon and in various mixtures of argon and CO2 at different dc current. SEM microstructural observations and EDS analysis, on quenched drops from GMAW experiments have allowed to characterize the physicochemical transformations of the anode wire, according to working conditions of the process, such as the nature of shielding gas, the wire feeding speed and the arc current. The presence of oxide gangue, comparable to slag in iron metallurgy, is effectively typical of globular transfer and short-arc. Its composition depends of the gas nature, with large oxide enrichment when CO2 is added. CO2 in shielding gas favours the gangue formation by a chemical oxidation-reduction reaction. This bad conducting gangue hinders the current transfer and the arc needs a larger attach zone. That explains the bell arc shape associated to globular transfer or short-arc. The thickness of this gangue is decreasing when current value increase or CO2 content decreases. So, it has not been observed in spray transfer mode for pure argon shielding gas and the plasma column has a well-defined conical shape, the curvature on the current lines is clearly modified.
We will discuss various solutions to reduce this phenomenon and decrease the transition current value from the globular to spray metal transfer mode.
Summary: In Gas Metal Arc Welding, the transition current between globular mode and spray mode increases with the CO2 percentage within the argon mixture. We believe that the oxyde gangue generated at the wire extremity during the metal transfert is responsible of the bad droplet detachment. We will discuss various ways to reduce this phenomenon and to decrease the transition current.
