Low oxidation potential shielding gas instability in GMA welding

Abstract

Gas metal arc (GMA) welding is one of the most important processes for joining metals. However, this process is characterised by a relatively large number of variables and high sensitivity to changes in operational conditions. Furthermore, GMA welding stability can be degraded by several factors. Although some of these instability problems have been studied extensively a few of them, particularly those linked to the arc transition regions, are still poorly known and studied. The present paper studies a strong process instability characterised by repulsive metal transfer, large variations in welding current and voltage, and the formation of large quantity of spatter. It was observed previously in the welding of low carbon steel with gas shielding of low oxidation potential, and was linked to alterations in electron emission mechanisms on the cathode surface. Welding current and voltage oscillograms, arc light intensity measurements, and high speed filming were used in this study. It was shown that this instability can also occur in the welding of austenitic stainless steels. Indications of its occurrence were also observed in Al. Changes in metal transfer, arc light intensity and welding current and voltage occur simultaneously during the instability. Unstable operation was also characterised by alterations in arc position and shape and by the lack of removal of the oxide layer on the base metal surface. The onset of instability was linked to the occurrence of a rapid short circuit between the electrode and the weld pool.