Effects of shielding gas flow properties on ultra‐high‐speed DCEN TIG welding of ultra‐thin aluminium sheets

Abstract

Summary Butt‐welding of O.3 mm ultra‐thin aluminium sheets using a conventional inverter‐controlled TIG welder at the ultra‐high welding speed of 12 000 mm/min has been developed by nozzle improvements. The following welding process features were found and nozzle improvements appropriately sought: By use of DCEN polarity and fine 2% Th‐W electrodes of 1.0 and 1.6 mm dia., arc spreading can be prevented and the heat responsible for base metal strain can be prevented from flowing into the zone not yet welded. Butt‐welding is performed by the technique of clamping the aluminium base metal between two 30 mm thick Cu plates and allowing the heat from the base metal to escape to the Cu plates. Butt‐welding becomes more difficult when the welding speed exceeds 6 000 mm/min. This is due to the fact that, as the welding speed increases, the arc is progressively dragged backwards (towards the bead formation zone), being dislodged from its position inside the stable shielding gas atmosphere, resulting in arc instability. To prevent this, the nozzle cross‐section is expanded into an elliptical shape. The nozzle can be subjected to a damping effect through insertion of several fine wire mesh sheets inside the nozzle. These sheets ensure a suitable flow distribution from the centre to the periphery and simultaneously facilitate a changeover of the turbulent flow of shielding gas passing through them to as near laminar flow conditions as possible. The results obtained during measurements of the pressure these trial‐manufactured nozzles supply to the base metal suggest that an intense gas flow is generated along the electrode from the electrode projection hole made at the mesh centre. This may well have the effect of focusing the arc and reducing arc dragging and oscillation. The gentle laminar flow passing through the mesh also ensures a suitable pressure distribution towards the periphery, thereby preventing air entrainment. The results obtained by inserting several different types of wire cloth inside the nozzle similarly suggest that butt‐welding can be performed at a welding speed of 12000 mm/min, the optimum wire cloth arrangement being the composite one with eight sheets of 100 and 250 mesh wire cloths alternately stacked. When the nozzle containing the composite arrangement of eight sheets of 100 and 250 mesh wire cloths alternately stacked and inserted in an elliptical nozzle with a double expanded cross‐section is used, it is found possible to perform butt‐welding at the ultrahigh speed of 18 000 mm/min, albeit within a narrow weldability range.