Variable polarity plasma arc welding of high strength aluminium alloys

Abstract

The VPP A W process has been evaluated and optimised for the joining of high strength aluminium alloys AA-2024 T351, AA-7150 W51 and AA-7150 T651. The initial experimentation was performed with conventional AC-GTAW power supplies with limited results. Failure of the control logic to stabilise the welding process or arc extinction became an issue. Not until a suitable VPP AW power source became available were sound welding conditions developed. Still, a great effort was placed in optimising the process. Factors such as the welding torch, the electrode composition, the plasma nozzle geometry, the pilot arc current, the electrode positive duration and amplitude, the shielding gas quality and the effect of gas backing had to be thoroughly studied to achieve sound welding conditions. Subsequent to the optimisation process, operational envelopes were generated to determine the most favourable welding conditions. Based on those, reliable techniques were developed to produce sound welds on butt welded assemblies. Further on, a special plasma welding torch was developed to weld complex structures such as dual stringer panels. A number of these panels were manufactured and surpassed the damage tolerance requirements of Airbus. Hardness profiles indicated a dip in hardness in the far heat affected zone. Microstructural observation of this area revealed significant coarsening of the precipitates related to the relatively high heat input conditions of the VPPAW process. Studies which involved the reduction of the heat input were carried out. Variables such as the addition of filler wire, the use of helium as shielding gas, the combined effect of both, and the use of an edge preparation were assessed. Temperature measurements were performed at different positions along the HAZ in order to relate the thermal history with the microstructural degradation by means of cooling curves and TTT diagrams. Similar investigations were performed under low heat input welding conditions with DC-GT A W helium shielding. I Variable polarity plasma arc welding of high strength aluminium alloys. Tensile tests were performed on selected specimens in order to asses the effect of temper condition, post weld heat treatment and heat input. The results of which indicated that low heat input conditions improved the tensile properties of the far heat affected zone. With regards to temper condition it was observed that the W51 temper was more advantageous. It was also noticed that VPPA welds offered similar 0.2% proof stress and ultimate tensile strength than that of MIG welds, together with, although still very low, improved elongation properties. This piece of research has involved the welding of high strength aluminium alloys which were considered difficult to weld or even unweldable in some cases. Prior attempts were performed with MIG welding but micro voids acting as initiation sites for cracks limited the damage tolerance of the assemblies. On the contrary VPPAW has proved its ability to produce extremely high quality welds where gas porosity or micro voids were almost inexistent.