| Summary: | 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.
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