Establishing welding procedure

After the hemisphere was machined,on each hemisphere one nozzle was welded in the specified location. These nozzles are specially machined to close tolerances.The nozzles have special threads so that once the gas is filled in to the bottle and closed by a plug the leakage from the gas bottle is almost zero.Two
hemispheres were tack welded and was ready for final welding.

We had to develop two types of gas bottles,namely,a smaller diameter gas bottle with a wall thickness of 5.5-0+0.5mm and a larger diameter gas bottle with a wall thickness of 8.5 -0+0.5mm

Customer had specified that the gas bottle should meet a certain stretch value at hydraulic test. This could only be achieved if weld test coupon had an elongation in excess of 8percent.
Welding procedure was established for a nominal thickness of 5.5mm and 8.5mm. When the test coupon was welded with matching composition filler wire.i.e,6Al-4V filler wire the desired elongation was not achievable.
When 5.5mm test coupon was welded with commercially pure titanium wire we could achieve the required elongation and the UTS as given in the specification. The weld bead picked up aluminium and vanadium from the parent plate which was responsible for meeting the required UTS and elongation.
8.5mm test coupon was welded with commercially pure titanium filler wire. The test coupon failed to develop the specified UTS. Detailed metallurgical analysis of the weld bead led to the conclusion that enough aluminium and vanadium was not picked up from the base plate as it happened in case of 5.5mm thickness plate. At the center of the weld bead the chemical composition was pure titanium and not an alloy of titanium.This was the cause for lower UTS observed. We realized that somehow we have to introduce aluminum and vanadium in to the weld without using 6Al-4V. We hit upon the idea of using two wires twisted together,one of commercially pure titanium and another of 6Al-4V and using the composite wire as filler wire for welding.When welded with this composite wire the test coupons passed all the tests.
We did extensive tests to ensure that the weld bead deposited with the combination wire did not lead to non uniformity of composition or gross segregation in weld bead.
We welded several gas bottles with this composite wire and the bottles passed all the tests.
Later the customer supplied TIG wire of composition Ti-3Al-2V which was used for welding large diameter bottles.