It’s welding, but not as we know it.
Only a bare sliver of glowing metal is visible during this ‘friction stir welding’. The result is a more robust titanium propellant tank produced more rapidly and cheaply than with traditional welding.
A rotating tool heats and softens the metal before mixing the two pieces together through mechanical pressure, like joining clay or dough. Requiring no external heat source, friction stir welding results in stronger joins.
“This type of welding is today widely used to join together aluminium components,” comments ESA metallurgist Andy Norman, “but the reliable welding of titanium demonstrated through this project is a new application for the technique.”
ESA’s Materials Technology section worked with welding specialist TWI in the UK, the original developer of the technique, as well as Airbus Defence and Space to produce a demonstration tank.
Titanium is a strong and highly resistant metal, but its very strength makes it hard to work with, requiring different pieces to be forged separately and then machined to the required thickness before being fusion-welded together. The result is resilient, but takes a long time to produce – in this case up to 12 months.
For this project, the pieces of the tank were instead cast – or set from molten metal – to near the required dimensions, requiring much less follow-up machining. It is possible to reach comparable levels of performance because friction stir welding imparts less stress to joints than standard fusion-welding.
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