English: A section of a friction-stir welded titanium tank being examined in an X-ray goniometer to detect residual stresses along welds.

A section of titanium fuel tank for a satellite produced through a novel welding technique has been subjected to detailed X-ray analysis to check its robustness for use in orbit.

ESA’s Materials and Electrical Components Lab used its X-ray goniometer to assess the residual stresses in the welds made by ‘friction-stir welding’ – cheaper, faster, greener and potentially stronger than traditional welding.

Friction-stir welded titanium tank

Fuel tanks such as this are at the heart of satellites and launcher upper stages, essential to store the highly energetic – and often corrosive – propellants used to shift orientation and position in space. A mission without fuel is essentially uncontrollable.

In 2016, ESA worked with welding specialist TWI in the UK and Airbus Defence and Space to produce a titanium tank with friction-stir welding: a rotating tool was used to heat and soften the metal parts before mixing them together, like kneading dough, with no external heat source necessary.

Because less energy is used, the welding process inflicts less residual stress on joints than standard ‘fusion welding’ – molten welds will pull on surrounding metal as they solidify. The result is that tanks made with friction stir welds will suffer less distortion, ending up with material performance comparable to unwelded metal.

Friction-stir welding

Friction-stir welding is today widely used to join aluminium components, but the reliable welding of titanium is a global first for the technique.

This follow-up testing by the Mechanical and Electrical Components Lab at ESA’s technical centre in Noordwijk, the Netherlands, is seeking to quantify the resulting weld strength and develop validation standards for this process to aid its future adoption by the space industry.

The results will be compared to previous material testing performed by the Agency’s new Advanced Manufacturing Lab at the Rutherford Appleton Laboratory (RAL) in Harwell, Oxfordshire, directly adjacent to ESA's own UK facility, which employed RAL’s ISIS neutron diffraction instrument.

More traditional but destructive follow-up testing will also take place, involving drilling tiny holes around the welds to measure residual stress.

X-ray goniometer

The project is supported through ESA’s General Support Technology Programme, bringing promising new technologies closer to the marketplace as part of the Agency’s Advanced Manufacturing cross-cutting initiative, helping the space sector to adopt novel production techniques.