Development of Friction Stir Welding for Hydrogen Fuel Storage Tanks
TWI Industrial Member Report 1190-2024 [pdf / 6,434KB]
By Pedro De Sousa Santos
PhD IEng AWeldI
Industrial Need
Liquid hydrogen (LH2) produced from renewable energy sources is now recognised as the most promising source of energy to achieve zero carbon emission targets set by many governments. To be used as an energy source, LH2 is currently stored in large metallic pressure vessels under high pressure due to its low ambient temperature density, resulting in a low energy per unit volume.
To support the decarbonisation of the transport sector, LH2 is required to be stored in a portable and lightweight pressure vessel. The most promising and safest pressure vessel type for this application is known as ‘type 3’ or a composite overwrapped pressure vessel with a metallic liner. A thin walled aluminium vessel liner with a carbon fibre reinforced polymer (CFRP) overlay could offer a lightweight solution for storing LH2. The external CFRP overlay acting as the main structural element, designed to withstand the tank high hoop and axial loads with the aluminium liner providing an impermeable barrier layer to contain the LH2.
As a non-load bearing part, the metallic liner wall thickness throughout the geometry of the vessels should be as thin as possible for light weighting purposes. The current joining process of choice for thin-walled aluminium liners is tungsten inert gas (TIG) welding. This approach has known limitations such as likelihood of porosity, significant distortion, and joint strength reduction due to the high heat input. Tungsten inert gas welding also relies upon the use of a shielding gas and, in some cases, a filler wire, which if from a dissimilar alloy, can compromise liner chemical compatibility to the fuel and shorten the tank lifecycle.
Due to the lack of a suitable alternative manufacturing processes for the fabrication of lightweight pressure vessels, significant research and development is required to mature LH2 storage solutions to make this the fuel of choice for transportation in the first half of this century.
Key Findings
- Consolidated welds can be achieved using Micro Friction Stir Welding (µFSW).
- A successful Stationary Shoulder Micro Friction Stir Welding (SS-µFSW) process for the welding of thin aluminium sheets was developed for flat and curved coupons.Welds produced using the SS-µFSW process developed in this project presented a smooth surface, without any flash or surface voids. This is ideal for the envisaged application.
- Cross section metallography of the SS-µFSW showed a defect free weld, with suitable material mixing and full penetration.
- Bend testing was performed across the face and the root of the SS-µFSW specimens up to 180°. No failures were reported, confirming the suitability of the process parameter combination selected.
- Transverse tensile testing of the baseline material and the welded specimens was performed at ambient temperature and -150°C. Weld efficiency values between 50 and 60% were obtained, with a consistent failure location on all welded specimens.
- Refill Friction Stir Spot Welding (Refill FSSW) showed promising potential as an exit hole mitigation strategy, with a flush weld being produced.
- Due to the use of a stationary shoulder, the SS-µFSW welds performed with retractable probe technology showed limited success since the indent could not be fully refilled at the end of the process.
Impact
In this project, process parameter combinations for µFSW and SS-µFSW were iteratively explored on flat sheet specimens of 0.8mm thickness AA2024-T3. Metallographic characterisation was performed along with mechanical testing to assess the weld quality and strength, respectively. To eliminate the exit hole feature, an intrinsic characteristic of the µFSW and SS-µFSW, the used of refill FSSW as well as welds performed using retractable probe technology were explored in this project. The design of bespoke fixtures for the welding of curved sheets was also performed to validate the weld process developed in this project and to produce technology demonstrators. The novel SS-µFSW process developed in this report should be considered as a suitable process for fabrication of aluminium liners. The process has the potential to reduce costs and improve quality compared with other joining processes.
Aluminium cylindrical liners produced using SS-µFSW.