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Welding Aluminium Liners for Liquid Hydrogen Storage Tanks

TWI has recently completed a Core Research Programme (CRP) project which focused on the development of a stationary shoulder - micro friction stir welding procedure for the fabrication of submillimetre thick aluminium liners for the storage of liquid hydrogen.

Introduction

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 III’ or a composite overwrapped pressure vessel with a metallic liner, as shown in Figure 1. 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 acts 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. TIG may also rely upon the use of a shielding gas and, in many cases (dependant on the application), a filler wire, which if from a dissimilar alloy, can compromise liner chemical compatibility to the fuel and shorten the tank life.

Due to the lack of a suitable alternative manufacturing process 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.

Originally invented at TWI in 1991, friction stir welding (FSW) is a solid state welding technique where a non-consumable rotating tool is used to generate frictional heating between the two components to be joined (Figure 2). Due to the solid state nature of the FSW process, the problems commonly associated with conventional fusion welding, such as solidification and liquation cracking, porosity and loss of lighter alloying elements does not occur. FSW is widely used to join very large (>2m diameter) tanks with minimum wall thickness of approximately 2-3mm in the space industry. However, when used to weld sheets thinner than 1mm, FSW is commonly referred to as ‘micro-friction stir welding’ (µFSW). This variant of FSW could contribute to improve the viability of hydrogen storage solutions by reducing vessel weight, improving joint strength and damage tolerance, whilst ensuring short and long-term cost savings.

The stationary shoulder - micro friction stir welding variant (SS-µFSW) was also originally developed by TWI and is characterised by a rotating FSW probe, which protrudes through a stationary shoulder (Figure 3). The stationary shoulder maintains a constant contact force with the material being joined and slides over its surface during welding, reducing the distortion of welded parts as well as the applied torque. The key benefit of this variant is the improved surface finish, eliminating flash and the typical stirring surface pattern, making it a more suitable process for composite overwrapped products.

Objectives

The main objectives for this project were:

  • Develop a SS-µFSW procedure to weld 0.8mm thick AA2024-T3 sheets in a butt weld configuration
  • Characterise the mechanical properties of the welds produced in flat sheet specimens
  • Establish the feasibility of welding longitudinal butt joints in thin-walled aluminium using SS-µFSW to manufacture tank liners
  • Develop strategies for the exit hole closure of SS-µFSW welds in thin aluminium sheets
  • Outline a new design and manufacturing approach for SS-µFSW tooling/jigging to enable large-scale manufacture of aluminium tank liners
Figure 1. Schematic cross-section of different types of cylindrical pressure vessels for hydrogen storage
Figure 1. Schematic cross-section of different types of cylindrical pressure vessels for hydrogen storage
Figure 2. The principle of the FSW process showing the rotating probe fully plunged into the interface between two plates
Figure 2. The principle of the FSW process showing the rotating probe fully plunged into the interface between two plates
Figure 3. Schematic of the SS-FSW technique
Figure 3. Schematic of the SS-FSW technique

Approach and Main Deliverables

In this project, process parameter combinations for SS-µFSW were iteratively explored on flat sheet specimens of 0.8mm thickness AA2024-T3. Metallographic characterisation was performed together with bend testing to assess the weld quality. Tensile testing was performed at room temperature and at negative 150°C, in accordance with BS EN ISO 4136:2022.

To eliminate the exit hole feature, an intrinsic characteristic of the µFSW and SS-µFSW, the use of refill friction stir spot welding (RFSSW) as well as welds performed using retractable probe technology were explored.

A representative technology demonstrator of a COPV was designed based on the expected manufacturing route for this application. Two half-shells, with an internal diameter of 420mm, 0.8mm thickness and length of 300mm, would be joined via SS‑µFSW at two locations to meet the requirements for Technology Readiness Level 3. The design and test of bespoke fixtures for the welding of curved sheets was also performed to validate the weld procedure that was developed and to produce technology demonstrators.

Conclusions and Future Developments 

The aim of this project was to develop a suitable SS-µFSW procedure for the joining of thin aluminium sheets for metallic liner components in type III pressure vessels. The following conclusions can be drawn from this investigation:

  • A successful SS-µFSW procedure for the welding of thin curved aluminium sheets was developed
  • Welds produced using the SS-µFSW procedure developed in this project presented a smooth surface, without any flash or surface voids. This is ideal for the envisaged application within COPV
  • 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
  • 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
  • The SS-µFSW procedure was successfully validated by performing butt welds on thin aluminium curved sheets using a bespoke fixture
  • Two technology demonstrators (Figure 4 and 5) were produced, meeting the requirements for Technology Readiness Level 3
Figure 4. SS-µFSW technology demonstrators
Figure 4. SS-µFSW technology demonstrators
Figure 5. Surface view of SS-µFSW weld
Figure 5. Surface view of SS-µFSW weld
Avatar Dr Pedro de Sousa Santos PhD IEng AWeldI Advanced Manufacturing Engineer, FWP

Pedro is working as an Advanced Manufacturing Engineer in TWI’s Friction Welding and Processing section, managing client relationships and the delivery of projects involving friction welding technologies, predominantly for the aerospace and space sectors. Due to his experience in engineering projects, Pedro is affiliated with the Engineering Council and The Welding Institute. As an active STEM ambassador, he frequently presents and supports workshops on engineering related topics to audiences ranging from Year one to sixth form students.

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