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Friction Stir Welding of Pressure Vessel Liners for the Storage and Transportation of Hydrogen

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Friction Stir Welding of Pressure Vessel Liners for the Storage and Transportation of Hydrogen

 

By Pedro de Sousa Santos

Background

Liquid hydrogen (LH2) produced from renewable energy sources is now recognised as the most promising source of energy to achieve zero carbon emission travel. However, due to its low ambient temperature density resulting in a low energy per unit volume, significant research and development is required to mature hydrogen as the fuel of choice for transportation in the first half of this century.

 

Thin walled aluminium vessels, lining carbon fibre reinforced polymer (CFRP), could offer a lightweight solution for storing LH2. The external CFRP 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. This type of storage vessel is referred to Composite overwrapped pressure vessel (COPV).

 

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; an approach with known limitations. The TIG process has some inconsistencies and can result in defects including porosity (a particular issue in Al alloys if cleanliness is inadequate; pores can form a significant proportion of the wall thickness, particularly when the wall thickness is under 3mm), distortion and joint strength reduction due to the high heat input. The high likelihood of potential flaws also contributes to a large dependency on non destructive inspection (NDI). TIG also relies upon the use of a shielding gas and, in many cases, a filler wire, which if from a dissimilar alloy, can compromise liner chemical compatibility to the fuel and shorten the tank lifecycle.

 

There is an increasing need to develop a reliable, consistent, high quality joining process for thin-walled aluminium liners that can be automated and employed for large-scale manufacture of hydrogen storage vessels. Combined with a reduced aluminium liner thickness, friction stir welding (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.

 

This report provides a review of the current manufacturing routes for production of pressure vessels and thin walled aluminium liners. Technical limitations of the established processes are also presented along with potential solutions to produce ultra-thin wall liners. The merits and development needs of friction stir welding (FSW) in this application are considered. 

 

Key Findings

  • Metallic liners for COPV made from aluminium alloys are generally preferred for their weldability via fusion welding processes, relatively high specific strength at low cost and being less permeable to hydrogen than other materials.
  • The established manufacturing routes to fabricate thin aluminium liners were identified in this review as: (i) machining from solid, (ii) drawing and spinning and (iii) welding formed sheets.
  • Liners manufactured by machining from solid have a high buy-to-fly ratio with a considerable amount of wasted material. The drawing and spinning is a highly efficient manufacturing route with the possibility for high volume production and automation. However, the limitations of the drawing process only makes this manufacturing route viable for small to medium size liners.
  • The welding manufacturing route is the most flexible with regard to liner size, thickness gauge and materials. This route commonly uses fusion welding methods, particularly TIG, which have some limitations on welding high strength aluminium alloys due to joint strength reduction and can result in defects including porosity and distortion. The possible need for filler wire, usually a dissimilar alloy, can also limit the minimum thickness of the liner and compromise the tank lifecycle.
  • The welding manufacturing route is the most flexible with regard to liner size, thickness gauge and materials. This route commonly uses fusion welding methods, particularly TIG, which have some limitations on welding high strength aluminium alloys due to joint strength reduction and can result in defects including porosity and distortion. The possible need for filler wire, usually a dissimilar alloy, can also limit the minimum thickness of the liner and compromise the tank lifecycle.
  • Recently published work have shown promising mechanical performance on aluminium sheets under 1mm in thickness, in excess of that possible with other welding methods in vessel liner use.
  • There are no fundamental intellectual property barriers to the development and uptake of this technology.
  • Further process development work needs to be performed to convert the current knowledge and laboratory-based practice from flat sheet specimens to fully formed cylindrical and spherical components, and to validate the use of this technology for the production of ultra-thin aluminium liners for COPV. 

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