Assessing the Durability of Flow-formed Aluminum 2195-T6 for Aircraft Cryotanks

Cryotanks need to withstanding tens to hundreds of thermomechanical cycles while supporting liquid hydrogen (LH2) powered launch vehicles. In contrast, using cryotanks for aviation applications require surviving thousands to tens-of-thousands of thermomechanical cycles in potentially degrading hydrogen environments. There is a need to understand and improve high-rate, advanced processing methods such as flow-forming to meet the growing demand for LH2 flight. This work studies the use of flow-forming in support of fabricating metallic cryotanks for aircraft using fatigue life experiments, fractography, and hydrogen embrittlement studies. Fatigue life experiments on flow-formed aluminum 2195 (Al-Li) resulted in near-equivalent performance as compared to wrought samples and demonstrated satisfactory fatigue life for use in aircraft. Additionally, the flow-formed Al-Li alloy demonstrated an approximate 40-fold increase in fatigue life in cryogenic temperatures as compared to room temperatures. Fractography of the failed flow-formed samples revealed a reduction in the number and size of grain delaminations as compared to wrought samples. Due to Al-Li's tendency to delaminate along grain boundaries, this observation suggests potentially improved durability outside of the laboratory. The work establishes preliminary durability data to enable the design and use of flow-formed metallic cryotanks for aircraft.