Liquation cracking of Al-6.3 Cu alloy propellant storage tank – A case study

Rings and sheets of aluminium alloy AA 2219 of Al–Cu series are being used for the fabrication of propellant storage tank. Under development phase, one such tank failed in the course of qualification. Detailed metallurgical analyses were carried out on this failed tank, which revealed the presence of large number of incipiently melted & solidified particles as a result of welding/repair welding, along the grain boundaries and also within the grains nearer to weld fusion line. This has resulted in the formation of continuous film along the grain boundaries of already coarser grains of the ring. Such film which formed by liquation at the weld fusion line/HAZ, under the influence of residual stresses due to welding under fixture constraints, especially near the weld repair region, initiated the crack formation. The crack once initiated, found easy propagation through film along the grain boundaries (GB). Electron probe micro analysis (EPMA) of these particles confirmed the film as eutectic, low-melting compound and their non-uniform distribution.This paper highlights the investigations carried out on the failed tank.