Metallurgical Investigation and Root Cause Analysis of Defects in a Titanium Alloy (Ti6Al4V) Propellant Tank Ring

Titanium alloy (Ti6Al4V) is extensively used in the fabrication of upper stage propellant tanks for satellite launch vehicles. These tanks are of welded construction and are made from three types of domes and 5 types of rings. This paper investigates the occurrence of X-ray radiography and fluorescent penetrant indications in a specific ring component of the tank assembly after machining to thickness of 1.7 mm. The affected ring, with dimensions OD-φ1400 mm, ID-φ1300 mm, and height 410 mm, exhibited a multitude of surface and embedded indications across eight sectors, spanning approximately 140 mm in height at the mid-section of the ring. To ascertain the root cause of these indications, a comprehensive metallurgical analysis was conducted, involving X-ray radiography, optical/electron microscopy, and mechanical testing. By correlating non-destructive observations with microscopic findings, this study establishes that the occurrence of defects can be attributed to a combination of raw material and deformation processing issues. The results contribute to a better understanding of the factors influencing the quality of Ti6Al4V propellant tank rings and aid in the development of improved manufacturing processes.