Initial work was performed to verify that the HI was not foreign material or caused by FSW pin tool debris.  That work determined that the HI were large copper precipitates, and a literature search on that subject determined that the HI particles were agglomerated Al₂Cu (θ phase), which is the strengthening precipitate in Al2219. 

Since this was detected in ET space flight hardware, an investigative study of the affect of agglomerated theta phase particles in FSW Al 2219/2195 was performed.  Numerous panels of various lengths were welded per normal ET weld procedures and radiographically inspected to determine if any HI was detected.  Areas that had HI were sampled for room temperature and cryogenic (-423F) tensile testing, and cyclic testing to determine the affect, if any, on mechanical properties. 

HI samples were examined post-fracture using Scanning Electron Microscopy (SEM) and Optical Microscopy (OM).  SEM examination and Energy Dispersive Spectroscopy (EDS) showed multiple regions of fractured θ phase agglomeration in the fracture face, but it was rarely involved with the fracture.  Other HI tensile specimens demonstrated secondary cracking through the agglomerated θ phases when present.  Like analyses were performed on nominal FSW specimens free of HI.