Comment by Polizeiposaune

Wasn't a bad weld; it was a bad interaction between liquid or solid oxygen and what were previously thought to be inconsequential defects in the composite-overwrapped pressure vessel the helium was loaded into.

Quoting from one of the press releases:

"The recovered COPVs showed buckles in their liners. Although buckles were not shown to burst a COPV on their own, investigators concluded that super chilled LOX can pool in these buckles under the overwrap. When pressurized, oxygen pooled in this buckle can become trapped; in turn, breaking fibers or friction can ignite the oxygen in the overwrap, causing the COPV to fail. In addition, investigators determined that the loading temperature of the helium was cold enough to create solid oxygen (SOX), which exacerbates the possibility of oxygen becoming trapped as well as the likelihood of friction ignition.

"The investigation team identified several credible causes for the COPV failure, all of which involve accumulation of super chilled LOX or SOX in buckles under the overwrap."

https://web.archive.org/web/20170216160231/http://www.spacex...