The study on defects in aluminum 2219-T6 thick butt friction stir welds with the application of multiple non-destructive testing methods

► Friction stir weld-defect forming mechanisms of thick butt-joints. ► Relationship between weld-defects and friction stir welding process parameters. ► Multiple non-destructive testing methods applied to friction stir welds. ► Empirical criterion basing on mass-conservation for inner material-loss defects. ► Nonlinear correlation between weld strengths and root-flaw lengths.