Friction stir welding (FSW) is rapidly penetrating the welding market in many materials and applications, particularly in aluminum alloys for transportation applications.  As this expansion outside the research laboratory continues, fitness for service issues will arise, and process control and NDE methods will become important determinants of continued growth.  The present paper describes research into FSW weld nugget flaw detection within aluminum alloy lap welds.  We present results for two types of FSW tool designs: a smooth pin tool and a threaded pin tool.  We show that under certain process parameters (as monitored during welding with a rotating dynamometer that measures x, y, z, and torque forces) and tooling designs, FSW lap welds allow significant non-bonded interface lifting of the lap joint, while forming a metallurgical bond only within the pin region of the weld nugget.  These lifted joints are often held very tightly together even though unbonded, and might be expected to pass cursory NDE while representing a substantial compromise in joint mechanical properties.   The phenomenon is investigated here via radiographic and ultrasonic NDE techniques, and also with a copper foil marking insert (as described elsewhere).   As one would expect, these results show that tool design and process parameters significantly affect plastic flow and this lifted interface.  NDE and mechanical strength ramifications of this defect are discussed.