Failure analysis of tool used in refill friction stir spot welding of Al 2099 alloy

Refill friction stir spot welding involves a 3-piece tool consisting of a clamping ring, with rotating inner sleeve and pin components made of tool steel to join thin sheets. During the welding process, the sleeve and pin components rotate to generate frictional heat, and weld two overlapping Al-alloy sheets. The present work examines the first application of the process to join Al 2099 alloy, which has a composition containing a rather high alloy content of 1.69 wt% Li. It was noted that after a few joints were produced, the tool abruptly overheated, glowing yellow while idling at a 300 RPM rotation speed between welds, and then remained seized once rotation stopped. Optical microscopy and secondary ion mass spectroscopy revealed that high concentrations of Li accumulated between the tool pin and sleeve components, which provided an opportunity for possible multi-phase reactions, and liquid metal embrittlement of the steel tooling. This led to damage of the tool sliding surfaces and catastrophic wear, followed by seizure of the tool.