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Weld nugget is core for the integrity of friction stir spot welds. Based on this understanding, the investigation of nugget
size, nugget rotation and weld failure mode of friction stir spot welded aluminium alloy of AA2219-O was examined.
Tapered pin and featureless pin-less tools were employed in welding the alloy at varying tool rotational speed, dwell
time and shoulder plunge depths. Intense nugget rotation and feeble nugget rotation were observed in pinless and
conical pin welds respectively. However, bonded size or nugget size was predominantly controlled by tool rotational
speed and plunge depth in both categories of weld; the percentage contributions of plunge depth in pinless and conical
pin welds are 83.97% and 41.68% respectively. Conical pin weld failed via nugget shear failure across the ring-like
weld nugget whereas nugget pull-out and interfacial weld failures were obtained in pinless weld.