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Abstract
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In the present paper, two-stage refilled friction stir spot welding is used to join mild steel sheets
with a thickness of 2 mm. Meanwhile, alumina and titanium oxide nanopowders are also
introduced to the nugget to achieve superior mechanical properties. Two non-consumable tools
are used with and without pins to weld and refill the keyhole, respectively. Before refilling, the
keyhole is filled with nanopowders which are then distributed by the pinless refilling tool.
Three parameters are investigated; the rotational speed of the welding tool, the rotational speed,
and the dwell time of the refilling tool. The tensile test is used to evaluate the strength of the
joints. The microhardness is measured in the welding zone to evaluate the powder distribution.
The results suggest an increase in the joint strength by 42% and 18% with alumina and TiO2
being used as a reinforcement, respectively. With a 31.94% contribution, the refilling tool
rotational speed is the most effective input parameter affecting the joint strength. Considering
the microstructure analysis and the microhardness test, the material flow pattern is mainly
downward which results in the accumulation of the reinforcing powder in the lower sheet
especially when a lower refilling tool rotational speed is used.
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