Abstract
Low carbon steel plates were joined by friction stir spot welding (FSSW) with lap configuration. The tool penetration depth exerted a strong effect on the failure mode of the joined samples and a weak effect on the joint shear strength. With increasing tool penetration depth, and consequently with increasing depth of the tool shoulder pressing into the top sample, the failure mode in a lap-shear test changed from brittle to ductile and concentrated near the pinhole located away from the weld towards the base metal. No mechanically mixed layer was formed between the top and bottom plates at the weld nugget due to the limited tool penetration and the fact that the pin height of the welding tool was less than the steel plate thickness. The region under the pin exhibited a fully recrystallized microstructure, with grain growth and a different texture.
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