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Effect of contamination on microstructure in friction stir spot welded DP590 steel

Abstract In this study, FSSW of DP590 steel was performed using both bare and TiC/TiN coated Si3N4 tool with Ar shielding, and effect of contamination on the microstructure during FSSW was examined. Defect free welds were successfully obtained at all welding conditions. Si, N and O contaminations were clearly found in the weld when bare Si3N4 tool was used without Ar shielding, but the coated tool and Ar shielding could suppress the contaminations. The contaminations caused higher hardness in the vicinity of the pin hole and reduced the weld strength. The present study showed that the prevention of the contaminations could improve the weld strength, especially the cross-tension strength, in FSSW of DP590 steel using Si3N4 tool.