Abstract
Friction stir spot welding (FSSW) was applied to interstitial free (IF) steel sheet and dual-phase high-strength steels for automobiles with an uncoated Si3N4 tool. The tool with TiN-coating and Ar shielding gas were also attempted. Defect-free joints were successfully obtained. Microstructure distribution of the joint and its evolution during FSSW were systematically examined as well as joint strength. The joint exhibited the heterogeneous microstructure distribution, which could be mainly explained by phase transformation from the different peak temperatures during FSSW. However, contaminations of oxygen, silicon and nitrogen were detected in an elliptical region observed near the pin hole, and strongly affected the microstructural evolution. Additionally, they caused the increase in hardness in the elliptical zone. On the other hand, by using coated tool and shielding gas, contaminations and hardness increase in elliptical region were suppressed and joint strength was improved.
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