Ryoji Ohashi

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.

Friction Spot Joining of High Strength Steel Sheets for Automotives

In this study, friction spot joining (FSJ) was applied to dual-phase steel DP980 using a newly-developed silicon nitride (Si3N4) tool with a threaded pin, after which microstructure and strength of the joint were examined. The cross-sectional microstructure was distinctly classified into four zones which experienced the different heat cycles and deformation histories during FSJ. Electron probe micro analyzer revealed that silicon, oxygen and nitrogen were involved in the alternate band patterns observed in the stir zone, which could be attributed to contamination from the tool. The contamination was prevented by coating of the tool surface and the use of Ar shielding. The mechanical tests showed that the contamination reduced the joint strength, especially the cross tensile strength.