Friction Stir Welding of 1.4 GPa-Grade Tempered Martensitic Steel

Thermal hysteresis in fusion welding causes significant weld deterioration in medium- and high-carbon steels. Therefore, the development of an effective alternative welding process is required. Friction stir welding (FSW) is a solid-state welding process performed in an atmosphere that reduces the risks associated with melting and solidification of metals, making it an effective alternative method. Furthermore, it facilitates a flexible in-process control of heat input, which can be achieved by controlling the welding parameters. Considering these, the authors conducted a series of studies to elucidate the characteristics of FSW for medium- and high-carbon steels, including high-strength tempered steels.

This paper presents the results of applying FSW to 1.4 GPa-grade tempered JIS-S55C steel plates. Five distinct weld types were created by varying the welding parameters, including tool rotation and welding speed. The temperature of the interface between the tool and in-process material was measured using a thermal imaging camera. The microstructure of the welds was evaluated using optical microscopy and field-emission scanning electron microscopy (FE-SEM) with an electron-backscatter diffraction (EBSD) measurement system. The mechanical properties of the welds were evaluated through Vickers hardness and tensile tests. Digital image correlation analysis was employed to analyze the local deformation during the tensile test.