Abstract
The thermal hysteresis in fusion welding causes serious deterioration of welds of medium to high-carbon steels, so the development of an effective alternative welding process are expected. Friction Stir Welding (FSW) is considered to be an effective alternative. FSW is a solid-state joining process in atmosphere, which reduces the risks associated with melting and solidification of metals. Another advantage is the in-process flexible controllability of heat input by controlling welding parameters. From this perspective, the authors are engaged in a series of studies to elucidate the characteristics of friction stir welded joints for medium- to high-carbon steels, including high-strength tempered steel.
This report describes the results of applying friction stir welding to 1.4 GPa-grade tempered JIS-S55C steel plates. Five types of joints with different welding parameters were obtained by varying the joining parameters e.g. tool rotation speed or welding speed. The temperature of the FSW tool and material interface during friction stir welding was measured using a thermal imaging camera. The microstructure of the friction stir welded butt joint was evaluated by optical microscopy and FE-SEM / EBSD. The mechanical properties of the welds were evaluated by Vickers hardness test and tensile test, and DIC analysis was applied to analyze the details of local deformation during the tensile test. The effects of joining parameters on microstructure, microstructure of welds and mechanical properties of welds were examined in detail by properly conducting FE-SEM micro-observations, EBSD measurements.
