Effect of tool rotational speed on microstructure and mechanical properties of friction stir welded DMR249A high strength low alloy steel butt joints for fabrication of light weight ship building structures

The friction stir welding (FSW) was employed for joining 5 mm thick plates of high strength low alloy steel (HSLA) of grade DMR249A in the development of light weight ship building structures. The main objective of this investigation is to study the effect of tool rotational speed (TRS) on microstructure and mechanical properties of friction stir welded (FSWed) DMR249A HSLA steel butt joints. The indigeneously developed Lanthanated Tungsten alloy (W99) tool was used for FSW of DMR249A HSLA steel plates. The TRS of W99 tool was varied from 500 rpm to 700 rpm while maintaining the remaining parameters at constant level. The microstructure of DMR249A steel joints was characterized using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron beam scatter diffraction (EBSD) techniques. The elemental analysis of stir zone (SZ) of DMR249A steel-joint was carried out using energy dispersive spectroscopy (EDS). The tensile properties, hardness and impact toughness of DMR249A steel joints were evaluated and compared to base metal (BM). Results disclosed that the DMR249A steel joints made using the TRS of 600 rpm exhibited superior tensile properties and impact toughness compared to other joints. The joints made using the TRS of 600 rpm revealed the greater tensile strength of 664 MPa, yield strength of 502 MPa, elongation of 19% and impact toughness of 48 J. It showed 8.85% and 14.61% improvement in tensile and yield strength along with 20.83% and 22.58% reduction in ductility and impact toughness compared to BM. It is attributed to the evolution of finer acicular ferritic and upper bainitic microstructure with no debris of W99 tool in SZ. © 2023 The Authors