Thermal Assisted Friction Stir Welding of HSLA Steel:A Novel Approach to Mitigate Lower Toughness and Ductility

Abstract

Friction stir welding (FSW) technology is well-known forits capability to join low softening metals and alloys such as aluminum and magnesium, conversely, high softening alloys like steel, titanium and nickel alloys is still a challenge due to tool material stringent property requirements and its availability. Presently, with the advancement in the development of tool materials,the joining of high softening alloysis possible. However, in case of high strength quench sensitive grade of steel, high cooling rate associated during FSW induces undesirable brittle martensitic microstructure and therefore, reduces the properties of the weld zone, particularly, the ductility and toughness. Therefore, in present investigation a novel approach to control the microstructure was investigated by employing induction pre heating source ahead of tool pin during FSW. The HSLA plate preheated at 300 °C and 600 °C respectively resulted in simultaneous increase of ductility and toughness because of widmanstatten ferrite and bainitic microstructure due to lower cooling rate. In case of FSW carried out without preheating the hard brittle zone was found, whereas the same was eliminated with preheating source. Optical and scanning electron microscope (SEM) with electron back scattered diffraction (EBSD) detector was utilized to characterize the microstructure of FSW nugget zone, and was correlated with mechanical properties.