FRICTION STIR WELDING OF SIMILAR AND DISSIMILAR METALS ALLOYS: MICROSTRUCTURAL CHANGES RELATED TO MECHANICAL PROPERTIES

Abstract

Lightweight alloys have attracted significant attention in the automotive industry to reduce environmental pollution and unnecessary energy waste. Among various lightweight materials, using aluminum (Al) alloys to replace steels is a popular way to achieve lightweight automobiles. Traditional fusion welding technologies, including resistance spot welding and gas tungsten arc welding, can induce various technical difficulties for welding Al alloys. Friction stir welding (FSW), which is a solid-state joining technology, can be considered as a promising substitute to replace the traditional fusion welding technologies. In this dissertation, the feasibility and effectiveness of friction stir spot welding (FSSW) on dissimilar metal materials are investigated. Besides, the effect of microstructure on the tensile properties and fatigue failure mechanism of FSW and friction stir lap welding (FSLW) of similar/dissimilar metal materials is studied. Firstly, FSSW of dissimilar S45C steel and 6061-T6 aluminum alloy in a butt configuration is experimentally investigated. In this study, butt spot welding is performed using a convex scrolled shoulder tool at different tool rotational speeds. The microstructures of the joints without significant defects are characterized using scanning electron microscopy and energy dispersive spectrometry. Microstructural analysis shows the presence of intermetallic compounds (IMCs) along the steel/aluminum interface. The results of tensile tests show that the tensile strength of the joint increase with increasing the tool rotational speed. The tensile performance of joints is strongly affected by the IMCs at the joint interface, such as the thickness or type of IMCs. Secondly, the microstructure, tensile properties, and fatigue behavior of FSW joints of multilayer AA3003-clad AA6013 are experimentally investigated. Linear butt welding is performed using a concave tool equipped with a columnar threaded pin at a rotating speed of 600 rpm and a transverse speed of 200 mm/min. The microstructures of FSW joints were observed using a field emission scanning electron microscope equipped with an electron backscatter diffraction system and an energy dispersive spectrometer. The microstructural characterization in the stir zone (SZ) reveals grain refinement, precipitate refinement, zigzag line, and the AA3003-clad layer fragments due to material mixing. The tensile test result of all-weld joints shows that the tensile strength of the SZ is significantly higher than that of base metal, while the cross-weld joints show a typical ductile fracture in the base metal. The result of fatigue tests shows that all the cross-weld joints fracture from the SZ. Analysis of the fatigue failure mechanism indicates that the crack causing the fatigue fracture originates from the AA3003-clad layer fragments in the advancing side of the SZ. Finally, the microstructural characteristics and mechanical properties of FSLW of Al-clad Al and Al-clad mild steel sheets are experimentally investigated. Optical microscopic observation confirms that the FSLW joints are successfully fabricated without noticeable joining defects, including cracks and hooks. The material flow is correlated with silicon (Si) element distribution in the stir zone (SZ). The tensile test result shows that all the FSLW joints fracture from the Al-clad Al sheet side. The fatigue test result shows that the fatigue life of the AS-loaded lap joint is lower than that of the RS-loaded lap joint. The tip of the non-bonding region on the AS, which is the fatigue failure initiation position of the AS-loaded lap joint, is located in the junction of the SZ and thermo-mechanically affected zone (TMAZ). The significant differences in the grain size and texture between the TMAZ and SZ are responsible for the poor fatigue performance of the AS-loaded lap joint. Besides, the higher dislocation value and smaller grain misorientation angle on the junction of SZ and TMAZ accelerate the degradation of fatigue performance. The fatigue failure of the RS-loaded lap joint occurs at the bottom of SZ due to the residual AA1050 layer cladded on the surface of the mild steel core.