Date of Award


Degree Name

Master of Science


Metallurgical and Materials Engineering


Luis Trueba


Self-reacting friction stir welding is the latest FSW tool configuration developed, it employs two shoulders--one on top and one on the bottom of the workpiece-- this tool configuration offers many advantages over conventional welding techniques. However, the current lack of information and specifications limits its understanding and application. This investigation analyzed the effect of process parameters on temperature distribution, microstructure evolution, and final mechanical properties of 6061-T651 SR-FS welds. A factorial experiment was carried out, in which three-process parameters were analyzed--rotational speed, travel speed, and tool plunge force-- three levels were analyzed for each parameter, except for a tool plunge force where four levels were investigated. A total of 36 welds were fabricated with one replicate for each set of parameters

The self-reacting friction stir welded joints revealed the same microstructural zones as conventional FSW; however, during SR-FSW a more symmetric microstructure between the crown and root of the weld was observed. The hardness of the weld was uniform through the thickness of the plate, apparently as a result of this microstructure symmetry. In contrast, during conventional FSW, the weld zone exhibits an inverted bell shape, with slightly higher hardness values on the root of the weld. During this investigation, the main microstrucural defects encountered were the lazy S defect and voids. Tensile properties such as yield strength, tensile strength and elongation were evaluated. High rotational speed was found to have a significant effect on defect formation and final mechanical properties. During tensile tests, failure occurred in the region that corresponds to the heat-affected zone for defect-free samples. The samples that exhibited the lowest ductility and tensile strength, exhibited severe internal voids and fracture occurred in the nugget region. The optimum parameters to maximize the strength and ductility of SR-FS welded joints were found to be 450 rpm, 20 ipm, and 1350 lbf.




Received from ProQuest

File Size

175 pages

File Format


Rights Holder

Monica Adriana Torres