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Friction stir welding (FSW) is one of the fastest developing welding technologies being implemented in the transport industry today. Fracture surfaces of AA5083-H111 specimens, which were friction-stir-welded using different parameters, were studied with a scanning electron microscope. The changes in the microstructure of the weld, the location and type of defects that accompanied each stirring process determined the magnitude of reduction in strength and soundness of joints for flawed samples. The paths of crack propagation and modes of fast fracture under static loads were also determined. The sharp edges of the defects were stress concentrators which aided crack initiation. Understanding of the effect of microstructure and defect on the failure of FSW joints will aid optimisation of the process variables, tool design, weld quality assurance and decision making. On the long run, it will boost the confidence of large and medium-scale manufacturers applying FSW for the adoption and implementation of this fairly new technology.