5052H34 Al-Mg plates were annealed and then friction stir processed at various rotation speeds ranging from 500 to 1500 rpm to investigate the tensile properties and vibration fracture resistance. The experimental results indicate that grain refinement could be observed at the stir zone with an average grain size varying from 5-16 μm. Based on the observed microstructure and tensile deformation resistance data, the k y slope value of the Hall-Petch equation can be determined. A refined grain size in the stir zone is a common feature of the friction stirred specimens. Different rotation speeds have different corresponding grain sizes and this can be attributed to dynamic recrystallization during friction stir processing (FSP). The effect of grain size on vibration fracture resistance in the stir zone was also examined. Results show that the vibration fracture resistance of the stir zone decreases with increasing the rotation speed. An increase in grain size due to higher rotation speed is detrimental to the vibration propagation resistance, and a small variation in grain size can result in significant changes in the duration of stage I of the D-N curves. The inward crack propagation behavior was found to be the main controlling factor on vibration fracture resistance. This result agrees with the variation in crack propagation rate.
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