| Summary: | 碩士 === 國立中正大學 === 機械工程所 === 96 === Tool size effects on microstructures and failure modes of friction stir spot welds (FSSW) in lap-shear specimens of aluminum 6061-T6 sheets were investigated based on experimental observations. A simple rule for the design of tool dimensions was developed and three types of tools with different dimensions, T1, T2 and T3 were used. The optimum processing parameters for FSSWs made by the three types of tools were obtained by design of experiments (DOE). Based on experimental results, the optimum processing parameters of these FSSWs are very similar to each other and their strengths appears to be linearly proportional to the tool dimensions. Micrographs of these FSSWs show that the nugget diameters appear to be directly proportional to the failure strengths. The failure mode of these FSSWs show the nugget pullout failure mode or mix failure mode. The micro hardness distributions of these FSSWs show that the micro hardness in heat affected zone (HAZ) gradually decreases when the location becomes closer to the thermal-mechanically affected zone (TMAZ). The minimum value of the micro hardness appears to be located near the boundary between HAZ and TMAZ. Then the micro hardness gradually increases to a local maximum value (Hv 75) when the location becomes closer to the central hole. Note that the initial failure points of these FSSWs usually occur near the location of the minimum value of the micro hardness. Scanning electron micrographs of the failed FSSWs show large amount of dimples on the fracture surfaces of the initial failure points which indicates the necking/shear failure. Finally, based on the dimensions of T3 tool, three types of new tools T4, T5 and T6 were made. The geometric effects of the tool on the plastic flow of FSSWs are well discussed.
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