PSI - Issue 13

Yanning Guo et al. / Procedia Structural Integrity 13 (2018) 806–812 Yanning Guo/ StructuralIntegrity Procedia 00 (2018) 000 – 000

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3.2. 2024 fracture morphology The specimens after dynamic loading are examined by the scanning electron microscope (SEM) in order to determine the effects of welding parameters on the evolution of the microstructure. During the FSW process, thermo-mechanical conditions vary across the weld, resulting in different microstructural zones. The solid-state stirring effect results in the fully recrystallized, equiaxed and fine grains in the stirred zone, due to the intense plastic deformation at elevated temperatures. Close to stirred zone, because of lower levels of deformation and temperature, the thermo-mechanically affected zone shows a typical elongated and recovered microstructure.

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Fig.5 Fracture morphology of 2024(1300 s -1 ): (a) 400rmp-150mm/min, stirred zone; (b) 400rmp-150mm/min, thermo-mechanically affected zone

Fig.6 is fracture morphology comparison of 2024 dynamic tensile specimen, which welding speed is 150mm/min, rotation speed is 400rmp. The fracture surface of thermo-mechanically affected zone exhibits small and shallow dimples which are generally indicative of a ductile fracture because of the void coalescence observed at higher magnifications. As the function of rotating tool, there have many coarse dish-shaped precipitates in the thermo mechanically affected zone. The precipitate structure in stirred zone is similar to that in thermo-mechanically affected zone, but it is evident that there are fewer dislocations left in this zone due to the dynamic recrystallization process. The lower dislocation density will increase the tensile ductility as it provides more space for further dislocation storage during tensile tests. It is in accordance with the results in Fig.2 and Fig.3.

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Fig.6 Fracture morphologies of 2024(400rmp-150mm/min): (a) and (b) thermo-mechanically affected zone; (c) and (d) stirred zone

The effect of different welding rotating speed on stirred zone fracture morphology is shown in Fig.7(a) and Fig.7(b). Compare to 400rmp, grain size of 600rmp is smaller and uniform. As the finer grain size help to enhance the strength, yield strength and fracture strain of 600rmp is obviously higher than 400rmp which is shown in Fig.3(c). The comparison of fracture morphology between different strain rates is shown in Fig.7(c) and Fig.7 (d). Many fine dimples and tear ridges distributing over the fracture surface can be observed at higher magnification. Dimple-like morphology of the fracture surfaces indicates that a brittle failure mode is dominant at two strain rates. Crack propagation developed through grain boundaries, which gave a typical intergranular fracture surface. With the increasing of strain rate, the dimples tend to become non-uniform in shape and size. The depth of big dimples increases significantly, resulting in large fluctuations of the tensile fractures. Thus, the yield strength under 2600s -1 is better than 1300 s -1 , which is shown in Fig.4.