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At least 1,207 records · Page 67
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Investigation of deformation twinning under complex stress states in a rolled magnesium alloy

We employed a specially designed semi-circular notch specimen in the current study to generate the various strain conditions, including uniaxial, biaxial, shear, and plane strains, which was utilized to explore the evolution of different deformation twinning systems under complex loading conditions. We found that when using in situ synchrotron X-ray diffraction mapping method, that the extensive double twins were activated during loading, while nearly no extension twinning activity was detected. After the formation of {10.1} and {10.3} compression twins, they transformed into {10.1}-{10.2} and {10.3}-{10.2} double twins instantaneously at the early stage of deformation. The lattice strain evolutions in different hkls were mapped at selected load levels during the loading-unloading sequence. Finally, the relationship between the macroscopic straining and microscopic response was established.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure evolution in the conventional single side and bobbin tool friction stir welding of thick rolled 7085-T7452 aluminum alloy

Highlights: • Comparison of microstructure evolution via EBSD of SS-FSW and BB-FSW was investigated. • Friction heat, strain rate and degree of recrystallization affected grain size. • Discontinuously dynamic recrystallization occurred in the weld nugget zone. • The degree of recrystallization was mainly controlled by the strain rate. - Abstract: The microstructure evolution during the conventional single side friction stir welding (SS-FSW) and the bobbin tool FSW (BB-FSW) of 7085-T7452 alloy thick plate was extensively studied by using the state-of-the-art materials characterization techniques. The Electron backscattering diffraction results reveal that a significant grain refinement and a larger percentage of high angle grain boundaries (HAGBs) exist in the weld nugget zone (WNZ) and the thermal-mechanical affected zone (TMAZ) compared with the base material. Meanwhile, more obviously inhomogeneous refined grains and HAGBs in WNZ along the thickness direction and a larger percentage of low angle grain boundaries (LAGBs) were observed in SS-FSW joint than that in BB-FSW joint. The change of the average grain size is the result of friction heat and strain rate of plastic deformation combined with a certain degree of recrystallization. Moreover, the recrystallization is mainly controlled by the strain rate during the FSW process. The presence of Cube and Cube ND after welding shows that the discontinuously dynamic recrystallization (DDRX) is a major factor in the final microstructural response. The research and findings help the understanding of the mechanism of microstructure homogenization and further optimizing the welding process for application.

36 MATERIALS SCIENCE↗