The variation of chemical compositions can affect the mechanical property of friction stir additive manufacturing (FSAM). Quantitative characterization of the relationship between the chemical composition and the mechanical property of FSAM components is key to control the quality of FSAM components. The effect of chemical composition on the mechanical property of 6xxx series aluminum alloy FSAM joint was studied by both experimental and numerical methods. A moving heat source model was established to simulate the heat transfer in FSAM process. The average grain size was calculated by Monte Carlo model, and the precipitate evolution model was used to calculate the hardness and constitutive stress-strain relationship. The validity of the numerical model was verified by experiments. Results indicate that the hardness and yield stress of 6xxx series aluminum alloy FSAW joint can be enhanced by increasing silicon or magnesium contents. By increasing the content of magnesium (silicon), the volume fraction and the mean radius of Mg 2 Si can be increased when the content of silicon (magnesium) is excessive. With the decrease in volume fraction, the average grain size can be increased. By changing the weight percentage of magnesium and silicon in different layers, the hardness and yield stress along the build direction can be controlled.
The metastable precipitate β″ plays a key role in material strengthening in Al–Mg–Si alloys. To investigate the mechanism on material strengthening of metastable precipitates, the interaction between β″ metastable precipitates and edge dislocations is studied using molecular dynamics. It is indicated in the results that the precipitate can be overcome by shearing without the formation of Orowan loops in case of smaller precipitate sizes. By increasing the temperature and reducing the shear strain rate, the critical resolved shear stress (CRSS) can be decreased. Larger precipitate sizes can lead to higher CRSS due to larger interaction area. Shear deformation can be clearly found on the precipitate when dislocations interact with the precipitate. When newly generated dislocations further interact with the sheared precipitate, the strengthening can be reduced due to the smaller cut width. This is the reason that strain hardening decreases with the increase of plastic deformation in macro scale.
For heat‐treatable aluminum alloys, solid solute elements play key role in material strengthening. Al–Mg–Si alloy is a typical heat‐treatable alloy; Cu and Si atoms are its main solid solution atoms. To reveal the strengthening mechanism, the interaction between the edge dislocations and the Cu and Si solute atoms of different concentration in aluminum matrix is investigated by molecular dynamics (MD) simulation. Results indicate that Cu atoms provide a more effective strengthening due to the stronger pinning effect. The increment of critical resolved shear stress (ΔCRSS) is a function of concentration of solid solute atoms. When more than two types of solid solution atoms coexist in matrix, the final increment of the ΔCRSS is determined by the interactive effects of the atoms instead of the direct sum of all items. The pinning of Cu solid solute atoms can lead to two Shockley partial dislocations merging to an edge dislocation.
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