Clarification of microstructure evolution of aluminum during friction stir welding using liquid CO2 rapid cooling

Publication date: 5 September 2017
Source:Materials & Design, Volume 129
Author(s): X.C. Liu, Y.F. Sun, H. Fujii
The microstructure evolution of aluminum during friction stir welding was reconstructed using tool "stop action" technique. Simultaneous liquid CO2 was used to "freeze" the weld microstructure and the transient microstructure after "stop action". Subsequent short-time annealing produced a situation similar to the normal cooling. The microstructure evolution during the deformation and annealing stages was investigated along the material flow and in the "frozen" weld zone, respectively, by high-resolution electron-backscatter-diffraction technique. The results showed that the base material evolved into microstructures containing large amount of low angle grain boundaries (44.8%) with strong (7.9 times) B/B− shear texture at the initial welding stage. With the increasing of welding strain, lamellar grain structure with strong (6.1 times) A/A− shear texture was developed. Next, continuous dynamic recrystallization via lattice rotation with the grain 〈101〉 orientation as the rotation axis occurred under the strain relaxation condition, leading to lamellar grains conversion into equiaxed grains. Meanwhile, the strong A/A− texture transformed into weak (2.9 times) β-fiber textures (dominated by B/B− and C components). At the cooling stage, preferred grain growth along {112}〈110〉 occurred, forming relatively strong (3.8 times) B/B− texture, which is generally observed during the friction stir welding of aluminum alloys at higher welding temperatures.

Graphical abstract

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