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

• Published in: Materials & design 2017-09, Vol.129, p.151-163
• Main Authors: Liu, X.C., Sun, Y.F., Fujii, H.
• Format: Article
• Language: English
• Subjects: Annealing; Friction stir welding; Grain structure; Microstructure evolution; Rapid cooling; Texture
• ISSN: 0264-1275; 1873-4197
• DOI: 10.1016/j.matdes.2017.05.013

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. [Display omitted] •Deformation stage and cooling stage during friction stir welding were separated.•Lamellar grain structure with strong A/Ā shear texture was developed at large strain and strain rate.•Continuous dynamic recrystallization via lattice rotation occurred under the strain relaxation condition.•At cooling stage, preferred grain growth along {112}⟨110⟩ occurred, forming a strong B/B− shear texture.