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

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) over bar shear texture at the initial welding stage. With the increasing of welding strain, lamellar grain structure with strong (6.1 times) A/(A) over bar 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) over bar texture transformed into weak (2.9 times) beta-fiber textures (dominated by B/(B) over bar and C components). At the cooling stage, preferred grain growth along {112} < 110 > occurred, forming relatively strong (3.8 times) B/(B) over bar texture, which is generally observed during the friction stir welding of aluminum alloys at higher welding temperatures.