Grain refinement and nanostructure formation in pure copper during cryogenic friction stir processing

Two different mechanisms in term of dislocation manipulation were identified for the microstructure evolutions in pure copper treated by cryogenic friction stir processing (CFSP) at different parameters. The final average grain size in the nuggets increased first and then decreased with increasing rotational speed. In the conditions of low rotational speed, the refined grains evolved from dislocation cells. In the conditions of high rotational speed, the refined grains transformed from the subdivision of nanoscale lamellar structures combined with the effect of dynamic recovery. The high proportion of nano grains in high rotational speed conditions should be attributed to the large strain from the strong constrain effect of “hard shell” structure around the nugget during CFSP.