2.2.5. Heat treatment

During the extrusion and expansion processes, the metal experiences severe plastic deformation, which is causing significant strain hardening. To aid in processing, it is necessary to restore ductility. Additionally, research in ARB, which is similar to AEB, requires intermediate annealing [61]. Initial annealing of the as-received material is performed to remove initial tempers of T6 for aluminum and H58 for copper. The as-received copper is annealed to 426 ◦C with a 1 h soak time and cooled at a rate of 426 ◦C/h. The as-received aluminum is annealed to 413 ◦C with a 2.5 h soak time and cooled at a rate of 28 ◦C/h. The initial annealing was selected to enhance ductility [62].

Intermediate annealing is performed after every iteration, and the same annealing as described above for annealing aluminum was employed on the bimetal copper-aluminum tubes produced. It was found that annealing was necessary in every iteration step. Multilayered bimetals were attempted with the annealing step omitted, and severe blistering and tearing occurred throughout the tubular wall.
