Impact of partial recrystallization on the performance of 6005A tube extrusions

Strain distribution in tube extrusion involves gradients not only across the thickness but also around the diameter in a cyclic fashion favouring recrystallization at the surfaces and weld seams where the peak strains occur. Recrystallized grains are coarse since only a small fraction of the potential nuclei are activated while the rest are pinned by the dispersoids. While coarse surface grain structure is typical in all 6005A tubes, only those with fully recrystallized weld seams have failed. Having been largely depleted of its defects during recrystallization, weld seams respond to a subsequent artificial ageing treatment favourably and enjoy a relatively higher hardness owing to the precipitation of the hardening precursor phases of the Mg2Si. The interface between the harder weld seams and the relatively softer fibrous zones is believed to be a weak link, a very favourable site for crack initiation. Once initiated, cracks propagate along this interface until they reach the recrystallized surface layer. The recrystallized weld seams are judged to be a serious threat to the integrity of hollow extrusions.