On Formation of Abnormally Large Grains in Annealing Prestrained Aluminum Alloy Multiport Extrusion Tubes
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Abnormally large grains (ALGs) appear after annealing of prestrained multiport extrusion (MPE) tubes, which will significantly degrade the mechanical properties of the tubes. In the present work, the underpinning mechanisms of ALGs’ formation are probed through experiments. MPE tubes made of A3102 alloy are prestrained by roll leveling with different thickness reduction ratios and are then subjected to annealing at 600 °C for different times. Microstructural evolutions during annealing are characterized through electron backscatter diffraction (EBSD), in terms of crystallographic orientation, grain size distribution, grain boundary characters, and residual plastic strain. Uniaxial tension tests are carried out on the annealed tubes to study the effect of annealing on tubes’ strength. The thickness reduction prior to annealing produces heterogeneous deformation in tubes. The hard zones formed near grain boundaries and triple junction (TJs) are responsible for recrystallized nucleation in annealing. The duration of the identified incubation period for grain growth is inversely proportional to the thickness reduction ratio. The strain-free nuclei formed in the incubation period can grow fast into ALGs by means of strain-induced boundary migration. Further grain growth is inhibited by grains’ impingement after the strained grains are exhausted. Grains consumed by the growing ALGs tend to establish special boundary relationships with the ALGs and can turn into island grains. The significant reduction of tubes’ strength after annealing is attributed to the diminishment of grain boundaries caused by the formation of ALGs.
The authors acknowledge the funding from the National Natural Science Foundation of China (Project Nos. 51705315, 51575346, and U1832183). One of the authors (DL) appreciates the support of the Materials Genome Initiative Center, Shanghai Jiao Tong University.
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