On Formation of Abnormally Large Grains in Annealing Prestrained Aluminum Alloy Multiport Extrusion Tubes
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INUM alloy multiport extrusion (MPE) tubes have been extensively applied as the flow channels of flat-tube heat exchangers.[1] The tubes are usually fabricated by hot extrusion to form their thin wall sections with multiple ports. A typical MPE tube manufacturing process consists of casting, homogenization, hot extrusion, roll leveling, and brazing.[2,3] The extruded MPE tubes are rolled as well as straightened in roll leveling to meet the rigorous dimensional requirement for being assembled with other components (fins and manifolds) of a heat exchanger in the subsequent
KAI LI, TIANXIA ZOU, and YINGHONG PENG are with the State Key Laboratory of Mechanical Systems and Vibration, Shanghai Jiao Tong University, Shanghai, 200240, P.R. China. DAYONG LI is with the State Key Laboratory of Mechanical Systems and Vibration, Materials Genome Initiative Centre, Shanghai Jiao Tong University, Shanghai, 200240, P.R. China. Contact e-mail: [email protected] DA SHU is with the Shanghai Key Lab of Advanced High-temperature Materials and Precision Forming Shanghai Jiao Tong University, Shanghai, 200240, P.R. China. Manuscript submitted February 4, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
brazing process. The prerolled MPE tubes are annealed in the brazing assembly at high temperatures, and abnormally large grains (ALGs) can come into being, which causes significant degradation of the mechanical properties of MPE tubes.[2,4] Formation of ALGs in MPE tubes results from the exaggerated growth of a few numbers of grains at the expense of others, i.e., abnormal grain growth (AGG).[5] AGG has been widely found in various metals, such as steels, bulk copper, nickel alloys, zirconium alloys, and aluminum alloys.[6–10] AGG is basically correlated with spatial or temporal variation of grain boundary mobility. AGG can be triggered by strong anisotropy of boundary properties, which are related to boundary misorientation,[11] or induced by the solute drag effect, depending upon the interaction between solute atoms and grain boundaries.[12] The variable pinning forces of the dissolving or coarsening precipitates enable larger grains to grow more easily.[13,14] The grains out of the primary texture component are prone to grow, while those in the primary texture component have a similar orientation and mobilities of their boundaries are limited.[15] When normal grain growth is inhibited by thermal grooving, the synergistic effect involving the size advantage of coarse columnar grains and surface energy
can also initiate AGG.[16] Omori et al.[17] reported that AGG results from subgrain structures formed during the phase transformation in a copper-based shape-memory alloy subjected to a cyclic heat treatment. Many efforts have been made to investigate AGG in aluminum processing. Kim et al.[18] reported that static abnormal grain growth (SAGG) in AA5052 sheets after primary recrystallization is attributed to the inhomogeneous distribution of cube-oriented grains. Na et al.[19] considered the occurrence of AGG in 5052 aluminum alloy as a result o
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