| dc.description.abstract | The 3003 aluminum alloy which contains Mn, Fe and Si as alloying elements is widely used in the container, packaging, and automobile industry, because of its excellent specific strength, corrosion resistance and formability. During solidification, most of the Mn atoms can be solid-dissolved in the aluminum matrix, which results in a supersaturated solid solution. This supersaturated solid solution decomposes via the precipitation of dispersed particles during the homogenization treatment prior to hot rolling or extrusion. Therefore, controlling the size, density and distribution of the precipitated particles, as well as quantity of Mn atoms in the solid solution during homogenization are very important. First, we study evolution of precipitation during different homogenization treatments in a 3003 aluminum alloy. The evolution of the precipitation of second phase particles dispersed in a DC cast 3003 aluminum alloy during different homogenization treatments was investigated. Eight kinds of homogenization conditions were designed. We conclude that the evolution of precipitated dispersed particles during homogenization is controlled by nucleation, growth, Ostwald ripening process and hetero-precipitation. Nucleation of the particles would occur first during the initial phase of homogenization. They would then undergo a process of growth, dissolution and coarsening, before reaching the final state of precipitation. Two-color particles usually appear at step-homogenization, which has a lower later temperature, 600℃x9h?460℃x3h, due to a hetero-precipitation behavior.
The mechanical properties of extrusion products are mainly determined by the final result of the extrusion recrystallization. Following the priority study, we used the four conditions which had the largest difference between the precipitation and the solution quantity in the eight designed conditions to study evolutionary behavior of recrystallization during the extrusion of Al-Mn alloys. The different solution quantities and precipitation states in a homogenized Al-Mn alloy, and the effects of these on recrystallization behavior during extrusion were investigated. Homogenization at a low temperature of 460℃ resulted in a plentiful precipitation, which acted to pin down dislocations, thus making the recrystallization more difficult. At a higher homogenization temperature of 600℃, the particles were more sparsely dispersed, causing a weaker obstruction effect and making recrystallization easier. There were almost no dispersed precipitates at the highest homogenization temperature of 630℃, but dislocations were held up by abundant solution atoms, causing weaker recrystallization than that at 600℃. Although the solution quantity was much less under step-homogenization (600℃x9h?460℃x3h) than that under the 600℃x9h condition, the recrystallization situation was very similar. Finally, the recrystallization could be distinguished as elongated grains or equi-axial grains.
The tubes of the automobile fin-tube heat exchangers are usually produced by extruding 3003 aluminum alloys, and are then combined with fins via brazing bonds at 600℃ for 10 minutes. In this high temperature, the extrusion recrystallization will change, and affect the final mechanical properties of the products. Therefore, the study of extrusion forming ability and brazing properties in 3003 aluminum alloys is very significant. We used the four conditions which had the better forming ability in the priority eight designed conditions to do this investigation. The effects of precipitation in homogenization treatments, recrystallization in extrusion and brazing on extrusion forming ability and final material properties are examined. At first, fine dispersoids were precipitated during the 460℃x9h homogenization treatment and coarse dispersoids were precipitated by homogenization treatments with 600℃x9h. Second, when the dispersoids were not plentiful and fine enough during extrusion, the amount of solution dominated the extrusion breakout pressure, and recrystallization was easier; on the contrary, the domination state was replaced by plentiful and fine dispersoids, and recrystallization became more difficult. Additionally, the hardness after extrusion was lower in the complete recrystallization position, and higher in the incomplete recrystallization position. Finally, in brazing, the sample under the 460℃x9h condition underwent full recrystallization with a reduction in strength; the local position of the edge of the sample under the 600℃x9h?460℃x3h condition exhibited a second recrystallization and a significant drop in hardness.
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