| Summary: | The effect of high temperature extrusion conditions on the microstructure of AA3003 aluminum alloys has been investigated. The extrusion trials were conducted using a laboratory scale fully instrumented extrusion press at Rio Tinto Alcan’s Arvida Research and Development Centre (ARDC). Direct Chill (DC) cast billets were homogenized using one of three different treatments (e.g. 8 h at 500°C, 8h at 550°C, and 24 h at 600°C) prior to extrusion. Extrusions were conducted using a range of extrusion temperatures (e.g. 350°C to 500°C), ram speeds (e.g. 2 mm/s to 32 mm/s), and extrusion ratios (e.g. 17:1 ER to 280:1 ER). This provided a full range of as-extruded microstructures from recrystallized to unrecrystallized, with a wide range of final grain sizes.
The microstructure of the extrudates was examined using optical microscopy and Electron Backscatter Microscopy (EBSD). The extrudate microstructures have been rationalized in comparison with the processing conditions from the extrusion trails. It was found that the extent of recrystallization is related to the homogenization treatment (i.e. dispersoid number density), and the ram speed (i.e. the temperature profile). It was also found that the ‘unrecrystallized’ deformed grain thickness could be approximated using a simple mass balance approach. The stability of ‘unrecrystallized’ as-extruded samples was investigated in post-extrusion annealing experiments, using a range of temperatures (e.g. 500°C to 550°C) and times (e.g. 10 min. at 550°C). It was found that most of the structures had enough stored energy to recrystallize.
In addition, high temperature compression tests were conducted using a Gleeble® 3500 Thermo-mechanical Simulator. These tests were conducted to further investigate the constituent behavior of the aluminum alloy with regards to transient strain rates. Tests were conducted at a temperature of 500°C using a constant strain of 1 s-¹ and 10 s-¹, and transient strain rates. The yield stress, flow stress and work hardening were fit to a physically based flow stress model developed by Kocks and Chen. It was found that the strain rate history had an effect on the flow stress of the material, and that the model could not fully capture the behavior.
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