Microstructure Evolution in 6XXX and 7XXX Aluminum Alloys During Extrusion and Related Heat Treatments
Extrusion is a hot forming process for aluminum alloys producing rods, tubes and complex profiles. The phase and grain structure evolution during extrusion is of great importance as it determines the mechanical properties of the product. In term of phase evolution, dissolution and precipitation of the precipitation hardening phase happens during extrusion as temperature changes. As for grain structure, recrystallization and grain growth can occur during or after extrusion as the result of deformation and high temperature. For better control of microstructure evolution during extrusion and improvement of properties of final product, the underlying mechanisms related to phase and grain structure evolution are studied fundamentally in the present dissertation. For 6XXX alloys (Al-Mg-Si), the Mg2Si phase is a critical factor determining the extrudability of the billet. In this work, the size and number density of Mg2Si particles in billets are varied using different homogenization cooling rates, the effect on compressive flow stress is analyzed, and the precipitation and dissolution behavior of Mg2Si during extrusion pre-heating is examined. Qualitative considerations are provided regarding to the optimization of extrusion pressure and age hardenability by modifying the size distribution and number density of Mg2Si in billet. The recrystallization and grain growth mechanisms of a 7XXX alloy (Al-Zn-Cu-Mg) are investigated using a laboratory-scale direct extrusion setup. The lab-scale setup enables rapid quenching of both extrudate and billet discard, which helps to capture the dynamic behavior of the material during extrusion. The static annealing process that happens at the die exit in industrial extrusion process is simulated by flash annealing experiments in a controlled manner. Subsequent solutionization is also carried out in lab. In the billet discards, deformation zones, transition of grain structure, and development of texture are characterized. In the extrudates, a <111> <100> double fiber texture is identified and an orientation dependence of recrystallization behavior is discovered and analyzed. Further grain structure evolution, including static recrystallization and abnormal grain growth during the subsequent flash annealing and solutionization treatments are also studied. With these results, the effects of extrusion parameters on grain structure are investigated. Small additions of Zr play an important role in controlling recrystallization in 7XXX alloys. A study is conducted using solidification methods of static casting and directional solidification to vary the concentration of Zr, as well as the size, number density and spatial distribution of the Zr-containing dispersoids. The corresponding recrystallization behavior is assessed after hot deformation and solutionization. Results show that even with a high Zr concentration, the dispersoids are only effective in inhibiting recrystallization when they are small in size and homogeneously distributed.
Johnson, Purdue University.
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