The origin of surface recrystallization in extrusion of 6xxx aluminum alloys.

@mastersthesis{vangeertruyden2004,
  title = {The origin of surface recrystallization in extrusion of 6xxx aluminum alloys.},
  author = {Van Geertruyden, William},
  year = {2004},
  abstract = {During extrusion of many aluminum alloys, surface imperfections may be present as a result of imposed processing parameters. One such surface imperfection is the presence of coarse grains at the surface, known as a Peripheral Coarse Grain (PCG) structure that has been known to affect mechanical properties and aesthetics of the extruded material. The objective of this research is to understand the origin of the PCG structure in 6xxx series aluminum alloys and further understand the effect of processing parameters on its appearance.;Physical simulation experiments were performed and included small-scale and industrial-scale extrusion as well as hot torsion testing at elevated values of strain and temperature compensated strain rate, Z. The results from small-scale extrusion experiments show that with increasing ram speed, extrusion ratio, starting billet temperature, and decreasing amount of Cr, the depth of the PCG structure increases. Upon further characterization of the extrusion discard, it has been concluded that the primary mechanism of the coarse grain formation in extrusion is the result of a static recrystallization process. In the absence of the coarse grain structure, a microstructure that consisted of fine, equiaxed grains was present in extrusion which was consistent with the one at the surface of samples after hot torsion to effective strains greater than 2.5. It was determined that due to the high cooling rate after hot torsion testing, that the fine, equiaxed microstructure is created as a result of a dynamic process such as continuous dynamic recrystallization or geometric dynamic recrystallization. Due to the presence of the fine, equiaxed high angle grain boundaries present after large strain deformation, the models that define stored energy from the measurement of subgrain size and subgrain misorientation should not be used to accurately predict the PCG structure. A theoretical model, based on the microstructural evolution in extrusion, has been developed here that relates the grain size after hot deformation to the occurrence of abnormal grain growth in a material that contains a homogeneous dispersion of fine particles. Future experiments have been suggested that will quantitatively predict the PCG depth in extrusion.},
  language = {English},
}