Isothermal Investigation and Deformation Behavior during Homogenization of 6063 Aluminum Alloy

The microstructural evolution during homogenization of aluminum 6063 billet has been investigated in this work. DSC was used to observe thermal transition from room temperature to 600 °C. Three transition temperature ranges, 218-353, 381-473, and 498-581 °C, were observed. These temperature ranges were assigned as three transformation stages occurring during homogenization. The finish transformation temperatures were selected for conducting isothermal treatments. In the first transformation stage, plate-like intermetallics (β) became thinner and fine black particles consisting of Mg and Si precipitated within matrix leading to Mg depletion from the matrix. The 353 °C heat-treated specimen showed lowest hardness and room temperature tensile strength probably due to Mg deficiency for solid solution strengthening. The 473 °C heat-treated specimen showed low hardness and room temperature tensile strength. In the third stage (581 °C isothermal treatment), globular intermetallics (α) distributed all over the matrix and most fine black particles underwent dissolution. Mg was found to dissolve back to the matrix. The 581 °C heat-treated specimen showed the highest hardness and room temperature tensile strength among all heat-treated specimens. All isothermally heat-treated specimens were also subject to tensile test at 470 °C to understand their behavior at the pre-heating temperature of extrusion commonly used as industrial practice. It was found that the specimens showed the flow stresses from the highest to the lowest as follows: the as-cast, the isothermally treated at 353, 473, and 581 °C, respectively. The flow stresses at 470 °C depended on the morphology and distribution of intermetallics due to isothermal temperatures. The isothermal treatment at 581 °C, producing globular intermetallics (α) all over the grains and lowest flow stress, is recommended for homogenization of aluminum 6063 billets. Causes responsible for such microstructural changes and mechanical properties during the three stages are discussed in this study.