Effect of Homogenization Cycle on Precipitation of Dispersoids and Abnormal Grain Growth in an Al-Mg-Si Alloy

AA6xxx alloys are used for various automotive and architectural applications where microstructural characteristics are critical to have acceptable final properties. Abnormal Grain Growth (AGG) in these alloys, industrially termed as Peripheral Coarse Grain (PCG) is undesirable and Mn, Cr based non-coherent dispersoids are used to control the extent of PCG. Homogenization soaking temperature and time along with heating rates determine the size, distribution with grains and volume fraction of these dispersoids. In this study the heating rates are varied in lab and industrial setting to assess the effect of aforementioned dispersoid features using SEM and digital microscopy. It is found that higher heating rates lead to coarser and lower area fraction of dispersoids which finally results in markedly large PCG in industrial extrusion. Observed dispersoid features were described based on basic kinetics and Thermo-Calc TM predicted trends of micro-segregation, fraction of dispersoids and fraction of potential nucleating sites (β’-Mg 2 Si) of dispersoids. A static recrystallization model was used to calculate the driving and retarding pressures based on substructural (EBSD analysis) and dispersoid features (SEM+ image analysis). The predicted recrystallisation response and PCG grain size was in close agreement with the observed values. This study highlights the significance of homogenization heating rates in addition to soaking time and temperature for PCG control.