Optimizing the homogenization of zirconium containing commercial aluminium alloys using a novel process model

A novel coupled model has been applied to predict Al 3Zr dispersoid formation in 7050, a high strength aluminium alloy. It has been shown that current homogenization practices do not lead to the optimum dispersoid distribution for inhibiting recrystallization. The model has been used to design a new, optimized, two step homogenization practice. The first step of this practice involves heating for 5 h below the final homogenization temperature. This results in the precipitation of the best dispersoid distribution to pin grain boundaries, particularly in the low zirconium regions close to the dendrite edges. Homogenization is then completed in the usual way, maintaining an improved dispersoid distribution. This is predicted to reduce the fraction of recrystallized grains after thermomechanical processing. To test the predictions, an experimental trial was conducted to compare the new two step homogenization practice with the standard treatment. High resolution scanning electron microscopy (SEM) was used to measure the difference in the number density of dispersoids after the two treatments. The effect of the two step practice on the final recrystallized fraction was determined by examining hot rolled and solution treated specimens. It was found that, in excellent agreement with the predictions, the new two step practice leads to an increase in the average dispersoid number density and decrease in particle size. The enhanced dispersoid distribution resulting from the two step practice leads to a reduction in the recrystallized fraction from 30 to 14% in laboratory processed plate.