Grain Growth Behavior and Hall-Petch Strengthening in Friction Stir Processed Al 5059

Friction stir processing (FSP) of Al 5059 is studied in which subsequent heat treatment is conducted to investigate its effects on grain size and hardness. It was found that mainly elongated and rhomboidal morphologies of Al-6(Mn,Fe) particles are present in the alloy both before and after FSP, where the rhomboidal particles are more effective in pinning grain boundaries during heat treatment. The stir zone reached a temperature of 705 K (432 A degrees C), and ThermoCalc modeling confirmed that the Al-6(Mn,Fe) particles will remain stable at this temperature. Negligible grain growth was observed during FSP of the Al 5059 due to low grain boundary mobility resulting from slow diffusion associated with a high Mg content in the alloy. During heat treatment at 448 K (175 A degrees C) grain growth could be correlated with time using a particle-controlled grain growth model. Microhardness values indicate that Hall-Petch behavior occurs in the processed alloy, while dislocation density and particle dispersion play a minor role in strengthening.