The effect of cold work on the precipitation kinetics of AA6111 aluminum

In the automotive industry, the drive to meet Corporate Average Fuel Efficiency (CAFE) standards has resulted in the need to reduce vehicular weight in new automobile designs. One effective method has been to use lighter materials such as aluminum alloys in outer body panels. Heat treatable AA6111 is one of the materials being employed in outer body panels due to its unique combination of formability, paint bake strengthening and superior corrosion resistance characteristics. For a formed and painted automobile panel, the final mechanical properties involve a combination of strength components arising from cold work, strain aging, precipitation and recovery. The challenge is to find a method of quantifying each of these strength components. In this study, the effect of cold work on the precipitation kinetics of AA6111 has been evaluated, by means of tensile testing, differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). The results show a considerable improvement in yield and tensile strength with increasing level of cold work. DSC showed acceleration in the precipitation kinetics of the alloy with increasing level of cold work. The Avrami-Johnson-Mehl approach, further developed by Mittemeijer and co workers was employed to determine the activation energies for dissolution of GPI zones and β″ formation. The activation energy of dissolution of GPI zones is observed to increase with increasing level of cold work while that for formation of β″ decreases with increasing level of cold work. As expected, TEM showed strong interaction of strengthening precipitates with dislocations. The density of dislocation tangles is shown to increase with increasing degree of cold work.