Physical Simulation and Processing Map of Aluminum 7068 Alloy

Abstract

Elevated temperature deformation of as-cast aluminum 7068 alloy was done to optimize its workability by physical simulation using Gleeble 3800. The microstructural evolution was traced using electron microscopy and electron back-scattered diffraction studies. Hot deformation involving uniaxial isothermal compression was done in the 300°C–475°C temperature range and 10−3–100 s−1 strain rate up to a true strain of 0.69. From the true stress–true strain plot it is observed that flow stress initially increases sharply because of the multiplication of dislocations during the initial phase of deformation; i.e., only work hardening is predominant during this phase. Subsequently, after attaining the peak value the flow stress increases, decreases, or remains constant. Because of dynamic restoration processes such as dynamic recovery and recrystallization, the flow stress decreases or remains constant. During this phase, the competition between work hardening and dynamic softening governs the slope of true stress–strain curve. The decrease in the Zener-Hollomon parameter with increasing temperature and decreasing strain rate follows the same trend as flow stress. Activation energy for this alloy is calculated as 206 kJ mol−1. The deformed microstructure shows serrated grains, and also well-formed subgrains are observed mostly inside the grains. Moreover, substructural strengthening is observed in this alloy, due to the presence of the high density of precipitates and subgrains. Microstructural analysis confirms the high power dissipation of the stable region may be mainly due to dynamic recovery. In the unstable region, flow instabilities may be due to adiabatic shear band formation, particle cracking, and debonding. The optimized working region is determined from the developed processing map.