Fatigue Behavior and Fracture Mechanism of a Hot Rolled AA7020 Aluminum Alloy

Fatigue tests of smooth specimens and CT specimens of a hot rolled AA7020 Aluminum alloy have been performed in laboratory at ambient temperature. Fatigue strength and fatigue crack propagation (FCP) characteristic were evaluated and fracture mechanism was discussed on the basis of crack initiation, small crack growth, and fracture surface analysis. The growth behavior of small and large cracks has been investigated on 7020 alloy.The FCP resistance was found significantly lower than that of other aluminum alloys. The crack growth measurements were performed in CT specimens at constant load ratios ranging from R = 0.1–0.5. The fatigue strength at 104 cycles was 110 MPa that led to a considerably fatigue ratio of 0.1, fatigue failure did not occur. There existed two different modes of crack initiation depending on applied stress level. Above 200 MPa, cracks initiated at the specimen surface in transgranular or intergranular manner due to cyclic slip deformation, while below that stress subsurface crack initiation took place. The growth of small cracks initiated at the surface coincided with the FCP characteristic after allowing for crack closure for large cracks, but the operative fracture mechanisms were different between small and large cracks.

Combined Physical Modeling and Monte Carlo Simulation of Recrystallization of Hot Deformed AA7020 Aluminum Alloy

In this research, recrystallization of AA7020 aluminum alloy after hot compression testing was predicted using a framework being a combination of physical modeling and Monte Carlo simulation. Stored energy was calculated as a function of subgrain size related to the Zener Hollomon parameter. The as-deformed grain structure was mapped into the Monte Carlo simulation from experimental results. Calculated stored energy was assigned to the mapped structure, considering the length scale of the simulation. Results were validated by comparing the microstructures obtained from the model predictions with those from experimental results and a reasonable agreement was reached. The predicted grain size was found to be 15 % smaller than the experimental values. Predicted fractions recrystallized showed a similar trend to the experimental results. However, a discrepancy between the model predictions and experimental results in terms of recrystallization kinetics was found, which was attributed to neglecting the effect of subgrain growth and resulting reduction of the stored energy during recovery on the recrystallization kinetics in the present simulation.

Optimization of the Chemical Composition and Homogenization Treatment of AA7020 Aluminum Alloy

Four variants of AA7020 aluminum alloy having different Zr and Cr contents were investigated aiming at reaching high recrystallization resistance during and after hot deformation. Isothermal homogenization treatments were performed at temperatures of 390-550 °C for 2 to 48 hours. The uni-axial hot compression tests were conducted at 450 °C and strain rate of 10 s-1 at a strain of 0.6. Thereafter, the samples were annealed at 550 °C for 10 min. It was found that the samples with the highest Zr and Cr contents showed the lowest volume fraction of recrystallized grains which was attributed to the highest volume fraction of Zr- and Cr-containing dispersoids formed during homogenization. The optimum homogenization treatment to achieve highest recrystallization resistance for these samples was 470 °C for 24 hours.