Effect of gradient microstructure on the mechanical properties of aluminum alloy

In the present work, the AA5052 with a gradient strain and microstructure was produced by the accumulative compression bonding (ACB) process. The microstructure and mechanical properties of initial and processed samples were investigated. A gradient microstructure was developed in the fourth pass sample. The grains were severely elongated perpendicular to the compression direction for the soft (fourth) layer. For the hard (first, second, and third) layers, the morphology of the microstructures did not maintain elongated grains due to the occurrence of dynamic recrystallization (DRX) and transformed into fine equiaxed grains. For the gradient sample, the microhardness was the highest in the top surface of the first layer with a value of 154.1 HV while this value was 88.4 HV for the bottom surface of the fourth layer. The fourth pass sample had a good combination of high strength (352.0 MPa) and high ductility (15.0%) owing to the formation of a gradient strain and microstructure. An interesting phenomenon called the recovery of strain hardening rate (RSHR) was observed in the curves of all processed samples. This phenomenon resulted in a remarkable improvement of strength without a serious decrease in the ductility of the processed samples. Finally, the fracture surfaces of the fourth pass sample exhibited the mixed fracture morphology, in which the first layer had a brittle/ductile fracture with low plasticity while the fourth layer had a ductile fracture with good plasticity. As the distance from the first layer increased, the size and depth of dimples in the fracture surface gradually were increased.