Electrochemical Polishing of 7075 Al Alloy in Phosphoric Acid

7075 aluminum (Al) alloy has been widely used in aircraft structures and other high-end electronic products owing to its excellent mechanical and chemical properties, while its damage-free and highly efficient surface finishing remains a challenge. Herein, we demonstrate a systematic study of the anodic behaviors of 7075 Al alloy during the electrochemical polishing (ECP) process in phosphoric acid under different applied potentials, and the changes of surface morphology, roughness, electric current and resistance are studied intensively. According to the surface morphology and current density, ECP of 7075 Al can be divided into 4 stages including the negative leveling stage, leveling and corrosion stage, levelling and brightening stage, and pitting and corrosion stage. Different factors influencing each stage and the effects of impurity phases in the ECP process are experimentally validated. Under optimized conditions, a mirror surface with a roughness (Ra) of 46.7 nm (decreased from an initial value of 153.2 nm) can be obtained by ECP for 10 min. The presented findings are of great value for the further development of ECP process of multiphase alloys.

Prediction of Flow Stress of Annealed 7075 Al Alloy in Hot Deformation Using Strain-Compensated Arrhenius and Neural Network Models

Hot compression experiments of annealed 7075 Al alloy were performed on TA DIL805D at different temperatures (733, 693, 653, 613 and 573 K) with different strain rates (1.0, 0.1, 0.01 and 0.001 s−1.) Based on experimental data, the strain-compensated Arrhenius model (SCAM) and the back-propagation artificial neural network model (BP-ANN) were constructed for the prediction of the flow stress. The predictive power of the two models was estimated by residual analysis, correlation coefficient (R) and average absolute relative error (AARE). The results reveal that the deformation parameters including strain, strain rate, and temperature have a significant effect on the flow stress of the alloy. Compared with the SCAM model, the flow stress predicted by the BP-ANN model is in better agreement with experimental values. For the BP-ANN model, the maximum residual is only 1 MPa, while it is as high as 8 MPa for the SCAM model. The R and AARE for the SCAM model are 0.9967 and 3.26%, while their values for the BP-ANN model are 0.99998 and 0.18%, respectively. All these reflect that the BP-ANN model has more accurate prediction ability than the SCAM model, which can be applied to predict the flow stress of the alloy under high temperature deformation.

Anisotropic Low Cycle Behavior of the Extruded 7075 Al Alloy

The quasi-static and low cycle fatigue tests of extruded 7075 Al alloy (Φ200 mm) were investigated in three directions: the extrusion direction (ED), the radial direction (RD), and 45° with ED (45°). Grain morphology analysis, texture measurement, and fatigue fracture characterization were conducted to discuss the relationship between microstructure and mechanical properties. The experimental results showed that the ED specimen had higher ultimate tensile strength (UTS) and low cycle fatigue (LCF) properties, which were mainly attributed to the following three causes. First, the grain boundaries (GBs) had an obvious effect on the crack growth. The number of GBs in the three directions was different due to the shape of the grains elongated along the ED. Second, the sharp <111> texture and the small Schmidt factor along the ED explained the higher ultimate tensile strength (UTS) of the ED specimens. Third, fatigue fracture observation showed that the ED specimen had a narrow fatigue striation spacing, which indicated that the plastic deformation of the ED specimen was the smallest in each cycle. In addition, two fatigue prediction models were established to predict the LCF life of extruded 7075 Al alloy, based on the life response behavior of the three directions under different strains.