The real stress-strain data of 7075 alloy aluminum sheet were obtained at forming temperatures from 573 to 723 K and strain rates from 0.01 to 10 s-1. The data were used as the basic model for calculating the triple criterion of strain rate sensitivity index (m value), power dissipation factor (η value), and instability criterion (ξ(ε.) value). The thermal processing diagram of 7075 aluminum sheet including strain was constructed by the triple criterion, and the microstructure of the specimen after deformation was observed to verify the thermal processing diagram and finally determine the best deformation parameters of the alloy in the test range.
The results show that the safety zone of 7075 aluminum plate hot working is concentrated in the high temperature and low strain rate zone, and the value of η increases gradually with the increase of strain; through the metallographic observation, in the stable deformation zone, the material refines the grain due to the dynamic recrystallization of deformation; in the unstable deformation zone, cracks are found along with the generation of flow dislocation zones, so the optimal process parameters determined by the hot working diagram including strain can be used to Therefore, the optimal process parameters can be determined from the strain-inclusive thermal processing map to ensure defect-free 7075 aluminum sheet forgings.
The microstructure of 7075 aluminum sheet heat deformation was observed by transmission electron microscopy (TEM). 7075 aluminum sheet experiments showed that under hot forging conditions, 7075 aluminum sheet is fully capable of dynamic recrystallization and small recrystallized grains are produced by dynamic recrystallization. Dynamic recrystallization is discontinuous dynamic recrystallization, nucleation mechanism for subcrystalline rotation, polymerization nucleation; its critical strain value and the number of processing, the more the number of passes, the lower the critical value. At the same Z value, the recrystallized grain size decreases with the increase of strain. Dispersed second-phase particles play an important role in the dynamic recrystallization process.
To investigate the process and properties of the composite method of chemical nickel plating plus gas nitriding on the surface of 7075 aluminum sheet and the interdiffusion process between 7075 aluminum sheet and Mg-Zn alloy. The structure and properties of the treated layer were investigated by means of optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD) and hardness testing, and the main findings are as follows:
(1) Chemical nickel plating can form a nickel layer on the surface of 7075 aluminum plate, and the thickness of the nickel layer increases with the increase of the plating time. Gas nitriding of 7075 aluminum plate with nickel layer surface can form nitrides such as Ni4N on the surface. In the process of gas nitriding, Al atoms in the matrix and Ni diffuse each other to form A13Ni, A13N2 compounds, with the extension of the nitriding time, the number of Ni4N, A13Ni, A13N2 increased. The presence of these compounds increases the number of aluminum plates for gas nitriding, which can form nitrides such as Ni4N on the surface. In the process of gas nitriding, Al atoms in the matrix diffuse with Ni to form A13Ni and A13N2 compounds, and the number of Ni4N, A13Ni and A13N2 increases with the extension of nitriding time. The presence of these compounds increases the surface hardness of the aluminum plate, and its hardness reaches up to 700 HV, which is 7-8 times of the hardness of the substrate.
(2) 7075 aluminum plate and Mg-Zn alloy in the annealing furnace for mutual diffusion found that, after annealing at 460 ℃, 10 min, the diffusion layer of Al-Mg-Zn alloy phase of different compositions formed in the diffusion zone, and its hardness can reach 240 HV, which is three times that of the aluminum plate matrix; after annealing at 470 ℃, 60 min, the diffusion layer organization is A1-Mg-Zn eutectic organization and Zn The microhardness of the diffusion layer was 77.2 HV, which was 20 HV higher than that of the aluminum plate matrix.