Study On Creep-age Hardening Behavior Of 7050 Aluminum Alloy With Sc Addition

7050 aluminum alloy is an important structural material in airspace industry. The microstructure and mechanical properties of aluminum alloys can significantly be improved with addition of rare earth element Sc. Creep-age forming technology is a new technology that can make alloy plates deformation under the joint action of stress and temperature during creep-age processing. The stress used in the creep-age forming technology is usually lower than the elastic limit of the alloy materials. So the use of this technology can effectively avoid the residual stresses that introduced by subsequent processing and improve performances of the alloys materials. 7050 aluminum alloy plates are mainly used to produce large-size panels. It belongs to heat-treatable alloy and has a higher alloying degree. However, the conventional aging process cannot meet the needs in industry production. At present, creep-age forming technology has successfully been used to manufacture large-size panels in developed countries. But the relevant reports have not been found in the domestic.Therefore, the study on creep-age hardening behavior of 7050 aluminum alloy with Sc addition has very important theoretical and real meaning. In this study, three kinds of 7050 aluminum alloys with different contents of Sc were prepared by using conventional cast process. The effect of Sc on the microstructures of 7050 alloy was firstly investigated. And then through processing the alloys by using conventional aging and creep-aging processes, the creep-age hardening behavior and mechanical performances of the 7050 alloys with Sc addition were comparatively studied. The main studies and results are as follows:1. The microstructures of the 7050 alloys with Sc addition in the different heat treatment states were investigated. The results show that the microstructure of as-cast 7050 alloy with no addition of Sc presents gross dendritic. But with the addition of Sc to the alloys, the dendritic structures obviously decreases and were transformed into equiaxed grains. The higher the content of Sc, the finer the grain of the alloys, and the best refinement effect can be achieved when the addition content of Sc is of 0.302wt%. Besides, the boundary segregation can also be improved with the addition of Sc.2. The conventional aging and creep-age hardening behaviors of the 7050 alloy with and without Sc addition were comparatively studied, and the effecting rule of Sc on the age hardening behaviors of the alloys was analyzed. The results reveal that the hardness of the alloys can be increased with the addition of Sc for the conventional aging process, and the anti-overaging capacity of the alloys can also be improved. The aging time of reaching peak hardness is shortened with the increase of aging temperature. The comprehensive analysis of the study results indicates the aging effect is better when aging temperature is of 150 oC. For the alloys age-processed in the stress states, the hardness of the alloys slightly decreases, and the aging time of reaching peak hardness also decreases. The stress states have some effect on the age hardening behaviors of the alloys. The hardness of the alloys in the compressive stress state is a little higher than that in the tensile stress state.3. The effect of Sc on the springback phenomenon of the 7050 alloys treated with the creep-age process was investigated. The results show there is some springback after creep-age processing for all of the 7050 alloy plates with and without Sc addition.. The higher the aging temperature and the longer the aging time, the lower the springback rate. Scandium has an obvious effect on the springback rate of the alloy plates treated by creep-age process. And the higher the Sc content, the lower the springback rate. This is beneficial to the deformation of the alloy plates.4. The effect of creep-age process on the tensile properties of the 7050 alloys with and without Sc addition was contrastively studied. The results reveal that for the alloy plates with same content of Sc treated by creep-age process at 150 oC for 9 hours,the yield strength of the specimens sampled in the different direction in relation to therolling direction shows no obvious difference, but the tensile strength of the specimens appears anisotropy. The specimen sampled at zero degree in relation to the rolling direction has the highest tensile strength, the specimen at 45 degree has the second and the specimen at 90 degree has the lowest. The elongation of the alloys also presents some anisotropy. For the specimens sampled in same direction, the content of Sc has little effect on the yield strength of the alloys plate, but has bigger effect on the tensile strength and elongation of the alloys plates. With the increase of the content of Sc, the tensile strength and elongation of the alloys increases.