Research On Low Treatment Thermomechanical Treatment And Serrated Yielding Of 7075 Aluminum Alloy Sheet

The high strength aluminum alloy sheet can not be formed in complex parts for high strength,poor plasticity and remarkable springback at room temperature.It is possible for aluminum alloy to form complex plate and shell components at room temperature through the process of?solution treatment—natural aging—cold deformation—artificial aging?.But the serrated yield behavior?PLC effect?will affect the surface quality and forming stability of the sheet metal which appear in the plastic forming process.7075 aluminum alloy T6 plate was taken as the research object.Comprehensive consideration of the low temperature deformation heat treatment process sheet forming performance and the effect of PLC effect,two process parameters were obtained according to the requirements working conditions of 7075 aluminum alloy plate low-temperature thermomechanical treatment were determined in this paper.The results show that the percentage total extension at maximum force of 7075aluminum alloy is not less than 21.5%,which is 90%higher than 7075 aluminum alloy T6;the yield strength is less than 180MPa,which is 60%lower than that of the raw material when the solution treatment parameters of 7075 aluminum alloy are T_S=480?and t_S=0.5h.The sheet has the best formability.The determination of natural aging parameters can be selected according to different production needs.Natural aging(t _NA?3h)can be selected for processing and forming for parts with low surface quality requirements;natural aging(t_NA?48h)can be selected for processing and forming for parts with high surface quality requirements.The sheet can restore mechanical properties completely after artificial aging?120?×12h?.The influences of solution treatment on the microstructure and macroscopic mechanical properties of the 7075 aluminum alloy sheet are investigated by optical microscopy?OM?,scanning electron microscopy?SEM?,energy dispersive spectroscopy?EDS?and Transmission electron microscope?TEM?.The 7075 aluminum alloy T6 plate's grains are a kind of small and compact equiaxed crystal and there are a large number of strengthening phase in the organization.The grains grow obviously and a large number of strengthening phases dissolve into the matrix,at the same time,the fracture form changes from brittle fracture to ductile fracture after solution treatment and natural aging.The grain size of the plate returned to 7075 aluminum alloy T6 state after artificial aging treatment.The critical strain of PLC effect gradually moves backward with the extension of natural aging.The average stress drop,reload time and drop time increase with the increase of natural aging time.The arithmetical mean deviation and the total height of profile of the surface waviness both decline as a power exponential function.The surface quality of tensile specimen is greatly improved.The PLC effect leads to the plastic instability and the serrated alternating stress,which leads to the similar fatigue fracture characteristics of the material.According to the shape of PLC bands,it can be divided into two types:static behavior with no propagation property and dynamic behavior with obvious propagation property.For the static behavior of the PLC bands,the PLC bands has a random position and short production cycle,which has a great influence on the surface quality of the specimen;For the dynamic behavior of the PLC bands,the propagation of the PLC bands causes a continuous and uniform thick displacement of the sheet surface,which has little effect on the surface quality.