Study On The Microstructural Evolution Of Cold-deformed AA7050 By Thermal Exposure And Its Influence On Tensile Propertie

7xxxx series aluminum alloys are widely used in aerospace,transportation and other fields.In order to meet the complex requirements,aluminum alloy components must be subjected to multi-frequency and multi-temperature hot-dip plating,which will cause the aluminum alloy to be in the’thermal exposure state’of over-aging heating and cause performance degradation.Therefore,it is of great significance to study the effect of thermal exposure on the microstructure and mechanical properties of 7xxx series aluminum alloy and improve the mechanical properties of 7xxx series aluminum alloy after thermal exposure for its safe application in the aviation industry.In this paper,7050 aluminum alloy（AA7050）is taken as the research object,and the effects of microstructure evolution on mechanical properties of 7050 aluminum alloy during peak aging and thermal exposure were explored.The main research contents and conclusions are as follows:1.After solution treatment,26%cold deformation and peak aging refined the grains and increased the dislocation density,which provided a large number of nucleation sites for the precipitates.After peak aging treatment,a high density ofη’phases are formed in the matrix of the cold deformed samples,which makes the yield strength of the cold deformed samples reach 601 Mpa,higher than the samples without cold deformation.2.After the peak aging samples were thermally exposed（110°C/2h→158℃/3h→cooling→115°C/10h→158℃/20h→cooling）,the microstructure evolution and its effect on mechanical properties were studied.The grain sizes of NPA,26PA and NOA samples hardly change after thermal exposure.The dislocation densities of NPA,26PA and NOA samples(19.2×10¹³/m²,7.58×10¹³/m²and8.65×10¹³/m²)almost did not change(18.9×10¹³/m²,6.90×10¹³/m²and 8.01×10¹³/m²)after thermal exposure.However,the precipitates are generally coarsened and the main strengthening phasesη’change to the equilibriumηphases,resulting in strength degradation.The high-density dislocations introduced by cold deformation provide a large number of channels for the rapid diffusion of solute atoms,resulting in the coarsening rate of precipitates and the transformation rate ofη’phase to equilibriumη’phase much faster than that of the uncold deformed sample,resulting in a significant decrease in strength.3.The evolution mechanism of the precipitates during thermal exposure was studied by means of TEM and EBSD.The results show that the enrichment degree of Zn,Mg and Cu solute atoms in large-size precipitates is higher than that in small-size precipitates.It was observed by TEM that Mg₂Zn₃with Zn/Mg=1.5 and higherηphase of Zn/Mg existed in one precipitate,which indicates that although the thermal exposure temperature is lower than the equilibrium precipitation temperature of theηphase.The solute atoms diffuse continuously to theη’phase.When the composition is close to the transformation requirement,theη’phase gradually transforms into the equilibriumηphase.4.The solid solution sample was studied.After the cold deformation layer was formed on the surface by surface ultrasonic rolling,the gradient structure from the surface ultrafine grain to the coarse grain near the center was observed by aging and thermal exposure conditions.There are differences in the nucleation and growth of precipitates in the surface ultrafine grain and coarse grain.Due to the low dislocation density in the surface ultrafine grain,the nucleation is mainly uniform nucleation,and the coarse grain boundary precipitates（Al₂Cu Mg）consumes a large amount of solute atoms,resulting in a low density of precipitates in the ultrafine grain.The defect density in coarse grains is high,both uniform nucleation and non-uniform nucleation,forming a high precipitation density.Therefore,the hardness of the surface gradient layer is low,and the hardness near the core of the sample gradually increases.After thermal exposure,the yield strength and tensile strength of the400PA-TE sample were improved and the plasticity decreased slightly compared with the unrolled sample.The tensile strength of 600PA-TE sample increased by about 30Mpa.The plasticity is also slightly improved.