Effect Of Forced Cooling On Microstructure Properties Of Fine-grained 7075 Aluminum Alloy Friction Stir Welding

7075 aluminum alloy is known as high-strength aluminum alloy and aviation aluminum alloy,which is widely used in flight manufacturing,shipbuilding heavy industry and civil fields.Isometric extrusion（ECAP）is an effective means to prepare high-strength fine-grained materials,which can significantly improve the mechanical properties of 7075 aluminum alloy,but due to the poor thermal stability of fine-grained7075 aluminum alloy materials,the high residual stress of traditional melting welding joints and the disadvantages of a variety of enhanced phase dissolution,its application range is limited.In this project,the fine-grained 7075 aluminum alloy prepared after ECAP deformation was connected by friction stir welding（FSW）technology to study the change of fine grain scale and microstructure properties of FSW joints under room temperature and liquid CO₂ cooling conditions,and the precipitation phase precipitation behavior and its mechanism for matrix strengthening under different cooling conditions were analyzed.ECAP technology was selected to extrude 7075 aluminum alloy at room temperature for four passes,and the grain structure of the material was refined from the original 26μm to 4.5μm after deformation,and it was distributed equiaxed.After ECAP deformation,the material undergoes continuous dynamic recrystallization,the degree of recrystallization is greatly increased,and the dislocation density decreases.The original 7075 aluminum alloy presents a typical rolled texture（β-fiber）,and after ECAP deformation,the strongest point is observed in the{111}crystal plane and many secondary strength points are derived,and the main texture type is Rotated Brass{111}<110>texture.After ECAP deformation,the strength and hardness of the material increased significantly under the coupling of grain boundary strengthening,dislocation strengthening and second phase strengthening,among which the yield strength increased by 166.1%,and it was calculated that grain boundary strengthening contributed the most to yield strength,followed by dislocation strengthening,and the precipitated phase strengthening was the smallest,with a ratio of 2.33:1.67:1.FSW connection of fine grain 7075 aluminum alloy under room temperature and CO₂ cooling conditions,welding formability is better under CO₂ cooling conditions,and the grain can be further refined in this welding environment,the grain size in the stir zone（SZ）is refined from the original fine grain base material from 4.5μm to2.9μm,and the grain size in the SZ region is 5μm at room temperature.A variety of DRX mechanisms can be observed in weld areas,among which the common mechanisms are continuous dynamic recrystallization（CDRX）and geometric dynamic recrystallization（GDRX）.The degree of recrystallization gradually decreases from the SZ region along the direction of the weld TD to the heat affected zone（HAZ）,and the degree of recrystallization under CO₂ cooling conditions is higher than that at room temperature,and the dislocation density is the opposite.By analyzing the anisotropic evolution,it can be seen that the texture rotation angle of each region is roughly the same under different cooling conditions,and the anisotropy is weak under room temperature conditions.Under CO₂ cooling conditions,the microhardness and strength of the joint were better than those at room temperature,and the welding efficiency reached 85%,which increased the tensile strength by 41.36%compared with the room temperature joint.By comparing the fracture morphology of the joint,it can be seen that the fracture dimple is deeper and smaller under CO₂ cooling conditions,and the Mg Zn₂ phase is found at the bottom of the dimple socket,which promotes the improvement of the tensile strength of the material.The CO₂ cooling condition decreases by about 105°C compared with the peak temperature at room temperature,and the cooling rate increases by 56.4%.By observing the morphology of the precipitated phase and comparing the crystal plane spacing by XRD,EDS and HRTEM,it can be determined that the 7075 aluminum alloy mainly includes Al₂Cu Mg,Al₂Cu Cu and Mg Zn₂ phases,and combined with the observation of the precipitated phase and the matrix interface lattice fringes,it can be determined that the Al₂Cu Mg and Al₂Cu phases have a co-lattice and semi-colatteric relationship with the matrix,and the Mg Zn₂ phase has a non-collattic relationship with the matrix.The dissolution sequence and TEM analysis of DSC precipitation phase showed that the SZ region was mainly Al₂Cu Mg and Mg Zn₂ phases at room temperature,the SZ region contained more Mg Zn₂ and Al₂Cu Mg phases under CO₂cooling conditions,the precipitated phases in the thermal mechanical affected zone（TMAZ）region under the two welding conditions were similar,both contained more Mg Zn₂ phases,and the size of the precipitated phase in the HAZ region was finer than that of other regions,mainly including Mg Zn₂ and Al₂Cu phases.According to the second phase strengthening mechanism,it can be seen that the coarsening of the precipitated phase can be effectively prevented under CO₂ cooling conditions,resulting in better Orowan strengthening effect and Zener nailing effect,which promotes the grain boundary effect of the precipitated phase pinning more obvious,and the CO₂cooling effect can also inhibit the grain boundary diffusion,shorten the PFZ distance,and effectively reduce the precipitation of Mg Zn₂ and improve the corrosion resistance of the material,so it is necessary to apply CO₂ cooling during FSW.