Study On Microstructure And Properties Of Al-Zn-Mg-Cu Aluminum Alloy During Friction Stirring Procession

7xxx aluminum alloy belongs to ultra-high strength aluminum alloy,with the advantages of light weight,specific strength and high specific stiffness.It is one of the aluminum alloys with better overall performance and is also the preferred material for aviation industry.However,as-cast Al-Zn-Mg-Cu alloy coarse grains,uneven microstructure,serious segregation of elements,while high-alloy aluminum alloy will generate a lot of unstable phase.So modifying it has become more and more important.Friction stir processing is a kind of newly simple and efficient plastic deformation technology which is evolved on the basis of friction stir welding.It is widely used in the modification of magnesium alloy.In this paper,the friction stir processing technology is analyzed to determine the reasonable stir head and friction stir processing parameters,the Al-Zn-Mg-Cu aluminum alloy for as-cast processing and heat treatment experiment,the use of a variety of modern materials characterization methods and numerical analysis theory.The microstructure subgrain structure,microcrystal strain and dislocation density of deformed specimens under different processing parameters were analyzed.The results show that the microstructure of the microstructure is refined obviously by the friction stir processing.The grain size is refined from 138?m to 12?m at the traveling speed of 100mm/min and the rotation speed is 600r/min.The microstructure of the microstructure uniformity has been significantly improved.At the same time,SEM,EDS and XRD were used to analyze the phase composition,morphology and amount of the multi-alloy in the materials before and after the friction stir processing.The results show that the as-cast Al-Zn-Mg-Cu aluminum alloy consists mainly of??MgZn₂?phase distributed in the grain and AlCu phase on the grain boundary.After the friction stir processing,the size of the AlCu is obviously refined,and uniformly distributed in the matrix.With the increase of rotating speed of friction stirring head,the diffraction peak of??MgZn₂?phase decreases gradually.Therefore,the??MgZn₂?phase is dissolved back after the friction stir processing.The matrix mainly consists of??Al?+?AlCu?phase+?phase?small amount?.The results of the hardness and tensile test of the samples with different parameters after friction stir processing show that the microstructure of the friction stir processing has obvious hardness when it is processed because of the dynamic recrystallization and the formation of fine equiaxed grains.The tensile strength and elongation of the material all increased significantly.When the traveling speed was 100mm/min and the rotation speed was 600r/min,the microhardness was 164.7HV,which was 40%higher than that of the base metal.The tensile strength up to 540MPa,50%higher than the base metal.After three passes,the tensile strength reached 609MPa.With the increase friction passes,the microstructure was more refined and the reinforcing phase was more dispersed.Tensile strength gradually increased.In addition,heat treatment experiments of different processes were carried out on the samples after the friction stir processing.The results show that the??MgZn₂?phase in the microstructure is evenly dispersed in the matrix after the heat treatment,and the irregular AlCu phase broken by the stirring head changes to a more stable spherical shape.The microstructure uniformity has been significantly improved.After the heat treatment,the hardness of the stirring zone is greatly increased,meanwhile,the performance unevenness caused by different regions is greatly improved due to the friction stir processing.For different friction stir processing parameters,the sample was solid-dissolved at 473?for 140 minutes and aged at 135?for 16 hours.After that,the tensile strength of the material was significantly improved.The traveling speed was 100 mm/min,and the rotation speed was 600 r/min,the tensile strength reached 752 MPa,which was 23.5%higher than that of the unheat-treated sample.