Research On Heat Treatment Regime And Corrosion Behavior Of Recycled Aviation Aluminum Alloys

With the rapid development of the aviation industry,the amount of highstrength aluminum alloys used continues to increase with the huge quantity of aviation aluminum alloy processing waste.Meanwhile,the number of retired aircraft is large.Therefore,the recovery and reutilization of scrap aviation aluminum alloys are important research directions in the field of recycled aluminum alloys.Due to the wide source and complex composition of scrap aviation aluminum alloys,the aviation aluminum technology which is low-cost,green and efficient is researched and developed to realize the recovery and reutilization of scrap aircraft aluminum alloys.In this way,the recycled aviation aluminum alloys can be obtained that can meet the standards of aviation aluminum alloys on components and properties to realize the recovery and reutilization of scrap aircraft aluminum alloys.For this purpose,various technical explorations have been proceeded on recycled aviation aluminum alloys.First,technologies for pretreatment,such as crushing,paint removal and sorting,have been carried out to realize the fine sorting of raw materials.Then,for alloy smelting,advanced smelting equipment were developed to reduce the rate of metal burning loss,and on-line control technology of melt composition was researched to achieve precise control of composition while deep purification technology for melt was explored to realize production of ultraclean recycled aviation aluminum alloys.Finally,for heat treatment process,the heat treatment regime is optimized to achieve precise control of the structure and properties of recycled aviation aluminum alloys.This paper studied the heat treatment system and the corrosion behavior of recycled aviation aluminum alloys in depth which focusing on the mechanism and technology of iron removal from the melt of recycled aviation aluminum alloys were discussed.And the preparation,microstructure evolution,properties and electrochemical behavior of recycled 7075 aluminum alloy plates were mainly researched.In addition,the heat treatment process of recycled 2024 aluminum alloys was optimized by response surface method(RSM).The main conclusions obtained are as follows:It was studied that the influence laws of Fe content on the microstructure and corrosion properties of recycled 7075 aluminum alloy ingots and the theory and process of borax iron removal.Fe content affected the microstructure and iron-rich phase morphology of recycled 7075 aluminum alloys.With the increase of Fe content,the iron-rich phase changed from skeletal shape to short rod shape and finally to long needle shape.The higher the Fe content,the higher the corrosion rate and the worse the corrosion resistance of the alloys.When Fe element reacted with B element to form Fe2B,the Gibbs free energy was less than 0,and the reaction can proceed spontaneously.The gravitational sedimentation process of Fe2B particles generated in the reaction of the recycled 7075 aluminum alloy melts conformed to Stoke’s law.The sedimentation rate was mainly affected by the melt density,melt viscosity,Fe2B density and particle size.Borax flux had a certain iron removal effect on recycled 7075 aluminum alloy melts with different Fe contents.When the Fe content of the melt was 1.06 wt.%,the iron removal rate reached the highest,which is 36.8%.Using scrap aircraft aluminum alloys as raw material,the non-aluminum impurities with high melting point were removed by controlling the smelting temperature according to the different melting points.And the elements below the standard were supplemented in the form of intermediate alloys and elemental metals,which can obtain 7075 aluminum alloy ingots that meet the brand standard.After homogenization,solid solution and aging treatment,the composition distribution of the extruded plates was more uniform,and the grains were finer,while the structure was more optimized.For recycled 7075-T6 and recycled 7075-T73 aluminum alloy extruded plates,the tensile strength was 558 MPa and 538 MPa,and the yield strength was 487 MPa and 463 MPa,while the elongation is 9.0%and 9.4%,respectively,which are all higher than that of the ASTM standard.Compared with single-stage aging of T6,the strength of recycled 7075 aluminum alloy plates after two-stage aging of T73 was slightly decreased.The corrosion rate of the recycled 7075-T6 aluminum alloy plates was 0.25788 mm/year,and the salt spray corrosion resistance grade was Class Ⅱ.The continuous neutral salt spray corrosion of recycled 7075-T6 aluminum alloy plates started from pitting corrosion and gradually developed into denudation.The heat treatment process of the recycled 2024 hot-rolled plates was optimized by the response surface method.By testing the tensile strength,yield strength and elongation of recycled 2024 aluminum alloy hot-rolled plates,the interaction of multiple factors was researched.The influence laws of the heat treatment process parameters for the recycled 2024 aluminum alloys to the tensile strength,yield strength and elongation were reflected by the CCD model with three factors and three levels.The optimized heat treatment process of recycled 2024 aluminum alloys was 497℃ for 98 min,and the quenching transfer time was 4.8 s.The tensile strength,the yield strength and the elongation of recycled 2024 aluminum alloy ot-rolled plates were 461.8 MPa,348.5 MPa,and 12.8%,respectively.The electrochemical behavior of recycled 7075-T6 aluminum alloys in NaCl solution was studied by potentiodynamic polarization technology and electrochemical AC impedance spectroscopy(EIS).Recycled 7075-T6 aluminum alloys in the NaCl solution of with different pH values had different electrochemical behaviors.At pH=3,the polarization curve mainly reflected the electrochemical behavior of the exposed alloy surface,while the activation affected both the cathodic and anodic reactions.At pH=7,the polarization curve mainly reflected the electrochemical characteristics of the oxide film on the alloy surface.The anodic reaction was affected by the pitting corrosion,while the cathodic reaction was affected by the oxygen diffusion process.In the NaCl solution at pH=3,there was only one capacitive on the EIS of the recycled 7075-T6 aluminum alloys,which was related to the charge transfer process.In the NaCl solution at pH=7,there were two capacitance arcs in the EIS of the recycled 7075-T6 aluminum alloy in the NaCl solution.Among them,one was the medium and high frequency capacitance arc which reflecting the electrochemical behavior of the oxide film,while the other one was the low frequency capacitance which reflecting the charge transfer process between the aluminum alloys and the oxide film interface.Moreover,there was a negatively linear relationship between the pitting potential(Epit)and the Clconcentration(lg[Cl-])of the recycled 7075-T6 aluminum alloys in NaCl solution,which means that Epit decreased with the increase of the Cl-concentration.The susceptibility of the passive oxide film formed by the recycled 7075-T6 aluminum alloys to the pitting corrosion process in the natural environment and anodic polarization was basically the same.In NaCl solution with pH 3,Cl-promoted the occurrence of alloy dissolution.In the NaCl solution with pH 7,the oxide film on the alloy surface was damaged to a certain extent by Cl-.This research realizes the recovery and reutilization of scrap aircraft aluminum alloys,and lays a good foundation for the industrialization of recycled aviation aluminum alloys.