Study On The Design Of 7XXX-series Aluminum Alloy Optimizing Stress Cracking Corrosion And Mechanism Research Based On Machine Learning

Recently,as the “energy conservation and emission reduction” has become a widespread concern in the public,the urgent need for “lightweight” high-speed trains has been further enhanced.7XXX-series aluminum alloy(Al-Zn-Mg-Cu series)are widely used in key parts of high-speed trains such as axle boxes due to the low density,high strength,toughness and excellent weldability.However,stress corrosion cracking(SCC)is a potential hazard when the alloy service.In this paper,based on the efficient global optimization(EGO)and Kriging surrogate model,the material learning loop is independently designed.The conductivity is regard as the primary goal of SCC,and the great performance alloy has been designed.Then,the mechanical properties and SCC resistance are further improved by the heat treatment system.The microstructure and rare-earth phase of the new EGO alloy are studied by a series of technologies of optical microscope(OM),scanning electron microscope(SEM),energy dispersive spectrometer(EDS)and X-ray diffraction(XRD).The reason of excellent SCC resistance is deeply explored,and the influence of rare earth elements Y and Ce on the SCC resistance is analyzed.The main contents are as follows:(1)By regarding the conductivity as the primary objective,based on the selfdesigned material learning loop,the higher conductivity aluminum alloy is designed by four iterations.The alloy composition is Al-6.05Zn-1.46Mg-1.32Cu-0.13Zr-0.02Ti-0.50Y-0.23 Ce,and the conductivity value is 42.152 %IACS.(2)After single-stage aging(T6),double-stage aging(T74)and retrogression reaging(RRA)system,the tensile strength of the new EGO alloy is 505 MPa,459MPa and 503 MPa respectively,and the yield strength is 428 MPa,365MPa and 432 MPa respectively.The rank of corrosion resistance is T74>RRA>T6.Support vector machine(SVM)is employed in the dual objective optimization with mechanical properties and SCC resistance.The calculated RRA parameters with best comprehensive properties of the alloy is 120?-24h+190?-40min+120?-24 h.At these parameters,the strength is 502 MPa and conductivity is 41.839% IACS.(3)The EGO alloy had better SCC resistance than the traditional 7N01 alloy.After single-aging and double-aging heat treatments,the Iscc values of the EGO alloy were significantly lower than the 7N01 alloy(6.82% and 1.15% compared to 23.55% and 14.31%).(4)The addition of rare earth elements Y and Ce formed the Al₈Cu₄(Y,Ce)and quadrilateral Al20Ti2(Y,Ce)phases,which improve the stress corrosion cracking(SCC)resistance of 7XXX Al alloys by decreasing the amount of ?(MgZn₂)on the grain boundaries,which disrupts the continuous anode channels;by forming rare earth oxide films,which help prevent hydrogen embrittlement;and by creating dense Cecontaining oxides and chlorides,which passivate against pitting damage.