| As a medium-strength weldable aluminum alloy,7N01 aluminum alloy is also an age-enhanced alloy.It has not only high strength but also good weldability.It has been widely used in welded structural parts such as high-speed rail and construction.For structural materials which used in high-speed trains,in addition to attention its static strength indexes such as yield strength,tensile strength,and elongation,fatigue properties including fatigue strength,fatigue life,and fracture toughness are also of concern,and the static and dynamic loads of materials are explored.The relationship between the fracture behavior under the action,the relationship between the static strength of the material and the fatigue strength,and the influence of the microstructure of the material on its fracture behavior have important scientific significance.In this paper,the industrially produced 7N01 aluminum alloy sheet was studied by metallographic microscope,scanning electron microscope,transmission electron microscope,tensile test,fatigue test and other systems.After 470℃ for 90 minutes,the holding furnace was cold(annealed),after 470℃ for 90 minutes,water quenching at room temperature for 4 days(solid solution quenching),after 470℃ for 90 minutes water quenching and then artificial aging treatment 120℃×1h,120 ℃×24 h,120℃×96 h.All of 5 heat treatment conditions which the influence of the fracture behavior was studied.Based on the study,the effects of pre-stretching deformation on the fatigue fracture behavior of the annealed state,quenched state and 120℃×1 h aging alloy sheet were studied.The main findings are as follows:(1)12.5 mm thick 7N01 hot rolled sheet after 470℃ for 90 minutes,the holding furnace is cooled(annealed state),470℃ for 90 minutes,water quenched at room temperature for 4 days(solid solution quenched state),470℃×90 minutes water after quenching,120℃×1h,120℃×24 h,120℃×96 h artificial aging treatment,the grain shape,the number of excess crystal phase particles,size and distribution are about the same.However,different heat treatments have a significant effect on the particle configuration of the precipitated phase of alloy sheet.No solid grain boundary or intragranular precipitation phase was observed in the solid solution water quenching state and the 120℃×1 h aging 7N01 aluminum alloy sheet;120℃×24 h and a large number of ellipsoidal precipitates were observed in the grains at 120℃ for 96 h.The aging time was prolonged,the particle size of the precipitated phase increased,and the dispersion increased.The PFZ width increased from approximately 65 nm at 120℃×24 h to about 81 nm at 120℃×96 h and the precipitated phase particle size in the annealed alloy sheet is between 0.1 μm and 0.4 μm.When the pre-stretching deformation increased from 0%to 20%for annealed state,solid solution water quenched state and 120℃×1 h aging 7N01 aluminum alloy sheet alloy phase particle shape,size,quantity,distribution has no significant effect.However,when the degree of tensile deformation exceeds 5%,there are a large number of dislocations in the grain boundary and the crystal of the alloy plate in the three states,and the degree of dislocation entanglement in the annealed state and the quenched alloy plate is serious.(2)For the 7N01 sheets with five heat treatment conditions,the yield strength and tensile strength of the 120℃×96 h aging sheets are 350.5 MPa and 371.2 MPa respectively,and the elongation is 27.9%.The yield strength and tensile strength of the annealed sheets are 145.6 MPa and 207.7 MPa respectively,and the best plastic elongation is 35.7%.With the increase of pre-tension deformation,the yield strength and hardness of the annealed state,the quenched state and the aged 7N01 aluminum plate at 120℃×1 h gradually increased.When the pre-stretching deformation increase to 20%,and the yield strength,macroscopic hardness and microhardness of the annealed sheet,quenched state and 120℃×1 h aging plate are undistorted by 140.1 MPa,60.5 HV,61.0 HV;241.1 MPa,99.3 HV,98.4 HV;181.0 MPa,95.9 HV,94.7 HV increased to 223.1 MPa,72.6 HV,74.1 HV;374.9 MPa,118.5 HV,120.1 HV;254.0 MPa,112.0 HV,117.1 HV.(3)For the five heat treatment states,when the fatigue stress of the 7N01 sheet is 150 MPa and the stress ratio is R=0,the fatigue life of the 120℃×1 h alloy sheet can be up to 9.23×105(cycles).The fatigue life of annealed alloy sheets is as short as 9.21×104(cycles).When the precipitated phase particles are configured as GP zone,the fatigue crack growth can be well hindered and the fatigue damage is small.(4)The fatigue crack sources of the 7N01 aluminum alloy sheets in the five heat treatment states were all generated at the excess crystal phase particles which have a size of about 10 μm.The surface of the fracture near the crack source area is uneven and there are many uneven crystallographic facets,tearing ribs and river-like patterns.In addition,the radial features are obvious.The surface morphology of the fatigue fracture zone of 120℃×1 h aging and solid solution water quenched alloy sheets is rough,the surface is mainly composed of wavy stripes and a small number of cleavage steps,the crack deflection is obvious and the crack deflection angle is large.The fracture indicates that the degree of undulation is large,the fatigue crack is dense,and the fatigue crease spacing is narrow.The surface morphology of the fatigue fracture extension zone of 120℃×24 h,120℃×96 h aging and annealed alloy sheets is smooth,crystallographic step-like features are obvious,and a large number of tearing edges appear near the steps and exhibit quasi-cleavage fracture.The degree of crack deflection is small and the surface undulation is low.The corresponding fatigue strips are long and flat and the distance between the strips is large.(5)With the increase of pre-tension deformation,the yield strength and hardness of the annealed state,the quenched state and the aged 7N01 aluminum plate at 120℃×1h gradually increased.The yield strength and hardness of the annealed sheet,the quenched state and the 120℃×1h aging plate are increased from the unstretched deformation of 140.1 MPa and 60.5 HV,241.1 MPa and 99.3 HV,181.0 MPa and 95.9 HV respectively to the 223.1 MPa and 72.6 HV,374.9 MPa and 118.5 HV,254.0 MPa and 112.0 HV which achieve 20%pre-stretching deformation.(6)When the annealed 7N01 aluminum plate is fatigued under the condition of stress level 155 MPa and stress ratio R=0,the fatigue life of the alloy sheet increases gradually with the increase of the pre-tension deformation amount.While the pre-tension deformation amount is 20%,the fatigue life can be up to 1.11×106(cycles)which is about 5.7 times of no pre-deformation.When the solid solution water quenched 7N01 aluminum plate is fatigued under stress level 180 MPa and stress ratio R=0,the fatigue life of alloy sheet increases first and then decreases with the increase of pre-tension deformation.While the pre-tension deformation is 12%,fatigue life can be up to 1.10×106(cycles)which is about 2.7 times without pre-deformation and 1.7 times at 20%pre-deformation;When the fatigue state of 7N01 aluminum plate at 120℃×1h is under stress level 175 MPa and stress ratio R=0,the fatigue life of the alloy sheet increases first and then decreases with the increase of pre-tension deformation.The fatigue life at 5%pre-deformation is up to 1.06×106(cycles)which is about 1.7 times that of no pre-deformation and 3.7 times that of 20%pre-deformation.The annealed state under 20%pre-deformation,the quenched state at 12%pre-deformation,and the 120℃ ×1h aging 7N01 aluminum plate under 5%pre-deformation hold the best dislocation configuration that hinders fatigue crack propagation. |
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