Study On Fatigue Properties Of 6005A Alloy Extruded Profiles For Rail Transit

Al-Mg-Si-Cu alloy has become the preferred material for rail transit vehicle structural components due to its excellent extrusion performance and absence of stress corrosion phenomenon,and its dosage and demand are increasing.Because of the special service conditions under alternating loads for a long time,the study on fatigue performance of rail transit vehicles structural components is particularly important.In this paper,the 6005A aluminum alloy of the Al-Mg-Si-Cu alloy producedby the company was selected as the research object,and its fatigue performance were studied.Firstly,stress-life data and crack length-life data were obtained by high-cycle fatigue test and fatigue crack propagation test.The S-N curves and a-N curves of the 6005A aluminum alloy were obtained by processing the test data.At the same time,by means of the detect methods such as metallographic microscope,X-ray diffraction（XRD）,scanning electron microscopy（SEM）,and energy spectrum（EDS）etc,the organizational structure,fracture morphology,and related fracture mechanisms of the 6005A aluminum alloy fatigue failure specimens were observed and analyzed..The main conclusions are as follows:（1）The median fatigue strength of the 6005A aluminum alloy extruded section under stress ratio R=0.1 is 164.5 MPa,and the S-N curve equation is expressed aslg N₅₀（28）10.696-0.024S.（2）The main cause of the difference in fatigue life under the same maximum stress of the6005A aluminum alloy used in the test is the fatigue crack initiation and initial expansion stage.The defects of holes in the materials,Al₂O₃ inclusions and the coarse second phase particles containing Fe elements have significant influence on the fatigue crack initiation and initial expansion stage,and the degree of influence is different.（3）The fatigue life of fatigue cracks in 6005A aluminum alloy on the Al₂O₃ inclusions is about one order of magnitude greater than the initiation life of cracks on coarse precipitated particles and about two orders of magnitude greater than the initiation lifetime at the defects in the pores.The effect on the crack growth rate shows a similar pattern.（4）Al₂O₃ inclusions have a great influence on the fatigue life of the 6005A alloy used in the test,not only shortening the life of the crack initiation,but also accelerating the crack growth rate after crack initiation.This effect is also related to the size of the Al₂O₃ particles,and the larger the size of the Al₂O₃ particles,the greater the effect on the fatigue life.The Al₂O₃ particle inclusion mainly initiates cracking through separation of the Al₂O₃from the matrix.（5）The main second-phase particles contained in the 6005A aluminum alloy used in the test were Mg₂Si,AlFeMnMgSi,Al₅FeSi,and Al₇（CrFe）.The second phase particles that trigger fatigue crack initiation are mainly coarse Al₅FeSi,and Al₇（CrFe）,and this effect is related to the shape,size and location of the second phase particles.