| 6xxx series aluminum alloy is a heat-treatable reinforced aluminum alloy.It is widely used in aerospace,rail transit and modern architecture and other fields because of its excellent comprehensive properties such as medium strength,corrosion resistance,welding property,forming property and high temperature resistance.The results show that the forming properties and mechanical properties of aluminum alloy sheet can be improved effectively by adjusting the alloy composition and improving rolling and heat treatment process,so that the aluminum alloy can be used more widely,but at the same time,accidents caused by its fracture failure often result in huge loss of life and property.According to statistics,fatigue failure accounts for about 80% of the mechanical failure,and fatigue failure is more and more prominent in engineering failure.Therefore,the mechanical properties and fatigue properties of 6xxx series aluminum alloy were studied by means of metallographic microscope,tensile test,fatigue test,scanning electron microscopy(SEM)and transmission electron microscopy(TEM).The high and low temperature tensile flow behavior and rolling process of 6xxx series aluminum alloy were studied.Through the combination of experiment and theory,the S-N curve of AlMgSiCu alloy sheet was established,the fracture characteristics and basic law of fatigue were explored,and the initiation and propagation of fatigue crack were clarified,the effects of different aging conditions on the fracture and fatigue properties of Al-Mg-Si-Cu alloy sheet were also studied,which laid a foundation for the study of fracture of 6xxx series aluminum alloy.The main conclusions obtained in this paper are as follows:(1)It is found that the flow stress decreases with the increase of temperature and increases with the increase of strain rate by high temperature tensile test and medium-low temperature tensile test on Al-0.75Mg-0.75Si-0.8Cu alloy and Al-0.75Mg-0.75Si-0.8Cu-0.7Zr alloy,respectively.The constitutive equations of high temperature tensile are established.The rolling process of Al-0.75Mg-0.75Si-0.8Cu-0.7Zr alloy is also studied,the tensile strength of the alloy reaches a maximum of 475 MPa and an elongation of 8.13% when the alloy rolled at 300 ?,accompanied by solution of 510 ?/80 min and aging of 195 ?/13 h,and then cold rolling.(2)The power function expression of the S-N curve of Al-0.75Mg-0.75Si-0.8Cu alloy sheet under different stress amplitudes is established,the slope of S-N curve decreases gradually with the decrease of stress level and then tends to be stable.Origin software is used to fit the measured data and the calculated S-N curve expression,and it is found that the two curves are basically in agreement with each other.(3)Fatigue cracks usually initiate at or near the free surfaces of the specimen or at the second phase particles,fatigue crack initiation show a trend of multi-source under high loading stress,which is due to the increase of loading stress,the particles are prone to self-rupture,resulting in micro-cracks,and because of the stress concentration at the crack tip,the crack is easy to propagate into the matrix.In the fatigue crack propagation region,the number of secondary cracks and holes in the fracture surface increases with the increase of the loading stress level,and the spacing of fatigue stration increases.(4)Under the condition of under-aging,peak-aging and over-aging,the strength of Al-Mg-Si-Cu alloy is improved as a whole during fatigue process,in the early,middle and late stages of the fatigue process,the yield strength of the alloys is similar,showing a trend of increasing first and then decreasing. |
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