Low-cycle Fatigue Behavior Of Rolled Al-5.4Zn-2.6Mg-1.4Cu Alloy Sheet

The Al-Zn-Mg-Cu systems alloys are extensively used in the automobile manufacturing,rail transit and aerospace industries owing to their high specific strength and excellent fromability.The mechanical properties of the Al-Zn-Mg-Cu systems alloys rolled sheet are closely related to their sampling direction.It is of great significance to investigate the anisotropy of mechanical properties of these alloy products.Therefore,the understanding of the anisotropy of fatigue behavior of these alloys has an important reference value for the safety and reasonable application of the Al-Zn-Mg-Cu systems alloys.In this paper,Al-5.4Zn-2.6Mg-1.4Cu alloy was prepared by melting,casting and rolling processes,successively.The microstructure observation shows that the alloy demonstrates a typical rolling deformation structure.In the plane parallel to the rolling direction(RD),the morphologies of grains are fibrous and obviously elongated along the rolling direction.While in the plane perpendicular to the rolling direction(TD),the grains are squashed due to the compressive stress in the previous step.Before and after heat treatment,the microstructure and precipitation behavior of the second phase of Al-5.4Zn-2.6Mg-1.4Cu alloy are different.The tensile test results show that the yield strength and ultimate tensile strength of the Al-5.4Zn-2.6Mg-1.4Cu alloy along the TD are higher than those in the RD under the same heat treatment conditions.The elongation of the alloy along RD are higher than that of the alloy in TD.The fractography of the tensile fracture shows obvious ductile fracture characteristics both along the RD and TD.The Al-5.4Zn-2.6Mg-1.4Cu alloy both along the RD and TD are reached the peak age condition while aged for 26 hours.The results of low-cycle fatigue experiments reveal that at the total strain amplitudes ranged from 0.4% to 0.8%,the alloy along both RD and TD exhibit the stable cyclic stress response behavior.At the same total strain amplitude,the cyclic stress amplitude of alloy along TD is higher than that along RD,while the fatigue life of alloy along RD is significantly longer than that along TD.For the Al-5.4Zn-2.6Mg-1.4Cu alloy sheet,the relationship between the plastic strain amplitude,elastic strain amplitude with the number of reversals to failure are linear.In addition,under the low-cycle fatigue test,the fatigue cracks initiate transgranularly at the free surface of fatigue samples and propagate transgranularly.