The Effect Of Pressure And Fe Content On Low Cycle Fatigue Behaviour Of Al-Zn-Mg-Cu Alloy

Squeeze Casting is an advanced near-net-shape forming technology which combined the characteristics of casting and plastic forming. Al-Zn-Mg-Cu alloys prepared by squeeze casting can be widely used in aviation and aerospace field. However, with the development of aerospace field, the requirement for the performance of high strength aluminum alloy is more and more strict, so as the requirement for impurity Fe content. Moreover, high strength aluminum alloys often serve in the condition of alternating stress or cyclic stress. Thus, it is of great significance to study the effect of pressure and Fe content on low cycle fatigue behavior of Al-Zn-Mg-Cu alloys.In this paper, Al – 7.1 wt% Zn – 2.4 wt% Mg – 2.1 wt% Cu alloys with 0.01 Fe, 0.33 Fe, 0.55 Fe were prepared by squeeze casting. The effect of pressure and Fe content on low cycle fatigue behavior of T4-treated alloys was investigated by the strain-controlled fatigue experiment, optical microscope, XRD, SEM, TEM. The experimental results are as followed:At the pressure of 75 MPa, the tensile strength and elongation of T4-treated Al-Zn-Mg-Cu alloys with 0.01 Fe, 0.33 Fe, 0.55 Fe are 493MPa、19.3%,472MPa、10.8%,464MPa、8.9%, respectively, which are much better than these of 0 MPa alloys.The existing forms of Fe and the influence of Fe on tensile properties of Al-Zn-Mg-Cu alloys were investigated. Fe-rich phases in as-cast alloys with three kinds of Fe contents are Al3 Fe, and these Fe-rich phases turn into Al7Cu2 Fe and Al6(Fe Cu) by diffusive transformation after T4 heat treatment. With the increase of Fe content, the amount of eutectic phases decreases, the amount and size of Fe-riched phases increase. The increase of Fe content is harmful to tensile properties of alloys, especially the elongation.The cyclic hardening behavior of Al-Zn-Mg-Cu alloys with three kinds of Fe contents prepared by squeeze casting was studied. The alloys exhibit the same cyclic hardening behavior: cyclic stabilization is presented at total strain amplitudes of 0.3% and 0.4%, while initial cyclic hardening at the first 200 cycles, then followed by cycling stabilization under total strain amplitudes of 0.5%. The maximum value of stress response curves increase with pressure at the same strain amplitides.The microscopic mechanism of cyclic hardening behavior was analyzed. Although there is nearly no macroscopical plastic strain at total strain amplitude of 0.3%, plastic deformation have been occurred in some grains, so as dislocation proliferation, dislocation entanglement, dislocation accumulation at second phases and grain boundary, but these interaction effect is so weak that cyclic stabilization is presented until cyclic fracture. There is a small plastic strain at beginning cycles when total strain amplitude is 0.5%, so the density of dislocation increase in the process of cyclic loading which lead to cyclic hardening. To a certain degree, the proliferation and annihilation of dislocation offset, so the alloys exhibit cyclic stabilization at the following cycles. What’s more, stress induced precipitation during cyclic loading is an other important factor to cyclic hardening.The effect of pressure and Fe content on fatigue life of Al-Zn-Mg-Cu alloy were studied. The fatigue life of Al-Zn-Mg-Cu alloy increase with the increase of pressure at the same Fe content. At the pressure of 0 MPa, the fatigue life of alloys has little change when Fe content increase from 0.01% to 0.55%. At the pressure of 75 MPa, the fatigue life of 0.33 Fe alloy is better than that of 0.55 Fe alloy at all specific total strain amplitudes. The fatigue life of 0.01 Fe alloy is better than that of 0.33 Fe alloy when the strain amplitudes are up to 0.5%, while the fatigue life of 0.01 Fe alloy is poorer than that of 0.33 Fe alloy when total strain amplitudes are below 0.5%.The fatigue fracture mechanism of Al-Zn-Mg-Cu alloys with the same Fe content and different pressures was analyzed. Fatigue cracks of 0MPa alloys dominantly initiate from shrinkage porosities and are easy to propagate along them, while the crack initiation sites for 75 MPa alloys are slip bands, big eutectic phases, big Fe-rich phases and inclusions at or near the free surface and a lot of broken eutectic phases and broken Fe-rich phases can be seen in the crack propagation zone.The fatigue fracture mechanism of Al-Zn-Mg-Cu alloys with the same pressure and different Fe contents was analyzed. When the pressure is 0 MPa, Fe content has little influence on fatigue crack propagation, the fatigue crack growth behavior dominately determined by casting defect, such as shrinkage porosities. At the pressure of 75 MPa, Fe content has great influence on fatigue properties of alloys, at low total strain amplitudes, small Fe-rich phases can alter the crack propagation path which is in favor of fatigue property. At total high strain amplitudes, big Fe-rich phases can easily broken or separate from matrix to become the crack initiation sites and fatigue cracks are inclined to propagation along them.