Investigations on Ratcheting and Post–ratcheting Tensile Behavior of a 7075-T6 Aluminum Alloy

Abstract

In this work, it has been aimed to study the ratcheting and post–ratcheting tensile behavior of a 7075-T6 aluminum alloy at room temperature which is potentially used in aerospace, automobile components etc. where deformation caused by ratcheting cannot be ignored. The T6 heat treatment was done on as-received aluminum alloy rods. The heat treated rods were then characterized for the microstructural features using an optical microscope along with hardness measurements. The tensile tests were carried out on specimen designed as per ASTM standard E8M using universal tensile testing machine (INSTRON 1195). The fatigue specimens as per ASTM standard E606 were prepared and fatigue tests were done using ±250 kN servo hydraulic universal testing machine (Instron: 8800R). The effect of stress parameters such as mean stress and stress amplitude were investigated on the ratcheting behavior of the selected aluminum alloy. In order to study the post ratcheting tensile behavior of the investigated alloy, tensile tests were done on all ratcheted samples using INSTRON 1195. Also the fractographic studies of all post ratcheting tensile samples were done using a scanning electron microscope (SEM). Further, for qualitative analysis of the phases present in unratcheted and post ratcheted tensile specimens are subjected to X-Ray diffraction (XRD) analysis using Cu Kα radiation. From the results of ratcheting tests for different combinations of mean stress and stress amplitudes it is observed that at constant mean stress, by increasing stress amplitude, ratcheting strain increases. Also saturation in strain accumulation takes places in the investigated material after around 10-20 cycles under all test conditions. The analyses of hysteresis loop generated during cyclic loading indicate that the material exhibits cyclic hardening in the initial fifty cycles which gets softened in further loading up to about 70-80 cycles and finally attains a steady state. From the post ratcheting tensile test it was observed that the yield strength of the material increases whereas the ultimate tensile strength decreases. The increase in ratcheting strain with stress parameters occurs due to increased deformation zone during cycling; also it can be correlated with increased remnant dislocation density. The cycling hardening followed by softening is correlated with characteristic precipitation features of the alloy.