Ratcheting and ratcheting-creep interaction in A356 aluminium
alloy

Abstract

In automobile sectors, particularly inner turbo frame, engine blocks, pistons etc. and in aerospace industries the components like pistons, brackets, pulleys, wheels etc. are fabricated from the cast A356 aluminium alloy. Most of these components are frequently exposed to cyclic loading at elevated temperature and the major cause behind failure of such components is fatigue as well as creep. Fatigue damage becomes much more severe when the cyclic loading is asymmetric in nature; this kind of loading causes build-up of plastic strain in the structure during each cycle and thus, causes premature failure. The phenomenon is commonly known as ratcheting. On the other hand, high temperature exposure may enforce pre-mature failure of such components due to creep deformation. Situation becomes more vulnerable when ratcheting and creep occur simultaneously. Considering the importance of static and cyclic plastic deformation in structural integrity of components, extensive research work has been carried out by several researchers to understand ratcheting fatigue, creep and fatigue-creep interaction of various materials. However, the study of ratcheting behavior of A356 Al alloy has not been reported. Also, most of the existing fatigue-creep interaction studies of various aluminium alloys are concentrated on the influence of dwell time in combination with fatigue. However, the effect of previous ratcheting on creep behavior of materials has not been studied so far. It was also noticed that, as per the best possible knowledge of the author, the use of Artificial Neural Network (ANN) based prediction tool to predict ratcheting, creep and ratcheting-creep interaction was not reported in open literature. In view of this, the primary aim of the present work was to understand the ratcheting behavior, creep behavior and ratcheting-creep interaction of the A356 aluminium alloy. In association, the extent of dislocation density and its character in differently deformed specimens were evaluated using X-ray diffraction (XRD) profile analysis. Finally, ANN technique was used to asses various properties of the alloy.

Ratcheting behavior of the as-received A356 alloy was studied at ambient temperature under different combinations of mean stress (σm) and stress amplitude (σa). The values of σm and σa were chosen in such a manner that the applied cyclic loading must be in tension-tension mode. Prior to the ratcheting tests, few basic physical and mechanical characterizations of the selected alloy were carried out. Substructural variations due to ratcheting were assessed using transmission electron microscope (TEM).


In addition, the effect of accumulated ratcheting strain (after 2000 cycles) on tensile properties of the specimens was studied together with fractographic examination. In order to understand ratcheting-creep interaction, impression creep tests were carried out on asreceived as well as pre-ratcheted specimens at different combinations of applied stresses and temperatures. Dislocation density was estimated from XRD profile analysis using modified Williamson-Hall approach. All the XRD scans were done from 20 – 110 degrees using very slow scan rate (0.2º / min) near all the well identified peaks. Finally, the ANN technique was used to predict the fatigue life and ratcheting-creep interaction of the investigated alloy within and beyond the experimental domain.

The highlights of this investigation can be summarized as: (i) increase in the magnitude of a and /or m resulted an increased accumulation of ratcheting strain. The observed increase in strain accumulation was correlated with increased cyclic damage as well as with increased dislocation density in the ratcheted specimens; (ii) the postratcheting tensile specimens exhibited higher tensile properties as compared to the asreceived one. The increase in yield strength and ultimate tensile strength was attributed to occur due to increased cyclic hardening due to ratcheting; (iii) all the ratcheted specimens showed higher creep rate as compared to that of the as-received ones due to the work softening that took place during impression creep test of ratcheted specimens. On the other hand, superior creep resistance of as-received alloy resulted due to work hardening during creep test. Also, among the ratcheted specimens, the specimen that accumulated lowest ratcheting strain showed highest creep rate; (iv) the dislocation density of the tensile deformed, ratcheted and crept specimens were more as compared to the as-received one. While on the other hand, the ratcheted + crept specimens exhibited lower dislocation density than the only ratcheted specimens. These observations were explained by dislocation annihilation owing to reverse loading during impression creep. Dislocation characters indicated screw dislocation dominance on the ratcheted specimen while edge dislocations were more in the crept and ratcheted + crept specimens; (v) predictive model based on ANN approach predict and simulate the fatigue, creep and ratcheting-creep interaction response of the A356 Al alloy successfully under various test conditions within and beyond the experimental domain.