Fatigue behavior of A356-T6 aluminum cast alloys. Part I. Effect of casting defects

Abstract

The influence of casting defects on the room temperature fatigue performance of a Sr-modified A356-T6 casting alloy has been studied using un-notched polished cylindrical specimens. The numbers of cycles to failure of materials with various secondary arm spacings (SDAS) were investigated as a function of stress amplitude, stress ratio, and casting defect size. To produce pore-free samples, HIP-ed and Densal™ treatments were applied prior to the T6 heat treatment. It was observed that casting defects have a detrimental effect on fatigue life by shortening not only the crack propagation period, but also the initiation period. Castings with defects show at least an order of magnitude lower fatigue life compared to defect-free ones. The decrease in fatigue life is directly correlated to the increase of defect size. HIP-ed alloys show much longer fatigue lives compared to non-HIP-ed ones. There seems to exist a critical defect size for fatigue crack initiation, below which fatigue crack initiates from other competing initiators such as eutectic particles and slip bands. A fracture mechanics approach has been used to determine the number of cycles necessary to propagate a fatigue crack from a casting defect to final failure. Fatigue life of castings containing defects can be quantitatively predicted using the size of the defects. Moreover, the fatigue fracture behavior of aluminum castings is well described by Weibull statistics. Crack originating from different defects (such as porosity and oxide films) can be readily identified from the Weibull modulus and the characteristic fatigue life. Compared with oxide films, porosity is more detrimental to fatigue life.