Porosity Control and Fatigue Behavior in A356-T61 Aluminum Alloy

Much work has been done, in recent years, to characterize the factors that cause porosity in aluminum castings, and to analyze the impact of porosity or microstructural parameters on fatigue life. What has yet to be done is systematically separate out the effects of dendrite arm spacing and largest pore size from each other, and from the underlying chemical and process parameters that influence both the microstructure and defect structure. This is the objective of the work described in this paper. The factors influencing porosity in A356 aluminum castings are reviewed. The results of the analysis of an A356 alloy fatigue test database, covering a wide range of microstructural scale, pore size and stress, are then presented. Degradation in fatigue life, caused by increasing pore size and secondary dendrite arm spacing, is quantified as a function of stress amplitude, and the relative impact of each is discussed. It is shown that the impact of both largest pore size and the dendrite arm spacing is most severe at high stress amplitudes, with a tendency for the dendrite arm spacing effect to die away toward lower stress amplitudes.