Effect of casting imperfections on the fatigue life of 319-F and A356-T6 Al-Si casting alloys

Casting imperfections, such as porosity, in cast aluminum components greatly influence their fatigue properties. The effect of porosity on the fatigue life of 319-F and A356-T6 aluminum alloys was studied, where the porosity characteristics on the fracture surfaces of fatigue-tested samples were examined using SEM and image analysis. The results show that porosity has the greatest detrimental effect on fatigue life: 92% of all tested samples fractured as a result of porosity which acted as the main crack initiation site. In the absence of casting imperfections, other microstructural aspects such as slip bands may be held responsible (4%). Porosity was investigated in terms of the pore size at the sample fracture surface. It was found that fatigue life decreases as the size of the surface pore increases. A comparison was made between the fatigue behavior of low-pressure-permanent mold-cast 319 alloy and lost foam-cast A356-T6 alloy. The results show that the 319 alloy provides greater fatigue strength compared to the 356 alloy, which may be explained by taking into consideration the nature of the surface porosity (single pore versus multiple shrinkage pores) that initiated the fatigue crack in the two alloys. The microstructural characteristics are of secondary importance in this regard.