Fatigue behavior of A356-T6 aluminum cast alloys. Part I. Eect of casting defects Q.G. Wang * , D. Apelian, D.A. Lados Metal Processing Institute (MPI) Worcester Polytechnic Institute (WPI), Worcester, MA 01602, USA Abstract The in¯uence of casting defects on the room temperature fatigue performance of a Sr-modi®ed 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 Densale treatments were applied prior to the T6 heat treatment. It was observed that casting defects have a detrimental eect 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 ®nal 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 dierent defects (such as porosity and oxide ®lms) can be readily identi®ed from the Weibull modulus and the characteristic fatigue life. Compared with oxide ®lms, porosity is more detrimental to fatigue life. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Aluminum castings; Fatigue life; Casting defects; Fracture mechanics; Weibull statistics 1. Introduction Cast aluminum alloys are seeing increasing uses in the automotive industry due to their excellent castability, corrosion resistance, and especially their high strength to weight ratio. The increasing use of high integrity shaped cast aluminum components under repeated cy- clic loading, has focused considerable interest on the fatigue properties of cast Al±Si alloys. Fatigue properties of cast aluminum components strongly depend on casting defects and microstructural characteristics. However, there are dierent opinions as to which are the critical microstructural characteristics. For example, some experimental data support the view that fatigue resistance, as with tensile ductility, is im- proved by re®ning the dendrite arm spacing and the size of the eutectic silicon particles [1,2]. However, the del- eterious eect of casting defects has also been recognized [3]. Liquid aluminum is prone to hydrogen adsorption and oxidation; gas porosity and oxide inclusions are inevitably found in aluminum castings. In addition, if the casting is not properly fed, shrinkage porosity re- sults, which is also quite deleterious to fatigue proper- ties. Gas pores are, typically, spherical, whereas shrinkage pores have an irregular three-dimensional shape. Both of these types of pores can also be associ- ated with aluminum oxide ®lms. A quantitative method for predicting the relation between fatigue life and defect size has only recently been developed [4±8]; however, it is still not possible to fully account for the eects of pore shape and defect type on fatigue life. A comprehensive understanding based on experimental data does not exist. In practice, a certain amount of porosity can be tolerated in castings; however, this varies with the application. It is important to identify the speci®c contributory roles of defects and other microstructural parameters on fatigue life. Fur- thermore, a quantitative understanding of the role of defects is crucial to establishing defect acceptance stan- Journal of Light Metals 1 (2001) 73±84 www.elsevier.com/locate/ligandmet * Corresponding author. Present address: General Motors Corpo- ration, Materials Engineering, Powertrain, Saginaw, MI 48605, USA. E-mail address: qigui.wang@gm.com (Q.G. Wang). 1471-5317/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 4 7 1 - 5 3 1 7 ( 0 0 ) 0 0 0 0 8 - 0