Characterization of microshrinkage casting defects of Al–Si alloys by X-ray computed tomography and metallography Gianni Nicoletto a,⇑ , Radomila Konec ˇná b , Stanislava Fintova b a Department of Industrial Engineering, University of Parma, Italy b Department of Materials Engineering, University of Z ˇ ilina, Slovakia article info Article history: Received 27 April 2011 Received in revised form 30 December 2011 Accepted 6 January 2012 Available online 17 January 2012 Keywords: Al–Si alloys Casting defect Fatigue behavior X-ray computed tomography Metallography abstract The statistical pore size characterization by metallography in the framework of Extreme Value Statistics (EVS) is presented and applied to different sets of cast AlSi7Mg specimens. Specimen production by sep- arate casting or by extraction from automotive cast parts is found to result in different SDAS and porosity (i.e. pore morphology and size) but did not influence the fatigue strength. The application of two equiv- alent pore size definitions (i.e. maximum Feret diameter and (Area) 1/2 ) combined with the EVS approach is discussed in terms of predicted critical pore sizes and observed fatigue strengths. The role of casting pore morphology on stress concentration is investigated using the X-ray computed tomography and the finite element method. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Cast Al–Si alloys are widely used in fatigue critical structural applications, such as engine blocks, cylinder heads, and chassis and suspension components, for their excellent combination of mechanical and technological properties and to improve automo- tive fuel economy [1]. Fatigue properties of aluminum castings are strongly dependent on the casting defects and little affected by chemical composition, heat treatment, or solidification time, as reflected by dendrite arm spacing and the shape and size of eutectic silicon and intermetallic phases [2–10]. Typical defects of casting are macro pores and micro pores and bifilms [5]. While macropores (i.e. larger than a few mm) can be identified by X-ray inspection during quality check, microp- ores and bifilms are invisible to this kind of inspection. Since the presence of casting defects and discontinuities is almost inevitable in cast aluminum alloys and aluminum alloys have no apparent fatigue endurance limit, a defect-tolerant approach to fatigue de- sign should be used, as proposed in [7–11]. Such an approach is based on the crack propagation life estimated from the crack growth rate law of the material and the initial crack size estimate. Typically, empirical models based on traditional linear elastic frac- ture mechanics (i.e. long crack behavior) are used although the short crack behavior has been also considered in [11]. The accuracy of the life prediction strongly depends on the prior knowledge of the defect population for a given material. The size of the largest defect has been recognized as the most important parameter in determining the fatigue properties of aluminum cast- ings. The larger the maximum defect size, the lower the fatigue strength. Therefore, any defect tolerant design approach for mate- rials containing defects should be based on a method to estimate the largest defect size distribution. In this context the approach and procedure developed by Murakami and coworkers [11,12], represents a fundamental starting point. In short, metallographic inspection of a selected material cross-section and determination of the largest pore size in many fields of view allows constructing a statistical description of the largest pore size using Gumbel’s extreme value distribution. Such a distribution is then used to esti- mate the largest pore size in realistic part cross-sections by extrap- olation [12,13]. The procedure has been applied by many researchers to a wide range of materials with appreciable success [11–14]. The Extreme Value Statistics (EVS) approach is based on two key assumptions: (i) the distribution of defects is uniform in the material, (ii) the pore size is well described by the parameter (Area) 1/2 where Area is the area of the largest pore measured on a metallographic section. However, the combination of complex part geometry and solid- ification process typical of Al–Si alloys is known to give different pore types and sizes in different sections of the casting [11]. The morphology of casting pores in Al–Si alloys is typically classified either as gas pore or microshrinkage pore [1]. While the former is 0142-1123/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijfatigue.2012.01.006 ⇑ Corresponding author. E-mail addresses: gianni.nicoletto@unipr.it (G. Nicoletto), radomila.konecna@ fstroj.uniza.sk (R. Konec ˇná). International Journal of Fatigue 41 (2012) 39–46 Contents lists available at SciVerse ScienceDirect International Journal of Fatigue journal homepage: www.elsevier.com/locate/ijfatigue