Microstructural Inclusion Influence on Fatigue of a Cast A356 Aluminum Alloy
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STRUCTURAL aluminum castings give a widely scattered distribution of fatigue data due to the variances of microstructures, defects, and inclusions present within the material. A widely used alloy is A356 aluminum in which the fatigue behavior was analyzed by Stephens et al.[1,2] In an effort to link the microstructure with varying stages of crack growth of A356 aluminum, Fan et al.,[3] Gall et al.,[4–6] Horstemeyer,[7] and Horstemeyer and Gokhale[8] performed micromechanical finite element simulations and microstructural analyses. Although many of the microstructure-property studies were focused upon porosity (void volume fraction) or silicon particle size, other aspects such as the interactions between the size effect of pores, nearest neighbor distances (NNDs), number of pores, and void volume fraction (porosity) on the fatigue life have not been elucidated. The dendrite cell size (DCS), which is analogous to the secondary dendrite arm spacing, has been a critical variable in fatigue life,[9–11] while J.B. JORDON, Assistant Research Professor, Center for Advanced Vehicular Systems (CAVS), and M.F. HORSTEMEYER, Professor, Center for Advanced Vehicular Systems (CAVS) and Department of Mechanical Engineering, are with Mississippi State University, Mississippi State, MS 39762. Contact e-mail: [email protected]. edu N. YANG, Materials Scientist, is with Sandia National Laboratories, Livermore, CA 94551-0969. J.F. MAJOR, Material Scientist, is with the Arvida R&D Centre, Rio Tinto Alcan, Jonquiere, QC, Canada G7S-4K8. K.A. GALL, Professor, School of Materials Science and Engineering, and D.L. McDOWELL, Professor, George Woodruff School of Mechanical Engineering, are with the Georgia Institute of Technology, Atlanta, GA 30332. J. FAN, Professor, is with the Department of Mechanical Engineering, Alfred University, Alfred, NY 14802. Manuscript submitted May 4, 2009. Article published online November 4, 2009 356—VOLUME 41A, FEBRUARY 2010
Major[12] and Kumai et al.[13] asserted it as the most important defect. It is important to note that the term DCS is a measure of microstructural scale, with the larger DCS corresponding to larger slip distances. Although fatigue crack growth thresholds increase with larger DCS,[11,13,14] the pore size also tends to increase with DCS.[12] Essentially, as the microstructure becomes more fatigue crack growth resistant, the opportunity for flaws large enough to start a crack decreases. In addition, we will show that DCS is not the only metric that should be used to relate to fatigue failure. The DCS is proportional to pore size, as Major[12] observed. Hence, pore size,[15–17] porosity level,[17–20] and casting defects[21] have also been a focus of fatigue failure. However, NNDs must also be considered along with second-phase particles and oxides as well. A porecontrolled fatigue defect sensitivity (FDS) map showed that the relationship between the applied stress amplitude, largest pore size, nearest pore spacing, and critical pore size can be related to cycles to failure.[14] While thes
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