fatigue life of samples failed by persistent slip bands in both coarse and fine microstructures with actual fatigue life showed the predicted fatigue life is in good agreement with the actual fatigue life. By considering the probabilistic distribution of characteristic microstructures (median +/- 3 sigma SDAS or grain size), the research- ers’ model accurately predicted the variation and scatter of the fatigue life data. In cast aluminum alloys, casting flaw and microstructure features such as porosity, oxides, eutectic particles and dendrite cell and grain structures are not uniform in the entire stressed volume. Tis is due to randomness in the nucleation and growth of casting flaw and microstructure features dur- ing local solidification. It is generally accepted that the characteristic flaw and microstructure features in alu- minum castings follow extreme value probabilistic distributions. Te upper
estimate of flaw sizes, which is difficult to obtain early in the product and process design cycle. Metallographic measurements of the pore population are one of the earliest characterizations available for new components. Unfor- tunately, random two-dimensional sections through pores do not provide good estimates of the flaw size without further data analysis. Pores observed on fracture surfaces are usually larger than those observed on the metallo- graphic planes regardless of alloy and casting process. Pores responsible for fatigue failures are normally the largest in the stressed volume, and can be 10 times larger than the “maximum pore size” measured in random metallo- graphic sections. In this case, the maximum pore
3
size in a cast component can be estimated from the metallographic data using EVS. In the research study, maximum pore size predictions by
Results and Conclusions In the presence of alumi-
num casting flaws, fatigue life can be predicted by models that require an accurate
Fig. 4. The fatigue crack was initiated from persistent slip bands in a sand-cast A356-T6 casting. (Image was taken using secondary electron imaging.)
tail populations in these distributions dominate the fatigue behavior. In the presence of second phase
particles, inhomogeneous deforma- tion during cyclic loading results in high internal stress in the particles due to dislocation pileups at particles. Te larger the particle, the higher the internal stress. When internal stress is greater than the particle fracture strength, the particle cracks. Otherwise, decohe- sion occurs when the internal stress is higher than interfacial strength
EVS treatment of 2D metallographic data agreed well with measurements of the initiation pore sizes from fracture surfaces. Both long and short crack models gave a reasonable lower bound fatigue life prediction when the upper bound EVS estimate of the maximum pore size was used as the starting flaw size. In the absence of casting flaws, the fatigue resistance of cast alumi- num alloys is significantly affected by characteristic microstructures. In coarse microstructures, particularly with no eutectic modification, large elongated eutectic particles located on the casting free surfaces often crack and initiate fatigue cracks. In fine microstructures, or when eutectic sili- con has been modified, small eutectic particles are more fracture resistant, and decohesion of eutectic particles and crystallographic shearing from persistent slip bands dominate the fatigue crack initiation. Te fatigue life of the cast aluminum alloys failed by various crack initiation mecha- nisms can be calculated using the presented MSF models together with
between the particle and matrix. Cracking or decohesion of second phase particles can happen in the first few cycles, particularly when the second phase particle is segregated and located in the high stress areas. If the cracked particle size or the debonded particle area is comparable to characteristic microstructure sizes such as dendrite cells (in coarse micro- structure) or grain size (in fine microstructure), then the fatigue
crack immediately propagates like a long crack. Otherwise, the fatigue crack behaves like a short crack. A final failure mode considered in this work is persistent slip bands, which can form in favorably oriented grains or dendrite cells, particularly close to the grain or boundaries near the free surfaces of the component. In this case, a substantial portion of fatigue life is spent in crack initiation and small crack propagation, which must be included in the total life estimates.
the microstructure characteristics. Like casting flaws, the characteristic microstructure dimensions can be estimated by the EVS. Te study showed good agreement
between the measured and calculated fatigue lives for cast aluminum alloys A356 and 319 over a range of micro- structural scales and flaw populations. Te developed MSF life models and methods to estimate the characteristic microstructure dimensions essential to the models are applicable to other alloys such as wrought aluminum alloys and magnesium alloys.
Tis article is based on research paper 13-1342 presented at the 2013 AFS Metalcasting Congress.
ONLINE RESOURCE
Find the original research paper with detailed explanations of the MSF life model equations used in this study at
www.moderncasting.com
May 2013 MODERN CASTING | 41
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