3
Results and Conclusions
In the presence of aluminum casting flaws, fatigue life can
be predicted by models that require an accurate estimate of flaw sizes, which is difficult to obtain early in the product and process design cycle. Metallographic measure- ments of the pore population are one of the earliest characteriza- tions available for new components. Unfortunately, random two-di- mensional 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 metal- lographic planes regardless of alloy and casting process. Pores responsi- ble 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 metallographic sections. In this case, the maximum pore
size in a cast component can be estimated from the metallographic data using EVS. In the research study, maximum pore size predic- tions by EVS treatment of 2D me- tallographic 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 fl aws, the fatigue resistance of cast alumi- num alloys is signifi cantly aff ected by characteristic microstructures. In coarse microstructures, particularly with no eutectic modifi cation, large elongated eutectic particles located on the casting free surfaces often crack and initiate fatigue cracks. In fi ne microstructures, or when eutec- tic silicon has been modifi ed, small eutectic particles are more fracture resistant, and decohesion of eutectic particles and crystallographic
May/Jun 2013 | METAL CASTING DESIGN & PURCHASING | 41
shearing from persistent slip bands dominate the fatigue crack initia- tion. T e fatigue life of the cast aluminum alloys failed by various crack initiation mechanisms can be calculated using the presented MSF models together with the microstructure characteristics. Like casting fl aws, the characteristic microstructure dimensions can be estimated by the EVS. The study showed good agree-
ment between the measured and calculated fatigue lives for cast alu- minum alloys A356 and 319 over a range of microstructural scales and flaw populations. The devel- oped MSF life models and meth- ods to estimate the characteristic
microstructure dimensions essential to the models are applicable to other alloys such as wrought aluminum alloys and magnesium alloys.
T is article is based on research paper 13-1342 presented at the 117th Metalcasting Congress.
AFS @
ONLINE RESOURCE
Find the original research paper with detailed explana- tions of the MSF life model equations used in this study at
www.metalcastingdesign.com
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