TESTING 1-2-3
Predicting the Fatigue Life of Aluminum Castings
Technical specialists with General Motors developed models that account for microstructural constituents and flaws. A MODERN CASTING STAFF REPORT
fatigue resistance strongly depends upon the presence of casting flaws, which significantly reduce fatigue crack initiation life. In the absence of casting flaws, crack initiation occurs at the fatigue-sensitive microstruc- tural constituents. Technical specialists Qigui Wang
T 1
and Peggy E. Jones, General Motors, Pontiac, Mich., studied the latest understanding of fatigue crack mecha- nisms in cast aluminum alloys. At the 2013 AFS Metalcasting Congress, they presented “Fatigue Life Predic- tion in Aluminum Shape Castings,” detailing multiscale fatigue (MSF) life models for aluminum castings.
Question How do multiscale flaws and
microstructural constituents affect the fatigue life of aluminum castings?
Background Cast aluminum alloys
have been used widely in automotive and aerospace industries because of their
strength-to-weight ratio, corrosion resistance and low manufacturing costs, with near-net shaping features. Te increasing use of cast aluminum components under cyclic loading has drawn considerable interest in their fatigue properties. Fatigue cracks (Figs. 1-4) have been observed to form at flaws and inhomogeneities such
he increasing use of aluminum shape cast- ings has drawn atten- tion to the fatigue properties of cast alumi- num components. Teir
as microporosity, oxides and eutectic particles. Cracking and decohesion of large silicon and iron-rich intermetal- lic particles, as well as crystallographic shearing from persistent slip bands in the aluminum matrix, play an impor- tant role in crack initiation. Te fatigue life of cast aluminum
alloys is determined by the maximum flaw size. Te larger the casting flaw, the lower the fatigue strength and the shorter the fatigue life. In the absence of large casting flaws, the largest microstructural weak link governs the
ADDING IT ALL UP
Breaking down the latest research is as easy as 1-2-3. “Fatigue Life Prediction in Aluminum Shape Castings”
1 2 3
Qigui Wang and Peggy E. Jones, General Motors, Pontiac, Mich. Background—Aluminum castings can contain flaws such as porosity and oxides that dominate their fatigue performance. In the presence of casting flaws, fatigue life can be predicted by long and short crack models which account for the flaw population. However, these models require an accurate estimate of flaw sizes, which is difficult to obtain early in the product and pro- cess design cycle. Technical specialists with General Motors sought to deter- mine models for accurately predicting aluminum shape casting fatigue life. Procedure—The researchers calculated the fatigue life of the cast aluminum alloys, failed by various crack initiation mechanisms, using the presented mul- tiscale fatigue (MSF) models together with the microstructure characteristics. Like casting flaws, the characteristic microstructure dimensions were esti- mated by the extreme value statistics (EVS). Both long and short crack models gave reasonable lower bound fatigue life predictions when the upper bound EVS estimate of the maximum pore size was used as the starting flaw size. Results and Conclusions—Good agreement between the measured and calculated fatigue lives was found for the cast aluminum alloys A356 and 319 over a range of microstructural scales and flaw populations. The 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.
May 2013 MODERN CASTING | 37
fatigue response. Like other engi- neering materials, the fatigue life of aluminum castings consists of two parts: fatigue crack initiation life and fatigue crack propagation life, which can be further divided into short and long fatigue crack propagation life. Te researchers reviewed the lat- est understanding in fatigue crack initiation, crack propagation and life prediction with special emphasis on the influences of short cracks and the uncertainty of multiscale flaws and microstructure constituents.
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