TESTING 1-2-3


PREDICTING THE FATIGUE LIFE OF ALUMINUM CASTINGS


Technical specialists with General Motors developed models that account for microstructure constituents and fl aws.


AN MCDP STAFF REPORT T 1


2 3


he increasing use of aluminum shape castings has drawn attention to the fatigue properties of cast aluminum components. Their 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 micro- structure constituents. Technical specialists Qigui Wang and Peggy E.


Jones, General Motors, Pontiac, Mich., studied the latest understanding of fatigue crack mechanisms in


ADDING IT ALL UP


“Fatigue Life Prediction in Aluminum Shape Castings” Qigui Wang and Peggy E. Jones, General Motors, Pontiac, Mich. Background—Aluminum castings can contain fl aws such as porosity and oxides that dominate their fatigue performance. In the pres- ence of casting fl aws, fatigue life can be predicted by long and short crack models which account for the fl aw population. However, these models require an accurate estimate of fl aw sizes, which is diffi cult to obtain early in the product and process design cycle. Technical specialists with General Motors sought to determine models for ac- curately predicting aluminum shape casting fatigue life.


Procedure—The researchers calculated the fatigue life of the cast alumi- num alloys, failed by various crack initiation mechanisms, using the pre- sented multiscale fatigue (MSF) models together with the microstruc- ture characteristics. Like casting fl aws, the characteristic microstructure dimensions were estimated 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 fl aw 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 fl aw popu- lations. The developed MSF life models and methods to estimate the characteristic microstructure dimensions essential to the mod- els are applicable to other alloys such as wrought aluminum alloys and magnesium alloys. 


38 | METAL CASTING DESIGN & PURCHASING | May/Jun 2013


cast aluminum alloys. At the 2013 AFS Metalcasting Congress, they presented “Fatigue Life Prediction in Aluminum Shape Castings,” detailing multiscale fatigue (MSF) life models for aluminum castings.


Question How do multiscale fl aws and micro-


structure constituents aff ect the fatigue life of aluminum castings?


1 Background


Cast aluminum alloys have been used widely in automo- tive and aerospace industries because of their strength-to-


weight ratio, corrosion resistance and low manufacturing costs, with near-net shaping features. T e increasing use of cast aluminum components under cyclic loading has drawn considerable inter- est in their fatigue properties. Fatigue cracks (Figs. 1-4) have been observed to form at fl aws and inhomogeneities such 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 important role in crack initiation. T e fatigue life of cast aluminum


alloys is determined by the maximum fl aw size. T e larger the casting fl aw, the lower the fatigue strength and the shorter the fatigue life. In the absence of large casting fl aws, the largest mi- crostructural weak link governs the fa- tigue response. Like other engineering materials, the fatigue life of aluminum


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