TESTING 1-2-3
Prevent Hot Tearing
Strain rate can predict defects in AZ 91 magnesium alloy castings. A MODERN CASTING STAFF REPORT
design stage of a product. Tere is no unique explanation of the root cause for this defect, although many predictive models exist. One of the reasons is that hot tearing is a material-specific problem. Researchers Milan Rakita, Qingyou Han and Z. Liu, Purdue University, Mechanical Engineering Technology, West Lafayette, Ind., compared simu-
H ADDING IT ALL UP Breaking down the latest research is as easy as 1-2-3.
“Strain Rate as a Predictor of Hot Tearing in AZ 91 Magnesium Alloy,” Milan Rakita, Qingyou Han and Z. Liu, Purdue University, Mechanical Engineering Technology, West Lafayette, Ind.
Background—It is a well known practical observation that hot tearing can be reduced or eliminated in controlled casting conditions that prevent the formation of large temperature and stress gradients. Procedure—The researchers tried to simulate similar conditions to earlier studies and determine what hot tearing criterion matches prior experimen- tal observations. To test the sensitivity of the prediction, AZ 91 alloy was simulated because it has relatively low susceptibility to hot tearing and a somewhat low pouring temperature, which sometimes help decrease the danger of hot tears. Results and Conclusions—As the results of this simple simulation showed, strain rate could be used as a reliable predictor of hot tearing in simulations of casting magnesium alloys.
38 | MODERN CASTING October 2014
ot tearing is one of the most frequent defects in castings, and it is impor- tant to predict its occur- rence as early as in the
lation results for magnesium alloy AZ 91 with prior experimental work. Tey found strain rate can be used in simulations to predict locations suscep- tible to hot tearing in this alloy. Teir paper, “Strain Rate as a Predictor of Hot Tearing in AZ 91 Magnesium Alloy,” provides their analysis of the study.
Question
Given the random appearance and extent of hot tearing, what reliable predictive model can be developed for magnesium alloy AZ 91?
and nonmechanical. Te basic assump- tion in many hot tearing models is that this defect occurs when feeding stops to accommodate the casting shrinkage. A combination of inadequate feed- ing and the strain rate has been quali- tatively described as a cause of hot tearing. Rappaz, Drezet, and Gremaud gave a quantitative description of conditions that can cause nucleation of hot tears, usually called RDG crite- rion. According to the model, if the pressure drop caused by applied strain rate goes below a critical value, a hot tear will nucleate. A modified form of RDG
1
criterion assumes strain rate to be independent of temperature. Tere, the limits of integration are taken to be the mass feeding temperature and the temperature when bridg- ing between dendrite arms occur. A study involving several hot tearing tests concluded the RDG model has the greatest potential of predicting hot tearing, but only on qualitative basis. Te strain rate-based hot tear- ing models are tested on aluminum alloys, frequently Al-4.5% Cu, known for its susceptibility to hot tearing.
Background Hot tearing criteria can be
roughly compiled in several main groups: stress-based, strain-based, strain rate-based
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