Stresses and Distortion
Stress related casting defects are as old as the casting process itself; note the cracked Liberty Bell in Philadelphia or the Bell of the Czar in the Kremlin that tore itself apart during cooling. In contrast to filling and solidification, stresses are initially invisible, contrary to the “predictable” behavior of filling and solidification. On top of that, they usually reverse during the cooling process, so that even today the almost desperate ques- tion is asked: “Why is it that exactly where I have cracks, I also find compressive stress values? How can that be?”
The developments regarding the prediction of hot tears, as well as the creation of residual stresses and distortion be- havior created much more transparency (Fig. 17). The pro- found experiences made in the arena of load simulations in mechanical engineering provided a lot of help. However, the topic of stresses in castings is much more complex, as we
need to deal with elastic-plastic or even visco-elastic ma- terial behavior, which need to be considered over a large temperature range. Additionally, the stress simulation has to consider not only the part itself, but also the impact of mold and cores, as they are often predominantly responsible for stress-related defects (Fig. 18 and 19).
The foundry engineer has learned this through experience and has attempted to avoid hot tears and cracks in castings by using various shrink factors, tear brackets (cooling fins) or even using straw in the mold material. Many of these options can now be evaluated quantitatively through stress simulation. However, the development of these tools is far from over. As already mentioned, P.N. Hansen wrote his Ph.D. thesis in 1975 on the prediction of hot tears in steel castings. Today, research continues on improving the simu- lation models to better predict the complex influence of cast- ing and mold material behavior on the stress development.
Figure 19 Prediction of potential cracks after machining. Simulation of stresses in castings can also consider the impact of machining. Stresses re-distribution can lead to stress concentration, which might lead to failures after machining. /8/
Time in s 16
Figure 20. Die-life prediction. Premature heat-checking on the die surface or cracks around cooling channels are reasons for premature failures of dies or at least lead to expensive repairs. Main contributor to these effects is the stress cycling during the casting process. The die surface expands during the filling and solidification process. This leads to compressive stresses. The subsequent spraying and cooling leads to a contraction of the die surface, which leads to tension. This leads to a stress hysteresis over multiple casting cycles. Simulation of these phenomena leads to conclusions regarding crack initiation and lifetime of a die.
International Journal of Metalcasting/Spring 10
Principal Stress in MPa
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