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Metal Casting Technologies : March 2006
Correlation of calculated and measured porosity In the past years, computer tomography (CT) has been increasingly used in the area of qual- ity assurance. As imperfections can be exactly localized and results are very accurate, the computer tomography is a great support for casting simulation (Fig. 14). During the development of a gear box cast- ing some cast prototypes were analyzed with the help of CT. At the same time, the casting process was simulated. The comparison of the results shows a vast agreement between the measured and the calculated pores. Local squeezing Shrinkage cavities are acceptable in some die castings; however, they generally cause problems during mechanical exposure and during machining of the casting. Shrinkage cavities always occur in heavy sections that are functionally necessary in many die castings. There are only a few possibilities to avoid shrinkage cavities, if the design of the casting can't be modified. One of those possibilities is local squeezing. The example in Figure 15 presents the di- mensioning of a squeezing system that should be used for the forced feeding of a critical area in the casting (Fig. 15). Purpose of optimization is to completely feed the critical heavy sections with a small volume squeezer (Fig. 16). Microstructure formation The microstructure formation is depending on the alloy and on the local solidification condi- tions. A consistent solidification structure or structure after heat treatment can only be assumed for castings with an even wall thick- ness. All castings with different wall thick- Figure 15 Figure 16 Figure 17 METAL Casting Technologies March 2006 ADVERTORIAL