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Metal Casting Technologies : September 2007
66 www.metals.rala.com.au TECHNICAL FEATURE One possible explanation is that the graphite at the surface nucleates and may start growing as flake graphite, which invariantly advances inwards by the continuous feeding of surface-active impurities: sulfur, oxygen, etc, from the interface. At the same time that such impurities contribute with atoms adsorption to the prism planes of graphite, they also bind and render some of the magnesium atoms present in the liquid state. This continues to happen until the presence of spheroidizing elements overrules impurity activity or impurity concentration is greatly reduced and compacted graphite becomes the preferred graphite morphology. Another aspect that cannot be ignored is the unexpected direction of the skin wedge. Although consistent, the trends observed were anticipated to be reversed in nature. As the molten metal fills the mold from the bottom and the casting starts to solidify, the space surrounding the upper edge of the cores results in an area of convergent heat. A hot zone of elevate temperature will remain for an extended time until the casting is completely brought to room temperature. Figure 7 shows a solidification profile for a cross sectional area of the penetration mold using NovaCast AB modeling software. The thermal modulus indicates that temperature gradients decrease with increasing distance from the sample corner. One might expect that the higher the temperature, the more graphite degradation; however, the experimental results prove otherwise. A possible explanation would be that heat convection currents cause fresh metal to circulate more intensely around the top portion of the cores, constantly renewing the chemistry until mold filling is complete; while the metal at the lower sections of the casting remains more stagnant and promote graphite degradation to a larger extent. Despite of the trend consistently observed for skin depth, it is critical to discuss the difficulty of controlling and measuring skin formation. The innumerous samples analyzed clearly indicate some results variation. Although in small magnifications, skin depth varied depending on sampling location, both from the inside or outside sample location of the casting. The majority of the inside samples demonstrated more pronounced skin than the samples from the outside location, inferring that slightly higher temperatures resulted in a deeper skin formation. Pouring temperature also had some implications; samples collected from the first mold developed slightly more skin than samples from the last mold, which was poured at slightly lower temperatures. Therefore, some variability is indeed expected. However, concern arises if it assumes higher magnitudes, and the sensitivity of the experiment can be affected by possible high variability. Others factors that are known to affect the casting process should also be controlled for they can directly interfere with mold-metal reactions and, consequently, skin development. Control of the molten metal chemistry is a key factor to guarantee casting properties. Graphite morphology is strongly sensitized by the balance between spheroidizing, primary and gaseous elements. For instance, while excess magnesium can cause severe spheroidal graphite formation, the presence sulfur of can limit both magnesium and rare earth efficiencies. Consistency during mold and core making processes, combined with the chemistry of systems and atmosphere used can also play a major role in the skin formation and should be closely monitored. Reproducibility of pouring practices, such as melt preparation and treatment, sampling technique, pouring temperature and operator skills, are undoubtedly added sources of variation and can greatly affect the results in this type of experiments. Therefore, successful examination of flake skin in CGI requires high control of process parameters to keep variation at small levels. Although all aforementioned factors are extremely critical, the test casting also deserves some attention. It needs to be robust enough to minimize negative effects related to the experiment. The use of a design in one dimension would probably eliminate the skin gradient seen with the current test piece and thus yield more homogeneous flake skin development. Although other issues may raise and some settings in the experiment may need to be adjusted, preliminary results indicate that a plate-like design could help solving the issue of skin wedge. This alternative geometry needs to be investigated in detail since the evaluation of different molding materials could apparently be conducted more accurately. Figure 7. NovaCast solidification modeling for penetration mold