surface porosity was reduced to 10% of baseline. Tis would indicate that other metallurgical factor(s) were dominant. Te Al-Cu alloy (A206) also
showed some, but not dramatic im- provement in tensile properties with VAP/SUP, even when fracture surface porosity was reduced to 10% of base- line. However, the baseline and VAP/ SUP fracture surface porosity were the highest values of the four alloys. Op- timization of the melting practice and VAP/SUP processing may give better results and should be pursued since the upside with A206 is significant. Te Al-Mg alloy (535) showed the
most dramatic improvements in tensile properties, a 70% increase in ultimate strength and 400% increase in strain at maximum stress. All castings were examined to
detect any indications of hot tearing or incomplete fill. Only one instance of suck-in shrinkage was observed in one of the baseline castings. Even the baseline castings did not hot
tear with A206 or 535 alloy. Te unbonded sand used in the lost foam process may provide enough elasticity to minimize hot tear inducing strain during solidification.
Presentation Shrinkage, Feeding and Riser
Design (14-002)
Author Geoffrey K. Sigworth, GKS Engineer-
ing Services, Dunedin, Fla.
Background Aluminum casting alloys un-
dergo a 5-7% volume change during solidification. To produce sound, high quality components, one must design the mold so liquid metal is fed to the casting while it freezes. Te feeding process and riser design are compli- cated by the high thermal conductivity of aluminum. In sand castings, the temperature differences inside the casting are usually only a few degrees,
whereas the freezing range of the al- loy is between 25C (77F) and 125C (257F). Tus, to understand fully the feeding process, one must consider the properties of the semi-solid "mush" in the mold. Engineering principles for mold and riser design in aluminum are reviewed and compared to those offered previously for steel castings. Different approaches to riser design are reviewed and compared.
Conclusions
It would be a good investment to buy a small data logger or computer data acquisition system that would allow you to place thermocouples in a mold and record the metal tempera- tures in the casting during solidifica- tion. If shrinkage problems exist in a casting, a few strategically placed thermocouples will quickly show what is happening. One can easily mea- sure the relative freezing times at the shrinkage location and in the riser and perhaps in the feed path in between.
May 2014 MODERN CASTING | 57
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