. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MetalCasting 101
How Solidification Affects Cast Metal Cast Metals Institute, Schaumburg, Illinois A
fter a met- alcaster has spent much ti me an d money to
prepare a proper molten metal bath, the metal must solidify into the desired shape in a mold. How the casting solidifies has a great effect on the final mechanical properties of the casting. Controlling the way
the liquid metal freezes in the mold, however, is a challenge, one that makes metalcasting unique. Be- cause liquids will natural- ly take the shape of their container, the process provides the world with many products unattain- able by other manufactur- ing methods. The metal cools be-
cause the temperature between objects—in this case, the hot liquid metal and the mold—naturally seeks to equalize. The difference in temperature creates thermal gradients, a determination of how much heat transfer will take place over a period of time. Simulation soft- ware has become readily available to the metal- caster, performing mil- lions of thermal gradient measurements based on each material’s physical properties in order to op- timize the casting process. Heat transfer takes
place through the liquid metal in direct physical contact with the mold via conduction. Temperature also is lost from the liquid
May/June 2011
metal through convec- tion currents (heat flow- ing through the liquid metal) and through radia- tion from the skin of the metal to air. The geometry of the
part also has a large in- fluence on how the cast- ing cools. Sections of the casting with a high ratio of cooling surface contact to volume will be able to transfer their heat more quickly and easily to the surrounding mold. Heavy, thick sections of the casting will trap heat in them longer. As the alloy yields its
The grain structure of this casting riser (used to feed extra metal to castings as they solidify and shrink) is visible to the naked eye.
Casting simulation software has made it possible for metalcasters to use thermal gradient measurements to optimize the casting process.
heat (or energy) to the mold, it undergoes a phase change. Soon, the solid state is atomically preferred, as the par- ticles arrange themselves into crystal structures. Crystal structures (or grains) simultaneously grow throughout the freezing liquid. Some al- loys naturally form grains without significant out- side influence. These alloys are known as hav- ing a narrow freezing range. Other alloys tend to freeze over a larger temperature range, typi- cally resulting in larger average grain sizes. The size and arrangement of the grains influence the strength of the casting beyond its physical prop- erties. Measuring and plotting the temperature of the alloy as it freezes helps the metalcaster evaluate the quality of the metal.
METAL Metal Casting Design & PurChasing 47
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