Machinery's Handbook, 31st Edition
1492 FLUIDITY OF MOLTEN METAL Test for Fluidity.—Although none is accepted universally, several tests have been devel- oped to quantify fluidity. One such test is the spiral test ( Fig. 3a), where the molten metal is made to flow along a channel at room temperature. The distance the metal flows before it solidifies (and stops flowing) is a measure of its fluidity. Obviously, this distance is a function of the thermal properties of both the metal and mold, as well as the design of the channel. Another test for fluidity is the vacuum test ( Fig. 3b), which measures the length the metal flows inside a narrow channel when sucked from a crucible by a vacuum pump. Such tests can be useful in simulating casting situations. B
Fig. 3. Test Method for Fluidity: a) Spiral Method; b) Vacuum Method Heat Transfer
An important consideration in casting is heat transfer during the complete cycle from pouring to solidification and cooling to ambient temperature. Heat flow at different loca - tions in the system is a complex phenomenon that depends on many factors relating to the casting material and mold and process parameters. For instance, when casting thin sec- tions, metal flow rates must be high enough to avoid premature chilling and solidification. However, the flow rate must not be so fast as to cause excessive turbulence, which can have detrimental effects. Solidification and Cooling of Metals At the macro level, solidification refers to the phase change of metal from liquid to solid. At the micro level, changes occur in the material as the disordered structure of the liquid transforms into an orderly arrangement of crystals. Once molten metal has been poured into the mold, it cools rapidly. When the tempera- ture of the liquid drops below the melting point of that metal or alloy, the solidification process begins. This usually takes less than a few minutes. As the temperature drops further, the molten metal loses energy, and crystals begin to form. (This process starts near the mold walls, where cooling occurs first.) These crystals eventually become grains within the final structure. Grain size refers to crystals ( dendrites ) formed during the solidification process. If the metal solidifies slowly, the grains are longer. If it cools quickly, the grains are visibly shorter. Crystals continue to form and harden, until the entire melt is solidified. Throughout the solidification process, the metal shrinks. It is important to compensate for such shrinking to ensure castings are free of voids and shrink defects. This is accom- plished by using risers. The cooling rate of a casting affects its microstructure, quality, and properties. The cool- ing curve illustrates how molten metals solidify. There is a fundamental difference be- tween the cooling curve observed during solidification of a pure metal and that of an alloy.
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