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CASTING PROPERTIES
1. Fluid Life
Fluid life more accurately defines the alloy’s liquid characteristics than does the traditional term “fluidity.” Molten metal’s fluidity is a dynamic property, changing as the alloy is delivered from a pouring ladle, die casting chamber, etc. into a gating system and finally into the mold or die cavity. Heat transfer reduces the metal’s temperature, and oxide films form on the metal front as this occurs. Fluidity decreases most rapidly with temperature loss, and it can decrease significantly from the surface tension of oxide films.
The absolute value of temperature is not the test of fluidity at a given moment. For example, some aluminum alloys at 1,200-1,400F (650-750C) have excellent fluid life. However, some molten steels at 3,000F (1,650C) have much shorter fluid life. In other words, a molten alloy’s fluid life also depends on chemical, metallurgical and surface tension factors.
Fluid life affects the design characteristics of a casting, such as the minimum section thickness that can be cast reliably, the maximum length of a thin section, the fineness of cosmetic detail (like lettering and logos) and the accuracy with which the alloy fills the mold extremities.
It is essential to understand that moderate or even poor fluid life does not limit the cost-effectiveness of design. Knowing that an alloy has limited fluid life tells the designer that the part should feature:
• softer shapes and larger lettering;
• finer detail in the bottom portion of the mold, where metal arrives first, fastest and generally hottest;
• coarser detail in the upper portions of the mold where the metal is slower to arrive and more affected by oxide films and solidification “skin” formation. Even an alloy with good fluidity, when overexposed to oxygen, may form a high surface tension oxide film that makes the fluidity die, “rounding off” the leading metal front as it flows.
• more taper toward thin sections.
Some alloys, like 356 aluminum, have been specifically designed metallurgically to enhance fluid life. In the case of 356, the high heat capacity of silicon atoms “revives” aluminum atoms as their fluid life begins to wane.
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