Flowability, also called fluidity, describes the ability of liquid metal to flow through a mold cavity and fill thin sections before it solidifies.
What Determines Flowability
Flowability is not the same as viscosity alone. It also depends on how the alloy solidifies: a narrow freezing range, as at the eutectic temperature, keeps the liquid channel open longer than a wide pasty range does, because there is no mushy two-phase zone to block the flow front early. Superheat above the liquidus temperature, oxide film formation on the melt surface, and the wall thickness of the mold cavity all add to or work against fluidity as well.
Measuring Flowability: the Spiral Test
The most common shop-floor method is the spiral fluidity test: molten metal is poured into a narrow spiral-shaped channel mold, and the length the metal reaches before it solidifies and stops flowing becomes the fluidity index. A longer spiral casting means higher flowability under the test conditions used. The method goes back to the early 20th century and has been adapted with many spiral geometries since; there is no single DIN standard that fixes one universal spiral shape or test procedure.
Flowability in Foundry Practice
Good flowability is what lets a foundry cast thin walls, fine detail, and long thin-walled sections without cold shuts or incomplete fill. This is one practical reason alloys are often kept close to the eutectic composition. The term also appears in a second, unrelated sense on the molding side: the flowability of the molding sand itself, meaning how freely the sand packs around the pattern during sand preparation. Both senses use the same word but describe different materials in the process.
Factors Affecting Metal Flowability
| Factor | Effect on Flowability |
|---|---|
| Superheat above liquidus | Higher superheat generally improves flowability |
| Freezing range | Narrower range (eutectic alloys) improves flowability |
| Oxide film / surface tension | Stronger film reduces flowability |
| Mold wall thickness | Thinner walls reduce achievable flow length |
Frequently Asked Questions About Flowability
Is flowability the same as viscosity?
No. Viscosity is one input to flowability, but freezing behavior matters just as much. Two alloys with similar viscosity can show different flowability if one solidifies over a wide pasty range and the other solidifies close to a single point.
Why do eutectic alloys generally have better flowability?
A eutectic alloy solidifies at one fixed temperature instead of over a range, so it stays fully liquid right up to the point of solidification, without an intermediate mushy zone slowing the flow front.
Can flowability be checked without lab equipment?
Yes. A spiral test casting is a simple, practical shop-floor check: pour the spiral mold under normal casting conditions and measure how far the metal ran before it stopped.
Standard Reference
No DIN standard was found that fixes a single spiral fluidity test procedure or geometry; foundries and test laboratories use their own established spiral molds. This point is flagged for review rather than stated as settled.
Further Terms
- Eutectic temperature – the composition and temperature that give the narrowest freezing range
- Sand preparation – where the unrelated meaning, mold sand flowability, applies
- Alloy – composition choice is one of the main levers over flowability