Sweat

Pearl-shaped exudates firmly bonded to the casting which occur in shrink holes, gas bubbles, cavities, but also often at the surface of castings.

The defect is clearly visible to the naked eye and can occur in all casting alloys containing components with a low melting point and/or eutectics, independent of the molding process.

Typical examples are copper-tin alloys, cuprous aluminum alloys or phosphorus-rich iron alloys. In contrast to spraying pearls, sweat generally has a different chemical composition and different structural components than the actual casting. For example, in flake graphite cast iron, sweat consists of a large phosphide eutectic part, and is then also referred to as phosphide pearls.

In copper-tin alloys, sweat mainly consists of segregatedtin, whereas in cuprous aluminum alloys, it has the composition of the Al-Al2-Cu eutectic. If sweat occurs at the surface of a casting, it can be pressed against the mold wall where it is crushed and then sticks to the casting surface like a plaster.

Sweat formation is a phenomenon of macro segregation (pressure segregation, see also Segregation).

Low melting point components which have remained liquid after solidification of the structural matrix are pressed out at the casting surface under the pressure of the shrinkage or the pressure of gases (s. Gas bubble), released during solidification or occurring due to mold material reactions. This case is the more likely to occur, the lower the melting point of the re-melted phases compared to the solidus temperature of the relevant alloy.

The formation of sweat can be prevented by rapid, directional solidification.

Additional references:
Casting defect
Casting defect
Mold sand-based casting defect
Spraying pearl
Cold shot

  • Fig. 1: Sweat in an extensive gas bubble on a real flake graphite cast iron casting
  • Fig. 2:  Graphite formation in the sweat highlighted in Fig 1, magnification 100:1, not etched
  • Fig. 3: Basic structure of the sweat highlighted in Fig. 1 , the phosphide eutectic is clearly visible, magnification 100:1, etched with HNO3
  • Fig. 4: Detailed SEM image of the phosphide eutectic from sweat with marked EDX measuring point
  • Fig. 5: EDX analysis of the section highlighted inFig. 4 in the phosphide eutectic
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