The primary cause of pinhole porosity in sand casting is the dissolution of hydrogen gases in the molten metal.
Understanding Pinhole Porosity
Pinhole porosity is a common defect found on the surface of sand castings. These are typically described as:
- Small holes: Roughly 2 mm in size.
- Surface appearance: Appearing on the surface of the finished casting.
This defect directly impacts the quality, strength, and finish of the cast product.
The Primary Cause: Dissolved Hydrogen
As noted in the reference, this defect happens because of the dissolution of the hydrogen gases in the molten metal. Here's a breakdown of the process:
- Hydrogen Absorption: Molten metals, especially aluminum and its alloys, can absorb significant amounts of hydrogen gas. Common sources of hydrogen include:
- Moisture (H₂O) in the sand mold, cores, or ladle linings, which decomposes at high temperatures.
- Humid atmosphere.
- Contaminated scrap metal or alloying additions.
- Wet fluxes or furnace fuels.
- Dissolution in Liquid Metal: Hydrogen dissolves much more readily in the liquid state of the metal than in the solid state.
- Solidification and Gas Release: As the molten metal cools and begins to solidify in the mold, the solubility of hydrogen decreases drastically. The excess hydrogen gas comes out of solution.
- Pinhole Formation: If this gas cannot escape through the solidifying metal or the mold, it gets trapped near the surface, forming small pores or pinholes. The gas bubbles are pushed towards the solidifying front, often ending up just beneath or on the surface.
Preventing Pinhole Porosity
Preventing pinhole porosity involves minimizing hydrogen pickup and facilitating its removal before solidification. Practical steps include:
- Proper Drying: Ensure sand molds, cores, and ladles are thoroughly dried to eliminate moisture.
- Degassing: Use degassing techniques (like fluxing, inert gas bubbling, or vacuum treatments) to remove dissolved hydrogen from the molten metal before pouring.
- Material Control: Use clean, dry charge materials and avoid contaminated scrap.
- Atmosphere Control: Control humidity in the casting area if possible.
- Pouring Temperature: Avoid excessively high pouring temperatures, as hydrogen solubility increases with temperature.
By addressing the sources of hydrogen and implementing effective degassing practices, casters can significantly reduce or eliminate pinhole defects.
| Defect Name | Primary Cause | Appearance |
|---|---|---|
| Pinhole Porosity | Dissolution of hydrogen gases | Small (~2mm) holes on the surface |
