Molten aluminum purification is essential to produce defect-free castings with excellent mechanical properties and surface finish. Impurities such as hydrogen, oxides, and non-metallic inclusions can cause porosity, defects, and reduced corrosion resistance. This article explores proven purification methods for industrial foundries and their practical applications.
Degassing: Removal of Hydrogen and Gases
Hydrogen is the most common gas dissolved in aluminum melts, which can cause porosity during solidification. Effective degassing methods include:
- Spin degassing: A rotating graphite impeller injects argon or nitrogen into the melt, creating fine bubbles that deliver hydrogen to the surface. This method can reduce hydrogen levels to <0.1 mL/100g Al.
- Tablet flux: Chlorine-containing tablets (e.g., hexachloroethane, C₂Cl₆) release bubbles to remove hydrogen and light inclusions.
- Vacuum degassing: Used in high-end applications (e.g., aerospace), molten aluminum is exposed to a vacuum to extract dissolved gases.
Best Practice: For best efficiency, degassing temperatures should be 710-750°C.
Flux Treatment: Removes Oxides and Slag
Flux chemically reacts with impurities to improve metal cleanliness:
- Covering flux (NaCl-KCl mixture) prevents oxidation during melting.
- Deslagging flux breaks down the oxide layer, making it easier to scrape off.
- Wall Cleaning flux removes oxide buildup in the furnace.
Note: Overdosing flux may introduce new impurities, so controlling usage is critical.
Aluminium Filtration: Captures Solid Inclusions
Filters remove non-metallic particles (oxides, carbides) that weaken castings:
- Ceramic Foam Filters (CFF): With pore sizes of 10-60 PPI (pores per inch), they capture inclusions while maintaining flow rates.
- Deep Bed Filters: Used in ultra-high purity aluminum (e.g., aluminum foil production).
- Electromagnetic Filtration: Effective for removing iron-rich intermetallic compounds in electrical grade aluminum.
Selection Tips: Gravity castings typically use filters with 10-30 PPI; low pressure castings may require finer filters (30-60 PPI).
Alloy Adjustment: Fine-tuning the Composition
After purification, alloying elements are added to achieve the desired properties:
- Grain refiners (Al-Ti-B rods) improve mechanical strength.
- Modifiers (Sr, Na) improve silicon distribution in aluminum silicon alloys.
- Fe neutralizers (manganese, chromium) reduce the deleterious effects of iron impurities.
Verification: Always check the composition with a spectrometer before pouring.
Conclusion: A systematic approach to purifying aluminum
For best results, a combination of degassing, flux handling, and filtration is recommended. The specific process depends on the specific application—automotive castings may prioritize hydrogen removal, while electronic components require extremely low inclusion levels.
By implementing these methods, foundries can consistently produce aluminum castings with minimal defects and high integrity.
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