Why Is Degassing So Important in Aluminum Casting, and How Do You Do It Right?

When producing aluminum alloy castings, an invisible enemy can quietly sabotage your product: hydrogen gas.

You won’t see it during melting, but it reveals itself later as porosity, surface defects, or leaks—critical issues in industries where quality is everything.

In this guide, you’ll learn:

• Why aluminum absorbs hydrogen
• What methods remove it effectively
• How proper degassing improves your casting performance

Why Does Aluminum Need Degassing?

Hydrogen Absorption in Molten Aluminum

Liquid aluminum is highly reactive with hydrogen, especially at elevated temperatures. At 700 °C, it can dissolve about 0.69 mL of hydrogen per 100 g of metal. However, once solidified, its hydrogen solubility drops dramatically to 0.036 mL/100 g, forcing hydrogen to escape as bubbles.

The result? Pinholes, subsurface voids, and casting porosity.

How Porosity Hurts Your Castings

Even small amounts of porosity can severely degrade casting performance:

• Reduced strength: Each 1% increase in porosity may reduce tensile strength by ~10%.
• Surface quality issues: Bubbles can cause blisters during electroplating or spots during anodizing.
• Leakage failure: In applications like engine blocks or hydraulic housings, porosity can cause leak test failures.

How to Degas Aluminum: 3 Proven Methods

Foundries choose degassing techniques based on cost, application, and performance needs.

Inert Gas Purging (Most Common)

This method bubbles argon or nitrogen into the molten aluminum—often using a rotary degassing unit.

• How it works: Gas bubbles bond with hydrogen atoms and carry them to the surface.
• Setup example: 1–2 L/min·kg for 10–15 minutes.

This is widely used in automotive and general-purpose foundries.

Chemical Degassing (Fast but Needs Control)

Uses tablets or powders (like hexachloroethane) that release reactive gases upon contact with the melt.

• Tip: Use only 0.1–0.3% by weight to minimize slag residue.
• Useful for smaller operations or quick fixes, but requires precise control to avoid contamination.

Vacuum Degassing (High-End Option)

Used in aerospace and high-spec parts where porosity tolerance is near zero.

• Removes even trace hydrogen to meet sub-0.10 mL/100 g targets.
• Costlier and more complex, but essential for ultra-high quality casting.

Key Benefits of Proper Degassing

Proper degassing dramatically improves casting quality:

• Stronger parts: Keeping hydrogen under 0.1 mL/100 g can increase tensile strength by 15–20% and elongation by 30%+.
• Smoother surfaces: Reduces “flyspeck” and bubble marks.
• More stable molding: Less pressure variation, reduced flash, and longer die life.
• Higher yield: In wheel production, degassing can reduce X-ray rejection rates from 5%+ to under 0.5%.

Real-World Applications of Degassing

• Automotive: Tesla’s gigacastings require ≤ 0.12 mL/100 g hydrogen, achieved through inline degassing + vacuum systems.
• Electronics: 5G heat sink manufacturers demand pores < 50 µm, using rotary gas injection.
• Aerospace: Airbus A380 components like wing joints use vacuum degassing + ultrasonic treatment for critical strength.

Best Practices for Consistent Degassing Results

✅ Control the Temperature

• Keep between 700–730 °C.
• Too low = poor gas removal.
• Too high = more hydrogen absorption.

✅ Maintain Equipment

• Rotary impellers should run at ≥ 400 rpm.
• Keep ceramic parts clean to avoid contamination.

✅ Monitor Hydrogen Content

• Use reduced pressure tests (RPT) or real-time hydrogen analyzers for precision control.

Final Thoughts

Degassing isn’t just a box to check—it’s a core quality control step in aluminum casting.

When done correctly, it can transform scrap-prone melts into strong, defect-free, high-performance parts. Most modern foundries now integrate degassing with filtration and grain refinement as part of a full melt treatment process.

Want better castings? Start with better melt quality.