Why Do the Pores Form During Solidification?

As aluminum cools:

Hydrogen solubility ↓ dramatically

When the hydrogen concentration exceeds its solubility in the solidifying alloy, hydrogen precipitates as gas.

If the metal is already becoming solid, the hydrogen cannot escape easily. It therefore forms small internal cavities.

A simplified process is:

Molten Aluminum
↓
Hydrogen dissolves in melt
↓
Solidification begins
↓
Hydrogen solubility decreases
↓
Hydrogen precipitates
↓
Gas bubbles + shrinkage
↓
Hydrogen porosity

What Causes Hydrogen Porosity in Aluminum Castings?

The basic mechanism is:

Hydrogen enters molten aluminum → remains dissolved → aluminum solidifies → hydrogen solubility drops sharply → hydrogen comes out of solution → pores form inside the casting.

1. Moisture in the melt

The primary source of hydrogen is water/moisture. Moisture can come from:

• Humidity in the air
• Wet charge materials or returns
• Damp tools, ladles, and crucibles
• Moist furnace linings
• Wet fluxes or additives
• Moisture on ceramic filters or refractory materials

At high temperatures, water reacts with molten aluminum and generates hydrogen, which dissolves into the melt.

2. Excessive turbulence during melting and pouring

Turbulent flow can continuously expose the molten aluminum to the atmosphere.

Processes such as:

• Excessive pouring height
• High pouring speed
• Poor runner design
• Splashing
• Strong mechanical stirring
• Improper furnace transfer

can increase hydrogen pick up and also trap oxide films and air in the melt.

3. Insufficient hydrogen removal

Even if hydrogen enters the melt, good degassing can remove much of it.

If the melt is not properly treated with:

• Rotary degassing
• Argon or nitrogen gas
• Proper degassing time
• Correct rotor design and gas flow
• Vacuum treatment where applicable

excess dissolved hydrogen remains before casting.

4. Oxide films can make porosity worse

Aluminum rapidly forms oxide films when exposed to air. Turbulent handling can fold these films into the melt, creating bifilms.

These oxide films can act as nucleation sites for hydrogen bubbles, making hydrogen porosity more likely.

Hydrogen Porosity vs. Shrinkage Porosity

This distinction is important for aluminum foundries.

In many real castings, hydrogen porosity and shrinkage porosity occur together, rather than as completely independent defects.

How to Reduce Hydrogen Porosity

The most effective approach is to control the entire melt-handling process:

1. Keep materials dry
→ Charge materials, tools, ladles, fluxes and refractories should be properly dried/preheated.

2. Minimize turbulence
→ Use controlled transfer and pouring practices.

3. Perform effective rotary degassing
→ Injecting argon or nitrogen through a properly designed rotor creates fine bubbles that absorb dissolved hydrogen and carry it to the surface.

4. Use ceramic filtration
→ Filtration removes non-metallic inclusions and oxide particles that can contribute to pore formation.

5. Control holding time and temperature
→ Avoid unnecessary high-temperature holding, which can increase hydrogen pickup and oxidation.

6. Verify hydrogen level before casting
→ A Reduced Pressure Test (RPT) or other hydrogen measurement method can help determine whether the melt is adequately degassed.

For an aluminum foundry, the key relationship is:

Moisture + molten aluminum + poor melt handling → hydrogen pickup → inadequate degassing → hydrogen porosity.