Aluminum Degassing Systems for Molten Aluminum Treatment

Hydrogen in molten aluminum is one of the leading causes of porosity, pinholes, blistering, and casting defects in aluminum products.

Proper aluminum degassing removes dissolved hydrogen before solidification, improving mechanical properties, surface quality, machinability, anodizing performance, and downstream processing reliability.

Modern aluminum casthouses rely on rotary degassing systems, ceramic foam filtration, and controlled molten metal transfer to produce clean aluminum for extrusion billet, slab casting, wheel manufacturing, aerospace components, and foundry applications.

the importance of degassing

Table of Contents

What Is Aluminum Degassing?

Aluminum degassing is the process of removing dissolved hydrogen from molten aluminum before casting.

Because liquid aluminum can dissolve significantly more hydrogen than solid aluminum, excess hydrogen becomes trapped during solidification and forms internal gas porosity.

Aluminum degassing systems use inert gases such as argon or nitrogen to extract dissolved hydrogen from the melt. Modern systems typically use rotary impeller degassing technology to generate fine gas bubbles that absorb hydrogen and carry it to the melt surface.

In addition to hydrogen removal, aluminum degassing also helps:

Reduce gas porosity
Improve casting density
Minimize blistering during anodizing
Improve mechanical properties
Reduce reject rates
Remove some oxide inclusions through flotation
Improve downstream rolling and extrusion performance

For high-quality aluminum production, degassing is usually combined with ceramic foam filtration and controlled molten metal transfer.

Online Degassing Purification

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Why Hydrogen Is Dangerous in Molten Aluminum

Hydrogen is the only gas with significant solubility in molten aluminum.

At approximately 720°C, liquid aluminum can dissolve around 0.77 ml H₂/100g aluminum. However, during solidification, hydrogen solubility drops dramatically to roughly 0.036 ml/100g.

This sharp reduction forces excess hydrogen out of solution.

The trapped gas forms:

Porosity
Pinholes
Internal voids
Surface blistering
Reduced density
Pressure leakage defects
Mechanical weakness

These defects are especially critical in:

Automotive wheel casting
Aerospace aluminum components
Extrusion billet production
Can stock rolling
Heat-treated aluminum alloys
Hydraulic castings

Why Hydrogen Is Dangerous in Molten Aluminum

Common Defects Caused by Hydrogen

Defect Type	Typical Cause	Production Impact
Gas porosity	Excess dissolved hydrogen	Reduced strength and density
Pinholes	Hydrogen precipitation during solidification	Poor surface quality
Blistering	Trapped hydrogen expands during heat treatment	Anodizing rejection
Leakage	Internal pores connect under pressure	Failed pressure tests
Reduced fatigue life	Internal voids initiate cracks	Premature failure

Without proper aluminum degassing, even well-controlled casting processes can produce unacceptable quality.

Where Hydrogen Comes From in Aluminum Casting

Hydrogen pickup occurs continuously during molten aluminum handling.

Even well-operated furnaces absorb hydrogen from moisture exposure.

The primary hydrogen sources in aluminum casting operations include:

Source	Mechanism	Relative Contribution	Mitigation Strategy
Atmospheric moisture	H₂O reacts with molten aluminum surface	High	Minimize exposed melt surface
Wet scrap charge	Moisture decomposes on contact	High	Preheat and dry scrap
Furnace combustion gases	Water vapor contacts melt	Medium	Reduce direct flame exposure
Refractory moisture	Furnace lining releases water vapor	Medium	Complete dry-out cycle
Ladles and tools	Cold surfaces introduce moisture	Medium	Preheat tools and launders
Fluxes and additives	Hygroscopic materials absorb water	Low	Controlled storage

Experienced casthouse operators understand that aluminum degassing equipment removes hydrogen already dissolved in the melt — but upstream melt practice determines how difficult the degassing task becomes.

For example:

A clean operation may deliver molten aluminum to the degassing station at 0.18 ml/100g hydrogen
A poorly controlled operation may exceed 0.40 ml/100g

The higher the inlet hydrogen level, the more difficult it becomes to achieve low final hydrogen content.

How Rotary Aluminum Degassing Works

Modern aluminum degassing systems primarily use rotary impeller technology.

The process combines inert gas injection with controlled rotor rotation to maximize hydrogen transfer efficiency.

Rotary Degassing Principle

A hollow graphite or ceramic shaft delivers inert gas into molten aluminum.

A submerged rotor spins at controlled speed and breaks the gas stream into thousands of micro-bubbles.

These bubbles provide a large gas-liquid contact surface area.

Because fresh argon or nitrogen bubbles contain almost no hydrogen, dissolved hydrogen in the surrounding molten aluminum diffuses rapidly into the bubbles.

The bubbles rise to the surface and release hydrogen into the atmosphere.

At the same time:

Oxide inclusions attach to rising bubbles
Inclusions float upward and enter the dross layer
Some alkali metals can be removed using chlorine-containing gas mixtures

This combination of hydrogen removal and inclusion flotation makes rotary aluminum degassing highly effective for industrial casting operations.

Benefits of Rotary Aluminum Degassing

Rotary degassing systems provide:

Stable hydrogen removal efficiency
Continuous inline operation
Low operating cost
Compatibility with automated casting lines
Improved metal cleanliness
Reduced reject rates
Better process consistency

Online Degassing vs Batch Degassing

The two primary aluminum degassing approaches are inline degassing and batch degassing.

Online Aluminum Degassing

Online degassing systems are installed between the furnace and the casting machine.

Molten aluminum flows continuously through the treatment chamber before entering filtration and casting.

Advantages of Online Degassing

Consistent treatment quality
Minimal hydrogen reabsorption
Continuous production capability
Better automation integration
Improved process repeatability
Lower defect variation

Online systems are widely used in:

DC casting
Billet casting
Slab casting
Automotive casting
Continuous foundry operations

Batch Degassing

Batch degassing treats a discrete ladle or crucible of molten aluminum.

The treatment itself may be effective, but the metal can reabsorb hydrogen during transport and pouring.

Typical Batch Degassing Challenges

Re-gassing during transfer
Surface turbulence
Variable treatment quality
Increased operator dependence
Longer exposure to atmosphere

Field measurements frequently show hydrogen increasing again after ladle transport.

For small foundries, batch degassing may still be economically practical. However, for medium and large-scale production, inline aluminum degassing generally delivers superior consistency.

Online aluminum Degassing Systems

Critical Parameters Affecting Aluminum Degassing Efficiency

Aluminum degassing performance depends heavily on process control.

Simply injecting gas into molten aluminum is not enough.

Rotor Speed

Rotor speed directly affects bubble formation.

Higher rotor speeds create smaller bubbles with greater surface area, improving hydrogen transfer.

However, excessive speed creates vortex formation.

A vortex pulls surface oxides back into the melt and increases turbulence.

In practice, most inline aluminum degassing systems operate between:

450–550 RPM for wrought alloys
400–500 RPM for foundry alloys

The optimal value depends on:

Alloy type
Throughput
Degassing chamber geometry
Rotor design

Gas Flow Rate

Gas flow must also remain within an optimal range.

Too little gas:

Insufficient hydrogen removal
Poor bubble distribution

Too much gas:

Bubble coalescence
Surface splashing
Oxide generation
Reduced efficiency

Recommended Aluminum Degassing Parameters

Alloy Group	Gas Type	Flow Rate (L/min)	Rotor Speed (RPM)	Target H₂ Output
1xxx / 3xxx alloys	Nitrogen or Argon	10–15	450–500	<0.10 ml/100g
6xxx extrusion alloys	Nitrogen or Ar/N₂	12–18	450–520	<0.12 ml/100g
High-Mg alloys	Argon	15–22	480–550	<0.10 ml/100g
Al-Si foundry alloys	Nitrogen or Argon	10–16	400–480	<0.15 ml/100g
Aerospace alloys	Argon	15–25	480–550	<0.08 ml/100g

High-magnesium alloys generally require argon because nitrogen may react with magnesium and form Mg₃N₂.

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Aluminum Degassing Equipment Selection Guide

Choosing the right aluminum degassing system requires more than simply comparing throughput ratings.

Equipment design strongly influences:

Hydrogen removal efficiency
Maintenance frequency
Operating cost
Process stability
Metal cleanliness

Rotor and Shaft Material

Rotor material is one of the most critical design decisions.

Graphite Rotors

Traditional graphite rotors are widely used but suffer from:

Oxidation
Abrasion
Magnesium attack
Thermal shock damage

Typical service life may range from several weeks to a few months depending on alloy conditions.

Silicon Nitride (Si₃N₄) Rotors

Advanced ceramic rotors offer significantly longer service life.

Benefits include:

Excellent oxidation resistance
Reduced wear
Improved dimensional stability
Longer maintenance intervals
Better bubble generation consistency

In many industrial applications, Si₃N₄ rotors achieve multi-year service life.

Degassing Box Design

The degassing chamber must resist:

Molten aluminum attack
Thermal cycling
Dross buildup
Mechanical cleaning damage

High-quality linings improve:

Thermal stability
Non-wetting performance
Process cleanliness
Service life

Heating System

Maintaining melt temperature is essential during treatment.

Integrated heating systems reduce:

Temperature loss
Metal freezing risk
Process instability

Sealing Performance

Poor sealing allows atmospheric moisture into the treatment chamber.

This directly reduces aluminum degassing efficiency because additional hydrogen enters the melt during treatment.

Complete Aluminum Melt Treatment Process

Aluminum degassing is only one stage of the total molten metal treatment chain.

A modern aluminum casting line typically follows this sequence:

Melting Furnace → Holding Furnace → Launder System → Inline Aluminum Degassing → Ceramic Foam Filtration → Casting Machine

Each stage serves a different function.

Furnace

The furnace:

Melts the alloy
Controls composition
Maintains target temperature

Launder System

The launder transfers molten aluminum between treatment stages while minimizing:

Temperature loss
Oxidation
Turbulence
Hydrogen pickup

Aluminum Degassing Unit

The degassing station removes:

Dissolved hydrogen
Some floating inclusions
Certain alkali impurities

Ceramic Foam Filter

Ceramic foam filters remove:

Oxide films
Non-metallic inclusions
Entrained particles

Degassing alone cannot fully replace filtration.

Casting Machine

The final casting stage solidifies treated molten aluminum into:

Billets
Slabs
Ingots
Wheels
Foundry castings

The best casting quality is achieved when all treatment stages work together.

AdTech Case Study: Improving Billet Quality with Inline Aluminum Degassing

A Turkish extrusion billet producer operating two DC casting pits struggled with excessive hydrogen levels and inconsistent billet quality.

The plant produced 6060 and 6063 extrusion billet for domestic construction markets but needed improved quality to meet European export standards.

Initial Problems

The plant experienced:

Hydrogen levels of 0.20–0.32 ml/100g
Billet surface defects
Anodizing blistering
High reject rates
Inconsistent casting quality

The root causes included:

Wet return scrap
Poor furnace sealing
Incomplete refractory dry-out
No inline aluminum degassing system

Installed Solution

The melt treatment upgrade included:

Dual-rotor inline aluminum degassing systems
Ceramic foam filtration
Insulated launders
Hydrogen measurement equipment
Operator training

Results After Aluminum Degassing Optimization

Parameter	Before	After	Improvement
Hydrogen level	0.20–0.32 ml/100g	0.08–0.11 ml/100g	60–65% reduction
RPT density index	8–14%	1.5–3.0%	>75% improvement
Billet surface quality	Moderate pitting	Smooth surface	Significant improvement
Anodizing blister rate	6–9%	<0.8%	>90% reduction
Export compliance	Failed	Passed	Full qualification

The customer achieved full compliance with European extrusion quality requirements and significantly reduced rejection costs.

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Best Practices for Aluminum Degassing

High-performance aluminum degassing requires more than equipment alone.

Best practices include:

Drying all scrap and tools before charging
Minimizing melt turbulence
Maintaining furnace seals
Controlling rotor speed and gas flow
Using proper filtration downstream
Monitoring hydrogen levels regularly
Preventing re-gassing during transfer
Performing regular rotor inspection and maintenance

Casthouses that combine good melt practice with optimized aluminum degassing systems consistently achieve superior metal quality.

Improve Casting Quality with Advanced Aluminum Degassing Systems

Hydrogen control is one of the most important factors in producing high-quality aluminum castings.

Modern inline aluminum degassing systems help casthouses achieve:

Lower hydrogen content
Reduced porosity
Better surface quality
Improved mechanical properties
Stable production quality
Lower rejection rates

AdTech provides complete molten aluminum treatment solutions including:

Inline aluminum degassing systems
Ceramic foam filtration
Launder systems
Ceramic fiber insulation
Hydrogen measurement support
Process optimization services

Our engineering team supports extrusion billet, slab casting, foundry, wheel casting, and aerospace aluminum applications worldwide.

Contact AdTech for aluminum degassing system recommendations, hydrogen control optimization, and complete molten aluminum treatment solutions.

FAQ

1. What is aluminum degassing?

Aluminum degassing is the process of removing dissolved hydrogen from molten aluminum before casting. Proper degassing helps reduce porosity, improve casting density, and enhance overall aluminum product quality.

2. Why is hydrogen harmful in molten aluminum?

Hydrogen is the only gas with significant solubility in molten aluminum. During solidification, excess hydrogen forms gas pores inside the casting, leading to porosity, pinholes, blistering, leakage defects, and reduced mechanical strength.

3. How does rotary aluminum degassing work?

Rotary aluminum degassing uses a rotating rotor to inject inert gas into molten aluminum. The rotor breaks the gas into fine bubbles that absorb dissolved hydrogen and float some oxide inclusions to the melt surface.

4. What gas is used for aluminum degassing?

Argon and nitrogen are the most common gases used for aluminum degassing. Argon is preferred for high-magnesium alloys, while nitrogen is widely used for standard extrusion and foundry alloys due to lower operating cost.

5. What is the difference between inline and batch degassing?

Online degassing continuously treats molten aluminum as it flows to the casting machine, providing more stable quality and less hydrogen reabsorption. Batch degassing treats molten aluminum in a ladle or crucible and may allow re-gassing during transfer.

6. Can aluminum degassing remove oxide inclusions?

Partially. Rising gas bubbles can help float some oxide inclusions to the surface, but aluminum degassing cannot fully replace filtration. Ceramic foam filters are still necessary for complete inclusion removal.

7. What hydrogen level is acceptable after degassing?

Acceptable hydrogen levels depend on the application. Standard aluminum castings may target below 0.15 ml/100g, while high-quality extrusion billet often requires below 0.10 ml/100g. Aerospace applications may require even lower levels.

8. How is hydrogen measured in molten aluminum?

Hydrogen in molten aluminum is commonly measured using the Reduced Pressure Test (RPT), ALSCAN, or Telegas systems. These methods help evaluate degassing efficiency and overall melt cleanliness.

9. Why does molten aluminum reabsorb hydrogen after degassing?

Molten aluminum can reabsorb hydrogen when exposed to atmospheric moisture, furnace gases, turbulence, or poorly sealed transfer systems. Proper melt handling and enclosed launders help minimize re-gassing.

10. How do I choose the right aluminum degassing system?

The correct aluminum degassing system depends on alloy type, casting throughput, hydrogen targets, and production process. Important factors include rotor design, gas type, treatment capacity, maintenance requirements, and compatibility with the full melt treatment line.