Making perfect products in aluminium processing is all about quality control measures—none more critical than degassing. So, what is degassing in aluminium, and why would manufacturers be interested? This guide breaks down the process, its purpose, significant techniques, and equipment such as the aluminum degasser, with a focus on applicable, achievable information.
1. What is Degassing in Aluminium? A Basic Definition
Degassing in aluminum is a process of extracting dissolved gases—mainly hydrogen—from liquid aluminium prior to solidification. One characteristic of molten aluminium is that it can dissolve 50 times more hydrogen than solid aluminium. This gas uptake occurs naturally during the melting process, and if neglected, produces lethal defects in the finished product.
Hydrogen is attacked first as it's the most common gas present in molten aluminium. Less problematic gases (like oxygen or nitrogen) will be driven out with hydrogen. The aim of degassing is to level off hydrogen content below 0.15 cm³/100g of aluminium—a threshold where defects like porosity or cracking no longer happen.
Degassing falls between melting and casting/forming in the production of aluminium. It's not to be compromised by industries like automotive, aerospace, or construction, where aluminium parts need to be strong, durable, and reliable.
2. Why Gases Enter Molten Aluminium: Main Sources
To have a handle on why degassing is crucial, you should first understand how hydrogen enters molten aluminium. The most common sources are:
2.1 Raw Materials' Moisture
Aluminium scrap, ingots, or alloys usually pick up moisture from humidity, rain, or poor storage. When heated to melting points (650–750°C), any such moisture (H2O) will react with molten aluminium and release hydrogen:
2Al + 3H2O → Al2O3 + 3H2
The hydrogen goes into solution in the molten metal, where it is locked until the aluminium has solidified.
2.2 Humid Air
There is water vapor in the atmosphere close to melting furnaces, especially where there are coastal or tropical regions. The hot surface of molten aluminium acts like a sponge, drawing hydrogen from the vapor. Rising temperatures speed up absorption—humid conditions presenting serious challenges to foundries.
2.3 Lubricants and Contaminants
Oils, lubricants, or cleaning agents on tools, molds, or scrap deplete upon contact with molten aluminium. When they deplete, hydrogen is released as a byproduct. Small amounts of these impurities can result in gas levels to go through the roof, and therefore pre-melting cleaning is as crucial as degassing per se.
3. What If You Don't Degas? Defects and Hazards
Poor degassing leads to costly, dangerous defects. And here's what you'll see in finished aluminium products:
3.1 Porosity
Most common flaw: tiny gas bubbles being trapped in solid aluminium. They appear as tiny holes (macroporosity, bare eye) or micro-voids (only visible under magnification). Porosity weakens metal by reducing its cross-sectional strength, lowering tensile ductility and fatigue life. For example, a porous aluminium car bracket would break up under stress, whereas an aerospace component with pores would fail totally.
3.2 Blisters
Large pockets of gas on the surface, typically formed during welding or heat treatment. Under heating, dissolved hydrogen expands and, with pressure on the aluminium surface, results in blisters. These ruin appearance quality—whereby parts are unable to be utilized in visible applications like architectural panels or consumer products.
3.3 Cracks
Gas bubbles are "stress points." In forming, machining, or service, the bubbles dilate to cracks. Microscopic pores may lead to catastrophic failure in load-carrying components, like building columns or aircraft structures.
3.4 Economic and Safety Costs
Chaotic flaws increase scrap quantities (wasting materials and labor) and reduce output. In regulated industries (like aerospace), rejected items lose a contract to the company. Worse, faulty parts lead to accidents—something no firm can afford.
4. How to Degas Aluminium: 3 Popular Techniques
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Inert Gas Degassing (Most Popular)
This action emulates inert gas—historically argon (Ar) or nitrogen (N2)—over molten aluminium. Dissolved hydrogen is removed by the bubbles of the gas, and it diffuses into the bubbles. When bubbles reach the surface, they release hydrogen into the atmosphere.
Success is dependent on three problems:
- Small bubbles: The larger the surface area, the higher absorption of hydrogen.
- Even distribution: Bubbles must move to all parts of the molten metal.
- Controlled flow: Not enough gas = not completely de-gassed; too much = turbulence (which re-introduces fuel).
Inert gas degassing is cheap, efficient, and well suited for continuous or batch processing. It's almost always paired with an aluminum degasser—equipment for optimizing bubble size and distribution.
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Chemical Degassing
Chemical degassers are solids or liquids that are added to molten aluminium. They combine with hydrogen to form stable compounds that either float as slag (solid waste) or stay dissolved with no defect.
Choices:
- Chlorine (Cl2): Produces HCl gas upon reaction with hydrogen (easily evaporates) but is toxic and corrosive.
- Hexachloroethane (C2Cl6): Less toxic than chlorine but still environmentally hazardous.
Chemical degassing works well with small batches but less common today due to safety and environmental concerns. It's often used in combination with inert gas degassing for "deep cleaning" high-contamination melts.
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Vacuum Degassing (High-Performance Applications)
Vacuum degassing places molten aluminum in a vacuum chamber and lets pressure drop. Lower pressure lowers the solubility of hydrogen, allowing gas to emit as bubbles. It's the ideal method—removing hydrogen to levels as low as 0.05 cm³/100g—but it's expensive and energy-intensive.
This method is reserved for high-stakes applications, such as medical instruments or aerospace alloys, where zero defects are non-negotiable.
5. The Aluminum Degasser: What It Is and Why It Matters
An aluminum degasser is advanced equipment that renders inert gas degassing both efficient and uniform. It's the cornerstone of contemporary aluminium processing lines—with it, inert gas degassing would be slow, erratic, and wasteful.
5.1 How an Aluminum Degasser Works
Most industrial degassers use a rotating shaft with a graphite or silicon carbide rotor. The molten aluminium envelops the rotor, and inert gas is supplied through the shaft. As the rotor rotates (300–600 RPM), it shears the gas into very small, uniform bubbles—exposing all surfaces of the melt to gas.
Key components:
- Rotor: Heat-resistant material (graphite) that can withstand 750°C temperatures.
- Gas supply: Regulates flow and pressure to avoid turbulence.
- Drive system: Controls rotor speed for optimal bubble size.
5.2 Types of Aluminum Degassers
Two most common types, designed to production size:
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Batch degassers
For small-to-medium production batches (e.g., foundries creating custom parts). Degasser is immersed in crucible/ladle, takes 5–10 minute cycle, and then extracted.
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Continuous degassers
For large-scale production (e.g., sheet or extrusion mills). Integration with casting lines, degassing liquid aluminium as it is pumped through a duct—no down time between batches of production.
5.3 Why You Need an Aluminum Degasser
- Repeatable results: Eliminates "dead zones" where hydrogen lingers.
- More processing: Saves 30–50% of degassing time compared to manual gas injection.
- Inert gas efficiency: Uses 20–30% less inert gas by generating small bubbles.
- Controllability: Adjust speed and gas flow for different alloys (e.g., magnesium-bearing alloys need slower speeds to avoid oxidation).
6. Significant Variables Affecting Degassing Efficiency
In spite of having an aluminum degasser, success depends on the regulation of these variables:
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6.1 Molten Aluminium Temperature
Hydrogen solubility increases with temperature—but also increases hydrogen diffusion into bubbles at higher temperatures. The ideal is 680–720°C: hot enough to enable rapid degassing, but not so hot as to raise energy costs or cause the alloy to oxidize.
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6.2 Processing Time
Degassing is a time function: the more time the melt spends under the influence of gas bubbles, the more hydrogen is removed. But beyond 10–15 minutes, hydrogen levels level out—more time yields nothing. Batch degassers typically run 5–8 minutes; continuous equipment varies time according to flow rate.
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6.3 Gas Flow and Pressure
For an aluminum degasser, flow rate is critical. Good rule of thumb: 0.5–1.0 L/min of argon per 100 kg of molten aluminium. Too little gas = too little degassing; too much = splashing (which dirties with new air).
Aluminium degassing is not a choice when producing high-quality, defect-free components. Whether you're using inert gas, chemicals, or vacuum methods, your best resource is the aluminum degasser—it provides efficiency, reproducibility, and cost-effectiveness. With information on sources of gas, controlling key parameters, and pairing the right degassing method with your application, you can avoid costly defects and deliver high-quality aluminium products every time.
For new degassing manufacturers, start with a starter batch aluminum degasser: it's affordable, easy to use, and works for most small- to medium-sized production needs. When your production level expands, upgrade to a continuous system to stay current—your bottom line (and your customers) will thank you.
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