Aluminum Degassing: Methods Compared and How to Check the Result

Aluminum degassing removes dissolved hydrogen from the melt before it can turn into gas porosity in the casting. This guide explains why the hydrogen is there, how the main methods compare, and how to prove on the shop floor that the treatment worked.

Porosity shows up late: at machining, at leak testing, or as blisters after heat treatment. By then the melt is long gone. That is why aluminum degassing and its check both belong in the routine, not only in the troubleshooting file.

Why hydrogen porosity forms in the melt

Liquid aluminum dissolves far more hydrogen than solid aluminum can hold. As the metal freezes, the excess hydrogen is rejected and forms pores. A paper in Archives of Foundry Engineering describes this drop in solubility as the main driver of gas porosity in aluminum castings.

The hydrogen comes from water in its many forms. Common sources on the shop floor include:

• Humid air over the bath, especially in wet seasons.
• Damp or oily scrap, returns and turnings.
• Tools, ladles and skimmers that were not dried or preheated.
• Fluxes that picked up moisture in storage.
• Combustion products from gas burners in direct contact with the metal.

Oxide films matter too. Researchers at Brunel University note that unbonded oxide films can nucleate pores with little difficulty. So a clean melt and a degassed melt are related goals. In practice, aluminum degassing and inclusion control should be planned together.

Aluminum degassing methods compared

The Archives of Foundry Engineering paper lists several methods studied over the past decades. These include vacuum, ultrasonic, spray, rotary impeller and tablet degassing. In most foundries, though, the real choice is between three options.

Rotary impeller

The Brunel study calls rotary degassing the method commonly used in foundries. A purge gas, usually argon, is bubbled into the metal. Hydrogen diffuses into each bubble and leaves with it at the surface. Smaller bubbles mean more surface area and faster removal. Rotor geometry and graphite grade are covered in our article on graphite rotors for aluminum degassing.

Lance

A plain lance produces large bubbles that rise fast and close to the injection point. Much of the bath sees little gas. It can work in small vessels when demand is low. However, results tend to vary from heat to heat.

Degassing tablets

Tablets based on hexachloroethane (C2Cl6) are one of the methods the same paper reviews. The Archives of Foundry Engineering authors describe tablet degassers as chlorine and fluorine containing salts that react with the aluminum. In their tests, argon degassing limited intermetallic growth on crucible furnace steel better than tablets did. Tablets also release fumes, so ventilation and exposure control matter.

Rotary aluminum degassing: settings that matter

According to the Brunel paper by Lordan and co-authors, rotary degassing typically needs an argon flow of 4 to 10 L/min. It also needs 15 to 30 minutes of treatment to reach hydrogen below 0.15 cm3/100 g. Those figures depend on bath size, so use them as a reference, not a recipe.

The same paper flags the main trade-off. Higher rotor speed makes smaller bubbles. However, it also creates a vortex and surface turbulence that drag oxide films into the melt. In other words, an aggressive cycle can trade gas porosity for inclusions.

Practical points for a stable aluminum degassing cycle:

1. Set rotor speed below the point where a vortex opens at the surface.
2. Immerse the rotor before starting gas flow, and keep gas on while withdrawing it.
3. Keep treatment time and flow constant per ladle size, and record both.
4. Inspect rotor and shaft wear on a schedule, and log hours of use.
5. Skim after degassing, once dross has floated, not during the cycle.

For a fixed station, a rotary degassing unit makes speed, gas flow and time repeatable. Spare graphite rotors and shafts keep the station running between rotor changes.

How to check results: reduced pressure test and density index

Without a test, aluminum degassing is a guess. The reduced pressure test (RPT) is the most common shop-floor check. A sample freezes under partial vacuum, so any hydrogen forms visible, enlarged pores.

Foundry Management & Technology describes a typical procedure. The sample solidifies for about 7 minutes at 25 to 28 inches of mercury. Under that vacuum, hydrogen porosity expands roughly ten times compared with normal pressure.

Reading the sample

Visual comparison against a chart is quick but subjective. Measuring specific gravity is more reliable. Weigh the sample in air and in water, then calculate. The same article uses alloy 356 as an example: book density 2.68, with 2.6 set as the minimum to pour.

Density index

The density index (DI) compares two samples: one frozen in air and one frozen under vacuum. The Brunel paper defines it as DI = (D air minus D vacuum) divided by D air. It cites an industry threshold of about 5% DI, linked to 0.15 cm3/100 g of hydrogen. Each plant should still set its own target by alloy and part.

Common mistakes after aluminum degassing

A good result at the station can fade before pouring. In the Brunel trial, DI fell to a minimum about 10 minutes after rotary treatment. It then climbed to 4.1% after 70 minutes of holding. The authors link this to surface turbulence and regassing.

Typical mistakes that undo the work:

• Long holding after treatment, with no repeat test before pouring.
• Turbulent transfer through launders or from height into the ladle.
• Cold, wet tools or additions entering a treated melt.
• Testing only at the start of the shift, not when the melt has waited.

Therefore, take the RPT sample as close to pouring as practical. If the melt waits, test again.

Aluminum degassing FAQ

Is nitrogen acceptable instead of argon?

Many foundries use dry nitrogen for cost reasons. Gas choice depends on alloy and quality targets, so confirm it with your metallurgist and track RPT results after any change.

How often should we run the reduced pressure test?

At minimum, once per treated ladle or furnace batch. Test again whenever the melt holds for a long time before pouring.

Can degassing fix dirty metal?

Only partly. Flotation removes some oxides, but filtration and good fluxing practice do most of the inclusion work.

Need equipment or consumables for your aluminum degassing line? AL-LINX quotes each item to your specification, and you can bundle several items in one request. Request a quote for the rotary degassing unit and graphite rotors.