A356 Melt Treatment Before Gravity Casting: Degassing, RPT and Release

Release Sequence for Every Melt

Every A356 or ZL114 melt passes the same steps before it reaches a gravity or tilt-pour die. If the reduced pressure test or the chemistry check fails, the melt is not released for production.

Rotary Degassing: The Settings We Use

We degas each melt for 15 to 20 minutes with the rotor at 480 to 520 rpm.

We started from published degassing research on cylinder-head castings and adjusted the settings to our own furnaces and parts. The time is longer than in small laboratory tests because a production furnace holds far more metal.

A rotary unit has two jobs. To clean the melt, a vortex is allowed to form so that treatment flux is drawn into the metal and collects oxides, which are then skimmed off as dross. To degas, a baffle suppresses the vortex, and the rotor breaks the purge gas into fine bubbles that carry dissolved hydrogen out of the melt. On our unit the baffle is raised for flux treatment, so the vortex can form, and lowered for degassing.

Our operators also watch the melt surface. When the rotor reaches speed, the bubble stream becomes fine and dense, and small flashes appear on the surface as the hydrogen carried up by the bubbles burns at the hot surface. Operators read this as a sign that degassing is under way, but it is not a release criterion; the reduced pressure test decides.

Published studies point in the same direction. A 2026 study on A356 compared rotor speeds of 200 and 500 rpm, treatment times of 5 and 10 minutes and nitrogen flows of 5 and 10 L/min in an 8 kg furnace; the best of the tested combinations was 500 rpm for 10 minutes at 10 L/min (Archives of Foundry Engineering, 2026). Laboratory results set the direction, not the production setting.

Reduced Pressure Test: Why Molten Aluminum Expands Under Vacuum and How We Read the Sample

If a molten aluminum sample swells, rises or pushes its surface up while it solidifies under vacuum, the usual cause is gas in the melt. Hydrogen dissolved in the liquid comes out of solution at low pressure, and the bubbles expand instead of staying dissolved.

A reduced pressure test shows whether the melt still holds enough gas to form pores. The sample solidifies under reduced pressure, so dissolved gas expands into pores that are easy to see.

We release the melt only when the top surface of the sample is slightly concave and the cut face shows no visible pores. A top surface that bulges upward points to a higher gas level.

Some foundries also calculate a density index (DI) from a pair of samples, one solidified in air and one under reduced pressure. A DI of about 2% or lower is often described as a good general level, but the limit should follow the porosity requirements of the casting (Giesserei Lexikon). The test also responds to oxide films, so it is a melt-quality check, not a hydrogen measurement alone.

Spectrometer Check Before Production

After the reduced pressure test, a sample from the same melt is analyzed on our Oxford optical emission spectrometer. Production starts only when the chemistry is within the limits of the specified alloy. A356 and ZL114 are the alloys we use for gravity and tilt-pour casting.

The chemistry check and the reduced pressure test answer different questions. A melt can have the right composition and still hold too much gas, so neither check replaces the other.

Temperature Control During Pouring

Each holding furnace has an immersion thermocouple in a silicon carbide protection tube, made to fit our furnaces, that shows the melt temperature continuously.

Quality inspectors spot-check die temperatures with infrared thermometers on the die cores and die walls. An infrared reading depends on the emissivity of the surface, so coated and bare steel read differently; readings should be compared at the same location and coating condition.

Pouring-temperature and die-temperature windows are set for each part, because the right window depends on the part's wall sections, size and die design.

What Each Defect in a Radiograph Points To

Different pores have different causes, so the corrective action depends on what the radiograph or the section shows. A single image cannot prove the cause; it tells us where to look first.

Results on a Long-Running Automotive Part

On a shock-absorber bracket that we have supplied to one automotive customer for about 20 years, our in-house inspection pass rate rose from 92–95% to between 98.27% and 98.94% after the melt-treatment and process-control changes described on this page. Castings that fail our inspection are scrapped before shipment.

For a large pump volute where melt control decided whether the castings passed CT inspection, see our pump volute case study.

What to Put in Your RFQ

State the porosity requirements on the drawing so that the melt treatment and the inspection plan can be matched to the part.

• Alloy and temper, for example A356-T6 or ZL114.
• Radiographic standard, acceptance grade and the zones where it applies.
• Leak-test pressure and hold time, and whether every part or a sample is tested.
• Machined surfaces where exposed porosity matters, such as sealing faces and bores.

For acceptance by zone, see our guide to porosity inspection and acceptance and the digital radiography RFQ guide. For pressure-tight parts, see the leak-test specification guide. Our inspection equipment is listed on the quality control page.

Sources

• Optimization of rotary degassing parameters in A356 aluminum alloy, Archives of Foundry Engineering, 2026.
• Low-pressure density test, Giesserei Lexikon.
• ASTM E155 film reference radiographs and ASTM E2422 digital reference images for aluminum castings, ASTM.