Metallurgy
Melt treatment for aluminium die casting: degassing, cleanliness and control
How hydrogen, oxides, iron and temperature affect casting quality, and the degassing, testing and handling practices that keep every melt consistent.
Hydrogen: the main source of gas porosity
Molten aluminium dissolves hydrogen from moisture in the air, damp charge material, tools and refractories. Solubility in the liquid is many times higher than in the solid, so hydrogen comes out of solution as the metal freezes and forms rounded pores. Higher melt temperatures and humid weather both increase pick up.
Degassing and melt quality checks
| Control | How it works | Typical practice |
|---|---|---|
| Rotary degassing | A spinning impeller disperses fine bubbles of nitrogen or argon that collect hydrogen and float oxides | A few minutes per treatment, repeated after holding periods |
| Reduced pressure test | A sample solidifies under partial vacuum (about 80 mbar) next to one solidified in air | Done after each treatment |
| Density index | (Density in air minus density under vacuum) divided by density in air, as a percentage | Limits set per part: tighter for pressure tight parts |
| Spectrometer analysis | Sparked sample measured against the specified standard | Every new batch, and after alloy additions |
Cleanliness: oxides, dross and flux
- Keep charge material clean and dry; preheat returns and ingots before charging.
- Skim dross gently; turbulent skimming folds oxide films back into the melt.
- Use cover and cleaning fluxes to separate oxides and keep furnace walls clean.
- Control the ratio of returns (runners, overflows, scrap) to fresh ingot, since returns carry oxides.
Grain refinement and modification
- Grain refinement: Al Ti B master alloy gives fine, equiaxed grains, better feeding and fewer hot tears, mainly in gravity castings.
- Modification: strontium, typically in the range of about 100 to 200 ppm, turns coarse silicon plates into fine fibres in hypoeutectic alloys such as LM25 and A356, improving elongation. In HPDC, the very fast cooling already refines silicon, so modification is less common.
Iron, manganese and sludge
Die casting alloys carry iron to stop soldering, but iron, manganese and chromium can combine into hard sludge that settles in holding furnaces and causes hard spots in castings. A common check is the sludge factor: Fe + 2 × Mn + 3 × Cr. Keeping it below about 1.6 to 1.8, and holding temperatures above the point where sludge forms, prevents it.
Temperature control
Hold metal no hotter than the process needs. Every extra degree increases hydrogen pick up, oxide growth and energy use. At Alcast, melting is by electric resistance furnaces, which give stable temperatures and no combustion gases in the melt.
Values are typical published ranges for guidance. Every part is different: process windows are set and proven for each component through trials, and the governing standard and your drawing take precedence. Reviewed by the Alcast engineering team, November 2026.
Questions engineers ask
What is a good density index for aluminium die casting?
It depends on the part. Pressure tight and structural parts need low values, often around 1 to 2%; general HPDC parts can accept more. The limit is set per part and checked after every treatment.
Nitrogen or argon for degassing?
Both work with a rotary impeller. Argon is fully inert and is preferred for magnesium bearing alloys and critical work; nitrogen is cheaper and widely used for general die casting.
What causes dross in aluminium melting?
Oxidation of the melt surface, turbulence when charging or skimming, and dirty or wet charge material. Dross is reduced by clean charge, gentle handling and flux.
What is the sludge factor?
Fe + 2 × Mn + 3 × Cr. When it is too high for the holding temperature, hard intermetallic sludge forms and settles in the furnace.