Degassing Aluminium: Why Hydrogen Porosity Forms and How to Prevent It

One of the most common reasons for aluminium castings failing leak tests or showing voids after machining is hydrogen porosity. Understanding its source is half the solution.

Where does the hydrogen come from?

Molten aluminium reacts with moisture in the air and in the charge; the released hydrogen dissolves in the melt. Hydrogen solubility in liquid aluminium is roughly 20 times higher than in solid aluminium, so during solidification the excess must leave the metal — and where it cannot escape, it forms gas pores.

Main sources of moisture and hydrogen:

• Wet or oily scrap, chips and returns,
• Undried ladles, crucibles, tools and refractories,
• High ambient humidity (especially in summer),
• Excessive melt temperature and long holding times,
• Frequent stirring of the surface, breaking the oxide film.

Cut the source first

Keeping the charge dry, preheating tools, avoiding unnecessarily high temperatures and keeping the furnace covered greatly reduce the load on degassing.

How does rotary degassing work?

The most common method injects an inert gas (nitrogen or argon) through a graphite rotor. The rotor breaks the gas into many small bubbles and stirs the melt. Hydrogen diffuses into these hydrogen-free bubbles and is carried to the surface. The smaller the bubbles and the longer they stay in the melt, the higher the efficiency. The same action helps float oxide inclusions to the surface.

Rotor speed, gas flow and treatment time are set according to metal quantity, ladle geometry and target quality. Too high a speed creates a vortex that can draw oxides and hydrogen back in.

How is melt quality measured?

The most widely used shop-floor method is the reduced pressure test (RPT) and the resulting density index. Two samples of the same metal solidify, one at atmospheric pressure and one under vacuum; the difference in density indicates the gas content. Measuring before and after degassing shows whether the treatment is really working.

Quick checklist

• Are charge and tools dry and preheated?
• Is the melt temperature higher than necessary?
• Are rotor speed and gas flow recorded?
• How long does the metal wait between degassing and pouring?
• Is the density index measured regularly?

OEM FURNACE degassing units are designed to work together with our transfer ladles and furnace systems.

Let us help you choose the right furnace. Send us your capacity, alloy and energy supply and we will prepare a clear, itemised quotation: talep@oemfurnace.com.tr · +90 533 945 27 94