There is a saying among experienced foundry metallurgists: you cannot cast quality into a component — you have to melt it in. What happens between the moment aluminium charge enters the furnace and the moment metal enters the die determines the quality ceiling of every casting produced that day. No amount of process optimisation downstream can compensate for a poor melt. Yet melt quality management remains one of the most inconsistently practised disciplines across small and medium aluminium foundries.
This article covers the three pillars of effective melt quality management — degassing, fluxing, and Reduced Pressure Testing — and explains why all three must work together as a system rather than as isolated interventions.
Understanding the Enemy — Hydrogen and Oxides
Two contaminants are responsible for the vast majority of melt-related casting defects: dissolved hydrogen and oxide inclusions. Hydrogen enters the aluminium melt from multiple sources — atmospheric moisture, damp or contaminated charge materials, wet or poorly dried fluxes, and furnace linings that have absorbed moisture during downtime. At melt temperatures above 660 degrees Celsius, hydrogen solubility in aluminium is relatively high. As the metal solidifies and cools, solubility drops sharply — and the excess hydrogen precipitates as gas bubbles that become trapped porosity in the casting.
Oxides form instantly when molten aluminium contacts oxygen. The aluminium oxide film is tenacious, thin, and nearly neutrally buoyant — which means it does not separate from the melt easily. When turbulence during melting, transfer, or pouring folds oxide films into the melt body, they become bifilm inclusions in the solidified casting — stress concentrators that reduce mechanical properties and fatigue life without necessarily being detectable by standard visual inspection.
Degassing — The Most Critical Melt Treatment Step
Rotary degassing using an inert gas — typically nitrogen or argon — is the most effective and efficient method for removing dissolved hydrogen from an aluminium melt. The principle is straightforward: a rotating impeller disperses fine bubbles of inert gas throughout the melt, and hydrogen diffuses from the metal into the bubbles as they rise, carrying it out of the melt and into the atmosphere above.
The critical variables are bubble size, distribution, rotor speed, gas flow rate, and treatment time. Larger bubbles rise quickly and contact less metal. Finer bubbles have more total surface area for hydrogen diffusion and remain in the melt longer. Modern FDU rotors are engineered specifically to produce the finest possible bubble distribution — maximising degassing efficiency per unit of treatment time.
Two common errors undermine degassing effectiveness in practice. First, insufficient treatment time — operators cutting the degassing cycle short under production pressure, leaving residual hydrogen in the melt. Second, contamination reintroduction — a properly degassed melt that sits exposed to a humid atmosphere, or is transferred turbulently in a damp ladle, will rapidly reabsorb hydrogen and undo the treatment. Degassing must be the last melt treatment step before pouring, and the interval between degassing completion and first pour should be minimised.
Fluxing — Cleanliness Beyond Hydrogen
While degassing targets dissolved hydrogen, fluxing addresses the other half of the melt quality problem — oxide films, dross, non-metallic inclusions, and alkali metal contaminants such as sodium and calcium that degrade alloy properties and casting surface quality.
Covering fluxes form a protective layer over the melt surface, limiting atmospheric oxidation during holding periods. Cleaning fluxes are worked into the melt to agglomerate fine oxide particles into larger masses that can be skimmed. Drossing fluxes react with the metallic aluminium trapped in dross, releasing it back into the melt and producing a drier, more easily removed dross that contains less aluminium — directly improving metal recovery and reducing raw material cost.
Flux selection is not trivial. Sodium-containing fluxes, while effective for many applications, are incompatible with alloys sensitive to sodium modification. COVERAL FREE and COVERAL PURE formulations address these requirements with sodium-free and sodium-and-calcium-free formulations respectively. The method of addition matters as much as flux selection — granular fluxes are consistently more effective than powder, penetrating the melt more efficiently and allowing addition rates to be reduced while maintaining treatment effectiveness.
RPT and Density Index — Verifying What You Cannot See
Degassing and fluxing are treatments. Reduced Pressure Testing and Density Index measurement are verification — and in a well-run foundry, treatment without verification is insufficient.
The Reduced Pressure Test is conceptually elegant. A small sample of molten aluminium is solidified under reduced atmospheric pressure — typically around 80 millibars. Under these conditions, dissolved hydrogen that would ordinarily remain in solution expands significantly, producing far more visible porosity than would appear in a standard atmospheric solidification. The sample is then sectioned and compared against a set of reference standards graded by porosity severity. RPT gives a fast, qualitative answer: is this melt acceptable, or does it need further treatment?
Density Index testing takes this further by quantifying hydrogen content numerically. Two samples are taken simultaneously — one solidified at atmospheric pressure, one under vacuum. Their densities are measured precisely and the percentage difference — the Density Index — provides a numerical measure of porosity tendency. A Density Index below three to five percent is generally acceptable for standard casting applications. Values above this indicate further degassing is required.
Building the System
The three elements — degassing, fluxing, and RPT verification — are most powerful when they operate as a defined, documented system rather than as ad hoc practices. A written melt treatment protocol that specifies flux type and addition rate, degassing rotor speed and treatment duration, and RPT acceptance criteria for each alloy and product family gives a foundry the consistency that separates good average quality from reliably excellent quality.
The investment in this system is modest compared to the cost of scrap castings, customer complaints, and the reputation damage that comes from inconsistent quality. It is one of the most high-return improvements available to any aluminium foundry — regardless of size.