Porosity in
Aluminium Castings.

Two distinct mechanisms produce porosity in aluminium die castings, and they require different countermeasures.

Gas porosity forms when hydrogen or entrapped air is trapped in the solidifying casting. Gas pores are typically spherical or near-spherical and distributed throughout the casting. In HPDC, air entrapment during the high-velocity injection phase is the primary source. Hydrogen comes from moisture in the die lubricant, the metal charge, or the atmosphere.

Shrinkage porosity forms as the casting solidifies and contracts. Aluminium shrinks approximately 6.6% by volume on solidification. Where feed metal cannot reach a solidifying region — typically in thick sections remote from the gate — a void forms. Shrinkage porosity tends to be irregular and jagged in cross-section, often concentrated at hot spots in the casting.

Air entrapment in HPDC is controlled primarily through: die venting (vent channels in the die parting line allow air to escape ahead of the metal front), vacuum-assisted die casting (a partial vacuum is drawn in the die cavity before injection, reducing trapped air), shot parameters (first phase slow shot speed allows the plunger to push air out of the shot sleeve before injection; controlled transition to fast shot minimises turbulence at the gate), and die lubricant volume (excess lubricant generates steam that contributes to gas porosity — correct lubricant dosing and drying time are critical).

Hydrogen porosity is controlled by: metal degassing before casting (rotary degassing with nitrogen or argon removes dissolved hydrogen), metal temperature management (higher superheat increases hydrogen solubility — metal should be poured at the minimum temperature consistent with good fill), and dry charge materials (recycled scrap with surface contamination or oxide should be limited).

Shrinkage porosity is primarily a die design problem. It is addressed by: gate location (the gate should be positioned so that metal flows from thin to thick sections, allowing the thicker sections to be fed while still liquid), runner and overflow design (overflows positioned at the last-to-fill sections create a reservoir of hot metal that feeds shrinkage as the casting cools), intensification pressure (in HPDC, a secondary intensification pressure is applied after the die fills to compensate for shrinkage — correct timing and pressure are critical), and die temperature control (temperature differentials between die zones control the solidification sequence — water or oil cooling channels in the die must be correctly positioned and controlled).

Porosity detection methods range from destructive to non-destructive.

X-ray (radiographic) inspection is the primary non-destructive method. It reveals both gas and shrinkage porosity and is specified to standards including ASTM E505 (aluminium castings) and EN 12681 (foundry radiography). X-ray is required for pressure-tight and structural components.

Leak testing involves pressurising the casting (air, helium, or water) to detect connected porosity that would allow fluid passage. This is specific to pressure-tight components like pump casings and valve bodies.

Cross-sectioning (destructive) is used during process validation to map internal porosity distribution and validate gate and overflow design before committing to production tooling.

Density measurement (Archimedes method) provides a statistical indicator of overall porosity level — useful for incoming material control of secondary alloy charges.

At SAPL, porosity control begins at the tool design stage. Our engineering team reviews gate location, overflow positioning, and die cooling circuit design before tool manufacture. For critical components, we conduct mould flow simulation to map fill patterns and identify potential shrinkage hot spots.

In production, real-time shot monitoring records first-phase speed, fast-shot velocity, gate velocity, and intensification pressure for every shot. This data is traceable to the casting batch. Deviation alarms flag shots where parameters fall outside the qualified window.

For components with specified X-ray or leak test requirements, we maintain the test equipment and qualified operators in-house. X-ray results are documented per batch and retained as part of the quality record.