Porosity is the most common and most consequential defect category in aluminium casting. It reduces mechanical properties, creates leak paths in pressure-containing components, produces surface defects that reveal themselves during machining, and is responsible for a disproportionate share of casting scrap and rework across the industry. Yet porosity is not a single defect — it is a category that encompasses at least two fundamentally different mechanisms, each with different causes, different morphologies, and different remediation strategies. Conflating gas porosity and shrinkage porosity leads foundries to apply the wrong fixes and perpetuate the problem. Getting the distinction right is the first step toward systematic porosity reduction.
Gas Porosity: The Hydrogen Problem
Gas porosity in aluminium castings arises primarily from dissolved hydrogen — the only gas with significant solubility in liquid aluminium. Hydrogen solubility in aluminium drops sharply at the liquid-solid transition: liquid aluminium at 700°C can dissolve approximately 0.65 ml/100g of hydrogen, while solid aluminium at the same temperature dissolves only about 0.034 ml/100g. When liquid aluminium solidifies, the hydrogen that was dissolved in the melt is rejected from the solid phase; if solidification is rapid and directional, hydrogen can escape through the remaining liquid. If solidification is too slow, or if hydrogen content is high, the rejected hydrogen nucleates as spherical or near-spherical pores distributed through the casting microstructure. Gas porosity pores are typically round in cross-section when viewed on a cut-and-polished surface, and they are distributed relatively uniformly through the affected section rather than concentrated in specific locations.
Sources of Hydrogen in the Melt
Hydrogen enters aluminium melts from moisture — the dissociation of water vapour at the melt surface liberates hydrogen, which is immediately absorbed. Moisture sources in a casting operation include wet or incompletely dried charge materials (ingot, returns, scrap), damp tools and ladles that contact the melt, high humidity in the foundry environment, and refractory linings or crucibles that have absorbed moisture. Wet flux additions and inadequately dried core binders that outgas into the die cavity during fill also contribute. Controlling hydrogen requires addressing all of these sources systematically: proper charge drying, preheated ladles and tools, degassing of the melt before pouring using rotary degassing equipment or solid degassing tablets, and careful management of melt holding time (longer holding allows more hydrogen absorption from the atmosphere).
Shrinkage Porosity: The Solidification Problem
Shrinkage porosity has a completely different origin. Aluminium contracts as it solidifies — typically by 3.5–8.5% by volume depending on alloy — and if this contraction is not compensated by feeding liquid metal from a riser or ingate into the solidifying section, voids form in the last regions to solidify. Shrinkage pores are morphologically distinct from gas pores: they are irregular, angular, and often described as spongy or dendritic in appearance when viewed on a polished section, reflecting the shape of the interdendritic spaces from which the final liquid was drawn as the dendrite network solidified. Shrinkage porosity is concentrated in specific, predictable locations — the last-to-solidify sections of the casting, which are determined by the thermal geometry of the part, die, and gating system.
Why Shrinkage Porosity Location Is Predictable
The location of shrinkage porosity is governed by solidification sequence, which in turn is governed by the thermal mass distribution of the casting and the heat extraction capacity of the die at each location. Thick sections solidify last; sections remote from the riser or ingate that cannot be fed from liquid metal reserves solidify into voids. Isolated thick sections — bosses, junctions where walls intersect, sections where internal cores create die-adjacent hot spots — are the canonical shrinkage porosity locations. Once a foundry has mapped the thermal geometry of a casting, the likely shrinkage locations can be predicted with reasonable accuracy before cutting the first trial casting. Thermal simulation software makes this prediction quantitative, but even manual analysis of the casting cross-section with attention to section thickness distribution provides useful guidance.
Diagnosis: Which Type Is Present?
Before attempting to reduce porosity, the foundry must identify which type is present. The key diagnostic tools are visual examination of polished cross-sections (round pores = gas; irregular/angular/interdendritic = shrinkage), X-ray or CT scanning of representative castings (which reveals the three-dimensional distribution and morphology), and correlation with process variables. Gas porosity tends to increase when melt hydrogen content is high — measurable with a reduced pressure test or a commercial hydrogen sensor — and when holding times are long or charge moisture is high. Shrinkage porosity is insensitive to melt hydrogen but responds to riser sizing, die temperature distribution, and gating changes. A process change that reduces one type may have no effect on or even worsen the other — which is why correct diagnosis precedes intervention.
Remediation Strategies
Gas porosity reduction requires melt quality improvement: rotary degassing to hydrogen levels below 0.10–0.12 ml/100g (verified by reduced pressure test density index below 2–3%), charge drying protocols, minimising holding time, and controlling furnace atmosphere. Shrinkage porosity reduction requires solidification engineering: riser resizing or repositioning to ensure feeding reaches the last-to-solidify sections, die temperature adjustment (local die heating at isolated thick sections delays their solidification, equalising the solidification front; local cooling at specific locations can redirect the solidification sequence), chilling of sections that currently solidify last to advance them in the solidification sequence, and in some cases redesign of the gating system to ensure the last-to-fill section is also the last to solidify. Both problems are solvable with systematic process discipline — but only when the correct diagnosis has been made first.