Porosity Control in Aluminum Casting: Degassing, Causes, and Solutions
Porosity is the most common and costly defect in aluminum casting. It can manifest as spherical gas pores, irregular shrinkage cavities, or a combination of both, and its presence drastically reduces mechanical properties, pressure tightness, and surface quality of parts. Understanding the causes, formation mechanisms, and control techniques is essential for any foundry aiming to produce high-integrity castings.
This guide covers the scientific fundamentals of porosity in aluminum alloys, available degassing methods, measurement techniques, and best practices for minimizing this defect.
Types of Porosity in Aluminum
Porosity in aluminum casting is classified into two fundamental categories, although in practice they frequently coexist and amplify each other:
Gas porosity (hydrogen)
Gas porosity originates from the precipitation of dissolved hydrogen during solidification. Gas pores are typically spherical or near-spherical, with smooth and shiny internal surfaces. Their size ranges from micropores (<0.1 mm) to macropores (>1 mm) depending on hydrogen content and solidification rate.
Shrinkage porosity
Shrinkage porosity occurs when liquid metal cannot adequately feed the last areas to solidify. Shrinkage pores are irregular, dendritic, with rough internal surfaces reflecting the dendrite morphology. They tend to be located in last-to-solidify zones: hot spots, section changes, and areas far from feeding gates.
| Characteristic | Gas porosity | Shrinkage porosity |
|---|---|---|
| Morphology | Spherical, rounded | Irregular, dendritic, branched |
| Internal surface | Smooth, shiny | Rough, dendritic |
| Distribution | Dispersed, uniform | Localized in hot spots |
| Typical size | 0.05 – 2 mm | 0.5 – 10+ mm |
| Root cause | Excess dissolved hydrogen | Insufficient feeding |
| Solidification rate effect | Higher rate = finer pores | Higher rate = less interdendritic shrinkage |
| X-ray detection | Rounded diffuse shadows | Irregular, clustered shadows |
| Primary solution | Metal degassing | Feeding system design |
In most real castings, porosity is mixed: hydrogen precipitates preferentially in zones where interdendritic microshrinkage already exists, amplifying the defect. Therefore, even with excellent feeding, a metal with high hydrogen will produce porosity. Conversely, a perfectly degassed metal can still exhibit shrinkage if feeding is inadequate.
Hydrogen Solubility in Aluminum: Sievert's Law
Hydrogen is the only gas with significant solubility in liquid aluminum. Solubility follows Sievert's Law, which states that the solubility of a diatomic gas in a metal is proportional to the square root of its partial pressure:
S = K × √(pH₂)
Where:
S = hydrogen solubility (mL/100g Al)
K = temperature-dependent constant
pH₂ = partial pressure of hydrogen above the bath
The critical aspect is the abrupt solubility drop at the solidification point. In pure aluminum, solubility goes from approximately 0.69 mL/100g in liquid state (just above 660 °C) to merely 0.036 mL/100g in solid state (just below 660 °C), a reduction of nearly 20:1. This rejected hydrogen during solidification is what forms the pores.
| Temperature (°C) | State | Solubility (mL H₂/100g Al) | Solubility (ppm by weight) |
|---|---|---|---|
| 750 | Liquid | 0.92 | 0.82 |
| 720 | Liquid | 0.82 | 0.73 |
| 700 | Liquid | 0.76 | 0.68 |
| 660 (above) | Liquid | 0.69 | 0.61 |
| 660 (below) | Solid | 0.036 | 0.032 |
| 600 | Solid | 0.025 | 0.022 |
| 500 | Solid | 0.012 | 0.011 |
In Al-Si casting alloys, hydrogen solubility in the liquid is lower than in pure aluminum (silicon reduces solubility). However, the wider solidification range of hypoeutectic alloys (such as A356/AlSi7Mg) favors mixed interdendritic porosity formation.
Hydrogen Sources in the Foundry
Hydrogen enters liquid aluminum through the decomposition of hydrogen-containing compounds (primarily water and moisture). The main sources are:
| Source | Mechanism | Relative contribution | Control measure |
|---|---|---|---|
| Atmospheric moisture | H₂O(g) → 2[H] + [O] at bath surface | High | Humidity control, covers |
| Wet metallic charge | Water trapped in returns/ingot | Very high | Preheat to >150 °C, dry storage |
| Tools and crucibles | Adsorbed water in refractories | Medium | Preheating, new crucible curing |
| Fluxes | Hygroscopicity of KCl/NaCl salts | Medium-High | Sealed storage, pre-drying |
| Combustion gas (NG/LPG) | Combustion products H₂O | Medium | Stoichiometric adjustment, oxidizing flame |
| Mold coatings/paints | Organic decomposition | Low-Medium | Complete curing, minimum thickness |
| Contaminated recycled aluminum | Oil, plastic, organics | Variable | Scrap selection and cleaning |
The reaction 2Al(l) + 3H₂O(g) → Al₂O₃ + 6[H] is extremely fast and exothermic. One gram of water can introduce enough hydrogen to create unacceptable porosity in several kilograms of aluminum. In humid climates (>60% RH), hydrogen absorption can double compared to dry conditions if control measures are not taken.
Degassing Methods
Degassing aims to reduce dissolved hydrogen content below the critical level that produces porosity. The fundamental principle is to provide nucleation sites (inert gas bubbles) through which hydrogen can diffuse and be removed from the bath.
Rotary degassing
This is the most widely used and efficient method in the industry. A graphite or SiC rotor spins submerged in liquid aluminum at 200–600 RPM while inert gas (argon or nitrogen) is injected through the shaft. The rotor fragments the gas into thousands of fine bubbles (1–3 mm diameter) that provide enormous surface area for hydrogen diffusion.
| Parameter | Typical range | Optimal |
|---|---|---|
| Rotor speed | 200 – 600 RPM | 350 – 450 RPM |
| Gas flow (Ar or N₂) | 5 – 25 L/min | 8 – 15 L/min |
| Treatment time | 5 – 20 min | 8 – 12 min per 500 kg |
| Rotor immersion depth | 2/3 of bath depth | ~2/3 of bath |
| Target bubble size | 1 – 5 mm | 1 – 3 mm |
| Preferred gas | Ar or N₂ | Ar (for alloys with Mg > 0.3%) |
| Achievable H₂ reduction | 50 – 80% | 60 – 75% in 10 min |
Nitrogen is more economical than argon and works well for alloys without magnesium or with Mg < 0.3%. However, in magnesium-containing alloys (A356, A357, 6xxx), nitrogen can react to form Mg₃N₂ (magnesium nitride), an undesirable compound that degrades metal quality. For these alloys, always use argon.
Tablet degassing
Hexachloroethane (C₂Cl₆) tablets were for decades the most common method in small foundries. When submerged in aluminum, they decompose generating chlorine and other gas bubbles that entrain hydrogen. However, their use is being phased out for environmental and health reasons: they generate dioxins, furans, and HCl, all highly toxic. Modern alternatives include fluoride and carbonate-based tablets that are significantly less toxic.
C₂Cl₆ tablets are banned or severely restricted in the European Union, Japan, and several North American jurisdictions. If you still use them, consider migrating to rotary inert gas degassing. The typical ROI on rotary equipment is recovered in 6–18 months through savings on rejects and consumables.
Vacuum degassing
Vacuum degassing reduces pressure above the aluminum bath, decreasing hydrogen partial pressure and forcing its release from the metal per Sievert's Law. It is very effective but requires specialized equipment (vacuum chamber, sealed crucible). It is used mainly in high-tech foundries for aerospace and high-integrity automotive components.
Porosity Measurement: RPT and Density Index
Continuous measurement and monitoring of hydrogen content and porosity tendency are fundamental for process control. The most widely used methods are:
Reduced Pressure Test (RPT / Straube-Pfeiffer)
The RPT (Reduced Pressure Test) is the most practical and widely used in-plant method. It consists of solidifying an aluminum sample under reduced pressure (typically 80 mbar / 60 mmHg) and comparing its density with that of a sample solidified at atmospheric pressure.
DI (%) = [(ρ_atm − ρ_vac) / ρ_atm] × 100
Where:
ρ_atm = density of sample solidified at atmospheric pressure
ρ_vac = density of sample solidified at reduced pressure
Typical acceptance criteria:
DI < 1% = Excellent – suitable for safety parts
DI 1–2% = Good – suitable for most applications
DI 2–4% = Acceptable – only for non-critical applications
DI > 4% = Unacceptable – requires additional degassing
| Dissolved H₂ (mL/100g) | H₂ (ppm) | Typical DI (%) | Expected quality |
|---|---|---|---|
| < 0.10 | < 0.09 | < 1.0 | Excellent – safety parts, pressure-tight |
| 0.10 – 0.15 | 0.09 – 0.13 | 1.0 – 2.0 | Good – quality general casting |
| 0.15 – 0.25 | 0.13 – 0.22 | 2.0 – 5.0 | Marginal – porosity risk in thick sections |
| 0.25 – 0.40 | 0.22 – 0.36 | 5.0 – 10.0 | Poor – visible porosity likely |
| > 0.40 | > 0.36 | > 10.0 | Unacceptable – metal not suitable for casting |
Direct hydrogen measurement
Equipment such as Alscan (ABB), Hyscan, and Telegas directly measure dissolved hydrogen content in liquid aluminum using electrochemical or gas recirculation sensors. They provide readings in mL H₂/100g Al with ±0.02 mL/100g precision. They are more accurate than RPT but more costly and require regular calibration.
Effect of Porosity on Mechanical Properties
Porosity acts as an internal stress concentrator that significantly reduces mechanical properties, especially elongation to fracture and fatigue resistance. The effect is nonlinear: a small increase in porosity can cause a disproportionate drop in properties.
| Volumetric porosity (%) | UTS (MPa) | Rp0.2 (MPa) | Elongation (%) | Fatigue life (relative) |
|---|---|---|---|---|
| 0 (theoretical) | 310 | 260 | 8–12 | 100% |
| 0.5 | 290 | 250 | 5–8 | 70% |
| 1.0 | 270 | 240 | 3–5 | 45% |
| 2.0 | 240 | 220 | 1.5–3 | 25% |
| 5.0 | 190 | 180 | <1 | 10% |
Elongation is the property most sensitive to porosity, followed by fatigue resistance. Yield strength (Rp0.2) is relatively less affected because porous zones have already yielded before the macroscopic yield stress is reached. For safety components (steering knuckles, engine brackets), the typical minimum elongation specification is 5–7%, which requires volumetric porosity < 0.5%.
Influence of Alloy Composition
Alloy composition affects porosity tendency through several mechanisms:
| Element | Effect on H₂ solubility | Effect on solidification range | Net porosity impact |
|---|---|---|---|
| Si (7–12%) | Reduces solubility in liquid | Reduces range (narrow eutectic) | Beneficial – lower tendency |
| Mg (0.3–0.5%) | Slightly increases | Widens range | Detrimental – higher tendency |
| Cu (1–4%) | Little effect | Significantly widens range | Detrimental – more microporosity |
| Fe (0.1–1.0%) | Little effect | Forms intermetallics that block feeding | Detrimental – interdendritic porosity |
| Sr (modifier, 200 ppm) | Increases H₂ absorption | Modifies eutectic | Detrimental if H₂ not controlled |
| Na (modifier) | Increases H₂ absorption | Modifies eutectic | Detrimental if H₂ not controlled |
| Ti+B (refiner) | No significant effect | Refines grain | Beneficial – reduces pore size |
Adding strontium (Sr) to modify eutectic silicon morphology is standard practice in Al-Si alloys. However, Sr increases the tendency for hydrogen absorption and can increase porosity if the metal is not well degassed. It is essential to degas BEFORE adding the Sr modifier, and perform a second RPT verification after the addition.
Best Practices for Porosity Control
Charge and melting control
- Preheat all metallic charge to a minimum of 150 °C to eliminate surface moisture. Returns (internal scrap) must be clean, dry, and oil-free.
- Use certified quality ingot with hydrogen < 0.12 mL/100g. Request density or hydrogen certificate from the supplier.
- Minimize return/virgin ingot ratio. Returns accumulate oxides and hydrogen with each remelt. Target: maximum 50% returns.
- Control furnace atmosphere. Slightly oxidizing flame to minimize H₂ generation from incomplete combustion.
- Avoid overheating the metal. Every 10 °C above the necessary temperature increases hydrogen absorption. Target: minimum operating temperature + 20 °C margin.
Degassing process
- Degas with inert gas rotor as standard practice. Minimum time: 8 minutes per 500 kg of metal.
- Verify with RPT before and after degassing. Target: DI < 2% for general casting, DI < 1% for safety parts.
- Maintain the rotor in good condition. A worn rotor produces large bubbles (>5 mm) that are inefficient. Replace the rotor when diameter reduces by more than 15%.
- Do not excessively agitate the surface. A deep vortex incorporates air and oxides. The surface should move gently without breaking the dross layer.
- Skim (dross removal) after degassing. Remove dross containing oxides and gas entrapments before pouring.
Pouring and solidification
- Pour at the lowest temperature possible that allows complete mold filling. Lower temperature = less dissolved hydrogen = less porosity.
- Design the feeding system for directional solidification. Risers must solidify last and feed the casting sections.
- Use ceramic filters in the gating system to trap oxides and inclusions that act as pore nucleation sites.
- Minimize turbulence during filling. Turbulent filling incorporates air and generates bifilms (folded oxides) that nucleate porosity.
- Apply pressure during solidification when possible (squeeze casting, low pressure, HPDC with intensification). Pressure suppresses pore nucleation.
Relationship Between Ingot Quality and Final Porosity
The quality of the feed ingot (primary ingot) is the starting point for porosity control. An ingot with high hydrogen content, internal oxides, or out-of-spec composition compromises quality from the start, requiring greater degassing effort with less predictable results.
| Parameter | Standard | Premium | Aerospace |
|---|---|---|---|
| Hydrogen (mL/100g) | < 0.20 | < 0.15 | < 0.10 |
| RPT Density Index (%) | < 3.0 | < 1.5 | < 0.8 |
| Oxides (inclusions per cm²) | < 0.5 mm²/cm² | < 0.2 mm²/cm² | < 0.05 mm²/cm² |
| Chemical analysis | Within standard | Center of range | Center ± 1/4 range |
| Certification | Chemical cert. | Cert. + H₂ | Cert. + H₂ + PoDFA/Prefil |
Conclusion
Porosity control in aluminum casting requires a comprehensive approach spanning from feed ingot selection to solidification conditions in the mold. Dissolved hydrogen is the primary cause of gas porosity, and its control through rotary inert gas degassing is the most effective tool available. However, degassing alone does not eliminate shrinkage porosity, which requires proper feeding system design.
Investment in measurement equipment (RPT, Alscan) and degassing (rotor) pays for itself quickly through reduced rejects, improved mechanical properties, and the ability to meet increasingly stringent specifications in the automotive and aerospace sectors. For ingot buyers, requiring hydrogen and density certification from the supplier is the first step toward cleaner metal and a more profitable foundry.
Inquire About Ingot Quality
Our engineering team is ready to help you find the ideal solution for your application.