Master Alloys for Aluminum: Grain Refinement and Eutectic Modification — Practical Guide
Introduction: The Critical Role of Master Alloys
In aluminum casting, the metallurgical quality of the final part depends heavily on two liquid metal treatments: grain refinement and eutectic modification. Both are achieved by adding master alloys to the metal bath before pouring. These concentrated titanium-boron and strontium alloys act at the microstructural level to transform aluminum solidification, producing parts with superior mechanical properties, better feeding, and reduced hot tearing tendency.
This guide covers the metallurgical fundamentals, practical differences between the most common master alloys, and best practices for dosing and quality control in foundries processing Al-Si casting alloys.
Why Does Grain Refinement Matter?
During aluminum solidification, crystal grains grow from nuclei in the liquid metal. Without treatment, these grains can grow as large columnar dendrites, especially in thick sections or slow cooling. A coarse grain structure leads to multiple problems:
- Lower mechanical strength: the Hall-Petch relationship establishes that yield strength increases inversely with the square root of grain size. A fine grain of 200 µm vs. a coarse one of 2000 µm can mean 15-25% higher yield strength.
- Greater hot tearing tendency: columnar grains create long interfaces where shrinkage stresses concentrate. Fine equiaxed grain distributes these stresses uniformly.
- Poorer feeding: a coarse dendritic structure blocks liquid metal flow toward shrinkage zones, increasing shrinkage microporosity.
- Anisotropic properties: columnar grains generate mechanical properties that vary by direction, unacceptable in structural components.
- Poorer surface finish: a coarse grain is visible after machining or anodizing, especially problematic in decorative parts.
ASTM E112 classifies grain size. For structural aluminum parts, a grain size of ASTM 5-7 (average diameter 60-200 µm) is typically targeted. Without refinement, Al-Si alloys can exhibit grains of 1-5 mm.
Grain Refinement Mechanism with AlTi5B1
The master alloy AlTi5B1 (5% titanium, 1% boron, balance aluminum) is the industry standard grain refiner. Its effectiveness relies on two complementary mechanisms:
Heterogeneous Nucleation on TiB₂ Particles
AlTi5B1 contains titanium diboride (TiB₂) particles of 1-5 µm diameter, uniformly dispersed in the aluminum matrix. These particles are thermodynamically stable in liquid aluminum (TiB₂ melting point: ~3225 °C) and act as heterogeneous nucleation sites for α-aluminum grains. The effectiveness of TiB₂ as a nucleant is due to its low crystallographic mismatch with aluminum (< 5% on the basal plane), which reduces the energy barrier for nucleation.
Al-Ti Peritectic Reaction
Excess dissolved titanium (above TiB₂ stoichiometry) generates an Al₃Ti layer on the surface of TiB₂ particles. This Al₃Ti phase is an even more potent nucleant than pure TiB₂, thanks to the peritectic reaction: Liquid + Al₃Ti → α-Al at 665 °C. Thus, each TiB₂ particle coated with Al₃Ti becomes an extremely efficient nucleation site.
In a typical addition of 1 kg/ton of AlTi5B1, approximately 10⁹ TiB₂ particles are introduced per kilogram of aluminum. However, not all nucleate a grain: actual efficiency is 1-5%, as constitutional undercooling limits how many nuclei can grow simultaneously. Still, this is sufficient to produce a fine equiaxed structure.
AlTi10 vs AlTi5B1: When to Use Each
AlTi10 (10% titanium, no boron) is a simpler composition master alloy used primarily as a titanium source to adjust alloy chemical composition. Unlike AlTi5B1, AlTi10 does not contain preformed TiB₂ particles, so its grain refinement capability is significantly lower.
| Characteristic | AlTi5B1 | AlTi10 |
|---|---|---|
| Composition | 5% Ti, 1% B, bal. Al | 10% Ti, bal. Al |
| Primary mechanism | TiB₂ nucleation + peritectic | Peritectic reaction only (Al₃Ti) |
| Relative refinement efficacy | High (reference) | Low-medium (30-50% of AlTi5B1) |
| Primary use | Grain refinement | Ti composition adjustment |
| Typical form | Cut rods, waffles | Waffles, small ingots |
| Dissolution rate | Fast (2-5 min at 730 °C) | Medium (5-10 min at 730 °C) |
| Fading effect | Moderate (TiB₂ settles slowly) | High (Al₃Ti dissolves rapidly) |
| Relative cost | Higher | Lower |
Use AlTi5B1 as primary grain refiner for all Al-Si casting alloys. Reserve AlTi10 for adjusting Ti content when the alloy specification requires >0.10% Ti (e.g., A356.2 specifying 0.04-0.20% Ti) or when you need to add Ti without introducing additional boron.
Eutectic Modification with Strontium (AlSr)
In hypoeutectic Al-Si alloys (5-12% Si), the eutectic Si phase constitutes 30% to 60% of the microstructure volume. Without modification, eutectic silicon solidifies as acicular plates (needle-like), which act as stress concentrators and drastically reduce ductility and fatigue resistance.
Adding strontium (Sr) to the aluminum bath changes eutectic silicon morphology from acicular to fibrous. Modified Si fibers are shorter, rounder, and interconnected, significantly improving mechanical properties:
| Property | Unmodified | Sr Modified | Change |
|---|---|---|---|
| Si morphology | Acicular (plates) | Fibrous | — |
| Tensile strength (MPa) | 160-180 | 190-220 | +15-25% |
| Elongation (%) | 2-4 | 5-9 | +100-150% |
| Fatigue limit (MPa) | 50-60 | 65-80 | +25-35% |
| Machinability | Good | Very good | Better finish |
| Eutectic temperature | 577 °C | 571-574 °C | -3 to -6 °C |
Modification Mechanism
Strontium modifies the Al-Si eutectic through a crystal growth poisoning mechanism. Sr atoms adsorb preferentially on growth steps of Si particles, blocking the orderly addition of silicon atoms and forcing frequent branching. This changes Si growth from faceted (flat plates) to non-faceted (curved fibers). The mechanism is known as IIT (Impurity Induced Twinning), where Sr promotes twin formation in Si that redirects crystal growth.
AlSr10 vs AlSr15: Dosing and Retention
The two most common strontium master alloys are AlSr10 (10% Sr) and AlSr15 (15% Sr). The choice between them depends on the casting process, production volume, and inventory control strategy.
| Characteristic | AlSr10 | AlSr15 |
|---|---|---|
| Sr content | 10% | 15% |
| Typical form | Cut rods, waffles | Cut rods, waffles |
| Dissolution temperature | 720-740 °C | 720-740 °C |
| Dissolution rate | Fast (3-5 min) | Slightly slower (4-7 min) |
| Typical dosing | 0.5-1.5 kg/ton | 0.3-1.0 kg/ton |
| Target residual Sr | 80-250 ppm | 80-250 ppm |
| Sr recovery | 80-90% | 75-85% |
| Main advantage | Higher recovery, more uniform dissolution | Less material per addition |
Excess Sr (> 300 ppm residual) causes formation of Al₂Si₂Sr intermetallic compounds that appear as coarse particles in the microstructure and can degrade mechanical properties. Additionally, high Sr levels increase hydrogen porosity. Keep residual Sr between 80-250 ppm depending on part type.
The Fading Effect and Re-addition Strategies
Both the grain refinement effect and eutectic modification degrade over time after addition to the bath. This phenomenon is known as fading and is one of the most important operational challenges in the foundry.
Grain Refinement Fading
TiB₂ particles, having higher density (4.52 g/cm³) than liquid aluminum (2.38 g/cm³), tend to slowly settle to the bottom of the furnace or crucible. Settling velocity depends on particle size and bath convection. In a holding furnace without agitation, the refining effect can decrease by 50% in 30-60 minutes. With periodic stirring (manual or with degassing rotor), the effect is maintained for 2-4 hours.
Strontium Modification Fading
Strontium is lost from the bath through two main pathways: (1) surface oxidation, as Sr has high oxygen affinity and forms SrO that passes to the dross, and (2) volatilization at holding temperatures. The loss rate is approximately 10-20 ppm/hour at 720 °C. Using cover fluxes and minimizing surface turbulence reduces this loss.
| Strategy | Application | Effectiveness |
|---|---|---|
| Late addition | Add master alloy 10-15 min before pouring | High — minimizes exposure time |
| Periodic stirring | Stir bath every 20-30 min with rotor or ladle | Medium — resuspends settled TiB₂ |
| Scheduled re-addition | Add 30-50% of initial dose every 1-2 hours | High — maintains constant levels |
| Cover flux | Flux layer on bath surface (NaCl-KCl) | Medium — reduces Sr oxidation |
| Continuous thermal analysis | Monitor eutectic ΔT to decide Sr re-addition | Very high — based on objective data |
| Minimum temperature | Maintain bath at ≤730 °C when possible | Medium — reduces Sr oxidation rate |
Interaction Between Grain Refinement and Modification
There is a known interaction between titanium and strontium that must be managed. At elevated Ti concentrations (> 0.15%), formation of Ti-Sr or Ti-B-Sr compounds can occur, reducing the efficacy of both the refiner and the modifier. This phenomenon is known as mutual poisoning and manifests as:
- Partial reduction of eutectic modification (silicon shows partially modified morphology)
- Decreased grain refiner efficacy (coarser grains than expected)
- Formation of complex sediments (sludge) at the furnace bottom
To minimize negative interaction: (1) add the grain refiner (AlTi5B1) first and allow 5-10 minutes for homogenization, (2) then add the modifier (AlSr), (3) keep total Ti < 0.15% and residual Sr < 250 ppm. Under these conditions, both treatments coexist without issues.
Dosing Guidelines
Correct dosing depends on the alloy, casting process, part geometry, and mechanical property requirements. The following tables provide starting ranges that should be adjusted through thermal analysis and metallographic evaluation.
| Process | Dosing (kg/ton) | Target residual Ti | Notes |
|---|---|---|---|
| Sand casting | 1.0-2.0 | 0.10-0.20% | Slow cooling requires more nucleants |
| Permanent mold (gravity) | 0.5-1.5 | 0.08-0.15% | Moderate cooling helps |
| High pressure die casting (HPDC) | 0.5-1.0 | 0.05-0.12% | Rapid cooling contributes to fine grain |
| Investment casting | 1.0-2.0 | 0.10-0.20% | Mold preheat reduces cooling rate |
| Continuous casting (billets) | 1.0-1.5 | 0.01-0.03% B | Continuous addition in trough |
| Alloy | Target residual Sr (ppm) | AlSr10 dosing (kg/ton) | Notes |
|---|---|---|---|
| A356 / AlSi7Mg | 100-200 | 0.5-1.2 | Standard for automotive |
| A357 / AlSi7Mg0.6 | 120-220 | 0.6-1.4 | Higher Mg consumes some Sr |
| AlSi10Mg | 150-250 | 0.8-1.5 | More Si requires more Sr |
| AlSi12 | 150-250 | 0.8-1.5 | Eutectic alloy, high response |
| AlSi5Cu3 | 80-150 | 0.4-0.8 | Cu reduces Sr response |
Quality Control: Thermal Analysis
Thermal analysis (cooling curve) is the most practical and rapid tool for verifying both grain refinement and eutectic modification on the production line. A metal sample is poured into an instrumented crucible with a thermocouple, and the temperature-time curve is recorded during solidification.
Cooling Curve Indicators
- Nucleation undercooling (ΔTn): difference between the minimum temperature before recalescence and the α-Al growth temperature. With good grain refinement, ΔTn < 0.3 °C. Without refinement, ΔTn > 1 °C.
- Eutectic temperature (TEut): with complete Sr modification, eutectic temperature drops 3-6 °C from unmodified eutectic (~577 °C to 571-574 °C in Al-7Si).
- Eutectic depression (ΔTEut): difference between unmodified and modified TEut. ΔTEut > 4 °C indicates complete modification; ΔTEut of 2-4 °C indicates partial modification.
- Solidification time: finer grain produces curves with less recalescence and shorter local solidification time.
Commercial systems such as ATAS (Novacast), MeltLab, and Thermolan offer automated cooling curve analysis with real-time interpretation of modification and refinement levels. The investment pays for itself quickly by reducing rejects and optimizing master alloy consumption.
Best Practices Summary
- Store master alloys in a dry location — moisture degrades AlTi5B1 rod quality and can introduce hydrogen to the bath.
- Pre-cut rods to the appropriate size for the planned addition. Do not partially submerge long rods.
- Add AlTi5B1 first, wait 5-10 min, then add AlSr. Never mix both master alloys outside the bath.
- Submerge rods into the bath with a bell or tongs that keep them submerged until completely dissolved.
- Perform thermal analysis at the start of each shift and after each significant re-addition.
- Record master alloy consumption per ton of aluminum processed — anomalous variations indicate process problems.
- Maintain bath temperature as low as possible (710-730 °C) to minimize Sr fading and reduce hydrogen absorption.
- In continuous production, schedule AlSr re-additions every 1-2 hours based on thermal analysis.
Conclusion
Master alloys for grain refinement and eutectic modification are indispensable tools for any aluminum foundry seeking to produce parts with consistent, reliable mechanical properties. The combination of AlTi5B1 for fine grain and AlSr10/AlSr15 for fibrous silicon, applied with correct dosing and verified through thermal analysis, is the foundation of quality production in Al-Si alloys.
At Transformación Puebla we offer the full range of master alloys in practical presentations for foundries of any size. Our technical team can advise you on optimal dosing for your specific process.
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