Silicon Metal for Aluminium Alloys: Grades and Casting Benefits

What Silicon Does in an Aluminium Melt

Silicon is the principal alloying element in the largest family of casting aluminium alloys, and it is added for a combination of metallurgical and process reasons that are easy to state and hard to substitute. In solid aluminium, silicon has limited solubility: the maximum solid solubility is about 1.65% at the eutectic temperature of roughly 577 degrees Celsius, and above that level the excess appears as silicon phase in the microstructure. The binary Al-Si system is therefore mildly eutectic, with the eutectic composition at approximately 12.6% Si, and the practical working range of commercial alloys extends from about 5% Si in filler and wrought material to 18% and above in hypereutectic casting alloys for wear-resistant components.

Silicon metal supplied for this purpose is elemental crystalline silicon, silver-grey in colour, delivered as lumps, granules or powder. It is charged to the melt either directly as silicon lumps or absorbed from a master alloy. In both cases the material must be sized for the charging method and must be chemically consistent, because iron and calcium in the silicon addition enter the melt and cannot easily be removed afterwards. That single fact drives most of the purchasing decisions in this application.

Grades of Silicon Metal Selected for Aluminium Alloying

Furnace silicon is graded by its iron, aluminium and calcium content under GB/T 2881, with the grade digits encoding those three elements. Aluminium alloy melting shops generally favour the lower-digit grades because iron and calcium are harmful rather than merely indifferent: iron forms brittle intermetallic phases, and calcium promotes gas porosity and dross. The table below shows the customary limits and the way different alloy families use them.

Grade (GB/T 2881 basis)	Fe max, %	Al max, %	Ca max, %	Typical use in aluminium melting
Si 553	0.50	0.50	0.30	Secondary die casting alloys where Fe is already balanced by Mn
Si 441	0.40	0.40	0.10	General casting alloys, gravity and low-pressure casting
Si 3303	0.30	0.30	0.03	Higher-integrity castings, low-iron specifications
Si 2202	0.20	0.20	0.02	Wrought and filler alloys where ductility matters
Si 1101	0.10	0.10	0.01	Critical filler wire and high-purity master alloys

Grade selection is not decided by the silicon level required in the final alloy but by the impurity budget. A foundry targeting 0.20% Fe in a gravity casting cannot use a silicon addition containing 0.5% Fe unless scrap dilution provides enough headroom. Working backwards from the iron target, the charge calculation and the silicon grade have to be chosen together.

Al-Si Alloy Families and Their Silicon Ranges

The commercial significance of silicon varies sharply across the alloy families, and understanding where a given silicon addition will be consumed helps in setting the specification. The following ranges are typical of the common designations rather than limits of any single standard.

Alloy family	Typical Si content, %	Why silicon is present
A356 type (Al-7Si-0.3Mg)	6.5-7.5	Castability plus response to heat treatment through magnesium
A360 type	9.0-10.0	Better fluidity and pressure tightness for die casting
A380 type	7.5-9.5	General purpose die casting with copper for strength
4043 filler (Al-5Si)	4.5-6.0	Lower melting point and improved weld pool fluidity
4047 filler (Al-12Si)	11.0-13.0	Near-eutectic brazing and welding, minimal hot cracking
A390 type (hypereutectic)	16.0-18.0	Primary silicon particles for wear resistance in cylinder bores

Two practical boundaries follow. Below roughly 5% Si, alloys remain ductile and are used in wrought products and filler wire, where low iron is essential. Above about 12.6% Si, the alloy becomes hypereutectic and primary silicon crystals form before the eutectic; those crystals provide wear resistance but must be refined in size or they become crack initiation sites.

Fluidity, Feeding and Casting Behaviour

The processing benefit of silicon is most obvious in the mould. Increasing silicon content lowers the liquidus temperature and narrows the freezing range of hypoeutectic alloys, which improves fluidity and allows thinner sections and more intricate shapes to fill before the metal freezes. Near the eutectic composition the alloy freezes over a very short interval, which improves feeding and reduces dispersed shrinkage porosity, though it increases the need for risers in heavy sections because the alloy contracts sharply on solidification.

Silicon also reduces the density of the liquid: pure aluminium is about 2.7 g/cm3 while a near-eutectic Al-Si melt is around 2.5 g/cm3. This matters in two ways. Less mass is poured for the same volume, and the density difference between the melt and the oxide and dross layer changes, which affects skim removal in the furnace and transfer ladle. Silicon additions also change the character of the surface oxide film and can influence the amount of dross generated, so melting practice should be adjusted when the silicon level is raised rather than assumed to remain unchanged.

Silicon in solid solution and as eutectic phase raises hardness and wear resistance and reduces thermal expansion, which is why silicon-rich alloys are chosen for pistons, cylinder heads and compressor parts. The trade-off is machinability and ductility: coarse silicon phase increases tool wear and reduces elongation, and the balance between these properties is set by silicon content and by the modification and refinement practice applied at the melt.

Eutectic Modification and Primary Silicon Refinement

In untreated hypoeutectic Al-Si alloys the eutectic silicon appears as coarse plates and needles that act as internal stress raisers. Modification is the deliberate addition of a small amount of a modifying element, normally sodium or strontium, to change the eutectic silicon morphology to a fine fibrous form. Modifier additions are small, typically in the region of a few thousandths of a percent to about two hundredths of a percent in the melt, and the exact level depends on the alloy, the cooling rate and the holding time, because strontium fades slowly while sodium fades quickly. A modified structure usually shows improved elongation and fatigue resistance at a small cost in fluidity.

In hypereutectic alloys the problem is different. Primary silicon crystals precipitate first and can grow to tens or hundreds of micrometres, forming hard particles that improve wear resistance but can crack under thermal cycling. Phosphorus is used here as a refiner: it forms aluminium phosphide nuclei that promote a larger number of smaller primary silicon crystals. Refinement practice and the level of phosphorus added are usually set against a metallographic acceptance criterion for maximum primary silicon particle size, so the foundry specification should be based on measured particle size rather than on the addition rate alone.

Silicon metal itself is supplied in commercial grades and does not carry a modifying or refining effect. Modification and refinement are added separately at the melt, and mixing up these three functions refinement, modification and alloying is a frequent source of casting defects.

Charging and Dissolution of the Silicon Addition

Silicon dissolves by contact with the melt, and dissolution is a time and temperature dependent process rather than an instantaneous one. In practice there are two charging routes. The first is direct addition of silicon metal lumps or granules into the melt, where the material is usually added near the end of the melt cycle, pushed beneath the surface, and given enough time and temperature for complete dissolution. The second is the use of an aluminium-silicon master alloy, commonly around 50% Si, which dissolves quickly and allows the silicon level to be trimmed accurately with a smaller temperature penalty.

When choosing between them, consider three factors. Charging method: bulk lump silicon is economical but requires a charging door and bath agitation, while master alloy suits small ladle additions and precise adjustment. Dissolution losses: fine silicon powder oxidises and can be lost to the dross, so the powder fraction should be limited unless the process is designed for it. Temperature practice: raising the bath temperature to dissolve large lumps increases oxidation loss and gas pickup, which is why sizing matters more than the price per tonne of the addition.

Sizing is usually agreed between buyer and supplier as a nominal range with a fines limit, and particle size is verified by sieving against the applicable particle size standard. For direct charging, lump sizes of roughly 10-100 mm are common, while 25-50 mm and finer fractions suit automatic feeding. Whatever the loose size range, the delivered material should be free of fines that blow out or float and of oversized pieces that will not dissolve within the melt cycle.

Iron, Calcium and the Impurity Balance

Iron is the impurity that limits how much silicon-grade flexibility a foundry has. In Al-Si alloys, iron forms the beta phase Al5FeSi, which appears as thin brittle needles that lower ductility and impact toughness and increase porosity in thick sections. Iron can be partially neutralised by adding manganese, which promotes the more compact alpha AlFeSi script morphology instead, but the neutralisation consumes manganese and adds its own cost and density penalty, and it does not remove the iron from the alloy. Where the scrap charge already carries iron, a lower-iron silicon grade is often the cheapest way to stay inside the iron limit.

Calcium and other alkali and alkaline earth metals in the silicon addition are associated with increased gas porosity and dross volume, which is why the calcium limit in the silicon grade matters as much as the iron limit for critical castings. Hydrogen pickup, which is fundamentally governed by melt treatment and degassing practice, can be aggravated by a wet or oxidised charge; silicon stored in damp conditions will introduce moisture directly into the melt. Aluminium and calcium in the addition are relatively benign for the alloy composition itself, but both consume refining effort and both indicate a less well-controlled furnace product.

Compositional acceptance of the finished alloy follows the applicable standards rather than the supplier certificate for the raw metal. Cast aluminium alloys are covered by GB/T 1173, die casting aluminium alloys by GB/T 15115, wrought aluminium alloys by GB/T 3190 and the corresponding international composition standard ISO 3522 for aluminium alloy castings. Each of these sets limits on silicon, iron, copper, magnesium and other elements, and the incoming silicon must be selected so that the final analysis fits inside them with margin for melt losses and additions.

Specifying and Verifying Silicon Metal for Aluminium Melts

A workable purchase specification for this application carries the following items: the GB/T 2881 grade designation together with iron, aluminium and calcium maxima; the minimum silicon content; total metallic impurity ceiling where relevant; delivery form and size range with a fines limit; packaging type, unit mass and lot marking; and the document set to be supplied with each lot.

Verification rests on sampling and analysis. Bulk sampling of ferroalloys and related materials follows the framework of GB/T 4010 and ISO 3713, which require increments taken from multiple positions across the lot and reduced by crushing and riffling to a laboratory sample; methods for the determination of iron, aluminium and calcium in silicon metal are covered by the GB/T 14849 series. Packaging practice, including liner and unit marking, follows GB/T 3650 for ferroalloys.

Three acceptance habits repay the effort. Keep the retained sealed sample from each lot so that any dispute can be settled by umpire analysis rather than by argument. Track incoming analysis against melt results over several lots, because iron and calcium trends in the silicon feed show up in casting rejects before they show up in any single analysis. Review the size distribution and fines content together with the chemistry, since an on-specification chemistry with excessive fines produces off-specification melt recovery rather than an off-specification alloy.

Finally, store the material under cover on a dry floor, in lined bulk bags with the liner sealed, away from chloride sources and away from ferrous scrap handling areas. Silicon metal is taken into the melt essentially whole, so anything that reaches the bag reaches the casting.

FAQ

Q: What silicon content marks the eutectic in aluminium alloys?
The Al-Si eutectic is at approximately 12.6% Si with a eutectic temperature of about 577 degrees Celsius, and the maximum solid solubility of silicon in aluminium is about 1.65% at that temperature. Alloys below the eutectic are hypoeutectic with primary aluminium; alloys above it are hypereutectic with primary silicon.

Q: Why is low-iron silicon metal preferred for aluminium alloys?
Iron forms the beta Al5FeSi phase, which appears as brittle needles that reduce ductility and impact toughness. Manganese additions can promote the more compact alpha morphology, but they do not remove the iron, so limiting iron in the silicon feed is often the most economical way to hold the alloy inside its iron limit.

Q: What size of silicon metal dissolves best in an aluminium melt?
Lump sizes of roughly 10-100 mm are common for direct charging, with narrower sized fractions such as 25-50 mm for automated feeding systems. The size range should be agreed with a fines limit, because fine material oxidises and is lost to the dross before it dissolves.

Q: Should silicon metal be dosed directly or as a master alloy?
Direct lump charging suits bulk melting where there is a charging door, bath agitation and enough time at temperature. An aluminium-silicon master alloy, commonly around 50% Si, dissolves faster and allows more accurate trimming of the silicon level in the ladle with less temperature penalty.

Q: How does modification differ from grain refinement of primary silicon?
Modification uses sodium or strontium in the region of a few thousandths to about two hundredths of a percent to convert coarse eutectic silicon plates into a fine fibrous form in hypoeutectic alloys. Phosphorus is used in hypereutectic alloys to refine primary silicon crystals, and acceptance is judged by measured maximum particle size rather than by the addition rate.

Q: Which standards govern the alloy composition that silicon additions must fit?
Cast aluminium alloys are specified under GB/T 1173, die casting aluminium alloys under GB/T 15115, wrought aluminium alloys under GB/T 3190, and aluminium alloy castings internationally under ISO 3522. The silicon metal itself is graded under GB/T 2881, sampled under GB/T 4010 and ISO 3713, analysed under the GB/T 14849 series and packaged under GB/T 3650.