Aluminum Silicon Strontium

Alloy Description:

No ImageAluminum Silicon Strontium master alloy is the alloy of Aluminium, Silicon and Strontium which is used as structural modifier of AlSi eutectic and hypo-eutectic casting alloy. AISiSr addition modifies the structure of eutectic and primary silicon and the modification mechanism is attributed to changes in phase equilibria, modified nucleation and growth kinetics. The chemical composition and physical properties of the Aluminium Silicon Strontium master alloy is given below:

Chemical composition:

Alloy Grade Chemical composition (%)
Sr Si Fe(max) Others (max) AI
AISiSr 5 4.5 - 5.5 6.5 - 7.5 0.25 0.5 Balance
AISiSr 10 9 - 10 14 - 16


0.5 Balance

Other impurities- Mn, Ca, Zn, Na, Si, Pb **above hardeners are produced from 99.7% (Min.) purity of Al.

Physical properties:

Alloy Grade Density (gm/cc) Melting point (°C)
AISiSr 2.64 - 2.70 700 - 710*

* melting point is not a relevant in normal use, the majority of master alloy and tablets produced by Minex Metallurgical Co. Ltd., dissolve in Aluminium rather than melt.

Addition technique:

Remove heavy dross from melt surface and add appropriate amount of AlSiSr master alloy (At T 720 - 770°C) to the molten metal. Addition can be made either at treatment furnace or transfer ladle. After the dissolution of alloy, good stirring of melt is required to maximize modification and recovery.

Modifiers are added to eutectic and hypo-eutectic Al-Si alloys to refine the eutectic Si phase from one of angular platelets to fine fibers. This change in microstructure results in improved mechanical properties particularly strength and ductility aiding refinement in porosity.

The first hypo-eutectic modifiers were based on Na, and are added as Na metal or encapsulated Na in Al foil. Due to high reactivity and low solubility of Na in Aluminum (recoveries low up to -20%) and gradual loss of Na with time (fade) results in process difficulties. The modifying action of Na practically disappears only after few remelts, where as modifying effect of strontium does not fade on holding at elevated temperatures.


Sodium Metal

Al-Si-Sr Alloy


Low Variable and operator dependent

High and consistent

Contact time

Fast (20-40 min)

Slow (>2 Hrs)


Fumes Refractory attack Clean and fume free


Possible due to


Potency Addition Level Ease Handling


difficulty of control Very High 50 ppm

Must be protected from moisture 

High 250 ppm Unreactive with moisture if <20% strontium master alloy

No ImageNo ImageIn unmodified alloy, the silicon grows as plates that can be lamellar or acicular. Lamellar structures are nearly parallel and thin plates, while acicular structures are randomly distributed plates with much massive structure. At an unmodified condition, thin silicon plates can be seen cutting through the aluminum matrix. (fig.1)

Figure 1: Unmodified alloy (0.0003%Sr) lamellar structure (a) 200x (b)1500x

No ImageNo ImagePartially modified structures are the coexistence of flake and fibrous silicon mixed, cellular or banded structure. They can be obtained by a combination of slow solidification rates (bet. 1 & 25 m/s) and low strontium concentration. At low strontium concentration and during slow growth, the eutectic is able to reject enough strontium ahead its interface to avoid transforming to fibrous morphology. (fig.2)

Figure 2: Partially modified structure (0.0026%Sr) (a) 200x (b)SEM deep itched 1500x

No ImageNo ImageWhen the addition of strontium exceeds the amount necessary to produce a fully modified structure, it exerts a deleterious effect on the mechanical properties of the alloy. Strontium overmiodification is much more subtle and difficult to identify by microstructural analysis. In overmodified alloys, elongated silicon particles that no longer grow as well as refined fibers can be observed. However the formation of Sr-rich phase is reported in strontium concentration above 0.03%

Figure 3: Fully modified structure (0.0075%Sr) (a) 200x (b)SEM deep itched 1000x

No ImageNo ImageIn well modified structure silicon grows in a coral-like form (fibrous) that looks like small individual round-shaped particles in polished surface. The modification treatment affects not only size and shape of the particle, but also its distribution. Ideal concentration of strontium results in a fibrous eutectic structure, well refined and homogenous. (fig.3)

Figure 4: Over modified structure (0.0130%Sr) (a) 200x (b)SEM deep itched 1000x

The average mechanical property results after the various treatments are shown in Fig. 12 these show that the melt treatments lead to improved mechanical properties in GDC A319 alloy. The addition of modifier gives an increase in UTS and elongation values, treatments have resulted in reduction in scatter as evidenced by an increases in Weibull modulus from 5.9 (untreated) to 17.6 (modified) (Fig.6). Similarly the Fig.5 shows an increased fracture toughness value for Sr-modified alloy A357 compared to unmodified alloy.

No ImageNo Image

Forms Std. Size (mm) Std. Weight (gms)
Piglet 60×45×45 250±10
Waffle plate   8 – 10
Piglet Waffle plate
25 Kg gunny bag/500 Kg Jumbo Bag (or as per customer requirement)

500 Kg strapping with palette (or as per customer requirement)

Use cool and dry place to store the material.

Material Safety Data Sheet can be supplied on demand. Material is not hazardous but normal safety precautions to be followed.