Aluminium-silicon casting alloys

 

The effects of silicon

• Silicon in aluminium alloys much improved their casting characteristics.
• Additions of silicon to pure aluminium very much improved fluidity, hot tear resistance, and feeding characteristics.
• That’s why the most widely used compositions of all casting processes are those of the aluminium-silicon alloys family.
• Commercial aluminium-silicon alloys ranges up to about 25% Si [1].

Aluminium-silicon alloys

Aluminum-silicon alloys are used when good castability and good corrosion resistsnce is needed. These alloys do not contain copper additions. Metallographic structures of pure components and intermediate compositions of these alloys are shown in Figure 1.

The intermediate compositions are mixtures that consist of

• aluminium cotaining about 1% Si in solid solution as continuous pfase
• many or few particles of pure silicon [1].

Figure 1 – Aluminium-silicon phase diagram [2]

Figure 1 – Aluminium-silicon phase diagram and cast microsttuctures of pure components and of alloys of various compositions [1].

Aluminium-silicon alloys are called:

• with less than 12% Si as “hypoeutectic”
• with close to 12% Si as “eutectic”
• with over 12% Si as “hypereutectic”.

Modifications of aluminium-silicon alloys

Strengh and ductility of aluminium-silicon alloys can be much improved be modification of Al-Si eutectic. Modification of hypereutectic Al-Si alloys is effectively achieved through the addition of a controlled amount of sodium or strontium and also calcium and antimony. These additions refines the eutectic phase.

An unmodified alloy contains large flakes of brittle silicon, which cause the casting to have poor ductility. Unmodified alloys often have elongations no more than a few percent, and the fracture surface is primarily brittle. With a successful modification treatment, the silicon assumes a fine, fibrous structure [4].

Modification is particularly advantageous in sand castings [3].

Figure 2 – Structure of nonmodified A356 alloy [3]

Figure 3 –  A356 alloy modified with 156 ppm Sr [3]

Most important commercial Al-Si casting alloys

Alloys with silicon as the major alloying constituent are the most important commercial casting alloys. The large number of alloys of this type that have been developed displays a broad range of properties:

• Binary aluminum-silicon alloys (443.0, 444.0, 413.0, andA413.0) are low-density, weldable, and resistant to corrosion. Although castings of these alloys are somewhat difficult to machine, good results are obtained with cutting fluids, sintered carbide tools, and chip breakers.
• Alloy 443.0 is used with all casting processes for parts in which strength is less important than ductility, resistance to corrosion, and pressure tightness.
• Permanent mold alloys 444.0 and A444.0 display high ductility and are used where impact resistance is a primary consideration.
• Alloys 413.0 and A413.0 are close to the eutectic composition and, as a result, have very high fluidity. They are useful in die casting and where large-area, thin-walled parts with cast-in  lettering or other high-definition details are required [3].

The effect of magnesium additions

• Magnesium additions make aluminium-silicon casting alloys heat-treatable.
• The hardening-phase Mg2Si displays a useful solubility limit corresponding to approximately 0,70% Mg.
• Common high-strength aluminum-silicon compositions specify magnesium in the range of 0,40 to 0,070% [2].

Aluminum-silicon-magnesium alloys including 356.0 and A356.0 have high casting characteristics and resistance to corrosion. Heat treatment provides combinations of tensile and physical properties for many applications including machinery, automotive, military, and aerospace parts.

Higher tensile properties are obtained with 357.0, A357.0, 358.0, and 359.0 alloys. The high properties of these alloys, attained by heat treatment to the fully hardened condition. Those are of special interest in structural applications [3].

 Sources:

1. Aluminum and Aluminum Alloys, ASM International, 1996
2. TALAT Lecture 1203 – Phase Diagrams / M H Jacobs – The University of Birmingham, UK
3. Aluminum Alloy Castings: Properties, Processes And Applications – J. Gilbert Kaufman, Elwin L. Rooy
4. Modification of Aluminum-Silicon Alloys / G.K. Sigworth, Alcoa Primary Metals // ASM Handbook, Volume 15: Casting – 2008