Aluminum-Silicon Alloys

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Introduction to Aluminum-Silicon Alloys and Their Applications

Castings represent the primary application of aluminum-silicon alloys, although some sheet or wire is manufactured for welding and brazing, and certain piston alloys are extruded for forging stock. In many brazing applications, only a cladding of aluminum-silicon alloy is used, with the core consisting of some other high-melting alloy.

The copper-free variants deliver low- to medium-strength castings with excellent corrosion resistance, while copper-bearing alloys provide medium- to high-strength castings for applications where corrosion resistance is less critical. Due to their outstanding castability, these alloys can produce reliable castings, even with complex geometries, in which the minimum mechanical properties obtained in poorly fed sections exceed those in castings made from higher-strength but lower-castability alloys. The alloys in this group fall within the following composition limits:

Table 1. Composition limits of aluminum-silicon alloys

Si	5-25%	Mn, Cr, Co, Mo Ni, Be, Zr	up to 3%
Cu	0-5%	Fe	up to 3%
Mg	0-2%	Na, Sr	< 0.02%
Zn	0-3%	P	< 0.01%

Composition and Microstructure

Silicon serves as the main alloying element, imparting high fluidity and low shrinkage, which results in excellent castability and weldability. The alloys' low thermal expansion coefficient makes them ideal for piston applications, while the high hardness of silicon particles provides superior wear resistance. The maximum silicon content in cast alloys typically reaches 22-24%, but alloys produced through powder metallurgy may contain as much as 40-50% silicon.

Sodium or strontium produces the 'modification' effect, while phosphorus nucleates the silicon to enable fine distribution of primary crystals. Iron represents the main impurity, and in most alloys, efforts are made to keep iron content as low as economically feasible due to its detrimental effects on ductility and corrosion resistance. In sand castings and permanent mold castings, the upper limit typically remains at 0.6-0.7% Fe. Some piston alloys may contain deliberately added iron, and die-castings can tolerate up to 3% Fe.

Cobalt, chromium, manganese, molybdenum, and nickel sometimes function as correctives for iron; their addition also improves high-temperature strength. Copper increases strength and fatigue resistance without reducing castability, though at the expense of corrosion resistance. Magnesium, especially after heat treatment, substantially increases strength but reduces ductility.

Zinc represents a tolerated impurity in many alloys, often up to 1.5-2% Zn, as it has no significant effect on room-temperature properties. Titanium and boron are occasionally added as grain refiners, although grain size in these alloys is not critical since properties are mainly controlled by the amount and structure of silicon, as affected by modification from sodium or phosphorus additions.

Scientists sometimes distinguish between dissolved and 'graphitic' silicon by dissolving the alloy in acids, wherein dissolved silicon transforms into SiO₂ while graphitic silicon remains uncombined. Prolonged or repeated heating tends to spheroidize the silicon. This spheroidization occurs faster in modified alloys and results in silicon coarsening to a size very similar to that of unmodified material. In copper-free alloys, iron usually appears in the Al-FeSiAl₅-Si eutectic as thin platelets interspersed with silicon needles or rods. When iron content exceeds 0.8%, primary FeSiAl₅ crystals appear.

Titanium and boron are typically added in amounts well within their solid solubility and do not form separate phases.

Iron reduces their solubility, so less is needed for grain refinement; 0.1-0.2% vanadium reportedly refines the FeMn compounds. When present together with magnesium, tin and lead tend to enter the Mg₂Si phase. All the phases formed typically concentrate at grain boundaries in complex eutectics of varying degrees of coupling.

Physical Properties

The lattice parameter decreases slightly with silicon in solution and somewhat more with copper; other elements have negligible effects. Thus, the parameter of these alloys ranges between a = 4.045 × 10⁻¹⁰ m and a = 4.05 × 10⁻¹⁰ m, depending on composition and treatment.

Silicon substantially reduces thermal expansion, while other additions have less pronounced effects, except for magnesium, which slightly increases expansion. Expansion coefficients at subzero temperatures are also 10-20% lower than those of pure aluminum. Reported reductions in expansion coefficient from titanium and zirconium additions are likely minimal. Powder metallurgy-produced alloys containing up to 50% Si exhibit even lower expansion coefficients. Precipitation of silicon, magnesium, and copper from solution causes permanent expansion, which may reach up to 0.15%.

Thermal conductivity typically ranges from 1.2-1.6 × 10⁻² W/m/K, with lower values observed in alloys cast in metallic molds or heat-treated to retain silicon, copper, or magnesium in solution.

Electrical conductivity primarily depends on the amount of silicon in solution; copper and magnesium also affect it. Values of 35-40% IACS for annealed materials and 22-35% IACS for solution-treated alloys are common. In the liquid state, resistivity is approximately 10-15 times higher than at room temperature. Manganese, chromium, titanium, and zirconium also reduce conductivity, as does modification.

Silicon, copper, and magnesium only slightly decrease magnetic susceptibility, which depends primarily on manganese content.

Mechanical Properties and Performance

Powder-prepared alloys demonstrate somewhat higher strengths, especially at elevated temperatures. Wrought products achieve ultimate tensile strengths of 200-400 MPa, with corresponding elongation from 20% down to 2-3%. Poor casting techniques may reduce properties, although aluminum-silicon alloys are among the least sensitive to variables such as gas content, casting design, cooling rate, and feeding. High purity and special treatments can improve properties by 10-20% above average, while secondary alloys typically exhibit lower ductility than primary ones. Pressure casting enhances properties toward those of forgings.

Increasing silicon content improves strength at the expense of ductility, though this effect is not dramatic. Sodium modification produces a limited strength increase but substantially improves ductility, especially in sand castings. With higher cooling rates typical of metal mold castings, silicon is already somewhat refined without modification, reducing the improvement from modification. Cell size and dendritic arm spacing have minimal effects on the mechanical properties of alloys with >8% Si, but in lower-silicon alloys where aluminum dendrites predominate, the effect follows normal patterns.

Iron may slightly increase strength but drastically decreases ductility, particularly above 0.7% Fe when not corrected by manganese, cobalt, or other elements. Beryllium, manganese, chromium, molybdenum, nickel, cobalt, and zirconium all slightly enhance strength; manganese, cobalt, nickel, and molybdenum can increase ductility when needed to correct for iron effects, otherwise, all reduce ductility. Beryllium reportedly also corrects iron effects. Copper and zinc increase strength at the expense of ductility, but magnesium is the most effective strengthener, especially after heat treatment, provided its amount and distribution are correct.

Grain refinement through titanium, boron, and zirconium additions has limited effects on mechanical properties. Silver additions reportedly increase elongation. Antimony, tin, lead, and cadmium decrease all properties, and antimony may reduce heat treatment response by combining with magnesium. Calcium may increase strength and decrease elongation in straight aluminum-silicon alloys but negatively affects piston alloys.

Compressive strength exceeds tensile strength by approximately 10-15%. Shear strength is approximately 70% of tensile strength.

Impact resistance is low, but so is notch sensitivity, as expected in alloys containing large amounts of hard, brittle second phase, often with sharp angles. Spheroidizing the silicon improves impact resistance.

The elastic modulus ranges from 85-95 GPa, varying with temperature as does tensile strength. Aging reportedly decreases damping capacity.

Properties at cryogenic temperatures surpass those at room temperature; little change occurs down to 170 K, but at 70 K, strength becomes approximately 20% higher than at room temperature, with minimal decline in ductility. Notch strength remains relatively stable at cryogenic temperatures. The effects of alloying elements on cryogenic properties are not well established but likely negligible.

At high temperatures, strength decreases while ductility increases. The decline is regular and more rapid than for other aluminum alloys except the aluminum-zinc-magnesium group. The slight strength increase shown by heat-treatable alloys, especially those naturally aged, is temporary; once overaging occurs, strength drops sharply and then declines regularly with temperature. Impact resistance increases with temperature. Elements with high melting points (copper, iron, manganese, nickel, cobalt, chromium, tungsten) somewhat mitigate strength decline at higher temperatures, though their effect is modest. Beryllium also reportedly improves high-temperature strength. Despite their poor high-temperature strength and fatigue resistance, aluminum-silicon alloys are extensively used for pistons due to their low expansion coefficient, good wear resistance, and excellent castability. Hypereutectic alloys with 2-3% additions of copper, nickel, iron, manganese, chromium, or magnesium are preferred, though hypoeutectic alloys and alloys low in heavy metals have also performed well. Zinc, lead, and tin decrease high-temperature strength. Modified alloys exhibit slightly lower high-temperature strength.

Creep resistance is moderate. Silicon increases aluminum's creep resistance less than most other alloying elements. Copper, iron, manganese, nickel, cobalt, chromium, magnesium, and rare earths enhance creep resistance, as expected.

Fatigue resistance is relatively low, especially with unmodified or heat-treatment spheroidized silicon. Cobalt and manganese may improve fatigue resistance. Pressure during solidification increases fatigue strength and wear resistance, while surface defects and complex loads reduce it, particularly at high temperatures. Fatigue strength decreases gradually with temperature in straight aluminum-silicon alloys, but aluminum-copper-silicon alloys show no decrease up to 500 K. These alloys are susceptible to thermal fatigue due to the substantial difference in expansion coefficient between the matrix and silicon particles.

Wear resistance is excellent, especially in hypereutectic alloys where hard silicon particles are well distributed either through phosphorus nucleation, powder metallurgy fabrication, or bismuth addition. The wear resistance of high-silicon alloys (20-25% Si) is ten times better than plain steel and comparable to surface-hardened steel. Friction in steel-against-aluminum-silicon alloy couples decreases with surface perfection and steel hardness; however, aluminum-silicon bearing alloys have been unsuccessful unless they contain substantial tin.

Corrosion Resistance and Fabrication Properties

Copper-free aluminum-silicon alloys demonstrate good corrosion resistance in most environments; only alkaline solutions that attack both silicon and aluminum cause poor performance. Copper significantly reduces corrosion resistance, as does iron unless corrected with manganese or chromium. Zinc up to 2-3% has no effect. Tin and calcium also negatively impact corrosion resistance. Porosity decreases corrosion resistance. Flowing water causes more rapid corrosion than still water, though of the same type. Aluminum-silicon alloys with iron and nickel exhibit particularly good resistance to high-temperature water or steam. In secondary alloys containing many elements in small amounts, zinc and manganese compensate for copper and nickel, resulting in corrosion resistance reportedly very close to that of primary alloys. Contact corrosion is especially problematic in aluminum-silicon-copper alloys, but even copper-free variants perform worse in this respect than 99.8% aluminum.

Machinability is poor because the extreme hardness of silicon combined with the relative softness of the matrix rapidly wears tools. In hypereutectic alloys, phosphorus additions that improve silicon distribution enhance machinability; however, in hypoeutectic alloys, phosphorus tends to reduce machinability, while sodium improves it. Copper further reduces machinability at the same silicon content, especially after heat treatment, but some low-silicon copper-silicon alloys may have machinability equal to or better than high-silicon, copper-free alloys. Iron, manganese, nickel, zinc, titanium, and similar elements do not decrease machinability.