Aluminum-Silicon Alloys


Castings are the main use of aluminum-silicon alloys, although some sheet or wire is made for welding and brazing, and some of the piston alloys are extruded for forging stock. Often the brazing sheet has only a cladding of aluminum-silicon alloy and the core consists of some other high melting alloy.
The copper-free alloys are used for low- to medium-strength castings with good corrosion resistance; the copper-bearing for medium- to high-strength castings, where corrosion resistance is not critical. Because of their excellent castability, it is possible to produce reliable castings, even in complex shapes, in which the minimum mechanical properties obtained in poorly fed sections are higher than in castings made from higher-strength but lower-castability alloys.

Castings are the main use of aluminum-silicon alloys, although some sheet or wire is made for welding and brazing, and some of the piston alloys are extruded for forging stock. Often the brazing sheet has only a cladding of aluminum-silicon alloy and the core consists of some other high melting alloy.

The copper-free alloys are used for low- to medium-strength castings with good corrosion resistance; the copper-bearing for medium- to high-strength castings, where corrosion resistance is not critical. Because of their excellent castability, it is possible to produce reliable castings, even in complex shapes, in which the minimum mechanical properties obtained in poorly fed sections are higher than in castings made from higher-strength but lower-castability alloys. The alloys of this group fall within the composition limits:

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%

Silicon is the main alloying element; it imparts high fluidity and low shrinkage, which result in good castability and weldability. The low thermal expansion coefficient is exploited for pistons, the high hardness of the silicon particles for wear resistance. The maximum amount of silicon in cast alloys is of the order of 22-24% Si, but alloys made by powder metallurgy may go as high as 40-50% Si.

Sodium or strontium produces the ’modification’ and phosphorus nucleates the silicon to permit of a fine distribution of the primary crystals. Iron is the main impurity and in most alloys efforts are made to keep it as low as economically possible, because of its deleterious effects on ductility and corrosion resistance. In sand castings and permanent mold castings the upper limit is usually 0.6-0.7% Fe. In some piston alloys iron may be added deliberately and in die-castings up to 3% Fe may be tolerated.

Cobalt, chromium, manganese, molybdenum and nickel are sometimes added as correctives for iron; their addition also improves strength at high temperature. Copper is added to increase the strength and fatigue resistance without loss of castability, but at the expense of corrosion resistance. Magnesium, especially after heat treatment, increases substantially the strength, but at the expense of ductility.

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

A distinction between dissolved and ’graphitic’ silicon is sometimes made by dissolving the alloy in acids, in which the dissolved silicon transforms in SiO2 whereas the graphitic remains uncombined. Prolonged or repeated heating tends to spheroidise the silicon. This spheroidising is faster in modified alloys and results in a coarsening of the silicon to a size very close to that of non modified material. In the absence of copper the iron is usually in the Al-FeSiAl5-Si eutectic as thin platelets interspread with the silicon needles or rods. If there is more than 0.8% Fe, primary FeSiAl5, crystals appear.

Titanium and boron are usually added in amounts well within their solid solubility and do not form any separate phase. Iron reduces their solubility, so that less is needed for grain refinement; 0.1-0.2% V is reported to refine the FeMn compounds. Tin and lead, if present together with magnesium, tend to enter the Mg2Si phase. All the phases formed tend to concentrate at the grain boundaries, in the form of complex eutectics, more or less coupled.

The lattice parameter is decreased slightly by silicon in solution and somewhat more by copper; none of the other elements affects it appreciably. Thus, the parameter of the alloys is between a = 4.045 x 10-10m and a = 4.05 x 10-10m, depending on composition and treatment.

Thermal expansion is reduced substantially by silicon and much less pronouncedly by all other additions except magnesium, which tends to increase it slightly. Expansion coefficients at subzero temperatures also are some 10-20% lower than those for pure aluminum. A reduction of expansion coefficient by titanium and zirconium additions is reported, but it is very doubtful that it can be appreciable. Alloys produced by powder metallurgy containing up to 50% Si have even lower expansion coefficients. Permanent expansion accompanies precipitation out of solution of silicon, magnesium and copper; the amount varies but maybe as high as 0.15%.

Thermal conductivity is of the order of 1.2-1.6 x 10-2W/m/K, the lower values being for the alloys cast in metallic molds or heat treated to retain silicon, copper or magnesium in solution.

Electric conductivity depends mostly on the amount of silicon in solution; copper and magnesium also affect it. Values of the order of 35-40% IACS for annealed materials and of 22-35% IACS for solution treated alloys are reported. In the liquid state resistivity is some 10-15 times the resistivity at room temperature. Manganese, chromium, titanium, zirconium also reduce conductivity, and so does modification.

Magnetic susceptibility is only slightly decreased by silicon, copper and magnesium, but depends mostly on manganese content.

Mechanical properties. Alloys prepared from powders exhibit somewhat higher strengths, especially at elevated temperatures. In wrought products ultimate tensile strengths of 200-400 MPa, with elongation correspondingly from 20 to 2-3% are obtained. Poor casting technique may reduce the properties, although the aluminum-silicon alloys are among the least sensitive to such variables as gas content, design of castings, rate of cooling and feeding. High purity find special treatments can produce properties some 10-20% better than average, and, conversely, secondary alloys tend to have lower ductility than do primary ones. Casting under pressure improves properties toward those of forgings.

Increasing silicon content increases strength at the expense of ductility, but this effect is not very marked. Modification by sodium produces a limited increase of strength, but the increase of ductility is substantial, especially in sand castings. At the higher cooling rates, normal with metal mold castings, the silicon is already somewhat refined without modification and the improvement from modification is reduced. The effect of cell size and dendritic arm spacing on mechanical properties of alloys with Si > 8% is not very marked, but in lower-silicon alloys, in which the aluminum dendrites predominate, the effect is normal.

Iron may slightly increase the strength, but drastically decreases the ductility, especially if above 0.7% Fe and not corrected by manganese, cobalt, etc. Beryllium, manganese, chromium, molybdenum, nickel, cobalt and zirconium all slightly increase the strength; manganese, cobalt, nickel and molybdenum, if needed to correct for the iron, can also increase the ductility; otherwise all of them reduce it. Beryllium is also reported to correct the iron effect. Copper and zinc increase the strength at the expense of ductility, but the most effective strengthener is magnesium, especially after heat treatment, provided that the amount and distribution of the magnesium are correct.

Grain refinement by titanium, boron and zirconium additions has only a limited effect on mechanical properties. Silver additions are reported to increase the elongation. Antimony, tin, lead and cadmium decrease all properties, and antimony, by combining with magnesium, may reduce response to heat treatment. Calcium may increase strength and decrease elongation in straight aluminum-silicon alloys, but it has a deleterious effect on piston alloys.

Compressive strength is higher than tensile by some 10-15%. Shear strength is approximately 70% of the tensile strength.

Impact resistance is low, but so is notch sensitivity, as is to be expected in alloys that contain a large amount of hard, brittle second phase, often with sharp angles. Impact resistance is improved by spheroidising the silicon.

The modulus of elasticity is of the order of 85-95 GPa, changing with temperature, as does tensile strength. A decrease in damping capacity with aging is reported.

Properties at cryogenic temperatures are higher than at room temperature; there is little or no increase down to 170 K, but at 70 K the strength has become some 20% higher than at room temperature, with little or no decline in ductility. Notch strength does not change substantially at cryogenic temperatures. The effect of alloying elements on cryogenic properties is not too well established, but probably it is negligible.

At high temperature the strength declines and the ductility increases. The decline is regular and more rapid than for other aluminum alloys except the aluminum-zinc-magnesium group. The slight increase in strength shown by heat treatable alloys, especially if only naturally aged, is only temporary, once the overaging stage is reached, there is a sharp drop and then the decline of strength with temperature becomes regular. Impact resistance increases with increasing temperature. At the higher temperatures elements with high melting points (copper, iron, manganese, nickel, cobalt, chromium, tungsten) reduce to some extent the decline in strength, although their effect is not substantial. Beryllium, too, is reported to improve the high-temperature strength. In spite of their poor high-temperature strength and fatigue resistance, aluminum-silicon alloys are used extensively for pistons because of their low expansion coefficient, good wear resistance and good castability. Hypereutectic alloys with up to 2-3% additions of copper, nickel, iron, manganese, chromium or magnesium are preferred, although good performance has been obtained also with hypoeutectic alloys and alloys low in heavy metals. Zinc, lead and tin decrease the high-temperature strength. Modified alloys have slightly lower high-temperature strength.

Creep resistance is not particularly good. Silicon increases the creep resistance of aluminum much less than do most other alloying elements. Copper, iron, manganese, nickel, cobalt, chromium, etc., increase it, as is to be expected, and so do magnesium and rare earths.

Fatigue resistance is relatively low, especially if the silicon is not modified or is spheroidised by heat treatment. Cobalt and manganese may improve the fatigue resistance. Pressure during freezing increases the fatigue strength and wear resistance; surface defects and complex loads reduce it, especially at high temperature. Fatigue strength drops gradually with temperature in straight aluminum-silicon, but there is no drop up to 500 K in aluminum-copper-silicon alloys. The alloys are susceptible to thermal fatigue because of the substantial difference in expansion coefficient of the matrix and silicon particles.

Wear resistance is very good, especially in hypereutectic alloys in which the hard silicon particles are well distributed either by phosphorus nucleation or by powder metallurgy fabrication, or in alloys to which bismuth has been added. Wear resistance of high-silicon alloys (20-25% Si) is 10 times better than that of plain steel and comparable with that of surface hardened steel. Friction in couples of steel against aluminum-silicon alloys decreases with surface perfection and hardness of the steel; however, aluminum-silicon alloys for bearings have not been successful unless they contain substantial tin.

Corrosion resistance. Aluminum-silicon alloys without copper have good corrosion resistance in most reagents; only in alkaline solutions which attack silicon as well as aluminum their performance is poor. Copper reduces appreciably the corrosion resistance and so does iron, unless corrected with manganese or chromium. Zinc up to 2-3% has no effect. Tin and calcium also have a deleterious effect on corrosion resistance. Porosity decreases corrosion resistance. Corrosion by flowing water is more rapid than in still water, but of the same type. Aluminum-silicon alloys with iron and nickel have particularly good resistance to high-temperature water or steam. In secondary alloys, where many elements are present in small amounts, zinc and manganese compensate for copper and nickel, and corrosion resistance is reported as very close to that of primary alloys. Contact corrosion is especially poor in aluminum-silicon-copper alloys, but even copper-free alloys are worse in this respect than aluminum 99.8%.

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

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