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.Get more news about
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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:
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.
