Aluminium – Copper Alloys

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Copper has been the most common alloying element almost since the beginning of the aluminum industry, and a variety of alloys in which copper is the major addition were developed.
In the cast alloys the basic structure consists of cored dendrites of aluminum solid solution, with a variety of constituents at the grain boundaries or interdendritic spaces, forming a brittle, more or less continuous network of eutectics. Wrought products consist of a matrix of aluminum solid solution with the other solible and insoluble constituents dispersed within it.

Copper has been the most common alloying element almost since the beginning of the aluminum industry, and a variety of alloys in which copper is the major addition were developed. Most of these alloys fall within one of the following groups:

  • Cast alloys with 5% Cu, often with small amounts of silicon and magnesium.
  • Cast alloys with 7-8% Cu, which often contain large amounts of iron and silicon and appreciable amounts of manganese, chromium, zinc, tin, etc.
  • Cast alloys with 10-14% Cu. These alloys may contain small amounts of magnesium (0.10-0.30% Mg), iron up to 1.5%, up to 5% Si and smaller amounts of nickel, manganese, chromium.
  • Wrought alloys with 5-6% Cu and often small amounts of manganese, silicon, cadmium, bismuth, tin, lithium, vanadium and zirconium. Alloys of this type containing lead, bismuth, and cadmium have superior machinability.
  • Durals, whose basic composition is 4-4.5% Cu, 0.5-1.5% Mg, 0.5-1.0% Mn, sometimes with silicon additions.
  • Copper alloys containing nickel, which can be subdivided in two groups: the Y alloy type, whose basic composition is 4% Cu, 2% Ni, 1.5% Mg; and the Hyduminiums, which usually have lower copper contents and in which iron replaces 30me of the nickel.
In most of the alloys in this group aluminum is the primary constituent and in the cast alloys the basic structure consists of cored dendrites of aluminum solid solution, with a variety of constituents at the grain boundaries or interdendritic spaces, forming a brittle, more or less continuous network of eutectics.

Wrought products consist of a matrix of aluminum solid solution with the other constituents dispersed within it. Constituents formed in the alloys can be divided in two groups: in the soluble ones are the constituents containing only one or more of copper, lithium, magnesium, silicon, zinc; in the insoluble ones are the constituents containing at least one of the more or less insoluble iron, manganese, nickel, etc.

The type of soluble constituents formed depends not only on the amount of soluble elements available but also on their ratio. Available copper depends on the iron, manganese and nickel contents; the copper combined with them is not available.

Copper forms (CuFe)Al6 and Cu2FeAl7, with iron, (CuFeMn)Al6 and Cu2Mn3Al20 with manganese, Cu4NiAl, and several not too well known compounds with nickel and iron. The amount of silicon available to some extent controls the copper compounds formed. Silicon above 1% favors the FeSiAl5, over the iron-copper compounds and (CuFeMn)3Si2Al15, over the (CuFeMn)Al6 and Cu2Mn3Al20 compounds.

Similarly, but to a lesser extent, available silicon is affected by iron and manganese contents. With the Cu:Mg ratio below 2 and the Mg:Si ratio well above 1.7 the CuMg4Al6 compound is formed, especially if appreciable zinc is present. When Cu:Mg > 2 and Mg:Si > 1.7, CuMgAl2 is formed. If the Mg:Si ratio is approximately 1.7, Mg2Si and CuAl2 are in equilibrium. With the Mg:Si ratio 1 or less, Cu2Mg8Si6Al5, is formed, usually together with CuAl2. When the copper exceeds 5%, commercial heat treatment cannot dissolve it and the network of eutectics does not break up. Thus, in the 10-15% Cu alloys there is little difference in structure between the as-cast and heat treated alloys.

Magnesium is usually combined with silicon and copper. Only if appreciable amounts of lead, bismuth or tin are present, Mg2Sn, Mg2Pb, Mg2Bi3 can be formed.

The effect of alloying elements on density and thermal expansion is additive; thus, densities range from 2 700 to 2 850 kg/m3, with the lower values for the high-magnesium, high-silicon and low-copper alloys, the higher for the high-copper, high-nickel, high-manganese and high-iron contents.

Expansion coefficients are of the order of 21-24 x 10-6 1/K for the 300-4000 K range and 23-26 x 10-6 1/K for the 300-700 K range, with the higher values for the high-magnesium, low-copper and low-silicon alloys, the lower ones for the higher silicon and higher copper contents. At subzero temperatures the coefficient decreases practically in the same way as that of pure aluminum. However, release of casting stresses or precipitation and solution of copper and magnesium produce changes in length of up to 0.2%, which may affect the dimensional accuracy of parts exposed to high temperature. Subzero treatment of castings to reduce warpage has been recommended.

Specific heat of the commercial alloys is practically the same as for the binary aluminum-copper. Thermal conductivity is little affected by alloying elements other than copper: for the commercial alloys with 4-12% Cu, < 4% other elements, it is approximately 70% of that of pure aluminum at room temperature, some 75-80% at 600 K and 30-35% at 200 K.

Electric conductivity is very sensitive to copper in solution, and to a much lesser extent to magnesium and zinc, but is little affected by alloying elements out of solution. In an alloy with 5% Cu in solution the conductivity is approximately half that of pure aluminum (30-33% IACS), but in the annealed state an alloy with 12% Cu and up to 5% other elements has a conductivity of 37-42% IACS, only 25-30% lower than that of pure aluminum.

The mechanical properties of the alloys vary over an extremely wide range, from those of the sand cast 8% Cu alloys, which are among the lowest in aluminum alloys, to those of durals or wrought 5% Cu alloys, which may reach values of up to 650 MPa.

Higher purity, special compositions, fabricating techniques or heat treatments may produce higher properties. Porosity, poor feeding of castings, excessive amounts of impurities, segregation and poor quality control in fabrication may reduce the properties well below the determined limits. Surface defects reduce the properties of castings more than internal ones. Prestrain or elastic strain during testing have no effect on properties. Ultrasonic vibration may reduce or increase them; and irradiation at cryogenic temperatures may slightly increase strength. Dynamic loading may produce strength and ductility values higher or lower, depending on the speed, but not at high temperature. Temperatures below room temperature increase strength and hardness, with some loss of ductility and a decrease in anisotropy.

Correspondingly, exposure to temperatures above room temperature eventually results in a decrease in strength and hardness with a decided increase in elongation. Heat treatment has a substantial effect: if the alloys are quenched from high temperature and only naturally aged, exposure to temperatures in the range up to 500-600 K may produce a temporary increase in hardness and strength due to artificial aging. Eventually this increase disappears, the faster the higher the temperature, and the normal decline sets in, as in alloys already aged to peak hardness. Prolonged heating (for up to 2 years) results in appreciable softening at all temperatures. For intermediate exposure times this softening is less if the materials are thermo-mechanically treated. In short-time tests fast heating to test temperature increases the strength.

Impact resistance is low, as for all aluminum alloys: in the Charpy test values range from a minimum of 2-3 x 104N/m for cast alloys with 7% Cu to a maximum of 30-40 x 104N/m for wrought products in the naturally aged temper. Notch sensitivity is usually low, especially in the wrought alloys, or in the cast alloys heat treated to maximum ductility. The plane strain fracture toughness ranges from 85 to 100% of the yield strength, depending on a variety of factors. Both impact resistance and notch toughness increase with increasing temperature, but the decrease with subzero temperatures is limited. In the softer alloys at 70 K the difference is within error of testing; only for the higher-strength alloys is the decrease appreciable.

Shear strength is of the order of 70-75% of tensile strength, even at high temperature; bearing strength is approximately 1.5 of tensile; compressive yield strength is 10-15% higher or lower than ultimate tensile strength.

Most alloying elements raise the modulus of elasticity of aluminum, but the increase is not substantial: for the aluminum-copper alloys the modulus of elasticity at room temperature is of the order of 70-75 GPa and practically the same in tension and in compression. It changes regularly with temperature from a value of 76-78 GPa at 70 K to a value of the order of 60 GPa at 500 K. The change during aging is negligible for practical purposes. The Poisson ratio is slightly lower and of the order of 0.32-0.34, and so is the compressibility. The Poisson ratio increases with increasing temperature.

Many of the cast alloys and of the aluminum-copper-nickel alloys are used for high-temperature applications, where creep resistance is important. Resistance is the same whether the load is tensile or compressive.

Wear resistance is favored by high hardness and the presence of hard constituents. Alloys with 10-15% Cu or treated to maximum hardness have very high wear resistance.

Silicon increases the strength in cast alloys, mainly by increasing the castability and thus the soundness of the castings, but with some loss of ductility and fatigue resistance, especially when it changes the iron-bearing compounds from FeM2SiAl8 or Cu2FeAl7, to FeSiAl5.

Magnesium increases the strength and hardness of the alloys, but, especially in castings, with a decided decrease in ductility and impact resistance.

Iron has some beneficial strengthening effect, especially at high temperature and at the lower contents (< 0.7% Fe).

Nickel has a strengthening effect, similar to that of manganese, although more limited because it only acts to reduce the embrittling effect of iron. Manganese and nickel together decrease the room-temperature properties because they combine in aluminum-manganese-nickel compounds and reduce the beneficial effects of each other. The main effect of-nickel is the increase in high-temperature strength, fatigue and creep resistance.

Titanium is added as grain refiner and it is very effective in reducing the grain size. If this results in a better dispersion of insoluble constituents, porosity and nonmetallic inclusions, a decided improvement in mechanical properties results.

Lithium has an effect very similar to that of magnesium: it increases strength, especially after heat treatment and at high temperatures, and there is a corresponding decrease in ductility. Zinc increases the strength but reduces ductility.

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