Aluminum-Zinc-Magnesium Alloys

All commercial aluminum alloys that comprise this group contain more zinc than magnesium. There are a few commercial alloys with Zn:Mg < 1, but they have been considered with the aluminum-magnesium alloys because their behavior and structure are much closer to those of aluminum-magnesium alloys than of aluminum-zinc-magnesium.

Table 1 shows the composition limits of the alloys in commercial use. The alloys can be used for both castings and wrought products, but because of the poor castability, the bulk is in the form of wrought products.

Zinc and magnesium are the main alloying elements: high Zn:Mg ratios produce the best strength and response to heat treatment, together with the highest susceptibility to stress corrosion. Low ratios produce the best weldability and the lowest quench sensitivity.

Copper has a limited strengthening effect and slightly reduces the stress corrosion susceptibility, but, by lowering the potential, allows of anodic protection with Cu-free Al-Zn alloys. Fe and Si are common impurities of the aluminum, and their effect is limited. Cr, Mn and Zr have some strengthening effect, but their main purpose is for control of stress corrosion. Ti and B are normal grain refiners.

The total amount of Zn + Mg + Cu controls the properties and consequently the uses. When the total is above 9%, high strength is paramount and corrosion resistance, formability and weldability are subordinated to it. With a total of from 6 to 8%, strength is still high, but formability and weldability are much better. Below a total of 5-6% fabricability becomes paramount and stress corrosion susceptibility tends to disappear.

As in most other aluminum alloys, aluminum solid solution forms the matrix, into which the other phases are dispersed. In alloys properly homogenized the bulk of the zinc, copper and magnesium are in solid solution and only small amounts of their phases are visible; most of the phases present are iron-, silicon-, manganese-, chromium- and zirconium-bearing ones.

In cast or annealed alloys, however, zinc, magnesium, copper phases are present, and their amount and distribution depend on cooling rate and minor alloying elements. The Zn:Mg ratio controls the zinc-bearing constituents: with a ratio of over 2, MgZn₂ is formed, with lower ratios - Mg₃Zn₃Al₂. Copper behaves similarly to zinc in the ratio and most of it dissolves in the two above compounds. Only exceptionally are CuMgAl₂, CuFe or CuMn compounds formed.

Lattice parameter is controlled by the main elements in solution: zinc, magnesium and copper.

The amounts of manganese, iron, silicon, chromium, etc., that dissolve are too small to have a noticeable effect. Thus, the data for the Al-Mg-Zn and Al-Cu-Mg-Zn systems can be used safely for the commercial alloys. Density is reduced by magnesium and increased by all the other elements: for commercial alloys it ranges between 2740kg/m³ and 2830kg/m³, with most alloys around 2800kg/m³.

Table 2 shows the strengths and percentage elongation of alloys within the normal ranges. Somewhat higher strengths can be obtained in alloys of the first group by artificially aging to peak hardness, but at this stage the alloys are extremely susceptible to stress corrosion. Shock hardening can also be used to increase strength. As for other aluminum alloys, high testing speed produces properties some 10% better. The size and shape of test specimens also affect properties; foils exhibit higher strength than sheet.

Alloys prepared by powder metallurgy, in which often very large amounts of alloying elements are incorporated, naturally reach much higher strengths, usually at the expense of ductility and stress corrosion resistance. Thus, ultimate tensile strengths of up to 900 MPa and yield strengths to 850 MPa are reported, but with elongations of not more than 1%. Similar properties with lower alloy contents are also obtained from splat-cooled powders.

Impact resistance is maximum in wrought alloys in the annealed and naturally aged tempers, in which it may reach values of 30-40 x 10⁴ N/m in the longitudinal and 5-10 x 10⁴ N/m in the transverse direction. It is lower in the artificially aged temper and the lowest in cast products, where values are of the order of 3-5 x 10⁴ N/m. For the high-strength alloys, artificially aged, shear strength is of the order of 300-400 MPa and bearing strength from 600 to 850 MPa. Radiation does not affect bearing strength, even at subzero temperatures. Compressive yield strength is from 500 to 650 MPa. The modulus of elasticity ranges from 70 to 75 GPa.

Above room temperature hardness, strength, fatigue resistance and moduli decline, and ductility and impact resistance increase. The decline, especially for the high-strength alloys is more rapid than for any other type of aluminum alloys, so that above 500 K the aluminum-zinc-magnesium alloys are less strong than most others. Rapid heating may reduce the properties. In most alloys the decline with temperature is somewhat time-dependent, because age hardening may counteract the decline for some time, especially in naturally aged alloys and at temperatures below 450 K; often a temporary increase may result. Properties determined after exposure to temperature are also time-dependent. Very short times (of seconds to minutes) have little or no effect although the exposure is cumulative; intermediate times produce the increases due to age hardening effects; longer times result in permanent softening, which is the more pronounced the better the original properties.

Creep resistance of the alloys is relatively low, especially for the high-strength alloys. Longer creep life can be achieved if the load is intermittently relieved and the material is allowed a period of rest. Residual stresses from working processes may reduce creep resistance and so does precipitation during creep.

Copper additions have a rather limited effect, which is approximately the same as that caused by the addition of the same amount of zinc. Thus, a copper-bearing alloy can be expected to have properties similar to those of a copper-free alloy with zinc content equal to the zinc plus copper content. This additive effect of copper applies up to 2.5% copper. Above this amount copper does not dissolve in the matrix or magnesium-zinc phases, but forms CuMgAl₂ mostly insoluble. Some improvement of the percentage elongation is reported for copper additions. An appreciable improvement of fatigue resistance from 54-57 MPa to 73-75 MPa through the addition of 0.25% Cu is reported on a cast alloy with 5% zinc, 0.5% manganese, 0.2-0.5% chromium. However, the effect may be due to increased fluidity, reducing the number and size of shrinkage cavities, than to an intrinsic effect of copper.

Iron produces a slight decrease in strength, elongation and fracture toughness in wrought products. In cast products the FeAl₃ needles are not broken up by working and thus can manifest their full embrittling effect; the decrease in ductility with increasing iron content is therefore much more pronounced. When the iron-bearing compounds are small and well dispersed, as in fast-cooled castings, a reduction in hot shortness and an increase in stress corrosion resistance may result from the addition of up to 1.5% Fe. Iron together with manganese may produce a slight increase in strength, accompanied by a limited decrease in elongation.

The effect of silicon additions varies somewhat, depending on the Mg:Zn ratio. If the ratio corresponds to either Mg₃Zn₃Al₂ or MgZn, silicon additions produce a decrease in strength and especially ductility. At other ratios the decrease may not be as pronounced and in some cases may even be unnoticeable.

Manganese, chromium, molybdenum and zirconium appear to have a strong strengthening effect, with a corresponding decrease in percentage elongation. An increase in fatigue strength from 55-60 to 65 MPa on a casting alloy with 5% Zn. 0.5% Mg is reported for chromium additions up to 0.4%, and some increase in creep strength from manganese is reported.

The effect of titanium and vanadium on the strength of wrought alloys appears to be negligible. In cast alloys titanium, and to some extent vanadium, may produce some improvement of properties through grain refinement.

Nickel and cobalt have an effect similar to that of iron plus manganese. With 2% Ni there is an increase in strength of the order of 5% and a more pronounced decrease in elongation.

Beryllium is mentioned as a grain refiner and a corrective for the embrittling effect of iron; but other reports deny these effects. A slight improvement of strength, at the expense of elongation, is mentioned for cerium or thorium additions, but all these effects are minor. More than 0.8% Ca reduces the mechanical properties, especially elongation. Hydrogen reduces substantially the properties of high-strength alloys.

Antimony additions to cast alloys reduce the tensile strength. Cadmium additions of the order of 0.05% have little or no effect on the properties of alloys with and without 1.5% Cu. Higher amounts of cadmium, as well as indium, weaken the grain boundaries. Alloys with 4-6% Cd are recommended for free machining and bearings.

Lead, bismuth and tin up to 0.1% have no significant effect on the room temperature properties, but decrease high-temperature ductility. Lithium and sodium additions, even in small amounts, produce a severe embrittling effect. Silver additions produce an increase in strength.

Generally, the corrosion resistance of Al-Zn-Mg alloys is good, especially of alloys with lower copper and zinc contents. As a class the corrosion resistance of aluminum-zinc-magnesium alloys falls between that of the corrosion resistant such as the aluminum-manganese, aluminum-magnesium, aluminum-magnesium-silicon alloys and the strong alloys such as the aluminum-copper, aluminum-copper-magnesium ones. This of course applies only to alloys treated so as to be not susceptible to inter-granular, stress corrosion or exfoliation. Unfortunately, all the alloys of this group are, at least to some extent, susceptible to these types of corrosion.

Stress corrosion cracks are usually at the grain boundaries but, especially at the latter stages when the failure is more mechanical than corrosive, they may take a transgranular path. The start of the cracks requires an indentation in the metal such as a deep pit or an etched grain boundary. The start is generally slow but then growth of the crack is rapid: indications are that 70-90% of the failure time is for nucleation of the crack. Tension is necessary for stress corrosion and in bent specimens cracking always starts in the tension side. Residual stresses may produce tension at the surface even in non-loaded material.