Aluminum-Magnesium (5000) Alloys

The magnesium in the commercial alloys ranges all the way from 0.5 to 12-13% Mg, the low-magnesium alloys having the best formability, the high-magnesium reasonably good castability and high strength. It is normal practice to prepare these alloys from the higher grades of aluminum (99.7 or better) to obtain maximum corrosion resistance and reflectivity; thus the iron and silicon contents are usually lower than in other aluminum alloys.

Iron and zirconium are sometimes added to increase the recrystallisation temperature; silicon to improve the fluidity; manganese or chromium to correct for the corroding effect of iron. Copper is added to reduce pitting corrosion by enhancing general corrosion; zinc has little or no effect on corrosion but enhances castability and strength.

In the early days antimony was added, and to its oxide was attributed the corrosion resistance to seawater but later experiments disproved antimony`s effectiveness. Titanium and titanium plus boron are often added as grain refiners; beryllium and sometimes lithium to reduce oxidation of magnesium at high temperature, and especially in the molten state. Lead has been added to improve machinability, supposedly without loss of strength or corrosion resistance.

The composition limits of commercial alloys are:

Mg	0.5-13%	Zn	up to 3%
B	up to 0.05%	Li	up to 3%
Si	up to 2%	Cr	up to 0.5%
Ni	up to 0.5%	Zr	up to 0.5%
Fe	up to 0.8%	Ti	up to 0.2%
Be	up to 0.01%	Mn	up to 2%
Cu	up to 0.2%	-	-

In the aluminum-magnesium commercial alloys solidification starts with the aluminum as primary crystals and usually growing as dendrites, with the other constituents segregating at the grain boundaries or between the dendrite arms. In alloys with more than 10% Mg and more than 0.5% Si, Mg₂Si crystals may be primary, in the form of cubes or hexagons.

If iron, iron plus manganese or iron plus chromium is above 1-2% (depending on magnesium content), primary crystals of FeAl₃, (FeMn)Al₆, (FeMn)₃Si₂Al₁₅, (FeCr)Al₇, or (FeCr)₄Si₄Al₁₃ may form. These primary crystals do not have a substantial effect on strength but affect appreciably the formability, fatigue resistance and surface finish. The claim that magnesium additions reduce the size of FeAl₃, and Co₂Al₉ primary crystals is doubtful.

The solid solubility of magnesium in commercial alloys ranges from 2% Mg at room temperature up to 14-15% at 720K. Therefore most magnesium is in solution and only nonequilibrium conditions or annealing produces Mg₅Al₈ as divorced eutectic at the boundaries in cast alloys, as globules in annealed or age hardened material.

Silicon usually forms Mg₂Si, mostly insoluble, especially in the alloys with more than 3-4% magnesium. Iron may form Fe₂SiAl₈ in low-magnesium, high-silicon alloys; FeAl₃ in the absence of chromium or manganese; (FeMn)Al₆ or (FeMn)₃Si₂Al₁₅ when manganese is present; (FeCr)Al₇ or (FeCr)₄Si₄Al₁₃ when chromium is present. Copper has been detected as CuMgAl₂ and Cu₂FeAl₇. Zinc is seldom out of solution and then forms Mg₃Zn₃Al₂; titanium, boron and beryllium are mostly in solution.

In most of the commercial alloys other elements are present only in small amounts, and their effect on physical properties is submerged by that of magnesium, so that the properties of the commercial alloys are within error of testing of those of the binary alloys.

Magnesium is the main factor that controls mechanical properties, but all other alloying elements contribute to it. Table 1 shows the properties as a function of composition. Heat treatment produces no substantial improvement in strength, but, especially in castings, a solution treatment followed by natural aging may more than double the %A.

The properties obtainable in alloys with 4-5% Mg and 1-3% Li after heat treatment are the same as those obtained in the 5-7% Mg alloys by cold working.

In wrought products the grain size has little effect on the strength. Properties of wrought products depend to some extent on the quality of the ingot from which they were made, especially thick plates or strip made from thin castings. High-pressure die-castings may have properties approaching those of wrought products.

Alloys containing more than 5% Mg are seldom used in the cold worked condition because they may be susceptible to stress corrosion. A stabilizing (stress relief) treatment is used, which has little effect on properties but substantially reduces susceptibility to stress corrosion. Properties in thin foils tend to be lower, especially ductility, but strain hardening is more pronounced. Ultrasonic irradiation increases the strength of the surface, but decreases it if applied in a corrosive environment. Neutron irradiation produces hardening, which disappears only after annealing above 500K. In lithium-bearing alloys radiation produces helium bubbles, which tend to reduce properties.

Compressive strength is approximately the same as tensile strength; shear strength is 70-80% of the tensile strength. Notch toughness is also minimum in cast materials, especially high-magnesium alloys sand cast: best in low-magnesium wrought products.

The modulus of elasticity is lowered by magnesium and raised by most other additions. The effect is cumulative and the result is a modulus little different from that of pure aluminum for alloys up to 5-6% Mg, slightly lower at higher magnesium contents.

Fatigue resistance increases proportionally to the strength with magnesium content. As in other aluminum alloys, fatigue resistance is very sensitive to testing methods, heterogeneity of the alloy, notches, holes, surface quality, corrosion and atmosphere.

The change in properties with temperature is the same as in other aluminum alloys: lower temperatures produce an increase in strength and fatigue resistance, with little or no decrease in ductility and notch toughness in the wrought alloys. In cast alloys, on the other hand, with more or less continuous brittle phases at the grain boundaries, ductility and notch toughness, already low, are reduced appreciably at subzero temperatures.

At higher temperatures the decline in strength, modulus and fatigue resistance is less rapid than for most other aluminum alloys. Because of this strength at high temperature, the use of Al-Mg alloys in automotive pistons has been tried, but with little success. Prolonged heating and temperature cycling reduce strength.

Creep resistance is also high but depends to an appreciable extent on the distribution of alloying elements. When they are in solution or in the form of fine precipitates, creep resistance is high; coarse precipitate particles do not improve creep resistance. Alloys produced by powder metallurgy have exceptionally high creep resistance, but it is due to the Al₂O₃ content rather than to the magnesium. Friction and wear decrease with increasing magnesium content.

Silicon in the amounts normally present in the alloys (< 0.2% Si) reduces slightly ductility and notch toughness without a compensating increase in strength. When added in larger amounts to improve castability, it substantially reduces the ductility. It has no appreciable effect on creep resistance.

Iron has a limited strengthening effect in the low-magnesium (< 2% Mg) alloys. In higher-magnesium alloys it is generally present as relatively coarse crystals, which only reduce ductility, creep and fatigue resistance.

Manganese and chromium have a strengthening effect with reduction of ductility on low-magnesium alloys, because they can dissolve in the matrix, but with higher magnesium their solubility decreases and their particle size tends to coarsen. Also, the amount that produces primary crystals decreases. Both increase slightly the low-temperature properties and creep resistance.

Copper in the amounts usually found in the alloys has little or no effect on mechanical properties. Zinc up to 1.5-2% slightly increases the strength, with little or no effect on creep resistance. Cerium, titanium, molybdenum, vanadium and zirconium do not have a direct strengthening effect but some hardening and some increase in creep resistance can result from the grain refining and increase in recrystallisation temperature that they produce. Beryllium, calcium, silver and antimony have no appreciable effect on mechanical properties.

The electrolytic potential of the commercial alloys is the same as that of the high-purity alloys. Zinc tends to increase the potential, copper to decrease it; none of the other elements has an appreciable effect.

Corrosion resistance of the commercial alloys also closely duplicates that of the binary ones; impurities that reduce it are usually present in limited amounts and their effect negligible. Thus, the alloys have excellent corrosion resistance to normal exposure, to water or steam, to seawater and marine atmospheres, and to many chemicals.

Susceptibility to intergranular and stress corrosion is present in commercial alloys, with a decided dependence on structure.

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