Aluminum-Magnesium (5000) Alloys

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Introduction to Aluminum-Magnesium Alloys

The magnesium content in commercial aluminum alloys ranges from 0.5% to 12–13%. Low-magnesium alloys demonstrate excellent formability, while high-magnesium variants offer reasonably good castability and high strength. Manufacturers typically prepare these alloys using higher grades of aluminum (99.7% purity or better) to maximize corrosion resistance and reflectivity. Consequently, iron and silicon contents in Al-Mg alloys are usually lower than in other aluminum alloy systems.

Various elements are added to enhance specific properties. Iron and zirconium increase the recrystallization temperature, while silicon improves fluidity. Manganese or chromium counteract the corroding effect of iron. Copper reduces pitting corrosion by enhancing general corrosion, and zinc, though having minimal effect on corrosion resistance, improves castability and strength.

Historically, antimony was added and its oxide was believed to contribute to seawater corrosion resistance, but later experiments disproved this effectiveness. Titanium and titanium-boron combinations serve as grain refiners. Beryllium and sometimes lithium reduce magnesium oxidation at high temperatures, particularly in the molten state. Lead additions can improve machinability without significantly compromising strength or corrosion resistance.

Composition and Microstructure

The composition limits of commercial aluminum-magnesium alloys are shown in Table 1 below.

Table 1. Composition limits of commercial aluminum-magnesium alloys

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 aluminum-magnesium commercial alloys, solidification begins with aluminum forming primary crystals that typically grow as dendrites. Other constituents segregate at grain boundaries or between dendrite arms. In alloys containing more than 10% magnesium and 0.5% silicon, Mg₂Si crystals may form as primary phases, appearing as cubes or hexagons.

When iron content—alone or combined with manganese or chromium—exceeds 1–2% (depending on magnesium content), primary crystals of FeAl₃, (FeMn)Al₆, (FeMn)₃Si₂Al₁₅, (FeCr)Al₇, or (FeCr)₄Si₄Al₁₃ may form. These primary crystals don't substantially affect strength but significantly impact formability, fatigue resistance, and surface finish. The claim that magnesium additions reduce the size of FeAl₃ and Co₂Al₉ primary crystals remains questionable.

The solid solubility of magnesium in commercial alloys ranges from 2% at room temperature to 14–15% at 720K. Consequently, most magnesium remains in solution, with Mg₅Al₈ appearing as divorced eutectic at grain boundaries in cast alloys or as globules in annealed or age-hardened material only under non-equilibrium conditions or after annealing.

Silicon typically forms mostly insoluble Mg₂Si, especially in alloys containing 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; and (FeCr)Al₇ or (FeCr)₄Si₄Al₁₃ when chromium is present. Copper has been detected as CuMgAl₂ and Cu₂FeAl₇. Zinc rarely precipitates out of solution, but when it does, it forms Mg₃Zn₃Al₂. Titanium, boron, and beryllium remain mostly in solution.

Mechanical Properties and Performance Characteristics

In most commercial alloys, other elements appear in small amounts, and their effects on physical properties are overshadowed by magnesium's influence. As a result, commercial alloy properties closely match those of binary alloys.

Magnesium is the primary factor controlling mechanical properties, though all alloying elements contribute to some degree. Table 2 shows properties as a function of composition. Heat treatment doesn't substantially improve strength, but solution treatment followed by natural aging may more than double the elongation percentage, especially in castings.

Table 2. Mechanical properties of commercial Al-Mg alloys

Alloy	Condition	Hardness (HV)	RM (MPa)	Rp0.2 (MPa)	Elong. A (%)
0.5-1.5% Mg	Annealing Stress relieved	25-35 60-80	100-150 200-300	40-80 150-250	20-40 5-15
1% Mg, 1% Mn	Annealing Stress relieved	35-50 65-90	150-200 250-350	50-100 200-300	20-30 5-8
2-3% Mg, 0-2% Zn	Sand cast PM cast Annealing Stress relieved	50-60 50-70 40-55 65-90	150-200 170-220 150-250 250-350	50-100 70-150 80-150 200-300	3-7 3-8 25-35 6-15
5-7% Mg	Sand cast PM cast Annealing Stress relieved	50-60 60-80 60-80 80-100	150-200 200-300 250-350 400-500	70-150 100-200 120-250 250-350	4-10 5-12 20-30 10-15
8-12% Mg	Sand cast PM die cast Annealing Stress relieved Heat treated	70-90 75-95 80-100 90-110 120-140	150-300 200-350 350-500 450-600 400-500	100-200 100-250 150-300 300-400 250-350	3-8 5-10 10-25 5-15 20-25

Alloys containing 4–5% magnesium and 1–3% lithium can achieve, after heat treatment, properties comparable to those of cold-worked 5–7% magnesium alloys.

In wrought products, grain size minimally affects strength. The properties of wrought products depend somewhat on the quality of the ingot from which they were made, particularly for thick plates or strips produced from thin castings. High-pressure die-castings may demonstrate properties approaching those of wrought products.

Alloys containing more than 5% magnesium are seldom used in the cold-worked condition due to potential stress corrosion susceptibility. A stabilizing (stress relief) treatment is commonly employed, which has minimal effect on properties but substantially reduces stress corrosion susceptibility. Properties in thin foils tend to be lower, especially regarding ductility, though strain hardening is more pronounced. Ultrasonic irradiation increases surface strength but decreases it when applied in corrosive environments. Neutron irradiation produces hardening that disappears only after annealing above 500K. In lithium-bearing alloys, radiation produces helium bubbles that tend to reduce properties.

Compressive strength approximately equals tensile strength, while shear strength is 70–80% of tensile strength. Notch toughness is lowest in cast materials, especially high-magnesium sand cast alloys, and highest in low-magnesium wrought products.

The elastic modulus decreases with magnesium additions but increases with most other elements. The cumulative effect results in a modulus similar to that of pure aluminum for alloys containing up to 5–6% magnesium, becoming slightly lower at higher magnesium contents.

Fatigue resistance increases proportionally to strength with magnesium content. As with other aluminum alloys, fatigue resistance is highly sensitive to testing methods, alloy heterogeneity, notches, holes, surface quality, corrosion, and atmospheric conditions.

Temperature Effects and Special Properties

The property changes with temperature follow patterns similar to other aluminum alloys. Lower temperatures increase strength and fatigue resistance with minimal decrease in ductility and notch toughness in wrought alloys. In cast alloys, which typically have continuous brittle phases at grain boundaries, the already low ductility and notch toughness decrease significantly at subzero temperatures.

At elevated temperatures, the decline in strength, modulus, and fatigue resistance occurs more gradually than in most other aluminum alloys. This high-temperature strength led to attempts to use Al-Mg alloys in automotive pistons, though with limited success. Prolonged heating and temperature cycling reduce strength.

Creep resistance is also high but depends significantly on alloying element distribution. When these elements remain in solution or form fine precipitates, creep resistance is high; coarse precipitate particles provide minimal improvement. Powder metallurgy-produced alloys exhibit exceptionally high creep resistance, primarily due to their Al₂O₃ content rather than magnesium. Friction and wear decrease as magnesium content increases.

Silicon in amounts normally present in these alloys (<0.2%) slightly reduces ductility and notch toughness without compensating increases in strength. When added in larger quantities to improve castability, it substantially reduces ductility and has negligible effects on creep resistance.

Iron provides limited strengthening in low-magnesium (<2%) alloys. In higher-magnesium alloys, it generally appears as relatively coarse crystals that only reduce ductility, creep resistance, and fatigue resistance.

Manganese and chromium strengthen low-magnesium alloys while reducing ductility because they can dissolve in the matrix. Their solubility decreases with higher magnesium content, and their particle size tends to coarsen. Both slightly improve low-temperature properties and creep resistance.

Copper, in typical amounts found in these alloys, has minimal effect on mechanical properties. Zinc up to 1.5–2% slightly increases strength with negligible effects on creep resistance. Cerium, titanium, molybdenum, vanadium, and zirconium don't directly strengthen the alloys, but can increase hardness and creep resistance through grain refinement and increased recrystallization temperature. Beryllium, calcium, silver, and antimony have negligible effects on mechanical properties.

Corrosion Resistance and Electrochemical Properties

The electrolytic potential of commercial alloys matches that of high-purity alloys. Zinc tends to increase this potential, while copper decreases it; other elements have minimal effect.

Commercial alloys demonstrate corrosion resistance closely mirroring that of binary alloys. Impurities that might reduce corrosion resistance are typically present in limited amounts with negligible effects. Consequently, these alloys exhibit excellent corrosion resistance to normal environmental exposure, water or steam, seawater and marine atmospheres, and many chemicals.

Susceptibility to intergranular and stress corrosion exists in commercial alloys, with a strong dependence on microstructure.