Aluminum-Zinc-Magnesium Alloys

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Introduction to Aluminum-Zinc-Magnesium Alloys

Aluminum-zinc-magnesium alloys are an important subset of high-strength aluminum alloys, commonly referred to as Al-Zn-Mg alloys. In all commercial alloys of this group, the zinc content exceeds that of magnesium. Although some commercial alloys have a Zn:Mg ratio of less than 1, these are often classified with aluminum-magnesium alloys because their structure and behavior are more similar to that group than to true aluminum-zinc-magnesium alloys.

Al-Zn-Mg alloys are used for both castings and wrought products. However, due to their poor castability, the majority of applications utilize wrought forms.

Chemical Composition and Phase Structure

Table 1. Composition Limits for Aluminum-Zinc-Magnesium Alloys

Chemical element	Amount (wt.%)
Zn	2-8
Mg	0.5-4
Cu	0-3
Fe	0.1-0.8
Si	0.05-0.3
Cr	0.0-0.5
Mn	0.0 -1.5
Ti	0.0-0.5
B	0.0-0.05
Zr	0.0-0.25
Ag	0.0-1.0
Be	0.0-0.10
Other elements	< 0.05 each

Zinc and magnesium are the primary alloying elements. High Zn:Mg ratios result in the best strength and heat treatment response but also increase susceptibility to stress corrosion. Lower Zn:Mg ratios improve weldability and reduce quench sensitivity.

Copper adds some strengthening effects and marginally decreases stress corrosion susceptibility. However, it also lowers the alloy's potential, enabling anodic protection when using copper-free Al-Zn alloys. Iron and silicon are common impurities with limited impact. Chromium, manganese, and zirconium primarily help control stress corrosion, while titanium and boron serve as grain refiners.

The total content of zinc, magnesium, and copper governs the alloy properties. When the combined total exceeds 9%, strength becomes paramount, with corrosion resistance, formability, and weldability being secondary. For totals between 6% and 8%, strength remains high, but formability and weldability improve. Below 5–6%, fabricability is prioritized, and stress corrosion susceptibility diminishes.

In most aluminum-zinc-magnesium alloys, the aluminum solid solution forms the matrix, dispersing the other phases. Properly homogenized alloys contain most of the zinc, magnesium, and copper in solid solution, with only minor visible phases. Most other phases are associated with iron, silicon, manganese, chromium, and zirconium.

In cast or annealed alloys, zinc, magnesium, and copper phases are present, with their distribution influenced by cooling rate and minor alloying elements. The Zn:Mg ratio determines the nature of zinc-bearing constituents: ratios above 2 form MgZn₂, while lower ratios produce Mg₃Zn₃Al₂. Copper behaves similarly to zinc in ratio and dissolves mainly in these compounds. CuMgAl₂, CuFe, or CuMn compounds form only in exceptional cases.

The lattice parameter depends on the main elements in solution: zinc, magnesium, and copper. Elements such as manganese, iron, silicon, and chromium dissolve in quantities too small to affect it significantly. Data for Al-Mg-Zn and Al-Cu-Mg-Zn systems can be reliably used for commercial alloys. Magnesium reduces density, while other elements increase it; commercial alloys typically have densities between 2740 kg/m³ and 2830 kg/m³, averaging around 2800 kg/m³.

Mechanical Properties of Al-Zn-Mg Alloys

Table 2. Mechanical Properties of Aluminum-Zinc-Magnesium Alloys

Alloy	Property	Annealed or as-cast	Naturally aged	Artificially aged
Wrought High strength (Zn+Mg+Cu ≥ 10%)	UTS (MPa)	250-350	450-550	550-650
	YS (MPa)	100-200	300-400	500-550
	Elongation (%)	10-20	10-15	5-10
Wrought Medium strength (Zn+Mg+Cu 7-9%)	UTS (MPa)	200-300	400-500	450-550
	YS (MPa)	80-150	250-350	300-400
	Elongation (%)	15-25	15-20	8-15
Wrought Low strength (Zn+Mg+Cu ≤ 6%)	UTS (MPa)	150-250	300-400	400-500
	YS (MPa)	60-120	200-300	300-400
	%	20-30	20-25	10-20
Cast	UTS (MPa)	100-150	150-200	180-250
	YS (MPa)	50-100	80-150	120-200
	Elongation (%)	1-3	2-5	0-2

High-strength alloys can achieve even higher strength through artificial aging to peak hardness, though this increases stress corrosion susceptibility. Shock hardening can also enhance strength. As with other aluminum alloys, higher testing speeds can improve measured properties by roughly 10%. The size and shape of test specimens affect results, with foils exhibiting greater strength than sheet.

Powder metallurgy alloys, which can incorporate higher amounts of alloying elements, reach ultimate tensile strengths up to 900 MPa and yield strengths up to 850 MPa, albeit with elongations below 1%. Splat-cooled powders offer similar properties with lower alloy content.

Impact resistance peaks in wrought alloys in annealed and naturally aged tempers, reaching 30–40 × 10⁴ N/m longitudinally and 5–10 × 10⁴ N/m transversely. Artificially aged tempers show lower impact resistance, and cast products the lowest, around 3–5 × 10⁴ N/m. For high-strength, artificially aged alloys, shear strength ranges from 300–400 MPa and bearing strength from 600–850 MPa. Radiation does not affect bearing strength, even at subzero temperatures. Compressive yield strength is typically 500–650 MPa, and the modulus of elasticity is 70–75 GPa.

Above room temperature, hardness, strength, fatigue resistance, and modulus decrease, while ductility and impact resistance increase. This decline is faster for high-strength Al-Zn-Mg alloys than for most other aluminum alloys, making them less strong above 500 K. Rapid heating may further reduce properties. Age hardening can temporarily counteract this decline, especially in naturally aged alloys at temperatures below 450 K. After exposure to elevated temperatures, mechanical properties are time-dependent: very short exposures have little effect, intermediate times can increase strength due to age hardening, and prolonged exposure results in permanent softening.

Creep resistance is relatively low, especially in high-strength alloys. Longer creep life is possible if intermittent loading allows the material to rest. Residual stresses from processing and precipitation during creep can reduce creep resistance.

Influence of Minor Alloying Elements

Copper additions up to 2.5% have an effect similar to that of zinc, providing additive strengthening. Above 2.5%, copper forms insoluble CuMgAl₂, limiting further benefits. Small copper additions can enhance elongation and fatigue resistance, especially in cast alloys. However, improvements may be due to increased fluidity reducing shrinkage cavities, rather than copper's intrinsic effect.

Iron slightly decreases strength, elongation, and fracture toughness in wrought products. In cast alloys, FeAl₃ needles can significantly reduce ductility. Fast cooling and small, well-dispersed iron compounds can reduce hot shortness and increase stress corrosion resistance with up to 1.5% iron. Iron combined with manganese may slightly increase strength but typically reduces elongation.

The effect of silicon depends on the Mg:Zn ratio. For ratios corresponding to Mg₃Zn₃Al₂ or MgZn, silicon decreases both strength and ductility. In other ratios, the impact may be minimal or negligible.

Manganese, chromium, molybdenum, and zirconium have a strong strengthening effect but reduce elongation. Chromium additions up to 0.4% can increase fatigue strength, and manganese may slightly enhance creep strength.

Titanium and vanadium have negligible effects on the strength of wrought alloys. In cast alloys, titanium, and to a lesser extent vanadium, can improve properties through grain refinement.

Nickel and cobalt behave similarly to iron and manganese. Adding 2% nickel increases strength by about 5% but also reduces elongation.

Beryllium is reported as a grain refiner and as a remedy for iron embrittlement, though some sources dispute these effects. Cerium or thorium may slightly increase strength at the expense of elongation, but these effects are minor. Calcium beyond 0.8% reduces mechanical properties, particularly elongation. Hydrogen significantly diminishes strength in high-strength alloys.

Antimony reduces tensile strength in cast alloys. Cadmium up to 0.05% has little effect, while higher amounts and indium can weaken grain boundaries. Alloys with 4–6% cadmium are suitable for free machining and bearings.

Lead, bismuth, and tin up to 0.1% have no notable effect at room temperature but reduce high-temperature ductility. Lithium and sodium, even in small amounts, cause severe embrittlement. Silver additions increase strength.

Corrosion Resistance and Stress Corrosion Cracking

Generally, aluminum-zinc-magnesium alloys have good corrosion resistance, especially those with lower copper and zinc content. Their corrosion resistance falls between that of the more corrosion-resistant aluminum-manganese, aluminum-magnesium, and aluminum-magnesium-silicon alloys, and the stronger but less corrosion-resistant aluminum-copper and aluminum-copper-magnesium alloys. This applies only when alloys are properly treated to avoid susceptibility to intergranular, stress corrosion, or exfoliation.

Stress corrosion cracks usually initiate at grain boundaries, but in later failure stages, cracks may propagate transgranularly. Crack initiation requires a surface indentation, such as a deep pit or an etched grain boundary, and is typically slow, with 70–90% of failure time spent on crack nucleation. Once initiated, crack growth is rapid. Tension is necessary for stress corrosion cracking, and in bent specimens, cracks always start on the tension side. Residual stresses can also create surface tension, causing cracking even in unloaded material.