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The Role of Elements in 7XXX Series Aluminum Alloys

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The 7xxx series aluminum alloy is a type of aluminum alloy that contains zinc as its main alloying element. It primarily consists of Al-Zn-Mg and Al-Zn-Mg-Cu series alloys.

Some 7xxx series alloys also contain trace amounts of other elements such as Mn, Cr, Zr, V, Ag, Ti, etc. These elements can improve the strength of the alloy through heat treatment.

There are approximately 80 different 7xxx series alloys in use, with Al-Zn-Mg-Cu series alloys accounting for 74.3% of all 7xxx series alloys.

Due to its high strength, hardness, good resistance to corrosion, and high initial properties, the 7xxx series aluminum alloy is a widely used and highly valued structural material.

Common 7xxx series alloys include 7A03, 7A04, 7A09, 7005, 7075, 7475, 7050, and 7055. These alloys can be processed into semi-finished products such as plates, bars, wires, pipes, and forgings and are primarily used for structural applications.

Related reading: 8 Types of Aluminum and Aluminum Alloy You Should Know

1. Al Zn Mg alloy

The main constituents of Al-Zn-Mg alloys are zinc and magnesium, with trace elements of manganese, chromium, copper, zirconium, and titanium added. Iron and silicon are the primary impurities found in these alloys.

There are around 20 commonly used Al-Zn-Mg alloys, including 7003, 7004, 7005, 7008, 7108, and others.

These alloys have excellent hot workability, a wide hardening range, and can achieve high strength when subjected to appropriate heat treatment.

Al-Zn-Mg is a high-strength, weldable aluminum alloy that offers good welding properties, resistance to corrosion, and has limited applications in corrosive environments. It is primarily used in aircraft and ship components, vehicle armor, military pontoon bridges, lifting vehicles, and other similar applications.

1.1 Main alloy elements

Zinc, magnesium:

In Al-Zn-Mg alloys, the zinc and magnesium content is usually not more than 7.5%.

The strength and hardness of these alloys are significantly enhanced as the content of zinc and magnesium increases, but this also leads to a reduction in plasticity, stress corrosion resistance, and fracture toughness.

The stress corrosion tendency of the alloy is dependent on the total content of zinc and magnesium.

An alloy with high magnesium and low zinc content, or high zinc and low magnesium, has good stress corrosion resistance as long as the total content of zinc and magnesium does not exceed 7.5%.

The content of zinc and magnesium not only affects the number of strengthening phases, but also determines the critical quenching speed, which in turn affects the self-quenching property and the property changes during aging.

Alloys with low zinc content (below 3%) have low strength, high elongation, and no noticeable strengthening during artificial aging.

The quenching-aging strength of alloys with w(Zn) = 4% to 6% and w(Mg) = 2% to 4% is very sensitive to the cooling rate during quenching.

Quenching in air will weaken the alloy and increase its stress corrosion tendency.

Al-Zn-Mg alloys tend to soften when the temperature rises, which increases the likelihood of stress corrosion and separation corrosion.

The extent of corrosion is also related to the content and proportion of zinc and magnesium in the alloy.

Zinc and magnesium have high solid solubility in aluminum, but as individual components, they cannot achieve high levels of strength due to their weak age hardening effect.

When Zn and Mg coexist, a series of new phases are formed, such as α-Al, η phase (MgZn2), T phase (Al2Mg3Zn3), etc.

The η phase and T phase have high solubility and a clear temperature relationship in aluminum, and they exhibit strong age hardening properties.

The solubility of the η phase in aluminum is 28% at 470°C, but only 4% at room temperature.

The solubility of the T phase in aluminum is 17% at 489°C and only 1% at room temperature, making it possible to strengthen the alloy through heat treatment.

To prevent crack tendency during casting, it is important to carefully control the alloy elements, keeping the content of copper and manganese within the lower limit of the standard range.

The magnesium content should be increased and the ratio of zinc to magnesium should be reduced.

The silicon content should be kept below 0.15%, and the iron content should not be too high. However, it is necessary for the iron content to be greater than the silicon content in order to narrow the solidus temperature range of the solidifying liquid and reduce the tendency for cracks.

The impact of the Zn/Mg value is also significant. For example, in the 7007 alloy (6.0-7.0% Zn, 1.4-2.2% Mg, Zn/Mg=3.6), when Zn+Mg is 8.6-9.5% and Zn/Mg is around 1.75, both properties reach their maximum.

In general, Zn/Mg values of 1.5-2.5 are considered reasonable. At this range, the mechanical properties of the alloy, the mechanical properties of welding, and the crack tendency coefficient are optimal.

1.2 Trace alloy elements

Copper:

A small amount of copper is added to the alloy to improve the aging process, strength, and quenching sensitivity. Copper contributes to the acceleration of the early aging process by being a core component of CuMgAl2, which in turn speeds up the GP area to become the intermediate phase.

Alloys that contain copper have a wider regression temperature range compared to copper-free alloys. The addition of a small amount of copper enhances the stress corrosion resistance and increases the tensile strength of the alloy, but it can negatively impact the weldability.

Studies have indicated that the properties of the alloy are determined by the total content of zinc, magnesium, and copper. This, in turn, determines the use of the alloy. When the total content exceeds 9%, the strength is high, but the corrosion resistance, formability, and weldability are poor. With a total content of 6% to 8%, the strength remains high, but the formability and welding performance are significantly improved. When the total content drops to 5% to 6%, the alloy has excellent machinability, and the stress corrosion sensitivity is nearly eliminated.

The effect of Cu in the ratio is similar to that of zinc. At the same time, most copper will also be dissolved in MgZn2 and Al2Mg3Zn3.

Manganese and chromium:

A small quantity of Mn and Cr have a noticeable impact on the structure and properties of the alloy. The presence of these two elements can result in the formation of dispersed particles during the homogenization annealing of ingots, which impedes the migration of dislocations and grain boundaries, thereby raising the recrystallization temperature and effectively preventing grain growth while refining the grain structure.

Furthermore, the addition of manganese and chromium can enhance the stress corrosion resistance of the alloy. If both elements are added simultaneously, the effect of reducing stress corrosion will be even more pronounced.

The effect of adding chromium is more pronounced than that of adding manganese. The ideal content of chromium is 0.1% to 0.2% by weight (Cr), and the manganese content should be 0.2% to 0.4% by weight (Mn).

Vanadium:

V forms the Al11V phase refractory compound in aluminum alloys, which is dispersed in the α(Al) crystal structure. The addition of V leads to a decrease in secondary dendrite spacing, followed by an increase. The number of the second phase at the grain and dendrite boundaries of the alloy increases with the addition of V, causing a noticeable change in its shape from a simple network of thin strips to a combination of thin strips, points, and block-like structures.

An appropriate amount of V added to the alloy improves its tensile strength and elongation. This is due to the reduction in secondary dendrite spacing, which leads to fine-grained strengthening, thereby enhancing the strength and plasticity of the alloy. The fine Al11V phase precipitated in the α(Al) matrix also has a strong pinning effect on dislocations, hindering their movement and increasing the shear stress required for dislocation slip, thereby contributing to precipitation strengthening.

However, if excessive V is added, it results in a decrease in the tensile strength and elongation of the alloy. This is because excessive V increases the secondary dendrite spacing again, reducing the strength and plasticity of the alloy. Additionally, the irregular block-like Al11V phase precipitated in the α(Al) matrix has a splitting effect, thereby reducing the mechanical properties of the alloy.

The addition of V leads to an increase in the number of eutectic phases and intercrystalline liquid films during solidification. This increase in the number of liquid films improves the compensation ability of the liquid phase at the end of solidification, allowing it to directly fill intercrystalline bridging pores. This promotes transverse growth of the intercrystalline bridging, reduces and eliminates intercrystalline bridging pores, improves intergranular adhesion, and decreases the hot cracking tendency of the alloy.

The compensation of shrinkage porosity during solidification enhances the compactness of the alloy structure and further decreases the hot cracking tendency of the alloy.

Zirconium:

The addition of trace amounts of zirconium can greatly enhance the weldability of the Al-Zn-Mg alloy system.

When 0.2% of Zr is added to the Al-5Zn-3Mg-0.35Cu-0.35Cr alloy, the occurrence of welding cracks is significantly reduced.

Zr also increases the final recrystallization temperature.

In the Al-4.5Zn-1.8Mg-0.6Mn alloy, when the concentration of Zr (w(Zr)) exceeds 0.2%, the final recrystallization temperature of the alloy exceeds 500℃, allowing the material to retain its deformed structure after quenching.

The addition of 0.1% to 0.2% Zr to Al-Zn-Mg alloys containing manganese can also improve their stress corrosion resistance, though the effect of zirconium is not as pronounced as that of chromium.

Titanium:

The addition of titanium to aluminum alloys can refine the grain structure and improve weldability, though its effect is not as pronounced as that of zirconium.

When both titanium and zirconium are added, the benefits are more pronounced.

In the Al-5Zn-3Mg-0.3Cr-0.3Cu alloy, the addition of 0.12% w (Ti) and a concentration of w (Zr) greater than 0.15% results in improved weldability and elongation, matching the effects seen with the addition of more than 0.2% w (Zr) alone.

Titanium also raises the recrystallization temperature of the alloy.

Scandium:

Sc (Scandium) has a significant impact on the structure and properties of aluminum alloys.

Al3Sc particles have a strong effect on precipitation hardening in aluminum alloys.

By adding the appropriate amount of Sc to Al-Zn-Mg alloys, Al3Sc particles that are fully coherent with the aluminum matrix can precipitate, refining the alloy structure and altering the size, shape, and distribution of the main strengthening phase η. This reduces the precipitation free width of the grain boundary and greatly improves the strength, plasticity, and high-temperature stability of the alloy.

Silver:

The addition of trace amounts of Ag (Silver) can enhance the age hardening effect of Al-Zn-Mg alloys and increase the level of age hardening.

Ag can also modify the aging precipitation process of some alloys, refine the transition phase η’ phase, and broaden the stable temperature range in the GP region.

Lithium:

Li (Lithium) is the lightest metal found in nature.

The addition of Li to aluminum alloys significantly raises the elastic modulus and decreases the density, positively impacting the lightweight and mechanical properties of Al-Zn-Mg alloys. However, the amount added must be carefully controlled.

1.3 Impurities

Iron:

Iron can negatively impact the corrosion resistance and mechanical properties of alloys, particularly those with a high manganese content.

For this reason, it is recommended to minimize the iron content and limit W (Si) to less than 0.3%.

The hardness, elongation, and fracture toughness of the deformed alloy will also decrease.

In casting, needle-like FeAl3 particles cannot be broken during processing deformation and retain their brittleness, leading to a significant decrease in plasticity with an increase in iron content.

The addition of Si can weaken the strength of the alloy, reduce its bending capabilities, and increase the risk of welding cracks.

In rapidly cooled castings, iron-containing compounds are both fine and dispersed, and a Fe content greater than 1.5% can reduce thermal brittleness while improving resistance to stress corrosion.

When iron and manganese are added simultaneously, the strength of the alloy increases slightly, but the elongation decreases.

Silicon:

The addition of Si can easily form Mg2Si with the Mg in the alloy, which reduces the main strengthening phases (η (MgZn2) and T (Al2Mg3Zn3)) and thus weakens the strength of the alloy.

This also results in decreased bending properties and an increased likelihood of welding cracks. To mitigate these effects, the content of W (Si) should be kept below 0.3%.

2. Al Zn Mg Cu alloy

The Al-Zn-Mg-Cu alloy is a heat-treatable material that provides strength through its main elements, Zn and Mg. Copper also contributes to the strength, but its primary function is to enhance the corrosion resistance of the alloy.

Additionally, trace elements such as manganese, chromium, zirconium, vanadium, titanium, and boron are present in small quantities. However, iron and silicon are considered impurities in this alloy.

2.1 Main alloy elements

Zinc, magnesium:

Zn and Mg are the primary strengthening elements that, when present together, form the (MgZn2) and T (Al2Mg3Zn3) phases.

The solubility of the T phase in aluminum is quite high and can vary significantly with changes in temperature.

An increase in the zinc and magnesium content can significantly enhance strength and hardness, but it also tends to reduce plasticity, stress corrosion resistance, and fracture toughness.

It is widely believed that when the content of Zn in Al Zn Mg Cu alloys exceeds 3%, and the content of Cu and Mg is greater than 1% each, with the content of Cu being higher than Mg, the S phase is formed.

(MgZn2) occurs when the ratio of Zn to Mg is greater than 2.2.

If the copper (Cu) content in the alloy is lower than magnesium (Mg), and the zinc (Zn) to Mg ratio is less than 2.2, the microstructure consists of only α (Al) + T eutectic.

For high-strength aluminum alloys, when the Zn content is 7% to 12% and the Mg content is 2% to 3%, and the Zn to Mg ratio is greater than 3.0, Zn and Mg form the main strengthening phase (MgZn2) in the alloy.

Additionally, the isothermal cross-section of the Al Zn Mg Cu system with an aluminum-rich angle at 480°C has been confirmed through calculations and experiments.

As the Zn and Cu content increases, the α (Al) phase area decreases and the α (Al) + S (Al2CuMg) phase area increases.

The impact of the Zn/Mg ratio on the mechanical properties of Al Zn Mg Cu alloys is also significant.

However, for comprehensive properties, it is advisable to reduce the Zn/Mg ratio appropriately, such as in the 7178 alloy where the Zn/Mg ratio is about 2.5, which is well balanced.

The same holds true for similar alloys.

Therefore, in the Al Zn Mg Cu alloy system, there is an optimal Zn/Mg ratio for any range of Zn + Mg.

In this range, both values are at their maximum and closest to each other, but at a minimum. When the Zn/Mg ratio is less than M, it increases as the Zn/Mg ratio increases, and the two values become closer to each other, reaching a minimum value.

When the Zn/Mg ratio is greater than M, it decreases rapidly as the Zn/Mg ratio increases.

For optimal matching, and in the actual production of alloys, the Zn/Mg ratio usually deviates slightly from the M value of the corresponding range, sacrificing some strength for improved properties.

Copper:

The addition of copper alloy elements can significantly enhance the precipitation phase’s dispersion and improve the intergranular structure.

When the weight ratio of Zn to Mg (w(Zn)/w(Mg)) is greater than 2.2 and the copper content exceeds the magnesium content, copper and other elements can form the strengthening phase S (Al2CuMg), thereby increasing the alloy’s strength. However, if the weight ratio is less than 2.2, the occurrence of the S phase is unlikely.

Copper also reduces the solid solubility of Zn and Mg and minimizes the potential difference between grain boundaries and grains.

With a copper content greater than 1%, the alloy’s tendency for intergranular cracking decreases and its stress corrosion resistance improves.

However, if the copper content exceeds 1.5%, the alloy’s corrosion resistance decreases.

When the atomic percentage of copper and magnesium (Cu/Mg) in the alloy is less than 1, most of the copper is dissolved in the T phase and a smaller amount in the α(Al) phase.

Moreover, Copper (Cu) can alter the precipitation phase structure and enhance the grain boundary precipitation phase, however, it has little impact on the width of the PFZ (Precipitation Free Zone).

It is important to note that when the content of Copper (w(Cu)) exceeds 3%, the corrosion resistance of the alloy decreases.

Copper has the ability to increase the supersaturation of the alloy, hasten the artificial aging process of the alloy between 100-200°C, broaden the temperature range of the GP zone, and improve the tensile strength, ductility, and fatigue strength.

Studies have indicated that within the range of copper content, an increase in copper content leads to improved cyclic strain fatigue resistance and fracture toughness, and a reduced crack growth rate in corrosive media.

However, adding copper can also lead to intergranular and pitting corrosion.

Additionally, the impact of Copper on fracture toughness has been found to be related to the value of w(Zn)/w(Mg). When this value is low, higher copper content leads to a decline in toughness. On the other hand, when the ratio is large, toughness remains good even with high copper content.

2.2 Trace alloy elements

Vanadium:

V forms an Al11V phase refractory compound in aluminum alloys, which is present in the α(Al) crystal structure.

With the addition of V, the secondary dendrite spacing initially decreases and then increases.

Upon the addition of V, the number of second-phase particles at the grain and dendrite boundaries of the alloy increases, and the shape of the phase is noticeably altered, transitioning from thin strips and simple networks to thin strips, points, and block structures.

Incorporating an appropriate amount of V can improve the tensile strength and elongation of the alloy.

The improvement is due to the fact that an appropriate amount of V reduces the secondary dendrite spacing, leading to fine-grained strengthening and improved strength and plasticity.

Additionally, fine Al11V phases are precipitated in the α(Al) matrix, providing a strong pinning effect on dislocations and hindering their movement.

This results in increased shear stress required for dislocation slip, precipitation strengthening, an increase in interdendritic bridging and dendritic deformation ability, a reduction in the possibility of intergranular bridging damage caused by solidification shrinkage stress, and an increase in the number of eutectic phases.

After the addition of excessive V, the tensile strength and elongation of the alloy decreases, mainly due to two reasons:

Firstly, excessive V increases the secondary dendrite spacing in the alloy, reducing its strength and plasticity.

Secondly, the addition of excessive V results in the precipitation of irregular blocky Al11V phase in the α(Al) matrix, which splits the matrix and reduces its mechanical properties.

The addition of V also increases the number of eutectic phases and intercrystalline liquid films during solidification. The increase in the number of liquid films can improve the compensation ability of the liquid phase at the end of solidification, filling intercrystalline bridging pores, promoting transverse growth and adhesion, and reducing hot cracking tendency. This enhances the compactness of the alloy structure and compensates for shrinkage porosity, further reducing the hot cracking tendency.

The smallest crack tendency is observed in Al-7.0Zn-2.5Mg-1.0Cu alloy when 0.1% V is added, and in Al-4.5Zn-1.0Mg-0.8Cu alloy when 0.05% V is added.

Manganese, chromium:

The incorporation of a small quantity of Mn, Cr, and other elements has a substantial impact on the microstructure and properties of the alloy.

During the homogenization annealing of ingots, these elements form dispersed particles that prevent the migration of dislocations and grain boundaries. This leads to an increase in the recrystallization temperature, restricts grain growth, and refines grain size, ensuring that the structure remains in a non-recrystallized or partially recrystallized state after hot processing and heat treatment. This results in improved strength and stress corrosion resistance.

In terms of stress corrosion resistance, the addition of chromium is more effective than manganese. The corrosion resistance life of an alloy with 0.45% w (Cr) is tens to hundreds of times longer than that of an alloy with the same amount of manganese.

The chromium content in the alloy is relatively low and mainly present in the form of (CrMn) Al13 and (CrFe) Al7, among other metal compounds. This strengthens the alloy, reduces its sensitivity to stress corrosion cracking, and increases its KIC value.

Zirconium:

Zirconium can significantly raise the recrystallization temperature of the alloy.

Both hot and cold deformation can result in an unrecrystallized structure after heat treatment.

Zirconium also enhances the hardenability, weldability, fracture toughness, and stress corrosion resistance of the alloy.

During the solidification of the alloy, Zr precipitates primary Al3Zr.

It is a highly promising trace additive in the Al Zn Mg Cu alloy.

Ti and B:

Titanium and Boron refine the grain structure of the alloy as cast and raise the recrystallization temperature of the alloy.

Ni:

Ni typically occurs as the Al3Ni phase in Al-Zn-Mg-Cu alloys, which accelerates aging and enhances strengthening. As the content of the Al3Ni phase increases, the value of A decreases while Rm increases.

Scandium:

The addition of Sc elements forms an Al3Sc phase in aluminum alloys, which has a significant grain refining effect. If the Sc content is not sufficient to form the Al3Sc phase, the grain refining effect will not be effective, and Al3Sc dispersed phases will be precipitated after homogenization annealing, inhibiting recrystallization.

Studies have shown that the strengthening effect of Sc in Al Zn Mg Cu Zr alloys is primarily due to fine grain strengthening, substructure strengthening, and precipitation strengthening. Sc also enhances the welding and corrosion resistance of the alloy.

During the homogenization treatment of the alloy, a large amount of fine, uniform, and dispersed secondary Al3Sc phases are precipitated, which are bean-shaped and coherent with the matrix. These particles strongly pin dislocations and grain boundaries, hindering recrystallization of the alloy and allowing the deformed substructure to persist after solution aging, resulting in a processed fiber structure.

The presence of substructure impedes the movement of dislocations in the crystal, leading to an improvement in the strength of the alloy.

Lithium:

The presence of Li elements has an impact on the alloy. It is widely accepted that Li elements can prevent the formation of GP regions and promote the formation of the metastable phase MgZn2 during natural aging.

Li elements play a crucial role in alloy lightweighting. When the Li content is below 1.7%, the mechanical properties are decreased because the (Al3Li) phase replaces the matrix. However, if the Li content is above 1.7%, it can prevent the uniform shape of Zn-rich phases in the matrix and the coarsening of these phases.

Therefore, the Li content of 1.7% is commonly considered as a threshold. When the Li content is at a moderate level, Li and vacancies form Li-V groups, slowing down the diffusion rate of Zn and Mg atoms, which is favorable for the dispersed distribution.

2.3 Impurities

Iron and silicon:

In the 7xxx aluminum alloy, iron and silicon are unavoidable impurities that can negatively impact the alloy’s quality. They form insoluble or refractory phases such as FeAl3, Al7Cu2Fe, AlFeMnSi, and other brittle compounds and eutectic mixtures.

The source of these impurities is primarily from the raw materials, tools, and equipment used during smelting and casting processes.

Additionally, these impurities also lead to the formation of coarse compounds like (FeMn) Al6, (FeMn) Si2Al5, and Al (FeMnCr) containing manganese and chromium.

While FeAl3 can improve grain refinement, it can also significantly decrease corrosion resistance.

As the content of insoluble phases and their volume fraction increase, these second phases become more prone to breaking and stretching during deformation, leading to a banded structure.

The particles are arranged in a straight line along the direction of deformation and form short, disconnected strips.

Due to the distribution of impurity particles both within the grain and at the grain boundaries, plastic deformation results in the formation of pores at some grain matrix boundaries, leading to the development of microcracks that eventually become macroscopic and contribute to the cracking of the alloy.

This has a very detrimental effect on elongation, particularly on the alloy’s fracture toughness.

Additionally, the inclusion of silicon (Si) is known to form Mg2Si with magnesium (Mg) in the alloy, which reduces the presence of the main strengthening phases, such as a (MgZn2) and T phase (Al2Mg3Zn3), ultimately lowering the strength of the alloy.

As a result, strict control over the content of iron (Fe) and silicon (Si) is maintained during the design and production of the new alloy.

In order to ensure the use of high-purity metal raw materials, various measures are taken during the smelting process to prevent the unwanted mixing of Fe and Si into the alloy.

To maintain optimal strength, the content of Fe and Si should be kept below 0.15%.

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