The Role of Elements in 7XXX Series Aluminum Alloys

7xxx series aluminum alloy is an aluminum alloy with zinc as the main alloy element, which is mainly composed of Al Zn Mg and Al Zn Mg Cu series alloys.

Some alloys also have a small amount of Mn, Cr, Zr, V, Ag, Ti, etc., which can be strengthened by heat treatment.

There are nearly 80 7xxx series commonly used, mainly Al Zn Mg Cu series alloys, accounting for 74.3% of the total.

Because of its high specific strength and hardness, good corrosion resistance and high initial properties, it has become the most important structural materials.

Among these alloys, 7A03, 7A04, 7A09 and 7005, 7075, 7475, 7050, 7055 are commonly used.

It can be processed into semi-finished products such as plates, bars, wires, pipes and forgings, mainly used for structural materials.

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

The Role of Elements in 7XXX Series Aluminum Alloys 1

1. Al Zn Mg alloy

The main alloying elements in Al Zn Mg alloy are zinc and magnesium, and the trace added elements are manganese, chromium, copper, zirconium and titanium.

The impurities are mainly iron and silicon.

There are about 20 commonly used Al Zn Mg alloys, such as 7003, 7004, 7005, 7008, 7108, etc.

Al Zn Mg alloys have good hot deformation ability, wide quenching range, and can obtain high strength under appropriate heat treatment conditions.

It is a high-strength weldable aluminum alloy with good welding performance, good corrosion resistance and certain application corrosion tendency, mainly used for aircraft and ship parts, vehicle armor, military pontoon bridge, lifting vehicles, etc.

1.1 Main alloy elements

Zinc, magnesium:

In Al Zn Mg alloys, the content of both Zn and Mg is generally not more than 7.5%.

With the increase of zinc and magnesium content, the strength and hardness of Al Zn Mg alloys are greatly improved, but the plasticity, stress corrosion resistance and fracture toughness are reduced.

The stress corrosion tendency of the alloy is related to the total content of Zn and Mg.

The alloy with high Mg and low Zn or high Zn and low Mg has good stress corrosion resistance as long as the total content of Zn and Mg is not more than 7.5%.

The content of Zn and Mg not only determines the number of strengthening phases, but also determines the critical quenching speed, thus determining the self quenching property and the property change during aging.

The alloy with low Zn content (below 3%) has low strength, high elongation and no obvious strengthening during artificial aging.

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

Quenching in air will reduce the alloy strength and have a high stress corrosion tendency.

Al Zn Mg alloys will soften when the temperature rises, which is prone to stress corrosion and separation corrosion.

The degree of corrosion is also related to the content and proportion of Zn and Mg in the alloy.

Zn and Mg have high solid solubility in aluminum, but as independent components, Zn and Mg cannot reach the high strength level due to 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.

η phase and T phase have high solubility and obvious temperature relationship in aluminum, and have strong age hardening effect.

The solubility of phase η in aluminum is 28% at 470 ℃, while it is only 4% at room temperature.

At 489 ℃, the solubility of T phase in aluminum is 17%.

With the temperature decreasing, the room temperature is only 1%, so the alloy can be strengthened by heat treatment.

In addition, to prevent crack tendency in the casting process, it is necessary to strictly control the alloy elements, and try to keep the content of Cu and Mn within the lower limit of the standard allowable range;

Increase Mg content and reduce the ratio of Zn and Mg.

The Si content should be controlled below 0.15%, while the Fe content should not be too high.

However, it is necessary to make Fe greater than Si, which is generally the case, in order to narrow the solidus temperature range of the solidification liquid and prevent the tendency of cracks.

In addition, the influence of Zn/Mg value is also prominent.

For example, 7007 (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 about 1.75, both and reach the maximum.

To sum up, in general, Zn/Mg values of 1.5-2.5 are reasonable.

At this time, the mechanical properties of the alloy, the mechanical properties of welding, and the crack tendency coefficient are ideal.

1.2 Trace alloy elements

Copper:

A small amount of copper is added to the alloy to accelerate the aging, improve the strength and enhance the quenching sensitivity.

Copper accelerates the early aging process, because CuMgAl2 can be the core, so that the GP area can be accelerated to become the intermediate phase.

The regression temperature range of copper containing alloys is wider than that of copper free alloys;

A small amount of Cu can improve the stress corrosion resistance and tensile strength of the alloy, but the weldability of the alloy is reduced.

However, some studies have shown that the total content of zinc, magnesium and copper determines the properties of the alloy.

Therefore, it also determines the use of the alloy.

When the total content is more than 9%, the strength is very high, but the corrosion resistance, formability and weldability are not good.

When the total content is 6% – 8%, the strength is still high, but the formability and welding performance are much better.

When the total content drops to 5% – 6%, it has excellent machinability, and the stress corrosion sensitivity almost disappears.

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 amount of Mn and Cr has obvious influence on the structure and properties of the alloy.

The two elements can produce dispersed particles during homogenization annealing of ingots, hinder the migration of dislocations and grain boundaries, thus increasing the recrystallization temperature, effectively preventing grain growth, and refining grains.

In addition, the addition of manganese and chromium can improve the stress corrosion resistance of the alloy.

If manganese and chromium are added at the same time, the effect of reducing stress corrosion tendency will be better.

The addition of Cr is more obvious than that of Mn.

The appropriate amount of chromium is 0.1% – 0.2% of w (Cr), and the manganese content is 0.2% – 0.4% of w (Mn).

Vanadium:

V forms Al11V phase refractory compound in aluminum alloy, which is distributed in α(Al) crystal.

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

After the addition of V, the number of the second phase distributed at the grain boundary and dendrite boundary of the alloy increases, and the shape of the phase changes obviously, from the original thin strip and simple network to the thin strip, point and block structural phase.

The addition of appropriate amount of V is conducive to improving the tensile strength and elongation.

The main reason is that an appropriate amount of V reduces the secondary dendrite spacing of the alloy, which plays a role in fine-grained strengthening, thus improving the strength and plasticity.

At the same time, fine Al11V phase is precipitated in the α(Al) matrix, which has a strong pinning effect on dislocations and hinders the movement of dislocations.

The shear stress required for dislocation slip is increased, which plays a role in precipitation strengthening.

After adding excessive V, the tensile strength and elongation of the alloy decrease, mainly due to two reasons;

On the one hand, excessive V increases the secondary dendrite spacing of the alloy again, reducing the strength and plasticity of the alloy;

On the other hand, when excessive V is added, irregular blocky Al11V phase is precipitated in the α(Al) matrix, which has a splitting effect on the matrix, thus reducing the mechanical properties of the alloy.

The addition of V increases the number of eutectic phases and the number of intercrystalline liquid films during solidification.

The increase in the number of liquid films can enhance the compensation ability of the liquid phase at the end of solidification, help to directly fill the intercrystalline bridging pores, promote the transverse growth of the intercrystalline bridging, reduce and eliminate the intercrystalline bridging pores, increase the intergranular adhesion, and reduce the hot cracking tendency of the alloy;

It is beneficial to compensate the shrinkage porosity during solidification, improve the compactness of the alloy structure, and reduce the hot cracking tendency of the alloy.

Zirconium:

The addition of trace zirconium can significantly improve the weldability of Al Zn Mg alloy system.

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

Zr can also increase the final temperature of recrystallization.

In Al-4.5Zn-1.8Mg-0.6Mn alloy, when w (Zr) is greater than 0.2%, the final recrystallization temperature of the alloy is above 500 ℃, so the material still retains the deformed structure after quenching.

The stress corrosion resistance of Al Zn Mg alloy containing manganese can also be improved by adding 0.1% – 0.2% w (Zr), but the effect of zirconium is lower than that of chromium.

Titanium:

The addition of Ti in aluminum alloy can refine the as cast grain and improve the weldability of the alloy, but its effect is lower than that of zirconium.

If titanium and zirconium are added at the same time, the effect is better.

In Al-5Zn-3Mg-0.3Cr-0.3Cu alloy with 0.12% of w (Ti), when the content of w (Zr) exceeds 0.15%, the alloy has good weldability and elongation, which can achieve the same effect as when adding more than 0.2% of w (Zr) alone.

Titanium can also increase the recrystallization temperature of the alloy.

Scandium:

Sc element has significant influence on the structure and properties of aluminum alloy.

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

Adding appropriate Sc into Al Zn Mg alloys can precipitate Al3Sc particles that are completely coherent with the aluminum matrix, which can significantly refine the alloy structure, change the size, shape and distribution of the main strengthening phase η, reduce the precipitation free width of the grain boundary, and significantly improve the strength, plasticity and high-temperature stability of the alloy.

Silver:

Trace Ag can accelerate the age hardening effect of Al Zn Mg alloy and improve the age hardening level.

Ag can also change the aging precipitation process of some alloys, refine the transition phase η ‘phase, and improve the stable temperature range in GP region.

Lithium:

Li is the lightest metal in nature.

The addition of Li to aluminum alloys can greatly increase the elastic modulus and reduce the density, which is of positive significance for the improvement of the lightweight and mechanical properties of Al Zn Mg alloys, but the addition amount should be strictly controlled.

1.3 Impurities

Iron:

Iron can reduce the corrosion resistance and mechanical properties of alloys, especially for alloys with higher manganese content.

Therefore, the content of iron should be reduced as much as possible, and the limit of W (Si) should be less than 0.3%.

In addition, the hardness, elongation and fracture toughness of the deformed alloy decrease.

The needle like FeAl3 in the casting cannot be broken during the processing deformation, and their brittleness is completely retained, so the phenomenon that the plasticity decreases with the increase of iron content is very significant.

Si can reduce the strength of the alloy, reduce the bending property and increase the welding crack tendency.

In the rapidly cooled castings, the iron containing compounds are both fine and distributed in a dispersion state, so the Fe content greater than 1.5%, on the one hand, reduces the thermal brittleness, on the other hand, improves the resistance to stress corrosion.

When iron and manganese are added at the same time, the strength of the alloy increases slightly, and the elongation also decreases.

Silicon:

The addition of Si can easily form Mg2Si with Mg in the alloy, reducing the main strengthening phase η (MgZn2) and T phase (Al2Mg3Zn3) in the alloy, thereby reducing the strength of the alloy.

The bending property decreases and the tendency of welding cracks increases. W (Si) shall be limited to less than 0.3%.

2. Al Zn Mg Cu alloy

Al Zn Mg Cu alloy is a heat treatable strengthening alloy. The main strengthening elements are Zn and Mg.

Cu also has a certain strengthening effect, but its main role is to improve the corrosion resistance of the material.

There are also a small amount of trace elements such as manganese, chromium, zirconium, vanadium, titanium and boron in the alloy. Iron and silicon are impurities in the alloy.

2.1 Main alloy elements

Zinc, magnesium:

Zn and Mg are the main strengthening elements. When they exist together, they will form (MgZn2) and T (Al2Mg3Zn3) phases.

The solubility of and T phase in aluminum is very large, and it changes dramatically with the rise and fall of temperature.

The increase of zinc and magnesium content can greatly improve the strength and hardness, but the plasticity, stress corrosion resistance and fracture toughness are reduced.

It is generally believed that in Al Zn Mg Cu alloys, when the content of Zn is greater than 3%, the content of Cu and Mg is greater than 1% respectively, and the content of Cu is greater than Mg, S phase is formed.

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

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

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

In addition, the isothermal cross-section of Al Zn Mg Cu system with aluminum rich angle at 480 ℃ has been verified by calculations and experiments.

With the increase of Zn content and Cu content, the α(Al) phase area shrinks and the α(Al)+S (Al2CuMg) phase area expands.

Also, the influence of Zn/Mg value on the mechanical properties of Al Zn Mg Cu alloys is also very important.

However, for the sake of comprehensive properties, the value of Zn/Mg should be appropriately reduced, such as 7178 alloy, where the value of Zn/Mg is about 2.5, which is in good coordination with each other.

So the same is true for similar alloys.

Therefore, in the Al Zn Mg Cu alloy system, no matter which range Zn+Mg is, there will be an optimal Zn/Mg value.

In this range, both values are the maximum, and the two values are the closest, but the minimum. When Zn/Mg value is less than M, it increases with the increase of Zn/Mg value, and these two values are closer to each other, which is the minimum value.

When the Zn/Mg value is greater than M, it decreases with the increase of Zn/Mg value, and the decrease is rapid, while it increases with the increase of Zn/Mg value.

For the sake of good matching between, and, in actual use or production of alloys, the Zn/Mg value generally slightly deviates from the M value of the corresponding range, sacrificing part of the strength to improve.

Copper:

The addition of Cu alloy element can significantly improve the dispersion of precipitation phase and improve the intergranular structure.

When w (Zn)/w (Mg) is greater than 2.2, and the copper content is greater than the magnesium content, copper and other elements can produce strengthening phase S (Al2CuMg) to improve the strength of the alloy, but in the opposite case, the possibility of the existence of S phase is very small.

Copper can also reduce the solid solubility of Zn and Mg, reduce the potential difference between grain boundaries and grains.

When its content is greater than 1%, it can also reduce the tendency of intergranular cracking, improve the stress corrosion resistance of the alloy.

When its content is greater than 1.5%, the corrosion resistance of the alloy decreases.

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

In addition, Cu can also change the precipitation phase structure and refine the grain boundary precipitation phase, but has little effect on the width of PFZ (grain boundary precipitation free zone).

However, when w (Cu) is greater than 3%, the corrosion resistance of the alloy decreases.

Copper can increase the supersaturation of the alloy, accelerate the artificial aging process of the alloy between 100-200 ℃, expand the temperature range of GP zone, and improve the tensile strength, plasticity and fatigue strength.

Some scholars have found that, within the range of copper content, with the increase of copper content, the cyclic strain fatigue resistance and fracture toughness will be improved, and the crack growth rate will be reduced in the corrosion medium.

However, the addition of copper tends to produce intergranular corrosion and pitting corrosion.

In addition, some studies have shown that the influence of copper on fracture toughness is related to the value of w (Zn)/w (Mg).

When the value is small, the higher the copper content, the worse the toughness.

When the ratio is large, even if the copper content is high, the toughness is still good.

2.2 Trace alloy elements

Vanadium:

V forms Al11V phase refractory compound in aluminum alloy, which is distributed in α(Al) crystal.

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

After the addition of V, the number of the second phase distributed at the grain boundary and dendrite boundary of the alloy increases, and the shape of the phase changes obviously, from the original thin strip and simple network to the thin strip, point and block structural phase.

The addition of appropriate amount of V is conducive to improving the tensile strength and elongation.

The main reason is that an appropriate amount of V reduces the secondary dendrite spacing of the alloy, which plays a role in fine-grained strengthening, thus improving the strength and plasticity.

At the same time, fine Al11V phase is precipitated in the α(Al) matrix, which has a strong pinning effect on dislocations and hinders the movement of dislocations.

It increases the shear stress required for dislocation slip, plays the role of precipitation strengthening, increases the number of interdendritic bridging and the ability of dendritic deformation, reduces the possibility of intergranular bridging damage caused by solidification shrinkage stress, and also increases the number of eutectic phases.

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

On the one hand, excessive V increases the secondary dendrite spacing of the alloy again, reducing the strength and plasticity of the alloy;

On the other hand, when excessive V is added, irregular blocky Al11V phase is precipitated in the α(Al) matrix, which has a splitting effect on the matrix, thus reducing the mechanical properties of the alloy.

The addition of V increases the number of eutectic phases and the number of intercrystalline liquid films during solidification.

The increase in the number of liquid films can enhance the compensation ability of the liquid phase at the end of solidification, help to directly fill the intercrystalline bridging pores, promote the transverse growth of the intercrystalline bridging, reduce and eliminate the intercrystalline bridging pores, increase the intergranular adhesion, and reduce the hot cracking tendency of the alloy;

It is beneficial to compensate the shrinkage porosity during solidification, improve the compactness of the alloy structure, and reduce the hot cracking tendency of the alloy.

When 0.1% V is added to Al-7.0Zn-2.5Mg-1.0Cu alloy, the crack tendency is the smallest.

When 0.05% V is added to Al-4.5Zn-1.0Mg-0.8Cu alloy, the crack tendency is the smallest.

Manganese, chromium:

The addition of a small amount of Mn, Cr and other elements has a significant effect on the microstructure and properties of the alloy.

These elements can produce dispersed particles during homogenization annealing of ingots, prevent the migration of dislocations and grain boundaries, so as to increase the recrystallization temperature, effectively prevent the growth of grains, refine grains, and ensure that the structure remains in the non recrystallized or partially recrystallized state after hot processing and heat treatment, so as to improve the strength and have good stress corrosion resistance.

In terms of improving the stress corrosion resistance, the effect of adding chromium is better than that of adding manganese, and the corrosion resistance life of adding 0.45% w (Cr) is tens to hundreds times longer than that of adding the same amount of manganese.

The content of chromium in the alloy is not high, mainly in the form of (CrMn) Al13 and (CrFe) Al7 and other metal compounds, which can strengthen the alloy, reduce the stress corrosion cracking sensitivity and increase the KIC value.

Zirconium:

Zr can greatly increase the recrystallization temperature of the alloy.

Whether hot deformation or cold deformation, unrecrystallized structure can be obtained after heat treatment.

Zirconium can also improve the hardenability, weldability, fracture toughness, stress corrosion resistance, etc. of the alloy.

Zr precipitates primary Al3Zr during alloy solidification.

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

Ti and B:

Ti and B refine the grains of the alloy as cast and increase the recrystallization temperature of the alloy.

Ni:

Ni usually exists as Al3Ni phase in Al Zn Mg Cu alloys, which has the effect of accelerating aging and strengthening.

When the content of Al3Ni phase increases, A decreases and Rm increases.

Scandium:

Sc element forms Al3Sc phase in aluminum alloy, which has a very obvious grain refining effect.

If the content of Sc is not enough to form Al3Sc phase, the grain refining effect cannot be carried out smoothly, and Al3Sc dispersed phase will be precipitated after homogenization annealing to inhibit recrystallization.

Some studies have shown that the strengthening effect of Sc in Al Zn Mg Cu Zr alloy mainly comes from fine grain strengthening, substructure strengthening and precipitation strengthening.

In addition, Sc can improve 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 phase is precipitated, which is bean shaped and coherent with the matrix.

This particle strongly pins the dislocation and grain boundary, hindering the recrystallization of the alloy, so that the alloy still maintains the deformed substructure after solution aging, that is, the processed fiber structure.

The existence of substructure will hinder the movement of dislocations in the crystal, so as to improve the strength of the alloy.

Lithium:

Li element has a certain influence on the alloy. It is generally believed that Li element can prevent GP region and promote the formation of metastable phase MgZn2 during natural aging.

Li element has an important influence on alloy lightweight. When the content of Li is less than 1.7%, because ‘(Al3Li) replaces’ phase, the mechanical properties are reduced.

When the content of Li is more than 1.7%, it can inhibit the uniform shape of Zn rich phase in the matrix and the coarsening of’ phase.

Therefore, the Li content of 1.7% is generally a dividing point.

As long as the Li content reaches a moderate amount, Li and vacancy form Li-V groups, slowing down the diffusion rate of Zn and Mg atoms, which is conducive to the dispersion distribution.

2.3 Impurities

Iron and silicon:

In 7xxx aluminum alloy, iron and silicon are unavoidable harmful impurities, which form insoluble or refractory FeAl3, Al7Cu2Fe, AlFeMnSi and other brittle phases and eutectic compounds in the alloy.

The impurities mainly come from raw materials, tools and equipment used in smelting and casting.

These impurities also form coarse compounds such as (FeMn) Al6, (FeMn) Si2Al5, Al (FeMnCr) with manganese and chromium.

FeAl3 can refine grains, but its corrosion resistance is greatly affected.

With the increase of the content of insoluble phase and the volume fraction of insoluble phase, these insoluble second phases will be broken and elongated during deformation, resulting in banded structure.

The particles are arranged in a straight line along the deformation direction, consisting of short unconnected strips.

Because the impurity particles are distributed in the grain interior and grain boundary, when plastic deformation occurs, pores occur on some grain matrix boundaries, resulting in micro cracks, which become macroscopic; The origin of the crack.

It has a very adverse effect on elongation, especially on the fracture toughness of the alloy.

In addition, the addition of Si is easy to form Mg2Si with Mg in the alloy, reducing the main strengthening phase a (MgZn2) and T phase (Al2Mg3Zn3) in the alloy, thereby reducing the strength of the alloy.

Therefore, during the design and production of the new alloy, the content of iron and silicon is strictly controlled.

In addition to using high-purity metal raw materials, some measures are also taken in the smelting process to prevent these two alloy elements from mixing into the alloy.

The content of Fe and Si should be controlled below 0.15%.

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