The Role Of Elements In 7A03, 7A04, 7A09 And 7005, 7075, 7475, 7050, 7055 Alloys

7xxx series aluminium alloys are aluminium alloys with zinc as the main alloying element, which are 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., belonging to the heat treatment can strengthen the aluminium alloy. There are nearly 80 commonly used 7xxx series, mainly Al-Zn-Mg-Cu series alloys, accounting for 74.3% of the total. Because of its high specific strength and hardness, better corrosion resistance and higher initial properties, it has become the most important structural material in this series of alloys. Commonly used grades are 7A03, 7A04, 7A09 and 7005, 7075, 7475, 7050, 7055 and other alloys. It can be processed into semi-finished products such as plates, rods, wires, pipes and forgings, which are mainly used for structural materials.

1. Al-Zn-Mg series alloy

The main alloying elements in the Al-Zn-Mg alloy are zinc and magnesium, the trace added elements are manganese, chromium, copper, zirconium and titanium, and the impurities are mainly iron and silicon. There are about 20 commonly used Al-Zn-Mg series alloys, such as 7003, 7004, 7005, 7008, 7108, etc. Al-Zn-Mg series alloys have good thermal deformation ability and a wide quenching range. Under proper heat treatment conditions, Lower can get higher strength. Good welding performance, generally good corrosion resistance, and a certain application corrosion tendency. It is a high-strength weldable aluminium alloy. It is mainly used for aircraft and ship parts, vehicle armour, military floating bridges, lifting vehicles, etc.

1.1 Main alloying elements

Zinc and magnesium: In Al-Zn-Mg series 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 series alloys are greatly improved, but the plasticity, Stress corrosion resistance and fracture toughness are reduced. The stress corrosion tendency of alloys is related to the sum of Zn and Mg contents. For alloys with high Mg and low Zn or high Zn and low Mg, as long as the sum of Zn and Mg contents is not more than 7.5%, the alloy has good stress corrosion resistance. The content of Zn and Mg not only determines the number of strengthening phases but also determines the critical speed of quenching, thus determining the self-quenchability and performance changes during aging. Alloys with low Zn content (below 3%) have low strength and high elongation, and no obvious strengthening occurs during artificial aging. The quenching aging strength of w(Zn)=4%-6% and w(Mg)=2%-4% alloy is very sensitive to the quenching cooling rate. Quenching in the air will cause the alloy strength to decrease and have high stress. Corrosion tendency. Al-Zn-Mg alloys will soften when the temperature rises, and are prone to stress corrosion and separation corrosion. The degree of corrosion is also related to the content and ratio of Zn and Mg in the alloy. Zn and Mg have high solid solubility in aluminium, but as independent components, Zn and Mg cannot reach a high level of strength due to the weak effect of age hardening. When Zn and Mg coexist, a series of new phases such as α-Al, η phase (MgZn2), T phase (Al2Mg3Zn3) and so on are formed. η phase and T phase have a higher solubility and obvious temperature relationship in aluminium and have a strong age-hardening effect. The solubility of the η phase in aluminium at the eutectic temperature of 470°C is 28%, but only 4% at room temperature, so the η phase has a high quenching aging effect. At 489℃, the solubility of the T phase in aluminium is 17%. As the temperature decreases, the room temperature is only 1%, so the alloy can be strengthened by heat treatment. In addition, to prevent cracking tendency during the casting process, it is necessary to strictly control the alloying elements, try to keep the content of Cu and Mn within the lower limit of the standard allowable range; increase the content of Mg and reduce the ratio of Zn to Mg. The Si content should be controlled below 0.15%, while the Fe content should not be controlled 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 cracking tendency. In addition, the influence of Zn/Mg value is also more prominent. Such as 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, the sum reaches the maximum value. In summary, in general, Zn/Mg values of 1.5-2.5 are more reasonable. At this time, the mechanical properties of the alloy, the mechanical properties of welding, and the crack tendency coefficient are more ideal.

1.2 Trace alloying elements

Copper: A small amount of Cu is added to the alloy to accelerate aging, improve strength, and increase quenching sensitivity. Copper accelerates the early aging process because CuMgAl2 can be the core to accelerate the GP zone into an intermediate phase. The return temperature range of copper-containing alloys is higher than that of non- The width of the copper alloy; adding a small amount of Cu to the alloy can improve the stress corrosion resistance and tensile strength, but the weldability of the alloy is reduced. However, studies have shown that the total content of zinc, magnesium, and copper determines the properties of the alloy. This also determines the purpose of the alloy. When the total content is greater than 9%, the strength is high, but the corrosion resistance, formability, and weldability are not good. When the total content is 6%-8%, the strength is still higher, but the formability and welding performance is much better. When the total content drops to 5%-6%, it has excellent processability and the stress corrosion sensitivity almost disappears. The role of Cu in the ratio is similar to that of zinc, and most of the copper will also be dissolved in the two compounds of MgZn2 and Al2Mg3Zn3.

Manganese and Chromium: A small amount of Mn and Cr has a significant effect on the structure and properties of the alloy. The two elements can produce dispersed particles during the homogenization and annealing of the ingot, hinder the migration of dislocations and grain boundaries, thereby increasing the recrystallization temperature, effectively preventing the growth of grains, and can refine the grains. In addition, adding manganese and chromium can improve the stress corrosion resistance of the alloy, and the effect is significant. If manganese and chromium are added at the same time, the effect of reducing stress corrosion tendency is better. And adding Cr is more obvious than adding Mn. The amount of chromium added is preferably w(Cr) 0.1%-0.2%, and manganese content is w(Mn) 0.2%-0.4%.

Vanadium: V forms an Al11V phase refractory compound in the aluminium alloy, which is distributed in the α(Al) crystal. With the addition of V, the secondary dendrite spacing of the alloy decreases first and then increases. After the addition of V, the number of second phases distributed at the alloy grain boundaries and dendrite boundaries increases and the shape of the phases changes significantly, from the original elongated strip and simple network to narrow strips, dots, and blocks. The structure phase of the shape. Adding an appropriate amount of V is beneficial to increase the tensile strength and elongation. The main reason is that the appropriate amount of V reduces the secondary dendrite spacing of the alloy and plays a role in fine-grain strengthening, thereby improving the strength and plasticity. At the same time, the fine Al11V phase is precipitated in the α(Al) matrix, which has a strong nail for dislocations. The piercing effect hinders the movement of the dislocation. 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 are reduced. There are two main reasons for this. On the one hand, excessive V re-enlarges the secondary dendrite spacing of the alloy and reduces the strength and plasticity of the alloy; On the other hand, when excessive V is added, the irregular block Al11V phase is precipitated in the α(Al) matrix, which will split the matrix and reduce the mechanical properties of the alloy. The addition of V increases the number of eutectic phases in the alloy and the number of intergranular liquid films during solidification. The increase in the number of liquid films can enhance the compensating ability of the liquid phase at the end of solidification, help directly fill the pores between the intergranular bridges, promote the lateral growth of the intergranular bridges, reduce and eliminate the pores between the intergranular bridges, increase the intergranular bonding force, and reduce the alloy The hot cracking tendency of the alloy; it is beneficial to compensate the shrinkage porosity generated in the solidification process, improve the compactness of the alloy structure, and reduce the hot cracking tendency of the alloy.

Zirconium: Adding a small amount of zirconium can significantly improve the weldability of Al-Zn-Mg alloys. When 0.2% Zr is added to Al-5Zn-3Mg-0.35Cu-0.35Cr alloy, welding cracks are significantly reduced. Zr can also increase the final temperature of recrystallization. In the 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°C. Therefore, the material still retains the deformed structure after quenching. When adding w(Zr) 0.1%-0.2% to the Al-Zn-Mg alloy containing manganese, the stress corrosion resistance of the alloy can also be improved, but the effect of zirconium is lower than that of chromium.

Titanium: The addition of Ti to the alloy can refine the crystal grains in the as-cast state 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 the Al-5Zn-3Mg-0.3Cr-0.3Cu alloy with w (Ti) content of 0.12%, when the w (Zr) content exceeds 0.15%, the alloy has better weldability and elongation, which can be compared with The same effect is obtained when w(Zr) is added separately above 0.2%. Titanium can also increase the recrystallization temperature of the alloy.

Scandium: Sc element has a significant effect on the structure and properties of aluminium alloy. Al3Sc particles have a very strong precipitation hardening effect on aluminium alloys. Adding an appropriate amount of Sc to the Al-Zn-Mg series alloy can precipitate Al3Sc particles that are completely coherent with the aluminium matrix in the alloy, which can significantly refine the alloy structure, change the size, shape and distribution of the main strengthening η phase, and reduce crystals. There is no precipitation broadband, which obviously improves the strength, plasticity and high-temperature stability of the alloy.

Silver: A small amount of Ag can accelerate the age-hardening effect of Al-Zn-Mg alloys and increase the level of age hardening. Ag can also change the aging precipitation process of some alloys, refine the transition phase η'phase, and improve the stable temperature range of the GP zone.

Lithium: Li is the lightest metal in nature. The addition of Li to aluminium alloys can greatly increase the elastic modulus and reduce the density. It has positive significance for the lightweight and mechanical properties of Al-Zn-Mg alloys, but the additional amount should be strictly controlled.

1.3 Impurity elements

Iron: Iron can reduce the corrosion resistance and mechanical properties of the alloy, especially for alloys with higher manganese content. Therefore, the content of iron should be reduced as much as possible, limited to W (Si) should be limited to less than 0.3%. In addition, even if the hardness, elongation and fracture toughness of the deformed alloy decrease. The needle-shaped FeAl3 in the casting cannot be broken during the processing and deformation process, and their brittleness is completely retained, so the phenomenon that the plasticity decreases with the increase of the iron content is very significant. Si can reduce the strength of the alloy and reduce the bending performance and increase the tendency of welding cracks. In fast-cooling castings, iron-containing compounds are both fine and dispersed. Therefore, Fe with a content of more than 1.5% reduces thermal embrittlement on the one hand and improves stress corrosion resistance on the other. 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 the Mg in the alloy, reducing the main strengthening phase η phase (MgZn2) and T phase (Al2Mg3Zn3) in the alloy, thereby reducing the strength of the alloy. And it reduces the bending performance and increases the tendency of welding cracks. w(Si) should 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 function is to improve the corrosion resistance of the material. There are also a small number of trace elements such as manganese, chromium, zirconium, vanadium, titanium and boron in the alloy. Iron and silicon are impurity elements in the alloy.

2.1 Main alloying elements

Zinc and magnesium: Zn and Mg are the main strengthening elements. When they coexist, they will form (MgZn2) and T (Al2Mg3Zn3) phases. The solubility of and T phase in aluminium is very large, and with the temperature rise and fall, the increase of zinc and magnesium content can greatly increase 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 series alloys, Zn content is greater than 3%, Cu and Mg content are each greater than 1%, and Cu is greater than Mg, S phase is formed. (MgZn2) appears when the ratio of Zn to Mg is greater than 2.2. If the Cu content in the alloy is less than the Mg content and the ratio of Zn to Mg is less than 2.2, the structure is only α(Al)+T eutectic. For ultra-high-strength aluminium alloys, when the Zn content is 7%-12%, the Mg content 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, some researchers have calculated and experimentally verified the 480℃ isothermal cross-section of the Al-Zn-Mg-Cu aluminum-rich angle. With the increase of Zn content and Cu content, the α(Al) phase area shrinks, and the α(Al)+S(Al2CuMg) phase area expands. And also the influence of Zn/Mg value on the mechanical properties of Al-Zn-Mg-Cu alloys is also very important. However, for overall performance, the Zn/Mg value should be appropriately reduced. For example, 7178 alloy, Zn/Mg is about 2.5, which is better for each other. So the same is true for similar alloys. Therefore, in Al-Zn-Mg-Cu series alloys, no matter which interval Zn+Mg is in, there will be an optimal Zn/Mg value. In this interval, and are the maximum, and these two values are the closest And the minimum value. When the Zn/Mg value is less than M, it increases with the increase of the Zn/Mg value. The closer these two values are the minimum value. When the Zn/Mg value is greater than M, it decreases with the increase of the Zn/Mg value, and the decrease is rapid, and it increases with the increase of the Zn/Mg value. In order to make the alloy and match well, in the actual use or production of the alloy, the Zn/Mg value generally deviates slightly from the M value in the corresponding interval, and a part of the strength is sacrificed to improve.

Copper: The addition of Cu alloying elements can significantly increase the dispersion of the precipitated 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 a strengthening phase S (Al2CuMg) to increase the strength of the alloy, but in the opposite case, the S phase is The possibility of existence is very small. Copper can also reduce the solid solubility of Zn and Mg, and reduce the potential difference between the grain boundary and the grain. When its content is greater than 1%, it can also reduce the tendency of intergranular cracking and improve the alloy's stress corrosion resistance. When the content is greater than 1.5 After %, the corrosion resistance of the alloy decreases. When the atomic percentage of Cu and Mg in the alloy is Cu/Mg<1, most of Cu is dissolved in-phase and T phase and a small amount is dissolved in α(Al). In addition, Cu can also change the structure of the precipitation phase and refine the grain boundary precipitation phase, but it has little effect on the width of the PFZ (no precipitation zone at the grain boundary). However, when w (Cu) is greater than 3%, the corrosion resistance of the alloy decreases instead. Copper can increase the supersaturation of the alloy, accelerate the artificial aging process of the alloy at 100-200℃, expand the temperature range of the GP zone, and improve the tensile strength, plasticity and fatigue strength. Some scholars have found that when the copper content is not too high, as the copper content increases, it will increase the fatigue resistance and fracture toughness of periodic strain, and reduce the crack growth rate in corrosive media, but the addition of copper will cause intergranular corrosion. And the tendency of pitting corrosion. In addition, studies have shown that the effect 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 higher, the toughness is still very good.

2.2 Trace alloying elements

Vanadium: V forms Al1 in aluminium alloy

1V phase refractory compounds are distributed in α(Al) crystals. With the addition of V, the secondary dendrite spacing of the alloy decreases first and then increases. After the addition of V, the number of second phases distributed at the alloy grain boundaries and dendrite boundaries increases and the shape of the phases changes significantly, from the original elongated strip and simple network to narrow strips, dots, and blocks. The structure phase of the shape. Adding an appropriate amount of V is beneficial to increase the tensile strength and elongation. The main reason is that the appropriate amount of V reduces the secondary dendrite spacing of the alloy and plays a role in fine-grain strengthening, thereby improving the strength and plasticity. At the same time, the fine Al11V phase is precipitated in the α(Al) matrix, which has a strong nail for dislocations. The piercing effect hinders the movement of the dislocation. Increases the shear stress required for dislocation slippage, plays a role in precipitation strengthening, increases the number of inter-dendritic bridges and dendrite deformability, reduces the possibility of intergranular bridge damage caused by solidification shrinkage stress, and can also increase the eutectic phase quantity. After adding excessive V, the tensile strength and elongation of the alloy are reduced. There are two main reasons for this. On the one hand, excessive V re-enlarges the secondary dendrite spacing of the alloy and reduces the strength and plasticity of the alloy; On the other hand, when an excessive amount of V is added, an irregular block Al11V phase is precipitated in the α(Al) matrix, which has a splitting effect on the matrix, thereby reducing the mechanical properties of the alloy. The addition of V increases the number of eutectic phases in the alloy and the number of intergranular liquid films during solidification. The increase in the number of liquid films can enhance the compensating ability of the liquid phase at the end of solidification, help directly fill the pores between the intergranular bridges, promote the lateral growth of the intergranular bridges, reduce and eliminate the pores between the intergranular bridges, increase the intergranular bonding force, and reduce the alloy The hot cracking tendency of the alloy; it is beneficial to compensate the shrinkage porosity generated in the solidification process, improve the compactness of the alloy structure, and reduce the hot cracking tendency of the alloy. When 0.1% V is added when casting Al-7.0Zn-2.5Mg-1.0Cu alloy, the crack tendency is the smallest. When adding 0.05% V when casting Al-4.5Zn-1.0Mg-0.8Cu alloy, the crack tendency is the smallest.

Manganese, chromium: adding a small amount of Mn, Cr and other elements have a significant effect on the structure and properties of the alloy. These elements can produce dispersed particles during the homogenization annealing of the ingot to prevent the migration of dislocations and grain boundaries, thereby increasing the recrystallization temperature, effectively preventing the growth of grains, refining the grains, and ensuring the organization After hot working and heat treatment, the non-recrystallized or partially recrystallized state is maintained, which improves the strength and has good stress corrosion resistance. In terms of improving the stress corrosion resistance, adding chromium has a better effect than adding manganese. Adding w (Cr) 0.45% will have a corrosion resistance life of dozens to hundreds of times longer than adding the same amount of manganese. The content of chromium in the alloy is not high, mainly in the presence of metal compounds such as (Crum)Al13 and (CrFe)Al7, 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 it is hot or cold deformation, the unrecrystallized structure can be obtained after heat treatment. Zirconium can also improve the alloy's hardenability, weldability, fracture toughness, and stress corrosion resistance. Performance, etc., Zr precipitates primary Al3Zr during alloy solidification. It is a very promising trace additive element in Al-Zn-Mg-Cu alloy.

Titanium and boron: Titanium and boron refine the crystal grains of the alloy in the as-cast state and increase the recrystallization temperature of the alloy.

Nickel: Ni is often present as Al3Ni phase in Al-Zn-Mg-Cu alloys. This phase has the effect of accelerating aging and strengthening. When the content of the Al3Ni phase increases, A decreases and Rm increases.

Scandium: Sc element forms Al3Sc phase in aluminium alloy, which has a very obvious grain refining effect. If the content of Sc is not enough to form the Al3Sc phase, the grain refining effect will not proceed smoothly and will precipitate after homogenization annealing. The dispersed phase of Al3Sc inhibits recrystallization. Studies have shown that the strengthening effect of Sc in Al-Zn-Mg-Cu-Zr alloy is mainly derived from fine-grain strengthening, sub-structure strengthening, and precipitation strengthening. In addition, Sc can also improve the welding and corrosion resistance of the alloy. During the homogenization process of the alloy, a large number of fine, uniform and dispersed secondary Al3Sc phases that are watercress-shaped and coherent with the matrix are precipitated. The particles strongly pin the dislocations and grain boundaries, hinder the recrystallization of the alloy, and make the alloy in After solution and aging, the deformed substructure organization is still maintained, that is, the processed fibre organization. The existence of substructure organization will hinder the movement of dislocations in the crystal. Thereby improving the strength of the alloy.

Lithium: Li element has a certain effect on the alloy. It is generally believed that the Li element can prevent the GP zone and promote the formation of metastable MgZn2 during natural aging. The Li element has an important effect on the lightweight of the alloy. When the content of Li is less than 1.7%, the'(Al3Li) replaces the' phase, which reduces the mechanical properties. When it is greater than 1.7%, the uniform shape and uniform shape of the Zn-rich phase in the matrix can be suppressed. 'Phase coarsening. Therefore, the general Li content of 1.7% is a demarcation point. As long as the Li content reaches a moderate amount, Li and vacancies form Li-V groups, which slow down the diffusion rate of Zn and Mg atoms, which is beneficial to the dispersion distribution.

2.3 Impurity elements

Iron and silicon: In the 7xxx series aluminium alloys, 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 and smelting. Tools and equipment used in casting. These impurities also form (FeMn)Al6, (FeMn)Si2Al5, Al(FeMnCr) and other coarse compounds with manganese and chromium. FeAl3 has the effect of grain refinement, but it has a greater impact on corrosion resistance. As the content of the insoluble phase increases and the volume fraction of the insoluble phase increases, these insoluble second phases will be broken and elongated when deformed, and a banded structure will appear. The particles are arranged linearly along the deformation direction and consist of short, unconnected strips. Because the impurity particles are distributed inside the grains and on the grain boundaries, when plastic deformation occurs, pores will occur on part of the grain-matrix boundary, resulting in micro-cracks, which become the origin of the cracks. It has a very adverse effect on elongation, especially the fracture toughness of the alloy. In addition, the addition of Si easily forms Mg2Si with the Mg in the alloy, reducing the main strengthening phase η phase (MgZn2) and T phase (Al2Mg3Zn3) in the alloy, thereby reducing the strength of the alloy. Therefore, in the design and production of the new alloy, the content of iron and silicon is strictly controlled. In addition to the use of high-purity metal raw materials, some measures have also been taken during the smelting process to prevent these two alloying elements from being mixed into the alloy. Fe, Si The content should be controlled below 0.15%.