Introduction of Al-Li alloy
Al-Li alloy is a new type of aluminum alloy with low density, high modulus of elasticity, high specific strength and high specific rigidity. It has wide application prospects in the aerospace field. Adding metallic lithium element to aluminum alloy, every 1% of metallic lithium added, its density is reduced by 3%, and the elastic modulus can be increased by 5% to 6%, and can ensure that the alloy has a significant hardening effect after quenching and artificial aging. Aluminum The material preparation and parts manufacturing process of lithium alloys are not much different from ordinary aluminum alloys, except that attention should be paid to ensuring that metal lithium is not oxidized by air. In general, the technology and equipment of ordinary aluminum alloys can be used. Compared with carbon fiber composite materials, aluminum-lithium alloys are easier to form and maintain than composite materials, and the cost is relatively low. Therefore, aluminum-lithium alloys have obvious prices Advantages and performance advantages are considered to be one of the most competitive lightweight high-strength structural materials for the aerospace industry in the 21st century.
Alloying of Al-Li alloy
In general, the addition of micro-alloying elements is mainly to improve the grain refinement, precipitation strengthening phase, control the speed and sequence of failure, reduce the width of precipitation-free precipitation zone and other factors as the main purpose.
At present, the commonly used additive elements in aluminum-lithium alloys include the main alloying elements Cu, Mg and trace elements Ag, Ce, Y, La, Ti, Mn, Sc, Zr, etc. Cu can significantly improve the strength and toughness of the Al-Li alloy and reduce the width of the precipitation-free precipitation zone, but when the content is too high, more mesophases will be generated, and these mesophases will cause the toughness and density of the aluminum-lithium alloy to decrease The Cu content is too low to weaken the local strain and reduce the width of the precipitation-free precipitation zone, so the Cu content in the Al-Li alloy is generally 1% to 4%. The T1 (Al2CuLi) phase precipitated in the form of fine flakes in the Al-Cu-Li alloy together with the δ 'phase as the main precipitation strengthening phase in the alloy, they can weaken the coplanar slip and make the strength index of the alloy significantly improved.
Mg has greater solid solubility in Al, and the addition of Mg can reduce the solid solubility of Li in Al. Therefore, it can increase the volume fraction of δ 'phase under a certain Li content. In addition, it can also form a T (Al2LiMg) stable phase and suppress the formation of δ phase. Adding Mg can produce solid solution strengthening effect, strengthen no precipitation precipitation zone, and reduce its harmful effects. When Cu and Mg are added to the aluminum-lithium alloy at the same time, the S '(Al2CuMg) phase can be formed. The S 'phase preferentially precipitates unevenly near defects such as dislocations, and its densely packed surface is not parallel to the densely packed surface of the α phase of the matrix. , And leave dislocation loops, so the S 'phase can effectively prevent coplanar slippage, and has a certain positive effect on improving the strength and toughness of the alloy. However, when the Mg content is too high, the T phase will preferentially precipitate at the grain boundaries, increasing brittleness. When the Mg content is less than 0.5%, the S 'phase is little, and the strength of the alloy is reduced. The appropriate Mg content has a certain good effect in improving the high-temperature performance of the aluminum-lithium alloy.
Ag has solid solution strengthening and aging strengthening effects on aluminum-lithium alloys. But the effect is not very obvious. Adding Ag and Mg at the same time will exert a synergistic effect and produce the best strengthening effect, which can greatly accelerate the aging rate of the aluminum-lithium alloy. Adding a small amount of Ag to the aluminum-lithium alloy with a high Cu / Mg ratio will significantly improve their aging strengthening effect, and the effect is very obvious. At the same time, it will also change the aging precipitation sequence of the AI-Li-Cu series alloy and promote the T1 phase and The nucleation of the Ω phase is precipitated as an intermetallic compound, and the T1 phase is evenly distributed in the alloy, and at the same time, the grain size can be made fine.
The solid solubility of Zr in Al alloy is very small. Adding 0.1% to 0.2% of Zr to Al-Li alloy can precipitate Al3Zr dispersed particles at the grain boundary or sub-grain boundary, pinning the grain boundary, inhibiting recrystallization and being able to refine the crystal grain, thus improving The strength and toughness of the alloy; in addition, Al3Zr can be used as the nucleation center of the δ 'phase, which accelerates the process of aging precipitation. However, when the Zr content is too high, a coarse precipitation phase will be formed at the grain boundary, which will destroy the firmness of the bond between the grain boundary and the matrix, which will greatly reduce the performance of the alloy.
The functions of several common alloy elements in aluminum-lithium alloys are introduced below.
1. Element Li (lithium)
Lithium is the lightest metal element with a density of only 0.536g / cm3. During the aging of lithium aluminum alloys, due to precipitation of δ '(Al3Li) phase, the strengthening effect can be described as: the supersaturated solid solution metastable phase δ' and δ phase are spherical, have a LI2 type structure, and the lattice constant is 0.4nm , Is the main strengthening phase of alloy aging, its interface energy is relatively low, about 0.014J / m2, so the nucleation activation energy of δ 'phase is small, and the precipitation speed is very fast, even if quenching is adopted, δ' cannot be effectively suppressed Phase generation. The mismatch between the δ 'phase and the matrix is only 0.08%. This co-lattice is liable to produce coplanar slip, which causes dislocations to accumulate at the interface between the slip plane and the grain boundary, causing stress concentration. The delta phase has a B32 (NaTi) type diamond-like structure. When aging is performed, the delta phase precipitates along the grain boundary of the expanded phase, which can lead to the reduction of Li atoms near the grain boundary and lead to lithium depletion, forming a low-strength precipitation-free zone (PFZ ). When the alloy undergoes plastic deformation, PFZ will preferentially generate cracks, and this area will also reduce the corrosion resistance of the alloy, so in actual production, the formation of δ phase should be suppressed as much as possible. The main factor affecting the strength and toughness of Al-Li alloy is the δ 'phase morphology and distribution in the alloy. As mentioned earlier, the δ 'phase is spherical and has a better strengthening effect on metals.
2. Element Mg (magnesium)
Adding Mg will move the solubility curve of the aluminum-lithium alloy upward, reduce the solid solubility of Li, and increase the volume fraction of the δ 'phase, which can effectively improve the strength of the alloy. It is generally believed that this is due to the combination of Mg and vacancies. The binding energy of Mg and vacancies is relatively large, about 0.25 eV. During quenching, supersaturated vacancies form Mg-vacancy clusters with Mg atoms. These clusters are δ The crystallization of the phase provides a nucleation center. The aluminum-lithium alloy is added with Cu and Mg at the same time. Due to the interaction between Mg and vacancies and Cu atoms, the alloy forms many Cu-Mg clusters after quenching, which becomes the nucleation site of the Cu (q '')-rich phase, which promotes Cu Atoms continue to diffuse into the nucleation area, forming a metastable phase S '. The S 'phase has a lath-like, orthorhombic structure, with lattice constants a = 0.40nm, b = 0.93nm, c = 0.72nm, and its habit plane is not parallel to the densely packed plane of the substrate, which can make the coplanar slip tendency Yu dispersion, effectively improve the strength and toughness of the alloy. The S 'phase preferentially precipitates unevenly at dislocations and other defects, which can reduce or eliminate the precipitation-free zone (PFZ). Due to the large nucleation energy of the S' phase, the gestation period of the S 'phase nucleation is longer during the aging process. Its precipitation also requires long-term heat preservation and aging to achieve.
3. Transition metal elements
1. Cu
The addition of copper to the aluminum-lithium alloy will precipitate the T1 phase. The T1 phase is the most important equilibrium phase of the Al-Li-Cu alloy. It is in the form of a disc or a plate, with a hexagonal structure, with lattice constants a = b = 0.50nm and c = 0.93nm. The T1 phase hinders the dislocation expansion, and also has a pinning effect on the dislocation, and the strengthening effect is more obvious than the δ 'phase. However, the T1 phase densely packed surface (0001) // (111) α and the densely packed direction [1010] // [110] cannot significantly reduce the coplanar slip, so there is no obvious improvement in the alloy's plasticity. The T1 phase is non-uniformly nucleated by stacking faults at crystal defects such as dislocations and sub-grain boundaries. The critical nucleation work is large and the precipitation is very slow. Appropriate amount of pre-deformation can make the T1 phase uniform, fine and dispersed, and can play the role of increasing the alloy dislocation density and increasing the nucleation site of the T1 phase. The study found that the growth of the T1 phase is controlled by the step mechanism. The mismatch between the T1 phase and the matrix is only 0.12%. The number of steps for the T1 phase to grow up on the matrix is limited, so the T1 phase has a low tendency to coarsen at a certain temperature, which can keep the mechanical properties of the alloy stable. However, when the temperature is increased to 200 ℃, the δ 'phase dissolves, the migration rate of Cu and Li atoms to the step is accelerated, the resistance of the step nucleus becomes smaller, the number of steps is doubled, and the T1 phase is significantly coarsened, resulting in a decrease in the mechanical properties of the alloy.
2. Mn
Adding Mn to the aluminum-lithium alloy can form Al6Mn phase and precipitate in the form of particles. The Al6Mn phase can effectively improve the anisotropy of the aluminum-lithium alloy. On the one hand, the Al6Mn dispersed particles themselves uniformly slip during the processing, which transforms the deformation of the alloy from coplanar sliding to uniform slip, so that the structure distribution of the aluminum-lithium alloy is more consistent; on the other hand, the Al6Mn dispersed particles are affected by { The dislocation density of the 111} plane enables the T1 phase to uniformly nucleate on the {111} plane. This feature can effectively reduce and improve the anisotropy of the alloy.
3. Zr
Zr is added to the aluminum-lithium alloy, and Zr and Al can form a metastable phase β '(Al3Zr), which is rod-shaped, has a LI2 structure, and has a lattice constant a = 0.41 nm. The binding energy of Zr atoms to vacancies is large (0.24eV), and it is easy to combine with vacancies during alloy solidification, resulting in the reduction of vacancies combined with lithium atoms, thereby preventing the precipitation of δ 'phase, but the δ' phase can nucleate at the β 'phase interface It grows to form a β '/ δ' composite structure phase, increasing the degree of mismatch with the matrix, and the β '/ δ' phase has a large hardness, and the dislocation is difficult to cut through, which can effectively suppress coplanar slip and improve the alloy Plasticity. Sc and Zr form a very fine ternary co-lattice phase Al3 (Sc1-xZrx). Usually Sc content is 0.07% ~ 0.03%, Zr content is 0.07% ~ 0.15%, the ratio of the two is kept at about 1: 1, which is expressed as Al3 (Sc, Zr). Al3 (Sc, Zr) has a similar structure to δ ', and can become the core of δ' non-uniform nucleation during aging, forming Al3Li / Al3 (Sc, Zr) composite particles.
4. Rare earth elements
Rare earth elements have shown beneficial effects in the process of smelting and solidification of ordinary aluminum alloys, including the effects of degassing, impurity removal and grain refinement of rare earths. The addition of rare earth elements can improve the superplasticity, heat deformability, corrosion resistance, weldability, etc. of ordinary aluminum alloys, and has the effect of reducing impurities. In view of this, scholars at home and abroad have carried out research work on adding trace amounts of Ce (cerium), Y (yttrium), La (lanthanum) and other rare earth elements to aluminum-lithium alloys. The research results show that all rare earth elements can improve aluminum to varying degrees. Lithium alloy structure and performance.
The rare earth elements Ce, Y, La, Sc, etc. can delay the recrystallization process of the aluminum-lithium alloy, and can reduce the recrystallization ratio and refine the recrystallized grain size, refine the precipitation phase and make it uniformly distributed in the alloy At the same time, it can also reduce the negative effects of impurity elements in the aluminum-lithium alloy. Therefore, the rare earth element is a kind of beneficial additive element for the aluminum-lithium alloy, even in the case of adding a trace amount, it can obviously play a relatively good role. At this point, metal scandium is a prominent example, especially when added with metal zirconium, it can make aluminum alloy and magnesium alloy have obvious effects.