The influence of rare earth on aluminum and aluminum alloy
The application of rare earth in casting aluminum alloy was carried out earlier abroad. Although the research and application of rare earth in casting aluminum alloy began in China in the 1960s, it developed rapidly. A lot of work has been done from mechanism research to practical application, and some achievements have been made. With the addition of rare earth elements, the mechanical properties, casting properties and electrical properties of aluminum alloy have been greatly improved. In the field of new materials, the rich optical, electrical and magnetic properties of rare earth elements also play an important role in the production of rare earth permanent magnet materials, rare earth luminescent materials, rare earth hydrogen storage materials, etc. ◆ mechanism of rare earth in aluminum and aluminum alloy
Rare earth has high chemical activity, low potential and special electronic layer arrangement, which can interact with almost all elements. Rare earth commonly used in aluminum and aluminum alloys includes La (LA), Ce (CE), y (yttrium) and SC (scandium). It is often added into aluminum solution with modifier, nucleator and degassing agent, which can purify melt, improve structure and refine grain.
Purification of 01 rare earth
Because of the large amount of gas and oxide inclusions (mainly hydrogen, oxygen and nitrogen) brought into the casting during the melting and casting, the defects such as pinholes, cracks and inclusions are produced (see Fig. 1a), which reduces the strength of the aluminum alloy. The purification of rare earth mainly shows that hydrogen content in aluminum solution is significantly reduced, pinhole rate and porosity are reduced (see Fig. 1b), inclusions and harmful elements are reduced. The main reason is that rare earth has a large affinity with hydrogen, can absorb and dissolve hydrogen in large quantities, and form stable compounds, and will not gather into bubbles, which will significantly reduce the hydrogen content and porosity of aluminum; rare earth and nitrogen form refractory compounds, most of which are removed in the form of slag in the smelting process, so as to achieve the purpose of purifying aluminum liquid.
It is proved that rare earth has the effect of reducing the content of hydrogen, oxygen and sulfur in aluminum and aluminum alloy. Adding 0.1% ~ 0.3% RE in aluminum solution can help to better remove harmful impurities, refine impurities or change their morphology, and make the grains finer and distribute evenly; in addition, re and low melting point harm impurities to form res, reas, repb and other binary compounds, which have high melting point and dense melting point The small size and stable chemical properties can float up to form slag and remove the aluminum solution, and the remaining micro particles become the heterocrystal nucleus of aluminum, thus refining the grains.
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Fig. 1 SEM morphology of 7075 alloy without re and w (RE) =0.3%
a. No re added; B. w (RE) =0.3% added
02 metamorphism of rare earth
The modification of rare earth mainly includes refining grain and dendrite, inhibiting the appearance of thick T2 phase, eliminating the coarse bulk phase distributed in the original crystal and forming spherical phase, which makes the bar and fragment compounds at the grain boundary significantly reduced (see Fig. 2). In general, the radius of rare earth atoms is larger than that of aluminum, and the properties are relatively active. It is easy to fill the surface defects of alloy phase when melting in aluminum liquid, which makes the surface tension on the interface of the old and new two phases decrease, and the growth speed of the crystal nucleus is improved; meanwhile, the surface active film can be formed between the grains and the molten liquid to prevent the grain growth and refine the alloy structure (see Fig. 2b).
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Fig. 2 microstructure of alloy with different RE addition
a. The RE addition is 0; b.re is 0.3%; C.re is 0.7%
The grain size of α (AL) phase becomes smaller after adding rare earth elements, which plays a role in refining the grain. The original coarse dendrite α (AL) phase transition becomes a smaller rose or rod shape. When the content of rare earth is 0.3%, the grain size of α (AL) phase is the smallest, and the grain increases with the increase of rare earth content. The experiment shows that there is a certain incubation period of rare earth metamorphism, and only when it is kept at high temperature will rare earth play the most metamorphism. In addition, the increase of the number of crystal nuclei in the metal crystallization of the compound formed by aluminum and rare earth also makes the alloy structure refined. The results show that rare earth has good modification effect on aluminum alloy.
Microalloying of 03 rare earth
Rare earth is mainly found in aluminum and aluminum alloys in three forms: solid solution in matrix A (AL); segregation in phase boundary, grain boundary and branch boundary; solid solution in compounds or in the form of compounds. The strengthening effect of rare earth in aluminum alloy mainly includes fine grain strengthening, finite solid solution strengthening and second phase strengthening of rare earth compounds.
The existence of rare earth in aluminum and aluminum alloy is closely related to its addition. Generally, when RE content is less than 0.1%, re mainly plays a role in fine grain strengthening and finite solid solution strengthening; when RE content is 0.25% ~ 0.30%, re and Al form a large number of spherical or short rod like intermetallic compounds, distributed in grains or grain boundaries, and a large number of dislocations and fine grain spheroidization and The dispersion of rare earth compounds will produce the effect of second phase strengthening and other microalloying.
◆ effect of rare earth on the properties of aluminum and aluminum alloy
The strength, hardness, elongation, fracture toughness and wear resistance of the alloy can be improved by adding some rare earth. The addition of 0.3% RE to cast aluminum ZL10 alloy increases σ B from 205.9mpa to 274mpa, Hb increases from 80 to 108; 0.42% SC is added to 7005 alloy, and σ B increases from 314mpa to 414mpa, σ 0.2 increases from 282mpa to 378mpa, plasticity increases from 6.8% to 10.1%, and high temperature stability is significantly enhanced; La and CE can significantly improve superplasticity of the alloy. 0.14% ~ 0.64% of al-6mg-0.5mn alloy is added La, the Superplasticity of La increases from 430% to 800% - 1000%. The systematic study of Al SC alloy shows that adding appropriate SC can greatly improve the yield strength and ultimate tensile strength of the alloy. Figure 3 shows SEM morphology of tensile fracture of al-si7-mg0.8 alloy. It shows that it is a typical brittle cleavage fracture without re, while after adding 0.3% RE, obvious dimple like structure appears in the fracture, indicating that it has good toughness and ductility.
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Fig. 3 tensile fracture morphology
a. Not joined re; B. added 0.3% RE
The effect of 02 rare earth on the high temperature properties of alloy
Adding a certain amount of rare earth into aluminum alloy can effectively improve the oxidation resistance of aluminum alloy at high temperature. The high temperature strength of Al Si co crystalline alloy was increased by 33%, the high temperature endurance strength (300 ℃ and 1000 hours) was increased by 44%, and the wear resistance and high temperature stability were significantly improved. The high temperature properties of al-8.4% fe-3.4% Ce alloy can be improved by adding La, CE, y and mixed rare earth to the cast Al Cu alloy; the al-8.4% fe-3.4% Ce alloy can be solidified rapidly, which can be long under 400 ℃ The working time greatly improves the working temperature of aluminum alloy. Adding SC to Al Mg Si alloy, forming the grain boundary of al3sc particles which is not easy to coarsening and co lattice with the matrix at high temperature makes the alloy maintain the recrystallized structure during annealing, and greatly improve the high temperature properties of the alloy.
The influence of 03 rare earth on the optical properties of alloy
Adding rare earth into aluminum alloy can change the structure of oxide film on its surface, and make the surface more bright and beautiful. When 0.12% ~ 0.25% RE is added to the aluminum alloy, the reflectivity of the re 6063 section which is oxidized and colored can reach 92%; when 0.1% ~ 0.3% RE is added to the Al Mg casting aluminum alloy, the best surface finish and luster durability can be obtained.
The influence of 04 rare earth on the electrical properties of alloy
Adding rare earth to high purity aluminum is harmful to the conductivity of the alloy, but the conductivity can be improved to some extent by adding some re to the industrial pure aluminum and Al Mg Si conductive alloy. The experimental results show that the conductivity can be increased by 2% - 3% by adding 0.2% RE to aluminum. Adding a small amount of yttrium rich rare earth into Al Zr alloy can improve the conductivity of the alloy. The alloy has been used by most domestic wire factories. The aluminum re foil capacitor is made by adding micro rare earth to high purity aluminum. In 25kV products, the capacitance index is increased by 1 times, the unit volume capacity is increased by 5 times, the weight is reduced by 47%, and the capacitor volume is significantly reduced.
The influence of 05 rare earth on the corrosion resistance of alloy
In some service environments, especially when chloride ions exist, the alloy is vulnerable to corrosion, crevice corrosion, stress corrosion and corrosion fatigue. In order to improve the corrosion resistance of aluminum alloy, many researches have been carried out. It is found that adding some rare earth to aluminum alloy can effectively improve its corrosion resistance. The results of three years immersion test showed that adding a small amount of rare earth in aluminum can improve the corrosion resistance of aluminum, and the corrosion resistance of aluminum in salt water and artificial seawater is 24% and 32% higher than that of aluminum respectively; the chemical gas phase method is used to add rare earth multi component penetrant（ La, CE, etc.) can form a layer of rare earth conversion film on the surface of 2024 alloy, which makes the electrode potential of aluminum alloy tend to be uniform, and the anti intergranular corrosion and stress corrosion resistance are improved; adding LA to high Mg aluminum alloy can significantly improve the corrosion resistance of the alloy; adding 1.5% ~ 2.5% nd to the aluminum alloy can improve the high temperature performance, air tightness and corrosion resistance of the alloy, and is widely used As aerospace materials.
◆ preparation technology of rare earth aluminum alloy
Rare earth is added in aluminum alloy and other alloys in the form of trace elements. Rare earth has high chemical activity, high melting point and easy oxidation and burning loss at high temperature. This has caused some difficulties in the preparation and application of rare earth aluminum alloy. In the long-term experimental research, people have been exploring the preparation methods of rare earth aluminum alloy. At present, the production methods of rare earth aluminum alloy mainly include mixed melting, molten salt electrolysis and aluminum thermal reduction.
The method of mixing melting is to add rare earth or mixed rare earth metal to high temperature aluminum solution in proportion to produce intermediate alloy or application alloy. Then, the intermediate alloy and the remaining aluminum with the remaining calculated allowance are melted, stirred and refined fully.
02 melting electrolysis
Molten salt electrolysis is to add rare earth oxide or rare earth salt to the industrial aluminum cell when electrolysis aluminum, and electrolysis with alumina to produce rare earth aluminum alloy. Molten salt electrolysis has developed rapidly in China. There are two ways, liquid cathode method and electrolytic eutectoid method. At present, it has been developed to directly add rare earth compounds into industrial aluminum electrolyzer, and produce rare earth aluminum alloy by eutectoid electrolysis of chloride melt.
03 aluminum thermal reduction method
Because of its strong reduction ability, aluminum can form many intermetallic compounds with rare earth. Therefore, aluminum can be used as reducing agent to prepare rare earth aluminum alloy. The main chemical reactions are as follows:
RE2O3 + 6Al→2REAl2 + Al2O3
Among them, rare earth raw materials can be used rare earth oxides or rare earth rich slag; reducers can be pure aluminum or silicon aluminum for industrial use; reduction temperature is 1400 ℃ ~ 1600 ℃. In the early stage, it was carried out under the condition of the existence of heat aid and flux, and many problems would arise in the high reduction temperature. In recent years, researchers have developed a new method of aluminum thermal reduction, which is completed in sodium fluoride and sodium chloride system at a lower temperature (780 ℃), which avoids the problem caused by the original high temperature.
Application progress of rare earth aluminum alloy ◆ 01 application of rare earth aluminum alloy in power industry
Because of its advantages of good conductivity, high current carrying capacity, high strength, wear-resistant, easy to process and long life, rare earth aluminum alloy can be used in the manufacture of cable, overhead transmission line, wire core, sliding wire and special purpose fine conductor. The addition of re in Al Si alloy system can improve the conductivity, because silicon is a impurity element with high content in aluminum alloy has a great influence on electrical properties, while adding some rare earth can improve the existing morphology and distribution of silicon in the alloy, and can effectively improve the electrical properties of aluminum; after adding a small amount of yttrium or yttrium rich mixed rare earth into the heat-resistant aluminum alloy wire, not only can the electric properties be improved The high temperature performance can also improve the conductivity; rare earth can improve the tensile strength, heat resistance and corrosion resistance of the aluminum alloy system. The cable and conductor with rare earth aluminum alloy can increase the span of the cable tower and prolong the service life of the cable.
Application of 02 rare earth aluminum alloy in construction industry
6063 aluminum alloy is widely used in the construction industry. Adding 0.15% ~ 0.25% rare earth can obviously improve the as cast structure and processing structure, and can improve extrusion performance, heat treatment effect, mechanical property, corrosion resistance, surface treatment performance and color tone. It is found that rare earth in 6063 aluminum alloy is mainly distributed in the phase boundary, grain boundary and dendrite of α - Al, they are solid soluble in the compound or in the form of compound, refine dendrite structure and grain, and make the size of insoluble eutectic and dimple size in dimple area decrease significantly, and the distribution is uniform and density increases, which improves the properties of alloy to different degrees, such as type The strength of the material is increased by more than 20%, the elongation is increased by 50%, the corrosion rate is more than doubled, the thickness of oxide film increases by 5% ~ 8%, and the coloring performance is improved by about 3%. Therefore, re-6063 alloy building profile has been widely used.
Application of 03 rare earth aluminum alloy in daily products
The mechanical properties, deep drawing properties and corrosion resistance can be improved by adding trace rare earth to the aluminum alloys such as pure aluminum and Al Mg system for daily use aluminum products. Compared with the aluminum alloy products without rare earth, the corrosion resistance of aluminum pot, aluminum pot, aluminum pan, aluminum plate, aluminum rice box, aluminum furniture bracket, aluminum bicycle and household electrical appliances parts made of Al Mg RE alloy is more than twice as high as that of aluminum alloy products without rare earth, with weight reduced by 10% ~ 15%, the finished product rate increased by 10% ~ 20%, and the production cost decreased by 10% ~ 15%, and it has better deep drawing and deep processing performance. At present, rare earth aluminum alloy daily necessities have been widely used, and the products have increased greatly, and they are sold well in domestic and foreign markets.
Application of 04 rare earth aluminum alloy in other aspects
In the most used aluminum silicon casting alloy, plus a few thousand rare earth, the mechanical processing performance of the alloy can be improved obviously. Many brands of products have been used in aircraft, ship, automobile, diesel engine, motorcycle and armored vehicle (piston, gearbox, cylinder, instrument and other parts). It is found that SC is the most effective element to optimize the microstructure and properties of aluminum alloy. It has strong dispersion strengthening, fine grain strengthening, solid solution strengthening and micro alloy strengthening, which can improve the strength, hardness, plasticity, toughness, corrosion resistance and heat resistance of the alloy. SC Al alloy has been applied in high-tech industries such as aerospace, warship, high-speed train, light vehicle and so on. The c557al mg Zr SC Al alloy developed by NASA has been applied to aircraft fuselage and aircraft structure with high strength and high temperature and low temperature stability. 0146al Cu Li SC alloy developed by Russia has been applied to low temperature fuel storage tank of spacecraft.