Application of Rare Earth in Aluminum Alloy

【China Aluminum Industry Network】Rare earth is an effective additive in the metallurgical industry. Rare earth metals have high chemical activity, low potential and special electronic shell structure, and can react with almost all elements. China's rare earth resources are very rich, full range, good quality, wide distribution, easy mining. The proven rare earth reserves of 370 million tons, accounting for 80% of the world's reserves, ranking first in the world. In recent years,

Rare earths have been widely used in fields such as metallurgy, machinery, petrochemicals, electronics, nuclear energy, medical treatment, agriculture, aerospace and defense industries. The application of rare earths in aluminum and its alloys started relatively late. Foreign countries began in the 1930s, but China began in the 1960s, but it has developed rapidly, especially in the role and application of aluminum and its alloys have been obtained. Obvious effect. This mainly focuses on aluminum-silicon-based casting alloys, aluminum-magnesium-silicon-silicon (zinc)-based deformed aluminum alloys, aluminum alloy wires, and piston alloys. Some progresses have also been made in the study of the influence of rare earths on aluminum and its alloys.

First, the role of rare earths in aluminum and its alloys The rare earth elements are very lively and easily react with gases (such as hydrogen), non-metals (such as sulfur), and metals to produce the corresponding stable compounds. The atomic radius of rare earth elements is smaller than common metals such as lead, magnesium, etc. The solid solubility in these metals is extremely low, and almost no solid solution can be formed. The rare earth element added to the aluminum alloy can play a role of micro-alloying; in addition, it has a strong affinity with hydrogen and other gases and many non-metals, and can generate a compound with a high melting point, so it has a certain degree of hydrogen removal, refining, Purification; At the same time, the chemical activity of rare earth elements is extremely strong. It can selectively adsorb on the already formed crystal grain boundaries and hinder the growth of crystal grains. As a result, the grain refinement and metamorphism are caused.

1. Metamorphism The metamorphism treatment refers to adding a small amount or a trace amount of a modificator to the metal and the alloy to change the crystallization conditions of the alloy to improve the structure and performance of the alloy. The modifier is also known as a grain refiner or inoculant. In general, the radius of rare earth atoms. Because the rare earth element is more active, it melts in the aluminum liquid and can easily fill the surface defects of the alloy phase, thereby reducing the surface tension on the interface between the old and new phases, and increasing the crystal growth speed. At the same time, it can form a surface active film between the crystal grains and the alloy liquid, preventing the growth of the generated crystal grains and refining the microstructure of the alloy. In addition, as a foreign crystal nucleus, a compound formed of aluminum and a rare earth causes the structure of the alloy to be refined due to a large increase in the number of crystal nuclei when the metal is crystallized. Rare earths mainly play a role in metamorphism in aluminum-silicon alloys, making needle-like and platy eutectic silicon into spherical particles, which reduces the size of primary silicon. With different metamorphic capacities of different rare earths, La and Eu have a strong metamorphic effect, while mixed rare earths and Ce have only moderate metamorphic ability. The metamorphic ability of lanthanides is closely related to their atomic radii. As the atomic radius decreases from 0.187 nm for La to 0.175 nm for Er, the metamorphism of lanthanides gradually decreases. In general, the atomic radius is less than 0.18 nm, and the metamorphism is reduced to a degree that has no practical significance. The metamorphic ability of different rare earth elements can be measured by the critical metamorphic cooling rate (Vc). The smaller the Vc is, the more obvious the metamorphic effect is. When V is less than Vc, any concentration of rare earth elements can not cause deterioration of the alloy. This is rare earth and other. One of the major differences in modifiers. The study of Al-Si system shows that the modification process directly affects the metamorphism of rare earth. The key to obtaining a stable metamorphic structure is to reduce the burning of rare earths and prevent the segregation of rare earths, so that the rare earths diffuse rapidly and evenly into the aluminum liquid; in order to obtain a stable metamorphic structure, the metamorphic temperature should be increased as much as possible, and the metamorphism should be allowed to stand still after deterioration. After refining, the slag is strictly controlled and, as far as possible, no refining and covering are performed with the halogen element flux. Rare earth metamorphism has a certain incubation period, and it must be maintained at a high temperature for a certain period of time. The rare earths will play a greater role in metamorphism.

2. Purification (1), degassing of rare earths and effect on pinhole rate During the melting and casting of aluminum and its alloys, a large amount of gas will dissolve into the aluminum liquid, which is mainly hydrogen (about the gas in the aluminum liquid). 85%), followed by oxygen and nitrogen. The source of hydrogen is mainly the water vapor in the charge, the oil and water in the aluminum ingot and scrap, and the “aluminum rust”—Al(OH)3 on the surface of the aluminum ingot. Hydrogen is the main cause of pinholes in aluminum castings and significantly reduces the strength of aluminum. The addition of rare earths to aluminum and its alloys can act as a degas. When the addition amount of rare earth is less than 0.3%, the effect of rare earth hydrogen removal is obvious, and the reduction rate of pinhole rate is also larger. When the rare earth content is greater than 0.3%, the decrease in hydrogen content slows as the rare earth content increases. If Y, La single rare earth is used, when the content of rare earth exceeds 0.3%, the increase of rare earth content causes the hydrogen content to rise again, and the change of pinhole rate also has the same pattern, but the change range is more obvious. The authors believe that the effect of dehydrogenation is Y>La>Re (refused rare earth) sequentially; from the amount of addition, the single rare earth content is preferably less than 0.3%. Rare earth with oxygen, nitrogen can generate a refractory compound Re2O3 and ReN2. In the smelting process, most of the slag is excluded; at the same time, when the temperature is less than 200° C., the rare earth can react violently with fluorine and chlorine to generate rare earth fluorides and rare earth chlorides to remove fluorine and chlorine from the aluminum. Therefore, rare earths can be used as purifiers in aluminum alloys.

(2) Rare earths to remove impurities and influence on inclusions Compounds distributed in aluminum and its alloy matrix and grain boundary are various intermetallic compounds, oxides, and aluminum oxides. The composition, morphology, distribution, and quantity of these compounds have a significant effect on the properties of aluminum and its alloys, especially the plastic processing properties. Inclusions in aluminum and its alloys are mainly non-metallic inclusions such as Al2O3. The presence of these inclusions not only degrades the processing and mechanical properties of the alloy, but also deteriorates the casting properties. When 0.2% rare earths are added to pure aluminum (as-cast and deformed forms), the massive massive phase in the original crystal disappears, the spherical rare earth phases form, and the stripe-like and fragmented compounds at the grain boundaries are significantly reduced. Formed to form a well-shaped uniform tissue. The addition of 0.2% rare earth to the aluminum alloy reveals that the coarse spherulites originally distributed in the crystals form a uniformly distributed spherical phase on the substrate, and at the same time eliminate brittle lumps and strips at the grain boundaries, forming Fine-point chain-like plastic compounds are distributed along the deformation direction. The composition of the constituents of the matrix, grain boundary compounds, intragranular coarse compounds, and spherical compounds in the aluminum and its alloy before and after the addition of the rare earth was measured. It was found that by adding rare earths, impurities such as iron and other elements in aluminum and its alloys are segregated toward the spherical phase of high rare earths, so that the impurity elements at the later solidified grain boundaries are greatly reduced, and the grain boundary is purified, making The brittle phase of the high-iron at the grain boundary decreases, the grain boundary strength increases, and the plasticity improves. The point-chain compound distributed at the grain boundary is a low-iron, low-earth component phase, and is a plastic compound that is close to the aluminum matrix, especially rare earth. The amount of segregation in the aluminum alloy spherical phase is larger than that in pure aluminum, so its distribution on the grain boundary of the aluminum alloy is very small, so the grain boundary of the aluminum alloy is thinner and purer than pure aluminum.

There are two reasons why rare earths can significantly reduce the number of inclusions:

(1) Rare-earth oxides have the characteristics of high melting point and high specific gravity, and their specific gravity is about 2.5 times higher than that of pure aluminum. The greater the proportion of oxides, the more the amount of inclusions sinks during the standing process, and the less the amount of inclusions remaining in the aluminum liquid;

(2) After the rare earth is added into the aluminum alloy liquid, the aluminum liquid is stable and does not generate intense boiling like other refining agents, and the secondary oxidation in the melting process is very small, so the amount of the regenerated oxide film (Al2O3) entrapped is very small. The deoxidation ability of rare earth elements is stronger than that of strong deoxidizers such as Al, Mg, Ti, etc. Trace rare earths can make [O] to <1×10-4%. The rare earth desulfurization ability is also quite strong, and RES or RE2S3 can be generated. The product mainly depends on the activity or solubility of rare earth and sulfur. Rare-earth elements in the molten metal can also react with O and S simultaneously to form RE2O2S-type sulfides. Rare earth elements can also be combined with P, Sn, As and other low-melting metal elements to produce compounds such as REP, RESn, and REAs. These rare earth compounds all have characteristics such as high melting point and light specific gravity. When their melting points are higher than the metal smelting temperature, they can float to a part of slag. Their tiny particles become heterogeneous crystal nuclei in the aluminum crystallization process and serve the purpose of removing impurities from aluminum and its alloys. Therefore, rare earths can redistribute impurities in aluminum, spheroidize coarse-grained compounds, and purify grain boundaries, improving the plastic processing properties of deformed aluminum and its alloys.

3. Alloying The strengthening effect of rare earths in aluminum alloys includes fine-grained strengthening, limited solution strengthening, and second phase strengthening of rare earth compounds. When rare earths are added in different amounts, the rare earths mainly exist in three forms in the aluminum alloy: solid solution in the matrix α(Al); segregation in the phase boundary, grain boundary and dendritic boundary; solid solution in the compound or in compound form exist. When the rare earth content is low (less than 0.1%), rare earths are mainly distributed in the two previous forms. The former forms play a role of limited solid solution strengthening. The second form increases the deformation resistance, promotes the proliferation of dislocations, and increases the strength. After adding rare earth, the grain size of the alloy in the as-cast microstructure of the alloy decreases significantly, the secondary dendrite spacing may decrease, and the intermetallic compounds formed by rare earth elements such as Al, Mg, and Si have spherical and short rod-like distributions. Within the grain boundaries or boundaries, there are a large number of dislocation distributions in the tissue. When the rare earth content is greater than 0.1%, the latter existing form begins to dominate. At this time, the rare earth and other elements in the alloy begin to form many new phases containing rare earth elements; at the same time, changing the shape and size of the second phase may cause the second phase to change from long strips and the like to short rods. The size of the particles also becomes finer and diffusely distributed. Most of the second phases containing rare earth elements have been characterized by particle, spheroidization and refinement. This change has strengthened the aluminum alloy to some extent.

Second, the application of rare earth in aluminum and its alloy rare earth unique physical and chemical properties have developed a large number of rare earth-containing alloy materials. Rare earth is not only widely used in military industry, agriculture, light industry, handicraft industry and transportation industry, but also widely used as building materials, household life appliances and sporting goods.

The application of rare earths in conductive aluminum alloys is a field in which China has a wide application area, mature technology, high industrial value, and good economic efficiency. The highly conductive rare earth aluminum alloy generally refers to an alloy in which rare earths are added to pure aluminum, aluminum-magnesium-silicon and aluminum-magnesium alloys, and aluminum-zirconium-bismuth alloys. Mainly used for the manufacture of overhead transmission lines, cable lines, slip wires, cores, general wires, special-purpose thin wires and special wires and other aluminum wires. Its rapid development has been extended from 3.15 million V to 5 million V high-voltage lines, with an area ranging from tens of mm2 to several hundred mm2, and the development of wire and cable to the conductive busbars. They have become new products in conductive aluminum alloys in China, with high strength, large current carrying capacity, long service life, wear resistance and easy processing. At present, there are more than 20 provinces and municipalities in China that produce rare earth aluminum alloy wires with an annual output of more than 100,000 metric tons.

1. The rare earth-aluminum alloy single rare earth metal has high chemical properties, is easily oxidized and burned during melting, is inconvenient to store and transport, has high costs, and has a high melting point and high density, and is not easily added to aluminum. Therefore, in most cases, prefabricated rare earth-aluminum intermediate alloys are used, which can not only reduce the oxidation loss, reduce the cost, but also facilitate the transportation. When adding, the operation is simple and safe, and the composition is easy to control. The alloy with stable composition and reliable quality can be obtained.

2. Application of Rare Earths in High Purity Aluminum Capacitors High purity aluminum special aluminum foils containing rare earths are the ideal new materials for producing low electrolytic capacitors. Some capacitor manufacturers believe that the addition of trace amounts of rare earths in high-purity aluminum can significantly increase the corrosion factor K of aluminum foils used in the anodes of aluminum electrolytic capacitors, while the strength and capacitance are greatly increased. The volume of the manufactured capacitor is obviously reduced, and the material has been used in batches in some capacitors and has achieved certain results. However, the mechanism of action of rare earths in it is different, and in-depth research is being conducted.

3. Application of rare earth in aluminum building profiles In a typical 6063 aluminum alloy for architectural building profiles, the content of added rare earth is preferably 0.17% to 0.25%. Excessive addition may adversely affect various properties. . Rare earth has obvious improvement effect on the mechanical properties and processing performance of 6063 series aluminum alloy. After deformation, the tensile strength of the profile can be increased by 5% to 10%, the hardness increased by 8%, and the elongation rate also increased. It can reduce the bar preheating temperature and increase the extrusion speed. The study also found that 6063 aluminum profiles with rare earth elements added after oxidation and coloring have a significant increase in film thickness, film hardness and gloss, and have improved resistance to acid, alkali and salt corrosion. In short, after adding appropriate amount of rare earth to 6063 aluminum, the microstructure of the alloy is refined, the color is uniform, beautiful and durable, and it is welcomed by users.

4. Application of rare earths in aluminum alloy window screens Rare-earth aluminum alloy screens are superior to the same types of aluminum alloy screens without added rare earth in terms of strength, corrosion resistance, lightness, air permeability, processability, and cost. A wide range of applications. For example, rare earth aluminum alloy screen sash produced by Anlu Window Screening Factory, Qijiang Piston Factory, etc. have all been sold at home and abroad.

5. Application of Rare Earths in Daily Aluminum Products Adding trace rare earths to pure aluminum and Al-Mg alloys for daily aluminum products can significantly improve mechanical properties, deep drawability, and corrosion resistance. The daily production of rare earth aluminum alloy pots, pots, lunch boxes, cups and dishes developed and produced by China Aluminum Products Factory has a specific radioactivity of 1.22×103 Bq/kg, lower than the national standard (1.85×104 Bq/kg) and a tensile strength increase of 20%. 45%, corrosion resistance increased 2.85 times, yield increased 7% to 20%, aluminum pot can be reduced by 15% to 20% under the same strength, improve the material utilization, and reduce costs. The Al-Mg-RE alloy sheet developed by Baotou Metallurgical Research Institute has a 30% to 40% higher strength, a 30% increase in elongation, and a 1X increase in corrosion resistance; it is suitable for the manufacture of a variety of deep-drawn and deep-processed articles for daily use. It can be reduced by 15% to 20% and weight is reduced by 14%. At present, the daily use of rare earth aluminum alloys has been widely used, and the products have been greatly increased, and they have been well sold both at home and abroad. In addition, at present, some manufacturers use rare earth aluminum alloys to produce washing machine linings, reflections of large-scale spotlights, aluminum bicycle parts and decorative frames, furniture supports, eyeglass frames, and other supplies and home appliance parts, and have achieved certain results. Look good.

6, the application of rare earth in other areas of the Ministry of Space 621 by adding an appropriate amount of rare earth elements in the HZL401 alloy, to obtain a good heat resistance of aluminum alloy material, is currently being used on the "Qi 7". The rare earth aluminum alloy zipper jointly developed by Inner Mongolia Rare Earth Research Institute and Jining Zipper Factory has exceeded its original LF10 alloy. The use of rare earths in aluminum alloys for pens has also been successful, namely, the modification of pure aluminum by domestic rare earths has replaced the original 99.99% high purity aluminum. Baotou Rare Earth Research Institute and Jining Aluminum Products Factory have developed and produced rare earth aluminum alloy printing trays for printing. It has the advantages of good surface quality, light weight, high strength and hardness, good plasticity, etc., which makes the pad printing effect is good, not easy to deform , fully able to meet the printing quality requirements.

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