The Effect of Rare Earth Elements on Aluminum and Aluminum Alloys

The application of rare earths in cast aluminum alloys has been carried out earlier abroad. Although research and application in this field began in China in the 1960s, it has developed rapidly, and 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 alloys 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. ◆ ◆ The Mechanism of Rare Earth Action in Aluminum and Aluminum Alloys ◆ ◆
Rare earth has high chemical activity, low potential and special Electron shell arrangement, and can interact with almost all elements. The commonly used rare earths in aluminum and aluminum alloys include La (lanthanum), Ce (cerium), Y (yttrium), and Sc (scandium). They are often added to the aluminum melt with modifiers, nucleating agents, and degassing agents, playing a role in purifying the melt, improving the structure, and refining the grain size.
01 Catharsis of Rare Earth
Due to the introduction of a large amount of gas and oxide inclusions (mainly hydrogen, oxygen, and nitrogen) during the melting and casting of aluminum alloys, defects such as pinholes, cracks, and inclusions are produced in the castings (see Figure 1a), reducing the strength of the aluminum alloy. The Catharsis effect of rare earth is mainly manifested in the obvious reduction of hydrogen content in molten aluminum, the reduction of pinhole rate and porosity (see Figure 1b), and the reduction of inclusions and harmful elements. Mainly due to the high affinity between rare earth elements and hydrogen, they can adsorb and dissolve hydrogen in large quantities, and form stable compounds that do not aggregate into bubbles, resulting in a significant decrease in the hydrogen content and porosity of aluminum; Rare earth and nitrogen generate refractory compounds, which are mostly eliminated in the form of slag during the melting process, thus achieving the goal of purifying aluminum liquid.
Practice has proven that rare earths have the effect of reducing the content of hydrogen, oxygen, and sulfur in aluminum and aluminum alloys. Adding 0.1% to 0.3% RE to aluminum liquid helps to better remove harmful impurities, refine impurities, or change their morphology, making their grains refined and evenly distributed; In addition, RE and harmful impurities with low melting point form Binary phase such as RES, REAs, REPb, etc. These compounds have the characteristics of high melting point, low density, and stable chemical properties, which can be floated up to form slag and removed, thus purifying the aluminum liquid; The remaining fine particles become heterogeneous nuclei of aluminum, thereby refining the grains.
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Figure 1 SEM Morphology of 7075 Alloy with and without RE Addition and w (RE)=0.3% Addition
a. RE not added; b. Add w (RE)=0.3%
02 Metamorphism of Rare Earth
The Metamorphism of rare earth is mainly manifested in refining grains and dendrites, inhibiting the appearance of coarse lamellar T2 phase, eliminating the coarse massive phase distributed in the primary crystal and forming spherical phase, so that the strip and fragment compounds at the grain boundary are significantly reduced (see Figure 2). In general, the Atomic radius of rare earth is larger than the Atomic radius of aluminum, and its properties are relatively active. Melting in aluminum liquid is very easy to fill the surface defects of alloy phase, which reduces the surface tension on the interface between new and old phases, and improves the growth rate of crystal nucleus; At the same time, it can also form a surface active film between the grains and the molten liquid, preventing the generated grains from growing and refining the alloy structure (as shown in Figure 2b).
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Figure 2 Microstructure of alloys with different RE additions
a. The amount of RE added is 0; b. The addition amount of RE is 0.3%; c. The addition amount of RE is 0.7%
After adding rare earth elements α The grain size of the (Al) phase began to decrease, playing a certain role in refining the grain size. Originally, the coarse dendritic structure α (Al) transforms into smaller rose or rod shaped phases, when the rare earth content is 0.3% α The grain size of the (Al) phase is the smallest, and as the rare earth content further increases, the grain size gradually increases. Experiments have proved that there is a certain latency of rare earth Metamorphism, and only when it is kept at a high temperature for a certain time, rare earth will play the largest Metamorphism. In addition, the compound formed by aluminum and rare earths significantly increases the number of crystal nuclei during metal crystallization, which also refines the alloy structure. Research has shown that rare earths have good modification effects on aluminum alloys.
Microalloying Effect of 03 Rare Earth Elements
Rare earths mainly exist in three forms in aluminum and aluminum alloys: solid solution in the matrix α (Al); Segregation at phase boundaries, grain boundaries, and branch grain boundaries; Solid solution in or in the form of a compound. The strengthening effects of rare earth in aluminum alloys mainly include grain refinement strengthening, limited Solid solution strengthening and the second phase strengthening of rare earth compounds.
The existence form of rare earth in aluminum and aluminum alloys has a great relationship with its addition amount. Generally, when RE content is less than 0.1%, the role of RE is mainly fine grain strengthening and limited Solid solution strengthening; When the RE content is between 0.25% and 0.30%, RE forms a large number of spherical or short rod-shaped intermetallic compounds with Al, distributed within the grains or grain boundaries, and exhibits a large number of dislocations, fine grain spheroidized structures, and dispersed rare earth compounds, resulting in microalloying effects such as second phase strengthening.
◆ ◆ Effect of Rare Earth Elements on the Properties of Aluminum and Aluminum Alloys ◆ ◆ 01 Effect of Rare Earth Elements on the Comprehensive Mechanical Properties of Alloys
The strength, hardness, elongation, Fracture toughness and wear resistance of the alloy can be improved by adding appropriate amount of rare earth. 0.3% RE is added to the cast aluminum ZL10 alloy, which σ B increased from 205.9MPa to 274MPa, and HB increased from 80 to 108; Add 0.42% Sc to alloy 7005, which σ B increased from 314MPa to 414MPa, σ 0.2 increased from 282MPa to 378MPa, plasticity increased from 6.8% to 10.1%, and high-temperature stability was significantly enhanced; La and Ce can significantly improve the superplasticity of the alloy. Adding 0.14%~0.64% La to the Al-6Mg-0.5Mn alloy increases its superplasticity from 430% to 800%~1000%; A systematic study was conducted on Al-Sc alloy and it was found that adding an appropriate amount of Sc can significantly improve the yield strength and ultimate tensile strength of the alloy material. Figure 3 shows the SEM morphology of the tensile fracture surface of Al-Si7-Mg0.8 alloy, indicating a typical brittle cleavage fracture without the addition of RE. However, after the addition of 0.3% RE, a clear dimple like structure appeared in the fracture surface, indicating its good toughness and ductility.
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Figure 3 Tensile fracture morphology
a. Not added to RE; b. Add 0.3% RE
The Effect of 02 Rare Earth Elements on the High Temperature Properties of Alloys
Adding a certain amount of rare earth elements to aluminum alloys can effectively improve their high-temperature oxidation resistance. Adding 1% to 1.5% mixed rare earths to the cast Al Si eutectic alloy resulted in a 33% increase in high-temperature strength and a 44% increase in high-temperature rupture strength (300 ℃, 1000 hours), as well as a significant improvement in wear resistance and high-temperature stability; Adding La, Ce, Y, and mixed rare earths to cast Al Cu alloy can improve the high-temperature performance of the alloy; The rapidly solidified Al-8.4% Fe-3.4% Ce alloy can work for a long time below 400 ℃, greatly improving the working temperature of aluminum alloys; By adding Sc to Al Mg Si alloy, Al3Sc particles that are not easily coarsened and aligned with the matrix at high temperatures are pinned to the grain boundaries, maintaining the non recrystallized structure of the alloy during annealing, greatly improving the high-temperature performance of the alloy.
The Effect of Rare Earth Elements on the Optical Properties of Alloy 03
Adding rare earth elements to aluminum alloy can change the structure of its surface oxide film, making the surface more bright and beautiful. When 0.12%~0.25% RE is added to the aluminum alloy, the reflectivity of the oxidized and colored rare earth 6063 profile is as high as 92%; Adding 0.1% to 0.3% RE to Al Mg cast aluminum alloys can achieve the best surface smoothness and gloss durability.
The Effect of 04 Rare Earth Elements on the Electrical Properties of Alloys
Adding rare earth elements to high-purity aluminum is harmful to the conductivity of the alloy, but adding an appropriate amount of RE to industrial pure aluminum and Al Mg Si conductive alloys can improve the conductivity to a certain extent. The experimental results show that adding 0.2% RE to aluminum can increase the conductivity by 2% to 3%. Adding a small amount of yttrium rich rare earth to Al Zr alloy can improve its conductivity, which has been adopted by most wire factories in China; Add trace rare earth elements to high-purity aluminum to produce Al-RE foil capacitors, which are used in 25kV products. The capacitance index is doubled, the unit volume capacity is increased by 5 times, the weight is reduced by 47%, and the capacitor volume is significantly reduced.
The Effect of 05 Rare Earth Elements on the Corrosion Resistance of Alloys
In some service environments, especially in the presence of chloride ions, alloys are vulnerable to corrosion, Crevice corrosion, stress corrosion and Corrosion fatigue. In order to improve the corrosion resistance of aluminum alloys, many studies have been conducted, and it has been found that adding an appropriate amount of rare earth to aluminum alloys can effectively improve their corrosion resistance. The samples made by adding different amounts of mixed rare earths (0.1% ～ 0.5%) to aluminum were soaked in brine and Artificial seawater for three consecutive years. The results show that adding a small amount of rare earths to aluminum can improve the corrosion resistance of aluminum, and the corrosion resistance in brine and Artificial seawater is 24% and 32% higher than that of aluminum, respectively; By using chemical vapor phase method and adding rare earth multi-component infiltration agents (La, Ce, etc.), a layer of rare earth conversion film can be formed on the surface of 2024 alloy, making the surface electrode potential of aluminum alloy more uniform and improving its resistance to intergranular corrosion and stress corrosion; Adding La to high Mg aluminum alloy can significantly improve the alloy's resistance to marine corrosion; Adding 1.5% to 2.5% Nd to aluminum alloys can improve their high-temperature performance, airtightness, and corrosion resistance, making them widely used as aerospace materials.
◆ Preparation Technology of Rare Earth Aluminum Alloy ◆ ◆
Rare earth elements are often added in the form of trace elements in aluminum alloys and other alloys, which have high chemical activity, high melting point, and are prone to oxidation and burning at high temperatures. This poses certain difficulties for the preparation and application research of rare earth aluminum alloys. In long-term experimental research, people continuously explore the preparation methods of rare earth aluminum alloys. At present, the main production methods for preparing rare earth aluminum alloys include mixed melting method, molten salt electrolysis method, and aluminothermic reduction method.
01 Blending method
The mixed melting method is to add rare earth or mixed rare earth metals in proportion to high-temperature aluminum liquid to produce intermediate alloys or application alloys. The intermediate alloy and the remaining aluminum according to the calculated surplus are then melted together, thoroughly stirred, and refined.
02 Melt electrolysis method
Molten salt electrolysis method is the process of adding rare earth oxides or salts to industrial aluminum electrolysis cells during aluminum electrolysis, and electrolyzing them together with aluminum oxide to produce rare earth aluminum alloys. The development of molten salt electrolysis method in China is relatively fast, and there are generally two ways: liquid cathode method and electrolytic eutectoid method. Currently, it has developed to the point where rare earth compounds can be directly added to industrial aluminum electrolysis cells and chloride melts can be electrolyzed using eutectoid method to produce rare earth aluminum alloys.
03 Aluminum thermal reduction method
Due to the strong reduction ability of aluminum metal and its ability to form various intermetallic compounds with rare earths, aluminum can be used as a reducing agent to prepare rare earth aluminum alloys. The main chemical reactions are represented by the following formula:
RE2O3+6Al → 2REAl2+Al2O3
Among them, rare earth raw materials can be rare earth oxides or rare earth rich slag; Reducing agents can use industrial pure aluminum or silicon aluminum, etc; The reduction temperature ranges from 1400 ℃ to 1600 ℃. In the early stage, it was carried out under the conditions of the presence of heat and flux, and high reduction temperatures can cause many problems; In recent years, researchers have developed a new aluminothermic reduction method. At a lower temperature (780 ℃), the aluminothermic reduction reaction is completed in the system of Sodium fluoride and sodium chloride, which avoids the problems caused by the original high temperature.
◆ ◆ Application progress of rare earth aluminum alloys ◆ ◆ 01 Application of rare earth aluminum alloys in the power industry
Due to the advantages of good conductivity, high current carrying capacity, high strength, wear resistance, easy processing, and long service life, rare earth aluminum alloys can be used for manufacturing thin wires for cable, overhead transmission lines, wire cores, sliding wires, and special purposes. Adding a small amount of RE in the Al Si alloy system can improve the conductivity, because the silicon in the aluminum alloy is an impurity element with a high content, which has a greater impact on the electrical properties. Adding an appropriate amount of RE can improve the existing morphology and distribution of silicon in the alloy, and can effectively improve the electrical properties of aluminum; Adding a small amount of yttrium or yttrium rich rare earth mixture to heat-resistant aluminum alloy wires not only maintains good high-temperature performance but also improves conductivity; Rare earth elements can improve the tensile strength, heat resistance, and corrosion resistance of aluminum alloy systems. The use of rare earth aluminum alloy cables and wires can increase the span of cable towers and extend the service life of cables.
The Application of 02 Rare Earth Aluminum Alloy in the Construction Industry
The most widely used aluminum alloy in the construction industry is 6063. Adding 0.15% to 0.25% rare earth can significantly improve the cast and processed microstructure, as well as improve extrusion performance, heat treatment effect, mechanical properties, corrosion resistance, surface treatment performance, and color tone. Research has found that rare earths are mainly distributed in 6063 aluminum alloy α－ Al neutralizes phase boundaries, grain boundaries, and interdendritic boundaries, which are solidly dissolved in compounds or exist in the form of compounds, refining the dendrite structure and grains, significantly reducing the size of undissolved eutectic and dimples in the dimple zone, distributing uniformly, and increasing density. This improves the various properties of the alloy to varying degrees, such as increasing the strength of the profile by more than 20%, increasing the elongation by 50%, reducing the corrosion rate by more than twice, and increasing the thickness of the oxide film by 5% to 8%, The coloring performance is improved by about 3%. Therefore, RE-6063 alloy building profiles have been widely used.
Application of 03 Rare Earth Aluminum Alloy in Daily Products
Adding trace amounts of rare earth elements to pure aluminum and Al Mg series aluminum alloys used in daily aluminum products can significantly improve mechanical properties, deep drawing resistance, and corrosion resistance. Compared with aluminum alloy products without rare earths, daily necessities such as aluminum pots, aluminum pots, aluminum plates, aluminum lunch boxes, aluminum furniture brackets, aluminum bicycles, and home appliance components made of Al Mg RE alloy have more than twice the corrosion resistance, reduced weight by 10% to 15%, increased yield by 10% to 20%, reduced production costs by 10% to 15%, and better deep drawing and deep processing performance. At present, rare earth aluminum alloy daily necessities have been widely used, with a significant increase in products and best-selling in domestic and foreign markets.
Application of 04 rare earth aluminum alloy in other fields
Adding a few thousandths of rare earth elements to the aluminum silicon series casting alloy with the highest usage can significantly improve the mechanical processing performance of the alloy. There are multiple brands of products used in aircraft, ships, automobiles, diesel engines, motorcycles, and armored vehicles (such as pistons, gearboxes, cylinders, and instrument components). In research and application, it is found that Sc is the most effective element to optimize the structure and properties of aluminum alloys. It has strong dispersion strengthening, grain refinement strengthening, Solid solution strengthening and microalloy strengthening effects on aluminum, and can improve the strength, hardness, plasticity, toughness, corrosion resistance, heat resistance, etc. of the alloy. Sc-Al series alloys have been applied in high-tech industries such as aerospace, ships, high-speed trains, and light vehicles. The C557Al Mg Zr Sc series scandium aluminum alloy developed by NASA has high strength and high and low temperature stability, which has been applied to