Effect of rare earth on copper and copper alloy
Copper and copper alloy are important industrial raw materials, which are widely used in electrical machinery, components, engineering, transportation and other fields. The development of high and new technology puts forward higher requirements for the properties of copper and copper base alloys. Rare earth is a metal element with active chemical properties, which can interact with many elements. The application of rare earth in nonferrous metals has attracted more and more attention of material workers. The results show that rare earth in copper and copper base alloy can Degas, change microstructure, improve machining performance, improve corrosion resistance and wear resistance.
The role of rare earth in copper and copper alloy
The metal activity of rare earth is second only to alkali metal and alkaline earth metal. Among the rare earth elements, scandium, yttrium and lanthanum increase in order of metal activity, and decrease from lanthanum to lutetium, that is to say, lanthanum is the most active. Adding rare earth to copper and its alloys has three main functions: purification, modification and alloying.
Deoxidation and degassing of rare earth
The affinity between rare earth and oxygen is very strong, rare earth oxide has good thermal stability, so the deoxidation product floats on the surface of copper liquid and enters the slag phase and is removed. Using rare earth metal to remove a small amount of oxygen from copper and its alloy can obtain obvious effect. Thermodynamic calculation shows that lanthanum, cerium, praseodymium and rubidium are more powerful deoxidizers than titanium, germanium and zirconium, and their deoxidization ability is obvious at high temperature It is higher than aluminum and zirconium, and also higher than beryllium and magnesium, but similar to calcium. Copper solution is basically insoluble for nitrogen, carbon dioxide and water vapor, but has strong dissolving capacity for O2, SO2 and H2. Hydrogen is dissolved in copper solution in atomic state, rare earth elements are easy to interact with atomic hydrogen, and form stable hydrides of Rh2 and rh3 type (R represents rare earth gold), which is a strong exothermic reaction. Thus, rare earth elements are very strong hydrogen absorbers, because of the general The amount of hydrogen dissolved in copper is small. After adding rare earth, hydrogen will be absorbed by rare earth preferentially, and dissolved in copper and alloy in the state of solid solution of R-H system. Although hydrogen content will not be reduced at this time, stable solid solution has been formed between hydrogen and rare earth, so the hydrogen disease of copper embrittlement caused by water vapor generated by hydrogen reducing oxide in copper under heating can be avoided, and the harm of hydrogen can be eliminated effect.
Rare earth metals react with hydrogen to form Reh type stable hydrides, which can control the hydrogen in copper solution and prevent the possibility of “hydrogen embrittlement” during plastic processing. In the process of copper processing, the addition of rare earth to the molten copper containing hydrogen can rapidly absorb and dissolve the hydrogen in the atomic state from copper, and react with it to generate hydride under certain conditions. This kind of hydride has small density and is easy to float on the surface of copper liquid. Under the action of high temperature, it decomposes again, exhausts hydrogen, or is oxidized into slag phase.
Rare earth has strong chemical activity and can combine with many fusible components to form binary or multi-component compounds. It can interact with low melting point elements such as sulfur, phosphorus, tin, tin and lead to form high melting point rare earth and metal compounds with various atomic ratios, such as ce3pb (1200 ℃), bice3 (1400 ℃). These high melting point rare earth compounds will keep solid state and be discharged from liquid copper together with slag, so as to achieve the purpose of removing impurities.
Modification of rare earth in copper and copper alloy
First of all, rare earth can eliminate dendrite area and refine grains rapidly, and the high melting point compound of rare earth can be distributed in dispersion state, so as to improve its plasticity and strength and reduce surface cracks and defects. The addition of rare earth element Jin to H62 brass can refine the as cast grains and improve the mechanical properties of H62 brass at room temperature. Secondly, rare earth can change some bar, sheet or even block impurities in metals and alloys into point or ball, so as to improve or improve their mechanical properties and processing properties. Finally, impurities can be evenly distributed in the whole crystal to improve the metal properties. To sum up, adding rare earth to copper and copper alloy can produce four effects:
- (1) It forms high melting point compounds with oxygen, sulfur, lead, lead, etc. and enters the slag, which plays a role of impurity removal and purification;
- (2) It can form stable compound with hydrogen and dissolve in copper in the state of solid solution. Although it can’t reduce the content of hydrogen, it plays the role of hydrogen fixation, so as to avoid “hydrogen embrittlement”;
- (3) The morphology and distribution of some impurities were changed, and the microstructure was improved;
- (4) It has obvious modification effect, so that the processing, mechanics, welding, wear resistance and corrosion resistance are improved.
Effect of rare earth on the structure of copper and its alloy
Refine the organization
The effect of rare earth on the microstructure of copper and its alloys is mainly manifested in the refinement of grains, the reduction or elimination of columnar crystals, and the expansion of equiaxed regions. There are three views on the mechanism of rare earth refining copper and copper alloy structure
- (1) . Forming new crystal nucleus and inhibiting grain growth. Rare earth can react with some elements in copper and its alloys to form high melting point compounds, often suspended in the melt with very fine particles, becoming the dispersed crystal core, making grains more and smaller; from the solidification principle and thermodynamics point of view, because a large number of rare earth gather in the liquid phase at the front of the solid-liquid interface, the composition of the alloy is supercooled during solidification, and solidified in a dendritic way At the same time, it produces fine neck and fuse at the branch node, increases the crystal core, and refines the grain.
- (2) . Microcrystallization. Because the atomic radius of rare earth elements (0.174nm-0.204nm) is 36% – 60% larger than that of copper (0.127nm), it is easy for rare earth atoms to fill up the surface defects of the new phase of the growing copper or copper alloy grains, and form a film that can hinder the continuous growth of grains, so as to refine into microcrystals;
- (3) . Alloying. The solubility of rare earth in copper is very small, generally only a few thousandths to a few thousandths, but rare earth and copper can produce a variety of intermetallic compounds. These intermetallic compounds dispersed in the matrix to refine the grains.
For example, when the mass fraction of rare earth elements is 0.05% ~ 0.07%, the microalloying effect of light rare earth Ce, La, nd can increase the GMR of cu80co15ni5 alloy. When 0.07% La is added to the Cu based memory alloy, the alloy has good single and two-way memory properties after grain refinement, and the hysteresis width of martensitic transformation decreases.
Effect of rare earth on the structure of inclusions
The effect of rare earth on the structure of inclusions is mainly to change the shape and distribution of impurities. Its main performance is as follows: (1) The dendrite and columnar crystals in the alloy structure can be reduced or eliminated, which is related to the formation and dispersion of refractory compounds by rare earth and some impurities.
- (2) The mechanical and processing properties of copper and its alloys are improved by transforming some impurities (such as Pb, Bi, etc., some of which can form eutectic with low melting point) into dots or balls (Fig. 1). This is due to the strong activity of rare earth metals, which can sharply reduce the wettability of some impurities such as Pb to copper. These impurities can be found in the Under the action of body surface tension, the volume is greatly reduced.
- (3) Some harmful impurities (such as s, P, Pb, Bi, etc.) in the alloy are changed from centralized distribution between dendrites or grain boundaries to more uniform distribution in the whole crystal, so that the impurities can redistribute on the micro volume of the metal, or affect the macro segregation of some impurities, resulting in the improvement of various properties.
- (4) . the compounds containing rare earth are adsorbed on the grain boundary of metal or alloy (Fig. 2), reducing the amount of harmful impurities with low melting point on the grain boundary of alloy, thus weakening the high temperature temper brittleness of alloy.
Figure 1. Morphology of impurities in copper alloy containing rare earth
Figure 2. Re rich phase precipitated at grain boundary
For example, before adding rare earth in beryllium copper alloy, inclusions are mostly irregular angular Cu2O and Cu2S. When rare earth increases to 0.05%, some inclusions have been spheroidized. When rare earth increases to 0.32%, all inclusions have been spheroidized, and rare earth inclusions replace Cu2O and Cu2S, which makes inclusions from solid solution to rare earth compounds.
Effect of rare earth on properties of copper and copper alloy
The casting properties of copper and its alloys can be improved by adding a proper amount of rare earth metals. For different kinds of copper alloy, the fluidity can be increased by 30% ~ 40% after adding rare earth. For high manganese aluminum bronze, when the addition of Y and La is less than 0.10% and 0.20% respectively, the fluidity increases with the addition of rare earth. Adding 0.5% ～ 1.0% mixed rare earth in high lead bronze (zqpb25-5) and 0.04% ～ 0.05% mixed rare earth in HPb59-1 lead brass can improve the segregation or reverse segregation of the alloy. The addition of 0.01% ~ 0.03% mixed rare earth can significantly improve the high temperature elongation of deformed lead brass, improve the hot working performance, and reduce or eliminate the hot rolling cracking. The removal of impurities by rare earth elements increases the strength of grain boundary and reduces the number of embrittlement phases, thus increasing the high temperature σ B and δ of copper alloy. The addition of rare earth can reduce the residual stress and improve the cold deformation ability of the material within a certain range of deformation (< 14%). Adding 0.03% ～ 0.05% rare earth elements to deformed lead brass can greatly improve its cutting performance, especially reduce the surface roughness, burr and tool wear. This is because rare earth elements play a “direct role” in refining lead particles and promoting more lead particles to exist in β phase, and also in reducing the width of columnar crystal area, thus improving the cutting performance Machinability. Rare earth additives have a good effect on improving the welding process performance of copper and its alloys. Impurities in weld metal such as trace Pb, Fe, Si, Bi can cause hot cracks. Adding rare earth elements will effectively prevent this tendency.
Mechanical and electrical properties
The influence of rare earth on the mechanical properties of copper and copper alloy mainly lies in hardness, strength and plasticity. When the content of rare earth in pure copper is 0.1% ～ 0.2%, the strength increases greatly, and when it is higher than 0.2%, the strength increases slowly. The influence of rare earth on the strength of H68 brass has two functions: on the one hand, the solid solution strengthening and purification of rare earth make the strength of the material increase; on the other hand, when the amount of rare earth addition exceeds a certain value, the harmful effect of rare earth covers up the beneficial effect, and the macroscopic performance is the strength drop. The mechanism of the influence of rare earth on the conductivity of copper and copper alloy is generally considered as follows: on the one hand, the refining effect of rare earth makes the copper grain refined, the grain boundary increased, the probability of electric scattering increased, the resistivity increased, and the conductivity decreased; on the other hand, the purification effect of rare earth reduces the impurities in copper, the lattice distortion weakened, the probability of electronic scattering reduced, and the conductivity improved. These two factors have the opposite effect on the conductivity at the same time, and their influence changes with the addition amount of rare earth. This theory agrees with the experimental results. For example, the resistivity of Cu Ni Si alloy changes with the change of rare earth content (Fig. 3), which is related to the change of alloy structure caused by the addition of rare earth to Cu Ni Si alloy.
Figure 3. Relationship between resistivity and rare earth content of alloy
Oxidation resistance and corrosion resistance
In order to solve the contradiction between oxidation resistance and high conductivity, rare earth metals were added as alloy elements of copper and copper alloy. It was found that the conductivity did not decrease but increased slightly when the appropriate amount of RE was added, and yttrium in copper could significantly improve the oxidation resistance. At 400 ℃, the law of the curve of oxidation weight increase with time changed from parabola relationship when rare earth was not added to 0.2% RE to logarithm relationship (Fig. 4). Si, Al and RE elements are infiltrated into copper alloy by chemical heat treatment at 850 ℃, and a surface film with enhanced oxidation resistance is formed on the surface of copper alloy (Fig. 5).
Figure 4. Relationship between oxidation weight gain and time at 400 ℃
Figure 5. Surface morphology of CE containing copper after oxidation at 850 ℃ for 22h
The results show that the corrosion resistance of copper and copper alloy is improved to some extent by adding rare earth
- (1) The purification effect of rare earth can eliminate impurities in copper matrix and reduce the number of primary batteries.
- (2) A compact oxide layer is formed on the surface of copper and copper alloy to prevent the matrix atoms from diffusing outwards and the external atoms from diffusing inwards.
- (3) . improve the corrosion potential of copper and copper alloy. The self corrosion potential of brass containing cerium in NaCl aqueous solution is higher than that of brass without cerium. The reason is that the β phase in H70 alloy will be reduced and the electrode potential of β phase will be lower, so the corrosion resistance of the alloy will be improved.
- (4) The addition of rare earth reduces the crystallization temperature range of copper alloy. For the zqsn10-1 alloy after adding rare earth, the liquidus and solidus temperature decreased during solidification, which reduced the dendrite segregation and promoted the further improvement of corrosion resistance.
The addition of mixed rare earth can not only improve the corrosion resistance of tin brass, but also change the corrosion morphology of tin brass (Fig. 6), which not only reduces the thickness of the easily falling layer, but also greatly reduces the thickness of the permeable layer.
Figure 6. Corrosion morphology of alloy sample
Rare earth and copper can form intermetallic compounds with high hardness and uniform distribution, which become the resistance of dislocation movement; moreover, rare earth can effectively improve the existence form and distribution of inclusions, reduce the possibility of weakening the grain boundary, reduce the probability of cracking along the grain boundary under load, and thus improve the wear resistance. The microstructure of cast brass containing rare earth is β phase and κ phase (FeAl) distributed on the α phase matrix, in which α phase is relatively soft, which ensures that the copper alloy has certain plasticity, while β phase and κ phase are relatively hard, which improves the strength and wear resistance of the alloy. At the same time, there is also a spherical intermetallic compound formed by the reaction of rare earth and impurities in the structure, which has high hardness. Therefore, the cast brass containing rare earth has high hardness, good plasticity and toughness, which can shorten the running in period and extend the stable wear period, so as to reduce the wear and prolong the service life of the workpiece. Adding CE and B to high manganese aluminum bronze can reduce dry friction and wear by about 20% and lubricating friction and wear by about 50%.
Source: China Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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