Application of Rare Earths in Nonferrous Metals

- Nov 16, 2017 -

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First, rare earth aluminum alloy

 

The study of rare earth in aluminum alloy began in the 1930s. During the Second World War, rare earth was used in the casting of aluminum alloy to improve the high temperature performance and casting performance of the cast aluminum alloy. In the 20 years after the 1950s, a great deal of work has been done on strengthening, deteriorating, purifying rare earth elements in aluminum alloys and improving process performance. To date, progress has been made in the following areas.

 

(1) strengthening effect

 

Ceralumin containing 0.15wt% Ce was first applied to high-temperature working parts of the engine block. It was subsequently found that the strength of alloys containing 11 wt% rare earth can be doubled at 427 ° C. The former Soviet Union has applied the rare earth-bearing АЦР1 and ЖП207 alloys to supersonic aircraft. It is one of the best high-temperature alloys available today and can work long-term below 400 ° C. Its long-term strength can be increased by 1 to 2 Times In addition, the Al-Cu-RE high-strength heat-resistant cast aluminum alloy has also been developed. For example, an aluminum alloy containing 2 wt% RE has a tensile strength of 160 MPa at 300 ° C and a strength of 350 MPa at room temperature.

 

In the early stage, adding 0.3 ~ 0.35 wt% Ce can improve the strength and hardness of Al-Cu and Al-Cu-Si alloy pistons and reduce the thermal cracking. Now, rare earth aluminum silicon eutectic and hypereutectic piston has been used in the production of rare earth nickel to replace the piston can improve the high temperature performance and wear resistance, thus making it significantly improved life expectancy.

 

The solubility of rare earth in solid aluminum is very small (<0.05wt%), so its solid solution strengthening effect is very weak. However, due to the chemical activeness of rare earth, it can form intermetallic compounds (AlCuCe, AlSiCe, etc.) with many elements in aluminum alloy, which are very stable under high temperature, high hardness and are distributed in the grain boundaries at the grain boundaries, Variable slippage, which played a role in strengthening the high temperature.

 

(2) metamorphism

 

Rare earth on the Al-Si eutectic and hypoeutectic alloys degenerate behavior, is to refine the α + Si eutectic structure, the silicon from the coarse sheet into thin strips and granular, so that the performance, especially plastic increased. Lanthanum metamorphism best, such as the magnesium content increased from 0.4wt% to 0.9wt%, the lanthanum metamorphism can be further enhanced. The range of effective metamorphism of lanthanum is between 0.03-0.18wt%. Using rare earth as a modifier requires a certain cooling rate, the critical cooling rate (Vc) of lanthanum is the smallest. When Vc> 22 ℃ / min, Can be effective. The metamorphism of rare earth also has long-lasting effect. For example, after 10 times of remelting, the concentration of lanthanum is reduced from 0.056wt% to 0.035wt%, and the content is still in the best metamorphism range at this time.

 

(3) Purification

 

Rare earth with strong hydrogen affinity, the formation of REH, etc., thus adding rare earth in the liquid aluminum, can absorb part of the hydrogen, thus reducing the precipitation due to precipitation of hydrogen caused by pinhole. Join 0.2wt% RE, can significantly reduce the pinhole.

 

In addition, high magnesium content of aluminum alloy in the liquid easily oxidized. In recent years, it has been found that when containing 0.001 wt% of Ce or RE, oxidation of an aluminum alloy containing 10 to 12 wt% of Mg can be prevented at a temperature of 600 to 760C.

 

Addition of rare earth can also make the aluminum alloy impurity phase FeAl, AlFeSi, etc. to form a plurality of metal compounds, which can change its morphology and improve the mechanical properties of aluminum alloy, for example, containing 1wt% Fe alloy, adding an appropriate amount of RE, Coarse iron phase refinement and even the ball.

 

(4) improve process performance

 

Addition of 0.2 wt% Ce to Al-Cu-Mn system can reduce the hot cracking tendency. Adding a small amount of La, Ce and Y into the Al-4wt% Cu alloy can reduce the solidus of the alloy and effectively reduce the thermal weakness zone. The lanthanum can obviously improve the strength of the alloy in the quasi-solid state. In addition, the rare earth elements are enriched in the interdendritic, generate LaAl, La (CuAl) and other intermetallic compounds, strengthening the grain boundary, thus reducing the hot cracking tendency.

 

In addition, the rare earth can also change the structure and properties of the oxide film on the surface of the aluminum alloy, so that the Al-Mg and Al-Zn-Mg alloys have better color anodizing properties, such as Al containing <0.5 wt% Ce Zn-Mg alloys are anodizing materials suitable for surface coloring. Rare earth also improve the corrosion resistance of Al-Si-Mg alloy, adding 0.15 ~ 0.2wt% La, can resist the performance of seawater corrosion.

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Second, casting copper alloy

 

(1) improve performance

 

The addition of 0.02-0.2 wt% mischmetal in lead bronze reduces the grain size (1/3 less than the original) and reduces surface wear by 3/4. In the nickel-free high manganese aluminum bronze, adding a small amount of rare earth and boron, can reduce the amount of wear 50%. In the lead bronze by adding 0.05 ~ 1.0wt% mixed rare earth, tensile strength increased by 30%, the elongation increased by 1 times. Addition of 0.045wt% rare earth to aluminum bronze increased the elongation by a factor of 3 under the condition of not decreasing the tensile strength.

 

Copper-lead alloy by adding 0.055 ~ 0.2wt% S, so that the lead-rich alloy tends to reticulate organization, adding 0.028 ~ 0.149wt% rare earth, the lead-rich phase isolated in the dendrite, with the sulfur content The fatigue strength decreases obviously; however, as the amount of rare earth increases, the fatigue strength obviously increases.

 

(2) improve process performance

 

Lead-rich low melting point lead-prone phase enrichment of the surface to form a reverse segregation. The more lead, the more serious this segregation. Adding 1wt% Ce, to eliminate segregation.

 

Silicochloro-bronze added 0.01 ~ 0.05wt% mixed rare earth, so that silicon and tin segregation improved.

 

In Ce-8wt% Sn alloys, the addition of cerium increases the effective partition coefficient of tin and, at the same time, the grain refinement, thereby suppressing the formation of segregation.

 

Adding 0.028wt% Ce into the high manganese aluminum bronze and 0.1wt% rare earth mixed into the tin phosphorus bronze can obviously improve the fluidity and increase about 30% ~ 40%.

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Third, casting magnesium alloy

 

Due to the advent of rare earth (including thorium) magnesium alloys, the use of magnesium alloys has been rapidly developed since the 1950s. The role of rare earth in magnesium alloy can be attributed to: improve creep resistance, improve room temperature and high temperature strength and improve process performance. Therefore, the world's rare earth-containing magnesium alloy has accounted for more than 50% of the cast magnesium alloy.

 

(1) high temperature creep resistance alloy

 

First of all, aero-engine is applied to the Mg-RE-Zr (Mg-3RE-0.1Zr) alloy to meet the 205 ℃ with high strength and creep resistance.

 

(2) high-strength rare earth Mg-Zn-Zr alloy

 

ZK51 (Mg-4.5Zn-0.6Zr) has a tensile strength of 280 MPa, but poor casting performance. After adding rare earth, the Mg-Zn-RE compound is presented in the foundry structure, and the separation type eutectic is distributed on the grain boundary so that the casting process performance is obviously improved.

 

ZE63A (Zn-6wt%, RE-2.5wt%, Zr-0.6wt%) has been used for the thrust reversers of the RB211 engine for many years. Its tensile strength up to 276MPa; yield strength of 186MPa; elongation of 5%.

 

(3) yttrium-containing rare earth magnesium alloy

 

Yttrium has a very good strengthening effect on magnesium alloys due to the fact that yttrium is dissolved in the matrix and the grain boundaries are sealed by heat-resistant compounds. Therefore, Y-Mg alloy has high thermal strength, and even reaches the high temperature performance of thorium-magnesium alloy. In addition, it also has excellent high temperature oxidation resistance. The magnesium alloy containing 9 wt% was heated to 510 ° C in humid air for 1 hour only for 98 hours while the magnesium alloy containing thorium weighed 15 mg.

 

Although the development and application of rare earths in non-ferrous metals in China began in the late 1960s, it was not until 1985 that the National Rare Earth Colored Application Network was established and there was only a breakthrough in organizing the promotion of application of rare earths in aluminum wire and cable , The amount increased year by year. 330 tons of REO in 1985, 600 tons of REO in 1994 and 1,000 tons of REO in 2003, with an average annual growth rate of over 13%. In addition to aluminum alloy applications, but also successfully used in copper alloys, hot dip galvanized alloys, cemented carbide, magnesium alloy. At present, the development and application of rare earths in non-ferrous metals and alloys have been proven by experiments to have obvious effects of aluminum, copper, magnesium, titanium, molybdenum, nickel, cobalt, niobium and platinum group metals. Rare earth metals in these non-ferrous metals and alloys in the amount of less than 0.5%, but the effect is extremely significant. Rare earth can play a role in purification, deterioration, grain refinement. Particularly worth mentioning is that the addition of rare earths to aluminum wires and cables eliminates the adverse effect of silicon. Its electrical conductivity is not only slightly higher than that of the International Electrotechnical Commission but also its strength is increased by 20% and its corrosion resistance is more than doubled. Grinding performance increased by 10 times, in one fell swoop changed the backwardness of China's aluminum wire and cable industry, rare earth aluminum wire and cable have become the provisions of national grid products, the annual production capacity of 450,000 tons, and has entered the international market, the technology has been Caused by foreign attention. These aluminum wires and cables are put into operation, saving 4 billion kilowatt hours each year for the country with a benefit of 2 billion yuan. The annual output of aluminum alloy and as-cast aluminum alloy can reach 33 ~ 340,000 tons. The annual output of rare earth copper and brass can reach 60,000 tons and the annual output of rare earth hot-dip galvanized steel products can reach 30,000 tons. The amount of rare earth in non-ferrous metals showed an increasing trend year by year.


Article from NdFeB Industry Network

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