EA013656B1 - A casting process and alloy mg-al-re composition resulting in cast parts with superior combination of elevated temperature creep properties, ductility and corrosion performance - Google Patents

Abstract

A process for casting a magnesium alloy consisting of 2.0-6.00 % by weight of aluminium, 3.00-8.00 % by weight of rare earth metals (RE-metals), the ratio of the amount of RE-metals to the amount of aluminium expressed as % by weight being larger than 0.8, at least 40 % by weight of the RE-metals being cerium, less than 0.5 % by weight of manganese, less than 1.00 % by weight of zinc, less than 0.01 % by weight of calcium less than 0.01 % by weight of strontium and the balance being magnesium and unavoidable impurities, the total impurity level being below 0.1 % by weight, wherein the alloy is cast in a die the temperature of which is controlled in the range of 180-340°C, the die is filled in a time which expressed in milliseconds is equal to the product of a number between 5 and 500 multiplied by the average part thickness expressed in millimeter, the static metal pressures being maintained during casting between 20-70 MPa and is subsequently intensified up to 180 MPa.

Description

The invention relates to a method for casting a magnesium alloy, consisting of 2.0-6.0 wt.% Aluminum, 3.0-8.0 wt.% Rare earth metals (REM), in which the ratio of the number of rare-earth metals and the amount of aluminum, expressed in wt. wt.%, exceeds 0.8, at least 40 wt.% REM falls on the share of cerium, less than 0.5 wt.% manganese, less than 1.0 wt.% zinc, less than 0.01 wt.% calcium, less than 0 , 01 wt.% Strontium, and the remainder includes magnesium and constant impurities, while the total content of impurities is less than 0.1 wt.%.

Magnesium-based alloys are widely used in cast parts used in the aerospace industry and the automotive industry. Magnesium-based alloy castings can be manufactured by conventional casting methods, which include injection molding, sand casting, chill casting and semi-permanent casting, gypsum casting and lost-wax casting.

Magnesium-based alloys have a number of particularly advantageous properties, which contributed to an increase in demand for cast parts made of magnesium-based alloys in the automotive industry. These properties include low density, high tensile strength to weight ratio, good casting properties, high machinability and good damping characteristics.

It is known that the most common magnesium-based alloys for injection molding, such as magnesium-aluminum alloys or magnesium-zinc alloys, lose their creep resistance at temperatures above 120 ° C. Designed for use at higher temperatures, magnesium-aluminum-silicon alloys provide only a limited improvement in creep resistance. The alloy system MD-A1-Ca and MD-A1-8t provides a further improvement in creep resistance, but their casting qualities are a significant drawback. This is especially true when the metal collides at high speed directly with the surface of the injection mold, the so-called water hammer.

It is known that the alloy AE48 (4% AR, 2-3% REM) has significantly improved properties at elevated temperatures and anti-corrosion properties.

Magnesium-aluminum alloys containing elements such as §t and Ca have even more improved creep characteristics, albeit due to the deterioration of casting qualities. The alloy system MD-A1-Ca and MD-A1-8t has an even higher creep resistance, but a significant drawback of these alloys is their casting quality. This is especially true for the case when the metal at high speed collides directly with the surface of the injection mold, the so-called water hammer.

In FIG. 1A and 1B schematically show injection molding machines with a cold and a hot pressing chamber, respectively, each of which has an injection mold 10, 20 with a hydraulic damping system 11, 21, respectively. The molten metal is introduced into the mold using a shock cylinder 12, 22 with a piston 13, 23, respectively. In a system with a cold pressing chamber, an auxiliary dosing system for supplying metal to a horizontal impact cylinder is required. In an injection molding machine with a hot pressing chamber (Fig. 1B), a system with a vertical piston (12, 23) is used directly in the molten alloy.

To obtain alloys Md-A1-REM with good characteristics, it is necessary that the casting takes place under conditions of extremely rapid cooling. This relates to a method of injection molding. The steel mold 10, 20 has an oil (or water) cooling system that controls the temperature in the range of 200-300 ° C. A prerequisite for high quality is the short filling time of the mold in order to avoid solidification of the metal during filling. The recommended duration of filling the mold is about 10 ^-2 s x the average thickness of the part (mm). For this purpose, the alloy is forcibly passed with a high speed in the range of 30-300 m / s through the gate. In order to obtain the desired volumetric flow rates with the required short duration of filling, rams of sufficient diameter with a speed of up to 10 m / s are used in the shock cylinder. Typically, a static pressure on the metal of 20-70 MPa is used, followed by an increase in pressure to 150 MPa. With this casting method, the resulting cooling rate is usually in the range of 10-1000 ° C / s, depending on the thickness of the molded part. For AE alloys, this is the main criterion in determining the properties, taking into account the high overall cooling rate of the part and, in particular, the extremely high cooling rate of the surface layer.

In FIG. Figure 2 shows the relationship between the curing interval and the microstructure. The solidification rate in ° C / s is plotted along the horizontal axis, the distances between the secondary axis of the dendrites in microns are shown on the vertical scale on the left, and the grain diameter in microns is shown on the vertical scale on the right. Line 30 denotes the obtained grain size, and line 31 denotes the obtained value of the distances between the secondary axes of the dendrites.

In injection molding, a reduction in grain size is provided by the cooling rate. As noted earlier, a cooling rate in the range of 10-1000 ° C./s is usually achieved. In this case, the grain size is usually 5-100 microns.

It is well known that low grit favors the ductility of the alloy. This relationship is illustrated in FIG. 3, which shows the relationship between grain size and relative

- 1 013656 extension. The grain size in microns is plotted on the horizontal axis, and elongation in% is plotted on the vertical axis. The diagram also shows two different compositions, pure MD, indicated by line 35, and ΑΖ91 alloy based on MD, indicated by line 36.

It is also well known that low granularity favors the yield strength when tensile alloy. This (Hall-Petch) relationship is shown in FIG. 4. The diameter of the grain 6 (-0.5) in microns is plotted on the horizontal axis, and the yield strength in tension in MPa is plotted on the vertical axis.

It follows that the low grit obtained at very high cooling rates, which are possible with injection molding, is necessary to ensure tensile strength and ductility.

The casting qualities of the alloy characterize the possibility of casting a finished product from it with the required functionality and properties. Usually they include three categories: (1) the possibility of manufacturing a part with all the desired geometric features and dimensions, (2) the possibility of manufacturing a dense part with the desired properties, and (3) the impact on tooling, foundry equipment and injection molding efficiency.

In the patent application of Germany 2122148 describes alloys of the type Md-A1-KE, mainly alloys Md-A1-REM with an REM content of less than 3 wt.%, Although alloys with a higher REM content are also considered. It is known that alloy AE42 (4% A1, 2-3% rare-earth metals) has significantly improved properties at elevated temperatures and anticorrosion properties. It is known from experience that the addition of a small amount of rare-earth metals to magnesium-aluminum alloys leads to a significant improvement in their anticorrosion properties, but to a deterioration in casting qualities, since cases of jamming of casting molds are becoming more frequent. In FIG. Figure 5 shows the areas of good, bad and very bad casting qualities of the MD-A1-RZM alloy system. The horizontal axis represents the amount of A1 in wt.%, And the vertical axis represents the amount of rare-earth metals in wt.%. Line 40 indicates the solubility of rare-earth metals at 680 ° C, and line 41 indicates the solubility of rare-earth metals at 640 ° C. Area 42 (dark) displays a composition with very poor casting qualities. Area 43 (intermediate) displays the composition with poor casting properties, and area 44 (light) displays the composition with good casting qualities. As shown in FIG. 5, casting quality deteriorates with an increase in the content of rare-earth metals in the alloy. However, as shown in FIG. 5, there is a region with a rare-earth metal content exceeding 3.5 wt.% (The upper limit limited by the solubility of rare-earth metals), A1 - from 2.5 to 5.0%, as well as a percentage ratio of rare-earth metals and A1 over 0.8, in which foundry high pressure casting qualities are good.

In this regard, the object of the present invention is to provide relatively inexpensive magnesium-based alloys with improved characteristics at elevated temperatures and improved casting properties.

Due to the formation of dispersed phases A1 _x -REM in the compositions proposed in the present invention, the volume content of the brittle phase Md ₁₇ A1 ₁₂ is minimized (the ratio of REM / A1 in the dispersed phase increases with increasing percentage of% KE /% A1 in the alloy). Due to the fact that the eutectic phase Md ₁₇ A1 ₁₂ melts at a temperature of about 420 ° C, conventional magnesium aluminum alloys such as AM50, AM60 and ΑΖ91 have a solidification interval of about 200 ° C, as shown in FIG. 6. In FIG. 6 along the horizontal axis shows the solids content (in wt.%), And the vertical axis shows the temperature (° C) for several alloys. Alloys MD-A1-REM with percentage ratios KE / A1 according to the present invention completely harden at a temperature of about 570 ° C, therefore, the solidification interval is only about 50 ° C.

Usually, with an increase in the aluminum content in MD-A1 alloys for injection molding, their casting qualities are improved. This is because magnesium-aluminum alloys have a wide range of solidification, which makes them difficult to cast by nature, unless the eutectic phase is present at the end of solidification. This can explain the good casting properties of ΑΖ91 ^, which corresponds to the cooling curves shown in FIG. 6. With a decrease in the A1 content to 6, 5, and 2% in AM60, AM50, and AM20, respectively, the remaining eutectic phase decreases to a level at which the filling of the mold is difficult at the last stages of solidification, which with respect to thick-walled parts means loosening and even the presence of larger voids . In the case of thin-walled parts, the ability to fill the mold in the last stages is less important (although the fluidity of the alloy becomes an important indicator), since volume shrinkage is partially compensated by a decrease in thickness due to shrinkage under the influence of the mold walls. The cooling characteristics of AE44 and AE35 alloys are significantly different from the characteristics of magnesium-aluminum alloys.

The hardening interval is significantly narrowed, which indicates the possibility of reducing the concentrated shrinkage friability during hardening. These alloys have good flow when filling the mold, which makes it easy to cast finished products from them with lesser casting defects. The casting properties of alloys AE44 and AE35 approximately correspond to the qualities of alloy ΑΖ91 ^.

Another feature associated with a narrow solidification interval is that this does not result in reverse segregation, which is usually observed in ΑΖ91 ^ alloy, as well as in AM alloys. This can be illustrated by the fact that alloys AE with a high content of rare-earth metals have a brilliant

- 2 013656 surface without seizures of the eutectic phase MD-A1. The surface layer hardens during and immediately after filling the mold, and the temperature quickly drops below the transition temperature to the solid state, which prevents molten metal from entering the mold surface after the start of shrinkage. This is useful to prevent reactions between the mold wall and molten metal, which can cause seizing.

In FIG. 7, using an example of a wall with a thickness of about 3 mm, three layers of AE44 with different microstructures are shown. A surface layer about 50 microns thick consists of equiaxed grains about 10 microns in size. This fine-grained nature is explained by the conditions of rapid cooling of the mold walls. The intermediate layer has a thickness of about 100 μm and is extremely fine-grained. Its morphology is different from the previous layer, and it has ΌΆ8 in the range of 2-4 microns. This can be explained by a change in the equilibrium melting point under pressure. As the pressure on the metal increases, the equilibrium melting point increases, i.e. the metal suddenly becomes insufficiently cooled. Theoretically, this applies to all MD alloys, but there remains a significant difference between their alloys in their solidification characteristics. The core consists of equiaxed grains ~ 20 μm in size. Core hardening is limited by the heat flux of their core in the direction of shape. As a result of heat transfer both through the already hardened layer and through the boundary of the casting and the mold, the cooling rate becomes lower than on the surface, due to which a coarser microstructure is formed.

With a low content of rare-earth metals or a low percentage of rare-earth metals and A1, as in the alloy AE42 or AE63, there is the possibility of the presence of the eutectic phase MD-A1, capable of segregation to the surface, which leads to seizure. This may explain why the AE42 alloy has the worst casting qualities.

At the top of FIG. 8 shows a box mold for casting. Below are micrographs of examples of unit 3 (near the gate) for alloys AM60, AM40, AE63, AE44 and AE35. Hot cracks are seen in AM40 and AE63 alloys.

As shown in FIG. 8, alloys AE44 and AE35 are less susceptible to hot cracking than AM alloys. This is due to the rather rapid solidification of the surface layer, resulting in a relatively fine-grained structure, as described above.

Partly due to the fine-grained structure and partly due to the absence of the brittle Md ₁₇ A1 ₁₂ phase, this layer becomes highly plastic and thereby able to deform when thermal deformations occur during solidification. A surface layer with larger grains, which is usually characteristic of alloys with a large solidification interval, and / or a layer with a high content of Md17A112 will have much less ductility and tend to crack and form hot cracks rather than deformation.

As shown by testing large (~ 1.5 m) thin-walled parts (~ 3 mm thick), alloys AE44 and AE35 have excellent mold filling characteristics, and since thin-walled parts are not required to be filled in a long interval, as indicated above, it is assumed that this alloy will become a competitive alternative to alloys of these types for which filling the mold is of utmost importance.

The properties of various AE alloys can be explained by the fact that hardening of the solid solution provides only A1, while rare-earth metals in combination with A1 form a dispersed phase in the regions of grain boundaries. In alloys AE44 and AE35, the dispersed phase (mainly A1 ₂ -REM) forms a continuous three-dimensional network that effectively prevents creep resulting from thermal activation and creep along grain boundaries. This is shown in FIG. 9, which shows images obtained by the 8EM-BEC method (Vaskksayeg E1ec! Gosh Soshroyup) illustrating the microstructure of alloys AE44, AE35 and AE63 (from left to right) for injection molding. Only A1 provides hardening of the solid solution, and rare-earth metals in combination with A1 form a dispersed phase in the regions of grain boundaries.

In FIG. 10 shows further magnified images 8EM-AE WEIGHT alloy 44, which also shows the lamellar phase structure A1 -RZM _{x y} of the alloy AE44. As shown in FIG. The 10 dispersed A1 _x KEy phases in AE alloys consist of an extremely thin-layered structure. This structure of layers less than a micrometer thicker reinforces grain boundaries, thereby preventing creep. At the same time, these layers are not brittle (or not so brittle as the MD-A1 eutectic phase), since the AE44 alloy for injection molding has plasticity as in the AE42 alloy. The mesh (mainly A111-RZM3) in the AE63 alloy becomes fragmented, and a significant amount of the MD-A1 eutectic phase, which impairs ductility and creep characteristics, may affect the grain boundary region. In the AE42 alloy, it is also likely that a significant amount of the MD-A1 eutectic phase is present, which limits the creep characteristics. The AE35 alloy has slightly worse ductility than the AE44 alloy, but still better than the AE63 alloy.

The following are many examples of mechanical properties, including ductility, tensile strength, creep resistance and anticorrosion properties of AE alloys. In FIG. 11 illustrates a unique combination of creep resistance and ductility compared to

- 3 013656 existing alloys. In FIG. 11 shows ductility (horizontal axis) depending on the creep resistance of several known alloys MD. Region 50 includes alloys AM, region 51 - alloys AE, region 52 - alloy ΑΖ91 and region 53 - other high-temperature alloys. Alloys AE according to the present invention are the only alloys for injection molding, which in this way combine ductility and properties at elevated temperatures, and, therefore, they provide many new and unexplored opportunities for designers and designers, especially in the automotive industry.

More precisely, an object of the invention is to provide relatively inexpensive alloys of magnesium, aluminum and rare-earth metals for injection molding, having good casting properties, creep resistance, tensile yield strength and ability to hold a bolt load, especially at elevated temperatures of at least 150 ° FROM.

Summary of the invention

Thus, the present invention proposes casting an alloy in a mold whose temperature is controlled in the range of 180-340 ° C., filling the mold over a time that in milliseconds is equal to the number of products from 5 to 500 times the average thickness of the part in millimeters, maintaining a static pressure on the metal during casting in the range of 20-70 MPa with a subsequent increase to 180 MPa.

By combining the specified alloy MD-A1-REM with a special casting method, products with good creep resistance at elevated temperatures, high ductility and, in general, good mechanical and anti-corrosion properties can be obtained.

Typically, several rare-earth metals, such as, for example, Ce, ba, N6 and / or Pr and mixtures thereof, can be used as an alloy element. However, significant amounts of cerium are preferably used since this metal provides the best mechanical properties. In order to improve the corrosion resistance, Mn is added, but its addition is limited by limited solubility.

The aluminum content is preferably from 2.0 to 6.0 wt.%, More preferably from 2.60 to 4.50 wt.%.

At a higher aluminum content, the formation of Md ₁₇ A1 ₁₂ phases can occur, which adversely affects the creep characteristics. Too low A1 content negatively affects the casting qualities.

As regards REM, it is preferred that the REM content is from 3.50 to 7.00 wt.%, With the upper limit being limited by the solubility of the REM in the MD-A1-REM system, as shown in FIG. one.

When the REM content exceeds 3.50 wt.%, The creep characteristics are significantly improved. The content of more than 7.00 wt.% Is impractical due to the limited solubility of rare-earth metals in liquid magnesium-aluminum alloys.

In addition, it is preferred that the REM / A1 ratio is greater than 0.9.

In specific applications, the composition of the alloy is selected so that the aluminum content is from 3.6 to 4.5 wt.%, And the content of rare-earth metals is from 3.6 to 4.5 wt.%, With an additional condition is the ratio of rare-earth metals / A1 over 0.9.

Alloys of this type can be used at temperatures up to 175 ° C, while maintaining good creep characteristics and tensile strength, in addition, their properties do not deteriorate due to aging and they have good casting qualities.

For use at temperatures above 175 ° C, the composition of the alloy is selected so that the aluminum content is from 2.6 to 3.5 wt.%, And the content of rare-earth metals exceeds 4.6 wt.%.

In addition to the good creep and tensile strengths, the properties of this alloy do not deteriorate due to aging.

Preferably, REM is selected from the group consisting of cerium, lanthanum, neodymium and praseodymium.

REMs facilitate the production of alloys, but also increase corrosion resistance, creep resistance and improve mechanical properties.

Preferably, the amount of lanthanum is at least 15% by weight, more preferably at least 20% by weight of the total REM content. Preferably, the amount of lanthanum is less than 35% by weight of the total REM content.

Preferably, the amount of neodymium is at least 7 wt.%, More preferably at least 10 wt.% Of the total REM content. Preferably, the amount of neodymium is less than 20% by weight of the total REM content.

Preferably, the amount of praseodymium is at least 2% by weight, more preferably at least 4% by weight of the total REM content. Preferably, the amount of praseodymium is less than 10% by weight of the total REM content.

Preferably, the amount of cerium is greater than 50% by weight of the total REM content, preferably 50 to 55% by weight.

It is known that calcium and strontium increase creep resistance, and the addition of at least 0.5 wt.% Calcium increases tensile strength.

At the same time, Ca and 8g should be avoided, since even in very small concentrations

- 4 013656 tion these elements create significant problems of seizing, which affects the casting properties of the alloy.

Further, the present invention is described in more detail in the following examples, which only illustrate the invention and should not be construed in any way limiting the described broad invention.

Example 1

In order to determine the influence of alloy elements, several MD alloys were obtained, the composition of which is given in the table below.

Several test rods were made from each alloy for testing, which are described in the following examples. The following tests were carried out.

Tensile strength and ductility

6 mm test rods were manufactured according to A8TM and the following test conditions were created.

Tensile testing machine brand Instron machine with a force of 10 kN.

Room temperature up to 210 ° С.

At least 5 comparative tests at each temperature.

Strain rate

1.5 mm / min with deformation up to 0.5%, mm / min with deformation over 0.5%.

Tests were conducted in accordance with Ι8Θ 6892.

Tensile Creep Test

The following test material was used for this test.

Diameter: 6mm

Measuring base: 32.8 mm

Radius of curvature: 9 mm

Grip Head Diameter: 12mm

Total length: 125 mm.

The tests were carried out in accordance with L8TM E 139.

Stress Relaxation Test

The test material diameter is -12 mm, length - 6 mm

Samples were cut from the arbitrary end of the creep rods.

The tests were carried out in accordance with L8TM E328-86

Anticorrosion properties

The tests were carried out in accordance with A8TM 117.

Example 2

The strength of several formulations was measured as a function of temperature.

The results are shown in FIG. 12, 13 and 14. They show the tensile strength in MPa along the y axis, and the temperature in degrees Celsius along the x axis.

Example 3

The creep strain of several compositions was measured as a function of temperature.

The results are shown in FIG. 15 and 16. The measurements shown in FIG. 15 was carried out at a temperature of 175 ° C with a force of 40 MPa, and the measurements shown in FIG. 16, at a temperature of 150 ° C with a force of 90 MPa.

The y-axis shows the percent creep strain, and the x-axis shows time in hours.

Example 4

The stress relaxation of several compositions from the table was determined, expressed in the remaining load as a function of time. The results are shown in FIG. 17-19.

On them, the y-axis shows the remaining load as a percentage of the initial load, and the x-axis shows time in hours.

Example 5

In accordance with A8TMB117, the anticorrosion properties of several compounds were determined. During this test, a large amount of data was used to determine the effect of the REM content as a function of A1 content. The results are shown in FIG. twenty.

The y-axis shows the REM content in wt.%, And the x-axis shows the A1 content also in wt.%.

The boundary lines between regions with different hatching represent lines of the same corrosion resistance.

From the above test results it follows that a method of casting a magnesium alloy is proposed, which allows to obtain products with an improved combination of creep characteristics at elevated temperatures, ductility and corrosion characteristics.

- 5 013656

Type of alloy BUT! νΛ% Mn \ ν (% Ζπ νΛ% but νΛ% Xie Sa Νά "one% Wg EE ν / ί% Ce / KE 1a / r> £ Ν8 / ΚΕ WG / KE ΑΖ91ϋ 8.93 0.17 0.73 A821B 2.11 0.08 1.01 0.09 AE35-24 3.23 0.29 2.49 1.73 0.94 0.28 5.44 45.77 31.80 17.28 5.15 AE42-15 3.89 0.15 1.31 0.79 0.37 0.16 2.64 49.85 30.08 14.15 5.92 AE44-24 4.12 0.29 2.11 1,53 0.75 0.23 4.62 45.67 33.12 16.23 4.98 AE63-4 6.31 0.18 1.42 1.35 0.40 0.13 3.30 43.03 40.91 12.12 3.94 ACE44 3.70 3.90 3.90 100.00 AMs144 3.90 2,50 2,50 100.00 Ake44 3.70 0.38 3.00 3.00 100.00 A1_aCe431 3.70 0.45 0.90 2,30 3.20 28.10 71.90 A.1.aCe413 4.00 0.28 2.40 0.90 3.30 72.70 27.30 A1aK1Y431 3.90 0.46 2.60 0.80 3.40 76.50 23.50 A1_aYS1413 3.70 0.42 1.10 1.60 2.70 40.70 59.30 ACEI <1431 4.70 0.27 2.60 0.80 3.40 76.50 23.50 ACEIc1413 4.40 0.32 0.90 1.00 1.90 47.40 52.60 | ΑΟεΝά422 3.60 1.50 1,50 3.00 50.00 50.00

CLAIM

Claims (18)

1. The casting method of the alloy MD-A1-KE for the manufacture of cast parts with an improved combination of creep characteristics at elevated temperatures, ductility and corrosion characteristics, consisting of 2.0-6.0 wt.% Aluminum, 3.0-8.0 wt. .% of rare earth metals (REM), in which the ratio of the number of rare-earth metals and the amount of aluminum, expressed in wt.%, exceeds 0.8, at least 40 wt.% of rare-earth metals is cerium, less than 0.5 wt.% manganese, less than 1.0 wt.% zinc, less than 0.01 wt.% calcium, less than 0.01 wt.% strontium, and the remainder includes magnesium and constant impurities, total e the content of which is less than 0.1 wt.%, while casting the alloy is carried out in a mold whose temperature is maintained in the range of 170-390 ° C, fill the mold for a time, which in milliseconds is equal to the product of a number from 5 to 500 times the average thickness of the part in millimeters, during casting, static pressure on the metal is maintained in the range of 20-70 MPa, with a subsequent increase to 180 MPa. 2. The method according to claim 1, in which the temperature of the form is maintained in the range from 180 to 340 ° C, preferably from 200 to 270 ° C. 3. The method according to claim 1 or 2, in which the form filling time in milliseconds is equal to the product of a number from 8 to 200, preferably from 5 to 50, more preferably from 5 to 20, multiplied by the average thickness of the part in millimeters. 4. The method according to any one of claims 1 to 3, in which during casting, static pressure on the metal is maintained in the range of 30-70 MPa. 5. The method according to any one of claims 1 to 4, in which the cooling rate after casting is in the range of 10-1000 ° C / s. 6. The method according to any one of claims 1 to 5, in which the aluminum content is from 2.5 to 5.5 wt.%, Preferably from 2.6 to 6.5 wt.%. 7. The method according to any one of claims 1 to 6, in which the content of rare-earth metals is from 3.5 to 7.0 wt.%. 8. The method according to any one of claims 1 to 7, in which the aluminum content is from 3.6 to 4.5 wt.%, The content of rare-earth metals is from 3.6 to 4.5 wt.%, And the ratio of rare-earth metals and aluminum exceeds 0.9. 9. The method according to any one of claims 1 to 8, in which the aluminum content is from 2.6 and 3.5 wt.%, And the content of rare-earth metals exceeds 4.6 wt.%. 10. The method according to any one of claims 1 to 9, in which REM is selected from the group consisting of cerium, lanthanum, neodymium and praseodymium. 11. The method according to claim 10, in which the amount of lanthanum is at least 15 wt.% The total content of rare-earth metals, preferably at least 20 wt.%. 12. The method according to claim 10 or 11, in which the amount of lanthanum does not exceed 35 wt.% The total content of rare-earth metals. 13. The method according to any one of claims 10 to 12, in which the amount of neodymium is at least 7 wt.% The total content of rare-earth metals, preferably at least 10 wt.%. 14. The method according to any one of claims 10 to 13, in which the amount of neodymium does not exceed 20 wt.% The total content of rare-earth metals. 15. The method according to any one of paragraphs.10-14, in which the amount of praseodymium is at least 2 wt.% The total content of rare-earth metals, preferably at least 4 wt.%. 16. The method according to any one of paragraphs.10-15, in which the amount of praseodymium does not exceed 10 wt.% The total content of REM. 17. The method according to any one of paragraphs.10-16, in which the amount of cerium exceeds 50 wt.% The total content of rare-earth metals, preferably is from 50 to 55 wt.%. 18. The method according to any one of claims 10-17, in which the amount of calcium and / or strontium does not exceed 0.01 wt.%.