PT1957221E - A combination of casting process and alloy compositions 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

1

DESCRIPTION

" A COMBINATION OF THE FOUNDATION PROCESS AND LEAD COMPOSITIONS THAT RESULT IN COATED PARTS WITH HIGHER COMBINATION OF FLUENCY PROPERTIES AT HIGH TEMPERATURES, DUCTILEITY AND CORROSION PERFORMANCE " The invention relates to a process for the melting of a magnesium alloy consisting of 2.0 - 6.00% by weight of aluminum, 3.00 - 8.00% by weight of rare earth metals (RE metals), the ratio of the amount of metals ER for the amount of aluminum, expressed as weight%, greater than 0.8, at least 40% by weight of ER metals being cerium, less than 0.5% by weight manganese, 00% by weight of zinc, less than 0.01% by weight of calcium less than 0.01% by weight of strontium and the remainder being magnesium and unavoidable impurities, the total level of impurity being less than 0.1% by weight .

Magnesium-based alloys are widely used as castings in the aerospace and automotive industries. Castings with magnesium-based alloys can be produced by conventional casting methods, which include die-casting, sand casting, casting with permanent and semi-permanent castings, cast-die casting and investment casting.

Magnesium-based alloys demonstrate a number of particularly advantageous properties that have instigated a growing demand for magnesium-based alloy castings in the automotive industry. These properties include low density, high strength-to-weight ratio, good meltability, easy machinability and good damping characteristics. 2

The most common magnesium fused alloys, such as Mg-Al alloys or Mg-Al-Zn alloys, are known to lose their creep resistance at temperatures above 120 ° C. Mg-Al- Si were developed for applications at higher temperatures and offer only a small improvement in creep resistance. The Mg-Al-Ca and Mg-Al-Sr system alloys offer a further improvement in creep resistance, but a major disadvantage of these alloys is the problems of melting. This problem is particularly evident with high metal speeds that directly collide with the mold surface, the so-called water hammer effect. It is known that the AE48 alloy (4% AP, 2-3% RE) offers a significant improvement in high temperature and corrosion properties.

Mg-Al alloys containing elements such as strontium (Sr) and calcium (Ca) offer further improvement in creep properties, although at the expense of a reduction in melting. The Mg-Al-Ca and Mg-Al-Sr system alloys offer a further improvement in creep resistance, but a major disadvantage of these alloys is the problems of melting. This problem is particularly evident with high metal speeds that directly collide with the mold surface, the so-called water hammer effect.

Attached Figures 1A and 1B are schematically shown in cold chamber and hot runner die casting machines, respectively, each machine having a die 10, 20 equipped with a hydraulic damping system 11, 21 respectively . The molten metal is introduced into the mold through a firing cylinder 12, 22 equipped with a piston 13, 23, respectively. In the cold chamber system an auxiliary system for measuring the metal is required for the horizontal firing cylinder 3. The hot-chamber machine (Fig. 1B) uses a vertical piston system (12, 23) directly in the molten alloy.

To obtain the excellent performance of the Mg-Al-Re alloys, it is imperative that the alloys are melted under extremely rapid cooling conditions. This is the case with the high pressure mold casting process. The steel mold 10, 20 is equipped with an oil (or water) cooling system which controls the mold temperature in the range of 200-300 ° C. A prerequisite for good quality is a short mold fill time to avoid solidification of the metal during filling. It is recommended a mold filling time in the order of 10 s x average piece thickness (mm). This is obtained by forcing the alloy through a gate with high speeds, typically in the range of 30 to 300 m / s. Piston speeds up to 10 m / s with sufficiently large diameters are used to obtain the desired flow volumes in the firing cylinder to achieve the short fill times required. Static metal pressures of 20-70 MPa and subsequent pressure intensification are often used up to 150 MPa. With this casting method, the resulting cooling rate of the component is typically in the range of 10 to 1000 ° C / sec, depending on the thickness of the component to be melted. For AE alloys this is a key factor in determining the properties, both because of the generally high cooling rate of the part, and in particular the extremely high cooling rate of the surface layer.

In Fig. 2, the relationship between the solidification interval and the microstructure is presented in the annex. The horizontal axis shows the solidification rate, expressed in ° C / s, and the vertical vertical scale shows the spacings between secondary dendritic arms, expressed in pm, while in the vertical scale of the right 4 the diameter of the grain, expressed in pm. Line 30 indicates the size of the grain obtained, while line 31 corresponds to the value obtained for the spacings between secondary dendritic arms.

With mold casting, grain refinement is achieved through the cooling rate. As mentioned above, cooling rates in the range of 10 to 1000 ° C / sec are usually attained. This typically results in grain sizes between 5 and 100 pm. It is well known that fine grain size benefits the ductility of an alloy. This relationship is illustrated in Fig. 3, attached, which represents the relationship between grain size and relative elongation. In the horizontal axis the grain size provided was expressed in pm, while the vertical axis gives the relative elongation expressed in%. In the graph two different compositions are shown, first pure Mg, line 35, and a Mg alloy designated AZ91, line 36.

It is also well known that fine grain size is beneficial to the yield of the tensile strength of an alloy. This relation (Hall-Petch) is shown in Fig. 4, attached. The horizontal axis shows the grain diameter, expressed in d (-0.5), in which it was expressed in pm, and the vertical axis shows the yield of the tensile force, expressed in MPa.

Thus, it is evident that the size of the fine grain provided by the very high cooling rates facilitated by the die casting process is a necessary condition to obtain tensile strength and ductility. The term fusibility describes the ability of an alloy to be fused to an end product with the requisite functionalities and properties. This usually contains 3 categories; (1) the ability to form a part having all desired geometric characteristics and dimensions (5), (2) the ability to produce a dense part with the desired properties, and (3) the effects on die cast tools, casting process and the efficiency of the mold casting process. German patent application 2122148 describes Mg-Al-RE type alloys, mainly Mg-Al-RE alloys with RE content < 3% by weight, although alloys with higher ER content are also discussed. It is known that the AE42 (4% Al, 2-3% RE) alloy offers a significant improvement of properties at elevated temperatures and corrosion. It has been shown that small additions of RE to Mg-Al alloys lead to a significant improvement in corrosion properties but to a deterioration of meltability, as problems with mold adhesion more often occur. In Fig. 5, the excellent, low, and very low melting zones are shown in the Mg-Al-Re system. In the horizontal axis is represented the amount of Al, expressed in% by weight, while in the vertical axis the amount of RE, expressed in% by weight, is represented. Line 40 represents the line indicating the solubility of RE at 680øC, while line 41 indicates the solubility of RE at 640øC. Region 42 (dark) represents the very low melting composition. The (intermediate) region 43 represents the low melting composition and the (clear) region 44 represents the compositions having excellent melting. As shown in Fig. 5, melting becomes worse as the ER content of the alloy increases. However, as Fig. 5 indicates, there is a region with RE> 3.5% by weight (the upper limit is restricted by the solubility of RE), Al between 2.5% and 5.0%, and in addition , described by a ratio of% RE /% Al higher than 0.8, wherein the die casting at high pressure is excellent.

Thus, it is an object of the present invention to provide relatively low cost magnesium based alloys with improved performances at high temperatures and improved melting.

Due to the formation of dispersed phases of AlxREy, the compositions of the present invention minimize the fraction of the fragile phase volume of Mg17Al12 (the RE / AI ratio in the dispersed phases increases with increasing the% content of RE /% AI in the alloy) . Because the eutectic phase of Mg17Al2 melts at about 420Â ° C, conventional Mg-Al alloys, such as AM50, AM60 and AZ91, will have a solidification range of about 200Â ° C, as shown in Fig. 6 in the Annex. Fig. 6 shows the solid fraction (expressed in wt.%) On the horizontal axis versus the temperature (° C) on the vertical axis for a number of alloys. Mg-Al-RE alloys with RE%% Al ratios as specified in the present invention will solidify completely at about 570 ° C, so the solidification range is only about 50 ° C.

In general, increasing the aluminum content in the injection-fused Mg-Al alloys improves cast meltability. This is due to the fact that Mg-Al alloys have a great extent of solidification, which makes them inherently difficult to melt unless a sufficiently large amount of eutectic is present at the end of the solidification. This may explain the good melting of AZ91 D consistent with the cooling curves shown in Fig. 6. As the Al content is reduced by 6, 5 and 2% in AM6 O, AM5 O and AM2 O respectively, The remaining eutectic layer decreases to a level where feed becomes difficult during the final solidification steps, which means that for thick walled parts, even greater microporosity and voids may be present. For thin walled parts, the ability to feed during the final steps is less important (while alloy fluidity becomes the relevant factor), since volume shrinkage is partly supported by the reduction of thickness due to the shrinkage of the mold walls. The AE44 and AE35 alloys exhibit very different cooling characteristics of the Mg-Al alloys. The solidification range is significantly lower, which indicates that the concentrated retraction porosity can be decreased during solidification. These alloys have a good flowability during mold filling, and therefore can be easily melted into final products with less casting defects. The fusibility of AE44 and AE35 is relatively equal to that of AZ91D.

An additional issue related to the narrow solidification interval is that the inverse segregation, often observed in both AZ91 D alloys and AM alloys, will not occur. This is illustrated by the fact that high ER AE alloys contain a bright surface without segregation of the eutectic phase of Mg-Al. The surface layer solidifies during and immediately after mold filling, and the temperature will decrease rapidly below the solidus temperature, thereby preventing the molten metal from being forced against the surface of the mold when the retraction begins. This will be beneficial in preventing reactions between the mold wall and the molten metal, which can lead to mold adhesion.

An example with a wall thickness of about 3 mm having three layers with a different microstructure in AE44 is shown in Fig. The surface layer, having a thickness of approximately 50 μm, consists of equiaxed grains having a size of about 10 μm. This is a relatively small grain size, which can be explained by the rapid cooling conditions in the injection wall. The intermediate layer is about 100 μm thick and consists of an extremely fine grain. The morphology is different from the previous one and a DAS is observed in a range between 2 and 4 pm. This observation can be explained by a change in equilibrium melting point due to pressure. When the metal becomes pressurized, the equilibrium melting point increases, i.e., the metal suddenly becomes subcooled. In theory, this is the same for all Mg alloys, but between the alloys a significant difference remains in the solidification characteristics. The core consists of ~ 20 andm equiaxed grains. The solidification of the core is restricted by the heat flow from the core to the mold. Both the heat transfer through the already solidified layer and the heat transfer over the cast / mold interface will provide a slower cooling rate than the skin and as such a coarser microstructure is formed.

When the ER content is low, or the% RE /% AI ratio is low, such as in AE42 or AE63, there will be a possibility that Mg-Al eutectic is present that can secrete to the surface, and lead to adhesion. This may be the reason why AE42 appears with less fundibility.

In Fig. 8 a mold box is shown in the upper part of the drawing. Micrographs of the examples of nodule 3 (near the gate) for the alloys AM60, AM40, AE63, AE44 and AE35, as shown below. Hot cracks can be seen in AM40 and AE63. Fig. 8 shows that AE44 and AE35 are less susceptible to hot tears than AM alloys. This is explained by the relatively rapid solidification of the surface layer, which results in the relatively fine grain structure as described above.

Partially due to the fine grain structure and partially due to the absence of the fragile phase of Mgi7Al12 this layer becomes very ductile, and is therefore capable of deforming if thermal stresses develop during solidification. A surface layer having coarser grains, as would typically appear in alloys with a greater solidification range, and / or in a layer rich in Mg7 Al2, will have much lower ductility and will tend to crack and form hot tears rather than deform. The large-walled test (~ 1.5m) with thin walls (thickness ~ 3mm) revealed that the injection fill characteristics of AE44 and AE35 are excellent, and since large-range feed is not required for parts with As discussed above, it is expected that this alloy will provide a viable alternative to these types of components, where injection fill is of prime importance.

The properties of various AE alloys are explained by the observations that Al alone provides the strengthening of the solid solution, whereas the RE combines with Al to form dispersed phases in the boundary regions of the grain. In the AE44 and AE35 alloys, the dispersoid phase (mainly A12RE) constitutes a continuous three-dimensional network, effectively preventing the creep arising from thermal activation and grain boundary slip. This is represented in Fig. 9 which represents SEM-BEC images (Backscatter Electronic Composition) showing the die casting microstructure of (from left to right) AE44, AE35 and AE63. While Al alone provides a strengthening of the solid solution, RE combines with AL to form dispersed phases in the peripheral regions of the grain.

An enlargement of the SEM-BEC images to AE 44 is shown in Fig. 10, which also shows the lamella structure of the AlxREy phases in AE44. As can be seen from Fig. 10 the dispersed phases of AlxREy in the AE alloys consist of an extremely thin lamella structure. This structure of submicron lamellae hardens the grain boundaries, thus preventing creep. On the other hand, these lamellae are not brittle (or not as brittle as the Mg-Al eutectic), since the injection-fused AE44 alloy experiences a ductility which is similar to that of AE42. In AE63, the lattice (mainly AlnRE3) becomes fragmented and the regions of the grain periphery are probably influenced by a substantial amount of eutectic Mg-Al, reducing ductility and creep properties. In AE42 there is probably also a significant amount of Mg-Al eutectic limiting creep properties. The AE35 alloy has a ductility slightly less than AE44, but still greater than AE63.

Numerous examples of mechanical properties including the ductility, tensile strength, creep resistance and corrosion properties of AE alloys will be presented later. The unique combination of creep and ductility resistance compared to existing alloys is shown in Fig. 11. In Fig. The ductility (horizontal axis) is shown as versus the creep resistance for a number of known Mg alloys. Zone 50 comprises AM alloys, AE zones 51 alloys, zone 52 alloy AZ91 and zone 53 other high temperature alloys. The AE alloys of the present invention are the only cast alloys that combine ductility and high temperature properties in this manner, and thus offer numerous new and unexplored opportunities for builders and designers, particularly in the automotive industry.

An example of the applicability in the AE 44 industry is given in &quot; Magnesium makes its engine cradle mark &quot; by N. Li, AEI Maerial Innovations, April 2005, pages 110-111, which describes the research and development of the engine cradle of the 2006 version of the Chevrolet Corvette Z06 built from low-weight magnesium.

Additional information on casting in magnesium cast and magnesium alloys can be found on 11

ASM Specialty Manual. &quot; Magnesium and Magnesium Alloys &quot; by Avedesian et al., May 2000, pages 66-77. For example, typical mechanical properties of separate injection-fused test bars for a spectrum of magnesium alloys such as AE 42 and AZ91 are shown in table 2 on page 67.

It is an even more particular object to provide relatively low cost cast magnesium-aluminum-rare-earth alloys with excellent melt strength, good creep resistance, good yield of tensile strength and good screw load retention, particularly at elevated temperatures of at least 150 ° C.

SUMMARY OF THE INVENTION

Thus, the present invention provides: The alloy is cast in a mold at a controlled temperature in the range of 180 to 340Â ° C. The mold is filled in a time which in milliseconds is equal to the product of a number between 5 and 500 multiplied by the average thickness of the part, expressed in millimeters,

The static pressures of the metal being maintained during melting at 20 to 70 MPa and subsequently increased to 180 MPa.

Using the combination of a specific Mg-Al-RE alloy with a special casting process, products with excellent creep resistance, high temperatures, high ductility, and generally good mechanical and corrosion properties could be obtained.

In general, a number of RE metals may be used as the alloying constituent, such as, for example, Ce, La, Nd and / or Pr and mixtures thereof. It is, however, preferred to use cerium in substantial amounts, since this metal provides the best mechanical properties. Mn is added to improve resistance to corrosion but its addition is restricted because of its limited solubility.

Preferably the aluminum content is between 2.0 and 6.00% by weight, more preferably between 2.60 and 4.50% by weight.

If higher amounts of aluminum are present, this can easily lead to the formation of Mg17Al12 phases which are detrimental to the creep properties. A too low AI content is detrimental to melting.

With reference to the ER metals it is preferred that the ER content is between 3.50 and 7.00% by weight, the upper limit being restricted by the solubility of RE in the Mg-Al-RE system, as indicated in Fig.

If there is more than 3.50% RE by weight, this provides a significant improvement of creep properties. More than 7.00% by weight is impractical because of the restricted solubility of RE metals in liquid magnesium aluminum alloys.

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

For specific applications the composition of the alloy is selected such that the aluminum content is between 3.6 and 4.5% by weight and that the RE content is between 3.6 and 4.5% by weight, with the additional restriction that the Re / Al ratio is greater than 0.9.

This type of alloys can be used for applications up to 175 ° C while maintaining excellent creep properties and tensile strength. Moreover, this alloy does not show any degradation of its properties due to aging, and has a good melting.

For applications above 175 ° C the composition of the alloy is such that the aluminum content is between 2.6 and 3.5% by weight, and the RE content is greater than 4.6% by weight. 13

In addition to the excellent creep properties and tensile strength, this alloy shows no degradation of properties due to aging.

Preferably the ER metals are selected from the group consisting of cerium, lanthanum, neodymium and praseodymium.

RE metals contribute to the formation of the alloy, but also increase corrosion resistance, creep resistance and improve mechanical properties.

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

Preferably the amount of neodymium is at least 7% by weight, and more preferably at least 10% by weight of the total metal content RE. Preferably the amount of neodymium is less than 20% by weight of the total content of ER metals.

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

Preferably the amount of cerium is greater than 50% by weight of the total content of RE metals, preferably between 50 and 55% by weight. It is known that calcium and strontium provide an increase in creep resistance, and that the addition of at least 0.5% by weight of calcium will improve tensile strength. However, Ca and Sr should be avoided because, even at very low concentrations, these elements lead to considerable adhesion problems thus influencing the fusibility of the alloy. The present invention is described in more detail with reference to the following example, which is intended merely to illustrate and should not be construed as indicating or implying any limitation of the invention presented and described herein.

Example 1

To compose, the influence of the elements constituting the alloy and a number of Mg alloys were prepared with the compositions given in Table 1.

For each purpose of the alloy a number of test bars were made to perform the tests described in the following examples. The tests performed are as follows: Tensile strength and ductility

6 mm test bars were made in accordance with ASTM standards, and the following

Test conditions were used:

• Instron 10 kN machine

• Ambient temperature 210 ° C • At least 5 parallel at each temperature • Voltage rating - 1.5 mm / min up to 0.5% voltage, - 10 mm / min above 0.5% according to ISO 6892 Tensile strength test

For this text the following test material is used: • Diameter: 6 mm • Standard measurement: 32.8 mm • Bend radius: 9 mm • Handle head diameter: 12 mm • Overall length: 125 mm The test is done according to ASTM E 139 standard Stress relaxation test • Test material - 12 mm in diameter, 6 mm in length - Arbitrary end of fluence bars 15 • Test according to ASTM E328-86 Corrosion Properties Corrosion is tested according to ASTM Standard 117. Example 2

For a number of compositions the strength was measured as a function of temperature.

The results are shown in Figures 12, 13 and 14. In these figures the y-axis represents the tensile strength, expressed in MPa, while the x-axis represents the temperature, expressed in degrees Celsius.

Example 3

For a number of compositions the creep stress was measured as a function of time.

The results are shown in Figures 15 and 16. In Fig. 15 the measurement is made at 175 ° C with a force of 40 MPa, and in Fig. 16 the measurement is made at 150 ° C with forces of 90 MPa.

In these figures the y-axis represents the creep stress, expressed as a percentage, while the x-axis represents the time, expressed in hours.

Example 4

For a number of compositions, according to Table 1, stress relaxation was defined, expressed as the remaining charge versus time. The results are presented in figures 17, 18 and 19.

In these figures the y-axis represents the remaining charge, expressed as a percentage of the initial charge, while the x-axis represents the time, expressed in hours.

Example 5

For a number of compositions the corrosion properties were defined according to ASTM standard B117. In this test a large amount of data was incorporated in order to define the influence of the ER content versus the AI content. The results are shown in Fig.

In this figure, the y-axis represents the ER content, expressed in% by weight, while the x-axis represents the Al content, also expressed as% by weight.

The boundary lines between the zones with different shading represent lines of equal resistance to corrosion.

From the test results it is evident that a magnesium alloy melting process has been provided, where the products are obtained with a superior combination of creep properties at high temperatures, ductility and corrosion performance. 17

Table 1

18

DOCUMENTS REFERRED TO IN THE DESCRIPTION

This list of documents referred to by the author of the present patent application has been prepared solely for the reader's information. It is not an integral part of the European patent document. Notwithstanding careful preparation, the IEP assumes no responsibility for any errors or omissions.

Patent documents referred to in the specification • WO 2122148 A [0016]

Non-patent literature referred to in the description • N. LI. Magnesium makes its engine cradle mark. AEI Maerial Innovations, April 2005, 110-111 Magnesium and Magnesium Alloys. AVEDESIAN et al. ASM Specialty Handbook. May 2000, 66-77 [0031]

Claims (10)

A process for melting a magnesium alloy consisting of 2.0 - 6.00% by weight of aluminum, 3.00 - 8.00% by weight of rare earth metals (RE metals), the ratio of the amount of metals RE for the amount of aluminum, expressed as a weight percent, greater than 0,8, at least 40% by weight of the metals ER cerium, less than 0,5% manganese, less than 1,00% weight of zinc, less than 0.01% by weight of calcium less than 0.01% by weight of strontium and the remainder being magnesium and unavoidable impurities, the total impurity level being less than 0.1% by weight, wherein the alloy is cast in a mold at a controlled temperature in the range of 180 to 340 ° C, the mold is filled in a period of time, expressed in milliseconds, is equal to the product of a number between 5 and 500 multiplied by the average thickness of the part, expressed in millimeters, - the static pressures of the metals being maintained during melting at 20 ° to 70 ° Pa and is subsequently intensified to 180 MPa. The process of claim 1, wherein the mold temperature is controlled to a temperature in the range of 200-270øC. The process according to claim 1 or 2, wherein the mold fill time in milliseconds is equal to the product of the average thickness of the part, expressed in millimeters, multiplied by a number 2 between 8 and 200, preferably between 5 and 50, more preferably between 5 and 20. The process according to any one of claims 1 to 3, wherein the static pressure of the metal during casting is maintained between 30 and 70 MPa. The process according to any one of claims 1 to 4, wherein the cooling rate after melting is in the range of 10 to 1000 ° C / sec. The process according to any one of claims 1 to 5, wherein the aluminum content is between 2.50 and 5.50% by weight, preferably between 2.60 and 4.50% by weight. The process according to any one of claims 1 to 6, wherein the RE content is between 3.50 and 7.00% by weight. The process according to any one of claims 1 to 7, wherein the aluminum content is between 3.6 and 4.5% by weight and the RE content is between 3.6 and 4.5% by weight , and that the RE-AI ratio is greater than 0.9. The process according to any one of claims 1 to 8, wherein the aluminum content is between 2.6 and 3.5% by weight and the RE content is greater than 4.6% by weight. The process according to any one of claims 1 to 9, wherein the ER metals are selected from the group consisting of cerium, lanthanum, neodymium and praseodymium. The process according to claim 10, wherein the amount of lanthanum is at least 15% by weight of the total content of metals ER, preferably at least 20% by weight. The process according to claim 10 or 11, wherein the amount of lanthanum is not more than 35% by weight of the total metal content RE. The process according to any one of claims 10 to 12, wherein the amount of neodymium is at least 7% by weight of the total metal content RE, preferably at least 10% by weight. The process according to any one of claims 10 to 13, wherein the amount of neodymium is not more than 20% by weight of the total metal content RE. The process according to any one of claims 10 to 14, wherein the amount of praseodymium is at least 2% by weight of the total metal content RE, preferably at least 4% by weight. The process according to any one of claims 10 to 15, wherein the amount of praseodymium is not more than 10% by weight of the total metal content RE. The process according to any one of claims 10 to 16, wherein the amount of cerium is greater than 50% by weight of the total metal content RE, preferably between 50 and 55% by weight. The process according to any one of claims 10 to 17, wherein the amount of calcium and / or strontium is less than 0.01% by weight.