CA2627491C

CA2627491C - A combination of casting process and alloy compositions resulting in cast parts with superior combination of elevated temperature creep properties, ductility and corrosion performance

Info

Publication number: CA2627491C
Application number: CA2627491A
Authority: CA
Inventors: Per Bakke; Westengen Haakon
Original assignee: Magontec GmbH
Current assignee: Magontec GmbH
Priority date: 2005-11-10
Filing date: 2006-09-19
Publication date: 2011-11-22
Anticipated expiration: 2026-09-19
Also published as: BRPI0618517B1; JP2009527637A; SI1957221T1; KR101191105B1; CN101528390A; KR20080066805A; CA2627491A1; JP5290764B2; HRP20120244T1; AU2006312743B2; WO2007054152A1; CN101528390B; EP1957221A1; AU2006312743A1; BRPI0618517A2; ATE538887T1; ES2379806T3; EP1957221B1; RS52267B; EA200801268A1

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

A combination of casting process and alloy compositions resulting in cast parts with superior combination of elevated temperature creep properties, ductility and corrosion performance.

The invention relates to 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.

Magnesium-based alloys are widely used as cast parts in the aerospace and automotive industries. Magnesium-based alloy cast parts can be produced by conventional casting methods, which include die-casting, sand casting, permanent and semi-permanent mold casting, plaster-mold casting and investment casting.
Mg-based alloys demonstrate a number of particularly advantageous properties that have prompted an increased demand for magnesium-based alloy cast parts in the automotive industry. These properties include low density, high strength-to-weight ratio, good castability, easy machinability and good damping characteristics.
Most common magnesium die-casting alloys such as Mg-Al-alloys or Mg-AI-Zn-alloys are known to lose their creep resistance at temperatures above 120 C.
Mg-AI-Si alloys have been developed for higher temperature applications and offer only a limited improvement in creep resistance. Alloys of the Mg-Al-Ca and Mg-Al-Sr system offer a further improvement in creep resistance, but a great disadvantage with these alloys is problems with castability. This is particularly a problem with high metal velocities impinging directly onto the die surface, the so-called water hammer effect.

It is known that the alloy AE48 (4% AP, 2-3 % RE) offers a significant improvement in elevated temperatures properties and corrosion.

Mg-Al alloys containing elements like Sr and Ca offer a further improvement in creep properties, however at the cost of reduced castability. Alloys of the Mg-Al-Ca and Mg-Al-Sr system offer a further improvement in creep resistance, but a io great disadvantage with these alloys are problems with castability. This is particularly a problem with high metal velocities impinging directly onto the die surface, the so-called water hammer effect.

In the annex Figures 1A and 1B there are schematically shown cold chambers and hot chambers die castings machines respectively each machine has a die 10, 20 provided with a hydraulic damping system 11, 21 respectively.
Molten metal is introduced into the die by means of a shot cylinder 12, 22 provided with a piston 13, 23 respectively. In the cold chamber system an auxiliary system for metering of the metal to the horizontal shot cylinder is 2o required. The hot chamber machine (Fig. 1 B) 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 mandatory that the alloys are cast under extremely rapid cooling conditions. This is the case for the high pressure die casting process. The steel die 10, 20 is equipped with an oil (or water) cooling system controlling the die temperature in the range of 300 C. A prerequisite for good quality is a short die filling time to avoid solidification of metal during filling. A die filling time in the order of 10-2 s x average part thickness (mm) is recommended. This is obtained by forcing the 3o alloy through a gate with high speeds typically in the range 30-300 m/s.
Plunger velocities up to 10 m/s with sufficiently large diameters are being used to obtain the desired volume flows in the shot cylinder for the short filling times needed. It is common to use static metal pressures 20-70 MPa and subsequent pressure intensification up to 150 MPa. With this casting method the resulting cooling rate of the'component is typically in the range of 10-1000 C/s depending on the thickness of the component being cast. For AE alloys this is a key factor in determining the properties, both because of general high cooling rate of the part, and in particular the extremely high cooling rate of the surface layer.
In the annexed Fig. 2 there is shown the relationship between the solidification range and the microstructure. On the horizontal axis there is shown the solidification rate expressed as C/S and on the left hand vertical scale the secondary dendrite arm spacings expressed in pm is shown, whereas the right hand vertical scale the grain diameter expressed in pm is shown. Line 30 io indicates the grain size obtained, whereas line 31 is the.obtained value for the secondary dendrite arm spacings.
With die casting grain refining is obtained by the cooling rate. As mentioned above cooling rates in the range of 10-1000 C/s is normally achieved. This typically results in grain sizes in the range of 5-100 m.
It is well known that fine grain size is beneficial for the ductility of an alloy. This relationship is illustrated in the annexed Fig. 3, in which the relationship between grain size and relative elongation has been shown. On the horizontal axis the arrange grain size has been represented expressed in pm, whereas the vertical 2o axis gives the relative elongation expressed in %. In the graph there are shown two different composition, first pure Mg, line 35 and a Mg-alloy designated AZ91, line 36.

It is also well known that fine grain size is beneficial for the tensile yield strength of an alloy. This relationship (Hall-Petch) is shown in the annexed Fig. 4. In the horizontal axis there is represented the grain diameter, expressed as d (-0,5), in which has been expressed in pm , and in the vertical axis there is shown the tensile yield strength expressed in MPa.

It is therefore evident that the fine grain size provided by the very high cooling rates facilitated by the die casting process is a necessity for obtaining tensile strength and ductility.

The castability term describes the ability of an alloy to be cast into a final product with required functionalities and properties. It generally contains 3 categories;
(1) the ability to form a part with all desired geometry features and dimensions, (2) the ability to produce a dense part with desired properties, and (3) the effects on die cast tooling, foundry equipment and die casting process efficiency.

The German Patent Application 2122148 describes alloys of the Mg-Al-RE type mainly Mg-Al-RE alloys with RE content < 3wt%, although alloys with higher RE
content are discussed as well. It is known that the alloy AE42 (4% Al, 2-3%
RE) io offers a significant improvement in elevated temperature properties and corrosion properties. It is experienced that small RE additions to Mg-Al alloys lead to a significant improvement in corrosion properties, but a deterioration in the castability as problems with die sticking occur more frequently. In the annexed Fig. 5 there is shown the regions of excellent, poor and very poor castability in the Mg-Al-Re system. In the horizontal axis the amount of Al expressed as % by weight is shown, whereas in the vertical axis the amount of RE expressed in % by weight is shown. The line 40 is the line indicating the solubility of RE at 680 C, whereas the line 41 indicates the solubility of RE
at 640 C. The region (dark) 42 represents the composition with very poor castability. The region (intermediate) 43 represents the composition with poor castability and the region 44 (light) represents the compositions with excellent castability. As illustrated in Fig. 5, the castability becomes worse as the RE
content of the alloy increases. However, as Fig. 5 indicates, there is a region with RE >3.5 wt% (the upper limit restricted by the solubility of RE), Al in the range 2.5% to 5.0% and furthermore described with a%RE/%AI ratio greater than 0.8 where the high pressure die castability is excellent.

It is therefore an object of the present invention to provide relatively low cost magnesium-based alloys with improved elevated-temperature performance and improved castability.

Due to the formation of AIxREy dispersoid phases, the compositions of the present invention minimise the volume fraction of the brittle Mg AI12 phase (The RE/Al ratio in the dispersoid phases increases with increasing %RE/%Al content in the alloy). Due to the fact that the eutectic Mg AI12 phase melts at around 420 C, the conventional Mg-Al alloys like AM50, AM60 and AZ91 will have a solidification range of nearly 200 C as shown in the annexed Fig. 6. Fig 6 shows the fraction solid (expressed in % by weight) on the horizontal axis versus 5 the temperature ( C) on the vertical axis for a number of alloys. The Mg-Al-RE
alloys with the %RE/%AI ratios as specified in the present invention will solidify completely at around 570 C, hence the solidification range is only approximately 50 C.

io In general, increasing aluminium content in Mg-Al die casting alloys improves the die castability. This is due to the fact that Mg-Al alloys have a wide solidification range, which makes them inherently difficult to cast unless a sufficiently large amount of eutectic is present at the end of solidification.
This can explain the good castability of AZ91 D consistent with the cooling curves shown in Fig. 6. As the Al-content is reduced to 6, 5 and 2% in AM60, AM50 and AM20, respectively, the remaining eutectic is decreasing to a level where feeding becomes difficult during the final stages of solidification which means, for thick walled parts, microporosity and even larger voids can be present.
For thin walled parts, the ability to feed during the final stages is less important (while alloy fluidity becomes the significant factor) since the volume shrinkage is partly taken up by thickness reduction due to shrinkage from the die walls.
The AE44 and AE35 alloys show very different cooling characteristics from Mg-Al alloys. The solidification interval is significantly smaller, indicating concentrated shrinkage porosity can be decreased during solidification. These alloys have good fluidity during mold filling, and can thus easily be cast into final products with less casting defects. The castability of AE44 and AE35 is relatively equal to that of AZ91 D.

A further issue related to the narrow solidification interval is the fact that the commonly observed inverse segregation occurring in AZ91 D as well as AM
alloys will not occur. This is illustrated by the fact that AE alloys with high RE
contents have a shiny surface without segregations of Mg-Al eutectic phase.
The surface layer solidifies during and immediately after die filling, and the temperature will rapidly decrease below the solidus temperature, thereby preventing molten metal to be forced towards the die surface when shrinkage starts. This will be beneficial to prevent reactions between the die wall and molten metal, which could lead to die sticking.

An example with a wall thickness of about 3 mm showing three layers with different microstructure in AE44 is given in the annexed Fig. 7. The surface layer, having a thickness of approx. 50 m, consists of equiaxed grains with size about 10 m. This is a fairly small grain size, which can be explained by the rapid cooling conditions on the die wall. The intermediate layer is about 100 m io thick and is extremely fine grained. The morphology is different from the former and DAS in the range of 2-4 m is observed. The change in equilibrium melting point due to pressure may explain this observation. When the metal becomes pressurized the equilibrium melting point increases, i.e., the metal suddenly becomes undercooled. In theory, this is the same for all Mg alloys, but there remains a significant difference in the solidification characteristics among the alloys. The core consists of equiaxed grains of -20 m. The solidification of the core is restricted by the heat flow out of the core to the die. Both the heat transport through the already solidified layer and the heat transfer over the casting/die interface will give a slower cooling rate than the skin and thus a coarser microstructure is formed.

When the RE content is low, or the %RE/%Al ratio is low like in AE42 or AE63, there will be a possibility that eutectic Mg-Al is present that could segregate to the surface, and lead to sticking. This may explain why AE42 shows up with a poorer castability.

In Fig. 8 there is shown a box die (upper) part of the drawing. Micrographs of examples from node 3 (close to the gate) for alloys AM60, AM40, AE63, AE44 and AE35 as shown below. Hot cracks are observed in AM40 and AE63.

Fig. 8, have demonstrated that AE44 and AE35 are less susceptible to hot tearing than AM alloys. This is explained from the fairly rapid solidification of the surface layer resulting in the relatively fine grained structure as described above.

Partly due to the fine grain structure and partly due to the absence of the brittle MgõAI12 phase this layer becomes very ductile, and is therefore able to deform when thermal strains are developing during solidifaction. A surface layer with coarser grains, as would typically appear in alloys with larger solidification interval, and/or a Mg17Al12 rich layer, will have a much lower ductility and would tend to crack and form hot tears rather than deform.

Testing of large (-1.5m) thin walled parts (-3mm thick) has shown that the die filling characteristics of AE44 and AE35 are excellent, and since long range io feeding is not necessary for thin walled parts as discussed above, this alloy is expected to be a viable alternative for these types of components, where die filling is of prime importance.

The properties of various AE alloys are explained from the observations that Al alone.provides the solid solution strengthening while RE combines with Al forming dispersoid phases in the grain boundary regions. In the alloys AE44 and AE35, the dispersoid phase (mainly AI2RE) constitutes a continuous 3D network, effectively preventing creep arising from thermal activation and grain boundary sliding. This shown in Fig. 9 which are SEM-BEC (Backscatter Electronic Composition) images showing the die cast microstructure of (from left to right) AE44, AE35 and AE63. While Al alone provides the solid solution strengthening, RE combines with AL forming dispersoid phases in the grain boundary regions.

A further enlargement of the SEM-BEC-images for AE 44 is shown in Fig. 10, which. also shows the lamellar structure of AI,REy phases in AE44. As seen from Fig. 10 the dispersoid AIxREy phases in the AE alloys consist of an extremely fine lamellar structure. This structure of submicron lamellas are stiffening the grain boundaries thereby preventing creep. On the other hand, these lamellas are not brittle (or not as brittle as the eutectic Mg-Al) as the die cast AE44 alloy experience a ductility that is similar to AE42. In AE63, the network (mainly AI>>RE3) becomes fragmented and the grain boundary regions are probably influenced by a substantial amount of eutectic Mg-Al, reducing the ductility and the creep properties. In AE42 there is probably also a significant amount of eutectic Mg-Al that limits the creep properties. The alloy AE35 has slightly lower ductility than AE44, but still higher than AE63.

Numerous examples of mechanical properties including ductility, tensile strength, creep resistance and corrosion properties of the AE alloys are shown later. The unique combination of creep resistance and ductility compared to existing alloys is illustrated in Fig. 11. In fig. 11 the ductility (horizontal axis) is shown as versus the creep resistance for a number of known Mg-alloys. The zone 50 comprises AM-alloys, zones 51 AE-alloys, zone 52 AZ91-alloy and io zone 53 other high temperature alloys. The AE alloys of the present invention are the only die casting alloys that combine ductility and elevated temperature properties in this way, and hence offer numerous new and unexplored opportunities for constructors and designers particularly in the automotive industry.
It is a more particular object to provide relatively low cost die casting magnesium-aluminum-rare earth alloys with excellent castability, good creep resistance and tensile yield strength and bolt-load retention, particularly at elevated temperatures of at least 1500 C.

SUMMARY OF THE INVENTION

The present invention therefore provides :
~ 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.

By using the combination of a specified Mg-Al-RE alloy with a special casting process, products could be obtained having excellent creep resistance, at elevated temperature, high ductility and generally good mechanical properties as well as corrosion properties.

In general a number of RE-metals can be used as alloying element, such as e.g.
Ce, La, Nd and or Pr and mixtures thereof. It is however preferred to use cerium in substantial amounts as this metal gives the best mechanical properties. Mn is added to improve the corrosion resistance but its addition is restricted due to limited solubility.

io Preferably the aluminium content is between 2,0 and 600% by weight, more preferably between 2,60 and 4,50 % by weight.

If higher amounts of aluminium are present, this can easily lead to the formation of a Mg AI1Z-phases which is detrimental for the creep properties. Too low Al is negative for the castability.
With respect to the RE-metals it is preferred that the RE-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.1.

If more than 3.50 % RE by weight is present, this gives a significant improvement of the creep properties. More than 7.00 % by weight is not practical because of the restricted solubility of RE-metals in liquid magnesium-aluminium alloys.

Furthermore, it is preferred that the RE/Al ratio is larger than 0.9.

For specific applications the composition of the alloy is selected in such a way that the aluminium content is between 3,6 and 4,5 % by weight and the RE-content is between 3,6 and 4,5 % by weight, with the additional constraint that the RE/Al ratio is larger than 0,9.

This type of alloys can be used for applications up to 175 C while still showing excellent creep properties and tensile strength. Moreover this alloy does not show any degradation of its properties due to ageing and has a good castability.

For applications above 175 C the composition of the alloy is such that the aluminium content is between 2,6 and 3,5 % by weight and the RE-content is greater than 4,6 % by weight.

5 Apart from the excellent creep properties and tensile strength this alloy does not show any degradation of properties due to ageing.

Preferably the RE-metals are selected from the group cerium, lanthanum, neodymium and praseodymium.
The RE-metals are contributing to the ease of alloying, but also increase the corrosion resistance, the creep resistance and improve the mechanical properties.

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

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

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 give an increase in creep resistance, and the addition of at least 0,5 % weight of calcium will improve the tensile strength.

However, Ca and Sr should be avoided because even at very small concentrations these elements lead to considerable sticking problems thereby influencing the castability of the alloy.

The present invention is described in more detail with reference to the following example which are for purposes of illustration only and are not to be understood as indicating or implying any limitation on the brood invention described herein.
io Example 1 In order to compose the influence of the alloying elements and a number of Mg-alloys have been prepared with the compositions as given in table 1.

Of each alloy purposes a number of test bars has been made to do the testing described in the following examples. The performed tests are the following Tensile strength and ductility Test-bars of 6 mm in accordance to ASTM have been made, and the following Test conditions has been used :
= 10 kN Instron machine = Room temperature to 210 C
= At least 5 parallels at each temperature = Strain rate - 1.5 mm/min up to 0.5 % strain, - 10 mm/min above 0.5 % strain = Testing in accordance with ISO 6892 Tensile creep testing For this text the following test material is used = Diameter : 6 mm = Gauge length : 32.8 mm = Radius of curvature : 9 mm = Grip head diameter : 12 mm = Total length : 125 mm The testing is done in accordance with ASTM E 139 Stress relaxation testing = Test material - 12 mm diameter, 6 mm length - Cut from arbitrary end of creep bars = Testing in accordance with ASTM E328-86 Corrosion properties The corrosion is tested according to ASTM 117.
Example 2 For a number of compositions the strength has been measured as a function of the temperature.
The results are shown in Figures 12, 13 and 14. In these figures the y-axis is representing the tensile strength expressed in MPa, whereas the x-axis is representing the temperature expressed in degrees Celsius.

Example 3 For a number of compositions the Creep strain has been measured as a function of the time.
The results are shown in Figures 15 and 16. In fig. 15 the measurement is done at 175 C whit a 40 MPa-force, and in fig. 16 the measurement is done at 150 C with a 90 MPa-forces.
In these figures the y-axis is representing the creep strain expressed in percentage, whereas the x-axis is representing the time expressed in hours.
Example 4 For a number of compositions according to table 1 the stress relaxation has been defined, expressed as the remaining load versus the time. The results are shown in fig. 17, 18 and 19.
In these figures the y-axis is representing the remaining load expressed in percentage of initial load, whereas the x-axis is representing the time expressed in hours.

Example 5 to For a number of compositions the corrosion properties have been defined in accordance to ASTM B117. In this test a great amount of data has been incorporated in order to define the influence of the RE-contest versus the Al-contest. The results are shown in Fig. 20.
In this figure the y-axis is representing the RE-content expressed in % by weight whereas the x-axis is representing the Al-content also expressed in %
by weight.
The border lines between the zones with different shades are representing lines of equal corrosion resistances.

2o From these test results it is clear that a process for casting a magnesium alloy has been provided whereby products are obtained with a superior combination of elevated temperature creep properties, ductility and corrosion performance.

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Claims

1. A process for casting a magnesium alloy consisting of 2.00 - 6.50 % 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 170-390°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.

2. The process according to claim 1, wherein the die temperature is controlled to a temperature in the range between 200 and 270 °C.

3. The process according to either claim 1 or 2, wherein the filling time of the die expressed in milliseconds is equal to product of the average part thickness expressed in millimeter multiplied by a number between 8 and 200.

4. The process according to claim 3, wherein the filling time of the die expressed in milliseconds is equal to product of the average part thickness expressed in millimeter multiplied by a number between 5 and 50.

5. The process according to claim 3, wherein the filling time of the die expressed in milliseconds is equal to product of the average part thickness expressed in millimeter multiplied by a number 5 and 20.

6. The process according to either claim 1 or 2, wherein the static metal pressure during casting is maintained between 30-70 Mpa.

7. The process according to either claim 1 or 2, wherein cooling rate after casting is in the range of 10-1000 °C/s.

8. The process according to either claim 1 or 2, wherein the aluminium content is between 2.50 and 5.50 % by weight.

9. The process according to either claim 1 or 2, wherein the aluminium content is between 2.60 and 4.50 % by weight.

10. The process according to either claim 1 or 2, wherein the RE content is between 3.50 and 7.00 % by weight.

11. The process according to either claim 1 or 2, wherein the aluminium 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 RE/Al-ratio in % by weight is larger than 0.9.

12. The process according to either claim 1 or 2, wherein the aluminium content is between 2.6 and 3.5 % by weight and the RE-content is greater than 4.6 % by weight.

13. The process according to either claim 1 or 2, wherein the RE-metals are selected from the group cerium, lanthanum, neodymium and praseodymium.

14. The process according to claim 13, wherein the amount of lanthanum is at least 15 %
by weight of the total content of RE-metals.

15. The process according to either claim 13 or 14, wherein the amount of lanthanum is at least 20 % by weight of the total content of RE-metals.

16. The process according to one of claims 13 to 15, wherein the amount of lanthanum is at most 35 % by weight of the total content of RE-metals.

17. The process according to one of claims 13 to 16, wherein the amount of neodymium is at least 7 % by weight of the total content of RE-metals.

18. The process according to one of claims 13 to 17, wherein the amount of neodymium is at least 10 % by weight of the total content of RE-metals.

19. The process according to one of claims 13 to 18, wherein the amount of neodymium is at most 20 % by weight of the total content of RE-metals.

20. The process according to one of claims 13 to 19, wherein the amount of praseodymium is at least 2 % by weight of the total content of RE-metals.

21. The process according to one of claims 13 to 20, wherein the amount of praseodymium is at least 4 % by weight of the total content of RE-metals.

22. The process according to one of claims 13 to 21, wherein the amount of praseodymium is at most 10 % by weight of the total content of RE-metals.

23. The process according to one of claims 13 to 22, wherein the amount of cerium is greater than 50% by weight of the total content of RE-metals.

24. The process according to one of claims 13 to 23, wherein the amount of cerium is between 50 and 55 % by weight of the total content of RE-metals.