DE1446364B2 - Process for the powder-metallurgical production of shaped bodies or compacted masses from magnesium or magnesium alloys and magnesium compounds dispersed therein - Google Patents

Description

The invention relates to a method for the powder-metallurgical production of shaped bodies or compacted masses of magnesium or magnesium alloys with remotely distributed magnesium compounds, starting from optionally anoxidized metal powder, which before, during and / or after pressing and optionally during the heating for plastic deformation of a suitable for the formation of these magnesium compounds Is exposed to a gas atmosphere at an elevated temperature.

The particular properties of materials with a composite structure of the metal-metal oxide type and in particular their advantageous mechanical strength at elevated temperatures are known. It is also known that these properties are due to the fineness of the distribution of the non-metallic phase can be influenced in the material. In particular, the resistance to mechanical stress increases increases with the fineness of the distribution or dispersion, while the elongation at break decreases.

In general, such compositions or materials are made by using metal powders goes out, which are subjected to an oxidation treatment before or during the compaction or afterwards. The thickness of this Oxide films formed on the powder particles depend on the conditions under which this oxidation occurs he follows.

In the case of masses or products made up of metallic magnesium and magnesium oxide as non-metallic Assemble components, it is difficult because of the great tendency of the metal to react with oxygen, the thickness of the magnesium oxide film must be strictly observed on the metal particles. This difficulty is natural especially large for very fine powder particles. On the other hand, it is - a sufficiently compact one To obtain the final product - necessary to heat the metal particles to temperatures which are sufficient are high to drive off the hydrogen brought in by water vapor and organic matter which can adhere, for example, to the surface of the particles; this in turn brings about the disadvantage of excessive oxidation.

The "controlled" oxidation of the magnesium particles is therefore very difficult. The oxidation by known Oxidizing agents lead to oxide films that are much too thick or even almost complete Oxidation of the particles or to an insufficient or very different oxidation within of the material, especially in the case of powders with very different grain sizes.

The products obtained by the previously known processes therefore show only slightly increased breaking strengths, because very finely divided powders, which in themselves provide the best mechanical values, are due to great difficulties in the controlled anoxidation of powders with a relatively high degree of division cannot be used; or the increase in breaking strength remains insufficient because of one Low oxidation; the products also generally show a low elongation at break in the tensile test, both in the cold and in the warm state, which is excessive, localized or is generally due to excessive oxidation; in addition, it is usually a strong one Scattering of the mechanical properties found within the material, due to a for the sufficient removal of the interfering hydrogen insufficient heating of the powder prior to compaction.

The invention eliminates the above-mentioned disadvantages and allows relatively easy masses Manufacture from magnesium or magnesium alloys - starting from powders of these metals,

ίο what excellent mechanical properties exhibit. The invention makes a controlled and easily monitored oxidation of the particles of the Powder possible, regardless of how large the dimensions of these particles are.

The freedom obtained in this way in the selection of the particle size ratios of a powder mixture and the thickness of the oxide film to be produced on these grains enables the production of products, which are characterized by relatively high breaking strength and elongation limits, and both at normal and at high temperature, the elongations at break being proportionate stay high. In addition, the temperature treatment - for powder of any fineness - at be carried out at a sufficiently high temperature at which the hydrogen is removed and the for example between 350 and 550 ° C; the heating time determines the thickness of the film that is forms on the particles of the powder.

On the other hand, products can be obtained according to the invention whose microscopic grain size or Structure remains remarkably stable after compression, even after prolonged heating higher temperature.

If the mass is to be rolled or extruded, it can be applied directly to the material required for this Temperature can be heated in a single process stage and does not need - as before - in successive Stages to be compressed at increasing temperatures.

The inventive method for powder metallurgical production of moldings or consolidated mass of magnesium or magnesium alloys with finely dispersed therein f Magnesiumver-

bonds, starting from possibly anoxydier- v. tem metal powder before, during and / or after pressing and possibly during heating for the plastic deformation of a gas atmosphere suitable for the formation of these magnesium compounds is exposed at elevated temperature, is characterized in that the shaped body or treated compacted masses in a gas atmosphere containing at least 5% carbon dioxide will.

According to the invention, a very thin non-metallic layer is obtained on the particles or grains of the powder by the action of CO ₂ on the magnesium, instead of the usual reaction of oxygen on the metal. This new process is based on the discovery that heating the magnesium in a CO ₂ atmosphere leads to the formation of a protective film which significantly slows down the rate of oxidation. This allows the magnesium powder to be heated to much higher temperatures than would be permissible in air or oxygen without the risk of excessive oxidation. This heating under carbon dioxide allows even particles of the greatest fineness to be added in this way

treat and create a film of the desired thickness on their surfaces.

Although the film that forms contains both magnesium oxide and / or magnesium carbonate can, possibly even free carbon - depending on the selected temperature and pressure of the Gas depends - in the following, for the sake of simplicity, we will always speak of an "oxide film" will.

The treatment according to the invention with carbon dioxide at a higher temperature can be carried out in different ways Make way.

You can z. B. heat the magnesium powder intended for the production of the masses by compression in a C0 ₂ atmosphere, the oxidation of the magnesium powder particles can be carried out by heating in a stream of carbon dioxide according to known methods customary for the treatment of powders with flowing gases; a very economical and safe method consists in placing the powder in a container with externally heatable walls and keeping it in systematic motion while dry carbon dioxide flows through the powder; the temperature, the throughput of carbon dioxide and the duration of the treatment are set to the respectively suitable or most favorable values.

Another embodiment consists in heating a more or less compact mass produced by prior compression of the powder in the presence of CO _2. The mass can have been produced by cold compression or by compression at a higher temperature; however, the temperature used should be below 450 ° C if possible. In this way one can use a compressed body of non-oxidized powder or of a powder which has been pre-oxidized by known methods or by the method according to the present invention only to the extent that the oxidation is below the value which one wishes to obtain .

A special application of the method according to the invention is the treatment of a compressed body with CO ₂ while this body is being heated for plastic deformation, for example by rolling or drawing or extrusion.

It has been found that a fine powder with grain sizes below 60 microns, which had previously been oxidized to a desired degree of oxidation and then ^{compressed in the cold state at 10 to 60 kg / mm 2} , undergoes severe oxidation when the compressed body is subsequently is heated in air to the need for extrusion temperature (450 to 55O ⁰ C). This disadvantage is avoided if the compressed body or object is heated for plastic deformation by rolling, drawing, extrusion, etc. in a CO ₂ atmosphere. One then achieves the advantage that the hot plastic deformation can be carried out at the same time as the oxidation treatment by means of the carbon dioxide up to the desired degree of oxidation, assuming a degree of oxidation that has not yet been oxidized or that has been previously up to a lower degree of oxidation than the pre-oxidized powder that is finally to be achieved.

For example, you can cold compress the powder in a hollow body with the dimensions of the transducer of an extrusion press, then after the compression put on a tightly fitting lid which has a line connection via which a vacuum can first be created in the container and then the carbon dioxide can be introduced. When applying such a protective measure, a single cold compression is sufficient under relatively low compression pressure (up to 2 kg / mm ²⁾ to form a compact product by extrusion molding at temperatures between 450 and 55O ⁰ C to

ίο received; the pressing pressure at the high pressing temperature during extrusion is greater than 30 kg / mm ² .

A particularly advantageous application of the present invention consists in oxidizing a powder to the desired degree of oxidation in a CO ₂ atmosphere, compressing it into a compact mass and still working under a CO ₂ atmosphere, and finally this mass for the purpose of shaping to be heated in order to give it the final, desired shape.

Depending on the fineness of the powder used as the starting material, the type of magnesium alloy from which this metal powder consists, the temperature and the pressure, the present invention can be used in a CO ₂ atmosphere alone or equally well in mixtures of CO ₂ and air or oxygen or carry out nitrogen and other gases. However, it is necessary that the proportion of CO ₂ in the gas mixture is sufficiently high; In particular, the mixture with air must contain at least 5 percent by volume, better still more than 50 percent by volume, of carbon dioxide. The preferred proportions of carbon dioxide to be used are of the order of magnitude from 90 to 100 percent by volume. It should be said that excellent results have been achieved with technical carbon dioxide with a degree of purity of 99 to 99.7 percent by volume.
The new process can in principle be carried out at different temperatures. It is possible to work from 100 ^° C. in each case, a slow, very superficial oxidation being obtained; the temperature can be increased to values slightly above the melting temperature of the metal. In any case, for reasons of rapid implementation of the oxidation treatment and the quality of the masses to be obtained, it is preferable to work in a temperature range between 250 and 600 ^° C., in particular between 350 and 500 ^° C.

When carrying out the process according to the invention, one can use non-oxidized or still Inadequately oxidized powders start out with very different grain size distributions, in particular of powders with grain dimensions in a range between 1 and 2000 microns. The weight percentage the non-metallic phase in the process product is between about 0.1 and 20%; this phase is fine and homogeneous over the entire mass in the form of more or less coherent surface films distributed, their thickness between approximately 0.01 and 10 microns lies; the structure of the coherent films corresponds to the structure of the powder.

The following examples of implementation of the process according to the invention are intended to improve the process

, explain in more detail; they are supposed to be the invention though in no way restrict.

5 6

τ. ·. _ · _ Ι t pressions or only a single compression through-

^{Betspiel τ} were performed.

With a magnesium powder of medium grain sizes, in the following compilation the numerical

ranging between 10 and 100 microns contain reasonable results from these tests.

two different tests were carried out: 5 The table gives the measured values for the mechanical

. Properties at normal temperature and at a

a) heating in air to 400 C; Temperature of 450 ° C again. In both cases

b) 4 hours of heating at 400 ° C in a dry place, the rods are made of a magnesium-CO ₂ . powder were produced that lasted for 4 hours

In the treatment under a) (heating in air) 10 treated _{with CO 2 at a temperature of 400 ° C}

the powder ignites. If you tried that had been.

Avoid inflammation by following the speed table I
if the temperature rise is limited, one sets

found that the smallest particles of the mixture are very measurements at room temperature

oxidize quickly and thereby the neighboring 15 _σ 33 kg / _mm 2

exposed particles in an undesirably violent manner. ^B ₂

heat. ^{One observes σ} o · ^{2 M} kg / mm even after this treatment

the presence of more or less large Zu- <5 (with a measuring length Z ₀ = γΕ7Ξ, 7 ° / o
assemblies, which consist of agglomerations strongly with 5 the cross-sectional area of oxidized particles, so that it is almost impossible.

is borrowed, the optionally still usable powder H _B 53

to be separated without the properties necessary for its (ball 5 mm; pressure 31.250 kg)

Use are decisive. _O

If one considers the agglomeration in the powder measurements at 45U C

leaves, they bring about a very substantial weighting 25 a _B 2.6 kg / mm ²

reduction of cohesion and the other _σ 2.0 kg / mm ²

Properties of the end product. * '..Λ,

In contrast, the treatment according to the invention according to b) allows the achievement of a regular E _s means:
or controlled oxidation without the finest 30 _ ",. ,.
Particles have a more or less rapid combustion ° b ~ ^ ugtestigkeit,
Experienced. The powder obtained looks perfectly »0.2 ⁼ yield point

evenly, there are no blackish oxide (load at an elongation of 0.2),

agglomerations or fusions metal- <5 = elongation at break,

lischer type available. Samples of the working 35 H _B = ball indentation hardness,
way b) treated powder were cold under

compressed to a pressure of 40 kg / mm ^2; she was- Example 2
then after appropriate heating in air

further compressions at 300, 400, 450 and 550 ° C to the influence of the thickness of the surface film

subject. 4 ° of the individual particles on the properties of the

To show on other samples of the final product after the process, three samples became one

wise b) treated powder was only a single powder of the same grain size as in Example 1 in the

Compression after heating to 525 ° C in charcoal-treated the following way:

made of a dioxide-containing atmosphere and then _a ^ 4 hours at 350 ° C in CO

the material is extruded (with the cross-section 45 _b) 4 _{hours at 400} o _{c in co} '·

ratio before and after the work process approx. _{c) 4 hours at 450} o _{c in co} ² '

16 was) to round this way point to ER ^J

keep. The mechanical properties of these rods after cold compression with a pressure of

were made by a tensile test with a tensile strength of 40 kg / mm ² , heating to 525 ° C under carbon dioxide

speed of 4 mm / min, checked. 50 and extrusion at this temperature with a

In both types of the area reduction factor of 16 according to the invention were on The rods produced by the method were the same as the rods with the following tensile properties measured; so the result was measured without trial, and that at normal temperature depending on whether four successive grain and at 450 ° C.

Messu
kg / mm ² ng at room temp
^σ ο.2
kg / mm ² Table II ^σ Β
kg / mm ² ft δ
% Oxidation
temperature 32.5
33.5
34 30.5
30th
31 erature
δ
Vo 1.8
2.6
2.7 14 ^:
12.5
13.5 350 ° C
400 ° C
450 ° C 6th
5
4th design at 450 ° <
^σ 0 2
kg / mm ² 1.5
2
2.5

This example shows quite well how treatment with carbon dioxide makes it possible to prevent oxidation to control or monitor precisely on the surface of the particles in order to focus on the essential To be able to act in the desired manner on properties of the end products.

Example 3.

Pattern of powders of different grain sizes were treated for 4 hours at 400 ⁰ C under carbon dioxide atmosphere, then compressed and extruded as in Example 2. Tensile tests of the extruded rods at ordinary temperature and at a temperature of 45O ⁰ C the following values were measured.

Powder grain Messunj
^a B
kg / mm ² Table III temperature
δ
° / o ° B
kg / mm ² M. measurement at 450
kg / mm ^{2 0} C δ
% 300 microns. . 23.5
30th at room temperature
^ö 0.2
kg / mm ² 9
5.5 1.2
2.3 . 0.8
1.8 27
14th 1500 to
300 to 50 microns 34.5 16.5
27 5 2.85 2.45 12.8 10 to 50 microns 33

This example shows how high levels of carbon dioxide can be achieved through the treatment in the case of oxidation Mechanical property values can be obtained using the finest possible powder; how It emerged from Example 1 that an oxidation of fine powders in air would either not be possible or would it would lead to heterogeneous and easily fragile products.

In example 3, it is above all the values of the elongation at break both at normal and at high temperature, which compared to the materials of this type, as they are previously known, essential could be increased in which the elongations at break are more than half below those Values are as shown in Table III.

Example 4

A rod was extruded under the following conditions:

a) the starting powder consisted of grains with dimensions between 10 and 100 microns;

b) the oxidation treatment under a carbon dioxide atmosphere lasted 4 hours at 400 ^° C .;

then cold compressed at 40 kg / mm ² ;

c) Heating under carbon dioxide during extrusion at 525 ° C. with a reduction in cross section from 16.

After extrusion, an examination of the microstructure shows that the number of grains per square millimeter of the area of a section parallel to the axis is between 25,000 and 50,000. After heating to 500 ^° C. for a thousand hours, the number of grains is still 12,000 to 25,000 per square millimeter. Under the same conditions shows a classic alloy of magnesium with 0.4% zirconium 1000-2000 grains / mm ² immediately after the extrusion, and 50 to 200 grains / mm ² after lOOOstündigem heated to 500 ⁰ C.

One can clearly see the advantage of the magnesium-magnesium oxide composition produced according to the invention, their properties both in the heat and in the cold state even after long Periods of intense heating remain essentially stable.

45

Claims (7)

Patent claims: 1. Process for the powder-metallurgical production of shaped bodies or compacted masses made of magnesium or magnesium alloys with magnesium compounds finely divided therein, starting from optionally anoxidized metal powder, which before, during and / or after Pressing and possibly during the heating for plastic deformation to form a these magnesium compounds exposed to a suitable gas atmosphere at elevated temperature is, characterized in that the moldings or compacted masses in a gas atmosphere containing at least 5% carbon dioxide. 2. The method according to claim 1, characterized in that the shaped bodies or compacted masses at a temperature between 250 and 600 ⁰ C, preferably between 350 and 500 ° C, are treated. 3. The method according to claim 1 or 2, characterized in that the shaped body or compacted Masses are exposed to the carbon dioxide-containing atmosphere for 2 to 7 hours. 4. The method according to any one of the preceding claims, characterized in that the Shaped bodies or compacted masses are exposed to a gas atmosphere that is at least Contains 50% carbon dioxide. 5. The method according to any one of the preceding claims, characterized in that a Metal powder with a grain size between 1 and 2000 μ is used. 6. The method according to any one of the preceding claims, characterized in that the Formation of the magnesium compound takes place before pressing and under carefully selected Conditions of time, temperature and carbon dioxide concentration for the formation of a magnesium compound layer predetermined average thickness on the metal grains leads. 7. The method according to any one of the preceding claims, characterized in that a Magnesium compound layer is formed, which is 0.01 to 10 μ thick, so that the weight fraction this non-metallic phase is between 0.1 and 20%. 009519/210