Light metal matrix composite material based on magnesium and method for producing same

Abstract

The invention relates to a metal matrix composite material, produced from a powder mixture with magnesium with a degree of purity of at least 95.0% and with a separating agent and with hexagonal boron nitride, graphene oxide, molybdenum sulphide and / or other particles having a hexagonal structure, with up to 2% by weight of the separating agent and up to 5% by weight of the hexagonal particles.

Description

[0001] Magnesium-based light metal matrix composite material and process for its production

[0002] The invention relates to the technical improvement of a magnesium-based metal matrix composite material made from a powder mixture with a composition comprising

[0003] Magnesium with a purity of at least 95.0% and particles with a hexagonal structure and release agent as well as a process for producing this metal matrix composite material.

[0004] Metal matrix composites are called metal matrix composites (MMC) in English-speaking countries. When magnesium is used as a metal component, they can be referred to as magnesium matrix composites.

[0005] Magnesium naturally possesses good corrosion resistance, but its mechanical properties are insufficient for technical applications. To improve its mechanical properties, magnesium is alloyed with various alloying elements, such as aluminum, copper, manganese, nickel, silicon, and / or zinc. A more complete list of alloying elements can be found in "DIN 17007-4: 2012-12 Material numbers - Part 4: Classification of main groups 2 and 3: Non-ferrous metals." Alloying improves mechanical properties but automatically reduces corrosion resistance.

[0006] A very good example of this relationship is provided by the most frequently used magnesium die-casting alloys AZ91 (tensile strength: 200-260 MPa, fracture strength: 1-6%) and AM50 (tensile strength: 180-230 MPa, elongation at break: 5-15%). The increase in strength is achieved with the addition of 8.5-9.1% aluminum in AZ91 and 5.7-6.3% aluminum in AM50. However, with increasing aluminum content, corrosion resistance is significantly reduced.

[0007] Another group of magnesium-based materials with significantly improved mechanical properties are magnesium alloys alloyed with rare earth metals (rare earth elements, REE).

[0008] As an example, two patent applications can be cited here, which have each improved the mechanical properties such as the yield strength Rp 0 , 2 , but also the creep resistance, with the addition of REE :

[0009] DE102009038449B4 relates to a magnesium material in the form of a magnesium alloy alloyed with REE, with a yield strength Rp 0 , 2 of at least 140 MPa .

[0010] DE102008039683A1 relates to a creep-resistant magnesium alloy based on a multi-component system with added REEs.

[0011] Both examples are designed for technical applications.

[0012] For medical applications, magnesium-based materials are used, which must have very good biocompatibility and be bioabsorbable.

[0013] The applications DE102018120093A1 and DE102011082210 Al are listed here as examples.

[0014] The mechanical properties are improved by the addition of REEs.

[0015] The extraction of rare earth elements (REEs) poses a sustainability challenge. Since REEs occur only in low concentrations in deposits, extracting them in industrially relevant quantities requires the movement of very large volumes of earth material. This is followed by processing, sometimes using corrosive and highly toxic substances, to extract the REEs in metallic form from the carrier material. This leads to significant environmental pollution.

[0016] Secondly, some REEs are themselves highly toxic and thus pose a significant health risk to personnel involved in mining and processing. Furthermore, rare earths often occur in combination with radioactive metals, which releases radioactive radiation during mining.

[0017] Technically, REEs have a higher density than magnesium. Scandium has the lowest density at 2.985g / cm 3 and yttrium with 4,472g / cm3 . Other SEE have a density of up to 9 , 84g / cm 3 These values ​​are far higher than the density of magnesium with 1,738 g / cm 3 , and consequently significantly increase the density of REE magnesium alloys.

[0018] Worldwide, the demand for REE raw materials is increasing significantly, as they are contained in almost all high-tech products in consumer electronics, components of green technologies, as well as coatings and paints.

[0019] The invention is based on the object of creating a light metal matrix composite material based on magnesium for technical and particularly for medical applications, which can be produced at least at the level of conventional, possibly improved properties such as corrosion resistance, strength, biocompatibility, biological absorption, environmental friendliness and sustainability, in a simplified manufacturing process using components that are harmless to health.

[0020] The above object is achieved by a magnesium-based light metal matrix composite material according to claim 1 and a method for its production according to claim 4. Advantageous embodiments are specified in the subclaims.

[0021] The magnesium-based light metal matrix composite material according to the invention has a composition consisting of magnesium with a purity of more than 95.0%, preferably more than 99.0% (in particular 99.5%) and up to 2% by weight of a release agent, in particular metal soap or stearic acid, as well as hexagonal boron nitride, graphene oxide, molybdenum sulfide and / or other particles with a hexagonal structure (and preferably only these components) - and is characterized according to the invention by a proportion of particles with a hexagonal structure (particularly preferably hexagonal boron nitride) of equal to or less than 5% by weight.

[0022] This light metal matrix composite material is produced according to the invention by mixing the magnesium, for example by means of water atomization or gas atomization, in particular without prior fractionation, regardless of particle size (particle size from 0 to 1 mm) or, particularly preferably, mechanically, from the magnesium in the form of granules with an initial granule size of 0 to 5 mm and with the other components.

[0023] In the process according to the invention, the mixture is preferably extruded after the mechanical mixing of the components according to the invention, whereby a tensile strength range of 100 MPa to over 450 MPa can be achieved immediately after extrusion even without prior or subsequent heat treatment. Other primary shaping processes, such as sintering or 3D printing [additive manufacturing] to form an ingot, semi-finished product or a component in the final contour, are inventive processing processes for the light metal matrix composite powder mixture according to the invention, preferably in such a way (implemented, controlled and / or regulated in the process parameters) that no melting phase is formed and in particular the melting point, in particular of magnesium, is not exceeded.

[0024] For the production and in particular also the shaping of the light metal matrix composite material based on magnesium according to the invention, extrusion suitable for large-scale production, in particular already concluding the production process, is particularly preferred. For most common extruded magnesium matrix composite materials, a forming temperature of at least 450 °C is required after preheating. Typically, a subsequent heat treatment takes place to fully densify the composite. For the shaping of the light metal matrix composite material based on magnesium according to the invention, no preheating and a significantly lower forming temperature is sufficient (in particular so that the melting point is not exceeded or only slightly exceeded) in order to be able to achieve an almost 100% final density and final strength. A subsequent heat treatment is not necessary.

[0025] Typically, an extrusion process for powdered metal-matrix composites and also powdered magnesium-matrix composites takes place using capsules or sleeves made from non-native materials. For the magnesium-based light metal matrix composite material according to the invention, the subsequent particularly preferred powder extrusion process takes place with capsules or sleeves made from native material or even without the use of capsules or sleeves.

[0026] For the production of the magnesium-based light metal matrix composite material according to the invention, pure magnesium, preferably with a purity of 99.5%, and in particular exclusively physiologically acceptable materials are used. The magnesium-based light metal matrix composite material according to the invention is fully and unrestrictedly recyclable. A further important advantage is the possible use according to the invention of the mechanical comminution of pure magnesium granules to Mg powder for the inventive mixture of the magnesium-based light metal matrix composite material. The invention enables the sustainable use of raw materials, resource efficiency, and environmental friendliness.Since the composite material consists in particular of up to 99.95% pure magnesium (possible purity of the Mg powder component according to the invention), it can be recycled back into the secondary market with great advantage without any loss of quality. No REE, no carcinogenic nanoparticles and no nanotubes are used in the manufacturing process. Only components that are harmless to the human body and are also environmentally friendly are used. The extraction of magnesium from salt water, be it from sea water or salt lakes, for example as a by-product of sea water desalination, for example in the production of drinking water, also plays a potentially advantageous role according to the invention: as well as being a possible environmentally friendly, sustainable and local raw material source for the magnesium used according to the invention.

[0027] The magnesium-based light metal matrix composite material according to the invention is ideally suited for technical, particularly medical, applications due to its strength which is three times higher than that of pure magnesium, its high creep resistance, and its very high purity, which is particularly advantageous for medical technology, and thus its very good corrosion resistance.

[0028] Magnesium can therefore - according to the invention - be produced as a material with the aid of hexagonal boron nitride and a release agent in particular. Handling and handling of magnesium as a material are technologically different from that of other light metals such as aluminium. For example, magnesium is many times more reactive than aluminium. The entire process chain according to the invention is therefore particularly preferably designed to be safe, for example with regard to the risk of explosion. While aluminium can be produced very cost-effectively from recycled aluminium, this is not the case with magnesium. In the case of aluminium, slight impurities lead to a slight - but acceptable - impairment of electrical and thermal conductivity. On the other hand, magnesium, particularly for medical applications, is particularly preferably kept free from any contamination throughout the entire process chain, from the raw material to the end material.The purity of the product is of particular importance here.

[0029] Similar to hexagonal boron nitride, other hexagonal particles such as graphene oxide (GO) or molybdenum sulfide can be used in accordance with the invention. However, graphene compounds pose a corrosion problem. While particles such as molybdenum sulfide may be technically interesting for their inventive function, they are toxic. Thus, biocompatibility and bioabsorbability would be compromised.

[0030] Therefore, the use of hexagonal boron nitride for the production of magnesium-based light metal matrix composites is particularly advantageous for technical and especially for medical applications.

[0031] The following exemplary compositions have proven particularly suitable as a mixture (especially with only these three components) especially for subsequent, and especially final, extrusion:

[0032] Mechanically crushed magnesium granules with a purity of over 95%, preferably over 99%, particularly preferably over 99.5%, in particular 99.7%, in a fissured grain shape and surface, with an average grain diameter of at most 5 mm, preferably at most 1 mm, particularly preferably at most 500 μm, in particular 0 to 100 μm; hexagonal boron nitride powder at most 5%, preferably at most 2%, particularly preferably at most 0.2%, in particular 0.05%, of the weight of the composition with a purity of at least 95%, preferably at least 97%, particularly preferably at least 98%, in particular 98.5%, with an average particle size of at most 50 μm, preferably at most 10 μm, particularly preferably at most 5 μm, in particular 2 μm and

[0033] Release agent powder, preferably metal soap or stearic acid, namely at most 2%, preferably at most 1%, particularly preferably at most 0.1%, in particular 0.01%.

[0034] The state of the art is to mechanically alloy composite materials using the highest possible energy in a high-energy ball mill. However, ball mills severely limit the production volume. Very small grinding bowls with a volume of up to 500 ml are filled to a maximum of 20% with the powdered material. This method only allows for laboratory testing. For large-scale production, this method is only feasible at very high costs.

[0035] Commercially available atomized spherical Mg powder with a strictly defined particle size and shape is suitable for large-scale applications, but is relatively expensive - and does not necessarily require a high shape quality. The invention has shown that although the highest possible purity of the magnesium starting material is particularly advantageous, the grain shape and particle size play only a minor role. This means that the complex and resource-intensive production of water-atomized or gas-atomized powders can be dispensed with in favor of simple mechanically crushed powders. On the one hand, this approach enables an economical and resource-saving end product, and on the other hand, the supply of raw materials is also ensured on a large scale. Mg granules with a widely distributed grain size of around 0 to 5 mm are considered a standard product.There are diverse applications in smelting metallurgy and organic chemistry. Magnesium is used, for example, as an additive, reducing agent, alloying element, or reactant. The inventive elimination of energy-intensive remelting processes generates additional savings. This elimination applies both to the final shaping and to the typical ingot remelting as a subsequent step in the smelting electrolytic production of pure magnesium. Hexagonal boron nitride and release agents, particularly metal soap powder, are also materials that are inexpensively available in large quantities.

[0036] For powder production and mixing, high-energy planetary ball mills are particularly suitable as a grinding system for smaller throughputs. For the large-scale production of magnesium matrix composite materials, high-energy vibrating mills, particularly eccentric vibrating mills, are suitable devices according to the invention.

[0037] In the aforementioned manufacturing process, magnesium granules (with a grain size of, for example, 0 to 5 mm) can be processed with all components into the final product (a magnesium-based light metal matrix composite) in a high-energy vibrating mill, particularly an eccentric vibrating mill, in a single process step. According to the invention, comminution and mechanical alloying are combined.

[0038] With regard to the powder processing according to the invention, there are many different shaping processes (particularly for the non-porous material structure according to the invention - porous systems are suitable, for example, as filter materials). They can basically be divided into methods with a melt phase (for example casting, selective laser melting, powder welding, electron beam welding, build-up welding) or without a melt phase (extrusion (particularly at high temperatures: hot extrusion), sintering processes (hot isostatic pressing - HIP), roller compaction (P / M), powder injection molding (PIM), spark plasma sintering (SPS), selective laser sintering (SLS), jet printing).

[0039] Sintering processes usually consist of a pre-compaction phase, the sintering process, and post-compaction to reduce porosity. In extrusion at high temperatures (hot extrusion), which is particularly preferred according to the invention, the powder is compacted in a single operation in the recipient by the press ram. The subsequent thermomechanical bonding (friction welding and connection of the individual particles of the MMC) can be created by the applied pressure in combination with the temperature (particularly preferably below the material's melting point) within the die and, according to the invention, lead to the shaping of a largely pore-free molded part (ingot, semi-finished product, or even finished component).

[0040] In 3D printing systems (particularly sintered phase systems), pre-compaction and densification can be achieved according to the invention via the structure and composition of the printing powder and / or a laser-based joining technique (particularly without a melt phase). Possible techniques differ in their work steps. On the one hand, there are pure green part manufacturing approaches (binder jetting), which sinter a product separately only after shaping; on the other hand, there are manufacturing approaches in which the sintering process is initiated locally by the laser directly during powder application. The latter process variant has a lower porosity than the former, but is usually at the cost of higher production requirements and lower production output.

[0041] According to the invention, shaping by extrusion at high temperatures represents a particularly suitable process for large-scale powder processing. The material throughput rates are comparable to those of conventional mass production processes in the metal industry.

[0042] Devices for this purpose are designed according to the invention, for example, as follows.

[0043] According to the invention, extrusion in conventional extrusion systems for solid material forming preferably includes a handling aid for powder systems according to the invention in order to transfer the compaction energy applied by the extrusion ram to the powder material. Possible methods, in addition to direct powder extrusion, include, for example, sintering the powder material to form an extrusion billet or the use of powder-filled metal sleeves based on the powder matrix material (for example, Mg powder in a Mg sleeve). Both variants close the die side of the recipient (thus preventing the powder material from being blown out through the die) and allow continuous degassing while the extrusion ram compresses the system and transfers the pressing pressure into the material for shaping.

[0044] Compared to the sintered billet, the sleeve variant offers greater flexibility, while requiring less labor and lowering costs. The sleeve system can produce a layer of sleeve material on the outer shell of the extruded profile, which requires subsequent removal by mechanical processing.

[0045] For example, the sleeve can be designed without weld seams between its bottom and sides. This prevents the weld seams, which are usually softer than the sleeve itself, from deforming during the actual extrusion process.

[0046] Both cover sides can be mechanically sealed and keep the powder in the sleeve during extrusion. An opening in the sleeve allows the powder to outgas. By using such a sleeve system, for example, the processing according to the invention can also correspond to a conventional extrusion process in conventional machines or systems.

[0047] Another inventive design of the sleeve is the use of the powder material itself (here, a magnesium-based light metal matrix composite) as the main material of the sleeve. In this case, subsequent mechanical processing to remove the sleeve is not necessary. This represents a cost-effective technology for large-scale industrial processes in which sleeve-free powder processing is not implemented.

[0048] The following figure shows in a diagram properties of the following composition as an embodiment of the invention ( "Mg + BN") in comparison to pure magnesium ("pMg") from the prior art:

[0049] Mechanically ground magnesium powder with a purity of 95% [and average grain diameters between 20 and 55 pm] ;

[0050] < 3 wt% hexagonal boron nitride powder;

[0051] < 2% by weight release agent.

[0052] The figure shows a stress / strain diagram.

[0053] According to the figure, the significantly higher stress curve of the magnesium-based metal matrix composite material according to the invention with peak values ​​for the tensile strength (at 100 °C) in the range of over 450 MPa, compared to the known pure magnesium with maximum values ​​in the range of about 150 MPa, is clearly visible.

Claims

Claims 1. Metal matrix composite material, produced from a powder mixture with magnesium having a purity of at least 95.0% and with release agent and with hexagonal boron nitride, graphene oxide, molybdenum sulfide and / or other particles with hexagonal structure, with up to 2 wt% of the release agent and up to 5 wt% of the particles with hexagonal structure.

2. Material according to claim 1, characterized in that the composition comprises no other material component than the magnesium and the particles with hexagonal structure and the release agent.

3. Material according to one of the preceding claims, characterized in that the release agent is metal soap or stearic acid.

4. Material according to one of the preceding claims, characterized in that the powder mixture has an initial size of 0 to 5 mm.

5. A method for producing a magnesium-based light metal matrix composite material according to one of the preceding claims, characterized by the steps of a) mechanically crushing the magnesium granules and b) mixing the material components and c) to process the mixture into an ingot, semi-finished product, or component by primary forming, extrusion, sintering, or 3D printing. A method according to claim 5, characterized in that the primary forming, extrusion, sintering, or 3D printing step is carried out in such a way that the melting limit of the magnesium-based light metal matrix composite is not exceeded. A method according to claim 5 or 6, characterized in that the step of mixing the material components is carried out simultaneously with or after the start of step a), and / or the particles with a hexagonal structure and the release agent are added to the mixture substantially simultaneously, namely for a maximum of 5 minutes or a maximum of 2 minutes consecutively, and / or comprises the steps of mechanically alloying the magnesium and the particles with a hexagonal structure and deagglomerating them before and / or after.A method according to any one of claims 5 to 7, characterized in that the steps of mechanically comminuting the magnesium powder and mixing the material components in powder form are carried out in the same device, in a mill, in a single operation. A method according to any one of claims 5 to 8, characterized in that the magnesium granules are mechanically comminuted to a particle size of at most 500 pm, or at most 100 pm, or at most 50 pm. A method according to any one of claims 5 to 9, characterized in that the powder of particles with a hexagonal structure constitutes at most 5%, or at most 2%, or at most 0.1% of their weight and / or. a purity of at least 95% or at least 97% or at least 98% and / or a mean particle size of at most 50 pm or 10 µm or at most 5 pm. A method according to any one of claims 5 to 10, characterized in that the separating agent constitutes at most 2.0% or at most 1.0% or at most 0.8% of the composition by weight.

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