WO2014044432A1

WO2014044432A1 - Production of a refractory metal component

Info

Publication number: WO2014044432A1
Application number: PCT/EP2013/065211
Authority: WO
Inventors: Mathias Sommerer
Original assignee: Siemens Aktiengesellschaft
Priority date: 2012-09-24
Filing date: 2013-07-18
Publication date: 2014-03-27
Also published as: DE102012217188A1

Abstract

The invention relates to a method (S1-S13) used for the production of a refractory metal component, said method having the following steps: provision (S4) of an extrusion mass (M) which contains a refractory metal powder consisting of at least one refractory metal and/or a compound thereof, in addition to at least one binding agent; and extrusion (S5) of the extrusion mass (M) to form at least one green body (17), a step for heat treating (S7) the at least one green body (17) following on from the step involving the extrusion (S5) of the extrusion mass (M). A refractory metal component was produced by means of this method (S1-S13). The invention can be used in particular on X-ray tubes or fusion reactors, in particular for a surface of an X-ray anode, or a wall or structural component of a fusion reactor.

Description

description

The invention relates to a method for producing a refractory metal component, the method comprising the following steps: providing an extrusion compound comprising a refractory metal powder of at least one refractory metal and / or a compound thereof and at least one binder ; and extruding the extrusion mass. The invention also relates to a refractory metal component produced by means of the method. The invention is particularly applicable to X-ray tubes or fusion reactors, in particular for a surface of an X-ray anode or a wall or structural components of a fusion reactor.

The plasma-facing surfaces of an X-ray anode or the surfaces and structural components of a fusion reactor in addition to high temperatures also see high mechanical see, thermocyclic loads that can lead to cracking or melting of the materials. In both applications, refractory metals, in particular tungsten, are used. For the production of planar components in tungsten heavy metal alloys, the film casting process for refractory metals is known from WO 2007/147792 A1. WO 2007/147792 A1 discloses a process for the production of planar, shaped articles from a tungsten or molybdenum heavy metal alloy, from which a slurry for film casting is produced, from which slurry a film is poured and the film is debinded after drying and sintered to to obtain the molded article. The term tungsten heavy metal alloy or molybdenum alloy in the sense of the present WO 2007/147792 A1 materials selected from the

Group consisting of tungsten heavy metal alloys, tungsten, tungsten alloys, molybdenum and molybdenum alloys. Tungsten heavy metal alloys are about 90% Wt .-% to about 97 wt .-% of tungsten or tungsten alloys. The remainder is binder metals. As metallic binder, the elements Fe, Ni and / or Cu in proportions greater than 1% by mass are preferred. The metallic binders provide simplified manufacturing processes through lower sintering temperatures, improved mechanical properties, particularly ductility, and improved machinability, such as better machinability. These materials are intended for use in radiation shielding applications, with a high density of alloys in the foreground.

GB 928 626 A discloses a method for producing a dense, substantially crack-free and distortion-free refractory metal component by means of cold rolling and sintering. For this purpose, pure tungsten powder can be mixed with organic binder and water and subsequently extruded under heat to provide a starting material for cold rolling. Thus, extrusion here represents only one step for producing a shapeless starting material for cold rolling. Only cold rolling represents the primary shaping step in which a shaped body is provided as a finished component or semifinished product. The cold rolled stock is subsequently air dried and then sintered.

It is the object of the present invention to overcome the disadvantages of the prior art, at least in part, and in particular to provide a more stable refractory metal component under point-type, thermal alternating loads. In particular, it is an object to obtain by an isotropic, fine-grained microstructure of the refractory metal component.

This object is achieved according to the features of the independent claims. Preferred embodiments are in particular the dependent claims.

The object is achieved by a method for producing a component (hereinafter also referred to as "refractory metal component"). the method comprises (at least) the following steps: providing an extrusion mass (feedstock) comprising a refractory metal powder of at least one refractory metal and / or a compound thereof ("refractory metal powder") and at least one binder; Extruding the extrusion mass to at least one green body; and heat treating the at least one green body.

By extruding a finished green body or a green body is thus provided as a semi-finished product. The green body as a semi-finished product can be further shaped, for example bent, cut to size, etc., but unlike GB 928 626 A is not a shapeless mass but a body with a defined, predetermined shape. No further primary shaping step is carried out between the extrusion and the heat treatment, in particular no rolling, in particular no cold rolling. Thus, the step of extruding can be followed by a step of shaping the green body. The method has the advantage that it can be used (eg, in contrast to, for example, a rolling process) to produce homogeneous, isotropic, fine-grained and low-stress microstructures of the final refractory metal component with a narrowly distributed and fine particle size distribution. This may in particular also be associated with an isotropic crystal orientation. Under certain circumstances, the setting, for example, a bimodal particle size distribution in terms of mechanical properties is desired and then implemented by the method. In addition, for example, in contrast to a rolling process, no textures, lower internal stresses and misorientations of the grains are achieved in the material. In addition, the grain boundary property and, in total, the fracture behavior under pointy, thermocyclic loading are influenced by the setting of the grain structure (distribution / size). In addition, the method makes it possible to produce large-area refractory metal components and more complex shaped structures and / or profiles. Under a refractory metal component may basically be understood to mean any body or workpiece that has been produced by means of the method. An extrusion composition may generally be understood to mean a solids-containing, viscous suspension with the refractory metal powder as a solid, which is suitable for carrying out the extrusion. The technique of extrusion is generally well known and need not be further explained here. In principle, all suitable extrusion processes are applicable.

In particular, one or more powders of one or more pure refractory metals (eg tungsten and / or molybdenum), alloys thereof (eg tungsten-rhenium), of a (refractory metal) powder comprising at least one refractory metal and / or a compound thereof may be used. WRe) and / or compounds thereof. The refractory metal powder may include, for example, tungsten, molybdenum, rhenium and / or tantalum and / or alloys thereof and / or compounds thereof.

It is a development that processing of the at least one refractory metal powder takes place in the absence of oxygen, e.g. under a protective gas atmosphere, reducing atmosphere or under vacuum. This prevents oxidation of the refractory metal powder.

The binder can in principle have any organic and / or non-organic binder or binder. The binder binds the refractory metal powder functionally similar to an adhesive. Preference is given to organic binders, for example Polvvenylbutyral. It is a development that the extrusion compound has additional additives such as dispersants, plasticizers, solvents, etc. In particular, a viscosity of the extrusion mass or of the feedstock and the intrinsic viscosity Shafts of the green body (eg its strength and / or deformation capacity) influence.

A dispersing agent ensures that the wetting behavior of the particles of the refractory metal powder is improved and agglomeration is prevented. The solvent, e.g. Ethanol and / or toluene, dissolves organic components, in particular the binder. The flexibility and strength of the green body and thus its manageability can be adjusted by adding a plasticizer. Various mixing and grinding processes produce a homogeneous extrusion mass. It may be necessary to degas the extrusion mass prior to extrusion to avoid blistering in the extrusion mass.

For the preparation of the extrusion mass, for example, a mixture of the individual powders in a tumble mixer, in ball mills, etc. take place. The extrudate may be dimensionally stable, in particular for further processing.

The shape of the green body is not limited and may include, for example, a profile (e.g., a pipe), a greensheet, etc.

It is an embodiment that the extrusion comprises extruding a green sheet. As a result, large-area semi-finished products or components can be produced without further aftertreatment (for example rolling). For example, an extruder die may be appropriately shaped and e.g. have a slot or gap-like discharge opening. However, the greensheet may also be made in other ways, e.g. by extruding a multilayer wall of a complex component.

It is also a development that a thickness of the (individually) extruded green body about twenty microns to about three Millimeters. Thereby, a sufficiently high thickness for accommodating a plurality of grains of the refractory metal powder can be provided. In addition, a sufficient homogeneity of the individual slip components can be ensured over the thickness. The thickness may be, for example, a layer thickness of a green sheet or a wall thickness of a pipe or a layer of the pipe. However, the layer thickness is basically not limited. It is a further development that a thickness of the (individually) extruded green body corresponds to at least approximately five times to ten times the largest particle of the at least one refractory metal powder and / or ceramic powder. This avoids that the thickness is built up only by a few grains. This in turn improves break resistance.

It is still a development that the extrusion mass / the feedstock is extruded onto a carrier film. This facilitates handling of the extrudate, in particular thin extrudate, for example its shaping and / or stacking. The carrier film can then be removed, for example, before a thermal treatment. It is also an embodiment that several (two or more) green sheets are stacked on each other (eg, laminated and / or isostatically pressed). The green sheets may be made separately or may be part of a more complex geometry, eg, a pipe. By means of the resulting layer stack, in particular large-area articles with a high layer thickness can be sintered in one operation. In addition, a high (basically unlimited) thickness of the refractory metal component can be achieved with a constant material density. It is possible to extrude several layers, also of different properties (chemical composition, density, etc.) by means of several extrusion dies / extruder on top of each other. It is an embodiment that at least two green sheets of the layer stack differ in their properties. In particular, the thermo-mechanical properties and the fracture behavior of the layer stack can be adapted constructively. Furthermore, such a layer stack enables the production of connection zones, which allow a connection of refractory metal to external components, such as an anode support or a carrier of plasma chamber components in the fusion reactor. Also, stresses can be influenced by different thermal expansion coefficients of the components or the reaction behavior at the interfaces. For example, the layer stack here may also represent a part of a more complex geometry, for example a multilayer wall of a green body, eg a pipe.

It is a continuing education that the green sheets of the

Layer stack have a gradient structure. A gradient build-up can be used to influence the crack propagation and stress gradient, for example. In particular, a property may include a content of refractory metal, a kind and / or composition of the refractory metal or a compound thereof (eg, a content of W; Ta; Re; Mo, etc.), a presence, a kind, and / or a content of ceramics , a microscopic structure (eg, a grain size distribution), and / or a macroscopic structure (eg, a size of the powder particles, a porosity, etc.). By way of example, a gradient build-up can be achieved by layering W layers with W / Re layers, or dense tungsten layers alternate with porous tungsten layers. The porosity can be adjusted, for example, via the sintering activity of the refractory metal powders. The gradient material may be characterized in particular by a gradual (in particular stepwise) change of at least one property of the slurry layers over the stack thickness of the layer stack. The layer stack can basically be flat or three-dimensionally shaped, eg curved. It is a further development to apply the extruded layers and components to components / carriers and to jointly guide them through the subsequent thermal processes. It is an embodiment that the extrusion compound is metal binder-free, that is, has no metallic binder. The absence of the low-melting metal as a binder can be realized in particular by a lack of metal, mixtures or alloys thereof as an independent powder in the extrusion mass. Such a configuration has the advantage that the material properties of the finished refractory metal component, in particular its high melting point and its breaking strength under thermal cycling, are not degraded by the metal or metals in the binder (which would otherwise be the case). As a result, in turn, a refractory metal component produced in this way can withstand higher temperatures without destruction and / or have a longer service life. The process is not or not significantly more expensive to perform than in the presence of a metallic binder.

It is an embodiment that the extrusion compound additionally comprises ceramic powder. This has, inter alia, the advantage that a recrystallization behavior and / or a strength of the following refractory metal component can be influenced by the addition of ceramic. The presence of ceramic stabilizes the grain boundaries of the refractory metal, in particular in the context of dispersion hardening, and in particular can suppress grain growth. This, in turn, gives the refractory metal component increased resistance to high temperature stress, particularly thermal shock (e.g., caused by thermal cycling).

A ceramic powder can be present in particular as a nanopowder or micropowder, that is to say not larger than 1 micron or 1 millimeter. A mixing of ceramic and metallic Powders can be carried out together with other components of the extrusion compound or can be achieved by an optional, preceding mixing and grinding process (eg in a ball mill, an attritor, etc.). In this case, among other things, a particle size distribution can be adjusted.

It is still an embodiment that the ceramic particles La ₂ 0 ₃ , Y ₂ 0 ₃ , Tic and / or HfC have or consist of. It is yet another embodiment that a median grain size of the particles of refractory metal powder, D50, is less than two microns. These small grain sizes suppress grain growth at high sintering temperatures because the use of such fine powder fractions enables high sintering reactivity and therefore lower final sintering temperatures.

It is also an embodiment that the refractory metal powder is a powder of pure tungsten, tungsten-rhenium, WRe, or tungsten-tantalum, WTa.

It is also an embodiment that the fraction of the refractory metal or the compound thereof on the extrusion composition is 70% by weight to 99% by weight.

The heat treatment may include a step of debinding the at least one green body. In this case, the at least one green body can be heated so much that the binder is removed (thermal debinding). Alternatively or in addition, debinding may be effected by chemical debindering, in which the organic constituents of the binder are generally dissolved out of the green body by means of solvents.

The heat treatment may also include a step of sintering the at least one green body. As a result, a compacted refractory metal component is obtained. The sintering may in particular follow the debinding. The sintering may in particular be a pressureless sintering. Debinding and sintering can be carried out in one step, for example in special combined sintering systems that allow for clean debinding and subsequent sintering. This avoids reacting the components and shortens the process time.

Particularly in the case of a green body of pure tungsten as a refractory metal, a continuous process in a reducing and carbon-free atmosphere is preferred in order to keep the carbon and oxygen content low. For this purpose, the process may be carried out under vacuum or reducing atmosphere (hydrogen). It is yet another development that sintering is not performed at maximum sintering temperature to achieve complete compaction immediately, but at lower sintering temperatures. Thus, grain growth can be inhibited, which supports a homogeneous and isotropic, fine-grained microstructure. It may be sufficient, in particular, for a closed porosity to set in the refractory metal component and no maximum density. Sintering in which the workpiece has a non-negligible (closed) porosity and which is followed by another heat treatment step may also be referred to as presintering.

In particular to achieve an even higher density (in particular in the range of a maximum theoretical density) at low working temperatures of previously presintered

Workpieces, it is also a development that the step of, in particular unpressurized, (pre-) sintering followed by a further (high-temperature) heat treatment step, e.g. an isostatic hot pressing.

The step of heat treatment can thus be a step of hot pressing, in particular hot isostatic pressing, of the at least one (pre) sintered refractory metal workpiece.

The step of heat treatment may alternatively or additionally comprise a step of so-called "spark plasma" sintering. The green semifinished product, the debinded and / or pre-sintered material at comparatively low temperatures (a closed porosity is not necessary here) is flowed through under high pressure by electric current and thus brought to the final density in a short time and at comparatively low temperatures. The combination of debindering and sintering in these systems is also possible. The step of heat treatment may alternatively or additionally comprise a step of microwave sintering. In this case, the green semifinished product, the debinded and / or pre-sintered at comparatively low temperatures material is irradiated with microwaves to bring it to low density at the final density. The combination of debindering and sintering in these systems is also possible.

It is thus an embodiment that the step of heat treatment has a step of sintering below a maximum sintering temperature to a density below the maximum density and following a heat treatment step of further compacting.

It is a preferred embodiment for producing a particularly stable, in particular thermoshock resistant, refractory metal component, that at least one green body becomes at least closed-pored by the heat treatment. By "at least closed-pored", a closed-cell or dense (in particular, maximum, dense) state can be understood.

The refractory metal components produced by the above process (plates or structures, eg pipes) can already represent the final product or as semifinished by conventional bonding techniques, such as soldering, applied to surfaces. Alternatively, green bodies, in particular green sheets, can be applied to components before the furnace processes. In this case, these components must undergo heat treatment of the green body.

The object is also achieved by a component (refractory metal component) or body, which has been produced by means of the method as described above. The component may in particular be designed analogously to the method and have the same advantages.

Thus, the refractory metal component ceramic, for example as a particle and / or as a ceramic phase. It is still a development that the ceramic La ₂ 0 ₃ , Y ₂ 0 ₃ , Tic and / or HfC has or consists of.

It is also a development that the refractory metal component consists of several (two or more) layers, which may differ in particular in their properties. In particular, the layers may have a gradient structure. It is also a development that the refractory metal component is a three-dimensional component.

It is still a development that the refractory metal component is a closed-pore component or a dense component.

It is a development that the component for X-ray tubes or fusion reactors is applicable, in particular as a surface of an X-ray anode or as a wall or structural component of a fusion reactor. For example, for temperature stability in these applications, the use of a low melting metallic binder would be very disadvantageous. The above-described characteristics, features and advantages of this invention, as well as the manner in which they are achieved, will become clearer and more clearly understood in connection with the following schematic description of a

Embodiment, which will be explained in more detail in connection with the drawings. In this case, the same or equivalent elements may be provided with the same reference numerals for clarity.

Fig.l shows a sequence of a method according to the invention in several variants; and

Fig. 2 shows an apparatus for carrying out the method.

Fig.l shows a sequence of a method for producing a refractory metal component by means of extrusion, in several variants. A first preparation step Sl for producing a

Extrusion mass M (see also FIG. 2) comprises providing a powder mixture of refractory metal powder in the form of two tungsten powders. The two tungsten powders differ in their mean grain size, D50, namely once at 0.7 micrometers and once at 1.7 micrometers.

A second preparation step S2 comprises providing ceramic powder in the form of hafnium carbide powder (HfC powder).

A third preparation step S3 comprises providing additives such as a dispersing agent (Hypermer KD1), solvents in the form of ethanol and toluene, and a binder in the form of polvvenyl butyral (Pioloform BR 18) and a plastidizer in the form of dibutyl phthalate.

For producing the extrusion compound M, the constituents provided are mixed in a fourth step S4. For this purpose, the refractory metal powders, the ceramic powder, the dispersant and the liquids are first mixed in a speed mixer for 3 min at 1400 1 / min. Subsequently, the binder, to which ethanol has already been added, and the plasticizer are added and mixed for 10 minutes in the Speedmixer at 1500 1 / min.

The dispersant ensures that the wetting behavior of the refractile-metallic powder particles and of the ceramic powder is improved and agglomeration is prevented. The

Solvents Ethanol and toluene dissolve the organic components, in particular the binder Pioloform BR18. By adding a plasticizer, the flexibility and strength of the urgeformten green body 17 (see also Figure 2) and thus its handling can be adjusted. Various other mixing and milling processes produce a homogeneous starting material or extrusion compound M, also called feedstock. In some cases, it may be necessary to degas the extrusion mass M or feedstock prior to extrusion to avoid blistering in the reformed green body 17. The aim is a weight fraction of 70% to 99% of metallic powder in the extrusion mass M.

In a subsequent step S5, the extrusion mass M is extruded. For this purpose, the extrusion mass M is introduced into a hopper 12 of an extrusion system 11, as shown in FIG. 2 is shown as a single screw plasticizing extruder. The extrusion mass M passes from the hopper 12 into a cylinder 13, in which a screw 14 rotates driven by a motor 15. The screw 14 conveys the extrusion compound M to a tip of the cylinder 13, to which a, optionally heatable, extrusion die 16 is located. From the extrusion die 16, the green body 17 is pushed out as an extrudate. The green body 17 or the extrudate can be designed in particular profile-like. With a correspondingly shaped extrusion die 16, the provision of, for example, plate-like, green sheets is also possible. In a following step S6, the green body 17 can be formed. For example, the green body 17 may be cut and / or shaped, in particular three-dimensionally shaped. A minimum thickness is limited in particular by the particle size of the starting powders and preferably corresponds approximately to 5 to 10 times the largest refractory metal particles. With the above starting powders (especially D50 = 1.7 microns), the lower limit of a thickness of at least one layer of green body 17 is about 60 microns. The maximum thickness is approximately 1.5 mm to 2.0 mm.

In a following step S7, the cut / shaped green body 17 is heat-treated to produce the finished refractory metal component.

In a first sub-step S8 of step S7, the main body 17 is debinded, in particular by a heat treatment. In a second sub-step S9, the debindered and possibly shaped main body 17 is sintered, specifically in a coherent, in particular pressureless, sintering operation at a correspondingly high sintering temperature until a dense or practically non-porous refractory metal component is present.

In an alternative procedure to step S9, the debinded and possibly shaped green body 17 is first sintered ("pre-sintered") in step S10, wherein it does not yet reach its dense state, but is porous (open-pored or closed-pored) remains .

In a following step Sil, the presintered refractory metal work piece is compacted by hot isostatic pressing to form the refractory metal component, in particular compressed without pores, in particular at least approximately to its maximum density. This has the advantage that the temperatures required for hot isostatic pressing lower are inhibited as the sintering temperature required in step S9 and thus grain growth (which increases with increasing temperature).

As an alternative or in addition to step S, a spark plasma sintering step S12 and / or a microwave sintering step S13 may be performed.

While the invention has been further illustrated and described in detail by the illustrated embodiment, the invention is not so limited and other variations can be derived therefrom by those skilled in the art without departing from the scope of the invention.

Claims

claims

1. Method (S1-S13) for producing a refractory metal component, the method comprising the following steps:

Providing (S4) an extrusion mass (M) comprising a refractory metal powder of at least one refractory metal and / or a compound thereof and at least one binder; and

- extruding (S5) the extrusion mass (M) to at least one green body (17);

in which

the step of extruding (S5) the extrusion mass (M) is followed by a step of heat treatment (S7) of the at least one green body (17).

The method (S1-S13) according to claim 1, wherein said extruding (S5) comprises extruding a greensheet.

A method (S1-S13) according to claim 2, wherein a plurality of green sheets are stacked on each other and at least two green sheets differ in their properties.

4. The method (S1-S13) according to any one of the preceding claims, wherein the extrusion mass (M) is metal binder-free.

5. The method (S1-S13) according to any one of the preceding claims, wherein the extrusion mass (M) additionally comprises ceramic powder.

6. The method (S1-S13) according to claim 5, wherein the ceramic powder La ₂ 0 ₃ , Y ₂ O ₃ , Tic and / or HfC has.

A method (S1-S13) according to any one of the preceding claims, wherein a median grain size, D50, of powder particles of the refractory metal powder is less than two microns.

A method (S1-S13) according to any one of the preceding claims, wherein the refractory metal powder is a powder of pure tungsten, WRe or WTa.

9. The method (S1-S13) according to any one of the preceding claims, wherein the proportion of the refractory metal or the compound thereof to the extrusion mass is 70 wt .-% to 99 wt -.%.

10. The method (S1-S13) according to any one of the preceding claims, wherein the step of heat treatment (S7) comprises debindering (S8) and / or sintering (S10-S13).

The method (S1-S13) according to claim 10, wherein the step of heat treatment (S7) comprises a step of sintering

(S10) below a maximum sintering temperature to a density below the maximum density and following a heat treatment step of further densification

(S11-S13).

12. The method (S1-S13) according to any one of claims 10 or 11, wherein at least one green body (17) is at least closed-pored by the heat treatment.

13. refractory metal component, which has been produced by the method (S1-S13) according to one of the preceding claims.