WO2008003290A2

WO2008003290A2 - Method for the production of metal foams, and metal foam

Info

Publication number: WO2008003290A2
Application number: PCT/DE2007/001140
Authority: WO
Inventors: Norbert BABCSÁN; Goarke Sanjeeviah Vinod Kumar; John Banhart; Budaraju Srinivasa Murty
Original assignee: Helmholtz-Zentrum Berlin Für Materialien Und Energie Gmbh
Priority date: 2006-07-03
Filing date: 2007-06-27
Publication date: 2008-01-10
Also published as: WO2008003290A3; ATE486971T1; DE102006031213B3; EP2044230B1; DE502007005568D1; EP2044230A2

Abstract

Disclosed is a method for producing metal foams comprising stabilizing particles in the metal matrix. Said method comprises at least the following steps: an expandable starting material is produced, and the starting material is expanded. According to the invention, the stabilizing particles are produced in an in situ reaction of molten reactive liquids and a metal bath during the production of the expandable starting material, the components of the submicroscopic particles or nanoparticles that are to be produced being added to the metal bath as fluoride salt, then being mixed and being heated to a temperature lying above the melting point of the components of the mixture. The inventive method makes it possible to produce metal foams containing stabilizing particles in a metal matrix. Said stabilizing particles are provided in the metal matrix at a proportion of less than 10 percent by volume, have a size of less than 1 μm, and are disposed at a contact angle ranging between 10 and 100°, while the metal matrix is formed from regularly arranged polygonal closed cells whose average diameter ranges from 2 to 10 mm, and the metal foam has a minimum porosity of 75 percent.

Description

description

Process for producing metal foams and metal foam

description

The invention relates to a process for the production of metal foams with stabilizing particles and metal foam.

Metal foams are of great interest as materials because of their possible combinations of different properties.

On the one hand, metal foams are very light on the one hand, and on the other hand have high rigidity and considerable strength. You can z. B. are used for thermal insulation, noise and vibration damping or compression element.

For the production of such metal foams different methods are known, for. As the direct foaming of melts by gas injection or by the addition of foaming agents or the foaming of solid starting materials.

For example, a slurry is prepared at room temperature to produce steel foam from steel powder, water and a stabilizer. Phosphoric acid is added to this mixture as a binding and blowing agent. Two reactions then take place in the slurry, leading to the formation of a stable foam structure. On the one hand, in the reaction between steel powder and acid, hydrogen gas bubbles are produced which cause foaming. On the other hand, a metal phosphate is formed, which solidifies the pore structure by its adhesive effect. The foam thus produced is dried and then sintered free of pollutants to the metallic composite.

A melt metallurgical process is described, for example, in EP 1 288 320 A2 by introducing gas bubbles into a melt. The size of the bubbles is controlled by the adjustment of the Einströmparameter of the gas.

In EP 1 419 835 A1, a method and an apparatus for producing flowable metal foam with a monomodal

Distribution of the dimensions of the cavities presented, which is also based on a melt metallurgical process. In this case, at least two adjacent identically dimensioned feed pipes protrude at a defined distance from one another into a vessel with a foamable molten metal. Bubbles are formed in each case in the regions of the projecting tube ends, a continuous foam formation being formed by juxtaposition of regions of the bubble surfaces and formation of particle-containing intermediate walls.

A disadvantage of these melt metallurgical processes that a

Metal melt in the pure state is not foamable. For the purpose of achieving a foamability, the melt must be mixed with a viscosity-increasing agent, for example an inert gas (GB 1, 287,994) or with ceramic particles (EP 0 666 784 B), prior to carrying out the foaming. Only the metal foam accumulated on the enamel surface is stable. Although this is favorable for a shaping processing of the metal foam, but may result in a lack of stabilization of the metallic walls to a partial collapse of the metal foam formed and thus to an uncontrollable formation of dense zones in the interior of such a created object. Further, a part of the bubbles formed or the dissolved gas during the solidification of a melt escape from this, so that a Containment of the released gas in the melt does not occur and therefore the porosity of the objects created by this method is low.

A powder metallurgical process for the production of porous metal bodies is presented in DE 101 15 230 C2, in which a mixture which comprises a pulverulent metallic material which contains at least one metal and / or one metal alloy and a gas-releasing propellant-containing powder is compacted to form a semifinished product. This semifinished product is foamed under the action of temperature, wherein a propellant-containing powder is used, in which the temperature of the maximum decomposition is less than 120 K below the melting temperature of the metal or the solidus temperature of the metal alloy.

In WO 2005/011901 A1 it is proposed that, for the production of metal parts with internal porosity, first a foamable semifinished product consisting of metal and at least one blowing agent releasing gas at elevated temperature is produced. The foamable semifinished product contains a substantially closed matrix in which propellant particles are embedded. An increased quality of a created metal foam body to be achieved with a semi-finished, in which the

Foam particle enclosing metal matrix is formed by diffusion and / or pressure welding of metal particles.

Another method for producing metal foam bodies is described in WO 2004/063406 A2. This method can be used as a powder metallurgy or as a melt metallurgical process. In this solution, when melting a feedstock under atmospheric pressure in an open crucible without pressure devices and a simultaneous and / or subsequent introduction of gas into the liquid phase of the feedstock, introduced by blowing agent or by gas introduction, a sufficient gas supply of the melt is achieved in order of the Solidification of the same can cause the formation of a metal foam body low density.

JP 01-1273131 (Abstract) also describes a process in which hydrogen, nitrogen, oxygen is introduced into the liquid metal analogously to the abovementioned solution under atmospheric pressure or propellant particles, such as nitride, hydride or oxide, by thermal cracking gas into the Release melt. The gasified liquid metal is placed in a mold and held under reduced pressure, 53.320 to 101, 308 kPa for a period of time.

In order to increase the stability of the metal foams, a method has been described in US 3,816,952, in which the viscosity of the melt is increased by direct oxidation, which is due to the bubble formation.

US 4,713,277 describes a process for producing a metal foam having a specific gravity of 0.2 to 0.8 and uniformly distributed polygonal cavities of an average size of 2 to 10 mm, in which first the starting materials for the molten metal (Al or alloys thereof) and Ca are mixed as a binder (to increase the viscosity) and heated to melting temperature, then a frother, titanium hydride with 1 to 3 wt .-%, is mixed in air.

A process for producing stabilized metal foams is disclosed in US 5,112,697. In this case, solid stabilizing particles are added to the starting material for the metal matrix, mixed and heated to a temperature greater than the liquidus temperature of the metal matrix. Subsequently, gas is added to the mixture and the mixture is cooled. The stabilizing particles are then incorporated into the metal matrix. In the US patent attention is drawn to the correct selection of these solid stabilizing particles, since these can lose their form and their identity by dissolution in the metal or chemical combination with the metal. As possible Materials for these stabilizing particles are called: metal oxides, carbides, nitrides or borides, in particular Al, TiB ₂ , Zr, SiC, SiN, which are contained in the metal foam to <25%, preferably 5 to 15%. The size of the particles is given as 0.1 to 100 μm. It should be noted that the right size selection is important because too small particles are very difficult to mix. On the other hand, particles that are too large precipitate. If the volume fraction of these stabilizing particles is too low, the foam stability is too weak; if the volume fraction is too high, the viscosity is too high. The method described produces a metal foam having typical cell sizes of 250 μm to 50 mm.

Such metal foams containing μm-sized stabilizing particles in their metal matrix are brittle and are difficult to cut clean with a saw. In addition, the starting materials are very expensive and a large amount of these particles is necessary to achieve good stability.

The object of the invention is to provide a further process for producing metal foams comprising stabilizing particles, in particular the amount of stabilizing particles necessarily used should be reduced and the metal foams produced should be easier to process.

The object is achieved for a method of the type mentioned in that the stabilizing particles are generated in the preparation of the foamable starting material in an in situ reaction of molten reactive liquids and a molten metal, wherein the molten metal, the components of the submicroscopic particles or Nanoparticles are added at least as fluoride salt, then mixed and heated above the melting temperature of the mixture components. The solution according to the invention, in which the stabilizing submicroscopic particles or nanoparticles are produced during the preparation of the foamable starting material by means of an in situ reaction, makes it possible to provide and cooperate with such elements which are required for the formation of an interface layer between the melt and the stabilizing particles a contact angle of 10 to 100 grd, preferably from 60 to 80 grd are necessary. As has surprisingly been found, this angle is essential for the formation of metal foams according to the inventive solution and depending on the alloys used and also essential for the grain refining during solidification.

In the metal foams produced by the process according to the invention needs to achieve the same stabilization in

Compared to the prior art known metal foams less material to be used, since the stabilizing particles produced in an in situ step have a larger specific surface area. The metal foams produced in this way are less brittle, since their metal matrix has better mechanical properties, and can be processed more easily.

In embodiments of the invention, it is provided that K _x AF _{y is} used as fluoride salt, where A is an element of Ti, B, Zr, Nb, V, W, Ta or Ce, preferably K ₂ TiF ₆ or KBF ₄ is used or both salts are used as a mixture.

In another embodiment, in addition to the fluoride salt, a further constituent of the stabilizing particles to be produced in the in situ reaction, for example carbon in the form of graphite, is added. The metal used for the melt is aluminum or an aluminum alloy.

In another embodiment, after the production of the foamable starting material containing the stabilizing particles, first all unwanted constituents are removed, then the starting material is cooled to 700 ⁰ C and for foaming the same powdered metal hydride, preferably TiH ₂ , from 1 to 3 wt. % added as an intumescent at 700 ° C.

However, reactive gases can also be used for foaming, for example CH ₄ , NH ₃ and O ₂ , preferably O ₂ in a concentration of 0.1 to 10 vol.%, In particular of 1 to 2 vol.%. Here, the stability of the metal foam is positively influenced. The temperature is in this case 600 to 1000 ⁰ C, preferably 700 ° C.

The foaming of the starting material can also be carried out by other methods known from the prior art, for example by means of a gas injection directly into the melt.

The invention also comprises a metal foam according to claim 15. Thereafter, the metal foam stabilizing particles in a metal matrix, wherein the stabilizing particles less than 10 vol. % are contained in the metal matrix, have a size of less than 1 μm and are arranged at a contact angle of 10 to 100 °, and the metal matrix is formed of uniformly arranged polygonal closed cells with an average diameter of 2 to 10 mm and the metal foam has a porosity of at least 75%, preparable by a process comprising the following steps: producing the stabilizing particles in an in situ reaction of molten reactive liquids and a molten metal, the molten metal being the constituents of the submicroscopic particles to be produced Particles or nanoparticles are added at least as a fluoride salt, then mixed and heated above the melting temperature of the mixture components to form an interfacial layer in which the stabilizing particles are arranged at a contact angle of 10 to 100 °, preferably 60 to 80 °, subsequently the enstandenen melt all unwanted constituents are removed and then the melt of metal and stabilizing particles is cooled to 700 ⁰ C and finally foamed.

In embodiments, it is intended to form the metal matrix from one of the elements Al, Fe, Ti or Cr. The stabilizing particles contain Ti or B or Zr or Nb or V or W or Ta or Ce or C or Al in combination with a second element of this group and are preferably formed from TiC or AIB ₂ or TiB ₂ .

The invention will be described in more detail in the following embodiments.

The figures show:

Fig. 1 is a SEM image of a cut Al foam cell whose

Surface is covered with TiC particles; Fig. 2 is an X-ray of the same AI foam.

Embodiment 1

First, aluminum is melted at 1200 ^° C. This melt is then added to produce the stabilizing particles, a mixture of graphite and K ₂ TiF ₆ and mixed. After the reaction, unwanted components are removed except for the Al-TiC mixture. This melt mixture is cooled to 700 ⁰ C and further processed, ie foamed. This can be done, for example, by adding 1.6% by weight. powdered TiH ₂ happen, the temperature is then maintained at 700 ⁰ C over a period of 50 s. Now a mixing process takes place over a duration of approx. 100 s. This produces homogeneously foamed aluminum with a porosity of about 75%, which contains as stabilizing particles TiC in a concentration of 6 vol.% And a size of less than 1 micron.

Fig. 1 shows an SEM photograph of a section through a cell of the AI foam, on its surface, i. on their cell wall, the TiC particles are very well recognizable in their dense and relatively homogeneous arrangement. In the X-ray photograph shown in FIG. 2, the entire sample of the same AI foam with TiC particles is shown. The diameter of this sample is 40 mm.

EMBODIMENT 2 A mixture of KBF ₄ and K ₂ TiF _{6 is} added to an Al melt produced at 800 ^° C. and mixed. After the reaction, the unwanted components are again removed except for the Al-TiB ₂ mixture. Cooling and foaming process is carried out as in Example 1. The Al foam thus produced with a porosity of about 75% contains TiB ₂ particles in a concentration of 4 vol.% And a size of less than 1 micron.

Embodiment 3

Al is melted at 800 ^° C. This melt is added to KBF ₄ and mixed again. After removal of the undesirable components after the reaction, the Al-AIB ₂ mixture - as described - cooled and foamed. The result of this process is once again an Al foam with a porosity of about 75%, the stabilizing particles of which are formed here from AIB ₂ and are contained in the foam in a concentration of 4.3% by volume.

Claims

claims

1. A process for producing metal foams with stabilizing particles in the metal matrix, comprising at least the process steps

Producing a foamable starting material and foaming this

Starting material, characterized in that the stabilizing particles are produced in the preparation of the foamable starting material in an in situ reaction of molten reactive liquids and a molten metal, wherein the molten metal, the components of the submicroscopic particles or nanoparticles to be generated at least added as a fluoride salt, then mixed and heated above the melting temperature of the mixture components.

2. The method according to claim 1, characterized in that is used as the fluoride salt K _x AFy, wherein A is an element of Ti, B, Zr, Nb, V, W, Ta or Ce.

3. The method according to claim 2, characterized in that K ₂ TiF ₆ or KBF _{4 is} used as the fluoride salt or both salts are used as a mixture.

4. The method according to claim 1, characterized in that in addition to the fluoride salt, a further constituent of the stabilizing particles to be produced in the in situ reaction is added.

5. The method according to claim 4, characterized in that as a further component carbon, preferably in the form of graphite, is used.

6. The method according to claim 1, characterized in that is used as the metal for the melt aluminum or an aluminum alloy.

7. The method according to claim 1, characterized in that the melt after in-situ generation of the stabilizing particles for foaming the starting material pulverfö shaped metal hydride of 1 to 3 wt.% Added as a foaming agent at 700 ⁰ C and the melt is foamed.

8. The method according to claim 7, characterized in that TiH _{2 is} used as the metal hydride.

9. The method according to claim 1, characterized in that for foaming the starting material, a reactive gas is passed through the melt.

10. The method according to claim 9, characterized in that the reactive gas is selected from CH ₄ , NH ₃ or O ₂ .

11. The method according to claim 10, characterized in that

O ₂ in a concentration of 0.1 to 10 vol.%, In particular from 1 to 2 vol.% Is used.

12. The method according to any one of claims 1 to 11, characterized in that used as the metal for the melt aluminum, this is melted at temperatures between 1150 ° C and 1250 ° C, then the melt is added to a mixture of graphite and K ₂ TiF ₆ and mixed and then all components except the Al-TiC melt are removed, this is cooled to 700 ⁰ C and foamed.

13. The method according to any one of claims 1 to 11, characterized in that used as the metal for the melt aluminum, this is melted at temperatures between 750 ⁰ C and 850 ⁰ C, then the melt is a mixture of KBF ₄ and K ₂ TiF. ₆ is added and mixed and then all components except the AI-TiB ₂ melt are removed, this is cooled to 700 ⁰ C and foamed.

14. The method according to any one of claims 1 to 11, characterized in that used as the metal for the melt aluminum, this is melted at temperatures between 750 ⁰ C and 850 ⁰ C, then the melt KBF _{4 is} added and mixed and then all components are removed except the AI-AIB ₂ melt, this is cooled to 700 ° C and foamed.

Metal foam containing stabilizing particles in a metal matrix, wherein the stabilizing particles are less than 10 vol.% In the metal matrix, have a size less than 1 micron and in one Contact angles of 10 to 100 ° are arranged, and the metal matrix is formed of uniformly arranged polygonal closed cells with an average diameter of 2 to 10 mm and the metal foam has a porosity of at least 75%, producible by a method comprising the following steps: generating the stabilizing particles in an in situ reaction of molten reactive liquids and a molten metal, wherein the molten metal is added to the constituents of submicron particles or nanoparticles to be produced at least as fluoride salt, then mixed and heated above the melting temperature of the mixture components, then from the resulting melt removing any unwanted components to form an interfacial layer in which the stabilizing particles are disposed at a contact angle of 10 to 100 degrees, and thereafter the melt of metal and stabilized Particles is cooled to 700 ⁰ C and finally foamed.

16. Metal foam according to claim 15, characterized in that the metal matrix is formed from aluminum.

17. Metal foam according to claim 15, characterized in that the stabilizing particles Ti or B or Zr or Nb or V or W or Ta or Ce or C or AI in combination with a second element of this group.

18. Metal foam according to claim 17, characterized in that the stabilizing particles of TiC or AIB ₂ or TiB _{2 are} formed.

19. Metal foam according to claim 15, characterized in that the stabilizing particles are arranged in the forming interface layer in a contact angle of 60 to 80 °.