EP1397223B1 - Production of metal foams - Google Patents

Abstract

The invention relates to a process for producing metal foams of controlled structure and to the metal bodies in foam form obtained in this way, wherein metals from group IB to VIIIB of the periodic system of the elements are added before and/or during the formation of the foam.

Description

The invention relates to a process for the preparation of structure-controlled metal foams of aluminum or aluminum alloy and the foam-like metal body obtained in this way.

1. 1. das Kompaktieren von Metallpulvern mit geeigneten Treibmitteln und Erhitzen der so gewonnenen Grünkörper auf Temperaturen oberhalb der Liquidustemperatur der Metallmatrix und oberhalb der Zersetzungstemperatur des verwendeten Treibmittels;
2. 2. das Lösen bzw. Einblasen von Treibgasen in Metallschmelzen;
3. 3. das Einrühren von Treibmitteln in Metallschmelzen;
4. 4. das Sintern metallischer Hohlkugeln;
5. 5. die Infiltration von Metallschmelzen in Füllkörper, die nach Erstarren der Schmelze entfernt werden.

The state of the art for the production of metal foams essentially comprises five basic procedures:

1. 1. the compacting of metal powders with suitable blowing agents and heating the green bodies thus obtained to temperatures above the liquidus temperature of the metal matrix and above the decomposition temperature of the blowing agent used;
2. 2. the dissolution or blowing of propellant gases into molten metals;
3. 3. the stirring of blowing agents in molten metals;
4. 4. the sintering of metallic hollow spheres;
5. 5. the infiltration of molten metal in packing, which are removed after solidification of the melt.

ad 1) DE 197 44 300 A is concerned with the production and use of porous light metal parts or light alloy parts, wherein the pressed from a powder mixture (light alloy or Al alloy and propellant) body in a heated closed vessel be heated with inlet and outlet openings to temperatures above the decomposition temperature of the blowing agent and / or melting temperature of the metal or alloy.

ad 2) JP 03017236 A describes a method for producing metal articles with cavities by reacting gases in a Dissolves molten metal and then initiates the foaming by sudden pressure reduction. Cooling of the melt stabilizes the resulting foam.

WO 92/21457 teaches the production of Al foam by injecting gas under the surface of a molten metal, wherein abrasives, such as. As SiC, ZrO _2, etc., are used as stabilizers.

ad 3) According to the teaching of JP 09241780 A, metallic foams are obtained under controlled release of propellant gases by first melting the metals at temperatures below the decomposition temperature of the propellant used. By subsequently dispersing the blowing agent in the molten metal and heating the matrix via the temperature then required for the release of propellant gases, a metal foam is established.

ad 4) The production of ultralight Ti-6Al-4V hollow ball foams is based on the sintering of hydrogenated Ti-6Al-4V hollow spheres at 600 ° C. at temperatures ≥ 1000 ° C. (Synth./Process.Lightweight Met. Mater. II, Proc Symp., 2nd (1997), 289-300).

ad 5) Foamed aluminum is obtained after infiltrating molten aluminum into a porous filler by removing it from the solidified metal (Zhuzao Bianjibu (1997) (2) 1-4; ZHUZET, ISSN: 1001-4977).

In addition, of particular interest are components with a cavity profile for reducing weight and increasing its rigidity. DE 195 01 508 A deals with a component for the chassis of a motor vehicle, which consists of die-cast aluminum and has a cavity profile in the interior of which is a core of aluminum foam. Of the integrated aluminum foam core is previously prepared by powder metallurgy, then fixed to the inner wall of a casting tool and then cast in the die-casting with metal.

DE-A-23 62 293 discloses a process for the preparation of metal foams wherein metals of group IB to VIIIB of the Periodic Table of the Elements are added before and / or during the foaming.

In appreciation of the state of the art, it should be noted that the processes which provide green bodies containing precompacting blowing agent are expensive and expensive and are not suitable for the production of bulk goods. In addition, this method has in common that the desired temperature difference between the melting point of the metal to be foamed and the decomposition temperature of the propellant used should be as low as possible otherwise takes place during compaction or later in the melting phase disturbing propellant decomposition. This consideration also applies to the introduction of blowing agents in molten metals.

The presintering preformed hollow spheres to a metallic foam comes at most academic interest, since the production of hollow spheres already requires a complex process technology.

From this point of view, the infiltration technique is also to be evaluated, in which it is necessary to remove the porous filler from the foam matrix.

The dissolution or blowing of propellant gases into molten metal is not suitable for the production of near net shape workpieces, since a system consisting of the melt with occluded gas bubbles, is not sufficiently stable in time to be processed in forming tools.

The mechanical properties of metal foams are essentially determined in addition to the choice of the metal or alloy used.

However, the coupled processes taking place in the production of porous metal bodies often do not yield the desired result of a uniform metal foam interspersed with globular cells of comparable dimension, particularly in the case of the method based on the use of chemical blowing agents. Associated with this is, for example, no isotropy of the bulk density, which could be desired for the later function of the metal foam in numerous structural components achieved. Instead, one observes irregularities in the form of thickened zones in the metal body (for example pronounced foot and / or edge zone formation and / or also connected cavities, which derive from the combination of individual gas bubbles resulting from cell membrane destruction). The occurrence of such irregularities may also be an indicator of relatively inefficient propellant use.

Thus, the object of the present invention is to find a technically usable method for targeted structure control of the metal foams produced with chemical blowing agents. Linked to this is to improve the use of propellant used (for example, a metal hydride).

The solution of the above object is therefore in a process for the production of aluminum or aluminum alloy foam, which is characterized in that one adds metal powder of group 3 to 12 of the Periodic Table of the Elements before and / or during the foam formation.

Surprisingly, it has now been found that metals of groups 3 to 12 of the Periodic Table of the Elements act as additives to hydride-loaded systems in the sense of the abovementioned object, and significantly increase the blowing agent efficiency. The added metals of groups 3 to 12 of the Periodic Table of the Elements can be applied either individually or in the form of a mixture of several metals.

The inventive method thus provides in a preferred embodiment, to expand the matrix consisting of aluminum or aluminum alloy and magnesium hydride as blowing agent with small amounts of titanium, copper, iron, vanadium and mixtures thereof. The metallic additives are used in amounts of from 0.001% by weight to 1% by weight, particularly preferably from 0.01% by weight to 0.1% by weight, based on the metal to be foamed, in particular on the light metal to be foamed used.

Particularly suitable blowing agent in the context of the present invention is autocatalytically produced magnesium hydride, whose preparation is known from the literature. In addition, this magnesium hydride is commercially available under the name Tego Magnan® from the applicant. Generally, the amount of blowing agent can be varied within the usual limits of from 0.1% to 5% by weight, preferably from 0.25% to 2% by weight.

The use of the observed phenomenon ensures the production of very regular foam structures and ensures the reproducibility of morphologically uniform metal foams required under application technology aspects. Substantially, the application of the method according to the invention in Foaming process help to suppress the process of cell membrane destruction.

Assessment criteria for the qualitative assessment of plastic foams and of metal foams are, in addition to the visually recognizable homogeneity, the expansion achieved and, associated therewith, the final density of the porous metal body.

Using the powder metallurgical route (mixing of light metal powder with hydridic blowing agent and optionally additives, precompacting and / or pressing the matrix into green bodies, heating the green bodies to temperatures above the melting point of the metal to be foamed), the generalizable principle of the present invention should be demonstrated here. The loading of a metal-hydride system according to the invention with the additives claimed here is, of course, not limited to the powder metallurgical method but also covers systems which must be included in melt metallurgy.

EXAMPLES Example 1:

500 g of aluminum powder with a purity of 99.5% were mixed with 1 wt .-% Tego Magnan ^® (magnesium hydride, 95% hydride content), based on the amount of aluminum powder, and 0.1 wt .-% titanium powder, based on the amount to aluminum powder and 0.01 wt .-% copper powder, based on the amount of aluminum powder, applied and mixed with stirring. From this mixture, cylindrical compacts were prepared by cold isostatic pressing. The degree of grain pilling of the compacts thus obtained was 94 to 97% of the theoretical density to be achieved.

In an induction furnace with an HF output power of 1.5 kW, the compacts were in the graphite crucible with a heating rate of 300 ° C / min. foamed freely. 30 seconds after the beginning of the foaming process, the foam bodies were cooled rapidly.

After sawing the samples was recognized in Fig. 1 homogeneously distributed globular cells with a mean diameter of 3 mm to the edge regions. The density achieved was 0.5 g / cm ³ .

Example 2:

Analogously to Example 1, 500 g of aluminum powder with 1% by weight of Tego ^Magnan® (magnesium hydride), based on the amount of aluminum powder, 0.1% by weight of titanium powder, based on the amount of aluminum powder and 0.01% by weight Vanadium powder, based on the amount of aluminum powder added. This mixture was compacted as described above. The degree of compaction of the cylindrical compacts thus obtained was 94 to 96%.

After foaming and sawing, a fine homogeneous cell structure was seen, which had a mean size of 1.5 to 2 mm at a density of 0.6 g / cm ³ .

The resulting foam structure is documented by FIG.

Example 3:

As in Example 1, 500 g of aluminum powder, 1% by weight of Tego ^Magnan® (magnesium hydride), based on the amount of aluminum powder, 0.1% by weight of titanium powder, based on the amount of aluminum powder and 0.01% by weight. Iron powder, based on the amount of aluminum powder, mixed, compacted and the foamed green body obtained foamed. After sawing, a homogeneous structure with a mean cell size of 5 mm was visible. The measured density was 0.7 g / cm ³ .

The resulting foam structure is documented by FIG.

Example 4:

Analogously to Example 1, 500 g of aluminum powder, 1 wt .-% Tego Magnan ^® (magnesium hydride), based on the amount of aluminum powder and 0.1 wt .-% titanium powder, based on the amount of aluminum powder, mixed and compacted. The degree of compaction was between 95 to 97% of the theoretically achievable density. The green bodies thus obtained were foamed and after sawing a homogeneous structure with a mean cell size of 3.5 to 4 mm was recognizable. The measured density was 0.3 g / cm ³ .

The resulting foam structure is documented by FIG. 4.

Reference Example 1:

Analogously to Example 1, 500 g of aluminum powder, 0.1 wt .-% titanium hydride, based on the amount of aluminum powder and 0.1 wt .-% titanium powder, based on the amount of aluminum powder, mixed, compacted and foamed freely. After sawing, a coarse, very heterogeneous foam structure was visible, with an average cell size of 8 mm. Several pore membranes were torn. The determined density was 0.7 g / cm ³ .

The resulting foam structure is documented by FIG. 5.

Reference Example 2:

Similar to Comparative Example 1, 500 g of aluminum powder, 0.1 wt .-% of titanium hydride, based on the amount of aluminum powder and 0.1 wt .-% copper powder, based on the amount of aluminum powder, mixed and compacted. After foaming and sawing, a torn, inhomogeneous structure with a mean pore size of 5.5 mm and a pronounced base formation appeared. The achieved density was 0.5 g / cm ³ .

The resulting foam structure is documented by FIG. 6.

It was clearly evident that the morphology and final density of the foamed metal bodies were significantly influenced by the addition according to the invention of small amounts of transition metals and / or mixtures thereof.

Claims (6)

1. Process for producing aluminium foam or aluminium alloy foam of controlled structure by heating aluminium or aluminium alloys with magnesium hydride as blowing agent, characterized in that 0.001 to 1% by weight, based on aluminium or aluminium alloys, of one or more metal powders from groups 3 to 12 of the periodic system is added before or during the foaming.
2. Process according to Claim 1, characterized in that the metal powder(s) added is/are titanium, copper, iron or vanadium.
3. Process according to Claims 1 and 2, characterized in that from 0.01 to 0.1% by weight of metal powder(s) is added.
4. Process according to Claims 1 to 3, characterized in that magnesium hydride is used in amounts of from 0.1 to 5% by weight, based on aluminium or aluminium alloys.
5. Process according to Claims 1 to 4, characterized in that the magnesium hydride is produced autocatalytically.
6. Process according to Claims 1 to 5, characterized in that the formation of foam is achieved by compacting the matrix made from aluminium or aluminium alloy powder and M_gH₂ and heating the preforms obtained in this way to temperatures which are higher than the liquidus temperatures of the metal matrix and higher than the decomposition temperatures of the blowing agent.

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