EP2143809B1 - Aluminium alloy metal foams, its use and method for its manufacture - Google Patents

Abstract

Claimed is an aluminium alloy for use as metal foam with the general formula AlMg4(+1)Si8(+1) % Wt. Further claimed are the use of the aluminium alloy as the core of an aluminium sandwich material and a process for production of the alloy using a suitable mixture of metal powder.

Description

The invention relates to a metal foam, a product thereof and a method for its production.

The prior art generally discloses the production of metal foams in a powder-metallurgical manner by first compacting a mixture of metal powder and blowing agent and then partially or completely melting it and cooling it again after pore formation has taken place. In the molten state, the gas released by the blowing agent forms the pores in the melt.

Such a method is for example in DE 101 15 230 A1 described. As an example, the production of a metal foam of the alloy AA6060 (AlMgSi) is given.

The homepage of the Fraunhofer Institute for Machine Tools and Forming Technology (http://www.iwu.fraunhofer.de/schaumzentrum/produkte.htm) lists further aluminum alloys such as AISi10 and also a 6000 aluminum alloy, namely AIMg1 Si0.5.

The prior art and scientifically studied 6000 AlMgSi alloys containing Si and Mg in small amounts (up to about 2%) have not given satisfactory results in obtaining metal foams in terms of maintaining a fine pore structure at high expansion capability. good mechanical properties and good corrosion resistance.

Other technical aluminum alloys currently used for metal foams are the cast alloy AlSi7 or the alloy AlSi6Cu6 (see homepage of the company alm GmbH on 03.06.08: http://www.almgmbh.de/html/produkte.html ).

Further efforts have been made to achieve the desired properties of aluminum foams by varying the Si and / or Cu content.

Although the latter AlSi (for example AlSi6) and AlSiCu casting alloys (partially with modified Cu content) have become established, the desired and above-mentioned properties have not yet been achieved with these alloys. Here it is assumed that the mentioned casting alloys have far too low Mg and Si contents in order to achieve a sufficiently large melt quantity at the beginning of the melting process. In addition, the formation of small amounts of melt at the beginning of the process is associated with the risk of channeling in the semifinished product and the associated loss of propellant gas at the start of the melting process.

Also well known in the art are metal foam sandwiches in which a metal foam core is arranged between two outer cover layers closing off to the outside.

Sandwich structures of this type can be produced by bonding the cover layer to the foam core layer, but also by metallically bonding the unfoamed core material to the cover layers by applying pressure in one method step (see, for example, US Pat EP 0 997 215 A2 ) and only then foamed by means of thermally activated blowing agent.

For the latter method for the production of metal foam sandwiches, the choice of materials for the metal foam core and for the cover layers is particularly important because the foaming process requires special temperature conditions. In DE 101 36 370 A1 The composite blank is formed into a semi-finished product and foamed by heating to a temperature which is simultaneously above the Ausgastemperatur of the blowing agent powder and within the solidus-liquidus region of the metal powder, to form a component. It is stated that in the case where both the core layer and the cover layers have the same material, e.g. As aluminum, is used, different melting temperatures can be adjusted by different alloying additives in powder and topcoat materials.

In general, it should be noted that the beginning of the melting temperature range of the known prior art metal foam alloys is well above the decomposition temperature of the commonly used blowing agent TiH ₂ .

The object of the invention is now to provide a metal foam based on an aluminum alloy of the type AlMgSi and a use of this alloy, wherein the metal foam should have a fine pore structure with high expansion capacity, good mechanical properties and good corrosion resistance. The object is further to provide a method for producing a metal foam having product.

The object is achieved for a metal foam based on a Alu.miniumlegierung according to the invention that this alloy is AlMg4 (± 1) Si8 (± 1) - stated in wt.% - And the metal foam Al, Mg, Si and manufacturing-related impurities or Al, Mg, Si and the metallic component of a blowing agent and manufacturing-related impurities.

It has been found that the previously technologically irrelevant AlMg4Si8 alloy within the technically feasible by the powder mixture tolerance of ± 1% excellent foaming properties and the resulting metal foam has a much finer compared to the prior art pore structure.

This can be attributed to positive effects of the contained Mg, such as the reduction of the surface tension of the melt and its strong tendency to oxidation - as rapid oxide formation stabilizes the cell walls of the resulting pores - and the increase of the melt viscosity, the drainage is reduced and also to the stability of the pore structure contributes in the liquid range.

The improved properties can also be attributed to the particular melting behavior of the alloy according to the invention, which is characterized by the function of the liquid volume fraction as a function of the temperature of the melt. During the foaming process, the alloy isothermally produces a proportion of about 50% ternary-eutectic melt at 560 ° C. and has a liquidus temperature of about 600 ° C., which enables precise setting of an optimum toughness of the melt for foam expansion.

Compared to the Cu-containing alloys mentioned above in the prior art, there is also the advantage of higher ductility and better corrosion resistance of the finished product.

According to the invention, the claimed alloy is used as a foamed core material in aluminum foam sandwiches.

In the process of producing the metal foam from the claimed alloy, a metal powder mixture for the alloy AlMg4 (± 1) Si8 (± 1) is first prepared and made into a foamable semifinished product compacted and then foamed this semifinished product by known means.

In the method according to the invention for producing the core material from the claimed alloy, a metal powder mixture for the alloy AlMg4 (± 1) Si8 (± 1) is first produced and compacted into a foamable core layer, after which this core layer is placed between two cover plates of a 6000 alloy and these Assembly is transferred to a solid metallic composite, then this composite is heated to a temperature slightly lower than the solidus temperature of the 6000 alloy and, upon reaching the desired thickness of the foamed core material, the foaming process is stopped by cooling below the solidus temperature of the core material.

The metal powder mixtures in the context of the invention mean mixtures of alloy powders, i. Powders of such materials that make up the proposed alloy, and in such proportions by weight of the individual components that lead to this alloy. It is irrelevant whether powder of the three alloying components individually or z. For example, powders already containing two alloy components to which the missing constituents are added can also be used.

In embodiments of the invention, it is therefore provided by way of example that a mixture of the individual alloy constituents is used as metal powder mixture for the alloy AlMg4 (± 1) Si8 (t1), in particular in the composition 50 wt.% AlMg8, 8 wt.% Si and 41 wt % Al or in elemental composition 88 wt% Al, 4 wt% Mg and 8 wt% Si. Another embodiment provides a metal powder mixture of 8% by weight of the two-component alloy powder AlMg50, 8% by weight of Si and 84% by weight of Al.

The use of an alloy powder mixture has the advantage that the unwanted burnup of the Mg content in the production and in the foaming process of the alloy according to the invention is prevented.

The optionally provided exclusion or removal of foreign gases (for example oxygen) and their compounds with the metal powders during the production of the foamable semifinished product or of the foamable core layer also prevents the unwanted burning off of the Mg fraction.

It has been found that the foaming process for the alloy according to the invention is successful both with and without blowing agent.

If, as in a further embodiment, a propellant is used, it is provided that the decomposition temperature of the propellant and the melting temperature of the metal powder mixture are as close as possible to each other, i. a few degrees below the decomposition temperature, so that a high-viscosity large amount of melt is available at the decomposition temperature. The use of a blowing agent has the advantage that the foaming process is easy to control, in particular via the temperature, and thus runs very cleanly.

The invention is illustrated in the following exemplary embodiments.

Fig. 1 bis 3:

die Porengrößenverteilung der bekannten Legierungen AlMg6Si6 und AlSi6 im Vergleich zur erfindungsgemäßen Legierung AlMg4Si8 entsprechend;

Fig. 4 :

die gemessene Expansion bei verschiedenen Heizleistungen für die erfindungsgemäße Legierung und die bekannten Legierungen AlMg6Si6 und AlSi6;

Fig. 5:

Schaumqualität der Legierung AlMg4Si8 in Abhängigkeit der Konzentration der Legierungselemente Magnesium und Silizium.

The figures show:

1 to 3:

the pore size distribution of the known alloys AlMg6Si6 and AlSi6 compared to the alloy according to the invention AlMg4Si8 accordingly;

4:

the measured expansion at different heating powers for the alloy according to the invention and the known alloys AlMg6Si6 and AlSi6;

Fig. 5:

Foam quality of the alloy AlMg4Si8 as a function of the concentration of the alloying elements magnesium and silicon.

1st example

For the production of a cylindrical component of aluminum foam of the alloy according to the invention, first a powder mixture of 1 wt.% TiH ₂ , 8 wt.% Si, 4 wt.% Mg and 87 wt.% Al is produced. This is then uniaxially compressed at a temperature of 400 ° C, a pressing pressure of 195 MPa and 300 s pressing time to a tablet-shaped semi-finished product, which is then heated in a cylindrical steel sheet mold until the metal powder mixture is completely melted. During this process, the alloy AlMg4Si8 forms from the individual metal powders. The foaming process takes place in a known manner by the decomposition of the blowing agent TiH ₂ , whereby gas bubbles are formed in the semifinished product. Once the foam has filled the cylindrical sheet steel mold, it is removed from the oven. The foaming process stops by cooling the mold below the solidus temperature of the melt.

The cylindrical AlMg4Si8 alloy component has a low density and a homogeneous pore structure as well as a good corrosion resistance and high ductility.

2nd example

For the production of an aluminum foam sandwich, first a metal powder mixture of 50% by weight of the aluminum alloy AIMgB, 8% by weight of Si and 41% by weight of aluminum is produced and then compacted to form a core layer. This core situation will be discussed in a next step Cover plates of a 6000 series hardenable alloy converted into a solid, metallic composite. This can be done by way of example by means of roll-plating or another known method. This composite is then heated until a slightly lower temperature, here 590 ° C, than the solidus temperature of the cover plates, which is at about 600 ° C, is reached, and thereby the foaming process starts. During foaming, the aluminum alloy AlMg4Si8 forms in the foam core layer.

Upon reaching the desired foam layer thickness of the foaming process is stopped by cooling below the solidus temperature of the foam core alloy, for example, to a temperature between 555 ° C and 560 ° C. Now, if required, a heat treatment of the produced aluminum foam sandwich can take place directly afterwards or at a later time.

Also, these aluminum foam sandwiches have a high degree of expansion of the foam core layer as well as good mechanical properties and good corrosion resistance.

The good quality of the metal foam produced from the alloy according to the invention is now to be shown on the basis of the two parameters pore size distribution and achieved expansion height in comparison to the known alloys AlSi6 and AlMg6Si6.

In the Figure 1 to 3 The pore size distributions for the materials AlSi6 and AlMg6Si6 as well as the alloy AlMg4Si8 according to the invention are shown as a result of a digital image analysis in bar graphs. Compared to the magnesium-containing foam samples, the magnesium-free AlSi6 alloy sample has a coarser pore structure. Since the difference is difficult to assess with the naked eye, the individual pore cross sections were measured and in size classes of 2 mm ² Sorted by width. In the bar charts of the FIGS. 1 to 3 Now the difference between the pore structures becomes clear. While the magnesium-containing alloys show an upper limit for the pore size with relatively sharp delimitation at around 20 mm ² , the pore size distribution of the AlSi6 alloy runs rather flat to higher pore sizes around 60 mm ² and there is no sharp upper limit.

In the in Fig. 1 to 3 For the pore size distribution, the bar graphs shown are only slightly worse for AlMg6Si6 than for AIMg4Si8, but very different for AlSi6.

In the Fig. 4 shown expansion for a mean heating rate of 2.6 K / s and 1.2 K / s is very similar for AlSi6 and the alloy according to the invention, however, a lower expansion has been measured for AiMg6Si6.

As already mentioned above, it has been found that it is advantageous for a good quality - namely, a high expansion and a fine-pored structure - of the metal foam, if a sufficient amount of melt for the inclusion of the released gas is generated at a constant temperature, since the propellant decomposition without simultaneous increase in temperature is very slow and thus gas losses are avoided by channels formed during melting. However, this must not be too large, since the remaining unmelted components of the melt due to the high viscosity in the 2-phase region of the alloy avoid unwanted effects (drainage, foam collapse). In practice, it has been shown that in binary AlSi alloys, the amount of isothermally produced melt is about 50%.

In the case of the AlMg4Si8 alloy according to the invention, it is now possible to produce this fraction through the ternary eutectic melt ↔ Al + Mg 2 Si + Si, resulting in both a fine pore structure and a high pore structure Expansion - and thus to a better foam quality compared to the prior art according to known alloys - leads.

In FIG. 5 is schematically the foam quality, which results from the expansion and the pore size distribution, shown as a function of the concentration of the alloying elements magnesium and silicon. When using the alloy AlMg4Si8 the foam quality shows a maximum. Even slight deviations from the composition of the alloy according to the invention lead to a noticeable loss of foam quality due to a decrease in the expansion and / or coarsening of the pore structure.

Claims (11)

1. A metal foam based on an aluminium alloy, characterised in that this aluminium alloy, given in wt. %, is AlMg4(±1)Si8(±1) and the metal foam comprises Al, Mg, Si and impurities which are formed as a result of the production process, or comprises Al, Mg, Si as well as the metal components of a propellant and impurities which are formed as a result of the production process.
2. A use of the alloy according to claim 1 as a foamed core material for the production of aluminium foam sandwiches.
3. A method for the production of the metal foam formed of an alloy according to claim 1, wherein

   - a metal powder mixture is first produced for the alloy AlMg4(±1)Si8(±1) and is compressed to form a foamable semi-finished product and

   - is then foamed by known means.
4. A method for producing the core material according to either claim 1 or claim 2, wherein

   - a metal powder mixture is first produced for the alloy AlMg4(±1)Si8(±1) and is compressed to form a foamable core layer,

   - this core layer is arranged between two cover layers of a 6000 alloy and this structure is converted into a solid metal composite,

   - this composite is then heated to a temperature which is slightly below the solidus temperature of the 6000 alloy for the foaming process, and

   - once the desired thickness of the foamed core material has been achieved the foaming process is stopped by lowering the temperature below the solidus temperature of the core material.
5. The method according to either claim 3 or claim 4, characterised in that a mixture of the alloy constituents is used as the metal powder mixture.
6. The method according to claim 5, characterised in that the metal powder mixture comprises the following alloy constituents: 50 wt. % AlMg8, 8 wt. % Si and 41 wt. % Al.
7. The method according to claim 5, characterised in that the metal powder mixture comprises the alloy constituents in the elemental composition 88 wt. % Al, 4 wt. % Mg and 8 wt. % Si.
8. The method according to claim 5, characterised in that the metal powder mixture is formed of 8 wt. % of the two-component alloy powder AlMg50, 8 wt. % Si and 84 wt. % Al.
9. The method according to claim 3, characterised in that foreign gases and compounds thereof are eliminated or removed with the metal powders during the production of the foamable semi-finished product or the foamable core layer.
10. The method according to either claim 3 or claim 4, characterised in that a propellant is used for the foaming process.
11. The method according to claim 10, characterised in that the melting point of the metal powder mixture is set a few degrees below the decomposition point of the propellant.

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