EP1266973B1 - Method for producing a lightweight moulded body and moulded body from foamed metal - Google Patents

Description

The invention relates to a lightweight molded body of metal foam consisting of a metal matrix, in which particles are embedded and which encloses a plurality of substantially spherical and / or substantially ellipsoidal cavities.

Molded metal foam naturally have a low density and, due to the structure, have special mechanical material properties. For example, such bodies can be given large deformations with upsetting degrees of up to 70% and more when applying two- or three-dimensional compressive stresses. These materials with special properties are in the technological application, for example as an energy absorber in automotive technology and the like, can be used with advantage.

When using such moldings for selected functions with certain parameters, it is important to ensure the same and reproducible characteristics of the materials.

From the EP 483 184 B For example, there has been known a method for producing a particle-reinforced metal foam, in which cell-forming gas is introduced into a melt of metal with finely divided reinforcing agents, foamed metal composite is formed, and from the surface of the molten material, the accumulated foam is removed and allowed to solidify. However, this metal foam has bubbles with uncontrolled size or size distribution, resulting in a highly blurred property profile of the foam or molded body results as well as functionally uncertainties.

Out T. Miyoshi, et al., Aluminum Foam, ALPORAS: The Production Process, Properties and Applications, Mat. Res. Soc. Symp. Proc. Vol.121, 1998, p.133-137 is a method for producing a molded body made of metal foam, in which molten aluminum is filled into a mold and TiH _{2 is} supplied in the mold and stirred in the melt. Due to the thermal decomposition of the TiH ₂ powder hydrogen is released, which bubbles to form the metal foam in the Melt forms. Thus, a shaped body can be produced with a bimodal distribution of the cell sizes in the metal foam.

Another lightweight metal body according to EP 545 957 B1 and US 5,221,234 a plurality of closed and isolated, generally spherical pores having sizes in the range of 10 to 500 microns. Although such small pores with large differences in diameter can lend a metal body formed with aluminum in comparison with the solid material lower specific gravity, a density of less than 1.0 g / cm ³ and degree of compression of over 60% of the material under defined conditions are usually not achievable ,

For the production of various forms of light bodies formed from metal foam, a number of sequential ( US 5,281,251 . DE 43 26 982 C1 ) and / or continuously ( US 5,334,236 ) EP 544 291 A1 . DE 43 26 982 C1 . WO 91/03578 ) working methods and devices have been proposed with which quite fundamentally functional objects can be produced, however, the mechanical properties of the same can not be adjusted with an often required accuracy.

It is an object of the invention to provide a generic molding whose structure has a largely precise deformation behavior as a function of the applied, in particular multi-dimensional, Druckspannng.

This object of the invention, in particular with regard to a desired material behavior under mechanical stress, is achieved in that the metal foam of the molding has a monomodal distribution of the maximum longitudinal extent of the cavities in the range between 1.0 and 30.0 mm when viewed spatially.

The advantages of a so-shaped lightweight molded body made of metal foam are essentially due to the fact that, as already stated in the beginning, favorable conditions are achieved with regard to the knot formation of the walls of the gas bubbles by a monomodality. In a bimodal, poly- or multimodal distribution of the cavities size are in the wall node mostly thickening with possibly small and / or very small pores and Lunkerungänzen ago, which on the one hand the specific gravity of the foam body increases and increases the cost of metal to form the same, on the other hand can interfere with the division of the force components, whereby a Wandflächenknickung is not clearly determined under load.

The advantages according to the invention of the effect of the action mechanisms in the component distribution of the compressive forces can be enhanced if at least partially enclosing the cavities enclosing walls planar areas.

If, as may further be provided in a favorable manner, in spatial consideration of the metal foam, the ratio of the maximum longitudinal extents of two different cavities on average at least 20 pairs is less than 45, are largely narrow load ranges, in which a collapse of the foam cavities begins to reach ,

In this case, the accuracy of the transition from an elastic deformation in a plastic deformation of the material as a function of the compressive stress can be further increased if, viewed spatially of the metal foam, the ratio of the maximum longitudinal extent of two different cavities on average over at least 20 pairs less than 30, preferably less than 15 and in particular less than 5, is. These values refer to created cavities, ignoring solidification voids in the molding.

Equally important for a metal foam production and for the behavior of the molded article under mechanical stress is the composition and structure of the liquid metal and that or those of the boundary walls of the cavities.

If the particles are embedded uniformly distributed in the metal matrix for reinforcement, there is a high and isotropic reinforcement of the base metal with regard to the mechanical stress. It is also advantageous if adjoining cavities are completely separated from one another by the metal matrix. Individual cracks, which can be caused by mechanical stress during cooling, are not effective for compression loads.

Particularly light shaped articles can be produced per se according to the invention if the metal matrix consists of a light metal, preferably of aluminum or an aluminum alloy.

Furthermore, if the particles incorporated in the metal matrix have a size of 1 to 50 μm, preferably 3 to 20 μm, a particularly advantageous weight / property ratio can be achieved.

As reinforcement or reinforcement of the base metal for a foaming and solidification of the same or for a structure of kinkungsverstärkten bladder walls have inclusions of non-metallic particles, preferably SiC particles and / or Al ₂ O ₃ - particles and / or those of intermetallic phases as unexpected proved favorable.

It has proven to be particularly advantageous if the volume fraction of the incorporated particles in the metal matrix is between 10% by volume and 30% by volume, preferably between 15% by volume and 30% by volume.

The favorable weight / property ratio of a lightweight molded article of the type according to the invention can be increased if the density of the metal foam is less than 1.05 g / cm ³ , preferably less than 0.7 g / cm ³ , in particular less than 0.3 g / cm ³ , is.

A lightweight molded article according to the invention is produced by a method in which a molten metal having a particle formed by introducing gas or gas mixtures into this, at least partially introduced into a mold and its liquid phase is solidified in this and a flowable metal foam with a monomodal distribution of the dimension of the cavities and a proportionately maximum longitudinal extent of the same in the range between 1.0 and 30.0 mm made, placed in a Gießforrn or mold and compacted therein in substantially under all-round pressure, wherein the cavities enclosing particles containing Metal melt boundary walls at least partially planar areas are granted and the solidification heat of the melt is dissipated. Thus, the inner body structure is formed such that the material has substantially accurate mechanical characteristics.

The monomodal distribution of the dimensions of the cavities in the metal foam provides a prerequisite for a material behavior at certain stress states. The proportional maximum diameter of the cavities is important for the height the elastic limit of the material and the tolerable specific surface load at a Druckspannungsbeaufschlagung of the part.

To create at least partially planar areas in the boundary walls, a substantially all-sided, optionally low pressure load of the flowable foam is required, resulting in several advantages. Of particular importance, however, is that such that the boundary walls and their node areas in the foam material for a mechanical support or buckling load are set or formed low. As a result, it can be achieved that in the case of an upper limit of a defined stress limit buckling of the foam walls or collapse of the pores and energy absorption take place with high degrees of deformation or upsetting while the lightweight body is solidified.

Both for a monomodal distribution of the dimensions of the cavities which can be produced within narrow limits and for an accurate adjustment of the proportionately maximum diameter of the cavities in the foam material, it has proved to be particularly advantageous if the gas is introduced through at least one to form the monomodal distribution of the dimensions of the cavities in the melt projecting einragendes entry pipe is introduced with a small frontal area.

Manufacturing technology, but also in terms of product quality, it may be beneficial if the compression of the flowable metal foam in a mold with internal dimensions that correspond to the desired dimensions of the molding is performed.

On the basis of images and diagrams made in a material testing the invention will be explained in more detail.

• Fig. 1 Schnittbilder von erfindungsgemäßen leichtgewichtigen Formkörpern
• Fig. 2 graphische Darstellung des Zusammenhanges von Dichte und Stauchspannung von Formkörpern
• Fig. 3 graphische Darstellung des Stauchgrades in Abhängigkeit von der Stauchspannung von Formkörpern
• Fig. 4 Schnittbilder A,B,C von Knotenformen in der Schaumwandung Fig. 5 Draufsicht - Bilder A,B,C von Schaumkörpern mit unterschiedlicher Raumdichte
• Fig. 6 graphische Darstellung der mittleren lokalen Dichte eines erfindungsgemäßen und eines Vergleichs - Schaumkörpers

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• Fig. 1 Sectional views of lightweight moldings according to the invention
• Fig. 2 graphical representation of the relationship between density and compression stress of moldings
• Fig. 3 graphical representation of the degree of compression as a function of the compression stress of moldings
• Fig. 4 Cross-sectional views A, B, C of node shapes in the foam wall Fig. 5 Top view - pictures A, B, C of foam bodies with different volume density
• Fig. 6 graphical representation of the mean local density of a comparative and a foam body according to the invention

In Fig.1 , Image A and Image B, a cavitation formation in an Al molding according to the invention is shown in each case on the basis of a sectional image. In the case of monomodal distribution of the dimension, the largest longitudinal extents of cavities in the range between 20 and 12 mm were found in the body of image A, with the proportional maximum extent being 17.2 mm. Although a compression of the flowable metal foam of only about 3.2% was made, significantly planar area are formed in the cavities enclosing boundary walls.

Out Fig. 2 is the dependence of the compression stress of a shaped body of the In the development work, it was found that a monomodal distribution of the largest longitudinal extension of the cavities and an increasing equality of the same affects narrowing the range of dependency. In other words: If there is a monomodal distribution of the cavities in the foam body and if the cavities have a certain size within narrow limits, then the beginning of the deformation or collapse is an exact material value when the pressure is applied to the same. As a result, the behavior of a foam component can be calculated precisely or, for certain functions, the design and construction of the foam part can be defined.

The stress as a function of the compression deformation is in Fig. 3 compared with test results of three moldings. The structure of the lightweight moldings 1 and 2 with a density of 0.091 gcm ^-3 and 0.114 gcm ^-3 was according to the invention, the comparison body 3 had a bimodal distribution of the dimension of the cavities with material concentrations in the nodes of the foam walls. At the compression curves of the bodies 1 and 2, an extremely low hardening of the same is noted up to a degree of compression of about 70%. The comparison body 3 shows up to a compression ratio of about 45% a significant hardening of the material, which increases even further from this value. This indicates an effect of the bimodal distribution of the cavity dimensions.

Fig. 4 shows nodule shapes in the foam wall of lightweight bodies based on sectional images.

Figure A shows a sharp-edged node formation of the wall between three cavities. Such knots are prone early to cracks and breaks in the connection area.

Picture B shows a thickened wall node. This node formation leads to an increased specific weight and an unfavorable formation of the force components in a compression load of the body.

Figure C shows a node with wall parts, where both the thickness of the walls and the node mass with a view to a high compression set with less Solidification of the body are designed low at high compression ratios.

In Fig. 5 represented formed metal foam body without compression in plan view, wherein the gas was introduced in each case with different separation parameters for the bubbles durach projecting into the melt projecting entry pipes. A monomodal distribution of the respective dimensions of the gas bubbles can be seen. In this case, the body has a specific weight of 0.1 gcm ^-3 according to image A, those according to image B and image C have one of 0.2 gcm ^-3 and 0.4 gcm ^-3 .

Computer tomography data sets can be used to calculate local density values (density mapping). An averaging process for calculating the local density makes it possible to determine the material distribution between the averaging volumes. Diagrams of the calculated density values of investigations can give information about the homogeneity of a lightweight molded article.

Out Fig. 6 the relative frequency of the mean local density in a molded body according to the invention determined by a computed tomography method is designated 1 and can be taken from a comparison body 2. The mean local density of the body 1 has a narrow frequency maximum at about 0.22 gcm ^-3 , demonstrating a monomodal distribution of the dimension of the cavities and a narrow range of the proportionately maximum longitudinal extent thereof. On the other hand, the multimodal comparison body is characterized by a marked decline of the broad mean local density values.

Claims (11)

1. Lightweight moulded body made of metal foam, consisting of a metal matrix in which particles are embedded and which surrounds a plurality of substantially spherical and/or substantially ellipsoid cavities, characterised in that in a three-dimensional view the metal foam of the moulded body has a monomodal distribution of the proportionate maximum longitudinal extensions of the cavities in the range between 1.0 and 30.0 mm.
2. Moulded body according to Claim 1, characterised in that boundary walls at least partially enclosing the cavities have planar regions.
3. Moulded body according to Claim 1 or 2, characterised in that in a three-dimensional view of the metal foam the ratio of the maximum longitudinal extensions of two in each case different cavities on average over at least 20 pairs is less than 45.
4. Moulded body according to Claim 1 or 3, characterised in that in a three-dimensional view of the metal foam the ratio of the maximum longitudinal extensions of two in each case different cavities on average over at least 20 pairs is less than 30, preferably less than 15 and in particular less than 5.
5. Moulded body according to one of Claims 1 to 4, characterised in that the particles in the metal matrix are embedded in a uniform distribution.
6. Moulded body according to one of Claims 1 to 5, characterised in that cavities adjoining one another are completely separated from one another by the metal matrix.
7. Moulded body according to one of Claims 1 to 6, characterised in that the metal matrix consists of a light metal, preferably aluminium or an aluminium alloy.
8. Moulded body according to one of Claims 1 to 7, characterised in that the particles embedded in the metal matrix have a size of 1 to 50 µm, preferably 3 to 20 µm.
9. Moulded body according to one of Claims 1 to 8, characterised in that non-metallic particles, preferably SiC particles and/or Al₂O₃ particles and/or particles consisting of intermetallic phases, are embedded in the metal matrix.
10. Moulded body according to one of Claims 1 to 9, characterised in that the volume fraction of the embedded particles in the metal matrix is between 10% by volume and 50% by volume, preferably between 15% by volume and 30% by volume.
11. Moulded body according to one of Claims 1 to 10, characterised in that the density of the metal foam is less than 1.05 g/cm³, preferably less than 0.7 g/cm³, in particular less than 0.3 g/cm³.

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