EP1915226B1 - Process for the powder metallurgy production of metal foam and of parts made from metal foam - Google Patents

Abstract

A method for a powder-metallurgical production of metal foamed material and of parts made of metal foamed material includes mixing a pulverulent metallic material including at least one of a metal and a metal alloy; pressing, under mechanical pressure, the mixed pulverulent metallic material so as to form a dimensionally stable semi-finished product; placing the semi-finished product into a chamber that is configured to be sealed pressure-tight; sealing the chamber; heating the semi-finished product to a melting or solidus temperature of the pulverulent metallic material; once the melting or solidus temperature has been reached, reducing the pressure in the chamber from an initial pressure to a final pressure so that the semi-finished product foams so as to form a metal foam; and lowering the temperature of the metal foam so as to solidify the metal foam.

Description

The invention relates to a method for powder metallurgy production of metal foam and of parts made of metal foam. Metal foam is also commonly called metal foam.

Aqueous solutions, plastics or glass can be foamed. There have been efforts in recent decades repeatedly to also foam metals and produce novel foams, which have a new property spectrum due to the combination of the typical foam morphology with the known benefits of metallic materials; Metal stands for elasticity, strength and temperature resistance; Foam stands for low weight, cushioning, high porosity and a large specific surface area.

Metal foam is a novel material with specifically introduced pore structure, it is non-flammable and has a high strength. Foams made of metal are airy materials that are light, stiff, but flexible and absorb a lot of energy in the event of a crash. Metal foam can also fulfill a wide range of other technical tasks and is particularly suitable for applications as thermal insulation, noise and vibration damping or as a compression element.

Metal foams can be up to 85 percent air and only 15 percent metal, which makes them very light. They look like conventional plastic foams, but are much firmer. The manufacturing processes were too expensive, too expensive and too difficult to control until a few years ago, and the results were therefore rarely reproducible. But there are now melting and powder metallurgical processes that promise a high quality of the foamed metal. For the production of metal foams, various methods are known and used. 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 harmful substances to the metallic composite.

A melt metallurgical process is used, for example, in EP 1 288 320 A2 described by gas bubbles are introduced into a melt. For this purpose, at least one gas introduction tube with a defined gas outlet cross section protrudes into the melt through which individual bubbles are blown into the melt. The size of the bubbles is controlled by the adjustment of the Einströmparameter of the gas.

In the 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 is 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 metallurgical vessel with a foamable molten metal. Bubbles are formed in the regions of the protruding tube ends, with juxtaposition of regions of the bubble surfaces and formation of particles containing intermediate walls a continuous foam formation is formed.

A disadvantage of these melt metallurgical processes is that a molten metal in the pure state can not be foamed. For the purpose of achieving a foamability, before carrying out the foaming, 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). Only the metal foam accumulated on the melt surface is free-flowing. 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 a created object. Furthermore, a part of the bubbles formed or the dissolved gas during the solidification of a melt can escape from this, so that an inclusion of the released gas in the melt does not take place and consequently the porosity of the objects created by this method is low. In addition, expensive devices are required for introducing the gas bubbles into the melt.

A powder metallurgical process for producing porous metal bodies is described in US Pat DE 101 15 230 C2 and DE 40 18 360 CI, in which a mixture containing a powdered metallic material containing at least one metal and / or a metal alloy and a gas-releasing propellant-containing powder is compacted into a semi-finished 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 the WO 2005/011901 A1 It is proposed that for the production of metal parts with internal porosity first a foamable semi-finished product consisting of metal and at least a high temperature gas evolving propellant, wherein the metal forms a substantially closed matrix in which propellant particles are incorporated. An increased quality of a created metal foam body is to be achieved with a semi-finished product, in which the propellant particles einschießende metal matrix is formed by diffusion and / or pressure-welding of metal particles. In a first step, metal particles and at least one at elevated temperature gas (e) donating agent, so-called blowing agents, mixed, whereupon in a second step, the mixture formed under elevated pressure and elevated temperature to a semi-finished part and this while maintaining the Pressurization below the decomposition or outgassing temperature of the propellant is allowed to cool or cooled. In a third step, a heating of the semi-finished part on the decomposition temperature of the blowing agent and in forming an internal porosity, a shaping of the semi-finished product to a metal foam part.

Another method for producing metal foam bodies is in WO 2004/063406 A2 described. 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 the solidification of the same can cause the formation of a metal foam body low density. This effect can be usefully exploited according to the described solution for producing a metal foam body desired shape when the liquid metal is first placed in a mold and then allowed to solidify in this at least temporarily reduced ambient pressure. By a consolidation of the Melt at reduced ambient pressure, preferably 0.03 bar to 0.2 bar, it comes in the melt to form a plurality of gas bubbles, which are included due to the onset or progressive solidification of the melt in this and so created metal foam body a low density.

In the JP 01-127631 (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 blowing agent particles, such as nitride, hydride or oxide, release gas into the melt by thermal cracking. The gasified liquid metal is placed in a mold and held under reduced pressure, at 400 to 760 mmHg for a period of time.

With such powder metallurgy methods, metal foam bodies of high quality can be provided. However, these methods are extremely complicated with respect to the material used and the required devices, because it is necessary to use at least two powder components, namely metal particles and fuel particles. Also, the individual powder components must be intimately mixed prior to heating and the powder grains are sintered together, for example by hot isostatic pressing, in order to achieve in the produced metal foam body pores with a homogeneous distribution as possible. A further disadvantage is that gas escapes from the blowing agent particles even before the metal melts and accumulates in cracks, defects, etc. This results in different sized and unevenly distributed pores in the metal foam.
Pore size and volumetric expansion are difficult to control during the process.

The object of the invention is to provide a method for the production of metal foam and parts made of metal foam, the easy to perform without the use of blowing agents and without expensive devices, the trapped pores are as small as possible pores, have a nearly the same volume and a homogeneous distribution. The metal foam parts produced by the process according to the invention should have a high dimensional stability.

This object is achieved by a method having the features of claim 1 by a powdered metallic material containing at least one metal and / or a metal alloy, mixed without the use of propellants and then under mechanical pressure and a temperature of up to 400 C. is pressed to form a dimensionally stable semifinished product. This semi-finished product is placed in a pressure-tight sealable chamber, which is then sealed pressure-tight and the semifinished product is heated at the selected initial pressure to the melting or solidus temperature of the powdered metallic material. After reaching the melting or solidus temperature of the powdered metallic material, the pressure in the chamber is reduced to a selected final pressure. The semifinished product foams up without the use of blowing agents and the resulting metal foam solidifies during the subsequent lowering of the temperature. The lowering of the temperature takes place after the beginning of the pressure reduction according to a predetermined gradient, wherein the selected final pressure is always achieved before the solidification of the powdery metallic material.

It has proved to be particularly advantageous that a gas pressure up to approximately 50 bar is generated before or during the heating of the semifinished product in the closed chamber. After reaching the melting or solidus temperature of the pulverulent metallic material, the pressure in the closed chamber is reduced from the initial pressure to a predetermined gradient down to the final pressure of 1 bar. Another alternative is that the heating of the semifinished product takes place in the closed chamber at an initial pressure of about 1 bar and after reaching the melting or solidus temperature of the powdered metallic material, the pressure in the closed chamber is reduced to a final pressure of about 0.1 to 0.01 bar after a predetermined gradient. But it is also possible to realize the pressure reduction after foaming to other end pressures, for example, from an initial pressure of up to 50 bar to a final pressure of> 1 bar or even to <1 bar.

In the closed chamber advantageously a certain gas atmosphere can be created, for example an oxygen atmosphere or an atmosphere of moist air.

To produce the dimensionally stable semifinished product, the pulverulent metallic material is preferably compacted at a gas pressure of between 1 and 50 bar and a mechanical pressure of 200-400 MPa and a temperature of up to 400 ° C.

It is advantageous if the pulverulent metallic material is pretreated before being compacted into the semifinished product by modifying the surface of the individual granules of the pulverulent metallic material, for example by oxidizing or moistening.

With the method according to the invention, dimensionally stable metal foam bodies can also be produced simply if, instead of any pressure-tight chamber, a pressure-tight sealable molding tool, which has the shape of the metal foam body to be produced, is used.

A reservoir provided in the molding tool ensures that the metal foam, which is excessively expanded by the foaming of the metal, can escape from the molding tool through an opening to the reservoir. This also ensures that the molding tool is completely filled with the metal foam. With the reduction of pressure is also the Lowered temperature, so that the metal foam solidifies in the mold and thereby assumes the shape of the molding tool. After solidification of the metal foam, the metal foam body can be removed from the molding tool.

Further advantageous embodiments of the invention can be taken from the subclaims.

The advantages of the method according to the invention are, in particular, that it is possible to produce metal foam or body made of metal foam, without complicated devices for introducing gas bubbles into the melt or the use of blowing agents, in a simple manner. A further advantage is that with the method according to the invention low-density metal foam can be produced in which the pores have small dimensions (volumes), are distributed almost uniformly and homogeneously throughout the metal foam. An additional advantage is that the pore size and the volume expansion can be adjusted within certain limits very easily and precisely by adjustable different pressure differences between the initial and final pressure or during the process, wherein there is a direct relationship between the pore size and the volume expansion. Ie. the pore size and the volume expansion can be predetermined by observing certain limit values by setting the initial pressure and the final pressure. But it is also possible that when observing the process, this can be terminated at any time upon reaching a desired pore size or volume expansion.

If the foaming of the semifinished product from the pulverulent metallic material does not take place in a simple chamber but in a molding tool, dimensionally stable metal foam bodies can be produced in a simple manner.

The invention will be explained in more detail below with reference to two selected exemplary embodiments:

In the first preferred process, a metal foam is made without the use of additional gas-imparting blowing agents. For this purpose, in a first process step aluminum powder (99.7) with an average particle size of about 20 microns in a metal cylinder at a gas pressure of 1 bar and at a mechanical pressure of 300 MPa and at a temperature of about 400 ° C over a Period of 15 min to a semi-finished uni-axially compacted.
Thereafter, this semi-finished product is placed in a pressure-tight chamber and heated under an air atmosphere at an initial pressure p ₁ = 10 bar to a temperature of about 700 ° C, which is thus slightly above the melting temperature of the aluminum of about 660 ° C. If this temperature is maintained for a sufficiently long time, the semifinished product melts. As soon as the semifinished product has completely melted, the gas pressure in the chamber is reduced from the initial pressure p ₁ = 10 bar to the final pressure p ₂ = 1 bar with a gradient of 0.2 bar / s, so that the gas enclosed in the semifinished product, in the same ratio as the gas pressure in the chamber is reduced, expands and thus causes the sample within about 45 s to foam. The average pore size is about 2 mm. Finally, the temperature in the chamber is reduced by about 5 K / s to below the melting temperature of the aluminum, so that the liquid aluminum foam solidifies and thus the aluminum foam is solid.

In a further comparative example, a process is illustrated by which an aluminum foam is made using small amounts of gas donating propellants.

In a first process step, powder of AlSi6Cu4 with an average particle size of about 20 microns with 0.5 wt.% TiH ₂ , which has an average particle size of about 10 microns, homogeneously mixed. This mixture is uni-axially compacted in a metal cylinder at a gas pressure of 1 bar and at a mechanical pressure of 300 MPa and at a temperature of about 400 ° C over a period of about 15 minutes to form a semifinished product. Thereafter, this semi-finished product is placed in a pressure-tight chamber and heated under an air atmosphere at an initial pressure of 8 bar to a temperature of about 550 ° C, which is thus slightly above the solidus temperature of AlSi6Cu4 of about 516 ° C. Already at temperatures above 400 ° C, the propellant begins to release hydrogen. The gas released and trapped in the molten aluminum of the semifinished product, due to the external pressure, forms very small pores having an average diameter of less than 0.1 mm. As soon as the semifinished product has completely melted, the gas pressure in the chamber is reduced from the initial pressure p ₁ = 8 bar by approx. 3 bar to a final pressure p ₂ = 5 bar with a gradient of 0.2 bar / s. The gas enclosed in the semi-finished product causes the sample to foam within 15 seconds. After the AlSi6Cu4 foam has reached the specified volume, the temperature is reduced by about 5 K / s to below the solidus temperature of AlSi6Cu4, so that the liquid AlSi6Cu4 foam solidifies and thus the foam solidifies.

An AlSi6Cu4 foam produced by this comparison method has pores which are homogeneously distributed in the metal foam, round and small, the average pore size being about 0.5 mm. The size of the pores can be easily adjusted by the chosen pressure difference between initial pressure and final pressure (Δp = p ₁ -p ₂ ) over two orders of magnitude from about 0.1 mm to about 10 mm diameter.

Claims (19)

1. A method for the powder-metallurgical production of metal foamed material and of parts made of metal foamed material, in which a pulverulent metallic material containing at least one metal and/or a metal alloy is mixed and pressed to form a dimensionally stable semi-finished product under mechanical pressure, characterized in that the semi-finished product is placed into a chamber that can be sealed pressure-tight, the chamber is subsequently sealed and then the semi-finished product is heated up to the melting or solidus temperature of the pulverulent metallic material and, once the melting or solidus temperature of the pulverulent metallic material has been reached, the pressure in the chamber is reduced from an initial pressure (p₁) to a final pressure (p₂), whereby the semi-finished product foams without the use of foaming agents and the metal foam thus formed solidifies during the subsequent lowering of the temperature.
2. The method according to claim 1, characterized in that the pulverulent metallic material is pretreated in that the surface of the individual powder grains is modified, for instance, through oxidation or moistening.
3. The method according to claim 1, characterized in that the powder grains of the pulverulent metallic material has dimensions that average about 1 µm to 100 µm on the.
4. The method according to claim 1, characterized in that the semi-finished product is compacted at a gas pressure between 1 bar and 50 bar as well as at a mechanical pressure ranging from 200 MPa to 400 MPa at a temperature of less than 400°C.
5. The method according to claim 1, characterized in that the semi-finished product is pretreated in that the surface is modified, for instance, through oxidation, electrolytic oxidation or moistening.
6. The method according to claim 1, characterized in that a defined gas atmosphere prevails in the sealed chamber.
7. The method according to claim 6, characterized in that an oxygen atmosphere prevails in the sealed chamber.
8. The method according to claim 6, characterized in that an atmosphere consisting of moist air prevails in the sealed chamber.
9. The method according to claim 1, characterized in that an initial pressure (p₁) of up to approximately 50 bar is generated in the sealed chamber before or while the semi-finished product is being heated up.
10. The method according to claim 1, characterized in that the semi-finished product is heat up in the sealed chamber at an initial pressure (p₁) of about 1 bar.
11. The method according to claim 9, characterized in that, once the melting or solidus temperature of the pulverulent metallic material has been reached, the pressure in the sealed chamber is reduced according to a prescribed gradient from the initial pressure (p₁) to the final pressure (p₂) of about 1 bar.
12. The method according to claim 1 or 10, characterized in that, once the melting or solidus temperature of the pulverulent metallic material has been reached, the pressure in the sealed chamber is reduced according to a prescribed gradient from the initial pressure (p₁) to the final pressure (p₂) of about 0.1 bar to 0.01 bar.
13. The method according to claim 1 and 9 or 10, characterized in that the pressure in the sealed chamber is reduced from the initial pressure (p₁) to the final pressure (p₂) within a time span of about 1 second to 1000 seconds.
14. The method according to claims 1, 10, 12 and 13, characterized in that the temperature in the chamber is only lowered after the beginning of the pressure reduction according to a prescribed gradient, whereby the solidification temperature of the pulverulent metallic material is only reached after the final pressure (p₂) has been reached.
15. The method according to claim 1, characterized in that the size of the pores in the metal foam can be systematically set within a range from approximately 0.1 mm to approximately 10 mm by selecting the pressure differential (Δp = p₁ - p₂) between the initial pressure (p₁) and the final pressure (p₂).
16. The method according to claims 1 and 15, characterized in that the increase of the pore size in the metal foam can be terminated at any time by terminating the pressure reduction and by subsequently lowering the temperature of the metal foam to below the solidification temperature of the pulverulent metallic material for example, once the desired pore size or volume expansion has been reached.
17. The method according to claim 1, characterized in that the volume expansion of the metal foam can be systematically set to about ten times the initial volume by selecting the pressure differential (Δp = p₁ - p₂) between the initial pressure (p₁) and the final pressure (p₂).
18. The method according to claims 1 and 17, characterized in that the volume expansion of the metal foam can be terminated by terminating the pressure reduction and by subsequently lowering the temperature of the metal foam to below the solidification temperature of the pulverulent metallic material for example, once a prescribed volume has been reached.
19. The method according to claim 1, characterized in that a dimensionally stable metal foam object can be produced.