WO2009144563A2

WO2009144563A2 - Methods for obtaining an open-pore metal foam, composite material and open-pore metal foam

Info

Publication number: WO2009144563A2
Application number: PCT/IB2009/005726
Authority: WO
Inventors: Enrique Mariano Castrodeza; Carlo Mapelli
Original assignee: Politecnico Di Milano
Priority date: 2008-05-29
Filing date: 2009-05-27
Publication date: 2009-12-03
Also published as: ITMO20080166A1; WO2009144563A8; WO2009144563A3

Abstract

A method for obtaining an open-pore metal foam (15), comprises the steps of : obtaining a molten metal material; - infiltrating said molten metal material into a grouping (1) of solid particles (2), wherein said solid particles have a transformation temperature from solid to liquid/gas that is not less than approximately 804°C at atmospheric pressure.

Description

Methods for obtaining an open-pore metal foam, composite material and open-pore metal foam

The invention relates to methods for obtaining an open-pore metal foam. The invention further relates to a composite material and an open-pore metal foam that are obtainable with at least one of said methods .

Open-pore metallic foams made of metals or alloys with a high melting point are known that are obtained through procedures that are rather different from one another.

Some of these methods use a polymeric foam, such as, for example, polyurethane (PU) foam, which is coated with a metal layer. Subsequently, the polymer foam is removed during thermal treatment. EP 0402738 discloses that the metal layer can be applied to the polymeric foam by chemical deposition of vapours containing metals (CVD - Chemical Vapour Deposition) . US 2007/0009401 discloses that the metal layer can be applied by impregnating the polymeric foam with a fluid containing metal powder, and drying the polymeric foam that is thus impregnated. The metal powder is then sintered, owing to which the polymeric foam is removed.

DE 10205021960 discloses a method for producing an open-pore metal foam wherein, instead of a polymeric foam, coating particles of polymeric foam, for example polystyrene (PS) , with metal powder is provided. The particles of polymeric foam are removed by pyrolysis and the metal powder is sintered. US 5976454 discloses a method for obtaining an open-cell sintered product wherein from a fluid mixture comprising sinterable metal material powder a fluid foam is produced through vaporisation of a vaporisable material found in the fluid mixture. The fluid foam obtained is subsequently dried to obtain an intermediate open-pore no longer fluid body that is subsequently sintered. The aforementioned methods do not enable an open-pore metal foam to be obtained directly from liquid metal. They are not therefore usable in a foundry, where metals or metal alloys are prepared in liquid state. Further, the shape, size and arrangement of the pores of the metal foam obtained are not controllable or reproducible.

Other known methods use a metal material in liquid state.

These known methods are based on investment casting technique. These methods, which form a mould made of refractory material in which to pour molten metal , are rather complex and long to follow. Further, such methods require dedicated plants and rather a large initial financial installation investment . US 6196307 discloses an open-pore metal foam made by expanding and solidifying a liquid metal saturated with a pressurised inert gas. Solidifying occurs when the saturated gas expands to atmospheric pressure.

With this method, as for those disclosed above, the shape, size and arrangement of the pores of the metal foam obtained is not controllable.

An object of the invention is to improve known methods for producing an open-pore metal foam.

Another object is to provide a method for obtaining an open- pore metal foam that enables the shape and size of the pores of the metal foam to be controlled and reproduced.

Still another object is to provide a method for obtaining an open-pore metal foam that can be used directly in the foundry . A further object is to provide a method for obtaining an open-pore metal foam that is rather simple and has relatively low costs.

Still a further object is to provide a method for obtaining an open-pore metal foam that does not require complex or costly plants to be actuated. Another object is to obtain an open-pore metal foam of metal material with a high melting point.

In a first aspect of the invention there is provided a method for obtaining an open-pore metal foam, comprising the steps of : obtaining a molten metal material; infiltrating said molten metal material into a grouping of solid particles, wherein said solid particles have a transformation temperature from solid to liquid/gas that is not less than approximately 804⁰C at atmospheric pressure .

Owing to this aspect of the invention it is possible to use a molten metal material prepared directly in the foundry. The infiltration of the molten metal material into the grouping of solid particles enables the steps of constructing a refractory mould, i.e. a mould that is resistant to temperatures that are at least equal to the melting point of the metal material, as occurs in the prior art, to be avoided. Owing to the temperature of the transformation from a solid to a liquid/gas that is not less than approximately 804⁰C at atmospheric pressure of the solid particles, it is possible to obtain a metal foam of metal material with a high melting point . In one embodiment, the solid particles have a substantially regular geometrical shape .

Owing to this embodiment, it is possible to control the shape of the pores of the obtained metal foam, as the shape of the pores depends on the shape of the solid particles used.

In a second aspect of the invention a composite material is provided comprising solid particles buried in a die of solid metal material, in which said solid particles have a transformation temperature from solid to liquid/gas that is not less than approximately 804⁰C at atmospheric pressure. In a third aspect of the invention there is provided an open-pore metal foam in which said pores have a substantially regular geometrical shape.

In a fourth aspect of the invention a method is provided for obtaining an open-pore metal foam comprising the steps of: obtaining a molten metal material; infiltrating said molten metal material in a solid open- cell structure, wherein said solid open-cell structure has a temperature of transformation from solid to liquid/gas that is not less than approximately 804⁰C at atmospheric pressure; preheating said solid open-cell structure to a temperature that is not less than a minimum preheating temperature, said minimum preheating temperature being substantially the same as the melting temperature of said molten metal material reduced by approximately 50⁰C.

Owing to this aspect of the invention, it is possible to obtain an open-pore metal foam directly in the foundry. The invention can be better understood and implemented with reference to the attached drawings that illustrate some embodiments thereof by way of non- limiting example, in which:

Figures 1 to 6 are schematic views of steps of a method for obtaining an open-pore metal foam,-

Figures 7 to 10 are schematic views of an alternative embodiment of the steps in Figures 1 to 4 of the method for obtaining an open-pore metal foam,-

Figure 11 is an enlarged schematic view of a section of an open-pore metal foam;

Figure 12 is an enlarged schematic view of a composite material that is obtainable as an intermediate process during the applying of the method for obtaining an open-pore metal foam,- Figure 13 is a table listing materials that are usable in the method according to the invention. With reference to Figures 1 to 6, preparing a bed or grouping 1 of solid particles 2 inside a container 3, for example a mould for metals, is provided.

The solid particles 2 can be arranged in the container 3 according to a preset order or can have a random arrangement .

The solid particles 2 can have a regular, or substantially- regular, for example spherical, shape. Further, the solid particles 2 can have an average size comprised between 1 mm and 8 mm. In particular, the solid particles 2 can be substantially the same as one another, not only in shape but also in size. The solid particles 2 can be insoluble in water. The solid particles 2 are made of a material having a temperature of transformation from solid to liquid/gas that is not less than approximately 804⁰C at atmospheric pressure .

In particular, the solid particles 2 can be, for example, grains of silica gel. As is known, the grains of silica gel are solid particles of amorphous silica obtained synthetically from the chemical reaction between sulphuric acid and sodium silicate. The grains of silica gel have great humidity adsorbing power and are commonly packaged in bags that are permeable to water and in this form they are used as drying agents to control the local humidity of products, such as, for example, food products, leather products and electronic products, which could be damaged by excessive humidity. The melting point of the silica gel lies within a temperature range around 1600⁰C.

The container 3 can be heated by a heating element 4, for example an inductor winding or an electric resistor that externally surrounds walls of the container 3. A block 5 of solid metal material is inserted through an opening 9 of the container 3 and arranged resting on the grouping 1. The metal material that forms the block 5 is a metal or a metal alloy having a lower melting point than the melting point of the material of which the solid particles 2 are made. In particular, the metal or the metal alloy may have a temperature of transformation from solid state to liquid state of not less than approximately 750⁰C.

The metal material and the solid particles 2 are chosen such that the solid particles 2 are physically and chemically inert with respect to the metal material when they are subjected to a temperature that is at least the same as the melting temperature T_f of the metal material . In other words, the solid particles 2 are physically and chemically inert with respect to the molten metal material . After the grouping 1 has been arranged in the container 3, the heating element 4 is activated in such a way as to provide heat to the container 3 until it reaches a process temperature T_p that is almost the same as the melting temperature T_f of the solid metal material of which the block 5 is formed.

In particular, the process temperature T_p reached by the molten metal material in the container 3 is approximately 100⁰C greater than the melting temperature Tf of the metal material . If the solid particles 2 are of silica gel, the metal material that forms the block 5 can be a metal or a metal alloy having a lowest melting point of approximately 1600⁰C. During heating, a part 7 of the block 1 that touches the solid particles 2 starts to melt and infiltrate through interstices 6 located between the solid particles 2 of the grouping 1.

On a further part 11 of the block 5, opposite the part 7 that has started to melt, a force is applied that is directed to a bottom 8 of the container 3 according to the arrow F shown in Figures 3 and 4. Owing to the force F exerted on the still solid further part 11, it is possible to maintain the solid particles 2 of the grouping 1 substantially stationary in relation to one another so as to maintain substantially unaltered the arrangement of the solid particles 2 in the container 3 during infiltration of the molten metal material. Further, the force F pushes the already molten metal material to the bottom 8 of the container 3 , so that the molten metal material reaches the interstices 6 located between the solid particles 2 in all the grouping 1.

Air in the interstices 6 can exit the opening 9 of the container 3 through passages provided between the block 5 and an internal side surface of the container 3. In addition, creating a certain degree of vacuum in the container 3 can be provided.

In this case, the opening 9 of the container 3 is closed substantially hermetically, for example by a cover, which is not shown, so as to insulate the inside of the container 3 from an environment that is outside the container 3. The air in the interstices 6 can be removed through a hole provided in the cover or through a further hole, which is not shown, provided in the container 3.

Alternatively, the container 3 can be arranged inside a chamber in which the vacuum is created. In one embodiment, creating a certain degree of vacuum can occur as an alternative to applying the force F. In fact, by creating a certain degree of vacuum in the container 3, the vacuum generated inside the container 3, with respect to an environment outside the container 3, enables the block 5 to remain in contact with the grouping 1, maintaining the solid particles 2 substantially stationary to one another during infiltration.

When the molten metal material has reached the bottom 8 by infiltration and has reached the interstices 6 between the solid particles 2 arranged on the bottom 8, the heating element 4 is deactivated so that it stops providing heat to the container 3.

The container 3 is cooled, for example in air. In this manner the molten metal material moves from liquid state to solid state.

When the molten metal material is solidified, in the container 3 a body 13 of composite material 12 is formed. The composite material 12 comprises a die 10 in which the solid particles 2 are buried. The die 10 comprises metal material with which the block 5 was initially made. The solid particles 2 inside the die 10 have an arrangement that is substantially the same as that of the grouping 1 initially arranged in the container 3. Subsequently, the body 13 of composite material 12 is extracted from the container 3, in particular it is removed from the mould (Figure 5) .

The body 13 has a shape that is substantially the same as that identified by internal walls of the container 3 and can be subjected to mechanical machining to be made into a desired shape. For example, surfaces of the body 13 can be levelled, turned or milled, or obtaining holes, slots, grooves, undercuts, in the body 13 can be provided. After the body 13 has been extracted from the container 3 and has possibly been machined, it is provided that the solid particles 2 are removed from the die 10 of solidified metal material, in particular via chemical reaction. The body 13 is brought into contact with a substance 14 that reacts chemically with the solid particles 2 but does not react with the metal material of the die 10. The substance 14 can be a solvent of the material with which the solid particles 2 are formed, in which the body 13 is immersed, as shown in Figure 6.

If the solid particles 2 are made of silica gel, the substance 14 can be a hydrofluoric acid (HF) or sodium hydroxide (NaOH) solution, in particular a highly concentrated solution. In this case, the metal material of the block 5 meets the following conditions: it has a melting temperature T_f that is lower than the melting point of the silica gel and a chemical resistance to hydrofluoric acid (HF) and/or to sodium hydroxide (NaOH), i.e. it is not attackable chemically by hydrofluoric acid (HF) or by sodium hydroxide (NaOH) . For example, the metal material of the block 5 can be copper (Cu) , silver (Ag) , lead (Pb) , gold (Au) or one of the alloys of the aforesaid metals, such as, in particular bronze, i.e. Cu-Sn, Cu-Al, Cu-Ni, Cu-Be alloy, brass that is Cu-Zn alloys, Pb-Sn alloy, Cu-Ag, Pb-Cu, or carbon steel, in particular non-alloy steel.

After the solid particles 2 have been removed from the body 13, an open-pore metal foam 15 is obtained. The shape, dimension and arrangement of the pores of the metal foam 15 depend on the shape, dimensions and initial arrangement of the solid particles 2 in the grouping 1. As it is possible to control the shape, the dimension and the arrangement of the solid particles 2, it is possible to obtain an open-pore metal foam 15 that is extremely homogenous and isotropic, having pores of an almost constant shape and dimension in the entire mass of the metal foam 15. Figure 11 shows schematically an enlarged section of an open-pore silver (Ag) metal foam 15 obtained according to the method disclosed above, using solid particles 2 of substantially spherical silica gel, having an average dimension of approximately 5 mm and a random arrangement of the solid particles 2 in the grouping 1. From Figure 11 it can be noted that the pores of the metal foam 15 have a substantially spherical shape. Pores 16, that have a diametrical plane on the section plane of Figure 11, are represented by circles having diameters that are substantially equal to one another. Pores having a diametrical plane outside the section plane of Figure 11 are shown by circles with a smaller diameter than the circles that represent the pores 16, as they correspond only to portions of pores and not to diametrical sections of pores of the metal foam 15.

A cell or pore 16 and a further cell or pore 17 next to the pore 16 are connected together. Between the pore 16 and the pore 17 an intercellular channel or space 18 is provided. The shape, the diameter and the length of the intercellular space 18 depends on the arrangement of the solid particles 2 in the container 3 and on the surface tension of the interface between the liquid metal material and the surface of the solid particles 2. This surface tension determines the wettability of the solid particles 2 by the liquid metal material .

It has been established experimentally that the wettability of grains of silica gel by numerous metal materials or metal alloys is generally partial. This enables the intercellular spaces 18 to be formed during infiltration of the molten metal material into the grouping 1 of solid particles 2 of silica gel. In fact, as shown in Figure 12, during infiltration of the molten metal material between adjacent solid particles 2a and 2b, the metal material forms a convex meniscus 19 between the two adjacent solid particles 2a and 2b, owing to the surface tension of the interface between the liquid metal material and the surface of the solid particles 2. The interstices 6 are not completely filled by the molten metal material during infiltration, this enabling the intercellular spaces 18 to be formed.

After the solid particles 2 have been removed, the intercellular space 18 between the two adjacent solid particles 2a and 2b is bound by two meniscuses 19 facing one another .

Owing to the partial wettability of the solid particles of silica gel, the open-pore metal foam obtained using grains of silica gel has an apparent density, defined as the ratio between the weight of the metal foam and the volume occupied thereby that is slightly lower than the apparent theoretically obtainable density.

If, for example, the solid particles 2 are substantially spherical and have dimensions that are substantially the same as one another, and if they are arranged in the grouping 1 in a random manner, a 65% fraction of the volume of the grouping 1 in the container 3 is occupied by the solid particles 2 and a 35% fraction is not occupied by the solid particles 2, i.e. it is occupied by air, the latter fraction defining the total volume of the interstices 6. The apparent density that is theoretically obtainable corresponds to a situation in which the liquid metal material completely fills the interstices 6. Experimentally, it has been established that the molten metal material occupies a fraction between approximately 25 and 30% of the volume of the grouping 1.

Similarly, if, for example, the solid particles 2 are substantially spherical and have dimensions that are substantially the same as one another and if they are packaged in the grouping 1 according to a closet packing arrangement that is face centred cubic (FCC) or hexagonal compact (HC) , 74% of the volume of the grouping 1 in the container 3 is occupied by the solid particles 2 and 26% is not occupied by the solid particles 2, i.e. it is occupied by air, this latter fraction defining the total volume of the interstices 6. In this case, it has been experimentally established that the molten metal material occupies approximately 20% of the volume of the grouping 1. Thus, the apparent density of the metal foam obtained is less than the theoretically obtainable density, whilst being very near the theoretical density value.

Before arranging the solid particles 2 in the container 3 treating the solid particles 2 to substantially eliminate a fluid located on the surface of the solid particles 2, for example a fluid adsorbed by the solid particles 2, can be provided . In particular, if the solid particles 2 are of silica gel, as the silica gel is a hydrophilic material, drying the solid particles 2 before proceeding to the infiltration of the molten metal material can be provided. In this case the solid particles 2 are thermally treated at a temperature comprised between approximately 100⁰C and approximately 130⁰C, to eliminate water that has possibly been adsorbed by the grains of silica gel. The grouping 1 of solid particles 2 can be dried directly in the container 3, or in another device that is different from the container 3.

Experimentally, the solid particles 2 of silica gel have demonstrated high chemical and physical stability at the process temperature T_p reached during infiltration, i.e. at a temperature approximately 100⁰C greater than the melting temperature T_f of the metal material. In fact the grains of silica gel at the process temperature T_p of the metal material do not decompose, do not melt, do not substantially undergo significant dimensional variations, do not bond chemically with the metal material, and enable the metal material to form the die 10 of the composite material 12. The composite material 12 can be defined as a mixture of solid particles in a metal solid.

Before infiltrating the grouping 1 of solid particles 2, preheating the grouping 1 to a temperature that is not less than a minimum preheating temperature T_Rmi_n can be provided. The minimum preheating temperature T_Rmin is substantially the same as the melting temperature T_f of the metal material reduced by approximately 5O⁰C. As the minimum preheating temperature T_Rmiri is generally greater than the temperature at which the grouping 1 is dried, drying and preheating can occur simultaneously, i.e. in the event that a fluid is found on the surface of the solid particles 2, for example a fluid adsorbed by the solid particles 2, this fluid is eliminated when the grouping 1 is taken from ambient temperature to the minimum preheating temperature T_Rmin. Preheating can occur directly in the container 3 , or in a device other than the container 3. Alternatively, drying can be provided before preheating. Figures 7 to 10 show an alternative embodiment of steps of the method in Figures 1 to 4.

In this embodiment arranging the block 5 of metal material on the bottom 8 of the container 3 is provided. The grouping 1 of solid particles 2 is positioned on the block 5. In the grouping 1 the solid particles are positioned according to a desired arrangement. The container 3 can be provided with a cover 20 that closes the opening 9.

Subsequently, activating the heating element 4 is provided until the metal material reaches the process temperature T_p, that is at least the same as the melting temperature of the metal material .

The cover 20 of the container 3 is removed (Figure 8) and on the grouping 1 a pressing element 21 is rested, for example a steel pressing unit, that applies the force F to the grouping 1 (Figures 9 and 10) directed from the opening 9 to the bottom 8 of the container 3.

The molten metal material penetrates the interstices 6 whilst the solid particles 2 descend inside the container 3 pushed by the force F. When the solid particles 2 have reached the bottom 8, the heating element 4 is deactivated to stop supplying heat to the metal material .

The weight of the block 5 can be calculated so that, at the end of the infiltration, the molten metal material has reached a thrust surface 22 of the pressing element 21. The subsequent steps are similar to those disclosed with reference to Figures 5 and 6.

In both embodiments, after removal of the solid particles 2 from the body 13 of composite material 12, thermally treating the obtained open-pore metal foam 15 is possible according to prior-art techniques of thermally treating metals or metal alloys. In both embodiments of the method disclosed above, the infiltration can be performed in a protective atmosphere of inert gas, for example argon Ar.

Both embodiments of the method disclosed above use solid metal material in the shape of a block 5.

In alternative embodiments, it is possible to introduce into the container 3 metal material that is already in liquid state instead of the block 5. Owing to this embodiment, it is possible to obtain an open- pore metal foam directly in a foundry using a liquid metal or a liquid metal alloy prepared for castings producing. In particular, the container 3 may comprise a mould in which the grouping 1 of solid particles 2 is arranged having a preset arrangement inside the mould. In this case, the liquid metal or liquid metal alloy is poured inside the mould according to known techniques for making castings, ingots or pigs, the mould being already largely occupied by the solid particles 2. The solid particles 2 must be maintained stationary with respect to one another when the liquid metal material is introduced into the mould such that the preset arrangement of the solid particles 2 inside the mould can define the desired arrangement of the pores of the metal foam to be obtained. It is possible, for example, to provide for the mould being substantially filled with solid particles 2, i.e. for the solid particles 2 to be closely packed together so that it is the walls of the mould that press against the solid particles 2 and maintain the solid particles in the reciprocal position when the liquid metal material is introduced into the mould and during infiltration.

In order to obtain a random arrangement of the closely packed solid particles 2 in the mould, vibrating the mould before infiltration can be provided. As the solid particles 2 have a temperature of transformation from solid to liquid/gas that is not less than approximately 804⁰C at atmospheric pressure, it is possible to use the solid particles 2 as a spacing element to obtain open-pore metal foams having a high melting temperature, i.e. having a relatively high melting point. In particular, the metal material can have a melting point that is not less than approximately 750⁰C.

In order to obtain open-pore metal foams made of metal materials that melt at a high temperature it is not sufficient to choose solid particles 2 only on the basis of the transformation temperature from solid to liquid/gas of the material with which the solid particles 2 are made. In fact, the solid particles 2 have to be chemically stable to the process temperature, i.e. they must not, for example, release gaseous substances. The solid particles 2 further have to be chemically inert in relation to the metal material at the process temperature, i.e. must not react with the molten metal material to bond chemically therewith. In addition, the solid particles 2 have to be physically and structurally stable to the process temperature, i.e. substantially they must not become deformed or collapse, for example when subjected to the force F. Further, the solid particles 2 have to meet a partial wettability condition with the liquid metal material. In fact, if the wettability of the solid particles 2 is too high, there exists a certain risk that the meniscuses 19, and thus the intercellular spaces 18, i.e. the open pores, do not form. If, on the other hand, the wettability is too low, the infiltration might not occur thoroughly, i.e. the liquid metal material might not reach all the interstices 6. The wettability of the solid particles 2 is further influenced by the surface regularity of the solid particles 2, i.e. by the presence of pores or impurities on the surface of the solid particles. Lastly, the solid particles 2 to have removed without the die 10 of metal material being substantially modified. Thus, if the solid particles 2 are removed chemically, the solid particles 2 have to react with a removing substance without the latter damaging the die 10 of metal material. The foregoing remarks illustrate that not all the solid materials with a granular shape can be used effectively as spacing materials in a metallic die to obtain an open-pore metal foam. By using silica gel grains it is possible to obtain an open- pore metal foam of metals or metal alloys with a high melting temperature, in particular having melting temperatures between approximately 750⁰C and approximately 1450⁰C. In addition to the silica gel other spacing materials in the shape of solid particles can be used. These materials are listed in the table in Figure 13. In the table there is further indicated the melting temperature of the spacing material and with which solvents the spacing material can be removed chemically from the die 10 of metal material.

By applying the method disclosed above and choosing as solid particles the spacing materials listed on the table, it is possible to obtain an open-pore metal foam made of a metal material chosen from a group comprising not only the metal materials indicated above with reference to the grains of silica gel, but also the following: titanium (Ti) and titanium alloys, for example alpha alloy, pseudo alpha, alpha-beta, beta, pseudo beta, Ti-Al, Ti-Al-Vn, stainless steel, nickel (Ni) and nickel alloys, such as, for example, nickel-chrome (Ni-Cr) alloy, nickel-chrome-molibdenum (Ni- Cr-Mo) alloy, nickel-chrome manganese (Ni-Cr-Mn) alloy, nickel-chrome-cobalt (Ni-Cr-Co) alloy.

In particular, the spacing material such as chromium oxide (Cr₂O₃) and alluminium oxide (Al₂O₃) can be used to obtain an open-pore metal foam made of titanium or of an alloy thereof .

In some cases, heating the substance 14 with which to remove the solid particles 2 can be provided. This is provided, for example, if the substance 14 is potassium permanganate (KmnO₄) for removing solid particles of chromium oxide (Cr₂O₃) , hydrogen sulphide (H₂S) for removing solid particles of alumina (Al₂O₃) , sulphuric acid (H₂SO₄) for removing solid particles of titanium dioxide (TiO₂) , hydrofluoric acid for removing solid particles of zirconium oxide (ZrO₂) . In general, the spacing materials indicated on the table in the shape of solid particles have an apparent density that is less than the density of the molten metal material. In one alternative embodiment, instead of the grouping 1 of solid particles 2 infiltrating a solid open-cell structure with liquid metal material with a solid open-cell structure can be provided.

The metal material can be chosen from those indicated above in the embodiments disclosed with reference to grouping 1. The solid open-cell structure is made of a material chosen between those indicated in the embodiments disclosed with reference to grouping 1 and indicated in the table in Figure 13. In particular, the solid open-cell structure is made of ceramic material, such as, for example, silicon carbide (SiC), mullite (3Al₂O₃.2SiO₂) , alumina (Al₂O₃), calcium silicates, or a combination of these materials. • The solid open-cell structure extends three-dimensionalIy and may have a ramifying shape.

The solid open-cell structure may be a ceramic foam. Solid open-celled structures of ceramic material are currently commercially available at relatively low costs. A solid open-cell structure that can be infiltrated according to the method disclosed above is, for example, a ceramic filter of known type that is used for metal materials in a foundry. The volume occupied by the cells in the solid open-cell structure is variable between approximately 67% and approximately 92% of the total volume of the solid open-cell structure, i.e. the ligaments that define the walls of the cells occupy a volume that is variable between approximately 8% and approximately 33%. The cells have an ellipsoid shape. The average between the axes of the equivalent ellipsoid, which roughly has the shape of the cell bounded by the ligaments of the solid open-cell structure, is variable between approximately 0.3 mm and approximately 15 mm. The thickness of the ligaments is comprised in the interval between approximately 0.3 mm and approximately 11 mm. The solid open-cell structure can be infiltrated by positioning the solid open-cell structure inside the container 3, similarly to what is disclosed with reference to Figures 1 to 6. The solid open-cell structure and the walls of the container 3 define a preform or mould into which to introduce the metal material which is to be melted or which has already been melted.

Alternatively, infiltrating the solid open-cell structure by inserting into the container 3 the block 5 and subsequently introducing the solid open-cell structure, similarly to what is disclosed with reference to Figures 7 to 10 can be provided .

When the molten metal material is solidified around the solid open-cell structure, in the container 3 a body of composite material is formed, in particular a metal-ceramic composite. The body of composite material comprises a metal die in which the solid open-cell structure is buried. The metal die is naturally formed of the metal material of which the block 5 was initially formed, in fact, the molten metal material does not react with the ceramic material of the solid open-cell structure during infiltration.

Similarly to the embodiments of the method that refer to the grouping 1, before infiltration of the solid open-cell structure there is provided preheating the solid open-cell structure to a temperature that is not less than a minimum preheating temperature T_Rmi_n, which is substantially the same as the melting temperature Tf of said metal material reduced by approximately 50⁰C.

After the body of composite material has been extracted from the container 3, it is provided that the solid open-cell structure is removed from the metal die, in particular by means of chemical reaction. The body is placed in contact with a substance that reacts chemically with the ceramic material of the solid open-cell structure, but does not react with the metal material of the metal die, similarly to what is disclosed with reference to Figure 6. The substance that removes the solid open-cell structure of the metal die is chosen from those indicated in table Figure 13 on the basis of the material from which the solid open- cell structure is formed. The shape, size and distribution of the pores of the metal foam obtained by using as a spacing material a solid open- cell structure depends on the morphology of the solid open- cell structure.

All the steps of the method disclosed with reference to grouping 1 of solid particles 2 and all the embodiments of this method can be applied similarly to the case of a solid open-pore structure .

Both by infiltrating a grouping of solid particles, and by infiltrating a solid open-cell structure of ceramic material, the metal foam obtained with the method disclosed above can be used as filters for fluids, in particular in the field of purifying water, electrodes in energy accumulators, active elements in heat exchangers, catalysts, decorative elements, structural elements in the automobile field, sailing and aeronautical field. The method disclosed above enables an open-pore metal foam to be obtained, in particular made of metals or metal alloys that have a high melting temperature, in a simple manner and at rather a low cost. In fact, complex apparatuses or costly materials are not necessary. Further, the method disclosed above is directly applicable in the foundry.

The foams produced are completely recyclable, being formed of metal or a metal alloy. Further, the method disclosed above enables the dimension and the morphology of the pores of the metal foam to be controlled.

Claims

1. Method for obtaining an open-pore metal foam (15), comprising the steps of: obtaining a molten metal material ; - infiltrating said molten metal material into a grouping (1) of solid particles (2), wherein said solid particles have a transformation temperature from solid to liquid/gas that is not less than approximately 804⁰C at atmospheric pressure.

2. Method according to claim 1, wherein, before said infiltrating, preheating said solid particles (2) to a temperature that is not less than a minimum preheating temperature (T_Rmi_n) is provided, said minimum preheating temperature (T_Rmi_n) being substantially the same as the melting temperature (Tf) of said molten metal material reduced by approximately 50⁰C.

3. Method according to claim 1, or 2 , said solid particles

(2) have an average dimension that is variable between approximately 1 mm and approximately 8 mm.

4. Method according to any one of claims 1 to 3 , wherein said solid particles (2) have a substantially regular geometrical shape.

5. Method according to claim 4, wherein said substantially regular geometrical shape comprises a spherical shape.

6. Method according to any one of claims 1 to 5, wherein said solid particles (2) have an apparent density that is less than said molten metal material .

7. Method according to any one of claims 1 to 6 , wherein said solid particles (2) are insoluble in water.

8. Method according to any one of claims 1 to 7, wherein said solid particles (2) are chosen from a grouping of solid materials comprising: silica gel, silicon carbide (SiC) , chromium oxide (Cr₂O₃) , zeolite, forsterite (MgSiO₄), olivine (Mg, Fe) ₂Si0₄, mullite (3Al₂O₃.2SiO₂) , alumina (Al₂O₃) , titanium dioxide (TiO₂) , silica (SiO₂) , calcium silicate, zirconium dioxide (ZrO₂) , magnesium oxide (MgO) .

9. Method according to any one of claims 1 to 8, wherein said metal material comprises a high-melting metal or a metal alloy.

10. Method according to claim 9, wherein said metal or high-melting metal alloy has a temperature of transformation from solid state to liquid state of not less than approximately 750⁰C.

11. Method according to any one of claims 1 to 10, wherein said metal material is chosen from a grouping of metals and metal alloys comprising: copper (Cu) and copper alloys, bronze (Cu-Sn, Cu-Al, Cu-Ni, Cu-Be) , brass (Cu- Zn) , gold (Au) , silver (Ag) , lead (Pb) , Pb-Sn alloy, Cu-Ag alloy, Pb-Cu alloy, titanium (Ti) and titanium alloys (alpha, pseudo alpha, alpha-beta, beta, pseudo beta) , Ti-Al alloy, Ti-Al-Vn alloy, non-alloy steel, stainless steel, nickel (Ni) and nickel alloys, Ni-Cr alloy, Ni-Cr-Mo alloy, Ni-Cr-Mn alloy, Ni-Cr-Co alloy.

12. Method according to any one of claims 1 to 11, wherein during said infiltrating applying pressure (F) to said grouping (1) is provided to obtain said solid particles

(2) substantially stationary in relation to one another .

13. Method according to any preceding claim, wherein said infiltrating occurs substantially simultaneously to said obtaining said molten metal material, said obtaining comprising melting a metal material in solid state.

14. Method according to any preceding claim, wherein after said infiltrating there is provided solidifying said molten metal material .

15. Method according to claim 14, wherein after said solidifying there is obtained a body (13) of composite material (12) comprising said solid particles (2) buried in a die (10) of said solidified metal material.

16. Method according to claim 14, or 15, wherein after said solidifying there is provided extracting said body (13) of composite material (12) from a mould (3) .

17. Method according to any one of claims 14 to 16, wherein after said solidifying there is provided actuating mechanical machining on said body (13) of composite material (12) .

18. Method according to any one of claims 14 to 17, wherein after said solidifying there is provided removing said solid particles (2) from said solidified metal material .

19. Method according to claim 18, wherein said removing comprises bringing said solid particles (2) into contact with a substance (14) chemically reacting with said solid particles (2) .

20. Method according to claim 19, wherein said substance

(14) comprises a solvent suitable for dissolving said solid particles (2) .

21. Method according to claim 19, or 20, wherein said substance (14) is chosen from a group comprising: hydrofluoric acid (HF) , hydrochloric acid (HCl) , sulphuric acid (H₂SO₄) , potassium permanganate (KMnO₄) solution, sodium chloride (NaCl) and lead oxide (PbO) solution, manganese dioxide (MnO₂) and potassium chlorate (KClO₃) , boric acid (H₃BO₃) , boric acid (H₃BO₃) and calcium chloride (CaCl₂) , potassium hydroxide

(KOH) , iron sulphate (FeSO₄) and ammonia (NH₃) , magnesium chloride (MgCl₂) , nitric acid (HNO₃) , sulphuric acid (H₂SO₄) , hydrogen sulphide (H₂S) , sodium hydroxide (NaOH) and potassium hydroxide (KOH) solution, molten borax (Na₂B₄O₇ 10 H₂O) , borax (Na₂B₄O₇ 10 H₂O) and alkali carbonate solution, potassium fluoride (KF) , hydrofluoric acid (HF) solution and potassium fluoride (KF) solution, molten alkali-metal sulphates, phosphoric acid (H₃PO₄) , hydrofluoric acid (HF) and ammonium fluoride (NH₄F) , ammonium fluoride (NH₄F) solution, alkali carbonate, potassium hydroxide (KOH) , sodium hydroxide (NaOH) .

22. Method according to any one of claim 19 to 21, wherein before said placing in contact heating said substance (14) is provided.

23. Method according to any preceding claim, and further comprising drying said solid particles (2) before said infiltrating, for substantially eliminating a fluid located on the surface of said solid particles (2) .

24. Method according to any preceding claim, wherein during said filtering there is provided applying a certain degree of vacuum to said grouping (1) .

25. Composite material comprising solid particles (2) buried in a die (10) of solid metal material, wherein said solid particles (2) have a transformation temperature from solid to liquid/gas that is not less than approximately 804⁰C at atmospheric pressure.

26. Composite material according to claim 25, wherein said solid particles (2) are not metallic.

27. Composite material according to claim 25, or 26, wherein said solid particles (2) have an average dimension that is variable between approximately 1 mm and approximately 8 mm.

28. Composite material according to any one of claims 25 to 27, wherein said solid particles (2) have a substantially regular geometrical shape.

29. Composite material according to claim 28, wherein said substantially regular geometrical shape comprises a spherical shape.

30. Composite material according to any one of claims 25 to

29, wherein between two solid particles (2a, 2b) that are adjacent to one another in said die (10) there is provided a region (18) devoid of said solid metal material .

31. Composite material according to any one of claims 25 to

30, wherein said solid particles (2) are chosen from a group of solid materials comprising: silica gel, silicon carbide (SiC) , chromium oxide (Cr₂O₃) , zeolite, forsterite (MgSiO₄), olivine (Mg, Fe) ₂Si0₄, mullite (3Al₂O₃.2SiO₂) , alumina (Al₂O₃), titanium dioxide (TiO₂), silica (SiO₂) , calcium silicate, zirconium dioxide (ZrO₂) , magnesium oxide (MgO) .

32. Composite material according to any one of claims 25 to 31, wherein said solid metal material is chosen from a grouping of metals and metal alloys comprising: copper (Cu) and copper alloys, bronze (Cu-Sn, Cu-Al, Cu-Ni, Cu-Be) , brass (Cu-Zn) , gold (Au) , silver (Ag) , lead (Pb) , Pb-Sn alloy, Cu-Ag alloy, Pb-Cu alloy, titanium (Ti) and titanium alloys (alpha, pseudo alpha, alpha- beta, beta, pseudo beta) , Ti-Al alloy, Ti-Al-Vn alloy, non-alloy steel, stainless steel, nickel (Ni) and nickel alloys, Ni-Cr alloy, Ni-Cr-Mo alloy, Ni-Cr-Mn alloy, Ni-Cr-Co alloy.

33. Open-pore metal foam wherein said pores have a substantially regular geometrical shape.

34. Metal foam according to claim 33, wherein said substantially regular geometrical shape comprises a spherical shape.

35. Metal foam according to claim 33, or 34, wherein between two adjacent pores (2a, 2b) of said metal foam there is provided a connecting region (18) that connects said adjacent pores and is devoid of metal material of said metal foam.

36. Metal foam according to any one of claims 33 to 35, and made of a metal material chosen from a group of metals and metal alloys comprising: copper (Cu) and copper alloys, bronze (Cu-Sn, Cu-Al, Cu-Ni, Cu-Be), brass (Cu- Zn) , gold (Au) , silver (Ag) , lead (Pb) , Pb-Sn alloy, Cu-Ag alloy, Pb-Cu alloy, titanium (Ti) and titanium alloys (alpha, pseudo alpha, alpha-beta, beta, pseudo beta), Ti-Al alloy, Ti-Al-Vn alloy, non-alloy steel, stainless steel, nickel (Ni) and nickel alloys, Ni-Cr alloy, Ni-Cr-Mo alloy, Ni-Cr-Mn alloy, Ni-Cr-Co alloy.

37. Method for obtaining an open-pore metal foam comprising the steps of : - obtaining a molten metal material; infiltrating said molten metal material in a solid open-cell structure, wherein said solid open-cell structure has a temperature of transformation from solid to liquid/gas that is not less than approximately 804⁰C at atmospheric pressure; preheating said solid open-cell structure to a temperature that is not less than a minimum preheating temperature (T_Rmin) , said minimum preheating temperature (T_Rmin) being substantially the same as the melting temperature (T_f) of said molten metal material reduced by approximately 50⁰C.

38. Method according to claim 37, wherein said solid open- cell structure is made from a material chosen from a group of solid materials comprising: silica gel, silicon carbide (SiC) , chromium oxide (Cr₂O₃) , zeolite, forsterite (MgSiO₄), olivine (Mg₇Fe)₂SiO₄, mullite (3Al₂O₃.2SiO₂) , alumina (Al₂O₃), titanium dioxide (TiO₂), silica (SiO₂) , calcium silicate, zirconium dioxide (ZrO₂) , magnesium oxide (MgO) .

39. Method according to claim 37, or 38, wherein said metal material comprises a metal or a high-melting metal alloy.

40. Method according to claim 39, wherein said metal or high-melting metal alloy has a temperature of transformation from solid state to liquid state of not less than approximately 750⁰C.

41. Method according to any one of claims 37 to 40, wherein said metal material is chosen from a group of metals and metal alloys comprising: copper (Cu) and copper alloys, bronze (Cu-Sn, Cu-Al, Cu-Ni, Cu-Be) , brass (Cu- Zn) , gold (Au) , silver (Ag) , lead (Pb) , Pb-Sn alloy, Cu-Ag alloy, Pb-Cu alloy, titanium (Ti) and titanium alloys (alpha, pseudo alpha, alpha-beta, beta, pseudo beta), Ti-Al alloy, Ti-Al-Vn alloy, non-alloy steel, stainless steel, nickel (Ni) and nickel alloys, Ni-Cr alloy, Ni-Cr-Mo alloy, Ni-Cr-Mn alloy, Ni-Cr-Co alloy.

42. Method according to any one of claims 37 to 41, wherein during said infiltrating there is provided applying pressure (F) to said solid open-cell structure to obtain an orientation of said solid open-cell structure that is substantially fixed with respect to walls of a container (3) .

43. Method according to any one of claims 37 to 42, wherein said infiltrating occurs substantially simultaneously to said obtaining said molten metal material, said obtaining comprising melting a metal material in solid state .

44. Method according to any one of claims 37 to 43, wherein after said infiltrating there is provided solidifying said molten metal material.

45. Method according to claim 44, wherein after said solidifying there is obtained a body of composite material comprising said solid open-cell structure buried in a die of said solidified metal material.

46. Method according to claim 44, or 45, wherein after said solidifying there is provided extracting said body of composite material from a mould.

47. Method according to any one of claims 44 to 46, wherein after said solidifying there is provided actuating mechanical machining on said body of composite material .

48. Method according to any one of claims 44 to 47, wherein after said solidifying there is provided removing said solid open-cell structure from said solidified metal material .

49. Method according to claim 48, wherein said removing comprises bringing said solid open-cell structure into contact with a substance chemically reacting with said solid open-cell structure.

50. Method according to claim 49, wherein said substance comprises a solvent suitable for dissolving said solid open-cell structure.

51. Method according to claim 49, or 50, wherein said substance is chosen from a group comprising: hydrofluoric acid (HF) , hydrochloric acid (HCl) , sulphuric acid (H₂SO₄) , potassium permanganate (KMnO₄) solution, sodium chloride (NaCl) solution and lead oxide (PbO) , manganese dioxide (MnO₂) potassium chlorate (KClO₃) solution, boric acid (H₃BO₃) , boric acid (H₃BO₃) and calcium chloride (CaCl₂) solution, potassium hydroxide (KOH) , iron sulphate (FeSO₄) and ammonia (NH₃) , magnesium chloride (MgCl₂) , nitric acid

(HNO₃) , sulphuric acid (H₂SO₄) , hydrogen sulphide (H₂S) , sodium hydroxide (NaOH) and potassium hydroxide (KOH) solution, melted borax (Na₂B₄O₇ 10 H₂O) , borax (Na₂B₄O₇ 10 H₂O) and alkali carbonate. solution, potassium fluoride (KF) , hydrofluoric acid (HF) and potassium fluoride (KF) solution, molten alkali-metal sulphate, phosphoric acid (H₃PO₄) , hydrofluoric acid (HF) and ammonium fluoride (NH₄F) , ammonium fluoride solution (NH₄F) , alkali carbonate, potassium hydroxide (KOH) , sodium hydroxide (NaOH) .

52. Method according to any one of claims 49 to 51, wherein before said contacting heating said substance is provided .

53. Method according to any one of claims 37 to 52, and further comprising drying said solid open-cell structure before said infiltrating, to eliminate substantially a fluid located on the surface of said solid open-cell structure.

54. Method according to any one of claims 37 to 53, wherein during said infiltrating applying a certain degree of vacuum to said solid open-cell structure is provided.

55. Method according to any one of claims 37 to 54, wherein the cells of said solid open-cell structure occupy a volume that is variable between approximately 67% and approximately 92% of the total volume of said solid open-cell structure.