Abstract:
A method for making foam structures suitable for use as mechanical energy absorbers, structural members, filters, catalyst carriers or the like. A composite rod comprising an outer shell and an inner core is formed of respective mixtures of powders. The mixture for the outer shell comprises a sinterable powdered structural material such as ceramics, metals, intermetallics, and a powdered binder such as paraffin, wax or polymer. The inner core comprises a powdered channel-forming filler material such as melamine or polymers, or soluble inorganic compounds or a metal that can differentially be removed from the structural material of the shell. The composite rod may be formed by extrusion. The composite rod is sectioned into a plurality of composite rod segments of predetermined length and a plurality of these segments is assembled in randomly oriented relationship to one another. The assemblage of rod segments is then consolidated, and the binder and filler are then removed, as by heating. The remaining structure of the outer shells, comprised of ceramic or metal, as the case may be, is then sintered to produce the foam structure. In certain embodiments, the material of the inner core may be removed by heating it in the course of heating the structure to perform the sintering step. In other embodiments, the binder and/or filler material may be removed by means of a suitable solvent.

Description:
The present invention relates to novel methods for producing cellular structures, referred to as foam structures, and to foam structures produced by such methods that are suitable for uses as absorbers of mechanical energy as, for example, in automobile components, and also as light weight structural elements in support systems, etc., 
     BACKGROUND OF THE INVENTION 
     There are a large variety of methods for producing metal and ceramic foams or similar porous metal structures starting from liquid or powdered metals [1]. Currently there are two ways for directly foaming metals. The first of them involves melting the Al matrix metal, adding reinforcing particles to the melt (5-20% SiC or Al 2 O 3 ) and injecting gas (air, nitrogen, argon) into the melt using a rotating impeller. The second technique for directly foaming melts is to add a foaming agent to the melt. The foaming agent decomposes under the influence of heat and releases gas, which then propels the foaming process [1-3]. Another method, which was developed some years ago in the Ukraine, exploits the fact that some liquid metals form a eutectic system with hydrogen gas. As the melt cools bubbles of hydrogen are released [4, 5]. 
     Metal and ceramic foams can also be fabricated using open porosity polymer foams as a starting point. The polymer foam is filled with a slurry of heat resistant material, e.g. a mixture of mullite, phenolic resin and calcium carbonate. After drying the polymer is removed and molten metal is cast into resulting open voids. After removal of the mold material (e.g. by water under high pressure) metallic foam is obtained, which is an exact image of the original polymer foam [1]. Polymer foams can also be used in a deposition technique. Metal is deposited on the polymer foam, then the polymer is removed by heating. 
     Another method for foam calls for casting around inorganic granules of hollow spheres of low density or by infiltrating such materials with a liquid melt [6]. Powder metallurgy methods [1, 7-8] include mixing powders with a foaming agent, compaction of the powder blend into a dense precursor material and foaming of the precursor material by heating it to its melting temperature. Foams can also be produced by preparing a slurry of metal or ceramic powder mixed with a foaming agent. The slurry becomes more viscous and starts to foam during drying in a mould at elevated temperature [1, 9-10]. 
     Most foaming techniques work well for lightweight low-temperature metals, predominantly aluminum and its alloys, but can not be used for fabrication of high-temperature metallic or ceramic foam. However, there is a need for a universal method, which could be applied to the fabrication of foams from any material—metals, ceramics, intermetallics, composites. The vast majority of existing techniques do not allow rigid control of cell shape and size. Thus there arises a wide variation of cell sizes, an uneven distribution of cells in the foam volume and, as a result, a wide scatter in mechanical characteristics. 
     REFERENCES 
     1. J. Banhart, “Production Methods for Metallic Foams”, Metal Foams/Fraunhofer USA Symposium “Metal Foam”, Stanton, Delaware, Oct. 7-8, 1997.Ed.: J. Banhart and H. Eifert.—Bremen: MIT-Verl., 1998, pp.3-11 
     2. J. Banhart, P. Weigand, “Powder Metallurgical Process for the Production of Metallic Foams”, Metal Foams/Fraunhofer USA Symposium “Metal Foam”, Stanton, Del., Oct. 7-8, 1997.Ed.: J. Banhart and H. Eifert.—Bremen: MIT-Verl., 1998, pp.13-22 
     3. J. Wood, “Production and Applications of Continuously Cast, Foamed Aluminum” Metal Foams/Fraunhofer USA Symposium “Metal Foam”, Stanton, Del., Oct. 7-8, 1997.Ed.: J. Banhart and H. Eifert.—Bremen: MIT-Verl., 1998, pp.31-36 
     4. A. Pattnaik, S. C. Sanday, C. L. Vold, and H. I. Aaronson, “Microstructure of Gasar Porous Ingot”,  Materials Research Society Symposium Proceedings, Vol.  371,  Advance in Porous Materials , December 1994, p. 371-376T. 
     5. J. M. Wolla and V. Provenzano, “Mechanical Properties of Gasar Porous Copper”,  Materials Research Society Symposium Proceedings, Vol.  371,  Advances in Porous Materials , December 1994, p. 377-382. 
     6. W. Thiele, German Patent, 1933321, 1971 
     7. J. Baumeister, U.S. Pat. No. 5,151,246, 1992, German Patent 4018360, 1990 
     8. J. Baumeister, J. Banhart, M. Weber, German Patent DE 4401630, 1997 
     9. J. Drolet, Int. J. Powder Met., 13, 223, 1977 
     10. S. Kulkarni, P. Ramakrishnan, Int. J. Powder Met., 9, 41, 1973 
     OBJECTS AND ADVANTAGES OF THE INVENTION 
     It is an object of the present invention to provide a novel method using powdered materials for producing foam structures comprised of materials such as ceramics, metals, intermetallics and polymers. 
     It is a further object to provide such method to produce foam structures suitable for making structures usable as light weight, structural components, filters, catalyst carriers, heat exchangers, etc. 
     The methods of the present invention enable the production of novel foam structures with cells of predetermined and controllable size and distribution. 
     The methods of the present invention for making foam allow control of the final porosity (from a few volume percent to more than 95 vol. % and more), cell size and interchannel wall thickness (from a few microns to a few millimeters) with small tolerance. 
     An object of the present invention is to demonstrate a novel low cost near-net-shape fabrication technology, which allows precise control of cell size and distribution in the metal and ceramic foams and makes possible a mass production of such foam structures. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a foam structure comprising a body of structural material having a plurality of cells therein is produced by forming a composite rod comprising an outer shell formed of a powdered form of the structural material and a binder material and an inner core formed of a powdered form of a removable channel forming filler material and a binder material. The composite rod is sectioned into a plurality of rod segments of predetermined length and a plurality of these segments are assembled in randomly oriented relationship to one another. The assembly of rod segments is then consolidated. The binder and the filler core material are then removed and the resulting structure is sintered to produce the final foam structure containing cells as defined by the removed filler material. The binder and filler core material may be removed before sintering, during the sintering process or after sintering. Such removal will depend upon the specific binder and filler materials that are used, and such removal may be accomplished by evaporation, decomposition, dissolution, infiltration, melting with following blow out, etc. 
     In one embodiment, the structural material is a sinterable ceramic powder, such as alumina; the channel forming filler of the core is melamine or urea or a polymer, such as polyethylene or polypropylene; and the binder of both the core and outer shell is paraffin or wax. 
     Preferably, the viscosity or yield points of shell and core mixtures at extrusion temperature should be as close as possible to one another. 
     In the preferred embodiment, the binder is removed by heating. The filler core material can also be removed by heating, and this can be accomplished during the application of the heat used to preform the sintering step, which will require higher temperature than the melting or boiling point of the filler material. 
     In another preferred embodiment, the structural material of the shell is formed of a powdered ferrous metal, such as iron or steel, and the channel forming filler material of the core is an organic powder, such as melamine. In this embodiment, the binder has a lower melting point than the core filler and may be paraffin or bees wax. 
     In a further embodiment, the consolidated assemblage of segments is placed between two plates, formed of metal powders, preferably iron, and a binder, and the sandwich of the two plates and consolidated assemblage is then die compressed and heated to an elevated temperature to remove the binder and channel forming filler material from the assemblage and to remove the binder from the plates as well prior to sintering, with the result that the final structure comprises a metal foam sandwiched between two metal plates. 
    
    
     DESCRIPTION OF THE FIGURES 
     FIG. 1 is a schematic representation of apparatus suitable for carrying out steps involved in producing a foam structures according to the present invention. 
     FIG. 2 is a SEM micrograph of an iron foam structure produced by the method of the present invention, pursuant to Example 2. 
     FIG. 3 is a schematic representation of another apparatus suitable for carrying out the steps involved in producing a multi-cell foam structure according to the present invention. 
     FIG. 3A is a schematic representation of another form of die extruder suitable to replace that shown in FIG.  3 . 
     FIG. 4 is a SEM micrograph of an alumina foam structure produced by the method of the present invention, pursuant to Example 3. 
     FIG. 5 is a schematic representation of still another apparatus suitable for carrying out steps involved in producing foam structures according to the present invention. 
     FIG. 6 is a schematic representation of a three-layer structure comprising a foam core sandwiched between two metal plates, produced pursuant to Example 5. 
     FIG. 7 is a schematic representation of apparatus for producing a composite material consisting of an iron foam infiltrated with magnesium, as per Example 7. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1 there is shown, schematically, apparatus for carrying out methods according to the present invention for producing novel foam structures. The material for producing the outer shell of the composite rod comprised of powder foam structure material and binder material is shown at  101 - 102 , respectively wherein iron powder  101  and binder powder  102  are shown at the input of a double planetary mixer  103  from whence the mixture produced thereby is fed to a granulating twin screw extruder  104  the output of which, in turn, shown as  105 , is shown being fed to a screw extruder  106 , the output of which is fed to the die  107 . 
     Also shown in FIG. 1 is the method for producing the inner core of the composite rod structure shown as filler powder  108  and binder  109  at the input of a double planetary mixer  110  from whence the mixture produced thereby is fed to another granulating twin screw extruder  111  the output of which  112  is shown passing to a screw extruder  113  the output of which is also fed to the die  107 . The die  107  which produces the composite rod structure with the inner core filler material and binder enclosed within the outer shell formed of the iron powder and binder surrounding it, is shown producing an output in the form of the composite rod which is shown at  114 , identified as bimaterial green fiber being fed in random orientation to a compaction die  116  where it is consolidated after having been cut into segments by the knife  115 . The die  116  consolidates the assemblage of the randomly oriented rod elements or segments. After the consolidation step these elements, now randomly oriented in the assemblage, are put in the furnace  117  for debinding through the application of heat, after which they are passed to a sintering furnace which, in fact, could be the same furnace  117  operated perhaps at a different temperature and under a different atmosphere, e.g. hydrogen, to sinter the remaining structure with the binder and inner core material having been removed so that the resulting foam consists only of the sintered outer shell material If necessary, or optionally, the sintered material may be subjected to sizing as shown by the schematic box designated  118 . In many or most instances, the foam product by the present invention will be in “net shape” and will not require further sizing. 
     FIG. 3 shows another apparatus for carrying out the invention, which is similar to that in FIG.  1 . In FIG. 3, similar components are identified with the same reference numerals as those of FIG.  1 . The distinguishing difference in FIG. 3 is that a plunger extruder  116 A is employed to produce a structure  121 , which is then placed in the debinding furnace  117  as in the case of the apparatus shown in FIG.  1 . In the extruder  116 A of FIG. 3, the randomly oriented green fiber segments are caused to align themselves in the tapered extrusion die by plunge extruder  116 A to produce the structure illustrated in FIG.  4 . 
     FIG. 3A shows a similar apparatus with a somewhat different extruder ( 116 B), wherein the extruder die produces a hollow shaped structure. Other profile assemblage may be produced with different dies. 
     In FIG. 5, a similar apparatus to that of FIGS. 1 and 3 is illustrated wherein the consolidation step is carried out in a rolling mill  116 C. 
     Example 3 illustrates a method carried out with the apparatus of FIG. 3. A foam structure produced with the apparatus of FIG. 5 is shown in Example 4. 
     FIG. 6 is described in Example 5. 
     FIG. 7 is a schematic illustration of a die employed to produce a composite material made by infiltration of iron foam by i molten magnesium. In this Figure, the die  300  is shown with the punch press  301  above it, which is employed to subject the contents of the die to pressure so that the molten magnesium is pressured through surrounding porous ceramic into the foam enclosed within it. This is further described in Example 7. 
     In addition to the materials and techniques described specifically in Examples 1-7, materials employed to produce the composite rods, which in this instance are randomly oriented prior to consolidation, used in applicants invention for producing the composite rods for multi-channel structures, as shown in his U.S. Pat. No. 5,774,779 issued Jun. 30, 1998. This patent disclosure is incorporated by reference with respect to the subject matter of this application particularly that concerning the composition and formation of the composite rod structures used to produce the multi-channel structures of that patent, but wherein the random orientation of such rod structures for producing a foam structure is neither disclosed nor contemplated nor suggested. 
     EXAMPLES 
     Example 1 
     A foam structure with nonporous interchannel walls was produced using apparatus of the type illustrated in FIG.  1 . 
     Bimaterial rods, consisting of the 3 mm outer diameter shell, which is comprised of a first mixture of carbonyl Fe powders with 44 vol. % binder (30% polyethylene wax, 65% paraffin wax, 5% stearic acid), and the 2.85 mm diameter core comprised of a mixture of melamine powder with 40 vol. % the same binder, were produced using 2 screw extruders and a 3 mm diameter die orifice. These rods were cut to segments 5 mm length and the segments were poured into a die of rectangular cross section 70×45×20 mm. 
     The green body was heated with temperature raised from 20° C. to 400° C. at a rate of 0.5° C./min. in order to remove the binder and melamine, then sintered by being heated in an atmosphere of H 2  from a temperature raised to 1350° C. at a rate of 10° C./min and held at 1350° C. for two hours. After sintering, the resulting foam iron structure with 90% channel porosity was produced. The density of this iron foam was 0.79 g/cm 3  that is 21% less than that of water. The foam contained the 5 mm length channels; the walls between the channels were nonporous. 
     Example 2 
     A foam structure with porous interchannel walls was produced using apparatus of the type illustrated in FIG.  1 . 
     Bimaterial rods, consisting of the 0.5 mm outer diameter shell, which is comprised of a first mixture of carbonyl Fe powders with 44 vol. % binder (30% polyethylene wax, 65% paraffin wax, 5% stearic acid), and of the 0.35 mm diameter core comprised of a mixture of melamine powder with 40 vol. % the same binder, were extruded using 2 screw extruders and a 0.5 mm diameter die orifice. The rods were cut to segments of 15 mm length that were poured into a die of rectangular cross section 70×45 mm and this assemblage of randomly oriented rods was consolidated at 50° C. at pressure 5 MPa into a prismatic green body 70×45×20 mm. 
     The green body assemblage was heated with temperature raised from 20° C. to 450° C. at a rate of 0.5° C./min. in order to remove the binder and melamine, and then sintered by being heated in an atmosphere of H 2  while the temperature was raised from ambient to 1000° C. at a rate of 10° C./min and held at 1000° C. for one hour. After sintering, the foam iron structure with 49% channel porosity was produced. At the same time, the porosity of the interchannel walls was 36% and the total porosity of the foam was 67%. The structure of the produced foam is shown in FIG.  2 . 
     Example 3 
     A foam structure with parallel direct cells and relatively low porosity interchannel walls was produced using apparatus of the type illustrated in FIG. 5 using the rolling mills  116 C. 
     Bimaterial rods, consisting of the 1 mm outer diameter shell, which is comprised of a first mixture of 85 weight % alumina powder with 15 weight % binder (30% polyethylene wax, 65% paraffin wax, 5% stearic acid), and of the 0.7 mm diameter core comprised of a mixture of graphite powder with 17 weight % of the same binder, were extruded using 2 screw extruders and a 1 mm diameter die orifice. The rods were cut to segments of 10 mm length that were randomly poured into a die, which had a container 60 mm in diameter and outgoing orifice 10 mm in diameter, end extruded. As a result, the 10 mm diameter green rod was produced. It had the structure of fiber reinforced composite with matrix consisting of mixture alumina powder and binder and with fibers oriented along the rod axis and comprising mixture of graphite powder with the binder. The mean diameter of the fibers was approximately 200 micrometers. 
     The procedure of the heat treatment included heating from 20° C. to 500° C. at a rate of 5° C./hr, then heating from 500° C. to 1100° C. at a rate of 60° C. hour, holding the upper temperature for 1 hour, then heating with a rate of 30° C. hour from 1100d to 1500° C., holding 1 hour. After sintering, the 14% shrinkage took place. As a result, the sintered 8.6 mm alumina rod having parallel ducts ˜180 micrometers in diameter and total porosity 53% was obtained. The interchannel walls had the porosity 6%. The produced structure is shown in FIG.  4 . 
     Example 4 
     A foam structure with parallel oriented channels and porous interchannel walls was produced using apparatus of the type illustrated in FIG.  3 . 
     Bimaterial rods, consisting of the 0.5 mm outer diameter shell, which is comprised of a first mixture of carbonyl Ni powders with 46 vol. % binder (30% polyethylene wax, 65% paraffin wax, 5% stearic acid), and of the 0.44 mm diameter core comprised of a mixture of sodium chloride powder with 45 vol. % the same binder, were extruded using 2 screw extruders and a 0.55 mm diameter die orifice. The rods were cut to segments of 3-4 mm length and plurality of these segments were rolled flat in one pass at 50° C. to 40 mm width and 2.5 mm thickness strip using rolling mill. After rolling the green body tape or strip comprised the (Ni+binder) matrix and (NaCl+binder) fibers predominantly oriented along the rolling direction. 
     The green strip was heated with temperature raised from 20° C. to 360° C. at a rate of 1° C./min, held for 3 hours and then sintered by being heated in an atmosphere of H2 from a temperature raised to 1050° C. at a rate of 10° C./min and held at 1050° C. for two hours. The sintered strip was placed into flowing water and held there for 12 hours. NaCl fibers were dissolved from the structure and left Ni foam with oriented channels ˜0.4 mm in diameter. The foam had the 77% channel porosity and 21% interchannel wall porosity, its density was 1.6 g/cm 3 , that corresponds to the relative density ˜18%. 
     Example 5 
     As illustrated in FIG. 6, a three (3) layer iron structure was produced wherein an inner iron foam layer  202  is sandwiched between two solid iron plates  201  and  201 . 
     Two plates  201  were 70×45×1.2 mm each in size and made of mixture of 56 vol. % carbonyl Fe powder with 44 col. % binder (30% polyethylene wax, 65% paraffin wax, 5% stearic acid) were rolled at 48° C. using rolling mill. A prismatic green body 70×45×20 mm, produced as described in the Example 2, was placed between these two plates and compressed in a die at 41° C. The obtained three-layer green body was heated with temperature raised from 20° C. to 450° C. at a rate of 0.5 D/min. in order to remove the binder and melamine from all three layers, and then sintered by being heated in an atmosphere of H 2 . During sintering, the temperature was raised from ambient temperature to 1280° C. at a rate of 10° C./min and held at 1280° C. for two hours. After sintering, the composite material, consisting of the inner iron foam layer with 44% channel porosity and two outer solid iron coatings, was produced. 
     Example 6 
     An iron foam structure was produced with an outer solid shell of iron. Bimaterial rods, consisting of the 0.5 mm outer diameter shell, which is comprised of a mixture of 56 vol. % carbonyl Fe powders with 44 vol. % binder (30% polyethylene wax, 65% paraffin wax, 5% stearic acid), and of the 0.35 mm diameter core comprised of a mixture of melamine powder with 40 vol. % of the same binder, were extruded using two screw extruders with a 0.5 mm diameter die orifice. The rods were cut to segments of 3 mm length that were poured into a cylindrical barrel made of the mixture of 56 vol. % carbonyl Fe powders with 44 vol. % binder. The barrel was produced by pressing in a die at the temperature 50° C. Its height was 40 mm; the outer diameter 50 mm; wall and bottom thickness 1.5 mm. The barrel was filled in with bimaterial segments up to 80% of its height. Then the barrel with the segments was consolidated at 50° C. at pressure 5 MPa into a cylindrical green body. After consolidation, all segments were closed in the barrel. 
     The produced green body was heated with temperature raised from 20° C. to 450° C. at a rate of 0.5° C./min. in order to remove the binder and melamine, and then sintered by being heated in an atmosphere in H 2  from a temperature raised to 1320° C. at a rate of 5° C./min and held at 1320° C. for 2 hours. After sintering, the iron foam structure with 43% channel porosity coated with a solid shell was produced. 
     Example 7 
     A porous iron foam structure was produced and then infiltrated with molten magnesium metal to produce a composite structure consisting of a foam skeleton corresponding to the cellular walls of the iron foam with solidified magnesium matrix filling the voids of the foam, as illustrated in FIG.  7 . 
     The sample of 40×30×20 mm made of the iron foam of 67% total porosity (49% channel porosity and 36% interchannel wall porosity) was produced as described in the Example 2. The sample was surrounded by a porous ceramic filter board, put in the die and squeeze cast by melt Mg (see FIG.  5 ). Porous ceramic serves as a filter, it removes oxides entrained in the melt. Melt Mg was heated up to 820° C. in argon prior to pouring. The iron foam was separately heated in argon atmosphere too at 700° C. and immediately transferred to a preheated at 600° C. die maintained. The melt Mg was immediately poured on top, and the ram speed in the hydraulic press was controlled at 5 mm/sex during die closure. The pressure of 10-25 MPa during the metal penetration and solidification stages was maintained. The entire casting operation involving pouring the metal and pressurizing the die with full solidification of the metal normally occurs 10-15 s. As a result, the composite consisting of iron skeleton fully infiltrated with magnesium was produced. 
     The various methods and materials for making composite rod structure disclosed in U.S. Pat. No. 5,774,779, issued on Jun. 30, 1998 to Lev J. Tuchinskiy, the present applicant for patent, may be used in making the composite rod structures that are produced in the course of making the cellular structures of the present invention. However, that patent does not disclose or contemplate the making of cellular structures with randomly oriented cells as disclosed and claimed herein.