A PROCESS OF MANUFACTURING PARTICLE REINFORCED METAL FOAM AND PRODUCT THEREOF

Abstract: Particle reinforced low cost metal foam is provided by a process of manufacturing metal foam based on foaming of molten composite material using finely dispersed cellulating gas.

Description: A process of manufacturing particle reinforced metal foam and product thereof The present invention relates to a process of providing met foam and more particularly to a process resulting in provisi of thin wall close cell particle reinforced metal foam. Foamed metals, as well as foamed ceramics and plastics, due their unique combination of properties and light weight a earning growing attention as engineering materials. There are several ways to produce foams. Different foaming tech niques are known such as incorporating hydrides in the molte metal or adding organic compounds which release gases on heat ing. Vapor deposition on polymeric substrates or casting o metal around granules which are then leached out leaving porous metal structure are other examples of providing metal with cellular structure. The process of foam or ation using blowing agents is affecte by the surface tension and viscosity of the actual melt. Th viscosity counteracts bursting of the cell walls during a pro gressive increase in the volume of the formed bubbles, while low surface tension will favour formation of thin bubble walls. The properties of foams being gas-in-solid dispersions are largely determined by their density, but the cell size, struc¬ ture and their distribution are also important parameters in- fluencing the properties. In general such foamed metals are produced by adding a gas evolving compound to the molten metal followed by heating of the resultant mixture to decompose the compound and to produce ex¬ panding cellulating gases. The foaming compound is usually metal hydride such as TiH 2 or ZrH 2 , and after the foaming step the mould is cooled to form a solid foam material. Cells of non- uniform structure and/or undesirably large size are experienced due to the difficulties with uniform distribution of the evolv¬ ing gas through the whole volume of the foamed metal. GB patent No. 1.287.994 discloses a process for preparation of metal foams applying a viscosity increasing agent comprising an inert gas or an oxygen containing material gaseous at the melt conditions and treating the thus produced viscous melt with a foaming agent. Air, nitrogen, carbon dioxide, argon and water are preferably used in the process as viscosity increasing agents in amounts from 1 to 6 grams per 100 grams of metal alloy. Metal hydrides are used as foaming agents (hafnium, titanium or zirconium hydrides) in amounts of from 0,5 to 1,0 grams per 100 grams of alloy. Preferably the increase in viscosity is enhanced by the presence of a promoter metal, e.g. from 4 to 7 weight% magnesium is used in aluminium alloys. A good mixing technique is required, the addition of foaming agents is usually carried out at a tempera¬ ture lower than addition of the viscosity increasing agent in a separate second vessel. The disclosed batchwise process, achiev¬ ing better foams with regard to uniform size and distribution of the cells, and claiming a certain reduction in the consumption of foaming agents, is a rather complicated time consuming and expensive process requiring several process steps and uni based on use of expensive heat decomposible gas evolving co pounds (hydrides) . European patent application No. 0 210 803 discloses a simil batchwise method of producing foamed metals based on use of fr 0,2 to 8,0 weight% metallic calcium as viscosity adjusting age and titanium hydride in amounts of from 1 to 3 weight% of t molten melt as foaming agent. Still another method of producing cellularized metal by decompo sition of a heat-decomposable gas evolving compound in molte metal is disclosed in US patent No. 3.297.431. The improvemen comprises addition of an intimately dispersed, finely divide powder to the metal prior to decomposition of the gas evolvin compound (carbonates or hydrides) , or dissolving of gas in th melt. The stabilizing powders may be metals or non-metals elements or compounds, and two wettable powders are preferen tially used where one of which forms a solid alloy with th metal. Usually the gas is dissolved at one pressure and the evolved at a second lower pressure. A drawback in common for the hitherto known processes is tha all of them are batchwise operating processes using either ex pensive gas evolving compounds or dissolved gases as cellulatin means and viscosity increasing or stabilizing additives t achieve quality metal foams. Furthermore, the prior art processes require a close contro with the temperature and pressure conditions at different step of the process. Consequently, so far there is no method operat ing on an industrial scale in an economical way offering a lo cost metal foam to compete with other engineering materials. Accordingly, it is an object of this invention to provide a simple low cost method for preparation of quality foams. Another object of the invention is to provide a method for up¬ grading of scrap metal material. Still another object of the invention is to provide a novel type of particle reinforced metal foam having improved mechanical properties. The invention in its various aspects will be described in details, and various other objects, advantages and additional features thereof will become more apparent from the following description and accompanying patent claims which are to be read in conjunction with the attached drawings. Fig. 1-4, where Fig. 1 shows schematically in the form of a flow-sheet the process of preparation of metal foam accord¬ ing to the invention. Fig. 2 displays a natural size contact print of the foamed metal sample prepared according to the invention, Fig. 3 shows an optical metallograph picture of the closed cell Al-foam structure. Fig. 4 illustrates graphically results from a compres¬ sion test conducted on foam samples. Referring to Fig. 1, illustrating schematically the process of metal foam preparation, it has been found that a metal foam of the closed cell type structure having a uniform density and cell structure can be provided simply by feeding of finely dispersed cellulating gas into a molten particle reinforced metal matri composite material (PMMC) . No special additives adjusting th viscosity of the melt or particular precautions with regard t the distribution of the cellulating gas bubbles through the mel were required. The gas bubbles rise to the top of the melt an form foam gradually increasing in volume. No tendency to burst ing of the foam cells when they reach the melt surface was ob served. This indicates a (highly) stabilized surface of the ga bubbles. The upper portion of the foam cake solidifies and ca be easily removed. Even foam which is not completely solidifie can be removed whithout changing the cell structure due to th thick consistency of the formed foam. This is a quite importan feature of the method according to the present invention, whic allows to run the process continuously by transfer of semi solidified foam to the moulds. There is even a possibility o subjecting the foam at this stage to certain forming operations something which offers a flexibility with regard to the fina shape of the resultant metal foam semiproducts. Example 1 30 kg of an eutectic aluminium alloy (Sil2MglNi2,5) was melte in an open crucible. The molten alloy kept at a temperature o 650°C was added silicon carbide particles of an average size o 12 / um, and simultaneously C0 2 gas was finely dispersed through th melt by means of a special treatment rotor as disclosed in U patent No. 4.618.427. During the feeding of a Cθ 2 surplus int the formed molten composite material bubbles started to rise t the top of the melt forming a raising foam layer. The upper por tions of the foam solidified with no sign of surface burst. Fig. 2 shows in natural size a photographic picture of th resultant foam sample removed as the solidified top part of th foam cake. The cross-section of the sample exhibits a unifor distribution of cells having a diameter in the range of from to 5 mm. The density of the sample was measured to 0,2 g/cm 3 . Example 2 20 kg of scrap PMMC material (A1 2 0 3 reinforced Al-alloy) was re- melted in an open crucible. Pressurized air was applied as source of cellulating gas in this case, finely dispersed and distributed as described in Example 1. Also in this case the resulting bubbles gave rise to a foamed structure when they reached the top of the melt in the crucible and were allowed to cool. The achieved pores (cells) are essentially spherical and closed providing the foamed metal with isotropic properties in all directions, especially with regard to energy adsorption. Metallographic examination of the structure on the samples achieved from Example 1 reveals an extremely thin walled foam structure, as illustrated in Fig. 3. The wall thickness in this metallograph picture, magnification of 20, is in order of the reinforcing SiC particle size approximately 12 7 um. The mechanical behaviour of the produced foam is represented in Fig. 4 illustrating the results from the testing of compressive stress conducted on the samples from Example 1. The achieved flat stress/strain curve from the samples having an initial height of 26 mm applying a crosshead velocity of 2 mm/min. is typical for this type of material as long as the cell structure did not collapse completely. The energy absorption of this foam was determined to be 2 kJ/1 foam, which is a very favourable value compared to the values reported in literature for commer¬ cially provided Al-foams. Obviously, the achieved improved mechanical properties of the resultant foams are a result of a beneficial influence from the reinforcing particles incorporated in the cell walls. Evidently, the above described novel method of preparation of foamed metals according to the present invention offers several advantages both with regard to the economics of the process an the characteristics of the resulting foams. First of all there is an opportunity to run the process con tinuously by continuous remelting or feeding of molten articl reinforced metal material using a variety of available gases a a cellulating gas, e.g. N 2 , Ar, C0 2 , He and even pressurized air which is normally easily available at low costs. There are no special requirements to temperatures, pressure o uniform distribution of gas bubbles during the foaming an solidification of the resultant foamed metal. The density and t a certain extent also the cell size are simply controlled b dispersion of the cellulating gas through the melt, preferen tially by applying the above special treatment rotor, but als other means ensuring finely dispersed bubbles can be applied The foam accumulated on the top of the melt can be directly fe into moulds for solidification in desired shapes and dimension or subjected to a certain grade of deformation/reshaping of th semisolidified foam. Furthermore, even if it is possible to prepare the molten par ticle reinforced alloy in a separate process step using a active gas and addition of reinforcing particles prior to apply ing of the cellulating gas, the biggest potential of the presen invention is an up-grading of low grade composite scra material. This constantly increasing volume of composite scra today represents a considerable problem since it can not simpl be remelted or incorporated to the recycled secondary aluminium.

Claims: Claims 1. A process of manufacturing particle reinforced metal foam, c h a r a c t e r i z e d i n t h a t the process is a continuous process comprising steps of providing molten composite metal material, feeding of cellulating gas into the melt, foaming of the melt and accumulation of foamed metal on the top of the melt, and finally removal and solidification of the foamed metal. 2. The process according to claim 1, c h a r a c t e r i z e d i n t h a t the molten composite material is provided by re- melting of particle metal matrix composite material. 3. The process according to claim 1 , c h a r a c t e r i z e d i n t h a t the composite material is formed in situ in the vessel by adding and distribution of reinforcing particles in the molten metal or alloy by means of an active gas. 4. The process according to claim 3, c h a r a c t e r i z e d i n t h a t the active gas is C0 2 gas and the particles are refractory particles. 5. The process according to one or more preceding claims , c h a r a c t e r i z e d i n t h a t the molten composite material is aluminium or aluminium alloy. - 6. The process according to claim 1, c h a r a c t e r i z e d i n t h a t * 5 the cellulating gas is air. 7. A close cell particle reinforced metal foam characterized by cell wall thickness from 10 to 20 / Um comprising reinforcing refractory par¬ ticles. 8. The reinforced metal foam according to claim 7, c h a r a c t e r i z e d i n t h a t the matrix metal is aluminium alloy reinforced by SiC particles. 9. The reinforced metal foam according to claim 8, c h a r a c t e r i z e d i n t h a t the foam exhibits a compressive strength of 0,2 kg/mm 2 at a density of 0,2 g/cm 3 .

Inventors: KIRKEVAAG BJOERN, RUCH WOLFGANG WALTER, KIRKEVAAG, BJOERN, RUCH, WOLFGANG, WALTER
Applicants: NORSK HYDRO AS, NORSK HYDRO A.S
Application year: 1990
Main classification: C22C1
IPC codes: C22C1