METHOD OF FORMING FOAMED METAL

BACKGROUND OF THE INVENTION 1. The Field of the Invention

This invention relates to the formation of foamed metals and more particularly to the formation of closed cell foamed metals. 2. The Relevant Technology

Efforts to produce foamed metals of acceptable quality and in commercial quantities have been ongoing. See: U.S. Patent 3,087,807 (Allen, et al); U.S. Patent 6,881,241 (Fuerst, et al.); US Patent 6,874,562(Knott, et al.); U.S. Patent 6,840,301 (Nichol, et al.); Joseph C. Benedyk, Production and Application of Aluminum Foam, Light Metal Age (April 2002) pg 24-29.

Foamed metals are now available commercially in both closed cell and open cell structures. For example, closed cell products include Alporas material available from Shiko Wire of Fukoka, Japan and Foaminal metals available from Institut Fertigungstechnik Materialforschung (JJFAM) of Dresden, Germany. Cymat Corporation of Mississauga, Ontario, Canada also supplies closed cell foamed metal products which include an intermixed silicon carbide ceramic material which changes the hardness of the foamed metal. Other known closed cell foamed metals include AIu Foam products from Neuman Aluminum of Verona,

Virginia, AluLight metals from Mepura of Braunau-Ranshofen, Austria, and FOAMCARP and FOAMGRJJP foamed metal from Cambridge University, Department of Materials Science and Metallurgy of Cambridge, United Kingdom. Open cell products include IncoFoam metals from Inco of Toronto, Canada, Duocel metal from ERG in Canada and metallic foams from Racemat of Krimpen aan den Ijssel, Netherlands.

Various methods have been used to form foamed metal. Cymat and Alcan International of Maison Alcan, Canada both report producing foamed aluminum by adding a dispersion of refractory material or ceramic material sized at about 0.5 to 25 microns such as silicon carbide, alumina, boron carbide, silicon nitride or boron nitride. A gas or a foaming agent (e.g., TiH2, CaCO3 ) is stirred into the melt with a dispersion impeller to produce a foam that is pulled off in sheets. The closed cells have a random or non uniform size that are believed to vary

randomly and widely so that the material is not uniform and in turn has characteristics and qualities that are not uniform. It appears that in production, the refractory material is wetted by the melt and is dispersed through out by a mixing process. The refractory material has a hardness higher than the metal and appears to incidentally increase the hardness of the foamed material so that it has been found to be more difficult to cut or saw than the metal itself. Powdered metals have been used in the formation of a foamed metals. However, powdered metals are typically more expensive to produce than other commercial foams noted above. In addition, powdered metals have been found to be unsuitable for controlling the size of cells or pores in the final product. In addition, use of hydrides leads to the generation of hydrogen during the formation process which of course introduces certain recognized hazards or risks.

Other methods for forming foamed metals include use of gas infiltration or injection as discussed above. Also foamed metals can be produced by mixing in foaming agents to produce gas bubbles distributed throughout. Foaming agents have not been found suitable largely because the resulting metal product is contaminated with remnants from the foaming agent.

While different foamed metals are available for commercial use, a relatively inexpensive method for producing foamed metals of consistent quality and grade for widespread and varied commercial use has not yet been presented.

BRIEF SUMMARY OF THE INVENTION

A method for forming a foamed metal includes first providing a melt of metal that has a melt surface. The melt is positioned in a crucible and is then allowed to cool to a temperature to form solidified particles or nuclei dispersed in the melt. An injector tube is positioned in the melt. The injector tube has a first end having a nozzle positioned thereat and a second end spaced from the first end. Pressure means is provided to urge the melt into the second end and through the injector tube and out through the nozzle.

A source of gas is provided and connected to a gas conduit to supply gas to the bubble conduit. As melt is urged through the injector tube, the gas is injected into it forming discrete bubbles of gas in the melt in the conduit. The melt and the bubbles are urged out of the nozzle and into the melt where they rise through the melt creating a closed cell foamed metal on the surface of the melt.

In a preferred method, the gas is heated and preferably heated to the melt temperature. In other methods, the gas may be heated to a temperature just below the liquidus temperature.

In alternate and preferred processes, the pressure means includes a pump positioned in the melt. The pressure means also includes drive means connected to the pump to operate the pump to urge melt into the injector tube. More preferably the pump includes a rotating housing with vanes attached to the interior to be rotated upon rotation of said housing. The drive means is preferably a shaft connected to the pump housing. The shaft extends away from the surface of the melt and has rotation means connected to the shaft to rotate the shaft at a speed desired to create a flow of melt through the injector tube. The rotation means is most preferably a motor and may optionally be a motor connected by a drive mechanism to the shaft.

In alternate processes, the melt crucible has temperature sensing means to sense the temperature of the melt at different locations about the melt crucible. Preferably, the melt crucible will be configured for induction heating of the metal in said crucible. Alternately, the melt crucible can be placed within an oven which includes heat generating means and control means connected to operate the heat generating means.

Desirably, the control means is connected to the temperature sensing means to receive signals reflective of the temperature of the melt in the melt crucible. The control means is configured to operate the heat generating means to supply heat to the melt to maintain the temperature of said melt at a pre-selected temperature.

In some arrangements formation means is provided to remove the foam from the melt surface and to form the foam into desired shapes. The formation means may have a first surface and a second surface positioned proximate the surface of the melt with the foam being urged between them. A third surface and fourth surface may also be provided to facilitate formation of desired shapes.

In various embodiments of the process, the gas maybe argon, nitrogen, or gases that are inert. In yet other embodiments, the gas may be air.

Systems may also be assembled for performing the various processes as outlined above and as disclosed hereinafter. These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

To further clarify the above and other advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

FIG 1 is a simplified block diagram illustrating steps of the method of the present invention; FIG. 2 is a simplified illustration of a system for practicing the method of the present invention;

FIG. 3 is a simplified cross sectional illustration of a pump with an injector tube of the system shown in FIG. 2;

FIG. 4 is a cross sectional view of pump with an injector tube of FIG. 3; FIG. 5 is an enlarged cross sectional view of an injector tube and gas conduit of the system of FIG. 2; and

FIG. 6 is a depiction of formation means for the system of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A foamed metal is formed by injecting gas into a melt of metal selected by the user. It is believed that any metal can be foamed with preferred metals presently including aluminum, magnesium, nickel or typical alloys thereof. A preferred aluminum alloy is comprised of aluminum, silica and magnesium. As discussed in more detail hereafter, a quantity of a selected metal is heated at least to its melting temperature or liquidus temperature to form a melt. The melt is later cooled to a temperature winch is above the solidus temperature and just enough below the liquidus so that tiny particles or nuclei of the solidified metal begin to form. Preferably the melt temperature is uniform and the tiny particles of solidified metal begin to form throughout and are dispersed in the melt. It is believed that the tiny particles of metal function to stabilize the foam. A gas is then injected into the melt to form a foam at or proximate the surface of the melt. The gas is preferably heated to the temperature of the melt or to a temperature just less (e.g., 10 degrees centigrade) than the temperature of the melt or even less than liquidus. The gas is then injected into an injector tube that has melt passing thorough it. That is, the melt is

urged into and through the bubble tube by a pressure means which, as seen hereinafter, is a pump.

As seen in FIGS. 1 and 2, a metal 12 is provided in an amount and size suitable for placement in a crucible 14. The metal 12 is heated by an oven 16 until it melts 17 in the crucible 14 to form a melt 18.

The melt 18 is then urged toward a second crucible 20 which is also positioned within an oven 52. The melt 18 may be transferred by any suitable means from the first crucible 14 to the second crucible 20. In the illustrated embodiment, the melt 18 is transferred in a heated conduit 15 by gravity. However it should be understood that transfer could be effected by pumps, ladles or any other means suitable to effect the transfer of a molten metal. Indeed, the transfer could even be avoided by placing the metal 12 directly in the second crucible 20 to avoid using multiple crucibles. As presently envisioned, however, a commercial process involves essentially continuous operations so that the system would include means to continuously supply new molten metal as foamed metal is created and removed as hereinafter discussed, hi other words, crucible 14 is but one means for providing an essentially continuous supply of melt 18 for the second crucible 20. Other means may be used as desired.

The illustrated crucibles 14 and 20 are shown in cross section. Crucible 14 is shown to be generally rectangular with crucible 20 having hemispherical 2OA portion and cylindrical 2OB portion. However, it should be understood that any suitable shape for a crucible may be used. Further the crucibles 14 and 20 may be made of any suitable material including a ceramic material.

The crucible 20 has means for detecting the temperature of the melt in different locations within the crucible 20. As here illustrated, a plurality of temperature detectors 22, 24, 26, and 28 are positioned at different locations on the interior surface 21 of the crucible 20 or appropriately disposed in the crucible to detect the temperature of the melt 30 in the second crucible 20. While four temperature detectors 22, 24, 26 and 28 are shown, it should be understood that the number and positioning of temperature detectors will vary based on the size and shape of the second crucible 20. Indeed, the second crucible 20 is here shown in cross section and in turn does not depict temperature sensors as they would be positioned in a suitable pattern on or proximate interior surfaces 21 of the crucible 20. The temperature detectors, such as detectors 22, 24, 26 and 28, are placed in a pattern about the crucible 20; or they may even be suspended in the melt 30 to monitor the temperature of the melt 30 and reflect any areas of the melt 30 that may be hotter or cooler than desired. Signals reflective of

the temperature are sent from the temperature detectors such as detectors 22, 24, 26 and 28 via conductors such as conductors 23, 25, 27 and 29 to a control means 56. The control means 56 is any suitable device configured to receive temperature signals and to supply control signals to heating means associated with the crucible 20. The control means 56, which may be a suitable microprocessor, processes the signals and supplies control signals to heaters 54 in the oven 52 via conductor means 55 to maintain the temperature of the melt 30 at a desired temperature above or below liquidus.

While conductor means 55 is here shown as a single conductor, it should be understood that the oven 52, if electric, has a plurality of heaters 54 positioned about the crucible 20 which may be controlled separately, in groups or in their entirety through a plurality of conductors that are not shown to simplify FIG. 2. Upon activation, the heaters 54 supply heat to the crucible 20 in general and in specific desired locations in the crucible 20 to in turn maintain the temperature of the melt 30 in its entirety, or portions 30A of the melt 30, at a desired temperature. While an electric oven 52 is depicted, other forms of ovens using, for example, inductive heating may be used.

The melt 30 in the second crucible 20 is cooled or more correctly allowed to cool 32 to a temperature just below the melt or "liquidus stage" so that some tiny solid particles or nuclei of the metal that is the melt 30 start to form. That is, the viscosity of the melt 30 increases with minute solid particles deployed throughout the melt 30 as depicted by dots 34. The solid particles 34 are at least proximate the surface 35 of the melt 30 but are preferably dispersed throughout melt 30 in the second crucible 20. While uniform cooling or lowering of the temperature of the melt 30 is described, it should also be understood that heat may be lost from the surface 35 of the melt 30 and in turn, the surface 35 and the volume proximate the surface (e.g., several inches) may cool sufficiently to form the metal particles or nuclei 34 dispersed in the melt 30 proximate the surface 35. Alternately, the gas 40 being injected may be injected at a temperature less than the temperature of the melt 30. Upon injection of the gas 40 at a temperature less than the temperature of the melt 30 in and through the injector tube 38, the cooler gas 40 will induce localized cooling of the melt and the formation of the particles or nuclei 34 of metal for dispersal in the area or volume 30A where the injector tube 38 discharges into the melt 30.

A source of gas and preferably a source of hot gas 36 supplies gas 40 into a conduit 42 that is connected to the source 36 at one end 42 A and to the injector tube 38 at the other end 42B. The gas 40 may be air, nitrogen or argon (or other inert gases) and is injected into the

melt 30 in the injector tube 38 as more fully discussed hereinafter. In some applications, the gas 40 could be heated to a temperature comparable to or slightly warmer than the temperature of the melt 30 to avoid localized cooling and the formation of solid or more viscous portions of the melt in the injector tube 38 and in vicinity 3OA of the nozzle 44 and thus adversely effect the homogeneity of the melt 30. In other applications, it is preferred to heat the gas to a temperature below liquidus and inject it into the melt 30 in the injector tube 38 to form bubbles 4OA, 4OB, 4OC (FIG. 5) which are believed to be more stable that bubbles formed with wanner gas 40.

Referring to FIGS. 2, 3 and 5, the injector tube 38 has a discharge or outlet end 38A which may have a separate nozzle affixed thereto. Alternately, the outlet end 38A may be formed to be a nozzle 44. The nozzle 44 functions to retain the shape of the air bubbles 4OA, 4OB, 40 C, 4OD and 4OE. Only a few of the bubbles 40 A-E are labeled or identified by number. The nozzle 44 may be of different shapes and sizes to regulate the size of the bubbles and in turn the size of the cells of the foamed metal. Square or rectangular nozzles have been found suitable with very small openings in the vicinity of 0.0625 inches by 0.020 inches. The conduit 38 is typically made of a metal like stainless steel and has a ceramic coating on the inside and the outside. The injector tube 38 may also be referred to as a manifold inasmuch as it has a tube connected to inject gas.

The conduit 38 has an inlet end 38B (FIG. 3) positioned within a pressure means which functions to provide a pressure to or in the melt 30 to urge melt 30 into the inlet end 38B of the injector tube 38. That is, any device or arrangement that can urge the melt 30 into the inlet end 38B and cause the melt 30 in the injector tube to flow is presently believed to be suitable, hi FIGS. 2 and 3, the pressure means is a pump 47 that is essentially round in cross section and operated by driving means to cause it to rotate. Blades or vanes 48A, 48B, 48C, 48D, 48E, 48F, 48G and 48H (not shown) all operate to pump or urge the melt 30 toward the outside wall 46A by centrifugal force, hi turn the pressure of the melt 30B at and proximate the outside wall 46A is higher than the static pressure of the melt 30 in the crucible 20 at and along the outside wall 46A. Operation of the pump 47 provides an increase of pressure proximate the interior of the outside wall 46A in the interior of the pump 47 that is several pounds per square inch and may be more than 25 pounds per square inch above the static fluid pressure of the melt 30 at and along the wall 46A. In turn, melt 30B is urged into the injector tube 38 which extends out of the pump inlet 46C. The pump 47 may have other inlets including one or more formed in the top 46D of the pump 47. However one lower pump inlet 46C has been found

sufficient for the embodiment discussed. To direct the melt 3OB into the injector tube 38, the inlet 38B is positioned inside the pump 47 to be between upper vanes like vanes 48 A and 48C and lower vanes like vanes 48B and 48D.

As better seen in FIG. 5, to help maintain homogeneity and to facilitate dispersion of the gas 40 in the melt 30 to form foam 51, the air 40 is introduced into the flowing melt 30C in the conduit 38. The pressure of the air 40 is selected to be higher than the fluid pressure in the conduit 38 while being low enough so that the air 40 does not retard or force the melt 3OC back toward the inlet 38B which is still able to proceed into the melt 30 in the area 30A. It is believed that as the air 40 enters the conduit 38, the melt 30C proceeds past the opening 42C of the gas conduit 42 and shears the air 40 into units that close off and become bubbles 40A-E. The pressures of the gas 40 and the melt 30C are such that the bubbles 40A-E are of substantially the same volume forming substantially uniform spherical bubbles in the melt 30 and in turn a foam 51 that has cells 5 IA of substantially uniform volume and shape and in turn a foamed metal when hardened that is substantially uniform. The pump 47 is rotated by drive means which may be any arrangement suitable to rotate the pump 47 to attain the pressure rise for the melt 3OB at the outer wall 46 A of the pump 47. As here depicted, the drive means includes a shaft 65 having a first end 65A attached to the top 46D of the pump 47 by any suitable means to effect a mechanically secure connection including welding, screws, bolts, glue and the like. The shaft 48 is rotated by rotation means which is any means having sufficient power to rotate the pump 47 at the necessary rate to achieve the desired pressures to urge the melt into the injector tube 38.

The drive means depicted in FIG. 2 is a suitable motor 49 (e.g., 1.5 horsepower DC Motor) configured to turn a pulley or drive gear 50 and a chain 50A or belt attached around a driven pulley or gear 50B on the shaft 48. Alternately, a DC drive motor 49 A (shown in phantom) may be directly mounted to the shaft 48. In operation, the motor 49 is activated so that it turns the shaft 48 and the pump 47 at a desired RPM to create pressure for the melt 30B at the inner surface of the outer wall 46B of the pump 47.

The foam 51 forming at the surface 30D of the melt 30 needs to be removed and formed or shaped into suitable form for use. The foamed metal 51 is formed at or near the surface 3OD and is urged upward and away from the melt 30 because the foamed metal 51 is less dense and in turn is buoyant in the melt 30. Positioned above the surface 30D is formation means to form the froth or foam 51 into a solid foamed metal plate or into any shape that is desired.

The formation means shown in FIG. 6 may be positioned proximate the surface 3OD. The formation means includes first and second opposing plates 60 and 62. They are placed facing each other and are essentially parallel. As shown, the opposing plates 60 and 62 are preferably made of a thin metal or screen that is somewhat flexible so that endless loops 64 and 66 may be formed and trained about respective guide rollers 68 and 70 and 72 and 74. One of the guide rollers 68 and 70 is a driving roller and the other is a driven or idler roller.

As the foam 51 pushes up between the plates 60 and 62, the plates 60 and 62 shape the foam 51 into a flat sheet as it passes out of the outlet 84.

As better seen in FIG. 6, the formation means may also include a third metal plate 80 and a fourth metal plate 82. The third metal plate 80 is part of an endless loop 84 trained about rollers 86 and 88 one of which is preferably a driving roller and the other is a driven roller. The fourth metal plate 82 is part of an endless loop 90 trained around rollers 92 and 94 one of which is driven and the other a driving roller.

In FIG. 6, the formation means is shown with the metal plates 60, 62, 80 and 82 and the endless loops 64 and 66 all spaced apart for better illustration. However, they may be preferably positioned closer to each other to form the 4 sides of a metal plate that is made of a closed cell metal foam. The foam 51 passing between the plates 60, 62, 80 and 82 may be said to be extruded because the plates 60, 62, 80 and 82 rotate to urge or pull the foam 51 out of the melt 30. It should be understood, however, that other formation means may be used to effect different geometric shapes and to draw the foam away from the surface 30D of the melt 30. The formation means need only function to create a commercially useful output to the user.

Referring back to FIG. 1, it can be seen that the oven 52 is operated to allow the temperature of the melt 30 to cool 32 just enough below the melt temperature or liquidus to allow tiny nuclei or particles 34 of the metal to form dispersed throughout the melt 30. The particles 34 are believed to be quite small. Some means to mix the melt 30 may be desired to facilitate uniformity of temperature in the melt 30 and in turn uniformity in the formation of particles 34 of solid metal dispersed in the melt 30. In some applications, the crucible 20 may be formed to allow heat loss at or near the surface 30D metal particle formation being generally at or above the outlet nozzle 44.

Of course, the formation means hereinbefore discussed functions 102 (FIG. 1) to remove the foam 51 from the exit port 84 of the crucible 20. The foamed metal 51 so formed has a plurality of cells 51 A that are substantially uniform in size. That is, the foamed metal 51

can be produced in an essentially continuous process while having more predictable structural and other physical characteristics (e.g., sheer, R factor, ductility, sawability) because the cell size is essentially controllable. In turn the foamed metal 51 is suitable for a wider range of commercial uses. Example

Alloy A-356.2 was obtained. It had 7.12 % by weight silicone (Si), 0.37% by weight magnesium (Mg), 0.13% by weight iron (Fe), 0.13% by weight titanium and other minor elements. A small amount (<0.05% by weight) of strontium (Sr) was present to improve machinability. The alloy was grain refined by adding 0.2% TiBloy obtained from Metallurg of Swedesboro, New Jersey. The solidus temperature (Ts) was determined to be 545 C and the liquidus temperature (TL) was determined to be 623 C. The melt temperature was raised above liquidus to a temperature of about 635 C.

The A-356.2 was melted in a Paragon DTC 800C programmable resistance furnace having a volume of about 1.7 cubic feet. A separate heating plate was used at the bottom to reduce heat loss through the base or bottom of the furnace. A silicon-carbide crucible was used to hold the molten metal or melt. Thermocouples were dispersed in the melt to monitor temperature distribution in the melt. A 1.5 horsepower DC motor was used to drive the shaft that was connected to a melt dispenser or pump that urged melt into an injector tube. Sprockets with a 2: 1 ratio were used with a chain drive to rotate the dispenser or pump at about 1000 RPM. The injector tube was used to introduce gas (air, argon (or other inert gas) or nitrogen or the like) at the rate of about 4 cubic centimeters per second heated to the melting point of the A356.2 into a flow of melt in the injector tube that was a 1/4 inch stainless steel tube coated inside and outside with a ceramic or graphite material. As the pump or melt dispenser was rotated and the hot gas injected, a foam of metal appeared at the surface of the melt that could be scooped away or otherwise removed by suitable means. The metal foam had cells relatively small in size and substantially uniform in size. The size of the cells appear to be controllable by the nozzle at the end of the injector tube. The nozzle may be round, but in this test, rectangular openings were used with three openings sizes: (1) 0.0625 inches by 0.030 inches; (2) 0.0625 inches by 0.020 inches, and (3) 0.0625 inches by 0.010 inches. Foamed metals with cells varying from about 4 millimeters in diameter to about 1 millimeter in diameter could be obtained with nozzles of the identified sizes.

The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all

respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.