Reticulated foam structures

Abstract: A method of making a metal foam object comprising the steps of mixing a gasifier with metal powder and subjecting the mixture to an elevated temperature T 1 and pressure P 1 to form a sintered sheet; placing a least a portion of the sintered sheet into a mould and subjecting the mould to a temperature T 2 where T 2 is greater than T 1 at which the metal melts and the gas is released from the gasifier; an quenching the metal foam object thus formed in the mould, in which the quenching is carried out by applying a cryogen to the object (10) as a high velocity mixture of gas and liquid droplets.

Description: Foam structures are known in industry and the number of applications for metallic foam structures is continually increasing. For example, aluminium foam metal having a continuously connected, open celled (reticulated) geometry is available and employed in :- a) Energy/ impact absorbers; b) heat exchangers; and c) lightweight composite panels. When used with heat exchanges the high surface to volume ratio allows for a compact design and the high specific stiffness, that is, high strength to weight ratio makes the material useful in aerospace and car applications. Typical mannfacturing methods are disclosed in DE-C1-4018360. Low-cost aluminium foam panels can be produced by a continuous casting process. The foam is machinable by common aluminium metal working techniques (sawing, drilling, milling) and maybe joined by brazing or adhesive bonding. As previously indicated aluminium foam produced by this method finds application as lightweight cores for sandwich panels and as components in energy absorbing structures. However, when irregular complex shapes are required then metal foams are formed typically by mixing small quantities of a gasifier e.g. titanium nitride with aluminium powder and subjecting the mixture to heat and pressure to form a sintered sheet. The sintered sheet or a portion thereof is then placed in a mould which is then heated to a higher temperature at which the metal melts and nitrogen is released from the titanium nitride to provide an even dispersion of bubbles. The hot metal is allowed to solidify and then shock heat treated by dropping it into a cryogen such as liquid nitrogen which causes small fractures to occur between adjacent bubbles so that the mass becomes reticulated. For example, in US-A-4614544 the quenching is effected by immersion into liquid nitrogen. This quenching process can be controlled by monitoring the temperature of the metal before it is quenched in the cryogen. However, the rate of cooling and the temperature difference may still be insufficient to produce the necessary reticulated structure. It is an aim of the present invention to add a further degree of control to the quenching process by employing the cryogen as a high velocity mixture of gas and liquid droplets. According to the present invention a method of making a metal foam object comprises the steps of :- a) mixing a gasifier with metal powder and subjecting the mixture to an elevated temperature T 1 and pressure P 1 to form a sintered sheet; b) placing at least a portion of the sintered sheet in a mould and subjecting the mould to a temperature T 2 where T 2 is greater than T 1 at which the metal melts and the gas is released from the gasifier; and c) quenching the metal foam object thus formed by the mould; d) in which the quenching is carried out by applying a cryogen to the object as a high velocity mixture of gas and liquid droplets. Preferably the cryogen is nitrogen, the gasifier is titanium nitride and the metal is aluminium. An embodiment of the invention will now be described, by way of example, reference being made to the Figure of the accompanying diagrammatic drawing which is a block diagram of an apparatus for quenching metal foam objects. As shown, the apparatus 1 includes a heat insulated pressure vessel 2 containing a cryogen, for example, liquid nitrogen. An inlet pipe 4 is in communication with the ullage space at the top of the liquid cryogen and an outlet pipe 6 is located at or adjacent the base of the vessel 2 as illustrated. The flow of liquid / gas from the vessel 2 and through the pipe 6 is controlled by a valve 8. In use, the vessel 2, is pressurised to 5 barg by passing a gas through the inlet pipe 4 and the liquid cryogen is then expanded through the valve 8 and the outlet pipe assembly 6 such that a high velocity mixture of gas and liquid droplets impinges upon the object 10 to be quenched. The high velocity gas liquid droplet stream will extract heat from the block 10 many times faster than dipping the block 10 in liquid nitrogen since the film boiling effect which prevents the liquid nitrogen from touching block is avoided. Conventional ways of achieving high heat transfer with liquid nitrogen involves the use of a low thermal conductivity coating such as a grease which enables the liquid nitrogen to wet the surface of the object without an intervening gas film being formed. However, this is impractical with very hot metals and the film boiling effect is aggravated resulting in lower and unpredictable heat transfer. The use of a high velocity gas / liquid droplet stream can be finely controlled by pressure and valve openings to give the optimum cooling rate.

Claims: A method of making a reticulated metal foam object comprising the steps of:- mixing a gasifier with metal powder and subjecting the mixture to an elevated temperature T 1 and pressure P 1 to form a sintered sheet; placing at least a portion of the sintered sheet into a mould and subjecting the mould to a temperature T 2 where T 2 is greater than T 1 at which the metal melts and the gas is released from the gasifier; and quenching the metal foam object thus formed in the mould, in which the quenching is carried out by applying a cryogen to the object characterized in that , the cryogen is a controllably expanded high velocity mixture of gas and liquid droplets to create said reticulated structure. A method as claimed in claim 1, in which the cryogen is nitrogen. A method as claimed in claim 1 or 2 in which the gasifier is titanium nitride and the metal is aluminium.

Inventors: GARRETT MICHAEL ERNEST, GARRETT, MICHAEL ERNEST
Applicants: BOC GROUP PLC, THE BOC GROUP PLC
Application year: 2000
Main classification: C22C1
IPC codes: B22F3