FILTRATION OF ALUMINIUM-LITHIUM ALLOYS

Abstract: Alumina oxide and other non-metallic compounds, such as lithium oxide, are removed from molten aluminium-lithium alloys by filtration through a porous ceramic body comprising silicon carbide. The porous ceramic body may be, for example, a honeycomb type of structure or a foam having interconnecting pores.

Description: * FILTRATION OF ALUMINIUM-LITHIUM ALLOYS This invention relates to the filtration of alum nium-lit ium alloys. It is well known, in order to remove alumini-um oxide from molten aluminium or aluminium 5 alloys to pass the aluminium or aluminium alloy through a filter such as a fibreglass cloth, a bed of refractory particles such as alumina balls or a ceramic foam made from a material such as alumina, an al umi nosi 1 i cate such as mullite, or cordierite. 10 Molten aluminium-lithium alloys which contain 1.0 to 4.5% lithium, usually 1.1 to 2.5%, contain other non-metallic compounds in addition to aluminium oxide, for example lithium oxide, which will produce inclusions if they are not removed before 15 the metal is cast. However because aluminium-lithium alloys are particularly reactive compared with most other aluminium alloys they will attack the materials commonly used for filtering molten aluminium alloys 20 so that to date no satisfactory method of filtering aluminium-lithium alloys has been found. Instead it is necessary to take special precautions to minimise oxidation of the lithium and thereby minimise the formation of non-metallic compounds, for example by 25 melting the alloy under an inert atmosphere or under a thick layer of flux, and before casting to remove the non-metallic compounds present by a method other than filtration. According to the present invention there is provided a method of filtering a molten aluminium- -lithium alloy which comprises passing the molten alloy through a porous ceramic body comprising silicon carbi de . The porous body may be for example a honey¬ comb type of structure having pores which extend from one face of the body to another face or a structure having interconnecting pores such as a ceramic foam. Foam structures are preferred and such structures may be made using a known method of making ■ a ceramic foam in which an organic foam, usually a polyurethane foam, is impregnated with an aqueous slurry of ceramic material containing a binder, the impregnated foam is dried to remove water and the dried impregnated foam is fired to burn off the organic foam to produce a ceramic foam. The production of ceramic foams by this method is described in United States Patent 3090094 and in British Patents 923862, 916784, 1004352, 1054421 , 1377691, 1388911, 1388912 and 1388913. It is preferred that the porous ceramic body used-as the filter in the present invention is made solely from silicon carbide apart from minor amounts of oxides which are present as residues from binders, for example phosphorous pentoxide from monoal u i ni urn phosphate, and/or from additives which may have been used in the production of the porous body, for example silica from clay which may be added to improve the rheological properties of the slurry when the body is produced by impregnation of an organic foam with a silicon carbide-containing slurry, However the body may also contain a proportion, usually not exceeding 45% by weight, of a refractory oxide such as alumina. The following example will serve to illustrate the invention:- A ceramic foam having the following com¬ position by weight:- silicon carbide 53.0% al umi na 34.2% silica 4.1% phosphorous pentoxide 8.7% was made by impregnating a reticulated polyurethane foam with an aqueous slurry containing silicon carbide, alumina, clay and aluminium orthophosphate as binder, and drying and then, firing the impregnated foam. In order to assess the suitability of the foam as a fi 1 er for aluminium-li hium alloys the foam was fixed to a refractory tube by means of a high temperature resistant adhesive, the tube was attached to a steel rod, and the foam was then immersed in molten LITAL aluminium-lithium alloy at 760-765°C. The foam was unaffected by the molten alloy after immersion for 60 minutes. Similar tests were also performed on other types of filter materials. Honeycomb type structures of mullite, cordierite and mul 1 i te/cordi eri te and ceramic foams of alumina and al umi nosi 1 i cate were all attacked and severely weakened after 10 minutes immersion in the molten aluminium-lithium alloy, while silica based cloth withstood 60 minutes immersion but was extremely brittle on removal .

Claims: C L A I MS 1. A method of filtering a molten aluminium alloy wherein the molten alloy is passed through a porous ceramic body characterised in that the alloy is an aluminium-lithium alloy and the porous ceramic body comprises silicon carbide. 2. A method according to claim 1 characterised in that the porous ceramic body has pores which extend from one face of the body to another face. 3. A method according to clai 1 characterised in that the porous ceramic body is a ceramic foam. 4. " A method according to claim 1 characterised in that the porous ceramic body contains a minor amount of phosphorus pentoxide. 5. A method according to claim 1 characterised in that the porous ceramic body contains a minor amount of silica. 6. A method according to claim 1 characterised in that the porous ceramic body contains alumina. 7. A method according to claim 6 characterised in that al umi na • content is not more than 45% by weight .

Inventors: CHILD NICHOLAS RONALD, CHILD, NICHOLAS, RONALD
Applicants: FOSECO INT, FOSECO INTERNATIONAL LIMITED
Application year: 1986
Main classification: C22B21
IPC codes: B01D39, C04B35, C04B38, C22B21