JOINING ALUMINIUM ARTICLES

Abstract: A coating (12) comprising filler material and flux material is applied to an aluminium article (10). The coating (12) is heated to cause it to form a brazed joint between the article (10) and a further article. The coating (12) comprises a mixture of particles (16) which consist solely of filler material and particles (26) which consist solely of flux material. The particles (16 and 26) both have a particle size of from 50 to 100 microns.

Description: JOINING ALUMINIUM ARTICLES This invention is concerned with joining aluminium articles, ie components made from aluminium or its alloys or which are clad with aluminium or its alloys. It is well-known that aluminium is difficult to braze. This problem was alleviated by the introduction of flux materials which remove oxides from the vicinity of the braze, such flux materials being used in conjunction with aluminium alloy filler materials. However, the application of the correct guantities to the correct locations remains a problem. Conventionally, a component to be brazed is dipped into or sprayed with a slurry containing the flux material having a particle size of about 5 microns and the filler is added separately. This almost inevitably leads to there being flux material in unwanted locations and does not ensure that the correct proportions of flux and filler materials are present. Attempts to solve this problem have involved use of particles of filler material which have particles of flux material embedded in them or the flux material forming a coating on the outside thereof. The complex particles are either adhered to the component by adhesive or are thermally sprayed on to the component so that they solidify thereon. These attempts have improved the possibility that the correct proportions of flux and filler material are present in approximately the correct locations but involve a considerable cost in the preparation of the complex particles. It is an object of the present invention to provide a method of joining together aluminium articles which enables the advantages obtained through the use of the above- identified complex particles to be obtained using simple particles. The invention provides a method of joining together aluminium articles, the method comprising applying a coating comprising aluminium alloy filler material and a flux material to at least part of one of said articles, placing the articles so that the coating is between them, and heating thereby causing the filler material and the flux material to melt and form a brazed joint joining the articles, characterised in that said coating is applied by applying an adhesive to said part of the article and applying a mixture of particles to the adhesive, the mixture comprising particles which consist solely of filler material and have a particle size of from 20 to 200 microns and particles which consist solely of flux material and have a particle size of from 20 to 200 microns. In a method according to the invention, particles of flux material which are much larger that the conventional 5 microns are used. This means that the flux material particles are comparable in size with the particles of filler material and can be successfully mixed therewith and adhered by adhesive to a component. The proportions of the particles in the mixture controls the proportions of the materials applied to the component and the location is controlled by the location of the adhesive. Although a method according to the invention reguires larger particles of flux material to be formed, this has the advantage that the particles can be more easily handled and is far easier and cheaper than forming complex particles. It is advantageous, to avoid layer separation of the particles, if the ratio of the volume of the particles which consist solely of filler material to the volume of the particles which consist solely of flux material is approximately egual to the inverse ratio of the densities of the materials. This ensures that all the particles are approximately of the same weight thereby avoiding layer separation. Thus, for example, if the filler material particles have a particulate volume and the flux material has a greater density than the filler material, the particles of flux material are made smaller. The filler material may contain between 9 and 13 wt% of silicon, advantageously being a eutectic alloy with about 11 wt% of silicon. The flux material may be conventional Nocolok (Registered Trade Mark) flux containing 40 to 50 wt% of potassium fluoride and 50 to 60 wt% of aluminium fluoride. In a method in accordance with the invention, the adhesive may be applied by printing, eg screen-printing, or spraying, eg hydraulic spraying. A suitable adhesive is an acrylic resin. The mixture of particles is preferably applied by metering particles of the filler material and of the flux material into a sieve from which the mixture drops on to the adhesive. The sieve may be oscillated to improve the mixing. Further sieves may be used to further improve the mixing. The particles of flux material may be prepared by spray drying. There now follows a detailed description, to be read with reference to the accompanying drawing, of two methods of joining together aluminium articles which are illustrative of the invention. In the drawings: Figure 1 is a diagrammatic view of a stage in the first illustrative method; Figure 2 is a view similar to Figure 1 but of the second illustrative method; and Figure 3 is a view taken in the direction of the arrow III in Figure 2. The first illustrative method is a method of joining together parts of a heat exchanger, in this case a vehicle radiator. One of these parts is a tube 10 made of an aluminium alloy. The illustrative method comprises applying a coating 12 to part of the tube 10, placing the tube 10 and a further aluminium alloy part (not shown) so that the coating 12 is between them, and heating in a brazing furnace under a nitrogen atmosphere, thereby causing the coating 12 to melt to form a brazed joint joining the tube 10 and said further part. The first illustrative method is conventional except as far as the constitution and application of the coating 12 are concerned so that only these items are described hereinafter. The coating 12 is applied by first applying a layer of adhesive 14 to said part of the tube 10. The adhesive is, in this case, an acrylic resin, specifically polymethylmethacrylate, which is dissolved in solvent, in this case toluene, and sprayed on to the tube 10. Alternatively, the adhesive may be printed on to the tube. A wide range of other adhesives and solvents could also be used, the criteria for use being ability to stick to the tube 10, remaining sticky long enough to receive to coating 12, and burning off in the heating stage of the illustrative method without leaving a significant residue. After the layer 14 has been applied thereto, the tube 10 is moved beneath an applicator station at which a mixture of particles is applied to said part of the tube 10 so that the particles adhere to the layer 14 of adhesive. The mixture comprised particles 16 which consisted solely of aluminium alloy filler material. In this case, the filler material had a composition of 11 wt% silicon and 89 wt% aluminium so that it was a near-eutectic alloy. The particles 16 had a particle size of from 20 to 200 microns. The particles 16 were delivered to a hopper 18 which was closed at its lower end by a rotatable drum 20 having rows of bristles 22 projecting radially therefrom at egual circumferential spacings. The drum 20 was arranged to be rotated (clockwise viewing the drawing) to release metered guantities of the particles 16 to fall into a sieve 24, the guantity of particles released depending on the speed of rotation of the drum 20 and the length of bristles. The mixture applied at the applicator station also comprised particles 26 which consisted solely of flux material. In this case, the flux material was Nocolok (Registered Trade Mark) having a composition of 40 to 50 wt% of potassium fluoride and 50 to 60 wt% of aluminium fluoride. This flux material was obtained as particles having a particle size of about 5 microns which were dissolved in a solvent and spray dried to obtain particles of from 20 to 200 microns in size. The particles 26 were delivered to a hopper 28 which was similar to the hopper 18 and was closed by a drum 30 having rows of bristles 32 which were similar to the drum 20 and the rows 22. Rotation of the drum 30 (anti¬ clockwise viewing the drawing) was effective to release metered guantities of the particles 26 into the sieve 24. Thus, controlled rotation of the drums 20 and 30 allowed the particles 16 and 26 to be introduced into the mixture in any desired proportions. The proportion of particles 26 in the mixture was between 1% and 50%. The sieve 24 was arranged to pass particles of 100 microns or less in size and was oscillated horizontally to mix the particles 16 and 26. From the sieve 24, the particles fell to two similar sieves 34 and 36 which were also oscillated horizontally, the sieves 24, 34, and 36 being arranged one above the other. From the sieve 36, the particles 16 and 26 fell on to the layer 14 of adhesive and adhered thereto forming the coating 12. In the coating 12, the particles were in a single layer with a substantially uniform proportion of the particles 16 and 26 in each area thereof. The second illustrative method is identical to the first illustrative method (and like parts are given the same reference numbers) except that the hopper 18, and the hopper 28 are disposed side by side and are closed by the same drum 20. A further hopper 40 is disposed to be closed by the drum 20 also. The hoppers 18, 20 and 40 are arranged in a row with the hopper 20 in the middle. The hopper 40 contains further particles 16 which consist solely of filler material. In this case, the rotation of the drum 20 causes particles 16 to fall from both the hoppers 20 and 40 into the sieves 24, 34, and 36 and also particles 26 to fall from the hopper 28 into the same sieves. The proportion of particles 16 and 26 in the mixture is determined by the sizes of the openings of the hoppers 18, 28 and 40.

Claims: CLAIMS A method of joining together aluminium articles (10) , the method comprising applying a coating (12) comprising aluminium alloy filler material (16) and a flux material (26) to at least part of one of said articles, placing the articles so that the coating is between them, and heating thereby causing the filler material and the flux material to melt and form a brazed joint joining the articles, characterised in that said coating (12) is applied by applying an adhesive (14) to said part of the article (10) and applying a mixture of particles to the adhesive, the mixture comprising particles (16) which consist solely of filler material and have a particle size of from 20 to 200 microns and particles (26) which consist solely of flux material and have a particle size of from 20 to 200 microns. A method according to claim 1, characterised in that the ratio of the volume of the particles (16) which consist solely of filler material to the volume of the particles (26) which consist solely of flux material is approximately egual to the inverse ratio of the densities of the materials. A method according either one of claims 1 and 2, characterised in that the filler material contains between 9 and 13 wt% of silicon. A method according to any one of claims 1 to 3, characterised in that the flux material comprises 40 to 50 wt% of potassium fluoride and 50 to 60 wt% of aluminium fluoride. A method according to any one of claims 1 to 4, characterised in that the adhesive (14) is printed or sprayed on to said part of the article. A method according to claim 5, characterised in that the adhesive is an acrylic resin. A method according to any one of claims l to 6, characterised in that said mixture of particles (16, 26) is applied by metering particles of the filler material and of the flux material into a sieve (24) from which the mixture drops on to the adhesive (14) . A method according to any one of claims 1 to 7, characterised in that said particles (26) of flux material are prepared by spray drying.

Inventors: MARKS KEVIN TREVOR, PERRIN CARL, TIDD JONATHAN LISTER, MARKS, KEVIN, TREVOR, PERRIN, CARL, TIDD, JONATHAN, LISTER
Applicants: MARKS KEVIN TREVOR, PERRIN CARL, T & N TECHNOLOGY LTD, TIDD JONATHAN LISTER, MARKS, KEVIN, TREVOR, PERRIN, CARL, T & N TECHNOLOGY LIMITED, TIDD, JONATHAN, LISTER
Application year: 1996
Main classification: B23K1
IPC codes: B23K1, B23K35