Method for producing an aluminium alloy foam by moulding

Abstract

A method for producing an aluminum alloy foam wherein an aluminum alloy, in molten form, infiltrates the interstices of a preform of silicon elastomer elements, by means of a conventional molding process, typically a low-pressure process, followed by the elimination of the preform broken down into silica powder during the molding cycle and/or an additional baking cycle.

Claims

1. A method for manufacturing an aluminium alloy foam having open cells with a porosity of from 60% to 80%, wherein the method comprises: extruding a silicone elastomer through a die to form silicone elastomer elements; cutting each of the extruded silicone elastomer elements into portions of silicone elastomer elements, each of said portions having a similar length; agglomerating the portions of silicone elastomer elements; polymerizing the agglomerated portions of silicone elastomer elements in a moulding tool to form a preform; removing the preform from the moulding tool; storing the preform at a temperature of from 80 C. to 150 C.; placing the preform into a mould; casting an aluminium alloy in the mould at a temperature of from 800 C. to 820 C., and at a pressure of from 700 mbar to 1.5 bar, to allow the alloy to penetrate into interstices of the preform, thereby forming an aluminium alloy foam and silica powder; and removing the aluminium alloy foam from the mould.

2. The method according to claim 1, wherein the portions are formed as substantially spherical balls before the agglomeration step.

3. The method according to claim 1, wherein the silicone elastomer elements extruded from the die have a circumscribed outside diameter of from 2 mm to 10 mm.

4. The method according to claim 1, wherein the portions have a length of from 2 mm to 10 mm.

5. The method of claim 1, wherein the portions are agglomerated by mixing the portions in the presence of a binder, and forming the preform in a core box mould.

6. The method according to claim 5, wherein the binder is a liquid silicone binder.

7. The method according to claim 6, wherein the liquid silicone binder is used in an amount of from 1% to 3% expressed as a percentage by mass.

8. The method according to claim 5, wherein the binder is a liquid polyurethane resin binder.

9. The method according to claim 8, wherein the liquid polyurethane resin binder is used in an amount of from 2% to 4% expressed as a percentage by mass.

10. The method according to claim 1, wherein the preform has a density of from 0.5 to 0.8.

11. The method according to claim 1, further comprising introducing a tube into the preform during the agglomerating step.

12. The method according to claim 11, wherein the tube is made from aluminium alloy or borosilicate glass.

13. The method according to claim 1, wherein the preform is preheated to a temperature of from 150 C. to 250 C. before being placed into the mould.

14. The method according to claim 1, wherein the preform has a minimum size of 50 mm50 mm, and a thickness of from 10 to 100 mm, and a maximum size of 350 mm350 mm, and a thickness of from 15 to 80 mm.

15. The method of claim 1, wherein the polymerizing step is a natural polymerizing step or a forced polymerizing step.

16. The method of claim 15, wherein the forced polymerizing step is carried out by stoving at a temperature of from 50 C. to 100 C.

17. The method of claim 1, wherein the portions are agglomerated by directly clamping in a core box mould tool or in a press forming tool.

18. The method of claim 1, comprising casting the aluminium alloy at a pressure of from 700 mbar to 1.0 bar.

19. The method of claim 1, further comprising stoving aluminium alloy foam at a temperature of from 400 C. to 450 C., and discharging the silica powder.

20. The method of claim 1, wherein the silica powder is discharged by a member selected from the group consisting of manual knocking out, vibrating, blowing, water under pressure, and a combination thereof.

Description

DESCRIPTION OF THE INVENTION

(1) The invention is based on the finding made by the applicant that silicone elastomer, well known to persons skilled in the art since it is used for producing tubes or cylinders as precursors of air conveying channels in moulds or cores, used themselves in aluminium alloy moulding, withstood the casting of said aluminium alloys without melting, that is to say at temperatures of the order of magnitude of 800 C., the metal solidifying in contact therewith, before decomposing essentially into silica powder under the effect of the heat produced during the casting and solidification.

(2) This material has therefore appeared to be particularly suited to the production of destructible preforms or cores instead of the salt or kaolin or salt paste ball preforms of the prior art for manufacturing aluminium foam by infiltration of aluminium alloy in the interstices left free in said preform, solidification and elimination of the silica powder.

(3) To this end, a silicone elastomer, for example known under the references SI 50 to 80 from the company Plastelec and preferably SI 70, with a hardness of 70 Shore, is used as the base material.

(4) It is formed, for example by extrusion, into elongate elements in very varied shapes, that is to say cylinders, tubes, with star or polygonal cross sections, solid or tubular, rods, etc.

(5) The circumscribed outside diameters of these elongate elements, that is to say with a cross section substantially perpendicular to the extrusion axis, are typically but not exclusively from 2 to 10 mm.

(6) Said elements are then cut, for example by means of a granulator, into portions with a length, typically, but not exclusively, from 2 to 10 mm, which will be referred to as elements constituting the preform.

(7) They may at this stage be used as they stand for the following step or fashioned, in particular in the case of non-hollow elements, in the form of balls, that is to say rounded, for example in a forming machine, that is to say, most usually, between two moving plates.

(8) Said constituent elements, optionally in the form of balls, can then be, according to a variant of the invention, agglomerated as they stand in a low-pressure clamping tool of the core box type.

(9) The polymerisation is then carried out naturally at ambient temperature or forced temperature by stoving at a temperature of typically 50 to 100 C.

(10) Another variant of the invention consists of mixing said elements, for example in a mixer of the plug mill type, in the presence of a thermodegradable organic binder. The latter may in particular be of the polyurethane type, for example of the Isocure type from Ashland, at a proportion typically, but not exclusively, of 2% to 4% as a percentage by weight, or of the liquid silicone type, for example RTV with a component from the company Plastelec, at a proportion typically, but not exclusively, of 1% to 3% as a percentage by mass.

(11) The mixture is next placed for example in a tool of the core box type with a clamping pressure that is conventional for this type of tool, and the polymerisation is then carried out, as above, naturally at ambient temperature or forced by stoving at a temperature typically of 50 to 100 C., preferably 80 C., for half an hour to three quarters of an hour.

(12) It should be noted that, before or during the placing in the clamping tool, other elements may be introduced among the elements constituting the preform, such as for example aluminium alloy (or other) tubes, which proves to be particularly advantageous in the context of the subsequent manufacture of tube-type heat exchangers, or made from glass of the PYREX glass type, for applications in the medical field.

(13) It is also possible to introduce, among the elements constituting the preform, cores made from agglomerated moulding sand or other preferably thermodegradable material in order to produce in the foam orifices or other empty shapes, that is to say free from metal.

(14) The preform is then extracted from the forming/clamping tool in order to evacuate the solvents, in ambient air for a few hours or in an oven, typically between 80 and 150, for half an hour to two hours.

(15) The preform is then ready for the operation of moulding and infiltration with the liquid aluminium alloy, which is preferably carried out by low-pressure casting, the overpressure of liquid metal obtained by this method, typically 700 mbar to 1.5 bar at the end of a rise ramp of one to two seconds, facilitating the penetration of the alloy in the interstices of the preform.

(16) Before it is placed in the mould, which may be of the permanent metal type or of the destructible sand type or mixed, the preform may be preheated, at a temperature typically of 150 to 250 C.

(17) Casting of the low pressure type is then proceeded with in a conventional fashion.

(18) The alloy usually used is of the AlSi.sub.7Mg.sub.0.6 type but any other type of moulding alloy having good castability can be used.

(19) In the first case, the casting temperature is typically 800 to 820 C. The tube and feed system are filled and then the pressure-rise ramp, typically from 700 mbar to 1.5 bar, and preferentially from 700 mbar to 1 bar, is applied with a time generally of one to two seconds.

(20) The part obtained is then extracted, either by simple removal from the mould in the case of a metal mould, or by destruction of the mould on a vibrating grid, an operation known to persons skilled in the art as knocking out.

(21) At this stage the deburring and dressing or machining of the faces of the part can be carried out.

(22) An additional decomposition of the silicone residues into silica powder can also be carried out at a temperature of around 400 to 450 C. if it is not wished to await the natural decomposition during cooling of the part or if the decomposition is not complete at the end thereof.

(23) The final discharge of the silica powder generally takes place by vibration and blowing compressed air, optionally by means of pressurised water.

(24) It should be noted that this method perfectly meets the stated problem and has numerous advantages compared with the prior art:

(25) The manufacture of the preform is completely easy and the latter is sufficiently strong to make it easier to handle, making it possible to obtain larger dimensions of foam than by the methods or the prior art.

(26) It is moreover possible to assemble several preforms, for example by adhesive bonding, in order to obtain foams with a larger size.

(27) Depending on the organisation of the elements constituting the preform and the choice and any mixing thereof, it is possible to obtain an isotropic or anisotropic open porosity, and this in a perfectly controlled fashion.

(28) Inserting, in the preform during manufacture thereof, tubes made from aluminium alloy, typically intended to serve as heat exchanger tubes, but also for example made from glass of the PYREX glass type, or cores, is entirely possible, whereas the method of manufacturing the preforms of the prior art and/or the need for a pyrolysis step, as in the case of the salt paste, at 500 C., compromise this type of operation.

(29) The destruction of the preform as a powder, during the infiltration/solidification or requiring only subsequent quick stoving, is also a highly appreciable advantage compared with the preforms of the prior art for which this step is often very detrimental.

(30) Finally, the moulding method used, by low-pressure casting, is entirely standard and widespread, without any special adaptation.

(31) In its details, the invention will be better understood by means of the following examples, which do not however have any limitative character.

EXAMPLES

Example 1

(32) The silicone elastomer known by the reference SI 70, with a hardness of 70 Shore, from the company Plastelec, was used as the base material.

(33) It was extruded into cylindrical tubes with outside diameter of 3 mm and inside diameter 1.7 mm.

(34) The elements constituting the preform were obtained by cutting by means of a granulator, portions with a length of 3 mm.

(35) The elements were mixed with a binder of the liquid silicone type, in this case RTV with a component from the company Plastelec, at a proportion of 2.2% expressed as a percentage by mass, that is to say 40 g of binder for 1.6 kg of hollow cylindrical granules.

(36) They were then placed in the cavity of a core box with dimensions of 233 mm233 mm40 mm, where they occupied the entire space.

(37) The polymerisation was effected in ambient air, with removal from the box after 3 hours.

(38) The preform obtained was stoved for 2 hours at 150 C. to discharge the solvents.

(39) The density of the preform obtained, of dimensions 233 mm233 mm40 mm, was 0.73.

(40) The preform was preheated to 150 C. and placed in a sand mould, the cavity of which had substantially the same dimensions.

(41) The AlSi.sub.7Cu.sub.0.6 alloy was cast in low pressure mode at 815 C., with filling of the tube and feed system, and then the mould was filled during the final pressure rise of 791 mbar, in 1.6 s.

(42) After solidification and cooling, the mould was knocked out on a vibrating grid, the part deburred, and the faces machined, and then the remaining silica powder was removed by vibration and final blowing with compressed air.

(43) The foam obtained had dimensions of 218 mm218 mm40 mm and a weight of 1.5 kg.

(44) Its calculated density was 0.8 and its open porosity 71%.

Example 2

(45) The same silicone elastomer as before was used.

(46) It was extruded in rods, that is to say solid cylinders, with a diameter of 5 mm.

(47) They were cut as before into portions with a length of 5 mm.

(48) The elements were mixed with a binder of the liquid silicone type, in this case RTV with a component from Plastelec, at a proportion of 2% expressed as a percentage by mass, that is to say 30 g of binder for 1.6 kg of solid cylindrical granules. They were then placed in the cavity of a core box with dimensions of 233 mm233 mm40 mm, where they occupied the entire space.

(49) Two cylindrical moulding cores, made from agglomerated sand, with a diameter of 35 mm and a length of 40 mm, that is to say over the entire thickness of the preform, and two aluminium alloy tubes of the AA 5086 type with an outside diameter of 12 mm and a thickness of 0.8 mm, in a direction perpendicular to the cores, were also placed at the heart of the assembly.

(50) Polymerisation was partly effected in an oven for hour at 80 C., then at ambient temperature, with removal from the box with separation after a total of 2 hours.

(51) The preform obtained was not stoved.

(52) The density of the preform obtained, of dimensions 233 mm233 mm40 mmm, was 0.73.

(53) The preform was preheated to 150 C. and placed in a sand mould, the cavity of which had substantially the same dimensions.

(54) The AlSi.sub.7Cu.sub.0.6 alloy was cast in low pressure mode at 809 C., with filling of the tube and feed system, and then the mould was filled during the final pressure rise of 720 mbar, in 1.4 s.

(55) After solidification and cooling, the mould was knocked out on a vibrating grid, the part deburred, and the faces machined, and then the remaining silica powder was removed by vibration and final blowing with compressed air.

(56) The foam obtained had dimensions of 225 mm225 mm40 mm and a weight of 1.4 kg.

(57) Its calculated density was 0.7 and its open porosity 74%.

(58) There are then the two orifices with a diameter of approximately 35 mm passing through the foam throughout its entire thickness, left by the cores, as well as the aluminium alloy tubes, in this case in a direction perpendicular to the cores, and over the entire length of the foam.