Sintered metal material having directional porosity and comprising at least one ferromagnetic part, and production method thereof

Abstract

The invention relates to a sintered metal material comprising at least one magnetic part, characterised by directional through-pores having a size of between 1 and 100 μm, said material having a density varying by less than 20% from one sample of 1 cm3 to another taken from a one-piece part made from the material.

Claims

1. A sintered metallic material having a lamellar structure and comprising at least one magnetic portion, said sintered metallic material comprising open-ended oriented porosities of dimension of between 1 and 100 μm, said sintered metallic material having a density varying by less than 20% from one 1 cm.sup.3 sample to another, taken from a single-piece part made from said sintered metallic material, wherein the sintered metallic material is denser than titanium.

2. The material as claimed in claim 1, comprising at least one non-magnetic portion in a volume proportion of less than or equal to 50% and optionally a ferrimagnetic part in a volume proportion of less than or equal to 50%.

3. The material as claimed in claim 2, wherein the non-magnetic portion comprises at least one of: aluminum, niobium, titanium, chromium, molybdenum, manganese, or magnesium.

4. The material as claimed in claim 1, wherein the magnetic portion is ferromagnetic and comprises at least one of: iron, nickel, cobalt, ferromagnetic alloy, or ferromagnetic steel.

5. A process for manufacturing a sintered metallic material, comprising: providing a mixture of metallic powder and of solvent, the powder comprising a magnetic portion, casting the mixture into a mold, applying a magnetic field parallel to within +/−10° to the gravitational field lines in order to cause the magnetic field lines to pass into the molded mixture, causing the displacement of the powder until the powder is flush with an upper surface of the mixture, cooling the molded mixture to a temperature below the solidification temperature of the solvent, sublimating the solidified solvent, sintering the powder to obtain the sintered metallic material, the pores of the sintered metallic material being aligned along the magnetic field lines, being open-ended and being of dimension of between 1 and 100 μm, the sintered metallic material having a density varying by less than 20% from one 1 cm.sup.3 sample to another, taken from a single-piece part made from said sintered metallic material.

6. The process as claimed in claim 5, wherein the mixture of powder and solvent further comprises a binder, the binder being eliminated during sintering, the proportion of binder relative to the solvent being preferably between 1 and 5% by weight.

7. The process as claimed in claim 5, wherein the solvent is chosen from the group consisting of: water, camphene, or tert-butyl alcohol and 2-methylpropan-2-ol.

8. The process as claimed in claim 5, wherein the magnetic field is determined such that, the powder consisting of grains, the grains become magnetized and oriented in the direction of the field while remaining in the mixture, the magnetic field being generated by a permanent magnet or a coil.

9. The process as claimed in claim 5, wherein the magnetic field is less than 30 mT.

10. The process as claimed in claim 5, wherein the solidified event is sublimated by lyophilization or vacuum pumping at a temperature below the triple point of the solvent.

11. The process as claimed in claim 5, wherein the powder has a mean particle diameter of between 0.5 and 50 μm and/or the powder has a particle diameter less than a value located in the range of 2 and 100 μm.

12. The process as claimed in claim 5, wherein the sintering is carried out under inert gas under a pressure of 0 to 0.25 bar for a duration of between 30 and 180 minutes, at a temperature less than ⅔ of the melting point of a metal of the metallic powder mixture having the lowest melting point expressed in Kelvin.

13. The process as claimed in, claim 5, wherein the sintering is carried out with a temperature rise and fall ramp of less than or equal to 10 K/minute.

14. The process as claimed in claim 5, wherein the sintering is followed by a deposition of another material on a surface of the sintered metallic material, by carburization, CVD, PVD, electrochemical or electrolytic deposition, or powder impregnation.

15. The process as claimed in claim 5, wherein the mixture is cast in an organized structure, obtained by additive manufacturing or by foundry, to provide mechanical strength.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will be better understood from the detailed description of some embodiments taken as non-limiting examples and illustrated by the appended drawings:

(2) FIGS. 1 to 4 are sectional extracts of samples of materials taken under an optical microscope,

(3) FIG. 5 shows an example of assembly for the process, and

(4) FIG. 6 is an optical micrograph of organized material surrounded by sintered metallic material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(5) Metallic materials with oriented porosity are of interest in numerous applications such as filtration or the diffusion of chemical species. The micropores can also be effectively infiltrated by a liquid, for example lubricant in parts for which it is desired to delay the mechanical wear. The large specific surface area of the materials with oriented porosity also makes them very good choices for porous electrodes or for catalysis applications. These materials are very good absorbers of thermal, mechanical or electromagnetic energy, which makes them of great interest for applications in heat exchangers, protection against impacts or shock waves, shields for electrical installations against interference due to parasitic radiation, etc.

(6) Moreover, carefully dimensioned materials with controlled porosity can accurately reproduce the anisotropic morphology of bones and therefore their mechanical properties. It would become possible to improve the quality and service life of biomedical prostheses and also osseointegration thereof.

(7) The phenomena involved in the invention can be explained in a simplified two-dimensional case in which the applied magnetic field is unidirectional, in the vertical direction and in which the powder is spherical.

(8) The powder is immersed in the solvent: each particle is subjected to gravity, which tends to cause it to sediment, and also Archimedean upthrust, which tends to cause it to float. The density and the size of the metallic and possibly non-metallic grains of powder are such that gravity overcomes Archimedean upthrust and the powder flows to the bottom of the mold.

(9) A magnetic field B.sup..fwdarw. is applied to the mixture. If this field is large enough, gradually each grain of magnetic powder becomes magnetized in the direction of the magnetic field B.sup..fwdarw.. Each grain of magnetic powder behaves locally like a small magnet. Through the action of forces of magnetic repulsion and attraction, the grains of powder will create chains in the direction of the magnetic field B.sup..fwdarw.. This movement will be greatly facilitated by the Archimedean upthrust exerted by the solvent.

(10) These strings of grains of powder will rise up to the solvent/air interface. The force generated by the magnetic field B.sup..fwdarw. tends to extend the alignment of the structures beyond this interface by causing grains of powder to leave the solvent. However, this crossing of the interface would expend energy:

(11) the absence of Archimedean upthrust makes the movement of the grains of powder much more difficult in the air,

(12) the air/water surface tension tends to prevent the crossing of the grains of powder.

(13) There is therefore an opposition between:

(14) the forces aiming to cause the interface to be crossed: the attractive force of the magnet and the repulsive forces between neighboring strings of powder,

(15) the forces aimed at confining the powder under the solvent/air interface mentioned above.

(16) A magnetic field B.sup..fwdarw. is applied in a precise range to create the final material: strong enough to magnetize the powder and create the alignment of the particles, but without exceeding the value for which the powder crosses the interface.

(17) In order to obtain the final material, the solvent is then solidified and then sublimed, and the part is sintered at high temperature.

(18) In a real three-dimensional system, lamellae of powder are obtained, and not columns. This is explained by the repulsive and attractive forces that would tend to form two-dimensional structures. If the magnetic field lines are not parallel in the space considered, as is the case if using a conventional magnet, zones in which the lamellae have the same orientation, corresponding to the local magnetic field, are observed. These lamellae all have a preferential orientation along the main axis of the magnet. The disorientation thereof provides mechanical strength to the assembly. Indeed, if all the lamellae of the sample were perfectly parallel, the assembly would be extremely fragile when stressed perpendicularly to the lamellae. Partial disorientation of the lamellae improves resistance to transverse stresses.

(19) The solvent/air interface confines the powder. The volume of the final part is thus easily controllable since it is equal to the volume of solvent. This also makes it possible to precisely vary the porosity of the part. All parameters being otherwise equal, if there is more solvent, the space between the chains of powder will be larger and the final part will therefore be more porous.

(20) The microstructure is composed of lamellae with open porosities oriented in the direction of the magnetic field, of a size of the order of ten microns. The choice of parameters enables pore sizes between 1 and 100 μm and a homogeneity of the material obtained such that the density varies by less than 20% from one 1 cm3 sample to another, taken from a single-piece part made from said material, especially a part of a volume of 4 cm3.

(21) A non-magnetic portion is possible in volume proportion of less than or equal to 50%. The non-magnetic portion comprises at least one from: aluminum, niobium, titanium, chromium, molybdenum, manganese, magnesium.

(22) A ferromagnetic portion is also possible in volume proportion of less than or equal to 50%.

(23) In the general case, the magnetic portion is ferromagnetic and comprises at least one from: iron, nickel, cobalt, ferromagnetic alloy, especially ferromagnetic steel.

(24) In a first test, a cylindrical nickel slug with oriented porosities was manufactured. An aqueous solution containing 3% by weight of organic binder, in this case polyvinyl alcohol (PVA), is prepared. Then 7.5 g of pure nickel powder, with a diameter of less than 3 μm, are mixed with 4 ml of aqueous solution. The mixture is poured into a Teflon pad 3 cm in diameter resting on a metal plate, then a ferrite magnet is placed a few centimeters above the pad using a non-magnetic strip fixed on lateral supports, see FIG. 5. The average field at the aqueous solution loaded with nickel powder is of the order of 2 mT. The magnet is of elongate form and arranged so as to have field lines perpendicular to the surface of the solvent. The field lines are vertical for a portion of the solution and rounded for other portions of the solution. The mounting assembly is then placed in a cooling chamber set at a temperature below the solidification temperature of the aqueous solution, for example −18° C., until complete solidification of the water.

(25) The solidified solution and the metal plate supporting it are then removed and mounted on a cooling plate set at −10° C. The plate is then covered with a suitable sealed cover and connected to a vacuum pump. The vacuum pump makes it possible to reach a vacuum of the order of a few tenths of a Pascal. The pressure is controlled by a pressure sensor. After 16 h, the water is entirely sublimated. In addition, a liquid nitrogen trap makes it possible to recondense the water vapor to protect the pump.

(26) Once the sublimation of the water is achieved, this gives a green body with oriented grains of powder, the strength of which is temporarily provided by the binder.

(27) The green body is subjected to a heat treatment of 60 minutes at 900° C. under a stream of argon to obtain the final sintered part which has a diameter of 2.5 cm, i.e. a shrinkage of 17%. The pressure is 0.25 bar. The rise in temperature is 10 K/minute. The fall in temperature is 10 K/minute.

(28) It has been found that the free surface obtained is relatively rough. To obtain flat and parallel surfaces, the two end faces can be polished. This creates a dense surface layer. The surface layer can be removed by immersing the part in a solution of nitric acid diluted to 50%. This step can also be avoided by using a mold for which the free surface does not correspond to a surface of interest. The sintered part has entangled nickel lamellae with an average pore size of 50 μm. The pores are open-ended. The Darcy permeability of the sample is 10-12 m2. The heat treatment has an influence on the density of the lamellae and on the shrinkage. The alignment of the pores on the magnetic field lines is preserved. The density is homogeneous.

(29) Furthermore, the Applicant manufactured other samples according to the table below with pure nickel powder with a mean diameter of 3 μm:

(30) TABLE-US-00001 Rise and Weight Volume % by Duration Duration fall ramp Sintering Porosity of of the weight of of of the temper- of the powder solution of freezing sintering oven ature part No. (g) (ml) PVA (days) (h) (° C./min) (° C.) (%) 1 7.5 4 2 2 1 10 900 72 2 7.5 3 2 2 1 10 900 64 3 7.5 5 2 2 1 10 900 74 4 7.5 5.5 3 2 1 10 900 75

(31) FIG. 1 shows one aspect of the sample no. 1 with a lower zone 1 having a slightly less porous zone over a small height of a few % of the height of the sample.

(32) The zone of interest 3 has an orientation of the nickel lamellae that is indeed parallel to the north/south axis of the magnet. The lamellae are therefore relatively close to the vertical near the peripheral zone 2 and increasingly inclined closer to the central zone 4 located under the magnet. The central zone 4 has a portion with heterogeneities. The average inter-lamellar space is 18.2 μm+/−2. The porosity content is 72%.

(33) From observing FIG. 1, the Applicant deduces the benefit in implementing a magnetic field with parallel field lines over the entire zone of the sample, for example by means of a magnet assembly or an electromagnetic coil. The sedimentation present in the lower zone 1 can also be reduced if necessary by the use of a sonotrode or an ultrasonic bath.

(34) Sample no. 2 illustrated in FIG. 2 presents a zone 11 with porosities oriented in a plane perpendicular to the plane of observation. This zone lacked solvent, limiting the ability to orient along the field lines. The sample has a zone of interest 12 with porosities oriented in a relatively parallel manner. The sedimentation effect in the lower zone is weaker than on the sample 1. The lamellae are tight and the density obtained is high, which corresponds to a lower porosity than for sample no. 1. The effect of decreasing the amount of solution between sample no. 1 and sample no. 2 is that of having smaller inter-lamellar spaces. The average inter-lamellar space is 10 μm+/−6. The porosity content is high, at 64%.

(35) Sample no. 3 has an appearance closer to sample no. 1 with a lower zone 1 having a slight sedimentation of the powder prior to sublimation, a central zone 4 having porosities that are parallel by zones. The average inter-lamellar space is 31 μm+/−4. The porosity content is high, at 74%. The sample has rough patches.

(36) The porosity is higher than for samples no. 1 and no. 2. The presence of zones 3 and 4 is observed, in which the lamellae are parallel to one another.

(37) In the embodiment of FIG. 4, sedimentation at the bottom of the sample is low. The inter-lamellar space is larger than for sample no. 3. The porosities are oriented virtually vertically. Surface rough patches are present. The average inter-lamellar space is 41 μm+/−7. The porosity content is very high, at 75%.

(38) The comparison between samples no. 1 to no. 4 makes it possible to study the influence of the parameter of the volume of the solution. An increase in the volume of the solution results in an increase in the porosity content and an increase in the inter-lamellar space.

(39) Finally, it is preferable to apply the magnetic field before placing in the cooling chamber in order to enable the powders to become placed along the magnetic field lines before solidification of the solution.

(40) FIG. 6 illustrates an example of sintered metallic material formed around a metallic structure of nominal density. Nominal density is intended to mean here the density of a microscopically solid metal or metal alloy, for example one which is cast or machined. The metallic structure here is a square mesh grid surrounded by a ring. The metallic structure has mechanical properties superior to the mechanical properties of the sintered metal material. The metallic structure is obtained by casting or by additive manufacturing. The sintered metallic material fills the square holes of the grid. In practice, provision is made either to keep the upper surface of the grid free, or to cover it with the sintered metallic material. The sintered metallic material is obtained using 7.5 g of Ni powder, 5 ml of solution, 2% of PVA with a sintering of 1 hour at 900° C., the other parameters being identical to those of the test no. 3. The interlamellar space obtained is 37 μm+/−2. The sintered metallic material is compatible with a mechanically strong structure.