Novel porous body and process for its preparation

The invention concerns the manufacture of a porous product based on nickel, hromium, tungsten, molybdenum, iron, cobalt or an alloy of these metals. The porous product is characterized on one hand by the fact that it comprises 85 to 99.5% by weight of the base material and 15 to 0.5% of a fusible auxiliary metal chosen from among tin, indium, gallium, germanium and mixtures and alloys of these metals and, on the other hand, by the fact that the base material is in the form of elementary particles such as powders, fibers or chips, bonded to each other by means of diffusion brazing.

BACKGROUND OF THE INVENTION 
The present invention concerns novel porous bodies and a process for their 
preparation. 
It is known to prepare metallic pieces which may be porous, by a diffusion 
process. The process, effected in the solid state, consists of contacting 
metallic particles with each other, pressing them and heating them so as 
to make diffusion of metals from one particle to the other possible. 
A process has also been described, in particular in the French patent 
applications No. 77.06149, filed on Feb. 24, 1977 and No. 78.01318, filed 
on Jan. 18, 1978, wherein stainless steel pieces are bonded together by 
interposing between between the faces of the pieces to be joined, a layer 
of a fusible and diffusible material, then heating the assembly so as to 
effect the fusion and diffusion of the two pieces of said fusible and 
diffusible material and finally cooling the assembly. The process may be 
described as a diffusion brazing process. 
SUMMARY OF THE INVENTION 
The present invention concerns a diffusion brazing process, which is 
applied to a base material in bulk--thus having a certain porosity prior 
to being joined together--and produces final materials of an essentially 
identical porosity. 
The diffusion brazing process according to the invention is applicable to 
base materials which may consist of nickel, iron, cobalt and various known 
alloys of these materials. The base materials, which constitute 
approximately 85 to 99.5% by weight of the final product, must be present 
in the form of elementary particles such as powders, fibers or chips. 
When the base materials are in the form of powders, they have an average 
grain size in keeping with the intended application of the material; when 
the base materials are in the form of fibers or chips, the bulk products 
used may have very low apparent densities; it is, however, preferable to 
initially compact the accumulations of fibers or chips so that these 
accumulations will have apparent densities of the same order as those 
obtained from the same material when present in the form of a powder. 
According to the invention, the particles of the base materials are bonded 
together by means of a low melting point auxiliary material selected from 
the group comprising tin, indium, antimony, gallium, germanium, or formed 
of a combination of these elements, by exposing the assembly to a heat 
treatment in a controlled atmosphere, at a temperature higher than the 
melting temperature of the auxiliary material, but lower than the solidus 
temperature of the base material in all cases. In the case wherein 
particles of base materials of a different nature are present, the 
temperature of the heat treatment will be controlled by the lowest 
solidus. The treatment is applied for a period of time sufficient to allow 
the migration of the auxiliary material and the formation of compounds or 
of solutions, which insure the diffusion brazing of the particles to be 
bonded. The amount of the auxiliary material to be used is between 0.5 and 
15% by weight with respect to the weight of the final product. 
The invention is in part the result of observations of the equilibrium 
diagrams of pairs of materials, one of which is the base materials of the 
particles and the other auxiliary material used in the process according 
to the invention. 
It is noted that for certain material pairs and at a certain heat treatment 
temperature compatible with the base material, on the one hand a solid 
solution and on the other, a liquid rich in the base material, will be 
present. 
As a function of the rise in temperature to the heat treating temperature, 
the successive formation of a phase rich in the auxiliary material, then 
less rich while the liquid becomes enriched in the base material. The 
liquid then disappears progressively under the effect of diffusion and the 
concentration in the auxiliary material of the solid solution of the base 
material declines at the rate at which the material is dispersed. 
The auxiliary material used in the process according to the invention is 
selected from the group of materials generally considered poisons because 
they degrade the ductility at elevated temperature. This degradation may 
either be avoided by the accurate dosage of the amount of the interfacial 
material, or it may be desirable to render the material fragile, for 
example in the manufacture of certain abradable and friable materials. The 
requirements relating to the choice of the material used in the process 
are governed by the fact that this material forms liquid alloys with the 
base material insuring brazing during the first stage of the process; the 
diffusion in the second stage involves the flow of the liquid alloys 
formed, together with an intermetallic diffusion leading to a solid state 
weld. 
Several criteria have been established for the choice of the auxiliary 
material and for the conduct of the heat treatment. 
Primarily, the auxiliary material must be such that: prior to the bonding 
temperature, there, there is at least one liquid phase, preferably with 
the principal element of the base material; vapor pressures are 
sufficiently low so that heating in a furnace under a controlled 
atmosphere is feasible; dispersion as uniform as possible may be achieved 
of the auxiliary material within the volume constituted by the particles 
of the base material. Various known methods may be used for this purpose. 
For example, physical mixing of the two powders (the auxiliary material on 
the one hand, and the base material on the other hand) by means of 
agitation may be sufficient; in the case wherein the base material is in a 
state of fibers or chips, other modes of dispersion are preferably 
considered, for example electrolytic deposition, cathodic atomization . . 
. on said fibers or said chips of the fusible metal. Materials such as 
tin, indium, antimony, gallium, germanium, satisfy these three conditions 
and may thus be used in the process of the invention. Calculations 
performed by applying the laws of diffusion and with consideration of 
thermodynamic equilibrium diagrams, showed that isothermal solidification 
times in a Ni or Co base material are shorter when elements such as tin, 
indium, antimony, gallium, germanium are used, when compared with those 
required for more conventional elements, such as boron. 
The temperature of the heat treatment depends on the nature of the 
auxiliary material selected, but in any case, it must be sufficient to 
permit the formation of intermetallic compounds or of sufficiently stable 
and strong solid solutions. Under these conditions, the heat treating 
temperature will always be higher than 1050.degree. C. if tin is used as 
the interface material, higher than 900.degree. C. for elements such as 
indium or gallium and higher than 1000.degree. C. for antimony and 
germanium. The limitation of the temperature to the solidus of the 
particles to be assembled is imposed by the need of not affecting the 
texture of the assembly in the zone of contact, which would affect 
detrimentally the characteristics of the bond. In certain cases, the upper 
range of the temperature must be further reduced to avoid irreversible 
transformations detrimental to the quality of the base metal, or to be 
compatible with heat treatments of said material. 
The duration of the heating will be comsidered sufficient when, for a given 
temperature, all of the auxiliary, low melting point material has been 
diffused inside the base material. It has been noted that, depending on 
the average diameters of the particles of the base material, the low 
melting point auxiliary material will diffuse most often into the total 
volume of said particles. 
The final novel porous material according to the invention is characterized 
by the fact that it comprises approximately 85 to 99.5% by weight of a 
base material chosen from the group comprising nickel, chromium, tungsten, 
molybdenum, iron, cobalt and various alloys of these metals, and 
approximately 15 to 0.5% by weight of a fusible metal selected from the 
group comprising tin, indium, gallium, germanium, antimony, together with 
the mixtures and alloys of said metals, by the fact that said base 
material is present in the form of elementary particles, such as powders, 
fibers or chips and by the fact that said particles are bonded to each 
other by diffusion brazing by means of said fusible metal. Resistance to 
oxidation of the final porous material is obtained by the addition of up 
to 2% of rare earths or of alkaline metals, or aluminum or magnesium. The 
porosity of the final product is essentially equal to the porosity of the 
physical mixture of the particles of the base metal and of the fusible 
metal prior to heating to effect the operation of diffusion brazing. Thus, 
the porosity will be substantially that of the untamped material when the 
base material is in the form of powder. It is possible to modify the 
porosity, either by tamping the particles of the base material or by 
adding particles of a material that volatilizes during heating, such as 
zinc.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
EXAMPLE 1 
An intimate mixture of 93 g chromium powder having an average grain size of 
between 400 and 800 microns and 7 g of an additive material with an 
average grain size between 100 and 200 microns, is prepared. Said additive 
material in the instant case consists of 70% nickel and 30% tin in the 
powder form. The mixture is poured into an Al.sub.2 O.sub.3 crucible, 
which is heated to 1125.degree. C., while a vacuum of 10.sup.-3 Pa is 
maintained over the mixture. The temperature of 1125.degree. C. is 
maintained for 15 minutes and the product obtained is removed from the 
mold. The product is in the form of a porous element wherein the tin has 
disappeared and the chromiums grains are bonded to each other. 
EXAMPLE 2 
Example 1 is reproduced by using 90 g of a NK 15 CAT alloy, 3 g tin and 7 g 
nickel; heating is for 15 minutes at 1100.degree. C. The example is 
equally valid with 97 g of the alloy NK 15 CAT and 3 g tin. 
EXAMPLE 3 
An abradable porous body may be prepared by operating as follows: 
(100-Y) of an 80/20 nickel-chromium alloy in the form of a powder with a 
grain size of 160 to 210 microns is mixed with Y g of a tin powder with a 
grain size of 125 to 200 microns; the mixture is introduced in an Al.sub.2 
O.sub.3 crucible and heated in a vacuum to 1125.degree. C. for 1 hour; 
cooling is under argon. A porous body is obtained; the erosion resistance 
of this body has been determined by the standard BS 1615; results are 
shown by the curve of FIG. 1 representing on the ordinate the loss of 
volume in mm.sup.3 for a testing period of 5 minutes, as a function of the 
weight of tin in % plotted on the abcissa. The results are also summarized 
in the following table: 
______________________________________ 
Y 0 1.5 2 4 6 10 
volume loss (in 
mm.sup.3 for a 5 mn test 
52 12 4 1.2 0.4 0 
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The porous body prepared with Y=4 g was exposed to a thermal balance, with 
a thermal shock every 6 hours; this test has shown that the porous body 
may be used to approximately 900.degree. C.; the oxidation resistance of 
the porous body may be improved for example by the addition to the mixture 
of 1% aluminum. These results are illustrated by the curve of FIG. 2, 
which represents the loss of mass in % plotted on the ordinate as a 
function of time in hours, plotted on the abcissa. It has been found that 
the pieces rubbing against such an abradable product, in particular in 
turbine engines, the "tongues" of the sealing labyrinth or the tips of the 
blade paddles made of superalloys, do not suffer any wear when operating 
at elevated temperatures, in relation to the products according to the 
invention, containing 0.5 to 6% tin. FIGS. 3 to 10 concern examinations 
performed on a product with 4% tin. The figures are at a scale of 
magnification of 400. 
In FIGS. 3 and 4, micrographic examination shows the homogeneous structure 
of the product and the microanalyses of FIGS. 5, 6 and 7, which concern 
respectively tin, nickel and chromium, show that tin diffuses practically 
into the core of the grains of the base material and further that no 
intermetallic compound which would degrade the quality of the bonds, 
appears; 
In FIGS. 8, 9 and 10, which are enlarged by 12, 175 and 400, respectively, 
scanning electron microscope examination shows the form of the bonds 
between the grains of the base material; these bonds, having the shape of 
"bridges", insure the cohesion of the final product. Finally, it was found 
that: 
the porous product obtained with Y=5 g has a remelting temperature 
(measured by direct thermal analysis) of 1260.degree. to 1360.degree. C., 
the porous product obtained with Y=7 g has a remelting temperature of 
1253.degree. to 1353.degree. C. 
The elementary particles used are a function of the expected wear of the 
porous body. Powders are especially suitable for porous bodies to be 
exposed to abrasion, such as those employed as sealing gaskets in turbine 
engines. In this particular case, the shape and dimensions of the powders 
may be varied to adjust the properties of the porous bodies. Fibers are 
used for example to prepare porous bodies used in filters. Chips, on the 
other hand, are particularly suitable for use as panels in heat 
exchangers.