Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Powder pretreatment
Reexamination Certificate
2000-07-07
2002-05-21
Jenkins, Daniel J. (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Powder pretreatment
C419S026000, C419S037000
Reexamination Certificate
active
06391252
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns a method using powder metallurgy for producing molded parts intended to be assembled by self-brazing to metal parts capable of receiving them and said receiving parts.
The present invention also concerns a method for assembling such molded parts to receiving metal parts.
As used in the present document, the term “self-brazing” means the autogenous brazing of the molded part on the receiving metal part, the elements effecting brazing being contained in the molded part.
The operation of self-brazing of the molded part on the metal part may comprise or be followed by a diffusion heat treatment in the solid state, thus constituting what is commonly termed a brazing-diffusion operation, which treatment is intended to homogenize the composition and the structure of the molded parts and of the area of connection by self-brazing.
2. Discussion of Background Information
European Patent No. 075,497 teaches a method for assembly by brazing-diffusion of metal parts, such as components made of superalloy for gas turbines, which consists in interposing between the surfaces to be assembled a connecting layer of overall composition corresponding to a superalloy, and in carrying out a brazing-diffusion treatment on this assembly.
The connecting layer consists of an intimate mixture of two alloy powders, the first powder, termed “basic powder”, being a superalloy powder, and the second powder being a brazing powder of Ni—Co—Si—B alloy the liquidus temperature of which is below the solidus temperature of the mechanical parts and of the basic powder.
The relatively low melting temperature of the brazing powder is provided by its content of Si and/or of B.
As used in the present document, the term “melting element” designates an element such as, by way of non-limiting example, silicon or boron, which substantially lowers the solidus temperature of the alloy into which it is introduced.
The brazing-diffusion operation is carried out at a temperature such that the brazing powder melts and, flowing between the basic powder grains, makes it possible to decrease the porosity very rapidly and to obtain a compact connecting layer independently of the volume of powder employed.
The application of the temperature is then maintained to diffuse the melting element or elements B and Si. The homogenization of the composition resulting from such diffusion raises the liquidus temperature of the melted areas which solidify while the temperature is maintained, diffusion continuing in the solid state.
The result at the end of the brazing-diffusion operation is a homogeneous and dense structure with an absence of discontinuity between the parts to be assembled.
European Patent No. 075,497 gives several examples of application to superalloys based on nickel or based on cobalt.
One of the examples concerns the reconditioning of a fixed turbine blade, made of cobalt-based alloy KC25NW according to the AFNOR designation, cracked by thermal fatigue. AFNOR is the French Standards Committee. According to the AFNOR designation, the alloy KC25NW is a cobalt alloy containing approximately 25% chromium based on the weight of cobalt, and lesser amounts of nickel and tungsten.
For that, a paste containing an intimate mixture of the basic powder of Co superalloy, the brazing powder of Ni—Co—Si—B alloy and a volatile binding agent is applied in the previously cleaned and/or widened crack.
The binding agent of the type consisting of a solution of acrylic resin in the monomer of the latter is eliminated by pyrolysis during the brazing-diffusion treatment at 1200° C.
European Patent No. 075,497 also provides for applying, instead of a paste, a strip or tape obtained by lamination of an intimate mixture of basic and brazing powders and of acrylic resin.
European Patent No. 075,497 also provides for adding on an elementary part of simple shape in the form of a pre-sintered blank obtained from the mixture of basic and brazing powders, the surface of the pre-sintered blank acting as the connecting layer.
The patent cites in particular as application the plugging of hub support holes on hollow turbine blades, made of alloy NK15CADT according to the AFNOR designation, coming from the foundry. According to the AFNOR designation, the alloy NK15CADT is a nickel alloy containing approximately 15% cobalt based on the weight of nickel, and lesser amounts of chromium, aluminum, molybdenum and titanium. For that, there is introduced into the hole a plug consisting of a pre-sintered blank containing 75% by weight of basic powder of alloy NK17CDAT according to the AFNOR designation, and 25% by weight of brazing powder of Ni—Co—Si—B alloy, and self-brazing is carried out at 1200° C. for 15 minutes. According to the AFNOR designation, the alloy NK17CDAT is a nickel alloy containing approximately 17% cobalt based on the weight of nickel, and lesser amounts of chromium, molybdenum, aluminum, and titanium.
The use of the method described in that patent is nevertheless subject to a certain number of limitations.
The use of pastes or tapes containing, in addition to the basic and brazing powders, a binding agent which it is necessary to decompose and the decomposition products of which must be eliminated during brazing-diffusion, requires the use of furnaces capable of eliminating large quantities of gas coming from the pyrolysis of the binding agent. Such furnaces are poorly suited to the brazing-diffusion operation which takes placed at around 1200° C. and generally under vacuum.
Another problem to be solved is the production of elementary parts in the form of pre-sintered blanks. European Patent No. 075,497 does not indicate any method for producing such blanks and envisages only simple shapes.
The shaping of the blanks may be envisaged by uniaxial cold compacting, but this method does not make it possible to produce very slender or thin shapes of homogeneous density because of friction between powder grains or between grains and walls of the compacting die. Moreover, in order to limit the friction, lubricants of the zinc stearate type or similar are used as additive to the powders; these lubricants are capable of introducing zinc into the blank, which element has a harmful effect on the service life of the superalloys.
Other methods for shaping the blanks are in fact used, such as plasma spraying onto a rotating substrate and cutting of tapes by laser.
Plasma spraying makes it possible to produce blanks of tubular shape generated by rotation, by spraying metal powder onto a rotating cylindrical mandrel. Such a method has a very low yield, around 90% of the sprayed powder being sprayed elsewhere than onto the rotating mandrel, which severely affects the cost of producing the blanks, taking into account the extremely high cost of the metal powders sprayed. Furthermore, this method does not ensure clean edges at the end of the blank, which necessitates re-cutting and further increases the cost of the parts.
The laser cutting of tapes obtained by slip casting, elimination of the solvent and sintering, makes it possible only to obtain relatively thin flat parts. The losses resulting from the laser cutting, however, are high and represent ⅔ rds of the material employed.
The consolidation of self-brazing pre-sintered blanks obtained by powder metallurgy is described in U.S. Pat. No. 4,937,042 which concerns the production of fixed facing friction parts of the fins of gas turbines. The blanks consist of a mixture of a first superalloy powder of the M—Cr—Al or M—Cr—Al—Y type not containing Si, and of a second powder of the M—Cr—Al—Si type containing around 10% by weight of Si, the element Si being the melting element and M representing the element Co or the element Ni or a combination of these two elements. The blanks are pre-sintered at a temperature below the solidus temperature of the second powder.
The consolidation brought about by such pre-sintering is very limited, taking into account the low sintering capacity of the superalloys. The blanks thus obtained are the
David François
Huchin Jean-Pierre
Leroy Yves
Malie André
Greenblum & Bernstein P.L.C.
Jenkins Daniel J.
Metal Process Systems
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