NET OR NEAR NET SHAPE POWDER METALLURGY PROCESS

PROCEDE DE METALLURGIE DES POUDRES A FORME NETTE OU PRESQUE NETTE

English Abstract

In a hot isostatic pressing process or hot uniaxial pressing process for
producing a net or near net shape product, a
diffusion filter comprising boron nitride is provided between a graphite
former and metal powder to be pressed thereagainst. The
diffusion filter allows a controlled amount of carbon to diffuse into the
surface of the pressed component. The boron nitride is
conveniently applied as an aqueous slurry by spraying. In order to obtain
adherence between the coating and the surfaces of the
former, one or more thin ghost coat layers of slurry are applied to the
surface of the graphite former before one or more layers of
normal strength slurry are applied. Each layer of coating is allowed to dry
before the next layer is applied, and the former may be
heated to dry each layer. Pressed components of length greater than 2m can be
processed, relative contraction of the component
and former during cooling of the component being accommodated by the boron
nitride coating on the former. The thickness of the
coating can be determined by trials to achieve a controlled diffusion of
carbon into the surface of the pressed component, and the
dimensions of the former chosen to accommodate that thickness of coating.

French Abstract

Dans une opération de compression isostatique à chaud ou une opération de compression uniaxiale à chaud pour la production d'un produit de forme nette ou presque nette, un filtre à diffusion comprenant du nitrure de bore est prévu entre un gabarit en graphite et une poudre métallique qui doit être comprimée contre ce dernier. Le filtre à diffusion permet la diffusion d'une quantité contrôlée de carbone dans la surface du composant comprimé. Le nitrure de bore est commodément appliqué par pulvérisation sous la forme d'une boue aqueuse. Pour obtenir une adhérence entre le revêtement et les surfaces du gabarit, on applique une ou plusieurs minces couches de boue de revêtement fantôme à la surface du gabarit en graphite avant d'appliquer une ou plusieurs couches de boue de résistance normale. On laisse sécher chaque couche de revêtement avant d'appliquer la couche suivante et le gabarit peut être chauffé pour sécher chaque couche. On peut traiter des composants comprimés d'une longueur supérieure à 2 m, la différence des retraits du composant et du gabarit pendant le refroidissement du composant étant reprise par le revêtement de nitrure de bore prévu sur le gabarit. L'épaisseur du revêtement peut être déterminée par essais de manière à obtenir une diffusion contrôlée du carbone dans la surface du composant comprimé, et les dimensions du gabarit sont sélectionnées de telle sorte qu'il puisse reprendre cette épaisseur de revêtement.

Claims

9
CLAIMS
1. A hot isostatic pressing process or hot uniaxial pressing process
for producing a net or near net shape product in which a diffusion filter,
comprising boron nitride is provided between a graphite former and
metal powder to be pressed thereagainst.
2. A process as claimed in claim 1 in which the diffusion filter is
configured to allow a controlled amount of carbon to diffuse into the
surface of the pressed component.
3. A process as claimed in claim 1 or 2 in which one or more layers
of a coating material comprising boron nitride are applied to the surface
of the graphite former prior to metal powder being arranged next to the
former.
4. A process as claimed in claim 3 in which the layer or layers are
applied to the surface of the graphite former by application of a slurry of
the coating material to the surface.
5. A process as claimed in claim 4 in which the slurry is an aqueous
slurry.
6. A process as claimed in claim 4 or claim 5 in which the slurry is
applied by spraying.
7. A process as claimed in any one of claims 4 to 6 in which one or
more thin ghost coat layers of slurry are applied to the surface of the
graphite former before one or more layers of normal strength slurry are
applied.

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8. A process as claimed in claim 7 in which each layer of coating is
allowed to dry/is dried before the next layer is applied.
9. A process as claimed in claim 8 in which the former is heated to
dry at least one of the layers.
10. A process as claimed in claim 4 or any one of claims 4 to 9 in
which the metal powder is a PM Nickel based alloy and in which the
layer or layers when dry are of thickness 1 µm to 2 µm.
11. A process as claimed in any one of the preceding claims used to
create a pressed component of length greater than 2m, relative
contraction of the component and former during cooling of the
component being accommodated by the boron nitride coating on the
former.
12. A process as claimed in any one of claims 1 to 9 in which the
alloy has the following composition:
C1.0, Cr15, Si 4.0, B3.5, Fe4.5 Ni Balance.
13. A process as claimed in any one of claims 1 to 9 in which the
alloy has the following composition:
C 0.13, Si 0.20, Mn 0.50, P 0.020, S 0.020, Mo 0.18, Ni 3.40
14. A graphite former for use in a hot isostatic pressing process or hot
unixial pressing process, the former being provided with a coating on a
surface thereof that presses against metal powder in use of the former,
the coating comprising boron nitride to act as a diffusion filter, the
thickness of the coating having been determined by trials to achieve a
controlled diffusion of carbon into the surface of the pressed component.

11
15. A component produced by a process in accordance with any one of
claims 1 to 10 in which the surface of the component incorporates a
controlled amount of carbon that has diffused into the surface from the
graphite former during pressing, the carbon content of the surface of the
finished component having been determined by trials of how the carbon
content varies with the thickness of the boron nitride coating on the
former, and by choosing the dimensions of the uncoated former in
accordance with the required coating thickness and required final
dimensions of the pressed component.

Description

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NET OR NEAR NET SHAPE POWDER METALLURGY PROCESS
Field of the Invention
This invention relates to a net or near net shape powder metallurgy process.
The invention relates particularly, but not exclusively, to the provision of
an
atomic diffusion filter between a graphite former, used to derive the
finished net or near net shape form used in the manufacture of near net
shape powder metallurgy components.
Background to the Invention
A known manufacturing method used for producing components and
materials utilises the consolidation of metal powders by Hot Isostatic
Pressing. A pre-consolidation of the metal powders may or may not be
used utilising Cold Isostatic Pressing.
In summary, metal powder is placed in a containment and a vacuum is
applied within the containment, and the containment is sealed. This then
may or may not be partially consolidated in a cold form by subjecting the
containment to a cold Isostatic process (CIPing). The contained powder
is then subjected to Hot Isostatic Pressing (HlPing).
The HIPing process utilises the application of heat at approximately, but
not essentially, 80% of solidus of the material of which the powder is
derived. This process subjects the metal powder to thermo mechanical
stress whereby the metal powders are mechanically deformed in a super
plastic condition. The resulting intimate contact and movement between
the powder particles results in a shear and compressive stresses being
placed upon them. As result of this process an atomic interaction
CONFIRMATION COPY

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(interdiffusion) between the particles takes place subsequently removing
all prior practical history thus creating a solid homogenous metal form.
There is a need with certain components to be able to create an accurate
and/or near accurate final shape to the component being manufactured.
This can be done using a graphite former machined to an accurate size.
We have appreciated that it is desirable to partially inhibit or limit
(filter)
the diffusion of carbon atoms from the graphite into the powdered metal
being processed.
Statements of Invention
According to one aspect of the invention in a hot isostatic pressing
process or hot uniaxial pressing process a diffusion filter is provided
between a graphite former and the metal powder to be pressed.
We prefer to apply the filter to. the former which has been accurately
machined.
Preferably a wet-sprayed deposit of an aqueous suspension of Boron
Nitride is used to create the barrier/filter. The overall thickness of the
coating, determined primarily by the number of coats can be developed to
control the amount of carbon diffusion desired or which can be tolerated.
The method of spraying is by way of hand spraying for general
application, or by the use of robotics in the case of high accuracy
requirements and in applications requiring precise repeatability.
Aqueous suspensions of boron nitride at different volume percentages can
be selected through a series of tests aimed at optimising the spraying
constitution and enabling accurate spraying to be undertaken.

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The boron nitride spraying is preferably applied substantially normally to
provide a plurality of thin multi layers. Great care is required to ensure
the thickness of the layers is controlled in order to provide the correct
overall filtering level of the coating.
In a preferred method of applying the boron nitride coating, adhesion of
the initial coating layers is undertaken by the use of thin ghost coats
applied by spraying. This helps to prevent the aqueous suspension from
weeping and helps to provide adhesion of the coating to the
carbon/graphite former is before the build up of secondary coats of
normal strength are applied.
This procedure is of particular importance with regard to large accurate
components up to and beyond 2 metres in length.
It may be necessary to heat the component to ensure the thin ghost
coating dries quickly before the aqueous based carrier weeps and runs to
carry away the boron nitride coating leaving the surface void of coating.
It may require a plurality of ghost coats; as many as three may be
required or more in some cases where a high surface finish has been
created on the carbon/graphite former.
Precise control of the thickness of the coating is essential in the case of
net-shape forming to ensure that the finished dimensions after
consolidation are accurate.
The precise number of secondary coats used is governed to essentially
control the level of carbon diffusion, but the accuracy of the finished
component is also influenced by the coating thickness.

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We prefer to tailor the dimensions of the carbon/ graphite former to
accommodate the precise thickness/number of coats of boron nitride
applied. This process can involve balancing the level of diffusion with
the required final accuracy requirement for the part.
The surface of the consolidated powder adjacent to the boron nitride
filter is essentially modified by the controlled diffusion of carbon from
the carbon/graphite former during consolidation. The activity of carbon
atoms is high at the consolidation temperature that is, in case of nickel
based alloys at or above 10000C.
The ability to modify the morphology of the surface of the consolidated
powder is of importance in many cases and enables the surface to be
tailored to specific application. For example to increase wear resistance
and/or stiffness whilst the subsurface layers may be structured to provide
increased toughness and/or corrosion resistance.
The chemical analysis of the pre-consolidated powders is preferably
adjusted to accommodate the diffusion of carbon. This is the case with
both nickel based alloys and ferrous alloys.
Surface modification may be utilised to enhance already structured
powder formed parts. This can provide an in situ operation which
requires no further diffusion processing and, particularly in the case of
some nickel based alloys, requires no further heat treatment processing to
achieve optimum hardness.
The thickness of the coating also controls/ influences the surface finish of
the consolidated interface of the component. Thick layers of boron
nitride have a high level of conformance to the interface powder during
consolidation and thereby the surface will bear the topography of the
powder particle shape. Thinner coatings, with subsequent higher levels

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of carbon diffusion, are less conformant and bear a closer surface
likeness to that of the carbon/graphite former. In this case, if a high
grade finish is applied to the former the consolidated powder will also
bear a similar surface finish.
5 It is essential to dry the coated graphite former thoroughly before
consolidation, when the boron nitride has been applied in a aqueous
suspension.
The boron nitride will consolidate during both HlPing and ClPing, and
when using combinations of both, and size predictions can be developed
from a series of tests.
This has the effect of increasing the surface hardness, and is particularly
advantageous since no further machining is required to reach final size.
Further machining of a hardened surface would otherwise be difficult.
The use of boron nitride on graphite formers can serve a further and very
important function. This is to allow the differential expansion between
the powdered metal and the carbon/graphite former. This is of major
importance during the cooling cycle when the two materials are cooling
from the consolidation temperature. This may, for example, be from a
temperature in excess of 1000 C, and the differential expansion typically
between a nickel superalloy and some graphite can be as high as 11 x
10 -6/ C. This expansion differential can become a major problem.
However, the presence of boron nitride can/does allow movement
between the two materials to take place and thereby prevent the work
from being destroyed or at best spoilt.

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This feature is particularly important in the case of long components such
as linear motors and/or pumps. In this case, components up to and
greater than 2 metres are produced by this method which would not be
possible without the utilisation of this technique. In particular when hard
materials and/or hard metal matrix composite powders are consolidated
no further machining and or shape forming can be done therefore the
incorporation of this type of technique is essential.
The coefficient of thermal expansion of graphite can vary from 4 x
10-'/'C to approaching 6 x10-6/ C which is a significant difference but
not as significant as the potential difference between the different types
of powdered metals that may be used in this process, which may vary
between 15 x 10-6/ C and 9 x 10-6/ C. It can be clearly seen that great
care is required to accommodate the CTE difference between the former
and the consolidated powders when cooling.
Examples in accordance with the invention
(A) In the case of graphite forms used to produce net/near-net-shapes in
PM Nickel based alloys containing for example Cr, Fe, B, Si, C 5 thin
aqueous layers of BN of between 1 m and 2 m per layer work
satisfactorily. This allows a controlled amount of carbon to diffuse into
the Ni alloy to depth of between 100 m and 500 m. This slightly
increases the size of the precipitated CrC within this 100 m band and
thereby increasing the macro hardness from nominally 55R, to
approximately 57R,. This slight increase in hardness increases the
abrasive wear resistance of the consolidated material whilst limiting
brittle behaviour. Coatings built of multi layers from 1 m to up to and
beyond 250 m have been applied to control and tailor the surface
morphology and properties of consolidated materials.

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Also in this particular application of the invention the BN layer also acts
as a release agent allowing the graphite former to be removed following
HIPing. Here the surface finish of the net shape part is important.
Therefore it is of additional importance to ensure that the BN layer is
deposited evenly and accurately.
(B) Thin shell net shape profiled bore walls can be produced for the high
performance automotive industry. These parts are required to be
manufactured in high quality ferrous steel alloy, and in this particular
application of the invention it is essential to control the level of carbon
diffusion into the surrounding steel part and to keep it as low as possible.
Whilst in this case the accuracy is not so important, the quality and
subsequent performance of the material is of great importance. The BN
diffusion barrier in this application is applied to a thickness that is so
chosen as to reduce carbon diffusion into the steel to an insignificant
level.
(C) Choice of Suitable Alloys for Surface Treatment
Typical materials suitable for surface modification by the diffusion of
carbon are nickel-based alloys containing Si, B, Fe, Cr and C in this case
the carbon content of the alloy is enhanced by the diffusion of further
carbon during the HIPing process. It may be desirable to adjust the
specified carbon and/or chromium content to optimise the post process
properties of the material.
It has been found beneficial to orchestrate the diffusion of carbon into a
number of nickel and ferrous-based materials but Alloy steels designed
specifically for carburising are in particular suitable for use in this
application.
A Nickel based alloy is typically:-

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C1.0, Cr15, Si 4.0, B3.5, Fe4.5 Ni Balance, by weight percent.
A Ferrous alloy is typically:
C 0.13, Si 0.20, Mn 0.50, P 0.020, S 0.020, Mo 0.18, Ni 3.40, by
weight percent.
In addition specific material composition can be compiled to optimise the
process potential for a particular requirement
The diffusion of carbon into other alloy steels followed by an appropriate
heat treatment may be beneficial to increase both stiffness and the surface
performance of components even though the materials are not ordinarily
treated in such a way.
In all cases the duration of the peak HIPing temperature can be adjusted
to optimise the depth of carbon diffusion; provided the increase in time
does not have a detrimental effect upon the overall morphology of the
consolidated material: For example increased grain growth and/or
undesirably affect the volume fraction or dimensions of precipitates.