Net or near net shape powder metallurgy process

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.

Description

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 Iso static process (CIPing). The contained powder is then subjected to Hot Isostatic Pressing (HIPing).

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 (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.

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.

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 1000° C.

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 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.

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 HIPing and CIPing, 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 11×10⁻⁶/° 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.

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 4×10⁻⁶/° C. to approaching 6×10⁻⁶/° 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 15×10⁻⁶/° C. and 9×10⁻⁶/° 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 μm and 2 μm per layer work satisfactorily. This allows a controlled amount of carbon to diffuse into the Ni alloy to depth of between 100 μm and 500 μm. This slightly increases the size of the precipitated CrC within this 100 μm band and thereby increasing the macro hardness from nominally 55R_c to approximately 57R_c. 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 μm to up to and beyond 250 μm have been applied to control and tailor the surface morphology and properties of consolidated materials.

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:

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.

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 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 in which the slurry is applied by spraying.

7. A process as claimed in claim 4 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.

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 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 claim 1 used to create a pressed component of length greater than 2 m, 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 claim 1 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 claim 1 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.

15. A component produced by a process in accordance with claim 1 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.