GB2418166A - Component fabrication using electron beam deposition welding - Google Patents

Abstract

A component 3 is made by melting and solidifying a deposited fusible material 5,6 using an electron beam gun 1. The relative positioning between the electron beam 2 and component 3 is manipulated in order to carry out the build-up welding process. The fusible material 5,6 is preferably a particulate powder. An inert gas shield, preferably in the form of an inert gas cyclone 10 around the electron beam 2, may be used to aid in confining the powder material while it is being blown onto the welding surface 4.

Description

COMPONENT FORMATION

The present invention relates to component formation and more particularly to component formation by deposition techniques from a possible powder or wire material.

It is known to form components by deposition techniques such as shaped metal deposition (SMD) which is principally based upon welding techniques or through direct laser deposition (OLD) in which a powder is exposed to a laser in order to create successive layers of deposition in order to form a component. Essentially, a raw material in the form of a fusible wire or powder is heated by an electrical arc for shaped metal deposition whilst similar heating is provided by a laser in directed laser deposition.

Clearly, there are limitations with respect to the physical capabilities of SMD or DLD. In particular, it may be desirable to provide further influence as to the alloy solidification dynamics and grain size by more controlled melting along with improvements in cost effectiveness, greater capability in terms metallurgy and/or flexibility by removing the necessity for a vacuum. Electron beams are a potential alternative source of melting for raw material, but generally in the past the physical size of electron beam guns has negated against their potential use with respect to deposition techniques. Low pressure electron beam techniques have been used to weld pipes where fast narrow profile roles are required. However, the pipes cannot be encapsulated in a vacuum chamber such that the process gives a lower beam quality than in a vacuum and therefore is considered a rather limited application. Without provision of the vacuum it is difficult to focus the electron beam over acceptable distances such that the beam becomes defocused; and therefore low pressure electron beam technology is generally regarded as a high cost but lower stnda'delectron beam weld and as such has been considered unacceptable. To maintain electron beam quality requires a vacuum chamber which, as indicated in view of the cost and size of the chamber for an electron beam has dissuaded consideration with respect to deposition techniques.

Electron beams can be used for deposition, but cooling, part manipulation and precise filler delivery are all limiting factors.

In accordance with the present invention there is provided a method of component fabrication comprising; a) Presentation of a fusible material for deposition; b) Presenting an electron beam to that material for deposition of a fusible layer as a component; and, c) Manipulation of the relative positioning of the electron beam and the component for definition of that component by deposition.

Normally, the fusible material is a particulate powder.

Typically, that powder is blown for presentation as the fusible material.

Typically, the electron beam is arranged to scan or raster relative to the fusible material.

Generally, definition of the component is for geometry or metallurgy.

Possibly, the electron beam is held under a reduced pressure.

Typically, an inert gas shield is projected about the electron beam to confine the fusible material.

Also in accordance with the present invention there is provided a component fabrication apparatus comprising means to deliver a fusible material for deposition means to present an electron beam towards a distribution of the fusible material and means to manipulate the relative position of the electron beam and the component for definition of that component by deposition.

Typically, the means for presentation of the electron beam allows scanning or raster of the electron beam across a lateral width in the direction of deposition of the fusible material.

Generally, the means to present the electron beam incorporates a reduced pressure or vacuum to permit the open projection distance to the presentation of fusible material.

Possibly, the apparatus includes means for convective cooling or forced convective cooling, such as a high flow rate cryogenic gas, are provided during deposition.

Normally, the means for presentation of the electron beam incorporates means to shield that electron beam.

Normally, that shielding comprises an inert gas such as argon. Generally, the shield is projected in a cyclone about the electron beam to contain the deposition of fusible material.

An embodiment of the present invention will now be described by way of example only and with reference to the accompanying drawing in which there is a schematic representation of a component fabrication apparatus.

As indicated above, previous techniques for material deposition in order to define a component have included shaped metal deposition (SMD) in which an electric arc is used in order to melt a fusible material and direct laser deposition (DLD) in which a laser is similarly used to cause deposition of a material by melting. An electron beam similarly can cause melting of fusible material in order to create a weld pool which then through successive depositions can create a component. The advantage of an electron beam is that it is easier to control, and so allows precise rapid manipulation of a heat source for consistent high thermal transmission.

Standard electron beam techniques require the work piece to be in a vacuum and so with regard to deposition, such an approach is only practically workable when there is a wire feed but even then will generally be uneconomical.

Powder fusible material cannot generally be used with standard electron beam techniques. However, low pressure electron beams can be used outside of a vacuum and therefore powder feed is possible. It will be understood that powders in a vacuum are very difficult to control due to fly-away dispersion. It will be understood that powders cannot be directed by gas jets or gas entrainment in vacuum systems.

With an electron beam, the molten pool geometry of the fusible material is more readily controlled than is generally possible with shaped metal deposition (SMD) through a combination of the high intensity heat source (electron beam) and the rapid scan rate of the electron beam. By a fast raster" across the surface, it is

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possible to create a very shallow, fast moving melt pool.

Such a melt pool is again not practicable at least in a power efficient manner using a laser due to problems associated with inefficient laser beam absorption by the possible material. By directed scatter presentation of fusible powder material of a chosen sieve fraction (diameter range) into the shallow fast moving melt pool, it will be possible to develop a parameter set by appropriate analysis whereby the cooling rate of the pool is such that the powder particles exposed to molten metal is such as to allow capillary wetting and fusion of the powder surface but not the core of some particles. Essentially, the problem would be relating to control of an absolute minimum level of superheating of the melt pool, that is to say the temperature above the melting point of the pool.

Additionally, control is necessary of the melt pool geometries to minimise the thermal mass at any point in time to minimise residence time for the fusible powder material at melting temperature. Fusible powder material particles would stay solid in the centre due to the thermal inertia of the energy barrier to diffusion. In such circumstances, metallurgical objectives could be more easily achieved by this approach, in particular by a combination of the electron beam with a precise powder scatter deposition.

By greater metallurgical prospects it will be understood that heavily alloyed materials, in particular nickel based super alloys which require fast cooling rates may be more readily achievable by an electron beam deposition process. Traditionally, such fast cooling rates have limited the forms, that is to say component shapes, that materials can be used in and tends to exclude fusion build up processes due to segregation and precipitation leading to solidification, strain age cracking and less than optimal precipitation formation. It will be understood that deposition processes are essentially a fusion process whereby successive layers of deposition are melted into fusion with each other by the deposition process. Clearly, those parameters which require careful consideration in order to achieve the desired temperature control for geometric as well as metallurgical advantages include the accelerating voltage for the electron beam, the electrical current used, the fusible powder material size, the electron beam raster rate and raster pattern. Typically, the control parameters which would be monitored include melting pool temperature and pool geometry.

As indicated above, electron beams rapidly deteriorate when outside of their vacuum projection environment. Thus, if some focusing of the electron beam is required it is necessary to achieve or maintain a low pressure in order to keep the beam projected appropriately. Clearly, vacuum chambers become increasingly expensive with size and so the vacuum chamber volume necessary in order to deposit a large component structure would be prohibitively expensive.

However, with regard to deposition, as indicated, the focus of the electron beam is of less importance provided sufficient energy can be provided to the fusible powder material to create melting for the weld pool and therefore the deposition desirable for component fabrication.

Nevertheless, in order to improve performance the present invention advantageously utilises a partial vacuum and/or an inert gas shield or chamber about the electron beam deposition process at the position of electron beam projection. In such circumstances, the bulky electron beam gun can remain outside of the reduced pressure/vacuum area or argon shielding/argon chamber with the projection end of that gun extending into the partial vacuum/inert gas shielding/chamber. In such circumstances, the electron gun will remain stable and therefore it is necessary to provide, again either in or projecting into the reduced partial vacuum/inert shielding/chamber, a manipulation device in order to move the component as it is built up by the deposition process rather than the electron beam. For comparison it will be appreciated that previously either the arc electrode or laser beam has been manipulated in order to move relative to the component. This is not practicable with an electron beam deposition process but the electron beam could raster across the area of the molten pool.

Generally the inert gas is argon in view of its cooling as well as inert nature allowing the electron beam to remain within acceptable parameters for deposition purposes.

Clearly, component manipulation is preferred as it will be generally cheaper, but it may also be possible to manipulate the electron beam gun, but generally this will be a far more expensive process.

As indicated above, utilization of an electron beam in comparison with previous SMD or DLD processes has particular advantages with respect to metallurgical control. It will be understood that through use of the electron beam and its more controllable energy input, there are greater possibilities for alloy solidification dynamics and grain size by more controlled melting of the fusible material.

The electron beam scan rate is electrically controllable and has the potential for far more rapid scanning than previous processes with laser beams. By careful selection of the processing parameters, as indicated above, and specifically the electron beam power and scan rate, a more controlled weld pool cooling rate can be achieved and therefore the desired metallurgical objective. Specifically, such control would be through weld pool geometry in terms of the pool surface size and shape and the substrate depth into the weld pool molten material. As indicated, it may be possible through control to provide for incomplete melting of the fusible powder to achieve a high density of randomly aligned solidification nuclei) and therefore allow grain size to be influenced.

Electron beam deposition should be more rapid than direct laser deposition (DLD) and therefore on a ''like for like" basis prove more cost effective than such DLD processes. DLD has a lower melt rate due to the limitations of the weld pool dynamics but as indicated, care must be taken with respect to the approach of electron beam techniques as conventionally there are limitations with respect to the possibility for fusible material feed into a vacuum, that is to say wire instead of easily dispersed powder, and the increasing cost of large vacuum chambers.

Furthermore, electron beam systems in vacuum suffer prolonged cooling due to the lack of convective cooling. / -

As indicated above, metallurgy can be more easily manipulated by electron beam processes due to the ability to more precisely control the energy input through the electron beam to the fusible material and by implication the melting S and cooling rates and isotherms within the weld pool as it solidifies.

The present electron beam process does not require a vacuum and so allows much simpler component manipulation as the manipulator itself does not require enclosure within a vacuum chamber.

Referring to the drawing illustrating schematically a component fabrication apparatus in accordance with the present invention. Thus, a low pressure electron beam gun l operates in accordance with known principles in order to generate an exited electron beam 2 directed towards a deposited component 3. The electron beam gun l incorporates appropriate switching and focusing coils in order to direct the beam 2 towards the component 3 such that a weld pool 4 of molten material is created which fuses successive deposited layers 5, 6 to form the component 3 and initially deposits upon a substrate 7. A hopper 8 stores and presents a fusible powder material through a feed mechanism 9 towards the gun l for presentation by a confined sprinkler or other process downward towards the component 3 and in particular for deposition after melting in the weld pool 4. The fusible powder material is directed and confined by an inert shielding gas cyclone lO towards the weld pool 4.

The electron gun l in order to create the necessary excitation electron beam 2 is coupled to a source of high voltage ll and due to the nature of such excitation there is a vacuum extraction 12 which generates an electron beam. It will be noted that the bulk of the electron gun l is outside of a shielded volume 13 containing a slight positive pressure, that is to say above atmospheric, of inert gas in order to provide cooling and facilitate stability of the electron beam.

Although fusible powder material is described with regard to this embodiment it will be appreciated that it may be possible to use other sources of raw fusible material including co-axial wire etc. provided appropriate melting into the weld pool 4 is possible with a low pressure electron beam.

It is part of the present approach that by careful selection of processing parameters, specifically power and scan rate, a process combination can be achieved for the incomplete melting of a fraction of larger fusible powder particles. Such partial melting of the fusible powder particles enables greater metallurgical control as incompletely molten they will act as solidification nuclei.

With solidification nuclei it would be possible to minimise the resultant grain size with positive implications for mechanical properties.

Although electron beam systems have a comparatively fast raster rate or scan laterally across the direction of deposition, such capabilities would ensure a weld pool 4 of adequate dimensions for melting the fusible powder and achieving the correct pool depth for structural stability without forming a key hole melting pool typical with welding type SMD deposition.

It is the greater controllability of an electron beam even when defocused by projection into a non vacuum environment which provides the benefits of greater deposition control in terms of geometric and metallurgical properties. Clearly, particular care must be taken with regard to the construction of certain materials such as the feed mechanism 9 to avoid stray electrical arcing. In such circumstances typically the feed mechanism 9 will be formed from a ceramic material. Similarly, as indicated above, an inert gas cyclone lO as well as an inert slightly positive locality chamber 13 will be created to facilitate electron beam/fusible material melting into the weld pool 4 for adequate deposition purposes.

Low pressure electron beam guns are available in the power range 3 to 120 kW but higher power guns are known but would probably not be required to achieve the desired deposition rates for component fabrication. Furthermore, it will be understood that higher electron beam power generally incurs higher costs due to the power electronics invo1 veH (A and without, as indicated, any significant additional benefits for deposition. The exitation voltage by which the electron beam is accelerated will typically be in the range to 60 kilo volts for a low voltage arrangement or 180 to 220 kilo volts for a high voltage arrangement. Generally there will be a welding current adjustment in the range 0.1 to 1,000 milliamps through the deposition process with a typical bombardment current in the range 10 to 60 milliamps.

Normally, the focusing current utilised in a focusing cascade 14 of the gun 1 will be in the range 400 to 1000 milliamps. However, all these parameters will be determined by actual operational requirements.

As indicated previously, an electron beam becomes defocused when projected from its vacuum environment. The beam is generated, focused by the cascade 14 and accelerated in a hard vacuum such that it projects beyond a final orifice towards the component 3. The electron beam passes through a series of orifices that separate graduated vacuum zones which are each individually pumped to varying vacuum degrees. The final orifice minimises gas, primarily argon leakage into the chamber 13, intake into the vacuum system while permitting an electron beam to exit towards the component 3. As the beam 2 begins to defocus upon exiting the final orifice the work piece component 3 must be appropriately positioned such that the defocused electron beam still provides operational effect with respect to melting the fusible material. Typically, with generally available low pressure electron beam guns, the work piece component 3 must be positioned within a range of up to 25 millimetres from the final orifice. Typically, the component 3 would be held at pressures in the range 0.1 to millibar. Such a coarse vacuum level can be readily achieved through local sealing on the component such that the entire work piece component need not be completely encapsulated in a reduced pressure chamber.

Of equal importance to the electron beam is the nature of the fusible material. As indicated, advantageously for deposition purposed, the fusible material in accordance with the present invention will be a powder. The choice of powder in terms of its particle size distribution will as indicated above be dependent upon metallurgical as well as geometric parameters. Nevertheless, generally it is envisaged that the bulk of the particles will be in the range 50 to 100 micro metres in size and with a desirable size distribution for metallurgical effect.

Operation of the electron beam deposition process as described above will as indicated comprise providing a successive deposition of layers 5, 6 in order to form a component of appropriate shape. Electron beam deposition should provide the ability to more rapidly deposit material to create those components in comparison with previous direct laser deposition such that the greater accuracy of such deposition in comparison with shaped metal deposition (SMD) techniques can still be retained but with greater commercial advantage.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims (17)

1. A method of component fabrication comprising; a) Presentation of a fusible material for deposition; b) Presenting an electron beam to that material for deposition of a fusible layer as a component; and, c) Manipulation of the relative positioning of the electron beam and the component for definition of that component by deposition.

2. A method as claimed in claim 1 wherein the fusible material is a particulate powder.

3. A method as claimed in claim 2 wherein the powder is blown for presentation of the fusible material for deposition.

4. A method as claimed in any of claims 1 to 3 wherein the electron beam is arranged to scan or raster relative to the fusible material.

5. A method as claimed in any preceding claim wherein definition of the component is for geometry or metallurgy.

6. A method as claimed in any preceding claim wherein the electron beam is held under a reduced pressure.

7. A method as claimed in any preceding claim wherein an inert gas shield is projected about the electron beam to confine the fusible material.

8. A method of component fabrication substantially as hereinbefore described with reference to the accompanying drawings.

9. A component fabrication apparatus comprising means to deliver a fusible material for deposition means to present an electron beam towards a distribution of the fusible material and means to manipulate the relative position of the electron beam and the component for definition of that component by deposition.

10. An apparatus as claimed in claim 9 wherein the means for presentation of the electron beam allows scanning or raster of the electron beam across a lateral width in the direction of deposition of the fusible material.

11. An apparatus as claimed in claim 9 or claim 10 wherein the means to present the electron beam incorporates a reduced pressure or vacuum to permit the open projection distance to the presentation of fusible material.

12. An apparatus as claimed in any of claims 9 to 11 wherein the means for presentation of the electron beam incorporates means to shield that electron beam.

13. An apparatus as claimed in claim 12 wherein that shielding comprises an inert gas such as argon.

14. An apparatus as claimed in claim 12 or claim 13 wherein the shield is projected in a cyclone about the electron beam to contain the deposition of fusible powder material.

15. An apparatus as claimed in any of claims 9 to 14 wherein the apparatus includes means for convective cooling or forced convective cooling during deposition.

16. A component fabrication apparatus substantially as hereinbefore described with reference to the accompanying drawing.

17. Any novel subject matter or combination including novel subject matter disclosed herein, whether or not within the scope of or relating to the same invention as any of the preceding claims.