This application claims priority from United Kingdom Patent Application No. 14 03 557.0 filed 28 Feb. 2014, the entire disclosure of which is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a method of forming a composite component, in which a plurality of feed materials are initially in a powdered state and a solid component is formed by the application of heat.
The present invention also relates to an apparatus for forming a composite component, of the type in which a plurality of feed materials are initially in a powdered state and a solid component is formed by the application of heat.
2. Description of the Related Art
Composite components are component parts made from two or more dissimilar materials that are consolidated to produce a single structure with characteristics different from the individual constituent materials. The composite structure may be preferred for a number of reasons for example in creating a multi-layer component having a hard exterior surface but a low density, light-weight interior.
JPH 06 7916 (Kiyadeitsuku Technol Service K) shows an example casting method to create this type of composite component in which a mixture of two powdered materials is added into a mould cavity and utilises an induction coil so as to melt the powder mixture so as to control the melting and solidifying and thus microstructure of the final component.
A variety of methods are known for producing a multi-layer component comprising one or more dissimilar materials, such as hard facing or laser cladding.
However these known methods incur a number of problems. Firstly, a disadvantage is experienced in the number of separate processes required to create the finished component. Secondly, such techniques of applying a layer of material to the exterior of a solid object result in a non-discreet join of the two materials, leading to a structural weakness at the interface.
US 2013/294901 A1 (Mironets et al) shows a method of component forming which includes positioning a metal powder into a cavity before being melted in an induction furnace. This document also suggests using component strengthening structures which are held in position by the component once the component has been formed. A further method is shown in EP 1 724 438 A2 (General Electric) in which a slip case is used in the casting method which separates the two different powdered materials. The slip case is then removed to allow the powdered materials to mix before solidification.
EP 0 072 175 A1 (Mowill) is an example of a hot isostatic process in which a basket is placed within an enclosure to provide two separate regions for different powders or alloys. The basket allows for diffusion of the two alloys as the alloys begin to melt. The basket may be removed once the component is formed or melted and evaporated.
According to a first aspect of the present invention, there is provided a method of forming a composite component from a plurality of dissimilar powdered feed materials comprising the steps of; obtaining a negative ceramic mould of a component defining a first aperture associated with a first region of the mould and a second aperture associated with a second region of the mould; deploying a first powdered feed material into said first region of the negative mould via the first aperture and a second powdered feed material into said second region of the negative mould via the second aperture; preventing diffusion of said first feed material with said second feed material by means of a partition separating said first region and said second region; increasing the temperature within said negative mould to a first temperature causing said first feed material to melt during a heating stage; and melting said partition to allow diffusion of said first feed material with said second feed material.
According to a second aspect of the present invention there is provided an apparatus for forming a composite component from a plurality of dissimilar powdered feed materials comprising; a negative ceramic mould defining a first aperture associated with a first region of the mould and a second aperture associated with a second region of the mould; wherein said first aperture is configured for insertion of a first powdered feed material into said first region of the mould and said second aperture is configured for insertion of a second powdered feed material into a second region of the mould; and said negative ceramic mould is suitable for heating to a first temperature during a heating stage, characterised in that: said negative ceramic mould further comprises a partition separating said first and said second regions, said partition configured to prevent diffusion of said first feed material with said second feed material when in a solid form and further configured to melt to allow diffusion of said first feed material with said second feed material.
A method of forming a composite component from powdered feed materials falling outside the claimed invention is illustrated in
At step 101, a negative mould 102 is obtained that defines the profile of a component to be produced. At step 103, the powdered feed materials, first feed material 104 and second feed material 105 are deployed into the negative mould. At step 106, heat is applied to the mould increasing the temperature and causing the powdered feed materials to melt during a heating stage. At step 107 the temperature is maintained where necessary to allow diffusion of first feed material 104 with second feed material 105. At step 108 the temperature is decreased, causing the molten feed materials to solidify during a cooling stage.
As discussed with reference to step 101 of the method of
A method of obtaining a negative mould is illustrated in
Negative mould 102 is then formed around the positive model to define a negative profile of the component. In a first embodiment, the negative mould comprises a layer of material having a melting point higher than that of first feed material 104 and second feed material 105, such as a very high temperature ceramic material. The ceramic shell is created by applying a plurality of layers of ceramic slurry to the exterior of the positive model, until the required wall thickness is achieved.
The sacrificial positive model material 201 is then evacuated from the negative mould 102 via aperture 202 using an appropriate removal technique, such as the application of heat and/or a solvent. This leaves a cavity 203 defining the exterior surface of the component.
Step 103 of the method of
In the embodiment illustrated in
Thus, in the illustrated embodiment first feed material 104 and second feed material 105 are deployed into the mould together via aperture 202 so as to fill cavity 203 within negative mould 102, and then heated as in step 106 during a heating stage. The temperature is increased by heating the negative mould 102 to a first temperature, approximately equal to the melting point of first feed material 104, thereby causing first feed material 104 to melt. However second feed material 105, having a melting point much higher than that of the first temperature is not melted and remains in its solid particulate form.
In alternative embodiments the heating stage described above is performed in a pressure controlled environment at a pressure other than atmospheric pressure of 1 bar. In a first embodiment the pressure is reduced to below atmospheric pressure by removing air from the pressure controlled environment. In a second embodiment the applied pressure is increased to a level above atmospheric pressure. Equally the heating stage may be performed under time-evolving pressure conditions.
A negative mould 401, substantially similar to negative mould 102, of
Feeder section 403 feeds additional liquefied material into negative mould 401 during cooling stage 108 as the molten material contained within negative mould 401 contracts in volume. When materials cool from their molten liquid phase, their volume reduces as the temperature decreases to the point at which they become solid. Therefore feeders are used to provide liquefied material to compensate for the shrinkage cavities that would otherwise form at one or more thermal centres in the interior of the casting. Therefore the volume of the liquefied material in feeder section 403 is determined by the requirement for sufficient liquid material to be provided in order to compensate for the volume reduction of the material as it cools. During solidification the material contracts typically by about 7% by volume and consequently an equal volume of liquefied material enters the component section from the feeder section.
The efficiency of the feeder section 403 is influenced by the static pressure in the feeder, resulting from the amount of liquefied material it holds and its vertical height. The static pressure head assists in forcing the liquefied material into the casting as it cools. Moulds in accordance with this preferred embodiment have one or more feeders that are sufficiently tall such that during production of the composite component, the height of liquefied material within the feeder remains above the tallest part of the component section of the mould. This allows sufficient pressure to be exerted on the molten material within the mould as to ensure that any fine features defined by the mould are reproduced.
As previously discussed with reference to
Unlike negative mould apparatus of the prior art, in the illustrated embodiment negative mould 401 is not hermetically sealed during heating stage 106 and cooling stage 108. Moulds of the prior art compensate for a reduction in volume of feed materials during temperature changes by plastically deforming. In an embodiment negative mould 401 is substantially rigid and is configured to be incompressible so as not to deform during heating stage 106 and cooling stage 108. In order to account for fluctuations in volume of the material within the negative mould during temperature changes, negative mould 401 is configured to be pervious to the atmosphere during use. Therefore negative mould 401 exchanges volume with the atmosphere via aperture 407 and feeder section 403. However, in an alternative embodiment negative mould 401 is hermetically sealed after deploying stage 103 and prior to heating stage 106, by capping feeder section 403 to prevent interaction with the atmosphere.
In alternative embodiments the cooling stage may be conducted under pressure conditions other than atmospheric pressure, or applied pressure may be varied as a function of time. Altering the pressure under which the material solidifies can influence the crystal structure of the casting. Solidifying during the cooling stage under increased pressure conditions is likely to result in a denser and more structurally uniform casting, enhancing the impact properties of the component.
The negative moulds 102 and 401 described with reference to
An example of such a mould is shown in cross section in
First feed material 608 is deployed into first region 602, via first aperture 603 and feeder section 606. Second feed material 609 is deployed separately to said first feed material 608 into second region 605, via second aperture 604 and feeder section 607. This results in a casting with different materials localised to different regions of the component. The feed materials are deployed into their respective regions at roughly the same rate therefore they meet at an interface 610 that is approximately equidistant between first aperture 603 and second aperture 604.
Negative mould 601 previously discussed with reference to
First feed material 608 is a metal powder with a melting temperature of 700° C. and second feed material 609 is a different metal powder with a melting temperature of 1000° C.
Heat is applied to negative mould 601 as indicated by the arrows, so as to raise its temperature to a first temperature, approximately equal to the melting point of the first feed material. Therefore first feed material 608 is melted within first region 602. The temperature is further increased to a second temperature approximately equal to the melting point of second feed material 609, thereby melting second feed material 609.
In an embodiment said first feed material 608 is joined to said second feed material 609 by selective alloying through solid state diffusion of the particles.
At this point both first feed material 608 and second feed material 609 are in their molten liquid states and thus diffusion occurs resultant of the Brownian motion of the particles. At the interface 610 some of the particles of first feed material 608 deployed in first region 602 drift rightwards into second region 605. Similarly particles of second feed material 609 drift leftwards from second region 605 into first region 602, thereby alloying said first feed material 608 to said second feed material 609 across an alloyed region 701 about the interface 610.
The extent to which said first feed material 608 is alloyed to said second feed material 609 is controlled by the diameter d of the alloyed region 701. The diameter d of alloyed region 701 is dictated by the distance to which the particles are allowed to diffuse into the adjacent region. This is dependent on the energy of the particles, itself a function of the temperature, and the time period for which the feed materials are in their molten state.
In the illustrated embodiment the second temperature is maintained during step 107, for a time period of 10 seconds, so as to allow diffusion of particles over an alloyed region of diameter d. When the desired degree of alloying is achieved the temperature of negative mould 601 is decreased to below the melting points of the feed materials thereby preventing excessive diffusion of the molten materials.
Therefore once solidified a component is produced with a non-uniform metallurgy along its length, with a discreet interface between the different regions of material. In this way the component enjoys the functional benefits of both first feed material 608 and second feed material 609, whilst maintaining the structural properties of a single casting.
An example of a component produced by a process embodying the present invention is illustrated in cross section in
Additionally negative mould 801 comprises a partition 808 thereby dividing first region 802 and second region 803. Partition 808 is a titanium sheath that is approximately 2 mm in thickness and completely separates first region 802 from second region 803.
When producing more complex components using powder pressing techniques a problem is experienced in terms of maintaining the positioning of the feed materials, to prevent gravitational sedimentation or incidental dispersion in the powder phase prior to melting. By including partition 808 first feed material 806 in first region 802 is prevented from migrating into second region 803 and diffusing with second feed material 807 until partition 808 is melted.
In this embodiment the object to be produced is a rack component corresponding to a rack and pinion arrangement, used for transforming the rotational motion of a toothed pinion gear into linear motion of the splined rack. The splined surface of the rack component is repeatedly subjected to a torque exerted by the pinion gear; therefore must be extremely hard so as to be resistant to material degradation. However, additionally the component must be relatively lightweight.
Thus, it is desirable to have a first region of the component composed of a substance that is sufficiently lightweight, such as titanium, but wherein the driving splines have an increased surface hardness for example by using a titanium carbide powder.
In the illustrated embodiment first feed material 806 second feed material 807 and partition 808 each have a melting point corresponding to a first temperature. Therefore upon application of heat to a first temperature, as in step 106 and described with reference to
As in step 107 the temperature is maintained for a period of time to allow a diffusion layer of sufficient diameter as to create a discreet interface between first feed material 806 and second feed material 807 to form.
When the diffusion layer forming the alloyed region is the desired diameter the temperature is decreased to below said first temperature, causing the casting to solidify during a cooling stage.
Although cooling at a natural rate may be suitable for some materials, others may require accelerated cooling. In an embodiment the temperature of the mould is decreased rapidly by forcing an inert gas such as argon or helium to flow over the negative mould 801.
A fifth example of a mould embodying the present invention is shown in
Similarly to
In an embodiment of the invention partition 903 is invested within the positive wax model of the component, as shown with reference to
In the illustrated embodiment first feed material 909 in first region 902 and second feed material 910 in second region 904 are metallic powders. First feed material 909 is a low density metal powder such as aluminium with superior thermal conductivity characteristics, whilst second feed material 910 is a high density metal such as steel with increased surface hardness. First feed material 909 and second feed material 910 both have a melting point corresponding to a first temperature.
Partition 903 comprises a third material such as nickel which is beneficial to the alloying process, and having a melting point corresponding to a second temperature, higher than the first temperature.
Therefore when the temperature of mould 901 is increased to the first temperature, both first feed material 909 and second feed material 910 are melted, however they are separated by partition 903 which remains in solid form. Further increasing the temperature of the negative mould 901 to the second temperature causes the partition 903 to be melted, thereby allowing diffusion of first feed material 909 with second feed material 910, creating an alloyed region comprising both first feed material aluminium particles, second feed material steel particles and partition material nickel particles.
By altering the composition of the material used to create partition 903 it is possible to influence the melting point and therefore alloying temperature of the various feed materials.
In an embodiment the partition 903 is formed of a ceramic material having a melting point far greater than the melting points of the feed materials. Therefore the alloying process is conducted at a greatly increased temperature, resulting in a more uniform crystalline structure of the alloyed region.
Although the invention has been described by embodiments present using only two powdered feed materials, it will be appreciated that in alternative embodiments any number of dissimilar feed powders may be used.
Equally components may be created with any number of different localised regions of dissimilar materials, accessed by any number of apertures for insertion of feed materials. Indeed in a yet further alternative embodiment of the present invention a multi-core rod component is formed, comprising ten regions separated by nine partitions into which ten dissimilar feed materials are deployed.
Apparatus for performing the processing method embodying the present invention in which moulds such as negative mould 901, containing powdered feed materials are processed to form a composite component, is illustrated in
The apparatus 1001 comprises a vacuum furnace 1002, creating a vacuum-tight chamber 1003 and having a door 1004 to allow loading and unloading of the chamber 1003.
In an embodiment vacuum furnace 1002 includes means for varying the temperature and pressure within. Vacuum furnace 1002 has a heat source for producing radiant heat such as resistance heating element 1005, which is connected to power supply 1006.
The apparatus 1001 also includes a vacuum pump 1007 connected to chamber 1003 for evacuating air from the chamber 1003 such that the pressure in the chamber may be reduced to below atmospheric pressure. Additionally a compressor 1008 is also provided so as to allow the pressure inside the chamber 1003 to be increased above atmospheric pressure.
Electronic controller 1009 is provided comprising temperature sensor 1010 and pressure sensor 1011 and associated wiring. Temperature sensor 1010 is located within chamber 1003 and is configured to provide signals indicative of the actual temperature within chamber 1003. Similarly pressure sensor 1011 is configured to provide an indication of the air pressure within the chamber 1003.
Electronic controller 1009 is configured to receive signals from temperature sensor 1010 and pressure sensor 1011, and modulate the power supply 1006 for the resistance heating element 1005, vacuum pump 1007 and compressor 1008 accordingly. In an embodiment, controller 1009 is a programmed computer system or microcontroller.
In an embodiment of the invention it is desirable that the cooling stage is accelerated to cause the molten feed materials to decrease in temperature and solidify more rapidly. Therefore compressor 1008 is used to force an inert gas, such as nitrogen into vacuum chamber 1003 whilst vacuum 1007 evacuates the spent gas, thereby creating a cooling convection current over negative mould 901.
An example of process steps 106-108 embodying an aspect of the present invention as illustrated in
Initially at time 1102 the chamber of vacuum furnace 1002 is at ambient temperature 1103. Electrical power is then supplied to the resistance heating coils, at time 1104 until time 1105 to raise the temperature within the chamber to a first temperature 1106, above the melting point of first feed material 806, second feed material 807 and partition 808, thereby establishing molten liquid within mould 801.
The increased temperature 1106 is stably maintained between times 1105 and 1107, thereby maintaining the feed materials in their molten state and enabling a degree of diffusion at the interface.
When the period of time that has elapsed is sufficiently long to allow a suitable amount of diffusion between first feed material 806 and second feed material 807, creating an alloyed region of desired diameter, power to the resistance coils is interrupted and heating stops.
At time 1107 the power is cut-off and the chamber is allowed to cool naturally, until at time 1108 the temperature has returned to ambient temperature 1103, the casting has solidified and may now be removed from the chamber.
A second example of process steps 106-108 embodying the aspect of the invention as illustrated in
Again at time 1102 the vacuum furnace chamber is at ambient temperature 1103. At time 1104 the heating stage begins and the temperature of the chamber increases until the first temperature 1106 at time 1105. At this stage first feed material 909 and second feed material 910, both having melting temperatures below first temperature 1106, have been melted and are in their molten state.
However as discussed with reference to
In this embodiment it is desirable that the cooling stage is accelerated. Therefore at time 1203 compressor 1008 and vacuum 1007 are configured to force a convection current over the negative mould 901, as described with reference to
Number | Date | Country | Kind |
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1403557.0 | Feb 2014 | GB | national |
Filing Document | Filing Date | Country | Kind |
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PCT/GB2015/000067 | 2/26/2015 | WO | 00 |