FIELD OF THE INVENTION

The present invention relates to a method of manufacturing a powder- metallurgical component having at least one longitudinally extending channel. In particular it relates to a method wherein the green body is dried by guiding a flow of gas along the channel before sintering or oxidizing of the green body.

BACKGROUND OF THE INVENTION

When manufacturing a powder metallurgical component using a wet binder, it has been found that a slow drying due to water evaporation is not optimal, because it can be difficult to ensure that the green body keeps the desired shape during drying. In particular, a non-uniform drying of the green body induces tensions in the component resulting in cracks or distortion of the components. This is especially the case for non-symmetric geometries, large components and thin- walled structures.

Therefore a controlled acceleration or, occasionally, deceleration of the in-depth drying is required. This is typically done by placing the green body to be dried in a space having a controlled temperature, such as in an oven. However, the use of heat only accelerates the evaporation on the outer surface of the green body, i.e. results in a non-uniform drying. Within ceramics manufacturing the drying is often done by use of microwaves, but that is not applicable for drying of components made from metal powder.

Hence, an improved method of manufacturing a powder-metallurgical component would be advantageous.

OBJECT OF THE INVENTION

Thus, it is an object of the present invention to provide a method of manufacturing a powder-metallurgical component with which it is possible to minimize the risk of deformation and damage occurring during drying of a green body before sintering or oxidizing.

It is another object of the present invention to provide a method of manufacturing a powder-metallurgical component, which method facilitates the manufacturing of more complex component geometries compared to known methods. This is related to the object of providing a method with less risk of damage caused by the drying, since such damage could arise due to an uneven drying of a complex geometry; e.g. be due to large variations in wall thicknesses of a component to be dried.

It is a further object of the present invention to provide an alternative to the prior art.

In particular, it may be seen as an object of the present invention to provide a method of manufacturing a powder-metallurgical component that solves the above mentioned problems of the prior art.

SUMMARY OF THE INVENTION

Thus, the above described object and several other objects are intended to be obtained in a first aspect of the invention by providing a method of manufacturing a powder metallurgical component, the component comprising at least one longitudinally extending channel, the method comprising the following steps:

- preparing a powder mixture comprising metal powder and a binder,

- transferring the powder mixture to a processing equipment comprising a die,

- forming the powder mixture into a green body by forcing it through the die which is adapted to form the component in a shape having the at least one longitudinally extending channel,

- drying the green body by guiding a flow of gas through the at least one longitudinally extending channel, and

- sintering or oxidizing the dried green body to bond the powder together and thereby form the powder metallurgical component. The at least one longitudinally extending channel may be closed along all side walls thereof. It may also be open along one of the sides in which case it may be necessary to close off the open side to obtain the necessary guiding of the flow of gas that is to provide the drying.

The processing equipment may e.g. be an extruder, such as a piston extruder.

The powder mixture may be in the form of a paste. By "paste" is meant a thick, soft, sticky substance made by mixing a liquid with a powder. In other words, pastes typically consist of a suspension of granular material in a background fluid. In the context of the present invention, the viscosity of the paste should be so that it allows for the necessary handling of the paste during the transfer from the device used for the mixing and to the processing equipment. It should also allow for the subsequent process steps; i.e. it should be low enough to allow for the forming and high enough to ensure that the green body keeps the desired geometry. The viscosity of a given paste can be determined by equipment and methods designed therefore, such as by use of a capillary rheometer which is typically used to measure shear viscosity and other rheological properties. However, since the viscosity is correlated to the hardness of the material, it will also be possible to use this parameter in the determination of whether a given paste is suitable for the manufacturing method or not. A possible related measure to use is the Shore Hardness which can be determined in accordance with ISO 868/ ASTM D2240. Another option is to use a special tool designed for clays; this has been used during the development of the present invention. This tool is similar to a Shore tester but has been adapted for the characterization of clays; such an instrument can also be referred to as a durometer for clays. The operating principle is based on the force exerted by the sample material on the penetration of the calibrated spring of the instrument, when a pin of the tool is pressed into the material being tested until the pin reaches a support. In this way, a steady force at a steady stroke is always applied to the instrument. It has a scale from 0 to 20 to use as a relative hardness reference parameter, and gram scale of applied force. With this tool, a penetration point is pressed into the paste when it comes out of the kneader. Then the maximum value indicated at the moment when the penetration point is inside the paste is measured. The maximum point is used instead of waiting for it to stabilize because it will eventually show a much lower value, maybe getting close to 0 as the penetration point would be forced through the paste. With this method, it has been found that values higher than 12 Shore are necessary to obtain a satisfactory result, at least for the geometries tested.

The metal may be any metal that is available as powder. A non-exhaustive list of possible metals include: 316L, FeCrAI, Inconel 625, Hastalloy X, 17-4PH, 430L, and 304L.

A binder or a binding agent is any material or substance that holds or draws other materials together to form a cohesive unit mechanically, chemically, by adhesion or cohesion. The binder is preferably organic, such as cellulose ethers, agarose or polyoxymethylene. Examples of binders are: methylcellulose, 25 poly(ethylene oxide), poly(vinyl alcohol), sodium carboxymethylcellulose (cellulose gum), alginates, ethyl cellulose and pitch.

The powder mixture may also comprise other constituents, such as ceramic powder or lubricant. A non-exhaustive list of possible ceramics include: AIO, SiO, ZiO, Alumina, Zirconia, Boron Nitride, Cordierite, and Silicon Nitride.

An effect of the step of drying as described is that a more uniform drying throughout the component can be obtained. Studies made as part of the development leading to this invention have shown that such a drying step makes it easier to ensure that the component maintains its intended shape without deforming or cracking. This is particularly relevant for complex geometries or small wall thicknesses, such as for a component having a large number of longitudinally extending inner channels, possibly separated by thin walls.

In some embodiments of the invention, the step of drying further comprises guiding a flow of gas along outer surfaces of the green body so that the drying also takes place from the outside due to this flow of gas.

In alternative embodiments, the step of drying further comprises covering outer surfaces of the green body, e.g. with plates, so that the drying takes place due to the flow of gas being through the at least one longitudinally extending channel only. Hereby evaporation of liquid from the outer surfaces can be prevented. This has been found to provide for a more uniform drying at least for some geometries of the component being manufactured. The choice of whether or not to cover outer surfaces can be used to control the drying process e.g. to avoid undesired deformation or cracking. This may e.g. depend on the geometry of the component being manufactured, including the thickness of the walls surrounding the longitudinally extending channel. In addition to the influence on the drying, plates or other elements used for the covering of the outer surfaces can provide structural support to the green body during drying. Hereby the supporting effect can be used to ensure that the green body keeps the desired shape during drying.

In some embodiments of the invention, the following steps precede the step of drying:

- providing a drying tool comprising:

- a first end comprising or being connectable to a gas flow generating device, and

- an opposite second end comprising a plurality of nozzles each in fluid communication with the first end so that gas can flow through each of the nozzles under the action of the gas flow generating device during use of the drying tool,

- arranging the drying tool in relation to the green body so that a nozzle of the drying tool extends into an end region of each of the at least one longitudinally extending channel of the green body,

- activating the gas flow generating device so that gas flows into each of the at least one longitudinally extending channel.

An example of a possible design of such a drying tool will be described in relation to the figures. Such a drying tool will be particularly advantageous for the drying of a green body having a plurality of longitudinally extending channels as such a geometry might otherwise be more difficult to dry uniformly.

In alternative embodiments of the invention and wherein the green body has a plurality of longitudinally extending channels, the method may comprise using a drying tool as just described, but the step of arranging the drying tool in relation to the green body is performed so that nozzles of the drying tool extend into an end region of at least some, such as a majority, of the plurality of longitudinally extending channels of the green body.

By "a majority" is preferably meant more than 50%, such as more than 70%, such as more than 90%.

By using a drying tool as described and letting the nozzles extend into each or a majority of the longitudinally extending channels, a uniform drying throughout the volume can be ensured. In addition, the nozzles may be shaped and dimensioned so that they provide structural support to the part of the walls of the at least one longitudinally extending channel that is in contact with the tool and thereby prevent deformation thereof. The advantage thereof is both that the green body remains undeformed and that the gas flow is not hindered as it could be by deformed, such as collapsed, longitudinally extending channels.

In some embodiments of the invention, the plurality of nozzles of the drying tool are arranged over the whole cross section of the green body to be dried. Hereby a uniform drying throughout the component may be ensured.

The nozzles may be shaped and dimensioned so that they provide structural support to the part of the walls of the at least one longitudinally extending channel that is in contact with the tool and thereby prevent deformation thereof.

An alternative to using such a drying tool comprising nozzles could be to use a drying tool having a connecting end for guiding the gas flow into the at least one longitudinally extending channel. Such a drying tool should preferably have at least one sealing or gasket to be placed in engagement with the green body or an opening end of the at least one longitudinally extending channel so that the gas is thereby guided into the channel. However, any suitable method of providing the guiding of the gas through the at least one longitudinally extending channel is covered by the scope of the claims. It may be provided by a blowing or a sucking action. The flow generating device may be any type of device which is adapted to provide a gas flow. It may e.g. be a gas fan, a vacuum pump, a reverse fan, or a compressor. The gas flow generating device may be an integrated part of the drying tool, or it may be an external device connected thereto. An auxiliary tool may be arranged at an opposite end of the green body as the one where the drying tool is arranged, the auxiliary tool being adapted to support the at least one longitudinally extending channel during drying and thereby prevent undesired deformation of the green body.

By "support" is preferably meant that the auxiliary tool supports part of the inner surfaces of the at least one channel and thereby prevents it from deforming, such as collapsing. This should preferably be done without significantly restricting the gas flow.

The step of drying may have a length being a predetermined time period or being determined by measurements of the humidity of the gas that has flown through the green body. It will depend on parameters including the material, the geometry, and the dimensions of the green body. Which measures to use may be determined experimentally, possibly assisted by computer simulations.

In some embodiments of the invention, the component being manufactured has a plurality of longitudinally extending internal channels, such as having a honeycomb structure.

The gas used for the drying may have a higher or a lower temperature than the surrounding air, and/or the gas may have a higher or a lower humidity than the surrounding air. The gas may e.g. be air. The drying may also be controlled by varying the speed of the flow of gas.

In any of the embodiments as described above, a step of debinding may precede the step of sintering or oxidizing, the debinding step typically comprising heating the green body to a temperature at which at least some, such as all, of the binder burns off. This debinding step is typically performed after the step of drying. Debinding is the process in which the binder is removed from the green body to ensure that no leftover carbon is present in the component during sintering. This debinding is typically done by heating the green body to a temperature between 200 to 750 degrees Celsius and allowing the binder to burn off. Different binders require different debinding temperatures. In embodiments using methylcelulose, the debinding is typically done in an oxidizing atmosphere, typically air, but it can also be done partially in the same atmosphere as the sintering atmosphere, if the final component is not ruined by the extra content of carbon. In order to ensure that the debound green body can still be handled, it may be necessary to oxidize the powder slightly together; these oxides will be removed in the sintering process.

A second aspect of the invention relates to a drying tool for drying a green body during the manufacturing of a powder metallurgical component before sintering or oxidizing, the drying tool comprising:

- a first end comprising or being connectable to a gas flow generating device, and

- an opposite second end comprising a plurality of nozzles each in fluid communication with the first end so that gas can flow through each of the nozzles under the action of the gas flow generating device during use of the drying tool, and

- the drying tool being suitable for drying a component obtained by a method according to the first aspect of the invention.

In such a drying tool, the plurality of nozzles may be arranged in a predetermined pattern, such as in a regular pattern of aligned rows and columns. They may e.g. be arranged to match a pattern formed by the mutual positions of longitudinally extending channels of a component being manufactured by a method according to the first aspect of the invention as described above. In some embodiments, there are at least three rows of nozzles, each row comprising at least three nozzles.

A drying tool according to the invention may comprise a plurality of fluid channels each extending between the first end and a nozzle. Hereby it may be facilitated to obtain a uniform flow of gas through all of the longitudinally extending channels when the drying tool is used in a method according to the first aspect of the invention. Hereby it may be easier to ensure a uniform drying of the component whereby the risk of deformations and cracking due to the drying can be avoided or minimized. At least some of the nozzles may comprise a closing mechanism for closing off the respective nozzle so that there is no flow of gas there-through during use of the drying tool. The mutual position of at least some of the nozzles may be adjustable. By a drying tool having one or both of these two features, it is obtained that a given drying tool can be adapted for the drying of components having different geometries and sizes, including different numbers of longitudinally extending channels.

The first and second aspects of the present invention may be combined. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE FIGURES

The method of manufacturing a powder-metallurgical component according to the invention will now be described in more detail with regard to the accompanying figures. The figures show one way of implementing the present invention and is not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

Figure 1 shows schematically a method of manufacturing a powder-metallurgical component according to the first aspect of present invention. Figure l.a shows the steps of preparing the powder mixture, transferring it into the processing equipment, and forming a green body. Figure l.b shows how a gas flow is guided through an internal channel of the green body having the outer surfaces covered by plates. Figure l.c shows the sintering.

Figure 2 shows schematically an example of a component having a plurality of longitudinally extending inner channels arranged in a regular pattern.

Figure 3 shows schematically an embodiment of a drying tool according to the second aspect of the present invention. Figure 3. a is a side view, and figure 3.b is three-dimensional partial view of the second end comprising nozzles. Figure 4 shows how the drying tool of figure 3 can be arranged with the nozzles being engaged with end sections of channels of a green body during drying.

Figure 5 is a cross-sectional view of the drying tool in figure 4.

Figure 6 shows schematically the drying tool in figures 3-5 having some of the nozzles closed by a plug.

Figure 7 shows schematically a drying tool wherein the mutual positions of nozzles are adjustable.

Figure 8 shows schematically a step of drying wherein an auxiliary tool is used to support the green body.

Figure 9 shows experimental results of tests performed during the development of the present invention.

DETAILED DESCRIPTION OF AN EMBODIMENT

Figure 1 shows schematically a method of manufacturing a powder-metallurgical component according to the first aspect of present invention. As shown in figure l.a, a powder mixture is prepared by mixing at least metal powder 11 and a binder 12. The powder mixture may comprise further constituents, such as ceramic powder or lubricant. The powder mixture is then transferred to a processing equipment 31 comprising a die 32; it may e.g. be an extruder, such as a piston extruder. The powder mixture is formed into a green body 20 by forcing it through the die 32. This step is done by applying a pressure P as shown schematically in the figure as an arrow. The die 32 is designed so that it is adapted to form the component 21 in a shape having the at least one longitudinally extending channel 22. In figure 1 the component has only one channel, but a component having a plurality of channels can be produced by a similar method by using another die.

Figure l.b shows schematically the step of drying the green body 20 by guiding a flow of gas G through the longitudinally extending channel 22. In the illustrated embodiment, the outer surfaces of the green body 20 are covered by plates 39 so that the drying takes place due to the flow of gas G being through the longitudinally extending channel 22 only. The gas may have a higher or a lower temperature than the surrounding air, and/or the gas may have a higher or a lower humidity than the surrounding air. The gas is typically air, but other gasses can also be used. As seen from figure l.b, in addition to ensuring that evaporation of water does not take place from the outer surfaces, the covering plates can also provide structural support to the green body during the drying.

The step of drying could have a length being a predetermined time period, e.g. determined experimentally. It could also be determined during the drying process from measurements of the humidity of the gas that has flown through the green body 20.

After drying, the final component 21 is obtained by sintering the dried green body as shown schematically in figure l.c. This may e.g. be done in a reducing atmosphere, in vacuum, or in an inert atmosphere. The sintering is typically performed in a furnace 34 at temperatures of 950 to 1430 degrees C. As explained in more details above, a step of debinding may precede the step of sintering or oxidizing, the debinding step typically comprising heating the green body 20 to a temperature at which at least some, such as all, of the binder burns off.

Figure 2 shows schematically an example of a component 21 having a plurality of longitudinally extending inner channels 22 arranged in a regular pattern, separated by walls 23. Such a component can be manufactured by a method as described in relation to figures 1 provided that a suitably designed die 32 is used.

Figure 3 shows schematically an embodiment of a drying tool 40 for drying a green body 20 before sintering or oxidizing. Figure 3. a is a side view illustrating that the drying tool 40 has a first end 41 comprising or being connectable to a gas flow generating device 43, and an opposite second end 42 comprising a plurality of nozzles 44. The nozzles 44 are in fluid communication with the first end 41 so that gas can flow through each of the nozzles 44 under the action of the gas flow generating device 43 during use of the drying tool 40. Figure 3.b is three- dimensional partial view of the second end 42 comprising nozzles 44. In the illustrated embodiment, the nozzles 44 are arranged in a regular pattern of aligned rows and columns. In the illustrated embodiment, the nozzles 44 have two different shapes, but the nozzles may all be identical, or there may be more different shapes and sizes of nozzles.

Figure 4 shows how the drying tool 40 of figure 3 can be arranged with the nozzles 44 being engaged with, such as extending into, end sections of the longitudinally extending channels 22 of a green body 20 during drying. By comparing figures 2 and 3, it can be seen that arrangement of the nozzles 44 of the drying tool 40 of figure 3 matches the arrangement of the inner channels 22 of the component in figure 2. However, when this is not the case, it will still be possible to use the drying tool 40 as will be shown in the following. It has to be ensured that the nozzles 44 do not damage the green body 20. When the nozzles 44 have been arranged, the gas flow generating device 43 is activated so that gas flows into each of the longitudinally extending channels 22. At the same time, the fact that the nozzles 44 extend into each of the inner channels 22 means that the nozzles both provide for a uniform drying and support the walls 23. Both measures lead to a minimization of possible deformation and damage of the green body 20.

Figure 5 is a cross-sectional view of the drying tool 40 in figure 4. It illustrates the drying tool 40 comprising a plurality of fluid channels 45 each extending between the first end 41 and a nozzle 44. Hereby a more uniform distribution of the gas into all of the nozzles 44 can be obtained compared to an embodiment wherein the middle section of the drying tool 40 is one open space or a lower number of fluid channels 45. However, such an embodiment would also be covered by the scope of protection.

Some of the nozzles 44 may comprise a closing mechanism 46 for closing off the respective nozzle 44 so that there is no flow of gas there-through during use of the tool. An example of such an embodiment is shown schematically in figure 6, wherein the nozzles 44 in the upper and lower rows of the drying tool 40 are shown as being closed with a closing mechanism 46, such as a removable plug. This may e.g. be relevant if the drying tool 40 is to be used for the drying of a green body 20 having a smaller cross-section or a green body 20 having outer regions without inner channels.

Figure 7 shows schematically a drying tool 40 wherein the nozzles 44 are in the form of flexible tubes so that the mutual positions of nozzles 44 is adjustable. Hereby it may be possible to use the drying tool 40 for different geometries of green bodies 20. In some embodiments of a drying tool 40 having this kind of adjustable nozzles 44, at least some of the nozzles 44 may comprise stiff end sections (not shown) adapted to support the walls 23 of the longitudinally extending channels 22 and thereby prevent them from deforming during the drying as described above.

Figure 8 shows schematically how an auxiliary tool 47 can be arranged at an opposite end of the green body 20 as the one where the drying tool 40 is arranged. The auxiliary tool 47 is used to support the longitudinally extending channel 22 during drying. In figure 8 this is shown schematically as small pins 48 protruding from an end surface of the auxiliary tool 47 so that they can extend into the longitudinally extending channels 22 of the green body 20 being dried.

Figure 9 shows results of some tests made to study the effect of using a drying tool according to the present invention. All the six components were made from the same materials and extruded to have a plurality of inner channels as shown in figure 2. The experiments were repeated three times as shown in figures 9. a, 9.b and 9.c, respectively. The lower components in the figures were left to dry without any forced flow of air, and the upper component were dried by guiding air through the inner channels by use of a drying tool according to the invention. The results clearly show how the use of a drying tool and method according to the present invention can be used to stabilize the component during drying and thereby prevent undesired deformation thereof.

Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Furthermore, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.