A METHOD OF MANUFACTURING A COMPOSITE COMPONENT WITH VARYING ELECTRIC RESISTIVITY ALONG A LONGITUDINAL DIRECTION

Abstract

The invention relates to a method of manufacturing a composite component (21) having a varying electric resistivity along a longitudinal direction of the component. At least a first paste (10a) having a first composition, and at least a second paste (10b) having a second composition are prepared. The pastes are transferred into a supply chamber (35) of a processing equipment (31), such as an extruder. A green body (20) is shaped by forcing the pastes from the supply chamber through a die (32), and the green body is then sintered or oxidized to form the composite component. The pastes may comprise metal powder, ceramic powder, and binder. The varying electric resistivity may be due to variations in one or more of the following parameters: the volume ratio between the metal powder and the ceramic powder, the size of the ceramic particles, and the type of the ceramic material.

Claims

1. Method of manufacturing a composite component having a varying electric resistivity (ρ) along a longitudinal direction, the method comprising the following steps: preparing a plurality of pastes comprising: at least a first paste having a first composition, and at least a second paste having a second composition, transferring the plurality of pastes into a supply chamber of a processing equipment, shaping a green body from the plurality of pastes by forcing the pastes from the supply chamber through a die of the processing equipment, and sintering or oxidizing the green body to obtain the composite component having the varying electric resistivity (ρ) along the longitudinal direction of the composite component, the longitudinal direction corresponding to the direction of movement of the pastes through the die, and the varying electric resistivity (ρ) resulting from the first composition being different from the second composition.

2. Method according to claim 1, wherein: the first paste comprises metal powder with a first alloy composition, ceramic powder, and a first binder, the second paste comprises metal powder with a second alloy composition and a second binder, and wherein the first alloy composition and the second alloy composition both consist of at least one chemical element, and wherein the chemical elements are chosen so that, for each of the chemical elements being present in an amount higher than 0.5 weight % in each of the alloy compositions, that chemical element is comprised both in the first and second alloy composition, and  for the chemical elements being present in the first alloy composition in amounts of up to 5.0 weight %, the amount of that chemical element differs by at most 1 percentage point between the first and second alloy compositions, and for the chemical elements being present in the first alloy composition in amounts of more than 5.0 weight %, the amount of that chemical element differs by at most 3 percentage point between the first and second alloy compositions.

3. Method according to claim 2, wherein the first binder and the second binder have similar or the same solvability.

4. Method according to claim 2, wherein the second paste further comprises a ceramic powder.

5. Method according to claim 1, wherein the different electric resistivities (ρ) are obtained by varying one or more of the following parameters: the volume ratio between the metal powder and the ceramic powder, the size of the ceramic particles, the shape of the ceramic particles, and the type of the ceramic material.

6. Method according to claim 2, wherein each of the metal powders of the first paste and of the second paste comprises one or more of the following chemical elements: iron, copper, chromium, aluminium, cobalt, nickel, manganese, molybdenum, vanadium, yttrium, and silicon.

7. Method according to claim 1, wherein the step of preparing a plurality of pastes comprises supplying material from at least two feeding chambers into a mixing chamber in varying amounts, and preparing the plurality of pastes in the mixing chamber.

8. Method according to claim 1, wherein a predetermined order in which the plurality of pastes are transferred into the supply chamber corresponds to the longitudinal direction of the composite component being manufactured.

9. Method according to claim 1, wherein the step of shaping a green body is performed by continuously forcing the pastes through the die.

10. Method according to claim 1, wherein the die has a pattern of outlets resulting in the green body having at least one longitudinally extending internal channel.

11. Method according to claim 10, wherein the die has a pattern of outlets resulting in the green body having a plurality of longitudinally extending internal channels arranged in a regular pattern, such as having a honeycomb structure.

12. Method according to claim 2, wherein a step of debinding precedes the step of sintering or oxidizing, the debinding step preferably comprising heating the green body to a temperature at which at least some, such as all, of the binder burns off.

13. Composite component having an electric resistivity (ρ) which varies along a longitudinal direction of the composite component, wherein the composite component has been manufactured by a method according to claim 1, so that the longitudinal direction corresponds to a direction of movement of the pastes through a shaping die during manufacturing of the composite component.

14. Composite component according to claim 13, wherein the composite component has been manufactured from pastes comprising metal powder and ceramic powder.

15. Composite component according to claim 14, wherein the varying electric resistivity (ρ) is due to variations in one or more of the following parameters: the volume ratio between the metal powder and the ceramic powder, the size of the ceramic particles, the shape of the ceramic particles, and the type of the ceramic material.

16. Composite component according to claim 13, wherein the electric resistivity (ρ) is substantially constant in cross-sections perpendicular to the longitudinal direction of the composite component.

17. Composite component according to claim 13, wherein the composite component has at least one longitudinally extending internal channel, such as wherein the composite component has a plurality of longitudinally extending internal channels, such as has a honeycomb structure.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0073] The method of manufacturing a composite 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.

[0074] FIG. 1 shows schematically the overall idea of having a composite component with varying electric resistivity along a length direction of the composite component.

[0075] FIG. 2 shows schematically how two pastes are extruded into a composite component having regions with different electric resistivities.

[0076] FIG. 3 shows schematically cross-sections of a composite component, the two cross-sections comprising different amounts of ceramic particles.

[0077] FIG. 4 shows a graph of how the electric resistivity varies as a function of the amount of the ceramic alumina.

[0078] FIGS. 5.a and 5.b shows schematically two examples of shapes of components that can be manufactured with a method according to the present invention.

[0079] FIG. 5.c shows schematically an example of a die that can be used for manufacturing of a component with an array of longitudinally extending inner channels.

[0080] FIG. 6 shows schematically a processing equipment that can be used in a method according to the present invention.

[0081] FIG. 7 shows a flow diagram of a method according to the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT

[0082] The present invention is in a first aspect related to the manufacturing of a composite component 21 having an electric resistivity which varies along a longitudinal direction of the composite component 21. FIG. 1.a shows schematically an example of such a composite component 21 which has four regions 21a, 21b, 21c, 21d with different electric resistivities along the longitudinal direction corresponding to a direction of movement of the pastes through a shaping die 32 (see FIG. 2) during manufacturing of the component 21. FIG. 1.b shows a curve of the electric resistivity ρ as a function of position along the length X of the component 21 in FIG. 1.a. In this illustrated embodiment, the electric resistivity varies in steps and with a constant increase rate in the narrow regions around the borders between the different regions 21a, 21b, 21c, 21d. However, the scope of protection also covers a non-constant increase rate. FIG. 1.c shows schematically an example of what could be an ideal curve for a given application where a smooth change in electric resistivity ρ would be desired. FIG. 1.d shows an example of an actual curve for a component to be used in the application having the ideal curve as in FIG. 1.c.

[0083] FIG. 2 shows schematically the overall steps in the method. FIG. 2.a shows the step of preparing a first paste 10a having a first composition, and a second paste 10b having a second composition. The step of preparing the pastes may be performed by kneading the materials in a kneader, such as a Z-blade kneader or sigma blade kneader. Such a type of mixer has a high torque and a specific geometry of the mixing blades which has been found suitable for obtaining a homogenous mixture of the type of paste as described above, which paste typically has a high viscosity. The first and second pastes 10a, 10b are then transferred into a supply chamber 35 of a processing equipment 31, which in FIG. 2.b is schematically shown as a piston extruder. The pastes 10a, 10b are forced from the supply chamber 35 through a die 32 of the processing equipment 31 to result in a green specimen 20 as shown in FIG. 2.c. By moving the piston 36 towards the die 32, typically at a constant speed, the green body 20 is formed by continuously forcing the pastes 10a, 10b through the die 32. As shown for this embodiment, the order in which the pastes 10a, 10b are transferred into the supply chamber 35 corresponds to the longitudinal direction of the component 21 being manufactured. In presently preferred embodiments, the step of shaping is performed by an extruder, and the extrusion is performed at room temperature and with the pastes having a temperature of at most 50 degrees Celsius, such as at most 40 degrees Celsius, preferably at most 30 degrees Celsius. Hereby the properties of the pastes may be easier to control over time, since no significant amount of water or other liquid present in the pastes will evaporate at these temperatures, and the binder will not reach its gelation temperature.

[0084] After this shaping, and possibly a further step of drying, the green body is sintered or oxidized to obtain the composite component 21 having a varying electric resistivity along a longitudinal direction of the composite component 21. The sintering may e.g. be done in a reducing atmosphere, in vacuum, or in an inert atmosphere. The sintering is typically performed in a furnace 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 to a temperature at which at least some, such as all, of the binder burns off.

[0085] As seen from FIG. 2, the longitudinal direction of the green body 20 and thereby the composite component 21 corresponds to the direction of movement of the pastes 10a, 10b through the die 32, and the varying electric resistivity ρ results from the first composition being different from the second composition. As illustrated in FIG. 2, the green body 20 obtains a shape matching the shape of the die 32. Apart from possible minor changes caused by the following processing steps, this shape also corresponds to the shape of the final composite component 21.

[0086] In preferred embodiments of the invention, the first paste 10a comprises metal powder with a first alloy composition, ceramic powder, and a first binder. The second paste 10b comprises metal powder with a second alloy composition and a second binder. The first alloy composition and the second alloy composition both consist of a plurality of chemical elements. Each of the metal powders of the first paste 10a and of the second paste 10b may comprise one or more of the following chemical elements: iron, copper, chromium, aluminium, cobalt, nickel, manganese, molybdenum, vanadium, yttrium, and silicon. Examples of alloys that have been used in the development work leading to the present invention are FeCrAl, TWIP, 316L, and 17-4PH. However, the invention can be used for many other alloys.

[0087] The second paste 10b typically also comprises a ceramic powder. The ceramic powder used for the first and second compositions typically comprises one or more of the following: Alumina, Zirconia, Boron Nitride, Cordierite, and Silicon Nitride.

[0088] The different electric resistivities ρ in the pastes 10a,10b are typically obtained by varying one or more of the following parameters: [0089] the volume ratio between the metal powder and the ceramic powder, [0090] the size of the ceramic particles, [0091] the shape of the ceramic particles, and [0092] the type of the ceramic material.

[0093] FIG. 3 schematically shows two examples of cross-sections of components having different volume fractions of ceramic 14. In FIG. 3 the ceramic particles are shown as black even though they are white in the real components. Due to the significant differences in electric resistivity between metal and ceramic materials, the different examples of volume fractions shown in FIG. 3 result in different electric resistivities. The characteristics of the material in relation to the ceramic particles, such as the parameters mentioned above as well as the distribution, can e.g. be analysed by microscopy of polished cross-sections of the components.

[0094] FIG. 4 shows results obtained during the development of the present invention. It shows how the electric resistivity ρ of a composite component varies as a function of the content of ceramic in the form of Alumina. The graph is based on experiments where the electric resistivity along a composite component made with a method as described above was measured. The electric resistivity was measured by applying a known current to the component and measuring the voltage drop with two probes arranged in contact with the component with a fixed distance between them. The experiments were made both at room temperature and at a higher temperature, and both showed varying electric resistivity. For some of the materials used for the development of the present invention, the electric resistivity is almost constant over the relevant temperature ranges. The composite component may e.g. be used in a heating system wherein electrical power is used to heat an electrically conducting component due to the electric resistivity of the metal and then the heated metal is used for the heating of another media, such as a fluid flowing along the metal. In such an application, an electric resistivity that is almost independent of the temperature makes the heating process stable and controllable, and it may be easier to avoid hotspots. An example of materials with almost constant electric resistivity is FeCrAl alloys which are used in a wide range of resistance and high-temperature applications. They have a resistivity of about 1.4 μΩ.Math.m and a temperature coefficient of +49 ppm/K (i.e. +49×10.sup.−6 K.sup.−1).

[0095] FIGS. 5.a and 5.b shows schematically two examples of the overall shapes of composite components 21 that can be produced with a method according to the present invention. FIG. 5.a shows a component 21 having one longitudinally extending internal channel 22. FIG. 5.b shows a component having a plurality of longitudinally extending internal channels arranged in a regular pattern and separated by walls 23. These geometries are obtained by using dies 32 having shapes and arrangements corresponding to the cross-sectional shapes of the components. FIG. 5.c shows an example of a possible design of a die 32 that can be used for the manufacturing of a component 21 having an array of longitudinally extending internal channels.

[0096] FIG. 6 shows schematically an example of a processing equipment having two extruders 21a, 21b each supplying material into one mixing chamber 37, in the form of a manifold, possibly in varying amounts, so that the plurality of pastes for the final extrusion into a green body 20 are prepared in the mixing chamber 37. By “prepared” is preferably meant that they are mixed into a homogeneous material. The mixing chamber may include a mixer to perform at least part of the kneading. From the mixing chamber 37, a continuous flow of pastes is transferred to the supply chamber 35 from where it is forced through a die 32 in order to form the green body 20. The supply chamber 35 can be a separate chamber, but it can also be the part of the mixing chamber 37 adjacent to the die 32. The processing equipment shown in FIG. 6 has one single-worm extruder 31b and one twin-worm extruder 31a, but it could also be two of the same type. By varying the speeds of the worms 38, it is possible to control the compositions of the pastes being prepared from material supplied from the two extruders. It would e.g. be possible to supply a material comprising ceramic powder with one extruder and material without ceramic with the other extruder. Then the amount of ceramic in the paste being prepared depends on the relationships between the speeds of the two extruders.

[0097] FIG. 7 shows a flow diagram of an embodiment of a method according to the invention. First a plurality of pastes 10a, 10b are prepared as described above. FIG. 7 shows two pastes, but there could be more. This preparation could be performed by kneading the materials in a kneader, such as a Z-blade kneader or sigma blade kneader. The pastes 10a,10b are then transferred into a supply chamber 35 of a processing equipment 31. In the corresponding step in FIG. 6, this transfer into the supply chamber 35 will cause some mixing so that there is not a sharp border between the pastes. The processing equipment 31 is used to shape a green body 20 from the plurality of pastes 10a,10b by forcing the pastes 10a,10b from the supply chamber 35 through a die 32 of the processing equipment 31 as also shown in FIG. 2. In the embodiment in FIG. 7, a step of debinding the green body is then performed; this step may be preceded by a not shown step of drying. Such a debinding step is optional and whether or not to include it will e.g. depend on the materials used. The debinding step typically comprises heating the green body 20 to a temperature at which at least some of the binder burns off. Different binders require different debinding temperatures, and typical debinding temperatures are between 200 to 750 degrees Celsius. Finally, the green body 20 is sintered or oxidized to obtain the composite component 21 having the varying electric resistivity ρ along the longitudinal direction of the composite component 21. A drying step is typically performed in a controlled atmosphere involving controlling the temperature and the humidity in which the green body is placed. It may further include passing a flow of gas, such as air, along the green body, and the speed of the flow of the gas may then also be controlled.

[0098] 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.