Patent Application
20160138147
The present invention relates to a method of manufacturing a metal matrix composite, and in particular to a method in which a reinforcement preform is infiltrated by molten metal matrix.
Metal matrix composites are used for a number of applications that can benefit from the combined properties of the metal matrix material and the reinforcement which is typically ceramic or metal. Often even a small percentage of added reinforcement results in significantly enhanced properties. The reinforcement is typically used to obtain mechanical properties without significantly increasing the density. Metal matrix composites are also used e.g. for applications where it is important to have a low coefficient of thermal expansion to ensure a high degree of dimension stability. This can be particularly important for components experiencing very large temperature variations, such as for space applications including mirrors and supports for sensitive equipment.
However, metal matrix composites are difficult to produce with good homogeneity and thereby uniform properties. These materials have up to now typically been produced with micron-sized ceramic reinforcement, such as SiC in aluminium. It is known that metal matrix nano-composites offer a better strength, stiffness and toughness, but the smaller size of the reinforcement makes the composites even more difficult to produce both with respect to homogeneity, wetting of the reinforcement, and bonding between the metal matrix and the nanoparticles; these parameters all influence the resulting mechanical and thermal properties of the composite.
According to the latest numerical models, metal matrix nano-composites are predicted to be superior to all other presently known materials. Unfortunately, in practise it is difficult to obtain an even distribution of nanosize powders in matrix materials as it does not wet so easily and tends to clump. This has been tried overcome by use of ultrasonic and electromagnetic stirring as well as liquid shearing in twin screws.
Melt infiltration has been described as a possible process for the manufacturing of magnesium matrix composites in U.S. Pat. No. 4,279,289 relating to reinforcement with ceramic whiskers or fibres. U.S. Pat. No. 4,995,444 discloses a method of producing metal or alloy casting composites reinforced with fibrous or particulate materials. Both these documents disclose manufacturing methods relating to the use of equipment dedicated for that specific purpose. None of these references explicitly mention nanoparticulate preforms.
Hence, an improved method of manufacturing metal matrix composites would be advantageous, and in particular a more efficient method would be advantageous.
OBJECT OF THE INVENTION
It is an object of the present invention to provide a method of manufacturing a metal matrix composite component without the need for equipment specifically dedicated solely for that method.
It is an object of at least some embodiments of the present invention to provide a method of manufacturing a metal matrix composite component with which nano-particle composites can be manufactured in an efficient manner.
It is an object of at least some embodiments of the present invention to provide a method of manufacturing a metal matrix composite component with which a more homogenous distribution of the reinforcement, also when in the form of nano-particles, than with known methods can be obtained.
It is an object of some embodiments of the present invention to provide a method of manufacturing a metal matrix composite component with which it is possible to obtain significantly higher volume fractions of reinforcement than what is possible with known methods.
It is an object of some embodiments of the invention to provide a method manufacturing metal matrix nano-composites with high volume fractions, wherein at least most of the nano-particles are wetted and disperse easily in the metal matrix.
It is a further object of the present invention to provide an alternative to the prior art.
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 metal matrix composite component, the method comprising the steps of:
“Porous” refers to the fact that the preform has a continuous coherent network of inner cavities through which the melted metal can infiltrate the preform. The shape of these cavities will depend on the actual type of reinforcement used to make the preform as will be more clear from the below explanation.
By “preform” is preferably meant an object that has been subjected to preliminary shaping or moulding before undergoing complete or final processing. In the present invention the actual process of making the preform depends on the type of reinforcement used; this will be explained in further details below. For some applications it may be useful to only have the preform in certain sections of the metal matrix composite component, such as in the crown of a piston. In this case, the preform has to be held in place in the first compartment during manufacturing, e.g. by holding members placed in the first compartment together with the preform.
By “evacuate” is preferably meant to remove air and/or gases from the container. The process of evacuating and sealing the container will be well known to a person skilled in the art.
The matrix metal is preferably in the solid state when being placed in the second compartment to ensure an easy handling. However, it may also be possible to place it in a semi-solid state if desired. The matrix metal might also be poured into the second compartment as a molten liquid where it would then be allowed to cool and solidify. The preform could then be loaded into the first compartment, the container welded up, evacuated and sealed. This method would allow the maximum metal reservoir.
The step of heating the container may be performed under vacuum or in an inert gas atmosphere whereby it is possible to minimize oxidation of the container. Oxides formed on the surface would be disadvantageous during the consolidation process as the container may rupture.
The container may be heated during evacuation so the preform, metal and container outgas lowering the oxygen and moisture level later during infiltration.
Furthermore, at least for some materials, heating of the container during outgassing should preferably be done in vacuum or inert gas prevents the container from corroding.
In preferred embodiments of the invention, the pressure may be applied in a hot isostatic pressure vessel as that is an efficient way of applying both increased temperature and high pressure at the same time and in a controlled manner. The pressure that can be applied to the preform through the use of such a hipping process can be very high, such as up to 300 MPa, whereby a fully dense composite can be obtained.
The high pressure may be applied also while the alloy is cooling, solidifying and contracting. An advantage would be to avoid any shrinkage porosity that typically occurs on alloy freezing.
The infiltration process can be extended in time to ensure that the preform is fully infiltrated. Infiltration requires the matrix metal to be molten while pressure is applied. Other known processes such as squeeze casting infiltrate in a chamber that is below the melting point of the metal so the matrix metal rapidly solidifies. In a method according to the present invention, the matrix metal can be kept molten for a much longer time. Furthermore, since the whole container and its content are heated, the preform can be at the same temperature as the metal so there is no cooling of the matrix metal during infiltration. Hereby it can be ensured that the whole preform is infiltrated without the metal solidifying and thereby potentially blocking the pores in part of the preform. Furthermore, the wetting of the reinforcement is enhanced due to its elevated temperature and the extended time in contact with the molten matrix material as compared to known manufacturing methods.
The passage between the first and the second compartments may represent or comprise a restriction or filter. By “restriction” is meant a reduction in cross-sectional area. This can be used to prevent oxide formed on top of the molten metal from reaching the preform, since clean metal is required for an efficient infiltration of the preform. By passing the melt through a restriction, the melt has to change shape, the oxide skin is ruptured and the oxide layer is left in the second compartment. The restriction could contain a filter that would act as a sieve to ensure that no oxide is transferred into the infiltration chamber.
A wall thickness of the second compartment may be smaller than a wall thickness of the first compartment. Hereby the second compartment deforms more easily than the first compartment under the influence of the increased temperature and pressure so that the flow of matrix metal is hereby imparted by the deforming walls of the second compartment.
The matrix metal may be magnesium, aluminium, beryllium, lithium or alloys thereof. The container needs to have a higher melting point than the matrix metal and have a good chemical compatibility therewith so that no undesired chemical reactions take place during the process. These requirements favour lower melting point matrix metals.
The present inventive method may be seen as being particularly advantageous for the manufacturing of components comprising magnesium. This is because magnesium and its alloys, e.g. Mg—Li, tend to evaporate under normal atmospheric pressure when the molten metal is heated above its melting point. When encapsulated within a container as in the present invention, the Mg and Li vapours are reduced and contained and cannot escape. This convenient containment also has health and safety advantages. As further advantage of the method is that magnesium and its alloys can be melted in a low carbon steel container without reaction, and therefore the manufacture of the container is easy. Aluminium does react with steel, and therefore a container for use in the manufacturing of aluminium composites needs to be manufactured from or lined with a more costly material, such as niobium. Another option would be to coat a lower cost metal container with boron nitride (BN) spray coat to minimise reaction.
Magnesium would typically be used for components that would benefit from the very low density provided that it should not be exposed to high temperatures, since magnesium has a relatively low melting point. The lightweight properties would be advantageous in transportation structures, such as aerospace and automotive structures, but it may also be advantageous for sports equipment, such as bicycle frames and rackets.
Lithium could be used as a matrix material e.g. for metal-air battery applications.
As mentioned, the container may be made from low carbon steel. This material is relatively cheap compared to other metals and can easily be made in a complex, near-net shape so that very little finishing work is to be performed on the manufactured component. When choosing the container material to be used for a given matrix material, it must be ensured that it is sufficiently ductile for an expected level of deformation. It must also be ensured that the material of the container and of the matrix does not react in an undesired way. E.g. low carbon steel can be used for magnesium, and niobium can be used for aluminium matrix systems.
The preform may be made from a ceramic or metal material. Possible ceramic materials include: SiC, Y2O3, ZrO2, Al2O3, MgO, Mg2Si, Sc2O3, WC, Si, B, graphene. Possible metal materials include titanium and beryllium.
The reinforcement preform may be made from one or more of the following: nanoparticles, microparticles, fibres, wires and 3D woven structure. When the preform is made from particles, it will typically be rather isotropic in particle size and porosity. When it is made from fibres, wires or 3D woven structure, it will typically be anisotropic, such as having different properties parallel and perpendicular to uni-directionally oriented fibres. Such an arrangement could e.g. be used to obtain a high stiffness along an axis of rotation. When fibres or wires are used, they can e.g. be in the form of a bundle of long fibres/wires or a 3D-woven mesh of fibres/wires. The preform has an at least partly coherent structure to ensure that is does not collapse during placement in the first compartment. The stiffness and strength of the preform should also be so that even at elevated temperatures, it is not deformed to an undesired content under influence from the flow of molten metal during the infiltration.
The present invention may be considered as particularly useful for the manufacturing of components reinforced with nanoparticles. This is due to the fact that in a process comprising infiltration of nano-particles, the pressure required to infiltrate a preform rises rapidly as the size of the pathway between the particles decreases.
In some embodiments of the invention, the preform may be produced by:
This process can be used both for nano- and microparticles.
Another possible method for making the preform is freeze casting, wherein a slurry is produced, poured into a mould, cooled by liquid nitrogen, and the water removed by freeze drying. Still another possible method is by gel casting, wherein the slurry is produced with a gelling agent and cast into a mould. It is air dried and then heated in stages to remove the gelling agent. There are many ways to make the slurry and different gelling agents. Such methods will be known to those skilled in the art.
The structure of the preform may be such that the volume fraction of reinforcement in the manufactured component is 5 to 75%, such as 5 to 20%, or 20 to 50%, or 20 to 75%. It would be hard to make a low volume fraction perform with nano-powder as it would fall apart, but it would be easy with fibres. For the high volume fractions of nano-particles, it may be advantageous to produce a concentrated slurry with e.g. 40 vol % nanoparticles which is then gel cast and made to shrink to e.g. 75 vol %.
A method as described above may further comprise a subsequent step of removing the container from the metal matrix composite component. This may e.g. be done by machining, such as milling or turning.
In alternative embodiments, the container is kept as an outer layer on the manufactured metal matrix composite component. An example could be to have a titanium outer casing, resulting from the container, and a Mg-composite on the inside, such that the titanium serves as a corrosion-protection layer.
A second aspect of the invention relates to the use of a component manufactured as described above for further processing including one or more of the following: extrusion, wire drawing, re-melting or re-casting.
A third aspect of the invention relates to the use of a component manufactured as described above for a grain refiner or dissolvable master-alloy in the manufacturing of composite materials or alloys.
A grain refiner is a small nano-to-micron-sized ceramic particle deliberately added to a liquid alloy in order to promote heterogeneous nucleation of crystals during solidification, with the ultimate objective of achieving a fine equiaxed grain structure in the fully-solidified alloy. A fine equiaxed grain structure in an alloy leads to good quality material with superior properties such as strength, ductility, fatigue, and corrosion resistance. Grain refiners typically used in the magnesium and aluminium casting industry are in the form of waffles, tablets and rods.
With some embodiments of the present invention, it is possible to produce a reinforced metal matric nano-composite (MMNC) master alloy block for subsequent dissolution in a large crucible of another alloy. A MMNC block which already contains a high concentration of pre-wetted nanoparticles would be more easily dissolved and would lead to more effective grain-refinement than what is possible with known methods. The reinforced MMNC master alloy block e.g. with high nanoparticle loading of 40 vol % could be added to a large crucible of alloy in order to produce a more dilute MMNC, say with only 2 vol % nanoparticles. This low loading of 2 vol % is typically enough to impart superior properties on the composite alloy. Thus, an advantage is that a small quantity of MMNC produced by the present invention can produce a large quantity of the final MMNC. The process requires preforms with sufficient strength to be handled and infiltrated so very low volume fractions of particles are not generally suitable for direct production.
The method can be used for the manufacturing of components for a number of other applications than those mentioned above, such as for engineering applications in space, aeronautics, automotive, and energy systems. It may also find use e.g. for sport equipment where the combination of high strength and relatively low density is advantageous.
The first, second and third aspect of the present invention may each be combined with any of the other aspects. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
The work leading to this invention has partially received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under contract number EC-FP7-NMP-280421.
The method of manufacturing a metal matrix 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.
The steps involved in a method of manufacturing a metal matrix composite component according to the present invention are shown schematically in
The container 1 and its content is heated to above a melting temperature of the matrix metal 7 at least until the matrix metal 7 has melted; this heating is illustrated in
The method may further comprise a subsequent step of removing the container 1 from the metal matrix composite component to arrive at a final component 9 as shown schematically in
For some embodiments it is preferred to keep the container 1 as an outer layer on the manufactured metal matrix composite component 9. It will most often still be necessary to remove the part of the container 1 comprising the second compartment 3; i.e. the compartment in which the metal matrix 7 was initially placed. In the embodiment in
Examples of possible matrix metal 7 are magnesium, aluminium, beryllium, lithium or alloys thereof.
The preform 6 is preferably made from a ceramic or metal material. It may be made from one or more of the following: nanoparticles 11, microparticles 12, fibres 13, wires 14 and 3D woven structure 15 typically made from fibres or wires; these are shown schematically and simplified in
The manufacturing of the preform may be done by a number of methods, such as the one shown in the flow-chart in
A component 9 manufactured as described above may be used for further processing including one or more of the following: extrusion, wire drawing, re-melting or re-casting. In this way a number of geometries can be obtained which are not readily obtainable by a method as described above.
A method according to the present invention may e.g. be used to produce a reinforced metal matric nano-composite (MMNC) master alloy block for subsequent re-melting and dissolution in a large volume of metal. A MMNC block which already contains a high concentration of pre-wetted nanoparticles would be more easily dissolved and would lead to more effective grain-refinement than what is possible with known methods. The reinforced MMNC master alloy block e.g. with high nanoparticle loading of e.g. 40 vol % could be added to a large crucible of alloy in order to produce a more dilute MMNC, say with only 2 vol % nanoparticles. This low volume MMNC would then be cast to components.
The following is an example of manufacturing a metal matrix composite component 9 by use of a method according to the invention; part of the process itself is not shown in figures. First the preform was prepared by gel casting, also called starch consolidation casting, of a silicon carbide ceramic, which process includes the following steps: Slurry preparation, cast gelling (starch consolidation), and preform firing (starch burnout).
A suspension for casting was prepared with 40 nm SiC 15 wt % (SkySpring Nanomaterials Inc), corn starch 11 wt % (Sigma-Aldrich), defloculant Dolapix CE64 3 wt % and distillate water 71 wt %. The slurry was hand mixed in a plastic bottle with the alumina balls for 20 minutes. A cylindrical metal mould with an internal diameter of 70 mm was preheated to 40° C. prior to casting. Then the mould was filled with the slurry, and it was put into a heat resistance furnace and kept at 75-80° C. for two hours (gelling stage). After gelling, the casting—i.e. the preform—was removed from the mould and naturally dried at room temperature for 24 hours. Then the preform was heated in the furnace with a stepwise increase in temperatures of 90, 120, 200, 350, 450 and 550° C. with one hour dwell at the intermediate temperatures. A heating rate of 2° C./min was used. The final preform was 47 mm in diameter and 24 mm in height after the firing. The mass of preform was 29 g; this corresponds to 22 vol %.
The gel cast preform 6 was then placed into a first compartment 2 of a shaped container 1 produced from a low carbon steel. A second compartment 3 of the container 1 was filled with a shaped billet 7 of magnesium alloy (AZ31). The billet weight was 130 g. The container 1 was then evacuated to a vacuum level of 10-2 mbar and sealed. The container 1 was then HIP processed in an EPSI Hot Isostatic Press 8. The schedule was:
1st stage: heating to 200° C. and increase pressure to 100 bar
2nd stage: heating to 640° C., 180 degree/hour, constant pressure 100 bar
3rd stage: heating to 710° C., 30 degree/hour, and increase pressure to 1200 bar
4th stage: dwelling 30 min
5th stage: cooling to 550° C., 300 degree/hour, constant pressure 1200 bar
6th stage: cooling to 200° C., 300 degree/hour, 200 bar
The geometry of the container 1 used in the above example is shown schematically in
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. Also, 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.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2013/062729 | 6/19/2013 | WO | 00 |
