GB2288189A - Ceramic reinforced metal-matrix composites. - Google Patents

Abstract

A method of producing a ceramic reinforced metal-matrix composite comprises the steps of dispersing a ceramic phase (of titanium diboride) in a liquid aluminium or aluminium alloy, mixing the ceramic phase with a cryolite or other fluoride flux powder and melting the mixture together with the aluminium or aluminium alloy phase at a temperature of between 700 and 1000 DEG C. <IMAGE>

Description

Ceramic Reinforced Metal-Matrix Composites This invention relates to the production of ceramic reinforced metal-matrix composites.

The benefits of light alloy materials for structural engineering applications have been realised for their strength, toughness and above all for specific modulus.

Consequently the aerospace and automotive industries have reaped a considerable incentive: fuel economy and longevity of components in service. In the last two decades or so, a new type of material has emerged which are based on the reinforcement by low density, high temperature ceramic materials: namely silicon carbide, alumina and carbon fibres. The reinforcement has been achieved with these materials either in the form of particulates or as fibres, resulting in a substantial reduction in the density, coefficient of thermal expansion and improvement in the value of Young's modulus. The combinatorial effect of properties of matrix and reinforcement is therefore observed in the metal-matrix composites. Based on laboratory-scale experiments, novel metal-matrix composite fabrication techniques namely spray-forming of Al-alloy/SiC, squeeze and infiltration casting of fibre reinforced metal-matrix composites, including powder mixing and extrusion processing techniques, have emerged.

These methods offer potential benefits both in terms of profitability and materials properties. Also the laboratory methods have now been available for small-scale commercial production of materials and hence the abovedescribed metal-matrix composite fabrication methods compete with each other.

Experimental data also point out to several problems leading to formation of defect structures such as void formation during liquid metal infiltration and fibremetal reaction, or fibre misorientation during squeeze casting. In the spray-forming process, which is a rapid quenching of a two-phase mixture, namely liquid metal and fine ceramics, the cost of material production is high.

Additionally, the spray-formed ingot requires further processing because it has a wide range of porosity, and the ingot cannot be formed into complex shapes during a spray-forming process. The cost comparison indicates that the powder metallurgy and extrusion route produces materials of prohibitively high cost. The new technology and materials engineering has nonetheless been used in the fabrication of a wide range of consumer sports items for which high production cost has so far been justified.

Using the above techniques, the cost of automotive and aerospace components has not yet been justified and for this reason the metal-matrix market for automotive, aerospace and other engineering applications still remain uncertain. The fabrication cost of automotive and aerospace structural engineering components however remains unfairly high, hence the market for these mmc components has been virtually non-existent.

Apart from the high production cost of materials by above routes, a much more fundamental problem, related to the long-term reliability of Al-SiC components remains unsolved particularly for high temperature applications.

With prolonged exposure to high temperature service condition, aluminium matrix has a tendency to react with SiC over a period of time. Aluminium carbide forms at the matrix-reinforcement interface which is brittle and is detrimental for high temperature toughness of the composite materials. The material toughness and fatigue, being the most important properties of engineering components in motion, suffer adversely due to the presence of the brittle aluminium carbide phase. This therefore leaves a question mark on the long-term high temperature structural reliability of aluminium/SiC and Al/carbon fibre composites.

Additional problems of recycling A1/SiC and Al/carbon composites also arise due to undesirable presence of silicon and carbon in the metallic phase. This is expected to create a stock-pile of non-recyclable aluminium alloy composites which will also contribute to the overall cost of composite materials.

More recently, titanium based materials have been recognised as useful in the formation of metal-matrix composites.

Titanium boride and carbide have been traditionally used for grain refinement in aluminium alloys. The ceramic phase is known to microstructurally adapt with the metallic matrix providing a significant improvement in the mechanical properties of the alloy unlike SiC and carbon fibre. The ceramic phase does not aggressively react with the liquid metal to produce any embrittled layer.

The diboride dispersion technology in aluminium alloy is a well-proven technique in aluminium industries for the fabrication of grain-refined master alloy. The grainrefining reaction is: 4 Alliq + TiB2 = A13Ti + A1B2 which is an important aspect of TiB2 and related ceramic phase dispersion in the metallic phase. Both A1B2 and/or mixed diboride (Al,Ti)B2, which form as a result of the grain-refining reaction, are isostructural with TiB2 and hence from the Hume-Rothery rule exhibit extended solubility.

These solid-solution boride phases are interfacially and crystallographically compatible with the alloy matrix.

This is one of the reasons that the grain-refined Alalloy exhibit better fatigue properties because of the interlocking of grain boundaries and dislocation by complex boride phase: a feature also commonly seen in high temperature superalloys. Because of the favourable interfacial reaction and lower solubility of complex borides in the matrix, Al-TiB2 is microstructurally a far superior composite material exhibiting better high and low temperature fatigue and fracture properties.

Titanium carbide favours the improvement in the properties in the same way but to a lesser extent.

London Scandinavian Metallurgical (LSM) Company has recently developed an in-situ ceramic dispersion technique GB-A-2257985. In this method, the inventors used K2TiF6 and KBF4 molten flux mixture in contact with metallic aluminium. The chemical procedure is an extension of grain refining reaction: K2TiF6 + 2KBF4 + 4A1 = TiB2 + (K3AIF6) + KF + 2AIR3 + iF2 In this in-situ technique, also known as the reactive casting technique, the ceramic phase (TiB2) forms via chemical reaction (2) and is subsequently dispersed in the molten alloy.

The inventors point out that the procedure has resulted in the development of cast aluminium/TiB2 product with a maximum of 9 volume percent ceramic phase. So far there has been no further reported improvement in the volume fraction of titanium diboride phase dispersion.

According to the present invention, there is provided a method of producing a ceramic reinforced metal-matrix composite, comprising the steps of dispersing a ceramic phase in liquid aluminium or aluminium alloy, mixing the ceramic phase with a flux and melting the mixture together with the aluminium or aluminium alloy phase for dispersion.

In the preferred embodiment, the dispersion of TiB2 ceramic phase in liquid aluminium alloys is achieved by a technique using molten flux, in particular fluorides (there are also oxide/fluoride flux mixtures which can be used for dispersing ceramic phase in molten aluminium alloys). In this technique, the ceramic phase is mixed with a suitable flux powder and melted together with the alloy phase for dispersion. The molten flux facilitates the dispersion of the ceramic phase in the molten aluminium by lowering the interfacial energy between the flux, metal and the ceramic phase. The technique can therefore yield a very high volume fraction of ceramic dispersion in the alloy matrix.

This technique offers a new method for casting and shaping of metal-matrix composite ingots with a range of volume fractions of the ceramic dispersion. In addition to the new technique operating ex-situ, based on molten fluoride flux technique, we have also developed a unique in-situ formation of ceramic phase, which can also improve its dispersion. The new in-situ technique radically differs from the reactive casting method developed at LSM.

In this new in-situ dispersion technique using molten flux, metallic calcium or magnesium, either dissolved in the alloy phase or in the molten flux, reduces KBF4 and K2TiF6 simultaneously to yield TiB2, KF and MgF2 and CaF2. The chemical reactions: 2KBF4 + K2TiF6 + 5(Ca)dissolved = 5 (CaF7) + 4 KF + TiB2 2KBF4 + K2TiF6 + 5{Mg)dissolved = 5 (MgF2} + 4 KF + TiB2 are thermodynamically more favourable than the reduction reaction of K2TiF6 and KBF4 with metallic aluminium as proposed in the LSM process. Aluminothermic reduction of fluorides is not a novel concept since this has existed in the aluminium alloy grain refining for the last 40 years. The LSM process is an extension of the grain refining reaction of aluminium alloys. A large favourable thermodymanic driving force for Mg and Ca reduction process will also enable control of the size of TiB2 crystals in the dispersed state.

The new methods, both ex-situ and in-situ for the ceramic phase dispersion in molten aluminium, can be readily employed to manufacture a wide range of engineering materials for automotive, aerospace and tribological applications.

The underlying principles and methods employed for achieving a range of volume fractions of the ceramic phase in a wide variety of aluminium alloys are illustrated in the exemplary methods described below, with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view of an example of water cooled crucible; and Figures 2a to 2c are micrographs of titanium diboride.

It is to be understood that any component values or ranges given herein may be altered and/or extended without losing the effects sought, as will be apparent to the skilled reader from the teachings herein.

The dispersion of titanium diboride particles in a range of molten aluminium alloy was achieved by adopting the following steps. The procedure was followed for both 20 gram and 1 kilogram batch size of molten aluminium alloy.

a) Several types of aluminium alloys namely commer cial 1000 xxx series, Al-Li (0-5 wt%), Al-Cu (0-5 wt%), Al-Mg (0-8 wtW) and Al-Si (0-10 wit%) were melted in an atmosphere of argon gas. The tempera ture was varied between 7000 and 10000C. The melting temperature could also be fixed from the liquidus temperature and the known casting temperature of a specific alloy composition.

b) While melting the alloy of specific composition, the titanium diboride powder was mixed with the fluoride flux, namely cryolite (3MF,AlF3,M: Li,Na and K).

The flux mixed with ceramic powder was melted with the alloy. Additional amount of ceramic powder was also added with the flux after the alloy was completely molten. This method permits a means to control the volume fraction of the dispersed phase.

The flux-assisted dispersion of the ceramic phase was carried out by melting various aluminium alloys and flux compositions in a low oxygen potential atmosphere by maintaining a stream of an inert gas such as Ar or Ar-4% H2 gas mixture in the melting chamber. On the other hand, the apparatus shown in Fig. 1 can be used for a continuous production of metal-matrix ingots. The crucible is preferably made of alumina.

c) After a period of homogenisation above melting point, which could be between 7000 and 10000C depending upon the alloy and flux composition, the liquid metal dispersed with ceramic phase was cooled either by pouring it out in a mould or leaving it in the melting pot to cool down slowly.

After casting, the ingots were examined to ascertain the volume fractions of the dispersed phase and the resulting properties of the metal-matrix composites.

In the method described above, the desired ceramic phase is mixed with a suitable flux, preferably a fluoride flux, that preferably has a finite solubility for alumi na. This alters the favourable interfacial tension between alumina and liquid metal to energetically more favourably interfacial tension between ceramic phase and metal i.e. (aAl/cercÁl/Alumina) for /Alumifor achieving maxi- mum dispersion. The interfacial tension condition sets constraints on the processing parameters and equipment used. The first and the foremost variable is the overall oxygen content of the flux, ceramic powder and metal determines the oxygen potential for the stability of impervious alumina layer. The presence of an impervious layer of alumina prevents the dispersion of the ceramic phase. If impurities such as water vapour and CO2 are present in the melting environment, the surface contamination of the ceramic powder by oxygen increases therefore resulting into a poor dispersion of ceramic phase in the liquid metal. Flux to be used therefore should be substantially free from moisture and oxygen-containing impurities.

The wettability between the ceramic phase and aluminium metal can be improved by having a flux that reacts with alumina to form a complex. Molten cryolite is one such flux which has a large solubility of alumina. The addition of cryolite as flux therefore improves the dispersion of TiB2. The dispersion of TiB2 in the presence of either hydrous or partially hydrous KBF4 and K2TiF6, as shown in reaction (2), was not very encouraging because these two fluorides also absorb a lot of moisture and consequently promote the formation of alumina at the flux-metal interface. The complex ion-forming tendency of cryolite and related fluoride fluxes rapidly change the interfacial energy between alumina and aluminium metal. The total concentrations of moisture and oxygenrelated impurities of fluoride flux used for dispersion should be less than the saturation solubility of oxygen (as dissolved alumina) in the flux. If this solubility limit is low for a particular type of fluoride flux, the precipitation of alumina from flux takes place as an interfacial barrier between the metal and molten flux.

This thin layer of alumina adversely affects the transport and dispersion of TiB2 in molten aluminium alloys.

The wettability aspect also determines the selection criterion for crucible material. Graphite crucible as containment material is only suitable for achieving dispersion preferentially on the surface of the metal.

This arises due to a lower value of Ál/TiB2/C than Ál/TiB2 in the presence of molten cryolite. Consequently ceramic dispersion only on the surface of the alloy ingot was achieved at all temperatures. So far we have not found any fluoride flux that provides extensive dispersion in entire volume of metal while being held inside a graphite crucible in spite of the fact that graphite is an oxygen-getter. Its role in reducing oxygen partial pressure by forming with CO2 or CO gas at the interface can be readily appreciated from the thermodynamic considerations. The removal of interfacial oxygen will therefore affect the interfacial tension which is then lowered, thus affecting the dispersion of the ceramic phase.

The use of alumina as crucible material, with cryolite as flux is proposed. This is based on the principles of interfacial energy described above. The use of alumina as a crucible material has resulted in a significant improvement in ceramic dispersion in molten aluminium.

The reason is aAlumina/cryolite interfacial tension dominates at the crucible wall-flux boundary region due to which the interfacial tension between alumina/flux/TiB2 is artificially raised. This rise in surface energy difference between aAlumina/cryolite and a,lumina/flux/TiB2 leads to the surface-induced migration of TiB2 from the alumina/flux/TiB2 boundary near the crucible wall to energetically more favourable Al/TiB2 boundary in the bulk metal.

Our understanding of interfacial energy between the ceramic and metal phase has been developed from the first principle that invokes the theory of interfacial bonding.

All alloying elements that reduce the surface energy of molten aluminium aid the dispersion process The presence of certain alloying elements achieves a higher dispersion of TiB2 in Al-alloys. The presence of an alloying element also has an implication on the selection of the matrix material for achieving a higher value of specific modulus. The alloying elements that exhibit a strong compound-forming tendency improve the wettability and dispersion of the ceramic phase in general in aluminium alloys. For this reason, we have particularly selected Al-Mg and Al-Li alloys as low-density matrix materials. On the basis of the reduction in interfacial energy due to the presence of an alloying element, it has been also demonstrated that Al-Cu alloy matrix is a less effective matrix material than Al-Mg system. In this respect, the presence of Li in liquid aluminium has been found to be more effective in achieving high dispersion volume of TiB2.

The dispersion of titanium diboride (TiB2) has also been achieved by using a mixture of fluoride flux based on KBF4, LiBF4, K2TiF6 and Li2 TiF6. The presence of lithi um in the molten fluoride flux can achieve copious nucleation of very fine TiB2 ceramic phase in molten aluminium alloy. Furthermore, this concept, based on our understanding of surface energy has also led to a development of dissolving alloying elements such as lithium-Mg and calcium in molten aluminium which cannot be easily dissolved in elemental forms.

Developed flux-assisted alloying element dissolution techniques (as discovered from our ceramic dispersion experiments) using two types of flux mixtures, namely (K2TiF6 - KBF4): 97 wtW and 3 wt LiF and (K2TiF6- KBF,)08 - (Li2TiF6 - LiBF4 } o . 2 have yielded 0.45 wtW and 4.5 wtW Li in commercially pure molten aluminium. The chemical analysis was performed on the solidified ingot after cleaning the flux from the ingot surface. Currently, large-scale trials using 500-lOOOgm aluminium are planned for making Al-Li with a range of compositions for structural applications. This method of ensuring high dissolution rate of Li and Mg in commercial aluminium alloy is particularly attractive for the production of a range of alloy compositions for structural applications.

In the preferred fluoride-assisted ceramic dispersion process, the surface-active alloying elements such as Li and Mg also contribute to the modification of the morphology of the in-situ formed TiB2 ceramic phase. Our results show that the presence of Mg in the alloy phase leads to the growth of faceted TiB2 crystals which disperse homogeneously in the Al-alloy. The segregation of TiB2 at the grain boundary is a minimum in the presence of Mg which contrasts with the presence of copper, see micrographs 2a to 2c. Figure 2a shows micrograph of TiB2 dispersed in Al-4.5 wt% Li alloy using an in-situ dispersion technique. Flux composition was 80 wtW K2TiF6-KBF4 and 20 wtW Li2TiF6-LiBF4. Submicrometre size of TiB2 grew and dispersed throughout the ingot.

The micrometre bar should be referred to for comparing the size of TiB2 particulates. Figure 2b shows extensive dispersion of faceted shape TiB2 in Al-Mg (8wit%) alloy using an in-situ dispersion technique. The flux used was 100wit K2TiF6-KBF4. Figure 2c shows dispersion of TiB2 via an ex-situ technique in Al-4.5wtW Cu alloy. The TiB2 particulates were dispersed externally in sodium cryolite flux.

The presence of lithium on the other hand, enhances the nucleation of TiB2 and submicrometre size TiB2 (f < 50nm to 500nm) particulates form. In designing Al-alloy mmc, the combined effect of the presence of lithium and magnesium in alloy phase for morphological engineering is strongly recommended. This can be effected by mixing lithium and magnesium fluoride fluxes with potassium fluoride fluxes.

The above principles can be adopted and applied to a wide range of ceramic phase dispersion in both aluminium and magnesium alloy matrix. For achieving higher volume fractions of ceramic dispersion in the metallic matrix, the following methods have been developed: a) Dispersion of the ceramic phase in the molten metal has been achieved by using a suitable fluoride flux.

This can be a cryolite or any other fluoride or non fluoride flux that satisfies the interfacial tension conditions outlined above. The melting of matrix alloy can be carried out using an induction coil, or a gas-fired furnace or a muffle furnace. Either after melting or during melting, the dispersion could be initiated using an appropriate flux as long as the conditions for maintaining oxygen partial pressure and interfacial tensions are met. After dispersing the ceramic phase, the two-phase mixture can be cast into a suitable geometry by adopting any commercial casting method. The dispersion can also be achieved via molten K2TiF6 and KBF4 mixture with TiB2 as a nucleation-promoting phase.

b) Direct arc melting using a hollow aluminium electrode can be adopted to build metal and flux volume in a water-cooled crucible. An example is shown in Figure 1. This way the benefits from directional solidification are also harnessed.

Referring to Figure 1, the apparatus shown includes a power supply 1 coupled to a hollow electrode, in this case of aluminium or aluminium alloy, and to a water cooled copper plate 5. A water cooled cruci ble 3 rests on a graphite plate 4 which in turn rests on the copper plate 5. Argon gas is fed into the crucible 3 by a delivery tube 6. Liquid ceramic mixture 8 and molten flux 9 are provided in the crucible, on an ingot 7.

The method proposed is similar to electro-slag refining or remelting procedure developed for the processing of high temperature alloys. The flux and ceramic phase can be injected in the molten metal through the hollow consumable aluminium alloy elec trodes. The ceramic injection in the metal phase will ensure the uniform distribution of particles.

Two main advantages of the process are: i) control of ceramic volume fraction and ii) directionally solidified microstructure. We also expect a higher volume production rate than the spray-forming proc ess by using this technique with a comparable cost of the finished product.

c) In-situ and ex-situ dispersion of ceramic phase in aluminium alloy can be concomitantly achieved via cryolite-calcium fluoride/Ca or Mg metal/TiB2 mix ture in the molten state. Alternatively either Al Mg rich or Al-Ca rich or Al-Li alloy phase can be melted with the above-named flux compositions (cryolite mixed with KBF4/K2TIF6 or any other varia tion of CaF2/cryolite and potassium lithium magnesi um fluoroborate/titanate flux) and TiB2 to achieve a high volume fraction dispersion.

The advantages of the preferred method are as follows: a) Dispersion of TiB2 in aluminium alloy matrix namely 1 xxx alloy, (Al-4wtiCu), (Al-Mg) and Al-Li can be achieved. The maximum volume percent achieved so far exceeds 50 volume t.

b) Both the in-situ and ex-situ techniques can be simultaneously effected for the fabrication of metal-matrix composites. Such an electro-flux remelting technique using a hollow aluminium alloy electrode will yield a new technique for continuous production of mmc ingots with a mechanism for controlling the morphology of the ceramic phase and their dispersed phase volume.

The disclosures in British patent application number 94/06513.3, from which this application claims priority, and in the abstract accompanying this application are incorporated herein by reference.

Claims (17)

1. A method of producing a ceramic reinforced metalmatrix composite, comprising the steps of dispersing a ceramic phase in liquid aluminium or aluminium alloy, mixing the ceramic phase with a flux and melting the mixture together with the aluminium or aluminium alloy phase for dispersion.

2. A method according to claim 1, wherein the ceramic phase includes titanium diboride.

3. A method according to claim 1 or 2, wherein the flux is a metallic calcium or metallic magnesium powder.

4. A method according to any preceding claim, wherein the flux is a fluoride flux.

5. A method according to claim 4, wherein the flux is a cryolite.

6. A method according to any preceding claim, wherein the aluminium alloy is melted in an atmosphere of argon gas or an argon/hydrogen gas mixture.

7. A method according to claim 6, wherein the aluminium alloy includes one or more of the following: commercial 1000 xxx series, Al-Li (0-5 wt%), Al-Cu (0-5 wt%), Al-Mg (0-8 wit%) and Al-Si (0-10 wit%).

8. A method according to any preceding claim, wherein the melting temperature is fixed from the liquidus temperature and the known casting temperature of a specific alloy composition.

9. A method according to any preceding claim, wherein the melting temperature is between substantially 7000C and 10000C.

10. A method according to any preceding claim, wherein an additional amount of ceramic phase is added with the flux after the aluminium or aluminium alloy becomes completely molten.

11. A method according to any preceding claim, wherein after a period of homogenization above melting point, the liquid metal dispersed with ceramic phase is cooled either by pouring it out in a mould or by leaving it in a melting chamber to cool down slowly.

12. A method according to any preceding claim, wherein the method uses a melting chamber formed from alumina or graphite.

13. A method according to any preceding claim, wherein the melting of matrix alloy is carried out using an induction coil, or a gas-fired furnace or a muffle furnace.

14. A method according to any preceding claim, wherein the metal and flux melt is produced by direct arc melting using a hollow aluminium or aluminium alloy electrode in a water-cooled crucible.

15. A method according to claim 14, wherein the flux and ceramic phase are injected in the molten metal through the hollow electrode.

16. A method according to any preceding claim, wherein the dispersion of the ceramic phase is assisted by providing lithium within the mixture.

17. A method of producing a ceramic reinforced metalmatrix composite substantially as hereinbefore described.