The present invention relates to a method for making an alioy comprising compacting primary particles coated with secondary particles.

A very wide range of metal alloys are used for different applications, each alloy offering a particular combination of properties, including strength, ductility, creep resistance, corrosion resistance, fatigue resistance and castability. For example, although pure titanium is highly resistant to corrosion, its corrosion resistance can be improved by forming an alloy with 0.15 wt% palladium. Likewise, Ti-6AI-4V is a popular titanium alloy which displays high strength, creep resistance, fatigue resistance and castability. The corrosion resistance of Ti- 6AI-4V may also be similarly improved by the addition of palladium.

The global production of titanium is small in comparison with other metals or alloys and the majority of titanium currently produced is for use in the aerospace industries. Other industries, however, have encountered difficulties in sourcing the material they require and have additionally found it undesirable to maintain a large stock of a range of different titanium alloys as a result of the high price of titanium.

The rapid manufacture of a desired alloy composition comprising a primary metal, such as titanium, and at least one secondary metal would allow a manufacturer to store a reduced inventory while enabling the manufacture of a range of alloys, as well as the articles produced therefrom. However, while the high price of titanium contributes to the problems associated with titanium alloy article manufacture, these problems are not unique to titanium alloys and the advantages associated with the rapid manufacture of specific alloy compositions on demand are equally applicable to a wide range of alloys.

The inventors believe that the ability to generate a tailored alloy composition with required properties (e.g. corrosion resistance) would encourage the use of those alloys and, in particular, the use of titanium alloys, in addition, the subsequent fabrication of articles from the alloys would accordingly be facilitated as the period of time within which the wrought alloy is purchased would be reduced or even eliminated.

Accordingly, the present invention provides a method for making an alloy comprising the steps of:

(a) coating primary particles with secondary particles;

(b) compacting the coated particles; and

(c) forming an alloy therefrom; wherein,

i the primary particles comprise at least one of titanium, molybdenum, tungsten, nickel or iron; the secondary particles comprise at least one of platinum, palladium, rhodium, ruthenium, iridium, osmium, silver, gold or aluminium; the primary particles are coated with secondary particles by low energy ball milling; and the coated particles are substantially spherical.

The primary particles are preferably substantially spherical and maintain their substantially spherical shape during the coating process. The production of substantially spherical coated particles is advantageous because the flowability of the coated particles is improved, which assists in downstream metallurgical processing.

The secondary particles may be coated onto the primary particles using simple physical methods, such as low energy ball milling or any other process that minimises the physical distortion of the primary particles. Alternatively, the secondary particles may be coated onto the primary particles using one of the chemical routines that would be known to those skilled in the art e.g. electroless plating or reductive chemical deposition.

The inventors have surprisingly found that low energy ball milling is a gentle method which, while not wishing to be bound by theory, results in a physical change in the primary and secondary particles whereby the particles are physically cojoined. When low energy ball milling is used as the means to apply the coating to the primary particles, the milling process may be controlled by various parameters including the speed at which the milling takes place, the length of milling time and/or the level to which the milling container is filled. Additionally, improvement in the coating process can be made by controlling the milling media such as the particle ratio, the size of the milling media and other parameters as are familiar to the skilled person.

Preferably, the speed at which the milling takes place is from about 10 rpm to about

350 rpm. More preferably, the speed is from about 75 rpm to about 300 rpm. Even more preferably, the speed is from about 80 rpm to about 140 rpm.

The length of milling time is preferably from about 3 hours to about 20 days, more preferably, about 6 hours to about 10 days and even more preferably, about 1 day to about 7 days.

Preferably, the milling container is charged to a level from about 10 vol% to about 50 vol%. More preferably, the level is from about 15 vol% to about 40 vol% and even more preferably, from about 20 vol% to about 30 vol%. Certain metals or metal alloys possessing a strong affinity for oxygen suffer from excessive surface oxide growth if the milling is carried out in the presence of oxygen. In particular, if the final, compacted article is to conform to a recognised specification for oxygen content, it may be required that an oxygen-deficient atmosphere is to be used. Accordingly, the low energy ball milling of the present invention may be carried out under an inert atmosphere for at least a proportion of the milling time. Within the context of the invention, an inert atmosphere is one which has limited or no ability to react with the primary and/or secondary particles. Preferably, the inert atmosphere comprises argon, nitrogen or a mixture thereof. Preferably, the low energy ball milling is carried out under an inert atmosphere for the same period of time as the milling process itself.

Alternatively, the low energy ball milling may be carried out under a reactive atmosphere for at least a proportion of the milling time. A reactive atmosphere is one which may react with at least a proportion of the primary particles and/or secondary particles in order to reduce surface oxides on the particles or to convert a non-metallic source to a metallic one. Preferably, the reactive atmosphere comprises hydrogen gas.

The low energy ball milling process may be carried out in a wet or dry environment. For example, at least one solvent may added to the substances being milled, which solvent can act to minimise particle welding. The addition of a solvent is particularly helpful if the substances being milling has agglomerated prior to use, in which case the solvent can assist with breaking down the agglomerates.

Solvents suitable for use within the claimed method include but are not limited to single solvents, e.g. tetrahydrofuran (THF) or hexane, or solvent mixtures, such as THF/hexane or alcohokwater mixtures, such as ethanokwater. The ratio of the alcohokwater may be from about 3:1. The alcohokwater mixtures may be suitable, for example, where the oxygen affinity of the primary particles is low.

The solvent may further comprise a soluble surface-active material, for example, a long-chain organic compound, such as stearic acid, linoleic acid and/or oleic acid. When utilised, the surface-active material may prevent particle agglomeration, as well as the presence of detrimental residues which can affect subsequent downstream processing, for example, in metal injection moulding (MIM).

The primary particles preferably comprise a single metal, an admix of metals, an alloy or a combination thereof. When the primary particles comprise a single metal, titanium is preferred. When the primary particles comprise an alloy, titanium alloys (e.g. Ti-6AI-4V) or iron alloys (e.g. steel and, in particular, stainless steel) are preferred. The secondary particles preferably comprise a single metal, an admix of metals, an alloy or a combination thereof. When the secondary particles comprise a single metal, the metal is preferably palladium or ruthenium. In a particularly preferred embodiment, the metal is palladium. When the secondary particles comprise an alloy, a preferred alloy is palladium and ruthenium.

In one embodiment, the primary particles preferably comprise titanium, e.g. commercially available titanium or titanium alloy, and the secondary particles preferably comprise palladium and/or ruthenium. In a particularly preferred embodiment, the primary particles comprise ^" TΪ-6AI-4V and the secondary particles comprise palladium. The inventors have found that the corrosion resistance of articles produced using a titanium/palladium alloy made according to this process is equal to the corrosion resistance of articles produced using commercial titanium/palladium alloys. This would enable a manufacturer of alloy articles to keep an inventory of titanium powder and palladium powder, and to coat the titanium powder with the required amount of palladium whenever the manufacturer needed to fabricate articles from a particular alloy composition. This is particularly advantageous for the manufacturer of small, intricate articles who does not manufacture in bulk and so cannot normally benefit from economies of scale.

The same inventory/convenience advantages would apply to a range of different alloy compositions. For example, in an alternative embodiment the primary particles may comprise tungsten and the secondary particles may comprise rhenium and/or copper.

The coating of the secondary particles on the primary particles may be in the form of a film or in the form of discrete particles. The form may vary depending on the method used to apply the secondary particles to the primary particles. The degree of coverage will depend on the ductility of the secondary particles, the length of time allowed for the coating process and/or the quantity of secondary particles present e.g. palladium may be added to titanium alloys in a proportion of about 0.05 wt% to about 0.25 wt%, which is recognisable as the levels of additions in ASTM/ASME Ti grades, 7, 11 , 16, 17, 24 and 25. The quantity of secondary particles can also affect one or more properties of the desired alloy formed. In this respect, when the quantity of Pd is increased in a Pd/Ti alloy, the corrosion resistance of the alloy to chloride-containing solutions (such as salt water) improves.

A binder may be used to assist the mixing or dispersion of the primary particles and/or secondary particles. Whether a binder is present during the mixing stage and/or during any other pre-compaction stages, the binder will not be present in the alloy formed by the claimed method nor will any degradation products of the binder be present. This is because the binders used are destroyed during compaction. The science of the use of binders and the processes by which binder removal occurs are well documented, for example, in "Injection Molding of Metals and Ceramics" by Randall M. German and Animesh Bose, MPIF Publishers, 1997 (ISBN No. 1-878-954-61 -X) which is hereby incorporated by reference in its entirety for all purposes, and which will not be discussed further other than to say that the removal of the binder systems can generally be achieved cleanly and readily. Table 4.3 on page 91 of the above reference lists 24 example binder formulations, many employing components such as stearic acid, glycerine, polymethylmethacrylate, paraffin wax or carnauba wax.

The primary particles may be formed from at least one metal compound before being coated with the secondary particles, during the coating of the secondary particles, prior to compacting the coated primary particles or during the compacting of the coated primary particles. Even if metal compounds are used in the claimed method, the alloy thereby formed will not substantially contain any of the metal compounds. Preferably, the primary particles are formed from at least one metal hydride, for example, titanium hydride.

In another embodiment, the secondary particles are formed from at least one metal compound prior to or during the compacting of the coated primary particles. Preferably, the secondary particles are formed from at least one metal oxide (for example, palladium oxide or ruthenium oxide), metal hydride (such as palladium hydride or Pd/Ti mixed hydrides).

In one embodiment, the primary particles will have an average diameter of about ≤1000 μm, more preferably about ≤800 μm and even more preferably, about ≤550 μm. The average diameter of the primary particles is dependent on the downstream processing envisaged for the coated particles. For example, when MIM is to take place, the average diameter of the primary particles is preferably ≤45 μm and more preferably, ≤10 μm. However, the secondary particles need not necessarily be substantially spherical in shape.

The coated particles are compacted and an alloy formed therefrom. Suitable methods for compacting the coated particles include Hot lsostatic Pressing (HIP-ing), Cold lsostatic Pressing (CIP-ing), Metal Injection Moulding (MIM) and high energy beam fabrication methods, such as Direct Laser Fabrication (DLF), and Electron Beam Melting. Despite the fact that articles produced by HIP-ing have an inhomogeneous distribution of the metal from the secondary particles, the inventors have found that the corrosion resistance of the alloy formed by the claimed method is independent of the method used to compact the particles, therefore, whichever technique best suits the article to be made from the alloy may be used.

However, the mechanical properties of the alloy formed by the claimed method do depend on the method used to compact the particles, therefore, the compaction technique must be carefully selected depending on the required mechanical properties of the final article to be made from the alloy. In a further aspect, the present invention provides substantially spherical coated particles, wherein the primary particles are coated with secondary particles; and wherein, the primary particles comprise at least one of titanium, molybdenum, tungsten, nickel or iron; and the secondary particles comprise at least one of platinum, palladium, rhodium, ruthenium, iridium, osmium, silver, gold or aluminium.

In yet another aspect, the present invention provides an alloy formed by the claimed method, and metal articles formed from such an alloy.

The invention will now be described by way of the following non-limiting examples and with reference to the following drawings in which:

Figure 1 is a backscattered electron image of a titanium alloy powder coated with 0.2 wt% Pd by a low energy ball milling process.

Figure 2 is a backscattered electron image of titanium alloy powder compacted structure that has been formed by first coating a titanium alloy powder with 2.0 wt% Pd via a low energy ball milling process and then subjecting the coated powder to a standard hot isostatic pressing cycle process (93O ⁰ C, 4 hours, 100MPa).

Figure 3 is a graph showing the polarisation curves for titanium alloy samples produced from a) Ti alloy powder coated with 0.15 wt% Pd via a low energy ball milling process and then compacted using a hot isostatic pressing cycle as per Figure 2, and b) a melt and cast, followed by hot isostatically pressed alloy article containing 0.15 wt% Pd.

Figure 4 is a graph comparing the corrosion potential (open circuit potential) vs. time for HIP-ed titanium alloy powders made by a) coating with 0.15 wt% Pd for 7 days and b) milling alone for 7 days.

EXAMPLE 1 HlP-ing of TJ-6AI-4V with Pd

Ti-6AI-4V powder (20Og) with a particle size of 150-500 μm is low energy ball milled with 0.15 wt% of palladium particles (surface area 19-26 m ² /g) for 7 days. A sample of the resulting palladium coated alloy particles (see Figure 1) are encapsulated in a mild steel container, which is evacuated and sealed prior to HlP-ing at 930 ⁰ C for 4 hours at 100 MPa. This process results in a heterogeneous distribution of palladium in which palladium is located preferentially within the beta phase at the particle boundaries of the primary particles. Palladium concentrations within areas that would have formed the interior of the original primary Ti particles are below the limits of detection.

An image of an alloy powder HIP-ed as described above but which contains 2.0 wt% palladium can be seen in Figure 2.

EXAMPLE 2 DLF of TJ-6A1-4V with Pd

A sample of the palladium coated particles produced by low energy ball milling in

Example 1 is fabricated into a solid article by direct laser fabrication using process parameters suitable for the manufacture of Ti-6AI-4V articles (laser power 750 W, laser scan speed 400 mm/min and step build height 0.3 mm). Melting of the particles during the DLF process results in a homogeneous distribution of palladium within the article formed.

EXAMPLE 3 Assessment of Corrosion Resistance

For corrosion experiments samples are mounted in cold setting resin and all of the tests were carried out in naturally aerated 2 M HCI at 37 ⁰ C using a 3-electrode cell (with the sample acting as the working electrode, a platinum counter electrode and a saturated calomel electrode (SCE) as reference). 2 M HCI was selected as the electrolyte to simulate aggressive environmental conditions that might exist within a crevice on the surface of an alloy particle.

Polarisation curve tests: Samples were prepared by grinding a) Ti powder compacts which had been low energy ball-milled with 0.15 wt% palladium then HIP-ed, and b) a cast alloy article containing 0.15 wt% palladium which had also been HIP-ed. Each sample was prepared by grinding to 1200 grit SiC, then washed in de-ionised water, rinsed in ethanol and dried and then tested immediately once dry. Figure 3 shows that the palladium still provides equivalent corrosion protection whether it is homogeneously distributed or inhomogeneously distributed within a titanium article.

Open circuit potential tests: Samples of HIP-ed Ti-6AI-4V powders a) low energy ball-milled with 0.15% Pd for 7 days and b) low energy ball-milled alone for 7 days, were polished using colloidal silica immediately prior to testing, then washed in de-ionised water, rinsed in ethanol and tested immediately once dry. Figure 4 shows that the sample containing palladium has a higher corrosion potential than the one not containing palladium.