Technical Field The present invention relates to titanium and its alloys and in particular to methods for producing titanium, titanium alloys and composites comprising titanium and other metal or non-metal materials by low temperature sintering. Background Art

Lightweight alloys based on titanium and other metals, with low density and high stiffness, melting point, strength, elasticity and other desirable properties are attracting interest from the aerospace, chemical equipment, construction and biomedical industries.

However, conventional ingot technology used to date still limits the number and concentration range of additives in titanium alloy.

Titanium, a metallic element with a lustrous, iron-like appearance, is used in the manufacture of aircraft and spacecraft parts, chemical equipment and biomedical equipment such as bone replacements . Titanium is a superior metal when compared even to platinum due to its chemical stability, or to other metals due to its ductile qualities. Titanium loses heat more rapidly than steel, having the lowest frictional constant, and is therefore well suited to high-temperature applications where frequent heating is required. It is also non-magnetic, which is an advantage for high speed processing. Due to its corrosion resistant and non-ionic properties, titanium is often used for prosthetic devices and orthopaedic implants because tissue grows on titanium quite readily.

Titanium is about half the density of iron but twice

2 as resilient to mechanical stress (30kg/mm ) with an

4 ? elastic modulus of 1.18 x 10 kg/m (40%-55%), a

2 Brinnell hardness of 120kg/mm and a fatigue limit of

2 33kg/mm . Titanium has a specific gravity of f.

4.51g/cm , thermal extension constant 8.09 x 10~ and a specific electric conductivity of 55 _/ .ohm.cm (20°C).

Titanium has a 2 and 4 valency atomic structure, is non-corrosive even in sea water and has a « - β modification of crystals at 882 C.

Above 800 C, however, titanium spontaneously reacts with oxygen, nitrogen, hydrogen and carbon and therefore all high temperature processes with titanium even from as low as 600 C must be carried out in a neutral atmosphere or vacuum. The metal is usually obtained by melting in a crucible at 1668°C or sintering at 1200°-1300°C in a compressed form in an inert atmosphere or vacuum.

In an effort to ameliorate the disadvantages of the prior art or at least provide a commercial alternative to the prior art it is proposed to provide a method for producing titanium, titanium alloys and composites which, at least in the preferred embodiments, is cheaper than the prior art methods but ^' which still produces an acceptable product.

Disclosure of the Invention

In a first aspect, therefore, the present invention provides a method of manufacturing titanium products comprising the steps of:- i) providing an amount of TiH-, powder, ii) compressing the TiH-, powder, iii) placing the compressed TiH„ powder in a chamber, and iv) heating the TiH,, powder sufficiently to cause liberation of H ₂ gas and fusion of particles of Ti while continually removing H ₂ gas from said chamber, said heating not exceeding 1100 C. In a first embodiment, the inventive method may

comprise the further step of:- v) applying a further compression to the heated TiH-, powder to control the porosity of the final product. Such an additional step may be carried out after step iv) or simultaneously with step iv) .

Titanium hydride or TiH„ is a grey powder with face centred cubic crystals and a specific gravity of 3.78g/cm . In the temperature range of 300-750°C titanium hydride dissociates into Ti and H-, gas. This reaction is reversible in the same temperature range. At 300 C, the specific heat and resistivity of titanium hydride are 7.19cal/ C per mole and 80 _/ .ohm cm respectively. The calculated value of the heat of formation of stoichiometric titanium hydride from titanium metal at 450 C is ΔH = -31.8kcal/mole very close to the value obtained experimentally.

Titanium hydride has previously been used chemically for producing suicides, nitrides and borides as a catalyst in hydration of organic compounds and to obtain extremely pure hydrogen. The mechanical properties of titanium hydride are of no practical use. In the inventive process, however, the compressed titanium hydride decays and produces hydrogen gas which is continually evacuated. The chemical process may be represented by the following equation:-

TiH ₂ = Ti + H ₂ + ΔH

where Δ H is exothermic heat arising from the Ti-H bond energy.

Although not wishing to be bound by any theory, it is believed the titanium particles then become easily fused for two reasons. Firstly, their surface has just been activated due to the hydrogen evacuation and secondly the heat ΔH liberated increases the local temperature at the

interface between the particles to a fusion level.

To explain, when the Ti-H bond is broken an amount of heat ΔH is produced. This heat increases the temperature of the titanium particles to a fusion level despite the fact that the vacuum chamber is only heated to a maximum of 1100 C whereas normal sintering of titanium takes place at 1200°-1300°C.

The heating of the vacuum chamber is sufficient to produce the desired reaction i.e. TiH„=Ti + H_ . The exothermic nature of this reaction is then sufficient to produce the remaining heat required to fuse the titanium particles .

Accordingly, it can be seen that one can exploit the exothermic properties of the titanium hydride - titanium and hydrogen gas reaction to avoid the need for costly heating normally required to sinter ( 1200°-1300°C) or melt (1668°C) titanium.

The 100% titanium product obtained by the inventive process can then be treated in a traditional manner as required eg. reshaping, grinding, polishing and thermal treatment.

It is preferred that cooling of the obtained product is also carried out under vacuum to protect the titanium from gas attack. The degree of vacuum during the process should be sufficient to evacuate the atmosphere from the initial heating and the subsequently produced hydrogen.

Alternatively, the subsequently produced hydrogen may be removed by entrainment with an inert gas, eg. argon, flowing through the chamber. Such an inert gas will not react with titanium at any temperature. The latter will gradually displace the amount of residual air from the sintering volume to a level that does not react with the heated titanium.

The inventive method may also be used to produce titanium alloys and their composites by simply using a

powder comprising titanium hydride in combination with other metal or non-metal powders . The inventive method may also be used to produce multilayer composite materials based on titanium/titanium alloys with metals and non-metals having a low sintering temperature such as copper, brass, bronze.

Once again, due to the hydrogen liberation from the compressed composite mixture and the local increase in temperature between the particles, the other metals and non-metals may fuse together into metallic bonds. The obtained product, based on titanium and other metals and non-metals may then be treated in a traditional manner.

The inventive method is suitable for the manufacture of a variety of products including titanium or composite alloy ingots, rods, tubes, lists and plates as well as final shape cast pieces of instruments, machines, engines, constructions, wheels, gears and biomedical applications such as implantable prosthetic devices and orthopaedic implants . The present inventive method may also be used to produce abrasive tools particularly titanium-diamond abrasive tools.

Diamond is the hardest substance known to man. It

3 has a specific gravity of 3.51g/cm and is brittle with a modulus of elasticity of 4 x 10 12 to 9.5 x 1012din/cm2 along different axes. The thermal extension coefficient of diamond is 1.2 x 10~ to 4.5 x 10~ which is very close to titanium and diamond has a thermal conductivity better than titanium. Unfortunately, at the conventional sintering (1200°-1300°C) and melting (1668°C) temperatures of titanium, diamond may be adversely affected. Diamond burns in air at 850 C-1000 C in an oxygen rich atmosphere at 750 C and in a high vacuum it graphitises from 900 C. Accordingly, conventional sintering

techniques for titanium may not be used with diamond since at these high temperatures the diamond will graphitise.

Diamond cutting tools such as wheels, drills, strips, saw blades, cutting wires, grinding and polishing tools are used in a wide range of industries because of the abrasive qualities of diamond to cut, grind and polish and drill materials with a high hardness index eg. stone, rock, ceramic, concrete, brick, glass, metal, silicon crystal etc. Normally, diamond grains are embedded mechanically in low melting point alloys such as bronze, or other composites of different metals and high-strength cross-linked organic composites. Composites based on mixtures of metal powders and organic material have recently become more attractive for such application. The visco-elasticity (non-brittleness) of the binding material is extremely important for the embedding stability of the diamond grains . Low melting-point additives such as tin traditionally help to reduce the melting temperatures of Ti, Cr, Fe, Zr, as a liquid phase in the process of their sintering, but binding systems become more brittle. The binding systems currently in use have become uneconomical for use with higher grades of diamond. Titanium is known as an important constituent in other metals/composites since it adheres with high melting point carbides and non-metallic additives such as graphite, quartz, ceramics, bases presumably by producing chemical bonding at these interfaces . Limited studies have proven the existence of chemical covalent bonding between titanium and diamond when at high temperatures the diamond surface becomes covered with a thin layer of carbon.

The applicants inventive low temperature sintering technique is particularly suitable for producing titanium-diamond abrasive tools to avoid graphitisation of the diamond yet still produce a strong mechanical/chemical

bond with the titanium.

In a second aspect, therefore, the present invention provides a method of producing a titanium-diamond composite material comprising the steps of:- i) providing a mixture of TiH_ powder mixed with a plurality of natural or synthetic diamonds, ii) compressing the mixture iii) placing the compressed mixture in a chamber, and iv) heating the mixture sufficiently to cause liberation of H _? gas and fusion of particles of Ti while continually removing H„ gas from the vacuum chamber, said heating not exceeding 1100 oC..

As with the abovementioned first aspect of the present invention, the inventive method of producing the titanium-diamond composite material may comprise a further step of applying further compression to the heated TiH ₂ powder/diamond mixture to control the porosity of the final production.

It is preferred to include a minimum concentration of 20% by weight of titanium hydride in order to produce satisfactory contact Between the titanium and diamond crystal surfaces and chemical bonding at the interface.

The titanium diamond segments obtained by the inventive method may be treated in a traditional manner and then brazed onto steel discs or tool bases if required. In another embodiment of the present invention special soldering thin foils or wires may be used to perform the brazing in a vacuum or an inert gas medium to prevent chemical gas attack of the titanium at the elevated temperatures required for brazing ie. above 300°C.

In yet another embodiment, solder plates of not less than 0.1mm thickness and containing at least 40-50% silver may be used to braze the titanium-diamond segments under an argon curtain.

Alternatively, vacuum brazing with solders or laser welding with or without solders and gas curtain may be used.

Such composite material may undergo a stress alleviating step by annealing the material preferably at a low temperature, eg. 200 -400 C for preferably not less than 3 hours. This annealing eliminates stresses between the composite layers.

The titanium-diamond segments obtained by the inventive method may be produced with diamond embedded in 100% titanium only or alternatively other metals and non- metals may be introduced for improvements of the required parameters of the use of the composite material for tools or for economic reasons. It should be noted, however, that additives with sintering temperatures higher than the temperatures used in the inventive process i.e. up to 1100°C, are still suitable for this method due to the local heat release between the particles discussed above.

Table 1 provides examples of titanium sintering alloys and titanium-diamond segments produced according to the present invention.

Brief Description of the Drawings

The present invention will now be described by way of example only with reference to the accompanying drawings in which:- Figure 1 is a side elevational view of an apparatus for producing titanium products according to an embodiment of the present invention, and

Figure 2 is a cross-sectional view of a surface portion of a titanium-diamond abrasive tool manufactured according to another embodiment of the present invention. Figure 3 is a graph showing the rate of hydrogen disorption from a titanium hydride composite.

Figures 4A-4D are micrographs of titanium alloys/composites produced in accordance with the present inventive method.

Examples of Ti sintering alloys, Ti-Diamond segments and their physical n prroopDeerrttiieess

Table 1 Titanium bonding alloys for metal powder application

Examples Content % Parameters of Alloys

Ti Al Mo Ni Cu Fe Co Sint. Temp., C HB

TICK) 100 850-870 120

T95A5 95 5 . 850-870

T90A5N5 90 5 5 850-870

T90A5M5 90 5 5 850-870

T85A5N5M5 85 5 5 5 830-860

T60C40 60 40 810-830 140

T60N28C12 60 28 12 840-850 156

T60F28N12 60 . . 12 28 840-870 230

T60N28F12 60 . 28 12 840-860 210

T60N28Col2 60 — _ 28 . . 12 840-860 156

Mode(s) for Carrying out the Invention

Turning firstly to Figure 1, a sample 1 is prepared by compressing a powder of titanium hydride (TiH~) optionally with other metal or non-metal powders. Experiments have been conducted with pure TiH powder being compressed at 6 tons/cm 2 , however, the compressive force applied to the powder may be altered to suit the required porosity and size of the pores of the final product. The compressed sample 1 is then placed on a ceramic tray 2 in an evacuated chamber 3, in this case a quartz tube.

The evacuated chamber 3 is then placed in a furnace 4 with thermocouple 10 connected to thermometer 8 to monitor the heat of the chamber 3. Evacuated chamber 3 is connected to vacuum pump 5 through tube 9 to remove air and other gases from the chamber 3. Preferably a cooling trap 6 is provided between the evacuated chamber 3 and vacuum pump 5 to cool the removed gases prior to evacuation through gas outlet 11. Meter 7 measures the vacuum in tube 9 and chamber 3. The apparatus of ^{^} Figure 1 operates as follows. Firstly, the compressed sample 1 is placed in evacuator chamber 3 and vacuum pump 5 turned on to remove all air from the chamber. Furnace 4 is then activated to heat chamber 3 to a maximum of 1100 C preferably up to 870 C. As shown in Table 1 the actual sintering temperature will depend upon the alloying components included in the sample. A pure titanium hydride sample will undergo sintering at 850-870 . On the other hand, as shown in Table 1, other metal and non-metal additives may reduce the required sintering temperature.

At this sintering temperature, the TiH ₂ = Ti+H„ reaction occurs with hydrogen gas being removed from the chamber via tube 9 and vacuum pump 5. Figure 3 shows the rate of hydrogen liberation from the TiH ₂ composite.

Curve 31 relates to a heating rate of 5°C per minute and curve 32 a heating rate of 10 C per minute. As evidenced by figure 3, from between approximately 300 and 750 C hydrogen gas is liberated from the titanium hydride composite. In order to prevent the hydrogen atoms from reattaching to the titanium, the hydrogen gas is evacuated from chamber 3.

The titanium particles of the sample 1 then become fused since their surfaces have been activated due to hydrogen evacuation and the increase in temperature of the particles to a fusion level arising from the exothermic heat produced by breaking the Ti-H bond.

This fusion of Ti particles produces a strong titanium or composite alloy product which may then be treated in a traditional manner as required. Preferably, cooling of the sintered product also takes place under vacuum to prevent gas attack of the titanium or titanium alloy.

By using the additional exothermic energy released from the Ti-H bond, the inventive sintering process is more economical as compared with prior art techniques and takes place at a much lower ambient temperature range.

The local increase in temperature of the particles in the powder sample also allows other metal and non-metal powder components to be fused despite the fact that there sintering temperatures may be much higher than 900 C. It is preferred, however, that the powder sample contain not less than 10% by weight of titanium hydride.

The sintering of titanium hydride with other metals and non-metals at relatively low temperatures, when the particles are fused but not molten, also allows a high packing of up to 1% porosity. Porosity cannot be controlled with the high temperature sintering techniques of the prior art since at such temperatures certain metals, eg. copper, bronze, brass, will melt. The

inventive method prevents these low melting-point powders, eg. under 1000 C, from melting during sintering thereby allowing control of the porosity. Further, in these conditions when the molten phase is not present, the different layers in the structures will possess thermal expansions very close to each other thereby minimising inter-layer stresses and thermally stimulated deformations and preventing delamination. Further stress alleviation may be accomplished by annealing the composite material. Such annealing is preferably at a low temperature, eg. 200 -400 C and for not less than 3 hours.

In addition, selected concentrations of titanium in composite alloys may reduce their expansion by about 3 times compared with any other metal alloy. This minimises shrinkage and distortion of manufactured casts and segments. Further, any desired porosity of up to 50% of the final product may be obtained and the size of the pores and their uniformity is controllable by altering the compactness of the sample. Turning now to Figure 2, the present inventive process is particularly suitable for producing a material for abrasive tools in which diamond is embedded in titanium or a titanium alloy or their composites.

The method for producing such a titanium-diamond composite is essentially the same as mentioned above apart from including natural or synthetic diamonds in the powder sample. Once the titanium-diamond segment has been sintered, a segment is produced with a plurality of diamonds 20 embedded in the titanium or titanium alloy composite 21.

The inventive method has several advantages over prior art techniques for producing a composite for abrasive tools. As mentioned above, graphitisation of diamonds occurs from 900 C and above. In air diamond burns at 850°C-1000 C and in an oxygen rich atmosphere

it burns at 750°C. At over 1000°C it will take no more than 1-2 minutes for the diamond grains to degrade in air or form a layer of graphite in a vacuum. It is virtually impossible to sinter titanium with diamond at temperatures above 1300 C. By providing low temperature sintering of titanium at a maximum of 870 C not only is graphitisation of the diamond avoided but a chemical bond between the titanium and diamond is produced. The diamond is, therefore both mechanically and chemically bonded to the titanium. This is particularly important in terms of the useful life of an abrasive tool formed from such a material. To explain, during cutting, grinding and polishing operations, the diamond grain 20 with a length d exposes a first face 22 with an exposed height h_ from the side of the force P (see Fig. 2). On the opposite side, a supportive wedge wall 23 of a height of d-h-. resists force P . As the height of this supportive wedge wall decreases the diamond grain may lose its stability and become dislodged from the metal. If the diamond-metal interface adhesion is strong then the increased wear resistance and low friction of the binding metal increases tool efficiency and life.

The present inventive method provides a process for producing a titanium-diamond composite material suitable for abrasive tools which avoids graphitisation of the diamond and provides good mechanical and chemical bonding of the diamond grain to the titanium or titanium composite alloy base.

In an embodiment of the second aspect of the present invention, the titanium-diamond abrasive powder segment may be compressed with a backing layer such as bronze to assist brazing the segment to the base metal. The titanium-diamond powder is compressed with bronze powder and then sintered. The resulting composite is then brazed onto steel or other base metal. A diamond free backing

layer may also be produced comprising a mixture of titanium hydride, at least 10% by weight, with other non-metal or metal powders having a concentration of not less than 50%. These other metal and non-metal powders may be easily brazed with steel. The obtained backing layer may then be treated in a traditional manner and act to assist in brazing the titanium-diamond segments to the steel, bronze etc discs or tool bases.

To avoid possible delamination of the diamond-titanium segment or separation from the backing layer due to different thermal expansion coefficients it is preferable to include at least 20% by volume of the Ti particles . The absence of molten phase of a metal also assists in this regard. Figures 4A-4D are micrographs of several different titanium/titanium alloy products/composites produced by the present inventive method.

Figure 4A is a micrograph of a Ti-Ni-Cu alloy with a bronze backing layer. The backing layer is at the top half of the diagram. The micrograph clearly shows the diffused microstructure at the interface between the Ti alloy and backing layer.

In contrast, Figure 4B shows a clear distinct boundary layer between the darkish thin bronze backing layer and the light Ti-Al-Ni-Mi alloy.

Figures 4C and 4D show a cutting segment made in accordance with the present inventive method. This cutting segment is a composite material comprising a plurality of diamond grains embedded in a Ti-Ni-Cu alloy as shown in Figure 4C.

The top 1/3 of Figure 4D is virtually identical to the composite material of Figure 4C, ie. it shows the dark diamond grains embedded in the light covered Ti-Ni-Cu alloy. The lowest layer, which is almost totally white in the

Figure, is a steel tool base. Extending between this steel base and the Ti-Ni-Cu alloy is a bronze backing layer.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodimentε are, therefore, to be considered in all respects as illustrative and not restrictive.