WO2001083837A1

WO2001083837A1 - Titanium alloy and method of manufacture

Info

Publication number: WO2001083837A1
Application number: PCT/AU2001/000488
Authority: WO
Inventors: Steven Martin Bossong; Radomir Vukcevic
Original assignee: WESTERLAND Pty Ltd
Current assignee: WESTERLAND Pty Ltd
Priority date: 2000-04-28
Filing date: 2001-04-30
Publication date: 2001-11-08
Anticipated expiration: 2002-10-28
Also published as: AUPQ717000A0

Abstract

A titanium nitride alloy of crystalline grain structure, and composite material of a titanium nitride alloy of crystalline grain structure over a titanium substrate, and method for forming alloys of titanium with various alloying materials including nitrogen (to form nitride alloy), the method comprising heating titanium metal in a reactant excluding atmosphere or in a vacuum to form molten titanium, and contacting said molten titanium with alloying material in said reactant excluding atmosphere or in said vacuum.

Description

"Titanium Alloy and Method of Manufacture"

Field of the Invention

This invention relates to a titanium nitride alloy, and a method of forming the same.

Background Art

Titanium metal has wide application in industry. While titanium is highly reactive with oxygen and other elements and compounds, a layer of titanium dioxide which forms on the metal prevents oxidation of underlying titanium metal under normal conditions. At room temperatures, titanium metal is not generally attacked by mineral acids (except in the presence of fluoride ions) or even hot alkaline solutions. However, at elevated temperatures, titanium metal will react with hot acid solutions.

To avoid the corrosive effects where titanium metal is contacted with, for example, hot acid solutions, it has been known to form interstitial compounds of titanium, with carbon, nitrogen, or boron. These compounds of titanium are very stable, hard and refractory. These compounds, can be formed as a surface treatment of titanium metal.

The inventors have found that typical vapour deposition techniques utilised to form a coating of titanium nitride or titanium carbide, formed only in a thin layer. This thin layer of carbide or nitride could be easily damaged in sen/ice, which could expose the titanium metal below to corrosive effects. An attempt using such prior art techniques to increase the thickness of the nitride or carbide layer invariably results in formation of a brittle surface which is prone to formation of fine cracks which can extend down to the underlying titanium metal. In pressure vessels, pipes, and valves carrying superheated sulphuric acid solution, any exposure of the titanium metal underlying the nitride or carbide layer would result in corrosion, and failure of the equipment. Furthermore, where the equipment also contacts a process slurry, abrasion of a thinner nitride or carbide layer could also expose the underlying titanium metal, leading to corrosion of the underlying titanium metal.

It is an object of this invention to provide a superior treatment method for titanium, and resultant titanium alloy or composite of titanium metal and titanium alloy.

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Disclosure of the Invention

In a particularly preferred embodiment, this invention concerns treatment of titanium by "nitriding" to form a substantial layer of titanium nitride alloy on the treated surface of the metal.

It should be understood that the term "titanium metal" and "titanium" used in this specification, includes alloys of other elements with elemental titanium.

In accordance with the invention there is provided a method of forming a titanium alloy, the method comprising melting titanium metal in a reactant excluding atmosphere or in a vacuum to form molten titanium, and contacting said molten titanium with alloying material in said reactant excluding atmosphere or in said vacuum. The term "reactant excluding" in this context means excluding undesirable reactants other than the alloying material.

Particularly, in accordance with one aspect of the invention there is provided a method of producing a titanium alloy on or in the surface of titanium metal, the method including the steps of heating titanium metal to form a molten region of the metal where the alloy is to be formed, and contacting said molten region with alloying material to form said titanium alloy in said region, the steps of heating the titanium metal and contacting the molten region being carried out in a reactant excluding atmosphere or in a vacuum. Preferably the steps of heating the titanium metal and contacting the molten region are carried out simultaneously.

Preferably said step of contacting said molten region comprises impinging said molten region with alloying material to form said titanium alloy in said region. The step of impinging said molten region is performed by directing said alloying material into said molten region (along with any shielding gas/reactant excluding atmosphere) with sufficient velocity to cause a depression in said molten region. This results in the formation of the alloy (or a mix of the alloy and titanium metal) to a greater depth in the titanium metal than any known surface treatment involving formation of surface coating alloys such as titanium nitride, titanium carbide, or titanium boride.

Preferably said molten region is formed in a localised area on and in said titanium metal, and said localised area is moved progressively so that the surface heating and impinging steps progressively cover the surface being treated.

Preferably said alloying material is selected from one or more of nitrogen, carbon, and boron, and compounds containing one or more of these elements.

Preferably said alloying material is nitrogen.

Preferably the step of heating the titanium metal is carried out with an electric arc in a reactant excluding atmosphere, said reactant excluding atmosphere being an inert gas toward which titanium is non-reactive.

Preferably said inert gas is an ionising gas such as argon or helium.

Alternatively the step of heating the titanium metal is carried out with a laser, and said reactant excluding atmosphere comprises said alloying material.

Also in accordance with the invention there is provided a composite material of a titanium nitride alloy overlying a titanium metal substrate, produced in accordance with the foregoing method. This composite material of said titanium nitride alloy overlying said titanium metal substrate is preferably of substantially uniform crystalline grain structure for a thickness down to a transition zone between said titanium nitride alloy and said titanium metal substrate. The transition zone is believed to have a relatively high N.Ti ratio at the interface with the titanium nitride alloy grading to a lower N.Ti ratio toward the titanium metal substrate. It should be appreciated that the structural aspects of the alloy itself, and the transition zone are not well understood, however it is understood that the alloy properties may be varied between relatively soft by using lower levels of nitrogen in the method, and quite hard by using high levels of nitrogen (or repeated passes). It will be appreciated that the properties of the transition zone must be dependent on the properties of the overlying alloy, although with a hard high N:Ti ratio alloy, the transition zone could equate to normal nitrided titanium.

As the key to the alloy appears to be the melting of the titanium at the surface during the alloy formation allowing mixing of nitrogen throughout the molten layer, the thickness of said titanium nitride alloy may be less than 0.2mm; however, preferably said titanium nitride alloy overlying said titanium metal substrate is of substantially uniform crystalline grain structure for a thickness of at least 0.2mm down to said transition zone. Preferably said thickness is a minimum of at least 0.5mm, 1 mm, or 2mm, up to a maximum of 5mm, 10mm, 25mm, 50mm or more.

The thickness of the titanium nitride alloy could extend throughout the titanium metal substrate, in which case the entire titanium metal substrate would be converted to titanium nitride alloy.

Brief Description of the Drawings

An embodiment of the invention will now be described in the following description, made with reference to the drawings, in which:

Figure 1 is a diagram of a plasma arc gun in operation forming a titanium nitride alloy;

Figure 2 is a photograph at 100x magnification showing the surface of the machined alloy at depth of 0.87 mm ; Figure 3 is a photograph at 23x magnification showing the surface of the machined alloy in cross section at the bond area between the alloy and the underlying titanium at depth of around 3.8 mm ;

Figure 4 is a photograph at 130x magnification showing the surface of the machined alloy after heavy grinding, at depth of 0.067 mm; and Figure 5 is a photograph at 160x magnification showing the surface of the machined alloy after heavy grinding, at depth of 0.545 mm.

Best Mode(s) for Carrying Out the Invention

Titanium nitride alloy according to the invention is formed by subjecting a piece of titanium to heating with a plasma arc gun, using argon as the inert and ionising gas. Once the titanium metal is melted by the arc, a stream of nitrogen gas is directed at the molten metal, so as to contact the melt where a depression is caused in the melt by either the nitrogen or the argon or both. It is believed that eddy currents set up by impingement of the nitrogen circulate titanium metal and titanium nitride in the melt, where molten titanium metal at the surface of the melt is available to react and form further titanium nitride. The plasma arc gun is moved progressively across the surface, forming alloy at the surface of the metal, and extending into the metal to a depth proximate to the depth the metal is melted. The process may be repeated to further increase the extent of alloying as required.

The plasma arc gun utilised is that generally used in a Plasma Transferred Arc welding system. The Plasma Transferred Arc welding system torch (or gun) has a number of orifices that are conventionally used to introduce metal powder into or adjacent to the plasma stream, during welding operations that the system is normally put to use in.

A plasma arc 9 is established between a tungsten electrode 1 and the titanium surface 7 by applying an electrical potential across them. This arc 9 produces temperatures sufficient to cause melting of the titanium 7. It is believed that the reaction between titanium and nitrogen is accelerated at the high temperatures developed by the plasma arc (up to 30,000°C). The plasma is transferred through an argon (or similar gas) based gas injected through pipe 3. When this gas flows against the molten titanium surface it causes a depression 11 in that surface. The depth of the depression 11 is controlled by the flow rate of gas introduced through pipe 3, and electrical energy applied to the plasma stream.

The plasma steam is constrained by a nozzle 4, and a gas shroud fitted around the nozzle and a shielding gas, predominantly argon (or a similar gas), is injected into the area constrained by the shroud through pipe 6, forming a predominantly inert shield 8 around the plasma stream 9. As an alternative, nitrogen can be used as a shielding gas as the reaction product of nitrogen and titanium in the shielding region and therearound, does not interfere with the alloy formation process.

A port 5 which is normally used to inject metal powder in conventional Plasma Transferred Arc system, is modified to inject nitrogen therethough. The nitrogen introduced through the port enters the plasma stream 9 and contacts the depression in the molten titanium 11 , to impinge in the titanium 7.

Excess nitrogen introduced through the port 5 mixes with the inert gas shield 8. The nitrogen reacts with the molten titanium within the depression 11 and forms a layer containing Titanium Nitride with a thickness corresponding to the depth of the plasma depression 11. Any excess nitrogen that is present within the gas shield 8 causes the formation of titanium nitride at the surface of the surrounding titanium. The torch and/or titanium work piece are manipulated relative to each other, to move the depression 11 across the titanium 7, and so to effect the area of titanium requiring conversion to titanium nitride.

In use, once the titanium melts, the nitrogen can be introduced. On introduction of the nitrogen, the reaction between the titanium and the nitrogen can be seen taking place. In numerous trials performed by the inventors, it was found that a second treatment of the treated titanium produced further formation of titanium nitride. Depending upon the amount of electrical energy, and the amount of nitrogen supplied, the resultant hardness of the alloy formed can be from 30 Hrc to 66 Hrc (Rockwell C scale), or higher (greater than 68Hrc, above the Rockwell C scale).

Samples produced by repeated application were hardness tested and macro analysed. From the analysis, it was found that titanium nitride hardness throughout the depression depth was uniform, and in the order of 58 to 62 Hrc. The region of alloy formation appears to have a grain structure, which is shown in figures 2 to 5, which minimises fracture propagation. This is to such an extent that even if the surface is scratched, that fractures were not observed to occur as a result. The region of alloy formation is crack free, and can be machined and/or ground, to achieve high surface finishes. The region of alloy formation appears to have good impact resistance, is erosion resistant, and has excellent corrosion resistance in both acidic and alkaline environments at high temperatures. The depth of alloy formation was typically in the order of 10mm, although depths of alloy formation from 2-5mm up to 50mm are readily achievable with the plasma arc gun described. It should be understood that an alloy comprising a very thin surface coating could be achieved, and similarly the maximum thickness of the alloy coating is dictated by the size of the plasma arc gun and the temperature of the arc, the transit time of the plasma arc gun across the work, and the number of passes that the plasma arc gun makes.

Envisaged applications of the alloy or alloyed/treated titanium include valve bodies and sealing elements, pump bodies and mechanical elements (impellers), pipes and pipe fittings, pressure vessels, tanks, and aerospace and automotive components.

It should be appreciated that the scope of the invention is not limited to the particular embodiment described herein.

Claims

The Claims Defining the Invention are as Follows

1. A method of forming a titanium alloy, the method comprising heating titanium metal in a reactant excluding atmosphere or in a vacuum to form molten titanium, and contacting said molten titanium with alloying material in said reactant excluding atmosphere or in said vacuum.

2. A method as claimed in claim 1 wherein the steps of heating the titanium metal and contacting the molten region are carried out simultaneously.

3. A method as claimed in claim 1 or 2 wherein said step of contacting said molten region comprises impinging said molten region with alloying material to form said titanium alloy in said region.

4. A method as claimed in claim 3 wherein the step of impinging said molten region is performed by directing said alloying material into said molten region (along with any reactant excluding atmosphere) with sufficient velocity to cause a depression in said molten region.

5. A method as claimed in claim 3 or 4 wherein said molten region is formed in a localised area on and in said titanium metal, and said localised area is moved progressively so that the surface heating and impinging steps progressively cover the surface being treated.

6. A method as claimed in any one of the preceding claims wherein said alloying material is selected from one or more of nitrogen, carbon, and boron, and compounds containing one or more of these elements.

7. A method as claimed in claim 6 wherein said alloying material is nitrogen.

8. A method as claimed in any one of the preceding claims wherein the step of heating the titanium metal is carried out with an electric arc in a reactant excluding atmosphere, said reactant excluding atmosphere being an inert gas toward which titanium is non-reactive.

9. A method as claimed in claim 8 wherein said inert gas is an ionising gas such as argon or helium.

10. A method as claimed in any one of claims 1 to 7 wherein the step of heating the titanium metal is carried out with a laser, and said reactant excluding atmosphere comprises said alloying material.

11. A method of producing a titanium alloy on or in the surface of titanium metal, the method including the steps of heating titanium metal to form a molten region of the metal where the alloy is to be formed, and contacting said molten region with alloying material to form said titanium alloy in said region, the steps of heating the titanium metal and contacting said molten region being carried out in a reactant excluding atmosphere or in a vacuum.

12. A method as claimed in claim 11 wherein the steps of heating the titanium metal and contacting the molten region are carried out simultaneously.

13. A method as claimed in claim 11 or 12 wherein said step of contacting said molten region comprises impinging said molten region with alloying material to form said titanium alloy in said region.

14. A method as claimed in claim 3 wherein the step of impinging said molten region is performed by directing said alloying material into said molten region (along with any reactant excluding atmosphere) with sufficient velocity to cause a depression in said molten region.

15. A method as claimed in claim 13 or 14 wherein said molten region is formed in a localised area on and in said titanium metal, and said localised area is moved progressively so that the surface heating and impinging steps progressively cover the surface being treated.

16. A method as claimed in any one of claims 11 to 15 wherein said alloying material is selected from one or more of nitrogen, carbon, and boron, and compounds containing one or more of these elements.

17. A method as claimed in claim 16 wherein said alloying material is nitrogen.

18. A method as claimed in any one of claims 11 to 17 wherein the step of heating the titanium metal is carried out with an electric arc in a reactant excluding atmosphere, said reactant excluding atmosphere being an inert gas. toward which titanium is non-reactive.

19. A method as claimed in any one of claims 18 wherein said inert gas is an ionising gas such as argon or helium.

20. A method as claimed in any one of claims 11 to 17 wherein the step of heating the titanium metal is carried out with a laser, and said reactant excluding atmosphere comprises said alloying material.

21. A titanium nitride alloy produced in accordance with the method as claimed in any one of claims 7 to 10.

22. A composite material of a titanium nitride alloy overlying a titanium metal substrate, produced in accordance with the method as claimed in any one of claims 17 to 20.

23. A composite material of a titanium nitride alloy overlying a titanium metal substrate, the titanium nitride alloy being characterised by being of substantially uniform crystalline grain structure for a thickness down to a transition zone between said titanium nitride alloy and said titanium metal substrate.

24. A composite material as claimed in claim 23 wherein said thickness is a minimum of at least 0.2mm.

25. A titanium nitride alloy characterised by being of substantially uniform crystalline grain structure.

6. A method of producing a titanium nitride alloy substantially as herein described, with reference to the description of the best mode for carrying out the invention, and the drawings.