WO1998048067A1

WO1998048067A1 - Process of direct forming titanium or titanium alloys

Info

Publication number: WO1998048067A1
Application number: PCT/AU1998/000205
Authority: WO
Inventors: Claude Fortin
Original assignee: Individual
Current assignee: Individual
Priority date: 1997-04-18
Filing date: 1998-03-26
Publication date: 1998-10-29
Anticipated expiration: 1999-10-18
Also published as: AUPO630797A0; ZA98604B

Abstract

The present invention concerns a process for forming titanium metal alloys directly from crushed titanium sponges or from powders. The process uses a low melting point bath containing bismuth, which allows the introduction under vacuum of titanium, as well as the addition of other elements and facilitates low temperature extrusion. The extruded semi-finish products then undergo degassing, grouping, homogenisation and diffusion treatment. The semi-finish products thus obtained can then be cut, drawn, rolled, etc. Degassing and diffusion create pores or micro-pores which can be eliminated during homogenisation treatment or which can, on the other hand, be expanded to produce porous and lighter materials. To obtain this result, the characteristics must necessarily be specially adapted and it is possible to add a suitable agent to the bath. Superficial refusion allows the formation of a skin.

Description

PROCESS OF DIRECT FORMING TITANIUM OR TITANIUM ALLOYS

The present invention relates to a process for direct forming of a semi-finished titanium product which is suitable for direct commercial use or further reprocessing. More particularly the present invention provides a process for direct forming of a semi-finished titanium product by drawing of titanium from liquid bismuth or low melting point alloy comprising bismuth and/or antimony and/or lead.

Although the present invention will be described with particular reference to semi-finished titanium products comprising titanium and bismuth, it is to be noted that the scope of the present invention is not limited to the described embodiment but rather the scope of the invention is more extensive so as to include semi-finished titanium products comprising titanium in combination with other metals or alloyed with other metals. Currently, commercial titanium products such as round sections, tubes and sheets are produced from titanium ingots and sponges using processes adapted from traditional metallurgical techniques. The ingots used in the processing are commonly obtained by refining titanium metal from titanium bearing ores by the Kroll process and less commonly, the Hunter process. The Kroll process is based on the reduction of titanium tetrachloride in the presence of magnesium at about 850°C to produce titanium sponges. The Hunter process is based on the reduction of titanium tetrachloride to titanium metal in the presence of sodium.

One of the problems associated with the traditional metallurgical techniques for commercial production of titanium products from titanium sponges and ingots is the reactive sensitivity of titanium, particularly at elevated temperatures. In particular, titanium is very reactive towards to oxygen and nitrogen. During processing of titanium sponges and ingots, it is necessary to take precautions to ensure that the titanium has not been contaminated by oxygen and nitrogen and other species which can introduce impurities. The precautions taken to deal with the problem of titanium reactivity contribute to production costs and place considerable limits on the economic possibilities of using titanium.

It has now been found that many of the problems associated with processing titanium sponges and ingots using metallurgical techniques of the past can be overcome by the process of the present invention for direct forming of semi-finished titanium products. The semi-finished titanium products of the present invention can be further worked by rolling, drawing, shaping, machining and so forth to provide finished titanium products. The process of the present invention is quicker, more flexible and more economical than traditional techniques even for relatively small production runs and the process can be adapted for continuous production.

Furthermore, despite teaching of the prior art that titanium powders cannot be drawn and that batch fusing of titanium powders can only be carried out by baking at high temperatures and in controlled atmospheres, the process of the present invention utilises both drawing and baking at relatively low temperatures using relatively mild processing parameters.

The present invention provides a process for direct forming of semi-finished titanium products comprising the steps of;

- introducing titanium into liquid bismuth or a liquid low melting point alloy comprising bismuth and/or antimony and/or lead, and

- drawing the titanium from the liquid to form the semifinished titanium product.

The titanium introduced to the liquid and/or the titanium drawn from the liquid may be in the form of titanium alloy.

Where used herein the term alloy refers to a mixture of combination of metals whether or not they are chemically joined.

During drawing the titanium or titanium alloy can be formed with one or more structures depending on the particular elements present and the level of the vacuum/pressure and temperature employed.

The present invention further provides semifinished titanium products produced by the process of the present invention. Typically the titanium introduced to the liquid bismuth or liquid low melting point alloy is in the form of particulate titanium such as titanium powder, ground titanium sponge. As stated above, alternatively the titanium may be in the form of titanium alloy, preferably a low melting point titanium alloy, which may be in particulate form. The titanium alloy may comprise or consist of impurities to be removed during processing. During continuous production of the semi-finished titanium product, titanium may be continuously supplied to the liquid low melting point alloy.

As stated previously the titanium is introduced to liquid bismuth or liquid low melting point alloy comprising bismuth and/or antimony and/or lead. Typically, where a liquid low melting point alloy is used, the alloy comprises bismuth and an element chosen from the group comprising selenium, antimony, zinc, lead, aluminium, tin and vanadium. More typically the low melting point alloy comprises bismuth and antimony.

The semi-finished titanium product may comprise titanium, or titanium and another metal or alloy.

Typically the semi-finished titanium product will comprise titanium and at least one of the components of the liquid low melting point alloy.

Typically the semi-finished titanium product will comprise particles of titanium or particles of titanium sponge. Other metals present in the semi-finished titanium product will be present as a mixture with the particles of titanium, as an alloy with the titanium and/or other metals or as part of an intermetallic compound with the titanium and/or other metals. For example, bismuth derived from the liquid bismuth or liquid low melting point alloy may be present in the semi-finished titanium product as a bismuth- titanium intermetallic species between the particles of titanium. This intermetallic compound sticks and binds the titanium particles together at low melting point.

Where a liquid low melting point alloy is used, metals (other than bismuth) which are present in the alloy may also be incorporated into the semi-finished titanium product. Accordingly, the composition of the low melting point alloy may depend on the desired composition of the semi-finished product. During continuous production of the semi-finished titanium product, it may be necessary to continuously replenish the component of the liquid low melting point alloy which is removed by incorporation into the semi-finished product. For example, typically the liquid low melting point alloy comprises metals such as aluminium, vanadium, molybdenum and tin which may be necessary for commercial products.

The use of a liquid low melting point alloy may be useful for sequestering or removing undesirable species which would otherwise be extruded or react with the titanium. For example, typically the liquid low melting point alloy comprises metals such as aluminium, vanadium or molybdenum which sequesters or react with oxygen and/or nitrogen which are present in the processing atmosphere or which are introduced with the titanium into the liquid low melting point alloy. Without wishing to be bound by theory- it is believed that the low melting point alloy can act like a "liquid sieve" which sequesters or reacts with gases such as nitrogen and oxygen, and other undesirable and/or reactive species. A particular element or species added to the low melting point alloy can modify the structure of the resultant semi-finished titanium product. Typically, additives such as selenium and zinc, together or separately will cause expansion of the pores in the semi-finished product such that the semi-finished product has an appearance like foam rubber. The drawing of the titanium from the liquid bismuth or liquid low melting point alloy may be carried out by many different processes. In one embodiment of the process of the present invention, the titanium is drawn using a vacuum column. Typically, the lower end of a vacuum column is partly immersed in a bath of liquid bismuth or low melting point alloy, and the pressure in the vacuum column is then reduced by opening the upper end of the column to a vacuum pump. Titanium particles gather at the immersed base of the vacuum column by virtue of the relatively low density of titanium. A combination of titanium and bismuth and possibly other elements, climbs to the upper, low pressure part of the column. The titanium and bismuth and other elements are thus drawn along the vacuum column to progressively form the semi-finished titanium product.

One of the advantages of using a vacuum column is that the width or volume of the column and the amount of vacuum applied can be varied to control the proportion and quantities of titanium, bismuth and other metals drawn out of the liquid and along the column.

The vacuum column facilitates segregation along its length. For example, without wishing to be bound by theory, it is believed that typically the TiBi at the bottom of the column will become Ti₃Bi . Furthermore, under vacuum and high temperature, the bismuth can evaporate and allow titanium crystals to grow the higher in the column they rises.

The semi-finished titanium product of the present invention can be drawn in any convenient form such as a tube, cylinder, or even a flat plate. When the semifinished product is a tube or rod, the tube or rod may be cut into sections, each section having the same or similar mass as an ingot of the prior art.

Preferably the drawing is carried out using a "toothed channel" extrusion column which has an internal passage which forms a toothed pattern on the inner and outer surfaces of the semi-finished titanium product. The toothed channel increases the surface area of the semifinished titanium product as compared with semi-finished product from a vacuum column having a smooth internal surface. The toothed pattern is particularly desirable if the drawing is followed by a degassing step because the increased surface area facilitates the exit of gas from the semi-finished product.

The semi-finished titanium product may be further worked. For example the semi-finished titanium product may be formed into a long tube by simple homeothetic reduction or rolled out to form round sections, bars, sheets or any other convenient form or shape.

The semi-finished titanium product may be directly used to manufacture a finished product. Alternatively the semi-finished product may be subjected to further processing steps which alter the composition, remove undesirable species or alter the physical characteristics (such as the porosity) of the semi-finished product. For example the semi-finished product may be subjected to further processing steps such as degassing and homogenisation.

In particular, further processing may be required to remove metal species such as magnesium or sodium from the semi-finished product. Typically, magnesium may be present in the semi-finished product if the titanium was produced using the Kroll process or sodium may be present if the titanium was produced using the Hunter process.

Typically the process of the present invention comprises a further degassing step, that is, a step which causes at least one species to diffuse out of the semifinished species as a gas. Degassing may be carried out by any convenient method but typically the degassing is carried out by heating the semi-finished product in a vacuum oven and reducing the pressure in the oven.

For example, where the semi-finished product comprises titanium in combination with bismuth, degassing may cause a certain amount of the bismuth to diffuse out of the product. It is believed that where the semi-finished product comprises titanium in admixture with bismuth, the degassing and diffusion of bismuth may permit the rearrangement of atoms in the semi-finished product to form an intermetallic compound or alloy such as Ti₃Bi. It is believed that similar rearrangements may occur in respect of other metals present in the semi-finished product.

Diffusion of gas out of the semi-finished product, may also increase the porosity or micro-porosity of the semi-finished product.

The process of the present invention may further comprise an expansion step in which the semi-finished product undergoes such an extensive diffusion of gas that the entire structure of the semi-finished product expands, thus converting the semi-finished product into an expanded porous product resembling foam. Where the semi-finished product is in the form of a tube, the expansion may be sufficient to fill the interior of the tube with porous expanded material. Typically, the extruded semi-finished product is submitted to a degassing step which causes partial degassing before the expansion step.

The expansion step may be carried out by many different methods but typically, expansion is carried out using a method similar to that used for degassing, but with higher power, vacuum and processing rates. Preferably the semi-finished product is pre-cooled before undergoing an expansion step. Typically, following the expansion step, the porous, expanded product is again subjected to a degassing step and the stabilized by being placed in a vacuum.

The process of the present invention may further comprise a step of homogenisation. During homogenisation the semi-finished product is held at high temperature, preferably under vacuum, until sufficient diffusion occurs within semi-finished product to provide a uniform composition and structure. The semi-finished product obtained by the process of the present invention has good high temperature properties which allows easy homogenisation. Homogenisation may be carried out by any convenient method known in the art such as moving an inductor along the semi-finished product, or moving the semi-finished product through an inductor. Alternatively, homogenisation may be achieved using laser or electronic bombardment methods .

Finally, the semi-finished product may be subjected to the step of superficial refusion by induction, application of a laser or any other convenient process.

The superficial refusion step causes the outermost pores to close over and seal, thus forming a skin on the surface of the semi-finished product. Where the semi-finished product has also been subjected to an expansion step, the product thus obtained is of particularly low density and has superior mechanical characteristics compared to products of the prior art, thus opening the way to new applications of titanium. The precise physical characteristics of the semi-finished product obtained will depend upon the exact processing parameters applied.

Most of the previously described steps can be achieved along the column but preferably in this case, the column is inclined upwards at an angle.

After the main production step using the inclined column, an open structured product is obtained. At this time, there is available the option of consolidating the structure (solid product) or expanding the structure further (foamed product) . The continuous product may then proceed under vacuum along a horizontal section of column where stabilization, degassing and skin formation may occur. The product may then continue along a downward sloping column which terminates in a second bath of low melting point alloy.

The second bath of low melting point alloy has two functions:

1. it contributes to ensuring the total integrity of the vacuum, and

2. it may allow cooling of the product

In a preferred embodiment of the process of the present invention, ground titanium sponge is introduced into a bath of liquid, low melting point alloy comprising bismuth. The lower end of a first inclined vacuum column is partly immersed in the liquid, and the pressure in the first vacuum column is then reduced by opening the upper end of the first vacuum column to a vacuum pump. Titanium gathers at the immersed base of the vacuum column by virtue of the relatively low density of titanium. Under pull of the vacuum, a combination of titanium particles and bismuth climbs to the upper, low pressure part of the column. The titanium and bismuth are thus drawn along the vacuum column to form a pre-consolidated semi-finished product which is extruded through a nozzle in the form of a tube and is later heated in a vacuum oven to de-gas. The degassed semi-finished product is then passed through an inductor to homogenise the structure and composition.

In a particularly preferred embodiment of the process of the present invention, titanium-bismuth alloy is introduced into a bath of liquid low melting point alloy comprising bismuth, zinc and selenium. The lower end of a first, inclined vacuum column is partly immersed in the liquid, and the pressure in the first, inclined vacuum column is then reduced by opening the upper end of the column to a vacuum pump. Segregation occurs along the first, inclined vacuum column such that at the top of the vacuum column, the titanium rich product is porous due to inductive heating having evaporated zinc, selenium, and bismuth present. The evaporated metals are condensed and recycled. This porous titanium product at high temperatures has insufficient strength to be pulled through the column. Hence a continuous piece of metal wire, preferably titanium wire, may be introduced through the first bath and the center of the column. This continuous titanium wire ensures sufficient strength for the product to proceed through the column and the process. The product then passes to a second, horizontal column where the product is degassed under vacuum.

The semi-finished product is degassed by being subjected to vacuum at elevated temperatures in a vacuum oven. The degassed extruded semi-finished product is then homogenised by being passing through an inductor and then exposed to a laser which causes superficial refusion, closing off pores at the surface and thus forming an outer skin on the semi-finished product. The process of the present invention will now be further described with reference to the following non- limiting examples.

Example 1 Titanium powder is introduced into a bath of liquid low melting point alloy comprising bismuth and zinc. The lower end of a vacuum column is partly immersed in the liquid, and the pressure in the column is then reduced by opening the upper end of the column to a vacuum pump. The low density of the titanium causes the powder particles to gather at the immersed base of the vacuum column and then a combination of titanium and bismuth climbs up the inside of the column to the uppermost part of the column. The titanium and bismuth is then drawn along the vacuum column to form the semi-finished titanium product.

Example 2

A semi-finished titanium product was produced by the process described in Example 1 was degassed by placing the semi-finished product in a vacuum oven at elevated temperatures. During degassing, the titanium and bismuth of the semi-finished product was formed into a continuous alloy of Ti₃Bi and residual bismuth diffused out of the structure.

The semi-finished titanium product of examples 1 and 2 differed in so far as the product of example 1 comprised an admixture of titanium and bismuth in combination with some intermetallic species while the product of example 2 was comprised almost entirely of Ti₃Bi.

Example 3

A semi-finished titanium product produced by the process described in Example 1 was placed in a vacuum oven at elevated temperatures such that the pores dilated and the product expanded substantially and increased in porosity until resembling foam. The method used for the expansion step was similar to the method used for degassing semi-finished titanium product but the power, vacuum and relative processing rates used were substantially higher than for the degassing performed according to Example 2. The semi-finished product of examples 2 and 3 differed in so far as the product of example 3 had an open, porous structure resembling foam and concomitantly lower density than the less porous structure of the product of example 2.

Example 4

A semi-finished titanium product was produced by the process described in Example 1 was placed in a vacuum oven at elevated temperatures and subjected to degassing, then passed through an inductor to homogenize the composition and then subjected to a final diffusion treatment under vacuum.

The semi-finished product of examples 1, 2 and 4 differed in so far as the product of example 4 had a lower level of impurities (and a higher proportion of titanium and bismuth) compared to the products of examples 1 or 2. Example 5

A semi-finished titanium product was produced by the process described in Example 4 and then subjected to superficial refusion.

The semi-finished product of examples 4 and 5 differed in so far as the product of example 5 comprised a continuous non-porous skin while the product of example 4 had a porous surface.

Example 6

Titanium powder is introduced into a bath of liquid low melting point alloy comprising bismuth and zinc. The lower end of an upwardly inclined vacuum column is partly immersed in the liquid, and the pressure in the column is then reduced by opening the column to a vacuum pump. The particles of titanium powder gather at the immersed base of the vacuum column and then a combination of titanium and bismuth climbs up the inside of the vacuum column to the uppermost part of the column. The semifinished titanium product at this point has a very open structure. The structure is further expanded until it has the appearance of foam by subjecting the product to expansion under high vacuum and high temperatures. The semi-finished titanium product is then drawn along a substantially horizontal vacuum column where it is subjected to degassing by application of vacuum, homogenisation by being passed through an inductor, and finally, superficial refusion. The semi-finished titanium product thus formed was suitable for direct supply to a commercial process.

While the invention has been explained in relation to its preferred embodiments it is to be understood that various modification thereof will become apparent to those skilled in the art upon reading the specification. Therefore, it is to be understood that the invention disclosed herein is intended to cover such modifications that fall within the scope of the appended claims.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A process for direct forming of semi-finished titanium product comprising the steps of: - introducing titanium into liquid bismuth or liquid low melting point alloy comprising bismuth and/or antimony and/or lead, and drawing the titanium from the liquid to form the semi-finished titanium product.

2. A process according to claim 1 wherein the titanium introduced to the liquid, low melting point alloy is in the form of titanium powder, ground titanium sponge and/or titanium alloy.

3. A process according to claim 2 wherein the titanium alloy is a particulate, low melting point alloy comprising bismuth.

4. A process according to any one of the preceding claims wherein the low melting point alloy further comprises a metal chosen from the group comprising selenium, antimony, zinc, lead, aluminium, vanadium, molybdenum, and tin or combinations thereof.

5. A process according to any one of the preceding claims wherein at least one of the components of the liquid low melting point alloy are drawn with the titanium to form the semi-finished titanium product.

6. A process according to any one of the preceding claims wherein the liquid low melting point alloy comprises structure modifying additives chosen from the group comprising selenium and zinc or a combination thereof.

7. A process according to any one of claims 3 to 6 wherein the semi-finished titanium product comprises titanium and bismuth.

8 A process according to any of the preceding claims wherein the titanium is drawn from the liquid low melting point alloy by vacuum.

9. A process according to claim 8 wherein the titanium is drawn from the liquid low melting point alloy by an inclined vacuum column.

10. A process according to claim 9 wherein the titanium is subsequently drawn along a horizontal vacuum column.

11. A process according to claim 9 or 10 wherein the titanium is subsequently drawn along a downward vacuum column into a second liquid low melting point alloy.

12. A process according to claim 8 wherein the titanium is drawn from the liquid low melting point alloy along a single vacuum column having a first upwardly inclined section, a second substantially horizontal section and third downward section wherein the titanium passes through the three sections of column into a second low melting point alloy.

13. A process according to any one of the preceding claims wherein a titanium wire passes along the inside of the column or columns .

14. A process according to any one of the preceding claims which further comprises a degassing step.

15. A process according to any one of the preceding claims which further comprises a homogenising step.

16. A process according to any one of the preceding claims which further comprises an expansion step.

17. A process according to any one of the preceding claims which further comprises a superficial refusion step.

18. A process according to any of claims 9 to 13 wherein the product in the inclined vacuum column or inclined section of vacuum column is subjected to an expansion step.

19. A process according to any of claims 9 to 13 wherein product in the horizontal or downward vacuum column or horizontal or downward sections of vacuum column are subjected to degassing and/or homogenisation and/or superficial refusion steps.

20. An apparatus for use in the process of claim 8 comprising a container for the liquid bismuth or liquid low melting point alloy comprising bismuth and/or antimony and/or lead and an upwardly inclined vacuum column for drawing the titanium from the liquid low melting point alloy, wherein the lower end of the first upwardly inclined section opens into liquid bismuth or liquid low melting point alloy held within the reservoir.

21. An apparatus for use in the process of claim 8 comprising a container for the liquid bismuth or liquid low melting point alloy, and a vacuum column for drawing the titanium from the liquid low melting point alloy, the vacuum column having a first upwardly inclined section, and a second substantially horizontal section and third downward section, wherein the lower end of the first upwardly inclined section opens into the reservoir.

22. An apparatus according to claim 21 wherein the three sections of the vacuum column are integral.

23. An apparatus according to claim 21 or claim 22 which further comprises a metal wire located inside the column.

24. A semi-finished titanium product produced by the process of any of claim 1 to 19.

25. A semi-finished titanium product produced by the process of any one of claims 1 to 18, characterized in that the product comprises a skin.

26. A process substantially as herein described with reference to the examples .

27. A semi-finished titanium product substantially as herein described with reference to the examples.