GB2575005A

GB2575005A - A process and method for producing titanium and titanium alloy billets, spherical and non-spherical powder

Info

Publication number: GB2575005A
Application number: GB201720892A
Authority: GB
Inventors: Machaka Ronald; Matheti Motsai Tebogo; Chikosha Silethelwe; Nyongesa Machio Christopher
Original assignee: Council for Scientific and Industrial Research CSIR
Current assignee: Council for Scientific and Industrial Research CSIR
Priority date: 2017-12-14
Filing date: 2017-12-14
Publication date: 2020-01-01
Anticipated expiration: 2037-12-14
Also published as: ZA202003526B; WO2019118991A1; GB2575005B; GB201720892D0

Abstract

A solid-state method of making titanium and titanium alloy billets or ingots by introducing titanium feedstock powder formed from titanium sponge, hydrogentated-dehydrogenated titanium, titanium hydride, commercially pure titanium and/or a titanium based alloy, optionally blended with alloying elements into a capsule and solid state sintering the encapsulated powder to produce either a dense billet/ingots or a frangible billet without pre-pressing the feedstock powder. Powder can be made by vacuum or electrode induction gas atomizing dense billets or by crushing, pulverizing and/or milling frangible billets. The crushed powder can be spheroidised using a plasma. Alternatively the billets/ingots can be rolled, extruded or forged. The capsule can be made of alumina, yttria, zirconia or graphite and can itself be encapsulated in steel, with sintering being in air, vacuum or argon.

Description

A PROCESS AND METHOD FOR PRODUCING TITANIUM AND TITANIUM ALLOY BILLETS, SPHERICAL AND ΝΟΝ-SPHERICAL POWDER

Field of the Invention

The invention relates to a process for producing titanium and titanium-based alloy billets, and spherical and non-spherical powders.

Background to the Invention

The current standard procedure for producing titanium and titanium alloy billets starts with titanium sponge fines and is shown in Figure 1.

In the current art, according to Titanium Metals Corporation of America (TiMet) USA Patent No. US4,728,364A, a titanium sponge is typically converted to titanium alloy products, including near net shape and mill products and powder products, by a series of processes.

The prior art process operations typically follow the following steps:

• Initially, the titanium sponge with alloying particles is provided in particle form. If needed, various alloying elements may be introduced in admixture with the sponge particles in desired amounts for producing titanium-base alloys. These unalloyed or admixtures are what we may call a starting feedstock material.

• The feedstock material is typically pressed to compact the particles to form compacts or briquettes that are joined by welding to produce a consumable electrode.

• The consumable electrodes are vacuum arc melted (VAR) to produce a dense titanium-based ingot. The ingots are typically re-melted once or twice (double or triple VAR) to produce a final ingot. The melting is commonly performed in a protective atmosphere such as vacuum and inert gas environments. The re-melting is commonly performed twice in order to achieve sufficiently high chemical homogeneity and low porosity levels. Alternatives to VAR melting processes include electro-slag, double electrode, electron beam, plasma melting and zone melting methods.

• The ingot resulting from either double or triple vacuum arc melting is then typically processed by thermomechanical processing (a series of forging and annealing steps) to form from ingot products such as billets - often known as the wrought billets.

• The wrought formed billet can either be forged or machined to a desired near-net shape product or worked further to produce mill products such as, but are not limited to, billets, bars, rods, wire, tubes, slabs, plates, sheets, and foils. In addition, wrought formed billet can be the atomized to produce spherical powders.

The current art for producing titanium billets is expensive, energy-intensive, and hazardous - all due to the need to work with metal melts and the re-melting of billets/ingots to/at temperatures in excess of 1700 °C.

It can be seen from Figure 1 that billets/ingots formed by thermomechanical processing are both products and starting workpieces for further processing.

Summary of the Invention

The invention provides a process for producing both frangible and dense billets.

The billets may be produced by sintering loose powder in a mould of suitable shape. The mould can be made from any suitable material, e.g. ceramic. Further, the mould can be encapsulated in an enclosure made from any suitable material, e.g. steel.

The sintering may be done in an open air or vacuum or argon furnace.

The frangible billets may be crushed and/or pulverized to non spherical powders. The non spherical powders of the desired particle size distributions can find powder metallurgical applications. Further, suitable particle size distributions of the non spherical powders can be spheroidized into highly spherical powders using any commercially available plasma spheroidization equipment.

The dense billets may be atomized into highly spherical powders using any commercially available atomization equipment. For all purposes, dense billets mean billets that can withstand handling without breaking. Their theoretical density can vary between 30 and 100%.

The powders so produced may possess similar characteristics to commercially available powders, but cost less, because the process described in this invention uses fewer steps than the current commercial process. The process is not limited to any particular powder feedstock: it can be used with powders of different morphologies, and is amenable, but not limited to, titanium, titanium and its alloys. More specifically, so long as alloys contain titanium as a principal or alloying element, typically processed by way vacuum arc melted (VAR) or in any other protective environment, materials or alloys to which an uncontrolled or noninert environment will introduce undesirable properties, other materials such as cermets and hybrid material systems.

Thus, in accordance with the invention, there is a process for producing billets that are crushable to produce irregular (non spherical) powders that along with some billets can be spheroidized or atomized into high quality but cheap highly spherical powders having similar properties to available commercial powders.

The invention provides a process for producing dense titanium and titanium alloy billets or ingots through a melt-free solid-state sintering process, wherein the process comprises:

• the introduction of a feedstock powder material of the Ti sponge, TiHDH, TiH2, CP Ti or their blends and/or blends of these with other alloying elements;

• the introduction of the feedstock powder material in to a mould capsule without pre-pressing of material. If needed, tapping or vibration can be done to the feedstock material in the mould to improve densification; and • subject the mould capsule to thermal processing in solid state i.e. sintering, to produce either dense billets/ingots or frangible billets without attaining melting of the feedstock material.

The mould capsule may be:

• a ceramic mould such as, but not limited to, AI2O3, Y2O3, ZrO2 moulds for sintering in inert environment; and/or • a graphite mould lined with titanium, molybdenum or any other suitable mould for sintering in inert environment; and/or • a ceramic mould further encapsulated in metal such as steel and sintered in open air environment and/or

The invention extends to a method for producing a variety of titanium and titanium-based alloy mill products such as sheets, wires, seamless tubes, welded tubes, bars and forgings from the billets/ingots above using thermomechanical processing (rolling, extrusion, forging, or otherwise).

The invention extends to a method for producing a variety of titanium and titanium alloy spherical powders from said dense billets/ingots, using vacuum induction melting and gas atomizing (VIGA), plasma atomizing, and electrode induction melting gas atomization (EIGA).

The invention extends to a method for producing a variety of titanium and titanium alloy shaped products, parts, or components from the dense billets/ingots using a machining processing such as turning or milling.

According to a further aspect of the invention, there is provided a method for producing frangible titanium and titanium alloy billets through a melt-free solidstate homogenization process; wherein the process comprises:

• introducing the feedstock powder material in to a suitable mould capsule without pre-pressing of material; and • subjected the feedstock mould capsule to thermal processing in solid state composition homogenization to produce either a fragile billet without attaining melting or sintering of the feedstock material.

According to a further aspect of the invention, there is provided a method for producing a variety of titanium and titanium alloy non-spherical powder from said frangible billets by way of crushing processes such as pulverizing, milling, mechanical alloying, etc.

The method for producing a variety of titanium and titanium alloy spherical powder from the non-spherical powder recovered from said frangible billets may use spheroidization or similar techniques, such as plasma-based spheroidization.

Description of Embodiments of the Invention

Embodiment 1: Method to produce dense titanium and titanium-based alloy billets/ingots through a melt-free solid-state sintering process;

Embodiment 2: Method for producing a variety of titanium and titanium-based alloy mill products such as, but are not limited to, billets, bars, rods, wire, tubes, slabs, plates, sheets, and foils, from the billets/ingots from Embodiment 1 using thermomechanical processing (rolling, extrusion, forging, or otherwise);

Embodiment 3: Method for producing a variety of titanium and titanium-based alloy spherical powder from the dense billets/ingots from Embodiment 1, using atomization techniques such as vacuum induction melting and gas atomizing (VIGA), plasma atomization and plasma rotating electrode process (PREP), and electrode induction melting gas atomization (EIGA);

Embodiment 4: Method for producing a variety of near net or net shaped titanium and titanium-based alloy products from the dense billets/ingots from Embodiment 1 using machining processing (e.g. turning or milling);

Embodiment 5: Method to produce frangible titanium and titanium-based alloy billets through a melt-free solid-state sintering-free homogenization process;

Embodiment 6: Method for producing a variety of titanium and titanium-based alloy non-spherical powder from the frangible billets/ingots from Embodiment 5, by way of crushing processes (such as pulverizing, milling, mechanical alloying, etc.); and

Embodiment 7: Method for producing a variety of titanium and titanium-based alloy spherical powder from the frangible billets/ingots from Embodiment 6 using plasma-based spheroidization.

Brief description of the drawings

While aspects of the present invention are described herein with reference to illustrative embodiments for sometimes particular applications, it should be understood that the invention is not only limited thereto. Those having ordinary skill in the art and having access to the teachings provided herein will recognize additional modifications, applications, embodiments, and circumventions within the scope thereof and additional fields in which the present invention would be of significant utility.

The aspects of invention described herein may be better understood by reference to Figures 1 and 2, wherein the current art and the current invention are illustrated. In Figure 2 the flow chart also shows some steps in the current art circumvented in the present invention. The current invention ends up with similar products to those produced by the current art.

The Figure 1 process illustrates an approach for processing a starting feedstock material (Article 10) such as a titanium sponge is typically converted to titanium alloy products (Articles 20), including near-net shaped products, mill products and powder products, by a series of processes. The process operations typically follow the following steps, the starting feedstock material a titanium sponge with either admixtures of alloy particles is provided in particle form if needed (Article 10). The feedstock material is typically pressed to compact the particles to form compacts or briquettes that are joined as by welding to produce a consumable electrode (Step 12). In Step 14, the pre-pressed consumable electrodes are vacuum arc melted (VAR) to produce a dense titanium-based ingot which is typically re-melted once or twice (double or triple VAR), in a protective atmosphere, to produce a final ingot. The resulting ingot is then typically processed by a series of forging and annealing steps (thermomechanical processing) to form ingot products such as billets - often known as the wrought billets (Step 16). The wrought formed billet can be processed further to produce a variety of titanium alloy products (Steps 18 and 20); i.e. either by forging or machining to a desired near-net shape product or cold or hot working further to produce mill products such as, but are not limited to, billets, bars, rods, wire, tubes, slabs, plates, sheets, and foils. In addition, wrought formed billet (Step 18) can be atomized to produce spherical powders.

The process of Figure 2 depicts an approach for processing a starting feedstock material (Article 22), this can be a Ti sponge, TiHDH, TiH₂, CP Ti, blends of (Ti sponge, TiHDH, TiH₂, CP Ti), Ti-based alloys or blends of these with other alloying elements. The feedstock material is introduced first in a suitably shaped mould (Step 24). While circumventing the conventional pre-pressing (Step 18) and said double or triple vacuum arc melting (Step 20), the feedstock material is then subjected to solid-state thermal processing (Step 30) in order to produce either dense billets/ingots (Article 32) or frangible billets (Article 34). The solid state thermal processing (Step 30) takes the form of sintering, above the beta transits temperature but below melting in the production of dense billet/ingot articles (32). The dense ingots/billets (Article 32) can further be converted into mill products, machine shaped parts (Articles 36), or atomized spherical powders (Articles 38) as known and demonstrated in Figure 1. While the frangible billet (Article 34) can produce non-spherical powder (Articles 40) by way of crushing, pulverizing, milling, mechanical alloying or any other appropriate way and spherical powders (Article 42) by way of plasma-based spheroidization of the said non-spherical powder (Articles 40), respectively.

For more illustrative examples, Articles 32, 34, 36, 38, 40 and 42 are also demonstrated in Figures (3 to 7) below, respectively.

In Figure 2, the current disclosure is in the process flowchart blocks 24, and 30 to 38, compared to the current art in process flowchart blocks 22, 26 and 28.

The flowchart, Figure 2, illustrates the implementations of the present invention and the current art. Figure 2 demonstrates the circumvented costly aspects, of pre-compaction and melting (VAR), used in the current art for producing titanium and titanium-based billets and spherical powder.

Noteworthy is the circumvention of the pre-compaction and melting (such as up to 3 times of vacuum arc re-melting, VAR and the freedom to starting the process with a custom feedstock powder material (Ti sponge, TiHDH, TiH₂, CP Ti, blends of (Ti sponge, TiHDH, TiH₂, CP Ti), Ti-based alloys or blends of these with other alloying elements) of choice.

In Figure 3, a titanium-based dense billet/ingot produced via solid state sintering route (Article 32) is shown.

In Figure 4, a frangible billet (Article 34) is shown in (4a) and (4b) shows the preparation of a titanium-based feedstock suitable for spheroidization.

In Figure 5, a titanium-based spherical powder (Article 36) produced via the EIGA atomization of the solid state sintered dense billet (Article 32).

In Figure 6, an example of a sheet, a mill product (Article 36) produced by cold rolling dense billet (24) is shown.

In Figure 7, a non-spherical titanium-based alloy powder (Article 38) (a) which is produced from the frangible billets/ingots (Article 34);Ti6AI4V powder (Article 38) is shown as well one produced via spheroidization of powder from frangible billet (Article 34) in (b).

Detailed description of Embodiments of the Invention

The first embodiment of this invention disclosure is the process for producing titanium and titanium alloy billets. Our approach to producing titanium and titanium alloy billets and spherical powder from the said billets is schematically illustrated in Figure 2. The implementation of the present invention specifically circumvents the compaction, vacuum arc re-melting and ingot breakdown forging steps which have become a standard in the titanium and titanium alloy production for more than 50 years and typically energy-intensive and requires a high-capital investment. The said solid billets are produced by loading powder feedstock with particle sizes of up to a few millimetres into a mould. The mould set-up is then placed in a furnace for heat treatment at temperatures ranging from 700°C to 1400°C for durations ranging from 15 minutes to 10 hours in a furnace with controlled environment, which can either be vacuum or argon. An open air furnace can also be used if the mould is encapsulated in a metal enclosure. Other environments can be used depending on the feedstock powders.

In the current example, the powder feedstock was titanium-based alloy. During the heat treatment, there is spontaneous loose titanium powder pressure-less solid-state sintering (or thermal processing) to produce two types of billets of the desired alloys; a dense billet (Article 32) and a frangible billet (Article 34). In line with the fore-mentioned production of mill products as illustrated in Figure 1, the mill products, defined here as rods, plates, sheets, billets, and bars can be produced therefrom via metal working fabrication processes as forging, rolling, 5 and extrusion - these metal working fabrication processes are well established in the field.

The second embodiment of the present invention disclosure is the production of titanium and titanium alloy spherical powder from the dense billets (32) in 10 Embodiment 1, via gas atomization processes. Gas atomization is a well-known process and will not be described here. The constraint is that the billet (32) should have a suitable diameter and length. Embodiment 1 is able to produce these billets (32) because of the versatility of the process. Figure 5 is an example of the spherical powders (Article 36 in Figure 2) that can be produced. The 15 powders shown are Ti6AI4V (wt. %). Table 1 shows that the powders had a Ti6AI4V chemical composition.

Table 1: Overall composition of TI6AI4V powder produced by the process described in Embodiment X

The Ti6AI4V powders ((Article 36), Figure 5) can be used for building components using additive manufacturing.

The third embodiment of the present invention disclosure is the production of titanium and titanium alloy spherical powder from the frangible billets (Article 34 in Figure 2) via the plasma spheroidization process. In this embodiment, Frangible Billet (Article 34) is crushed to form a feedstock material (Article 22) that is an irregular powder of up to 10mm. The irregular powder is then pulverized in a suitable pulveriser for between 1 minute and 1 hour to reduce size to a suitable value, of between 5microns and 1000 microns. This powder is then subjected to a plasma in a plasma spheroidization facility to produce powders with a spherical shape (morphology) ((Article 38) in Figure 2). The Frangible Billet ((Article 34), Figure 3)) is produced as described for the Dense Billet (Article 32).

The benefits of the current invention are, but not limited to,

1. Reduced steps to achieving a similar product from conventional processing

2. Ability to use a wide range of powders in terms of type (sponge, cp-Ti, TiH2, TiHDH or blends of), morphology and particle size

3. Ability to use different environments for thermal processing such as argon, vacuum and open air furnace

4. Expected cost reduction due to the absence of melting

Claims

1. A versatile process for producing dense titanium and titanium alloy billets or ingots through a melt-free solid-state sintering process; wherein the process comprises:

• the introduction of a feedstock powder material of the Ti sponge, TiHDH, TiH2, CP Ti their blends and/or blends of these with other alloying elements;

• the introduction of the feedstock powder material in to a mould capsule without pre-pressing of material; and • subject the mould capsule to thermal processing in solid state (sintering) to produce either a dense billet/ingots or a frangible billet without attaining melting of the feedstock material.

2. A process as claimed in claim 1, wherein regarding the mould capsule:

• a ceramic mould is used such as, but not limited to, AI2O3, Y2O3, ZrO2 moulds for sintering in inert environment;

• a graphite mould is lined with titanium, molybdenum for sintering in inert environment; and • a ceramic mould is further encapsulated in metal such as steel and sintered in open air environment.

3. A process as claimed in claim 1, wherein a variety of titanium and titanium-based alloy mill products such as sheets, wires, seamless tubes, welded tubes, bars and forgings from the billets/ingots are produced using thermomechanical processing.

4. A process as claimed in claim 1, wherein a variety of titanium and titanium alloy spherical powders are produced from the dense billets/ingots using one or more of vacuum induction melting and gas atomizing (VIGA), plasma atomizing, and electrode induction melting gas atomization (EIGA).

5. A process as claimed in claim 1, wherein a variety of titanium and titanium alloy shaped products, parts, or components are produced from the dense billets/ingots using machining processing.

6. A method for producing frangible titanium and titanium alloy billets through a melt-free solid-state homogenization process; wherein the process comprises:

• introducing the feedstock powder material in to a suitable mould capsule without pre-pressing of material; and • subjecting the feedstock mould capsule to thermal processing in solid state composition homogenization to produce either a fragile billet without attaining melting or sintering of the feedstock material.

7. A method as claimed in claim 6, wherein a variety of titanium and titanium alloy non-spherical powder are produced from the frangible billets by way of crushing processes selected from the group including pulverizing, milling, and mechanical alloying.

8. A method as claimed in claim 7, wherein a variety of titanium and titanium alloy spherical powder are produced from the non-spherical powder recovered from the frangible billets using spheroidization or similar techniques.

9. A method as claimed in claim 8, wherein the spheroidization technique is plasma-based spheroidization.