TITANIUM ALLOYS

Field of the Invention

The present invention relates to a method of forming metal pipes and relates particularly to forming seamless metal pipes. More particularly, the invention relates to forming pipe of titanium and/or titanium alloys.

Background

One method for forming metal pipes involves forming metal plate, and rolling the plate to bring opposite edges together to form a tube . The edges are welded together forming a seam. This technique is not expensive and there is little waste produced. Pipes are typically formed of steel due to its tensile strength which provides pipes with a high band strength . This means that steel pipes are well suited to high pressure applications. Steel is also used due to its low cost relative to other metals and alloys and due to its weldability which makes steel well suited to forming pipes with seams by the method of rolling and welding plates .

Weldability is also important in the context of joining lengths of pipe together because pipe is typically manufactured off-site in discrete lengths, then

transported and assembled on-site. Steel can be welded in the open atmosphere , so in a practical sense the assembly process is relatively simple. Steel, however, is not well suited to highly corrosive environments , such as in chemical processing plants , and must be replaced in regular maintenance operations . Replacing steel pipe can disrupt plant operations and can have a significant impact on overall plant productivity . Accordingly, forming pipes of materials that are more corrosion resistant reduces the frequency of maintenance operations to replace pipe. For this reason, titanium or titanium alloys can be chosen to replace steel .

Manufacturing titanium or titanium alloy is difficult because titanium and titanium alloy has a high affinity for oxygen and readily forms surface oxides. This means that welding titanium or titanium alloy plate to form pipe is difficult because welding must be performed in an inert atmosphere to prevent oxides occurring in the weld seam which will degrade the strength of the weld. If the weld is compromised by oxides, the pipe will not have

sufficient band strength to be suitable for high pressure applications. In addition titanium and titanium alloys have a hexagonal atomic structure and is more difficult to roll into tubes than steel .

Accordingly, forming titanium and titanium alloy pipe by rolling and welding is costly due to the steps required to ensure that the weld seam is free of oxides. In addition, the same problem applies to joining together sections of titanium or titanium alloy pipe by welding in order to form lengths required in, for example, chemical processing plants .

A current alternative method for forming titanium and titanium alloy pipe without welding involves punching a core out of a solid bar of titanium or titanium alloy to leave a seamless pipe. Being a seamless pipe, there is no weld to compromise the band strength. However, forming pipe by this method requires specialised equipment to generate the considerable forces required to punch through a bar of titanium or titanium alloy . The equipment is costly and, therefore, contributes to the overall cost of the pipe. Accordingly, titanium or titanium alloy pipe produced by this method is

considerably more costly than steel pipe.

The cost is also affected by the waste produced in forming the pipe. Specifically, the core of titanium or titanium alloy must be melted down and reformed as a solid bar before being punched again to form another pipe . This process is on-going as cores are melted and bars are reformed. Processing of cores adds to the overall cost of the final formed pipe .

An alternative method for forming seamless titanium or titanium alloy pipe, albeit in short sections, is

disclosed in International patent application

PCT/AU2009/000276 in the name of Commonwealth Scientific and Industrial Research Organisation ("CSIRO") . The contents of the CSIRO International application are incorporated herein by this reference.

This alternative method involves spraying fine particles of titanium or titanium alloy onto a cylindrical

substrate . The particles are sprayed at supersonic velocities and bond together upon impact to build up a layer of titanium or titanium alloy. The extent of the build up determines the wall thickness of the pipe .

An advantage of this method is that the layer is

continuous over the length of the cylindrical substrate so that the layer forms a seamless pipe . The band strength of the pipe is , therefore , well suited to high pressure applications . While the pipe is seamless, the overall length of the pipe is limited by the length of the substrate, typically about a metre. Accordingly, this alternative method does not produce titanium or titanium alloy pipe in lengths that are practical as a replacement for long sections of steel pipe. Short sections of titanium or titanium alloy pipe may be used to form an overall longer pipe. However, there is considerable cost involved in assembling a longer pipe because the short sections are difficult to weld together. While this alternative method can produce titanium or titanium alloy pipe more cost effectively than other methods, this cost benefit is off-set by the cost

associated with assembling lengths of pipe from a series of shorter sections . There is a desire to produce longer sections of titanium or titanium alloy pipe to reduce the practical

disadvantages of assembling short sections .

Summary of Disclosure

In a first aspect, a method of forming sections of seamless titanium or titanium alloy pipe is provided. The method comprises the steps of : (a) providing an elongate substrate for forming a pipe;

(b) spraying particles of titanium or titanium alloy onto the substrate to cause the particles to bond together and to begin forming a pipe; and

(c) moving formed pipe longitudinally relative to the

pipe-forming substrate to remove formed pipe from the pipe-forming substrate and continuing to spray titanium or titanium alloy particles to cause further pipe to form continuously and integrally with the formed pipe, thereby enabling formation of a seamless titanium or titanium alloy pipe of a desired length. The term "pipe-forming substrate" is a reference to a surface portion of a substrate. An underlying portion of the substrate may be formed of a different material , include heating or cooling structures or may be hollow .

The method enables formation of titanium or titanium alloy pipe to a desired length because the pipe is continuously formed and removed from the substrate . For practical purposes, the pipe may be formed in lengths suitable for transport, such as up to 16 metres or longer, or may be continuously formed and cut to predetermined lengths after the desired length has moved from the substrate during the forming process .

The method, in conjunction with the spray-forming method disclosed in the CSIRO International application, also enables pipes to be formed with a diameter in the range of lmm to 1000mm (typically) . In addition, the method, in conjunction with the spray-forming method disclosed in the CSIRO International application, enables pipes to be formed with a wall thickness in the range of 0.1mm to 50mm (typically) . The applicant recognises that an important aspect of moving the formed pipe relative to the pipe-forming substrate is the extent to which the formed pipe bonds to the pipe-forming substrate. Without wishing to be bound by any particular theory, experimental work carried out by the applicant suggests, and it is the belief of the applicant, that bonding is affected by the following factors :

(a) differential thermal expansion of the formed pipe and the substrate;

(b) surface roughness of the substrate; (c) chemical bonding between the formed pipe and the substrate; and (d) titanium or titanium alloy "particle relaxation" .

It is not clear to what extent each of these aspects contribute to overall bonding between the formed pipe and the substrate .

In view of this belief, the applicant anticipates that movement of the formed pipe relative to the pipe-forming substrate may be achieved by controlling the extent of bonding between the formed pipe and the pipe-forming substrate. It should be understood, however, that

alternative options that enable movement of the formed pipe relative to the pipe-forming substrate are

encompassed by the subject invention. With the above factors in mind, the method may involve controlling the extent of bonding between formed titanium or titanium alloy pipe and the pipe-forming substrate to enable formed pipe to be moved relative to the substrate. In regard to factor (a) , it is believed that thermal bonding occurs when the titanium or titanium alloy

particles are sprayed onto the pipe-forming substrate. In particular, it is believed that thermal bonding occurs if the pipe-forming substrate expands more than the formed pipe in the course of being exposed to the spray carrier gas which is at an elevated temperature .

One option for counteracting the thermal bonding effect may involve controlling the extent of bonding by heating the formed titanium or titanium alloy pipe to cause differential thermal expansion of the formed pipe relative to the pipe-forming substrate, thereby releasing the formed pipe from the pipe forming substrate and enabling the formed pipe to be moved relative to the pipe-forming siαbstrate . The thermal differential may be caused

preferentially by heating the formed titanium or titanium alloy pipe . Alternatively , the thermal differential may be caused by cooling the pipe-forming substrate causing a thermal differential between the formed pipe and the pipe forming substrate relative to the pipe . Another option for counteracting the thermal bonding effect may involve controlling the extent of bonding by selecting a substrate having a co-efficient of thermal expansion that is less than the co-efficient of thermal expansion of the titanium or titanium alloy.

The substrate may be ceramic, glass, metal or composite.

As an example suitable substrates may be : Pyrex®

(borosilicate glass) , fused silica, diamond or tungsten.

In regard to factor (b) , the applicant believes that surface morphology of the pipe-forming substrate affects the extent of bonding with the formed pipe. In particular, the applicant believes that the titanium or titanium alloy particles fill surface relief on the pipe-forming

substrate with the effect that, at least in the

longitudinal direction along the substrate, the formed pipe and the pipe-forming substrate mechanically

interlock .

Accordingly, the applicant further believes that an option for reducing the impact of surface morphology and, hence overall bonding, may involve controlling the extent of bonding by selecting a substrate having a surface

roughness to reduce mechanical bonding between the formed pipe and the substrate. Typically the surface roughness may provide low mechanical bonding in a longitudinal direction of the substrate to enable formed piped to be removed from the substrate. The average surface roughness may be R _a < l.Oum.

Preferably, the surface roughness may be R _a < 0.5μm.

In regard to factor (c) , the applicant believes that bonding may be affected by the chemical affinity of the formed pipe to the pipe-forming substrate.

Accordingly, it is believed that an option for reducing the effect of chemical bonding may involve controlling the extent of bonding by selecting a substrate having a low potential for chemically bonding with titanium or titanium alloy.

The substrate may be formed of a material that has little or no chemical potential for bonding with titanium or titanium alloy.

In regard to factor (d) , the applicant believes that bonding is affected by mechanical reactions of titanium or titanium alloy alloys particles impacting on the pipe- forming substrate or on a section of forming pipe. Again, without wishing to be bound by any particular theory, the applicant believes that the titanium or titanium alloy particles elastically deform on impact by flattening to an extent. For example, generally spherical particles deform to produce a disc or elongated shape. It is though that, while in that deformed shape, the particles are impacted with and bind with other particles that are also

elastically deformed. After impact and binding, the resiliency of the elastic particles provides a tendency to revert to their original shape. The particles, however, bind together while in an expanded shape so the resilience manifests as a contraction of the formed pipe about the siαbstrate .

For convenience, this effect will be referred to by the term "particle relaxation" .

In order to counteract the effect of particle relaxation, the method may involve initially spray forming pipe on a first pipe-forming substrate dimensioned to form the pipe with a predetermined diameter and transitioning the particle spray to a second pipe-forming substrate that is smaller than the first pipe-forming substrate so that the second pipe-forming substrate is in contact with the forming and formed pipe in only a section of a side wall of the forming and formed pipe, whereby rotation of the forming pipe and the second pipe-forming substrate cause divergence between them.

As the second pipe-forming substrate is dimensioned to be smaller than the pipe, particle relaxation will not result in the formed pipe contracting onto the pipe-forming substrate . The tensile strength of the forming pipe as it is integrally formed with the formed pipe causes the forming pipe to lift off the second pipe-forming substrate as they diverge .

The advantage of this option is that thermal and particle relaxation factors are reduced or resolved. In accordance with this option, the first pipe-forming substrate is moved longitudinally away from the second pipe-forming substrate in order to continuously form pipe on the end of the formed pipe . Additional pipe thickness may be built up by a plurality of particle spraying sources operating simultaneously. The method may involve forming a layer of titanium or titanium alloy as a transition section between an

initiating substrate and the pipe-forming substrate such that the tensile strength of the transition section is greater than the bond strength between the titanium or titanium alloy and the pipe-forming substrate.

The method may involve selecting the initiating substrate to cause the transition section to bond to the initiating substrate, whereby moving the initiating substrate relative to the pipe-forming substrate causes pipe formed on the pipe-forming substrate to move relative to the pipe-forming substrate, thereby removing formed pipe from the pipe-forming substrate.

Spraying particles of titanium or titanium alloy in steps (b) and (c) may be in accordance with a cold-spray process disclosed in the CSIRO International application in order to form titanium or titanium alloy pipe .

In a second aspect, there is provided a titanium or titanium alloy pipe formed in accordance with the

continuous forming method defined above . The titanium or titanium alloy pipe may have a composition comprising: titanium: 99.8 wt%; and

the balance comprising incidental impurities .

The titanium alloy pipe may alternatively have a

composition comprising: titanium: 90 to 94 wt%; and

aluminium and vanadium: 6 to 10 wt% ; and the balance comprising incidental impurities . This does not exclude other alloys of titanium where titanium is greater than any other single element either by atom or by weight. The pipe may be formed by spraying particles selected to have different compositions . For example , pipe may be formed in accordance with the first aspect from particles having one or more different alloy compositions .

Alternatively, the pipe may be formed of particles of titanium and particles of one or more different alloy compositions . In these circumstances , the pipe may be formed with a generally homogenous composition or the pipe may be formed with a composition that is graded or otherwise varies along the length of the pipe .

In a third aspect, there is provided a pipe-forming substrate for spray-forming titanium or titanium alloy pipe on the substrate, the substrate being formed of material for and having properties for controlling the extent of bonding between titanium or titanium alloy pipe and the pipe-forming substrate to enable formed pipe to be removed from the pipe-forming substrate by relative movement of the formed pipe and the pipe-forming

substrate .

The pipe-forming substrate may have a co-efficient of thermal expansion that is less than the co-efficient of thermal expansion of the titanium or titanium alloy. The pipe-forming substrate may have a surface roughness such that low mechanical bonding occurs between the formed pipe and the substrate.

The pipe-forming substrate may have a low potential for chemically bonding with titanium or titanium alloy. The pipe-forming substrate may be elongate. Optionally, the pipe-forming substrate is in the form of an elongate cylinder . Brief Description of the Drawings

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a schematic cross section view of a pipe- forming substrate and spraying arrangement before movement of the titanium or titanium alloy pipe relative to the pipe-forming substrate.

Figure 2 is a schematic cross section view of the pipe- forming substrate and spraying arrangement in Figure 1 during movement of the titanium or titanium alloy pipe relative to the pipe-forming substrate .

Figure 3 is a schematic side view of a pipe-forming substrate and spraying arrangement according to an alternative method for forming seamless titanium or titanium alloy pipe .

Figure 4 is a schematic end view of the pipe-forming substrate and formed pipe in Figure 3.

Detailed Description of Embodiments

The description of embodiments of the invention that follow is in the context of producing a seamless titanium alloy pipe from titanium alloy particles. However, it will be appreciated that the embodiments invention enable production of seamless pipe of titanium and/or titanium alloys and the description should not be interpreted as limiting the embodiments to producing titanium alloy pipe only.

One embodiment for forming seamless pipe of titanium alloys involves providing a pipe-forming substrate, in the form of a mandrel 10, and an initiating substrate, in the form of a starter pipe 30.

The starter pipe 30 is a tube having an internal diameter that closely receives the mandrel 10. The starter pipe 30 can be moved from a position with the mandrel 10 located within the starter pipe 30 (Figure 1) and in a direction that is co-axial with a longitudinal axis of the mandrel 10. During operation, the starter pipe 30 is moved away from the mandrel 10 so the starter pipe 30 and the mandrel 10 are remote from each other.

Titanium alloy pipe 40 is formed by placing the mandrel 10 within the starter pipe 50 and rotating both in the same direction and at the same rate of rotation. Spraying of titanium alloy particles from a nozzle 20 is commenced when the nozzle 20 is positioned such that a deposition zone 14 for the titanium alloy particles coincides with a position wholly on and adjacent to the end of the starter pipe 30.

The nozzle 20 is connected to a spray apparatus 22 that supplies a source of inert carrier gas and titanium alloy feed particles. The apparatus 22 and nozzle 20 used for spaying the titanium alloy particles is likely to be of conventional form and, in general terms, the basis of the equipment is as described and illustrated in US patent 5,302,414. The titanium alloy particles are entrained in the carrier gas and pass through a series of stages to accelerate the carrier gas and particles to supersonic velocities. Accordingly, the spray 12 exiting the nozzle comprises a jet of carrier gas and entrained titanium alloy particles .

The spraying conditions, such as spray angle, temperature, velocity, particle size and shape and distance between the nozzle 20 and the deposition zone 14, are in accordance with the spray forming method disclosed in the CSIRO

International application referenced herein and the spraying conditions are incorporated herein by this reference. For example, the spraying conditions may be

• Equipment: CGT Kinetic 3000 or 4000

• Number of supersonic nozzles: one or more

• Mandrel speed: up to 600 RPM

• Stand-off: 20 - 100 mm

• Spray angle: within 15° from normal to the surface of the mandrel 10

• Spray material: CP Titanium and/or titanium alloy powder

• Particle diameter: 10 - 30 microns

• Gas pressure: 10 - 40 bar

• Gas: Helium, nitrogen, argon or air

• Carrier gas: Helium, nitrogen, argon or air or

mixtures thereof

• Powder feed rate: 10 - 200 g/min

• Traverse rate: 10 - 100 mm/min

The nozzle 20 is typically positioned opposite the mandrel 10. The mandrel 10 is formed of a material and has properties selected to facilitate release of formed pipe

40 from the mandrel 10. The mandrel 10 is formed of Pyrex® (borosilicate glass) and has a hardness of 418kg/mm ² (Knoop 100) and a surface roughness R _a < 0.5μm. The diameter of the mandrel 10 is 10cm, but is selected according to the desired internal diameter of the formed pipe. The position of the starter pipe 30 and mandrel 10 are moved relative to the nozzle 20, or vice versa, to extend the layer of titanium alloy from the position adjacent the end of the starter pipe 30 to over the end of the starter pipe 30 and onto the substrate 10. This movement causes a transition pipe 32 , in the form of a continuous layer of titanium alloy, to be created over the end of the starter pipe 30 and on the mandrel 10. Such movement of the starter pipe 30 and mandrel 10, or the nozzle 20, continues until a small section of titanium pipe 40 is formed on the mandrel 10 and the deposition zone 14 is spaced a small distance from the transition pipe 32 , as in Figure 1.

At this point, the starter pipe 30 is moved in the direction of arrow A (Figure 2) and the longitudinal position of the nozzle 20 relative to the mandrel 10 is maintained. However, movement of the starter pipe 30 is understood to cause formed titanium pipe 40 that is integral with the transition pipe 32 to move also in the direction of arrow A. This movement slowly drags formed titanium pipe 40 out of or to the edge of the deposition zone 14. The movement of the pipe 40 in the direction of arrow A is performed continuously at a slow rate that is equivalent to the rate of deposition titanium alloy particles required to build up the desired wall thickness of the titanium alloy pipe 40. In this manner, titanium pipe 40 is formed continuously and can be formed in any desired length provided the formed pipe 40 continuously moves in the direction of arrow A and spray 12

continuously impinges upon the mandrel 10. An alternative to using the ^Λ starter pipe' is to mechanically grab a section of pipe on the mandrel and moving this

longitudinally. The length of the formed pipe 40,

therefore is independent of the length of the mandrel 10. The ability to drag the titanium pipe 40 out of the deposition zone 14 and from the mandrel 10 depends upon the extent to which the titanium pipe 40 bonds with the mandrel 10.

Without wishing to be limited to any particular theory , the applicant believes that the primary cause of bonding between the mandrel 10 and the pipe 40 is thermal bonding caused by thermal expansion of the mandrel 10 relative to the formed pipe 40. The applicant also believes that surface roughness of the mandrel 10, particle relaxation and chemical bonding of the titanium pipe 40 to the mandrel 10 contribute to bonding. However, the applicant is not aware of the extent to which mechanical and chemical bonding and particle relaxation contribute to overall bonding of the titanium pipe 40 to the mandrel 10.

In view of this belief, it is thought that thermal bonding may be controlled by selecting a material for the mandrel 10 that has a co-efficient of thermal expansion that is less than the co-efficient of thermal expansion of titanium alloy. Expansion of the mandrel 10 is caused by spray 12 which is at an elevated temperature. Although not confirmed, it is thought that a steel mandrel is heated to a temperature of around 300 ⁰ C. Steel has a coefficient of thermal expansion that is greater than the co-efficient of thermal expansion of titanium alloy and, therefore, a steel mandrel 10 will expand more than the titanium alloy pipe 40, thereby causing thermal bonding. It is anticipated that forming the mandrel 10 of materials that have a co-efficient of thermal expansion that is less than the co-efficient of thermal expansion for titanium alloys will reduce the impact of thermal bonding and enable the titanium alloy pipe 40 to be pulled from the mandrel 10. Alternatively the mandrel 10 may be cooled so that it contracts away from the alloy pipe 40. Alternatively, removal of the titanium alloy pipe 40 may be assisted or caused by heating the titanium pipe 40 in preference to the mandrel 10. The heating causes the titanium alloy pipe 40 to expand more than the mandrel 10, thereby releasing the titanium alloy pipe 40 from the mandrel 10. Such removal of titanium alloy pipe 40 is suitable in the circumstances that the mandrel 10 has a coefficient of thermal expansion greater than that of the titanium alloy pipe 40.

Surface roughness is thought to impact on bonding between the mandrel 10 and the titanium alloy pipe 40 because the titanium alloy pipe 40 fills localised relief in the surface of the mandrel 10 thereby mechanically

interlocking with the mandrel 10. Reducing surface roughness is anticipated to reduce the effect of

mechanical bonding. It is though that the abrasive effect of spraying titanium alloy particles onto the mandrel 10 will cause erosion of the surface of the mandrel 10. The erosion is expected to increase surface roughness over time. The mandrel 10 may be formed of hard, wear resistant material to reduce surface erosion . Such material may be glass, ceramic or metal. It is also anticipated that chemical bonding plays a role in the overall process, but the material of the mandrel 10 may be selected to reduce or eliminate the effect of chemical bonding. When the desired length of formed pipe 40 is reached, the starter pipe 30 is cut away from the remainder of the formed pipe 40. The opposite end is removed from the mandrel 10 and is finished by removing any partially formed pipe .

An alternative method of forming seamless titanium alloy pipe to ameliorate the effects of thermal bonding and particle relaxation involves adopting a mandrel 50 having a smaller circumference than the inner circumference of the tube 40 to be formed (Figures 3 and 4) . This is shown in Figure 4 with the mandrel 50 having an outer radius R _M less than the inner radius of the formed pipe Rp .

In this alternative, an axis of rotation of the mandrel 50 is off-set from an axis of rotation of the formed titanium alloy pipe 40 so that the outer surface of the mandrel 50 is in contact with or is closely spaced from an inside surface of the formed titanium alloy tube 40 in the deposition zone 14. In other regions, the mandrel 50 is spaced from the inside surface of the formed titanium alloy tube 40.

The smaller size of the mandrel 50 is selected to reduce, and preferably avoid, the bonding effects produced by thermal bonding and particle relaxation. The size of the mandrel is selected to be smaller than the size of the formed pipe 40 after particle relaxation and after thermal contraction of the formed pipe 40.

Accordingly, it is believed that particles sprayed into the deposition zone 14 will bond onto a portion at the end of the formed pipe 40, thus extending the titanium alloy pipe. The tensile strength of the bound particles is thought to be greater than the bond strength between the particles and the mandrel 50. Accordingly, as formed pipe 40 rotates, the portion moves out of the deposition zone 14 and the mandrel 50 and the formed pipe 40 diverge. The tensile strength of the bound particles is expected to pull the bound particles free of the mandrel 50. The end of the formed pipe 40, therefore, is not bound to the mandrel 50 and can be advanced away from the deposition zone to enable continuous build-up of further titanium alloy pipe . In operation, the mandrel 50 and the formed pipe 40 are rotated at respective rates so the surface of the mandrel 50 passes through the deposition zone 14 at the point of contact between the formed pipe 40 and the mandrel 50 generally at a circumferential speed that is the same as the inner circumferential speed of the formed pipe 40.

Although, this process is described in the context of using formed pipe 40 with the mandrel 50, the process may be commenced by a starter pipe 30 such as described above in respect of Figures 1 and 2.

Additionally, the process may involve preferential heating of the bound particles in a heating zone 60 to assist with separation of the bound particles from the mandrel 50.

Many modifications may be made to the preferred embodiment of the present invention as described above without departing from the spirit and scope of the present invention.

It will be understood that the term "comprises" or its grammatical variants as used in this specification and claims is equivalent to the term "includes" and is not to be taken as excluding the presence of other features or elements .

It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.