GB2586486A

GB2586486A - Structural material for fabricating a vehicle component

Info

Publication number: GB2586486A
Application number: GB1911985.8A
Authority: GB
Inventors: Blake Paul
Original assignee: Jaguar Land Rover Ltd
Current assignee: Jaguar Land Rover Ltd
Priority date: 2019-08-21
Filing date: 2019-08-21
Publication date: 2021-02-24
Anticipated expiration: 2039-08-21
Also published as: GB201911985D0; GB2586486B

Abstract

A spring 100, e.g. for forming a vehicle component such as a spring is provided. The spring 100 comprises a structural core portion 110 and a structural sheath portion 120. The core portion 110 comprises a metal such as steel as a majority constituent by weight percent; and the sheath portion 120 comprising a metallic material such as titanium or a titanium alloy. A method of forming a spring and a vehicle including such a spring are also disclosed.

Description

STRUCTURAL MATERIAL FOR FABRICATING A VEHICLE COMPONENT

TECHNICAL FIELD

The present disclosure relates to a structural material for fabricating a vehicle component. In particular, but not exclusively, the present disclosure relates to a hybrid wire or rod material having a core/sheath structure. Aspects of the invention relate to a hybrid wire or rod, to a component such as a spring formed from the hybrid wire or rod, to a method of manufacture of the hybrid wire or rod, and to a vehicle including a component comprising the hybrid wire or rod.

BACKGROUND

Vehicle suspension springs in the form of coil springs are typically manufactured from coiled steel wire or rod. The present inventor recognised that these components may be subject to reduced service life and degraded performance due to stress corrosion cracking or pitting corrosion.

It is an aim of the present invention to address one or more disadvantages associated with the prior art.

SUMMARY OF THE INVENTION

Aspects of the invention provide a hybrid wire or rod, a spring, a vehicle component, a method of manufacture, and a vehicle, all as claimed in the appended claims.

According to an aspect of the present invention there is provided a hybrid wire or rod comprising a structural core portion and a structural sheath portion.

In some embodiments the core portion may comprise a metallic material, such as a metal or metal alloy, e.g. a steel (for instance, a high strength steel) as a majority constituent thereof by weight percent. Alternatively or additionally, the metallic material of the core portion may comprise, as a majority or minority constituent, one or more other metals or metal alloys, e.g. aluminium, nickel.

In some embodiments the sheath portion may comprise a metallic material, such as a metal or metal alloy, e.g. titanium or a titanium alloy. Alternatively or additionally, the metallic material of the sheath portion may comprise one or more other metals or metal alloys, e.g. aluminium, steel (e.g. stainless steel).

Thus, in some embodiments the metallic material of the core portion and the metallic material of the sheath portion may be different from one another, or at least the respective majority constituent metals or metal alloys of the core portion and the sheath portion may be different from one another. Alternatively or additionally, in other embodiments each or either of the metallic material of the core portion and the metallic material of the sheath portion may have at least one metal, metal alloy or metallic constituent that is also present in the other thereof.

The structural core portion may be in the form of a wire or rod, for example which is formed by drawing, extrusion, rolling, casting, or any other suitable method.

In some embodiments the core portion may comprise a steel in a proportion of from about 50 or 51 or 55 or 60 or 70 up to about 100 % by weight, optionally from about 75 or 80 or 85 up to about 99.7 or 99.8 or 99.9 % by weight, further optionally from about 90 or 95 up to about 96 or 97 or 98 or 99 or 99.5. % by weight of the total material of the core portion.

The elemental components of which the steel material of the core portion is comprised may include Fe and one or more, especially a plurality of, additional steel-forming elements, which may be metallic or non-metallic, and all of which may be present in any suitable amounts. Such additional elements may include, for instance, one or more metals or non-metals selected from the group consisting of: Cr, V. Mn, Nb, Mo, Ni, Cu, Al, B, Si, P, S, 0 and C. Suitable amounts of each of any one or more such additional elements, apart from Fe, in the steel material of the core portion may be such that all such additional elements are collectively present in an amount up to about 1 or 2 or 3 or 4 or 5 or 7 or 8 or even possibly up to about 10% by weight of the total material of the core portion. Fe may thus make up the balance of the steel content of the material of the core portion.

The material of the core portion may comprise minor amounts of one or more adjunct elements or compounds, in addition to Fe, C and any other steel-forming elements, such as one or more stabilisers, one or more property-modifying substances, and/or any unavoidable impurities resulting from the steel's manufacturing process, each of which adjuncts may be present in any suitable or tolerable amount, especially an amount that does not adversely affect the mechanical properties of the core portion in the resulting hybrid wire or rod. Such amounts of any one or more such adjunct elements or compounds in the material of the core portion may be such that all such adjunct elements or compounds are collectively present in an amount of up to about 0.1 or 0.2 or 0.3 or 0.4 or 0.5 or 0.7 or 0.8 or 1 or 1.5 or 2 or 3 or 4 or 5% by weight of the total material of the core portion.

In embodiments any suitable grade or composition of steel may be used for the steel of the core portion. Practical examples of suitable steels include for instance chromium-vanadiumrich steels and silicon-chromium-rich steels, among others.

In some embodiments the sheath portion may comprise a titanium alloy, especially an alloy of titanium with one or more other metallic elements selected from the group consisting of: Al, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Sn, Ta, Pd, Ru.

In some embodiments the sheath portion may comprise titanium, present as either Ti metal or as a component of a Ti alloy, in an amount of from about 50 or 60 or 70 % by weight up to about 95 or 96 or 97 or 98% by weight, optionally from about 80 or 85% by weight up to about 92 or 94 % by weight of the material of the sheath portion.

In some embodiments the core portion may have a diameter in the range from about 1 to about 20 or 24 or 25 mm, optionally in the range from about 5 to about 14 or 15 mm, further optionally in the range from about 5 to about 10 mm.

In some embodiments the sheath portion may have a wall thickness in the range from about 1 to about 10 mm, optionally in the range from about 1 to about 7 mm, further optionally in the range from about 3 to about 5 mm, e.g. around 4 mm.

In some embodiments an overall diameter or width of the hybrid wire or rod may be in the range from around 5 or 10 or 15 up to around 20 or 24 or 25 or 30 mm.

In some embodiments the hybrid wire or rod may have one or more mechanical features comprising raised protrusions and/or indentations which are provided on a surface of the core portion and/or on a surface of the sheath.

In some embodiments the hybrid wire or rod may further comprise at least one interlayer or interfacial coating between the core and sheath portions.

In some embodiments the hybrid wire or rod may comprise at least one interlayer or interfacial coating between the core and sheath portions, in which: (i) the interlayer or interfacial coating may comprise a coating on the core portion, or (ii) the interlayer or interfacial coating may comprise a layer formed within an interfacial or surface region of the body of the material that forms either or both of the core and/or the sheath.

In some embodiments a radial thickness of the sheath portion as a proportion of the radius of the core portion may be any of: from about 5 to about 10 %, from about 10 to about 15 %, from about 15 to about 20%, from about 20 to about 25%, from about 25 to about 30%, from about 30 to about 35 %, from about 35 to about 40 %, from about 40 to about 45 % , from about 45 to about 50, from about 50 to about 60 %, from about 60 to about 70 %, from about 70 to about 80 %, from about 80 to about 90 %, from about 90 to about 95 %, or from about 95 to about 100%.

In some embodiments a radial thickness of the core portion as a proportion of the radius of the sheath portion may be any of: from about 5 to about 10 %, from about 10 to about 15%, from about 15 to about 20%, from about 20 to about 25 %, from about 25 to about 30%, from about 30 to about 35 %, from about 35 to about 40 %, from about 40 to about 45 %, from about 45 to about 50 %, from about 50 to about 60 %, from about 60 to about 70 %, from about 70 to about 80 %, from about 80 to about 90%, or from about 90 to about 95%.

In some embodiments of the hybrid wire or rod, the material forming the sheath portion may have a lower density than the material forming the core portion.

The hybrid wire or rod may be used to form a vehicle component, such as a spring, e.g. a coil spring or a torsion bar. In some embodiments the hybrid wire or rod may be a vehicle chassis spring such as an anti-roll bar torsion spring or a suspension compression spring.

Such a spring, e.g. a coil spring, may be formed by coiling the hybrid wire or rod to form the said spring.

However, in some alternative embodiments the core portion may initially be formed into a helical coil spring before forming the structural sheath portion around the core portion.

According to another aspect of the present invention there is provided a method of forming a hybrid wire or rod, comprising: providing a structural core portion; and forming a structural sheath portion over the core portion; optionally wherein: (i) the core portion comprises a metallic material selected from a steel as a majority constituent thereof by weight percent, with or without one or more other metals or metal alloys as a majority or minority constituent thereof; and/or (ii) the sheath portion comprises a metallic material selected from titanium or a titanium alloy, with or without one or more other metals or metal alloys.

In embodiments of the above method, features of the core and sheath portions and the overall hybrid wire or rod itself may be as defined therefor in the context of any embodiment of the hybrid wire or rod aspect of the invention.

In some embodiments of the above method, the method may comprise: placing the structural core portion in a container; supporting the core portion substantially coaxially within the container by means of support means; and filling the container with sheath particles to be formed into the sheath portion; wherein supporting the core portion substantially coaxially within the container comprises coupling support means in the form of at least one support element to the core portion or providing support means on or in a wall of the container.

In some embodiments of the above method, the method may comprise: providing the sheath portion in the form of sheath particles to be formed into the sheath portion around the structural core portion; and subjecting the core portion and sheath particles to a hot isostabc pressing (HIP) process to form and densify the sheath portion; optionally wherein the sheath portion has a porosity of less than 10 %, optionally less than 5%.

In other embodiments of the above method, the method may comprise: providing the sheath portion in the form of sheath particles to be formed into the sheath portion around the structural core portion; and subjecting the core portion and sheath particles to a sintering process.

In some embodiments of the above method, the method may comprise: comprising forming the sheath portion around the core portion by: extrusion of the sheath portion around the core portion; or extrusion of a green body sheath portion around the core portion and subsequently subjecting the green body sheath portion to a heat treatment to form the sheath portion; optionally wherein extrusion of the sheath portion or green body sheath portion around the core portion is carried out either: (i) by directly extruding the sheath portion or green body sheath portion around the core portion, or (H) by forming the sheath portion or green body sheath portion by extrusion and subsequently introducing the core portion into the sheath portion or green body sheath portion.

In some other embodiments of the above method, the method may comprise: forming the sheath portion around the core portion by casting.

In some other embodiments of the above method, the method may comprise: forming the sheath portion around the core portion by co-sintering with or into one or more pre-formed metallic, optionally titanium or titanium alloy, shells.

In some embodiments of the above method, the method may further comprise a subsequent step of coiling the hybrid wire or rod to form a coil spring.

In some embodiments of the above method, the method may further comprise a subsequent step of forming the core portion into a helical coil spring before forming the structural sheath portion around the core portion.

In some embodiments of the above method, the method may comprise forming an interlayer between the core and sheath portions.

In some embodiments of the above method, the method may comprise forming a graded interface between the core and sheath portions, the graded interface comprising a layer having a through-thickness graded composition.

Examples of, and example features of, such interlayers and/or graded interfaces will be discussed further below in the context of some specifically described embodiments of the invention in its various aspects.

According to another aspect of the present invention there is provided a vehicle including a vehicle component, for example a spring, which comprises a hybrid wire or rod according to an above-defined aspect of the invention or any embodiment thereof, or optionally comprises a hybrid wire or rod formed by any above-defined method of the invention or any embodiment thereof.

Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

For the avoidance of doubt, it is to be understood that features described with respect to one aspect of the invention may be included within any other aspect of the invention, alone or in appropriate combination with one or more other features.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the invention in various of its aspects will now be described, by way of example only, with reference to the accompanying drawings, in which: FIGURE 1 shows (a) a hybrid coil spring according to an embodiment of the present invention and (b) a free end of the coil spring shown in (a); FIGURE 2 is a schematic illustration of a container in which is provided a core rod and sheath particles ready for insertion in a hot isostatic pressing (HIP) furnace; FIGURE 3 is a schematic illustration of a furnace for performing HIP processing; FIGURE 4 is a schematic illustration of a motor vehicle having suspension coil springs according to an embodiment of the present invention; FIGURE 5 is a flow diagram of a method according to an embodiment of the present invention; 30 and FIGURE 6 shows support means according to embodiments of the present invention in the form of (a) a wire support element and (b) a spider washer support element.

DETAILED DESCRIPTION

In the following description all values, quantities and % amounts are to be considered as being approximate only, unless stated to the contrary or non-applicable to the feature or parameter

S

in question.

As noted above, the present inventor recognised that components manufactured from coiled steel wire or rod may be subject to reduced service life and degraded performance due to stress corrosion cracking or pitting corrosion. The use of a different material with improved corrosion resistance such as titanium is possible in theory, but it would require the coil spring to be made of material having a greater diameter in order to obtain a spring of comparable stiffness, thereby rendering such components unsuitable for many applications and excessively costly to produce.

The present inventor devised the concept of forming a hybrid wire or rod having a core formed from a first, relatively stiff, material and a sheath formed from a material of enhanced corrosion resistance. The sheath may be formed from a material of lower density than the core, enabling a spring or the like of reduced weight to be fabricated, in some embodiments.

In one embodiment, a hybrid rod is provided having a core formed from a steel material and a sheath formed from a titanium alloy. The titanium alloy sheath provides a corrosion resistant cladding to the core. It is to be understood that by the term hybrid wire or rod is meant that the sheath makes a non-negligible contribution to the mechanical properties, including Young's modulus, of the wire or rod, in contrast to the contribution made by a relatively thin coating. Thus, the sheath provides structural strength to the hybrid rod in addition to the core and thus the core and sheath may be described as a structural core and a structural sheath.

FIG. 1 illustrates a hybrid wire or rod 100 according to an embodiment of the present invention, which is approx. 1 m in length and is coiled to form a motor vehicle coil spring 10. FIG. 1(a) is a view of the complete coil spring, whilst FIG. 1(b) is a close-up view of a free end of the coil spring, showing more clearly the core+sheath structure. The rod 100 has a core portion (or "core") 110 formed from steel bar or wire or rod 110 (e.g. a chromium-vanadium-rich rolled steel bar or a silicon-chromium-rich cold-drawn steel wire), and a sheath portion (or "sheath") 120 formed from a titanium-aluminium-vanadium alloy with atomic proportions in the ratio 1:6:4. Such Ti alloys are conventionally termed "Ti-6A1-4V" alloys. Other relative ratios/proportions of Ti, Al and V in Ti-Al-V alloys may be possible and useful in some embodiments, such as in the ratio 1:3:2.5. In the embodiment shown, the outer diameter Dc of the core 110 is 5 mm and the overall outer diameter/width Ds of the sheath 120 (with rod 100 therewithin) is 16 mm. Thus the radius of the core is 2.5 mm and the radial wall thickness of the sheath 120 is 5.5 mm.

It is to be understood that the core and sheath are structural portions of the hybrid rod 100 in that they each have a non-negligible contribution to one or more structural properties of the hybrid rod 100, including stiffness (e.g. as defined by Young's modulus) and torsional rigidity. The contribution of the core portion 110 and sheath portion 120 to a stiffness of the coil spring 10 formed from the hybrid rod 100 is also determined in a non-negligible manner by the core portion 110 and sheath portion 120. This is in contrast to the case where the sheath portion 120 is instead a thin coating of titanium or a material containing titanium such as an alloy thereof. Such a coating would be expected to make a substantially negligible contribution to the stiffness of a coil spring 10 formed from the resulting coated core portion 110.

In the present embodiment, the hybrid wire or rod is formed by hot isostatic pressing (HIP) of Ti6AI4V titanium alloy powder (containing Ti, Al and V in the ratio 1:6:4). The sheath particles in the form of a powder are provided in a sealed elongate hollow steel cylindrical container 180, in which the steel rod 110 is provided substantially coincident with a longitudinal cylinder axis of the container 180. The container 180 may be referred to as a "can". Prior to its placing in the can 180, the steel rod 110 (together with a preformed sheath 120, if that is being employed), may have its/their surfaces pre-cleaned as appropriate, e.g. so as to be substantially rust-and impurity-free. FIG. 2 is a schematic illustration of a cylindrical hollow steel container or can 180 in which cylindrical steel rod 110 is suspended, the cylinder axes of the rod 110 and can 180 being substantially coincident. The container 180 has a basal end face 180FB and a removable end face 180FR having a suction pipe 182 coupled thereto, for connection to a suction pump. This feature enables an interior of the container 180 to be placed under vacuum in order to assist compaction and consolidation of the Ti6AI4V powder during the subsequent HIP process. As shown in FIG. 2, the container 180 has been packed with Ti6AI4V powder and is ready for HIP processing.

In some embodiments the removable end face and/basal end face 180FB may be arranged to provide support to the rod 110 in order to hold in a substantially fixed position within the container 180. For example, in some embodiments the removable end face 180FR and/or basal end face 180FB may be provided with a support means comprising an annular ridge portion 180FRR/180FBR (shown in dotted outline in FIG. 2) having an internal diameter corresponding to that of the rod 110, the annular ridge portion being provided on an inner face of the respective end face 180FR, 180FB substantially concentric with a cylinder axis of the container 180 in use.

FIG. 3 shows a HIP arrangement in which the container 180 shown in FIG. 2, containing steel rod 110 at its core and packed with the Ti6AI4V powder, has been placed in a HIP furnace 190. The suction pipe 182 of the container 180 has been coupled to a suction pump 194. The furnace 190 is coupled to a pressurised inert gas supply, in the present embodiment an argon (Ar) gas supply 192G via inlet pipe 192.

In the present embodiment, following vacuum or gas purging of the furnace chamber, the HIP furnace 190 is programmed to heat the container 180 to a sintering temperature of 950°C +/-5 °C for a period 60 mins in an Ar gas atmosphere at a pressure of 100 MPa. Pressurisation to the required pressure may be achieved by use of appropriate compressor(s) and/or by the increase in temperature itself. Throughout the heat treatment process the container is subject to suction by means of suction pump 194, which is configured to draw a vacuum of approx.

10-4 Pa.

(More particularly, in the present example embodiment: In a first step, the container 180 is heated to vaporise any volatile components/contaminants/air/moisture and extract these under vacuum. Then, the can 180 is sealed such that the vacuum is retained thereinside. In a second step, the can 180 is then placed in an HIP environment, in which the heat softens the can 180 and the product materials and the 100MPa (or other value) pressure squeezes the can 180 and its contents together, thereby collapsing the materials because of the low(er) pressure (i.e. vacuum) inside the can 180. If desired or appropriate, the heating may be increased up to a diffusion bond temperature. These two process steps may for example be combined into one overall process stage, as described above. It may be noted that a temperature of 950°C may not be essential, as this temperature may depend on the metals/alloys used. It may further be noted that the foregoing process may not "sinter" as such, since sintering is generally defined as involving a liquid phase process where surface tension pulls material together to densify the parts. Diffusion bonding on the other hand does not reach a molten state and the particle grain structure may be retained, which may in some example instances offer better performance.) It is to be understood that, in the above-described embodiment process, during the high temperature heat treatment, the Ti6AI4V powder may be subject to non-melt diffusion bonding, resulting in a sheath density typically of at least about 95 up to about 100%, due to the pressure of the HIP process.

It is to be understood that various parameters of the heat treatment process, such as precise values of process conditions including maximum heating temperature, maximum pressure during heating, rate of temperature rise and any temperature fall, dwell times during temperature increases and decreases, and composition of the gas atmosphere, may all be independently selected, controlled and/or adjusted as appropriate for optimising the process for any given practical embodiment using particular steels and/or Ti alloys for the core and sheath materials.

It is to be understood that, rather than subjecting the hybrid rod 100 to coiling following removal from the container 180 after the high temperature heat treatment, in some alternative embodiments the container 180 itself may be subjected to coiling prior to the heat treatment, the container being subjected to HIP treatment in the HIP furnace 190 in the form of a coil. This feature has the advantage that a risk of cracking of the hybrid rod 100 during coiling following removal from the container 180 may be reduced. Furthermore, a risk of oxidation of the surface of the sheath 120 due to any heating applied to facilitate coiling may also be reduced by performing coiling of the container 180 prior to the HIP process.

In some embodiments, coiling of the spring 10 may be conducted following the HIP process but before removal of the rod 100 from the container 180.

In some embodiments, the container 180 may be formed from titanium or other material that remains part of the hybrid rod 100 following HIP treatment (or other treatment in embodiments where HIP treatment is not performed). In some embodiments the container 180 may be formed from material of substantially the same composition as the material within the container that forms the sheath 120. It is to be understood that in some embodiments the composition of the material from which the container 180 is formed (and/or the material within the container 180 from which the sheath 120 is to be formed) may be of a different stoichiometry to the desired final composition of the sheath 120, in order to compensate for changes in composition during processing, for example loss of one or more elements during processing.

Thus in embodiments in which the rod 100 is coiled to form a coil spring 10, the container 180 may form part of the coil spring 10 rather than being removed.

Following the HIP process at 950°C, the container (or "can') 180 is removed from the furnace as quickly as possible and quenched to room temperature in fresh 5W/30 engine oil at room temperature. The container is then subjected to an ageing treatment at a temperature intermediate the sintering temperature of 950°C and room temperature. Ageing treatments of between 30 mins and 8 hours at temperature of from around 300°C to around 700°C may be particularly useful. In the present embodiment ageing is conducted for 4 hours at a temperature of 540°C +/-5°C. It is to be appreciated that the heat treatment can be sequential to the HIP process, or could be carried out some time after.

Following removal of the hybrid rod from the can 180, the rod is subjected to a coiling process to form the coil spring 10 shown in FIG. 1(a).

FIG. 4 shows a vehicle 200, such as a motor vehicle, according to an embodiment of an aspect of the present invention, which vehicle includes within its suspension system one or more, especially a plurality of, vehicle components in the form of coil springs according to the embodiment of FIG. 1(a).

FIG. 5 is a flow diagram illustrating a method of making the automotive coil spring 10 shown in FIG. 1(a) and described above.

At step S101 a suitable wall thickness is determined for the container 180, which may be in the form of a tube, in which the hybrid rod 100 is to be fabricated. The optimising of the container wall thickness to a suitable wall thickness allows for sufficient shrinkage for powder densification. This may be done empirically, by trial and error, and/or by numerical calculation, optionally involving computer simulation of deformation of the container 180, e.g. depending on predicted/calculated powder shrinkage as a result of densification during the heat treatment procedure.

At step S102, a container 180 having the wall thickness determined at step S101 is selected. The steel rod 110 is placed in the container with its cylinder axis substantially coincident with that of the container 180 and titanium sheath powder (Ti6AI4V) is packed around the rod to a known green state packing density. Optionally, one or more spacer elements may be provided within the container 180 to hold the steel rod 110 substantially concentric with the container 180 whilst the powder is added. The spacer elements may for example be formed from wire, such as a wire support element 175 of titanium, aluminium or vanadium or an alloy of any thereof, such as the support element 175 shown in FIG. 6(a) by way of example, which may be arranged to loop around the steel rod core 110. Other alternative examples of forms of spacer elements may be envisaged, such as thin sheets or plates such as spider washers. By way of example, FIG. 6(b) shows a sheet 176 of metallic material, which is suitable for holding the rod 110 coaxial with the container 180. The sheet may have a circular aperture portion 176A in the form of a circular aperture 176A that is not fully circular but is a little over approx. 180 degrees of arc, as illustrated in FIG. 6(b), in order to permit a snap-fit to the rod 110, or it may be less than or substantially equal to 180 degrees of arc, or any suitable proportion thereof, or optionally it may be a fully circular aperture through which the rod 110 may be threaded. As noted above, removable end face 180FR and/or fixed basal end face 180FB may be provided with an annular ridge portion 180FRR/180FBR to assist location and support of the rod 110. Other types of support means, e.g. based on a tubular configuration, may be employed instead, if desired or appropriate.

At step S103, removable end face 180FR of the container 180 is attached to the container 180 to seal the container 180. In some embodiments the end face 180FR is crimped to the container 180 whilst in some embodiments the end face 180FR is welded. In some embodiments the end face 180FR is attached by means of a screw thread. Other means for coupling the removable end face 180FR may be useful in some alternative embodiments.

In some embodiments a preformed green state sintered tube formed from the Ti6AI4V powder may be introduced into the container 180 instead of loose powder.

Optionally, step S103 may include heating in an argon (Ar) gas atmosphere before sealing the removable end face 180FR, in order to permit outgassing of organic compounds trapped in the container 180. The sealing of the end face 180FR may be done by welding, or an automated crimping process, or another process.

At step 5104 the container 180 (together with other containers, if appropriate) is loaded into a HIP pressure vessel 190 for high pressure heating and vacuum suction via pipe 182 in order to consolidate/compact the Ti6AI4V powder to form the sheath 120.

At step 6105, following removal from the HIP pressure vessel 190 the containers 180 may be subject to an ageing and/or other heat treatment.

At step S106 the formed hybrid rod 100 is removed from the container 180 and subject to cold coiling to form an automotive coil spring.

It is understood that instead of cold coiling at step S106, the automotive coil spring may be formed by hot coiling, i.e. at elevated temperature. The temperature used may be selected to allow coiling with reduced risk of adverse structural effects on the hybrid rod 100, for example it is sufficiently hot to reduce a risk of any cracking whilst not being too hot to cause excessive oxidation. Following hot coiling, the spring 10 may if desired or appropriate be subjected to a surface treatment in order to remove oxide that has formed during hot coiling.

In some further embodiments the hybrid rod 100 may be subject to hot coiling before removal from the container 180, i.e. the container 180 itself may be hot coiled. The container 180 may then be removed by acid etching or any other suitable method.

It is to be understood that the rod 100 may be subjected to heat treatment following cold coiling to form a coil spring 10.

In some embodiments, the steel core rod 110 may be subject to cold coiling (or hot coiling) prior to insertion into the container 180, which is formed to have a corresponding helical shape. Following filling of the container 180 with Ti6AI4V powder and HIP processing as described in steps S101 to S105, the container 180 may then be removed by acid etching or any suitable etch process.

As described above, in one aspect the present invention provides a method of hot coiling a hybrid rod without oxidation, by hot coiling the rod whilst contained within a container 180 which is subsequently removed, optionally by etching with a suitable etchant such as an acid, such as an aqueous acid solution.

It is to be understood that the container 180 may be formed from any suitable material such as steel, titanium, titanium alloy, or any other suitable material. Some embodiments may in particular employ steel as the container material.

In some embodiments, the hybrid rod 100 may not have an exposed core at either end, in order to reduce a risk of corrosion. Thus, in some embodiments the core 110 of the hybrid rod may be fully encapsulated at each end, optionally with an extension of the sheath material 120 or with a different, corrosion-resistant, material. For example, in some embodiments end caps may be welded onto the ends of the hybrid rod 100 in some embodiments. In practising some other embodiments, the hybrid rod 100 may be suspended within container 180 with sheath material covering one or both opposed ends of the rod core 110, in order to fully encapsulate the core 110 during the formation of the sheath 120.

Optionally, in yet other embodiments, the container 180 may have a barrier coating on an inner surface thereof to prevent diffusion of container material into the sheath 120, where it may be undesirable for the material of the container 180 to diffuse into the sheath 120.

Optionally, a coating may be provided that enhances removal of the hybrid rod 100 from the container 180 and/or enhances a surface finish of the hybrid rod 100.

It is to be understood that one or more features and/or elements may be provided at one or both free ends of the hybrid rod 100 and/or at one or more positions between the free ends of the rod 100, along a length thereof. Such features and/or elements may facilitate attachment of the rod 100, once it has been formed into a component such as a coil spring, to another component such as a coil mount, and/or it/they may facilitate the attachment of damping and/or anti-squeal element(s) or material thereto.

In some such embodiments, the rod core 110 may be subject to a shape forming operation to form a feature or to have an element added thereto, in order to form the desired feature or element of the hybrid rod 100. By way of example, in some embodiments, the rod 100 may be shaped to form a hollow loop at one or both its ends to facilitate attachment to e.g. a coil mount, or one or more chassis links, control arms or anti-roll bars. Alternatively, one or more features may be provided on or in or to the rod core 110 by casting, extrusion and/or addition by a joining operation such as by welding or other joining operation, or any other suitable method.

In some embodiments, instead of forming the hybrid rod 100 by a HIP process the rod 100 may be formed by casting the sheath material over the core material, such as by casting the titanium alloy sheath 120 over the steel rod core 110.

In some embodiments, forming the sheath 120 by sintering a powder to form the sheath 120 over the core 110 may be performed without a HIP process being employed.

In some embodiments the sheath 120 may be formed over the core 110 by an additive layer manufacturing process to encapsulate the core 110.

In some embodiments the sheath 120 may be formed by co-sintering extruded steel wire or rod with or into one or more pre-formed titanium or titanium alloy shells, e.g. half-shells. These shells may be tubular or segmented to provide coverage around the rod core 110 and for the full length of the rod core 110. The shells may have a length of from around 5 mm to around 2.5 m or more in some embodiments. The shells may be bonded to the steel wire or rod by subjecting the assembly to high temperature and/or pressure processes such as a HIP process, forging, extrusion or sintering.

It is to be understood that preformed shells may be formed from virgin material or recycled material.

It is to be understood that preformed shells may be cast, pressed, machined, extruded, shaped from solid bar or powder or formed by any other suitable technique.

In some embodiments the formed sheath 120 of the hybrid rod 100 may have a resulting porosity of less than 10%, optionally less than 5%.

It is to be understood that the properties of a hybrid spring 10 according to an embodiment of the present invention may be further tailored to suit individual or specific requirements by the application of a secondary heat treatment either integral to the bonding process of the core 110 and sheath 120, or alternatively after that process.

The hybrid wire or rod 100 may be further enhanced through the use of a permanent or sacrificial outer covering which prevents oxidation of the sheath and/or damage due to mechanical handling (such as when coiling) during high energy processing. The outer covering may be metallic and removed by mechanical or chemical means, such as by aqueous or other acid etching. In some embodiments the container 180 may provide such a sacrificial covering, as described below.

It is to be understood that in some embodiments bonding between the core 110 and sheath 120 may be enhanced by mechanical features, e.g. interference or engagement features, on one or more surfaces of the core 110 and/or sheath 120. For example, raised protrusions or indentations, e.g. dimples or recesses, or other surface modification features on the surface of the core 110, such as partially spherical nodules or dimples, may be useful in some embodiments.

It is to be understood that in some embodiments bonding between the core 110 and sheath 120 may be enhanced by means of a graded interface formed from combining differing ratios of steel and titanium (or titanium alloy) powder and sintering or forming same by a high energy process such as a HIP process.

It is to be understood that in some embodiments bonding of the core 110 to the sheath 120 may be enhanced by the introduction, application or formation (e.g. by spraying, plating or an appropriate means of deposition) of one or more interlayers (or "interfacial layers") or interfacial coatings -such as of copper or nickel or stainless steel (but others may be useful in addition or instead) -which act to improve bonding and/or load transfer between the core and sheath materials and/or to improve corrosion resistance of such a triple-(or multi-) layer/component system. Interfacial coatings, e.g. formed on or applied to the core 110, may themselves provide an interlayer between the core 110 and sheath 120. Alternatively an interlayer may be formed within an interfacial or surface region of the body of the material that forms either or both of the core 110 and/or the sheath 120. In some embodiments compositionally graded interfacial layers may be employed in order to enhance bonding between the core 110 and sheath 120. In some embodiments interfacial layers may reduce a risk of cracking of an interface between the core 110 and sheath 120.

It is to be understood that chemical and/or mechanical processing may be used to pre-treat the core 110 and/or sheath 120 materials to enhance the surface reaction during bonding. For example, an etchant such as acid etchant may be used to remove oxide such as titanium oxide from sheath material and processing such as heat treatment may be undertaken under vacuum or inert gas conditions.

It is to be understood that coiling may be performed prior to or after processing to create a mechanical and chemical bond between the two materials. For example, a sheath preform (green body preform prior to heat treatment) may surround the wire in a tube which is then coiled and subsequently subjected to HIP processing or sintered to consolidate the sheath material and then heat treated. Coiling before processing to bond the core and sheath may reduce the risk of interface damage being introduced during coiling. The relatively loose assembly may be cold coiled, and then subject to a high temperature process to densify the sheath material and form a core/sheath chemical and/or mechanical bond. It is to be understood that only one high temperature process may be required if sintering and heat treatment (HT) or HIP and HT can be combined.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", means "including but not limited to", and is not intended to (and does not) exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

Claims

CLAIMS: 1. A spring comprising a structural core portion and a structural sheath portion, wherein: (i) the core portion comprises a metallic material selected from a steel as a majority constituent thereof by weight percent, with or without one or more other metals or metal alloys as a majority or minority constituent thereof; and (ii) the sheath portion comprises a metallic material selected from titanium or a titanium alloy, with or without one or more other metals or metal alloys.
2. A spring according to claim 1, wherein the core portion comprises a steel in a proportion of from 50 or 51 or 55 or 60 or 70 up to 100 % by weight, optionally from 75 or 80 or 85 up to 99.7 or 99.8 or 99.9% by weight, further optionally from 90 or 95 up to 96 or 97 or 98 or 99 or 99.5. % by weight of the total material of the core portion.
3. A spring according to any preceding claim, wherein the sheath portion comprises an alloy of titanium with one or more other elements selected from the group consisting of: Al, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Sn, Ta, Pd, Ru.
4. A spring according to any preceding claim, wherein the sheath portion comprises titanium, present as either Ti metal or as a component of a Ti alloy, in an amount of from 50 or 60 or 70 % by weight up to 95 or 96 or 97 or 98 % by weight, optionally from 80 or 85 % by weight up to 92 or 94% by weight of the material of the sheath portion.
5. A spring according to any preceding claim, wherein the core portion has a diameter in the range from 1 to 20 or 24 or 25 mm, optionally in the range from 5 to 14 or 15 mm, further optionally in the range from 5 to 10 mm.
6. A spring according to any preceding claim, wherein the sheath portion has a wall thickness in the range from 1 to 10 mm, optionally in the range from 1 to 7 mm, further optionally in the range from 3 to 5 mm.
7. A spring according to any preceding claim, wherein one or more mechanical features comprising raised protrusions and/or indentations are provided on a surface of the core portion and/or on a surface of the sheath.
8. A spring according to any preceding claim, further comprising at least one interlayer or interfacial coating between the core and sheath portions.
9. A spring according to claim 9, wherein: (i) the interlayer or interfacial coating comprises a coating on the core portion; or (ii) the interlayer or interfacial coating comprises a layer formed within an interfacial or surface region of the body of the material that forms either or both of the core and/or the sheath.
10. A spring according to any preceding claim, wherein: either (i) a radial thickness of the sheath portion as a proportion of the radius of the core portion is: from 5 to 10%, from 10 to 15%, from 15 to 20%, from 20 to 25%, from 25 to 30 %, from 30 to 35%, from 35 to 40 %, from 40 to 45, from 45 to 50, from 50 to 60%, from 60 to 70 %, from 70 to 80 %, from 80 to 90 %, from 90 to 95%, or from 95 to 100 %; or (ii) a radial thickness of the core portion as a proportion of the radius of the sheath portion is: from 5 to 10 %, from 10 to 15%, from 15 to 20%, from 20 to 25%, from 25 to 30 %, from 30 to 35%, from 35 to 40 %, from 40 to 45%, from 45 to 50%, from 50 to 60%, from 60 to 70%, from 70 to 80%, from 80 to 90%, or from 90 to 95%.
11. A spring according to any preceding claim, wherein the material forming the sheath portion has a lower density than the material forming the core portion.
12. A spring according to any preceding claim, wherein the spring is a coil spring or a torsion bar.
13. A vehicle component comprising a spring according to any preceding claim.
14. A method of forming a spring, comprising: providing a structural core portion; and forming a structural sheath portion over the core portion; optionally wherein: (i) the core portion comprises a metallic material selected from a steel as a majority constituent thereof by weight percent, with or without one or more other metals or metal alloys as a majority or minority constituent thereof; and (ii) the sheath portion comprises a metallic material selected from titanium or a titanium alloy, with or without one or more other metals or metal alloys.
15. A method according to claim 14, comprising: placing the structural core portion in a container; supporting the core portion substantially coaxially within the container by means of support means; and filling the container with sheath particles to be formed into the sheath portion; wherein supporting the core portion substantially coaxially within the container comprises coupling support means in the form of at least one support element to the core portion or providing support means on or in a wall of the container.
16. A method according to claim 14 or claim 15, comprising: providing the sheath portion in the form of sheath particles to be formed into the sheath portion around the structural core portion; and subjecting the core portion and sheath particles to a hot isostatic pressing process to form and densify the sheath portion; optionally wherein the sheath portion has a porosity of less than 10 %, optionally less than 5 %.
17. A method according to claim 114 or claim 15, comprising: providing the sheath portion in the form of sheath particles to be formed into the sheath portion around the structural core portion; and subjecting the core portion and sheath particles to a sintering process.
18. A method according to claim 14, comprising forming the sheath portion around the core portion by: extrusion of the sheath portion around the core portion; or extrusion of a green body sheath portion around the core portion and subsequently subjecting the green body sheath portion to a heat treatment to form the sheath portion; optionally wherein extrusion of the sheath portion or green body sheath portion around the core portion is carried out either: (i) by directly extruding the sheath portion or green body sheath portion around the core portion, or (ii) by forming the sheath portion or green body sheath portion by extrusion and subsequently introducing the core portion into the sheath portion or green body sheath portion.
19. A method according to claim 14, comprising: either forming the sheath portion around the core portion by casting; or forming the sheath portion around the core portion by co-sintering with or into one or more pre-formed metallic, optionally titanium or titanium alloy, shells.
20. A method according to any one of claims 1410 19, further comprising a subsequent step of coiling the spring to form a coil spring.
21. A method according to any one of claims 14 to 19, further comprising a subsequent step of forming the core portion into a helical coil spring before forming the structural sheath portion around the core portion.
22. A method according to any one of claims 14 to 21, comprising forming an interlayer or interfacial coating between the core and sheath portions.
23. A method according to any one of claims 14 to 21, comprising forming a graded interfacial layer between the core and sheath portions, the graded interfacial layer comprising a layer having a through-thickness graded composition.
24. A vehicle comprising: (i) a vehicle component according to claim 13; or (ii) a spring formed by a method according to any one of claims 14 to 23; or (iii) a vehicle component comprising a spring formed by a method according to any one of claims 14 to 23.