WO1999005332A1

WO1999005332A1 - Titanium materials containing tungsten

Info

Publication number: WO1999005332A1
Application number: PCT/US1998/015425
Authority: WO
Inventors: Stanley Abkowitz; Susan M. Abkowitz; Paul F. Weihrauch; Harold Heussi; Walter Zimmer
Original assignee: Dynamet Technology Inc
Current assignee: Dynamet Technology Inc
Priority date: 1997-07-25
Filing date: 1998-07-24
Publication date: 1999-02-04
Anticipated expiration: 2000-01-25

Abstract

A titanium containing material including tungsten and other additional metals including tantalum, molybdenum, and niobium. The titanium containing material having a density ranging from approximately 4.8 to 11.5 kg/m3 and improved mechanical properties including elastic modulus, strength, hardness, and wear resistance.

Description

TITANIUM MATERIALS CONTAINING TUNGSTEN

FIELD OF THE INVENTION

The present invention relates to titanium and to titanium-based alloys where additions enable the density of the resulting materials to be controllably increased up to 11.48 kg/m³. The major alloying additions employed in the titanium and titanium-based alloys are tungsten or tantalum.

DESCRIPTION OF RELATED ART Until recently, the principal structural advantage of titanium alloys was their superior specific strength, that is, the tensile yield strength divided by density. Alloying elements which added strength without a substantial increase in the resultant titanium alloy density were widely developed and applied throughout the aerospace industry. These structural titanium alloys have now become more widely used in sporting goods, such as mountain bicycle frames and golf club heads, based upon the same structural efficiency design criteria. However, as a result of these structural titanium alloys now being used in new areas, there is a growing need for these alloys to have similar or improved mechanical properties and a controllable density of up to approximately 11.48 kg/m³.

For example, the materials employed in modern golf clubs have rapidly evolved to incorporate advanced composite materials in the shaft and in the head components. The replacement of traditional hardwood and ferrous club head materials with advanced alloys has been constrained principally by restrictions on club head weight. All categories of golf clubs, including drivers, fairway woods, irons, wedges and putters, have become progressively more refined in geometrical shape, in the sophistication of the materials used, and in their methods of construction.

The materials from which drivers are constructed have evolved from a hardwood club head with a metal striking face, to a hollow cast head of conventional stainless steel ("metal woods"), and recently to cast titanium drivers. Because titanium alloys, such as Ti-6AI-4V, possess some of the highest strength-to-weight ratios of all structural metals and they exhibit a useful modulus of elasticity, they are attractive alternatives to the traditional ferrous driver materials. By substituting lower density titanium for the higher density steel materials, the dimensions of the driver can be proportionately increased. Specifically, the surface area of the striking face including both width and height can be enlarged without exceeding the weight restrictions of a "regulation" club. As an example of the benefit of this material substitution, the enlarged striking face area, or "sweet spot", permits golfers to achieve greater consistency in play with a club of prescribed swing weight.

The same trend toward enlarged club heads has occurred in wedges or irons. Forged or cast stainless steel has been replaced by titanium in part or all of the club head. Further improvement in the performance, specifically responsiveness and consistency through both design and material selection, is of ongoing interest.

As the mechanics of energy transfer between the club face, for example, of a driver or an iron, and the golf ball, are examined in more detail, there appears to be several advantages to weighting specific areas of the golf club head in order to control the locus of the center of gravity of the club head with respect to the desired point of impact with the ball. Using a monolithic club head, that is a club head made of one material, such as 17-4 PH stainless steel or Ti-6AI-4V, the control of the center of gravity can be achieved by controlling the thickness of various regions of the club head.

In the case of the iron, which is basically a plate inclined to the axis of the shaft of the golf club, a dimensionally enlarged face can be produced by redistributing the thickness of the plate. By thinning the club face to the minimum required for responsiveness, the remaining material is distributed around the perimeter of the golf club face as a thicker "picture frame." Not only does this perimeter-weighted iron design increase the size of the "sweet spot" (i.e., zone of most responsive and repeatable impact behavior) on the face of the iron and increase the rotational stability of the iron about the axis of the shaft, but it also provides a means to control the distance between the center of gravity of the club and the intended point of impact on the face of the club.

In the case of a driver which is a hollow shell, typically formed by joining two cast subsections, the center of gravity is controlled in three dimensions by the thickness of each element of the cast shell. During driver- ball impact, the mechanics of elastic energy transfer, including stiffness and deflection, can be used to determine the optimum design thickness of the intended impact region. Some club designs seek to shift the center of gravity of the club head towards the sole plate, the portion of the driver that lies at the greatest distance from the grip end of the club. To achieve this sole-plate- weighting, a higher density material, such as copper-infiltrated tungsten or bronze, is mechanically-attached or joined, for example by welding or brazing, to the rest of the club. The result is no longer a monolithic material structure, but a combination of at least two components of two dissimilar materials with differing densities.

Other recreational goods which incorporate structurally efficient titanium alloys and a geometry optimized to control the center of gravity of a component in motion, include such things as lacrosse sticks, tennis rackets, and sail boat keel weights. Aircraft subcomponents, such as rudders and flaps, also require control of center of gravity and moments of inertia, and often involve mounting counterweights in specific locations. Missile wings and fins, satellite structures, and control surfaces for submarine and aerodynamic applications require the same combination of titanium structural alloy properties and control of center of gravity.

The hardness of these structurally efficient titanium alloys is inadequate for many mechanical wear environments. Increased hardness and thus improved wear-resistance would render these titanium materials more useful in cutting blade and abrasive wear applications including ski edges, knife blades and ice skate blades. Further, biomechanical devices such as implanted hip and knee prostheses could be produced from these materials once the hardness and wear-resistance were improved. To achieve design center of gravity values, both monolithic material and multiple material solutions, that is, segmented subcomponents of dissimilar metals, are employed. A notable example of dissimilar materials solutions is the use of depleted uranium in aircraft counterweights.

Thus, there now appears to be a need for a "heavy" titanium-based alloy, with mechanical properties, including tensile yield strength and elongation to failure, near those of other titanium alloys, with titanium-like chemical compatibility, but with controllable density up to 11.48 kg/m³. There also appears to be a need for a titanium alloy with higher specific strength and modulus properties. There further appears to be a need for a titanium alloy combining the above attributes with enhanced hardness and wear- resistance.

SUMMARY OF THE INVENTION

The present invention is directed toward materials comprising titanium and tungsten having a density in the range of from approximately 4.8 to 11.5 kg/m³. Preferably the alloy comprises titanium and partially dissolved tungsten particles. The alloy can further include, as optional ingredients, metals selected from the group consisting of tantalum, molybdenum, niobium, ferrotungsten, and mixtures thereof. These additional metals are preferably particulates which are partially dissolved in the titanium. The resulting alloy has a density ranging between approximately 4.8 and 11.48 kg/m³.

By controlling the density of these alloys, it is possible to facilitate control over the center of gravity of the titanium-based structural components thus formed. It is also possible to facilitate control over the center of gravity of systems (or assemblies) which incorporate titanium components (or subcomponents) by the substitution of these higher density titanium materials. The alloys can be made by a combination of powder metallurgy (P/M) and subsequent processing including hot isostatic pressing, extrusion, forging, rolling, melting (and subsequent reduction to powder), diffusion bonding, and casting. These compositions also create increases in the strength, specific strength, modulus of elasticity and wear resistance of the structural components so generated. The construction of a structural assembly with controlled center of gravity, produced from titanium alloys and integrating metallurgically-bonded substructures containing higher density titanium alloys in specific locations within the structure, is technically feasible through the processes and compositions described below.

BRIEF DESCRIPTION OF THE DRAWINGS

The above object and feature of the present invention will be apparent from the following description of the preferred embodiments with reference to the accompanying drawings.

Figure 1A is a photomicrograph at 100X of Commercially Pure (CP) titanium with 30.5 weight per cent tungsten produced from a P/M preform after forging, air cooling, polishing and chemical etching to reveal uniform microstructure.

Figure 1 B is the same material shown in Figure 1A at 500X showing islands of undissolved tungsten surrounded by tungsten-rich (white) beta phase. Darker etching impurities mark the grain boundaries. The intermixed (grey) areas contain fine alpha titanium (white) and impurities.

Figure 2A is a photomicrograph at 100X of Commercially Pure (CP) titanium with 10.2 weight per cent tungsten produced from a P/M preform after forging, air cooling, polishing and chemical etching to reveal uniform microstructure. Islands of tungsten surrounded by tungsten-rich beta phase (white). Darker etched regions are impurities intermixed with alpha phase titanium.

Figure 2B is the same material shown in Figure 2A at 500X showing a typical (white) tungsten-rich island of beta titanium. Thin white regions in the surrounding area are alpha phase titanium.

Figure 3 is a photomicrograph at 50X of "micro-macro" composite, composed of a layer of CP Ti with 10.2 weight per cent tungsten (white layer), integrally bonded to a Ti-6AI-4V layer (grey) by forging a double-layered P/M perform at 1750F. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

To achieve these and other objects of the invention there is provided an alloy comprising titanium and tungsten having a density in the range of from approximately 4.8 to 11.5 kg/m³. Preferably the alloy comprises titanium and partially dissolved tungsten particles. The alloy can further include, as optional ingredients, metals selected from the group consisting of tantalum, molybdenum, niobium, ferrotungsten, and mixtures thereof. These additional metals are preferably particulates which are partially dissolved in the titanium.

The above heavy or high density titanium materials can be produced in a form near to the net shape of the final article based upon powder metallurgy ("P/M") technology. Such an embodiment consists of blending commercially pure titanium powder with powders of pure tungsten, and then consolidating the blended powders into a structural solid ("green compact") through a controlled sequence of processes. This sequence of processes, includes blending the powders, either die pressing or cold isostatic pressing the powders into a free-standing solid or green compact, vacuum sintering the free-standing solid or green compact at an elevated temperature, and if necessary, hot isostatic pressing or hot forging the sintered solid.

Subsequent to the formation by powder metallurgy of a free standing solid, the material may be further processed through traditional thermomechanical operations including hot isostatic pressing, extrusion, forging, rolling and diffusion bonding. The objective of these operations is near-net shape making and microstructural control. The powder metal solid may be alternatively melted and cast into final shape. In addition, subsequent to melting, the alloy can be reduced to a powder form by a number of processes, including a rotating electrode process, pulverizing by hydriding/dehydriding, and atomization.

The P/M sequence results in a near-net shape P/M preform composed of a titanium alloy matrix, wherein the high density tungsten powder constituent is partially or totally dissolved. The desirable compositions of this "heavy" titanium alloy, which has been designated as "DensiTi"™, covers alloys with theoretical densities of 4.80 to 11.48 kg/m³ or 0.173 to 0.415 lb/in³ achieved by the addition of 10.0 to 79.33 weight percent tungsten to titanium. This corresponds to a 2.82 to 49.99 atomic percent addition of tungsten.

Selected compositions within the density ranges described above are shown in Table I, together with an approximation of the resultant density as unalloyed elemental metals.

Illustrated in Figures 1 , 2 and 3 are microstructures of alloys or composites illustrating embodiments of the invention produced by the preferred process of cold isostatic pressing, then vacuum sintering, followed by hot forging at 1750F. Figure 1 is a 200X photomicrograph of Commercially Pure (CP) titanium with 30.5 weight per cent tungsten after forging, air cooling, polishing and chemical etching to reveal its microstructure. Figure 2 is a 200X photomicrograph of Ti-6AI-4V with 10 weight per cent tungsten, forged at 1750F and air cooled. Figure 3 is a 200X photomicrograph of "micro-macro composite", composed of a layer of CPTi with 10.2 weight per cent tungsten, integrally bonded to a Ti-6AI-4V layer. This two-layered puck was produced by forging a double-layered preform at 1750F. The mechanical response of a given DensiTi™ alloy, its mechanical properties and structural integrity, are determined by the degree to which the tungsten alloying constituent is homogeneously dissolved within the titanium matrix. Mechanical properties of typical heavy titanium alloys in the hot isostatically pressed condition, without forging, are presented in Table II. The materials of the present invention exhibit increased values of density, hardness, strength and specific strength and can also be expected to show increased wear resistance and modulus of elasticity.

TABLE II. Mechanical Properties of Heavy Titanium Alloys

* Tensile Properties at room temperature from hot-isostatically pressed material

** Hardness measurements in forged then air-cooled condition.

The above powder metallurgy processing route, even without providing adequate time, temperature or pressure to form a homogeneous solid solution of the tungsten in titanium does achieve the desired increase in density and more than a commensurate level of strengthening (i.e., an unexpected increase in specific strength) as shown in Table II. The resulting incomplete state of alloying, wherein a precursor tungsten particle remains only partially dissolved in the surrounding tungsten-titanium matrix, constitutes a "non-equilibrium metal composite," with distinct load sharing attributes typical for composite materials. This composite state can be expected to display a proportionate increase in elastic modulus as compared to pure titanium, reflecting the volume fraction contribution of the pure tungsten constituent with its high modulus. It is noteworthy that, regardless of the state of dissolution, these heavy titanium alloys possess an increased density due to the addition of tungsten.

At the other extreme, tungsten is completely soluble in titanium upon melting into the liquid phase, such as by consumable arc melting a P/M titanium-tungsten electrode. Although after solidification the concentration of tungsten within the titanium alloy matrix is not perfectly uniform throughout the solid, the resultant concentration gradients are generally lower than those achievable through sintering a P/M solid in the solid phase.

Alternatively, with sufficient time at high temperatures in vacuum, solid state diffusion can distribute the tungsten more uniformly throughout the titanium. If plastic flow conditions are imposed at elevated temperatures such as by forging or extrusion, a more homogeneous distribution of tungsten occurs even more quickly than by solid state diffusion at elevated temperature, as seen in Figure 1. Superimposing pressure but limiting flow at elevated temperatures such as by hot isostatic pressing, also accelerates solid state diffusion, reduces porosity in the P/M solid, and creates a more uniform distribution of tungsten in titanium.

Once homogeneous distribution has been attained, whether through diffusion in the liquid state or the solid state, the tendency of the titanium- tungsten alloy is to form a microstructure of metastable beta (body centered cubic, Im3m lattice), with varying amounts of alpha, alpha prime, alpha double prime and omega phases present. Phase Diagrams of Binary Titanium Alloys, Joanne L. Murray, Editor, ASM International, Metals Park, OH 44073, 1987, pp. 328-332 describes the different phases of the Ti-W alloy.

This invention recognizes the difference in the degree of dissolution of the tungsten alloy attained, and encompasses titanium materials of this composition regardless of the end state of the alloying elements, i.e., either as a non-equilibrium metal composite, a homogeneous solid solution alloy, or a combination of both. Substitution of tantalum, molybdenum, or niobium powder for part of the tungsten may be made. Like tungsten, the elements tantalum, molybdenum, or niobium are completely soluble in the beta phase of titanium, that is, they are beta isomorphous. Thus the same P/M consolidation process, with subsequent treatments such as hot isostatic pressing, forging, extrusion, or rolling, creates fully dense, heavy titanium alloys when these elements are substituted for tungsten.

The addition of tungsten may be accomplished by adding tungsten powder, or a precursor alloy powder such as ferrotungsten, a commercially available source of tungsten containing 78% W with the balance iron. In certain applications chemical compatibility issues, rather than economics, may dictate the substitution of tantalum, molybdenum, or niobium for tungsten as powder starting material, but the processing sequence, final near net shape forms, and intent to control center of gravity with density/location are essentially the same.

Addition of elemental powders (a chemical element in powdered form, e.g. iron powder) or master alloy powders (an alloy of two or more elemental metals melted together then converted to powder) such as those containing nickel, iron, copper, vanadium, aluminum, which facilitate transient liquid phase formation upon heating during the sintering cycle and which freely diffuse within the titanium matrix at sintering temperatures, are limited to 15 weight per cent of the total alloy. The objective of these alloy additions is to facilitate the densification of the material during sintering, to reduce the sintering temperature and/or time and to enhance specific material properties.

In the above discussion, elemental titanium serves as the matrix continuum into which tungsten particulate along with Ta, Mo, and Nb can be added to increase density. Alternatively, the same tungsten, tantalum, molybdenum, niobium particulates could be added to improve density and mechanical properties of titanium-based matrix alloys, including the following classes: alpha, including commercially pure titanium containing less than 1% deliberate alloy addition such as ASTM B 348-94 Grades 1 , 2, 3, 4, 7, and 14; near alpha, including alloys such as Ti-6AI-2Sn-4Zr-2Mo-0.08Si; alpha-beta alloys, including alloys such as Ti-6AI-4V (ASTM B 381-93 Grade F-5, ASTM B 348-94 Grade 5) as demonstrated in Table II and Figure 3, and Ti-6AI-6V- 2Sn; beta-rich alloys, including alloys such as Ti-13 Zr-13Nb; and metastable beta alloys, including alloys such as Ti-1AI-8V-5Fe, Ti-15Mo-3AI-2.5Nb, and Ti-6.8Mo-4.5Fe-1.5AI.

Furthermore, particulate-reinforced, titanium-alloy-matrix composites such as those designated CermeTi® and described in U.S. Patents #4,731 ,115, #4,968,348, and #5,102,451 , hereby incorporated by reference, may serve as a compositional basis for creating "heavy" titanium alloys with tungsten, tantalum and niobium additions. The CermeTi® composites are created using powder metallurgical methods by adding particulate TiC, TiB, TiB₂, and TiAI to titanium alloy matrix materials.

The above alloys with particulate additions of TiC, TiB, TiB₂ and TiAI, formulated by P/M methods, may be further thermomechanically processed or cast into shapes. The addition of these reinforcing particulates, which further harden and stiffen the alloy, to the heavy titanium alloys of this invention, forming a multi-compositional ceramic or intermetallic particulate-reinforced metal matrix composite, adds another dimension, beyond selective density control or center of gravity control. For example, a composite of titanium carbide and tungsten particles within a matrix of Ti-6AI-4V, serves to increase the elastic modulus, extend the service temperature capability, and improve the wear-resistance and enhance the corrosion resistance.

Another form of composite, termed a macro-micro composite, is produced from a multilayered P/M preform. As an example, a layer(s) of P/M titanium-tungsten alloy can be alternated with layers of P/M Ti-6AI-4V, then cold isostatically pressed, vacuum sintered, then forged to form an integral, fully dense component with a microcomposite of P/M Ti-W interlayered with Ti-6AI-4V, as shown in Figure 3. The same layering or patterning of materials can be accomplished in radial fashion through extrusion, or in a planar fashion through rolling.

The P/M preform or component described above may serve as a finished object such as an architectural fitting or golf club striking face insert, a semi-finished solid such as a machining preform requiring metal removal to produce a dimensioned surface, a consumable arc melting electrode such as feedstock for melting and investment casting, or as an intermediate solid preform, shaped specifically for further processing by forging, rolling or extrusion.

In addition, the following articles can be made from the invention: sporting and recreational goods, including golf club heads, lacrosse sticks, and tennis rackets; counter weight applications, including sail boat keel weights, aircraft and missile subcomponents, such as rudders, flaps and fins, and aerodynamic or submarine control surfaces; and wear resistant and cutting blade applications, including ski edges, knife blades, ice skate blades and the wearing surfaces of implanted hip and knee joint prosthetic devices.

The inventors have further determined that the center of gravity of a titanium or titanium alloy component such as golf club head or wing flap assembly may be controlled by adding predetermined amounts of tungsten and/or tantalum powder to specific regions of the component by any of the following processing methods.

The first processing methods include forging, extruding, or rolling a monolithic P/M preform. Each of the processes begins with a P/M preform, produced by consolidation and sintering of one powder blend. For the forging process, the P/M preform would have a three-dimensional geometry dictated by the forging tooling cavity, hot flow behavior of the Ti-W matrix alloy, and desired final shape of the finished article. In the extrusion process, elevated temperatures can be used to consolidate the preform, creating a fully dense solid with an extended linear dimension. By rolling the P/M preform a fully dense sheet or plate can be created. Finally, the hot isostatic pressing process can be employed after sintering to consolidate the powder preform into a more fully dense solid using high temperatures and pressures.

The second processing methods include forging, extruding, rolling, or hot isostatically pressing a macro-micro composite P/M preform. The process begins with a cold-isostatically pressed preform composed of several powder metallurgy precursor materials. As shown by Figure 3, it is metallurgically possible to build up a P/M preform, by placing material of prescribed density in specific locations within the preform. The three-dimensional geometry of a macro-micro composite forging preform is dictated by the forging tooling cavity, hot flow behavior of the various materials, and desired final shape of the finished article. During forging the dense material flows to specific, predetermined regions of the forging, creating a structural solid with specific solid density in specific regions. Hot extrusion of a macro-micro composite preform can create a linear solid product, yet with higher density material distributed in specific regions within the cross-sectional area. An example of this could be an extruded bar for flywheel applications with a Ti-6AI-4V core and a higher density Ti-W periphery. Rolling of a macro-micro composite P/M preform can create a sheet or plate with an internal layered structure or lattice-patterned distribution, of increased density regions. Hot isostatic pressing can be employed after sintering to consolidate a macro-micro composite into a free- form solid using high temperatures and pressures.

The third processing method includes diffusion bonding titanium alloy subcomponents. A structural component of a titanium alloy such as a wing flap with an integral weight can be produced by diffusion bonding a structural member, such as wing skin/spar assembly to a "heavy" titanium counterweight. Because both components are titanium based alloys, conventional titanium diffusion-bonding at elevated temperatures will permit joining of the two components with a structurally sound interface. The "heavy" titanium component could be produced by P/M processing or could be investment cast preferably from a P/M electrode starting stock.

The term "alloy" as used herein is not limited to single phase or multiple phase microstructures and includes mixtures of pure materials such as commercially pure titanium and tungsten particles, whether or not the particles have diffused into the surrounding material.

The present invention has been disclosed in terms of preferred embodiments. The scope of the invention is not limited thereto but is determined by the appended claims and their equivalents.

Claims

WHAT IS CLAIMED IS:

1. An alloy comprising titanium and tungsten, said alloy having a density in the range of from approximately 4.8 to 11.5 kg/m³.

2. The alloy of claim 1 wherein said alloy comprises titanium and partially dissolved tungsten particles.

3. The alloy of claim 1 further comprising additional metals selected from the group consisting of tantalum, molybdenum, niobium, ferrotungsten, and mixtures thereof.

4. The alloy of claim 3 wherein the alloy comprises titanium, partially dissolved tungsten particles, and partially dissolved particles of said additional metals.

5. The alloy of claim 1 further comprising particles selected from the group consisting of TiC, TiB, TiB₂, and mixtures thereof.

6. The titanium-based alloy of claim 1 wherein the titanium is selected from the group consisting of alpha alloys, near alpha alloys, alpha- beta alloys, beta-rich alloys, and metastable beta alloys.

7. The alloy of claim 6 wherein the titanium is selected from the group consisting of Ti-6AI-4V, Ti-6AI-6V-2Sn, Ti-6AI-2Sn-4Zr-2Mo-0.08Si, Ti- 1AI-8V-5Fe, Ti-15Mo-3AI-2.5Nb, and Ti-6.8Mo-4.5Fe-1.5AI.

8. The product of claim 1 made by a powder metallurgy process wherein the powder metallurgy process comprises the steps of blending, pressing, and sintering to form said product.

9. The product of claim 8 subsequently formed by a thermomechanical process selected from forging, extrusion, rolling, hot isostatic pressing, or combinations there of.

10. The product of claim 8 subsequently formed by a melting process.

11. The product of claim 10 subsequently reduced to a powder form.

12. An alloy comprising titanium and tungsten, said alloy having a density in the range of from approximately 4.8 to 11.5 kg/m³ made by a powder metallurgy process comprising the following steps: blending powders comprising titanium and tungsten; pressing said blended powders into a green compact; and sintering said green compact.