Patent Application
20160201165
1. Field of the Technology
The present disclosure relates to high strength alpha-beta titanium alloys.
2. Description of the Background of the Technology
Titanium alloys typically exhibit a high strength-to-weight ratio, are corrosion resistant, and are resistant to creep at moderately high temperatures. For these reasons, titanium alloys are used in aerospace, aeronautic, defense, marine, and automotive applications including, for example, landing gear members, engine frames, ballistic armor, hulls, and mechanical fasteners.
Reducing the weight of an aircraft or other motorized vehicle results in fuel savings. Thus, for example, there is a strong drive in the aerospace industry to reduce aircraft weight. Titanium and titanium alloys are attractive materials for achieving weight reduction in aircraft applications because of their high strength-to-weight ratios. Most titanium alloy parts used in aerospace applications are made from Ti-6Al-4V alloy (ASTM Grade 5; UNS R56400; AMS 4928, AMS 4911), which is an alpha-beta titanium alloy.
Ti-6Al-4V alloy is one of the most common titanium-based manufactured materials, estimated to account for over 50% of the total titanium-based materials market. Ti-6Al-4V alloy is used in a number of applications that benefit from the alloy's advantageous combination of light weight, corrosion resistance, and high strength at low to moderate temperatures. For example, Ti-6Al-4V alloy is used to produce aircraft engine components, aircraft structural components, fasteners, high-performance automotive components, components for medical devices, sports equipment, components for marine applications, and components for chemical processing equipment.
Ductility is a property of any given metallic material (i.e., metals and metal alloys). Cold-formability of a metallic material is based somewhat on the near room temperature ductility and ability for a material to deform without cracking. High-strength alpha-beta titanium alloys, such as, for example, Ti-6Al-4V alloy, typically have low cold-formability at or near room temperature. This limits their acceptance of low-temperature processing, such as cold rolling, because these alloys are susceptible to cracking and breakage when worked at low temperatures. Therefore, due to their limited cold formability at or near room temperature, alpha-beta titanium alloys typically are processed by techniques involving extensive hot working.
Titanium alloys that exhibit room temperature ductility generally also exhibit relatively low strength. A consequence of this is that high-strength alloys are typically more costly and have reduced gage control due to grinding tolerances. This problem stems from the deformation of the hexagonal close packed (HCP) crystal structure in these higher-strength beta alloys at temperatures below several hundred degrees Celsius.
The HCP crystal structure is common to many engineering materials, including magnesium, titanium, zirconium, and cobalt alloys. The HCP crystal structure has an ABABAB stacking sequence, whereas other metallic alloys, like stainless steel, brass, nickel, and aluminum alloys, typically have a face centered cubic (FCC) crystal structures with ABCABCABC stacking sequences. As a result of this difference in stacking sequence, HCP metals and alloys have a significantly reduced number of mathematically possible independent slip systems relative to FCC materials. A number of the independent slip systems in HCP metals and alloys require significantly higher stresses to activate, and these “high resistance” deformation modes are activated in only extremely rare instances. This effect is temperature sensitive, such that below temperatures of several hundred degrees Celsius, titanium alloys have significantly lower malleability.
In combination with the slip systems present in HCP materials, a number of twinning systems are possible in unalloyed HCP metals. The combination of the slip systems and the twinning systems in titanium enables sufficient independent modes of deformation so that “commercially pure” (CP) titanium can be cold worked at temperatures in the vicinity of room temperature (i.e., in an approximate temperature range of −148° F. (−100° C.) to 392° F. (+200° C.)).
Alloying effects in titanium and other HCP metals and alloys tend to increase the asymmetry, or difficulty, of “high resistance” slip modes, as well as suppress twinning systems from activation. A result is the macroscopic loss of cold-processing capability in alloys such as Ti-6Al-4V alloy and Ti-6Al-2-Sn-4Zr-2Mo-0.1Si alloy. Ti-6Al-4V and Ti-6Al-2-Sn-4Zr-2Mo-0.1S alloys exhibit relatively high strength due to their high concentration of alpha phase and high level of alloying elements. In particular, aluminum is known to increase the strength of titanium alloys, at both room and elevated temperatures. However, aluminum also is known to adversely affect room temperature processing capability.
In general, alloys exhibiting cold deformation capability can be manufactured more efficiently, in terms of both energy consumption and the amount of scrap generated during processing. Thus, in general, it is advantageous to formulate an alloy that can be processed at relatively low temperatures.
Some known titanium alloys have delivered increased room-temperature processing capability by including large concentrations of beta phase stabilizing alloying additions. Examples of such alloys include Beta C titanium alloy (Ti-3Al-8V-6Cr-4Mo-4Zr; UNS R58649), which is commercially available in one form as ATI® 38644™ beta titanium alloy from Allegheny Technologies Incorporated, Pittsburgh, Pa. USA. This alloy, and similarly formulated alloys, provides advantageous cold-processing capability by decreasing and or eliminating alpha phase from the microstructure. Typically, these alloys can precipitate alpha phase during low-temperature aging treatments.
Despite their advantageous cold processing capability, beta titanium alloys, in general, have two disadvantages: expensive alloy additions and poor elevated-temperature creep strength. The poor elevated-temperature creep strength is a result of the significant concentration of beta phase these alloys exhibit at elevated temperatures such as, for example, 500° C. Beta phase does not resist creep well due to its body centered cubic structure, which provides for a large number of deformation mechanisms. Machining beta titanium alloys also is known to be difficult due to the alloys' relatively low elastic modulus, which allows more significant spring-back. As a result of these shortcomings, the use of beta titanium alloys has been limited.
Lower cost titanium products would be possible if existing titanium alloys were more resistant to cracking during cold processing. Since alpha-beta titanium alloys represent the majority of all alloyed titanium produced, cost could be further reduced by volumes of scale if this type of alloy were maintained. Therefore, interesting alloys to examine are high-strength, cold-deformable alpha-beta titanium alloys. Several alloys within this alloy class have been developed recently. For example, in the past 15 years Ti-4Al-2.5V alloy (UNS R54250), Ti-4.5Al-3V-2Mo-2Fe alloy, Ti-5Al-4V-0.7Mo-0.5Fe alloy, and Ti-3Al-5Mo-5V-3Cr-0.4Fe alloy have been developed. Many of these alloys feature expensive alloying additions, such as V and/or Mo.
Ti-6Al-4V alpha-beta titanium alloy is the standard titanium alloy used in the aerospace industry, and it represents a large fraction of all alloyed titanium in terms of tonnage. The alloy is known in the aerospace industry as not being cold workable at room temperatures. Lower oxygen content grades of Ti-6Al-4V alloy, designated as Ti-6Al-4V ELI (“extra low interstitials”) alloys (UNS 56401), generally exhibit improved room temperature ductility, toughness, and formability compared with higher oxygen grades. However, the strength of Ti-6Al-4V alloy is significantly lowered as oxygen content is reduced. One skilled in the art would consider the addition of oxygen as being deleterious to cold forming capability and advantageous to strength in Ti-6Al-4V alloys.
However, despite having higher oxygen content than standard grade Ti-6Al-4V alloy, Ti-4Al-2.5V-1.5Fe-0.250 alloy (also known as Ti-4Al-2.5V alloy) is known to have superior forming capabilities at or near room temperature compared with Ti-6Al-4V alloy. Ti-4Al-2.5V-1.5Fe-0.250 alloy is commercially available as ATI 425® titanium alloy from Allegheny Technologies Incorporated. The advantageous near room temperature forming capability of ATI 425® alloy is discussed in U.S. Pat. Nos. 8,048,240, 8,597,442, and 8,597,443, and in U.S. Patent Publication No. 2014-0060138 A1, each of which is hereby incorporated by reference herein in its entirety.
Another cold-deformable, high strength alpha-beta titanium alloy is Ti-4.5Al-3V-2Mo-2Fe alloy, also know as SP-700 alloy. Unlike Ti-4Al-2.5V alloy, SP-700 alloy contains higher cost alloying ingredients. Similar to Ti-4Al-2.5V alloy, SP-700 alloy has reduced creep resistance relative to Ti-6Al-4V alloy due to increased beta phase content.
Ti-3Al-5Mo-5V-3Cr alloy also exhibits good room temperature forming capabilities. This alloy, however, includes significant beta phase content at room temperature and, thus, exhibits poor creep resistance. Additionally, it contains a significant level of expensive alloying ingredients, such as molybdenum and chromium.
It is generally understood that cobalt does not substantially affect mechanical strength and ductility of most titanium alloys compared with alternative alloying additions. It has been described that while cobalt addition increases the strength of binary and ternary titanium alloys, cobalt addition also typically reduces ductility more severely than addition of iron, molybdenum, or vanadium (typical alloying additions). It has been demonstrated that while cobalt additions in Ti-6Al-4V alloy can improve strength and ductility, intermetallic precipitates of the Ti3X-type also can form during aging and deleteriously affect other mechanical properties.
It would be advantageous to provide a titanium alloy that includes relatively minor levels of expensive alloying additions, exhibits an advantageous combination of strength and ductility, and does not develop substantial beta phase content.
According to a non-limiting aspect of the present disclosure, an alpha-beta titanium alloy comprises, in weight percentages: an aluminum equivalency in the range of 2.0 to 10.0; a molybdenum equivalency in the range of 0 to 20.0; 0.3 to 5.0 cobalt; titanium; and incidental impurities. Aluminum equivalency, as defined herein, is in terms of an equivalent weight percentage of aluminum and is calculated by the following equation, in which the content of each alpha phase stabilizer element is in weight percent:
[Al]eq=[Al]+⅓[Sn]+⅙[Zr+Hf]+10[O+2N+C]+[Ga]+[Ge].
Molybdenum equivalency, as defined herein, is in terms of an equivalent weight percentage of molybdenum and is calculated by the following equation, in which the content of each beta phase stabilizer element is in weight percent:
[Mo]eq=[Mo]+⅔[V]+3[Mn+Fe+Ni+Cr+Cu+Be]+⅓[Ta+Nb+W].
According to another non-limiting aspect of the present disclosure, an alpha-beta titanium alloy comprises, in weight percentages: 2.0 to 7.0 aluminum; a molybdenum equivalency in the range of 2.0 to 5.0; 0.3 to 4.0 cobalt; up to 0.5 oxygen; up to 0.25 nitrogen; up to 0.3 carbon; up to 0.4 of incidental impurities; and titanium. The molybdenum equivalency is provided by the equation:
[Mo]eq=[Mo]+⅔[V]+3[Mn+Fe+Ni+Cr+Cu+Be]+⅓[Ta+Nb+W].
An additional non-limiting aspect of the present disclosure is directed to a method of forming an article from an alpha-beta titanium alloy. In a non-limiting embodiment, a method of forming an alpha-beta titanium alloy comprises cold working a metallic form to at least a 25 percent reduction in cross-sectional area, wherein the metallic form does not exhibit substantial cracking during cold working. In a non-limiting embodiment, the metallic form comprises an alpha-beta titanium alloy comprising in weight percentages: an aluminum equivalency in the range of 2.0 to 10.0; a molybdenum equivalency in the range of 0 to 20.0; 0.3 to 5.0 cobalt; titanium; and incidental impurities. Aluminum equivalency is in terms of an equivalent weight percentage of aluminum and is calculated by the following equation, in which the content of each alpha phase stabilizer element is in weight percent:
[Al]eq=[Al]+⅓[Sn]+⅙[Zr+Hf]+10[O+2N+C]+[Ga]+[Ge].
Molybdenum equivalency is in terms of an equivalent weight percentage of molybdenum and is calculated by the following equation, in which the content of each beta phase stabilizer element is in weight percent:
[Mo]eq=[Mo]+⅔[V]+3[Mn+Fe+Ni+Cr+Cu+Be]+⅓[Ta+Nb+W].
Another non-limiting aspect of the present disclosure is directed to a method of forming an article from an alpha-beta titanium alloy. In a non-limiting embodiment, forming an alpha-beta titanium alloy comprises providing an alpha-beta titanium alloy comprising, in weight percentages: 2.0 to 7.0 aluminum; a molybdenum equivalency in the range of 2.0 to 5.0; 0.3 to 4.0 cobalt; up to 0.5 oxygen; up to 0.25 nitrogen; up to 0.3 carbon; up to 0.2 of incidental impurities; and titanium. The method further includes producing a cold workable structure, where the material is amenable to cold reductions of 25% or more in cross-sectional area without resulting in substantial cracking, as defined herein.
It is understood that the invention disclosed and described in this specification is not limited to the embodiments summarized in this Summary.
Various features and characteristics of the non-limiting and non-exhaustive embodiments disclosed and described in this specification may be better understood by reference to the accompanying figures, in which:
The reader will appreciate the foregoing details, as well as others, upon considering the following detailed description of various non-limiting and non-exhaustive embodiments according to the present disclosure.
Various embodiments are described and illustrated in this specification to provide an overall understanding of the structure, function, operation, manufacture, and use of the disclosed processes and products. It is understood that the various embodiments described and illustrated in this specification are non-limiting and non-exhaustive. Thus, the invention is not limited by the description of the various non-limiting and non-exhaustive embodiments disclosed in this specification. Rather, the invention is defined solely by the claims. The features and characteristics illustrated and/or described in connection with various embodiments may be combined with the features and characteristics of other embodiments. Such modifications and variations are intended to be included within the scope of this specification. As such, the claims may be amended to recite any features or characteristics expressly or inherently described in, or otherwise expressly or inherently supported by, this specification. Further, Applicant reserves the right to amend the claims to affirmatively disclaim features or characteristics that may be present in the prior art. Therefore, any such amendments comply with the requirements of 35 U.S.C. §112, first paragraph, and 35 U.S.C. §132(a). The various embodiments disclosed and described in this specification can comprise, consist of, or consist essentially of the features and characteristics as variously described herein.
All percentages and ratios provided for an alloy composition are based on the total weight of the particular alloy composition, unless otherwise indicated.
Any patent, publication, or other disclosure material that is said to be incorporated, in whole or in part, by reference herein is incorporated herein only to the extent that the incorporated material does not conflict with existing definitions, statements, or other disclosure material set forth in this disclosure. As such, and to the extent necessary, the disclosure as set forth herein supersedes any conflicting material incorporated herein by reference. Any material, or portion thereof, that is said to be incorporated by reference herein, but which conflicts with existing definitions, statements, or other disclosure material set forth herein is only incorporated to the extent that no conflict arises between that incorporated material and the existing disclosure material.
In this specification, other than where otherwise indicated, all numerical parameters are to be understood as being prefaced and modified in all instances by the term “about”, in which the numerical parameters possess the inherent variability characteristic of the underlying measurement techniques used to determine the numerical value of the parameter. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter described in the present description should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.
Also, any numerical range recited in this specification is intended to include all sub-ranges of the same numerical precision subsumed within the recited range. For example, a range of “1.0 to 10.0” is intended to include all sub-ranges between (and including) the recited minimum value of 1.0 and the recited maximum value of 10.0, that is, having a minimum value equal to or greater than 1.0 and a maximum value equal to or less than 10.0, such as, for example, 2.4 to 7.6. Any maximum numerical limitation recited in this specification is intended to include all lower numerical limitations subsumed therein and any minimum numerical limitation recited in this specification is intended to include all higher numerical limitations subsumed therein. Accordingly, Applicant reserves the right to amend this specification, including the claims, to expressly recite any sub-range subsumed within the ranges expressly recited herein. All such ranges are intended to be inherently described in this specification such that amending to expressly recite any such sub-ranges would comply with the requirements of 35 U.S.C. §112, first paragraph, and 35 U.S.C. §132(a). Additionally, as used herein when referring to compositional elemental ranges, the term “up to” includes zero unless the particular element is present as an unavoidable impurity.
The grammatical articles “one”, “a”, “an”, and “the”, as used in this specification, are intended to include “at least one” or “one or more”, unless otherwise indicated. Thus, the articles are used in this specification to refer to one or more than one (i.e., to “at least one”) of the grammatical objects of the article. By way of example, “a component” means one or more components, and thus, possibly, more than one component is contemplated and may be employed or used in an implementation of the described embodiments. Further, the use of a singular noun includes the plural, and the use of a plural noun includes the singular, unless the context of the usage requires otherwise.
As used herein, the term “billet” refers to a solid semi-finished product, commonly having a generally round or square cross-section, that has been hot worked by forging, rolling, or extrusion. This definition is consistent with the definition of “billet” in, for example, ASM Materials Engineering Dictionary, J. R. Davis, ed., ASM International (1992), p. 40.
As used herein, the term “bar” refers to a solid product forged, rolled or extruded from a billet to a form commonly having a symmetrical, generally round, hexagonal, octagonal, square, or rectangular cross-section, with sharp or rounded edges, and that has a length greater than its cross-sectional dimensions. This definition is consistent with the definition of “bar” in, for example, ASM Materials Engineering Dictionary, J. R. Davis, ed., ASM International (1992), p. 32. It is recognized that as used herein, the term “bar” may refer to the form described above, except that the form may not have a symmetrical cross-section, such as, for example a non-symmetrical cross-section of a hand rolled bar.
As used herein, the phrase “cold working” refers to working a metallic (i.e., a metal or metal alloy) article at a temperature below that at which the flow stress of the material is significantly diminished. Examples of cold working involve processing a metallic article at such temperatures using one or more techniques selected from rolling, forging, extruding, pilgering, rocking, drawing, flow-turning, liquid compressive forming, gas compressive forming, hydro-forming, flow forming, bulge forming, roll forming, stamping, fine-blanking, die pressing, deep drawing, coining, spinning, swaging, impact extruding, explosive forming, rubber forming, back extrusion, piercing, stretch forming, press bending, electromagnetic forming, and cold heading. As used herein in connection with the present invention, “cold working”, “cold worked”, “cold forming”, and like terms, and “cold” used in connection with a particular working or forming technique, refer to working or the characteristic of having been worked, as the case may be, at a temperature no greater than about 1250° F. (677° C.). In certain embodiments, such working occurs at a temperature no greater than about 1000° F. (538° C.). In certain other embodiments, cold working occurs at a temperature no greater than about 575° F. (300° C.). The terms “working” and “forming” are generally used interchangeably herein, as are the terms “workability” and “formability” and like terms.
As used herein, the phrase “ductility limit” refers to the limit or maximum amount of reduction or plastic deformation a metallic material can withstand without fracturing or cracking. This definition is consistent with the definition of “ductility limit” in, for example, ASM Materials Engineering Dictionary, J. R. Davis, ed., ASM International (1992), p 131. As used herein, the term “reduction ductility limit” refers to the amount or degree of reduction that a metallic material can withstand before cracking or fracturing.
Reference herein to an alpha-beta titanium alloy “comprising” a particular composition is intended to encompass alloys “consisting essentially of” or “consisting of” the stated composition. It will be understood that alpha-beta titanium alloy compositions described herein that “comprise”, “consist of”, or “consist essentially of” a particular composition also may include incidental impurities.
A non-limiting aspect of the present disclosure is directed to a cobalt-containing alpha-beta titanium alloy that exhibits certain cold-deformation properties superior to Ti-6Al-4V alloy, but without the need to provide additional beta phase or further restrict the oxygen content compared to Ti-6Al-4V alloy. The ductility limit of the alloys of the present disclosure is significantly increased compared to that of Ti-6Al-4V alloy.
Contrary to the current understanding that oxygen additions to titanium alloys reduce the formability of the alloys, the cobalt-containing alpha-beta titanium alloys disclosed herein possess greater formability than Ti-6Al-4V alloy while including up to 66% greater oxygen content than Ti-6Al-4V alloy. The compositional range of cobalt-containing alpha-beta titanium alloy embodiments disclosed herein enables greater flexibility of alloy usage, without adding substantial cost associated with alloy additions. While various embodiments of alloys according to the present disclosure may be more expensive than Ti-4Al-2.5V alloy in terms of starting materials costs, the alloying additive costs for the cobalt-containing alpha-beta titanium alloys disclosed herein may be less than certain other cold formable alpha-beta titanium alloys.
The addition of cobalt in the alpha-beta titanium alloys disclosed herein has been found to increase the ductility of the alloys when the alloys also include low levels of aluminum. In addition the addition of cobalt to the alpha-beta titanium alloys according to the present disclosure has been found to increase alloy strength.
According to a non-limiting embodiment of the present disclosure, an alpha-beta titanium alloy comprises, in weight percentages: an aluminum equivalency in the range of 2.0 to 10.0; a molybdenum equivalency in the range of 0 to 20.0; 0.3 to 5.0 cobalt; titanium; and incidental impurities.
In another non-limiting embodiment, an alpha-beta titanium alloy comprises, in weight percentages an aluminum equivalency in the range of 2.0 to 10.0; a molybdenum equivalency in the range of 0 to 10.0; 0.3 to 5.0 cobalt; and titanium. In yet another non-limiting embodiment, an alpha-beta titanium alloy comprises, in weight percentages an aluminum equivalency in the range of 1.0 to 6.0; a molybdenum equivalency in the range of 0 to 10.0; 0.3 to 5.0 cobalt; and titanium. For each of the embodiments disclosed herein, aluminum equivalency is in terms of an equivalent weight percentage of aluminum and is calculated by the following equation, in which the content of each alpha phase stabilizer element is in weight percent:
[Al]eq=[Al]+⅓[Sn]+⅙[Zr+Hf]+10[O+2N+C]+[Ga]+[Ge].
While it is known that cobalt is a beta phase stabilizer for titanium, for all embodiments disclosed herein, molybdenum equivalency is in terms of an equivalent weight percentage of molybdenum and is calculated herein by the following equation, in which the content of each beta phase stabilizer element is in weight percent:
[Mo]eq=[Mo]+⅔[V]+3[Mn+Fe+Ni+Cr+Cu+Be]+⅓[Ta+Nb+W].
In certain non-limiting embodiments according to the present disclosure, the cobalt-containing alpha-beta titanium alloys disclosed herein include greater than 0 up to 0.3 total weight percent of one or more grain refinement additives. The one or more grain refinement additives may be any of the grain refinement additives known to those having ordinary skill in the art, including, but not necessarily limited to, cerium, praseodymium, neodymium, samarium, gadolinium, holmium, erbium, thulium, yttrium, scandium, beryllium, and boron.
In further non-limiting embodiments, any of the cobalt-containing alpha-beta titanium alloys disclosed herein may further include greater than 0 up to 0.5 total weight percent of one or more corrosion inhibiting metal additives. The corrosion inhibiting additives may any one or more of the corrosion inhibiting additives known for use in alpha-beta titanium alloys. Such additives include, but are not limited to, gold, silver, palladium, platinum, nickel, and iridium.
In further non-limiting embodiments, any of the cobalt-containing alpha-beta titanium alloys disclosed herein may include one or more of, in weight percentages: greater than 0 up to 6.0 tin; greater than 0 up to 0.6 silicon; greater than 0 up to 10 zirconium. It is believed that additions of these elements within these concentration ranges will not affect the ratio of the concentrations of alpha and beta phases in the alloy.
In certain non-liming embodiments of an alpha-beta titanium alloy according to the present disclosure, the alpha-beta titanium alloy exhibits a yield strength of at least 130 KSI (896.3 MPa) and a percent elongation of at least 10%. In other non-limiting embodiments, the alpha-beta titanium alloy exhibits a yield strength of at least 150 KSI (1034 MPa) and a percent elongation of at least 16%.
In certain non-liming embodiments of an alpha-beta titanium alloy according to the present disclosure, the alpha-beta titanium alloy exhibits a cold working reduction ductility limit of at least 20%. In other non-liming embodiments, the alpha-beta titanium alloy exhibits a cold working reduction ductility limit of at least 25%, or at least 35%.
In certain non-liming embodiments of an alpha-beta titanium alloy according to the present disclosure, the alpha-beta titanium alloy further comprises aluminum. In a non-limiting embodiment, the alpha-beta titanium alloy comprises, in weight percentages: 2.0 to 7.0 aluminum; a molybdenum equivalency in the range of 2.0 to 5.0; 0.3 to 4.0 cobalt; up to 0.5 oxygen; up to 0.25 nitrogen; up to 0.3 carbon; up to 0.2 of incidental impurities; and titanium. The molybdenum equivalency is determined as described herein. In certain non-limiting embodiments, alpha-beta titanium alloys herein comprising aluminum may further comprise one or more of, in weight percentages: greater than 0 to 6 tin; greater than 0 to 0.6 silicon; greater than 0 to 10 zirconium; greater than 0 to 0.3 palladium; and greater than 0 to 0.5 boron.
In certain non-liming embodiments of an alpha-beta titanium alloy according to the present disclosure comprising aluminum, the alloys may further include greater than 0 up to 0.3 total weight percent of one or more grain refinement additives. The one or more grain refinement additives may be, for example, any of the grain refinement additives cerium, praseodymium, neodymium, samarium, gadolinium, holmium, erbium, thulium, yttrium, scandium, beryllium, and boron.
In certain non-limiting embodiments of an alpha-beta titanium alloy according to the present disclosure comprising aluminum, the alloys may further include greater than 0 up to 0.5 total weight percent of one or more corrosion resistance additives known to those having ordinary skill in the art, including, but not necessarily limited to gold, silver, palladium, platinum, nickel, and iridium.
Certain non-liming embodiments of the alpha-beta titanium alloys disclosed herein comprising cobalt and aluminum exhibit a yield strength of at least 130 KSI (896 MPa) and a percent elongation of at least 10%. Other non-limiting embodiments of the alpha-beta titanium alloys herein comprising cobalt and aluminum exhibit a yield strength of at least 150 KSI (1034 MPa) and a percent elongation of at least 16%.
Certain non-limiting embodiments of the alpha-beta titanium alloys disclosed herein comprising cobalt and aluminum exhibit a cold working reduction ductility limit of at least 25%. Other non-liming embodiments of the alpha-beta titanium alloys herein comprising cobalt and aluminum exhibit a cold working reduction ductility limit of at least 35%.
Referring to
In a specific embodiment, cold working 102 the metallic form comprises cold rolling the metallic form.
In a non-limiting embodiment of a method according to the present disclosure, the metallic form is cold worked 102 at a temperature less than 1250° F. (676.7° C.). In another non-limiting embodiment of a method according to the present disclosure, the metallic form is cold worked 102 at a temperature no greater than 575° F. (300° C.). In another non-limiting embodiment of a method according to the present disclosure, the metallic form is cold worked 102 at a temperature less than 392° F. (200° C.). In still another non-limiting embodiment of a method according to the present disclosure, the metallic form is cold worked 102 at a temperature in the range of −148° F. (−100° C.) to 392° F. (+200° C.).
In a non-limiting embodiment of a method according to the present disclosure, the metallic form is cold worked 102 between intermediate anneals (not shown) to a reduction of at least 25% or at least 35%. The metallic form may be annealed between intermediate multiple cold working steps at a temperature less than the beta-transus temperature of the alloy in order relieve internal stresses and minimize chances of edge cracking. In non-limiting embodiments, an annealing step (not shown) intermediate cold working steps 102 may include annealing the metallic form at a temperature in the range of Tβ−36° F. (Tβ−20° C.) and Tβ−540° F. (Tβ−300° C.) for 5 minutes to 2 hours. The Tβ of alloys of the present disclosure is typically between 1652° F. (900° C.) and 2012° F. (1100° C.). The Tβ of any specific alloy of the present disclosure can be determined using conventional techniques by a person having ordinary skill in the art without undue experimentation.
After the step of cold working 102 the metallic form, in certain non-limiting embodiments of the present method, the metallic form may be mill annealed (not shown) to obtain desired strength and ductility and the alpha-beta microstructure of the alloy. Mill annealing, in a non-limiting embodiment, may include heating the metallic form to a temperature in a range of 1112° F. (600° C.) to 1706° F. (930° C.) and holding for 5 minutes to 2 hours.
The metallic form processed according to various embodiments of the methods disclosed herein may be selected from any mill product or semi-finished mill product. The mill product or semi-finished mill product may be selected from, for example, an ingot, a billet, a bloom, a bar, a beam, a slab, a rod, a wire, a plate, a sheet, an extrusion, and a casting.
A non-limiting embodiment of the methods disclosed herein further comprises hot working (not shown) the metallic form prior to cold working 102 the metallic form. A person skilled in the art understands that hot working involves plastically deforming a metallic form at temperatures above the recrystallization temperature of the alloy comprising the metallic form. In certain non-limiting embodiments, the metallic form may be hot worked at a temperature in the beta phase field of the alpha-beta titanium alloy. In one specific non-limiting embodiment, the metallic form is heated to a temperature of at least Tβ+54° F. (Tβ+30° C.), and hot worked. In certain non-limiting embodiments, the metallic form may be hot worked at a temperature in the beta phase field of the titanium alloy to at least a 20 percent reduction. In certain non-limiting embodiments, after hot working the metallic form in the beta phase field, the metallic form may be cooled to ambient temperature at a rate that is at least comparable to air cooling.
After hot working at a temperature in the beta phase field, in various non-limiting embodiments of a method according to the present disclosure, the metallic form may be further hot worked at a temperature in the alpha-beta phase field. Hot working in the alpha-beta phase field may include reheating the metallic form to a temperature in the alpha-beta phase field. Alternatively, after working the metallic form in the beta phase field, the metallic form may be cooled to a temperature in the alpha-beta phase field and then further hot worked. In a non-limiting embodiment, the hot working temperature in the alpha-beta phase field is in a range of Tβ−540° F. (Tβ−300° C.) to Tβ−36° F. (Tβ−20° C.). In a non-limiting embodiment, the metallic form is hot worked in the alpha-beta phase field to a reduction of at least 30%. In a non-limiting embodiment, after hot working in the alpha-beta phase filed, the metallic form may be cooled to ambient temperature at a rate that is at least comparable to air cooling. After cooling, in a non-limiting embodiment, the metallic form may be annealed at a temperature in the range of Tβ−36° F. (Tβ−20°) to Tβ−540° F. (Tβ−300° C.) for 5 minutes to 2 hours.
Referring now to
The molybdenum equivalency of the cobalt-containing, aluminum containing, alpha-beta titanium alloy is provided by the following equation, in which the beta phase stabilizers listed in the equation are weight percentages:
[Mo]eq=[Mo]+⅔[V]+3[Mn+Fe+Ni+Cr+Cu+Be]+⅓[Ta+Nb+W].
In another non-limiting method embodiment of the present disclosure, the cobalt-containing, aluminum-containing, alpha-beta titanium alloy is cold worked to a reduction in cross-sectional area of at least 35 percent.
In a non-limiting embodiment, cold working 204 the cobalt containing, aluminum-containing, alpha-beta titanium alloy to a reduction of at least 25%, or at least 35%, may take place in one or more cold rolling steps. The cobalt containing, aluminum-containing, alpha-beta titanium alloy may be annealed (not shown) intermediate multiple cold working steps 204 at a temperature less than the beta-transus temperature in order relieve internal stresses and minimize chances of edge cracking. In non-limiting embodiments, an annealing step intermediate cold working steps may include annealing the cobalt containing, aluminum-containing, alpha-beta titanium alloy at a temperature in the range of Tβ−36° F. (Tβ−20°) to Tβ−540° F. (Tβ−300° C.) for 5 minutes to 2 hours. The Tβ of alloys of the present disclosure is typically between 1652° F. (900° C.) and 2192° F. (1200° C.). The Tβ of any specific alloy of the present disclosure can be determined by a person having ordinary skill in the art without undue experimentation.
After cold working 204, in a non-limiting embodiment, the cobalt containing, aluminum-containing, alpha-beta titanium alloy may be mill annealed (not shown) to obtain the desired strength and ductility. Mill annealing, in a non-limiting embodiment, may include heating the cobalt containing, aluminum-containing, alpha-beta titanium alloy to a temperature in a range of 1112° F. (600° C.) to 1706° F. (930° C.) and holding for 5 minutes to 2 hours.
In a specific embodiment, cold working 204 of the cobalt-containing, aluminum-containing, alpha-beta titanium alloy disclosed herein comprises cold rolling.
In a non-limiting embodiment, the cobalt-containing, aluminum-containing, alpha-beta titanium alloy disclosed herein is cold worked 204 at a temperature of less than 1250° F. (676.7° C.). In another non-limiting embodiment of a method according to the present disclosure, the cobalt-containing, aluminum-containing, alpha-beta titanium alloy disclosed herein is cold worked 204 at a temperature no greater than 575° F. (300° C.). In another non-limiting embodiment, the cobalt-containing, aluminum-containing, alpha-beta titanium alloy disclosed herein is cold worked 204 at a temperature of less than 392° F. (200° C.). In still another non-limiting embodiment, the cobalt-containing, aluminum-containing, alpha-beta titanium alloy disclosed herein is cold worked 204 at a temperature in a range of −148° F. (−100° C.) to 392° F. (200° C.)
Prior to the cold working step 204, the cobalt-containing, aluminum-containing, alpha-beta titanium alloy disclosed herein may be a mill product or semi-finished mill product in a form selected from one of an ingot, a billet, a bloom, a beam, a slab, a rod, a bar, a tube, a wire, a plate, a sheet, an extrusion, and a casting.
Also prior to the cold working step, the cobalt-containing, aluminum-containing, alpha-beta titanium alloy disclosed herein may be hot worked (not shown). Hot working processes that are disclosed for the metallic form hereinabove are equally applicable to the cobalt-containing, aluminum-containing, alpha-beta titanium alloy disclosed herein.
The cold formability of the cobalt-containing, alpha-beta titanium alloys disclosed herein, which includes higher oxygen levels than found, for example, in Ti-6Al-4V alloy, is counter-intuitive. For example, Grade 4 CP (Commercially Pure) titanium, which includes a relatively high level of up to 0.4 weight percent oxygen, is known to be less formable than other CP grades. While the Grade 4 CP alloy has higher strength than Grades 1, 2, or 3 CP, it exhibits a lower strength than embodiments of the alloys disclosed herein.
Cold working techniques that may be used with the cobalt-containing, alpha-beta titanium alloys disclosed herein include, for example, but are not limited to, cold rolling, cold drawing, cold extrusion, cold forging, rocking/pilgering, cold swaging, spinning, and flow-turning. As is known in the art, cold rolling generally consists of passing previously hot rolled articles, such as bars, sheets, plates, or strip, through a set of rolls, often several times, until a desired gauge is obtained. Depending upon the starting structure after hot (alpha-beta) rolling and annealing, it is believed that at least a 35-40% reduction in area (RA) could be achieved by cold rolling a cobalt-containing, alpha-beta titanium alloy before any annealing is required prior to further cold rolling. Subsequent cold reductions of at least 20-60%, or at least 25%, or at least 35%, are believed possible, depending on product width and mill configuration.
Based on the inventor's observations, cold rolling of bar, rod, and wire on a variety of bar-type mills, including Koch's-type mills, also may be accomplished on the cobalt-containing, alpha-beta titanium alloys disclosed herein. Additional non-limiting examples of cold working techniques that may be used to form articles from the cobalt-containing, alpha-beta titanium alloys disclosed herein include pilgering (rocking) of extruded tubular hollows for the manufacture of seamless pipe, tube, and ducting. Based on the observed properties of the cobalt-containing, alpha-beta titanium alloys disclosed herein, it is believed that a larger reduction in area (RA) may be achieved in compressive type forming than with flat rolling. Drawing of rod, wire, bar, and tubular hollows also may be accomplished. A particularly attractive application of the cobalt-containing, alpha-beta titanium alloys disclosed herein is drawing or pilgering to tubular hollows for production of seamless tubing, which is particularly difficult to achieve with Ti-6Al-4V alloy. Flow forming (also referred to in the art as shear-spinning) may be accomplished using the cobalt-containing, alpha-beta titanium alloys disclosed herein to produce axially symmetric hollow forms including cones, cylinders, aircraft ducting, nozzles, and other “flow-directing”-type components. A variety of liquid or gas-type compressive, expansive type forming operations such as hydro-forming or bulge forming may be used. Roll forming of continuous-type stock may be accomplished to form structural variations of “angle iron” or “uni-strut” generic structural members. In addition, based on the inventor's findings, operations typically associated with sheet metal processing, such as stamping, fine-blanking, die pressing, deep drawing, and coining may be applied to the cobalt-containing, alpha-beta titanium alloys disclosed herein.
In addition to the above cold forming techniques, it is believed that other “cold” techniques that may be used to form articles from the cobalt-containing, alpha-beta titanium alloys disclosed herein include, but are not necessarily limited to, forging, extruding, flow-turning, hydro-forming, bulge forming, roll forming, swaging, impact extruding, explosive forming, rubber forming, back extrusion, piercing, spinning, stretch forming, press bending, electromagnetic forming, and cold heading. Those having ordinary skill, upon considering the inventor's observations and conclusions and other details provided in the present description of the invention, may readily comprehend additional cold working/forming techniques that may be applied to the cobalt-containing, alpha-beta titanium alloys disclosed herein. Also, those having ordinary skill may readily apply such techniques to the alloys without undue experimentation. Accordingly, only certain examples of cold working of the alloys are described herein. The application of such cold working and forming techniques may provide a variety of articles. Such articles include, but are not necessarily limited to the following: a sheet, a strip, a foil, a plate, a bar, a rod, a wire, a tubular hollow, a pipe, a tube, a cloth, a mesh, a structural member, a cone, a cylinder, a duct, a pipe, a nozzle, a honeycomb structure, a fastener, a rivet, and a washer.
The unexpected cold workability of the cobalt-containing, alpha-beta titanium alloys disclosed herein results in finer surface finishes and a reduced need for surface conditioning to remove the heavy surface scale and diffused oxide layer that typically results on the surface of a Ti-6Al-4V alloy pack rolled sheet. Given the level of cold workability the present inventor has observed, it is believed that foil thickness product in coil lengths may be produced from the cobalt-containing, alpha-beta titanium alloys disclosed herein with properties similar to those of Ti-6Al-4V alloy.
The examples that follow are intended to further describe certain non-limiting embodiments, without restricting the scope of the present invention. Persons having ordinary skill in the art will appreciate that variations of the following examples are possible within the scope of the invention, which is defined solely by the claims.
Two alloys were made having compositions such that limited cold formability was anticipated. The compositions of these alloys, in weight percentages, and their observed rollability are presented in Table 1.
The alloys were melted and cast into buttons by non-consumable arc melting. Subsequent hot rolling was conducted in the beta phase field, and then in the alpha-beta phase field to produce a cold-rollable microstructure. During this hot rolling operation the non-cobalt containing alloy failed in a catastrophic manner, resulting from lack of ductility. In comparison, the cobalt-containing alloy was successfully hot rolled from about 1.27 cm (0.5 inch) thick to about 0.381 cm (0.15 inch) thick. The cobalt-containing alloy was then cold-rolled.
The cobalt-containing alloy was then subsequently cold rolled to a final thickness of below 0.76 mm (0.030 inch) with intermediate annealing and conditioning. Cold rolling was conducted until the onset of cracks exhibiting a length of 0.635 cm (0.25 inch) was observed. The percent reduction achieved during cold working until edge cracks were observed, i.e., the cold reduction ductility limit, was recorded. It was surprisingly observed in this example that a cobalt-containing alpha-beta titanium alloy was successfully hot and then cold rolled, without exhibiting substantial cracks, to at least a 25 percent cold rolling reduction, whereas the comparative alloy, which lacked a cobalt addition, could not be hot rolled without failing in a catastrophic manner.
The mechanical performance of a second alloy (Heat 5) within the scope of the present disclosure was compared with a small coupon of Ti-4Al-2.5V alloy. Table 2 lists the composition of Heat 5 and, for comparison purposes, the composition a heat of a Ti-4Al-2.5V (which lacks Co). The compositions in Table 2 are provided in weight percentages.
Buttons of Heat 5 and the comparative Ti-4Al-2.5V alloy were prepared by melting, hot rolling, and then cold rolling in the same manner as the cobalt-containing alloy of Example 1. The yield strength (YS), ultimate tensile strength (UTS), and percent elongation (% El.) were measured according to ASTM E8/E8M-13a and are listed in Table 2. Neither alloy exhibited cracking during the cold rolling. The strength and ductility (% El.) of the Heat 5 alloy exceeded those of the Ti-4Al-2.5V button.
The cold rolling capability, or the reduction ductility limit, was compared based on alloy composition. Buttons of alloy Heats 1-4 were compared with a button having the same composition as the Ti-4Al-2.5V alloy used in Example 2. The buttons were prepared by melting, hot rolling, and then cold rolling in the manner used for the cobalt-containing alloy of Example 1. The buttons were cold rolled until substantial cracking was observed. Table 3 lists the compositions (remainder titanium and incidental impurities) of the inventive and comparative buttons, in weight percentages, and the cold working reduction ductility limit expressed in percent reduction of the hot rolled buttons.
From the results in Table 3, it is observed that higher oxygen content is tolerated without loss of cold ductility in the alloys containing cobalt. The inventive alpha-beta titanium alloy heats (Heats 1-4) exhibited cold reduction ductility limits that were superior to the button of the Ti-4Al-2.5V alloy. For comparison, it is noted that Ti-6Al-4V alloy cannot be cold rolled for commercial purposes without the onset of cracking, and typically contains 0.14 to 0.18 weight percent oxygen. These results clearly show that the cobalt-containing alpha-beta alloys of the present disclosure surprisingly exhibited strengths and cold ductility that are at least comparable to Ti-4Al-2.5 alloy, strengths that are comparable to Ti-6Al-4V alloy, and cold ductility that is clearly superior to Ti-6Al-4V alloy.
In Table 2, the cobalt-containing alpha-beta titanium alloys of the present disclosure exhibit greater ductility and strength than a Ti-4Al-2.5V alloy. The results listed in Tables 1-3 show that the cobalt-containing alpha-beta titanium alloys of the present disclosure exhibit significantly greater cold ductility than Ti-6Al-4V alloy, despite having 33-66% more interstitial content, which tends to decrease ductility.
It was not anticipated that cobalt additions would increase the cold rolling capability of an alloy containing high levels of interstitial alloying elements, such as oxygen. From the perspective of an ordinarily skilled practitioner, it was unanticipated that cobalt additions would increase cold-ductility without reducing strength levels. Intermetallic precipitates of Ti3X-type, where X represents a metal, typically reduce cold ductility quite substantially, and it has been shown in the art that cobalt does not substantially increase strength or ductility. Most alpha-beta titanium alloys contain approximately 6% aluminum, which can form Ti3Al when combined with cobalt additions. This can have a deleterious effect on ductility.
The results presented hereinabove surprisingly demonstrate that cobalt additions do in fact improve ductility and strength in the present titanium alloys compared with Ti-4Al-2.5V alloy and other cold deformable alpha+beta alloys. Embodiments of the present alloys include a combination of alpha stabilizers, beta stabilizers, and cobalt.
Cobalt additions apparently work with other alloying additions to enable the alloys of the present disclosure to have high oxygen tolerance without negatively affecting ductility or cold processing capability. Traditionally, high oxygen tolerance is not commensurate with cold ductility and high strength simultaneously.
By maintaining a high level of alpha phase in the alloy, it may be possible to preserve machinability of cobalt-containing alloys compared with other alloys having a greater beta phase content, such as, for example, Ti-5553 alloy, Ti-3553 alloy, and SP-700 alloy. Cold ductility also increases the degree of dimensional control and control of surface finish achievable compared with other high-strength alpha-beta titanium alloys that are not cold-deformable in mill products.
It will be understood that the present description illustrates those aspects of the invention relevant to a clear understanding of the invention. Certain aspects that would be apparent to those of ordinary skill in the art and that, therefore, would not facilitate a better understanding of the invention have not been presented in order to simplify the present description. Although only a limited number of embodiments of the present invention are necessarily described herein, one of ordinary skill in the art will, upon considering the foregoing description, recognize that many modifications and variations of the invention may be employed. All such variations and modifications of the invention are intended to be covered by the foregoing description and the following claims.
