Patent Application
20190169713
The present disclosure relates to titanium (Ti) alloys. In particular, alpha-beta Ti alloys having an improved combination of mechanical properties achieved with a relatively low-cost composition are described as well as methods of manufacturing the Ti alloys.
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Ti alloys have found widespread use in applications requiring high strength-to-weight ratios, good corrosion resistance and retention of these properties at elevated temperatures. Despite these advantages, the higher raw material and processing costs of Ti alloys compared to steel and other alloys have severely limited their use to applications where the need for improved efficiency and performance outweigh their comparatively higher cost. Some typical applications which have benefited from the incorporation of Ti alloys in various capacities include, but are not limited to, aeroengine discs, casings, fan and compressor blades; airframe components; orthopedic components; armor plate and various industrial/engineering applications.
A conventional Ti-base alloy which has been successfully used in a variety of applications is Ti-6Al-4V, which is also known as Ti 6-4. As the name suggests, this Ti alloy generally contains 6 wt. % aluminum (Al) and 4 wt. % vanadium (V). Ti 6-4 also typically includes up to 0.30 wt. % iron (Fe) and up to 0.30 wt. % oxygen (O). Ti 6-4 has become established as the “workhorse” titanium alloy where strength/weight ratio at moderate temperatures is a key parameter for material selection. Ti 6-4 has a balance of properties which is suitable for a wide variety of static and dynamic structural applications, it can be reliably processed to give consistent properties, and it is comparatively economical.
Recently, the design of new aircraft engines has been driven by airline demands for reduced atmospheric emissions and noise, reduced fuel costs, and reduced maintenance and spare part costs. Competition between engine builders has caused them to respond by designing engines with higher bypass ratios, higher pressures in the compressor, and higher temperatures in the turbine. These enhanced mechanical properties require an alloy that has a higher strength than Ti 6-4, but the same density and near equivalent ductility.
Other alloys, such as TIMETAL® 550 (Ti-4.0Al-4.0Mo-2.0Sn-0.5Si) and VT 8 (Ti-6.0Al-3.2Mo-0.4Fe-0.3Si-0.15O), gain approximately 100 MPa of strength compared to Ti 6-4 from the inclusion of silicon in the alloy. However, these alloys have a higher density and a higher production cost, compared to Ti 6-4, because they use molybdenum as the main beta stabilizing element, as opposed to vanadium. The cost premium arises not only from the greater cost of molybdenum relative to vanadium, but also because the use of Ti 6-4 turnings and machining chip as a raw material is precluded in those alloys.
Therefore, there is a need in the industry to provide a cost-effective alloy that has a higher strength, finer grain size, and a particularly improved Low Cycle Fatigue Life with a comparable density when compared to Ti 6-4.
In one form of the present disclosure a method of manufacturing a titanium alloy part is provided. The method comprises performing a first heat treatment on an ingot of a titanium alloy at a temperature between 40 and 200 degrees Centigrade above the beta transus temperature of the titanium alloy, forging the ingot to break down the cast structure of the ingot, and then cooling the forged ingot. A second heat treatment is performed on the forged ingot at a temperature between 30 and 100 degrees Centigrade below the beta transus temperature, the forged ingot is rolled to a plate, bar, or billet, and the plate, bar or billet is annealed at a temperature below the beta transus temperature. In weight percent (wt. %) the titanium alloy comprises: aluminum from about 6.0 to about 6.7; vanadium from about 1.4 to about 2.0; molybdenum from about 1.4 to about 2.0; silicon from about 0.20 to about 0.35; oxygen from about 0.18 to about 0.23; iron from about 0.16 to about 0.24; carbon from about 0.02 to about 0.06; and balance titanium with incidental impurities. In some aspects of the present disclosure, the forged ingot is reheated to a temperature between 50 and 150 degrees Centigrade above the beta transus temperature to allow recrystallization of the beta phase. In the alternative, or in addition to, the forged ingot is reheated to a temperature between 30 to 150 degrees Centigrade above the beta transus temperature to allow recrystallization of the beta phase and then forged to a strain of at least 10 percent and water quenched.
In some aspects of the present disclosure, the titanium alloy comprises at least one of: aluminum from about 6.3 to about 6.7; vanadium from about 1.5 to about 1.9; molybdenum from about 1.5 to about 1.9; silicon from about 0.34 to about 0.38; oxygen from about 0.18 to about 0.21; iron from about 0.1 to about 0.2; and carbon from about 0.01 to about 0.05. In such aspects, the alloy may consist of at least one of: aluminum at about 6.5; vanadium at about 1.7; molybdenum at about 1.7; silicon at about 0.36; oxygen at about 0.2; iron at about 0.16; and carbon at about 0.03. Also, the maximum concentration of any one impurity element present in the alloy is 0.1 wt. % and the combined concentration of all impurities is less than or equal to 0.4 wt. %. The titanium alloy may have at least one of a molybdenum equivalence (Moeq) of 2.6 to 4.0 and an aluminum equivalence (Aleq) of 10.6 to about 12.9, when the molybdenum equivalence is defined as: Moeq=Mo+0.67V+2.9Fe and the aluminum equivalence is defined as: Aleq=Al+27O.
In some aspects of the present disclosure, the annealed plate, bar or billet of the titanium alloy has an ultimate tensile strength (UTS) greater than 950 MPa; a tensile yield strength of at least 1,000 MPa; and/or an elongation of at least 10%. In the alternative, or in addition to, the annealed plate, bar or billet of the titanium alloy has a V50 ballistic limit that is at least 80 feet per second greater than a base V50 ballistic limit measured for a T-64 alloy when a 0.616 inch thick plate is tested against a 12.7 mm diameter steel fragment simulating projectile, and/or a reduction of area (RA) of at least 25% of an original cross-sectional area of a tensile sample of the annealed alloy after fracture when evaluated using ASTM E8 standard. In other aspects of the present disclosure, the annealed plate, bar or billet of the titanium alloy has a UTS greater than 1,100 MPa; a tensile yield strength of at least 1,000 MPa; and an elongation of at least 10%. In the alternative, or in addition to, the annealed plate, bar or billet of the titanium alloy has a V50 ballistic limit that is at least 60 feet per second greater than a base V50 ballistic limit measured for a T-64 alloy when a 0.616 inch thick plate is tested against a 12.7 mm diameter steel fragment simulating projectile, and/or a RA of at least 25% of an original cross-sectional area of a tensile sample of the annealed alloy after fracture when evaluated using ASTM E8 standard.
In some aspects of the present disclosure, the annealed plate, bar or billet of the titanium alloy comprises a room temperature longitudinal low cycle fatigue (LCF) maximum stress of at least about 950 MPa over about 20,000 cycles and/or a room temperature transverse LCF maximum stress of at least about 970 MPa over about 25,000 cycles. The annealed plate, bar or billet of the titanium alloy has a density between 4.4 g/cm3 (0.161 lb./in3) and 4.55 g/cm3 (0.164 lb./in3). Also, the titanium alloy has beta transus temperature between 1010° C. and 1040° C.
In some aspects of the present disclosure, the annealed plate, bar or billet of the titanium alloy comprises a microstructure with a primary alpha phase in a background of beta phase. In the alternative, or in addition to, the annealed plate, bar or billet of the titanium alloy comprises a microstructure with a primary alpha phase in a background of beta phase.
In another form of the present disclosure, a method of manufacturing a titanium alloy part includes performing a first heat treatment on an ingot of a titanium alloy at a temperature between 40 and 200 degrees Centigrade above the beta transus temperature of the titanium alloy and forging the ingot to break down the cast structure of the ingot and then cooling the forged ingot. The titanium alloy comprises, in weight percent (wt. %): aluminum from about 6.0 to about 6.7; vanadium from about 1.4 to about 2.0; molybdenum from about 1.4 to about 2.0; silicon from about 0.20 to about 0.35; oxygen from about 0.18 to about 0.23; iron from about 0.16 to about 0.24; carbon from about 0.02 to about 0.06; and balance titanium with incidental impurities. A second heat treatment is performed on the forged ingot at a temperature between 30 and 100 degrees Centigrade below the beta transus temperature and the forged ingot is rolled to a plate, bar, or billet. The plate, bar, or billet is annealed at a temperature below the beta transus temperature and the annealed plate, bar, or billet comprises a UTS greater than 950 MPa, a tensile yield strength of at least 1,000 MPa, an elongation of at least 10%, and/or a reduction of area (RA) of at least 25% of the original cross-sectional area of a tensile sample of the annealed alloy after fracture when evaluated using ASTM E8. In some aspects of the present disclosure, the annealed plate, bar, or billet has a V50 ballistic limit that is at least 80 feet per second greater than a base V50 ballistic limit measured for a T-64 alloy when a 0.616 inch thick plate is tested against a 12.7 mm diameter steel fragment simulating projectile. In the alternative, or in addition to, the annealed plate, bar, or billet has a V50 ballistic limit that is at least 60 feet per second greater than a base V50 ballistic limit measured for a T-64 alloy when a 0.616 inch thick plate is tested against a 12.7 mm diameter steel fragment simulating projectile.
In still another form of the present disclosure, a method of manufacturing a titanium alloy part includes performing a first heat treatment on an ingot of a titanium alloy at a temperature between 40 and 200 degrees Centigrade above the beta transus temperature of the titanium alloy, forging the ingot to break down the cast structure of the ingot and then cooling the forged ingot. A second heat treatment of the forged ingot is performed at a temperature between 30 and 100 degrees Centigrade below the beta transus temperature of the titanium alloy and the forged ingot is rolled to a plate, bar, or billet. Also, the plate, bar or billet of the titanium alloy is annealed at a temperature below the beta transus temperature. The titanium alloy comprises, in weight percent (wt. %): aluminum from about 6.3 to about 6.7; vanadium from about 1.5 to about 1.9; molybdenum from about 1.5 to about 1.9; silicon from about 0.34 to about 0.38; oxygen from about 0.18 to about 0.21; iron from about 0.1 to about 0.2; carbon from about 0.01 to about 0.05; and balance titanium with incidental impurities. In some aspects of the present disclosure, the plate, bar or billet has a UTS greater than 950 MPa, a tensile yield strength of at least 1,000 MPa, an elongation of at least 10%, and a reduction of area (RA) of at least 25% of the original cross-sectional area of a tensile sample the annealed alloy after fracture when evaluated using ASTM E8 standard. Also, the plate, bar or billet may have a V50 ballistic limit that is at least 80 feet per second greater than a base V50 ballistic limit measured for a T-64 alloy when a 0.616 inch thick plate is tested against a 12.7 mm diameter steel fragment simulating projectile, and/or a V50 ballistic limit that is at least 60 feet per second greater than a base V50 ballistic limit measured for a T-64 alloy when a 0.616 inch thick plate is tested against a 12.7 mm diameter steel fragment simulating projectile.
In various aspects of the present disclosure an aviation component, for example a fan blade, is formed according to the teachings of the present disclosure. Also, the Ti alloy disclosed in this specification provides a comparative alternative to conventional Ti 6-4 alloys while meeting or exceeding mechanical properties established by the aerospace industry for Ti 6-4.
Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.
In order that the disclosure may be well understood, there will now be described various forms thereof, given by way of example, reference being made to the accompanying drawings, in which:
The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.
The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.
Exemplary Ti alloys having good mechanical properties which are formed using reasonably low cost materials are described. These Ti alloys are especially suited for use in a multitude of applications including aircraft components requiring higher strength and low cycle fatigue resistance when compared to Ti 6-4, such applications include, but are not limited to, blades, discs, casings, pylon structures or undercarriage. Additionally, the Ti alloys are suited for general engineering components using titanium alloys where higher strength to weight ratio would be advantageous.
The Ti alloy comprises, in weight percent, about 6.0 to about 6.7% aluminum, about 1.4 to about 2.0% vanadium, about 1.4 to about 2.0% molybdenum, about 0.20 to about 0.42% silicon, about 0.17 to about 0.23% oxygen, maximum about 0.24% iron, maximum about 0.08% carbon and balance titanium with incidental impurities. Preferably, the Ti alloy comprises, in weight percent, about 6.0 to about 6.7% aluminum, about 1.4 to about 2.0% vanadium, about 1.4 to about 2.0% molybdenum, about 0.20 to about 0.42% silicon, about 0.17 to about 0.23% oxygen, about 0.1 to about 0.24% iron, maximum about 0.08% carbon and balance titanium with incidental impurities. More preferably, the alloy comprises about 6.3 to about 6.7% aluminum, about 1.5 to about 1.9% vanadium, about 1.5 to about 1.9% molybdenum, about 0.33 to about 0.39% silicon, about 0.18 to about 0.21% oxygen, 0.1 to 0.2% iron, 0.01 to 0.05% carbon, and balance titanium with incidental impurities. Even more preferably, the Ti alloy comprises, in weight percent, about 6.5% aluminum, about 1.7% vanadium, about 1.7% molybdenum, about 0.36% silicon, about 0.2% oxygen, about 0.16% iron, about 0.03% carbon and balance titanium with incidental impurities.
Aluminum as an alloying element in titanium is an alpha stabilizer, which increases the temperature at which the alpha phase is stable. Aluminum can be present in the Ti alloy in a weight percentage of about 6.0 to about 6.7%. In particular, the aluminum is present at about 6.0, about 6.1, about 6.2, about 6.3, about 6.4, about 6.5, about 6.6, or about 6.7 wt. %. Preferably, the aluminum is present in a weight percentage of about 6.4 to about 6.7%. Even more preferably, the aluminum is present at about 6.5 wt. %. If the aluminum concentration were to exceed the upper limits disclosed in this specification, the workability of the alloy significantly deteriorates and the ductility and toughness worsen. On the other hand, the inclusion of aluminum levels below the limits disclosed in this specification can produce an alloy in which sufficient strength cannot be obtained.
Vanadium as an alloying element in titanium is an isomorphous beta stabilizer which lowers the beta transformation temperature. Vanadium can be present in the Ti alloy in a weight percentage of about 1.4 to about 2.0%. In particular, the vanadium is present in about 1.4, about 1.5, about 1.6, about 1.7, about 1.8, about 1.9, or 2.0 wt. %. Preferably, the vanadium is present in a weight percentage of about 1.5 to about 1.9%. More preferably, the vanadium is present at about 1.7 wt. %. If the vanadium concentration were to exceed the upper limits disclosed in this specification, the beta-stabilizer content of the alloy will be too high resulting in an increase in density relative to Ti 6-4. Also, if the vanadium concentration were to increase relative to the molybdenum content, the primary alpha grain size of the alloy would tend to increase. On the other hand, the use of vanadium levels that are too low can result in a deterioration in the strength and ductility of the alloy as the alloy tends toward near-alpha, rather than a true alpha-beta alloy.
Molybdenum as an alloying element in titanium is an isomorphous beta stabilizer which lowers the beta transformation temperature. Using the appropriate amount of molybdenum to cause refinement of the primary alpha grain size can provide improved ductility and fatigue life compared to an alloy using only vanadium as the beta stabilizing element. Molybdenum can be present in the Ti alloy in a weight percentage of about 1.4 to about 2.0%. In particular, the molybdenum is present in about 1.4, about 1.5, about 1.6, about 1.7, about 1.8, about 1.9, or about 2.0 wt. %. Preferably, the molybdenum is present in a weight percentage of about 1.5 to about 1.9%. Even more preferably, molybdenum is present at about 1.7 wt. %. If the molybdenum concentration were to exceed the upper limits disclosed in this specification, there is a technical disadvantage of increased density relative to Ti 6-4, and there is an economical and industrial consequence because the preeminence of Ti 6-4 as an industrial titanium alloy results in most of the scrap available for incorporation into ingots having that composition. Since the total beta stabilizer content of the alloy is limited to control the density, the proportion of beta stabilizers added as molybdenum is limited in order to optimize the economics of manufacture. On the other hand, the use of molybdenum levels below the limits disclosed in this specification can result in a deterioration in the strength and ductility of the alloy as the alloy tends toward near-alpha, rather than a true alpha-beta alloy.
Silicon as an alloying element in titanium is a eutectoid beta stabilizer which lowers the beta transformation temperature. Silicon can increase the strength and lower the density of titanium alloys. Additionally, silicon addition provides the required tensile strength without a major loss of the ductility, particularly when the molybdenum and vanadium balance is optimized. Furthermore, the silicon provides elevated temperature tensile properties relative to Ti 6-4 and similar to TIMETAL® 550. Silicon can be present in the Ti alloy in a weight percentage of about 0.2 to 0.42%. In particular, the silicon is present in about 0.20, about 0.22, about 0.24, about 0.26, about 0.28, about 0.30, about 0.32, about 0.34, about 0.36, about 0.38, about 0.40, or about 0.42 wt. %. Preferably, the silicon is present in a weight percent of about 0.34 to 0.38%. More preferably, the silicon is present at about 0.36 wt. %. If the silicon concentration were to exceed the upper limits disclosed in this specification, ductility, and toughness of the alloy will be deteriorated. On the other hand, the use of silicon levels below the limits disclosed in this specification can produce an alloy which has inferior strength.
Iron as an alloying element in titanium is a eutectoid beta stabilizer which lowers the beta transformation temperature, and iron is a strengthening element in titanium at ambient temperatures. Iron can be present in the Ti alloy in a maximum weight percentage of 0.24%. In particular, the iron can be present in about 0.04, about 0.8, about 0.10, about 0.12, about 0.15, about 0.16, about 0.20, or about 0.24 wt. %. Preferably, the iron is present in a weight percentage of about 0.10 to about 0.20%. More preferably, iron is present at about 0.16 wt. %. If the iron concentration were to exceed the upper limits disclosed in this specification, there will potentially be a segregation problem with the alloy and ductility and formability will consequently be reduced. On the other hand, the use of iron levels below the limits disclosed in this specification can produce an alloy that fails to achieve the desired high strength, deep hardenability, and excellent ductility properties.
Oxygen as an alloying element in titanium is an alpha stabilizer, and oxygen is an effective strengthening element in titanium alloys at ambient temperatures. Oxygen can be present in the Ti alloy in a weight percentage of about 0.17 to about 0.23%. In particular, the oxygen is present at about 0.17, about 0.18, about 0.19, about 0.20, about 0.21, about 0.22, or about 0.23 wt. %. Preferably, the oxygen is present in a weight percent of about 0.19 to about 0.21%. More preferably, oxygen is present at about 0.20 wt. %. If the content of oxygen is too low, the strength can be too low and the cost of the Ti alloy can increase because scrap metal will not be suitable for use in the melting of the Ti alloy. On the other hand, if the oxygen content is too great, ductility, toughness and formability will be deteriorated.
Carbon as an alloying element in titanium is an alpha stabilizer, which increases the temperature at which the alpha phase is stable. Carbon can be present in the Ti alloy in a maximum weight percentage of about 0.08%. In particular, the carbon is present in about 0.01, about 0.02, about 0.03, about 0.04, about 0.05, about 0.06, about 0.07, or about 0.08 wt. %. Preferably, the carbon is present in a weight percent of about 0.01 to about 0.05%. More preferably, the carbon is present at about 0.03%. If the content of carbon is too low, the strength of the alloy can be too low and the cost of the Ti alloy can increase because scrap metal will not be suitable for use in the melting of the Ti alloy. On the other hand, if the carbon content is too great, then the ductility of the alloy will be reduced.
The alloys according to the present disclosure may consist essentially of the recited elements. It will be appreciated that in addition to those elements, which are mandatory, other non-specific elements may be present in the composition provided that the essential characteristics of the composition are not materially affected by their presence.
The Ti alloy can also include incidental impurities or other added elements, such as Co, Cr, Cu, Ga, Hf, Mn, N, Nb, Ni, S, Sn, P, Ta, and Zr at concentrations associated with impurity levels for each element. The maximum concentration of any one of the incidental impurity element or other added element is preferably about 0.1 wt. % and the combined concentration of all impurities and/or added elements preferably does not exceed a total of about 0.4 wt. %.
The density of the Ti alloy is calculated to be between about 0.1614 pounds per cubic inch (lb/in3) (4.47 g/cm3) and about 0.1639 lb/in3 (4.54 g/cm3) with a nominal density of about 0.1625 lb/in3 (4.50 g/cm3).
The Ti alloy has a beta transus of about 1850° F. (1010° C.) to about 1904° F. (1040° C.). The microstructure of the Ti alloy is indicative of an alloy processed below the beta transus. Generally, the microstructure of the Ti alloy has a primary alpha grain size at least as fine as, or finer than, Ti 6-4. In particular, the microstructures of the Ti alloy comprise primary alpha phase (white particles) in a background of transformed beta phase (dark background). It is preferable to obtain a microstructure in which the primary alpha grain size is as fine as possible, in order to maintain ductility as the strength of the alloy is increased by varying the composition. In some aspects of the present disclosure, the primary alpha grain size may be less than about 15 μm.
The Ti alloy achieves excellent tensile properties. For example, when analyzed according to the ASTM E8 standard, the Ti alloy has a tensile yield strength (TYS) of at least about 145 ksi (1,000 MPa) and an ultimate tensile strength (UTS) of at least about 160 ksi (1,103 MPa) along both transverse and longitudinal directions. Additionally, the Ti alloy has an elongation of at least about 10%, and a reduction of area (RA) of at least about 25%.
The Ti alloy has a molybdenum equivalence (Moeq) of 2.6 to 4.0, wherein the molybdenum equivalence is defined as: Moeq=Mo+0.67V+2.9Fe. In a particular application, the Moeq is 3.3.
The Ti alloy aluminum equivalence (Aleq) of 10.6 to about 12.9 wherein the aluminum equivalence is defined as: Aleq=Al+27O. In a particular application, the Aleq is 11.9.
Additionally, the Ti alloy maintains its strength advantage over Ti 6-4 at high strain rates while exhibiting equivalent ductility to Ti 6-4. Furthermore, ballistic testing has shown that the Ti alloy exhibits resistance to fragment simulating projectiles which is equal to or greater than that of Ti 6-4. In particular, the Ti alloy demonstrates a V50 of at least 60 fps in ballistic testing performed using 0.50 Cal. (12.7 mm) Fragment Simulating Projectiles (FSP). In particular applications, the Ti alloy demonstrates a V50 of at least 80 fps. Also the Ti alloy exhibits comparable fracture toughness when compared to Ti 6-4. As is the case for Ti 6-4, the Ti alloy is recognized to be capable of a range of property combinations, dependent on the processing and heat treatment of the material.
The Ti alloy can be manufactured into different products or components having a variety of uses. For example, the Ti alloy can be formed into aircraft components such as discs, casings, pylon structures or undercarriages as well as automotive parts. In a particular application, the Ti alloy is used as a fan blade.
Also disclosed is a method for manufacturing a Ti alloy having good mechanical properties. The method includes melting a combination of source materials in the appropriate proportions to produce the Ti alloy comprising, in weight about 6.0 to about 6.7% aluminum, about 1.4 to about 2.0% vanadium, about 1.4 to about 2.0% molybdenum, about 0.20 to about 0.42% silicon, about 0.17 to about 0.23% oxygen, about 0.1 to about 0.24% iron, maximum about 0.08% carbon and balance titanium with incidental impurities. Melting may be accomplished in, for example, a cold hearth furnace, optionally followed by remelting in a vacuum arc remelting (VAR) furnace. Alternatively, ingot production may be accomplished by multiple melting in VAR furnaces. The source materials may comprise a combination of recycled and virgin materials such as titanium scrap and titanium sponge in combination with small amounts of iron. Under most market conditions, the use of recycled materials offers significant cost savings. The recycled materials used may include, but are not limited to, Ti 6-4, Ti-10V-2Fe-3Al, other Ti—Al—V—Fe alloys, and CP titanium. Recycled materials may be in the form of machining chip (turnings), solid pieces, or remelted electrodes. The virgin materials used may include, but are not limited to, titanium sponge, aluminum-vanadium; aluminum-molybdenum; and titanium-silicon master alloys, iron powder, silicon granules, or aluminum shot. Since the use of Ti—Al—V alloy recycled materials allow reduced or no aluminum-vanadium master alloy to be used, significant cost savings can be attained. This does not, however, preclude the use and addition of virgin raw materials comprising titanium sponge and alloying elements rather than recycled materials if so desired.
The manufacturing method can also include melting ingots of the alloy and forging the Ti alloy in a sequence above and below the beta transformation temperature followed by forging and/or rolling below the beta transformation temperature. In a particular application, the method of manufacturing the Ti alloy is used to produce components for aviation systems, and even more specifically, to produce plates used in the manufacture of fan blades.
A flowchart which shows an exemplary method of manufacturing the Ti alloys is provided in
After preparation, the raw materials are melted and cast to produce an ingot in step 110. Melting may be accomplished by, for example, VAR, plasma arc melting, electron beam melting, consumable electrode skull melting or combinations thereof. In a particular application, double melt ingots are prepared by VAR and are cast directly into a crucible having a cylindrical shape.
In step 120, the ingot is subjected to initial forging or rolling. The initial forging or rolling is performed above the beta transformation temperature. If rolling is performed at this step, then the rolling is performed in the longitudinal direction. In a particular application the ingot of the titanium alloy is heated to a temperature between about 40 and about 200 degrees Centigrade above the beta transus temperature and forged to break down the cast structure of the ingot and then cooled. Preferably, the ingot of the titanium alloy is heated to a temperature between about 90 to about 115 degrees Centigrade above the beta transus. Even more preferably, the ingot is heated to about 90 degrees above the beta transus.
In step 130, which is optional, the ingot is reheated below the beta transformation temperature and forged to deform the transformed structure. In a particular application, the ingot is reheated to a temperature between about 30 and about 100 degrees Centigrade below the beta transus. Preferably, the ingot is reheated to a temperature between about 40 to about 60 degrees Centigrade below the beta transus. More preferably, the ingot is reheated to a temperature about 50 degrees Centigrade below the beta transus.
Next, in step 140, which is optional, the ingot is reheated to a temperature above the beta transus temperature to allow recrystallization of the beta phase, then forged to a strain of at least 10 percent and water quenched. In a particular application, the ingot is reheated to a temperature between about 30 and about 150 degrees Centigrade above the beta transus temperature. Preferably, the ingot is reheated to a temperature between about 40 and about 60 degrees Centigrade above the beta transus temperature. Even more preferably, the ingot is reheated to a temperature about 45 degrees Centigrade above the beta transus temperature.
In step 150 the ingot is subject to further forging and/or rolling to produce a plate, bar, or billet. The wrought ingot produced by step 120, or by optional steps 130 or 140, if performed, is reheated to a temperature between about 30 and about 100 degrees Centigrade below the beta transus and rolled to plate, bar, or billet of the desired dimensions, with the metal being reheated as necessary to allow the desired dimensions and microstructure to be achieved. In a particular application, the ingot is reheated to a temperature between about 30 and about 100 degrees Centigrade below the beta transus temperature. Preferably, the ingot is reheated to a temperature between about 40 and about 60 degrees Centigrade below the beta transus temperature. More preferably, the ingot is reheated to a temperature about 50 degrees Centigrade below the beta transus temperature.
Rolling of plate is typically (but optionally) accomplished in at least two stages, so that the material can be rotated through 90 degrees between stages, in order to promote the development of the microstructure of the plate. The final forging and rolling is performed below the beta transformation temperature with rolling being performed in the longitudinal and transverse directions, relative to the ingot axis.
The ingot is then annealed in step 160 which is preferably performed below the beta transformation temperature. The final rolled product may have a thickness which ranges from, but is not limited to, about 0.020 inches (0.508 mm) to about 4.0 inches (101.6 mm). In some variations, the annealing of plates may be accomplished with the plate constrained to ensure that the plate complies to a required geometry after cooling. In another application, plates may be heated to the annealing temperature and then leveled before annealing.
In some applications, rolling to gages below about 0.4 inches (10.16 mm) may be accomplished by hot rolling to produce a coil or strip product. In yet another application, rolling to thin gage sheet products may be accomplished by hot rolling of sheets as single sheets or as multiple sheets encased in steel packs.
Additional details on the exemplary titanium alloys and methods for their manufacture are described in the Examples which follow.
The examples provided in this section serve to illustrate the processing steps used, resulting composition and subsequent properties of Ti alloys prepared according to the teachings of the present disclosure. The Ti alloys and their associated methods of manufacture which are described below are provided as examples and are not intended to be limiting.
Several Ti alloys having compositions outside the elemental ranges disclosed in this specification were initially prepared to serve as comparative examples. In evaluating the effectiveness of the elements contained in the proposed alloy, two series of 200 g buttons were melted and then (β then α/β) rolled to 13 mm square bars. The results are summarized in Table 1 below.
Table 1 provides the tensile test results from five alloys including Ti 6-4. Table 1 demonstrates that comparable tensile test results were obtained when vanadium was substituted with molybdenum. Specifically, when the proportions of molybdenum and vanadium were varied between 1% to 2.6%, only minor changes in tensile strength compared to Ti 6-4 were observed (compare Alloys A, B, D, and E).
Table 1 also shows that the inclusion of 0.5% silicon resulted in a significant strength increase compared to an alloy without this element (compare Alloy C with Alloy B). Alloys A, B, D, and E were given a 2 stage heat treatment typically applied to Ti 6-4. Alloy C was heat treated under different conditions compared to the other alloys because of the inclusion of silicon. This heat treatment was selected because the prior art alloys that contain Si, such as TIMETAL® 550, suggested that the optimum properties of such alloys is typically attained when the final step of heat treatment is an aging process in the temperature range 400 to 500° C.
In titanium alloys, as for other metallic materials, the grain size has an influence on the mechanical properties of the material. Finer grain size is typically associated with higher strength, or with higher ductility at a given strength level.
Table 2 provides an additional set of eight buttons (nominal compositions) along with their tensile test results.
The results reported in Table 2 demonstrate the strengthening effect of including silicon m alloy compositions. For example, adding silicon to a Ti 6-4 base resulted in a substantial increase in tensile strength (compare Alloy F with Alloy G). Table 2 also shows that for any given base composition, the inclusion of 0.5% Si compared to 0.35% Si resulted in a higher strength (compare H, J, and L with I, K, and M, respectively).
Table 2 also shows the effects of varying the amount of molybdenum and vanadium in the alloys. Alloys that contained 2% Mo and 2% V had a higher strength and ductility compared to alloys that contained 1.5% Mo and 1.5% V (compare I and J with L and M, respectively).
Additionally, decreasing the oxygen content resulted in a lower strength for a given base composition (compare M with I). Furthermore, Table 2 shows that the elastic modulus varies little over the range of compositions analyzed.
The experimental work provided evidence that the principles of alloy design in
Additional experiments were performed to evaluate the chemical composition, calculated parameters, tensile properties, and ballistic properties of the Ti alloy. In particular, six ingots were melted as 8 inch (203 mm) diameter double VAR containing the compositions shown in Table 3 below. The material was converted to 0.62 inch (15.7 mm) plate with final subtransus rolling of 40% reduction in thickness in each direction.
Using the average chemical analysis results for the Ti alloy (Ti 639; Heat V8116), the beta transus was calculated to be 1884° F. (1029° C.). This value was confirmed using metallographic observation after quenching from successively higher annealing temperatures.
The density of an alloy is an important consideration where the alloy selection criterion is (strength/weight) or (strength/weight squared). For an alloy which is proposed to be a substitute for Ti 6-4, it is particularly useful for the density to be equal to that of Ti 6-4 since this would allow substitution without design change where higher material performance is required.
Density calculations for each of the tested alloys is reported in Table 3. Using the rule of mixtures, the density for V8116 (Ti-6.5Al-1.8V-1.7Mo-0.16Fe-0.3Si—0.2O-0.03C) was calculated as 0.1626 lbs in−3 (4.50 g cm−3). When calculated on the same basis, the density of Ti 6-4 was 0.1609 lbs in−3 (4.46 g cm−3). Therefore, the density of V8116 is greater than that of Ti 6-4 by a factor of only about 1.011.
Prior to determining the tensile properties of each alloy, the plates were heat treated to the solution treated plus overaged (STOA) condition as follows: Anneal 1760° F. (960° C.), 20 minutes, air cool (AC) to room temperature, then age 1292° F. (700° C.) for 2 h, AC.
Tensile property results are provided in Table 4. The Ti 6-4 baseline (V8111) exhibited typical properties for this formulation and heat treatment condition. The specific UTS and specific TYS of the Ti alloy (V8116) were approximately 9% and 12% higher, respectively, than that of the similarly processed Ti 6-4.
Lab-scale ingots of the comparative compositions identified in Table 3 were melted and converted to 0.62 in (15.7 mm) cross-rolled plate. Tensile and ballistic evaluations were performed in the solution treated plus overaged condition as follows: Anneal 1760° F. (960° C.), 20 minutes, air cool (AC) to room temperature, then age 1292° F. (700° C.) for 2 h, AC.
Ballistic property results are provided in Table 3. Ballistic testing was performed using 0.50 Cal. (12.7 mm) Fragment Simulating Projectiles (FSP). Three plates were tested: V8111 (Ti 6-4), V8113 (Ti-6.5Al-1.8V-1.4Mo-0.16Fe-0.5Si-0.2O-0.06C), and V8116 (Ti-6.5Al-1.8V-1.7Mo-0.16Fe-0.3Si-0.2O-0.03C).
The ballistic results for V8116 were favorable demonstrating a V50 at 81 feet per second (fps) above the base requirement; localized adiabatic shear was not a dominant failure mechanism; and no secondary cracking occurred. The last observation is especially important because it indicates that the 0.03 wt % C and 0.3 Si wt % did not have a deleterious effect on the impact resistance. The overall ballistic performance for V8116 for these particular test conditions was found to be similar to that of Ti 6-4 (V8111). Therefore, the benefit of the higher strength of the V8116 composition can be realized without suffering a decrease in impact resistance.
In contrast, heat V8113, which had tensile properties similar to V8116 but had higher Si (0.5 vs. 0.3 wt %) and higher C (0.06 vs. 0.03 wt %), had a low V50 value (92 fps below the base requirement) and exhibited severe cracking that resulted in the plate breaking in half during the testing. The cracking of V8113 occurred even with shots of relatively low sectional impact energies. Additionally, V8113 exhibited cracking both between shots and to the corner of the plate; this behavior was not observed for Ti 6-4 (V8111) or V8116.
The combination of high strength (167 ksi UTS and 157 ksi), high elongation (11%), and good ballistic and impact properties observed for V8116 (Ti-6.5Al-1.8V-1.7Mo-0.16Fe-0.3Si—0.2O-0.03C) was very favorable considering that it avoids large alloy additions which would tend to increase density and cost that are normally associated with this strength level in Ti alloy plate.
1Density estimated using rule of mixtures. Tβ (beta transus) calculations based on binary equilibrium phase diagrams. Aleq = Al + 27O Moeq = Mo + 0.67V + 2.9Fe
2Average of 2 L and 2 T specimens for 0.5 in plate El = using (5.65√So)
In order to verify the properties of the Ti alloy (designated Ti 639) on an industrial scale, a 30 inch (760 mm) diameter ingot, nominal weight 3.4 MT, designated FU83099, was manufactured by double VAR melting. This ingot was then converted to plate in accordance with the processing principles laid out in
Room temperature tensile tests were also performed in order to further evaluate the characteristics of Ti 6-4 fan blade plate compared to the Ti alloy plate according to ASTM E8. Chemical compositions of the plates are shown in Table 4 along with the RT tensile test results.
The results from Table 4 further demonstrate that the Ti alloy is stronger than Ti 6-4. Comparison of the results from FU83099A and B demonstrates the greater anisotropy of properties in the material when the rolling is executed along a single axis, compared to cross rolling.
Samples taken from FU83099B were heat treated according to a schedule designed to simulate the manufacture of hollow titanium fan blades, and then subjected to a range of mechanical tests.
In the interest of clarity, the following terms and acronyms are defined as provided below.
Unless otherwise expressly indicated herein, all numerical values indicating mechanical/thermal properties, compositional percentages, dimensions and/or tolerances, or other characteristics are to be understood as modified by the word “about” or “approximately” in describing the scope of the present disclosure. This modification is desired for various reasons including industrial practice, manufacturing technology, and testing capability.
The description of the disclosure is merely exemplary in nature and, thus, variations that do not depart from the substance of the disclosure are intended to be within the scope of the disclosure. Such variations are not to be regarded as a departure from the spirit and scope of the disclosure.
As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A OR B OR C), using a non-exclusive logical OR, and should not be construed to mean “at least one of A, at least one of B, and at least one of C.”
This application is a divisional of U.S. application Ser. No. 13/349,483, filed on Jan. 12, 2012, now U.S. Pat. No. 10,119,178 issued on Nov. 6, 2018. The disclosure of the above application is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13349483 | Jan 2012 | US |
| Child | 16182122 | US |
