Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C14/00—Alloys based on titanium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/16—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of other metals or alloys based thereon
  • C22F1/18—High-melting or refractory metals or alloys based thereon
  • C22F1/183—High-melting or refractory metals or alloys based thereon of titanium or alloys based thereon

Definitions

• the invention relates to a method for processing titanium alloys and, more particularly, beta titanium alloys.
• the method of the present invention includes cold working a beta titanium alloy and subsequently direct aging the alloy For less than 4 hours.
• titanium alloys allow their use in applications requiring high corrosion resistance, high strength, and low material weight.
• applications requiring corrosion resistance often utilize low-strength unalloyed titanium mill products.
• Unalloyed titanium may be fabricated into equipment used in, for example, chemical processing, desalination, and power generation.
• high performance applications often utilize high-strength titanium alloys in a very selective manner depending on several design factors including weight, strength, ductility, and reliability requirements.
• alloys intended for high performance applications normally are more stringently processed, with resulting additional cost, than titanium for corrosion service.
• a titanium alloy may be classified as one of several metallurgical types, for example, alpha, near-alpha, alpha-beta, or beta. Beta titanium alloys are particularly useful in aerospace structures. Hot worked beta titanium alloys may be cold worked to final or near final form. The cold working process imparts high strength levels and/or a favorable ductility/strength relationship in the alloys.
• AMS 4957A and AMS 4958A define recommended processing conditions for the beta titanium alloy Ti-3AI-8V-6Cr-4Zr- 4Mo (referred to herein as Ti-38-644 alloy) to produce round bar or wire primarily for use as aerospace coil springs.
• Ti-38-644 alloy includes, by weight, 3.0 to 4.0% aluminum, 7.5 to 8.5% vanadium, 5.5 to 6.5% chromium, 3.5 to 4.5% molybdenum, 3.5 to 4.5% zirconium, maximum 0.14% oxygen, maximum 0.05% carbon, maximum 0.03% nitrogen, and remainder titanium.
• AMS 4957B requires certain additional restrictions on alloy composition, including maximum 0.30% iron, maximum 0.10% palladium, maximum 300 ppm hydrogen, maximum 50 ppm yttrium, and maximum total residual elements 0.40%. According to the AMS specifications, the alloy is aged by heating to a temperature within the range of 850°F to 1050°F (454°C to 566°C) and held at the selected temperature ⁇ 10°F (6°C) for six to twenty hours.
• the required minimum tensile properties depend on the nominal diameter of the round bar or wire final product, but in no case are to be less than minimum tensile strength of 180 ksi, minimum elongation of 8%, and minimum reduction of area ("RA") of 20%.
• a titanium alloy is of the alpha, near-alpha, alpha beta, or beta metallurgical type is influenced by the chemical composition of the alloy, the applied heat treatment, and other factors.
• the metallurgical type designations refer to the predominant crystalline phase present in the microstructure of the alloy at room temperature. Titanium metal has a close packed hexagonal crystal structure ("hep"), referred to as "alpha", at room temperature. This structure may be transformed to a body-centered cubic (“bcc”) crystal structure ("beta”) at elevated temperatures. The temperature at which this transformation occurs is referred to as the "beta transus temperature".
• the beta transus temperature for a commercially pure titanium alloy is approximately 1625°F (885°C).
• alpha stabilizers elements that favor the alpha structure
• beta stabilizers elements that favor the beta structure
• Aluminum for example, is an alpha stabilizer and, therefore, adding aluminum to a titanium alloy increases the beta transus temperature.
• Chromium, iron, molybdenum, and vanadium are beta stabilizers, and their addition lowers the beta transus temperature, stabilizing the beta structure at lower temperatures.
• the relative amounts of alpha and beta stabilizers in an alloy and the heat treatment applied to the alloy determine whether the microstructure of the alloy is predominantly single alpha phase, single beta phase, or a mixture of alpha and beta phases over a particular temperature range.
• the properties of a titanium alloy are related to its microstructure.
• Two-phase alpha-beta alloys generally exhibit tensile strengths greater than single-phase alpha alloys or single-phase beta alloys.
• alpha-beta alloys can be further strengthened by heat treatment because the microstructure may be manipulated by controlling heating, quenching, and aging cycles.
• beta titanium alloys are alloyed with more than one beta stabilizer. With sufficient quantities of beta stabilizer, and suitable control over heat treatment and cooling, beta phase may be retained at relatively low temperatures, below the alloy's normal beta transus temperature. For example, beta phase may be retained in a titanium alloy by rapid cooling from above and through the beta transus temperatures, such as by quenching. However, the titanium alloy must have sufficient quantities of beta stabilizers to prevent the beta phase from transforming to alpha phase by martensitic transformation. Titanium alloys containing beta stabilizers in quantities sufficient to reduce the alloy's martensitic transformation temperature to below room temperature but not sufficient to reduce the beta transus to below room temperature are known as "metastable" beta titanium alloys. Metastable beta titanium alloys may maintain at least a portion of beta structure after heat treatment and cooling to room temperature. As used herein, references to a beta titanium alloy are to a metastable beta titanium alloy as described above.
• An embodiment of the present invention comprises processing a beta titanium alloy by a method including the steps of cold working the alloy and then direct aging the alloy for a total aging time of less than 4 hours.
• the beta titanium alloy may be, for example, Ti-38-644 alloy.
• the method may include fabricating the alloy into an article of manufacture such as, for example, a bar, wire, a coil spring.
• Another embodiment of the present invention is a method for producing a spring or other article of manufacture from a beta titanium alloy.
• the beta titanium alloy may be, for example, the alloy comprising, by weight, 3.0% to 4.0% aluminum, 7.5 to 8.5% vanadium, 5.5 to 6.5% chromium, 3.5 to 4.5% molybdenum, 3.5 to 4.5% zirconium, and titanium.
• the alloy is hot worked, cold worked to provide a 5% to 60% reduction, and direct aged for a total time of less than 4 hours.
• cold working is defined as various working processes performed at a temperature below an effective aging temperature of the alloy. Cold working of a titanium alloy, therefore, may be performed at temperatures below the beta transus temperature of the alloy. Cold working permanently deforms the work piece, which does not return to its original shape when the load causing the deformation is removed.
• the degree of cold working typically, is determined by the percentage reduction in cross-sectional area of the work piece.
• a 5% reduction achieved by cold working refers to a reduction of 5% in the cross-sectional area of the work piece upon cold working.
• Any cold working technique may be used in embodiments of the present invention. Useful cold working techniques include, but are not limited to, compression processes, drawing, wire drawing, tube drawing, deep drawing, rolling, contour forming, extruding, cold heading, swaging, coining, forging, tension processes, stretch forming, and spinning.
• Cold working may be used to improve the mechanical properties of an alloy including hardness, yield strength, and tensile strength.
• Ductility may be reduced during cold working.
• Ductility is a measure of the ability of a material to deform plastically without fracture. Elongation or RA in tensile testing typically is used as a measure of ductility of material.
• the method of the present invention may be used to increase the strength of beta titanium alloys while also maintaining good ductility and significantly increasing the aging response of the alloy.
• a beta titanium alloy was prepared and processed according to the method of the present invention. Its properties were then compared with the same alloy composition processed using a conventional method including cold working and heat treating steps. This testing is described in greater detail below.
• a melt of a Ti 38-644 alloy was prepared and cast into an ingot. The alloy had the average composition, in weight percentage, shown in Table 1.
• a first ingot was hot rolled at a temperature not to exceed 1750°F, annealed and air cooled.
• Table 1 Composition of first Ingot A portion of the hot rolled, annealed, and air cooled ingot was processed by the method of the present invention. Another portion of the hot rolled, annealed, and air cooled ingot was processed in a conventional manner for comparison purposes. The portion processed in the conventional manner was hot worked, then solution heat treated, and subsequently aged. The heat treatment parameters were varied to assess the impact on mechanical properties.
• solution heat treating is a heat treatment step wherein an alloy is heated to a suitable temperature and held at the temperature for a time period sufficient to cause one or more constituents of the alloy to enter solid solution. The alloy is then cooled rapidly so as to hold the one or more constituents in solution. Solution heat treatment typically is performed on an alloy to improve ductility at a given strength.
• Table 2 includes the results of room temperature tensile testing of the alloy of Table 1 processed by the conventional heat treatment process under various conditions. All tensile properties reported in Table 2 were determined in accordance with ASTM E 8. Tensile testing was used to determine ultimate tensile strength ("UTS"), 0.2% yield strength, elongation, and RA of the test pieces. RA and elongation are measures of ductility of the test pieces. Elongation is the amount of extension of a test piece when stressed. In tensile testing, elongation is the increase in gage length, measured after fracture of the specimen with the gage length, usually expressed as a percentage of the original gage length as marked on the test piece.
• UTS ultimate tensile strength
• RA and elongation are measures of ductility of the test pieces. Elongation is the amount of extension of a test piece when stressed. In tensile testing, elongation is the increase in gage length, measured after fracture of the specimen with the gage length, usually expressed as a percentage
• the test pieces listed in Table 2 were hot rolled from 4 inch diameter billet to 0.569 inch diameter bar and solution heat treated prior to aging.
• the data in Table 2 clearly shows that long aging times, more than 8 hours, are required to achieve high strength, greater than 180 ksi, in the alloy.
• the conventional process required more than 8 hours of aging to achieve the minimum tensile strength for Ti-38-644 bar and wire specified in AMS 4957A and AMS 4958B.
• AMS 4958A specifies that the beta titanium alloy must receive no more than 5% cold work reduction after hot rolling and solution heat treating.
• AMS 4958A also requires that the alloy be subjected to aging temperatures for at least 12 hours. Additionally, due to solution heat treatment and aging at elevated temperatures, an oxide layer may form on the alloy surface. AMS 4958A requires an acid pickling step to remove this layer.
• the aging time of an alloy may be measured and expressed by different criteria. For example, the length of the aging process may be measured as the total time the alloy is exposed to the aging temperature in a furnace, or as the total time that the surface or an internal portion of the alloy is maintained within the aging temperature range. Unless otherwise noted, all aging times reported herein for embodiments of the present invention are the total time that the alloy is exposed to an environment at approximately the desired aging temperature. Aging of the test piece samples listed in the examples was performed in a laboratory oven. More efficient means for heating an alloy, such as, for example, a production convection oven, may allow faster heat transfer to the alloy and thereby reduce the minimum aging time necessary to impart desired properties to the alloy.
• the method of the present invention is not limited to the embodiments described herein, including the particular aging equipment used, but includes various other embodiments. Thus, the embodiments of the present invention presented herein are only examples of the invention, and the scope of the invention is not so restricted.
• An embodiment of the process of the present invention includes direct aging a beta titanium alloy for less than 4 hours after a step of cold working.
• the beta titanium alloy Prior to cold working, the beta titanium alloy may be hot worked. After hot working, and prior to cold working, the alloy also may be annealed.
• a preferred annealing temperature for beta titanium alloys is 1425°F (774°C). Strength and ductility has been shown to be nearly identical for test pieces that have been annealed and test pieces that have not been annealed prior to cold working and aging by the process of the present invention.
• Figure 1 is a graph depicting the effect of aging time on UTS, 0.2% yield strength, elongation, and RA of a Ti-38-644 alloy subject to 13% or 15% cold work reduction and aged at 950°F (510°C);
• Figure 2 is a graph depicting the effect of aging time and aging temperature on UTS of a Ti-38-644 alloy subjected to 13% or 15% cold work reduction and aged at 950°F (510°C), 1000°F (538°C) and 1050°F (566°C); and
• Figure 3 is a graph depicting the effect of aging time and aging temperature on the RA of a Ti-38-644 alloy subjected to 13% or 15% cold work reduction and aged at 950°F (510°C), 1000°F (538°C), or 1050°F (566°C).
• Test pieces of the alloy of Table 1 were processed according to the method of present invention.
• a relatively high strength beta titanium alloy may be produced in a relatively short time while maintaining ductility.
• Embodiments of the present invention are listed in Tables 3-9.
• the test pieces were direct aged for a total aging time of less than 4 hours after a cold work step.
• Direct aging an alloy includes aging the alloy after working without an intermediate heat treating step, such as solution heat treating.
• Direct aging does not preclude other processing steps from being performed after cold working the alloy and prior to aging the alloy. These processes may be, for example, mechanical processes, such as shaving, or chemical processes, such as, pickling.
• the tables list the process steps employed and the mechanical properties of the processed alloy samples derived from tensile testing at room temperature.
• Tables 3-9 list embodiments of the method of the present invention applied to the beta titanium alloy having the composition of Table 1.
• the amount of cold work may be to any degree and, preferably, in an embodiment of the method of the present invention the beta-titanium alloy is cold worked from at least a 5% reduction to a 60% reduction. Even more preferably, cold working the beta-titanium alloy comprises less than a 35% reduction. More preferably, an embodiment of the method of the present invention includes cold working the beta-titanium alloy to a reduction between 15% and 35%.
• Table 3 the test pieces were hot rolled, cold drawn to provide 8% reduction and then direct aged at the temperatures and for the times shown in the tables. The test pieces described in Table 3 also were annealed and centerless ground prior to cold drawing.
• the embodiments listed in Table 3 produced high strength (UTS greater than 170 ksi) and maintained ductility (greater than 8% elongation and greater than 20% RA) with less than four hours of direct aging.
• UTS values greater than 180 ksi and as high as 199 ksi were realized in the listed embodiments.
• the highest UTS values were realized at aging temperatures of 950°F (510°C), at which a UTS of 199 ksi was achieved at a total aging time of only 166 minutes.
• the highest ductility as measured by elongation and RA was realized at the higher aging temperature of 1050°F (566°C).
• Table 4 lists embodiments of the present invention wherein test pieces were hot rolled, cold drawn to 13% reduction, and direct aged.
• the embodiments described in Table 4 were annealed and centerless ground after hot rolling and prior to cold drawing.
• the embodiments of the method of the present invention in Table 4 displayed significantly increased strength after only 20 minutes of total aging time. With further aging at aging temperatures of 950°F (510°C) and 1000°F (538°C) the strength increased to a value greater than required by AMS 4958A and 4957B specifications.
• the test pieces aged at 1050°F (565°C) did not obtain strength as high as the test pieces aged at lower aging temperatures.
• the test pieces aged at 1050°F (565°C) did maintain a greater degree of ductility as measured by elongation and RA.
• Table 5 lists embodiments of the present invention wherein test pieces were hot rolled, cold drawn to 13% reduction, and direct aged, in a fashion similar to the embodiments shown in Table 4. However, the test pieces listed in
• Table 5 were not annealed and centerless ground prior to cold drawing.
• the embodiments of the invention listed in Table 5 produced test pieces that exhibited high strength and ductility.
• the embodiments of Table 5 produced very high strength (UTS above 190 ksi) in the beta titanium alloy when aged for as short as 69 to 72 minutes.
• the results show that the annealing step may be excluded in embodiments of the present invention without significant affect on mechanical properties when the invention is applied to the beta titanium alloy of Table 1.
• Table 5 Tensile Testing Results for Embodiments of the Present Invention with 13% Cold Work Reduction and Without Anneal Table 6 lists embodiments of the present invention wherein test pieces were hot rolled, cold drawn to 15% reduction, and direct aged. Additionally, the test pieces of Table 6 were not annealed and centerless ground prior to cold drawing. Certain embodiments of the present invention in Table 6 included aging times less than 60 minutes. The embodiments including cold working to 15% reduction showed higher strengths than the embodiments including cold working to only 8% reduction, without a corresponding loss of ductility.
• the embodiments cold worked to 15% reduction achieved UTS greater than 190 ksi after aging for only 45 minutes of total aging time at 900°F (482°C) and 950°F (510°C), and achieved UTS greater than 200 ksi after aging for only 60 minutes of total aging time at the same temperatures.
• Table 6 Tensile Testing Results for Embodiments of the Present Invention with 15% Cold Work Reduction
• Table 7 lists embodiments of the present invention wherein test pieces were hot rolled, cold drawn to 19%, and direct aged. Additionally, the embodiments described in Table 7 were annealed and centerless ground prior to cold drawing.
• Table 8 lists embodiments of the present invention wherein test pieces were hot rolled, cold drawn to 20% reduction, and direct aged. Additionally, the test pieces of Table 8 were not annealed and centerless ground prior to cold drawing.
• the embodiments of the present invention in Table 8 produced an increase in UTS of approximately 5% and an increase in 0.2% yield strength of 6% over the embodiments employing a cold work of 15% reduction. Cold working to 20% reduction reduced ductility by either 5% (as measured by elongation) or 9% (as measured by RA).
• Table 9 lists embodiments of the present invention wherein test pieces were hot rolled, cold drawn to 25% reduction, and direct aged.
• Figure 1 graphically depicts the effect of aging time on samples of Ti-38-644 beta titanium alloy having the composition shown in Table 1 and wherein the method included a step of cold working to a 13 or 15% reduction.
• the UTS and 0.2% yield strength increase rapidly for at least the first 60 minutes of total aging time.
• UTS of the test pieces reached 180 ksi in approximately 30 minutes of total aging time.
• Production aging furnaces would likely more efficiently heat articles and, therefore, in a production furnace it is expected that total aging times in the method of the present invention necessary to reach high strength (180 ksi, for example) may be reduced, possibly by two thirds or more in some cases.
• the aging of the beta titanium alloy may be conducted at an temperature below the beta transus.
• the aging of the beta titanium alloy occurs at a temperature between 800°F (427°C) and 1100°F (538°C).
• the aging of the beta titanium alloy may occur between 800°F (427°C) and 1000°F (538°C), and more preferably between 900°F (482°C) and 1000°F (538°C).
• ductility of the test pieces decreased with total aging time.
• ductility decreases slowly with total aging time, and UTS of over 200 ksi was achieved while maintaining relatively good ductility.
• Automobile production lines may include installations for winding and aging springs as required for production.
• the springs may be, for example, wound and then aged on a conveyer belt as the belt passes through an aging furnace.
• aging of the beta-titanium alloy will be for a period of less than 3 hours.
• the aging of the beta-titanium alloy will be for a period of less than 2 hours, and even more preferably for some time sensitive applications the aging will be for a period of less than 1 hour or more preferably for less than 45 minutes. Alloys produced by the present invention may also be useful in other applications than springs, such as, for example, in the biomedical industry for surgical instruments or implants.
• Figure 2 depicts the effect of aging time and temperature on UTS of test pieces of the beta titanium alloy of Table 1 processed by embodiments of the present invention including cold working to 13% or 15% reduction.
• the embodiments of the present invention employing aging at lower temperatures achieved higher UTS. This may be expected due to crystalline growth at higher temperatures and the lower volume of alpha phase present in the alloy as a result of the processing conditions, which both may adversely effect the strength of a beta titanium alloy.
• Figure 3 depicts the effect of aging time and temperature on ductility of test pieces of the beta titanium alloy of Table 1 , as measured by reduction in area, using embodiments of the present invention including cold working to 13% or 15% reduction.
• the embodiments of the present invention utilizing aging at higher temperatures produced higher ductility in the test pieces over time. This may be expected due to crystalline growth at higher temperatures which, although adversely effecting strength, enhances ductility of the beta titanium alloy.
• a second titanium ingot was produced and processed according to method of the present invention.
• the composition of the second ingot at three locations is shown in Table 10.
• the composition of the alloy was tested in three locations to verify the composition and ensure a fairly consistent composition throughout the ingot.
• the second ingot was processed according to the method of the present invention.
• the second ingot was hot rolled at a temperature not to exceed 1825°F (996°C), annealed and air cooled.
• Table 11 test pieces produced from the second ingot were hot rolled, cold drawn to provide 16.5% reduction and then direct aged at the temperatures and for the times shown in the table.
• the test pieces described in Table 11 also were annealed at a temperature not to exceed 1450°F (774°C) and air cooled prior to cold drawing.
• the embodiments listed in Table 11 produced higher strength (UTS greater than 190 ksi) and maintained ductility (greater than 8% elongation and greater than 20% RA) with less than 30 minutes of direct aging.
• UTS values greater than 200 ksi and as high as 220 ksi were realized in the listed embodiments. Again, the highest UTS values were realized at the lower aging temperatures, 900°F
• test pieces produced by an embodiment of the process of the present invention as described in Table 11 achieved higher tensile strengths with shorter aging times than the test pieces produced by the embodiments of the process of the present invention as described in Tables 3 to 9.
• the ductility of the test pieces described in Table 11 were lower. It is believed that the higher hot rolling temperature experienced by the second ingot produced the lower ductility since the higher processing temperatures favored a larger prior beta grain size. The higher strength is thought to be associated with slower cooling after the anneal which allowed for some aging prior to cold working.
• Table 12 shows the results of Rotating Beam Fatigue Testing on articles prepared by the method of the present invention wherein the articles were hot rolled, cold drawn to 15% reduction, and direct aged at 950°F (510°C) for one hour.
• Invention comprising 15% Cold Work Reduction and direct aging at 950°F (510°C) for one hour.
• Table 13 shows the results of Load Controlled Axial Fatigue Testing on articles prepared by the method of the present invention wherein the articles were hot rolled, cold drawn to 15% reduction, and direct aged at 950°F (510°C) for one hour.
• the results indicate the number of cycles experienced for each specimen prior to failure.
• Specimen prepared using different conditions in the method of the present invention, such as, a longer aging time, different aging temperature or different degree of cold working, for example, may result in an increase in the number of cycles prior to failure in the fatigue tests.
• Table 13 Load Controlled Axial Fatigue Testing Results for Embodiments of the Present Invention comprising 15% Cold Work Reduction and direct aging at 950°F (510°C) for one hour.
• the method of the present invention is described above with respect to beta titanium alloys of certain compositions, it is believed that the method of the present invention has wider application, to the processing of other beta titanium alloys.
• some additional commercially available beta titanium alloys that may benefit from the present invention are titanium alloys having the following nominal compositions, in weight percentages.
• Ti-12Mo-6Zr-2Fe an alloy comprising 12% molybdenum, 6% zirconium, 2% iron and titanium, and which is available commercially in at least one form as ALLVAC TMZF alloy
• Ti-4.5Fe-6.8Mo-1.5AI an alloy comprising 4.5% iron, 6.8% molybdenum, 1.5% aluminum and titanium, and which is available commercially in at least one form as TIMETAL LCB alloy
• Ti-15Mo-2.6Nb-3Al-0.2Si an alloy comprising 15% molybdenum, 2.6% niobium, 3% aluminum, 0.2% silicon and titanium, and which is available commercially in at least one form as TIMETAL 21 S alloy
• Ti-15V-3Cr-3Sn-3AI an alloy comprising 15% vanadium, 3% chromium, 3% tin, 3% aluminum and titanium, and which is available commercially in at least one form as ALLVAC 15-3 alloy
• Ti-11.5Mo- 6Zr-4.5Sn an alloy comprising 11.
• compositions of the alloys presented above are nominal compositions, and the content of each element may vary by at least as much as 2% or more and the alloys may also include additional components. It is to be understood that the present description illustrates those aspects of the invention relevant to a clear understanding of the invention. Certain aspects of the invention 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 the present invention has been described in connection with certain embodiments, those 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.

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