DIE CASTTITANIUM ALLOY ARTICLES

Abstract: A die cast article composed of titanium alloy is disclosed, and is preferably characterized by a transformed beta microstructure, a relatively fine average grain size and an absence of flow lines. Typical alloys include Ti 6Al-4V and Ti 6Al-2Sn-4Zr-2Mo. Exemplary articles include gas turbine engine components. The articles may be used in place of corresponding forged articles, and have at least comparable mechanical properties.

Description: DIE CAST TITANIUM ALLOY ARTICLES CROSS REFERENCE TO RELATED APPLICATION Some of the material disclosed in this application is described and claimed in co-pending application titled "Method Of Making Die Cast Articles of High Melting Temperature Materials", filed on even date herewith and expressly incorporated by reference herein. BACKGROUND OF THE INVENTION The present invention relates generally to articles fabricated from titanium, and relates more particularly to articles fabricated from titanium alloys. Titanium and titanium alloys are employed in applications which require light weight and high strength-weight ratios. These alloys exhibit good corrosion resistance, and are generally useful up to relatively high temperatures, e.g., up to about 1200 °F/ 650 °C. Broadly, the term "titanium alloys" is intended to include alloys composed of at least about 25 at. % titanium. In gas turbine engines for example, titanium alloys are employed in the compressor section of the engine, including but not limited to airfoils such as blades and vanes, as well as structural components such as intermediate and compressor cases and compressor disks. A titanium alloy widely utilized in gas turbine engines is Ti 6-4, which includes about 6 w/o (percent by weight) aluminum, about 4 w/o vanadium, balance generally titanium, and is used in environments up to about 600° F. For higher temperature applications, e.g., in environments up to about 1200° F/650 °C, and where improved creep and other high temperature properties are needed, Ti-6-2-4-2 may be employed, and includes about 6 w/o Al, about 2 w/o Sn (tin), about 4 w/o Zr (zirconium) and about 2 w/o Mo (molybdenum), balance generally titanium. Other titanium-base alloys may also be employed, such as Ti 8-1-1, which includes about 8 w/o Al, 1 w/o Mo and 1 w/o V (vanadium), balance generally titanium, as well as titanium aluminides which are broadly composed of titanium and aluminum in stoichiometric amounts such as TiAl and TiA . In addition to the above-discussed properties, these materials must at least be capable of being formed into relatively complex, three dimensional shapes such as airfoils, and must be oxidation resistant - particularly at elevated temperatures. Titanium and titanium alloys (except titanium aluminides) have in the past typically been precision forged to produce parts having a fine average grain size and a balance of high strength, low weight and durability or high cycle fatigue resistance. In the gas turbine engine industry, forging is a preferred method used to produce parts having complex, three-dimensional shapes such as blades and vanes. When properly produced, forged parts do exhibit a balance of high strength, low weight, and durability. Briefly, in order to forge a part such as an airfoil, an ingot of material is converted into billet form, typically cylindrical for blades and vanes, and is then thermomechanically processed, such as by heating and stamping several times between dies and/or hammers that are shaped progressively similar to the desired shape, in order to plastically deform the material into the desired component shape. The forging dies typically may be heated. Each component is typically heat treated to obtain desired properties, e.g., hardening/strengthening, stress relief, resistance to crack growth and a particular level of HCF resistance, and is also finished, e.g., machined, chem-milled and/or media finished, if necessary to provide the component with the precise shape, dimensions and/or surface features. The production of components by forging is an expensive, time consuming process, and thus is typically warranted only for components that require a particular balance of properties, e.g., high strength, low weight and durability, both at room temperature and at elevated temperatures. With respect to obtaining material for forging, certain materials require long lead times. Forging typically includes a series of operation, each requiring separate dies and associated equipment. The post- forging finishing operations, e.g., machining the root portion of a blade and providing the appropriate surface finish, comprise a significant portion of the overall cost of producing forged parts, and include a significant portion of parts which must be discarded. During component forging, much of the original material (up to about 85% depending upon the size of the forging) is removed and does not form part of the finished component, e.g., it is waste. The complexity of the shape of the component produced merely adds to the effort and expense required to fabricate the component, which is an even greater consideration for gas turbine engine components having particularly complex shapes. Titanium alloys may also some springback, e.g., the material is resilient, and the springback must be taken into account during forging, i.e., the parts must typically be "over forged". As noted above, finished components may still require extensive post forging processing. Moreover, as computer software is used to apply computational fluid dynamics to analyze and generate more aerodynamically efficient airfoil shapes, such airfoils and components have even more complex three-dimensional shapes. It is more difficult or impossible to forge titanium alloys precisely into these advanced, more complex shapes, e.g., due in part to the slightly resilient nature many materials exhibit during forging, which adds further to the cost of the components or renders the components so expensive that it is not economically feasible to exploit certain advances in engine technology, or to utilize particular alloys for some components. Forged components may contain forging imperfections that tend to be difficult to inspect. Moreover, precise reproducibility is also a concern - forging does not result in components having dimensions that are precisely the same from part to part. After inspection, many parts must still be re- worked. As a general rule, forged parts must be scrapped or significantly re-worked about 20 % of the time. Moreover, newer, more advanced titanium based alloys will be increasingly difficult (if not impossible) and correspondingly more expensive to forge. These concerns will only intensify as more complex three-dimensional airfoil geometries are employed. Casting has been extensively used to produce relatively near-finished-shape articles. Investment casting, in which molten metal is poured into a ceramic shell having a cavity in the shape of the article to be cast, can be used to produce such articles. However, investment casting produces extremely large grains, e.g., ASTM 1 or larger (relative to the small average grain size achievable by forging), and in some cases the entire part comprises a single grain. Moreover, since an individual mold is produced for each part, this process is expensive. Reproducibility of very precise dimensions from part to part is difficult to achieve. In addition, molten material is typically melted, poured and/or solidified in air or other gas, which can result in parts having undesirable properties such as inclusions and porosity, particularly for materials containing reactive elements such as titanium or aluminum. Permanent mold casting, in which molten material is poured into a multipart, reusable mold and flows into the mold under only the force of gravity, has also been used to cast parts generally. See, e.g., U.S. Pat. No. 5,505,246 to Colvin. However, permanent mold casting has several drawbacks. For thin castings, such as airfoils, the force of gravity may be insufficient to urge the material into thinner sections, particularly so where high melting temperature materials and low superheats are employed, and accordingly the mold does not consistently fill and the parts must be scrapped. Dimensional tolerances must be relatively large, and require correspondingly more post casting work, and repeatably is difficult to achieve. Permanent mold casting also results in relatively poor surface finish, which also requires more post cast work. Die casting, in which molten metal in injected under pressure into a re-usable die, has been used successfully in the past to form articles from materials having relatively low-melting temperatures, e.g., below about 2000 °F/ 1095 °C. As set forth, for example, in U.S. Pat. Nos. 2,932,865, 3,106,002, 3,532,561 and 3,646,990, a conventional die casting machine includes a shot sleeve mounted to one (typically fixed) platen of a multiple part die, e.g., a two part die including fixed and movable platens which cooperate to define a die cavity. The shot sleeve is oriented horizontally, vertically or inclined between horizontal and vertical. The sleeve communicates with a runner of the die, and includes an opening on the sleeve through which molten metal is poured. A plunger is positioned for movement in the sleeve, and a driving mechanism moves the plunger and forces molten metal from the sleeve into the die. In a "cold chamber" type die casting machine, the shot sleeve is typically oriented horizontally and is unheated. Casting usually occurs under atmospheric conditions, i.e., the equipment is not located in a non-reactive environment such as a vacuum chamber or inert atmosphere. The drawbacks of such machines are also discussed in U.S. Pat. Nos. 3,646,990 and 3,791,440, both to Cross, particularly in connection with the inability to use such machines to cast higher melting point materials. In conventional machines the atmosphere in the shot sleeve is not evacuated, and the plunger also forces any air from the sleeve into the die resulting in porosity of die cast articles, a condition that is both undesirable and impermissible particularly where the article is to be used in demanding applications such as aerospace components. Accordingly, in order to avoid injecting bubbles with the molten material the shot sleeve must be filled as completely as possible, or is inclined such that any air in the molten material migrates away from the die before injection. Moreover, since the shot sleeve is unheated, a skin or "can" of molten metal solidifies on the inside of the shot sleeve, and in order to move the plunger through the sleeve to inject the molten metal into the die, the plunger must overcome the resistance of the solidified metal, scraping the skin off of the sleeve and thereby "crushing the can". However, the can forms a structurally strong member, e.g., in the form of cylinder which is supported by the sleeve, the plunger and/or associated structure for moving the plunger can be damaged or destroyed due to the resistance to plunger motion. Where the plunger is thermally distorted and fails to match the sleeve shape or the sleeve is thermally distorted altering the clearances between the sleeve and plunger, the passage of metal between plunger and sleeve ("blowback") may occur and/or bind the plunger, all of which detrimentally affects the resultant articles. See also U.S. Pat. No. 3,533,464 to Parlanti et al. Despite extensive efforts, the conventional "cold chamber" die casting apparatus have not been used successfully to produce articles composed of high melting temperature materials, such as titanium alloys and superalloys. As used herein, superalloys generally refer to those materials characterized by high strength and which maintain high strength at high temperatures. Such materials are also characterized by relatively high melting points. Past attempts to die cast high melting temperature materials such as titanium alloys and superalloys has resulted in inoperable die casting machinery, as well as articles characterized by inferior qualities such as impurities, excessive porosity, and relatively poor strength and fatigue properties. It is an object of the present invention to provide die cast articles composed of high melting temperature materials, such as titanium alloys. It is an additional object of the present invention to provide high quality articles composed of alloys having a significant content of reactive elements, such as titanium and aluminum. It is another object of the present invention to provide die cast titanium alloy articles having properties comparable to corresponding forged articles. It is a more specific object of the present invention to provide titanium alloy articles that have strength, durability and fatigue resistance comparable to corresponding forged titanium alloy articles. It is still another object of the present invention to provide such articles having complex, three dimensional shapes (such as gas turbine engine blades and vanes) that are difficult if not impossible to forge. Additional objects will become apparent to those skilled in the art based upon the following disclosure and drawings. SUMMARY OF THE INVENTION According to one aspect of the invention, a die cast titanium alloy article is disclosed. The article, for example an airfoil for a gas turbine engine, has a transformed beta microstructure and preferably also has a fine average grain size for a cast article, e.g., ASTM 1 or smaller, and an absence of flow lines. Exemplary titanium alloys include Ti 6AI-4V (composition including about 4 - 8 w/o Al and 3 - 5 w/o V, balance generally titanium) or Ti 6Al-2Sn-4Zr-2Mo (composition including about 5.5 - 6.5 w/o Al, about 1.75 - 2.25 w/o Sn (tin), about 3.5 - 4.5 w/o Zr, about 1.8 - 2.2 w/o Mo, balance generally titanium). Other titanium alloys have also been successfully die cast, such as Ti 8AI-IM0-I V and titanium aluminides. The articles have both yield and ultimate tensile strengths at both room and elevated temperatures that are at least comparable to forged parts composed of the same material, and also have similar fatigue properties. The present invention is advantageous in it obviates any need to prepare specially tailored billets of material. Accordingly, the time required to produce a part, from ingot to finished part, is reduced significantly. Die casting broadly can be performed in a single operation, as opposed to multiple forging operations. In die casting, multiple parts can be produced in a single casting. Die casting enables the production of parts having more complex, three dimensional shapes than forging, thereby enabling new software design technology to be applied to and exploited in areas such as gas turbine engines and enabling production of more aerodynamically efficient airfoils and other components. Die casting enables the production of such articles utilizing materials that are difficult or impossible to forge into such shapes. Die cast parts are produced nearer to their finished shape, and with a superior surface finish, thereby minimizing post forming finishing operations, and reducing the cost of producing such parts. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view of a die cast titanium alloy article in accordance with the present invention. FIG. 2 is a photomicrograph illustrating the microstructure of an airfoil composed of die cast Ti 6-4 in accordance with the present invention. FIG. 3 is a photomicrograph illustrating the microstructure of a test bar composed of die cast Ti 6-4 in accordance with the present invention. FIG. 4 is a photomicrograph illustrating the microstructure of an airfoil composed of forged Ti 6-4. FIGS. 5 and 6 illustrate a comparison of properties for die cast Ti 6-4 in accordance with the present invention and forged Ti 6-4. FIGS. 7 and 8 illustrate fatigue properties of die cast Ti 6-4 and corresponding forged articles. FIG. 9 is a photomicrograph illustrating the microstructure of an airfoil composed of die cast Ti 6-2-4-2 in accordance with the present invention. FIG. 10 is a photomicrograph illustrating the microstructure of a test bar composed of die cast Ti 6-2-4-2 in accordance with the present invention. FIG. 1 1 is a photomicrograph illustrating the microstructure of an airfoil composed of forged Ti 6-2-4-2. FIG. 12 illustrates a comparison of properties for die cast Ti 6-2-4-2 in accordance with the present invention and forged Ti 6-2-4-2. FIG. 13 is a photomicrograph illustrating the microstructure of an article composed of die cast Ti 8-1-1. FIGS. 14 and 15 are schematic views of a die casting machine in accordance with the present invention. FIG. 16 is a flow diagram illustrating a process of die casting high melting temperature materials in accordance with the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Turning now to FIG. 1 , a die cast titanium alloy article in accordance with the present invention is indicated generally by the reference numeral 10. In the illustrated embodiment, the article is a compressor blade 10 for a gas turbine engine, and includes an airfoil 12, a platform 14, and a root 16. The present invention is broadly applicable to titanium alloys used for various applications, and is not intended to be limited to gas turbine engine parts. One titanium alloy widely used for aerospace applications (and also used in many other applications) is Ti-6A1-4V ("Ti 6-4"), which broadly includes about 4 - 8 w/o (weight percent) Al, 3 - 5 w/o V. Ti 6-4 typically also includes some impurities, and by way of example may also include up to about 0.5 w/o Fe, up to about 0.25 w/o O, up to about 0.20 w/o C, up to about 0.1 w/o N, up to about 0.02 w/o H, up to about 0.01 w/o Y, up to about 0.4 w/o other elements. For higher temperature applications, where improved higher temperature properties are needed, Ti 6Al-2Sn-4Zr-2Mo ("Ti 6-2-4-2") is used, and broadly includes about 5 - 7 w/o Al, about 1.5 - 2.5 w/o Sn (tin), about 3.0 - 5.0 w/o Zr, about 1.5 - 2.5 w/o Mo. Ti 6-2-4-2 typically also includes some impurities, and by way of example may also include about 0.05 - 0.15 w/o Si, up to about 0.2 w/o Fe, up to about 0.25 w/o O, up to about 0.15 w/o Cu, up to about 0.1 w/o N, up to about 0.02 w/o H, up to about 0.010 Y, up to about 0.4 w/o other elements. Other Ti alloys include Ti 8-1-1 and titanium aluminides, which are composed of stoichiometric amounts of titanium and aluminum. Ti 8-1-1 broadly includes about 7-8.5 w/o Al, 0.5- 1.5 w/o Mo and 0.5-1.5 w/o V, balance generally titanium. Ti 8-1-1 typically also includes some impurities, and by way of example may also include up to about 0.22 w/o Si, up to about 0.4 w/o Fe, up to about 0.15 w/o O, up to about 0.1 w/o C, up to about 0.25 w/o Sn, up to about 0.15 w/o Cu, up to about 750 ppm N, up to about 200 ppm H, up to about 50 ppm B, up to about 75 ppm Y, and traces of other elements. Broadly, titanium aluminides are composed primarily of titanium and aluminum in stoichiometric amounts, having compositions such as TiAI and TiA . Titanium aluminides are discussed for example in U.S. Pat. Nos. 4,294,615 and 4,292,077 both to Blackburn et al., which are hereby expressly incorporated by reference herein. In accordance with the present invention, articles prepared in accordance with the present invention are characterized by a stable, predominately transformed beta microstructure, in contrast to the alpha + beta microstructure typically produced in forged titanium articles to date. While investment cast articles also typically exhibit a transformed beta microstructure, such microstructure typically consists both of larger grains and coarser alpha/beta laths than die cast article of the present invention. The particular, preferred average grain size and maximum allowable grain size will depend upon the application and cross sectional thickness of the part, e.g., whether the article is intended for use in a gas turbine engine versus other application, rotating vs. non-rotating parts, operating in lower temperature versus higher temperature environments. For gas turbine engine components, such as compressor blades and vanes, the average grain size should be ASTM 1 or smaller, more preferably ASTM 3 or smaller. The titanium alloy articles, such as blades and vanes, in accordance with the present invention are preferably characterized by an absence of flow lines. It should be noted that the articles may be thennomechanically processed after casting, if desired. In other words, the die cast articles may subsequently serve as pre-forms for use in a forging operation. In order to maximize cost savings associated with the present invention, we prefer that the die cast articles be cast to near net shape, so as to minimize post-casting work and associated expense performed on the articles. In addition, the die cast articles may be processed to heat any residual casting porosity that may be present, such as by isostatic pressing operations such as hot isostatic pressing (HIP). Careful selection of HIP parameters, such as temperature, pressure and time is required to heal any porosity without altering the fine grain, transformed beta microstructure. The temperature must be sufficiently high to enable closing of the porosity under pressure, e.g., to enable creep, but not so high as to enable recrystallization of the material, e.g., below the beta transus temperature of the titanium alloy. In the case of Ti 6-4, the HIP temperature preferably should not exceed 1750° F/ 950 °C, and is more preferably between about 1550 - 1650 ° F/ 845 - 900 °C. Ti 6-4 may also be annealed at about 1550 F in a non-reactive environment, preferably argon or vacuum, for at least two hours after HIP'ing. In the case of Ti 6-2-4-2, the HIP temperature should not exceed 1850° F/ 1010 °C, and is more preferably between 1650 - 1750 ° F/ 9900 950 °C. Ti 6-2-4-2 may be heat treated at about 1100 F/ 595 °C in a non-reactive environment, preferably argon or vacuum, for at least 8 hours Additional post cast processing may also be performed, such as chemical milling of the surface to remove surface contaminants, media processing to improve surface finish, and additional thermal cycles to achieve a particular balance of mechanical properties. Such additional processing will vary on factors such as alloy composition and desired properties. Die cast articles composed of Ti 6-4 were prepared in accordance with the present invention, as discussed in more detail below. The articles included compressor airfoils and test bars, and also included the above-post cast processing. Exemplary microstructures of a test bar and of an airfoil are illustrated in FIGS. 2 and 3. The microstructure of a corresponding airfoil composed of forged Ti 6-4 is illustrated in FIG. 4. Testing of the articles confirmed that properties were comparable to those of corresponding forged articles. While the particular properties required will depend upon the use to which any particular die cast article is to be put, die cast articles to be used in place of forged articles must have properties at least comparable to those of corresponding forged articles. The following values were obtained by testing standard specimens machined from oversize test bars. Results from the die cast articles were compared to results from specimens machined from corresponding forged articles, and the results are shown in FIGS. 5, 6, 7 and 8. As indicated by FIGS. 5 and 6, the die cast articles are characterized, at room temperature and at about 300° F/ 150 °C, by comparable 0.2% yield strengths, ultimate tensile strengths, elongation at failure and impact strengths. In the case of compressor airfoils, die cast airfoils have at least strength and impact properties equivalent to those exhibited by corresponding forged articles. Compressor airfoils composed of Ti 6- 4 should have a 0.2% yield strength at room temperature of at least 100 ksi/ 700 MPa and more preferably at least 1 10 ksi/ 770 MPa and most preferably at least 125 ksi/ 875 MPa: and at yield strength at 300° F/ 150 °C of at least 90 ksi/ 630 MPa and more preferably 100 ksi/ 700 MPa and most preferably at least 105 ksi/ 735 MPa. Such articles have a ultimate tensile strength at room temperature of at least 1 10 ksi/ 770 MPa and more preferably at least 125 ksi/ 875 MPa and most preferably at least 135 ksi/ 945 MPa; and an ultimate tensile strength at 300° F/ 150 °C of at least 100 ksi/ 700 MPa and more preferably 1 10 ksi/ 770 MPa and most preferably at least 120 ksi/ 840 MPa. Elongation (in 4D) at failure at room temperature is preferably at least 10% and more preferably at least 15%; and at 300° F/ 150 °C is preferably at least preferably at least 13% and more preferably at least 15%. Impact strength at room temperature is preferably at least 15 ft-lbs/ 2.1 mg-m and more preferably at least 17 ft-lbs/2.35 kg- ; and at 300 F/l 50 °C is preferably at least 15 ft-lbs/ 2.1 kg-m and more preferably at least 22 ft-lbs/3.04 kg-m. In addition, such components have equivalent durability properties, such as fatigue strength, and particularly high cycle fatigue capability. Fatigue tests also compared die cast Ti 6-4 and corresponding forged parts, and as indicated in FIGS. 7 and 8 the die cast articles exhibit smooth and notched fatigue lives comparable to forged articles. The above values are at least comparable to corresponding forged articles in compliance with AMS 4928 (Rev. N, Apr. 1993). Other relevant specifications include AMS 4967, 4965 and 4930. which are all incorporated by reference herein. Again, the specific required values will differ depending upon the particular use to which the articles are being put. Die cast articles composed of Ti 6-2-4-2 were also prepared in accordance with the present invention, as discussed in more detail below. The articles included compressor airfoils and test bars, and also included the above-post cast processing. Exemplary microstructures of a test bar and of an airfoil are illustrated in FIGS. 9 and 10. The microstructure of a corresponding airfoil composed of forged Ti 6-2-4-2 is illustrated in FIG. 1 1. Turning generally to FIG. 12, die cast articles composed of Ti 6-2-4-2 to be used as compressor airfoils have strength and impact properties at least equivalent to those exhibited by corresponding forged articles produced for this application. Compressor airfoils composed of Ti 6-2- 4-2 should have a yield strength at 900 F/480 °C of at least 55 ksi/ 385 MPa and more preferably 65 ksi/ 455 MPa and most preferably at least 72 ksi/ 504 MPa. Such articles should have an ultimate tensile strength at 900 ° F/ 480 °C of at least 75 ksi/ 525 MPa and more preferably 85 ksi/ 595 MPa and most preferably at least 95 ksi/ 665 MPa. Elongation (in 4D) at failure at 900 F/ 480 °C is preferably at least preferably at least 10% and more preferably at least 13%. In addition, such components must have equivalent durability properties, such as fatigue strength, and particularly high cycle fatigue capability. Fatigue tests also compared die cast Ti 6-2-4- 2 and corresponding forged parts, and as indicated in FIG. 12 the die cast articles exhibit properties comparable to forged articles. The above values are comparable to corresponding forged articles in compliance with AMS 4976 (Rev. E, July 1994); other relevant specifications include AMS 4975, which are all incorporated by reference herein. The above examples support and illustrate that die casting may be used to produce articles composed of a broad range of titanium alloy compositions. In further support, articles have also been die cast from Ti 8-1-1. Exemplary microstructure of die cast Ti 8-1-1 is illustrated in FIG. 13. Fatigue tests comparing die cast Ti 8-1-1 and corresponding forged parts are expected to indicate that the die cast articles exhibit comparable properties to forged articles. The above values will differ depending upon the particular use to which the articles are being put. The above values are comparable to corresponding forged articles in compliance with AMS 4973 (Rev. D, Oct. 1990); other relevant specifications include AMS 4972, which are all incorporated by reference herein. Turning to FIGS. 14, 15 and 16, we prefer to use a die casting machine (FIGS. 14-15) of the type having an unheated shot sleeve ("cold chamber") to produce articles in accordance with the present invention. Generally, a charge of material is prepared (FIG. 16, step 44), and the material to be die cast is melted (step 46 - FIG. 16) in the apparatus 18. As is generally known, molten titanium is an aggressive material, and attacks the material in which it is melted. Accordingly, we prefer to melt the titanium by induction skull remelting or melting (ISR) 24, for example in a unit of the type manufactured by Consarc Corporation of Rancocas, NJ which is capable of rapidly, cleanly melting a single charge of material to be cast, e.g., up to about 25 pounds of material. In ISR, material is melted in a crucible defined a plurality of metal (typically copper) fingers retained in position next to one another. The crucible is surrounded by an induction coil coupled to a power source 26. The fingers include passages for the circulation of cooling water from and to a water source (not shown), to prevent melting of the fingers. The field generated by the coil heats and melts material located in the crucible. The field also serves to agitate or stir the molten metal. A thin layer of the material, e.g., the titanium, freezes on the crucible wall and forms the skull, thereby minimizing the ability of molten titanium to attack the crucible. By properly selecting the crucible and coil, and the power level and frequency applied to the coil, it is possible to urge the molten material away from the crucible, further reducing attack of the crucible wall by the molten material. By melting only a single charge rather than maintaining a large container of molten alloy, we ensure that components having relatively low melting points relative to the alloy as a whole are not vaporized and lost prior to casting. Where reactive materials, such as titanium and aluminum and alloys containing these materials, are to be cast it is important to melt the materials in a non-reactive environment, to prevent reaction, contamination or other condition which might detrimentally affect the quality of the resulting articles. Since any gasses in the melting environment may become entrapped in the molten material and result in excess porosity in die cast articles, we prefer to melt the material in a vacuum environment rather than in an inert environment, e.g., argon. More preferably the material is melted in a melt chamber 20 coupled to a vacuum source 22 in which the chamber is maintained at a pressure of less than 100 μm, preferably less than 50 μm. While we prefer to melt single, or smaller charges of titanium material using an ISR unit, the material may be melted in other manners, such as by vacuum induction melting (VIM) and electron beam melting, so long as the material being melted is not significantly contaminated. Moreover, we do not rule out melting bulk material, e.g., several charges of material at once, in a vacuum environment and then transferring single charges of molten material into the shot sleeve for injection into the die. However, since the material is melted in a vacuum, any equipment used to transfer the molten material must typically be capable of withstanding high temperatures and be positioned in the vacuum chamber, and consequently the chamber must be relatively large. The additional equipment adds cost, and the correspondingly large vacuum chamber takes longer to evacuate thus affecting the cycle time. Since some amount of time will necessarily elapse between material melting and injection of the molten material into the die, the material is melted with a limited superheat - high enough to ensure that the material remains at least substantially molten until it is injected, but low enough to ensure that rapid solidification occurs upon injection enabling formation of small grains and also to minimize the thermal load upon the die casting apparatus (particularly those portions of the apparatus which come into contact with the molten metal). We have melted titanium alloys to a controlled, limited superheat, e.g., we have successfully used superheats within about 100° F to 200° F/ 37 - 95 °C above the melting temperature of the alloy and more preferably within about 50° F to 100° F/ 10 - 37 °C, preferably using a ceramic free melting system such as an inducto-skull melting unit. The material is sufficiently superheated to ensure that it remains molten until injected into the mold, but the amount of superheat is low enough to enable rapid solidification of the molten material after injection. Molten alloy is then transferred to a horizontal shot sleeve of the machine, which is preferably located in a vacuum environment and the molten material is injected under pressure into a reusable mold. We have found that the process of pouring and injecting the molten material in one or two seconds works well in a die casting machine having an unheated shot sleeve. In order to transfer molten material from the crucible to a shot sleeve 30 of the apparatus (step 48 - FIG. 16), the crucible is mounted for translation (arrow 31 in FIG. 15), and also for pivotal movement (arrow 33 of FIG. 14) about a pouring axis, and in turn is mounted to a motor (not shown) for rotating the crucible to pour molten material from the crucible through a pour hole 32 of the shot sleeve 30. Translation of the crucible occurs between the melt chamber 20 in which material is melted and a position in a separate vacuum chamber 34 in which the shot sleeve is located. The pour chamber 34 is also maintained as a non-reactive environment, preferably a vacuum environment with a pressure level less than 100 μm and more preferably less than 50 μm. The melt chamber 20 and pour chamber 34 are separated by a gate valve or other suitable means (not shown) to minimize the loss of vacuum in the event that one chamber is exposed to atmosphere, e.g., to gain access to a component in the particular chamber. While the illustrated embodiment includes separate melting and pouring chambers, it is also possible to perform melting and pouring in a single chamber. We prefer to use separate chambers in order to minimize the loss of vacuum environment in the event that a given component must be exposed to atmosphere, e.g., to service the melting unit or the shot sleeve or to remove a casting. As noted above, the molten material is transferred from the crucible 24 into the shot sleeve 30 through a pour hole 34. The shot sleeve 30 is coupled to a multipart, reusable die 36, which defines a die cavity 38 A sufficient amount of molten material is poured into the shot sleeve to fill the die cavity, which may include one part or more than one part. We have successfully cast as many as 12 parts in a single shot, e.g., using a 12 cavity die. The illustrated die 36 includes two sections, 36a, 36b (but may include more sections), which cooperate to define the die cavity 38, for example in the form of a compressor blade or vane for a gas turbine engine. The die 36 is also preferably coupled directly to the vacuum source and also through the shot sleeve, to enable evacuation of the die prior to injection of the molten metal. The die may be located in a vacuum chamber, instead of or in addition to being coupled directly to a vacuum source. One section of the two sections 36a, 36b of the die is typically fixed, while the other part is movable relative to the one section, for example by a hydraulic assembly (not shown). The die preferably includes ejector pins (not shown) to facilitate ejecting solidified material from the die. The die may also include a stripper mechanism (not shown) for removing casting material from the die while the material is still hot, to further reduce thermal loads on the die. The die may be composed of various materials, and should have good thermal conductivity, and be relatively resistant to erosion and chemical attack from injection of the molten material. A comprehensive list of possible materials would be quite large, and includes materials such as metals, ceramics, graphite, ceramic matrix composites and metal matrix composites. Each of various die material has attributes, e.g., ease of machining, strength at elevated temperatures and compromises of the two, that makes it desirable for different applications. For titanium, we currently prefer to use dies composed of mild carbon steel, e.g., 1018, due to its low cost and ease of machining. Coatings and surface treatments may be employed to enhance apparatus performance and the quality of resulting parts. The die may also be attached to a source of coolant such as water or a source of heat such as oil (not shown) to thermally manage the die temperature during operation. In addition, a die lubricant may be applied to one or more selected parts of the die and the die casting apparatus. Any lubricant should generally improve the quality of resultant cast articles, and more specifically should be resistant to thermal breakdown, so as not to contaminate the material being injected. Molten metal is then transferred from the crucible to the shot sleeve. A sufficient amount of molten metal is poured into the shot sleeve to partially fill the sleeve, and subsequently to fill the die. Preferably, the sleeve is less than 50% filled, more preferably less than about 40% filled, and most preferably less than 30% filled. An injection device, such as a plunger 40 cooperates with the shot sleeve 30 and hydraulics or other suitable assembly (not shown) drive the plunger in the direction of arrow 42, to move the plunger between the position illustrated by the solid lines and the position indicated by dashed lines, and thereby inject the molten material under pressure from the sleeve 30 into the die cavity 38 (step 50 - FIG. 15). In the position illustrated by solid lines, the plunger and sleeve cooperate to define a volume that is substantially greater than the amount of molten material that will be injected. Preferably, the volume is at least twice the volume of material to be injected, more preferably at least three times. Accordingly, the volume of molten material transferred from the crucible to the sleeve fills less than one half and most preferably less than about one third of the sleeve volume. Since the sleeve is only partially filled, any material or skin that solidifies on the sleeve forms only a partial cylinder, e.g., an open arcuate surface, and is easily scraped or crushed during metal injection, and reincorporated into the molten material. For injection, we have used plunger speeds of between about 30 inches per second (ips) and 300 ips (with a shot sleeve having an inner diameter of about 3 inches), and currently prefer to use a plunger speed of between about 50 - 175 inches per second (ips)/ 1.2 - 4.5 m/s. The plunger is typically moved at a pressure of at least 1200 psi/8.4 MPa, and more preferably at least 1500 psi/ 10.5 MPa. As the plunger approaches the ends of its stroke when the die cavity is filled, it begins to transfer pressure to the metal. It may then be desirable to intensify the pressure to ensure complete filling of the mold cavity, the particular intensification parameters will depend upon the desired result. Intensification is performed to minimize porosity, and to reduce or eliminate any material shrinkage during cooling. We have used intensification above 1500 psi/ 10.5 MPa with satisfactory results. After a sufficient period of time has elapsed to ensure solidification of the material in the die, the ejector pins (not shown) are actuated to eject parts from the die (step 52 - FIG. 16). As is known in the art, cast articles typically include some porosity, generally up to a few percent. Accordingly, and particularly where such articles are used in more demanding applications, such as compressor airfoils for gas turbine engines, there is a need to reduce and preferably eliminate porosity and otherwise treated as needed (step 54 - FIG. 16). The parts are therefore preferably hot isostatically pressed (HIP'd) as described above to reduce and substantially eliminate porosity in the as cast parts. For titanium alloy articles, we generally prefer to HIP at a temperature of above about 1500 - 1600 F/ 815 - 870 °C (and below the beta transus temperature of about 1850 F/ 1010 °C where it is desired to maintain existing beta phase), at a pressure of at least 14 ksi/ 98 MPa, more preferably above 14.5 ksi/ 101.5 MPa, and for at least 2 hours. If desired, the articles may then be heat treated. As noted above, for airfoils composed of die cast Ti 6-4, the articles may be heated to a temperature of between about 1500-1600 VI 815 - 870 °C in an inert environment, e.g., argon or vacuum, for at least 2 hours. For airfoils composed of die cast Ti 6-2-4-2, the articles are preferably heated to a temperature of between about 1000-1200 F, more preferably about 1 100 F/ 590 °C in an inert environment, e.g., argon or vacuum, for at least 8 hours. Actual heat treatment and HIP parameters may be varied depending upon the desired application for the article and target cycle time for the process, however the temperature, pressure and time used during HIP must be sufficient to eliminate substantially all porosity in the cast articles, but not to allow significant grain growth and coarsening of any alpha and the beta phases in the transformed beta microstructure. The parts are inspected (step 56 - FIG. 16) using conventional inspection techniques, e.g., by fluorescent penetrant inspection (FPI), radiographic, visual, and after passing inspection may be used or further treated/re-treated if necessary (step 58 - FIG. 16). As a result of our work with titanium alloys, we believe that several conditions are important to produce good quality castings. The melting, pouring and injection of material, particularly for reactive materials such as titanium alloys, must be performed in a non-reactive environment, and we prefer to perform these operations in a vacuum environment maintained at a pressure preferably less than 100 μm and more preferably less than 50 μm. The amount of superheat should be sufficient to ensure that the material remains substantially and completely molten from the time it is poured until it is injected, but also to enable rapid cooling and formation of small grains once injected. Due to the relatively low superheat, molten metal transfer and injection must be rapid enough to occur prior to metal solidification. The resulting microstructure such as grain size appears to correspond to the sectional thickness of the part being cast as well as the die materials utilized and the superheat used, i.e., thinner sections tend to include smaller grains and thicker sections (particularly internal portions of thicker sections) tend to include larger grains. Higher thermal conductivity die materials result in articles having smaller grains, as does use of lower superheats. We believe that this results from relative cooling rates. The rate at which the plunger is moved, and correspondingly the rate at which material is injected into the mold appears to affect the surface finish of the articles as cast, although the design of the gating as well as the die material may also play a role in combination with the injection rate. Die casting provides other significant advantages over forging. From the standpoint of required equipment, forging requires the production of multiple dies to make a new part, at significant cost. In contrast, only a single die set is required per part, at significantly reduced expense relative to forging. The time required to produce a part, from ingot to finished part, is reduced significantly, since there is no need to prepare specially tailored billets of material, and casting broadly is performed in a single step, as opposed to multiple forging operations. In die casting, multiple parts can be produced in a single casting. Die casting enables production of parts having more complex three dimensional shapes, thereby enabling new software design technology to be applied to and exploited in areas such as gas turbine engines and enabling production of more efficient airfoils and other components. We believe that die casting will enable the production of articles having complex shapes utilizing materials that are difficult or impossible to forge into those shapes. Moreover, die cast parts can be produced nearer to their finished shape, and with a superior surface finish, which minimizes post forming finishing operations, all of which also reduces the cost of producing such parts. While the present invention has been described above in some detail, numerous variations and substitutions may be made without departing from the spirit of the invention or the scope of the following claims. Accordingly, it is to be understood that the invention has been described by way of illustration and not by way of limitation. What is claimed is:

Claims: 1. A die cast titanium alloy article (10). 2. The article of claim 1, wherein the microstructure is characterized by a transformed, beta microstructure. 3. The article of claim 1, wherein the microstructure is characterized by an absence of flow lines. 4. The article of claim 1, wherein the article comprises a gas turbine engine component. 5. The article of claim 4, wherein the article is a compressor component. 6. The article of claim 1, wherein the article has an average grain size smaller than about ASTM 0. 7. The article of claim 6, wherein the grain size is smaller than about ASTM 3. 8. The article of claim 1 , wherein the article is composed of at least about 25 at. % titanium. 9. The article of claim 1, wherein the article is composed of about 4 - 8 w/o Al and 3 - 5 w/o V, balance essentially Ti. 10. The article of claim 1 , wherein the article is composed of: 5 - 7 w/o Al, 3.5 - 4.5 w/o V, up to about 0.5 w/o Fe, up to about 0.25 w/o O, up to about 0.20 w/o C, up to about 0.1 w/o N, up to about 0.02 w/o H, up to about 0.01 w/o Y, balance essentially titanium. 1 1. The article of claim 8, wherein the article has an ultimate tensile strength of at least 120 ksi/840 MPa and a 0.2% yield strength of at least 110 ksi/ 770 MPa. 12. The article of claim 1 1, wherein the article has an ultimate tensile strength of at least 135 ksi/ 945 MPa and a 0.2% yield strength of at least 125 ksi/ 875 MPa 13. The article of claim 1, wherein the article is composed of about 5 - 7 w/o Al, about 1.5 - 2.5 w/o Sn (tin), about 3.0 - 5.0 w/o Zr, about 1.5 - 2.5 w/o Mo, balance generally titanium. 14. The article of claim 1, wherein the article is composed of: 5.5 - 6.5 w/o Al, about 1.75 - 2.25 w/o Sn (tin), about 3.5 - 4.5 w/o Zr, about 1.8 - 2.2 w/o Mo, up to about 0.2 w/o Fe, up to about 0.15 w/o C. up to about 0.25 w/o O, up to about 0.1 w/o N, up to about 0.02 w/o H, and up to about 0.01 Y, balance essentially titanium. 15. The article of claim 13, wherein the article has an ultimate tensile strength of at least 75 ksi/ 525 MPa at about 900 F/ 480 °C. 16. The article of claim 15, wherein the article has an ultimate tensile strength of at least 95 ksi/ 665 MPa at about 900 F/480 °C. 17. The article of claim 1, wherein the article is composed of about 7-8.5 w/o Al, 0.5-1.5 w/o Mo and 0.5-1.5 w/o V, balance generally titanium. 18. The article of claim 1 , wherein the article is composed of: 7.35-8.35 w/o Al, 0.75-1.25 w/o Mo and 0.75-1.25 w/o V, up to about 0.15 w/o Si, up to about 0.4 w/o Fe, up to about 0.15 w/o O, up to about 0.1 w/o C, up to about 0.25 w/o Sn, up to about 0.15 w/o Cu, up to about 750 ppm N, up to about 200 ppm H, up to about 50 ppm B, up to about 75 ppm Y, balance titanium and traces of other elements. 19. The article of claim 18, wherein the article has an ultimate tensile strength of at least 1 15 ksi/ 805 MPa and 0.2% yield strength of at least 100 ksi/ 770 MPa, at about room temperature, and an ultimate tensile strength of at least 75 ksi 525 MPa and 0.2% yield strength of at least 60 ksi/ 420 MPa, at about 800 F/ 425 °C. 20. The article of claim 19, wherein the article has an ultimate tensile strength of at least 125 ksi/875 MPa and 0.2% yield strength of at least 1 10 ksi/ 770 MPa, at room temperature, and an ultimate tensile strength of at least 80 ksi/ 560 MPa and 0.2% yield strength of at least 70 ksi/ 490MPa, at about 800 F/ 425 °C. 21. A die cast gas turbine engine component composed of titanium alloy and having a transformed beta microstructure with an absence of flowlines. 22. The component of claim 21, wherein the component is a compressor component. 23. The component of claim 22, wherein the average grain size is smaller than about ASTM 3. 24. The article of claim 21, wherein the article is composed of about 4 - 8 w/o Al and 3 - 5 w/o V, balance generally Ti. 25. The article of claim 24, wherein the article has an ultimate tensile strength of at least 120 ksi/ 840 MPa and a 0.2% yield strength of at least 1 10 ksi/ 770 MPa. 26. The article of claim 24, wherein the article has an ultimate tensile strength of at least 135 ksi/ 945 MPa and a 0.2% yield strength of at least 125 ksi/ 875 MPa. 27. The article of claim 21, wherein the article is composed of about 5 - 7 w/o Al, about 1.5 - 2.5 w/o Sn (tin), about 3.0 - 5.0 w/o Zr, about 1.5 - 2.5 w/o Mo, balance generally titanium. 28. The article of claim 27, wherein the article has an ultimate tensile strength of at least 75 ksi/ 525 MPa at about 900 F/ 482 °C. 29. The article of claim 21, wherein the article is composed of about 7-8.5 w/o Al, 0.5-1.5 w/o Mo and 0.5-1.5 w/o V, balance generally titanium. 30. The article of claim 29, wherein the article has an ultimate tensile strength of at least 1 15 ksi. 805 MPa and 0.2% yield strength of at least 100 ksi/ 700MPa, at about room temperature, and an ultimate tensile strength of at least 75 ksi/ 525 MPa and 0.2% yield strength of at least 60 ksi/ 420 MPa, at about 800 F/ 425 °C. 31. The article of claim 1 , wherein the material is composed of titanium and aluminum in stoichiometric amounts, whereby the resulting article is composed of TiAl and TiAb. 32. The article of claim 21, wherein the material is composed of titanium and aluminum in stoichiometric amounts, whereby the resulting article is composed of TiAl and TiAb. 33. The article of claim 24, wherein the article has strength and fatigue resistance comparable to the values for forged components in accordance with AMS 4928. 34. The article of claim 27, wherein the article has strength and fatigue resistance comparable to the values for forged components in accordance with AMS 4976. 35. The article of claim 29, wherein the article has strength and fatigue resistance comparable to the values for forged components in accordance with AMS 4973.

Inventors: ANDERSON DAVID WILLIAM, BORG CHRISTOPHER ANTHONY, SCHIRRA JOHN JOSEPH, ANDERSON, DAVID, WILLIAM, BORG, CHRISTOPHER, ANTHONY, SCHIRRA, JOHN, JOSEPH
Applicants: ANDERSON DAVID WILLIAM, BORG CHRISTOPHER ANTHONY, SCHIRRA JOHN JOSEPH, UNITED TECHNOLOGIES CORP, ANDERSON, DAVID, WILLIAM, BORG, CHRISTOPHER, ANTHONY, SCHIRRA, JOHN, JOSEPH, UNITED TECHNOLOGIES CORPORATION
Application year: 1999
Main classification: C22C
IPC codes: C22C1, C22C14