BR102013006917A2 - Method of processing an intermetallic titanium aluminide composition - Google Patents

Abstract

Method of processing an intermetallic titanium aluminide composition. These are methods for processing compositions containing titanium and aluminum, especially intermetallic titanium aluminide (thiai intermetallic) compositions based on the thiai (gamma) intermetallic compound. The methods provide processing steps that include an isostatic hot (hip) pressing cycle and a heat treatment cycle that can be performed in a single container. The thiatrial intermetallic compositions processed in this manner preferably exhibit a duplex microstructure containing equiaxial and lamellar morphologies.

Description

“METHOD OF PROCESSING AN INTERMETAL TITANIUM ALUMINUM COMPOSITION” Cross Reference to Related Applications This application claims the benefit of Provisional Application No. 61 / 614,751, filed March 23, 2012, the content of which is incorporated herein by reference. reference.

Background of the Invention The present invention generally relates to compositions containing titanium and aluminum and the processing thereof. More particularly, this invention relates to methods of processing molten titanium aluminide intermetallic compositions that impart isostatic hot pressing and heat treatment to close porosity and yield a desirable microstructure.

Because high temperature strength and weight are primary considerations in gas turbine engine designs, there is a continuous effort to create relatively light weight alloys / compositions that have high strength at high temperatures. Titanium based alloy systems are well known in the art as having mechanical properties that are suitable for relatively high temperature applications. The high temperature capacities of titanium based alloys have been increased through the use of titanium intermetallic systems based on titanium aluminide compounds Ti3AI (alpha-2 (a-2) alloys) and TiAI (gamma alloys (γ)). These titanium aluminide (or, for convenience, TiAI intermetallic) intermetallic compounds are generally characterized as being of relatively light weight, although they are known to be capable of exhibiting high strength, creep force, and fatigue strength in general. high temperatures. Chromium and niobium additions are known to promote certain TiAI intermetallic properties, such as oxidation resistance, ductility, strength, etc. As a non-limiting example, Huang U.S. Patent 4,879,092 discloses an intermetallic titanium aluminide composition having an approximate formula of Ti46-5oAl46-5oCr2Nb2, or nominally about Ti-48AI-2Cr-2Nb. This alloy, referred to herein as alloy 48-2-2, is considered to have a nominal temperature capacity of up to about 1,400 ° F (about 760 ° C), with useful but diminishing capacities of up to about 1,500 ° F (about 815 ° C). On gas turbine engines used in commercial aircraft, alloy 48-2-2 is suitable for low pressure turbine blade (LPTB) applications. The production of TiAI intermetallic components is complicated by their relatively low ductilities and the typical desire for such compositions to be extrudable, forged, laminatable and / or fusible. Isostatic hot pressing (HIP) is commonly performed to eliminate internal voids and microporosity in titanium aluminide intermetallic castings. Because uncontrolled cooling rates typically performed following HIP are not effective to generate a desired microstructure, the responsiveness to post-HIP heat treatments is another desirable feature in order to obtain microstructures and mechanical properties necessary for specific applications.

HIP cycles are typically separated from the heat treatment cycle in foundry processing. As an example, the desired mechanical properties and microstructures have been obtained in 48-2-2 alloy castings by the use of a process shown in Figure 3.

Following casting production, a pre-HIP heat treatment is performed at a temperature within a range of about 1,800 to about 2,000 ° F (about 980 to about 1,090 ° C) and for a duration of about five till twelve o'clock. Thereafter, the smelter is cooled and transferred to a HIP chamber and then subjected to a high pressure HIP step (eg 25 ksi (about 1,720 bar) or more) at about 2,165 ° F for a duration of about three hours. The HIP smelter is then cooled, removed from the HIP chamber, and then subjected to post HIP solution treatment at a temperature of about 2,200 ° F for a duration of about two hours. This sequence requires the use of at least two different containers and loading and unloading the foundry three times from these containers. In addition to entailing additional cost and cycle time, this process has been linked to loss of aluminum from the casting surface, which leads to reduced mechanical and / or environmental properties.

Unexpectedly, end-form castings that were produced, for example, by 48-2-2 alloy spin casting to produce low pressure turbine blades did not respond well to the heat treatment process described above, or to other processes employed. with conventional TiAI castings such as gravity casting and material overcasting. In particular, 48-2-2 alloy form castings processed by form cast cast methods do not develop a desirable duplex microstructure containing equiaxial and lamellar gamma TiAI morphologies that improve cast ductility, particularly when the cast fraction is cast. The volume of the lamellar structure is about 10 to about 90 percent, particularly if the volume fraction of the lamellar structure is about 20 to about 80 percent, and ideally about 30 to about 70 percent. Figures 1 and 2 are photomicrographs showing desirable duplex microstructures present in two conventional TiAI foundries.

In view of the above, a method that is capable of processing TiAI intermetals is required, including but not limited to 48-2-2 alloy final shape geometries, to yield a duplex microstructure that contains equiaxial morphologies. and lamellar. It would be even more desirable if such a method did not require a sequence in which a foundry does not require to be transferred between multiple different containers.

BRIEF DESCRIPTION OF THE INVENTION The present invention provides methods capable of processing titanium and aluminum-containing compositions, and especially intermetallic titanium aluminide (TiAI intermetallic) compositions based on TiAI intermetallic compound (gamma), to yield desirable microstructures.

The methods have the additional ability to be performed in a single container, which results in a less complicated process than conventional methods used to produce compositions requiring void space closure (eg by HIP process) and heat treatment.

According to a first aspect of the invention, a method of processing an intermetallic titanium aluminide composition includes hot isostatic pressing of the composition to a temperature of at least 1,260 ° C (about 2,300 ° F), cooling of the composition to a not less than 1,120 ° C (about 2,050 ° F), heat treating the composition at a temperature of about 1,150 to about 1,200 ° C (about 2,100 to about 2,200 ° F), and then cooling the composition to room temperature. Following the above procedure, the intermetallic titanium aluminide composition exhibits a desirable duplex microstructure that contains gamma TiAI phase equiaxial and lamellar morphologies.

According to a second aspect of the invention, an alternative method of processing an intermetallic titanium aluminide composition includes isostatic hot pressing of the intermetallic titanium aluminide composition, cooling the composition, heat treating the composition to a temperature of at least 1,260 ° C (about 2,300 ° F) for about 2.5 to about 5 hours, cooling the composition to a temperature of not less than 1,120 ° C (about 2,050 ° F), maintain the composition at a temperature of maintaining about 1,150 to about 1,200 ° C (about 2,100 to about 2,200 ° F) for a duration of about two to about six hours, and then cooling the composition to room temperature. Following this procedure, the intermetallic titanium aluminide composition exhibits a desirable duplex microstructure that contains gamma TiAI phase equiaxial and lamellar morphologies.

A technical effect of the invention is the ability to produce desirable duplex microstructures in TiAI intermetals which may otherwise be difficult to obtain, particularly if produced by final form casting methods such as spin casting and possibly certain other castings. casting techniques. Another technical effect is the ability to take advantage of the energy available for phase equilibrium during cooling of a HIP step to aid in subsequent heat treatment, which has been determined to eliminate the requirement for pre- and post-cycle conventional treatments. heat that can cause aluminum to be lost from the casting surface as well as imply additional cost and cycle time. These advantages have been particularly observed with final form castings produced by final form casting methods, such as spin casting, in the aforementioned alloy 48-2-2, although other intermetallic TiAI compositions also benefit from the processing methods provided by present invention.

Other aspects and advantages of this invention will be further appreciated from the following detailed description.

BRIEF DESCRIPTION UAS HGURAS

Figures 1 and 2 are photomicrographs showing the microstructures of two foundries formed of an intermetallic TiAI composition with a desirable duplex microstructure. Figure 3 is a flowchart depicting a method of processing foundries formed of TiAI intermetallic compositions according to a prior art heat and HIP treatment process.

Figures 4 and 5 are flow charts depicting two methods of processing foundries formed in TiAI intermetallic compositions in accordance with embodiments of the present invention.

Figures 6 and 7 are microphotographs showing the microstructures of two foundries formed of the same TiAI intermetallic composition, wherein the foundry of Figure 6 was processed according to the prior art heat and HIP treatment process of Figure 3 and FIG. The smelter of Figure 7 was processed according to the heat and HIP treatment process of Figure 4.

Detailed Description of the Invention Figures 4 and 5 contain flowcharts depicting two related methods whereby intermetallic TiAI compositions, which include but are not limited to alloy 48-2-2, can be processed to yield a desirable duplex microstructure with added benefit of avoiding the disadvantages of the prior art process summarized in Figure 3.

In particular, the methods of Figures 4 and 5 prevent vacuum heat treatments pre and post HIP which are believed to be capable of promoting aluminum loss in intermetallic TiAI compositions. The invention also takes advantage of the high gas pressures and (inert) protective atmospheres used during HIP, the combination of which is believed to be capable of reducing aluminum loss in an intermetallic TiAI composition. In addition, each of the methods summarized in Figures 4 and 5 provides interrupted cooling of a HIP step (Figure 4) or a temperature believed to be able to take advantage of unbalanced phase distribution in TiAI intermetallic compositions following HIP (Figure 5) to generate (during subsequent heat treatment) microstructures that are capable of providing desirable mechanical properties, especially if the TiAI intermetallic composition is a foundry that uses a final form casting process, such as die casting. rotation or other means.

As indicated above, the processes summarized in Figures 4 and 5 are believed to be capable of being particularly beneficial to alloy 48-2-2, whose composition is based on the gamma intermetallic compound (TiAI). Alloy castings 48-2-2 exhibit enhanced ductility and other desirable properties if they contain a duplex microstructure that contains equiaxial and lamellar gamma phase morphologies. Figures 6 and 7 are representative of LPTB castings produced from alloy 48-2-2. Both foundries were produced by spin casting, the casting in Figure 6 was processed by a heat treatment and HIP procedure that corresponds to that shown in Figure 3, and the casting in Figure 7 was processed by a heat treatment procedure. and modified HIP corresponding to that shown in Figure 4. The heat-treated casting microstructure shown in Figure 6 has an excessive amount of equiaxial gamma phase and an inadequate amount of lamellar phase (less than 10% lamellar phase volume fraction) . Such a microstructure would yield a component with creep force under insufficient high temperature. The heat-treated casting microstructure shown in Figure 7 has acceptable amounts of the equiaxial gamma phase and the lamellar phase (about 20% volume fraction of the lamellar phase), with the only exception being the outermost surface of the foundry where Titanium levels are depleted.

However, the outermost surface may be removed by conventional techniques such as abrasive blasting or chemical grinding, with the result that the remaining total cast would contain acceptable amounts of the equiaxial gamma phase and the lamellar phase.

Although the invention has been shown to yield particularly advantageous results with the 48-2-2 alloy, it is believed that the invention is more generally applicable to titanium aluminide intermetallic compositions, particularly modified TiAI (gamma) intermetallic compositions with elements which are suitable for are intended to promote various properties.

For example, the invention has also been shown to be effective with tantalum-containing TiAI intermetallic compositions. Particular compositions that have been correctly evaluated include TiAI compositions containing chromium, niobium and / or tantalum, for example about 1.8 to about 2 atomic percentage of chromium, up to about 2 atomic percentage of niobium, and up to about 2%. about 4 atomic percentage of tantalum. Specific compositions that were correctly evaluated contained atomic percentage: about 47.3% aluminum, about 1.9% chromium, about 1.9% niobium and the titanium content and incidental impurities (which corresponds to high to alloy 48-2-2); or about 47.3% aluminum, about 1.8% chromium, about 0.85% niobium, about 1.7% tantalum and the titanium content and incidental impurities; or about 47.3% aluminum, about 2.0% chromium, about 4.0% tantalum and the titanium content and incidental impurities. More generally, the titanium and aluminum levels in these TiAI intermetallic compositions are selected to yield a smelter whose predominant constituent is TiAI (gamma) intermetallic compound. Although all compositions evaluated contain about 47.3 percent atomic aluminum and about 46.7 to 48.9 percent atomic titanium, those skilled in the art will find that titanium and aluminum levels beyond these amounts can be used. to yield a foundry that is wholly or predominantly the intermetallic TiAI compound, and such variations are within the scope of the invention. Furthermore, those skilled in the art will recognize that other alloying constituents could be included to modify the properties of the TiAI intermetallic compound, and such variations are also within the scope of the invention.

During investigations leading to the present invention, solidification modeling was conducted and suggested that areas of low pressure turbine blade (LPTB) foundries formed by final form casting, including spin casting, solidified in less than a few seconds. It has been found that, compared to other casting methods and / or other types of casting, such rapid solidification rate can modify the route through the Ti-AI phase diagram that the alloy / composition takes during solidification and can lead to unexpected responses to conventional heat treatments that are subsequently performed in the foundries. These unexpected results negatively impact uniformity of the end-form casting microstructure and heat-treated components, such as chemistry and uniformity of the microstructure along the middle chord and the end-form TiAI airfoil amplitude. The process shown in Figure 4 combines a HIP cycle with a non-cooling heat treatment to room temperature between them, which reestablishes phase balances that are capable of developing the duplex microstructure that provides desirable mechanical properties. The process of Figure 4 generally confers preparation of the intermetallic TiAI composition. A preferred but non-limiting example gives the rotational casting an appropriate melt containing the desired constituents of the TiAI intermetallic composition. The composition (cast) is then loaded into a suitable HIP chamber and heated in a protective atmosphere (eg argon or other inert gas) to a temperature at which the cast should pass the HIP process. According to a preferred aspect of the invention, the temperature of HIP (THipi) is at least 2,300 ° F (about 1,260 ° C), more preferably at least 2,350 ° F (about 1,290 ° C), and even higher. preferably in a range of from about 2,375 to about 2,425 ° F (about 1,300 to about 1,330 ° C). The pressure applied to the foundry during the HIP cycle is designed to eliminate internal voids and microporosities in the foundry. To this end, pressures of at least 15 ksi (about 1,030 bar) are believed to be sufficient, with pressures of about 18 ksi (about 1,240 bar) and above believed to be particularly preferred. The duration of the HIP cycle may vary depending on the particular composition and pressure used, but it is believed that suitable results will be able to be obtained with HIP cycles lasting from about 2.5 to about 5 hours, and particularly about from 2.5 to about 3.5 hours.

Following the HIP cycle, the casting is cooled to a temperature of not less than 2,050 ° F (about 1,120 ° C), more preferably not less than 2,100 ° F (about 1,150 ° C), and most preferably about 2,100 to about 2,150 ° F (about 1,150 to about 1,175 ° C). The cooling rate may vary, but it has been found that rates of about 5 to about 20 ° F / min (about 3 to about 11 ° C / min) are acceptable. Without the need to be removed from the HIP chamber, the foundry is then heat treated at a temperature of about 2,100 to about 2,200 ° F (about 1,150 to about 1,200 ° C), for example about 2,100 to about 2,150 ° F (about 1,150 to about 1,175 ° C). The duration of such heat treatment may vary depending on the particular HIP composition and treatment used, but it is believed that adequate results will be obtained with heat treatment cycles lasting from about two to about six hours, and especially about 4.5 to about 5.5 hours.

Following heat treatment, the casting can be cooled directly to room temperature (about 20 to about 25 ° C) at any desired rate. As a result of this process, the TiAI intermetallic casting preferably exhibits a duplex microstructure of the type seen in Figure 7. From above, it should be apparent that the casting is not required to be removed from the HIP chamber during the steps identified in Figure 4, and that the smelter may be continuously exposed to the inert atmosphere of the HIP chamber throughout the process shown in Figure 4. The process shown in Figure 5 differs from that shown in Figure 4 by allowing complete cooling (to room temperature). between the HIP cycle and the heat treatment. The process of Figure 5 further involves heating the melt to THipi temperature prior to heat treatment. This process is believed to allow for more flexibility in the temperature used for the HIP cycle, where the HIP process is not required to be performed at the THipi temperature of Figure 4, but may instead be at a temperature (designated as THip2 ) which may be higher or lower than temperatures within the ranges stated above for Thipi.

In view of the above, the process shown in Figure 5 generally gives the HIP process an intermetallic TiAI composition (typically a foundry) at a suitable temperature (THip2), which may be followed by cooling of the foundry. up to essentially any temperature (including room temperature). Thereafter, the foundry is heat treated at Thipi temperature (e.g. at least 2,300 ° F (about 1,260 ° C)) for a sufficient time to ensure that the entire foundry is Thipi. The smelter can then be cooled to a suitable range (for example, about 5 to about 20 ° F / minute (about 3 to about 11 ° C / minute)) to a temperature of no less than 2,050 ° F ( about 1,120 ° C), more preferably not less than 2,100 ° F (about 1,150 ° C), and even more preferably about 2,100 to about 2150 ° F (about 1,150 to about 1,175 ° C). The casting can then be subjected to the same heat treatment as described for the process of Figure 4, after which the casting can be cooled directly to room temperature (about 20 to about 25 ° C). As a result of this process, the TiAI intermetallic casting preferably exhibits a duplex microstructure of the type seen in Figure 7. As with the process of Figure 4, it should be apparent that the casting is not required to be removed from the HIP chamber for any purpose. that the casting can be continuously exposed to the inert atmosphere of the HIP chamber throughout the process shown in Figure 5.

Although the invention has been described in terms of particular embodiments, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention should be limited only by the following claims.

Claims (20)

1. METHOD OF PROCESSING AN INTERMETAL TITANIUM ALUMINUM COMPOSITION, based on an intermetallic tial compound to yield a duplex microstructure containing thia gamma phase equiaxial and lamellar morphologies, the method comprising: isostatic hot pressing of titanium aluminide intermetallic composition at a temperature of at least 1,260 ° C; cooling the intermetallic titanium aluminide composition to a temperature of not less than 1 120 ° C; heat treating the intermetallic titanium aluminide composition at a temperature of about 1,150 to about 1,200 ° C; and then cooling the intermetallic titanium aluminide composition to room temperature; wherein the titanium aluminide intermetallic composition exhibits the duplex microstructure following the cooling step of the titanium aluminide intermetallic composition to room temperature. A method according to claim 1, wherein the isostatic hot pressing step is conducted at a pressure of at least 1,030 bar. A method according to claim 1, wherein the isostatic hot pressing step is conducted at a pressure of at least 1,240 bar. A method according to claim 1, wherein the isostatic hot pressing step is conducted at a temperature of at least 1,290 ° C. The method according to claim 1, wherein the isostatic hot pressing step is conducted at a temperature of about 1,300 to about 1,330 ° C. The method of claim 1, wherein the isostatic hot pressing step is conducted for a duration of from about 2.5 to about 5 hours. A method according to claim 1, wherein the intermetallic titanium aluminide composition is cooled to a temperature of not less than 1,150 ° C during the cooling step. A method according to claim 1, wherein the intermetallic titanium aluminide composition is cooled to a temperature of 1,150 to about 1,175 ° C during the cooling step. A method according to claim 1, wherein the heat treatment step is carried out at a temperature of about 1,150 to about 1,175 ° C. The method of claim 1, wherein the heat treatment step is performed for a duration of about two to about six hours. A method according to claim 1, wherein the intermetallic titanium aluminide composition consists of titanium and aluminum in amounts to yield the TiAI intermetallic compound, one or more of chromium, niobium, tantalum and incidental impurities. A method according to claim 1, wherein the intermetallic titanium aluminide composition consists of atomic percentage from about 1.8 to about 2% chromium to about 2% niobium to about of 4% tantalum, titanium and aluminum in quantities to yield TiAI intermetallic compound, and incidental impurities. A method according to claim 12, wherein the intermetallic titanium aluminide composition contains about 46.7 to 48.9 in atomic percentage of titanium. The method of claim 12, wherein the intermetallic titanium aluminide composition contains about 47.3 percent atomic aluminum. The method of claim 12, wherein the intermetallic titanium aluminide composition contains atomic percentage about 1.9% chromium, about 1.9 atomic percentage niobium, and no intentional amount. of tantalum. The method of claim 12, wherein the intermetallic titanium aluminide composition contains atomic percentage about 1.8% chromium, about 0.85 atomic percentage niobium, and about 1%. , 7% tantalum. The method of claim 12, wherein the intermetallic titanium aluminide composition contains atomic percentage about 2% chromium, about 4% tantalum, and no intentional amount of niobium. 18. METHOD OF PROCESSING AN INTERMETAL TITANIUM ALUMINUM COMPOSITION, based on an intermetallic tial compound to yield a duplex microstructure containing thia gamma phase equiaxial and lamellar morphologies, the method comprising: isostatic hot pressing of titanium aluminide intermetallic composition; cooling of the intermetallic composition of titanium aluminide; heat treating the intermetallic titanium aluminide composition at a temperature of at least 1,260 ° C for about 2.5 to about 5 hours; cooling the intermetallic titanium aluminide composition to a temperature of not less than 1,120 ° C; maintaining the intermetallic titanium aluminide composition at a maintenance temperature of about 1,150 to about 1,200 ° C for a duration of about two to about six hours; and then cooling the titanium aluminide intermetallic composition to room temperature; wherein the titanium aluminide intermetallic composition exhibits the duplex microstructure following the cooling step of the titanium aluminide intermetallic composition to room temperature. A method according to claim 18, wherein the intermetallic titanium aluminide composition is cooled after the heat treatment step to a temperature of not less than 1,150 ° C prior to the maintenance step, and the maintenance temperature. is from 1,150 to about 1,200 ° C. A method according to claim 18, wherein the intermetallic titanium aluminide composition consists of titanium and aluminum in amounts to yield the TiAI intermetallic compound, one or more of chromium, niobium, tantalum and incidental impurities.