JPH11293432A - Production of gamma-titanium alloy product by powder metallurgy - Google Patents

Abstract

PURPOSE: To enable to mold into a complex shape, particularly thin part by charging a mixture of Al3 Ti powder with Ti3 Al, which is in a TiAl composition as the whole composition, to a mold, solidifying to form a premolding, heating to work into a final shape and homogenizing under a condition to accelerate the mutual diffusion and the formation of TiAl. CONSTITUTION: The whole composition is the composition of TiAl, which is constituted substantially of preferably the molding formed from Al3 Ti powder and Ti3 Al powder. A sheet formed by using a precursor solidified in the mold, having <=0.1 in thickness and the whole composition of TiAl and free from breaking is provided. For example, the TiAl composition having γ-structure is obtained by rolling and forming the homogeneous premolding, which is obtained by charging into the mold, hot-pressing in vacuum to solidify and hot-forging, at 2200 deg.F and next, heating the molded product having the final shape at 2400 deg.F in vacuum for 10 hr to carry out the mutual diffusion between Al3 Ti and Ti3 Al.

Description

Description: TECHNICAL FIELD The present invention relates to a powder metallurgy process for gamma titanium (TiAl) alloy.

(Prior art) Titanium and its alloys have the unique properties of low density and high melting point, and are therefore widely used in advanced technology applications, especially in gas turbine engines.

 Numerous titanium alloys have been proposed. These generally contain at least about 80% titanium, the balance being other additives such as aluminum, vanadium, chromium, zirconium. Widely used commercial alloys of this type are either alpha or beta structures, both of which are essentially solid solutions of titanium.

Research has also been directed at the use of various titanium-based titanium intermetallic compounds. Among them, Al ₃ Ti, Ti ₃ Al, and TaAl are included. The composition of TiAl is of interest for the present invention. It has a high melting point of about 2600 ° C and a lower density than titanium itself due to the presence of large amounts of aluminum. One disadvantage of TiAl is that it lacks useful ductility. To overcome this problem, various alloying methods have been employed with some success. U.S. Pat. No. 4,294,615, filed by the same assignee (applicant) as the present invention, discloses that adding a small amount of vanadium increases the ductility of a TiAl-type composition, and adding a small amount of carbon. It has been published that the creep rupture strength of some materials is increased. This patent also mentions some of the earlier work in the TiAl system. This patent is incorporated herein by reference.

 Because of its high strength, low or medium ductility, and high melting point, intermetallic compounds have always been very difficult in the past to make TiAl type alloys. Molding was always done at elevated temperatures, usually at about 2400 ° F. or higher for ductility reasons. This requirement has created problems in the production of certain thin-profiled alloys, especially sheet materials.

The sheet material is formed by rolling, but when a thin sheet is formed, the material between the rolls quickly loses its heat due to the heat extraction capability of the rolls, causing the material to lose its heat. If it gets too cold, it will crack. The obvious countermeasure is to heat the roll to the hot roll temperature, but this is not practical for the temperatures used. To the inventor's knowledge, there has been no example of a Ti Al sheet having a thickness of 0.1 inch or less and having no cracks.

 Similar difficulties are expected when forming TiAl type materials into different thin-walled shapes by different methods such as forging.

In the case of other titanium-aluminum compounds, such a large lack of ductility is not a serious problem. Al ₃ -Ti and _Ti 3 Al shows a particularly useful ductility.

As used herein, the terms Ti ₃ Al, Al ₃ Ti and TiAl contain small amounts of alloying elements that do not substantially alter the crystal structure of the alloy phase.

These terms also refer to materials containing up to about 10% by volume for Ti ₃ Al and Al ₃ Ti and up to about 20% by volume for TiAl. That is, a structure composed of 85% by volume of TiAl, 5% by volume of Al ₃ Ti, and 10% by volume of Ti ₃ Al is regarded as TiAl.

(Problems to be Solved by the Invention) An object of the present invention is to provide a practical method for forming a complicated shape, particularly a thin portion having a TiAl type composition.

(Means for Solving the Problems) The prior art clarifies a wide range of TiAl type compositions and adds them to alloys of this type mainly for various purposes such as improving ductility, mainly for the purpose of improving ductility. Some possible alloying additives that can be added are discussed in some detail. However, as far as the inventor of the present application knows, we always consider forming or manufacturing a TiAl structure from a homogeneous preform, and the origin of the preform is essentially It is a cast product having a TiAl composition.

The present invention is a method using powder metallurgy to produce a TiAl composition from a mixture of powders (starting material), one based on Al ₃ Ti and the other based on Ti ₃ Al. Those skilled in the art will readily imagine that it is possible to produce a mixture of such powders whose overall net composition is in the region of the TiAl gamma phase. .

In accordance with the present invention, such a mixture is compacted in a mold to produce a preform of uniform mass of substantially full density. The resulting preform has useful ductility, especially if the time of exposure to high temperatures is minimized. This preformed body can be heated to a high temperature, and can be formed into a useful shape by, for example, rolling even if the heat is removed by the rollers. The molded article having the substantially final shape is then heated to an elevated temperature for a time sufficient to allow diffusion between the Al ₃ Ti and Ti ₃ Al components, thereby obtaining the desired TiAl composition. The finished product has an overwhelming amount of TiAl and can have useful ductility due to alloying additives added to the powdered precursor.

 The above and other features and advantages of the present invention will become more apparent from the following description and accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION According to the present invention, the shape of a TiAl material that is difficult to form is a ratio of Al ₃ Ti and Ti ₃ Al in which TiAl is formed by a subsequent thermal diffusion process. It is molded from a preform made of powder.

 The inventor used a powder made by rotary atomization, which also provided a very rapid cooling rate from the liquid. However, it is not necessary that the material be in a highly spherical shape if the cooling rate is not critical for the invention to be successful. The present inventor believes that powders made by other methods can be used in the present invention as well.

The inventor believes that the useful powder particle size range applicable to the present invention is between about -40 and -120 on a US standard sieve. Powders falling within this size range should be formed from Al ₃ Ti type compositions and Ti ₃ Al compositions. The desired alloying additives to enhance ductility or other alloy properties can be made according to either or both alloy types.

The proportion of powder to reach the desired TiAl composition depends somewhat on the alloying additives that have already been made. Gamma-type alloys, although based on intermetallics, have a fairly wide composition range of aluminum components (without other alloying elements) of about 35-45% by weight. One of ordinary skill in the art will readily be able to determine the proportions of Al ₃ Ti and Ti ₃ Al material required to achieve the desired composition within the gamma range. The mixture of suitable powders is placed in a sheet metal container, such as a stainless steel can, at a temperature in the range of 1600-2000 ° F, such as 1800 ° C.
At 20 ° F., for 20 to 40 ksi, for example 30 ksi, solidification can take place for about 30 minutes to 10 hours. This results in a powder compact having a density of at least 95% of the theoretical density.

 Next, the can is removed by mechanical or chemical means.

Next, the molded body can be forged or extruded into an intermediate shape. Forging or extrusion can be performed at a temperature of about 1700-2100 ° F. A typical deformation rate would be 0.1 inches / inch / min.

 What I would like to reconfirm is that both starting materials are reasonably ductile, but they react by diffusion after solidification, resulting in a TiAl structure that is much stronger but much less ductile than the starting material. Is to form. Therefore, processing should be performed in a timely manner at the lowest possible temperature, consistent with producing the desired results. As a result, the formation of TiAl during the processing can be minimized, and possible disturbance of the deformation can be prevented.

 The primary product for the purposes of the present invention is a thin sheet material that may be used, for example, in the manufacture of honeycombs. The forged or extruded intermediate product is put back into the can, but this time it is made of a strong alloy such as columbium.

The choice of alloy for the can in this case is determined by the desire to use a can that has the same resistance to deformation as the compact at the temperature chosen for the preform. To minimize diffusion and / or adhesion between the can and the preform, yttria (yttria: Y ₂ O ₃ ) can be used between the preform and the can. The canned material can then be hot rolled to the desired thickness at an elevated temperature between 2000 and 2400 ° F, and the can can then be removed.

The hot rolled material is then heat treated to promote complete diffusion interaction between Al ₃ Ti and Ti ₃ Al to form a TiAl gamma structure. The time and temperature of this thermal diffusion treatment will depend to some extent on the size of the initial powder used.

If the particle size of the powder is large, the diffusion distance will be large and the boundary area will be small, so it is necessary to increase the time and increase the temperature. For a useful range of powder sizes, conditions of 2200 to 2500 ° F. for 2 to 20 hours are typical.

 The present invention will be better understood from the following description of specific embodiments.

Example 100 grams of titanium containing -100 mesh Al-25 at% were mixed with 194 grams of niobium containing -50 mesh Ti-24 at% and aluminum -11 at%, respectively. Mixing was carried out in a V blender for about 2 hours. Next, the mixed powder was transferred into a stainless steel container and vacuum hot pressed within a vacuum of 0.0001 Torr. The powder was first soaked at 1700 ° F. for 1 hour and then solidified at 1700 ° F. for 2 hours at a pressure of 30 ksi. The resulting compact was about 1.5 inches in diameter and about 4 inches long.

 The can was removed by mechanical means. Next, the compact was forged with a 500-ton vacuum press. The compact was soaked at 1850 ° F for 1 hour and then forged at 1850 ° F using a molybdenum die at a deformation rate of 0.1 inch / inch / min. The resulting pancake was about 0.25 inch thick and had a fine structure as shown in FIG.

To make a rolled preform, the bread cake is first cut into rectangles and placed on a flat picture frame can made of columbium alloy C103. Before transferring the pancakes to the can space, all inner surfaces are coated with yttria (Y ₂ O ₂ ) to prevent reaction between the titanium material and the niob can. The can assembly was TIG welded along the seam and the front end was chamfered to assist in the initial rolling process.

 Rolling was performed on a normal roll mill at a temperature of 2200 ° F, but this relatively low temperature was chosen because the starting material did not change any further in order to obtain a stronger TiAl material. . The material was first soaked for 20 minutes and then rolled using a pass schedule as shown in Table 1 with intermediate reheats.

(Pass means that the deformed metal passes between the rolling rolls in rolling the metal.) We know that this rolling schedule is quite drastic, but the intent was to prevent the formation of TiAl as much as possible. X-ray diffraction analysis following this step showed that the rolled material had not yet completely transitioned to the gamma structure. The rolling was completely successful despite a significant reduction in the pass process. A final thickness of about 0.07 inches was achieved. As far as the inventor of the present application knows, it is the thinnest sheet among all sheets having a TiAl composition that has been successfully manufactured so far. FIG. 2 shows the microstructure after rolling.

 The rolled sheet was then subjected to a diffusion treatment at a temperature of 2400 ° F. for 10 hours in a vacuum so that the starting material was completely converted to TiAl. It surrounded a stock rolled with commercially available pure titanium sheets and prevented the material from absorbing oxygen. After homogenization, X-ray diffraction analysis confirmed that most of the material had been converted to a gamma structure.

The microstructure of the homogenized sheet is shown in FIG. The layered structure is typical of the gamma microstructure. is alpha ₂ phase (alpha two phase) _Ti 3 Al were also found in X-ray analysis.

Since only say nominal composition of the entire selected (overall nominal composition) is enriched slightly titanium to tolerate the presence of some alpha ₂ phase at equilibrium, not exposed to this very surprising.

It is generally believed that such a titanium-rich gamma alloy is even more difficult to manufacture than a completely pure TiAl structure.

 Although the present invention has been shown and described in detail in the embodiments, it will be understood that the form and details of the present invention can be variously changed without departing from the spirit of the present invention and the scope of the invention described in the claims. It will be understood by those skilled in the art.

[Brief description of the drawings]

 FIG. 1 shows a micrograph of the forged metal material. FIG. 2 shows a micrograph of the rolled metal material. FIG. 3 shows a micrograph of the homogenized metal material.

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Claims (3)

[Claims] 1. A) A mixture of Al ₃ Ti powder and Ti ₃ Al powder whose entire composition is in the TiAl phase region is put into a mold and solidified to form a preform, and b) The preform is finally finished. And c) homogenizing the heat-processed product under conditions that promote interdiffusion and formation of TiAl. 2. The composition as a whole comprises Ti ₃ Al powder and TiA
l ₃ is essentially the composition of TiAl which consists of shaped moldings by powder, precursors hardened placed in the mold. 3. An unbreakable sheet material having a thickness of less than about 0.1 inch and an overall composition of TiAl.