EP1407056A2

EP1407056A2 - Moulded piece made from an intermetallic gamma-ti-al material

Info

Publication number: EP1407056A2
Application number: EP02759850A
Authority: EP
Inventors: Andreas Dr. Hoffmann; Heinrich Dr. Kestler
Original assignee: Plansee SE
Current assignee: Plansee SE
Priority date: 2001-07-19
Filing date: 2002-07-12
Publication date: 2004-04-14
Anticipated expiration: 2022-07-12
Also published as: AT5199U1; WO2003008655A2; WO2003008655A3; EP1407056B1; US20040094242A1; DE50204409D1; US6805759B2

Abstract

The invention relates to a moulded piece made from a selection of gamma TiAl alloys with excellent mechanical properties, which, according to the invention, may be produced particularly economically, by a series of process steps.

Description

MOLDED PART FROM AN INTERMETALLIC γ-TiAl MATERIAL

The invention relates to a molded part made of an intermetallic γ-TiAl

Material (γ-titanium aluminide) with 41 - 49 atom% AI and a process for the same

Production.

γ-TiAl materials are often also referred to as "near-γ-titanium aluminides".

The metal structure of these consists mainly of TiAl phase (γ phase) with a small proportion of Ti ₃ Al (α ₂ phase). With some

Multi-component alloys can also still have a small proportion of β-phase, this phase being composed of elements such as chromium, tungsten or

Molybdenum is stabilized.

According to JW Kim (J. Met. 41 (7), p 24-30, 1989, J. Met. 46 (7), p 30-39, 1994), individual groups of advantageous alloying elements in γ-TiAl alloys can generally be like are described as follows (in atomic%): Ti-AI ₄ 5-48- (Cr, Mn, V) ₀ _ ₃ - (Nb, Ta, Mo, W) o_ ₅ - (Si, B) o_ι. Niobium, tungsten, molybdenum and to a small extent tantalum improve the resistance to oxidation, chromium, manganese and vanadium have a ductile effect.

Due to their high strength / density ratio, the high specific elastic modulus, as well as the oxidation resistance and creep resistance, intermetallic γ-TiAI materials are of interest for a large number of applications. These include, for example, turbine components as well as engine or transmission components of automobiles. The prerequisite for large-scale application of γ-TiAl is

Availability of a technically reliable molding method that enables inexpensive production of molded parts with properties that meet requirements.

Building on the experience with the casting of titanium, great efforts have been made in recent years to establish an investment casting technique for γ-TiAl materials. It was shown that the coarse cast structure usually formed is extremely disadvantageous for the mechanical properties of each γ-TiAl. Molded parts made of intermetallic γ-TiAl materials, based on Ti - 45 atom% AI - 5 atom% Nb, which were produced by means of investment casting, had an undesirably coarse structure with an average grain size of> 500 μm, with the minimum and maximum grain size also spread over a wide range. Also a molded part produced by investment casting with the alloy composition 44 atom% AI - 1 atom% V - 5 atom% Nb -

1 atom% B, balance Ti, an alloy according to EP 0 634 496, has an average grain size in the range of 550 μm, likewise with a wide range of scatter.

The following are representative of the multitude of attempts to achieve a fine-grained structure both in terms of alloys and processes.

US Pat. No. 5,429,796 describes a cast molded part made of a titanium aluminide material, consisting of 44-52 atom% of aluminum, 0.05-8 atom% of one or more elements from the group consisting of chromium, carbon, gallium, molybdenum, manganese and niobium , Nickel, silicon, tantalum, vanadium and tungsten and at least 0.5% by volume of a boride phase which has a yield strength of 55 ksi and an elongation at break of at least 0.5%. In the case of the alloys preferably produced there according to the processes mentioned, Ti - 47.7 atom% AI - 2 atom% Nb - 2 atom% Mn - 1 vol.% TiB ₂ , Ti - 44.2 atom% AI -

2 atom% Nb - 1, 4 atom% Mn - 2 vol.% TiB ₂ and Ti - 45.4 atom% AI - 1, 9 atom% Nb - 1, 6 atom% Mn - 4.6 vol.% TiB ₂ , the achievable average grain sizes were 50 to 150 μm, ie the structure was comparatively fine. When designing an alloy with Ti - 45.4 atom% AI - 1.9 atom% Nb - 1.4 atom% Mn - 0.1 vol.% TiB ₂ , the average grain size was 1000 μm, ie the structure was comparatively coarse , The two alloys with a high proportion of TiB ₂ phase, however, tend to form coarse boride deposits at the grain boundaries with slow cooling after the casting process, which have a very disadvantageous effect on the mechanical properties. A high cooling rate cannot be used because in these cases cracks occur due to thermally induced stresses. The borides are added to the master alloy in the molten state. In order to keep the inevitable coarsening of the borides in the melt as low as possible, the time between casting and solidification has to be kept short as a further manufacturing difficulty. In addition to the process-technical difficulties, high boron contents, which on the one hand appear to be suitable for effective refinement, and on the other hand their mechanical properties.

It is known to convert intermetallic γ-TiAl materials into a more fine-grained state by means of annealing treatments, see for example

US Pat. No. 5,634,992, US Pat. No. 5,226,985, US Pat. No. 5,204,058, and US Pat. No. 5,653,828. Refining is achieved by means of the anneals described therein, the grain size of the cast structure specifying the grain size that can best be achieved by annealing treatment. A sufficient level of refinement is ultimately not possible with a microstructure produced using casting technology.

In addition to the coarse structure, cast pores / blowholes also adversely affect the mechanical properties of γ-TiAl manufactured using casting technology, so that post-compression processes such as e.g. hot isostatic pressing or forming processes must be used.

Because of the difficulties described above, the production of molded parts from intermetallic γ-titanium aluminides by means of the usual casting processes, such as e.g. Investment casting, so far no large-scale implementation.

As an alternative to the production using casting technology, near-net-shape molded parts, molded parts with final shape, but also primary material for further metal-forming processing using conventional powder metallurgy

Processes such as, for example, hot isostatic pressing, see, for example, US Pat. No. 4,917,858, US Pat. No. 5,015,534 and US Pat. No. 5,424,027. In these cases, powders which are usually produced by spray technology are used as starting material. Shaped parts manufactured using powder metallurgy are significantly more fine-grained than those produced using the casting process. However, material made from powder metallurgy has pores filled with gas - usually the protective gas argon used in spray powder production. The pores have a disadvantageous effect both on the creep behavior and on the fatigue behavior.

In the case of molds made of γ-TiAl, satisfactory refinement can be achieved using specially developed forming processes such as extrusion, forging, rolling and combinations of these processes. On an industrial scale today, γ-TiAl alloys are therefore usually made from VAR (Vacuum Arc Remeiting) raw material, which is converted into a fine-grained state by means of forming and annealing, the actual shaping following the hot working using complex mechanical, predominantly Machining is carried out.

The entire production route for such molded parts is therefore expensive and limits the possible variety of applications for cost reasons.

It is an object of the present invention, measured using the prior art described above, to provide a fine-grained, as pore-free and ductile molded part based on intermetallic γ-TiAl by means of a comparatively economical process technology.

This object is achieved according to the invention by a molded part made of an intermetallic γ-TiAl alloy with 41-49 atom% AI, which has a grain size d _θ5 <300 μm and a pore volume <0.2 vol.% And its production at least the following Process steps include: - Manufacture of a semi-finished product including a forming process, the degree of deformation being> 65%, - Forming of the semi-finished product in the solidus-liquidus phase state

Alloy in a mold with at least temporary application of mechanical shaping forces. The subclaims contain preferred designs of the molded part according to the invention.

Processing an alloy in the solidus-liquidus phase state is a semi-solid process. Partially liquid masses are usually processed in a thixotropic state in a semi-solid process. Thixotropy is the property of a material to behave highly viscous in the absence of external forces, but to assume a viscosity that is several orders of magnitude lower under the action of shear forces. Thixotropic behavior is limited to certain alloy compositions and those temperature ranges in which both solid and liquid phase components are present in the alloy. The aim is to achieve a semi-solid phase in which there are regular, i.e. globular grains in the solid phase that are evenly surrounded by the melt. The shaping of an alloy using the semi-solid process is known as such.

In the course of this process, molten alloys are usually slowly cooled to a temperature in the solidus-liquidus two-phase range using one of the known stirring techniques, such as MHD (Magneto-Hydrodynamic-Stirring) or mechanical stirring. Stirring dendrites are destroyed by stirring. The material is given thixotropic properties and the formation of globular primary crystals in the solid phase is promoted. This method is described in US Pat. No. 5,358,687 for intermetallic materials, TiAl being mentioned among others, but in contrast to the present invention a further shaping including mechanical ones

Hot forming steps is not mentioned. The achievable grain size was> 50 μm.

However, this technique applied to γ-TiAl does not allow economical production. The mechanical stirrer wear is too high with TiAl.

Also in the past, a structure was created in semi-finished products from individual steel alloys using extrusion on a laboratory scale, which during subsequent processing in the solidus-liquidus two-phase area had thixotropic properties (dissertation H. Müller-Späth, RWTH Aachen, 1999). However, no encouraging quality and / or cost targets could be achieved there.

Unlike steel alloys, however, intermetallic materials are difficult to handle in terms of forming technology. Especially with γ-TiAl, the structure consolidation that can be achieved is not very satisfactory. This is expressed in the fact that the reshaped and dynamically recrystallized structure regularly has a line structure and chemical inhomogeneities resulting from segregation.

For the person skilled in the art, it was therefore not foreseeable that, according to the invention, γ-TiAl alloys formed into semifinished products in a first hot-forming process section, after heating to a temperature in the solidus-liquidus phase region, exhibit thixotropic behavior for the further shaping processing. However, a degree of deformation of> 65% is a prerequisite, this value being defined as follows:

Degree of deformation = {(cross-sectional area before forming - cross-sectional area in the deformed state) / cross-sectional area before forming} x 100 [%]. The thixotropic behavior is unsatisfactory at lower degrees of deformation.

The advantages described were demonstrated by means of a process route which is described in more detail in the examples for individual different γ-TiAl alloys. Γ-TiAl primary material produced by means of VAR (Vacuum Are Remelting) was preferably formed by extrusion with a degree of deformation> 65%. Then the semi-finished product was inductively heated to a temperature between solidus and liquidus in the form of a roughly shaped bolt. In this state, the semifinished product had a sufficiently high "handling" strength to be shaped by thixo casting. For this purpose, it was placed in the filling chamber of a die casting machine and pressed into the adjacent mold with the casting piston. The resulting shear stress formed the alloy as a flowable suspension that could be used to form complex components to be free of flow turbulence in the material so that the material spreads free of pores and voids in the mold. This shaping process made it possible to dispense with mechanical machining, or to greatly reduce it, so that, in addition to excellent structural and mechanical properties, the molded parts according to the invention were also highly economical to manufacture. Compared to molded parts cast directly from the melt into a final shape, the advantage according to the invention lies in the much more fine-grained structure and the high degree of pore freedom.

As a measure of the grain sizes of the molded parts manufactured in this way

Grain size distribution determined using the line cutting method and the dgs value. This means that 95% of the grains evaluated have a diameter that is smaller than the specified value. It should be noted that the dg grain size results in a significantly higher numerical value than is the case in the form of the average grain size. However, the dg ₅ value is the more meaningful value, especially for structures with a large grain size range. Depending on the composition of the γ-TiAl material and the applied semi-solid process, the achievable dg ₅ grain sizes are between <100 μm and <300 μm. Such molded parts produced for comparison purposes by means of investment casting and not further processed by hot forming have a structure which is at least 5 times coarser than the molded parts produced according to the invention.

The grain size difference is particularly pronounced if, according to a preferred embodiment of the invention, alloys with a niobium content of between 1.5 and 12 atom% are used. These alloys show a micro-fine structure by a factor of 7 up to a factor of 16 compared to conventional investment casting.

The best results were achieved with γ-TiAI alloys with a niobium content of 5 to 10 atom%. An additional refining effect was achieved by the alloying elements carbon and boron in amounts of up to

0.4 atomic% achieved. Thixo forging and thixo cross extrusion, each a technique that is already known and tried and tested, have proven to be useful alternative forming or shaping processes for the γ-TiAl alloys according to the invention in the solidus-liquidus phase state. In thixo forging, the partially liquid bolt is inserted into an open tool or die tool. The shaping is carried out by a subsequent tool movement, for example in a forging press. Thixo cross extrusion is a modification of thixo casting. The bolt pushed by a punch is deflected by an angle of 90 ° on its way from the casting chamber to the mold or to the forming tool.

The invention is explained in more detail below on the basis of production examples.

example 1

The primary casting of an alloy with the composition titanium - 46.5 atom% Al - 2 atom% Cr - 1, 5 atom% Nb - 0.5 atom% Ta - 0.1 atom% boron was carried out using vacuum arc melting (VAR) , The casting block was remelted twice in order to achieve satisfactory homogeneity. The ingot diameter was 210 mm, the ingot length 420 mm.

The ingot was extruded in the known state according to previously known process conditions, the degree of deformation being 83%. A bolt section with a length of 110 mm was then heated to a temperature in the solidus-liquidus phase range of the alloy from 1460 to 1470 ° C. and in this state was pressed in a servo-hydraulic press into a closed die-casting tool made of a molybdenum alloy. The molded part produced in this way, a cylindrical component with an average diameter of 40 mm, a length of 100 mm, a flange on the side and a recess of 35 mm x 35 mm x 35 mm in the cylindrical part was examined metallographically. The grain size ds was 120 μm. The relative density was determined using the buoyancy method and was 99.98%.

For comparison, the grain size dg _{5 of} the remelted investment casting was 1400 μm.

Example 2

Analogous to the process control in Example 1, an ingot of the alloy composition titanium - 45 atom% Al - 5 atom% Nb - 0.2 atom% C - 0.2 atom% boron was produced by vacuum arc melting (VAR) and remelted twice. The ingot diameter was 210 mm, the ingot length 420 mm.

The ingot was extruded in the known state by conventional methods, the degree of deformation being 83%. A bolt section with a length of 110 mm was heated to a temperature of 1460 - 1480 ° C, the alloy was brought into the solidus-liquidus phase area and in this state pressed into a closed die-casting tool made of a molybdenum alloy in a servo-hydraulic press. The molded part produced in this way, a cylindrical component with an average diameter of 40 mm, a length of 100 mm, a flange on the side and a depression of 35 mm x 35 mm x 35 mm in the cylindrical part was examined metallographically. The grain size dgs was 75 μm. The relative density was 99.99%.

The grain size dg _{5 of} the investment casting produced at the beginning had been 1200 μm.

Example 3

Analogous to the process of Example 1, a primary casting blank of the alloy titanium - 46.5 atom% Al - 2 atom% Cr - 0.5 atom% Ta - 0.1 atom% boron was produced by vacuum arc melting (VAR) and remelted twice. The ingot diameter was 170 mm, the ingot length 420 mm. The ingot was extruded in the known state, the degree of deformation being 83%. A bolt section with the length of 110 mm was heated to a temperature of 1440-1470 ° C and in a servo-hydraulic Press pressed into a closed die casting tool made of a molybdenum alloy.

The molded part produced in this way, a cylindrical component with an average diameter of 40 mm, a length of 100 mm, a flange on the side and a depression of 35 mm x 35 mm x 35 mm in the cylindrical part was examined metallographically. The grain size dgs was 220 μm. The relative density was 99.99%. The grain size dg of the investment casting had been 1500 μm.

Example 4 A primary casting block of the alloy titanium -46.5 atom% Al - 10 atom% Nb was produced in accordance with the process steps of Example 1 using vacuum arc melting (VAR) and remelted twice. The ingot diameter was 170 mm, the ingot length 420 mm. The ingot was extruded in the known state, the degree of deformation being 83%. A bolt section with a length of 110 mm was heated to a temperature of 1440-1470 ° C and pressed in a servo-hydraulic press into a closed die-casting tool made of a molybdenum alloy. The molded part produced in this way, a cylindrical component with an average diameter of 40 mm, a length of 100 mm, a flange on the side and a depression of 35 mm x 35 mm x 35 mm in the cylindrical part was examined metallographically. The grain size dgs was 90 μm. The relative density was 99.98%. The grain size dg _{5 of} the investment casting had been 1300 μm.

Example 5

The primary casting block of the alloy titanium - 46.5 atom% Al - 10 atom% Nb was produced in accordance with Example 1 by means of vacuum arc melting (VAR) and remelted twice. The ingot diameter was 170 mm, the ingot length 420 mm.

The ingot was extruded in the known state, the degree of deformation being 72%. A bolt section with the length of 110 mm was heated to a temperature of 1440-1470 ° C and in a servo-hydraulic Press pressed into a closed die casting tool made of a molybdenum alloy.

The molded part produced in this way, a cylindrical component with an average diameter of 40 mm, a length of 100 mm, a flange on the side and a depression of 35 mm x 35 mm x 35 mm in the cylindrical part was examined metallographically. The grain size d _g5 was 170 μm. The relative density was 99.98%. The grain size dg _{5 of} the investment casting had been 1300 μm.

The invention is not limited to the aforementioned exemplary embodiments. Preferred areas of application for moldings according to the invention are

Automotive industry, e.g. Transmission and engine parts, but also parts for stationary gas turbines and for aerospace, e.g. Turbine components.

Claims

Patent claims

1. Shaped part made of an intermetallic γ-TiAI alloy with 41-49 atom% AI, with a grain size dg ₅ <300 μm and a pore volume

<0.2 vol.%, The production of which comprises at least the following process steps:

- Manufacture of a semi-finished product using a hot forming process, the degree of forming being> 65%,

- Forming the semi-finished product in the solidus-liquidus phase state of the alloy in a molding tool with at least temporary application of mechanical shaping forces.

2. Shaped part made of an intermetallic γ-TiAI alloy according to claim 1, characterized in that the alloy is in the thixotropic state during molding.

Shaped part made of an intermetallic γ-TiAI alloy according to claim 1 and 2, characterized in that the solid component of the alloy in the solidus-liquidus phase state has a globular structure during molding.

Shaped part made of an intermetallic γ-TiAI alloy according to claims 1 to 3, characterized in that the semi-finished product is shaped in a die form by means of thixo forging.

5. molded part made of an intermetallic γ-TiAI alloy according to claim 1 to 3, characterized in that the shaping is carried out by means of thixoflowing of the semi-finished product in a mold.

6. molded part made of an intermetallic γ-TiAI alloy according to claim 1 to 5, characterized in that the manufacture of the semi-finished product in

An extrusion process is included.

7. molded part made of an intermetallic γ-TiAl alloy according to claims 1 to 6, characterized in that it has a grain size d ₉₅ <200 microns.

8. molded part made of an intermetallic γ-TiAl alloy according to claims 1 to 7, characterized in that it has a grain size dgs <150 microns.

9. molded part made of an intermetallic γ-TiAI alloy according to claim 1 to 8, characterized in that the alloy contains 43 - 47 atom% AI and 1 - 5 - 12 atom% niobium.

10. molded part made of an intermetallic γ-TiAI alloy according to claim 9, characterized in that the niobium content is 5 - 10 atom%.

11. molded part made of an intermetallic γ-TiAI alloy according to claim

9 or 10, characterized in that the alloy further contains the following constituents: boron: 0.05-0.5 atom%, carbon: 0-0.5 atom%, chromium: 0-3 atom%, Ta: 0-2 Atom%.

12. molded part made of an intermetallic γ-TiAI alloy according to claim

11, characterized in that the carbon content is 0.1-0.4 atom% and the boron content is 0.1-0.4 atom%.

13. molded part made of an intermetallic γ-TiAl alloy according to claim 1 to 7, characterized in that the hot forming process with a

Degree of deformation> 80%.

14. Molded part made of an intermetallic γ-TiAI alloy according to claims 1 to 13 for use as an engine or transmission component in automobiles.

5. molded part made of an intermetallic γ-TiAI alloy according to claim 1 to 14 for use as a component in stationary and non-stationary gas turbines.