EP1407056B1 - Process for producing a moulded piece made from an intermetallic gamma-ti-al material - Google Patents

Description

The invention relates to a method for producing a molded part from an intermetallic γ-TiAl Material (γ-titanium aluminide) with 41-49 atom% Al.

γ-TiAl materials are often referred to as "near-γ-titanium aluminides". In these, the metal structure consists mainly of TiAl phase (γ-phase) with a small proportion of Ti ₃ Al (α ₂ -phase). In the case of some multicomponent alloys, there may still be a small proportion of β-phase, this phase being stabilized by elements such as chromium, tungsten or molybdenum.

According to JW Kim (J. Met 41 (7), p24-30, 1989, J. Met. 46 (7), p30-39, 1994), individual groups of advantageous alloying elements in γ-TiAl alloys can be generally as follows (in atom%): Ti-Al _45-48 - (Cr, Mn, V) _0-3 - (Nb, Ta, Mo, W) _0-5 - (Si, B) _0-1 . Niobium, tungsten, molybdenum and, to a lesser extent, tantalum improve oxidation resistance, while chromium, manganese and vanadium have a ductile effect.

Due to their high strength / density ratio, high specific modulus of elasticity, as well as oxidation and creep, intermetallic γ-TiAl materials are of interest for a variety 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 the availability of a technically reliable forming method that enables the cost-effective production of molded parts with requirements-oriented properties.

On the experience with the casting technology of titanium building on it, great efforts have been made in recent years to to establish a precision casting technique for γ-TiAl materials.

It was found that the commonly formed coarse cast structure for mechanical properties of each γ-TiAl is highly disadvantageous. Molded parts intermetallic γ-TiAl materials, based on Ti - 45 atom% Al - 5 atom% Nb, which were produced by investment casting, had an undesirable coarse microstructure with a mean particle size of> 500 microns on, where also scatter the minimum and maximum grain sizes in a wide range. Also a molded part produced by investment casting with the Alloy Composition 44 At% Al - 1 At% V - 5 At% Nb - 1 atom% B, balance Ti, an alloy according to EP 0 634 496, has a average grain size in the range of 550 microns on also with a wide Spread.

From the multitude of trials, both alloying and procedurally to achieve a fine - grained structure, the Subsequent representative called.

No. 5,429,796 describes a cast molding made of a titanium aluminide material consisting of 44-52 atom% aluminum, 0.05-8 atom% of one or more elements of the group chromium, carbon, gallium, molybdenum, manganese, niobium , Nickel, silicon, tantalum, vanadium and tungsten and at least 0.5% by volume of a boride phase having a yield strength of 55 ksi and an ultimate elongation of at least 0.5%. In the case of the alloys produced there preferably by the abovementioned processes Ti - 47.7 atom% Al - 2 atom% Nb - 2 atom% Mn - 1 vol.% TiB ₂ , Ti - 44.2 atom% Al - 2 atom% Nb - 1.4 atom% Mn - 2 vol.% TiB ₂ as well as Ti - 45.4 atom% Al - 1,9 atom% Nb - 1,6 atom% Mn - 4,6 Vol.% TiB ₂ , the achievable mean Grain sizes at 50 to 150 microns, ie, the structure was relatively fine. In an alloy design with Ti - 45.4 at.% Al - 1.9.% Nb - 1.4 at.% Mn - 0.1 vol.% TiB ₂ , the average particle size was 1000 .mu.m, ie the structure was comparatively coarse ,
The two alloys with a high proportion of TiB ₂ phase, however, tend with slow cooling after the casting process to form coarse boride precipitates at the grain boundaries, which have a very adverse effect on the mechanical properties. A high cooling rate can not be used because in these cases cracks occur due to thermally induced stress. The borides are added to the master alloy in the molten state. In order to minimize the unavoidable coarsening of the borides in the melt, the time between cast and onset of solidification must be kept short as a further production obstacle. In addition to the process engineering difficulties, high boron contents, which on the one hand appear to be suitable for effective grain refining, on the other hand degrade their mechanical properties.

It is known to intermetallic γ-TiAl materials by annealing treatments in to convert a finer-grained state, see for example US 5,634,992, US 5,226,985, US 5,204,058 and US 5,653,828. There by annealing described achieves a grain refining, the grain size the cast structure, the lowest achievable by means of annealing Specifies grain size. A grain refinement that is sufficient from the user's point of view is included Ultimately not possible with a microstructure made by casting technique.

In addition to the coarse microstructure also influence cast pores / Gusslunker the mechanical properties of γ-TiAl made by casting disadvantageous, so that for the production of technically useful moldings Post-compaction process, such as hot isostatic pressing or Forming must be applied.

Because of the difficulties described above, the production of Moldings of intermetallic γ-titanium aluminides by means of the usual Casting method, such as Investment casting, so far no large-scale implementation Experienced.

As an alternative to the production by means of casting technology, moldings close to the final shape, Moldings with final shape, but also starting material for another Forming technical processing by means of conventional powder metallurgical Methods, e.g. hot isostatic pressing, prepared, see for example US 4,917,858, US 5,015,534 and US 5,424,027. In these cases, as Starting material commonly used by spraying powder.

Powder metallurgically produced moldings are much finer-grained than after Casting manufactured. However, powder metallurgically produced material has filled with gas pores on - usually in the spraying Powder production used inert gas argon. The pores have an effect disadvantageous both on the creep behavior, as well as on the Fatigue behavior.

In the case of γ-TiAl casting molds, a satisfactory grain refinement can be achieved by means of 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 of VAR (Vacuurn-Arc-Remelting) starting material, which is converted by forming and annealing in a feinkömigen state, the actual shaping following the hot working by means of complex mechanical, predominantly machining processing takes place.
The entire production route for such molded parts is therefore expensive and limits the possible variety of applications for cost reasons.

The production of fine-grained molded parts from γ-TiAe with a lower Porsity is e.g. from Semiatin et al. "Processing of Intermetallic Alloys" - Sructural Intermetallics 1997 - Pages 263-276, known.

It is therefore an object of the present invention, measured at the top described prior art, a fine-grained, as possible porentrelen and ductile molding based on intermetallic γ-TiAl, by means of a To provide comparatively economical process technology.

• Fertigen eines Halbzeuges unter Einbeziehung eines Umformprozesses, wobei der Umformgrad > 65 % beträgt,
• Ausformen des Halbzeuges im Solidus-Liquidus Phasenzustand der Legierung in einem Formwerkzeug unter zumindest zeitweisem Aufbringen von mechanischen Formungskräften.

This object is achieved according to the invention by a method for producing a molding of a γ-TiAl intermetallic alloy with 41-49 atom% Al, which has a particle size d ₈₅ <300 microns and a pore volume <0.2 vol.%, And at least the following method steps include:

• Producing a semi-finished product including a forming process, the degree of deformation being> 65%,
• Forming the semifinished product in the solidus liquidus phase state of the alloy in a mold with at least temporary application of mechanical shaping forces.

The dependent claims contain preferred embodiments of the molding according to Invention.

The processing of an alloy in the solidus-liquidus phase state is a semi-solid process. Typically, in a semi-solid process, partially liquid masses are processed in a thixotropic state. Thixotropy is the property of a material to behave highly viscous in the absence of external forces, but under the action of shear forces to assume a viscosity several orders of magnitude lower. Thixotropic behavior is limited to certain alloy compositions and those temperature ranges where both solid and liquid phase portions are present in the alloy. Here, a semi-solid phase is sought, in which regular, that is as globular as possible grains in the solid phase portion, which are uniformly surrounded by melt.
The shaping of an alloy by means of a semi-solid process as such is known.
Usually, in the course of this process, molten alloys are slowly cooled to a temperature in the solidus-liquidus two-phase region using one of the known stirring techniques, such as MHD (Magneto-Hydrodynamic Stirring) or mechanical stirring. Stirring destroys dendrites leaving the melt. Thixotropic properties are imparted to the material 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, mention being made, inter alia, of TiAl, but in contrast to the present invention, there is no mention of further shaping involving mechanical hot forming steps. The achievable grain size was> 50 microns.
However, this technique, applied to γ-TiAl, does not allow for economical production. With TiAl the mechanical stirrer wear is too high.

Also earlier, a microstructure was produced in semifinished product from individual steel alloys by means of extrusion on a laboratory scale, which had thixotropic properties in a subsequent further processing in the solidus-liquidus two-phase region (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 by forming technology. Especially in the case of γ-TiAl, the achievable microstructural consolidation is unsatisfactory. This is evidenced by the fact that the deformed and dynamically recrystallized structure regularly exhibits a row-like structure and segregated chemical inhomogeneities.

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 exhibit thixotropic behavior after being heated to a temperature in the solidus-liquidus phase region for the further shaping processing. However, the prerequisite is a degree of deformation of> 65%, this value being defined as follows: Degree of deformation = {(cross-sectional area before forming - cross-sectional area in deformed state) / cross-sectional area before forming} x 100 [%]. At lower degrees of deformation, the thixotropic behavior is unsatisfactory.

The proof of the described advantages succeeded by means of a process route, which is described in more detail in the examples for individual different γ-TiAl alloys.
By means of VAR (Vacuum Arc Remelting) generated γ-TiAl starting material was preferably formed by extrusion with a degree of deformation> 65%. Then the semi-finished product in the form of a coarsely shaped bolt was heated inductively to a temperature between solidus and liquidus. In this state, the semifinished product had a sufficiently high "handling" strength in order to shape it by thixocasting. For this purpose, it was placed in the filling chamber of a die-casting machine and pressed with the casting piston in the adjacent mold. In the shear stress occurring, the alloy formed as a flowable suspension, which could be used to form complex designed components. This impressions must be made slowly and free of flow turbulence in the material, so that the material propagates free of pores and voids in the mold.
By this shaping process, a mechanical, machining could be omitted or greatly reduced, so that in addition to excellent microstructural and mechanical properties of the moldings according to the invention also high efficiency was given in their production. In comparison to moldings cast directly from the melt into a final mold, the advantage according to the invention lies in the substantially finer-grained microstructure and the high degree of freedom from pores.

As a measure of the particle sizes of the molded parts produced in this way, the particle size distribution was determined by means of the linear section method and the d ₉₅ value. By this is meant that 95% of the grains evaluated have a diameter smaller than the specified value. It should be noted that the d ₉₅ grain size gives a significantly higher numerical value than is the case with the indication in the form of the mean grain size.
However, the d ₉₅ value is the more meaningful value especially for structures with a high particle size distribution range. Depending on the composition of the γ-TiAl material and the applied semi-solid process, the achievable d ₉₅ grain sizes are values of <100 μm to <300 μm.
Such, for comparison purposes manufactured by investment casting and not further treated by hot forming moldings show an at least a factor of 5 coarse-grained structure than inventively produced moldings.

Particularly pronounced is the grain size difference, if according to a preferred embodiment of the invention, alloys are used with a niobium content between 1.5 and 12 atom%. These alloys show a finer grain size by a factor of 7 up to a factor of 16 than in conventional precision casting.
The best results were achieved with γ-TiAl 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 atom% each.

As useful alternative molding methods for the γ-TiAl alloys of the present invention in the solidus-liquidus phase state, thixoforging and thixocross extrusion molding, each of which is a well-known and well-proven technique, have been proven. When Thixoschmieden the partially liquid bolt is inserted into an open tool, or die tool. The shaping takes place by a subsequent tool movement, for example in a forging press.
The Thixoquerfließpressen represents a modification of Thixogießens. The pushed by a punch bolt is deflected on its way from the casting chamber to the mold or the forming tool by an angle of 90 °.

In the following, the invention will become more apparent with reference to manufacturing examples explained.

example 1

The production of the primary casting of an alloy of the composition titanium - 46.5 at% Al - 2 at% Cr - 1.5 at% Nb - 0.5 at% Ta - 0.1 at% Boron was achieved by vacuum arc melting (VAR) , To achieve satisfactory homogeneity, the ingot was remelted twice. 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, wherein the degree of deformation was 83%. A 110 mm length of stud was then heated to a solidus-liquidus phase temperature range of the 1460-1470 ° C alloy and, in this state, pressed in a servo-hydraulic press into a closed die cast molybdenum alloy die.
The molded part thus produced, a cylindrical member having an average diameter of 40 mm, a length of 100 mm, a laterally mounted flange and a recess of dimensions 35 mm x 35 mm x 35 mm in the cylindrical part was examined metallographically. The grain size d ₉₅ was 120 μm.

The relative density was determined by buoyancy method and was 99.98%.
For comparison, the grain size d _{95 of} the twice remelted investment casting was 1400 microns.

Example 2

Analogously to the process procedure 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, wherein the degree of deformation was 83%. A 110 mm length of stud was heated to a temperature of 1460-1480 ° C, the alloy was thus brought into the solidus-liquidus phase region and, in this state, pressed in a servo-hydraulic press into a closed die-casting mold made of a molybdenum alloy.
The molded part thus produced, a cylindrical member having an average diameter of 40 mm, a length of 100 mm, a laterally mounted flange and a recess of 35 mm × 35 mm × 35 mm in the cylindrical portion was examined metallographically. The grain size d ₉₅ was 75 microns.
The relative density was 99.99%.
The grain size d _{95 of} the initially produced investment casting had been 1200 μm.

Example 3

Analogously to the process of Example 1, a primary casting blank of the alloy titanium-46.5 at% Al-2 at% Cr-0.5 at% Ta-0.1 at% 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, wherein the degree of deformation was 83%. A 110 mm length of stud 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 thus produced, a cylindrical member having an average diameter of 40 mm, a length of 100 mm, a laterally mounted flange and a recess of 35 mm × 35 mm × 35 mm in the cylindrical portion was examined metallographically. The grain size d ₉₅ was 220 microns.
The relative density was 99.99%.
The grain size d _{95 of} the precision casting had been 1500 μm.

Example 4

A primary cast ingot of the alloy titanium -46.5 at.% Al-10 at.% Nb was fabricated and remelted twice in accordance with the process steps of Example 1 by vacuum arc melting (VAR). The ingot diameter was 170 mm, the ingot length 420 mm.
The ingot was extruded in the known state, wherein the degree of deformation was 83%. A 110 mm length of stud 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 thus produced, a cylindrical member having an average diameter of 40 mm, a length of 100 mm, a laterally mounted flange and a recess of 35 mm × 35 mm × 35 mm in the cylindrical portion was examined metallographically. The grain size d ₉₅ was 90 microns.
The relative density was 99.98%.
The grain size d _{95 of} the precision casting had been 1300 μm.

Example 5

The primary cast ingot of the alloy titanium - 46.5 at.% Al - 10 at.% Nb was manufactured 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, wherein the degree of deformation was 72%. A 110 mm length of stud 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 thus produced, a cylindrical member having an average diameter of 40 mm, a length of 100 mm, a laterally mounted flange and a recess of 35 mm × 35 mm × 35 mm in the cylindrical portion was examined metallographically. The grain size d ₉₅ was 170 μm.
The relative density was 99.98%.
The grain size d _{95 of} the precision casting had been 1300 μm.

The invention is not limited to the aforementioned embodiments. Preferred application areas for molded parts according to the invention are the Automotive industry, e.g. Transmission and engine parts, but also parts for stationary Gas turbines and aerospace, e.g. Turbine components.

Claims (15)

1. A process for preparing a moulded part made from an intermetallic γ-TiAl alloy with 41 - 49 atom-% Al, with a particle size d₉₅<300 µm and a pore volume <0.2 vol-%, which comprises at least the following process steps:

   producing a semi-finished product in a thermoforming process, wherein the degree of deformation is >65 %

   giving the final shape to the semi-finished product in the solidus-liquidus phase of the alloy in a mould with the at least occasional application of mechanical thermoforming forces.
2. A process for preparing a moulded part in accordance with Claim 1, characterised in that the alloy is in the thixotropic state during the final shaping process.
3. A process for preparing a moulded part in accordance with Claims 1 and 2, characterised in that the solid constituent of the alloy in the solidus-liquidus phase has a globular structure during the final shaping process.
4. A process for preparing a moulded part in accordance with Claims 1 to 3, characterised in that the semi-finished product is given its final shape by thixotropic forging in a compression mould.
5. A process for preparing a moulded part in accordance with Claims 1 to 3, characterised in that the semi-finished product is given its final shape by thixotropic flow moulding in an ingot mould.
6. A process for preparing a moulded part in accordance with Claims 1 to 5, characterised in that the semi-finished product is produced in an extrusion process.
7. A process for preparing a moulded part in accordance with Claims 1 to 6, characterised in that the part has a particle size d₉₅<200 µm.
8. A process for preparing a moulded part in accordance with Claims 1 to 7, characterised in that the part has a particle size d₉₅<150 µm.
9. A process for preparing a moulded part in accordance with Claims 1 to 8, characterised in that the alloy contains 43 - 47 atom-% Al and 1.5 - 12 atom-% niobium.
10. A process for preparing a moulded part in accordance with Claim 9, characterised in that the niobium content is 5 - 10 atom-%.
11. A process for preparing a moulded part in accordance with Claim 9 or 10, characterised in that the alloy also contains the following constituents: boron: 0.05 - 0.5 atom-%, carbon: 0 - 0.5 atom-%, chromium: 0 - 3 atom-%, Ta: 0 - 2 atom-%.
12. A process for preparing a moulded part in accordance with Claim 11, characterised in that the carbon content is 0.1 - 0.4 atom-% and the boron content is 0.1 - 0.4 atom-%.
13. A process for preparing a moulded part in accordance with Claims 1 to 7, characterised in that the thermoforming process takes place with a degree of deformation >80 %.
14. A process for preparing a moulded part in accordance with Claims 1 to 13 for use as engine or drive components in automobiles.
15. A process for preparing a moulded part in accordance with Claims 1 to 14 for use as components in stationary and non-stationary gas turbines.