EP3077557B1 - Method for producing titanium-aluminum components - Google Patents

Description

The invention relates to a method for producing a component from a titanium-aluminum base alloy, which can be used as a component for turbocharger units of internal combustion engines.

Generic high-temperature lightweight materials, which have a high specific gravity at a low density, find particular application to devices for energy conversion to achieve required increases in efficiency increased use. For example, because of their high specific strength, titanium-aluminum base alloys are also used at high temperatures and their additional very good corrosion resistance in the production of components of gas turbines, aircraft engines or turbocharger units of modern internal combustion engines in motor vehicles.

The ever increasing number of motor vehicles and the shortage of energy resources requires the reduction of harmful emissions, noise and fuel consumption of motor vehicles. The development of efficient exhaust gas turbochargers represents a possible solution.
Through the use of exhaust gas turbochargers internal combustion engines can be made smaller while maintaining power. In an exhaust gas turbocharger formed from a turbine and a compressor, the turbine runner is set in rotation with the energy of the exhaust gas flow. A shaft transmits the torque to the compressor wheel, which compresses the air flowing into the combustion chamber and introduces into the engine. As a result of the oxygen excess produced by the compressed air flow, the fuel in the engine is almost completely burned and harmful emissions are reduced. The exhaust gases of a diesel engine reach temperatures up to about 850 ° C, while the exhaust gases of gasoline engines even temperatures of about 1050 ° C have. The high temperatures of the exhaust gases lead to a large thermal load of the arranged in the exhaust stream components. In order to meet the requirements of the rotating components, in particular in the exhaust stream formed components, such as the turbine wheel, the development of high-temperature lightweight materials and their manufacturing and processing technologies are driven. In this case, intermetallic titanium aluminum alloys, also referred to as TiAl alloys or titanium aluminides, based on the γ-TiAl phase with a low density and a high specific strength at high temperature, have great potential. Titanium aluminides are known from the prior art as multiphase TiAl alloys whose complex structure consists of γ-TiAl, α ₂ -Ti ₃ Al and a small proportion of β ₀ -TiAl phase. Through a specific combination of heat treatment and hot working, the mechanical properties of the alloys are optimized, which is mainly due to the lower lamellar distance within the α ₂ / γ colonies.

In the EP 2 386 663 A1 discloses a thermally tempered component and a method for producing the component from a TiAl-based alloy. In order to achieve homogeneous mechanical properties, in particular a high ductility and creep resistance with high strength and high temperature of a material, in a first process step, the starting material is hot isostatically pressed. In a second process step, the blank is subjected to rapid solid forming. In a subsequent third step, a fine grain formation takes place with the phases γ, β, α ₂ by annealing in the region of the eutectoid temperature of the alloy. The component is subsequently annealed and / or stabilization annealed to adjust the microstructure and the mechanical material properties in a final step with dimensions close to the final dimensions.

The necessary step of hot isostatic pressing, also referred to as HIP, serves to reduce or remove internal porosities. The hot isostatic pressing, the annealing for fine grain formation and the subsequent annealing or the stabilizing annealing cause a great deal of time and are associated with increased costs.

From the DE 10 2007 051 499 A1 For example, a titanium-aluminum-based alloy material for a gas turbine component, a method of manufacturing the gas turbine component, and a gas turbine component are disclosed. In the region of room temperature, the material has the phases β / B2-Ti, α ₂ -Ti ₃ Al and γ-TiAl with a proportion of the β / B2-Ti phase of not more than 5% by volume and in the eutectoid temperature range Phases β / B2-Ti, α ₂ -Ti ₃ Al and γ-TiAl with a proportion of β / B2-Ti phase of at least 10 vol .-% on.
The method for producing the gas turbine component comprises the following steps: providing a semifinished product of an aforementioned material and forging or massive forming of the semifinished product at a forming temperature in the range between the reduced by 50 K eutectoid temperature and plus 100 K to the alpha transus temperature of the material.

The object of the present invention is to provide a method, which is improved over the prior art, for producing components from a TiAl-based alloy. The component produced by the method should be made of a material having homogeneous mechanical properties, in particular a high creep resistance at high strength, especially in high-temperature applications. The method should be less costly and less time consuming than the methods known in the art, the component with dimensions close to the final dimensions should be economically manufacturable.

• Erwärmen von gegossenem Einsatzmaterial innerhalb des (α+β)-Phasenfeldes auf eine Temperatur, bei welcher die β-Phase mindestens 5 Vol.-% aufweist,
• Schmieden des erwärmten Einsatzmaterials zum Schmiederohling durch Schlagumformung mit einer Formänderungsgeschwindigkeit von mindestens 19 1/s, wobei der Schmiederohling nach dem Schmieden gratlos ausgebildet ist,
• Entnahme des Schmiederohlings aus dem Gesenk und kontrolliertes, gleichmäßiges Abkühlen, wobei der Schmiederohling nach der Abkühlung auf Raumtemperatur einen Anteil an β/B2-Ti-Phase von maximal 10 Vol. % aufweist, sowie
• Weiterverarbeitung des Schmiederohlings zum Endprodukt.

The object is achieved by an inventive method for producing a component made of a titanium-aluminum base alloy, in particular as components for turbocharger units of internal combustion engines. According to the concept of the invention, the method has the following steps:

• Heating cast feed material within the (α + β) phase field to a temperature at which the β phase has at least 5 vol.%,
• Forging the heated feed to the forging blank by hammering with a rate of deformation of at least 19 l / s, the forging blank being flashless after forging,
• Removal of the forging blank from the die and controlled, uniform cooling, wherein the forging blank after cooling to room temperature has a proportion of β / B2-Ti phase of not more than 10 vol.%, As well
• Further processing of forging blank to the final product.

The feed is forged as a semi-finished as cast, the feed is taken directly from the cast and processed in the forging process. Since no further processing steps or method steps, such as a heat treatment, are required between the cast state of the semifinished product and the process of heating, the process chain of the production of the component is greatly shortened in comparison to the prior art.
As a result of the targeted preheating in the (α + β) phase field, an uncontrolled grain coarsening within the feedstock during the preheating time is avoided, moreover, a high volume fraction of disordered, ductile β-phase is set for the subsequent forming. The high volume fraction of disordered, ductile β-phase ensures good ductility.

In impact forming with a strain rate of ≤ 19 1 / s, the heated feed is reformed at a very high rate. The feedstock is preferably forged at a ram speed of ≥ 1.4 m / s.
The rate of change of shape results from the change in the dimensions of the component per unit of time, that is from the ratio of the dimension of the component after each forming operation to the dimension of the component before the forming process per unit time or as average strain rate from the dimension of the forging blank after forging in proportion to measure the component before forging per time.
To determine the rate of deformation, the so-called degree of deformation is used as the natural logarithm of the rate of deformation cp = In (h ₁ / h ₂ ), where h _{1 is} the dimension after deformation and h _{2 is} the dimension before deformation. Since the shape change of the component does not take place in one direction only, the largest of the shape changes is taken into account for the calculation of the deformation change ratio.
The rate of change of shape defined as the first derivative of the change in shape over time is dependent on the speed of the forming tool, also referred to as ram speed, or on the forming force causing the deformation, which in turn is given by the forming machine: φ '= 1 / h · ie / dt = v / h, where h represents the instantaneous dimension or height of the component to be formed.
In forming machines with a moving tool, which acts directly on the component to be formed, the speed of the forming tool corresponds to the tool speed, which is determined by the forming machine.

In conventional burr-forged components, excess material is forced out of the forging. Subsequently, the burr formed from the excess material is removed, for example by punching. The component is deburred.
The flashless forging according to the invention saves in comparison to methods from the prior art, on the one hand, further process steps of the post-processing or the finishing of the forging blanks up to the end product. As a result of the formed with very small wall thickness transition from the actual forging member to the burr, components made of TiAl materials have strong embrittlement in the transition zone, which continue partially and uncontrollably in the forging. The flashless training of forging blanket thus prevents the other hand, the disadvantage of the emergence of strong embrittlement.

Following the controlled, uniform process of cooling the forging blank to room temperature, the microstructure of the material of the component with the mechanical material properties is already set such that advantageously no further process step to change the microstructure is necessary. Under a controlled, uniform cooling is a process step or process to understand in which the forging blank after the process of forging without draft evenly cooled in the atmosphere. For cooling, the forging blank is exposed only to the air of the atmosphere, instead of being placed in a heated oven or the like. Depending on the size of the components, compressed air can also be used for cooling.

After impact molding of the heated feedstock with a rate of deformation of at least 19 l / s and cooling of the forging blank to room temperature, preferably in air, this has a structure which is formed of lamellar α ₂ / γ colonies, at whose colony boundaries β- Phase exists. The proportion of β-phase is clearly below 15% by volume. The occurrence of grain refining processes is not significant. Also, only an insignificantly small amount of globular γ-phase occurs along the α2 / γ-colony boundaries.
Surprisingly, these advantageous properties of the microstructure of the cooled component produced according to the invention are only achieved during a forming process with a strain rate of ≥ 19 1 / s.

According to a preferred embodiment of the invention, the forged blank after cooling to room temperature by machining or by other methods, such as chemical removal methods, brought into its final form. It is advantageous that additional process steps between the cooling and the removal of the forging blank can be omitted in its final form.

The end product is preferably formed as a rotationally symmetrical component, wherein the ratio of height to largest outer diameter is advantageously in the range of 0.8 to 1.1. The height of the component preferably has values in the range of 50 mm to 55 mm.

According to an advantageous embodiment of the invention, the feed before the process step of heating in the isostatically hot pressed state. The feed was then poured and then hot isostatically pressed.

According to a development of the invention, the feedstock was cast, isostatically hot pressed and pre-formed. The feed is thus in a cast, isostatically hot pressed and pre-formed state before being fed to the heating step.

According to a preferred embodiment of the invention is used as the feed material powder metallurgy, which after sintering is preferably present in rod form and subdivided for further processing in predetermined section lengths and thus shortened.

The feedstock advantageously has the following chemical composition in% by weight: - aluminum (Al) 26.00 to 33,00 - niobium (Nb) 2.00 to 12,00 - Tantalum (Ta) to 10.00 - molybdenum (Mo) 1.00 to 8.00 - iron (Fe) to 4.00 - Chrome (Cr) to 4.00 - Vanadium (V) to 3.00 - manganese (Mn) to 2.00 - boron (B) 0.02 to 0.05 - silicon (Si) to ..1,00 - zirconium (Zr) to 1.00 - carbon (C) to ..0,50

According to a further advantageous embodiment of the invention, the process of forging takes place at a temperature between 1100 ° C and 1400 ° C, wherein the range of temperature between 1260 ° C and 1360 ° C is particularly preferred.

During the process according to the invention, the heated feedstock is preferably shaped by means of drop forging to the forging blank. The process of drop forging is advantageously carried out in one stage in a suitably executed die. This process can be done alternatively, but also in several stages.

It is particularly advantageous that the die has a temperature in the range of 140 ° C to 250 ° C. The die is therefore only moderate heated, wherein the temperature of the die is significantly lower than the temperature usual in Isothermschmieden or hot die forging. In isothermal forging, the tool or die is heated approximately to the temperature of the forging blank, so that at forging temperatures of about 1,300 ° C and the die has a temperature of about 1,300 ° C. The high temperatures of the tools, however, require the use of high-melting materials for the tools, which makes the process extremely uneconomical.

According to a development of the invention, the heated feedstock during the process of drop forging with a force in the range of 140 t to 1,000 t is transformed. The applied force is dependent on the forging geometry.

The advantageous embodiment of the invention enables the use of the component produced by the method according to the invention as a component for turbocharger units of internal combustion engines, wherein the component is rotationally symmetrical and has a ratio of height to largest outer diameter of 0.8 to 1.1.

• nach dem Schmiedeverfahren sind die angestrebten mechanischen Werkstoffeigenschaften eingestellt, sodass keine weitere Wärmebehandlung des Schmiederohlings oder des fertigen Bauteils mehr notwendig ist, was im Vergleich zu Verfahren aus dem Stand der Technik zu einer deutlichen Einsparung von aufzuwendender Energie führt,
• der sehr kurze und kompakte Prozess zur Einstellung der mechanischen Werkstoffeigenschaften weist nur eine geringe Anzahl an Verfahrensschritten auf, was zu deutlich geringeren Anfälligkeiten in Bezug auf Schwankungen innerhalb des Prozesses im Vergleich zum Stand der Technik führt,
• die sehr kurze Prozesskette ist zudem kostengünstig und wenig anfällig in Bezug auf Prozessstörungen.

The method according to the invention has the following advantages in summary:

• After the forging process, the desired mechanical material properties are set, so that no further heat treatment of the forging blank or the finished component is more necessary, which leads to a significant saving of energy to be expended compared to prior art methods,
• The very short and compact process for adjusting the mechanical material properties has only a small number of process steps, resulting in significantly lower vulnerabilities in terms leads to fluctuations within the process compared to the prior art,
• The very short process chain is also cost-effective and not very prone to process disturbances.

Fig. 1:

Schaubild einer Gefügeausbildung von Titan-Aluminium-Basislegierungen abhängig von der Temperatur und der Aluminiumkonzentration,

Fig. 2a:

Gefügestruktur eines TiAl-Bauteils nach dem Schmiedevorgang mit einer geringen Formänderungsgeschwindigkeit,

Fig. 2b:

Gefügestruktur eines lediglich gegossenen und isostatisch heißgepressten TiAI-Bauteils,

Fig. 2c:

Gefügestruktur eines TiAI-Bauteils nach dem Schmiedevorgang mit einer hohen Formänderungsgeschwindigkeit,

Fig. 3:

Einfluss der Gefügestruktur auf die statischen mechanischen Eigenschaften anhand der Abhängigkeit der Streckgrenze R_p0,2 sowie der Dehnung von der Temperatur und

Fig. 4a, 4b:

Ausführungsformen eines Schmiedebauteils.

Further details, features and advantages of the invention will become apparent from the following description with reference to the accompanying drawings. Show it:

Fig. 1:

Diagram of a microstructure of titanium-aluminum base alloys depending on the temperature and the aluminum concentration,

Fig. 2a:

Microstructure of a TiAl component after the forging process with a low rate of deformation,

Fig. 2b:

Microstructure of a cast and isostatically hot pressed TiAl device,

Fig. 2c:

Microstructure of a TiAI component after the forging process with a high strain rate

3:

Influence of the microstructure on the static mechanical properties on the basis of the dependence of the yield strength R _p0.2 as well as the strain on the temperature and

4a, 4b:

Embodiments of a forging component.

In Fig. 1 is a diagram of a microstructure of titanium-aluminum base alloys depending on the temperature and the aluminum concentration shown.

In order to achieve homogeneous, mechanical properties, in particular high creep resistance with high strength of a material, especially in high-temperature applications, the feedstock is subjected to forming in the direct casting state.

The feedstock can be cast as a rod, which can be turned off after the desired external dimensions. After cutting to a predetermined length, the feed is reformed at a strain rate ≥ 19 1 / s.
After a controlled process of cooling, the structure of the material with the desired mechanical material properties is set. The forging blank is further processed by forging process with a strain rate ≥ 19 1 / s without additional process steps, such as heat treatments, by cutting or chemical processes to the final product.

Before the feed material, also referred to as raw material or preform, is forged or supplied to the impact deformation, it becomes in the (α + β) phase field according to Fig. 1 preheated. During the preheating operation, the temperature is adjusted to achieve a volume fraction of the β-phase of at least 5% by volume in order to avoid uncontrolled grain coarsening during the preheating time. The feedstock is heated to a temperature ≥ 1,320 ° C.

The use of an oxide-based protective layer which is conventionally used to protect the base material from being oxidized during the heating phase, or the like, is omitted since, on the one hand, a protective layer thus formed thermally insulates the base material during the cooling phase, so that the base body can be compared to Training without the protective layer cools much slower. For optimum adjustment of the structural properties, however, a predetermined cooling rate is required under existing ambient conditions. On the other hand, the oxide layer grows very little due to the very short time process with few steps compared to methods known from the prior art with about 800 μm and is negligible.

Subsequently, the heated to a temperature of about 1260 ° C to 1360 ° C feedstock, for example, by impact forming by drop forging is formed. The rate of deformation is in the range of 19 1 / s to 50 1 / s. The process step of drop forging takes place in a suitably executed die, which has a temperature in the range of 140 ° C to 250 ° C. Depending on the size and geometry of the component to be formed, the process of forging takes place in one or more stages. The forging geometry dependent forging force for forming is in the range of 140 t to 1,000 t.
After completion of the forging process, the forging blank is removed from the die and cooled uniformly and controlled in ambient air. The cooling of the forging blank is carried out according to Fig. 1 at a constant composition of the material and passes through different phases.

After the forging blank has cooled to room temperature, it has a structure which is formed of lamellar α ₂ / γ colonies, at the colony boundaries of which β-phase is present. The proportion of β-phase is well below 15 vol .-%.

The Fig. 2a and the Fig. 2c show comparative microstructures of a TiAl component after the forging process. Out Fig. 2a In this case, the structure of a forged with a low rate of deformation rate component emerges while Fig. 2c discloses the structure of a forged high rate of change component. The microstructure out Fig. 2a shows a low to no formed texture. The microstructure out Fig. 2c on the other hand has a distinct texture.

In particular in areas of higher degrees of deformation of the forging blank, the influence of the rate of deformation is according to Fig. 2c seen. The Occurrence of grain refining processes is not significant. Also, only an insignificantly small amount of globular γ-phase occurs along the α ₂ / γ-colony boundaries.
The proportion of γ-phase in the α ₂ / γ colonies and in the β-phase surrounding the colonies and the lamellar spacing in the α ₂ / γ colonies are close to the thermodynamic equilibrium. The microstructure is thermally stable in the component insert. The proportion of globular γ grains at the colony boundaries is not significant with 0 to a maximum of 3% by volume and thus has no negative influence on the creep resistance in high-temperature use.

The mechanical properties of the TiAl component produced by the process are determined by the expression and the lamellar spacing of the α ₂ / γ colonies of less than 1 μm and the proportion of β-phase. The value of the creep resistance of the forging blank is, for example, above the value of a cast / HIP starting material.

From the comparison of Fig. 2b and Fig. 2c the structure of the TiAI component, which has been deliberately changed by the forging process, becomes visible. While out of the Fig. 2c As already mentioned, the microstructure after the forging process shows a high rate of deformation, shows Fig. 2b the microstructure of a merely cast and isostatically hot pressed TiAI component.
In Fig. 3 the influence of the microstructure on the static mechanical properties is shown by the dependence of the yield strength R _p0,2 and the strain on the temperature. The microstructures correspond on the one hand to the structure after the forging process with a high rate of deformation change in accordance with Fig. 2c and the structure of the cast only and isostatically hot pressed TiAl component according to Fig. 2b ,
Out Fig. 3 It becomes clear that the component forged with a high rate of deformation has a much higher yield strength R _p0.2 than the cast, isostatically hot pressed and non-forged part, which can be seen by the upper solid line and the dashed line below it.
In addition, will be out Fig. 3 the significant difference in the dependence of the elongation of the different structures having components of the temperature significantly. While the differences in elongation up to a temperature of about 700 ° C are still low, they increase very strongly at temperatures above 700 ° C. The difference in elongation at various microstructures increases greatly with increasing temperature, with the elongation in the forged component is always lower than the un-forged component.

The Fig. 4a and 4b show alternative embodiments of the rotationally symmetrical forging component produced by the method according to the invention.
The TiAl components are designed as components for turbocharger units of internal combustion engines. The component off Fig. 4a has a maximum outside diameter of 48 mm and a height of 53 mm. In the Fig. 4b shown component is formed with a largest diameter of about 66 mm and a height of about 55 mm. The details of the numerical values are to be understood as examples. The production can also be transferred to components with much larger dimensions.

Claims (9)

1. Method for producing a component from a titanium-aluminium base alloy, in particular as component parts for turbocharger units of internal combustion engines, comprising the following steps:

   - heating cast starting material within the (α+β) phase region to a temperature at which the β phase comprises at least 5% by volume,

   - forging the heated starting material into the forged blank by means of impact forming at a rate of form change of at least 19 1/s, wherein the forged blank has no burr after the forging,

   - removing the forged blank from the die and cooling the forged blank in a controlled, uniform manner, wherein after the cooling to room temperature the forged blank has a proportion of β/B2 Ti phase of at most 10% by volume, and

   - further processing the forged blank into the final product.
2. Method according to Claim 1, characterized in that the final product is a rotationally symmetrical component.
3. Method according to Claim 2, characterized in that the rotationally symmetrical component has a ratio of height to greatest outside diameter of 0.8 to 1.1.
4. Method according to one of Claims 1 to 3, characterized in that the starting material is in the hot isostatically pressed state.
5. Method according to Claim 4, characterized in that the starting material is in the preformed state.
6. Device according to one of Claims 1 to 3, characterized in that a powder-metallurgical preliminary material is used as the starting material.
7. Method according to one of Claims 1 to 6, characterized in that the starting material has the following chemical composition in % by weight: - aluminium (Al) 26.00 to 33.00 - niobium (Nb) 2.00 to 12.00 - tantalum (Ta) to 10.00 - molybdenum (Mo) 1.00 to 8.00 - iron (Fe) to 4.00 - chromium (Cr) to 4.00 - vanadium (V) to 3.00 - manganese (Mn) to 2.00 - boron (B) 0.02 to 0.05 - silicon (Si) to ..1.00 - zirconium (Zr) to 1.00 - carbon (C) to ..0.50
8. Method according to one of Claims 1 to 7, characterized in that the heated starting material is formed into the forging blank by means of die forging.
9. Method according to Claim 8, characterized in that the die has a temperature in the range from 140°C to 250°C.

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