Patent Application
20150104318
The present invention relates to a turbine rotor for an exhaust gas turbine and to a method for producing the turbine rotor.
Such a turbine rotor consists of a turbine wheel and a rotor shaft as a structural unit and is for example part of the running gear of an exhaust gas turbocharger and serves for the conversion of exhaust gas energy, contained in the exhaust gas of an internal combustion engine, into rotational energy of the running gear and for the transmission of this rotational energy to a compressor wheel connected to the turbine rotor, with the aid of which the rotational energy is used for generating an increased pressure of the air supply to the internal combustion engine, and consequently for increasing the output and efficiency of the internal combustion engine.
Instead, there may also be coupled to the turbine rotor for example a generator, with the aid of which the rotational energy is converted into electrical energy, which in turn can be used variously.
However, the main area of use at present concerns exhaust gas turbochargers for internal combustion engines in motor vehicles, for which reason, whenever useful for better understanding, hereafter reference is made in the description to exhaust gas turbochargers.
Exhaust gas turbochargers are being used increasingly for increasing the output in motor vehicle internal combustion engines. This is taking place increasingly frequently with the aim of reducing the internal combustion engine in overall size and weight, with the same output or even increased output, and at the same time reducing the consumption, and consequently the emission of CO2, with regard to increasingly stringent legal specifications. The operating principle is that of using the energy contained in the stream of exhaust gas to increase the pressure in the induction tract of the internal combustion engine and thus bring about better filling of the combustion chamber with air-oxygen, and consequently be able to convert more fuel, petrol or diesel, in each combustion process, that is to say increase the output of the internal combustion engine.
An exhaust gas turbocharger has for this purpose a turbine arranged in the exhaust-system branch of the internal combustion engine, with a turbine rotor driven by the stream of exhaust gas and a compressor arranged in the induction tract, with a compressor impeller building up the pressure. The turbine rotor wheel and the rotor shaft are connected to one another in a material-bonded manner and thus form a structural unit. The compressor impeller is fastened to the end of the rotor shaft of the turbine rotor opposite from the turbine rotor wheel for rotation with said shaft, the rotor shaft being rotationally mounted in a bearing unit arranged between the turbine and the compressor. Consequently, with the aid of the mass flow of exhaust gas, the turbine rotor, and via the rotor shaft in turn the compressor impeller, is driven and the exhaust gas energy is thus used for building up pressure in the induction tract.
During operation, the turbine wheel is in the hot stream of exhaust gas, and is consequently exposed to very great temperature fluctuations, peak temperatures up to over 1000° C. being reached. At the same time, the turbine rotor rotates at very high rotational speeds of up to 300 000 rpm, whereby the turbine rotor wheel, and in particular the turbine wheel blading, is exposed to very high mechanical loads due to the high centrifugal forces occurring. Furthermore, particularly the mass of the turbine wheel is very important for the dynamic response of the turbine, which is hindered if the turbine rotor wheel is designed with a high mass to match the high loads.
Therefore, highly heat-resistant metal alloys, such as for example titanium-aluminum alloys (TiAl alloys or titanium aluminide) or Ni-based alloys, which are distinguished in particular by their high specific strength at high temperature and a nevertheless low relative density, are being used increasingly for the turbine rotor wheels. In addition, the coefficient of thermal expansion of these highly heat-resistant metal alloys comes very close to that of metals that are usually used in turbine construction, which helps to avoid problems caused by differing heat expansion. In practice, intermetallic mixtures with a main proportion of titanium and aluminum or nickel are used.
As also known for example from DE 10 2007 048 789 A1, in the case of the TiAl alloys the specific alloy compositions may well vary and also contain further constituents, and are typically characterized by a proportion of titanium of between 50 and 60% (proportion by weight) and a proportion of aluminum of greater than 25% (proportion by weight). Further constituents may be for example Cr, Nb, B, C or Mo. TiAl alloys form what is known as a γ-TiAl phase (gamma titanium aluminide) with a tetragonal crystal structure and, depending on the proportion of other different phases, are referred to as gamma, duplex or lamellar alloys.
The Ni-based alloys are for example Inco 713 C, Inco 713 LC, MAR-M 246 MAR-M 247, B 1964, IN 100 or GMR-235.
In the following explanations, all of these alloy structures are subsumed altogether under the term “highly heat-resistant metal alloys”.
On the other hand, the rotor shaft is part of the mounting system of the turbine rotor and must be able to withstand a high alternating bending load and must have a sufficiently hardened outer layer, at least in the mounting region, to avoid seizing of the bearings. On the other hand, the rotor shafts are not exposed to the same extreme high temperatures as the turbine rotor wheel.
Correspondingly suitable for this use are materials such as steel, in particular structural steel, low- or high-alloy heat-treatment steel, such as for example 42CrMo4(1.7225), X22CrMoV12-1(1.4923) or X19CrMoNbVN11-1(1.4913), or else superalloys such as Inconel or Incoloy (see also DE 10 2007 048 789 A1). These materials are referred to in the following explanations simply and altogether as steel.
In order to be able to use the respective advantages of the corresponding materials, the turbine rotors are therefore produced from the aforementioned components, the turbine rotor wheel of highly heat-resistant metal alloy and the rotor shaft of steel, and must as a consequence be advantageously joined together by means of a material-bonded connection to form a structural unit.
In the case of material-bonded connections, the elements being connected are held together by means of atomic or molecular forces and are inseparable connections that can only be released again destructively. In this context, material-bonded connections are in particular welded connections and brazed connections.
As known for example from DE 697 24 730 T2, the friction welding method known in this context in connection with other material combinations can only be used to a restricted extent. The reason for this is that, if a friction welding method is used, for example the transformation of the steel at the time of cooling down from austenite to martensite causes an expansion of the steel, which brings about a residual stress, and, even if the material of the turbine rotor wheel has a high rigidity, the formability at room temperature is approximately at a low 1%, and therefore rupturing of the wheels can occur. Furthermore, there may be a reaction of TiAl with the carbon, C, in the steel, whereby titanium carbide is formed at the connection interface, whereby the strength at the interface falls to a critical degree.
In the case of welding methods generally, the high temperatures, up to beyond the melting point of the materials to be connected, and the internal stresses occurring during cooling down increasingly cause crack formation in the region of the weld, and consequently weakening of the connection.
To avoid these problems, DE 697 24 730 T2 proposes a brazing method in which a brazing material that has for example an austenitic structure is inserted between the two elements to be connected, the turbine rotor wheel and the rotor shaft.
According to DIN 8505 “Soldering and brazing”, brazing is a thermal process for joining materials by material bonding, a liquid phase being produced by melting a brazing filler and a connection being created by diffusion of the brazing filler at the boundary surfaces. A further major difference from welding is that the solidus temperature of the base materials of the elements being joined is not reached thereby.
Consequently, this process takes place at lower temperatures than welding and fewer internal stresses are produced in the joint. Furthermore, the use of a corresponding brazing filler as an intermediate material between the elements being connected makes it possible to prevent the formation of microstructures that are detrimental to strength. According to DE 697 24 730 T2, primarily nickel-, copper-, silver- or titanium-based metal alloys are advantageously used as brazing materials.
One specific problem with these connecting processes is that of controlling the thickness of the layer of brazing filler between the two elements to be connected, and consequently controlling the overall length of the finished turbine rotor.
A further problem is that, even with the lower brazing temperatures, the austenite temperature of the steel used for the rotor shaft is possibly exceeded, and as a result a softening of the steel takes place. This problem is all the more serious the wider the heating region around the brazed connection extends, possibly into the bearing regions of the rotor shaft. This is the case in particular with the methods that are usually used for heating, by means of burners, induction coils or even heating ovens. As a result, renewed subsequent, cost- and time-intensive reworking and hardening of the rotor shaft is unavoidable. This is disadvantageous in particular for industrial mass production.
The present invention is therefore based on the object of providing a turbine rotor, consisting of a turbine rotor wheel of a highly heat-resistant metal alloy and a steel rotor shaft connected thereto by a brazing method, for an exhaust gas turbine, in which the width of the brazing gap, and consequently the exact length of the finished turbine rotor, and the hardening, in particular of the bearing regions, of the rotor shaft are defined, without requiring additional reworking. The object is also that of providing a method for producing such a turbine rotor that can be used at low cost industrially, in mass production.
This object is achieved by a turbine rotor with the features according to patent claim 1 and by a method for producing this turbine rotor with the features according to patent claim 5. Advantageous forms and developments that can be used individually or, as long as they are not mutually exclusive alternatives, in combination with one another are the subject of the dependent claims.
The turbine rotor according to the invention for an exhaust gas turbine has a turbine rotor wheel with a rotor wheel hub and a rotor shaft with a rotor shaft end facing the rotor wheel base. The turbine rotor wheel consists of a highly heat-resistant metal alloy and is preferably produced in a customary precision casting process. It has a main body with blading on the front side, and a rotor wheel hub in the form of a portion of a cylinder arranged concentrically on the rear side of the main body.
The rotor shaft consists of steel and is preferably finished for later use and hardened at least in the region of the later bearing locations.
The rotor wheel hub and the rotor shaft end are connected to one another in a metallurgically bonded manner by means of a brazed connection, a brazing gap filled with a brazing alloy being arranged concentrically in relation to the axis of rotation of the turbine rotor between the end faces of the rotor wheel hub and the rotor shaft end. Advantageously used as brazing materials are primarily nickel-, copper-, silver- or titanium-based metal alloys. The turbine rotor according to the invention is distinguished in particular by the fact that the brazing gap width is predetermined by material-removing machining, running around circularly and extending from the outer periphery over only part of the radius, on the end face of the rotor wheel hub or the end face of the rotor shaft end, and in that the brazed connection has been created by means of electron-beam brazing methods. The fact that the brazing gap is arranged concentrically and is formed by a removal of material running around circularly on one of the end faces, while the removal of material, and consequently the brazing gap, does not extend over the entire radius of the respective end face, means that part of the original end face remains, so that the removal of material produces a defined soldering gap when the end faces of the rotor wheel hub and the rotor shaft lie against one another. The corresponding removal of material may optionally take place both on the end face of the rotor wheel hub and on the end face of the rotor shaft end or on both faces.
The advantages of the turbine rotor according to the invention are in particular that a defined and optimized brazing gap width can be ensured in any event and independently of the applied forces when the two workpieces are joined to one another. This contributes to the constant quality of the brazed connection and its strength. Nevertheless, the spatially delimited heat input has the effect that the hardening of the rotor shaft is not impaired in the region of the bearing locations and there is no need for an additional hardening process. There are also no crack formations in the connecting region on account of the altogether lower temperatures. These are essential preconditions for use of the turbine rotor according to the invention in mass-produced products, such as for example in turbochargers for internal combustion engines in motor vehicles.
In an advantageous configuration of the turbine rotor according to the invention, a TiAl alloy or an Ni-based alloy is used as the highly heat-resistant metal alloy of the turbine rotor wheel and a low-alloy or high-alloy heat-treatment steel or an austenitic steel is used for the rotor shaft. This has the advantage that the optimum combination can be put together from a large multitude of known materials.
An advantageous configuration of the turbine rotor according to the invention is characterized in that the removal of material running around circularly forms an annular offset with a certain offset height, or a conical surface inclined at a certain gap angle α outwardly toward the respective workpiece, in such a way as to form an outwardly open brazing gap and a circular, end-face abutting surface adjoining thereto in the direction of the axis of rotation of the turbine rotor, which lies directly against the opposing end face. The removal of material therefore takes place from the outer circumference of the rotor shaft or the rotor wheel hub in the direction of the axis of rotation, over only part of the radius, so that part of the original end face remains in the respective center and forms the abutting surface for the respectively opposing workpiece.
As a result, a brazing gap with a defined width and length is predetermined, and consequently the connecting surface area is defined. This produces constant strength values of the brazed connections in mass production. At the same time, the turbine rotors have a constant overall length.
In a development of the aforementioned advantageous configuration, the rotor wheel hub or the rotor shaft end has in the respective end face a centrally arranged blind-hole bore, which acts as a thermal choke at the transition between the turbine rotor wheel and the rotor shaft. In this case, the diameter of the blind-hole bore is that much smaller than the diameter of the end-face abutting surface that an annular abutting surface with a ring width of at least 0.5 mm is formed. The blind-hole bore may be arranged both in the same workpiece, the turbine rotor or the rotor shaft, as that from which material has been removed or optionally also in the respectively other workpiece, from which material has not been removed. In the second case, the end-face abutting surface only lies against the opposing workpiece in the region in which the abutting surface overlaps the blind-hole bore.
This configuration has the advantage that a defined brazing gap width can be ensured in spite of the arrangement of the blind-hole bore as a thermal choke in one of the end faces of the rotor wheel hub or the rotor shaft.
In a continuation of the first-mentioned advantageous configuration of the turbine rotor according to the invention, the offset height of the annular offset is chosen as between 0.05 mm and 0.15 mm or the gap angle α is chosen such that the brazing gap does not exceed a brazing gap width of 0.20 mm at its outer circumference. With brazing gap widths or geometries within the aforementioned ranges, the connecting joints between the turbine rotor wheel and the rotor shaft have the best strength values.
The method according to the invention for producing the turbine rotor described above is characterized by the following method steps:
One possibility for carrying out the heating and temperature-maintaining operation is for example that the electron beam is focused in the form of a spot on one portion of the brazing gap, and the turbine rotor, that is to say the turbine rotor wheel and the rotor shaft together, is turned at a predetermined rotational speed about its axis of rotation.
The advantages of the method according to the invention for producing the turbine rotor according to the invention are in particular that a brazed connection of a constant quality with a defined brazing gap width, and consequently a defined overall length of the turbine rotor, can in any event be produced. The rapid and spatially delimited introduction of heat allows short process times to be achieved, and no subsequent operation of hardening the rotor shaft is required. These are essential preconditions for use of the method according to the invention in mass production, such as for example in turbochargers for internal combustion engines in motor vehicles.
An advantageous development of the method for producing a turbine rotor according to the invention is characterized in that, in an additional method step, a centrally arranged blind-hole bore is introduced into the rotor wheel hub or the rotor shaft end in such a way that the diameter of the blind-hole bore is that much smaller than the diameter of the end-face abutting surface that an annular abutting surface with a ring width of at least 0.5 mm is formed.
The blind-hole bore introduced acts as a thermal choke between the turbine rotor wheel and the rotor shaft and reduces the heat transfer to the rotor shaft during operation. At the same time, a brazing gap defined in length and width can be achieved, and the quality of the brazed connection can be increased as a result.
Summarized in brief, the invention relates to a turbine rotor for an exhaust gas turbine and to a method for producing such a turbine rotor, the turbine rotor having a turbine rotor wheel of a TiAl alloy and a rotor shaft of steel, and the rotor wheel hub and the rotor shaft end being connected to one another in a material-bonded manner by means of a brazed connection. A brazing gap filled with a brazing alloy is arranged concentrically in relation to the axis of rotation of the turbine rotor between the end faces of the rotor wheel hub and the rotor shaft end, the brazing gap width being predetermined by a removal of material, running around circularly, on the end face of the rotor wheel hub or the end face of the rotor shaft end, and the brazed connection being produced by means of electron-beam brazing methods.
Specific exemplary embodiments of the invention are explained in more detail below on the basis of the representations in the drawing, in which:
Items that have the same function and designation are provided in the figures with the same reference signs.
In
The connecting joint between the turbine rotor wheel and the rotor shaft is shown in a “broken-away” representation and identified as detail X, which in the following
In
The brazed connection is produced by means of electron-beam brazing methods.
In the enlarged representation of the detail Y from
The examples of the possible arrangements and combinations of the removal of material and the blind-hole bore that are shown should be understood as merely examples of the further possible combinations and further geometries of the removal of material or fashioning of the brazing gap.
This is illustrated once again in
The device has a clamping device 20 and an electron beam source 17 with a focusing device 18. The clamping device 20 has the following functional units:
The turbine rotor wheel 2 provided, prepared in a way corresponding to the first method steps, is clamped in a centered manner in the rotor wheel clamping chuck 12; the arrows 22 show the clamping movement of the individual clamping jaws that is required for this. Likewise, the rotor shaft provided, prepared in a way corresponding to the first method steps, is clamped in a centered manner in the rotor shaft clamping chuck 13; the arrows 23 show the clamping movement of the individual clamping jaws that is required for this. This is followed by the bringing together of the turbine rotor wheel 2 and the rotor shaft 4, which are aligned with one another in a centered manner, by way of a linear movement of the clamping slide 14, which is indicated in
All of the procedures described can be carried out in an automated manner with the aid of corresponding drive devices and a central programmable open-loop/closed-loop control device. The arrangement of further functional units also allows the foregoing method steps, such as for example the production of the circular, concentric removal of material and the application of a brazing material, to be carried out at least partially in the device described.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102012205043.4 | Mar 2012 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2013/055824 | 3/20/2013 | WO | 00 |
