KR20120042728A - Process for producing a turbine wheel for an exhaust gas turbocharger - Google Patents

Abstract

The present invention relates to a method for manufacturing a turbine wheel for exhaust gas turbocharger by metal powder injection molding, comprising the steps of: (a) providing a feedstock comprising a metal powder and a binder, (b) metal powder injection molding of a turbine wheel Providing a tool comprising a negative mold of a turbine wheel to be manufactured for the purpose of (c) introducing a rotationally symmetric core comprising a binder into the negative mold of the tool provided in process step (b) and Aligning the cores such that the cores are symmetrically aligned with respect to the axis of rotation of (d) forming a green body by metal powder injection molding of the feedstock provided in process step (a) around the core, (e) performing a binder removal step to remove the binder from the raw material to obtain a turbine wheel-shaped molded body, and (f) sintering the molded body.

Description

Manufacturing method of turbine wheel for exhaust gas turbocharger {PROCESS FOR PRODUCING A TURBINE WHEEL FOR AN EXHAUST GAS TURBOCHARGER}

The present invention relates to a method for producing a lightweight turbine wheel for an exhaust gas turbocharger of an internal combustion engine by metal powder injection molding.

A turbocharger for an internal combustion engine comprises an exhaust gas turbine provided in an exhaust gas stream of the internal combustion engine and connected by a shaft to a compressor in the suction region of the internal combustion engine. The turbine is adapted to rotate by the exhaust gas stream of the internal combustion engine and drives the compressor wheel. The compressor wheel increases the pressure in the intake zone of the internal combustion engine, allowing more air to enter the cylinder during the intake phase than in the case of a suction engine. Thus, more oxygen can be used for combustion of correspondingly higher amounts of fuel. Turbine wheels on the “hot” side exposed to the exhaust gas stream have a complex geometry and are generally made of high temperature resistant materials by precision casting and connected to the shaft by friction welding. The compressor wheel is installed at the opposite end of the shaft, for example by screw connection. During turbocharger operation, the shaft rotation speed becomes very high with two wheels up to about 300,000 rpm. In order to realize a very fast response of the turbocharger, the rotor inertia must be very low.

One of the documents related to the weight reduction of turbocharger parts is Japanese Laid-Open Patent Publication JP 2007-120409. This document discloses saving material by removing the core of the turbine wheel for weight reduction. The turbine wheel with the core removed is manufactured by a precision casting process. However, the disadvantage is that the disclosed precision casting process is complicated and expensive.

Metal powder injection molding (MIM) is known as a method of mass production of metal parts, in particular parts of a type having dimensions close to the final dimension. The MIM process enables the mass production of intricately shaped parts from small to medium size in a low cost and automated manner.

The MIM process involves plasticizing a metal powder having a spherical or irregular shape (the particle size of the powder is generally less than 100 μm) by means of a binder to provide a feedstock. After homogenization of the feedstock is carried out in a kneader, the feedstock is introduced into an injection molding machine. The binder (eg suitable wax) portion is melted in the heating zone to obtain a melt. The screw then transports this melt into a splittable mold. After the mold filling is complete, the melt is solidified so that the part can be removed from the mold. The binder is removed by a binder removal step before sintering. Depending on the binder, the binder is removed from the part in various ways.

In the case of binder removal, there is generally a difference between heated binder removal (binder decomposition or melting through a gas phase), solvent extraction and catalytic binder removal. A sintering process is carried out after the binder removal step, in which the component is densified to at least 95%, preferably at least 98%, of the theoretical density by diffusion treatment.

The use of metal powder injection molding in the manufacture of exhaust gas turbocharger parts has so far failed because it has not been economical enough to replace the precision casting process which is a conventional manufacturing process.

It is an object of the present invention to provide a novel and economical method for producing a turbine wheel for an exhaust gas turbocharger of an internal combustion engine by means that can be produced in a simple manner for a lightweight turbine wheel for an exhaust gas turbocharger.

This object is achieved by a method for producing a turbine wheel for exhaust gas turbocharger by metal powder injection molding, comprising steps a) to f):

a) providing a feedstock comprising a metal powder and a binder,

b) providing a tool comprising a negative mold of the turbine wheel to be manufactured for metal powder injection molding of the turbine wheel,

c) introducing a rotationally symmetric core comprising a binder into the engraved mold of the tool provided in process step (b) and aligning the core so that the core is symmetrically aligned with respect to the axis of rotation of the turbine wheel to be manufactured,

d) forming a green body around the core by metal powder injection molding of the feedstock provided in process step (a),

e) performing a binder removal step to remove the binder from the raw material, and at the same time removing the core to obtain a turbine wheel-shaped molded body, and

f) sintering the shaped body.

The turbine wheel for exhaust gas turbocharger having a hollow space with limited internal structure can be produced simply and inexpensively by the method of the present invention. The internal structure defining the hollow space is formed by removing the core in the binder removal step carried out in process step (e). For this reason, the turbine wheel which has been manufactured as a solid part by the precision casting method until now can be manufactured in large quantities as a hollow part, and it is economical, As a result, a turbocharger can be significantly reduced in weight. This reduction in weight can lead to faster reactions associated with lower fuel requirements, increased efficiency of internal combustion engines and significant savings in materials. In addition, the method of the invention makes it possible to produce turbine wheels for exhaust gas turbochargers of particularly delicate design, in contrast to the conventionally known precision casting process, having a wall thickness in the range of 0.1 to 1 mm.

In the present invention, the term "feedstock" generally refers to a composition comprising a sinterable metal or ceramic powder and a binder and is suitable for use in metal powder injection molding. Such compositions are known to those skilled in the art. The term "metal powder" refers to a powdered metal or powdered metal alloy or mixtures thereof in the present invention. Metals that may be included in powder form in the feedstock include, for example, iron, cobalt, nickel, chromium, titanium, molybdenum, niobium, and aluminum; The alloy is, for example, a nickel-based alloy or a titanium-based alloy. As the alloy, for example, a nickel-based alloy available under the trade name Inconel ^? 713 is preferable; These include 74 weight percent nickel, 12.5 weight percent chromium, 4.2 weight percent molybdenum, 2 weight percent niobium, 6 weight percent aluminum, 0.8 weight percent titanium and 0.12 weight percent carbon. In addition, in the case of nickel-based alloys, an alloy available under the trade name Inconel ^? 718 is preferable. The base alloy is 50-55 wt% nickel, 17-21 wt% chromium, <24 wt% iron, 2.8-3.3 wt% molybdenum, 4.8-5.5 wt% niobium, 0.2-0.8 wt% aluminum , 0.7-1.1 weight percent titanium and less than 0.08 weight percent carbon. In the case of a nickel-based alloy, NIMONIC ^? 90 is preferable. NIMONIC 90 contains less than 0.13% carbon, 2-3% titanium, 1-2% aluminum, less than 1.5% iron, 15-21% cobalt, 18-21% chromium And the rest is nickel. As the nickel-based alloy, HASTELLOY ^? X is more preferable. HASTELLOY ^？ X is 0.05 to 0.15 wt% carbon, less than 0.5 wt% aluminum, 0.5 to 2.5 wt% cobalt, 8 to 10 wt% molybdenum, 17 to 20 wt% iron, 20 to 23 wt% chromium And the rest are alloys containing nickel. Further suitable alloys are alloys comprising about 15 wt% chromium, about 10 wt% iron, 5 wt% molybdenum, 2 wt% titanium, niobium and nickel. The proportion of metal powder in the feedstock can vary widely and is generally 40-70% by volume, preferably 45-60% by volume, based on the feedstock.

Suitable “binders” herein are in principle all systems known from the prior art, which are suitable for use in metal powder injection molding. The proportion of binder in the feedstock can vary widely and is generally 10-60% by volume, preferably 30-50% by volume, based on the feedstock. Suitable binders are generally thermoplastics such as polystyrene, polypropylene, polyethylene and ethylene-vinyl acetate copolymers. Such a binder can be removed from raw material by, for example, heating to a temperature of 300 to 500 ° C. for 3 to 8 hours. Here, the binder is thermally decomposed. Further suitable binders are those that can be removed from the dough by solvent extraction. Polyoxymethylene based binders are also suitable and are removed by treating the dough in a gaseous acid containing atmosphere. Acids used in these processes are generally protic acids, ie acids that dissociate into protons (hydrated) and anions upon reaction with water.

In a preferred embodiment of the invention, the feedstock is

A) 40-90% by volume sinterable flour metal or powder metal alloy or mixtures thereof,

B) 10-60% by volume of a mixture of the following B1) and B2):

B1) 80 to 98% by weight, in particular 85 to 98% by weight, based on B), of polyoxymethylene homopolymer or copolymer, and

B2) 2-20% by weight of polyolefins or polyolefin mixtures, in particular 2-15% by weight.

Polyoxymethylene homopolymers or copolymers are known per se to those skilled in the art and are disclosed in the literature. In general, homopolymers are prepared by polymerization of formaldehyde or trioxane, preferably in the presence of a suitable catalyst. Preferred polyoxymethylene copolymers contain, in addition to the repeating unit —OCH ₂ —, the repeating unit of the following (I) at most 50 mol%, preferably 0.1-20 mol%, particularly preferably 0.3-10 mol%.

Where

R ¹ to R ⁴ are each independently a hydrogen atom, a C ₁ -C ₄ -alkyl group or a halogen substituted alkyl group having 1 to 4 carbon atoms, and R ⁵ is —CH ₂ —, —CH ₂ —O—, C ₁ -C ₄ - alkyl or C ₁ -C ₄ -? oxymethylene group to a methylene group or a corresponding substituted by haloalkyl and n is 0 3. These groups may be advantageously introduced into the copolymer by ring opening of the cyclic ethers. Preferred cyclic ethers are compounds of formula (II)

Wherein R ¹ to R ⁵ and n are as defined above. Purely by way of example, as cyclic ethers, ethylene oxide, 1,2-propylene oxide, 1,2-butylene oxide, 1,3-butylene oxide, 1,3-dioxane, 1,3-dioxolane and dioxepane and As the copolymer, mention may be made of linear oligoformal or polyformal such as polydioxolane or polydioxepan.

Further polymers suitable as component B1) are, for example, oxymethylene terpolymers prepared by the reaction of a trioxane, which is one of the cyclic ethers disclosed above, with a third monomer, preferably a bifunctional compound of formula (III)

Wherein Z is a chemical bond, -O- or -ORO- (R = C ₁ -C ₈ -alkylene or C ₃ -C ₈ -cycloalkylene).

Preferred monomers of this kind include ethylene diglysid, glycidyl ether and glycidyl in a molar ratio of 2: 1 and diethers of formaldehyde, dioxane or trionic acid, and also 2 moles of glycidyl compound and 2 carbon atoms? Diethers derived from one mole of eight aliphatic diols, such as ethylene glycol, 1,4-butanediol, 1,3-butanediol, cyclobutane-1,3-diol, 1,2-propanediol and cyclohexane-1, Diglycidyl ethers of 4-diol are exemplified.

Methods of making such homopolymers and copolymers are known to those skilled in the art and are disclosed in the literature, so further details are not required here. Preferred polyoxymethylene homopolymers and copolymers have a melting point of 150 ° C. or higher and a molecular weight (weight average) of 5000 to 150,000. Preferably it is 7000-60,000.

Component B2) comprises a polyolefin or mixtures thereof. As polyolefins, mention may be made of those having from 2 to 8 carbon atoms, in particular from 2 to 4, and copolymers thereof. The particularly preferred polyethylene and polypropylene and copolymers thereof as known in the art and, for example, under the trade name ^Lupolen? And ^Novolen? As is available from BASF SE.

As component C), the binder used in the process of the invention may comprise 0-6% by volume, preferably 1-5% by volume of dispersant. Purely examples include oligomeric polyethylene oxides, stearic acid, stearamides, hydroxystearic acids, fatty alcohols, fatty alcohol sulfonates and average copolymers of ethylene oxide and propylene oxide having an average molecular weight of 200 to 600.

In addition, the binder may further comprise conventional additives and processing aids which have a desirable effect on the properties of the mixture during molding.

In general, the feedstock is prepared by melting component B), preferably in a twin-screw extruder, preferably at a temperature of from 150 to 220 ° C, in particular from 170 to 200 ° C. The metal powder A) is then introduced in the required amount into the melt stream of the binder (component B) at the same range of temperatures.

In process step (b) of the method of the invention, a tool is provided which comprises a negative mold of a turbine wheel to be manufactured. According to the invention, it is suitable for metal powder injection molding of turbine wheels. Such tools are known to those skilled in the art and are not disclosed in more detail here. In general, a tool is a tool whose core is missing.

After providing the tool comprising the negative mold of the turbine wheel to be manufactured, the rotationally symmetric core is introduced into the negative mold of the tool of process step (c) of the present invention. The rotationally symmetric core is an aid, whereby the hollow space structure is introduced into the turbine wheel. According to the invention, the core is centered in the intaglio mold in such a way that it is symmetrical about the axis of rotation of the turbine wheel to be manufactured. By "symmetrical about the axis of rotation" it is meant that the core is aligned in the intaglio mold in such a way that it does not cause an imbalance of the turbine wheel produced as a result of the alignment. The core comprises the binder as defined above, or consists of the binder as defined above, as a result of which the process of the invention is completely removed from the turbine wheel during the manufacture of the turbine wheel, symmetrically disposed on the axis of rotation. The hollow space structure remains. According to a general embodiment of the invention, the core is introduced into the tool on a suitable receiving device to maintain its position. The receiving device may for example be a rod or pin on which the core is located. The binder or constituents of the binder that make up the core can then be diffused from the hollow space left in the receiving material in the dough during the binder removal step.

In process step (d) of the present invention, the feedstock provided in process step a) is injected into the intaglio mold around the core to form raw material. Conventional screw or piston injection molding machines can be used to effect injection molding in process step (d). In general, the molding of the feedstock is performed at an injection pressure of 300 to 2000 bar and a temperature of 60 to 200 ° C in a tool having a temperature of 60 to 150 ° C. This produces a green fabric that includes a core made of a binder and has a structure of a turbine wheel to be manufactured.

In order to remove the binder from the feedstock and remove the core from the raw material, a binder removal step, that is, process step (e), is carried out to obtain a turbine wheel-shaped molded body. The binder removal step is performed according to the selected binder. The progress of the binder removal step can be monitored by one skilled in the art, for example, by measuring the weight change of the dough. When using a preferred binder comprising components A), B) and optionally C), the binder removal step is generally carried out for 20 to 180 for 0.1 to 24 hours, preferably 0.5 to 12 hours in a gaseous acid containing atmosphere. It is carried out at a temperature in the range of ℃. Suitable acids for such treatments are inorganic acids which can be gaseous at room temperature or at least vaporized at treatment temperatures. For example, hydrogen halides and HNO ₃ can be monitored. Suitable organic acids are those having a boiling point of less than 130 ° C. at atmospheric pressure, such as formic acid, acetic acid, or trifluoroacetic acid or mixtures thereof. Further suitable acids are adducts with BF ₃ and organic ethers thereof. The required treatment time very generally depends on the concentration of acid in the treatment atmosphere and the treatment temperature. If a carrier gas is used, it generally passes first through the acid and the acid is loaded. The loaded carrier gas is brought to a treatment temperature, which is advantageously higher than the loading temperature to avoid acid condensation. It is preferable to mix the acid into the carrier gas via a metering device, and the mixture is heated to the extent that the acid can no longer condense.

In addition, the binder removal step can be carried out in two steps, for example. The treatment of the first step is carried out until the polyoxymethylene component B1) of the binder is removed to at least 80% by weight, preferably at least 90% by weight. This is easily recognized by the weight reduction of the dough. The molded article obtained in this manner is then heated at 250 to 500 ° C, preferably 350 to 450 ° C, for 0.1 to 12 hours, preferably 0.3 to 6 hours, to remove substantially all of the remaining binder.

In the binder removal step, the molded body from which the binder is removed can be converted into a metal molded body by sintering in a general manner. In the sintering process, the compacts are densified and shrunk to form parts with final geometrical properties. During sintering, the shaped bodies are correspondingly smaller, with the dimensions uniformly shrinking in all three directions in the space. Depending on the binder content, the linear shrinkage is generally 10-20%. Sintering can be performed under various protective gases or reduced pressure. Process step (f) is generally carried out at a temperature in the range from 250 to 1500 ° C. The sintering time is generally in the range of 1 to 12 hours, preferably in the range of 2 to 5 hours. In a preferred embodiment of the invention, a holding device for supporting the shaped body during sintering is used in the sintering process in process step (f) to at least prevent most deformation of the part. In an embodiment of the invention, the holding device is fixed to the part in the form of a mandrel. In a particularly preferred embodiment of the invention, at least one holding device is used during sintering whose material composition and wall thickness match the composition and wall thickness of the turbine wheel to be manufactured. This allows the shaped body to be sintered and the corresponding holding device to be strengthened and retracted in the same range during sintering. One surface of each holding device is coated at least in cross section to avoid reaction or diffusion of the part and the holding device in the sintering process to avoid sintering of the part and the holding device together. The surface is coated at least at the cross section in contact with the shaped body to be sintered. The holding device can also be coated on all sides. Of course, the coating used depends on the material or material composition of the shaped body to be sintered. Preference is given to using titanium nitrite coatings or ceramic coatings for holding devices.

In a preferred embodiment of the invention, the core introduced in process step (c) comprises the same binder contained in the feedstock. This is advantageous in that the removal of the binder and the core contained in the feedstock can be carried out in the same process step.

The size and / or geometry of the rotationally symmetric core introduced into process step (c) can vary widely. In general, the size of the core is chosen to have a volume of about 5 to 60% by volume, preferably 45 to 55% by volume of the turbine wheel. The introduction of the core and the hollow space structure resulting therefrom results in a turbine wheel which can be manufactured in a simple manner and which is significantly reduced in weight compared to conventionally known turbine wheels. It is also possible to manufacture a turbine wheel having a lost core by the process of the invention.

Introducing the core into the central region of the turbine wheel, which has significant thickness and accumulated mass, avoids commonly occurring voids and defects. The core geometry can be selected by one skilled in the art according to the geometry of the turbine wheel. Cores with conical geometry, spherical geometry (spherical geometry), elliptical geometry, cylindrical geometry or very common rotationally symmetrical geometry are generally suitable. In a preferred embodiment of the invention, the geometry of the core can be selected as the core, which can roughly reproduce the geometry of the turbine wheel, as a result of which the wall can withstand the forces acting on the turbine wheel in the course of operation. It is possible to obtain turbine wheels of selected thickness, in particular weight optimized.

After manufacture of the turbine wheel of the present invention, it is generally balanced after connecting the turbine wheel to the shaft by friction welding or direct injection molding.

In one embodiment of the invention, the turbine wheel obtained in process step (f) is connected to the shaft by metal injection molding in further process step (g).

The invention is illustrated by the following figures and examples.

1 shows a sectional view of a turbine wheel 1 for an exhaust gas turbocharger of an internal combustion engine.

The turbine wheel 1 for the exhaust gas turbocharger of the internal combustion engine shown in FIG. 1 has a hollow space structure 2 formed by the process of the present invention. The hollow structure is located at the center of the turbine wheel and is symmetrical about the axis of rotation of the turbine wheel 1.

Example

As a feedstock, under the trade name ^Catamold?, Heat-resistant nickel-base superalloy is used for the injection moldable pelletized material for the manufacture of a sintered molded article of (2 DIN 4632), available from BASF SE as N90.

The feedstock was processed on an Engel ES80 / 10 thermoplastic injection molding machine.

The settings in the molding machine are as follows:

Prior to injection molding, a core containing a binder and about 6 cm ³ in volume was introduced into the negative mold of the tool.

HNO ₃ in an atmosphere of the binder removal was performed at 110 ℃. Heraeus VT 6060 MU2 binder removal oven was used for this purpose with a volume of 50 L and 30 ml / h of acid introduction and 500 L / h of flushing gas stream (nitrogen). The binder removal process was completed after 7.7% of the binder was lost based on the starting weight of the dough.

Sintering was performed under 100% argon atmosphere. Argon used was cleaned and dried (99.98%, dew point <-80 ° C). The sintering cycle was as follows.

Room temperature-5 K / min-60 ° C., held for 1 hour,

600 ° C-5 K / min -1325 ° C, hold for 3 minutes,

Furnace cooling.

In order to make the turbine wheel very high, the parts were held at a temperature of 1185 ° C. for 4 hours at a pressure of 1000 bar.

In order to further optimize the strength properties, a two step heat treatment was subsequently carried out. In step 1 the turbine wheel was heated under reduced pressure at 1080 ° C. for 8 hours under argon of 900 mbar. In step 2, the workpiece was treated under reduced pressure at 705 ° C. for 16 hours under 900 mbar of argon.

The result is a turbine wheel with a volume of 7.5 cm ³ and one third lighter than a solid turbine wheel.

Claims (9)

a) providing a feedstock comprising a metal powder and a binder,
b) providing a tool comprising a negative mold of the turbine wheel 1 to be manufactured for metal powder injection molding of the turbine wheel 1,
c) introducing a rotationally symmetric core comprising a binder into the engraved mold of the tool provided in process step (b) and aligning the core so that the core is symmetrically aligned with respect to the axis of rotation of the turbine wheel 1 to be manufactured,
d) forming a green body around the core by metal powder injection molding of the feedstock provided in process step (a),
e) performing a binder removal step to remove the binder, and at the same time to remove the core to obtain a shaped body of the turbine wheel 1 shape, and
f) sintering the shaped bodies
The manufacturing method of the turbine wheel (1) for exhaust gas turbocharger by metal powder injection molding containing a. A process according to claim 1 wherein a core comprising a binder included in the feedstock provided in process step (a) is introduced in process step (c). The method according to claim 1 or 2,
(A) 40-90% by volume of sinterable powdered metals or powdered metal alloys or mixtures thereof,
(B) 10 to 60% by volume of the mixture of the following (B1) and (B2) as a binder:
(B1) 80 to 98% by weight polyethylene homopolymer or copolymer,
(B2) 2 to 20% by weight of polyolefins or polyolefin mixtures; And
C) A process for preparing a feedstock comprising 0-5% by volume dispersant in process step (a).  The process according to any one of claims 1 to 3, wherein a feedstock comprising a nickel-based alloy or a titanium-based alloy as the metal powder is provided in process step (a). The method according to any one of claims 1 to 4, wherein the shaped body is installed in one or more holding devices in process step (f). 6. The process according to claim 1, wherein a core having a volume of 5 to 60% of the volume of the turbine wheel is introduced in process step (c). 7. The process according to any one of claims 1 to 6, wherein process step (e) is carried out at a temperature in the range from 20 to 180 ° C. The process according to any one of claims 1 to 7, wherein process step (f) is carried out at a temperature in the range from 250 to 1500 ° C. Turbine wheel for exhaust gas turbocharger having a hollow space structure symmetrical about the axis of rotation of the turbine wheel and having a volume of 5 to 60% of the volume of the turbine wheel.

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