U.S. patent application number 13/263135 was filed with the patent office on 2012-02-09 for process for producing a turbine wheel for an exhaust gas turbocharger.
This patent application is currently assigned to BASF SE. Invention is credited to Martin Bloemacher, Andreas Kern, Franz-Dieter Martischius, Markus Steffen, Johan Ter Maat, Hans Wohlfromm.
Application Number | 20120034084 13/263135 |
Document ID | / |
Family ID | 42289418 |
Filed Date | 2012-02-09 |
United States Patent
Application |
20120034084 |
Kind Code |
A1 |
Kern; Andreas ; et
al. |
February 9, 2012 |
PROCESS FOR PRODUCING A TURBINE WHEEL FOR AN EXHAUST GAS
TURBOCHARGER
Abstract
The invention relates to a process for producing a turbine wheel
for an exhaust gas turbocharger by metal powder injection molding,
which comprises the following steps: (a) provision of a feedstock
comprising a metal powder and a binder, (b) provision of a tool
which comprises a negative mold of the turbine wheel to be produced
for metal powder injection molding of the turbine wheel, (c)
introduction of a rotationally symmetrical core comprising a binder
into the negative mold of the tool provided in process step (b) and
alignment of the core so that it is aligned symmetrically about the
axis of rotation of the turbine wheel to be produced, (d)
production of a green body by metal powder injection molding of the
feedstock provided in process step (a) around the core, (e)
carrying out of a binder removal step to remove the binder from the
green body in order to obtain a molding in the shape of the turbine
wheel, and (f) sintering of the molding.
Inventors: |
Kern; Andreas; (Mannheim,
DE) ; Bloemacher; Martin; (Meckenheim, DE) ;
Martischius; Franz-Dieter; (Neustadt, DE) ; Steffen;
Markus; (Maikammer, DE) ; Ter Maat; Johan;
(Mannheim, DE) ; Wohlfromm; Hans; (Mannheim,
DE) |
Assignee: |
BASF SE
Ludwigshafen
DE
|
Family ID: |
42289418 |
Appl. No.: |
13/263135 |
Filed: |
April 1, 2010 |
PCT Filed: |
April 1, 2010 |
PCT NO: |
PCT/EP10/54400 |
371 Date: |
October 6, 2011 |
Current U.S.
Class: |
416/204A ;
419/5 |
Current CPC
Class: |
F01D 5/048 20130101;
F05D 2230/211 20130101; B22F 3/225 20130101; F05D 2230/23 20130101;
F02C 6/12 20130101; B22F 5/009 20130101; B22F 2001/0066
20130101 |
Class at
Publication: |
416/204.A ;
419/5 |
International
Class: |
F01D 5/02 20060101
F01D005/02; B22F 3/12 20060101 B22F003/12; B22F 5/10 20060101
B22F005/10 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 9, 2009 |
EP |
09157695.9 |
Claims
1-9. (canceled)
10. A process for producing a turbine wheel for an exhaust gas
turbocharger by metal powder injection molding, the process
comprising: a) providing a feedstock comprising (A) from 40 to 90%
by volume of a sinterable pulverent nickel-based alloy or of a
titanium-based alloy, (B) from 10 to 60% by volume of a mixture of
(B1) from 80 to 98% by weight of a polyethylene homopolymer or
copolymer, and (B2) from 2 to 20% by weight of a polyolefin or a
mixture of polyolefins as binder, and (C) from 0 to 5% by volume of
a dispersant; b) providing a tool which comprises negative mold of
the turbine wheel to be produced for metal powder injection molding
of the turbine wheel; c) introducing a rotationally symmetrical
core comprising a binder into the negative mold of the tool
provided in process step (b) and alignment of the core so that it
is aligned symmetrically about the axis of rotation of the turbine
wheel to be produced; d) producing a green body by metal powder
injection molding of the feedstock provided in process step (a)
around the core; e) removing the binder and at the same time
removing the core in order to obtain a molding in the shape of the
turbine wheel; and f) sintering the molding.
11. The process according to claim 10, wherein a core comprising
the binder which is comprised in the feedstock provided in process
step (a) is introduced in process step (c).
12. The process according to claim 10, wherein the molding is
mounted in at least one holding device in process step (f).
13. The process according to claim 10, wherein the core introduced
in process step (c) has a volume from 5 to 60% of the volume of the
turbine wheel.
14. The process according to claim 10, wherein process step (e) is
carried out at a temperature in the range from 10 to 180.degree.
C.
15. The process according to claim 10, wherein process step (f) is
carried out at a temperature in the range from 250 to 1500.degree.
C.
16. A turbine wheel for an exhaust gas turbocharger having a hollow
space structure which is symmetrical about the rotational axis of
the turbine wheel and has a volume of from 5 to 60% of the volume
of the turbine wheel, produced by a process according to claim 10.
Description
[0001] The invention relates to a process for producing a
reduced-weight turbine wheel for an exhaust gas turbocharger in
internal combustion engines by means of metal powder injection
molding.
[0002] A turbocharger for an internal combustion engine comprises
an exhaust gas turbine which is arranged in the exhaust gas stream
of the internal combustion engine and is connected by a shaft to a
compressor in the intake tract of the internal combustion engine.
The turbine is set into rotation by the exhaust gas stream of the
internal combustion engine and drives the compressor wheel. The
compressor wheel increases the pressure in the intake tract of the
engine so that a larger amount of air goes into the cylinder during
the intake phase than in the case of a suction engine. As a result,
more oxygen is available for the combustion of a correspondingly
greater amount of fuel. The turbine wheel of the "hot" side exposed
to the exhaust gas stream, which has a complex geometry, is usually
produced from a high-temperature-resistant material 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 means of a screw connection. During operation of the
turbocharger, extremely high rotational speeds of the shaft
together with the two wheels of up to about 300 000 rpm are
reached. To achieve a very rapid response of the turbocharger, the
inertia of the rotating parts should be very low.
[0003] One document which is concerned with reducing the weight of
turbocharger components is the Japanese first publication JP
2007-120409. This document discloses the decoring of a turbine
wheel and thus saving of material for reducing the weight. The
decored turbine wheel is produced by means of a precision casting
process. However, a disadvantage is that the precision casting
process described is complicated and expensive.
[0004] Metal powder injection molding (MIM) is known as a process
for the mass production of metallic components, in particular for
components of this type with close-to-final dimensions. The MIM
process allows small to medium-sized parts having a complex shape
to be produced inexpensively and automatically in large
numbers.
[0005] The MIM process comprises plasticization of metal powders
having a spherical or irregular morphology (particle sizes of the
powder are generally less than 100 .mu.m) by means of a binder to
give a feedstock. Homogenization of the feedstock is carried out in
a kneader and the feedstock is subsequently introduced into an
injection molding machine. Parts of the binder (for example
suitable waxes) are melted in a heated zone to give a melt. A screw
then conveys the melt into a dividable too mold. After filling of
the mold is complete, the melt resolidifies and makes it possible
for the component to be taken from the mold. Removal of the binder
is effected by means of a binder removal step preceding sintering.
Depending on the binder, the binders are removed from the component
in various ways.
[0006] In the case of binder removal, a distinction is generally
made between thermal binder removal (melting-out or decomposition
of the binder via the gas phase), solvent extraction and catalytic
binder removal. The binder removal step is followed by the
sintering process in which densification of the component to over
95%, preferably even over 98%, of the theoretical density is
achieved by means of diffusion processes.
[0007] The use of metal powder injection molding in the production
of exhaust gas turbocharger components has hitherto failed because
the process was not sufficiently economical in order to displace
the conventional production process of precision casting.
[0008] It is an object of the invention to provide a novel
economical process for producing a turbine wheel for an exhaust gas
turbocharger of an internal combustion engine, by means of which
reduced-weight turbine wheels for exhaust gas turbochargers can be
produced in a simple way.
[0009] This object is achieved by a process for producing a turbine
wheel for an exhaust gas turbocharger by metal powder injection
molding, which comprises the following steps: [0010] a) provision
of a feedstock comprising a metal powder and a binder, [0011] b)
provision of a tool which comprises a negative mold of the turbine
wheel to be produced for metal powder injection molding of the
turbine wheel, [0012] c) introduction of a rotationally symmetrical
core comprising a binder into the negative mold of the tool
provided in process step (b) and alignment of the core so that it
is aligned symmetrically about the axis of rotation of the turbine
wheel to be produced, [0013] d) production of a green body by metal
powder injection molding of the feedstock provided in process step
(a) around the core, [0014] e) carrying out of a binder removal
step to remove the binder from the green body and at the same time
to remove the core in order to obtain a molding in the form of the
turbine wheel, and [0015] f) sintering of the molding.
[0016] The process of the invention makes it possible to produce
turbine wheels for exhaust gas turbochargers, which have a hollow
space of defined internal structure, simply and inexpensively. The
internal structure which defines a hollow space is formed by
removal of the core in the binder removal step carried out in
process step (e). This makes it economical to produce turbine
wheels, which have hitherto been manufactured as solid components
by means of precision casting, as hollow components in large
numbers, as a result of which a significant weight reduction can be
achieved in the turbocharger. This weight reduction leads to a more
rapid response associated with a lower fuel requirement and an
increase in the efficiency of the internal combustion engine and
also a considerable saving of material. Furthermore, the process of
the invention makes it possible, in contrast to the precision
casting process known in the prior art, to produce turbine wheels
for exhaust gas turbochargers in a particularly fine design with
wall thicknesses in the range from 0.1 to 1 mm.
[0017] For the purposes of the invention, the term "feedstock"
generally refers to a composition which comprises 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, for the
purposes of the invention, to a pulverulent metal or a pulverulent
metal alloy or a mixture thereof. As metals which can be comprised
in powder form in the feedstock, mention may be made by way of
example of iron, cobalt, nickel, chromium, titanium, molybdenum,
niobium and aluminum; alloys are, for example, nickel-based alloys
or titanium-based alloys. The alloys are preferably nickel-based
alloys which can be obtained, for example, under the trade name
Inconel.RTM. 713; these comprise 74% by weight of nickel, 12.5% by
weight of chromium, 4.2% by weight of molybdenum, 2% by weight of
niobium, 6% by weight of aluminum, 0.8% by weight of titanium and
0.12% by weight of carbon. Preference is likewise given in the case
of the nickel-based alloy to an alloy which can be obtained under
the trade name Inconel.RTM. 718. This base alloy comprises from 50
to 55% by weight of nickel, from 17 to 21% by weight of chromium,
<24% by weight of iron, from 2.8 to 3.3% by weight of
molybdenum, from 4.8 to 5.5% by weight of niobium, from 0.2 to 0.8%
by weight of aluminum, from 0.7 to 1.1% by weight of titanium and
less than 0.08% by weight of carbon. Preference is likewise given
in the case of the nickel-based alloy to NIMON IC.RTM. 90. NIMONIC
90 comprises less than 0.13% by weight of carbon, from 2 to 3% by
weight of titanium, from 1 to 2% by weight of aluminum, less than
1.5% by weight of iron, from 15 to 21% by weight of cobalt, from 18
to 21% by weight of chromium, with the balance being nickel. The
nickel-based alloy is more preferably HASTELLOY.RTM. X.
HASTELLOY.RTM. X is an alloy comprising from 0.05 to 0.15% by
weight of carbon, less than 0.5% by weight of aluminum, from 0.5 to
2.5% by weight of cobalt, from 8 to 10% by weight of molybdenum,
from 17 to 20% by weight of iron, from 20 to 23% by weight of
chromium and nickel as balance. A further suitable alloy is an
alloy which comprises about 15% by weight of chromium, about 10% by
weight of iron, 5% by weight of molybdenum, 2% by weight of
titanium, niobium and nickel. The proportion of metal powder in the
feedstock can vary over a wide range and is usually from 40 to 70%
by volume, preferably from 45 to 60% by volume, based on the
feedstock.
[0018] Herein suitable "binders" are in principle all systems which
are known from the prior art and are suitable for use in metal
powder injection molding. The proportion of binder in the feedstock
can vary over a wide range and is usually from 10 to 60% by volume,
preferably from 30 to 50% by volume, based on the feedstock.
Suitable binders are thermoplastic resins in general, e.g.
polystyrene, polypropylene, polyethylene and ethylene-vinyl acetate
copolymers. Such binders can be removed from the green body by, for
example, heating to temperatures of from 300 to 500.degree. C. over
a period of from 3 to 8 hours. Here, the binder is thermally
dissociated. Further suitable binders are those which can be
removed from the green body by extraction with a solvent. Binders
based on polyoxymethylene are likewise suitable and are removed by
treatment of the green body in a gaseous, acid-comprising
atmosphere. Acids used in these processes are usually protic acids,
i.e. acids which on reaction with water are dissociated into a
proton (hydrated) and an anion.
[0019] In a preferred embodiment of the invention, the feedstock
comprises [0020] A) from 40 to 90% by volume of a sinterable
pulverulent metal or a pulverulent metal alloy or a mixture
thereof, [0021] B) from 10 to 60% by volume of a mixture of [0022]
B1) from 80 to 98% by weight, in particular from 85 to 98% by
weight, based on B), of a polyoxymethylene homopolymer or copolymer
and [0023] B2) from 2 to 20% by weight, in particular from 2 to 15%
by weight, of a polyolefin or a mixture of polyolefins.
[0024] The polyoxymethylene homopolymers or copolymers are known
per se to those skilled in the art and are described in the
literature. The homopolymers are generally prepared by
polymerization of formaldehyde or trioxane, preferably in the
presence of suitable catalysts. Preferred polyoxymethylene
copolymers comprise, in addition to the repeating units
--OCH.sub.2--, up to 50 mol %, preferably from 0.1 to 20 mol % and
particularly preferably from 0.3 to 10 mol %, of repeating
units
##STR00001##
where R.sup.1 to R.sup.4 are each, independently of one another, a
hydrogen atom, a C.sub.1-C.sub.4-alkyl group or a
halogen-substituted alkyl group having from 1 to 4 carbon atoms and
R.sup.5 is a CH.sub.2--, --CH.sub.2--O--, a methylene group
substituted by C.sub.1-C.sub.4-alkyl or C.sub.1-C.sub.4-haloalkyl
or a corresponding oxymethylene group and n is in the range from 0
to 3. These groups can advantageously be introduced into the
copolymers by ring opening of cyclic ethers. Preferred cyclic
ethers are those of the formula (II)
##STR00002##
where R.sup.1 to R.sup.5 and n are as defined above. Purely by way
of example, mention may be made of ethylene oxide, 1,2-propylene
oxide, 1,2-butylene oxide, 1,3-butylene oxide, 1,3-dioxane,
1,3-dioxolane and dioxepane as cyclic ethers and linear
oligoformals or polyformals such as polydioxolane or polydioxepane
as copolymers.
[0025] Further polymers suitable as component B1) are oxymethylene
terpolymers which are prepared, for example, by reaction of
trioxane, one of the cyclic ethers described above and a third
monomer, preferably a bifunctional compound of the formula
(III)
##STR00003##
where Z is a chemical bond, --O-- or --ORO--
(R.dbd.C.sub.1-C.sub.8-alkylene or C.sub.3-C.sub.8-cycloalkylene).
Preferred monomers of this type are ethylene diglycide, diglycidyl
ether and diethers of glycidyls and formaldehyde, dioxane or
trioxane in a molar ratio of 2:1 and also diethers derived from 2
mol of glycidyl compound and 1 mol of an aliphatic diol having from
2 to 8 carbon atoms, e.g. the diglycidyl ethers of ethylene glycol,
1,4-butanediol, 1,3-butanediol, cyclobutane-1,3-diol,
1,2-propanediol and cyclohexane-1,4-diol, to name only a few
examples.
[0026] Processes for preparing the above-described homopolymers and
copolymers are known to those skilled in the art and are described
in the literature, so that further details are superfluous here.
The preferred polyoxymethylene homopolymers and copolymers have
melting points of at least 150.degree. C. and molecular weights
(weight average) in the range from 5000 to 150 000, preferably from
7000 to 60 000.
[0027] Component B2) comprises polyolefins or mixtures thereof. As
polyolefins, mention may be made of those having from 2 to 8 carbon
atoms, in particular from 2 to 4 carbon atoms, and also copolymers
thereof. Particular preference is given to polyethylene and
polypropylene and copolymers thereof, as are known to those skilled
in the art and are commercially available, for example under the
trade names Lupolen.RTM. and Novolen.RTM. from BASF SE.
[0028] As component C), the binders used in the process of the
invention can comprise from 0 to 6% by volume, preferably from 1 to
5% by volume, of a dispersant. Mention may here be made, purely by
way of example, of oligomeric polyethylene oxide having an average
molecular weight of from 200 to 600, stearic acid, stearamide,
hydroxystearic acid, fatty alcohols, fatty alcohol sulfonates and
block copolymers of ethylene oxide and propylene oxide.
[0029] In addition, the binders can further comprise customary
additives and processing aids which favorably influence the
rheological properties of the mixtures during shaping.
[0030] The feedstock is usually produced by melting the component
B), preferably in a twin-screw extruder, at temperatures of
preferably from 150 to 220.degree. C., in particular from 170 to
200.degree. C. The metal powder A) is subsequently introduced in
the required amount into the melt stream of the binder (component
B)) at temperatures in the same range.
[0031] In process step (b) of the process of the invention, a tool
which comprises a negative mold of the turbine wheel to be produced
is provided. According to the invention, this is suitable for metal
powder injection molding of the turbine wheel. Such tools are known
to those skilled in the art and do not have to be described in more
detail at this point. In general, the tool is a tool which allows
cores to be withdrawn.
[0032] After provision of the tool which comprises the negative
mold of the turbine wheel to be produced, a rotationally
symmetrical core is introduced into the negative mold of the tool
in process step (c) of the process of the invention. The
rotationally symmetrical core is an auxiliary by means of which a
hollow space structure is introduced into the turbine wheel. This
core is, according to the invention, aligned in the negative mold
in such a way that it is present symmetrically about the axis of
rotation of the turbine wheel to be produced. "Symmetrically about
the axis of rotation" means that the core is arranged in the
negative mold in such a way that it can cause no imbalance in the
turbine wheel produced as a result of its alignment. The core
comprises the above-defined binder or consists of a binder as
defined above, as a result of which it is removed completely from
the turbine wheel during production of the turbine wheel after the
process of the invention has been carried out and leaves behind a
hollow space structure which is arranged symmetrically to the
rotation axis. According to a general embodiment of the invention,
the core is introduced on a suitable accommodation device into the
tool and held in position. The accommodation device can be, for
example, a pin or a rod onto which the core is positioned. The
binder or constituents of the binder of which the core is composed
can then diffuse out of the hollow space which the accommodation
device leaves behind in the green body during the binder removal
step.
[0033] In process step (d) of the process of the invention, the
feedstock provided in process step a) is injected into the negative
mold around the core to produce a green body. Conventional screw or
piston injection molding machines can be used for carrying out
injection molding in process step (d). The shaping of the feedstock
is generally carried out at temperatures of from 60 to 200.degree.
C. and injection pressures of from 300 to 2000 bar in tools which
have a temperature of from 60 to 150.degree. C. This produces a
green body which has the structure of the turbine wheel to be
produced and comprises the core made from the binder.
[0034] To remove the binder from the feedstock and to remove the
core from the green body, the binder removal step, viz. process
step (e), is carried out in order to obtain a molding having the
shape of the turbine wheel. The binder removal step is carried out
as a function of the binder selected. The progress of the binder
removal step can be monitored by a person skilled in the art by,
for example, determining the weight change of the green body. If
the preferred binder comprising the components A), B) and
optionally C) is used, the binder removal step is generally carried
out at temperatures in the range from 20 to 180.degree. C. for a
period of from 0.1 to 24 hours, preferably from 0.5 to 12 hours, in
a gaseous acid-comprising atmosphere. Suitable acids for the
treatment are inorganic acids which are gaseous at room temperature
or at least can be vaporized at the treatment temperature. Hydrogen
halides and HNO.sub.3 may be mentioned by way of example. Suitable
organic acids are those which have a boiling point of less than
130.degree. C. at atmospheric pressure, for example formic acid,
acetic acid or trifluoroacetic acid or mixtures thereof. Further
suitable acids are BF.sub.3 and its adducts with organic ethers.
The required treatment time depends quite generally on the
treatment temperature and the concentration of the acid in the
treatment atmosphere. If a carrier gas is used, this is generally
passed beforehand through the acid and loaded with the latter. The
loaded carrier gas is then brought to the treatment temperature
which is advantageously higher than the loading temperature in
order to avoid condensation of the acid. The acid is preferably
mixed into the carrier gas via a metering device and the mixture is
heated to such an extent that the acid can no longer condense.
[0035] The binder removal step can also be carried out, for
example, in two stages. The treatment in the first stage is carried
out until the polyoxymethylene component B1) of the binder has been
removed to an extent of at least 80% by weight, preferably at least
90% by weight. This can easily be recognized from the decrease in
weight of the green body. The molding obtained in this way is
subsequently heated at from 250 to 500.degree. C., preferably from
350 to 450.degree. C., for from 0.1 to 12 hours, preferably from
0.3 to 6 hours, in order to remove virtually all the remaining
binder.
[0036] The molding which has been freed of the binder in the binder
removal step can be converted in the usual way into a metallic
molding by sintering. During sintering, moldings are densified and
shrunk to form components having the final geometric properties.
During sintering, the molding accordingly becomes smaller, with the
dimensions having to shrink uniformly in all three directions in
space. The linear shrinkage is, depending on the binder content,
generally from 10% to 20%. Sintering can be carried out under
various protective gases or under reduced pressure. Process step
(f) is generally carried out at temperatures in the range from 250
to 1500.degree. C. The sintering time is generally in the range
from 1 to 12 hours, preferably in the range from 2 to 5 hours. In a
preferred embodiment of the invention, a holding device which
supports the molding during sintering in order to at least largely
prevent distortion of the component is used during sintering in
process step (f). In an embodiment of the invention, this holding
device is fastened in the form of a mandrel to the component. In a
particularly preferred embodiment of the invention, one or more
holding devices whose materials composition and wall thickness are
matched to the materials composition and wall thickness of the
turbine wheel to be produced are used during sintering. This
ensures that the molding to be sintered and the corresponding
holding device are densified and shrink to the same extent during
sintering. To avoid reaction or diffusion between component and
holding device during sintering and thus avoid sintering together
of component and holding device, one surface of the respective
holding device is coated at least in sections. The surface is
coated in at least those sections in which the holding device is in
contact with the molding to be sintered. The holding device can
also be coated on all sides. Of course, the coating used depends on
the material or materials composition of the moldings to be
sintered. The use of a ceramic coating or a coating of titanium
nitrite for the holding device is preferred.
[0037] In a preferred embodiment of the invention, the core
introduced in process step (c) comprises the same binder which is
comprised in the feedstock. This advantageously ensures that the
removal of the core and of the binder comprised in the feedstock
can be carried out in an identical process step.
[0038] The size and/or geometry of the rotationally symmetrical
core introduced in process step (c) can be varied over wide ranges.
In general, the size of the core is selected so that it has a
volume which is from about 5 to 60% of the volume of the turbine
wheel, preferably from 45 to 55% of the volume of the turbine
wheel. The incorporation of the core and the hollow space structure
resulting therefrom gives a turbine wheel which can be produced in
a simple manner and has a significantly reduced weight compared to
the turbine wheels known from the prior art. Furthermore, the
process of the invention allows turbine wheels having a lost core
to be produced.
[0039] The introduction of the core into the central region of the
turbine wheel which has considerable thicknesses and accumulations
of mass makes it possible to avoid voids and defects which usually
occur. The geometry of the core can be selected by a person skilled
in the art as a function of the geometry of the turbine wheel.
Cores which have a cone geometry, ball geometry (spherical
geometry), elliptical geometry, cylindrical geometry or very
generally a rotationally symmetrical geometry are generally
suitable. As core, it is also possible, in a preferred embodiment
of the invention, to select a core whose geometry approximately
reproduces the geometry of the turbine wheel, as a result of which
particularly weight-optimized turbine wheels whose wall thicknesses
are selected so that they withstand the forces acting on them
during operation can be obtained.
[0040] After production of the turbine wheel of the invention, the
latter is usually joined by friction welding or direct injection
molding to a shaft and subsequently balanced.
[0041] In one embodiment of the invention, the turbine wheel
obtained in process step (f) is connected by means of metal
injection molding to a shaft in a further process step (g).
[0042] The invention is illustrated by the figures and examples
below.
[0043] FIG. 1 shows a sectional view of the turbine wheel 1 for an
exhaust gas turbocharger for internal combustion engines.
[0044] The turbine wheel 1 for an exhaust gas turbocharger for
internal combustion engines which is shown in FIG. 1 has a hollow
space structure 2 which has been created by means of the process of
the invention. The hollow space structure is located in the center
of the turbine wheel and is symmetrical about the rotational axis
of the turbine wheel 1.
EXAMPLE
[0045] As feedstock, use was made of an injection-moldable
pelletized material for producing sintered moldings of a
heat-resistant nickel superalloy (DIN 2 4632), which is marketed by
BASF SE under the trade name Catamold.RTM. N90.
[0046] The feedstock was processed on an Engel ES80/10
thermoplastics injection molding machine.
[0047] The settings on the machine were as follows:
TABLE-US-00001 Barrel temperature Zone 1 Zone 2 Zone 3 Die
160.degree. C. 170.degree. C. 180.degree. C. 190.degree. C. Tool
surface temperature 128.degree. C. Rotational speed of screw 50
min.sup.-1 Injection rate 10 cm.sup.3/s Maximum injection pressure
2000 bar Hold time 3 s Banking-up pressure 0 bar Cooling time in
the tool 30 s
[0048] Before carrying out injection molding, a core comprising the
binder and having a volume of about 6 cm.sup.3 was introduced into
the negative mold of the tool.
[0049] Binder removal was carried out at 110.degree. C. in an
HNO.sub.3 atmosphere. A Heraeus VT 6060 MU2 binder removal oven
having a volume of 50 l and provided with acid introduction at 30
ml/h and a flushing gas flow (nitrogen) of 500 l/h was used for
this purpose. The binder removal process was complete after a
binder loss of 7.7%, based on the starting weight of the green
body, had been achieved.
[0050] Sintering was carried out under a 100% argon atmosphere. The
argon used was clean and dry (99.98%, dew point<-80.degree. C.).
The sintering cycle was as follows
Room temperature--5 K/min--60.degree. C., hold for 1 h, 600.degree.
C.--5 K/min--1325.degree. C., hold for 3 h, furnace cooling.
[0051] To achieve a very high density of the turbine wheel, the
component was held at a temperature of 1185.degree. C. for a period
of 4 hours at a pressure of 1000 bar.
[0052] To optimize the strength properties further, a two-step heat
treatment was carried out subsequently. In step 1, the turbine
wheel was heated under reduced pressure at 1080.degree. C. for a
period of 8 h under 900 mbar of argon. In step 2, the workpiece was
treated under reduced pressure at 705.degree. C. for 16 h under 900
mbar of argon.
[0053] This gave a turbine wheel which had a volume of 7.5 cm.sup.3
and was one third lighter than a solid turbine wheel.
* * * * *