U.S. patent application number 10/506416 was filed with the patent office on 2006-10-19 for method for producing alloy ingots.
Invention is credited to Matthias Blum, Anita Chatterjee, Helmut Clemens, Heinz Danker, Willy Furwitt, Rainer Gerling, Volker Guther, Georg Jarczyk, Friedhelm Sasse, Frank-Peter Schimansky.
Application Number | 20060230876 10/506416 |
Document ID | / |
Family ID | 7705996 |
Filed Date | 2006-10-19 |
United States Patent
Application |
20060230876 |
Kind Code |
A1 |
Blum; Matthias ; et
al. |
October 19, 2006 |
Method for producing alloy ingots
Abstract
The invention relates to a method of producing metallic and
intermetallic alloy ingots by continuous or quasi-continuous billet
withdrawal from a cold wall crucible, which is characterized in
that the alloy material is continuously or quasi-continuously
supplied in a molten and pre-homogenized state to a cold wall
induction crucible.
Inventors: |
Blum; Matthias; (Budingen,
DE) ; Jarczyk; Georg; (Grosskrotzenburg, DE) ;
Chatterjee; Anita; (Nurnberg, DE) ; Furwitt;
Willy; (Furth, DE) ; Guther; Volker;
(Burgthann, DE) ; Clemens; Helmut; (Leoben,
AT) ; Danker; Heinz; (Geesthacht, DE) ;
Gerling; Rainer; (Reinbek, DE) ; Sasse;
Friedhelm; (Handorf, DE) ; Schimansky;
Frank-Peter; (Geesthacht, DE) |
Correspondence
Address: |
MCGLEW & TUTTLE, PC
P.O. BOX 9227
SCARBOROUGH STATION
SCARBOROUGH
NY
10510-9227
US
|
Family ID: |
7705996 |
Appl. No.: |
10/506416 |
Filed: |
November 13, 2002 |
PCT Filed: |
November 13, 2002 |
PCT NO: |
PCT/EP02/12668 |
371 Date: |
January 27, 2006 |
Current U.S.
Class: |
75/10.18 ;
148/421; 75/10.21 |
Current CPC
Class: |
B22D 23/10 20130101;
B22D 7/00 20130101 |
Class at
Publication: |
075/010.18 ;
148/421; 075/010.21 |
International
Class: |
C22C 14/00 20060101
C22C014/00; C22B 4/06 20060101 C22B004/06 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 16, 2001 |
DE |
101 56 336.1 |
Claims
1. A method of producing metallic and intermetallic alloy ingots by
continuous or quasi-continuous billet withdrawal from a cold wall
induction crucible, wherein the alloy material is supplied in a
molten and pre-homogenized state continuously or quasi-continuously
to a cold wall induction crucible.
2. A method according to claim 1, wherein inter-metallic
.gamma.-TiAl-based alloy ingots are produced.
3. A method according to claim 1, wherein the alloys are described
by the following summation formula:
Ti.sub.xAl.sub.y(Cr,Mn,V).sub.u(Zr,Cu,Nb,Ta,Mo,W,Ni).sub.v(Si,B,C,Y).sub.-
w with the concentrations of the alloying constituents being within
the following ranges (in atomic percent): x=100-y-u-v-w y=40 to 48,
preferably 44 to 48 u=0.5 to 5 v=0.1 to 10 and w=0.05 to 1.
4. A method of producing metallic and intermetallic alloy ingots of
high homogeneity and low porosity of any adjustable diameter
according to claim 1, comprising the following method steps: (i)
producing electrodes by customarily mixing and compressing the
selected starting materials; (ii) at least once remelting the
electrodes obtained in step (i) in a conventional
fusion-metallurgical process; (iii) inductively melting off the
electrodes obtained in steps (i) and (ii) in a high frequency coil;
(iv) homogenizing the pre-homogenized, molten material obtained in
step (iii) in a cold wall induction crucible; and (v) withdrawing
the melt, solidified by cooling, from the cold wall induction
crucible of step (iv) in the form of solidified ingots of freely
adjustable diameters and lengths.
5. A method according to claim 1, comprising the following method
steps: (i) producing electrodes by conventionally mixing and
compressing the selected starting materials; (ii) at least once
melting the electrodes obtained in step (i) by a conventional
fusion-metallurgical method; (iii) producing a pre-homogenized,
molten material of the electrode material obtained in step (ii) by
melting off in a cold crucible plasma furnace; (iv) homogenizing
the pre-homogenized, molten material obtained in step (iii) in a
cold wall induction crucible; and (v) withdrawing the melt,
solidified by cooling, from the cold wall induction crucible of
step (iv) in the form of cylindrical ingots of freely adjustable
diameters and lengths.
6. A method according to claim 1, wherein the melting process for
producing the pre-homogenized, molten material takes place in a
high frequency field of a frequency in the range of 70 to 300
kHz.
7. A method according to claim 1, wherein the temperature of the
pre-homogenized, molten material ranges between 1400 to
1600.degree. C.
8. A method according to claim 4, wherein the electrodes (iii) used
for producing the molten, pre-homogenized material by means of an
induction coil rotate preferably at a speed between 2 and 5
rpm.
9. A method according to claim 1, wherein the method is executed
quasi-continuously by one or several electrodes, in case of
inductive melting, being quasi-continuously fed while an ingot is
simultaneously withdrawn from the cold wall induction crucible.
10. A method according to claim 4, wherein homogenization in the
cold wall induction crucible in step (iv) takes place at a
temperature of 1400 to 1700.degree. C.
11. A method according to claim 4, wherein homogenization in the
cold wall induction crucible in step (iv) takes place in a range of
frequency of 4 to 20 kHz.
12. A method according to claim 4, wherein cooling the melt upon
ingot withdrawal in step (v) takes place by the aid of water-cooled
copper segments.
13. A method according to claim 4, wherein the diameter of the
ingots withdrawn in step (v) is in the range of 40 to 350 mm.
14. .gamma.-TiAl-based alloy ingots produced according to claim 1,
comprising (a) a length to diameter ratio of>12; (b) homogeneity
related to local macroscopic fluctuations of the aluminum and
titanium of maximally .+-.0.5 atomic percent; further metallic
alloying constituents of maximally .+-.0.2 atomic percent;
non-metallic alloying additions (boron, carbon, silicon) of
maximally .+-.0.05 atomic percent.
15. A method according to claim 5, wherein the electrodes (iii)
used for producing the molten, pre-homogenized material by means of
an induction coil rotate preferably at a speed between 2 and 5
rpm.
16. A method according to claim 5, wherein homogenization in the
cold wall induction crucible in step (iv) takes place at a
temperature of 1400 to 1700.degree. C.
17. A method according to claim 5, wherein homogenization in the
cold wall induction crucible in step (iv) takes place in a range of
frequency of 4 to 20 kHz.
18. A method according to claim 5, wherein cooling the melt upon
ingot withdrawal in step (v) takes place by the aid of water-cooled
copper segments.
19. A method according to claim 5, wherein the diameter of the
ingots withdrawn in step (v) is in the range of 40 to 350 mm.
Description
[0001] The invention relates to a novel fusion-metallurgical method
of producing, at a low cost, ingots of metallic and intermetallic
alloys of high chemical and structural homogeneity, in particular
ingots of .gamma.-TiAl.
[0002] In aeronautics and astronautics and in motor racing,
.gamma.-TiAl-based intermetallic alloys have proceeded from a
laboratory development stage to industrial application since 2000.
Advantageously combined high-temperature and low-weight properties
ensure their use in aeronautics and astronautics. High-temperature
and corrosion resistance recommend the material for use in rapidly
movable components of engines, for example valves in combustion
engines or blades in gas turbines. The properties of this material
depend on chemical and structural homogeneity to an extent not
known so far in structural materials. Consequently, manufacturing
ingots of correspondingly high quality is technically highly
complicated and costly. Homogeneous ingots are needed in various
process routes for the manufacture of further semi-finished
products or components of TiAl as starting material (see H. Clemens
and H. Kestler (2000), Advanced Engineering Materials 9, 551; Y.-W.
Kim (1994), JOM 46 (7), 30; and P. A. Bartolotta and D. L. Krause
(1999) in Gamma Titanium Aluminides, ed. Y.-W. Kim, D. M. Dimiduk
and M. H. Loretto, (TMS Warrendale, Pa., USA 1999), 3-10).
[0003] The .gamma.-TiAl-based technical alloys presently used are
of multiphase structure, containing the ordered hexagonal
.alpha..sub.2Ti.sub.3Al, typically at a proportion of 5 to 15
volume percent, in addition to ordered tetragonal .gamma.-TiAl as a
main phase. Refractory metals as alloying elements can lead to the
formation of a metastable body-centered cubic (bcc) phase which
appears either as a .beta. phase (disordered) or as a B2-phase
(ordered). These alloying additions improve oxidation resistance
and creep strength. Inferior quantities of Si, B and C serve for
increased strength of the cast structure (see B. Inkson and H.
Clemens (1999), MRS Symp. Proc. 552, KK3.12; S. Huang, E. Hall, D.
Shuh (1991), ISIJ Internatioal 31 (10), 1100 and Y.-W. Kim and D.
M. Dimiduk (1991), JOM 8, 40). Corresponding C contents can lead to
precipitation hardening (see V. Guther, A. Otto, H. Kestler and H.
Clemens, (1999) in Gamma Titanium Aluminides, ed. Y.-W. Kim, D. M.
Dimiduk and M. H. Loretto, (TMS Warrendale, Pa., USA 1999),
225-230). The alloying elements Cr, Mn and V increase room
temperature ductility of the otherwise very brittle TiAl. Depending
on the field of application, alloy development has led to a number
of alloy variants which will be described in detail below.
[0004] TiAl alloys are customarily produced as ingots by multiple
remelting in a vacuum-arc furnace (see FIG. 1) (VAR--vacuum arc
remelting). A pressed electrode, which includes all alloying
constituents, is melted off, expanding in diameter. Fundamental
problems result from occurring inhomogeneities in the alloy
composition of the .gamma.-TiAl ingot. A comparison of the Al
contents in twice or triple remelted .gamma.-TiAl ingot material
reveals that local fluctuation of the Al contents in the range of
.+-.2 atomic percent are still observed in twice remelted
.gamma.-TiAl ingot (see FIG. 2). Triple remelting in the VAR mill
is necessary for obtaining sufficient alloy homogeneity (see V.
Guther, A. Otto, H. Kestler and H. Clemens, (1999) in Gamma
Titanium Aluminides, edl Y.-W. Kim, D. M. Dimiduk and M. H.
Lorettok, (TMS Warrendale, Pa., USA 1999), 225-230; V. Guther,
Properties, processing and applications of .gamma.-TiAl, Proc.
9.sup.th Ti World Conference, O8 Nov. 06, 1999, St. Petersburg and
V. Guther, H. Kestler, H. Clemens and R. Gerling, Recent
Improvements in .gamma.-TiAl Ingot Metallurgy, Proc. Of the Aeromat
2000 Conference and Exhibition, (Seattle, Wash., June 2000).
[0005] Contrary to titanium alloys (ingot diameter up to 1.5 m),
workable diameters in the case of .gamma.-TiAl are distinctly
restricted by reason of limited formability. Presently, there is a
predominant demand on the market for ingots of only approximately
200 mm of diameter.
[0006] By using VAR technology, there is an increase in diameter of
approximately 40 mm per remelting job. Aiming at a final diameter
of approximately 200 mm, this means that the process must proceed
from pressed electrodes of about 60 mm of diameter maximally, the
porosity of which is in the range of 40 percent. The small diameter
limits the strength of the pressed electrode, and thus the possibly
employable length, to approximately 1.5 m (which corresponds to a
total mass of approximately 18 kg). The smaller the diameters of
the first pressed electrode, the higher the cost of manufacture,
because less material can be melted per melting cycle. A triple
remelted VAR ingot of a diameter of 180 mm and a length of 1000
mm--corresponding to prior art industrial embodiment--will need an
overall of 10 melting jobs (6 initial melting jobs, 3 second
melting jobs, 1 third melding job), which causes some costs. The
loss of material (piping etc.) per ingot is presently 35 percent.
Moreover, the conventional manufacturing method does not offer any
flexibility in the choice of ingot diameter.
[0007] Alternatives of manufacturing titanium alloy ingots are cold
hearth electron-beam melting and plasma arc cold hearth melting
(PACHM). While electron-beam melting (see FIG. 3 top) has been used
solely for pure unalloyed titanium, the PACHM method (see FIG. 3
bottom) is used for the production of titanium alloys and also of
.gamma.-TiAl ingots. In this case the starting material is melted
in a cold crucible by a plasma torch and the liquid melt is
supplied via a plasma-torch heated channel system to an equally
plasma heated billet discharge. This method has led to insufficient
alloy homogeneity, which may be due to the limits of the method
(see W. Porter, Proceedings of 3.sup.rd Int. Symp. Structural
Intermetallics, ed. K. J. Hemker et al., TMS Warrendale 2001, page
201). Even the addition of an induction coil for improved
homogenization of the melt in the plasma heated billet discharge
did not show the desired results (see M. Loretto, Titanium 95,
Science and Technologies; A. L. Dowson et al., in Gamma Titanium
Aluminides (1995), ed. Y.-W. Kim, R. Wagner and M. Yamaguchi (TMS
Warrendate, Pa., USA 1995), 467-474; M. Volas, Industrial
Initiatives in Wrought Orthorhombic and Gamma TiAl Mill Products;
Proc. Of the Aeromat 2000 Conference and Exhibition, Seattle,
Wash., June 2000).
[0008] Furthermore, the production of .gamma.-TiAl-based alloys by
means of ingot casting from a cold wall induction or plasma
furnace, or by means of inert gas spraying from a cold wall
crucible, to .gamma.-TiAl powder and powder metallurgical
processing has been put into practice technologically. These
alternatives have so far resulted in insufficient microstructure
(porosity upon ingot casting) and excessive costs (powder
metallurgy).
[0009] Reference is made to U.S. Pat. Nos. 5,846,351, 5,823,243,
5,746,846 and 5,492,574, standing in for the state of the art in
VAR technology.
[0010] It is an object of the present invention to embody a method
of reproduceably producing .gamma.-TiAl ingots of high chemical
homogeneity and little porosity, which can be put into practice
more easily and at a lower cost than the VAR method specified above
which needs numerous steps of melting for obtaining the desired
high homogeneity and inferior porosity. Moreover, the method is
intended to offer the possibility of arbitrarily dimensioning the
alloy ingots within a range of what is technically reasonable by
elusion of the VAR-method restrictions specified above.
[0011] This object is attained in a method of producing metallic
and intermetallic alloy ingots by continuous and quasi-continuous
billet discharge from a cold wall induction crucible with the alloy
material, in a molten or pre-homogenized state, being continuously
or quasi-continuously supplied to a cold wall induction crucible
(see FIG. 4). The method of continuous casting for the production
of metallic and intermetallic alloy ingots of high homogeneity and
inferior porosity is characterized by the following chronological
steps: [0012] (i) producing electrodes by customarily mixing and
compressing the selected starting materials; [0013] (ii) at least
once remelting the electrodes obtained in step (i) by a
conventional fusion-metallurgical process; [0014] (iii) inductively
melting off the electrodes obtained in steps (i) and (ii) in a high
frequency coil; [0015] (iv) homogenizing the melt obtained in step
(iii) in a cold wall induction crucible; and [0016] (v) withdrawing
the melt by cooling from the cold wall induction crucible of step
(iv) in the form of solidified ingots of freely adjustable
dimensions.
[0017] By alternative, the following sequence in the continuous
casting method for the production of metallic and intermetallic
alloy ingots of high homogeneity and inferior porosity can be put
into practice (see FIG. 5): [0018] (i) producing electrodes by
conventionally mixing and compressing the selected starting
materials; [0019] (ii) at least once remelting the electrodes
obtained in step (i) by a conventional fusion-metallurgical method;
[0020] (iii) producing a pre-homogenized, molten material from the
electrode material obtained in step (ii) by melt-off in a cold
crucible plasma furnace; [0021] (iv) homogenizing the melt obtained
in step (iii) in a cold wall induction crucible; and [0022] (v)
withdrawing the melt, solidified by cooling, from the cold wall
induction crucible of step (iv) in the form of cylindrical ingots
of freely adjustable diameters and lengths.
[0023] The method is preferably used for the production of
intermetallic .gamma.-TiAl based alloy ingots, the alloys being
generally specified by the following summation formula:
Ti.sub.xAl.sub.y(Cr,Mn,V).sub.u(Zr,Cu,Nb,Ta,Mo,W,Ni).sub.v(Si,B,C,Y).sub.-
w
[0024] The concentrations of the alloying constituents are
customarily within the following ranges (in atomic percent): [0025]
x=100-y-u-v-w [0026] y=40 to 48, preferably 44 to 48 [0027] u=0.5
to 5 [0028] v=0.1 to 10 and [0029] w=0.05 to 1.
[0030] Inductive melting of the electrodes in step (iii) takes
place in a high frequency field of a frequency of preferably 70 to
300 kHz, in particular 70 to 200 kHz, and preferably at
temperatures of 1400.degree. C. to 1700.degree. C., in particular
1400.degree. C. to 1600.degree. C. For uniform dropping to be
obtained, the electrode is rotated, preferably at a speed of 4 rpm.
The withdrawal speed of the electrode is continuously variable from
0 to 200 mm/min.
[0031] In the case of inductive melting, the method is preferably
performed quasicontinuously, by one or several electrodes being fed
quasi-continuously while an ingot is simultaneously withdrawn from
the cold wall induction crucible.
[0032] Homogenization of the melt in the cold wall induction
crucible of step (iv) takes place preferably by overheating at 10
to 100 K, preferably 40 to 60 K. This corresponds to temperatures
of 1400.degree. C. to 1750.degree. C., preferably 1450.degree. C.
to 1700.degree. C., depending on alloy composition. The frequency
range of the coil is 4 to 20 kHz, preferably 4 to 12 kHz.
[0033] Cooling the melt upon withdrawal of the ingots in step (v)
preferably takes place by the aid of water-cooled copper segments,
the diameters of the ingots preferably being in a range of 40 to
350 mm, by special preference 140 to 220 mm.
[0034] The withdrawal rates are adjustable between 5 to 10 mm/min.
The withdrawal rate must be adapted to the dropping rate (step iii)
which can be in the range of 50 kg/h.
[0035] The present method according to the invention enables novel
intermetallic .gamma.-TiAl-based alloy ingots to be produced which
excel by a novel combination of dimensions on the one hand and
homogeneity on the other. Therefore, the invention also relates to
intermetallic .gamma.-TiAl-based alloy ingots which are
characterized by [0036] (a) a length to diameter ratio of >12;
[0037] (b) homogeneity related to local fluctuations of the
aluminum and titanium of <.+-.0.5 atomic percent; further
metallic alloying constituents: .+-.0.2 atomic percent;
non-metallic alloying additions (boron, carbon, silicon): .+-.0.05
atomic percent.
[0038] The gist of the method according to the invention resides in
the continuous or quasi-continuous supply of a pre-homogenized melt
of the alloying material to a cold wall induction crucible (KIT).
Within the scope of the present invention, it has surprisingly been
found that melting off the electrode material that serves for the
production of metallic and intermetallic alloy ingots occasions
considerable homogenization of the material so that a single
subsequent step of homogenization in the cold wall induction
crucible will do to obtain, by means of these two steps, as far as
possible a degree of homogenization, which can be accomplished only
by a comparatively great number of remelting steps in the VAR
method. Consequently, the method according to the invention is
substantially less complicated and costly than the VAR method used
so far.
[0039] The KIT loses its principal prior art function, namely
melting material that is always supplied to the KIT in a solid
state. An essential advantage of the method according to the
invention resides in that segregation phenomena as a reason for
inhomogeneities of the final material, which are always observed
when solid alloys of multiphase structure are melted in the KIT, do
not occur because the material arrives in a liquid state in the
KIT.
[0040] Another advantage resides in that the frequency range of the
induction coil, which is favorable for homogenization of the molten
alloy, exceeds the frequency range that is favorable for melting a
solid alloy. Surprisingly, this helps considerably reduce surface
porosity of the ingot withdrawn from the solidifying melt in the
KIT, improving ingot quality.
[0041] A special advantage of the method according to the invention
resides in that any required dimensions of the alloy ingot can be
put into practice by the dimensions of the cold wall induction
crucible being freely selectable within a technologically
reasonable scope, which is not ensured by VAR technology.
[0042] Vacuum or protective-gas execution of the method is
preferred, and nonpolluted production waste can be returned to the
process. In accordance with the invention, material loss amounts to
12 percent as compared to 35 percent in conventional VAR
technology.
[0043] The method according to the invention enables local
(macroscopic) fluctuations of the main alloying elements, aluminum
and titanium, of <.+-.0.5 atomic percent to be put into practice
throughout the ingot; further metallic alloying constituents:
.+-.0.2 atomic percent; strength increasing elements (boron,
carbon, silicon): .+-.0.05 atomic percent.
[0044] The scope of the invention also comprises novel combinations
of prior art sub-processes, known per se, which ensure a continuous
or quasi continuous supply of liquid, pre-homogenized material to a
cold wall induction crucible with the aim of continuous or quasi
continuous billet withdrawal from the KIT.
[0045] This relates in particular to the combination of an
inductively heated melt-off device for alloy rods and alloy
electrodes (inductive drop melting), a KIT with a billet withdrawal
equipment, and the combination of a plasma cold wall furnace with a
heated channel system, of an overflow in the form of a skull,
comprising said KIT and said billet withdrawal equipment. Both
combinations of methods according to the invention will be
described in detail below, taken in conjunction with exemplary
embodiments.
[0046] Important steps of these combinations of methods according
to the invention, such as inductive melting of electrodes, the
PACHM method, the fusion of alloys in a cold wall induction
crucible, and billet withdrawal of alloys from ceramic as well as
cold wall induction crucibles, have been known and employed,
accompanied with distinctly varying boundary conditions, aims and
materials.
[0047] The inductive fusion of metals has been described for
example in U.S. Pat. Nos. 4,923,508, 5,003,551 and 5,014,769.
Moreover, inductively melting off electrodes has also been
described in connection with the manufacture of titanium alloyed
powder by the so-called EIGA (electrode induction melting gas
atomization) method (cf. DE-A-41 02 101, DE-A-196 31 582). In this
method, an alloy electrode dips into an HF coil which is insulated
by ceramics against arc-over. As in the present case, the electrode
is completely melted by a surface melting process. Further
treatment of the melt takes place in a gas jet where the drops are
atomized. This method serves exclusively for the production of
powder and not for the production of ingots. In the present
description, the melt is subjected to further homogenization in a
KIT prior to billet withdrawal (production of ingots).
[0048] As regards any prior art concerned with the fusion of
materials in a cold wall induction crucible, reference is made to
U.S. Pat. Nos. 5,892,790 and 6,144,690. Neither patent deals with
the production of ingots, which is completely different with
patents DE-A-198 52 747 and DE-A-196 50 856. The decisive
difference between DE-A-198 52 747 and DE-A-196 50 856 and the
present invention resides in the supply of material. While
pre-homogenized, molten material is supplied to the KIT in the
present case, solid material is fed to the KIT in the mentioned
patent. This means that, in the present case, energy input into the
KIT serves exclusively for further homogenization and keeping the
material liquid whereas, in the patent specified, melting,
homogenizing and solidifying occur at the same place -the KIT. This
increases the probability of segregation.
[0049] Ingot withdrawal is also known from the state of the art, in
particular withdrawal from the ceramic crucible. The prior art
patents predominantly relate to ingot withdrawal of non-ferrous
metals (Cu, brass). The above-mentioned patents DE-A-198 52 747 and
DE-A-196 50 856 however comprise ingot withdrawal from the cold
wall induction crucible, with the material being fed in solid form,
and not as pre-homogenized, molten material, to the KIT from which
the ingot is withdrawn. This can lead to differences in
homogeneity--as described above--in the material that is withdrawn
as an ingot.
[0050] Electrode production takes place preferably by pressing
and/or sintering powdery or granulated alloying constituents (cf.
DE-A-196 31 582 to -584, DE-A-198 52 747).
[0051] In the drawings,
[0052] FIG. 1 is an illustration of the VAR process for multiply
remelted .gamma.-TiAl ingots: (1) electrode feed, (2) furnace
chamber, (3) air-cooled current supply, (4) cable collecting duct,
(5) electrode guide, (6) water-jacket crucible, (7) part of the
vacuum arrangement, (8) XY_adaption, (9) pressure pick-up;
[0053] FIG. 2 is an illustration of deviations of Al contents in
the longitudinal direction of the ingot after double (black
symbols) and triple (gray symbols) VAR remelting;
[0054] FIG. 3 is a diagrammatic view of cold wall electron beam
melting (top) and cold wall plasma melting (bottom);
[0055] FIG. 4 is an illustration of the method according to the
invention (Example 1) for the fabrication of chemically homogeneous
.gamma.-TiAl ingots of variable dimensions: (1) rotating electrode,
(2) inductive HF coil, (3) cold wall induction crucible, and (4)
cooling arrangement and ingot withdrawal;
[0056] FIG. 5 is an illustration of the method according to the
invention (Example 2) for the fabrication of chemically homogeneous
.gamma.-TiAl ingots of variable dimensions: (1) charging slope, (2)
plasma torch, (3) cold hearth, (4) cold wall induction crucible
(KIT), and (5) cooling arrangement, and (6) ingot withdrawal.
[0057] In conclusion, the method according to the invention deals
with fusion-metallurgical technology for the production of
chemically and structurally homogeneous alloy ingots, in particular
of .gamma.-TiAl ingots as ingot material for the molding route or
remelter stocks for the casting route. The technology comprises a
combination of: [0058] the production of pre-homogenized, molten
material by the aid of inductive melting in an HF coil or of the
PACHM method. In both cases, the starting material comprises the
sum of all alloying constituents which are however only
insufficiently homogeneously distributed; [0059] the supply of
molten material to a cold wall induction crucible; [0060] the
further homogenization of the liquid (melted) material in the cold
wall induction crucible (KIT); and [0061] the preferably continuous
withdrawal from the KIT.
[0062] The individual steps of the method will be described in
detail in the following.
[0063] Producing the electrodes takes place first. By the aid of a
conventional fusion-metallurgical method, for example by VAR
technology, pressed electrodes, which include all alloying
constituents (Ti sponge, Al granules, pre-alloying granules), are
melted off by enlargement of diameter, forming rods of a diameter
of for instance 150 mm. These are rods of low chemical homogeneity
and of a certain porosity. They serve as electrodes for the ensuing
billet withdrawal.
[0064] The first technological step can be illustrated in two
alternative ways--by inductive melting or the PACHM method. Bother
methods aim at the production of pre-homogenized, molten
material.
[0065] In the case of inductive melting, the electrode, which has
been melted off by a customary method, is inductively melted by the
aid of an HF coil (according to the EIGA method, see DE-A-41 02
101, DE-A-196 31 582) in a KIT. The coil/drop-material system and
the shape of the coil interact closely. In accordance with minimum
demands on melting rates and ingot diameters, the
outer-oscillating-circuit frequency range amounts to 70 to 300 kHz.
When high-frequency induction fields are employed, a pronounced
skin effect must be expected to occur in the melting electrode. In
combination with the comparatively low heat conductivity of
titanium aluminides, this effect leads to local overheating in the
boundary layer and, subsequently, to aluminum evaporization that
cannot be sensed quantitavely. Since the pronounced current flow in
the skin layer is an essential feature of high-frequency
alternating current fields and can therefore not be avoided, the
only possibility of reducing aluminum evaporization resides in
reducing the dwell time of the material in the electromagnetic
field. Uniform pre-heating of the dropping electrode by means of
inductive heating (average frequency of approximately 500 Hz to 4
kHz) to temperatures below the melting point of the alloy will
reduce the energy and capacity that is required in the field for
melting by the amount of energy already inputted. This either
reduces the dwell time in the a.c. field of a single volume element
and in total of the entire dropping electrode, as a result of which
the melting performance increases, or lower overall capacities of
the HF coil will be needed. The requirements and consequences
explained show that design and dimensioning of the outer
oscillating circuit and the HF frequency make sense only in close
interaction with the designed electrode pre-heating capacity.
Electrode feed rates must be adjustable within a scope that will
allow dropping rates corresponding to mass flow rates of at least
50 kg/h in the case of electrode diameters of 150 mm.
[0066] In the case of the PACHM method, the melting process is put
into practice by plasma torches, which have two functions: melting
the starting material and keeping constant ambient conditions
during ingot discharge. Starting material in the form of
mechanically comminuted pre-alloyed compacts is charged
successively via a hydraulic platform into the melting chamber.
Finally, the material is melted by the aid of the plasma torches in
the water-cooled cold wall crucible of copper. The cold wall
crucible (cold hearth) serves as an instrument for the elimination
of undesired high-density (furnace bottom) and low-density
inclusions (floating slag) of the melt and as a reservoir for the
supply with molten material of the crucible/ingot-withdrawal
system. The amperages of the plasma torches above the cold hearth
range between 275 to 550 A, but may vary depending on the type and
number of plasma torches used.
[0067] In the ensuing step, the melt is fed to the cold wall
induction crucible. In the KIT, which is equipped with a movable
bottom, the homogeneity of the melt in a greater molten volume that
is largely kept constant is further improved by the agitating
effect of the electromagnetic field. The dwell time of the melt in
the crucible amounts to approximately 20 min to 45 min. Skull
melting in the cold wall induction crucible (KIT) is a technology
that has become industrially established for years. Electromagnetic
induction in a water-cooled copper crucible produces a field that
is used for heating and melting the materials. Simultaneously, the
occurring Lorentz forces partially squeeze the melt off the
crucible walls, establishing in the melt a circulating flow that
will consequently lead to excellent mixing of the melting phase. In
the vicinity of the crucible bottom and in the bottom part of the
crucible wall, a specific, solid skull will develop, conditioned by
the form of the electromagnetic field. In combination with the free
surface produced by the Lorentz forces, this skull prevents any
direct contact of the melt with the crucible, eliminating any risk
of contamination throughout the melting phase and ensuring mill
safety.
[0068] In the case of inductive melting, continuous supply of the
KIT with melt is ensured by the connected electrode depot which can
take up several electrodes at a time, which are then successively
melted off. In the case of the PACHM method re-charging of
mechanically comminuted, pre-alloyed material takes place by way of
a hydraulic platform.
[0069] The bottom skull, the thickness and habit of which depend
directly on the form of the induction field, offers a point of
departure for the possibility of semi production. If the bottom is
lowered in the course of the process, the system reacts in such a
way that a renewed state of equilibrium tends to form by another
layer growing on the previous bottom skull. Continuously lowering
the bottom will lead to a system of steadily adapting states of
equilibrium and, consequently, to an almost continuously growing
bottom layer. Since the base of the bottom skull is defined by the
bottom of the crucible, the growing of further layers will result
in a semi-finished product (ingot) originating. However, the steady
output of mass from the KIT also requires a supply of further
molten material.
[0070] Cooling the melt upon ingot withdrawal preferably takes
place by the aid of water-cooled Cu segments.
[0071] Ingot withdrawal from the KIT produces a chemically
homogeneous and largely nonporous ingot. In this method, the
diameter of the KIT is freely selectable to a major extent,
offering variable selection of ingot diameters. Withdrawal rates
will preferably be in a range of 0 to 50 mm/min.
[0072] The products manufactured according to the invention can be
used for various purposes. Primarily, semi-finished products are
made from them in a first step of transformation (extrusion), which
are then used for being further worked in the transformation route
(forging, rolling). Ingots of high structural and chemical quality
are needed for the production of .gamma.-TiAl-based components via
the transformation route. These components may for instance be
valves and turbine blades which must comply with a demand for
excellent quality and highest requirements.
[0073] Furthermore, the products according to the invention may
also serve as remelter stocks for the manufacture of cast blanks by
precision casting and centrifugal casting. Remelter stocks are
needed as starting material for the precision and centrifugal
casting route. Chemical and structural quality does not
predominate, because the material is melted once again--as opposed
to ingots. Therefore, step (ii) can be omitted in the method
according to the invention and the pressed electrodes can directly
be melted inductively or, respectively, pre-mixed compacts can be
melted by the PACHM method. The precision casting route serves for
the production of components of complicated design and complex
requirements. The .gamma.-TiAl-based turbo charger, which has been
commercialized, is mentioned here by way of example. Centrifugal
casting is a method of manufacturing at a low cost mass-produced
components (for example valves) of simple design and requirements.
Producing remelter stocks by the method according to the invention
results in products that are distinctly more homogeneous than
corresponding prior art products and, owing to the ingot
withdrawal, can be manufactured in any cylindrical dimensioning,
whereas the method used so far depends on the dimensions of the
available mold. The method according to the invention enables the
diameter and length of the remelter stocks to be freely selected,
which is a simple way of making direct account of customer
demand.
[0074] The ensuing examples of concrete embodiments of the
invention are given by way of explanation.
EXAMPLE 1 (see FIG. 4)
[0075] The example explains the production of a continuously cast
ingot of a .gamma.-TiAl-based alloy of a composition of Ti-46.5Al-4
(Cr,Nb,Ta,B) (indicated in atomic percent) with a diameter of 180
mm and a length of 2,600 mm.
[0076] The first step consists in the production of four
once-VAR-melted electrodes of a diameter of 150 mm and a length of
1,000 mm from pressed electrodes that contain all the alloying
constituents in the form of Ti sponge, Al granules and suitable
pre-alloys for Cr, Nb, Ta and B. The rods, which are not yet
homogeneous, serve as electrodes for the manufacture of
pre-homogenized, molten material by inductive melting in an HF
coil. The electrodes are conical at the foot, the set angle being
approximately 45.degree..
[0077] Upon inductive melting, an electrode is supplied from the
depot that holds all the four electrodes to the HF melting coil of
likewise conical design, and inductively melted. The melt
originates on the entire surface of the cone, at the tip of the
cone collecting in a melt stream in which the material is
pre-homogenized. By the force of gravity, the melt arrives in the
cold wall induction crucible which is located below the melting
coil. The frequency at the outer oscillating circuit of the melting
coil is 80.6 kHz. Uniform pre-heating of the dropping electrode by
inductive heating (mean frequency approximately 500 Hz to 1 kHz) by
way of an auxiliary coil, which is mounted above the melting coil,
to temperatures below the melting point of the alloy (approximately
1300.degree. C.) helps obtain increased melting capacity of more
than 50 kg/h. The electrode is rotated at a speed of 4 rpm, the
withdrawal speed is approximately 12 mm/min.
[0078] The pre-homogenized molten material drops into a cold wall
induction crucible with a bottom that can be drawn off downwards.
The diameter of the crucible is 180 mm. The melt solidifies in the
bottom area of the crucible and is continuously withdrawn
downwards. Cooling of the melt upon ingot withdrawal takes place by
means of water-cooled copper segments. The withdrawal rate amounts
to approximately 1 mm/min. The average dwell time of the melt for
homogenization in the cold wall induction crucible is approximately
20 min, which corresponds to a bath height of approximately 160 mm.
The bath temperature is in the range of 1580.degree. C. and the
frequency of the induction coil that surrounds the crucible amounts
to 12 kHz.
[0079] Once the first electrode has melted off, the second
electrode is moved into the required position and heated for
melting, billet withdrawal being interrupted for this period. Then
the process is continued as specified until all the four electrodes
of the depot have melted off.
[0080] Vacuum as well as protective gas execution of the method is
conceivable.
[0081] The ingot obtained has a diameter of approximately 180 mm
and a total length of 2,600 mm and excels by excellent chemical and
structural homogeneity. Local aluminum and titanium fluctuations
are less than .+-.0.5 atomic percent, those of the elements Cr, Nb
and Ta being less than .+-.0.2 atomic percent and the fluctuation
of B being less than .+-.0.05 atomic percent.
EXAMPLE 2 (see FIG. 5)
[0082] Example 2 differs from Example 1 by the kind and way of
production of the molten material and supply to the KIT. The
process is carried out under He protective gas. The PACHM process
(plasma arc cold hearth melting) offers an alternative of inductive
melting. In the present embodiment, the starting material in the
form of once-VAR-melted electrodes corresponding to Example 1 is
melted by an He plasma torch (150 kW) in a water-cooled copper
crucible and led on via a water-cooled channel which is equally
heated by an He plasma torch (150 kW). The amperage of the plasma
torches above the cold hearth is approximately 500 A. The liquid
alloying melt flows in the skull of its proper material towards an
overflow above the KIT, from where it flows continuously into the
KIT. The starting material is continuously re-charged via a
hydraulically triggered slope. The cold crucible has two principal
functions: in addition to working as a reservoir for
pre-homogenized molten material, it serves as a place of deposit of
undesired high-density and ceramic inclusions.
[0083] The process continues analogously to Example 1.
[0084] The technical data given in the Examples do not restrict the
scope of the invention in any way. In particular the number, type
and capacity of the plasma torches, the cold crucible material,
capacity and frequency ranges of the induction coils, diameters of
the KIT, bath height of the melts in the KIT and feed and
withdrawal rates can be varied within the scope of the prior art
without any negative effect on the invention.
* * * * *