U.S. patent number 3,902,544 [Application Number 05/487,030] was granted by the patent office on 1975-09-02 for continuous process for forming an alloy containing non-dendritic primary solids.
This patent grant is currently assigned to Massachusetts Institute of Technology. Invention is credited to Merton C. Flemings, Robert Mehrabian, Rodney G. Riek.
United States Patent |
3,902,544 |
Flemings , et al. |
September 2, 1975 |
Continuous process for forming an alloy containing non-dendritic
primary solids
Abstract
A process is provided for continuously forming a homogeneous
mixture of a liquid-solid metal composition, wherein said solid
comprises discrete degenerate dendrites or nodules, from a first
metal composition which, when frozen from its liquid state without
agitation forms a dendritic structure. The first metal composition
is maintained molten in a first zone and then is passed into at
least one agitation zone connected to said first zone. The first
zone and agitation zone are sealed to prevent entrainment of gas
into said agitation zone. In the agitation zone, the metal is
vigorously agitated and cooled to solidify a portion thereof and to
form primary solids comprising discrete degenerate dendrites or
nodules while preventing the formation of interconnected dendritic
networks. The primary solids comprise up to about 65 weight percent
of the liquid-solid metal composition. The liquid-solid metal
composition is removed from the agitation zone at about the same
rate that the melted first metal composition is passed into said
agitation zone. The metal removed from the agitation zone then can
be cast.
Inventors: |
Flemings; Merton C. (Lexington,
MA), Mehrabian; Robert (Arlington, MA), Riek; Rodney
G. (Stoneham, MA) |
Assignee: |
Massachusetts Institute of
Technology (Cambridge, MA)
|
Family
ID: |
23934121 |
Appl.
No.: |
05/487,030 |
Filed: |
July 10, 1974 |
Current U.S.
Class: |
164/485; 164/122;
164/437; 164/900; 420/590 |
Current CPC
Class: |
C22C
1/005 (20130101); Y10S 164/90 (20130101) |
Current International
Class: |
C22C
1/00 (20060101); B22d 027/08 () |
Field of
Search: |
;164/71,77,122,123,133,82,281 ;266/34A,38 ;23/295,31SP |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Husar; Francis S.
Assistant Examiner: Hampilos; Gus T.
Attorney, Agent or Firm: Smith, Jr.; Arthur A. Santa; Martin
M. Lorusso; Anthony M.
Government Interests
The invention herein described was made in the course of work
performed under a contract with the Department of the Army.
Claims
We claim:
1. The method for forming a homogeneous mixture of a liquid-solid
metal composition, wherein said solid comprises discrete degenerate
dendrites or nodules, from a first metal composition which, when
frozen from its liquid state without agitation forms a dendritic
structure, which comprises heating said first metal composition to
melt said first metal composition in a first zone, passing the
melted first metal composition into at least one agitation zone
connected to said first zone, said first zone and agitation zone
being sealed to prevent entrainment of gas into said agitation
zone, vigorously agitating and cooling the melted first metal
composition to solidify a portion thereof and to form primary
solids comprising discrete degenerate dendrites or nodules while
preventing the formation of interconnected dendritic networks in
said agitation zone, said primary solids comprising up to about 65
weight percent of the liquid-solid metal composition and removing
said liquid-solid metal composition from said agitation zone at
about the same rate that the melted first metal composition is
passed into said agitation zone.
2. The method of claim 1 wherein the heated metal composition is
cooled to form between 10 and 55 weight percent primary solids.
3. The method of claim 1 wherein the metal composition removed from
the agitation zone is cooled to solidify the liquid remaining after
the primary solids are formed.
4. The method of claim 3 wherein the heated metal composition is
cooled to form between 10 and 55 weight percent primary solids.
5. The method of shaping a homogeneous mixture of a liquid-solid
metal composition, wherein said solid comprises discrete degenerate
dendrites or nodules, from a first metal composition which, when
frozen from its liquid state without agitation forms a dendritic
structure, which comprises heating said first metal composition to
melt said first metal composition in a first zone, passing the
melted first metal composition into at least one agitation zone
connected to said first zone, said first zone and agitation zone
being sealed to prevent entrainment of gas into said agitation
zone, vigorously agitating and cooling the melted first metal
composition to solidify a portion thereof and to form primary
solids comprising discrete degenerate dendrites or nodules while
preventing the formation of interconnected dendritic networks in
said agitation zone, said primary solids comprising up to about 65
weight percent of the liquid-solid metal composition, removing said
liquid-solid metal composition from said agitation zone at about
the same rate that the melted first metal composition is passed
into said agitation zone and casting the liquid-solid mixture
removed from the agitation zone.
6. The method claim 5 wherein the heated metal composition is
cooled to form between 10 and 55 weight percent primary solids
prior to being cast.
7. The method of claim 5 wherein the mixture removed from the
agitation zone is held in a nonagitated state so that it exhibits
thixotropic properties and is liquid-solid in form, and applying
force to the thixotropic solid composition, thereby transforming it
into a material having properties more nearly that of a liquid to
cast said material.
8. The method of claim 5 wherein the liquid-solid mixture is
removed continuously from the agitation zone and is passed
continuously through a cooling zone to solidify liquid in said
mixture wherein said mixture and the solid obtained by solidifying
said mixture is removed continuously from said cooling zone.
9. The method of claim 6 wherein the liquid-solid mixture is
removed continuously from the agitation zone and is passed
continuously through a cooling zone to solidify liquid in said
mixture wherein said mixture and the solid obtained by solidifying
said mixture is removed continuously from said cooling zone.
10. The method of claim 5 wherein an amount of the liquid-solid
metal composition corresponding to about the amount of said
composition to be shaped is removed from the agitation zone and
placed in a holding chamber adapted to maintain said composition in
a liquid-solid state prior to casting said composition.
11. The method of claim 10 wherein the composition in the holding
chamber is cooled to form a solid and said solid is reheated to a
temperature wherein the composition is thixotropic or a
liquid-solid mixture prior to casting said composition.
12. The method of claim 6 wherein an amount of the liquid-solid
metal composition corresponding to about the amount of said
composition to be shaped is removed from the agitation zone and
placed in a holding chamber adapted to maintain said composition in
a liquid-solid state prior to casting said composition.
13. The method of claim 12 wherein the composition in the holding
chamber is cooled to form a solid and said solid is reheated to a
temperature wherein the composition is thixotropic or a
liquid-solid mixture prior to casting said composition.
14. The method of claim 10 wherein the liquid-solid composition is
removed from the holding chamber and is die cast.
15. The method of claim 11 wherein the reheated composition is
removed from the holding chamber and is die cast.
16. The method of claim 12 wherein the liquid-solid composition is
removed from the holding chamber and is die cast.
17. The method of claim 13 wherein the reheated composition is
removed from the holding chamber and is die cast.
18. The method of claim 10 wherein the metal composition removed
from the agitation zone is placed in a heat-resistant sleeve within
said holding chamber, removing said sleeve and metal composition
from the holding chamber and die casting said composition.
19. The method of claim 18 wherein the composition in the holding
chamber is cooled to form a solid and said solid is reheated to a
temperature wherein the composition is thixotropic or a
liquid-solid mixture prior to casting said composition.
20. The method of claim 6 wherein the metal composition removed
from the agitation zone is placed in a heat-resistant sleeve within
a holding chamber, removing said sleeve and metal composition from
the holding chamber and die casting said composition.
21. The method of claim 20 wherein the composition in the holding
chamber is cooled to form a solid and said solid is reheated to a
temperature wherein the composition is thixotropic or a
liquid-solid mixture prior to casting said composition.
Description
The present invention relates to a continuous process for making a
solid or a liquid-solid metal composition containing non-dendritic
primary solids and to the processes for shaping such
compositions.
Prior to the present invention, alloys have been prepared
containing non-dendritic primary solids by a batch method as
disclosed in copending application Ser. No. 379,991, filed July 17,
1973 in the names of Robert Mehrabian, Merton C. Flemings, and
David B. Spencer. As disclosed in that application, a metal alloy
composition is formed by heating an alloy to a temperature at which
most or all of it is in the liquid state and vigorously agitating
the alloy, usually while cooling the alloy, to convert any solid
particles therein to degenerate dendrites or nodules having a
generally spheroidal shape. The degree of agitation must be
sufficient to prevent the formation of interconnected dendritic
networks and to substantially eliminate or reduce dendritic
branches already formed within the alloy due to cooling. After the
primary solids have been formed, the liquid remaining in the alloy
composition can be allowed to cool to form a dendritic solid
surrounding the primary solids.
It has been found that the compositions formed by the process of
the invention described in the copending application provide
substantial advantages in casting methods as compared to the prior
existing casting methods in which a molten metal is poured or
forced into a mold. A number of problems exist when casting molten
alloy including the fact that when the liquid changes to the solid
state, metal shrinkage normally is encountered and the cooling
process is fairly long. Furthermore, a large number of liquid
alloys are highly errosive to dies and molds and the high
temperature of the liquids and their errosive characteristics make
difficult or impossible the casting of such alloys as copper or
iron alloys. By casting a liquid-solid slurry containing the
non-dendritic primary solids, the severity of these problems is
substantially reduced or eliminated since the casting is contacted
with a metal composition having a relatively low temperature
thereby reducing the errosive problem, cooling times and metal
shrinkage.
The processes specifically described in the above-identified
application produce products which have substantial advantages over
the prior art. However, the processes specifically disclosed in
that application are batch-type processes and have some
disadvantages as compared to the continuous process disclosed
specifically herein. In these batch processes, the entire
liquid-solid composition is subjected to vigorous agitation
including a top surface of the composition which is in direct
contact with a gaseous surrounding atmosphere. Due to the vigorous
agitation, there is some gas entrainment into the composition being
formed which is undesirable since the entrapped gas may adversely
affect articles which are formed therefrom. In addition, the batch
technique generally is slow and temperature control generally is
more difficult.
SUMMARY OF THE INVENTION
The present invention provides a process for forming a metal
composition containing degenerate dendritic primary solid particles
homogeneously suspended in a secondary phase having a lower melting
point than the primary solids and having a different metal
composition than the primary solids wherein both the secondary
phase and the solid particles are derived from the same alloy. The
present invention is based upon the discovery that these
compositions can be formed continuously or semi-continuously be
vigorously agitating a solid-liquid mixture, which mixture is
separated from any gaseous atmosphere by an alloy, in the molten
state from which the liquid-solid mixture is derived. It has been
discovered that by operating in this manner, the molten alloy can
be directed continuously to an agitation zone which is maintained
at a temperature at which the alloy becomes partially solid without
entrainment of gas and in a manner such that control of the portion
solid in the agitation zone can be maintained easily. The
liquid-primary solid mixture then is passed from the agitation zone
at about the same rate as the liquid entering the agitation zone
which can be continuous or semicontinuous. The mixture can be cast
or passed through a forming zone adjacent the agitation zone. The
resultant composition can exit from the forming zone either as 100
percent solid or as a liquid-solid mixture. In either case, the
composition comprises non-dendritic primary solids homogeneously
dispersed in a second phase, which second phase can either be solid
or liquid. When the secondary phase is liquid, the compositions
so-formed can be allowed to cool or can be formed such as be
casting. When the final product is entirely solid, it can be formed
at a later time merely by heating it back to the liquid-solid
temperature range wherein it may be thixotropic and rendered
formable when subjected to shear forces.
DESCRIPTION OF SPECIFIC EMBODIMENTS
By the term "primary solid" as used herein is meant the phase or
phases solidified to form discrete degenerate dendrite particles as
the temperature of the melt is reduced below the liquidus
temperature of the alloy into the liquid-solid temperature range
prior to casting the liquid-solid slurry formed. By the term
"secondary solid" as used herein is meant the phase or phases that
solidify from the liquid existing in the slurry at a lower
temperature than that at which the primary solid particles are
formed after agitation ceases. The primary solids obtained in the
compositions prepared by the process of this invention differ from
normal dendrite structures in that they comprise discrete particles
suspended in the remaining liquid matrix. Normally solidified
alloys, in absence of agitation, have branched dendrites separated
from each other in the early stages of solidification i.e. up to 15
to 20 wt. percent solid, and develop into an interconnected network
as the temperature is reduced and the weight fraction solid
increases. The structure of the compositions prepared by the
process of this invention on the other hand prevents formation of
the interconnected network by maintaining the discrete primary
particles separated from each other by the liquid matrix even up to
solid fractions of 60 to 65 wt. percent. The primary solids are
degenerate dendrites in that they are characterized by having
smoother surfaces and less branched structures which approaches a
spherical configuration than normal dendrites and may have a
quasi-dendritic structure on their surfaces but not to such an
extent that interconnection of the particles is effected to form a
network dendritic structure. The primary particles may or may not
contain liquid entrapped within the particles during particle
solidification depending upon severity of agitation and the period
of time the particles are retained in the liquid-solid range.
However, the weight fraction of entrapped liquid is less than that
existing in a normally solidified alloy at the same temperature
employed in present processes to obtain the same weight fraction
solid.
The secondary solid which is formed during solidification from the
liquid matrix subsequent to forming the primary solid contains one
or more phases of the type which would be obtained during
solidification of a liquid alloy of identical composition by
presently employed casting processes not employing vigorous
agitation. That is, the secondary solid can comprise dendrites,
single or multiphase compounds, solid solutions, or mixtures of
dendrites, compounds and/or solid solutions.
The size of the primary particles depends upon the alloy or metal
compositions employed, the temperature of the solid-liquid mixture
and the degree of agitation employed with larger particles being
formed at lower temperature and when using less severe agitation.
Thus, the size of the primary particles can range from about 1 to
about 10,000 microns. It is preferred that the composition contain
between about 10 and 55 wt. percent primary particles since they
have a viscosity which promotes ease of casting or forming without
causing heat damage to the forming or casting apparatus.
As employed herein, the terms "agitation" or "vigorous agitation"
as applied to the process of this invention mean that the
liquid-solid composition is subjected to an agitation force
sufficient to prevent the formation of interconnected dendritic
networks and to substantially eliminate or reduce dendritic
branches already formed on the primary solid particles.
In accordance with this invention, a metal alloy is rendered molten
in a first zone which is in communication with an agitation zone.
The agitation zone is connected to the first zone and is sealed to
prevent entrainment of gas into the metal composition therein. The
agitation zone is provided with means for cooling the metal
composition therein and for vigorously agitating the metal
composition therein. The degree of agitation in the agitation zone
must be sufficient to prevent the formation of interconnected
dendritic networks from the metal composition while it is cooled.
The particular means employed for providing the degree of agitation
is not critical so long as the interconnected dendritic networks
are not formed and the primary solids are formed while the metal
composition therein is cooled. The primary solids content of the
metal composition in the agitation zone can comprise up to about 65
wt. percent of the liquid-solid metal composition. The liquid-solid
metal composition is removed from the agitation zone through an
outlet at about the same rate as the molten composition is passed
into the agitation zone. The liquid-solid metal composition can be
cooled to form a solid which can be subsequently reheated to a
liquid-solid range for subsequent forming or casting at any time or
the liquid-solid composition can be cast upon removal from the
agitation zone. It is not critical to this invention that a
particular mode of casting be employed. However, the continuous
embodiment of the process of this invention affords casting
techniques not available in the prior art since the liquid-solid
mixtures continuously produced herewith have a degree of structural
strength which is not a characteristic of molten metal. This degree
of structural strength affords the use of unique means for
transporting and subsequently forming of the liquid-solid mixtures.
The casting techniques afforded by this invention will be described
below in greater detail.
Any metal alloy system or pure metal regardless of its chemical
composition can be employed in the process of this invention. Even
though pure metals and eutectics melt at a single temperature, they
can be employed since they can exist in liquid-solid equilibrium at
the melting point by controlling the net heat input or output to
the melt so that, at the melting point, the pure metal or eutectic
contains sufficient heat to fuse only a portion of the metal or
eutectic liquid. This occurs since complete removal of heat of
fusion in a slurry employed in the casting process of this
invention cannot be obtained instantaneously due to the size of the
casting normally used and the desired composition is obtained by
equating the thermal energy supplied, for example by vigorous
agitation and that removed by a cooler surrounding environment.
Representative suitable alloys include magnesium alloys, zinc
alloys, aluminum alloys, copper alloys, iron alloys, nickel alloys,
cobalt alloys and lead alloys such as lead-tin alloys,
zinc-aluminum alloys, zinc-copper alloys, magnesium-aluminum
alloys, magnesium-aluminum-zinc alloys, magnesium-zinc alloys,
aluminum-copper alloys, aluminum-silicon alloys,
aluminum-copper-zinc-magnesium alloys, copper-tin bronzes, brass,
aluminum bronzes, steels, cast irons, tool steels, stainless
steels, superalloys such as nickel-iron alloys,
nickel-iron-cobalt-chromium alloys, and cobalt-chromium alloys, or
pure metals such as iron, copper or aluminum.
This invention will now be discussed upon reference to accompanying
drawings, in which:
FIG. 1 is an elevational view of an apparatus having three
agitation zones useful for effecting the process of this
invention.
FIG. 2 is an elevational view of an apparatus having one agitation
zone.
FIG. 3 is a cross-sectional view of the apparatus of FIG. 3 taken
along line 2--2.
FIG. 4 is a reproduction of a photomicrograph showing the structure
of a copper-10 percent tin-2 percent zinc casting made employing
the teaching of this invention.
FIG. 5 is a reproduction of a photomicrograph showing the structure
of a tin-15 percent lead casting made employing the teaching of
this invention.
FIG. 6 is a reproduction of a photomicrograph showing the structure
of a cast iron casting containing 2.48 percent carbon and 3.12
percent silicon.
FIG. 7 is an elevational view showing a means for continuously
casting the liquid-solid composition obtained by the process of
this invention.
FIG. 8 is a schematic view of a means for casting discrete portions
of the liquid-solid composition obtained by the process of this
invention.
FIG. 9 is a schematic view of an alternative means for casting
discrete portions of the liquid-solid composition obtained by the
process of this invention.
Referring to FIG. 1, a metal alloy in the liquid state 1 is
retained within container 2. The metal alloy 1 can be heated
conveniently to the liquidus state or maintained at or above the
liquidus temperature by means of induction heating coils 3 which
surround container 2. Container 2 is provided with three openings
4, 5 and 6 the size of which are regulated by baffles 7, 8 and 9.
Agitation zones 10, 11 and 12 are located adjacent each opening 4,
5 and 6 respectively and are joined to the bottom surface of
container 2 in a manner to prevent gas from becoming admixed with
the metal alloy within either container 2 or agitation zones 10, 11
and 12. Augers 16, 17 and 18 are provided within agitation zones
10, 11 and 12 respectively and are mounted to rotatable shafts 20,
21 and 22 which are powered by any suitable means (not shown). Each
of the agitation zones 10, 11 and 12 is provided with induction
heating coils 25, 26 and 27 and with a cooling jacket 28, 29 and 32
in order to control the amount of heat and the temperature of the
metal alloy within the agitation zones 10, 11 and 12. Each cooling
jacket is provided with a fluid inlet 30 and a fluid outlet 31. The
distance between the inner surface 35 of agitation zone 12 and the
outer surface 36 of auger 18 as well as the corresponding distances
between surfaces 37 and 38 and surfaces 39 and 40 are maintained
sufficiently small so that high shear forces can be applied to a
liquid-solid mixture in the agitation zones 10, 11 and 12
sufficient to prevent the formation of interconnected dendritic
networks while at the same time allowing passage of the
liquid-solid mixture through the respective agitation zones 10, 11
and 12. Since the induced rate of shear in the liquid-solid mixture
at a given rotational speed of the auger is a function of both the
radius of the agitation zone and the radius of the auger, the
clearance distance will vary with the size of the auger and
agitation zone. To induce the necessary shear rates, increased
clearances can be employed with larger augers and agitation zones.
The bottom surface of agitation zones 10, 11 or 12 are each
provided with an opening 40, 41 or 42 respectively so that the
liquid-solid mixture in the agitation zones can be removed
conveniently by gravity or, if desired by establishing a pressure
differential between the upper surface of molten metal 1 and
openings 40, 41 and 42. The effective opening 40, 41 or 42 can be
controlled easily be raising or lowering the shaft 20, 21 or 22 so
that the bottom end of the auger 44, 45 or 46 can fit into all or a
portion of the respective openings 40, 41 or 42.
The operation of the apparatus shown in FIG. 1 will be described
with respect to one auger shown therein. A metal alloy is
introduced into container 2 wholly molten, partially solid or or
wholly solid. In any event, the metal alloy is rendered molten in
container 2 by heat induction coils 3, if necessary. After the
molten alloy is formed, the baffles 7 are opened to admit the
molten alloy into agitation zone 10. The baffles 7 also minimize
migration of primary solids from the agitation zone 10 into the
container 2. Meanwhile, rotation of shaft 20 and auger 16 is
initiated, for example at a rotational speed between about 100 and
about 1000 r.p.m. The heat in agitation zone 10 is removed
therefrom by heat exchange with a fluid, such as air or water, that
enters jacket 28 through inlet 30 and exits through outlet 31. Heat
induction coils 25 are provided for process control in the event
the metal composition in agitation zone 10 is cooled to a fraction
solid content above that desired, e.g. above about 65 weight
percent. The molten metal 1 is passed continuously through the
opening 4 into the agitation zone 10 wherein the desired quantity
of heat content of the alloy is removed to render it partially
solid and partially liquid wherein the solid portion comprises
primary solids. The rate at which the liquid-solid mixture exits
agitation zone 10 depends upon the effective opening in hole 40
controlled by the position of the end 44 of the auger 16. The heat
exchange within agitation zone 10 can be controlled easily by
controlling the flow rate and temperature of the cooling fluid in
jacket 28, controlling the power input in induction coils 3, and
the flow rate of metal through agitation zone 10 by controlling the
size of the opening 4 baffles 7 and the size of the opening 40 with
the end 44 of auger 16. Thermocouples (not shown) can be provided
along the length of agitation zone 10 and at the end thereof in
order to monitor the temperature of the liquid-solid mixture within
agitation zone 10. By operating in this manner, the molten metal in
zone 2 serves to seal the liquid-solid mixture within zone 10 from
the outside gaseous atmosphere thereby preventing undesirable
random entrainment of gas within the liquid-solid mixture in zone
10.
Referring to FIGS. 2 and 3, an alternative apparatus design is
shown. Molten metal 50 is maintained in the heated zone 51 having
an opening 52 in the bottom surface thereof. A rotatable shaft 53
extends through heated zone 51 and into an agitation zone 54
wherein an auger 55 is located. The auger 55 has splines 56
extending the length of the auger 55 through the agitation zone 54,
determined by the outer auger diameter 67 and the inner wall
diameter 62. The agitation zone 54 is surrounded by a cooling
jacket 58 having an inlet 59 and an outlet 60. In addition, the
agitation zone 54 is surrounded by heat induction coils 61 so that
the cooling jacket in combination with the induction coils 61 serve
to regulate the heat outflow from the metal alloy composition
within agitation zone 54. As best shown in FIG. 3, typical useful
sizes of the mixing apparatus comprises an agitation zone with a
diameter of 1 1/4 inches, an auger having a diameter of 1 to 1 1/8
inches and grooves between the splines of 1/16 inch. It is to be
understood that these dimensions are only exemplary and that larger
or smaller sizes can be employed so long as a high shear rate on
the metal can be maintained. The size of opening 52 can be
regulated by moving the rotating shaft 53 and auger 55 vertically
to open or close opening 52 with baffle 63 located on shaft 53. The
heating zone 51 is surrounded by heat induction coils 64 to provide
heat to the metal composition 50 therein. The agitation zone 54 is
provided with an outlet 66 for removal of the liquid-solid
composition containing the primary solid therefrom for subsequent
forming.
FIG. 4 is a reproduction of a photomicrograph taken at 50 times
magnification of an oil-quenched copper-10 percent tin-2 percent
zinc alloy (copper alloy 905). This alloy was formed with the
apparatus shown in FIGS. 2 and 3 but with only one auger. The
temperature in heating zone 51 was maintained above the liquidus
temperature of the alloy, i.e. 999.degree.C. The conditions of
temperature and heat in the agitation zone 54 were maintained so
that the liquid-primary solid mixture contained about 45 weight
percent primary solids. The sample was taken at about 925.degree.C.
The spheroidal primary solid metal formations 70 and dendritic
secondary solids 71 show an overall metal formation quite different
from the normal dendritic network observed upon cooling this alloy
without agitation. The black portion 72 of the primary solids 70
comprise liquid which was entrapped within the primary solids
during their formation.
FIG. 5 is a photomicrograph taken at 100 times magnification of a
casting made of 10-15 percent lead which was agitated in the
apparatus of FIG. 1 but employing only one auger so that the
liquid-primary solid mixture contained about 55 weight percent
primary solids. The sample was taken at about 191.degree.C. As can
be readily observed, the non-dendritic primary solids 73 are
surrounded by a secondary solid portion 74 which is dendritic in
nature.
FIG. 6 is a reproduction of a photomicrograph taken at 100 times
magnification of cast iron containing 2.48 percent carbon and 3.12
percent silicon. The alloy was formed with the apparatus shown in
FIGS. 2 and 3. The condition of temperature and heat were
maintained so that the liquid-primary solid mixture contained about
35 weight percent primary solids. The sample was taken at about
1280.degree.C. The spheroidal primary solid metal formations 75 are
surrounded by dendritic secondary solids 71a. The black portion 72a
of the primary solids 75 is entrapped graphite which precipitated
during cooling while the darker grey portion 73a comprises liquid
which was entrapped within the primary solids during their
formation.
Referring to FIG. 7, a convenient means for continuously casting
the liquid-solid compositions formed by the process of this
invention is shown. The process shown in this figure provides
substantial advantage over continuous casting processes of the
prior art which continuously cast molten metal alloys. Due to the
heat of fusion in the molten metals and their higher temperature,
than the liquid-primary solid compositions, they must be rendered
solid by heat extraction therefrom at a lower rate than with the
liquid-primary solid compositions. If the heat is extracted too
quickly from the molten metals, undesirable cracking of the cast
product is observed frequently. This results in an undesirably
lower throughput rate of metal in the continuous casting apparatus.
In addition, undesirable long range segregation (macrosegregation
of the alloy constitutents) is obtained when continuously casting
molten metals. In contrast, when continuously casting the
liquid-primary solid compositions of the present process, there is
far less heat of fusion available therein which must be removed and
therefore much faster throughput rates can be attained without
metal cracking. Furthermore, due to the presence of the primary
solids, long range segregation is minimized or eliminated. The
liquid-solid mixture 76 exiting from the agitation zone 10 is
directed to a cooling zone formed by generally cylindrical cooling
jacket 77 provided with a cooling fluid outlet 78 and a cooling
fluid inlet 79. The agitation zone 10 is constructed and operated
in the manner described above such as described with reference to
FIG. 1 or FIGS. 2 and 3. The final rod-shaped or cylindrical-shaped
solid product 80 containing the primary solids homogeneously
dispersed therein is formed by initially providing a plate along
the bottom surface of the jacket 77, as indicated by the dotted
line 81 in order to initially form a solid within the jacket 77.
After the solid is formed, the plate is removed and the solid 80
allowed to move by gravity out of the casing 77. Once this process
has been initiated, and interface between the solid 80 and the
liquid-solid mixture 82 as indicated by line 83 is formed.
Subsequent to formation within the casing 77, the solid 80 is
directly subjected to a spray of cooling liquid as indicated by the
arrow 84.
Referring to FIG. 8, an alternative means for collecting and
subsequently forming e.g. casting, the products formed by the
process of this invention is shown schematically. This process can
be used on a batchwise or a continuous basis to form discrete
portion of the liquid-primary solid mixture. At or near the exit 40
of the agitation zone, a holding chamber 90 provided with a heating
means such as induction heated coils 91 is provided. The agitation
zone is constructed and operated in the manner described above such
as described with reference to FIG. 1 or FIGS. 2 and 3. Within the
holding chamber 90, a generally cylindrical sleeve 92 formed from a
heat resistant material is placed to retain a discrete portion of
the liquid-primary solid mixture. A discrete portion of the
liquid-solid mixture exiting from opening 40 is directed into
sleeve 92 as composition 93. In order to maintain the desired
fraction solid in the alloy 93, the heating coils 91 are activated
in order to maintain the desired temperature. Once the desired
amount of metal 93 has been metered into the sleeve 92, it can be
formed or cast in any manner desired. Thus, this apparatus provides
a convenient means for metering a desired amount of metal which is
easily transportable to a subsequent process. For example when, it
is desired to form or cast the composition 93, the sleeve 92 and
the holding chamber 90 are rotated 90.degree. so that the sleeve 92
can be removed from holding chamber 90 easily while retaining the
composition 93 therein. Because of the mechanical characteristics
of the liquid-primary solid mixtures formed by the process of this
invention, the use of a sleeve 92 eliminates the need for a shot
sleeve normally employed in casting and therefore eliminates the
problems associated with shot sleeves which result from the need to
avoid undue temperature gradients in the metal contained in the
shot sleeve. The liquid-primary solid mixture is sufficiently
mechanically stable so that when sleeve 92 is removed from holding
chamber 90, the liquid-primary solid mixture is removed therewith
without substantial leakage. Furthermore, when sleeve 92 is placed
in a horizontal position so that the open ends thereof are
unsupported, the liquid-primary solid mixture will not leak
therefrom. The sleeve 92 and composition 93 are then located
between a mold 95 and a pneumatically actuated piston 96 housed
within a piston guide 97. The piston 96 can be pneumatically
actuated at the desired time, for example by means of air cylinder
98. When actuated, the piston 96 forces the composition 93 within
the interior 99 of mold 95 to form the desired product. In one
embodiment, a plurality of holding chambers 90 and associated
sleeve 92 can be located below the agitation zone exit 66 on a
support table (not shown) and they can be progressively indexed
below the exit 66 for filling and processed for subsequent casting
of the metal therein.
Referring to FIG. 9, an alternative means for casting the
compositions formed by the process of this invention is shown
schematically. This particular means can be employed batchwise or
on a continuous basis to form discrete portions of liquid-primary
solid mixtures formed by the process of this invention. As shown in
FIG. 9, as liquid-primary solid mixture 100 is exited from the
opening 54 of the agitation zone (not shown). Portions of the
liquid-solid mixture 101 would then break off on the main portion
100 by virtue of the gravitational forces thereon and allowed to
fall between the die halves 102 and 103. When portion 101 is
located between die halves 102 and 103, the die halves 102 and 103
are closed on the composition 101 by actuating the pistons 104 and
105 pneumatically. The pistons 104 and 105 can be actuated by any
suitable electronic means such as a photosensitive detector through
which the portion 101 passes prior to being positioned between the
die halves 102 and 103. After the composition 101 has been formed
by cooling, the die halves 102 and 103 are pulled apart and the
desired product formed from composition 101 is removed therefrom. A
plurality of die halves similar to 102 and 103 can be passed under
opening 54 continuously to capture subsequently formed discrete
compositions 101 and to cast them in the manner described.
The liquid-solid mixture can, when the desired ratio of
liquid-solid has been reached, be cooled rapidly to form a solid
slug for easy storage. Later, the slug can be raised to the
temperature of the liquid-solid mixture, for the particular ratio
of interest, and then cast, as before, using usual techniques. A
slug prepared according to the procedure just outlined may possess
thixotropic properties depending upon the reheating temperature and
the time it is maintained as a liquid-solid either before the slug
is fully frozen or after the frozen slug is reheated. Increased
time that the slug is maintained as a liquid-solid promotes
thixotropic increased behavior of the slug. It can, thus, be fed
into a modified die casting machine or other apparatus in
apparently solid form. However, shear resulting when this
apparently solid slug is forced into a die cavity causes the slug
to transform to a material whose properties are more nearly that of
a liquid.
Liquid-solid mixtures were prepared employing apparatus like that
shown in FIG. 2 and at speeds of about 500 RPM for the auger.
Temperature control at the exit 66 of agitation zone 54 was
monitored by using a thermocouple. The temperature of the
liquid-solid at 50 percent solid for various alloys is given below:
Sn -- 10% Pb 210.degree.C Sn -- 15% Pb 195.degree.C Al -- 30% Sn
586.degree.C Al --4.5% Cu 633.degree.C
Variations greater or less than the 50 percent primary solid-liquid
mixture will result from changes in the temperature values
given.
Casting of the partially solidified metal slurry or mixture herein
disclosed can be effected by pouring, injection or other means; and
the process disclosed is useful for die casting, permanent mold
casting, continuous casting, closed die forging, hot pressing,
vacuum forming (of that material) and others. The special
properties of these slurries suggest that modifications of existing
casting processes might usefully be employed. By way of
illustration, the effective viscosity of the slurries can be
controlled by controlling fraction of primary solid; the high
viscosities possible when the instant teachings are employed,
result in less metal spraying and air entrapment in die casting and
permits higher metal entrance velocities in this casting process.
Furthermore, more uniform strength and more dense castings result
from the present method.
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